chemical and microbial remediation of hexavalent chromium from contaminated soil and...

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Journal of Hazardous Materials 250–251 (2013) 272–291 Contents lists available at SciVerse ScienceDirect Journal of Hazardous Materials jou rnal h om epa ge: www.elsevier.com/locate/jhazmat Review Chemical and microbial remediation of hexavalent chromium from contaminated soil and mining/metallurgical solid waste: A review B. Dhal a , H.N. Thatoi b , N.N. Das c , B.D. Pandey a,a Metal Extraction & Forming Division, CSIR-National Metallurgical Laboratory, Jamshedpur 831 007, Jharkhand, India b Department of Biotechnology, College of Engineering & Technology, Bhubaneswar 751 003, Orissa, India c Department of Chemistry, North Orissa University, Baripada 757 003, Orissa, India h i g h l i g h t s The review focuses on the Cr speciation, uses, toxicity and remediation aspects. Assessed amount of hexavalent Cr in soils/metallurgical/other wastes and sites. Chemical and bioremediation processes for Cr(VI) are summarised in detail. Strategy for bio-remediation of Cr(VI) in different solid wastes is suggested. Capability of microbes for reducing Cr(VI) contamination described. Mechanism of microbial reduction of Cr(VI) to Cr(III) is included. a r t i c l e i n f o Article history: Received 4 October 2012 Received in revised form 7 January 2013 Accepted 21 January 2013 Available online xxx Keywords: Cr(VI) contaminated soil Chromium toxicity Cr(VI) reduction Cr(VI) resistance Bioremediation a b s t r a c t Chromium is a highly toxic non-essential metal for microorganisms and plants, and its occurrence is rare in nature. Lower to higher chromium containing effluents and solid wastes released by activities such as mining, metal plating, wood preservation, ink manufacture, dyes, pigments, glass and ceramics, tanning and textile industries, and corrosion inhibitors in cooling water, induce pollution and may cause major health hazards. Besides, natural processes (weathering and biochemical) also contribute to the mobility of chromium which enters in to the soil affecting the plant growth and metabolic functions of the living species. Generally, chemical processes are used for Cr- remediation. However, with the inference derived from the diverse Cr-resistance mechanism displayed by microorganisms and the plants including biosorption, diminished accumulation, precipitation, reduction of Cr(VI) to Cr(III), and chromate efflux, bioremediation is emerging as a potential tool to address the problem of Cr(VI) pollution. This review focuses on the chemistry of chromium, its use, and toxicity and mobility in soil, while assessing its concentration in effluents/wastes which becomes the source of pollution. In order to conserve the environment and resources, the chemical/biological remediation processes for Cr(VI) and their efficiency have been summarised in some detail. The interaction of chromium with various microbial/bacterial strains isolated and their reduction capacity towards Cr(VI) are also discussed. © 2013 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 2. Industrial uses of chromium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 3. Transformations of chromium in soil: mobility and bio-availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 3.1. The chromium cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 3.2. Speciation of Cr(VI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 3.3. Oxidation/reduction reactions in soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 4. Chromium toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 Corresponding author. Tel.: +91 657 2345242; fax: +91 657 2345213. E-mail addresses: bd [email protected], [email protected] (B.D. Pandey). 0304-3894/$ see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.01.048

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Page 1: Chemical and microbial remediation of hexavalent chromium from contaminated soil and mining/metallurgical solid waste: A review

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Journal of Hazardous Materials 250– 251 (2013) 272– 291

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous Materials

jou rna l h om epa ge: www.elsev ier .com/ locate / jhazmat

eview

hemical and microbial remediation of hexavalent chromium fromontaminated soil and mining/metallurgical solid waste: A review

. Dhala, H.N. Thatoib, N.N. Dasc, B.D. Pandeya,∗

Metal Extraction & Forming Division, CSIR-National Metallurgical Laboratory, Jamshedpur 831 007, Jharkhand, IndiaDepartment of Biotechnology, College of Engineering & Technology, Bhubaneswar 751 003, Orissa, IndiaDepartment of Chemistry, North Orissa University, Baripada 757 003, Orissa, India

i g h l i g h t s

The review focuses on the Cr speciation, uses, toxicity and remediation aspects.Assessed amount of hexavalent Cr in soils/metallurgical/other wastes and sites.Chemical and bioremediation processes for Cr(VI) are summarised in detail.Strategy for bio-remediation of Cr(VI) in different solid wastes is suggested.Capability of microbes for reducing Cr(VI) contamination described.Mechanism of microbial reduction of Cr(VI) to Cr(III) is included.

r t i c l e i n f o

rticle history:eceived 4 October 2012eceived in revised form 7 January 2013ccepted 21 January 2013vailable online xxx

eywords:r(VI) contaminated soilhromium toxicityr(VI) reduction

a b s t r a c t

Chromium is a highly toxic non-essential metal for microorganisms and plants, and its occurrence israre in nature. Lower to higher chromium containing effluents and solid wastes released by activitiessuch as mining, metal plating, wood preservation, ink manufacture, dyes, pigments, glass and ceramics,tanning and textile industries, and corrosion inhibitors in cooling water, induce pollution and may causemajor health hazards. Besides, natural processes (weathering and biochemical) also contribute to themobility of chromium which enters in to the soil affecting the plant growth and metabolic functionsof the living species. Generally, chemical processes are used for Cr- remediation. However, with theinference derived from the diverse Cr-resistance mechanism displayed by microorganisms and the plantsincluding biosorption, diminished accumulation, precipitation, reduction of Cr(VI) to Cr(III), and chromate

r(VI) resistanceioremediation

efflux, bioremediation is emerging as a potential tool to address the problem of Cr(VI) pollution. Thisreview focuses on the chemistry of chromium, its use, and toxicity and mobility in soil, while assessingits concentration in effluents/wastes which becomes the source of pollution. In order to conserve theenvironment and resources, the chemical/biological remediation processes for Cr(VI) and their efficiencyhave been summarised in some detail. The interaction of chromium with various microbial/bacterialstrains isolated and their reduction capacity towards Cr(VI) are also discussed.

© 2013 Elsevier B.V. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2732. Industrial uses of chromium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2743. Transformations of chromium in soil: mobility and bio-availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274

3.1. The chromium cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274

3.2. Speciation of Cr(VI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.3. Oxidation/reduction reactions in soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4. Chromium toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +91 657 2345242; fax: +91 657 2345213.E-mail addresses: bd [email protected], [email protected] (B.D. Pandey).

304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2013.01.048

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277

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B. Dhal et al. / Journal of Hazardous Materials 250– 251 (2013) 272– 291 273

5. Assessment and chemical treatment of chromium in different solid waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2775.1. Chromite ore processing residue (COPR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2775.2. Leather tannery contaminated soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2805.3. Electroplating sludge and contaminated soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2815.4. Contaminates from metallurgical and industrial waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2815.5. Contamination from mining waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2815.6. Contamination from municipality solid waste and contaminated soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282

6. Biological treatment of chromium in different solid waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2826.1. Chromite ore processing residue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2826.2. Leather tannery contaminated soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2826.3. Electroplating sludge and contaminated soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2846.4. Contaminates from metallurgical and industrial waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2846.5. Contamination from mining waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

7. Bacterial interaction with chromium in soils/solid waste. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2847.1. Bacterial reduction of hexavalent chromium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

8. Cr(VI) bioremediation practices with soil/solid waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2859. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287Appendix A. Supplementary data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

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ments and dyes, wood preservation, is a hazardous contaminant[6], because it readily spreads beyond the site of initial contamina-

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

The heavy metal contamination is a serious problem to thenvironment, because the anthropogenic activities from mining,rocessing and applications of these metals have increased enor-ously during the past few decades and have become a challenge

or life on earth. Hence their removal/remediation has become allhe more necessary. Over a few years, the discarding solid and/oriquid waste products containing heavy metals emanating fromhe industrial processes has received a lot of attention, and con-equently legislation for the protection of the environment hasradually become more rigid [1,2].

Chromium is one of the most frequently used metal contami-ants and is considered to be one of the top 20 contaminants onhe Superfund priority list of hazardous substances for the past 15ears [3]. Chromium bearing ores are found in many forms, buthe economically extractable form is the mineral chromite. Min-ral chromite in its spinel form is inert and insoluble in water.ts total world mine production in 2010 and 2011 was 23,700 and4,000 × 103 metric tons respectively (gross weight of marketable

6

hromite ore). Total world reserve is estimated at >480 × 10 metricons of shipping grade chromite ore (Table 1) with ∼45% Cr2O3nd major resources are located (×106 tons) in Kazakhstan (220),outhern Africa (200), India (54) and United States (0.62).

able 1orld reserves (shipping grade) and production (ore and concentrate) of chromium

by principal countries) [4].

Country Reserves Production (In ‘000 tons)

2007 2008 2009 2010 2011

World: Total(rounded)

>480,000 23,900 23,600 18,700 23,700 24,000

Indiaa 54,000 4873 3980 3372 3800 3800Kazakhstan 220,000 3687 3552 3333 3830 3900South Africa 200,000 9647 9683 6865 10,900 11,000USA 620 – – – – –Finland – 556 614 247Brazil – 628 700(e) 700(e)Russia – 777 913 416Turkey – 1679 1886 1770Zimbabwe – 614 442 194Other countries NA 1439 1829 1803 5170 5300

ource: World Mineral Production, 2005–2009 and Mineral Commodity Summaries,010, USGS (US Geological Survey), 2012.a Production of chromite in India in 2007–2008, 2008–2009 and 2009–2010 was

.9 million tons, 4.1 million tons and 3.4 million tons, respectively.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

Chromium can exist in several oxidation states, ranging fromCr2+ to Cr6+, but in soils the most stable and common forms aretrivalent, Cr(III) and hexavalent, Cr(VI) species [5], which displayquite different chemical properties and affect organisms in differ-ent ways. Hexavalent chromium is water soluble in the full pHrange, while trivalent chromium tends to be adsorbed on soil sur-face or precipitate as chromium hydroxide in a slightly acidic andalkaline environment. Therefore, Cr(III) has the high potential forenvironmental contamination, especially of aquifers and surfacewater. In its hexavalent form, the U.S. Environmental ProtectionAgency (EPA) has classified chromium as a Group ‘A’ human car-cinogen and is one of the main pollutants (Fig. 1). In both of itsprevalent forms, trivalent and hexavalent chromium it can causeallergic contact dermatitis.

Worldwide chromate, which is the most prevalent form of Cr(VI)present in solid/liquid waste due to human activities, such as elec-troplating, steel and automobile manufacturing, mining, leathertanning, cement, metal processing, textile, production of paint pig-

tion through aquatic systems and groundwater. The most mobileforms of chromium in soils are CrO4

2− and HCrO4− ions. They

Fig. 1. The frequencies of the most common contaminants at NPL sites [13](Based on data available for 976 National Priorities List (NPL) Sites with fiscal year1982–2003. Records of Decision (RODs). A site may contain one or more of these con-taminants. Source: U.S. EPA, Office of Emergency and Remedial Response, OERCLIS,June 2003).

