HAZARDOUS WASTE & HAZARDOUS MATERIALSVolume 11, Number 4, 1994Mary Ann Liebert, Inc., Publishers

Removal of Cr(VI)from Chromium Contaminated Sites

by Washing with Hot WaterVICTOR OSOSKOV and JOSEPH W. BOZZELLI

Department of Chemical Engineering, Chemistry, and Environmental ScienceNew Jersey Institute of Technology

Newark, NJ 07102

ABSTRACT

Extraction of chromium from a mixture of ore processing residue and soil (slag) is studied by hotwater and dilute alkali (0.01M NaOH) washing. We report that hexavalent chromium, which is the mostdangerous and mobile form of chromium, can be effectively removed (95-99%) from the slag by washingwith these extraction solutions. The extract can be reused after removing the CrfVT). ions by ion-exchange technologies. Trivalent chromium is strongly adsorbed by clay minerals and remains in theresidue after this mild extraction. The hot water or mild hot alkali washing is recommended as a rapidand inexpensive treatment for chromium contaminated sites to prevent chromium(VT) migration throughsoils into groundwater or dispersion as a dust into the atmosphere. Cr(IH) remaining in the residue maythen be left on site preferably with the addition of a reducing agent, to prevent conversion to CifvT)until further clean up is mandated. CrfTTT) could be also separately extracted by use of hot concentratedacid solution. The observed facile removal with these mild solutions between 25 and 100°C indicatesthat CrfVT) may be able to migrate through soils relatively easily.

INTRODUCTION

From 1905 to 1976 Hudson County, NJ, was a center for chromate and dichromate chemicalmanufacturing. Chromite ore was shipped to the county from around the world. The ore, containing 45-50% chromium was mixed with lime and soda ash and heated to convert insoluble CrfTfT) compounds tothe more soluble hexavalent form, which was then easily leached out with water. The remaining mudwas reprocessed a second time before being discarded as residue, and contained between 2 and 7%chromium. It is estimated that the total amount of this processing residue was in the range of 2 to 3million tons and the US EPA has identified over 130 chromium contaminated sites in Jersey City andKeamy, New Jersey/1/.

Chromite ore processing residue (slag) presents a serious environmental problem when improperlydisposed of, because it continues to leach chromate salts for decades. This is attributed to the fact thatthe slag contains a variety of chromium salts which have low solubility in water. The pH of this residueis usually 10-11 and these chromate salts have been observed to concentrate in the uppermost fewcentimeters of surface soil/1/. Chromium present in the residue, may exist in both trivalent andhexavalent forms. CrfTII) in weak alkali medium is precipitated and/or strongly adsorbed and is

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practically immobile. The CrfVT) state is not immobile however, and is considered to pose the greatesthuman risk, because it is more toxic and more soluble than CrfTn)/2/. Under natural conditions thetransformation of CrfVT) to CrfTII) occurs as a result of reduction - often by: ferrous iron solutions,ferrous iron minerals, reduced sulfur compounds or soil organic matter. While there are many potentialmechanisms for the reduction of CrfVT) to CrfJLU), the mechanism for the oxidation of CrfTir) to Cr(VT)in soil matrices appears to be limited to oxidation by manganese oxides/3/. It is practically impossible topredict the potential likelihood and the rate of REDOX reactions between Cr(TIT) and CrfVT) in real soiland solid wastes because of matrix complexity and the myriad of parameters affecting the process.

In order to remove almost all trivalent chromium from the chromite ore processing residue, it isnecessary to carry out the extraction with hot acids at high strength (pH less than l)/4/. As a result ofsuch treatment, other metals (Ca,!Fe, AI, Mg etc.) are also extracted from the matrix and slag loses up toone half its original weight. Treatment with other common extracting reagents (EDTA, oxalates etc.)will also produce similar effects (plus be much more costly). This is because properties of CrfTÍT), AKüT)and Fe(nr) ions are all similar in extraction processes. ChromiumfTIT) is usually strongly bound withclay minerals and soil organics, hence requiring a vigorous extraction process. One proposal/5/ has beenforwarded to treat the acid extract from tannery sludges containing CrfTfl) and other metals by hydrogenperoxide treatment at pH 9 in order to oxidize CríTíT) to CrfVI). The soluble CrfVT) can then beseparated from Al, Fe and partially from Ca and Mg by filtration. The CrfVT) is then reduced to Cr(III)and utilized. The problem with this hydrogen peroxide process is that this oxidation is rather expensiveand the price of chromium is too low to warrant the recovery from slag by this operation.

