WHC-SA-28 1 0-FP

Laboratory Evaluation of the Potential for In Situ Treatment of Chromate- Contaminated Groundwater by Chemical Precipitation

Prepared for the U.S. Department of Energy Off ice of Environmental Restoration and Waste Management

Westinghouse Hanford Company Richland, Washington

Hanford Operations and Engineering Contractor for the U .S. Department of Energy under Contract DE-AC06-87RL10930

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Abstract - A995

LABORATORY EVALUATION OF THE POTENTIAL FOR IN SITU TREATMENT OF CHROMATE- CONTAMINATED GROUNDWATER BY CHEMICAL PRECIPITATION

Edward C. Thornton, Mark A. Beck, and Cindy A. Jurgensrneier Westi nghouse Hanford Company P.O. Box 1970 Richland, WA 99352

Laboratory testing activities have been undertaken to evaluate the potential of in situ remediation of chromate-contaminate" groundwater through the introduction of ferrous and sulfide ions. Initial tests with chromate-solutions indicated that ferrous sulfate can quantitatively reduce and precipitate hexavalent chromium if applied at a Fe(ll):Cr(VI) mole ratio of 9 under slightly alkaline conditions. Verification tests were also conducted with chromate-contaminated Hanford Site groundwater and soil from the Hanford Site. Ferrous sulfate and combinations of ferrous sulfate and sodium sulfide were reacted with groundwater/soil mixtures in this set of experiments to assess the capability of removal of chromium from groundwater through reduction and precipitation. The application of 7 equivalents of reductant per equivalent of chromium was found to provide satisfactory treatment results. A ferrous sulfate solution or ferrous/sulfide solution with a concentration of about 10 equivalents (Le., a Fe(ll):Cr(VI) mole ratio of :30), however, is recommended for the system associated with this study since the soil exhibited a tendency to react with the reductants to a moderate extent. This corresponds to the injection of about 1200 pnoles of Fe2+ per liter of treatment solution (67 ppm Fez+) for remediation of grpundwater containing 21 00 ppb hexavalent chromium.

The results of these tests suggest that in situ remediation of chromate-contaminated groundwater by injection of solutions containing ferrous or ferroushlfide ions is feasible. This approach should be considered as a method that could enhance the efficiency of pump and treat operations.

A995

Laboratory Eva lua t i on o f t h e P o t e n t i a l f o r I n S i t u Treatment o f Chromate-Contaminated Groundwater by Chemical P r e c i p i t a t i o n

Edward C. Thornton Westinghouse Hanford Company

Richland, WA 99352 P.O. BOX 1970 MSIN H6-06

Mark A. Beck Westinghouse Hanford Company

Richland, WA 99352 P.O. BOX 1970 MSIN T6-09

Cindy A. Jurgensmeier Westinghouse Hanford Company

Richland, WA 99352 P.O. BOX 1970 MSIN S3-28

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INTRODUCTION

The current techno1 ogy commonly uti1 ized for remedi ation of contaminated groundwater systems consists primarily of the pump and t r e a t method. groundwater i s pumped t o the surface, contaminants removed, and the t reated water returned t o the aquifer o r otherwise disposed of. removing contamination i n the early stages o f operation, an extended Period of pumping may be required t o reduce contamination t o acceptable levels. a t t r ibu ted t o contaminant sorpt i on/desorption processes and the heterogeneous nature of aquifer systems.

In this approach,

While generally e f fec t ive for

T h i s may be

In situ chemical treatment of metal o r radionuclide contaminants w i t h i n groundwater systems has been recognized as a potential.' remediation approach tha t warrants development and may provide a viable a l te rna t ive t o the pump and t r e a t method.213 The objective of i n situ chemical treatment is t o immobilize contaminants w i t h i n the aquifer by precipi ta t ion as a re la t ive ly insoluble compound. chromium or uranium, for example, may be achieved by lowering the oxidation potential of the system, t h u s causing the valence of the metal t o drop t o a lower oxidation s t a t e . metal i s precipi ta ted as a component of a sol id phase.

The objective of this paper is t o present the resu l t s of a series of small-scale batch t e s t s performed t o assess the effectiveness o f chemical precipi ta t ion i n the remedi ation of chromate-contaminated groundwater. chromate solut ions w i t h ferrous and su l f ide ions. t h a t involved treatment of mixtures of chromate-contaminated groundwater and uncontaminated soil w i t h the ferrous i o n . A combination of ferrous su l f a t e and sodium su l f ide was also tes ted i n the groundwater treatment t e s t s , since this approach has4 been shown t o be an e f f i c i en t method for t rea t ing electroplat ing wastewaters.

