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Oxidation and fate of chromium in soilsJae Gon Kim b a & Joe B. Dixon aa Department of Soil and Crop Sciences , Texas A&M University , College Station, TX,77843-2474, USAb Department of Earth System Sciences , Yonsei University , 134 ShinchondongSeodaemoongu, Seoul, 120-749, KoreaPublished online: 22 Nov 2011.

To cite this article: Jae Gon Kim & Joe B. Dixon (2002) Oxidation and fate of chromium in soils, Soil Science and PlantNutrition, 48:4, 483-490, DOI: 10.1080/00380768.2002.10409230

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Soil Sci. Plant Nutr., 48 (3), 483-490,2002 483

Oxidation and Fate of Chromium in Soils

Jae Gon Kim! and Joe B. Dixon

Department of Soil and Crop Sciences, Texas A&M University, College Station, TX., 77843-2474 USA

Received July 12,2001; accepted in revised form March ll, 2002

Land disposal of chromium-containing wastes constitutes an environmental risl{ where oxi­dation of Cr(lll) to Cr(VI) may occur. The oxidation rate of Cr(lll) to Cr(VI) and different forms of Cr(VI), labile (adsorbed and solution) and solid forms were determined, in 11 moist soil samples collected from four Texas soil series (Silawa, Boonville, Burleson, and Pledger soils). The Cr oxidation rate increased with increasing content of reactive manga­nese (Mn) oxides and decreasing pH of the soils as follows: Pledger> Silawa > Boonville == Burleson. Concentration of labile Cr(VI) in the soils was apparently controlled by barium chromate (BaCr04) without barite (BaS04) or by Cr-substituted barite [Ba(S,Cr)041 in the presence of barite. The Boonville soil contained barite and a much lower concentration of labile Cr(VI) than the other soils. Based on the Cr oxidation rate and forms of Cr(VI), soils with a low content of reactive Mn oxide, high pH, and barite was recommended for field testing as safe land disposal sites of Cr-containing wastes to minimize Cr oxidation and mobility. Boonville soil was the best soil among the four soils for safe land disposal of wastes containing Cr.

Key Words: Ba, barite, labile and solid forms of Cr(VI), pH, reactive Mn oxide.

Chromium (Cr) is the tenth metal of the earth crust in abundance and the fifth metal in economic importance (Katz and Salem 1994). It has been discharged into the environment from various industrial sources including pigment production, magnetic tape production, electro­plating, leather tanning, gas and oil well drilling, etc. The added Cr can be a non-hazardous material or a toxic and carcinogenic pollutant depending on its oxidation state.

The most stable oxidation states of Cr under the earth surface conditions are trivalent and hexavalent forms (Cotton and Wilkinson 1988). Chromium(III) is, in gen­eral, much less toxic and mobile in soils than Cr(VI). A Cr(III)-picolinic acid species is essential in sugar metab­olism. Chromium(III) is cationic and forms stable com­plexes with organic and inorganic ligands that can be soluble or insoluble depending on the molecular weight and type of ligand on the pH (Cotton and Wilkinson 1988). The common cationic species of Cr(IIl) in aquat­ic environment are Cr3+, Cr(OH)" +, CrlOH)24+, and CriOH)66+. However, Cr(OH)3° and Cr(OH).j- occur at

1 To whom correspondence should be addressed. Present ad­dress: Department of Earth System Sciences, Yonsei University, 134 Shinchondong Seodaemoongu, Seoul. 120-749 Korea. E­mail: jgkim@yonsei.ac.kr

high pH. Inorganic Cr(m) is precipitated as inert and stable chromium hydroxide [Cr(OH)3J or chromium oxide [Crp3J or chromium oxide (Crp3) at pH> 5.5. Chromium hydroxide and Cr20 3 in soils are not readily absorbed by living organisms and are not transmitted in the food chain. However, Cr(Ill) treated with organic acids such as citric and fulvic acids remains soluble and mobile in soils even at pH > 8 (James and Bartlett 1983a).

