Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240

Modeling of equilibrium biosorption processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

Journal of Environmental Chemical Engineering 2 (2014) 239259

Received 29 October 2013

Accepted 20 December 2013

Keywords:

Biosorbent

Metal ions

Sequestration

Surface modication

Desorption

Recovery

m industrial activities pose a signicant threat to the environment and public

health due to their reported toxicity even at trace levels. Although there are several available methods to

treat or remove heavy metals from water and wastewater, the research focuses on development of

technological solutions which sound environmental friendly and economically feasible, able to reduce

the costs and maximize the efciency. In this framework, the biosorption process, which uses cheap and

non-pollutant materials, may be considered as an alternative, viable and promising, technology for

heavy metal and metalloid ions sequestration and ultimately removal technology in the waste water

treatment. However, there is as yet little data on full-scale applications for the design and testing of

adsorption units using single biosorbents and their combinations to sequester heavy metal ions from

multi-metal systems. Immediate research and development is hence earnestly required in this specic

direction to further make progress this blooming technology and widen its scope of application to real

situations needing heavy metal pollution remediation. This review provides a comprehensive appraisal

of the equilibriummodeling of a number of biosorption processes as well as the structural, chemical and

morphological modications and activation of biosorbents. Further the relative merits of the methods

used to recover sequestered heavymetal ions and regenerate biosorbents through desorption routes and

their future applications are discussed.

2013 Elsevier Ltd. All rights reserved.

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering

journa l homepage: www.e lsev ier .com/ locate / jeceNon-timber products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Modications and activation of biosorbents. . . . . . . . . . . . . . . . . . .Equilibrium adsorption isotherms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

Kinetic studies and models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

Pseudo-rst-order models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

Pseudo-second-order model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

Thermodynamic analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

Biomass and biosorption of metal ions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

Agro biosorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

Organometallic compounds biosorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

Weed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

Medicinal herbs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247Article history: Heavy metals released froBiomass-derived biosorbents for metal ions sequestration: Adsorbentmodication and activation methods and adsorbent regeneration

Ravindra Kumar Gautam a, Ackmez Mudhoo b, Giusy Lofrano c, Mahesh Chandra Chattopadhyaya a,*a Environmental Chemistry Research Laboratory, Department of Chemistry, University of Allahabad, Allahabad 211 002, IndiabDepartment of Chemical and Environmental Engineering, Faculty of Engineering, University of Mauritius, Reduit, MauritiuscDepartment of Environment, Salerno Province, 84132 Fisciano, SA, Italy

A R T I C L E I N F O A B S T R A C TModication techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

* Corresponding author. Tel.: +91 9307663844; fax: +91 532 2541786.

E-mail addresses: ravindragautam1987@gmail.com (R.K. Gautam), ackmezchem@yahoo.co.uk (A. Mudhoo), glofrano@unisa.it (G. Lofrano),

mcchattopadhyaya@gmail.com (M.C. Chattopadhyaya).

2213-3437/$ see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jece.2013.12.019

Activation of adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

Activated carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

Activated seaweed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

Activated sawdust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

Activated bagasse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

Activated carbon from agro-waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

New activated biosorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

Recovery of metals, regeneration and desorption of biosorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252

Outlooks and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254

Concluding notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

R.K. Gautam et al. / Journal of Environmental Chemical Engineering 2 (2014) 239259240Table 1Signicant anthropogenic sources of heavy metal in the environment.

Industry Metals Pollution arising References

Electroplating Cr, Ni, Zn, Cu Liquid efuents from plating processes [22,23]

Batteries Pb, Sb, Zn, Cd, Ni, Hg Waste battery uid, contamination of soil and groundwater [24]

Paints and pigments Pb, Cr, As, Ti, Ba, Zn Aqueous waste from manufacture, old paint deterioration and soil pollution [25]

Landll leachate Zn, Cu, Cd, Pb, Ni, Cr, Hg Landll leachate, contamination of ground and surface water [26,27]

Electronics Pb, Cd, Hg, Pt, Au, Cr, As, Ni, Mn Aqueous and solid metallic waste from manufacturing and recycling process [28]

Metalliferous mining Cd, Cu, Ni, Cr, Co, Zn, As Acid mine drainage, tailings, slag heaps [29,30]

Fertilizers Cd, Cr, Mo, Pb, U, V, Zn Run-off, surface and groundwater contamination, plant bioaccumulation [31,32]

Manures sewage sludge Zn, Cu, Ni, Pb, Cd, Cr, As, Hg Land spreading threat to ground and surface water [31,33]

Specialist alloys and steels Pb, Mo, Ni, Cu, Cd, As, Te, U, Zn Manufacture, disposal and recycling of metals. Tailings and slag heaps [34,35]

Paper and pulp Zn, Cu, Cd, Pb, Ni, Fe, Mn Wastewater efuents [36]Introduction

Heavy metal contamination of aqueous media and industrialefuents is one of the signicant environmental problems due tothe toxic nature and accumulation of these metal ions in thefood chain, because they are non-biodegradable [1,2]. Althoughseveral adverse health effects of heavy metals have been knownfor a long time, exposure to heavy metals continues, and is evenincreasing in some parts of the world, in particular in lessdeveloped countries, though emissions have declined in mostdeveloped countries over the last 100 years. For example,mercury is still used in gold mining in many parts of LatinAmerica. Arsenic is still common in wood preservatives, andtetraethyl lead remains a common additive to petrol, althoughthis use has decreased dramatically in the developed countries.Heavy metal contamination exists in wastewater of manyindustries such as metal plating, mining operations, surfacenishing industry, tanneries, paper and pulp industries, chlor-alkali, fertilizer and pesticide industry, radiator manufacturing,smelting, energy and fuel production, aerospace and atomicenergy installation, alloy industries, electroplating and batteriesindustries [311].

While many of the heavy metals are needed by biologicalsystems at the micronutrient level, higher concentrations areknown to produce a range of toxic effects. A high exposure to lead(Pb) causes encephalopathy, cognitive impairment, behavioraldisturbances, kidney damage, anemia and toxicity to the repro-ductive system [12]. Chromium (Cr) is widely accepted to exerttoxic effects in its hexavalent form due to its strong oxidationproperties [13,14]. Human exposure to Cr(VI) compounds isassociated with a higher incidence of respiratory cancers [15,16].After it reaches the blood stream, it damages the kidneys, causesirritation and corrosion of skin, the liver and blood cells throughoxidation reactions [17]. Cadmium (Cd) has been established as avery toxic heavy metal. Due to its acute toxicity, Cd has recentlyjoined lead and mercury in the most toxic Big Three category ofheavymetals with the greatest potential hazard to humans and theenvironment [18]. Symptoms of acute poisoning include head-aches, nausea, vomiting, weakness, pulmonary edema anddiarrhea. A disease known as ItaiItai in Japan is specicallyassociatedwith cadmiumpoisoning, resulting inmultiple fracturesarising from osteomalacia. High dose of copper (Cu) concentrationscan lead to weakness, lethargy, anorexia and damage to thegastrointestinal tract [19]. Human exposure to highly nickel (Ni)polluted environments can cause skin allergies, lung brosis, andcancer of the respiratory tract [20,21]. The exact mechanisms ofnickel-induced carcinogenesis are not known and have been thesubject of various epidemiologic and experimental investigations.Table 1 summarizes the anthropogenic sources of heavy metals inthe environment.

There are several methods for removing heavy metal ions fromaqueous solutions and mainly consist of physical, chemical andbiological techniques and technologies. Conventional methods forremoving toxic metal ions from aqueous solution have beenrecommended, such as chemical precipitation [37], ltration [38],ion exchange [39], electrochemical treatment [40], membranetechnologies [41], oatation [42,43], adsorption on activatedcarbon [44,45], evaporation and photocatalysis [4648,50].Table 2 shows the treatment technologies for the removal ofheavy metals from wastewaters and related advantages anddisadvantages.

However, chemical precipitation and electrochemical treat-ment are ineffective, especially when metal ion concentrations inaqueous solution are low, and also alongside these removaltechniques produce large quantity of sludge which require furthertreatment [10]. Ion exchange, membrane technologies andactivated carbon adsorption process are extremely expensivewhen treating large amount of industrial efuent and wastewatercontaining heavy metal ions in low concentration, they cannot beused at large industrial scale. Despite its extensive use in the waterand wastewater treatment technologies, activated carbon remainsan expensive material.

socia

t

R.K. Gautam et al. / Journal of Environmental Chemical Engineering 2 (2014) 239259 241Recently, magnetic nanoparticles have been investigated fortheir potential to remove heavymetal ions from aqueous solutions[57,58]. Polyrhodanine-coated g-Fe2O3 magnetic nanoparticleshave been synthesized by one-step chemical oxidation polymeri-zation and were applied to the process of removal of heavy metalions from aqueous solution [59]. These materials however have alimited application in the removal of heavy metal ions because oftheir magnetic properties. With time, the demand of safe drinkingwater and need of economical methods for the elimination ofheavy metals from wastewater has dictated research interest

Table 2Treatment technologies for the removal of heavy metals from wastewaters and as

Technology Advantages

Chemical precipitation Process simplicity

Not metal selective

Inexpensive capital cost

Ion exchange Metal selective

Limited pH tolerance

High regeneration

Coagulationocculation Bacterial inactivation capability

Good sludge settling and dewatering

Characteristics

Flotation Metal selective

Low retention times

Removal of small particles

Membrane ltration Low solid waste generation

Low chemical consumption

Small space requirement

Possible to be metal selective

Electrochemical treatment No chemical required can be

engineered to tolerate suspended solids

Moderately metal selective

Magnetic separation and

purication technique

Induced separation and purication of

magnetically inuenced contaminants

Adsorption Wide variety of target pollutants

High capacity

Fast kinetics

Possibly selective depending on adsorbentoward the production of low cost alternative bio-based adsor-bents in preference to commercially available activated carbon.