Precipitation o f

The reduced metal species are l e s s soluble and, consequently, the

These tests involved treatment of In addition, t e s t s were conducted

The chemical reactions t h a t govern the redox level of this system can be presented as follows:'

4 H,O t CrO$- + 3 Fez+ + 4 OH- + 3 Fe(OH)3(ppt) + C r ( OH 3(ppt)

17 H,O + 8 Cr0$' + 3 HS- 3 SO:- + 8 Cr(OH)3(ppt) + 12 OH-.

The above have been written as basic solution reactions since groundwater systems are commonly somewhat a lkal ine i n nature (pH -8).

The reduction o f hexavalent chromium, as indicated i n reaction 1, requires a mole r a t i o of Fe(II):Cr(VI) of a t l e a s t 3 t o completely reduce chromium. Reaction 2 requires a minimum r a t io o f S(-II):Cr(VI) o f only 3:8.

S o l u t i o n samples obtained during these t e s t s were analyzed f o r hexavalent chromium spectrophotometrically using diphenylcarbazide. Solution sa.mples were f i l t e r e d o r centrifuged t o remove s.ediment or precipi ta tes , and analyzed for t o t a l chromium by inductively coup1 ed plasma atomic emission spectroscopy (ICP) . was also performed on most o f the solution t e s t samples.

Measurement of pH

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FERROUS SULFATE TESTS

To evaluate the effectiveness of ferrous i r o n in reducing and precipi ta t ing chromium, ferrous su l fa te was added t o solutions containing 500 ppb hexavalent chromium. The r e su l t s of these t e s t s are'summarized in Figure 1, which i l l u s t r a t e s the r a t e of decrease i n the concentration of hexavalent chromium remaining i n solution during treatment. a t a mole r a t i o of Fe(II):Cr(VI) of 3, indicate tha t the extent of reduction increases as solution pH i s lowered. Thus in t e s t l A , which was conducted under. acidic conditions, hexavalent chromium decreased t o l e s s than 50 ppb within the f i r s t several hours. In t e s t 2A, which was almost neutral , hexavalent chromium was approximately 100 ppb throughout the t e s t . In t e s t 4A, which was conducted a t a pH of about 8, Cr(V1) concentrations declined t o only 381 ppb a f t e r 120 hours o f reaction.

Data from t e s t s l A , 2A, and 4A, which were a l l conducted

Comparison of the resu l t s of t e s t s 2A and 3A indicates t ha t the extent of hexavalent chromi um reduct i on increases as the mol e r a t i o of Fe ( I I ) : Cr (VI) a t a mole r a t i o of 3 in tesk 2A, Cr(V1) decreased t o about 100 ppb while i n t e s t 3A, which was conducted a t a mole r a t i o of 9, Cr(V1) was quant i ta t ively reduced. 5A and 6A were conducted with so i l a t a soi l t o water mass r a t i o of 0.1. Comparison of the r e su l t s from these two t e s t s fur ther i l l u s t r a t e s t h a t an increase i n the mole r a t i o o f Fe(II):Cr(VI) great ly enhances reduction of hexavalent chromium.

i ncreases . Thus , Tests

Comparison o f the data from t e s t s 2A, 4A, and 5A i l l u s t r a t e s tha t introduction o f so i l r e su l t s i n l e s s chromium reduction. solution was measured in t e s t 5A, while the pH o f the reacted s o l u t i o n of t e s t 2A was 5.8. with the calcareous component of the soi l used in the t e s t . t e s t appears t o have resulted in l e s s reduction of hexavalent chromium. conclusion i s supported by the s imilar i ty of the r e su l t s o f t e s t 5A with those of t e s t 4A, in which the solution pH was adjusted t o 8. reducing agent may have been consumed i n t e s t 5A by interaction o f the ferrous i o n with oxide coatings associated with the s o i l .