Page 3: Chemical and microbial remediation of hexavalent chromium from contaminated soil and mining/metallurgical solid waste: A review

274 B. Dhal et al. / Journal of Hazardous Materials 250– 251 (2013) 272– 291

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an be taken up by plants and easily be leached out into theeeper soil layers, leading to ground and surface water pollution7]. Hence, to reduce/remove Cr(VI), industrial solid waste/sludgend soil are treated by various physical/chemical methods such aseduction, precipitation, adsorption, ion exchange, reverse osmo-is and electro-dialysis, or disposed through landfill. Recentlyarrera-Díaz et al. [8] reviewed the chemical, electrochemicalnd biological methods for aqueous Cr(VI) reduction. Most ofhese approaches are economically expensive and have disadvan-ages like incomplete metal removal, high reagent consumptionnd energy requirements, while contaminating the ground waterecause of generation/disposal of toxic sludge/secondary wastes.ome of the treatment methods/processes have major limitationuch as its economic viability only at high or moderate concen-rations of metals and not at low concentrations (1 to 100 mg L−1)9,10]. On the other hand bioremediation appears to have widermplications to tide over the problem of low concentration of heavy

etal in detoxification. As a result, researchers have great inter-st in metal-microbe interactions to remove, recover or stabilizeeavy metals in soil and effluents [11]. Bioremediation technologyas been reported to be more effective for removal of soluble andarticulate forms of metals especially from dilute solutions, andence bioaccumulation and therefore, microbe-based technologiesan provide an alternative to the conventional techniques of metalemoval/recovery [12].

Since several microorganisms possess the capability to reducer(VI) to relatively less toxic Cr(III), bioremediation gives immensepportunities for the development of technologies to detoxifyr(VI)-contaminated soils as an alternative to the existing physico-hemical technologies [14]. In this review, some of the importantfforts made to use microbes for potential Cr(VI)-bioremediationf soils/sludge/mine and metallurgical waste, will be summarized.

. Industrial uses of chromium

Chromium is widely used in industry. The Cr(VI) compounds aresed in the metallurgical industry for chrome alloy and chromiumetal production, and in chrome plating. In the chemical industry it

s used as oxidizing agents and in the production of other chromiumompounds. Between 80–90% of leather is tanned with chromiumhemicals [15], out of which about 40% of chromium used is dis-harged in the effluent as Cr(VI) and Cr(III) [16]. Cr(III) salts are usedess widely, being employed in textile dyeing, in the ceramics andlass industry, and in photography.

Chromium and its compounds are useful in common life as well.rom the total chrome ore production 90% is used in metallurgi-al industries for steel, alloy and nonferrous alloy production. Only

% each has been used in refractory (iron & steel, cement, glass,eramics and machinery) and chemical (leather tanning, plating,ood preservation and pigment) industries (Fig. 2). Chromium is

esistant to ordinary corrosive agents at room temperature, which

um in different industries.

accounts for its use in electroplating for protective coating. Ferrousalloys, mainly stainless steels, account for most of the consumption.These steels have a wide range of mechanical properties, besidesbeing corrosion and oxidation resistant. Cast iron may containchromium from 0.5% to 30% which provides hardness and tough-ness, and corrosion and wear resistance. Chromium is also used innonferrous alloys in combination with nickel, iron-nickel, cobalt,aluminum, titanium and copper [17].

As regards the chromium chemicals the largest amount is con-sumed to manufacture pigments for use in paints and inks. Otherapplications include leather tanning, metal corrosion inhibition,drilling mud, textile dyes, catalysts, wood and water treatment.Chromite is used in the refractory industry to make bricks, mortar,and ramming and gunning mixes. Chromite enhances their ther-mal shock and slag resistance, volume stability and strength [18].Chromium is widely distributed in rocks, fresh water and seawater[19]. The distribution and concentration of Cr in various environ-mental samples (soils/solid waste) are given in ‘Supplementarymaterial’.

3. Transformations of chromium in soil: mobility andbio-availability

Chromium, a steel-grey, lustrous, hard, and brittle metal, occursin nature in the bound form that constitutes 0.1–0.3 mg kg−1 of theEarth’s crust. As mentioned above, Cr(III) and Cr(VI) present in soilshave opposite chemical and physical characteristics. The formeris relatively immobile because it has a strong affinity for negativecharged ions and colloids in soils, and gives sparingly soluble com-pounds such as Cr(OH)3. Such products dominate in the pH range4–8 [5].

3.1. The chromium cycle

A look at the chromium cycle (Fig. 3) infers that Cr(VI) is themost oxidized, mobile, reactive, and toxic form of chromium, andit would be the only existing form if all chromium were to be inthermodynamic equilibrium with the atmosphere [20]. Small con-centrations can be the result of oxidation of natural Cr(III), but largerconcentrations usually are the result either of pollution with Cr(VI)or the oxidation of Cr(III) [20]. Soils and sediments in partial equilib-rium with atmospheric oxygen contain both oxidized manganeseand reduced carbon. The oxidation of Cr(III) to Cr(VI) by manganeseoxides Eq. (1) [21] and the reduction of Cr(VI) to Cr(III) by soil carboncompounds are both thermodynamically spontaneous reactions Eq.

(2) [22]. The interesting aspect of the chromium cycle in soil is thatoxidation and reduction can take place at the same time. Some ofthe Cr(III) added to a sample of an aerobic soil will be oxidised, andsome of the Cr(VI) added to the same soil will be reduced.
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B. Dhal et al. / Journal of Hazardous Materials 250– 251 (2013) 272– 291 275

in the

s

C

(

mip8Chw(

datcoTttapioUhb

and the persistence of Cr(VI). Therefore, steps for developing reme-diation processes of chromium contaminated soils should considerthe possibility that certain forms of Cr(III) can be more favourably

Fig. 3. The natural chromium cycle

Oxidation [21] of Cr(III) to Cr(VI) by manganese oxides throughurface oxidation under neutral pH conditions proceeds as (1):

r3+ + 1.5MnO2 + H2O → HCrO4− + 1.5Mn2+ + H+ (1)

Whereas reduction by organic compounds, e.g. hydroquinonewith formation of quinine) [22] proceeds as:

C6H6O2 + CrO42− + 2H2O → 0.5Cr2O3 + 1.5C6H4O2 + 2.5H2O

+ 2OH− �G298◦ = −427 kJ mol−1 (2)

The characteristics of Cr(III) forms limit their bioavailability andobility in waters and soils. The concentrations of soluble Cr(III)

n equilibrium with insoluble compounds are <10−9 M (0.05 partser billion) in water at pH value 6 to less than 10−15 M at pH value

[23]. Cr(VI) is more soluble and bio-available than Cr(III) in soil.r(VI) is an anion form under most environmental conditions. Atigher pH values >6.4, it is primarily present as chromate (CrO4

2−)hereas below pH 6.4, it is present (Fig. 4) principally as bichromate

HCrO4−) [24].

The reduction of Cr(VI) to Cr(III) is easier compared to the oxi-ation of Cr(III) to Cr(VI) by oxidizing agents in soils [25]. Bartlettnd James [26] have experimentally studied the oxidation of Cr(III)o Cr(VI) in many field soils containing manganese oxides. Theyoncluded that the soils containing oxides of manganese readilyxidise Cr(III) to Cr(VI) than the soils without manganese oxides.he amount of oxidized Cr(III) is proportional to the Mn oxide con-ent of soil and also to the reduced manganese oxide [26]. However,he oxidation of Cr(III) is directly related to its concentration in soilnd strongly depends on the forms of Cr(III) [6]. The influence ofH on the oxidation and reduction reactions of chromium in soils

s a complex process, but generally high pH values enhance the

xidative power while low pH values enhance reduction reactions.nder laboratory conditions (soils with near-neutral pH values,igh levels of Mn oxides and optimal aeration conditions), it haseen observed that soluble and freshly precipitated forms of Cr(III),

environment (modified from [20]).

such as CrCl3 and Cr(OH)3 added to soil, may be oxidised up to 15%[24].

Potential Chromium Oxidation Score (PCOS) developed by Jameset al. [27] in order to design remediation process by reductionstrategies is based on four interacting parameters such as solubil-ity and form of Cr(III), reactive soil manganese, soil potential forCr(VI)-reduction and soil pH as a modifier of the first three param-eters. Such parameters can be quantified and ranked numerically;the sum of their values gives the PCOS. The PCOS ranges from 10 to40, high scores indicate an elevated probability for Cr(III) oxidation

Fig. 4. Eh-pH diagram for the system Cr-O2-H2O, assuming that the concentrationof �Cr = 10−6 mol kg−1 at solid/liquid boundaries (the highlighted lines are modifiedfrom FactSage).

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276 B. Dhal et al. / Journal of Hazardous Materials 250– 251 (2013) 272– 291

/reduc

ottoSotoa([C(K

3

HCpi1

H

H

2

troi

3

pibsh

Fig. 5. The chromium oxidation

xidised to Cr(VI). Moreover, caution should be taken regardinghe use of hydrogen peroxide for in situ remediation of soils con-aminated with chemically complex wastes, because mobilisationf Cr(VI) could be a dangerous consequence of such a process [28].oils contaminated with chromium from chromate ore processingr from electroplating waste release large amounts of Cr(VI) afterreatment with hydrogen peroxide [28]. In soils the ionic formf chromium that is absorbed by plants is Cr3+ and Cr6+; Cr3+ isbsorbed more rapidly than Cr6+. The different oxidation statesstable/unstable) found in environment (Cr compounds) are: 0Cr(CO)6]; +1 (unstable); +2 (chromous) (unstable) [Cr(CH3COO)4,rO, CrSO4]; +3 (chromic) (stable) [CrCl3, Cr2O3, Cr2(SO4)3]; +4unstable) CrO2; +5 (unstable) [CrF5], and +6 (stable) [K2Cr2O7,2Cr2O4, CrO3] [29].

.2. Speciation of Cr(VI)

Cr(VI) can form several species, namely Cr(VI) is found as CrO42−,

CrO4− or Cr2O7

2− depending on both pH of the medium and totalr(VI) concentration. H2CrO4 is the strong acid [30] and at pH >1 de-rotonated form of Cr(VI) is seen. Above pH 7 only CrO4

2− ions existn solution throughout the concentration range. In the pH between

and 6, HCrO4− is predominant species [31,32].

2CrO4 → H+ + HCrO4−, K1 = 10−0.75 (3)

CrO4 → H+ + CrO42−, K2 = 10−6.45 (4)

HCrO4− → Cr2O7

2− + H2O, K3 = 10−2.2 (5)

As mentioned earlier Cr(VI) compounds are quite soluble andhus mobile in the environment. However, Cr(VI) oxyanions areeadily reduced to trivalent forms by electron donors such asrganic matter or reduced inorganic species which are ubiquitousn soil, water and atmospheric systems [33].

.3. Oxidation/reduction reactions in soil

The fate of chromium in soil is partly dependent on the redoxotential and pH. In most soils, chromium will be present predom-

nantly in the Cr(III) state. Under reducing conditions, Cr(VI) wille reduced to Cr(III) by redox reactions with aqueous inorganicpecies, electron transfers at mineral surfaces, reaction with nonumic organic substances such as carbohydrates and proteins, or

tion cycle in soil (source: [47]).

reduction by soil humic substances. The reduction of Cr(VI) to Cr(III)increases with decrease in pH. In aerobic soils, the reduction ofCr(VI) to Cr(III) is possible even at slightly alkaline pH, if the soilcontains appropriate organic energy source to carry out the redoxreaction [34]. Sub-soils have much lower organic matter than thesurface soils, and may be less likely to inhibit chromate mobility.However, since pH is a key factor that affects the rate and the extentof Cr(VI) reduction in subsoils, Cr(VI) may be effectively reduced inacidic sub-soils simply because acidic conditions enhance the rateof release of Fe(II) species from soil minerals for reaction with aque-ous Cr(VI) species, and also increase the rate of Cr(VI) reduction byorganic matter [35–37] (Fig. 5). The reduction of Cr(VI) by abioticreductants such as Vitamin C [38] and nano-materials (UV/TiO2)[39] has been found to quite effective in the entire pH range.