Since environmental protection and stewardship is becomingan important global concern and urging one and all to developgreener remediation techniques and pollutant sequestration and/or degradation routes, biosorption has become a promisingtechnique for metal removal, and heavy metal and metalloid ionsremoval. Biosorption has been dened as the property of certainbiomolecules to bind and concentrate selected ions or othermolecules from aqueous solutions [18]. As opposed to a muchmore complex phenomenon of bioaccumulation based on activemetabolic transport, biosorption by dead biomass is passive andbased mainly on the afnity between the sorbent and sorbate.Materials such as various bacteria, yeast, fungi, algae, chitin andchitosan have been used to prepare several types of biosorbents.Biosorption of heavy metal ions has become a popular environ-mentally driven research topic and it represents only oneparticular type of a concentration-removal aspect of the sorptionprocess [10]. In recent years, applying the biosorption technologyin controlling and removing heavy metal pollution has beenrewarded much attention, and has gradually become a keystone inenvironmental remediation. Findings of studies on the develop-ment and potential use of different biosorbents have revealed themerits of biosorption as an effective technique for cleaning toxicmetal ion-bearing efuents [60].

Over the past decades, many new processes have been testedsuccessfully, a lot of them have gone into operation and a greatnumber of papers have been published in this eld. We hope thatthis review will help provide a better understanding andorientation in the important and interesting eld of biomassbased activated biosorbents and biosorption of heavy metals. Themethodology of studying biosorption is based on an interdisci-plinary approach to it, whereby the phenomenon can be studied,examined and analyzed from different angles and perspectives by chemists, chemical engineers as well as by environmentalists.

Modeling of equilibrium biosorption processes

ted advantages and disadvantages.

Disadvantages References

Large amount of sludge containing metals

Sludge disposal cost

High maintenance costs

[49]

High initial capital cost

High maintenance cost

[39]

Chemical consumption

Increased sludge volume generation

[49]

High initial capital cost

High maintenance and operation costs

[42]

High initial capital cost

High maintenance and operation costs

Membrane fouling

Limited ow-rates

[51,52]

High initial capital cost

Production of hydrogen (with some processes)

Filtration process for ocs

[53,54]

The collection of particles depends strongly on

the creation of large magnetic eld gradients,

as well as on the particle size and magnetic

properties.

[55]

Performance depends on type of adsorbent

Physical or chemical activation to improve its

sorption capacity

[56]Assessment of a solidliquid sorption system is usually basedon two types of investigations: equilibrium batch sorption testsand dynamic continuous-ow sorption studies. Equilibriumisotherm model equations such as Langmuir [61] and Freundlich[62] are used to describe experimental adsorption data in batchmode [63,64]. It is important to nd best-t isotherm to evaluatethe efcacy of the prepared adsorbent to develop suitableindustrial adsorption system designs.

Equilibrium adsorption isotherms

Equilibrium isotherm models are usually classied into theempirical equations and the mechanistic models. The mechanisticmodels are based on mechanism of metal ion biosorption, whichare able not only to represent but also to explain and predict theexperimental behavior. Some empirical models for single solutesystems are listed in Table 3. The Langmuir model (based onmonolayer adsorption of solute) and the Freundlich model(developed for heterogeneous surfaces) are the most widelyaccepted and used in literatures [6569]. The BrunauerEmmettTeller (BET) model [70] describes the multi-layer adsorption at thebiosorbent surface and assumes that the Langmuir isothermapplies to each layer. These models can provide information ofmetal uptake capacity and difference in metal uptake betweenvarious species.

Langmuir adsorption isotherms are widely used to describe therelationship between the amount of adsorbate adsorbed onadsorbent and its equilibrium concentration in aqueous solution.

and

ot

V

qe=

VS

KRh

R.K. Gautam et al. / Journal of Environmental Chemical Engineering 2 (2014) 239259242Langmuir equation relates the coverage of molecules on a solidsurface to concentration of a medium above the solid surface at axed temperature. Langmuir isotherms are based on threeassumptions: (i) the surface of the adsorbent is in contact witha solution containing an adsorbate which is strongly attracted tothe surface; (ii) the surface has a specic number of sites where thesolute molecules can be adsorbed; (iii) the adsorption involves theattachment of only one layer of molecules to the surface, i.e.monolayer adsorption [71,72].

At lower concentrations, an alternate isotherm developed byHerbert F. Freundlich frequently describes the data better. TheFreundlich isotherm is an empirical equation. This equation is oneamong the most widely used isotherms for the explanation ofadsorption equilibrium. Freundlich isotherm is capable of describ-ing the adsorption of heavy metals on a wide variety ofbiosorbents. On average, a favorable adsorption tends to haveFreundlich constant n between 1 and 10. Larger value of n (smallervalue of 1/n) implies stronger interaction between biosorbent andheavy metal while 1/n equal to 1 indicates linear adsorptionleading to identical adsorption energies for all sites [72].Sometimes, these empirical models do not reect anymechanismsof sorbate uptake and hardly have a meaningful physicalinterpretation for biosorption. It has been mentioned that theresults from empirical models cannot be extrapolated and nopredictive conclusions can be nd out for biosorption of heavymetals from aqueous solutions operating under different environ-mental conditions [10]. Both simple basic models (Langmuir andFreundlich models) also do not incorporate the effects of any

Table 3Summary of widely used isotherms for biosorption systems with their advantages

Isotherm Functional form Linear form Pl

Langmuir qe qmKLCe1KLCeCeqe 1KLqm

1qm

CeCeqe

Freundlich qe KFC1=ne ln qe lnKF 1n lnCe ln

Temkin qe RTb lnKTCe qe RTb lnKT RTb lnCe qe

Redlich

Peterson (RP)

qe KRPCe1aRPCbe

ln KRPCeqe

1

h i ln aRP b lnCe lnexternal variable environmental factors, although they are capableof describing many biosorption isotherms in most cases. Themechanistic conclusions from the good t of the models aloneshould be avoided. Moreover, biosorption isotherms may exhibitan irregular pattern due to the complex nature of both thebiosorbents and its varied multiple active sites, as well as thecomplex solution chemistry of some metallic compounds.

For instance Mohan and Singh [44] evaluated the behavior ofmodied bagasse, a waste product in sugar rening industry, forthe removal of aqueous cadmium and zinc from wastewater. Theadsorption studies were carried out both in single- and multi-component systems. The data were better tted by the Freundlichisotherm as compared to Langmuir in both the single- and multi-component systems. The Freundlich and Langmuir constantsobtained at 40 8C for Cd(II) and Zn(II) were 5.43 (mg g1),49.07 (mg g1), and 6.04 (mg g1), 54.00 (mg g1); respectively.Equilibrium isotherms had been used to obtain the thermody-namic parameters and the obtained result showed that theadsorption was endothermic in nature.Rao et al. [73] have studied the removal of Cr(VI) and Ni(II) fromaqueous solution using bagasse and y ash as low-cost potentialadsorbents. Raw bagassewas pretreatedwith 0.1 NNaOH followedby 0.1 N CH3COOH before its application. The equilibrium sorptiondata were correlated with Langmuir, Freundlich and Bhattacharyaand Venkobachar adsorption models. The efciencies of adsorbentmaterials for the removal of Cr(VI) and Ni(II) were found to bebetween 56.2% and 96.2% and 83.6% and 100%, respectively.

In some cases the isotherm model such as Langmuir orFreundlich fails to describe the biosorption behavior of metalions from aqueous solutions onto the biosorbent, and in mostcases, more than one model have been applied to illustrate thebiosorption mechanism. Several models initially developed for gasphase adsorption can be implemented to correlate heavy metalsbiosorption processes [74,75]. Some of these equations containtwo tting parameters such as Temkin isotherm [76], FloryHuggins [77], and DubininRaduskevich equations [78], whereasothers can have more than two parameters RedlichPaterson [79]and Sips isotherms [80]. Some other workers have given detailsabout these models [60,72,81].

Kinetic studies and models

Adsorption equilibria studies are important to conclude theefcacy of adsorption. In spite of this, it is also necessary to identifythe adsorption mechanism type in a given system. Kinetic modelshave been exploited to evaluate the mechanisms of biosorption ofheavy metals and its potential rate-controlling steps that include

disadvantages.

Advantages Disadvantages

S Ce Has Henry law and nitesaturation limit so valid

over a wide range of

concentration

Based on monolayer

assumption

VS lnCe Simple expression andhas parameter for surface

heterogeneity

Does not have Henry law

and no saturation limit,

not structured, not

applicable over wide

range of concentration

lnCe Simple expression Same as Freundlich.

It does not have correct

Henry law limit and nite

saturation limit, not

applicable over wide range

of concentrationPCeqe

1

iVS lnCe Approaches Freundlich

at high concentration

No special advantagesmass transport and chemical reaction processes [82]. In addition,information on the kinetics ofmetal uptake is required to select theoptimum condition for full-scale batch metal removal processes.Predicting the rate of adsorption for a given system is amongthe most important factors in adsorption system design, as thesystems kinetics determines adsorbate residence time and thereactor dimensions [72]. As previously noted that various factorsgovern the adsorption capacity, i.e., initial heavy metals concen-tration, temperature, pH of solution, biosorbent particle size, heavymetals nature, a kinetic model is only concerned with the effect ofobservable parameters on the overall rate.

Several adsorption kinetic models have been established tounderstand the adsorption kinetics and rate-limiting step. Theseinclude pseudo-rst and -second-order rate model, Weber andMorris sorption kinetic model, AdamBohartThomas relation,rst-order reversible reaction model, external mass transfermodel, rst-order equation of Bhattacharya and Venkobachar,Elovichs model and Ritchies equation. The pseudo-rst- and -second-order kinetic models are the most well liked model to

R.K. Gautam et al. / Journal of Environmental Chemical Engineering 2 (2014) 239259 243In order to t above equation to experimental data, theequilibrium sorption capacity, qe, must be known. Inmany cases qeis unknown and as chemisorption tends to become unmeasurablyslow, the amount sorbed is still signicantly smaller than theequilibrium amount. In most cases in the literature, the pseudo-rst-order equation of Lagergren does not t well for the wholerange of contact time and is generally applicable over the initial2030 min of the sorption process [84].