Note tha t a pH value o f 8 fo r the reacted

The higher pH of t e s t 5A may be at t r ibuted t o interaction of the solution The higher pH of t h i s

T h i s

In addition, a portion.of the

The data collected in these t e s t s generally indicate tha t equilibrium i s attained within the f i r s t 24 hours, as evidenced by the approach of Cr(V1) concentrations t o constant values. Tests 4A and 5A, however, indicate tha t higher pH values and soil interact ion e f f ec t s may lengthen required reaction time somewhat. be minimized by increasing the mole r a t i o o f Fe(II):Cr(VI), as demonstrated by the r e su l t s of t e s t s 3A and 6A.

Reaction time can

Solutions from a portion of these t e s t s were also analyzed for to t a l chromium. Comparison of the difference between to ta l chromium and hexavalent chromium provides a measure of the t r i va l en t chromium remaining in solution a f t e r treatment. data indicated t h a t even though most o f the hexavalent chromium was reduced t o t r i va l en t chromium during the t e s t s , re la t ive ly l i t t l e of the t r i va l en t chromium was precipitated under neutral t o acidic conditions or i n the t e s t s conducted a t a Fe(II):Cr(VI) mole r a t i o of 3.

This

The e f f ec t of using a mole r a t i o of Fe(II):Cr(VI) o f 9 on chromium reduction and precipi ta t ion was evaluated fur ther in two additional t e s t s . conducted a t a somewhat higher pH (8 t o 8.5). conducted without soi l , essent ia l ly complete reduction and precipitation t o o k place. In the second t e s t , which included s o i l , 96% reduction and 80% precipitation of

These t e s t s were also In the f i r s t t e s t , which was

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chromium was achieved. stoichiometric amount i s required t o achieve quantitative reduction of hexavalent chromium under a lkal ine conditions. Once reduction occurs, however, the reduced chromium readi ly precipi ta tes as a hydroxide sol id phase: The high degree of immobilization observed i s probably related t o the precipi ta t ion of f e r r i c hydroxide generated by oxidation of the ferrous su l fa te during the reaction. This prec ip i ta te may have enhanced removal of chromium by coprecipitation o f i r o n and chromium, and by adsorption of chromium ionic species by the hydroxide

These resu l t s suggest tha t ferrous -ion in excess of the

SODIUM SULFIDE TESTS

Sodium su l f ide was added t o 500 ppb hexavalent chromium so lu t ions t o assess the potential o f using sulf ide t o reduce and precipi ta te chromium. t e s t s conducted under near neutral pH conditions (2D-1, 2D-2, 2D-3, and 2D-5) , the solutions were t reated with sodium sulf ide a t mole r a t io s o f S(-11): Cr(V1) of 6 , 9 , and 12. reaction 2. Note tha t increasing the mole r a t i o used resulted in only a limited improvement in the reduction of hexavalent chromium i n the solutions. Test 2D--5 also contained soil a t a so i l t o water mass r a t i o of 0 . 1 and was treated a t a mole r a t i o o f 12. The degree o f Cr(V1) reduction tha t occurred in the f i r s t 24 hours was similar t o the other t e s t s in t h i s s e r i e s . The cause o f the reversal i s uncertain b u t may be due t o oxidat-ion of the su l f ide by the so i l or, a l te rna t ive ly , t o a gradual increase in solution pH i n response t o dissolution of the calcareous fract ion of the s o i l .

In one se r i e s o f

This corresponds t o 16, 24, and 32 stoichiometric equivalents according t o The resu l t s of these t e s t s are presented in Figure 2.

An apparent reversal, however, was observed a t 48 hours.

An additional t e s t was conducted t o obtain data related t o the degree of treatment and immobilization of hexavalent chromium tha t would occur -in groundwater t rea ted with sodium su l f ide a t a moderately high mole r a t io . employed i n t h i s t e s t and solution pH was adjusted t o 10 by addition of NaOH. The r e su l t s of t h i s t e s t were re la t ive ly favorable, achieving 97% reduction and 20% preci pi t a t i on of chromi urn.

A mole r a t i o of 50 was

GROUNDWATER TREATMENT TESTS

The r e su l t s of the t e s t s presented above indicate tha t ferrous su l f a t e should be e f fec t ive i n remediating groundwater systems contaminated with hexavalent chromium. Additional t e s t s were undertaken t o fur ther verify t h i s by t rea t ing contaminated groundwater and so i l mixtures with ferrous ion.. combinations of ferrous and su l f ide ions t o assess the effectiveness of ferrous/sulf ide in s i t u treatment o f groundwater. during separate weeks. val ues of corresponding sol ut i on sampl es coll ected a t 168 hours from these dupl i cate " t e s t s .