Reducing agents such as ferrous sulfate and sulfides are commonly used in water treatmenttechnology for Cr(VT) removal. They are effective in acid media (pH<3) with subsequent addition ofalkali and precipitation/6/. Transformation of this water treatment technology to use in soil flushing willlikely cause the leaching of many cations and some organic compounds. Adsorption of CrfVT) anions onsoil increases at low pH/7,8/ in contrary to adsorption trends for most other toxic, heavy metals cations.The rate of heterogeneous REDOX reactions on soil particles is also significantly lower than in water.Use of these water technologies for ex-situ soil treatment reduction of CrfVT) to Cr(III) will involvesignificant time and reagent consumption. Acid wastes containing CrfTII) and other toxic metal cationswill then need to be neutralized for their precipitation, and the sludge which is formed needs to betreated and disposed.

The essence of our approach is to develop an inexpensive CrfVT) extraction process from the alkaliore processing slags and other contaminated soils. Cr(IIT) and other metals will be filtered from theextract, possibly treated with reducing agent for stabilization and left on site. The future oxidation ofCrfTII) to CrfVT) even in the absence of added reducing agent is unlikely because of the presence offerrous compounds and the comparatively low concentrations of manganese in the slag. FefTf) saltscould be also added to the residues after extraction to insure that CrfTJT) remains in the reduced form.

In earlier experiments/4/ it was reported that the part of chromium, present in hexavalent form, isreadily removed from chromite ore processing residue by extraction with warm water. Katz/9/ alsoreported the successful extraction of the spiked hexavalent chromium from soil with 3% sodiumcarbonate -1% sodium hydroxide solution. Furtman and Seifert/10/ used extraction by NaOH at pH 12for rough estimation of CrfVT) concentration in soil. CrfTII) however is not readily extracted in hotwater or weak base solution/4/. The negative charge of Cr(VI) ions limits its adsorption on soil mineralsrelative to adsorption of CrfTfl) and other cations. Adsorption of Cr(VT) by clays and soils is alsodecreased at increased pH/7,8/. Hanson et al./l 1/ recently have demonstrated the effective removal ofCrfVI) from laboratory application of ^CrçOy solutions to several types ofNew Mexico soils. Studieswere conducted at subsoil temperatures, in column beds where extraction time run up to 30 hours. Up to99% removal was demonstrated with the major fraction CrfVT) removal in the first several hours.However some CrfVI) salts are relatively insoluble in cold water compared to potassium dichromate.

The objectives of this study are to evaluate extraction parameters associated with the removal ofCrfVI) from ore processing slag by washing with water and alkali solution. After extraction, the extractis analyzed for Cr(VT) colorimetrically. Total chromium is analyzed using the same method afteroxidizing a sample of the extract. A problem exists in determining hexavalent chromium in slags or soils

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using standard digestion procedures, because REDOX reactions between CrfTfl) and CrfVI) can occurin this extraction. Barlett/12/ proposed extraction using a mixture of KH2PO4 and K2HPO4 at pH =

7.2 (temperature of extraction was not mentioned) for determination of CrfVT) in soil. Non-extractablechromate was considered immobilized, i.e., it is either precipitated or was tightly bound via adsorption.It is not clear, however, that it is possible to remove all the hexavalent chromium from an ore processingresidue by this procedure. In this study we demonstrate that most (95-99%) of the CrfVT); all of themobile and therefore dangerous hexavalent chromium, can be washed from the slag by 30-60 minextraction with hot water or alkali (pH=l 1) solution.