Chromium(VI) occurts as chromate (CrO/-) or di­chromate (Cr20 7"-) anionic complexes or as moderately to sparingly soluble chromate salts such as BaCrO.), CaCrO.j, PbCr04 , and ZnCr04 (Katz and Salem 1994; James 1996). The anionic Cr(VI) complexes are rela­tively mobile in soils and can be adsorbed on iron (Fe) oxide and edge of clay minerals. Chromium(VI) is a Class A human carcinogen and an acute irritant to plant and animal cells. Thus, the contamination of soil and groundwater by Cr(VI) is an environmental concern due to its toxicity and mobility.

Land disposal of wastes results in the contact of Cr, a redox-reactive material, with oxidants and reductants in soils. The redox-reactions resulting from the contact may enhance or reduce the risk and rate of subsequent toxicity and mobility. The main oxidants in soils, which can oxidize Cr(III) to Cr(VI), are Mn oxides (Bartlett

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484 J.G. KIM and J.B. DIXON

and James 1988). On the other hand, ferrous iron (Fe2+) (Buerge and Hug 1997), organic matter (Wittbrodt and Palmer 1995), and reduced sulfur (S2-) (Patterson et al. 1 997) can reduce Cr(VI) to Cr(lll). A soil with a higher Mn oxide content and a lower organic matter content displays a higher Cr oxidation capacity than a soil with a lower Mn oxide content and higher organic matter content (Kim 1998).

Land disposal of Cr-containing wastes such as drilling fluid and fly ash has been practiced. The land disposal practice may cause an environmental hazard when the applied CrUll) is oxidized. The objective of this study was to determine the oxidation rate of added Cr(ID) and forms of Cr(VI), labile and solid forms, in four Texas soils to provide guidance for the selection of safe land disposal sites.

MATERIALS AND METHODS

Soil sampling and storage. Eleven moist soil samples were collected from four Texas soils in the USA: Silawa soil (Fine-loamy, siliceous, semi active , thermic Ultic Haplustalfs); Boonville soil (Fine, smec­titic, thermic Ruptic-vertic Albaqualfs); Burleson soil (Fine, smectitic, thermic Udic Haplusterts); Pledger soil (Very-fine, smectitic, hyperthermic Typic Hapluderts). Three or four soil samples of each soil series were col­lected at various depths (0 to about 100 em) except for the Pledger soil (one sample at 0 to 15 cm), which was relatively homogeneous to a depth of about 100 cm (personal communication with P.R. Owens who was working on the soil and provided a fresh sample). The collected soil samples were stored in double polyethyl­ene bags with moist paper towels between the layers in a refrigerator at 4°C to minimize the changes of the soil redox properties (Bartlett and James 1993).

Analysis of soil samples. Moisture content of the collected soil samples was determined by the weight difference between moist and oven-dried (l05T) sam­ples. Water-extractable organic carbon (WEOC) was ex­tracted with distilled water at room temperature (22T) (Candler and Cleve 1982). A mixture of 3 g of moist soil (oven-dry weight base) and 40 mL of distilled water was shaken for 24 h. After shaking, the mixture was filtered with a filter paper (Fisher Filter Paper, Q2; Fisher, Chi­cago, IL, USA). The concentration of organic carbon in the filtrate was determined with an 01 Analytical Total Organic Carbon Analyzer (01 Instrument, College Sta­tion, TX, USA).

A portion of each soil sample was air-dried and gently ground to pass a 2 mm sieve for the determination of the pH, cation exchange capacity (CEC), Mn oxide content, iron (Fe) oxide content, and contents of Cr and other

selected cations such as barium (Ba), calcium (Ca) , cobalt (Co), lead (Pb), and zinc (Zn). The pH and CEC were determined using a 1 soil: 10 water (w / v) method and a Na-acetate centrifugation method, respectively (Soil Survey Staff 1996). Manganese was extracted with 0.1 M hydroxylamine hydrochloride in 0.01 M HN03 for the determination of the total Mn oxide content (Chao 1972) and 0.002 M hydroquinone for the determination of the reactive Mn oxide content (Sherman et al. 1942). The mixture of soil and extracting solution (0.5 g of finely ground soil for total Mn oxide or 2.5 g of soil for reactive Mn oxide and 25 rnL of extracting solution) was shaken for I h. After shaking, the mixture was cen­trifuged at 10,000 rpm for 10 min and then the superna­tant was collected and stored for further analysis. Iron oxide content was determined by the dithionite-citrate­bicarbonate method (Jackson 1974). The concentrations of Fe and Mn in the solutions were determined with a Perkin Elmer 3100 atomic absorption spectrometer using air-acetylene flame (Perkin Elmer, Oak Ridge, TN, USA).