Under certain circumstances, Cr(III) may be oxidized to Cr(VI),a process that can lead to serious environmental consequences[20,26,28,40–44]. The oxidation of Cr(III) chelated by low molec-ular weight organic compounds is slower than freshly precipitatedCr(III) (less crystalline) [42,43]. The oxidation process appears to belimited to oxidation by oxygen or manganese oxides [26,42]. Oxy-gen does not react appreciably with Cr(III) according to Eary andRai [41]. The oxidation of Cr(III) may be correlated with the amountof hydroquinone reduced manganese in the soil [22,26] and doesnot occur in dry soils. The drying of the soil alters the manganesesurface decreasing its ability to oxidise Cr(III).

The oxidation of Cr(III) to Cr(VI) is also limited by the concentra-tion of water-soluble chromium, pH, initial available surface area,and the ionic strength [21,41]. A large portion of chromium in soilwill not be oxidized to Cr(VI) even in the presence of manganeseoxides and favorable pH conditions, due to the unavailability ofmobile Cr(III) [21]. Cr(VI) is reduced to Cr(III) in reduction reactions,or Cr(III) is oxidised to Cr(VI) in oxidation reactions [45] (Table 2).The pH, oxygen concentration, the presence and concentration ofreducers are important in these processes. James and Bartlett sug-gested that Cr(VI) mobile forms that are HCrO4

− and CrO42−, can

be reduced by different inorganic reducers such as Fe(II) or S2−,this process called ‘dechromification’ is quite important. In theabsence of such a process, atmospheric oxygen could be converted

into chromate which would pose a threat to life on earth [46]. Inpresence of oxidising agents such as Mn and Pb, the oxidation pro-cesses can oxidise Cr(III) to Cr(VI) in presence of H2O and free O2[47].
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B. Dhal et al. / Journal of Hazardous Materials 250– 251 (2013) 272– 291 277

Table 2Different oxidation and reduction reactions of chromium and its compound [45].

Oxidation E (V) Reduction E (V)

2Cr3+ + 5H2O + 3O3 ⇔ 2CrO42− + 10H+ + 3O2 0.87 HCrO4

− + 3V2+ + 7H+ ⇔ Cr3+ + 3V3+ + 4H2O 1.452Cr3+ + 2H2O + 3H2O2 ⇔ 2CrO4

2− + 10H+ 0.58 HCrO4− + 3Fe2+ + 7H+ ⇔ Cr3+ + 3Fe3+ + 4H2O 0.56

3MnO2 + 2Cr(OH)3 ⇔ 3Mn2+ + 2CrO42− + 2H2O + 2OH− 1.328 2HCrO4

− + 3H2S + 8H+ ⇔ 2Cr3+ + 5H2O + 3S 1.182Cr3+ + 3H2O + 2MnO4

− ⇔ Cr2O72− + 6H+ + 2MnO2 0.35 2HCrO4

− + 5H+ + 3HNO2 ⇔ 2Cr3+ + 5H2O + 3NO3− 0.35

2Cr3+ + 7H2O + 6Mn3+ ⇔ Cr2O72− + 14H+ + 6Mn2+ 0.18 2HCrO4

− + 5H+ + 3HSO3− ⇔ 2Cr+3+ + 5H2O + 3SO4

2− 2.1152Cr3+ + H2O + 3PbO2 ⇔ Cr2O7

−2 + 2H+ + Pb2+ 0.13 2CrO42−/Cr2O7

2− + R3CH ⇔ Cr3+ + R3COH2CrO4

2−/Cr2O72− + RCH3 ⇔ Cr3+ + RCH2OH

2CrO42−/Cr2O7

2− + RCHO ⇔ Cr3+ + RCOOH2− 2− 3+

4

cm[madatcetnbbrhtrC

bCcb[awanat(tmttTrma

d[(aarff

. Chromium toxicity

The greatest anthropogenic sources of Cr(VI) emissions are: (1)hromite mining and processing, (2) chrome plating, (3) chemicalanufacturing of chromium, and (4) evaporation cooling towers

48]. Chromium is an essential micro-nutrient in the diet of ani-als and humans, as it is indispensable for the normal sugar, lipid

nd protein metabolism of mammals [49]. Its deficiency in theiet causes alteration to lipid and glucose metabolism in animalsnd humans. Chromium is included in the complex named glucoseolerance factor (GFC). On the other hand, no positive effects ofhromium are known in plants and microorganisms [25,50]. How-ver, elevated levels of chromium are always toxic, although theoxicity level is related to the oxidation state of chromium. Cr(VI) isot only highly toxic to all forms of living organisms, mutagenic inacteria, mutagenic and carcinogenic in humans and animals [51],ut is also involved in causing birth defects and the decrease ofeproductive health [52]. Cr(VI) may cause death in animals andumans, if ingested in large doses [53]. The LD50 (dose that causeshe death of 50% of a defined animal population) for oral toxicity inats is from 50 to 100 mg kg−1 for Cr6+ and 1900–3000 mg kg−1 forr(III) [54].

Cr(VI) toxicity is related to its easy diffusion across the cell mem-rane in prokaryotic and eukaryotic organisms and subsequentr(VI) reduction in cells, which gives free radicals that may directlyause DNA alterations as well as toxic effects [55–57]. Cr(III) haseen estimated to be from 10 to 100 times less toxic than C(VI)54], because cellular membranes appear to be quite imperme-ble to most Cr(III) complexes. Nevertheless, intracellular Cr(III),hich is the terminal product of the Cr(VI)-reduction, forms in vivo

mino acid nucleotide complexes, whose mutagenic potentiality isot fully known [14]. The positive and negative effects of Cr(III)nd Cr(VI) for humans are described by Bielica et al. [17]. Posi-ive effects include participation of Cr(III) in glucose metabolismglucose tolerance factor), decreasing the body fat, cholesterol andriglyceride levels, activating enzyme reactions, and increasing the

uscle mass, whereas dermatitis, allergies, cancers, mutations anderatogenic impacts, are the negative effects of Cr(VI). It is knownhat prokaryotes are more resistant to Cr(VI) than eukaryotes [6].oxic effects of Cr(VI) on bacteria, algae and plants have beeneviewed by many authors [6,25,58]. On the contrary, scant infor-ation is available about the impact of chromium on the structure

nd diversity of soil microbial communities [59,60].The federal maximum concentration level (MCL) for total Cr in

rinking water is 100 �g L−1 [61], the California MCL is 50 �g L−1

7]. The National Institute for Occupational Health and SafetyNIOSH) recommends an exposure limit of Cr(VI) of 1 �g L−1 andn exposure limit for Cr(0), Cr(II) and Cr(III) of 500 �g L−1 for

10-h workday, 40-h week. Approximately 35% of chromiumeleased from all anthropogenic sources is Cr(VI). Human activityurther contributes to chromium in the environment (air, sur-ace water, groundwater and soil). Due to the toxicity concerns,

2CrO4 /Cr2O7 + PAH ⇔ Cr + Quinones2CrO4

2−/Cr2O72− + 3HCOOH ⇔ 2Cr3+ + 3CO2

2CrO42−/Cr2O7

2− + 2CH3CH2OH ⇔ 2Cr3+ + 2CH3COOH

in the USA concentration of total Cr are regulated at 0.1 mg L−1 indrinking water, 5 mg L−1 leached from solids in the toxicity char-acteristic. The typical ratio of chromium in plants to chromiumin soil is estimated at 0.0045 (or 0.45%). EPA has set a limit of100 �g Cr(III) and 50 �g Cr(VI)/L of drinking water. The Occupa-tional Safety and Health Administration (OSHA) has set limits of500 �g water-soluble Cr(III) compounds per cubic meter of work-place air (500 �g L−1), 1000 �g L−1 for metallic chromium(0) andinsoluble chromium compounds and 52 �g L−1 for chromium(VI)compounds for 8-h work shifts and 40-h work weeks [62].

In many studies, it has been difficult to assess the toxicityof chromium to soil microorganisms, because the environmentsexamined were often polluted at the same time with organicpollutants and/or different heavy metals [63]. In a soil chron-ically polluted with chromium (about 5000 mg kg−1 of soil) byleather tannery activity, the oxygenic phototrophic microorgan-isms and heterotrophic bacterial communities are both affectedby chromium [59]. The size of the cultivable heterotrophic bac-terial community is generally not affected by chromium pollution,but there is a relationship between the percentage of chromate-tolerant bacteria and the level of chromium in the soil. Someauthors have found that Gram-positive bacteria are more chro-mate tolerant than Gram-negative bacteria [59,60,64]. Shi et al.[65], established that chromium negatively affected the soil micro-bial activity and led to the accumulation of soil organic carbon. Speiret al. [66] have found that short-term Cr(VI)-exposure inhibits soilbiological properties such as phosphatase and sulphatase activities,and decreased microbial biomass [67].

5. Assessment and chemical treatment of chromium indifferent solid waste

Pollution of chromium in solid waste/residue has been noticedfrom different sources and sites due to the processing and use. Avariety of the wastes is generated from the processing of ore to theend use of chromium. In unpolluted soils, the relatively insolubleand less mobile Cr(III) predominates in the form of hydroxidesand oxides and adsorbed onto clay particles, soil organic matter,metal oxyhydroxides and other negatively charged surfaces. Forthe remediation of Cr(VI)-contaminated sites several innovativeclean up technologies have been described including permeablereactive barriers [68–70] and Na-dithionite liquid injections [71].Among the most cost-effective approaches to manage subsoilcontamination are the bioremediation and monitored naturalattenuation [72]. Here some major chromium contaminated sitesare listed and their treatment methodologies are discussed inTable 3.

5.1. Chromite ore processing residue (COPR)

Chromite ore processing residue is present in surface soils atmany commercial and industrial sites of USA, UK, China, Japan, India

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278 B. Dhal et al. / Journal of Hazardous Materials 250– 251 (2013) 272– 291

Table 3Assessment and chemical treatment of chromium in different solid waste (site, concentration, treatment and efficiency).

Contaminated site Pollutants (mg kg−1) Treatment reduction/leaching Efficiency Reference

Chromite Ore Processing Residue (COPR)Industrial waste site at Jinan,

China.PS 0.25 mm, dried at 80 ◦C

Cr(T) 43200Cr(VI) 5900

Nanoscale zero-valent iron (Fe0) Cr(VI) diminished < 0.01 g kg−1

in 24 h at 31–36% WC, 4% nanoFe0 ratio

[85]

Hunan Province, China. PS0.9 mm

Cr(III) 19700, Cr(VI) 2670 (WS)and Cr(VI) 5880 (AD)

After water washing for 8 h,leached with HF with diff., PS(0.180–0.098 mm)

Completely removed Cr,leaching followed diffusioncontrol model

[82]

Hunan Province, China. Cr(III) 19700Cr(VI) 10700

Hydrothermal reduction by starchpowder (85.9% starch) + H2SO4 atStarch (x = 0.375) with H2SO4

(H+/COPR = 10.80 mole kg−1) at180.1 ◦C for 120 min

Reduction met CEPA & TCLPlimits (1.5 mg L−1 & 4.5 mg L−1

Cr respectively

[92]

Qingdao Red Star chemicalplant, Shandong, China. PS2 mm, pH 8.2

Cr(VI) 432.6 Leaching distilled water 99.98% Cr(VI) leached at S/L1:40; 200 rpm and 50 ◦C in105 min.

[76]

SA7, gray-black (GB); reddishbrown (HB), Dundalk MarineTerminal in Baltimore, (MD).

Cr(III) 13500 (GB) and 23300(HB)Cr(VI) 6500 (GB) and 8700(HB)

– – [142]

7(SA7) in Jersey city, NewJersey, USA. pH 12.03–12.25

Cr(VI) 4575–6530 forPS < 475–0.01 mm

Batch: FeSO4.7H2O (25% w/w dry

COPR) and acid (H2SO4)(0.31 L kg−1 COPR)

<2 mg kg−1 for all PS, TCLP of Cr– 0.55 mg L−1 with 0.01 mm PStreated with acid in 1 h.