In addition, t e s t s were r u n with

A l l test:; were r u n in duplicate The analytical resu l t s presented below represent averaged

Chromate-contaminated groundwater from the Hanford unconfined aquifer was obtained from well D5-15 of the 100-D Area for use in these t e s t s . Analysis o f the groundwater indicated a to ta l chromi um content of abou t 2100 ppb, w i t h essenti a1 l y a l l o f the dissolved chromium in the hexavalent oxidation s t a t e (Table 1). Uncontaminated so i l was also collected from the 100-D Area of the Hanford S i t e f o r tes t ing purposes. Table 1 presents the resu l t s obtained by combining the soil and groundwater a t several soil/water. mass r a t io s . has a r e l a t ive ly oxidized character since l i t t l e or no reduction of hexavalent chromium occurred during the 168 hours associated with the t e s t s .

These data indicate tha t the so i l

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Ferrous treatment t e s t s were performed a t three levels i n terms of so i l amount and two levels o f ferrous iron concentration. t e s t ing was t o verify t h a t remedi ation o f chromate-contaminated groundwater can be achieved by the addition of ferrous i o n and t o determine the oxidizing capacity of the so i l r e l a t i v e t o the ferrous i o n . ferrous iron, groundwater, and soil i n the amounts shown i n Table 2, where so i l /water i s a mass r a t i o and Fe/Cr' i s a mole r a t io .

The objective of this portion of the

These t e s t s were conducted by combining

Tests A, B y and C were conducted a t a Fe/Cr mole r a t i o of 2.64, w h i c h i s s l i gh t ly l e s s than the stoichiometric r a t i o required for reduction (Fe/Cr = 3) . The results o f these tests indicate t h a t the soil i s oxidizing r e l a t ive t o ferrous ion, w i t h an oxidizing capacity of about 0.17 pequivalents per gram of soil .

The r e s u l t s o f tests D , E , and F indicate tha t essent ia l ly complete reduction and precipi ta t ion of hexavalent occurred when a Fe/Cr mole r a t i o o f 21.16 was employed ( i .e. , about 7 stoichiometric equivalents o f reductant). of so i l up t o a soil /water mass r a t i o of 0 .2 d i d not s ignif icant ly a f f ec t the treatment r e su l t s .

Furthermore, the addition

Ferrous/sulfide treatment tests were performed a t two leve ls i n terms o f so i l amount and th ree leve ls o f ferrous and su l f ide concentrations. The primary objective of this p o r t i o n o f the tes t ing was t o determine i f remediation o f chromate-contaminated groundwater can be more readi ly achieved by the addition of b o t h ferrous and su l f ide ions than by the use o f the ferrous treatment alone.' These t e s t s were conducted by combining ferrous iron, sulf ide, groundwater, and soil i n the amounts shown i n Table 2 , where soil./water i s a mass r a t i o and Fe/Cr and S/Cr a re mole r a t io s . Components were added i n the order ferrous su l fa te , sodium sulf ide, and so i l t o the groundwater sample.

Tests G and H were conducted w i t h a t o t a l o f 0.55 stoichiometric equivalents of reductants a t a Fe/S mole r a t i o o f about 2. Minimal reduction o f hexavalent chromium was observed. In tests I and J, a to ta l o f 7 stoichiometric equivalents of reductants was added a t a Fe/S mole r a t i o o f about 0.5. occurred, a1 though, the addition o f so i l interfered w i t h treatment. The r e su l t s of Tests K and L indicate t h a t essent ia l ly complete reduction and precipi ta t ion o f hexavalent occurred when 7 stoichiometric equivalents of reductants was applied a t a Fe/S mole r a t i o of 2 . The addition of soil up t o a soil /water mass r a t i o of 0.2 did n o t s ign i f icant ly affect the treatment resu l t s .