Barium, Ca, Co, Pb, and Zn in the soil samples were extracted using USEPA SW-846, Method 3050 (HN03-

Hp" extraction) (USEPA 1986). The concentrations of Ba, Ca, Co, Pb, and Zn in the solutions were determined by inductively coupled plasma spectroscopy (Spectro Analytical Instruments, Fitchburg, MA). Chromium(VI) in the soil samples was extracted with a carbonate­hydroxide solution (0.28 M Na1C03 in 0.5 M NaOH) (James et al. 1995). To a 2.5 g sample, 50 rnL of the ex­tracting solution was added in a glass beaker, along with 400 mg of MgCl2 and 0.5 mL of 1 .0 M phosphate buffer (0.5 M K2HP04 / 0.5 M KH2PO~, pH 7). The soil suspen­sion was stirred for 10 min and then heated to maintain a temperature of 90°C for 1 h. After gradual cooling to room temperature, the digest was filtered with a 0.45 m membrane filter and thc pH was adjusted to 7 with 5.0 M

HN03 . Chromium(VI) concentration in the solution was determined with by the diphenylcarbazide (DPC) chro­magen method (James and Bartlett 1983b).

Oxidation of chromium in the soils. Two and half grams (oven-dry weight base) of moist soil and 25 rnL of a fresh 120 mg L -1 Cr(lll) solution prepared with CrCI3 • 6Hp were placed in 40 rnL polyethylene centrifuge tubes. After the head-spaces were filled with Nl gas, the suspensions were shaken for 1 to 23 d. After shaking, 1 rnL of 0.25 M K2HPO.j / KH2P04 , buffered at pH 7.2 was added and the mixture was shaken addition­ally for 1 h to extract labile Cr(VI) (solution and ad­sorbed forms) (James and Bartlett 1983c). Then, the mixture was centrifuged at 10,000 rpm for 15 min and the supernatant was collected and stored in a plastic bot­tle for Cr(VI) determination. After collection of the supernatant, the mixture was washed 3 times with dis-

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Oxidation and Fate of Chromium in Soils 485

Table 1. Characteristics of the soil samples.

Soil Depth (cm) pH" CECh WEOC' TMnd RMn' Fe2O/ TCrIl Cr(VlY' Moisture;

Silawa A 0-36 6.3 2.7 0.14 109 2.28 1.2 6.87 1O.l Bl 46-66 6.1 11.9 0.13 14.3 0.25 8.2 27.0 15.6 B2 103-123 6.l 4.4 0.l9 15.6 0.34 3.1 14.9 0.1 13.3

Boonville A 0-25 5.9 2.3 0.19 9.62 0.39 1.1 5.41 12.2 E 25-46 5.9 1.1 0.19 4.24 0.30 1.0 8.00 14.1 Bl 46-76 6.0 30.8 0.42 2.82 0.62 11.9 41.4 22.8 B2 76-96 6.1 21.6 0.23 2.33 0.37 3.4 23.7 15.5

Burleson Al 0-15 7.7 35.7 0.22 680 1.29 0.1 43.2 23.0 A2 15-45 8.3 38.7 0.20 645 1.20 0.1 46.3 21.1 B 45-75 8.3 34.7 0.18 501 0.92 2.1 48.2 0.1 21.2