[86]

Hudson Country, New Jersey,USA

Cr(VI) 4000 Reduction with FeSO4.7H2O(2x = 16.62% w/w) + Asphalt(40–60% w/w)

91% with asphalt withoutpretreatment with FeSO4

.7H2Oin 16 months not met UTS of0.6 mg L−1; Completed in 5months with FeSO4.7H2O andafter 16 month got a TCLP ofCr(VI) was ∼22 mg L−1.

[87]

Yuxing chemical company inJinan, China; sewage sludge– Minhang municipal watertreatment plant, Shanghai,China

Cr(T) 31600Cr(VI) 3400

Distilled water at S:L 1:10 for 24 hshaking; dried at 105 ◦C and passedthrough 2 mm sieve.

Reduction from 3384 to<30 mg kg−1 treated at 600 ◦Cfor 30 min for <500 �m PS and0.1 Sludge/COPR ratio

[77]

Qingdao Hongxing chemicalplant, China

– Sequential water extraction; weekacid extractable; reducible andoxidizeble

>80% Cr(VI) extracted by weekacid; water extractable washigher than acid extractable.

[78]

The mid-Atlantic Coast of USA. Cr(T) 2000–40000 Mobilization with H2SO4 and HNO3 >2800 mg kg−1 Cr(VI) was inCOPR and ANC is very high(8 mol HNO3 kg−1 COPR)

[84]

pH 11.2–12.2 Cr(VI) 4900–7700 10 g of COPR (2–2.36 mm) reactedwith 100 ml of 0.5 N acid (H2SO4 &HNO3) S/L 1:10 for 1–72 h at30 rpm.

SA7 deposition site of JerseyCity, NJ, USA.

Cr(VI) 3720–11600 in sixsamples

– – [143]

New Jersey, USA Cr(T) 16800–23400Cr(VI) 588–2100

Ferrous sulfate in two dosages (5Xand 8X) at pH 12.1–12.6.

Treatment failed to met Cr(VI)regulatory limit (240 mg kg−1)in 240 days.

[88]

A chemical works in south-eastGlasgow, UK

Cr(T) 39000 (dry wt.) CaSx in S/L ratio 0:1; 0.88:1and4.4:1 for 4 days.

< 10 mg kg−1 of Cr(VI) with4.4:1(S/L) for all pH (12.2,10.87 & 10.03) in 4 days.

[89]

Cr(VI) 3120–7934 (dry wt.) In column (ID-2.5 cm): 23 g COPRcontain Cr 7210 mg kg−1, PS < 2 mm

Complete reduction withaddition of Ca and S in 2.5 h

COPSW – ferrochromeproduction site of MacalloyCorporation, NorthCharlestron. pH 9.68

Cr(VI) 550 and waterextractable Cr was 74

23 reductants such as: Na2SO2O4;FeCl2; FeSO4; Citric acid;hydroxylamine sulfate; peerlessiron and mixture of one another at100 rpm and 23 ◦C for 24 h.

100% with 0.05 MFeSO4 + 0.05 M Na2S2O4 andfield trail at 5700 mg kg−1

(0.07 M FeSO4 + 0.07 MNa2S2O4); Alone FeCl2 andFeSO4 showed 100% reductionin 24 h.

[90]

Former playing field insouth-east of Glasgow. pH11–12

Cr(T) 133484 Batch: 20 mM FeSO4, 1 mM NaCland 1 mM NaCl + 20 mM Na2SO4

100% after 25 pore volume,treated with FeSO4 (20 mM)+NaCl (1 mM) at pH 3.

[91]

Cr(VI) 40040 Column: ID 2.5 cm having 10 g acidwash sand in lower part.

6 cm depth Liberty State Parkin Hudson country, NewJersey USA. pH 6.8

Cr(T) 5450–28390Cr(VI) 2600

Leaching with simulated rainwater, COPR (PS <180 �m,1–10 g L−1)

Maximum at pH 4.5 andremain constant up to pH 12

[80]

Glencairn Football club,Rutherglen, Glasgow. pH9.7–10.8

Cr(T) 2500–25000 Cr(VI) absorption using Eclays(25 g L−1)

10.12 mg g−1 absorbed in Eclay [79]

Cr(VI) 1000–5000 No absorption with bentonitefrom 480 mg L−1 conc.

Rutherglen and Cambuslang tothe south-east Glasgow,Scotland, pH 7.1–12.5

Cr(III) 5800–36400Cr(VI) 290–8500

– Assessment of chromium [144]

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B. Dhal et al. / Journal of Hazardous Materials 250– 251 (2013) 272– 291 279

Table 3 (Continued )

Contaminated site Pollutants (mg kg−1) Treatment reduction/leaching Efficiency Reference

9 soil samples from Hudsoncountry, New Jersey. pH8.88–12.22

Cr(T) 1936–25573Cr(VI) 29.2–4800

Vitrification: heated at 1590 ◦Cfor 3 h, transferred to 1350 ◦Cfor 1 h, then 1250 ◦C for 1 h andfinally to 1000 ◦C for 1 h andslow cooling at room temp.

<detection level in all samples [83]

Chromite processing site,Northern New Jersey. pH:11.1

Cr(T) 7970Cr(VI) 2300

Batch: Tap water + H2SO4

(56.25 ml) + fresh cowmanure + COPR (4.1 kg) (0–7%)

<100 mg kg−1 at 7% cowmanure, Tap water 3 L, H2SO4

56.25 ml, COPR 4.1 kg (IC2000 mg kg−1 Cr(VI)) in 1 year.

[81]

130 sites in Hudson County,New Jersey, New York City

15% of sites containsCr(T) 10000 & Cr(VI) (1–50%)100–5000.

– – [73]

Soil from metallurgical site ofsteel, super-alloys,aluminum and titaniumalloys production unit,France. PS < 2 mm and pHH2O

6.3 ± 0.1

Cr(III)113 and Cr(VI) 28 withMn 921, Ni 280, Co 40, Zn 138,Cu 29, and Pb 87

Column leaching of metals (ID1.5 cm; H 5 cm); Chelants:EDTA and citric acid at2.5 mmol L−1 and 18.2 ± 0.2 mLh−1 flow rate.

Cr leached more with citricacid (250 �g L−1) than EDTA(200 �g L−1); For Ni leachingEDTA > citric acid

[116]

Steel slag samples fromInsichuan Province, China

Cr(T) 650 Absorption of Pb using slag An absorption rate constant13.26 g mg−1 min−1 wasachieved

[117]

Steel slag samples from LinzDonawitz

Cr(T) 1916 Ultrapure water at S/L 10 & 100for 15 and 30 days, 10 rpm inroom temp

Low Cr leached over 30 days [145]

Feed coal, bottom ash & fly ashfrom a power plant burningwestern Canadian mediumvolatile bituminous coal.

Cr 51.71 in feed coal, 343.67 inbottom ash, 137 in fly ash,192.5 in fabric filter

Cr speciation is mainlyCr3+ > 95%

Cr associated illite phase andonly 0.71 kg d−1 was emittedout from input 123.3 kg d−1 Cr

[146]

Soil and sediments fromformer industrial complex ofPorto-Romano, Albania.

Cr(T) 1130–24409Cr(VI) 12200

Leaching with CascadeLeaching Test (CLT)

∼2230 mg kg−1 Cr(VI) leachedat neutral pH and Cr(III)leached in acidic pH

[118]

Ferrochromium smeltersample (Slag, fly ash(FA),bottom ash(BA), nearbyagricultural soil(AS), pollutedsoil (PS))

Cr 2989 in slag material and526 in agricultural soil

DIN 38414-S4; TCLP, SPLP andalkaline digestion leachingmethods

310 times concentrationgreater than the maximumallowed value 160 mL kg−1 inagricultural soil

[119]

Ferrochromium dust fromCarbure and Affine,Kazkshstan

Cr(T) 242100 & 238000 inCarbure and Affine dustrespectively.

DC transferred plasma arcreactor

High recovery of Cr (90–96%)and Fe (+96%), the specificenergy consumption was 1250kWh ton−1

[122]

Steel manufacturing wastefrom centre of Torino, NW,Italy.

Assessment of chromium withdiff. lixivant

Aquaregia digestion, acetic acid(0.5 M), KH2PO4(0.015 M) andDH2O at S/L 1:10 (g mL−1)

Cr leached with Aquregia(2230), acetic acid (1.3),KH2PO4 (2.9) and with DH2O(0.9) mg kg−1

[120]

Slag from scrap basedproduction of low alloyedsteel in an electric arcfurnace, Sweden

Cr 7760 and Mn 39300 Ultrapure water and 0.1 mMHNO3 were used in batchcolumn (at L/S–5 for 24 h)

<0.0005 to 0.086 mg L−1 Cr wasfound in all leaching tests

[121]

Chrome plating contaminantsThe edge of escarpment formed

by glacial deposits adjacentLittle River, New England.

Cr(T) 380 & 14800; Cr(VI) 30 &7800 for GW and HA soilrespectively with highFe(25900)

29% calcium polysulfide inratio of 1× (156 mL kg−1 soil)and 2× (312 mL kg−1). PS 2 mmand time 0–365 d

Substantial leached up to 60dand than declined in 1× and2× treatments and reachednon-detectable value(<10 �g L−1) in 365 d.

[114]

Sludge from Tamilnadu, India Cr(T) 85000, Zn 20100 and Mn5600

Used for cement and cementfly ash

Compressive strength ofcement based binder > cementfly ash based binder system.TCLP < 5 mg L−1 in 28 d.

[109]

National Chromium, Inc.Putnam, CT), pH 5.4

Cr(T) and Cr(VI) was Oxidation and solubility of Crusing H2O2 (0–24 mM)

200 �M released over 24 hleachate containing 24 mMH2O2

[28]

COPR from Kearney, NJ, pH(8.8)

61000 & 71

Tannery waste soil from MA,pH 6.7

8600 & 914

Mine waste from northwest ofBaltimore, pH 6.5

1300 & <0.052500 & <0.05

Mining contaminated soilErvedosa mine in Northeastern

PortugalCr(T) 74–237 Sequential extraction by

leachants: DH2O, 1 M NH4

acetate, 0.2 M ammoniumoxalate, 35% H2O2, KCLO3

+12 M HCl + 4 M HNO3 andHF + HNO3 + HClO4

Water soluble fraction is verylow for Cr in all samples

[131]

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280 B. Dhal et al. / Journal of Hazardous Materials 250– 251 (2013) 272– 291

Table 3 (Continued )

Contaminated site Pollutants (mg kg−1) Treatment reduction/leaching Efficiency Reference

Leather tannery contaminantsIgualadina de Depuracio i

Recuperacio (IDR),Catalunya, Spain. pH 7.25

Cr(T) 8041 Saponin (3%) with 2 g oftannery sludge, pH 2, 210 rpmat room temp for 6–72 h.Another test using H2O2 foroxidation (Cr III–VI) andwashing with H2SO4 at roomtemp.