Furthermore, one has to nd some means of extrapolating theexperimental data to t =1 or treat qe as an adjustable parameter tobe determined by trial and error. For this reason, it is necessary touse trial and error to obtain the equilibrium sorption capacity, qe, inorder to analyze the pseudo-rst-order model kinetics.

The pseudo-rst-order kinetic model has been used exten-sively to describe the sorption of metal ions onto biosorbents[85104]. The main disadvantages of this model are (i) that thelinear equation does not give theoretical qe values that agree withexperimental qe values, and (ii) that the plots are only linear overthe rst 30 min, approximately. Beyond this initial 30 min periodthe experimental and theoretical data do not correlate well.Various authors have made proposals for these deviations. Onesuggestion for the differences in the qe values is that there is atime lag, possibly due to a boundary layer or external resistancecontrolling at the beginning of the sorption process [105]. Thistime lag is difcult to quantify and does little to help rationalizethe differences in experimental and theoretical qe values. In termsof the short time correlation between experimental andtheoretical data for the pseudo-rst-order model, it has beenproposed that another rst-order reaction supersedes the rst orwenot found equal to the intercept of a plot of log(qe qt) against t,hereas in a true rst-order sorption reaction log(qe) should bequal to the intercept of a plot of log(qe qt) against t.ii. Tstudy the biosorption kinetics of heavy metals and quantify theextent of uptake in biosorption kinetics.

Pseudo-rst-order models

The Lagergren rst-order rate expression based on solidcapacity is generally expressed as follows [83]:

dq

dt k1qe q

where q and qe are amounts of adsorbate adsorbed (mg g1) at

time, t (min) and at equilibrium, respectively, k1 is the rateconstant of adsorption (l min1). Integration of the above equationwith the boundary conditions: t = 0, q = 0, and t = t, q = q, gives

lnqe q ln qe k1twhich can be represented as follows:

q qe1 expk1tor it can be expressed as:

logqe qt logqe k1

2:303t

The equation applicable to experimental results generally differsfrom a true rst-order equation mainly in two ways [84].

i. The parameter k1(qe qt) does not represent the number ofavailable sites.he parameter log(qe) is an adjustable parameter which is oftenanother reaction of another order becomes predominant[83,106108]. No mechanisms have been proposed for this stageby any authors. Several researchers [109113] have tested theLagergren pseudo-rst-order reactionmodel for Cu(II), Cr(VI) andCu(II), Pb(II), Cu(II) and for Cu(II). In general, most of the ts aremoderate to poor. In studies on the sorption of cadmium bychitosan the pseudo-second-order kinetic model was much moresuccessful [114116]. However, it should be noted that thelimited model by Lagergren is generally restricted to only theinitial 20% to 40% of the adsorption capacity. The equation needsfurther modication for longer sorption times and higherfractional surface coverages. Hence, when the experimental dataare inspected several sets of results have very few data points inthe initial 2040% period and therefore the rejection of a pseudo-rst-order reaction model on the grounds of poor model ttingmay not be the correct reason. A more logical approach would beto analyze the likely mechanism of a rst-order metal ionsorption uptake process on biosorbents, and compare this with amodel where involving a second-order or higher process seemsmore likely. Nevertheless, there are several examples of the use ofthe pseudo-rst-order kinetic analysis for other systems, such as,Fe(II) on wollastonite, or Cd(II) and Ni(II) on peanut hull carbonby copper-coated moss [117].

Pseudo-second-order model

Pseudo-second-order model is derived on the basis of thesorption capacity of the solid phase, expressed as [111,112,118120,252]:

dq

dt k2qe q2

Integration of above equation with the boundary conditions t = 0,q = 0, and at t = t, q = q, results in

1

qe q 1

qe k2t

This equation can be expressed as

t

q t

qe 1k2q2e

where k2 is the equilibrium rate constant of pseudo-second-order adsorption (g mg1 min1). The pseudo-second-order rateconstants can be determined experimentally by plotting t/qversus t. As such, in comparison to pseudo-rst-order kineticthis model is considered more appropriate to represent thekinetic data in biosorption systems [72]. The pseudo-rst and-second-order rate expressions have been and still in wide-usefor studying the biosorption of heavy metals from aqueoussolutions. In chemisorption process, the pseudo-second order issuperior to pseudo-rst order model as it takes into account theinteraction of adsorbentadsorbate through their valency forces[72].

The pseudo-second-order kinetic model has been the mostwidely celebrated model for the biosorption of metal ions fromaqueous solutions [112]. There are several examples where thepseudo-second-order model was best tted for the biosorption ofmetal ions onto biosorbents, i.e., copper [109,110,113,122],cadmium [114116,123], and lead [124]. All the literature tsusing this method for chitosanmetal ion systems provide anexceptionally high degree of correlation between the modelresults and the experimental data over the whole range of thesorption uptake capacity. The main deciency of this extremely

R.K. Gautam et al. / Journal of Environmental Chemical Engineering 2 (2014) 239259244useful model stems from the fact that it is a pseudo-kineticmodel, so a specic but different rate constant is obtained foreach change in system variable [84]. So it is important to developan equation for correlating the pseudo-rate constant with eachvariable [122].

The Weber and Morris sorption kinetic model [125] wasinitially employed by Pasavant et al. [126]. The equation can beexpressed as:

q KWMpt

where q and KWM are the amount adsorbed at time t, and Weberand Morris intraparticle diffusion rate constant, respectively. Htrepresents the square root of time t.

According to them, the biosorption by Caulerpa lentilliferabiomass for Cu(II), Cd(II), Pb(II), and Zn(II)was regulated by twomain mechanisms, i.e., intraparticle diffusion and external masstransfer. The intraparticle diffusion can be estimated with

D p8640

dpKWMqe

2

where D and dp is the intraparticle diffusion coefcient and meanparticle diameter, and KWM and qe is the amount adsorbed at time tand amount of solute adsorbed at equilibrium condition;respectively.

The external mass transfer process was determined by

dq

dt K 0LAC Cis

where C and Cis is the liquid phase concentration of sorbate inthe bulk solution at time t and concentration of sorbate in theinner pore of sorbent, and KL and A is the liquidsolid masstransfer coefcient and specic surface area of biomass;respectively.

They observed that the external mass transfer coefcients canbe ordered from high to low values as Cu(II) > Pb(II) > Zn(II) > C-Cd(II), while the intraparticle diffusion coefcients were asfollows: Cd(II) > Zn(II) > Cu(II) > Pb(II).

Thermodynamic analyses

Gibbs free energy (DG), change in enthalpy (DH), and changein entropy (DS). Negative DG indicates the spontaneity of theadsorptionprocess.DH is used to identify thenature of adsorption[127]. A positive value of DH indicates the reaction is endother-mic, and the negative value of DH shows that the reaction isexothermic [15,127]. A positive value of DS indicates increasedrandomness of adsorbate molecules on the solid surface than insolution [128]. The free energy of adsorption (DG) can be relatedwith the Langmuir equilibrium constant by the followingexpression [129]:

DG RT lnKLEnthalpy and entropy changes are also related to the Langmuirequilibrium constant by the following expression:

lnKL DSRDH

RT

Thus, a plot of ln KL versus 1/T should be a straight line.DH andDSvalues could be obtained from the slope and intercept of this plot.The thermodynamic parameters such as changes in standard freeenergy (DG), enthalpy (DH), and entropy (DS) were determinedby using the following equation [130132]:

DG DH TDS

The slope and the intercept of the plot ofDG versus Twere used todetermine theDS andDH values [133]. Malik et al. [134] reportedthat the sorption of Hg2+ onto sunower stem was exothermic andthey observed DH value of 32 kJ mol1. The negative entropychange, DS, conrmed the decreased randomness at the solidsolution interface during biosorption and reversibility of mercurybiosorption [135]. The results showed that the biosorption capacityof Carica papaya for Hg2+ decreased with increase in temperature.The saturated monolayer biosorption capacity, qmax, was found todecrease from 185.62 to 119.56 mg g1 for an increase in solutiontemperature from 293 to 323 K. That was a clear indication that thebiosorption of Hg2+ on C. papaya biosorbent was an exothermicprocess. Conversely, the Langmuir isotherm constant decreasedfrom 0.00812 to 0.00502 L mg1, as temperature was varied from293 to 323 K. Yasemin and Zek [130] have calculated the values ofstandard free energy (DG) for the biosorption of Pb2+, Cd2+, and Ni2+

onto sawdust ofwalnut. The values ofDG (calmol1) for biosorptionof Pb2+, Cd2+, and Ni2+ ions at temperatures 25, 45 and 60 8C were1675.72, 1699.40, 1024.38; 1914.55, 2104.11, 1503.84;and 2071.03, 2547.43, 2077.65, respectively. The values ofDS(calmol1 K) and DH (calmol1) for Pb2+, Cd2+, and Ni2+ attemperature 45 8C were 11.33, 24.01, 29.76 and 1696.80, 5479.30,7879.60, respectively. Positive values of DH suggested theendothermic nature of the adsorption and the negative values ofDG indicated the spontaneous nature of the adsorption process.However, the negative values ofDGwith an increase in temperatureindicated that the spontaneous nature of adsorption was inverselyproportional to the temperature. The positive values ofDS showedthe increased randomness at the solidsolution interface during theadsorption process [136]. It was stated that the adsorbed watermolecules, which were displaced by the adsorbate species, gainmore transitional energy than was lost by the adsorbate ions, thusallowing the prevalence of randomness in the system. Theenhancement of adsorption at higher temperatures may beattributed to the enlargement of pore size and/or activation of theadsorbent surface [137].