Pledger A 0-15 7.9 40.5 0.23 261 2.75 7.5 62.5 22.8

"Determined by the I soil: 10 water method, bcation exchange capacity (cmol, kg-I), 'water-extractable organic carbon (mg g-I), "total Mn extracted with 0.1 M NHpH· HCl in 0.01 M HNO, (mg kg-I), 'reactive Mn extracted with 0.002 M hydroquinone-extract-able Mn (mg kg-I), firon oxide content extracted with dithionite-citrate-bicarbonate (mg g-I), qotal Cr determined by the HF-aqua regia digestion method (mg kg-I), h Cr(Vl) extracted with carbonate-hydroxide solution (mg kg-I), ; moisture content of field soils (%).

tilled water and the solid form of Cr(VI) was extracted with the carbonate-hydroxide solution as indicated above (James et al. 1995). The concentration of Cr(VI) in the solutions was determined by the DPC method (James and Bartlett 1983b). All the solutions were pre­pared with oxygen-free water made by boiling for 20 min. All the Cr oxidation tests were conducted with duplicate samples.

The multiple regression between the amount of total oxidized Cr(VI) and soil parameters such as reactive Mn oxide content, WEOC. and Fe oxide. pH. and CEC was determined using a statistical computer program. SPSS@ (SPSS Inc., Chicago. IL).

RESULTS

Characteristics of soil samples Burleson soil showed the highest total Mn oxide con­

tent and only a relatively small fraction of it was reac­tive (Table 1). Indicating that most of the Mn oxide in Burleson soil occurred as nodules and only a small frac­tion of the total was reactive in Cr oxidation determined by measurements under laboratory conditions. The Fe­Mn nodules were confirmed by observation in the field and in the laboratory (White and Dixon 1996). Surface horizons of Silawa and Pledger soils showed a relatively high content of reactive Mn oxide. A trace amount of native Cr(VI) was extracted from samples of Silawa B2 horizon and Burleson B horizon. The soil samples con­tained significant amounts of Ba and Ca and trace amounts of Co, Pb, and Zn except for the Burleson and

Table 2. Concentrations of barium (Ba), calcium (Ca), cobalt (Co). lead (Pb). and zinc (Zn) in the soil samples determined by the HNOJ -HP2 extraction method.

Concentration (mM) Soil

Ba Ca Co Pb Zn

Silawa A 0.28 16.2 0.06 om 0.01 Bl 0.49 49.1 0.12 0.06 om B2 0.21 25.4 0.05 0.02 0.50

Boonville A 0.17 9.4 0.Q3 0.02 0.01 E 0.12 13.7 0.03 0.01 om Bl 0.23 62.5 0.09 0.08 0.l4 B2 0.11 64.6 0.08 0.05 0.12

Burleson AI 1.86 983 0.45 0.12 0.02 A2 1.84 1,168 0.46 0.12 0.64 B 1.73 1,491 0.45 0.11 om

Pledger A 0.98 53.6 0.28 0.l2 0.17

Pledger soils that contained larger amounts of these ele­ments (Table 2).

Oxidation rate of added chromium(III) in the soil samples

The Cr oxidation rate, total oxidation ratio of added Cr(III), in the moist soil samples reached a steady state or declined after 16 d of reaction except for the Boon­ville E and B 1 horizons and Pledger soil, which showed a slight increase (Fig. 1). The highest oxidation rate of added Cr(III) observed in the Pledger soil [A: 149.9 mg kg-I of Cr(VI); 12.1 % of added Cr(III)] and the lowest

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486 J.G. KIM and J.B. DIXON

oxidation rate in a subsoil sample of the Burleson soil [B: 41.2 mg kg -1 of Cr(VI); 3.4 % of added Cr(III)] after 16 d of reaction. In general, the surface soil samples showed a higher Cr oxidation capacity than the subsur­face soil samples in a soil series. For integrated Cr oxi­dation rate for each soil series, the Boonville and Burleson soils exhibitd the lowest Cr oxidation capacity and the Pledger soil showed the highest Cr oxidation capacity as follows: Pledger soil> Silawa soil> Boonville soil == Burleson soil.