24% extracted with saponin atpH 2, performing multiplewash of 6 h, at 33 ◦C. Buttreatment with H2O2 extract70% of Cr < 4 h at pH 2 in roomtemp (21 ◦C)

[147]

Tasman Sheepskin Tannery,Near Brisbane, Australia

Cr(T) 35050–65000 ViroFlowTM Technologytreatment using ViroChromeTM

reagent

Leachablechromium < regulatory limit of5 mg L−1 in 5d mixing time

[105]

53 soil samples from Jajmauand Unnao, Kanpur, UttarPradesh, India

Cr 162–6228 Assessment of Cr All the samples examined fellinto class 4, ranging from0.22–5.04 mg kg−1

[99]

Municipality Solid wasteCompost soil, Henchir Lihoudia

Tunis of Norththen TunsniaCr(T) 29.21–32.41Cr 78.87

– Cr content in plants0.06–2.24 mg/plant

[138]

Kodungaiyur and Perungudi ofChennai, India

Cr 140 ± 40 with Pb 86, Zn 284and Cu 113

TCLP and acid digestionleaching and water extracted

In leachates Cr present5–200 �g L−1

[140]

Chromium Contaminated soilBeijing Academy of Agriculture

and Forestry sciencesCr(T) 4.96 Analysis of plant accumulation

of Cr Switch grassAccumulated Cr in root 872.5and shoot 52 mg kg−1 from thesoil

[75]

Balanagar industrial area,Hyderabad, India

Analyzed as Cr 371.98 butpreviously reported as84–2264

Assessment of Cr – [141]

Notation: WS–water soluble; WC–water content; AD–acid digested; Cr(T)–total chromium; rpm–revolution min−1; min–Minute; h–hour; S–solid; L–liquid;temp.–temperature; PS–particle size; HF–hydrofluoric; CEPA; TCLP–toxicity characteristic leaching procedure; wt.–weight; DH2O–distilled water; x–dose; ID–internald IC–int ing pr

aae[acpwTct2b

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ttdocbbllem

swpfhFc

iameter; COPSW–chromite ore processing solid waste; M–molar; mM–millimolar;reatment standards; CaSx–calcium polysulfide; SPLP–Synthetic precipitation leach

nd many other countries. Two important contaminated COPR sitesre Hudson County located in northeastern New Jersey on the west-rn shore of the Hudson river opposite New York City (>100 sites)73] and Shawfield in southeast Glasgow (>15 sites) [74]. COPR isn industrial waste material generated by the manufacturing ofhromate and bichromate chemicals from chromite ore. The ore isulverized in a ball mill to less than 100 mesh size (150 �m), mixedith soda ash and lime, and roasted in rotary kilns at 1100–1150 ◦C.

he mix does not fuse, but the molten soda ash reacts with thehromite to form water-soluble sodium chromate. It is estimatedhat the total amount of processing residue produced is in the range–3 million tons. The reaction for chromate production process cane generalized as Eq. (6) [73].

FeCr2O4 + 8Na2CO3 + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8CO2 (6)

The potential for human exposure and adverse effects due tohe mobility of Cr(VI), and environmental fate of chromium fromhe chromium-contaminated sites near New Jersey City, have beenescribed by Bruke et al. [73]. Different treatment processes (bothn bench and field scale) were suggested for the remediation ofhromium from COPR, some of these processes are described inrief here. The leaching and solubility of Cr(VI) from COPR haveeen studied using distilled water/ultrapure water [75–79], simu-

ated rain water [80] and tap water [81]. Also investigated were theeaching of chromium by chemical acids such as HF [82], week acidxtractable, reducible and oxidizable form of chromium [78], andobilization with H2SO4 [83,84] and HNO3 [84].The reduction of Cr(VI) in COPR has been reported using nano-

cale zero-valent iron (Fe0) [85], reduction with FeSO4 and leachedith H2SO4 [86], FeSO4 and asphalt [87], and FeSO4 [88]. Calciumolysulfide (CaSx) has also been used for the reduction of Cr(VI)

rom COPR [89]. The effect of mixed reductant on reduction of COPRas been investigated with FeSO4, sodium dithionate (Na2S2O4),eCl2, citric acid, hydroxylamine sulfate, peerless iron (a commer-ial zero-valent iron) and mixture of one another [90], and FeSO4,

itial concentration; H–height; ANC–acid neutralizing concentration, UTS–universalocedure.

NaCl, NaCl + Na2SO4 in batch and column [91]. Hydrothermalreduction of Cr(VI) in COPR was carried out using starch and leach-ing with H2SO4 [92]. Meegoda et al. [83] studied the extraction ofchromium by high temperature vitrification technology (makingglass out of chromium containing wastes, by three immobilizingmechanisms: encapsulation, replacement with silicon atom, andbonding to nonbridging oxygen), where 2.5 kg of soil (1/4 inchcontains 5% moisture) from each site were mixed with 25% (w/w)of sand (as a silica source) and 2 g of carbon (a reducing agent) toincrease the electrical conductivity of the soil mixture.

5.2. Leather tannery contaminated soil

Among the numerous waste pollutants, the chromium contain-ing tannery solid wastes known as shavings are of major concerndue to their high toxicity. The utilization of chromium in tan-ning procedures accelerates the mobility and transport rates ofchromium, which by far exceeds the rates of natural cycle pro-cesses resulting in serious problems in countries involved in thetanning industry [93]. Although the tanning industry is environ-mentally important as a principal user of meat industry waste, theindustry is perceived as a consumer of resources and a producer ofpollutants. Processing one metric ton of raw hide generates 200 kgof final leather product (3 kg Cr), 250 kg of non-tanned solid waste,200 kg of tanned waste (3 kg Cr), and 50,000 kg of wastewater (5 kgCr) [94]. Thus, only 20% of the raw material is converted into leather,and more than 60% of the chromium is in the solid and liquid waste.

In India, around 2500 units are scattered all over the country, butthe main areas of their concentration are TamilNadu, Uttar Pradesh,and West Bengal with an annual capacity of processing 0.7 milliontons of hides and skins [95]. With 80% of tanneries engaged in the

chrome tanning process [95], the consumption of basic chromiumsulphate (BCS) has increased from 25,000 to 40,000 t per annum[96]. Major problem concerns the possibility of oxidation of Cr(III)into Cr(VI) in gentle conditions by air in the wide range of pH.
Page 10: Chemical and microbial remediation of hexavalent chromium from contaminated soil and mining/metallurgical solid waste: A review

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rincipally, oxidation can be realized after the following equations97]:

Cr2O3 + 8OH− + 3O2 = 4CrO4− + 4H2O (in alkali medium) (7)

Cr2O3 + 3O2 + 2H2O = 2Cr2O7− + 4H+ (in acid medium) (8)

Soil pollution by tannery effluent and waste sludge is essen-ially different from air or water pollution because the persistencef heavy metals in soil is reportedly much longer than in otherompartments of the biosphere. Removal of heavy metals from pol-uted soil is difficult due to versatile composition and content. Onceeposited on the soil certain metals such as lead and chromiumay be virtually permanent [98]. The remediation of contaminates

rom tannery waste polluted soil has been studied using differ-nt reduction and extraction technologies. For the assessment ofhromium and other heavy metals, 53 soil samples were collectedrom Jajmau and Unnao leather tanning industrial areas of Kanpur,ttar Pradesh, India and were analyzed using wavelength disper-

ive X-ray Florescence spectrometer [99]. Chromium recovery fromannery chrome shavings was investigated by Erdem [100]. Thencineration at 850 ◦C (for organic removal) was followed by recov-ry of chrome (99.5%) using Na2O2 as an oxidising agent. Shen et al.101] have employed mineral acids for extraction of Cr. Macchit al. [102] reported the separation of Cr(III) with sulfuric acid andecovery by oxidation to Cr(VI). Chuan and Liu [40] investigated theffect of pH on leaching behavior of chromium from tannery sludge.nother study showed the oxidation of Cr(III) to Cr(VI), and Cr(VI)as further extracted using a organic solvent [103]. Other workerssed microemulsion system and have treated the dried, ground andieved tannery sludge by the solid-liquid extraction process [104].

pilot scale treatment of tannery sludge has been employed byirotec Global Solution Company, known as ViroflowTM technologysing ViroChromeTM reagent where the chromium concentrationapidly lowered below the regulatory limit (5 mg L−1) in 5d mix-ng period [105]. Generally these methods have good extractionields, however application of chemical extraction methods areimited and expensive due to operational difficulties, high chemicalonsumption and requirement of long processing time.

.3. Electroplating sludge and contaminated soil

Metal finishing and electroplating industries generate largeuantities of toxic waste sludge containing Cr, Pb, Ni and Zn that areainly disposed on landfill, causing real threat to the environment

106,107]. The electroplating sludge particularly those from thehrome plating units that settles in the bath contains 7–11% Cr and–5% Fe along with other metals such as Ni, Co, Zn, Cd, Mo, V, Cu etc.

n variable amounts depending on the type of substrate used for theurface treatment [108]. Sludge generated from the electroplatingndustries is highly complex due to the presence of metal ions andther inorganics, which pose serious problem on disposal. Solid-fication/stabilization (S/S) was used for stabilization of metals inolid waste like portland cement and portland cement with fly ash.n S/S processes with cement, water added reacts chemically withement to form hydrated silicates and aluminates resulting in anggregate mass of concrete; TCLP showed lower than the standardor chromium (5 mg L−1) [109]. Thus, optimum cement to wasteatio is required in order to produce a waste form with high physicaltability [110]. Rossetti et al. [111] summarized the optimizationf S/S system for Cr, Pb, Zn and Li in aqueous solution. Asavapisitnd Chotklang [112] investigated the potential for utilization oflkali-activated pulverized fly ash (PFA) as solidification binder

or the treatment of electroplating sludge. Espinosa and Tenório113] studied the possibility of partially substituting cement’s raw

aterial with sludge obtained from the chrome plating for cementroduction. While leaching chrome plating contaminated soil of

ials 250– 251 (2013) 272– 291 281

particle size ∼2 mm for substantial time (365d) using calciumpolysulfide (29% CaSx), reduction of Cr(VI) up to <10 �g L−1 wasobserved in much shorter time of 28 or 60 days depending uponthe concentration of the reductant [114]. Rock et al. [28] studiedthe oxidation and solubility of electroplating waste with hydrogenperoxide (0–24 mM), where 200 �M additional Cr(VI) was releasedover 24 h with leachate containing 24 mM H2O2. About 92% ofchromium was extracted from electroplating sludge using 30%H2O2 at 60 ◦C [115].

5.4. Contaminates from metallurgical and industrial waste

Generally, the metal containing wastes and byproducts of themetallurgical industries are in the form of slag, sludge, dust, tail-ings and byproducts generated either in the end or as intermediateproduct [108]. Besides, the formation of flue dust and sludge fromthe blast furnace, flue dusts/sludges from the electric arc furnaceparticularly that of Fe-Cr and alloying units, are rich in several non-ferrous metals depending on the type of metal and alloys produced.

As the slags of different origin contain good amount ofnonferrous metals and not only chromium, they should beremoved/extracted (leached) before dumping on soil. A few lit-eratures are available for the leaching of metals from suchmetallurgical wastes. Column leaching (ID 1.5 cm; hight 5 cm) ofNi and Cr using EDTA and citric acid (2.5 mmol) was studied fromsoil contaminated with steel, super alloys, aluminum and titaniumalloy production units. Chromium leached more with citric acid(250 �g L−1) than EDTA (200 �g L−1) and Ni leached more withEDTA than citric acid [116]. Liu et al. [117] found that the leach-ing of chromium is very low (in 30 d) with ultrapure water at theratio of 1:10 and 100 for 15 and 30 days, respectively, 10 rpm androom temperature. A high Cr(VI) leaching test has been carried outusing Cascade Leaching Test (CLT) in de-mineralized water (acidi-fied to pH 4 with a drop of diluted HNO3), where ∼2230 mg kg−1 ofCr(VI) was leached at neutral pH and Cr(III) was leached in acidicpH range [118].