Biomass and biosorption of metal ions

Of the many types of biosorbents (i.e. fungi, bacteria, yeasts,weeds, and industrial and agricultural wastes) recently investigat-ed for their ability to sequester heavymetal ions, low-cost biomasshave proven to be highly effective as well as reliable andpredictable in the removal of, for example, Pb2+, Cu2+, Cd2+, Cr6+,and Zn2+ ions from aqueous solutions. Uptake of metals bydifferent low-cost biosorbents is summarized in Table 4.

Agro biosorbents

Recently Schiewer and Patil [96], investigated pectin-rich fruitwastes as biosorbents for heavy metal ion removal. According totheir studies pectin-rich fruitmaterials and citrus peelswere foundto be most suitable for cadmium biosorption. Equilibrium kineticswas achieved within 3090 min, depending upon particle size. Apseudo-second-order model was found to be more suitable than arst-ordermodel to describe the kinetics. Isotherm studies showedas the data were well tted to Langmuir model. It was alsoobserved that the metal uptake decreased with decreasing pH,indicating competition of protons for binding to acidic sites.

Batch adsorption experiments were performed by Dhir andKumar (2010) [143] to study adsorption potential of agricultural

Table 4Recently reported biosorption studies of biomass materials for the removal of heavy metals and metalloids.

Biomass material Metal Biomass type Biosorption

capacity

(mgg1)

pH Reaction

time (h)

Adsorbent

concentration

(g L1)

Remarks References

Garcinia cambogia As Plants 704.11 6 0.5 5 Immobilized beads of a tropical tree fruit native to India [148]

Waste crab shells As Animal 8.3 2.51 24 2.0 Ground into particles, then washed with 1M HCl [149]

Rice husk ash Se Crop 2.007 28 100 110 The rice husk ash obtained from burning of rice husk in electricaloven at 700 8C for 3h

[138]

Saccharomyces cerevisiae

(brewers yeast) (from brewery)

Hg Fungi (yeast) 133.3 6.0 2.0 Magnetically modied Brewers yeast cells [150]

Penicillium oxalicum var. Armeniaca (residue

from fermentation industry)

Hg Fungi 269.3 56.2 72 0.1, 0.2 Treated with 10MNaOH under boiling; ground and passed through

125mm screen[151]

Tolypocladium sp. (residue

from fermentation industry)

Hg Fungi 161.0 7 72 0.1, 0.2 Ground and passed through 125mm screen [151]

Cystoseira baccata Hg Algae 329 6.0 4 2.5 Ground and passed through 0.51mm screen [152]

Spirulina sp. (commercially available) Cd Bacteria 99.5 7 0.5 1 Lyophilized bacterial cells [153]

Saccharomyces cerevisiae (waste brewers yeast) Cd Fungi (yeast) 15.4 4 3 2 Ground and passed through 150mm screen [154]Bakers yeast (lab cultured) Cd Fungi (yeast) 11.63 0.5 1 Modied by crosslinking cystine with glutaraldehyde [155]

Phomopsis sp. (lab cultured) Cd Fungi 291 6.0 24 2.0 Mycelium boiled with 0.5M NaOH, dried at 60 8C, ground andpassed through 100mm screen

[156]

Ulva onoi Cd Algae 61.9 7.8 12 1 Ground and passed through 14mm screen [157]

Ulva onoi Cd Algae 90.7 7.8 12 1 Soaked in 0.1M NaOH for 1h, then ground and passed through

14mm screen

[157]

Gelidium sesquipedale Cd Algae 18.0 0.2 5.3 1 2 Ground and sieved (0.251mm in size) [158]Olive pomace Cd Plants 5.5 0.1 5 10 Dried, ground and washed by distilled water [159]Azolla liculoides Cd Plants 111132 5.5 10 2 2.0mm in size; activated by alkali and CaCl2/MgCl2/NaCl (2:1:1,

molar ratio); experiments conducted at 1040 8C[160]

Phragmites australis shoot Cd Plants 10.2 0.3 3 1 Ground and passed through 90mm screen, then treated with 0.1MNaOH; optimal pH for adsorption: near neutral

[161]

Parthenium hysterophorous Cd Weed 27 34 0.333 10 Weed can be utilized for the treatment of Cd(II). Recovery of 82%

Cd(II) in the solution of 0.1M HCl had been obtained

[146]

Oryza sativa Cd Plants 20.70 6 0.166 16 Dried rice straw was grinded and sieved to pass 4060 mesh. The

biomass was mixed with urea in 1:2 by mass and irradiated in a

microwave oven for a period of 12min

[251]

Spirodela polyrhiza (L.) Schleiden biomass Cd Aquatic plant 36.0 6.0 120 0.1 The huge biomass generated from Spirodela polyrhiza can be

successfully utilized for wastewater treatment

[255]

Orange peels Pb Plants 400 35 0.51.5 1 Protonation was carried out by suspending the sorbent material in0.1M HNO3 for 3h, rinsing, and drying for 12h at 40 8C

[162]

Orange peels Pb Plants 1.22 4.56.0 1 1.67 Effects of alkaline saponication, different concentrations of citric

acid and different temperatures on the preparation of orange peel

biosorbents were investigated

[163]

Gloeocapsa gelatinosa Pb Bacteria 256.41 4 0.5 0.1 Lyophilized cells with capsular polysaccharide (CPS) [164]

Saccharomyces cerevisiae (waste brewers yeast) Pb Fungi (yeast) 85.6 4 3 2 Ground and passed through 150mm screen [154]Penicillium chrysogenum (lab cultured) Pb Fungi 204 5.5 6 1 Modied with polyethylenimine and crosslinked with

glutaraldehyde

[165]

Penicillium oxalicum var. Armeniaca

(residue from fermentation industry)

Pb Fungi 47.4 5 72 0.1, 0.2, 0.3 Treated with 10MNaOH under boiling; ground and passed through

125mm screen[151]

Sargassum sp. Pb Algae 303 5 6 1 Treated with 0.2% formaldehyde [166]

Azolla liculoides Pb Plants 264297 5.5 10 2 2.0mm in size; activated by alkali and CaCl2/MgCl2/NaCl (2:1:1,

molar ratio); experiments conducted at 1040 8C[160]

Phragmites australis shoot Pb Plants 17.2 3 1 Ground and passed through 90mm screen, then treated with 0.1MNaOH

[161]

Streptomyces rimosus Pb Bacteria 135 24 3 3 Streptomyces rimosus biomass treated with 0.1MNaOHwas used in

granulated form

[167]

Sargassum sp. Pb Algae 266 27 2 4 The brown seaweed Sargassum sp. was harvested from the sea,

washedwithdistilledwater to remove particulatematerial, and oven

driedat 343K for 24h.Driedbiomasswas cut and sieved. The fraction

with 0.30.7mm was selected for use in the sorption studies.

[168]

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Table 4 (Continued )

Biomass material Metal Biomass type Biosorption

capacity

(mgg1)

pH Reaction

time (h)

Adsorbent

concentration

(g L1)

Remarks References

Spirodela polyrhiza (L.) Schleiden biomass Pb Aquatic plant 137 4.0 120 0.1 The huge biomass generated from Spirodela polyrhiza can be

successfully utilized for wastewater treatment

[255]

Saccharomyces cerevisiae

(waste brewers yeast)

Ni Fungi (yeast) 6.34 4 3 2 Ground and passed through 150mm screen [154]

Penicillium chrysogenum (lab cultured) Ni Fungi 551 5.5 6 1 Modied with polyethylenimine and crosslinked withglutaraldehyde

[165]

Sargassum sp. Ni Algae 71.6 5 6 1 Treated with 0.2% formaldehyde [166]

Azolla liculoides Ni Plants 70.380.1 5.50.2 10 2 2.0mm in size; activated by alkali and CaCl2/MgCl2/NaCl (2:1:1,molar ratio); experiments conducted at 1040 8C

[160]

Phragmites australis shoot Ni Plants 7.92 0.06 3 1 Ground and passed through 90mm screen, then treated with 0.1MNaOH

[161]

Rice husk ash Zn Crop 9.588 28 1 110 The rice husk ash obtained from burning of rice husk in electricaloven at 700 8C for 3h.

[138]

Phomopsis sp. (lab cultured) Zn Fungi 10.3 0.3 5 24 2 Mycelium boiled with 0.5M NaOH, dried at 60 8C, ground andpassed through 100mm screen

[156]

Ulva onoi Zn Algae 74.6 7.8 12 1 Soaked in 0.1M NaOH for 1h, then ground and passed through

14mm screen

[157]

Azolla liculoides Zn Plants 64.184.4 5.50.2 10 2 2.0mm in size; activated by alkali and CaCl2/MgCl2/NaCl (2:1:1,molar ratio); experiments conducted at 1040 8C

[160]

Phragmites australis shoot Zn Plants 5.75 0.07 3 1 Ground and passed through 90mm screen, then treated with 0.1MNaOH; optimal pH for adsorption

[161]

Magnetic biochar Zn Empty food

branch

1.4 10 2 0.09 Empty food branch was grinded to 150mm and treated by ferricchloride with impregnation ratio of 0.5

[250]

Oedogonium hatei Cr Algae 31.0 14 0.172.67 0.11 Algal biomasswaswashed in running tapwater followed byMilli-Q

water 45 times, kept on a lter paper to reduce the water content,

then sun dried for four days followed by drying in an oven at 70 8Cfor 24h

[169]

Oedogonium hatei Cr Algae 35.2 14 0.172.67 0.11 Raw biomass of Oedogonium hatei was treated with 0.1M HCl and

then stirred the mixture at 200 rpm for 8.0h, centrifuged, washed

with the physiological saline solution and dried in an oven at 60 8C.