Signif icant reduction

DISCUSSION

The r e su l t s o f the ferrous and sulfide treatment. t e s t s conducted w i t h chromate solutions c l ea r ly indicate tha t ferrous treatment i s superior t o sulf ide treatment in the reduction o f hexaval ent chromi urn. groundwater w i t h ferrous i o n s d u r i n g this study further demonstrated t h a t the application o f ferrous i o n a t a suf f ic ien t ly h igh r a t i o of Fe/Cr may be a feas ib le remediation approach. I t i s suggested tha t an elevated molar r a t i o serves t o overcome the lower reaction r a t e s associated w i t h the moderately alkal ine character o f the groundwater. Once reduced, the chromium readily precipi ta tes as a hydroxide phase under these conditions. treatment should be effect ive i f applied a t a moderately elevated value o f t o t a l reductant t o chromate r a t i o and a mole r a t i o o f Fe/S of 2.

The treatment of chromate-cohtaminated

I t also appears t ha t combined ferrous/sulf ide

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The so i l sample u t i l i zed i n t h i s study has a generally oxid-izing character, as demonstrated by the interaction with ferrous ion. reductant r a t i o o f applied ferrous ion or ferrous/sulfide to chromate would be required t o achieve quant i ta t ive reduction of chromium i f treatment solutions were injected in to the groundwater system being considered. The r e su l t s of these t e s t s indicate t h a t the soi l sample has an oxidizing capacity of about 0.17 pequivalents per gram o f so i l re la t ive t o the ferrous ion. I f we assume tha t the soil /water mass r a t i o o f the aquifer i s 5 and one pore volume of treatment solution i s injected, a ferrous su l f a t e solution w i t h a concentration of 870 pmoles/L (49 ppm Fe2') would be requi.red t o overcome the inherent oxidizing capacity of the aquifer. Additional ferrous iron must also be added t o reduce the hexavalent chromium present. r e su l t s of t h i s study suggest tha t a Fe/Cr mole r a t io o f 9 -is adequate for e f f i c i e n t reduction of chromate (three times the stoichiometric r a t i o ) . Since the hexavalent chromium concentration of the aquifer i s abou t 2100 ppb i n t h i s case, an additional 363 ,moles/L of ferrous ion would be needed. Thus, injection of a ferrous su l f a t e solution a t a concentration of about 1200 pmoles/L (67 ppm f:e2+) s h o u l d be suf f ic ien t t o achieve complete reduction and immobilization of chromate present i n the groundwater system.

Thus , a somewhat higher t o t a l

The

Application of the in s i t u chemical precipitation approach t o aquifer remediation could be conducted in conjunction with a pump and t r e a t program. That i s , pumped groundwater could be t reated in ex s i t u f a c i l i t i e s t o remove contamination, the treatment agent added, and the groundwater reinjected i n an upgradient position within the aquifer. contamination would serve t o reduce and precipi ta te hexavalctnt chromium w i t h i n the aquifer. reducing the amount of time required t o achieve cleanup goals.

Subsequent movement of the treatment agent through the zone of

This approach could potent ia l ly provide s ignif icant cost savings by

CONCLUSIONS

The r e su l t s o f a se r ies o f small-scale batch t e s t s indicate tha t inject ion of ferrous or ferrous/sulf ide solutions may be a viable approach t o remediation o f many chromate-contaminated groundwater systems. amended with ferrous iron i s par t icular ly a t t rac t ive since the s o l u t i o n s would be nonhazardous in nature and shou ld be simple t o prepare and in jec t . The use of ferrous/sulf ide combinations may also be effect ive i n some s i tua t ions , b u t may require negiot ia t ions with regulatory s t a f f owing t o the potent ia l ly hazardous nature of sul f i de .

The t e s t r e su l t s presented in t h i s study indicate tha t application o f a s o l u t i o n with a Fe/Cr mole r a t i o o f 9 or greater i s required t o achieve quant i ta t ive reduction and precipi ta t ion of hexavalent chromium. This r a t i o may need t o be increased somewhat, however, depending on the capacity of associated aquifer sediments t o oxidize the treatment agent. s imilar t o those presented here be conducted as a means of determining appropriate treatment solution concentrations for remediation o f other chromate-contaminated groundwater systems.

The application o f di luted s o l u t i o n s

I t i s recommended t h a t bench-scale t e s t s

ACKNOWLEDGMENTS

Funding f o r t h i s work was provided by the Westinghouse Hanford Company Development Steering Board, the Westinghouse Hanford Company Environmental Restoration Program, and the U.S. Department of Energy's Office of Technology Development un-der the In Si tu Remediation Integrated Program.

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REFERENCES

1. C . C . Travis and C . B . Doty, "Can contaminated aquifers a t Superfund s i tes be remedi ated?" Envi ron. Sci . Technol . , 24( 10) : 1464-1466 (1990).