Different leaching methods were adopted for the estimation ofchromium concentration in ferrochrome smelter sample (Slag, flyash, bottom ash) and nearby agricultural and polluted soil [119].In the nearest vicinity of the factory, soil had chromium content7 to 1527 mg kg−1, which was 310 times greater than the max-imum allowed concentration. Chromium leached in presence ofdifferent lixivants as with aqua regia was ∼2230 mg kg−1, aceticacid ∼1.3 mg kg−1, KH2PO4 ∼2.9 mg kg−1, and with distilled water0.9 mg kg−1 of chromium from steel manufacturing waste [120].Fällman [121] reported the leaching of chromium from the scrapbased slag of low alloy steel production using ultrapure water and0.1 mM HNO3 in batch and column (diameter 0.19 m and height0.9 m) at L/S (liquid/solid) 5 for 24 h. While utilizing the Fe-Crdust in smelting in a DC transferred plasma arc reactor at a spe-cific energy consumption of 1250 kWh ton−1, the high extractionof chromium (90–96%) and iron (>96%) was achieved [122]. Gomesand Pinto [123] studied the leaching of chromium from EAF andladle slags from a Portuguese steel producer using DIN 38414-S4method (100 g L−1 S/L for 24 h, particle size <10 mm). They foundthat the cumulative leaching values were very low even for thetreatment of over 1 year, thus complied with the non hazardousstandard (10 mg L−1).

5.5. Contamination from mining waste

Mining can impact water and soil quality depending on the

chemical composition of the ores and waste dump materials. Opencast mining and the creation of overburden dumps can do harmmore than reducing forest yields. Overburden dumps are often hos-tile to plant growth due to the existence of various stress conditions
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2 Mater

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ike low nutrient content, pH imbalance coupled with toxic andeavy metal ion accumulation, and poor water holding capacity124]. In the last few decades, the amount of chromium in bothquatic and terrestrial ecosystems has increased as a consequencef anthropogenic activities [16,125,126]. The mine wastes are theajor source of contamination of heavy metals like Zn, Hg, Al, and

r, etc.Many reports are available on the Cr(VI) contamination from

hromite mine area around the world. Contamination fromhromite mines and ferrochrome production plants from dustepositing around an open pit chromite mine in Kemi, Finlandesulted in higher levels of Cr and other heavy metals in pineree bark [127]. Unfortunately, no studies were reported on howhese pollution levels affected the health of plants and animals. Inimbabwe, ferrochrome furnace dust emitted from a smelter stackolluted soils with chromium even up to 700 m away from the fur-ace, where 700 mg Cr kg−1 soil was measured [128]. The furnacemitted an average of 54.6 tons of chromium per year from the onetack (of six). In the Hunan Province of China, a ferroalloy plant dis-osed slag directly on the ground and also discharged wastewater

nto the sewage system that run through agricultural land [129].his resulted in long-term soil and vegetable contamination. In theoil, total chromium content ranged from 90 to 6200 mg kg−1 andr(VI) ranged from 0.1 to 252 mg kg−1.

The Sukinda Valley of Jajpur district, Orissa, India, comprises of98% of India’s chromite ore reserve. The open cast mining activi-

ies in this region generate around 7.6 million tons of solid wastesn the form of rejected minerals, overburden materials/waste rock,nd low grade ore [130]. Because of the leaching of these solidastes, the water bodies in and around the mines are contami-ated with Cr(VI) [131]. The mining of the Sukinda chromites isne of the most polluting activities mainly because of the naturalxidation of chromite from the overburden dumps, which affectshe nearby areas with soluble Cr(VI) species. As per estimates, 1 tonhromite mining generates around 10 tons of overburden whichs diverse in its chemical and mineralogical characteristics [130].avas et al. [132] studied the sequential leaching to leach the solubleraction of Cr(VI) using leachants such as distilled water, 1 M ammo-ium acetate, 0.2 M ammonium oxalate, 35% H2O2, KClO3 + 12 MCl + 4 M HNO3 and HF + HNO3 + HClO4. From the leaching tests itas found that soluble fraction was very low in the samples.

.6. Contamination from municipality solid waste andontaminated soil

The presence of toxic heavy metals in municipal solid wasteomposts (MSWC) raises serious concerns about the adversenvironmental impact as a result of excessive application to agri-ultural lands [133–135]. Heavy metals originate mostly in nonource-separated municipal solid wastes from a variety of sources:atteries, electronic appliances, newspapers, paint chips, foils,otor oils, and plastics that can all introduce metal contaminant

nto the compostable organic fraction [136]. High and excessiveccumulation of heavy metals in soil and other media may eventu-lly contaminate both human and animal food chain [137].

Recently, some researcher studied the leachability of metalsresent in MSW using different certified leaching methods.yari et al. [138] reported the chromium content (0.06–2.24 mger plant) in plants of municipality soil when compost/manureontaining 78.87 mg kg−1 chromium was used. Sequential leach-ng method was followed with distilled water for municipalityolid waste compost (653–678 mg kg−1) and cow manure com-

ost (51.8–54.5 mg kg−1). Results showed that the chromiumas insoluble as because it was bound with large molecules,umic compounds [139]. Esakku et al. [140] reported chromium5–200 �g L−1) in leached solutions particularly in TCLP test and in

ials 250– 251 (2013) 272– 291

acid digestion. The water soluble chromium was very low from achromium content of 140 mg kg−1 in the MSW.

Some contaminated sites were assessed for the chromium con-tent in the waste dump created by industrial activities. Machenderet al. [141] assessed the chromium level (371.98 mg kg−1) in soilsamples of Balanagar industrial area, Hyderabad, India. A Switch-grass (Panicum virgatum L.) plant accumulated 827.5 mg kg−1 of Crin shoots and roots while grown on soil containing 600 mg kg−1

chromium.

6. Biological treatment of chromium in different solidwaste

6.1. Chromite ore processing residue

The attempts made on biological reduction of Cr(VI) from differ-ent solid wastes are summarized in Table 4. COPR has been treatedusing reductants like gastric fluid [148], prior leaching with tapwater followed by H2SO4 and reduction with cow manure [81],acetate amended/un-amended microbes (microcosms) [149], Fe(II)containing COPR with microcosms [150], microbial reduction inpresence of HCl [151] and microbial reduction by Bacillus sp., Micro-coccus sp., and Rhodococcus sp. [152]. In situ physicochemical and exsitu microbiological, organic matter, sulfate reducing bacteria (SRB)and FeSO4 treatment were also examined for the remediation ofchromium from COPR [74]. For reducing Cr(VI) from COPR, somesecondary biological materials such as sucrose, starch and wheatflour were used [153]. Cr(VI) absorption on Eclay has been studiedfrom the leached solution of COPR [79].

6.2. Leather tannery contaminated soil

In the last decade the increasing environmental concern and theregulatory norms imposed by governments have forced industryto search for eco-friendly bioremediation approach to the pollutedsoil, which has become one of the key research areas in environ-mental science and technology. In the light of above, microbialbiosurfactants that are produced extracellularly or as a part ofthe cell membrane by bacteria, and plant derived bio-surfactantshave been found to be of particular interest in environmentalapplications. These bio-surfactants are also promising non-toxicwashing agents [154] due to their low toxicity, biodegradablenature, enhancing biodegradation characteristics, easy productionand possibility of reuse. Moreover, they do not cause sec-ondary pollution even if they are leaked and discharged to theecosystem.

Microbial reduction of Cr(VI) from the tannery contaminatedsoils and sediments has been studied by many authors. Theseare microbial reduction [155], microbial reduction in presence ofmolasses [156], use of microbes present in cow manure [157] andnative bacterial isolates of the sediments of tannery effluent accu-mulated site for Cr(VI) leached solution [158]. Zhou et al. [159]and Wang et al. [160] have employed bioleaching methods to thetannery sludge using the mixture of indigenous iron- and sulfur-oxidizing bacteria. Compared to the mineral acids consumingchemical approach, the sulfur-oxidizing bacteria using microbi-ological method have distinct advantage [101,161–166]. Sincesulfur-oxidising bacteria play an important role in the removal ofchromium and other metals from tannery sludge and other wastes,interest in bacterial treatment of tannery sludge has arisen. How-ever, bacterial leaching of chromium from tannery sludge using

Acidithiobacillus thiooxidans is time-consuming (from 8 to 25 days)[162–166] compared to chemical leaching besides, requiring highamounts of elemental sulfur (up to 30 g L−1) also [162,163] or pre-acidification with sulfuric acid plus addition of elemental sulfur
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B. Dhal et al. / Journal of Hazardous Materials 250– 251 (2013) 272– 291 283

Table 4Assessment and biological treatment of chromium in different solid waste (site, concentration, treatment and efficiency).

Contaminated site Pollutants (mg kg−1) Treatment Reduction/leaching Efficiency Reference

Chromite Ore Processing Residue (COPR)Disposal site in Jinan, China.

PS < 200 �mCr(T) 57570 Cr(VI)19490 Synthetic GF (0.15 M NaCl + 1%

(w/v) pepein) 180 mL + 1.2 gCOPR at 37 ◦C, pH 1.8 (withHCl) for 1 h; food products(dough milk, 9 g) and organicacids (ascorbic, 0.5 g; citric0.5 g; lactic 0.42 mL; malic 0.5 gand acetic acid 0.5 mL)

Cr(VI) and Cr(III) was 53.8% and22.6%, in GP and 42.9% & 3.0%in IP respectively; decreasedfrom 27.7 to 0.5 mg L−1 at theend of IP.

[148]

Glacial valley filled withalluvial deposits, NorthEngland. pH 10.5

Cr(T) 162-12716 Cr(VI) aq.279–287 �M

Acetate amended andunamended leaching andmicrocosms reduction

100% in 120 d with acetateunamended microcosms and ∼56.44% removed in amended.

[149]

Glacial valley filled withalluvial deposits, NorthEngland. pH 4.31–12.51

Cr(T) 10162–12716 Cr(VI)(leachable) 0.46–119.10

Fe(II) containing COPR withmicrocosms

100% removed in 30 d; Ehdecreased (−77 ± 10 to−135 ± 40) for activemicrocosms

[150]

A site between a canal and ariver in North England.

Cr(III) 600–3000 Cr(VI)2000–3400 mg L−1 (1:1) leachsolution in weathered andunweathered sample

Microbial and HCl basedreduction. Microbial speciesviz. Firmicutes (82%);Actinobactria (3%);Alpha-Proteobacteria (3%);Gamma-Preoteobacteria (3%)and unidentified (9%) at pH8.5–9.5

100% in HCL amended in 0 day(pH 4–5) 100% in Microcosmsamended in 14 days

[151]

Zhenxing chemical plant,Henan, China

Cr(T) 39889 Cr(VI) 10700 Sucrose: starch and wheatflour at S:R of 1:2.5; 1:2; 1:1.5;1:1 and 1:0.5

100% for all reductants of 1:2.5(S:R) in 10 min at 600 ◦C

[160]

Whites chemical works atShawfield in South eastGlasgow

Cr(T) 65100 Cr(VI) 15600 Exsitu physiochemical, Insitumicrobiological, Organicmatter and SRB & FeSO4

treatment

94% reduction with in situmicrobiological treatment in a3.0 cm depth (from3000 mg kg−1 Cr(VI) conc)

[74]

Tailings from East coast of USA. Cr(VI) 250 Microbial reduction usingBacillus sp.; Microcococcus sp.and Rhodococcus sp.

6.9 mg Cr(VI) L−1 h−1 frominitial conc. of 750 mg L−1

[152]

Pyrite concentrate fromchalocopyrite tailing atBoliden plant in Aitik and 5steel slags from several steelplants in Sweden

Cr 100 in pyrite conc. Cr<10.0–13,100 in 5 steel slags

Bioleaching of metals usingLeptospirillium ferroxidants, At.Caldus, At thiobacillus,Sulpholobacillus sp. andFerroplasma sp.