[169]

Spirulina sp. Cr Bacteria 185 7 0.5 1 Lyophilized cells [153]

Saccharomyces cerevisiae (waste brewers yeast) Cr Fungi (yeast) 12.8 4 18 2 Ground and passed through 150mm screen [154]Bengal gram (Cicer arientinum) seed husk Cr Plants 91.64 2.0 5 10 Available in tropical countries; dried at 105 8C for 24h [170]Soybean hulls Cr Plants 582 3 24 10 Modied by crosslinking with dimethyloldihydroxyethylene urea

and quaternary amine

[171]

Sugarcane bagasse Cr Plants 1039 3 24 10 Modied by crosslinking with dimethyloldihydroxyethylene ureaand quaternary amine

[171]

Corn stover Cr Plants 8410 3 24 10 Modied by crosslinking with dimethyloldihydroxyethylene ureaand quaternary amine

[171]

Mucor hiemalis Cr Fungi 53.5 2 1.5 1 M. hiemalis was cultured in potato dextrose broth and the biomass

was harvested after 7 days. Harvested biomass was washed

thoroughly andwas killed by autoclaving at 121 8C and a pressure of15 lb.

[15]

Spirulina sp. Cu Bacteria 196 7 0.5 1 Lyophilized cells [153]

Sphaerotilus natans Cu Bacteria 60 6 0.5 3 Lyophilized cells [172]

Aspergillus niger Cu Fungi 26 6 24 2 Treated with 1% formalin; particles (0.751.0mm in size) [173]

Sargassum sp. Cu Algae 87.1 5 2 1 Treated with 0.2% formaldehyde [166]

Olive pomace Cu Plants 101 5 10 Dried, ground and washed by distilled water [159]Phragmites australis shoot Cu Plants 9.91 0.06 3 1 Ground and passed through 90mm screen, then treated with 0.1M

NaOH; optimal pH for adsorption

[161]

Acrylic acid functionalized

poly(N isopropylacrylamide)

hydrogel

Cu Biomass based material 67.25 5.0 0.5 Poly(N isopropylacrylamide-co-acrylic acid) hydrogels can be

utilized for copper ion adsorption from aqueous solutions

[256]

Quercus coccifera shell Co Plants 33 311 1.5 4 A mixture of formalin 30% and 0.1N HCl was added to the biomass

to obtained formaldehyde treated biomass

[174]

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R.K. Gautam et al. / Journal of Environmental Chemical Engineering 2 (2014) 239259 247residues viz. rice straw, wheat straw and Salvinia plant biomass forremoval of heavy metals such as Cr, Ni, and Cd. Heavy metalremoval efciency was more at low metal concentration(35 mg L1). Salvinia biomass possessed higher efciency forremoving heavy metals such as Cr, Ni and Cd followed by acombination where three materials (rice straw, wheat straw,Salvinia biomass) were taken together in comparison to othercombinations. An attempt at the use of rice husk ash, anagricultural waste, as an adsorbent of Zn(II) and Se(IV) from theiraqueous solution was carried out by El-Said et al. [138] as afunction of contact time, metal ion concentration, adsorbent dose,and pH at 25 8C. Its adsorption capability and adsorption rate wereconsiderably higher and faster for Zn(II) ions than for Se(IV) ions.Zn(II) removal was found fast reaching equilibrium within 1 hwhile Se(IV) adsorption was slow reaching equilibrium within100 h. The authors used the Bangham equation to express themechanism of adsorption. The adsorption of metal ions increasedwith an increase in the biosorbent dosage from 1 to 10 g L1 andwith a decrease in initial metal ions concentrations. The afnity ofRice husk ash for Zn(II) ions was greater than that for Se(IV) ions.Equilibrium adsorption of Zn(II) and Se(IV) ions by rice husk ashfollowed typical adsorption isotherms and ts both the Langmuir,Freundlich, and Temkin adsorption isotherms.

Ho and Ofomaja [139] used coconut copra meal, a wasteproduct of coconut industry for its potential use as a biosorbent forcadmium ions from aqueous solution. They examined a compari-son of linear least-squares method and a trial and error non-linearmethod of three widely used isotherms, Langmuir, Freundlich, andRedlichPeterson. Langmuir isotherm parameters obtained fromthe four Langmuir linear equations by using linear method werenot similar, but were the samewhen nonlinear methodwas used.The biosorption process was spontaneous and exothermic processin nature.

Recently, Schiewer and Iqbal [140] investigated the role ofpectin in Cd2+ metal binding by citrus peels, native orange peels,protonated peels, depectinated peels, and extracted pectic acid.Kinetic experiments showed that equilibrium was achieved in1 h. The magnitude of the negative surface charge determinedfrom potentiometric titrations decreased in the orderPP > PrP > DP > NP, showing that carboxyl groups of pectinwas a major contributor to the surface charge. Mostly, metalbinding experiments have been carried out at optimized pH 5.Langmuir isotherm model provided the best t. Metal bindingkinetics was better described by the rst-order model than by thesecond-order model.

Vaghetti et al. [141] reported the feasibility of pecan nutshell(Carya illinoensis) as an biosorbent to remove Cr(III), Fe(III) andZn(II) metallic ions from aqueous solutions. The adsorptionpotential of pecan nutshell to remove these metallic ions wasinvestigated by using batch mode. The effects of severalparameters, such as pH and the biosorbent dosage on theadsorption capacities of pecan nutshell were studied. Five kineticmodels were tested; the adsorption kinetics being the better ttedone to the fractionary-order kinetic model. Taking into account astatistical error function, the data were best tted to Sips isothermmodels. Themaximumbiosorption capacity of pecan nutshellwere93.01, 76.59, and 107.9 mg g1 for Cr(III), Fe(III), and Zn(II),respectively.

Organometallic compounds biosorption

A few biosorbents have been reported for the adsorption ofheavy metals not only in the form of metallic ions but alsoorganometallic compounds. Saglam et al. [142] have prepared thebiosorbents by the biomass of Phanerochaete chrysosporiumwhichadsorbed inorganic mercury and alkylmercury species with anafnity of CH3HgCl > C2H5HgCl > Hg2+, with the maximum

sorption capacities of 79, 67, and 61 mg g1, respectively.

Weed

TheefciencyofPartheniumhysterophorousweedfor the removaland recovery of Cd(II) ions from wastewater have been studied byAjmal et al. [146]. The authors reported that the kinetics data for theadsorption process obeyed second-order rate equation. Theadsorption process was found to be endothermic and spontaneousinnature. ThemaximumadsorptioncapacityofCd(II) ionswas99.7%in the pH range 34. The desorption studies conrmed 82% recoveryof Cd(II) when 0.1 M HCl solution was used as eluent.

Medicinal herbs

Rao et al. [147] have tested the biosorption potential of Fennelbiomass (Foeniculum vulgari) a medicinal herb for the effectiveremoval of Cd(II) ions. The biosorption was maximum (92%) at pH4.3. Maximum biosorption capacities of Cd(II) at 30, 40 and 50 8Ctemperatures were 21, 24 and 30 mg g1, respectively. Thebiosorption of Cd(II) was concentration dependent and increasedfrom 0.49 to 9.3 mg g1 with increase in concentration from 5 to100 mg L1. Biosorption followed Freundlich isotherm at 50 8C.Mean free energies at different temperatures were in between 7.1and 11.95 kJ mol1 indicating chemical nature of biosorptionprocess. Kinetics studies showed that pseudo second order kineticsmodel was applicable to the data. The process was endothermicand spontaneous in nature, the spontaneity of the processincreases with increase in temperature.

Non-timber products

Lalhruaitluang et al. [121] have applied activated charcoalprepared from Melocanna baccifera raw charcoal, the mostabundant and economically important non-timber product inthe tropical countries, by chemical treatment and used in variousexperiments to test its functions as adsorbents for removal of Ni(II)and Zn(II) from aqueous solution. Freundlich and Langmuirisotherms constants (mg g1) for the adsorption of Ni(II) andZn(II) onto Melocanna baccifera raw carbon and M. bacciferaactivated carbon were 2.93, 9.45 and 28.53, 52.91, and 3.87, 4.723and 27.01, 40.485, respectively. The correlation coefcients (R2) ofNi(II) for the pseudo-second-order kineticsmodel ontoM. bacciferaraw carbon andM. baccifera activated carbonwere 0.999 and 0.999,respectively and the experimental qe values were agreed well withthe calculated qe values. Similarly, the correlation coefcients (R

2)of Zn(II) for the pseudo-second-order kinetics model onto M.baccifera raw carbon and M. baccifera activated carbon were 0.987and 0.999, respectively and the experimental qe values were alsoagreed well with the calculated qe values. On the other hand, thecorrelation coefcients of the pseudo-rst-order kinetics modelwere lower than the pseudo-second-order kinetics model in M.baccifera raw carbon and M. baccifera activated carbon for Ni(II)and Zn (II) and the calculated qe values from pseudo-second-orderkinetics were agreed well with the experimental qe values than thepseudo-rst-order. Therefore, it had been concluded that theadsorption system followed a pseudo-second-order reaction ratherthan pseudo-rst-order reaction.

Modications and activation of biosorbents

Various factors such as specic surface area, pore-sizedistribution, pore volume and presence of surface functionalgroups inuence the adsorption capacity of adsorbents prior totheir modication; so that theymay be tailored to have the desired

physical and chemical attributes to enhance their afnities towardmetal ion uptake from aqueous solutions. Usually, adsorptioncapacity increases with increase in specic surface area due to theavailability of a number of adsorption sites, while pore size andmicropore distribution are closely related to the composition of theadsorbents and the type of biomass rawmaterial supplied for theirsynthesis [175].

Modication techniques

Carbon surface can be modied to develop desirable physico-chemical properties by adequate choice of activation procedures. Itis even possible to prepare carbons with designated proportions ofmicro-, meso-, and macropores. The techniques of modication ofcarbon/activated carbon can be categorized into three broadgroups: modication of chemical, physical and biological char-

Table 5Technical advantages and disadvantages of existing modication techniques.