2. D . E . Sanning and M.I. Black, Protectinq the Environment a t Superfund S i t e s throuqh Chemistry, EPA/600/D-87/22ZY U. S. Environmental Protection Agency, Hazardous Waste Engineering Research Laboratory, Cincinnati, 1987, pp 5-11.

3. E .C . Thornton, C.A. Jurgensmeier, and M.A. Baeehler, Laboratory Evaluation of the In S i tu Chemical Treatment Approach for Remediation o f Cr(V1)-Contaminated Soi l s and Groundwater, WHC-SP-0704, Westinghouse Hanford Company, Richland, WA, 1991, pp 1-33.

4. J.M. Beller, GiS. Carpenter, R.E. McAtee, e t a1 ., Full-scale Implementation o f the Sodium Sulfide/Ferrous Sulfate Treatment Process, ESL-TI?-89-08, Engineering and Services Laboratory, Air Force Engineering and Services Center, Tyndall AFB, FL, 1989, pp 1-30.

5. L. E ; Eary and D. Rai , "Chromate removal from aqueous wastes by reduction w i t h ferrous ion," Env i ron . Sci. Technol., 22(8):972-977 (1988).

6 . T.E. Higgins and V . E . Satar, Treatment o f Electroplatinq Wastewaters bv Alkaline Ferrous Reduction o f Chromium and Sulfide Precipitation, ESL-TR-83-21, Engineering and Services Laboratory, Air Force Engineering and Services Center, Tyndall AFB, FLY 1983, pp 3-8.

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Tabl e 1. Groundwater analysis and soi 1 /groundwater test results.

Test I D Soil/Water Cr(V1) , ppb Cr(total), PH m b

D5-15 0 2114 2118, 8.2 Soil blank-1 0.1 2148 2113 8.0 Soilblank-2 ' 0.2 2078 2063 8.2

Table 2. Ferrous treatment of chromate-contaminated groundwater and soi 1 /groundwater mixtures.

~~

Test I D Soil/Water Fe/Cr Cr(VI), ppb Cr(total), PH DD b

A 0 2.64 232 23 2 8.3 B 0.1 2.64 574 569 8.0 C 0.2 2.64 83 4 777 8.0

D 0 21.16 t 5 0 10 6.9 E 0.1 21.16 t 5 0 15 8.1 F 0.2 21.16 t 5 0 25 8.1

Tabl e 3. Ferrous/sul fide treatment o f chromate-contaminated groundwater and soil /groundwater mixtures.

Test ID Soil/Water Fe/Cr S/Cr Cr(VI), Cr(total), pH

G 0 0.31 0.17 1871 ' 1873 8.2 H 0.2 0.31 0.17 1874 1790 7.9 I 0 1.23 2.47 113 . 144 8.1 J 0.2 1.23 2.47 367 43 2 7.9 K 0 4.23 2.12 t 5 0 10 8.2 L 0.2 4.23 2.12 t 5 0 17 7.8

PPb PPb

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400

300

9 e e

> 6 200

4 Y

100

0

e Test 4A: Molar Ratio Fe:Cr = 3, pH=: 8 e ' -

0

Test 5A (with soil): Molar Ratio Fe:C:r =3, pH = 8 e I .

0 - .

Test 2A: Molar Ratio Fe:Cr = 3, pH :: 5.8

Test 1A: Molar Ratio Test 3A and 614: Fe:Cr = 3, low pH Molar Ratio FcCr = 9

I 1 - -

0 10 20 30 40 50 Time, hrs

H9211018.4

Figure 1. Change in concentration o f hexavalent chromium w i t h time following treatment of a 500 ppb solution o f Cr(V1) w i t h ferrous su l f a t e .

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400

300

P e e & 200

0' > v

100

0

Test 2D-1 Molar Ratio = 6

Test 2D-2 Molar Ratio = 9

Test 2D-5 (with soil) Molar Ratio.= 12

I

Test ##4 Molar Ratio = 50 / *

Test 2D-3 Molar Ratio = 12

0 7

I I I I

0 10 20 30 40 Time, hrs

50

H9211018.8

Figure 2. Change i n concentration o f hexavalent chromium w i t h time f o l l o w i n g treatment o f a 500 ppb. Cr(V1) s o l u t i o n w i t h sod.ium su l f ide .