EXPERIMENTAL PROCEDURE

Total chromium concentration in the slag was determined by an atomic absorption spectroscopy(AAS) method after digestion. Digestion procedures followed EPA method 3050 for heavy methodremoval from soil/13/. The residue was separated by gravimetric filtration and then rinsed with 50%nitric acid and water, respectively. Samples of the slag were'analyzed after digestion and dilution fortotal chromium using a Thermal Jarrel Ash AAS model 1200 at a wavelength of 357.9nm with an

acetylene/air flame and Smith-Hieftje background correction. Chromium standards were prepared bydiluting a purchased standard 1000 ppm ammonium dichromate solution. The content of other selectedmetals extracted from the slag was estimated by inductive couple plasma mass spectrometry using a VG-PQ-2 ICPMS, following digestion and dilution. The composition of hot distilled water extract was alsodetermined by this method.

Distilled/deionized water and 0.0 IM sodium hydroxide solution were used for extraction. The dryslag (0.6g) was mixed with 50ml of boiling water or alkali solution. Mild heat, just boiling (100°C) andstirring (magnetic stirrer - low setting) was utilized. If the time of extraction exceeded 20min, some

boiling water was added to compensate for evaporation loss. Upon completion of extraction, thecontents of the beaker were cooled, filtered and rinsed. The extracts were diluted 1:50 in volume andanalyzed for hexavalent chromium. If two-stage washing was performed the first extract was dried,placed into the beaker and another 50ml of the extraction solution added.

ChromiumfVI) in extracts was determined colorimetrically by reaction with diphenyl carbazide (DFC)in acid media according to Standard Methods/14/. Addition of the excess of DFC yields the red-violetproduct and its absorbance was measured at 540nm using a Varían DMS 300 spectrophotometer. Totalchromium in the extract was determined by oxidizing the extract in a boiling solution of potassiumpermanganate in acid media, before reacting with DFC as recommended by Standard Methods/14/.

Concentration of CrfVI) in the slag sample was evaluated by DFC analysis after two-stage extraction(60 min with boiling water and 60. min with 0.0 IM NaOH solution). The increase of extraction time orthe application of other extractants (0.1 M NaOH, mixtures of 3% Na2C03 - 1% NaOH) did notincrease the hexavalent chromium content in the extract.

SLAG CHARACTERISTICS

A sample, containing ca 10 kilograms of chromium-laden industrial slag mixed with soil was received(via cooperation with the NJ Department of Environmental Protection) from a site in Keamy NJ as a

representative chromium-contaminated material. The sample was dry screened through 1/8" porestainless steel screen. Undersize material (about 85% by weight) was placed in a polyethylene pail andmixed for uniformity. Typical total chromium concentration in the slag was 21,000 ppm or 2.1% byweight as determined by AAS following acid digestion and dilution. The content of some metals in theslag after performing digestion was estimated by ICPMS and is illustrated in Table 1.

Particle size analysis was done on a dried 200g sample in analytical sieve shaker with standard Tylerwire mesh screens: Numbers 12; 20; 60; 100; 200; 325 corresponding to 1700; 850; 250; 150; 75 and45um opening diameters respectively. Total organic extractables were determined by Soxlet extraction

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TABLE 1. Concentration of Selected Metals in the Slag, g/kg.Ca Fe Mg Al Cr Ti Mn V Ni Pb Zn Cu

222 66 61 60 21 2.9 0.9 0.5 0.3 0.2 0.2 0.2

using dichloromethane solvent. The water fraction was estimated by placing a sample of material in theoven to dry overnight at 105°C. The weight loss was used to calculate water fraction. pH was measuredusing a glass electrode in a slag-O.lM KC1 slurry (ratio slag to solution 1:20). The characteristics of theslag studied are shown in Table 2.

TABLE 2. Slag CharacteristicsParticle Size

Sieve* 12 12 20 60 100 200 325Wt.% 0.02 1.4 18.7 12.3 20.0 13.0 34.5% Finer - 99.98 98.6 79.9 67.6 47.6 34.5

Water Fraction Organic Soluble Fraction pH30.6% 0.1% 9.6

RESULTS AND DISCUSSION

The fraction removal of chromium (R) from the slag was calculated as:

R=(l-m/m") 100%where m and m" are the masses of the total chromium in the sample after and before washingrespectively.rn" has been determined by AAS after acid digestion and for this investigated slag is 21.0 g/kg (n=3,S=0.08), where n - number ofdeterminations; S - standard deviation.

Concentration of ChromiumfVI) in the slag sample was evaluated after two stage washing withboiling water and 0.01M NaOH solution one hour each by DFC analysis of the extract. The averageCr(VT) concentration in the slag was 1.53g/kg (n=3, S=0.07) or about 7.3% of the total chromiumcontent.