Modication Treatment Advantages

Chemical characteristics Acidic Increases acidic functiona

carbon surface enhances

metal species

Basic Enhances uptake of organ

Impregnation of

foreign materials

Enhances in-built catalyti

Physical characteristics Heat Increases BET surface are

Biological characteristics Bioadsorption Prolongs activated carbon

oxidation of organics by b

material can occupy adso

R.K. Gautam et al. / Journal of Environmental Chemical Engineering 2 (2014) 239259248acteristics. Among these three methods, modication withchemical compounds has been more frequently employed toincrease the adsorption and hence removal capacity of activatedcarbon or similar biosorbents. In order to facilitate a more focuseddiscussion, only studies onmodicationwith chemical compounds

[(Fig._1)TD$FIG]Fig. 1. Biosorbents preparation routes from biomass using chemical and physicalmethods.of activated carbons have been visited in this review. Table 5 listsand compares the advantages and disadvantages of existingmodication techniques with regard to technical aspects.

As the biosorption process involves sequestration of metal ionson the cell surface, themodication of cell wall can greatly alter thebinding of metal ions. A number of methods have been employedfor cell wall modication of microbial cells in order to enhance themetal binding capacity of biomass and to elucidate themechanismof biosorption. The physical treatments include heating/boiling,freezing/thawing, drying and lyophilization. The various chemicaltreatments used for raw biomaterials modication includewashing the biomaterial with detergents, acid or alkali treatment.The pretreatments couldmodify the surface characteristics/groupseither by removing or masking the groups or by exposing moremetal binding sites [176]. Physical methods include vacuum andfreeze-drying, boiling or heating, autoclaving and mechanicaldisruption. Chemical methods include treatment with variousorganic and inorganic compounds, such as acid and caustic,methanol, formaldehyde, etc. Fig. 1 shows the sequence of steps inthe preparation of different types of biosorbents from biomasssources.

Activation of adsorbents

Activated carbon

Activated carbon biosorbents constitutes one of the mostimportant types of industrial carbon and is prepared by modica-tion and activation of a large number of raw biomass. Because of itshigh adsorption capacity it is used extensively in environmentalapplications for the removal of impurities from gases and liquids, itneeds to possess a well developed pore structure, which isrecognized to be the most important property of activated carbon[177,178]. The high adsorptive capacities of activated carbons aremainly associated with their internal pore properties such as poresurface area, pore volume, and pore size distribution [179181].Generally, activated carbons are mainly microporous, but inaddition to micropores they contain meso and macropores, whichare very important in facilitating access of the adsorbatemolecules

Disadvantages

l groups on activated

chelation ability with

May decrease BET surface area and pore volume

ics May, in some cases, decrease the uptake of

metal ions

c oxidation capability May decrease BET surface area and pore volume

a and pore volume Decreases oxygen surface functional Groups

bed life by rapid

acteria before the

rption sites

Thick biolm encapsulating activated carbon

may impede diffusion of adsorbate speciesto the interior of the carbon particle and in many of the adsorptionapplications in liquid phase [182].

Most of the activated carbons are produced by a two-stageprocess carbonization followed by activation [124]. The rst step isto enrich the carbon content and to create an initial porosity andthe activation process helps in enhancing the pore structure.Basically, the activations are two different processes for thepreparation of activated carbon: physical activation and chemicalactivation. There are two important advantages of chemicalactivation in comparison to physical activation. One is the lowertemperature in which the process is accomplished. The other isthat the global yield of the chemical activation tends to be greatersince burn off char is not required. The structure and volume ofmicropores and transitional pores developed during activation

depend upon the nature of the startingmaterial and the conditionsof activation. The changes that occur in micropore volume, pore

Table 6The classication of pretreatment methods for the production of activated biosorbents

Physical methods Chemical and Physicochemical me

Milling:

- Ball milling

- Two-roll milling

- Hammer milling

Irradiation:

- Ultrasound irradiation

- Gamma-ray irradiation

- Electron-beam irradiation

- Microwave irradiation

Others:

- Hydrothermal

- High pressure steaming

- Extrusion

- Pyrolysis

Explosion:

- Steam, Ammonia, CO2, SO2, Acids

Alkali:

- CaO, ZnCl2, NaOH, NH3, (NH4)2SO

Acid:

- H2SO4, HCl, HNO3 and H3PO4 aci

Gas:

- ClO2, NO2, SO2Oxidizing agents:

- H2O2- O3-Wet oxidation

Solvent extraction of lignin:

- Ethanolwater extraction

- Benzenewater extraction

- Butanolwater extraction

R.K. Gautam et al. / Journal of Environmental Chemical Engineering 2 (2014) 239259 249size, and surface area during activation of various types of carbonhave been reviewed by several authors [183].

Knowledge of different variables during the activation processis very important in developing the porosity of carbon sought for agiven application. Among the numerous dehydrating agents, zincchloride in particular is the widely used chemical agent in thepreparation of activated carbon for the removal of metal ions fromaquatic environment [124,184]. Chemical activation by zincchloride improves the pore development in the carbon structure,and because of the effect of chemicals, the yields of carbon areusually high [185]. Table 6 shows the classication of pretreatmentmethods for the production of activated biosorbents. Pretreatmentaction may results in modication of the structure, increasedsurface area, increased pore sizes, partial hydrolysis of hemi-celluloses etc. Fig. 2 shows the pretreatment action on thebiomaterials. Extensive drying of the lignocellulosic materialsshould be avoided because it may results to pore shrinking whichmay leads to limits the diffusion of metal ions. Severalsophisticated instruments such as SEM, TEM, IR spectroscopy,FTIR, XRD, BET surface area analyzer, XPS, TGADTA, DSC, AFM,NMR, Element analyzer, etc. have applied for the characterizationof biomass activated biosorbents. Fig. 3 shows the SEM images ofraw and activated biomass based biosorbents. Activate carbon[(Fig._2)TD$FIG]Fig. 2. Action of pretreatment on biomass materials for the production ofbiosorbents.have been developed by treating raw mustard husk with conc.sulfuric acid in 1:1 ratio (Fig. 4).

Alkaline treatments on biomass have been reported to enhancethe metal ion adsorption capacity in many cases [192198].Calciumoxide have been used as activating agents for preparationof activated carbon biosorbents and used as a nonconventionaladsorbent for the removal of Cu(II) and Ni(II) ions from aqueoussolutions [199]. The introduction of alkali or alkaline earthmetalson the surface of the adsorbent provides basic sites that have ahigh afnity for adsorption. CaO has a low charge to radius ratioand can provide strong basic sites to the surface of the adsorbent[200].

The purpose of activation of charcoal is to create pores anddevelop larger surface area in the carbon material and therebyincrease the adsorptive capacity. The adsorption capacity ofchemically activated charcoal biomass from Bamboo with variousconcentrations of KOH had been investigated by Lalhruaitluanget al. [121]. The KOH concentration of 50% and 60% treatmentshowed highest adsorption of Ni(II) and Zn(II) from aqueoussolutions, respectively. Among the alkali metal salts, KOH is themost effective activating agent in producing activated carbonmaterials. The KOH activation was reported to be effective forincreasing micropore volume [201,202]. The reaction betweenKOH and carbon precursor can result in the formation of functionalgroups such as OK using oxygen of the alkali salt. The presence ofsuch potassium and oxygen bond in the char leads to the oxidationof cross linking carbon atoms in the adjacent lamella during theprocess of activation. Surface functional groups can be created at

. Data has been compiled from Refs. [129,144,145,186191,247,257].

thods Biological methods

3

ds

Fungi and actinomycetes:

- Lignin peroxidase, manganese peroxidase, laccase

- White-rot and brown-rot fungithe edges of the lamella. As a result of removal of cross linkingbetween adjacent lamella and also the formation of new functionalgroups on individual lamella, the lamellas of the crystallite weredisturbed from their normal form into a slightly wrinkled or foldedor puckered form. Also the potassium metal produced in theprocess of activation, in situ, intercalates in to the lamella of thecrystallite. At the same time, the lamella cannot return to theiroriginal state, creating interlayer voids. The lamella remains apartcausing porosity and yielding high surface area carbon [203]. Thusduring activationwith KOH, amorphous carbon as well as silica areremoved from the carbon precursor resulting in porous structureand a corresponding increase in the surface area.

The oxidative treatment in an oxidized gas atmosphere or areaction with strong acid such as H2SO4, HNO3 and HCl couldincrease the total amount of acidic functional groups on biomasssurface [157,204,205]. It is worthwhile to mention that everytreatment for improving surface chemistry of activated carbonmay change its textural characteristics [197]. For example,impregnation of urea may enhance the development of carbon

porosity [206], and oxidation may decrease specic surface areaand pore volume [207,208].

Activated seaweed

The adsorption capacity for metal ions was signicantlyimproved with acidbase modication [157,209,210]. Both SBETand surface chemistry were changed dramatically due to acid andacidbase treatment, which resulted in extremely differentadsorption preference [197]. Suzuki et al. [157] have removedthe heavy metals from aqueous solution by non-living Ulvaseaweed as biosorbent. The dried biomass was pretreated with0.1 MHCl for 1 h and rinsed with distilled water until the pH of therinsing water was more than 4.5. In the case of the alkali-pretreated material, the dried biomass was soaked in 0.1 M NaOHfor 1 h, and rinsed with distilled water until the pH of the rinsingwaterwas less than 10. The prepared biomassmaterialwas appliedfor the adsorption of Cd from aqueous solution in batch mode. Ithas been showed that treated Ulva have excellent ability foradsorption of heavy metals. The isothermmodel was well tted tomost celebrated monolayer Langmuir model with sorptioncapacity of 6090 mg g1.

Activated sawdust

Sawdust is one of the promising materials which can be used to

[(Fig._3)TD$FIG]

Fig. 3. SEM images of biomass based biosorbents, (A) raw mustard husk, (B)activated mustard husk, (C) activated Pinus cone, and (D) activated Ficus fruit. All

the activated carbon had been obtained by treating concentrated sulfuric acid (ratio

of biomass with conc. sulfuric acid was 1:1).