Based on the statistical analysis, the content of reac­tive Mn oxide and pH were the main factors controlling the Cr oxidation capacity of the soils (Table 3). More Cr was oxidized in the soils with higher content of reactive Mn oxide, compared with total Mn oxide. and a lower pH. The equation derived for the amount of total oxi­dized Cr(VI) after 16 d of reaction and the amount of reactive Mn oxide and pH of the soils was as followings:

Cr(VI)total = 44.00 Mnreac'ive - 0.02 pH2 + l.49

Table 3. Statistical data for multiple regression analysis between the amount of total oxidized chromium(Vn in the soil sam­ples and soil parameters: reactive Mn oxide, water extractable organic carbon (WEOe) , Fe oxide, pH, and cation exchange capacity (CEC).

Variables

Mn Mn,pH" Mn,pH",WEOC Mn, pH", WEOC,

Fe-oxide Mn, pW, WEOC,

Fe-oxide, CEC

Adjusted R2 P value

0.633 0.003 (Mn) 0.748 0.001 (Mn), 0.035 (pW) 0.744 0.001 (Mn). 0.037 (pH~), 0.383 (WEOC) 0.728 0.002 (Mn), 0.078 (pH"), 0.281 (WEOC).

0.370 (Fe-oxide) 0.679 0.006 (Mn) , 0.709 (pHl), 0.606 (WEOC),

0.487 (Fe-oxide), 0.787 (CEC)

Equation

33.70 Mn + 0.67 44.00 Mn ~ 0.G2 pH~ + 1.49 43.7 Mn ~ 0.02 pH" ~ 1.29 WEOC + 1.79 42.46 Mn ~ 0.G2 pH2 ~ 2.12 WEOC +

0.03 Fe-oxide + 1.78 42.14 Mn ~ 0.01 pH~ ~ 1.54 WEOC +

0.04 Fe-oxide ~ 7.70 X 10-3 CEC + 1.42

Mn, reactive Mn (mM kg-l); WEOC, water-extractable organic carbon (mg g-l); Fe-oxide, DCB-extractable Fe-oxide (mg g-'); CEC, cation exchange capacity (cmole kg-l).

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Oxidation and Fate of Chromium in Soils 487

where, Cr(VI\otal is the total oxidized Cr(VI) (mM) after 16 d of reaction, Mnreactive corresponds to 0.002 M hydro­quinone-extractable Mn (mM), and the pH is the value determined by the 1: 10 soil: water method. The amount of oxidized Cr(VI) in the soils and the predicted amount of Cr(VI) determined with the above equation

3.0,----------------7\

R'2 O.BO 0

2.5

~ • ~

2.0

'" U

'" 1.5 :a .."! "tl

'" > 1.0 '" '" '" • Silawa soil .0 '" Boonville soli a • Burleson soli

0.5 0 Pledger soil

0.0.jL,..-~~-~~-~--~----.....j 0.0 0.5 1.0 1.5 2.0 2.5 3.0

Predicted labile Cr(VI) (rnM)

Fig. 2. Observed values of total oxidized chromium(VI) in the soils vs. the predicted values of chrornium(VI) using the equation, Cr(VI)total = 44.00 Mn.eactive - 0.02 pH2 + 1.49, deter­mined by statistical analysis using SPSS®.

Silawa soil 100 100

80 80

~ e..- ~ o

<: 60 <: 60 ..9 0

ti ti .. .. 40 u: 40 u:

20 20

0 0 A 81 82 A

Burleson soil

100 100

80 80

~ 0 ~ <: 60 <: 60

..9 ..9 ti ti .. .. u: 40 u: 40

20 20

0 0 A1 A2 8

showed a high correlation (R2 = 0.80) (Fig. 2). The WEOC and CEC of the soils had a negative effect and the Fe oxide content had a positive effect on the Cr oxi­dation capacity. However, the contents of WEOC and Fe oxide, and CEC were not statistically significant in rela­tion to Cr oxidation indicated, as by the high p values (> 0.05).

~ ~ U ~ :c co

;::. Cl .2

-3,-----------------,

-4

-5

-6

-7 3 4 5

• • Silawa soil

Boonville soil t::,.

lID

6 pH

7

Pledger soil o

• • Burleson soli

B 9

Fig. 4. Concentration of labile Cr(VI) in the soil samples. The solubility of BaCr04 (solid line) was cited from Rai et al. (1986).