Approximately 16% of the Crcontent in the slag dissolved(55 mg L−1) in 16 d

[177]

Chrome plating contaminantsFour electroplating effluent

contaminated soil fromRothak city, Haryana, India

Cr(T) 1172–3240 with Zn1376–3112; Fe 683–2228; Cu263–374 and Ni 234–335

Assessment of native metaltolerance plant species

Only plant species Amaranthusviridis accumulated1000 mg kg−1 Fe

[230]

Soils from an activeelectroplating facility ofnortheastern corner ofConnecticut

Max Cr(VI) 218.0 leached withmineral salt solution in 128 d

Indigenous bacterial strainswith diff media (DH2O,DH2O + Glucose, TSB, Mineralsalt(MS), MS + Glucose)incubated in dark at 20–22 ◦Cfor 128 d

64.5% reduction withMS = Glucose treat medium in128 days

[175]

Mining contaminated soilSukinda chromite mine

overburden, Odisha, IndiaCr(T) 23,468–136,842 Cr(VI)100–500

Bacterial reduction usingnative isolates

98% of Cr(VI) of the sample in10 h at 20% pulp density and7.0 pH

[178]

Cr(VI) contaminated soil fromSukinda mine area, Odisha,India

Cr(T) 3000–5480 Cr(VI) 12–66 Bacterial reduction 70.53% reduction from100 mg L−1 soln at pH 8.0, temp30 ◦C for 24 h withAcinrtobactor clcoaceticus

[179]

Leather tannery contaminantsSoil samples from Pallavaram

TamilNadu, IndiaCr 65600 and Cr(VI) 2520 Reduction by Bacillus sp.,

(20 mL) with glucose from92.5 mg L−1 Cr(VI)

100% Cr(VI) removed at pH7,temp 37 ◦C and. in 60 h

[155]

Sediments of tanneryaccumulated sites of Velloredistrict, TamilNadu, India.

Cr 47.4–682.4 mg L−1 Bacterial isolation andreduction

Bacterial species tolerant100–3000 mg L−1 Cr(III) andCr(VI) in solution

[158]

Soil samples from 7 differentsites of Ranipet, TamilNadu,India. pH 9.5–10.5

Cr(T) 9000 ±12 and Cr(VI)2500–5500

Microbial reduction withinoculums (15 mg g−1)andmolasses (34 mg g−1).

97% reduced in 20 d at pH 7and 35 ◦C

[156]

Soil from long term tannerywaste site of Mount Barker.Particle size <2 mm and pH8.4

Cr(T) 23000, Cr(VI) 5.75 and Cr(exchangeable) 10.9

150 g of soil in plastic cups(11 cm high x 7 cm wide,capacity 330 mL), Cow manure(6.6% w/w) incubated at 20 ◦C

In 20d all values were found innon-detectable range with cowmanure flooded.

[157]

Notations: GF/P – gastric fluid/phase; IF/P – intestinal fluid/phase.

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166]. Massara et al. [167] reported that the rhamnolipids have theapability of extracting 25% portion of the stable form of chromiumrom contaminated kaolinite and could be beneficial for long-erm conversion of Cr(VI) to Cr(III). Plant-derived bio-surfactantescin [168] and saponin have been used in aqueous solutionsor remediation of heavy metal contaminated soil [169], kaolin170] and incinerator fly ash. The treatment with saponin extracted4% of Cr from tannery sludge (8041 mg kg−1Cr) at pH 2 in 6 h at3 ◦C [147].

.3. Electroplating sludge and contaminated soil

The bio-treatment of electroplating waste sludge and contam-nated soil is also being investigated for leaching of metals andemediation of Cr(VI). Recently, Bayat and Sari [171] reported theioleaching of plating sludge in a completely mixed batch reactorCMBR) of 3.0 L capacity using A. ferrooxidans. The results showedhe low recovery of chromium (34%) along with reasonably goodeaching of other metals (97% Zn, 96% Cu, 93% Ni, 84% Pb and7% Cd) in 20 days time at 20 g L−1 pulp density, 2.0 pH and 25 ◦Crom the dewatered metal plating sludge with no sulphide orulphate compounds. The hybrid process involving bio-chemicalpproach was suggested for the treatment of a wastewater sludgeontaining chromium and other metals [172,173]. The chemicaleaching of the sludge in a 5.2 L continuous stirred tank reactorCSTR) with the bio-oxidized Fe(III) using A. ferrooxidans dissolvedbout 49% Cr along with 63% Cd, 71% Cu and 80% Zn. Earlier,ujii et al. [174] reported the potential application of a bacterialsolate (A. ferrooxidans) from the sludge of the plating and pickling

astewater pit of a steel works (Nippon Steel) for the oxidationf Fe(II). The oxidizability of Fe(II) from such waters containingeavy metals and chloride ions may be used to realize activatedludge treatment of the steel works water and for subsequentetal recovery by a suitable method. Turick et al. [175] reported

eaching of 218 mg kg−1 Cr(VI) with mineral salt solution from theoil contaminated with a active electroplating industry. Only 64.5%eduction was achieved due to growth of native bacterial strainsith mineral salt in glucose medium treated for 128 days.

.4. Contaminates from metallurgical and industrial waste

The biological treatment of slag and dust of ferrochrome andteel industries was also carried out to meet the environmentaltandards (Table 4). Higher bio-recovery of iron and heavy metalsrom the BF sludge and flue dust by A. ferrooxidans as compared tohat of a fungus was reported by Banerjee [176]. The BF slag cane used as a neutralizing agent (to pH 1.5) in place of slaked limeuring pyrite oxidation (75–80%) by a mixed mesophilic culturet 35 ◦C and the chromium leaching achieved was only 16% of itsontent in the AOD slag while dissolving 55 mg L−1 Cr in 16 days177].

.5. Contamination from mining waste

The microbial remediation of hexavalent chromium was alsonvestigated using native bacterial species [130] having a tolerancef 2000 mg L−1 Cr(VI) in Luria Bertani broth incubating at 35 ◦C,H 7 and 100 rpm. The chromate reduction rate was 3.22 × 10−2

o 6.5 × 10−3 h−1 for Cr(VI) concentration of 10 to 500 mg L−1.amuel et al. [126] reported the bacterial reduction of Cr(VI) usingukinda mine area isolates with the reduction rate varying from.199–0.477 mg L−1 h−1 to 0.5–1.16 mg L−1 h−1 at 5–20 mg L−1 of

nitial Cr(VI) concentration. In a recent study for the reduction ofr(VI) in chromite overburden/soil containing 500 mg Cr(VI) kg−1,acillus sp. (4.05 × 10 cell mL−1) reduced more than 98% of Cr(VI) inhe sample in 10 h at 20% pulp density and 7.0 pH [178]. Mishra et al.

ials 250– 251 (2013) 272– 291

[179] have also reported the reduction of Cr(VI) in solution usingbacterial strains isolated from 12 to 66 mg kg−1 Cr(VI) containingmine soil.

7. Bacterial interaction with chromium in soils/solid waste

Chromium contaminated soil has a low microbial populationdue to chromium toxicity. The bacterial species are able to growin the toxic conditions and are generally assumed to be toler-ant/resistant to chromium [59]. The terms resistance and toleranceare often used interchangeably, but their significance is different.Gadd [180] defined “resistance” as “the ability of a microorganismto survive toxic effects of metal exposure by means of a detoxifica-tion mechanism produced in direct response to the metal speciesconcerned” and defined tolerance as “the ability of a microorgan-ism to survive metal toxicity by means of intrinsic properties andor environmental modification of toxicity”. The first Cr(VI) reducingstrain, a Pseudomonas sp. was isolated from industrial wastewaterby Romanenko and Korenkov [181]. Since then, several researchershave isolated various Cr(VI) resistant microorganisms from dif-ferent sources. Although, many microbes in the environment aresensitive to Cr(VI) with few exceptions, bacteria isolated fromCr(VI) contaminated sites are reported to be highly resistant. A vari-ety of bacterial species isolated and reported for reducing Cr(VI) inhigher to lower concentration are summarised in the ‘supplemen-tary material’ of this paper. The details on isolation source, bacterialspecies, Gram character, Cr(VI) tolerance and efficiency of Cr(VI)reduction are also summarized.

7.1. Bacterial reduction of hexavalent chromium

As discussed above the toxicological effects of Cr(III) and Cr(VI)on living cells vary. Cr(III) is less toxic than Cr(VI) owing to its imper-meability into cell membrane as one of the possible reasons forinsolubility of Cr(III) that facilitates its precipitation and removal.Cr(III) inside the living cell is very toxic, as it disrupts DNA andforms stable bonds with S, N and O [182]. During the biotransfor-mation of Cr(VI) to Cr(III) in cells, the intermediate oxygen radicalsmay be formed and intermediate oxidation states of Cr, such asCr(V) and Cr(IV) may have more toxic effect than Cr(III) [182]. Butmicroorganisms are unique, specially Cr tolerant bacterial specieshave some alternative mechanism such as chromate resistant plas-mid [183] and iron efflux system [184] to counter such problems incellular mediated Cr(VI) to Cr(III) reduction and may be consideredas an alternative process for treating Cr(VI) contaminated wastes[25,185,186].

The mechanism of bacterial reduction for Cr(VI) varies fromstrain to strain depending upon their bio-geochemical activi-ties and nutrient utilization patterns which directly affect theresistance/tolerance to chromate [187,188]. All the microbial reme-diation of Cr(VI) follows either or a combination of the threereduction mechanisms. These are: (i) in aerobic conditions, it isassociated with soluble chromate reductases that use NADH orNADPH as cofactors, (ii) under anaerobiosis, some bacteria, likeDesulfotomaculum reducens [189], can use Cr(VI) as an electronacceptor in the electron transport chain, and (iii) reduction ofCr(VI) may also take place by chemical reactions associated withcompounds present in intra/extra cellular such as amino acids,nucleotides, sugars, vitamins, organic acids or glutathione. Cr(VI)-reduction may also follow plasmid resistance mechanism whichrepresents a potentially useful detoxification process for several

bacteria [25,190–192]. Thus, the bacterial property, which is par-ticularly useful for an effective bioremediation approach, is one thatcombines high tolerance/resistance with the ability to reduce Cr(VI)to less toxic Cr(III). It was recently observed that the microbes can
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B. Dhal et al. / Journal of Hazardous Materials 250– 251 (2013) 272– 291 285

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xhibit activation of several protective systems such as superoxideismutase (SOD) and catalase enzymes to accompany the metaltress on cell at higher concentration [193,194]. Thus, microbialr(VI)-reduction may be a result of stress strain response occurringhrough enzymatic activity [40,185] or indirectly through produc-ng a compound that can reduce Cr(VI) [195,196]. The bacterialeduction mechanism was previously described and presented inerobic and anaerobic conditions as shown in Fig. 6 [197].

The direct Cr(VI) reduction mechanism has been found in manyacterial genera including Pseudomonas, Micrococcus, Bacillus,chromobacter, Microbacterium Arthrobacter and Corynebacterium130,187,191,198–200]. Some bacterial species are capable of bothnaerobic and aerobic Cr(VI) reduction, others in either anaero-ic or aerobic conditions. The mechanisms through which bacterialtrains reduce Cr(VI) to Cr(III) are variable and species dependent198]. Anaerobic bacteria may use chromate as a terminal-lectron acceptor or reduce chromate in periplasmatic space byydrogenase or cytocrome c3 [189,201,202]. In aerobic bacteria,r(VI) reduction may take place by cellular reducing agents (therimary reductant is glutathione) and NADH-dependent chromateeductase [203–205]. It is yet unknown, although some hypothesesave been formulated, as to whether enzymatic or non-enzymaticeduction of chromate is dominant in bacterial cells under aerobiconditions, and it also remains unsolved whether the NADH-ependent reductases are specific to chromate only. Moreover, it

s also unclear whether anaerobic bacterial growth is supported athe expense of chromate as the only electron acceptor. The mech-nisms for Cr(VI) reduction might be a secondary utilisation oro-metabolism as suggested for Shewanella onoidensis MR-1 [206].herefore, under anaerobic conditions, Cr(VI)-reduction may ben activity of the reductase enzymes that have evolved on otherubstrates [6]. Losi et al. [51] isolated 20 Cr(VI) resistant bacte-ial strains from organic amended and Cr(VI) acclimatized soils,ut of which 9 strains showed good reducing ability for Cr(VI).acillus megaterium strain TKW3 isolated from multiple-metal-ontaminated marine sediments of Tokwawan was found to beacultative aerobic in nature which reduced chromate (0.2 mMr6+) and selenate (0.53 mM) in 360 h [207]. An enrichment con-

ortium and an isolate of sulfate-reducing bacteria (SRB) have beensed for the reduction of Cr(VI) [208].