R.K. Gautam et al. / Journal of Environmental Chemical Engineering 2 (2014) 239259250adsorb heavy metals. The sorption characteristics of sawduststrongly depend on its charging status controlled by the types andnumbers of functional groups on its surface [253].

Formaldehyde treated sawdust of Pinus sylvestris have beenstudied for the removal of Cd(II) and Pb(II) from aqueous solutions[211]. Biosorption of cadmium from aqueous solution usingMulberry wood sawdust treated with hydrochloric acid has beenstudied [205]. The Mulberry sawdust was chemically treated with1.0 mol L1 HCl solutions for 24 h. The biosorption of Cd(II) ionswas pH dependent and maximum biosorption was achieved at pH6 after 30 min shaking. The maximum biosorption capacity ofchemically treated sawdust obtained from Langmuir isothermwas403.73 (mg g1) for Cd(II) ions, which showed a great adsorptioncapacity of the treated biomass. The experimental data showedthat the pseudo-second-order model was the best tted kineticmodel for description of biosorption of Cd(II) ions on treatedMulberry sawdust. The same authors have reported that they have

[(Fig._4)TD$FIG]

Fig. 4. FTIR image of activated mustard husk. The mustard husk was activated byapplying concentrated sulfuric acid and the ratio of mustard husk with

concentrated sulfuric acid was 1:1.

egg shell was very low. The hen egg shells had been applied as anactivating agent for commercial carbon via modication of theirsurface chemistry [219].

A vast array of functional groups such as carboxylates, phenolicand aliphatic hydroxyls, and carbonyl groups in biomaterials havethe ability to adsorb metal ions. To enhance the heavy metal ionadsorption of biomass, the biomaterial was chemicallymodied byintroducing different complexing groups such as aminoalkyl [220],2,2-diaminoethyl, and amidoxime [221], or an ionicmoiety such asphosphate [222], thiolate [223], carboxy [224], and carboxymethyl[225,226].

New activated biosorbents

Adsorption of Pb(II) and Cu(II) onto diethylenetriamine-bacterial cellulose (EABC) was performed by Shen et al. [227].EABC was synthesized by amination with diethylenetriamine onbacterial cellulose. The amount of adsorption of Pb(II) and Cu(II)onto EABC was maximum in acidic medium at pH 4.5. This wasdue to the protonation of the amino groups at the acidic conditions.But when the pH was >4.5 the adsorption capacity declinedsimultaneously. The results showed that the adsorption rate waswell tted by pseudo-second-order rate model, and adsorptionisotherm was well described by the Langmuir model. In thisexperiment the adsorption uptake of metal ions was good but ittakes a too longer contact time. For keeping in mind the practical

[(Fig._5)TD$FIG]R.K. Gautam et al. / Journal of Environmental Chemical Engineering 2 (2014) 239259 251recoveredmaximumcadmium (92.79%) from loaded biosorbent byusing 1.5 mol L1 HCl solution.

Natural bamboo sawdust with celluloselignin polymericstructure was used by Zhao et al. [253,254] as a raw adsorbentto remove heavy metal in water. In contrast to most other plantswhich need to grow for 10100 years, bamboo is a fast-growingand renewable resource; which becomes mature in 46 years.Additionally, the wood processing in industrial and agriculturalproduction often generates a massive amount of sawdust thatneeds to be reused. The analysis of surface properties showed thathigh proton afnity sites were mainly composed of phenolic andalcohol hydroxyl, while low proton afnity sites mainly consistedof carboxylic acid, silicon hydroxyl, aluminum hydroxyl and somelow densities of sulfhydryl and phosphoryl groups.

Activated bagasse

Adsorption of heavy metal ion from aqueous solution bymodied sugarcane bagasse has been well recognized [212214].The sugarcane bagasse has been modied with succinic anhydrideto introduce carboxylic functions to sugarcane bagasse and thechemical introduction of commercial linear polyamine via theformation of amide functions [212]. It has been well establishedthat polyamines have powerful chelating properties, mainlytoward ions such as Cu2+, Zn2+, and Pb2+ [215,216].

Gurgel and Gil [217] have described the preparation of two newchelating materials, MMSCB 3 and 5, derived from succinylatedtwice-mercerized sugarcane bagasse (MMSCB 1). MMSCB 3 and 5were synthesized from MMSCB 1 using two different methods. Inthe rst method, MMSCB 1 was activated with 1,3-diisopropyl-carbodiimide and in the second with acetic anhydride and laterboth were reacted with triethylenetetramine in order to obtainMMSCB 3 and 5. The capacity of MMSCB 3 and 5 to adsorb Cu2+,Cd2+, and Pb2+ from aqueous single metal ion solutions wasevaluated at different contact times, pH, and initial metal ionconcentrations. Adsorption isotherms were well tted by Lang-muirmodel. Maximumadsorption capacities ofMMSCB 3 and 5 forCu2+, Cd2+, and Pb2+ were found to be 59.5 and 69.4, 86.2 and 106.4,158.7 and 222.2 mg g1, respectively.

Activated carbon from agro-waste

Sorption of Cu(II) and Ni(II) ions from aqueous solutions usingcalcium oxide activated date (Phoenix dactylifera) stone carbonhave been performed by Danish et al. [199]. The results revealedthat calcium oxide activated date stone has a honeycomb likesurface morphology with large mesoporous surface area(645.5 cm3 g1) for adsorption and removal of copper and nickelwas followed the pseudo-second-order kinetics and Langmuirmodel of isotherms. The trend of adsorption amount of Cu(II) andNi(II) was appeared same at the equilibrium, and both ionsadsorption increased as the initial metal ion concentrationincreased. The greater electronegativity of Cu(II) have been giventhe reason to enhance the binding capacity of copper toward thenegatively charged adsorbent surface, which resulted in a slightlyhigher adsorption capacity for Cu(II) than Ni(II).

Lalhruaitluanga et al. [218] have performed the removal ofPb(II) adsorption from aqueous solutions by raw and activatedcharcoals ofM. baccifera Roxburgh (bamboo). They have conductedbatch experiments for under varying range of pH (2.06.0), contacttime (15360 min) and metal ion concentrations (5090 mg L1).The optimum conditions for Pb(II) biosorption were almost sameforM. baccifera raw charcoal andM. baccifera activated charcoal pH5.0, contact time 120 min. The Langmuir and Freundlich adsorp-tion capacity of Pb(II) onto M. baccifera raw charcoal and M.baccifera activated charcoal were 10.66 mg g1 and 7.730 mg g1,and 53.76 mg g1 and 46.32 mg g1, respectively. However, theactivated biosorbents ofM. bacciferawas found to bemore suitablethan raw biosorbent of M. baccifera for the development of anefcient adsorbent for the removal of Pb(II) from aqueoussolutions.

Vasu et al. [190] have synthesized the activated carbon bycarbonizing the coconut shells in amufe furnace at a temperatureof 400 8C for 1 h. Then, the obtained carbon was oxidized withconcentrated nitric acid, hydrogen peroxide and ammoniumpersulfate. The purpose of the oxidizing agent modication ofthe carbon surface was to enhance the density of the surfacefunctional groups (Fig. 5). The pHzpc for coconut shell carbon (CSC),nitric acid treated coconut shell carbon (CSCN), hydrogen peroxidetreated coconut shell carbon (CSCH), and ammonia persulfatetreated coconut shell carbon (CSCA)were 7.45, 6.72, 6.76, and 6.40,respectively. Several authors have been used hen egg shell asadsorbent for heavy metal sequestration or as activating agent forcarbon surface [219]. However, the adsorption capacity of raw hen

Fig. 5. Effect of oxidation on the surface charges of coconut shell carbon (CSC); nitricacid treated coconut shell carbon (CSCN); hydrogen peroxide treated coconut shell

carbon (CSCH); and ammonia persulfate treated coconut shell carbon (CSCA) [190].

R.K. Gautam et al. / Journal of Environmental Chemical Engineering 2 (2014) 239259252applications of synthesized biomaterial the same authors haveperformed desorption experiments and regeneration of adsorbentby using 0.1 M EDTA and HCl solutions. The desorption efciencyof EDTA was quite high (99%) in comparison of HCl (90%) solution.

Konjac glucomannan (KGM) is a neutral polysaccharide whichconsists of mannose and glucose in a molar ratio of 1.6:1 with a b-1,4-linkage, derived from the tubers of Amorphophallus konjac[228]. On the other hand, its dissolution is very high in watershowing increased water absorbency [229]. This limitationprevents the KGM for being used as adsorbent in separationscience. Niu et al. [230] chemically modied the KGM bycrosslinking agent to overcome this problem and functionalizedwith carboxymethyl groups for removal heavy metal ions fromaqueous solutions. Because of low degree of substitution ofcarboxymethyl groups on crosslinked KGM, the removal capacityof crosslinked carboxymethyl KGM biomaterial for heavy metalions was not high. Liu et al. [231] synthesized carboxylic acidfunctionalized deacetylated konjac glucomannan by free radicalgraft copolymerization of methyl acrylate (MA) andmethylmetha-crylate (MMA) onto the backbone of deacetylated konjacglucomannan with subsequent chemical activation of the estergroups in the side chains of the resulting graft copolymer bysodiumhydroxide. The KGM showed a high adsorption capacity forCu2+ and Pb2+ ions. The maximum adsorption capacity ontocarboxylic acid functionalized deacetylated konjac glucomannanwas found to 64.5 mg g1 and 191.3 mg g1 for Cu2+ and Pb2+ ions,respectively. The adsorption isotherm data was well described byLangmuir model and the kinetic data were tted well to pseudo-second-order rate model.

