Bruce R. James

Hexavalent chromium extraction from soils : a comparison of five methods

A quantitative extraction method for total Cr(VI) (soluble and insoluble forms) in soils is needed to assess potential hazards from this heavy metal to humans and ecological systems. A laboratory study was conducted to measure native levels of Cr(VI) and recovery percentages of soluble and insoluble CrO 4 2- matrix spikes in four diverse soil materials following equilibration with commonly used extractants : distilled water (pH 5.7), phosphate buffer (5 mM K 2 HPO 4 /5 mM KH 2 PO 4 , pH 7.0), carbonate-hydroxide extractant (0.28 M Na 2 CO 3 /0.5 M NaOH ; pH 11.8) with and without heating, and hydroxide solution (0.1 M NaOH ; pH 13) with sonication. The heated carbonate-hydroxide solution was the most effective extractant for operationally defining total Cr(VI) in soils that contained native Cr(VI) or in soils that had a sufficiently high redox status to maintain Cr as Cr(VI), More than 90% of a soluble K 2 CrO 4 spike [1.0 g of Cr(VI) kg -1 ] was recovered using the heated carbonate-hydroxide extraction method on a redox-inert quartz sand, loamy-textured A horizon soil (Ultic Hapludalf), and chromite ore processing residue-enriched soil (HighHex COPR) containing 29% of its total Cr(VI) (1.4 g kg -1 ) in insoluble forms. No Cr(VI) was recovered by any of the methods after spiking an anoxic sediment, as predicted by the reducing conditions of this soil for Cr(VI). Lead chromate (K sp 1.8 x 10 -14 ) was completely recovered by the heated carbonate-hydroxide extractant in the absence of a soil matrix, after being spiked into the loam soil containing no native Cr(VI), and after being spiked into another COPR soil (LowHex COPR) with 61% of its native Cr(VI) in insoluble forms. In contrast, BaCrO 4 (K sp 1.6 x 10 -10 ) was completely recovered by the carbonate-hydroxide extractant in the absence of a soil matrix, but 63 and 78% recoveries were measured after spiking into the loam and LowHex COPR soils, respectively. Method-induced reduction of soluble Cr(VI) was not observed under the conditions of the heated alkaline extraction.

Peer Reviewed: The Challenge of Remediating Chromium-Contaminated Soil

The complex chemistry of chromium compounds presents unique measurement and regulatory challenges.

Oxidation‐reduction chemistry of chromium: Relevance to the regulation and remediation of chromate‐contaminated soils

Developing health‐protective clean‐up standards and remediation strategies for chromium‐contaminated soils based on the hexavalent forms of this heavy metal is a complex and controversial issue, because certain forms of Cr(III) can oxidize to Cr(VI) and Cr(VI) can be reduced to Cr(III) under diverse soil conditions. The extent of oxidation of Cr(III) in soils amended with wastes is based on four interacting parameters: (1) solubility and form of Cr(III) related to oxidation (waste oxidation potential, or WOP), (2) reactive soil Mn(III,IV) (hydr)oxide levels (soil oxidation potential for Cr(III), or SOP), (3) soil potential for Cr(VI) reduction (soil reduction potential, or SRP), and (4) soil‐waste pH as a modifier of the first three parameters (pH modification value, or PMV). Each of these four parameters can be quantified with laboratory tests and ranked numerically; the sum of which is the Potential Chromium Oxidation Score (PCOS) for assessing the relative hazard of a waste‐soil combination. The PCOS v...

System for categorizing soil redox status by chemical field testing

Abstract Useful interpretation of the mixed redox electrode potentials measured in soils and natural waters in partial equilibrium with atmospheric O2, is nearly impossible because most oxidized species of critically important N, S, Mn, Fe, and C are not electroactive. We propose that simple chemical tests be used in the field to group soils into the following soil redox status categories: Superoxic, Manoxic, Suboxic, Redoxic, Anoxic, and Sulfidic. These names connote status of oxidation and also indicate electron lability levels, respectively, of Very Low, Low, Medium, Medium High, High, and Very High. Tests involve qualitative measurement of tetramethylbenzidine oxidation colors, oxididation of added Cr, reduced Fe, easily reducible Fe, sulfide and other odors, and indicator pH.