In preliminary experiments it was noticed that the efficiency of ChromiumfVI) removal from slagincreased significantly at higher solution temperature (Table 3).

TABLE 3. Effect ofTemperature on CrfVT) Extraction with Water.

Temperature,°C 22 22 55 55 100 100Time, min 15 60 15 60 15 60Removal, % of total Cr 1.8 2.3 3.9 4.7 6.3 6.9Removal, % of CrfVT) 24.6 31.5 53.4 64.4 86.3 94.5

The solubility of chromium compounds is seem to increase as well as the rate of solution anddesorption processes with temperature. Extraction experiments on all samples in this study wereperformed at 100°C.

Data on the removal of ChromiumfVI) and of total chromium extraction (%) with boiling water and0.01M NaOH solution is presented in Table 4. The rate of extraction with alkali solution is higher thanthat with water. The washing process is complete (about 95% for water and 98% for alkali solution) in30 minutes. Taking into account errors of analytical determinations, we conclude that results for

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TABLE 4. Extraction of Chromium(\T) and Total Chromium from the Slag with Water,0.0 IM NaOH, and 2% H2S04 at 100°C.

Extractant Time,(min) Removal,%Cr(VT) Total Cr

Water 5 79.4 6.0Water 15 86.3 6.3Water 30 91.8 6.8Water 60 93.1 6.8Water 120 95.9 7.1

O.OlMNaOH 5 91.8 6.6O.OlMNaOH 15 94.5 7.0O.OlMNaOH 30 98.6 7.1O.OlMNaOH 60 98.6 7.3O.OlMNaOH 120 98.6 7.3

2%H2S04 30 61.6 93.3

chromium removal after washing with water as well as with alkali solution is the CrfVT) and constituteseffectively all of the CrfVT) in the sample. CrfTfl) is not removed. This is verified by adding dilutedpotassium permanganate solution to water or alkali extract (after adjusting pH) and observing that it didnot discolor (to any significant degree) after boiling. Consequently CrfTfi) did not oxidize to CrfVT) andit concentration in this extract was very small (less than 0.01%). Data in Table 4 also show that removalof hexavalent chromium from the slag with hot 2% sulfuric acid is less than that with hot water. Thetotal amount of chromium extracted (mostly CrfTfl) now) is however much greateríTable 4). Reductionof some CrfVI) to CrfHT) during acid extraction is possible because of large iron concentrations in theslag, which in its divalent form is the reducing agent.

Two stage removal of CrtVT) with water or with 0.01M NaOH does not result in any significantimprovement in the extraction efficiency (Table 5).

TABLE 5. Two Stage Extraction of CrfVI) from the Slag with Water and 0.01MNaOHatlOO°C.

Extractant Time (minute) Cr (VI) Removal, %Stage 1 Stage 2 Stage 1 Stage 2 Total

Water 5 5 76.4 6.8 83.2Water 30 30 91.8 4.1 96.2O.OlMNaOH 5 5 91.8 2.7 94.5O.OlMNaOH 30 30 98.6 2.7 101.3

The composition of selected metal species in the extract after 30 min washing with boiling water(ratio solvent to slag=83) as determined by ICPMS is shown in Table 6.

TABLE 6. Concentration of Selected Metals, ppm, in Hot Water Extracts.

Ca Cr Al Mg Fe Ti Sr Zn Cu54.1 26.8 3.1 0.5 0.3 0.2 0.09 0.03 0.02

Concentrations of heavy metals in the extract which are not listed above, are less than 0.02 ppm. Ona practical scale the only toxic metal present in this extract is chromium and it is present primarily as

CrfVT) anions. pH of the extract is approximately 9.6. CrfVT) from these water or dilute NaOH liquidextracts can be removed by absorption on an anion-exchange resin, then eluted and reused; for example,

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in an electroplating process. The application of ion-exchange for CrfVT) removal from various wastes/6/and alkali solutions/15/ is described in the literature. After CrCVT) has been removed the extract can bereused for further treatment of slag.