Boonville soil

E 81 82

solid Cr(Vl) labile Cr(VI)

Fig. 3. Fractions of labile (ad-sorbed and solution forms) and solid forms of Cr(VI) in the soils

A after 16 d of reaction.

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488 J.G. KIM and J.B. DIXON

Solid versus labile chromium(VI) The solid form of Cr(VI) was dominant in the Silawa

and Boonville soils and subsurface soils of the Burleson soil (A2 and B) which the labile form (solution and adsorbed forms) was dominant in the surface soils of the Burleson Al horizon and Pledger A horizon (Fig. 3). The solid fraction of oxidized Cr(VI), in general, in­creased with the reaction time (data not presented in this report). The concentration of labile Cr(VI) in the soils except for the Boonville soil and Silawa B 1 horizon agreed well with the solubility of BaCrO.j and apparent­ly was controlled by BaCrO.j (Fig. 4). The concentration of labile Cr(VI) in the Boonville soil and Silawa B 1 horizon was much lower than that in the other soil sam­ples.

DISCUSSION

Oxidation rate of added chromium(III) in the soils. The added Cr(ill) was slowly oxidized to Cr(VI) during the first week and then rapidly for the next 9 d before the highest rate was reached and a steady state or slight decline usually occurred. Decline and steady state in the changes of the total oxidized Cr(VI) level of the soils after 16 d of reaction (Fig. 1) was probably due to the reduction of Cr(VI) to Cr(III) and the decrease in the availability of Cr(Ill) at Mn oxide surfaces. In the pH range of the soils (5.9 to 8.3), the added fresh aqueous Cr(Ill) was precipitated as fine particles of Cr(OH)3 (Cotton and Wilkinson 1988). The Cr(OH)3' the surface of which had positive charges in the pH range of this experiment, was presumably ad­sorbed on the negative charges of the soil particles in­cluding Mn oxides, clay minerals, and organic matter (J ames and Bartlett 1983b). The adsorbed Cr(OH)3 par­ticles on the Mn oxide surface were readily oxidized but the adsorption of the Cr(OH)3 particles on clay minerals and organic matter reduced the availability of Cr(III) for the oxidation by Mn oxide. The oxidation and reduction of Cr occurred at the same time as when Cr was added to the soil. As the reaction continued after addition of Cr(Ill) to soil, the availability of Cr(Ill) for the oxida­tion decreased and thus, the relative rate of oxidation versus reduction decreased. Therefore, the level of Cr(VI) in soils decreased after a long period of time.

The Cr oxidation capacity of a soil can be predicted using the reactive Mn oxide content and pH (Fig. 2 and Table 3). The 0.002 M hydroquinone-extractable Mn oxide, the reactive Mn oxide, may represent dispersed rather than nodule forms. All the Mn oxides in the nod­ules may not be available due to the low accessibility of Cr(III) and cementation with other minerals such as Fe oxide and clay minerals, which form nodules with a low

porosity (White and Dixon 1996). With increasing pH, the solubility of Mn oxide and Cr(OH)3 decreases (Rai et al. 1986; Krishnamutri and Huang 1988). The lower solubility of Mn oxide and Cr(OH)3 at higher pH reduced the reactivity of Mn and Cr leading to decrease of Cr oxidation by Mn oxides.

Soil organic matter is a well known reductant for Cr and it also has a high CEC (Wittbrodt and Palmer 1995). A soil with a high CEC shows a high capacity for adsorption of Cr(III) cations and positively charged Cr(OH)3' which may lead to a low availability of Cr(Ill) for the oxidation by Mn oxide in the soil. Consequently, the reduced availability of Cr(III) associated with the high CEC reduced the Cr oxidation capacity of the soil. The positive effect of Fe oxide on the Cr oxidation capacity of the soils observed in this study was opposite to the results obtained by Fendorf et al. (1998). Showing that goethite reduced the Cr oxidation capacity of bir­nessite by adsorption of Cr(III) on goethite in a simple birnessite-goothite-Cr(III) solution system at <700 h of reaction and did not show any effect at a longer reaction time. A possible explanation for the positive effect of Fe oxide on the Cr oxidation capacity of the soils is that Cr(III) is oxidized to Cr(VI) by ·OH radicals from pho­tolysis of Fe(OH)+2 complexes (Zhang and Bartlett 1999).