Redox transformations and plant uptake of selenium resulting from root-soil interactions

Batch studies were conducted with Mn oxides (birnessite-hausmannite mixture, BHM) and samples of four soil series from the Mid-Atlantic region of the USA to determine effects of reducing organic acids, similar to those found in the rhizosphere, on the SeO3/SeO4 distribution. Jackland (Typic Hapludalf), Myersville (Ultic Hapludalf), Christiana (Aeric Paleaquult), and Evesboro (Typic Quartizipsamment) A and B horizon soil samples with and without prior Mn oxide reduction were incubated aerobically for 10 d with 0.1 mmol kg-1 SeO3 and 0 or 25 mmol kg-1 of ascorbic acid, gallic acid, oxalic acid, or citric acid. Selenite was also added to BHM (10 mmol kg-1) with 0 or 0.1 mmol kg-1 ascorbic acid. The availability of Se for plant uptake as a result of root-soil interactions was examined using growth chamber studies with barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) seedlings grown in 150-mL cone-shaped containers to maximize root-soil surface interactions and to create ‘rhizosphere’ soil throughout the root zone. In the BHM system ascorbic acid increased oxidation of SeO3 to SeO4 to 33% of added SeO3. In the presence of ascorbic and gallic acids and Mn oxides, oxidation of SeO3 to SeO4 occurred in the B horizons of all the soils and in the A horizons of Jackland and Myersville soils. Removal of Mn oxides decreased the oxidation in some samples. Wheat and barley plants were able to accumulate up to 20 μmol Se kg-1 from the Jackland soil when soluble Se was not measurable. The root-soil interactions in the Jackland soil with barley and wheat provided the plant with Se from insoluble sources. The results also indicate that Mn oxides coming in contact with reducing root exudates have a greater ability to oxidize SeO3 to SeO4. Thus, rhizosphere processes play an important role in the availability of Se for plant uptake.

Oxidation-reduction transformations of chromium in aerobic soils and the role of electron-shuttling quinones.

Oxidation of Cr(III) and reduction of Cr(VI) can occur simultaneously in aerobic soils, but the mechanisms involved are not well-understood, especially how electron shuttling by redox-active organic acids is involved. A and B soil horizons from three topohydrosequences from the Coastal Plain and Piedmont physiographic provinces of Maryland were chosen to investigate oxidation-reduction transformations of Cr under field moist conditions. Reduction of added Cr(VI) to Cr(III) was observed in all 18 samples, and 11 demonstrated enhanced reduction with added anthraquinone-2,6-disulfonate (AQDS) acting as an electron shuttle in 24 h quick tests under aerobic conditions. Oxidation of Cr(III) to Cr(VI) was observed in 12 samples, with 7 demonstrating diminished oxidation with AQDS added. Cr(VI) was undetectable after 11 d of incubation when lactic acid was added as a reductant for Cr(VI) to the Watchung soil A horizon. This reduction occurred in the presence of AQDS and a high salt background to suppress microbial growth, suggesting abiotic reduction was the dominant pathway. The results of this study demonstrate that in field-moist, aerobic soils, the electron shuttle, AQDS, enhanced Cr(VI) reduction and inhibited Cr(III) oxidation. This suggests redox-active organic C amendments and electron shuttles can be important in enhancing rates and extent of Cr(VI) reduction, while inhibiting Cr(III) oxidation in the in situ remediation of Cr(VI)-contaminated soils.

Cr(vi) soil analytical method: A reliable analytical method for extracting and quantifying Cr(vi) in soils

Chromium has been used in the industrialized world in many applications for more than a century. Chromium is a trace metallic element found in the Earths crust, and when it is found in concentrated ore deposits it is principally as FeCr2O4 (chromite ore). In the environment, chromium is typically found in the trivalent and hexavalent states. These two oxidation states have differing toxicities and mobilities. Hexavalent chromium [Cr(VI)] is classified as a known human carcinogen (via inhalation) and is rather mobile, whereas trivalent chromium [Cr(III)] is comparatively benign (it is an essential dietary element for humans) and relatively immobile. This significant toxicological and geochemical disparity between the two valence states necessitates that environmental investigators be able to quantitatively distinguish between these two forms in solid environmental media. Several regulatory‐approved analytical techniques exist for the quantitative differentiation between Cr(VI) and Cr(III) in aqueous solut...