Results of these experiments indicate that ex-situ extraction of the most mobile and dangerouschromium compounds from mixture of ore processing residue and soil is possible by simple andinexpensive washing with hot water or weak alkali solution. If the slag also contains toxic volatileorganics they may be partly volatized as a result of hot water treatment. Measures for controlling theiremissions to the atmosphere (e.g. charcoal filter) should be considered here. For ex-situ extractionpreliminary grinding of the matrix up to 20 mesh might be helpful. CrfVI) will be almost completely (95-99%) removed by washing, because chromite ore processing residues have high pH values(9.5-11.5).Since many CrfVI) salts in these residues are soluble in hot water and their adsorption by soil mineralsin alkali conditions is low it is recommended that this be done before the CrfVT) is allowed to migratethrough the soil. CrfllT) and other heavy metals will remain in the slag or soil and can be filtered fromthe solution and left on site. FefTT) salts or other reducing agents could be added to the residue before itis filtered and dried to insure that CrfTfl) remains in the reduced form. ChromiumfVT) from the extractcould be concentrated and utilized.

For in-situ extraction (soil flushing) additional procedures and extraction experiments including soilheating and/or fracturing in order to increase temperature and permeability may need to be developed.

Cr(VT) might not be effectively removed from all kind of soils and solid wastes by hot waterwashing. Preliminary laboratory experiments should be performed to choose the optimal extractant andextraction conditions for a concrete matrix.

CONCLUSIONS

Hexavalent chromium can be rapidly (less than 1 hour) and effectively (95-99%) removed from themixture of chromite ore processing residue (slag) by washing with hot water or hot 0.01 M alkalisolution. Trivalent chromium and other cations of heavy metals are not extracted at these conditions.The extract can be reused after removing CrfVT) by ion-exchange process.

The hot water or hot mild alkali extraction is recommended as rapid and inexpensive on-site washingin order to prevent CrfVI) migration into groundwater and atmosphere. CrfTII) remaining in slag is lefton site preferably with addition of a reducing agent to prevent its oxidation to hexavalent form. Therelatively facile removal observed with water and mild NaOH solutions between 25 and 100°C indicatesthat Cr(VT) may readily migrate through soil.

REFERENCES

1. Burke T., Fagliano J., Goldoft M., Hazen R., Iglewitz R., and McKee T., Environ. Health Persp. 92,131./1991/.

2. Chromium in the Nature and Environments, ed. by J.O.Nrigau, and N.Nieboer, Wiley Sons Inc.,NY., 1988.

3. Palmer CD., and Wittbrodt PR.,. Environ. Health Persp. 92, 25, /1991/.

4. Ososkov V., Gotlieb E., Bozzelli J. W., Gotlieb I., and Stevenson E., Intern. J. Environ. Studies,44, 285, /1993/.

5. Macchi B., Pagano M., Petine M., Santori M., and Tirovani G, Water Res., 21,1019, /1991/.

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6. Patterson J.W., Industrial Wastewater Treatment Technology, Butterworth Publ., Salem, NH, 1985.

7. Zachara J.M., Ainsworth C.C., Cowan CE., and Resch T., Soil Sei. Amer. Journal, 53, 418, /1989/.

8. Hsieh, H.N., Raghu D., Liskowitz J. W., and Grow J., Hazard. Ind. Waste, 21, 651, /1989/.

9. Katz S., Environ. Health Persp. 92, 13, /1991/.

10. Furtman K., and Seifert D., Frez. J. Anal. Chem., 338, 73, /1990/.

11. Hanson A.T., Dwyer B., Samani Z.A., and York D., Journal ofEnviron. Eng., 119, 825, /1993/.

12. Barlett R.J., Environ. Health Persp., 92, 17, /1991/.

13. Test Methods for Evaluating Solid Waste. US EPA, SW-846, 3rd ed., US EPA, 401 M. St.Washington, DC, Nov. 1986.

14. Standard Methods for the Examination Water and Wastewater, 17-th ed., APHA-AWWA-WCPE,Washington, DC, 1992.

15. Sengupta A.K., Clifford D., and Subramonian S., Water Res., 20, 1177, /1986/.

Adress reprints to:Joseph W.Bozzelli

Department of Chemical Engineerimg,Environmental Science and Chemistry,New Jersey Institute of Technology,University Heights, Newark, NJ07102

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