Hydroxyapatite/chitosan (HApC) composite was synthesizedby precipitation method for the adsorptive removal of Pb2+, Co2+

and Ni2+ by hydroxyapatite/chitosan composite from aqueoussolution [232]. They have investigated the effect of variousphysicochemical parameters such as pH, adsorbent dose, contacttime, initial metal ion concentration and temperature onadsorption of metal ions onto HApC composite. The adsorptionprocess followed pseudo-second order kinetics and intra-particle diffusion model. The Langmuir constant for Pb2+, Co2+

and Ni2+ removal onto HApC was 12.04, 10.63 and 8.54 mg L1,respectively. Equilibrium data were tted well in the Langmuirand Freundlich isotherm models. The effect of differentconcentration of Pb2+, Co2+ and Ni2+ on the adsorption hadbeen investigated at 303 K, Pb2+, Co2+ and Ni2+ adsorptioncapacities of HApC were given as a function of equilibriumconcentration. It was clear that Pb2+, Co2+ and Ni2+ adsorptioncapacities of HApC increased with increase of equilibriumconcentration. The Increase in adsorption capacity with increasein equilibrium metal ion concentration for different metal ionswas in the order Pb2+ > Co2+ > Ni2+.

Recovery ofmetals, regeneration and desorption of biosorbents

In favor of material cycling and safe post-treatment of metalloaded biosorbent, it is necessary to remove and recover metalsfrom the metal-loaded biomass that has been used in thetreatment of metal-contaminated wastewater. The metalsadsorbed on the surface of biosorbents can be recovered bydesorption with adequate eluents. Recent research on recovery ofmetals from metal-loaded biosorbents using chemical reagentsand regeneration of the biosorbents are summarized in Table 7.Heavy metals and metalloids can be effectively removed frommetal-laden biosorbents using dilute acids (e.g., HCl, HNO3, andH2SO4). Even though HCl has a very high capacity to desorbmetals,several studies have shown that it would also decrease metaladsorption abilities of the regenerated biosorbents because ofhydrolysis of the polysaccharides on the biomass surface as metalbinding sites [233,234]. EDTA can be used as an eluent of metalcations, although it is less effective as compared with dilute acids.

El-Said et al. [138] have conducted batch desorption experi-ments and the desorption efciencies were compared, a series of250 mL Erlenmeyer asks containing 50 mL of aqueous solution ofHCl, H2SO4, HNO3, and CH3COOH of known concentration wascontacted with metal-loaded Rice husk ash (0.5 g) at 25 1 8C. Themixtures were agitated at 100 rpm for 5 h in the orbital shaker. Thenafter, the mixture was centrifuged and the supernatant was analyzedfor metal ions released into the solvent. Desorption of Zn(II) andSe(IV) using of mineral acids, HCl, H2SO4, and HNO3 compared withCH3COOH acid showed maximum desorption efciency of 27% forZn(II) and 10% for Se(IV). On the other hand,mineral acids, HCl, H2SO4,and HNO3 showed almost equal and higher recovery efciency 27%for Zn(II). However, CH3COOH proved to be the best for recovery ofSe(IV).

Regeneration and reuse of a seaweed-based biosorbent in singleandmulti-metal systems have been performed by Bakir et al. [235].The regeneration of the biosorbent using an acid wash resulted inthe release of high metal concentrations during multiple desorp-tion cycles. For desorption experiments, 0.1 M HCl was selecteddue to its high metal desorption properties and its lower cost ofoperation [236240], the metal loaded biomass was exposed to acontinuous feed of a 0.1 M HCl solution at 25 mL min1 for180 min. Maximum desorption efciencies of 183%, 122% and 91%were achieved for Zn(II), Ni(II) and Al(III), respectively, forsubsequent metal loading cycles, signicantly exceeding thedesorption rates observed for conventional sorbents. The regener-ation of the sorbent was accomplishedwith very little loss inmetalremoval efciency for both single andmulti-metal systems. Valuesof 92%, 96% and 94% removal efciency were achieved for Zn(II),Ni(II) and Al(III), respectively, for the 5th sorption cycle in singlemetal aqueous solutions. A slight decrease was observed for thesame metals in multi-metal systems with maximum removalefciencies of 85%, 82% and 82% for Zn(II), Ni(II) and Al(III),respectively.

Bernardo et al. [241] have studied the sorption and desorptionof Cr(III) by agro-waste biosorbents such as sorghum straw, oatsstraw, and agave bagasse. Biosorption capacity for Cr(III) uptakewere conducted in batch experiments at pH 3 and 4, at 25 8C and35 8C. The Cr(III) loaded biosorbents were added to 100 mL ofHNO3 (1.0 N or 0.1 N), NaOH (1.0 N or 0.1 N) or EDTA (0.1 M or0.05 M) solutions at different concentrations and temperatures(25 8C, 35 8C, and 55 8C). These eluents were selected aiming at anion exchange process, between H+ and Na+ with Cr(III) species, orcomplexation (EDTA-Cr (III)). The partially saturated agro-wastematerials were efciently regenerated by an EDTA solution at 55 8Cwithout apparent modications on the biosorbents. The higherchromium desorption with 1.0 M EDTA obtained at 55 8Csuggested that the interaction Cr(III)-EDTA involved a chelateformation and an increase in temperature facilitated the chemicalinteraction between EDTA and Cr(III). In addition, the initial weightlost of acid-washed materials due to the regeneration with 0.1 MEDTA was much lower (09%) than what was obtained when 1.0 NHNO3 or 1.0 N NaOH was used as eluent. This fact suggested thatEDTA solutions produced fewer structural modications in thebiosorbents than the acid or the alkaline solutions. Acid andalkaline solutions attacked the carboxyl groups conversely to EDTAsolutions. In addition, alkaline and acid regeneration producedhydrolysis of the agro-waste materials conrmed by the initialweight lost, and the biosorbents color change. Therefore, based onthe chromium desorbed and the initial weight lost, EDTA was thebest option to regenerate the chromium-loaded agro-wastematerials.

Das et al. [81] have studied the removal of Zn(II) by untreatedand anionic surfactant treated dead biomass of isolated yeast

Table 7Recovery of heavy metals and metalloids from metal-loaded biosorbents reported recently.

Metal Biosorbent material Biosorbent type Eluent Number of

adsorption

desorption

cycles

Elution

mode

Elution

time (h)

Recovery (%) Remarks References

Cd Streptomyces clavuligerus Bacteria 1M Na2SO4 (pH about 10) Filtration >95 50-fold increase in Cd conc. in eluate as compared with

the original Cd solution. The regeneratedmaterial had a

higher maximum adsorption capacity than the original

one

[248]

Bakers yeast Fungi (yeast) 0.1M HCl >6 Batch 1 95 Modied by crosslinking cystine with glutaraldehyde [155]

Phragmites australis shoot Plants 0.1M HCl >3 Batch 1 98.5 Ground and passed through 90mm screen, then treatedwith 0.1M NaOH; optimal pH for adsorption: near

neutral

[161]

Pyrolyzed coffee residue Plants Distilled water 5 Batch 0.5 88.091.9 Pyrolyzed at 500 8C; mixed with clay as a binder(pyrolyzed coffee residue:clay=80:20 [in weight]);

glanular (10mm in length, 4mm in diameter)

[249]

Pb Bakers yeast Fungi (yeast) 0.1M HCl >6 Batch 1 95 Modied by crosslinking cystine with glutaraldehyde [155]

Gloeocapsa gelatinosa Bacteria 0.01M EDTA Batch 0.253 49.7 Lyophilized cells with capsular polysaccharide (CPS);recovery of metal was repeated 3 times

[164]

Phragmites australis shoot Plants 0.1M HCl >3 Batch 1 90.5 Ground and passed through 90mm screen, then treatedwith 0.1M NaOH; optimal pH for adsorption: near

neutral

[161]

Sargassum sp. Algae 0.05M HNO3; 0.10M HNO3;

0.05M HCl; 0.10M HCl;

0.05M Na2EDTA; 0.10M Na2EDTA

Batch >95 High value of the conditional formation constant of the

complex Pb(II)EDTA (K0 f=3.851011 at pH=5.0),which favors the desorption of lead, >95 lead ions was

recovered by Na2EDTA

[168]

Cr Oedogonium hatei Algae 0.1M NaOH 5 Batch 1 75 Raw biomass of Oedogonium hatei was treated with

0.1M HCl and then stirred the mixture at 200 rpm for

8.0h, centrifuged, washed with the physiological saline

solution and dried in an oven at 60 8C.

[169]

Cr Mucor hiemalis Fungi 0.1N NaOH 5 Batch 1.5 99 Desorption data showed that nearly 99% of the Cr(VI)

adsorbed on M. hiemalis was desorbed using 0.1N

NaOH.

[15]

R.K.Gautam

etal./Jo

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nmen

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icalEngineerin

g2(2014)239259

253

R.K. Gautam et al. / Journal of Environmental Chemical Engineering 2 (2014) 239259254species viz. Candida rugosa and Candida laurentii in both batch andcolumnmode. Themaximum removalwas 65.4% and 54.8% of Zn atpH 6.0 in presence of 90 mg L1 Zn(II) at 30 8C in batch system for C.rugosa and C. laurentii. Remarkable increase in Zn(II) removal wasnoted using dead yeast biomass treated with anionic surfactantsodium dodecyl sulfate (SDS). The experimental data wereanalyzed using two, three and four parameter isotherm models.Freundlich isotherm model was well tted for the adsorption ofZn(II) from aqueous system. Four cycles of desorption and sorptionwas performed to study the reusability of the biosorbent. Adecreased breakthrough time and exhaustion time were observedas the regeneration cycles progressed. The elution efciency andpercentage removal of Zn(II) were found to decrease in thesuccessive cycles. This behavior is primarily due to continuoususage of the biosorbent.

Outlooks and discussion

Several advantages in using low cost biosorbents for removaland recovery of heavymetals from aqueous solutions are discussedbelow together with the main drawbacks.

The metal ions removal rates from aqueous solution bybiosorption are generally faster than those by other metabolicallymediated processes. This property of biosorption would make itthe most effective in treatment of voluminous water bodiescontaining low concentrations of metals, such as the nalwastewater treatment process to meet regulation standards ofmetals before being discharged to the environment.

The costs for formulating biosorbe