Hexavalent Chromium Reduction by Tartaric Acid and Isopropyl Alcohol in Mid-Atlantic Soils and the Role of Mn(III,IV)(hydr)oxides

Chromium is a naturally occurring transition metal and a soil contaminant in the Cr(VI) oxidation state, but reduction of Cr(VI) to Cr(III) mitigates its toxicity. Tartaric acid reduces Cr(VI) via a termolecular complex with isopropyl alcohol and Cr(VI), but its efficacy in soils has not been demonstrated. Five Mid-Atlantic soils from Maryland, U.S. were examined for their potential to enhance the reduction of Cr(VI). A control treatment (no soil +12 mM tartaric acid + 0.29 M isopropyl alcohol) reduced 0.37 mM Cr(VI) (19%) in 99 h. Reduction was enhanced to 1.97 mM (99%) with addition of a Russett Ap soil horizon (fine-loamy, mixed, semiactive, mesic Typic Hapludult). With a half-life of 18.7 h, the rate of reduction of Cr(VI) with the Russett soil sample was 20 times faster than with no soil (371 h). Soil Mn was solubilized in this reaction and plays a role in the enhanced reduction of Cr(VI). Mn(III/IV)(hydr)oxide-coated quartz sand reduced 1.24 mM (62%) Cr(VI), with all of the Mn(III,IV)(hydr)oxides solubilized. The addition of isopropyl alcohol and tartaric acid to soils enhances the reduction of Cr(VI), and this reduction is further enhanced by the catalytic behavior of Mn(II) from easily reducible Mn(III,IV)(hydr)oxides in soil.

Hexavalent chromium reduction in solution and in chromite ore processing residue-enriched soil by tartaric Acid with isopropyl alcohol and divalent manganese as co-reductants.

Chromite ore processing residue (COPR), the solid waste product from the high-temperature alkaline processing of ferrochromite (FeO·CrO), contains Cr(VI) in soluble and insoluble compounds formed in the roasting process. This research investigated tartaric acid in combination with Mn and isopropyl alcohol (IPOH ) as co-reductants for reagent- and COPR-derived Cr(VI). The reduction of Cr(VI) by tartaric acid alone at pH 5.0 or greater was negligible; however, in the presence of Mn or IPOH, reduction occurred in hours. Isopropyl alcohol enhanced Cr(VI) reduction, probably via formation of a termolecular complex with the alcohol, tartaric acid, and Cr(VI). In aqueous solutions of reagent-derived Cr(VI) at pH 4, 12 mmol L tartaric acid with 1.0 mmol L Mn or 1.0 mmol L Mn and 0.29 mol L (2% v/v) IPOH reduced 1.0 mmol L Cr(VI) in 48 h. The same treatments at pH 5.5 reduced 0.60 and 0.58 mmol L Cr(VI) (60%) in 96 h, respectively. A minimum half-life of 10.2 h was calculated from first-order rate constants obtained from Mn and IPOH-Mn co-reductant treatments with tartaric acid at pH 4. The most COPR-derived Cr(VI) reduced in suspension was by IPOH and Mn at high acidity (pH 5.8), which reduced 0.52 mmol L (52%) of the COPR-derived soluble Cr(VI) at 96 h. The enhanced reduction of soluble Cr(VI) by tartaric acid by the addition of Mn proceeds within a complex formed by an esterification reaction between tartaric acid and Cr(VI) with Mn bound to tartaric acid. The combined treatment of tartaric acid, IPOH, Mn, and a strong acid to lower the pH of COPR-enriched soils would be effective in field applications of this chemistry. By creating a slurry of the field soil with these amendments, mass transfer limitations would be overcome, and Cr(VI) would be reduced to Cr(III) in days.

Metazoans from a sandy aquifer : dynamics across a physically and chemically heterogeneous groundwater system

We investigated the relationship between groundwater metazoans and their physical and chemical environment in a shallow Atlantic Coastal Plain aquifer adjacent to the Chesapeake Bay, Maryland, USA. Average abundance of the groundwater organisms over a 1 % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSGaaeaaca% aIXaaabaGaaGOmaaaaaaa!3776!\[{\raise0.7ex\hbox{