The chromte reduction was also reported with Bacillus mega-erium isolated from marine sediments which showed a novel

robic (lower) conditions SR and MR represent soluble and membrane -associatedified from [197]).

aerobic membrane-associated reductase with Cr6+ induced expres-sion as revealed by peptide mass fingerprints (PMF) [209]. In manybioreduction experiments, it has been found that after bacterialreduction of Cr(VI) to Cr(III), the end product is not detected tobe Cr(OH)3 even at the neutral to alkaline pH. Some authors havealso reported that the chromium(VI) after bacterial reductionforms organo-Cr(III) complexes, which are possibly an integralpart of the biogeochemical cycle of chromium [210]. Dhal et al.[130] have reported that after chromate reduction by Bacillus sp.at pH 7, the Cr(III) was present as chromium hydrogen phosphate,CrH2P3O10.2H2O as characterized by PXRD phase identificationof dried reduced product. A blue pigment-producing bacterium,Vogesella indigofera, was isolated and quantified for Cr(VI) reduc-tion with emphasis on the pigment production during Cr(VI)stress. The blue pigment production was directly correlated withthe Cr(VI) concentration which may be useful for the detectionand monitoring of environmental Cr(VI) pollution [211].

Therefore, before using a selected microorganism or anindigenous microbial mixed-culture for devising bioremediationstrategies for Cr(VI) contaminated soils, there is a need to under-stand Cr(VI) resistance mechanisms in these microorganisms. Forexample Shewanella sp., among dissimilatory metal reducing bacte-ria, is identified as potential strain for ex situ as well as in situtreatment of chromate contamination [212,213]. It has also beendemonstrated that Cr(VI) is toxic even at low concentrations(0.015 mM) to Shewanella oneidensis MR-1, a good Cr(VI) reducer[214]. It may be noted that chromate resistance and reduction arenot necessarily interrelated, Cr(VI) may be reduced by both bacte-rial Cr(VI)-sensitive and resistant strains and not all Cr(VI)-resistantbacteria reduce Cr(VI) to Cr(III). The reduction of Cr(VI) can alsooccur indirectly by bacterial activity. For instance, Fe(II) and HS-,metabolic end products of iron and sulphate-reducing bacteria, cancatalyse the reduction of Cr(VI) [215–218].

8. Cr(VI) bioremediation practices with soil/solid waste

Bioremediation of Cr(VI) contaminated soils/solid waste is

essential due to continuous natural leaching of Cr(VI) to the envi-ronmental water streams and percolation to the ground watertable. The physical and chemical approaches have several disad-vantages and also produce secondary contaminants, which suggest
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evelopment of other alternative treatment methods to complyith the environmental norms. The processes that are essential

o consider are excavation or pumping of contaminated mate-ial, followed by the addition of reducing chemicals leading to therecipitation and/or sedimentation of reduced chromium [Cr(III)],hich is less toxic than Cr(VI) and is greatly insoluble [219]. Accord-

ng to above considerations, naturally occurring microbes adaptedo the contaminated site have a greater potential for Cr(VI) reme-iation strategies.

As discussed above, the ability of several microbial groupsbacteria) to reduce Cr(VI) to Cr(III) would be of interest in ordero clean up soil/solid waste polluted with chromate. The Cr(VI)ioremediation of soils can be performed in situ or ex situ using aioreactor for treatment of soils or soil wash effluents [6,152,188].he bioremediation approach offers some advantages compared toraditional techniques [219], viz. (i) it can be performed in situ with-ut excavation of contaminated soils, (ii) can be applied to sites withigh water table, and (iii) can offer a continuity in Cr(VI) reductionrocess over the period, and (iv) it does not destroy the site that iso be detoxified [67].

Generally in contaminated soils with hexavalent chromium,ome indigenous bacteria are able to reduce Cr(VI), but the rates ofatural attenuation (that is to say without any human interference)f Cr-toxicity are slow and, therefore, unacceptable to devise reme-iation strategies [220]. Thus, there is a need to improve the rate of

n situ bioreduction process for catalyzing the reduction of Cr(VI).he Cr(VI) remediation efficiency can be enhanced by introducingelected strains in soils with intrinsic properties, such as high Cr(VI)esistance and reduction capability (bio-augmentation) or stimu-ating the activity of indigenous reducers (bio-stimulators). Bothases have a strong limitation as the contaminated sites are usu-lly lacking in nutrients and do not permit rapid growth of selectednd/or indigenous bacteria and, therefore, their potential bioreme-iation activities are not fully expressed. A strategy to stimulatehe metabolism and proliferation of bacterial Cr(VI) reducers in situ

ay be the addition of nutrients to the environment [2,221,222].arbon sources such as organic acids, manure, molasses, have beenroposed to improve Cr(VI)-reduction that otherwise, would benacceptably slow [81,220,223]. Reddy et al. [222] have demon-trated that under laboratory conditions the nutrient amended soileduce Cr(VI) much faster.

Recent attempts made to bioremediate the chromium con-aminated soil using native microbial species are presented here.athiravan et al. [155] studied the bioremediation processes usingacillus sp. in batch and continuous operations. Bacillus sp. was iso-

ated from tannery effluent contaminated soil and the optimum pHnd temperature were found to be 7 and 35 ◦C respectively wherelucose was used as an electron donor. Concentration of Cr(VI)as found to be 2.5 mg g−1 of soil. In packed bed column exper-

ments, time required for complete Cr(VI) reduction was foundo be 120, 112, 96 and 100 h for 2, 3, 4 and 5 ml min−1 flow rateespectively at an average viable cells in the range of 3.2 × 108 to.9 × 108 colony forming units (cfu) mL−1. In another study biore-ediation of Cr(VI) has been studied using seven native isolates

156]. Among these isolates H1 strain showed a high Cr(VI) reduc-ion rate (0.12 mg Cr6+ mg−1 of cells h−1) which was selected forioreactor operations. It was found that at a bacterial concentra-ion of 14 ± 1.0 mg g−1 of soil (wet weight), 50 mg of molasses g−1

f soil as carbon source was required for the maximum reductionf Cr(VI). Bioreactor operated under these conditions could reducentire Cr(VI) (5.6 mg Cr(VI) g−1 of soil) in 20 days.

It has been proposed that the organic aromatic pollutants might

erve indirectly as carbon sources for microbial Cr(VI) reduction inresence of a mixed culture of appropriate taxa [224]. However, theddition of nutrients to Cr(VI)-contaminated soils is a laborious andxpensive approach and it may cause problems, because it results

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in the production of considerable biomass [225,226]. Approachesshould be made between a high level expression of a gene and theneed for rapid growth, giving maximal expression under conditionsof slow growth. This innovative technology could also be appliedto Cr(VI)-reduction in bioremediation of soil [226].

Several studies on ex situ Cr(VI) bioremediation of soils or soilwash effluents were also reported using bioreactors or biofilms[152,227,228]. The possibility to use bioreactor systems for Cr(VI)-bioremediation is cost-effective, though it has been limited tolarge-scale applications only [6]. Native isolated pure cultures ormicrobial consortia are found to be more effective for Cr(VI) biore-mediation in both ex situ as well in situ studies. Sulphate-reducingbacteria (mixed-culture) were used to treat Cr(VI) contaminatedwaterways and soils [227]. This suggests that this system can beused to recover Cr(III) from the reduction and precipitation ofCr(VI). Turick et al. [229] used a bioreactor for Cr(VI) reductionaided by an inoculum consisting of enrichment cultures of indige-nous microorganisms from Cr(VI) contaminated soils. A membranebioreactor of Pseudomonas cells were immobilized in agar-agarfilms on the surface of synthetic membranes and was used todecrease the toxic action of high chromate concentrations [230].

Bacterial chromium (VI) reduction was reported under bothaerobic and anaerobic conditions in presence of different elec-tron acceptors, such as oxygen, nitrate, sulphate and ferric iron.The optimum bioremediation of Cr(VI) was achieved in aerobiccondition at higher and anaerobic condition at lower Cr(VI) concen-trations [225]. It has been reported that under anaerobic conditions,low concentrations of Cr(VI) can accelerate the growth and activ-ity of sulphate-reducing bacteria, obligate anaerobic heterotrophs,and thereby induce the indirect Cr(VI)-reduction by evolved sul-phide [6]. The reduction of Cr(VI) to Cr(III) is enhanced in situusing sulphide produced by sulphate-reducing bacteria throughthe addition of sulphate and nutrients. The amount of sulphideis also to promote the reoxidation of Cr(III) by Mn oxides. More-over, sulphate-reducing bacteria are found to be more sensitiveto Cr(VI) toxicity compared to other bacterial populations [218].This is in accordance with Marsh et al. [231] for sandy sedimentswith 0.5 mM Cr(VI) where no production of sulphide by sulphate-reducing bacteria was noticed. Data obtained by Marsh et al. [231]and Arias and Tebo [218] may be considered for devising bioreme-diation strategies for Cr(VI)-contaminated soils.

9. Conclusion

This review is focused on the chromium production, use, cycleand speciation, toxicity and assessment of different chromiumcontaminated sites such as chromite ore processing residue(COPR), leather tanning waste sludge/soil, mining and metallurgi-cal slag/dust and chromium contaminated MSW soil. The currenttechnologies that are being used to effectively remediate Cr(VI)by means of chemical and biological methods are also discussed.The chemical treatment technologies may work well for specificcontaminated site, but has a drawback of transforming certaincompounds while producing byproducts/secondary contaminantsthrough redox cycle which may still have toxic effects. On theother hand microbial Cr(VI) bioremediation has a great promise toformulate an acceptable strategy because of microbial reduction ofCr(VI) to Cr(III), which is relatively insoluble and considerably lesstoxic, environmental friendly and safe, and may be economicalas well. The process development for better treatment of soil isneeded at the laboratory/bench scale followed by pilot studies

and in situ studies. With the emphasis on extracting and use ofthe bioactive compounds from metal tolerant/resistant microbes,faster kinetics of the processes may be realised. The bioremediationtechnology may focus on two important strategies, One is to deeply
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nderstand the mechanisms involved in the process of strain resis-ance and how some abiotic factors (initial chromate concentration,H, temperature, carbon sources, electron acceptors) affect the ratef Cr(VI)-reduction. Secondly, the capability of indigenous bacterian reducing Cr(VI) to Cr(III) is to be quantified and the optimal con-itions are to be defined in order to improve the ability of specificacterial strains to play their role under stressful conditions as wells those in polluted-environments. Also, native isolates (species)rom different contaminated sites can be used for specific solidaste treatment after series of adaptation with nutrient media inresence of such contaminants from lower to higher quantity.

cknowledgements

The authors are thankful to the Director, National Metallurgicalaboratory, Jamshedpur for his permission to publish the paper.ne of the authors (B. Dhal) is grateful to Council of Scientific &

ndustrial Research for awarding Senior Research Fellowship.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.jhazmat.013.01.048.

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