Solid versus labile chromium(VI). Among the moderately to sparingly soluble chromate compounds that contain cations such as Ba, Ca, Co, Pb, and Zn, significant amounts of Ba and Ca were extracted by HN03-HP2 (Table 2). Barium chromate (BaCr04)

apparently controlled the concentration of labile Cr(VI) (adsorbed and solution forms) in the soils investigated except for the Silawa B 1 horizon and Boonville soil (Fig. 4). The solubility of BaCrO.) is pH-dependent at pH < 6 but is pH-independent at pH > 6 since HCr04 -

or Cr042- is the dominant species, respectively (Rai et

al. 1988). For the Boonville soil and Silawa B 1 horizon samples, the concentration of labile Cr(VI) was much lower than the solubility of BaCr04. The lower concen­tration of labile Cr(VI) in these soil samples may be controlled by Cr-substituted barite [Ba(S,Cr)O.)] which shows a much lower solubility than BaCr04 (Rai et al. 1988). The solubility of Ba(SO.99,CrO.01 )04 was 9 X 10-7

M and it decreased with decreasing Cr(VI) content in Cr-substituted barite. The concentration of labile Cr(VI) at 1.9 X 10-6 M and the presence of barite in the Boon­ville soil, which was identified by X-ray diffraction in the parallel sample of this soil, indicated that the con­centration of labile Cr(VI) was controlled by Cr-substi­tuted barite. The concentration of labile Cr(VI) in the soils was not controlled by CaCrO.\ (7.1 X 10-4 M),

CoCr04 (insoluble), PbCr04 (1.8 X 1O-7 M), and ZnCrO.j (insoluble) (Katz and Salem 1994) even though

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Oxidation and Fate of Chromium in Soils 489

their cations were present in the soils (Table 2), presum­ably due to the availability of these cations for the for­mation of the chromate salts and the high solubility especially for CaCr04 . The results for forms of Cr(VJ) imply that the disposal of Cr-containing waste on the Boonville soil containing barite can minimize the toxici­ty and mobility of oxidized Cr(VI). Authigenic barite has been reported in soils (Lynn et al. 1971) and barite is used as a weighting material in drilling fluids for oil and gas wells where Cr is also utilized. The interaction of Cr and barite requires more of detailed investigations in soils where both occur.

Conclusion. The chromium oxidation rate in the soils decreased or reached a steady state after 16 d of reaction. Chromium oxidation capacity of the soil sam­ples was mainly controlled by the content of reactive Mn oxide and pH. More Cr(Ill) became oxidized to Cr(VJ) in the soils with a higher content of reactive Mn oxide and a lower pH. Cation exchange capacity and content of WEOC had a negative effect on the Cr oxida­tion capacity of the soil samples but the content of Fe oxide had a positive effect. However, the latter three val­ues were not found to be statistically significant in their relationship to Cr oxidation. The concentration of labile oxidized Cr(VI) (adsorbed and solution forms) appar­ently was controlled by BaCrO.) without BaSO.j or by Ba(S,Cr)O.) in the presence of barite in the soils. Soils with a low content of reactive Mn oxide, high pH, and BaS04 are recommended for field testing as safe land disposal sites for Cr-containing wastes to minimize the oxidation, mobility of oxidized Cr(VI) and subsequent toxicity. Among the soils tested in this study, the Boon­ville soil with a low Cr oxidation capacity and low con­centration of labile Cr(VI) was considered to be the best site for safe land disposal of Cr-containing wastes.

Acknowledgments. This study was supported by the Interdis­ciplinary Research Program and the Texas Agricultural Experi­mental Station, Texas A&M University, College Station. TX. USA. The authors thank Dr. Tony Provin and Ms. Vicky Gergeni at Soil Testing Lab., Texas A&M University for their help in acid digestion of the soil samples and in using ICP. The authors also thank Mr. Phillip Owens for providing a fresh Pledger soil sam­ple for this study.

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