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In Situ Reduction of Chromium(VI) in Heavily Contaminated Soils through Organic Carbon Amendment

^a Lawrence Berkeley National Laboratory, Berkeley, CA 94720
^b University of California, Berkeley, CA 94720
^c University of Chicago, Chicago, IL 60637

View larger version (30K): [in a new window]   Fig. 1. Soil column design.

View larger version (27K): [in a new window]   Fig. 2. K-edge X-ray absorption near-edge structure (XANES) spectra of Cr(VI) and Cr(III) in a matrix of Altamont soil.

View larger version (29K): [in a new window]   Fig. 3. Redox potentials in soil columns (referenced to the standard H electrode). Symbols denote average redox potentials in the +0, +800, and +4000 mg L-1 organic carbon (OC)–amended columns, and bars denote ranges encompassed by ±2 standard deviations. No +4000 mg L-1 OC treatment was included in the control [no Cr(VI)] soils. The cases where lactate served as the OC treatment are indicated by an "L" next to the data. Also shown are ranges of redox potentials associated with Cr(VI)–Cr(III), Mn(IV)–Mn(II), Fe(III)–Fe(II), and S(VI)–S(-II) transformations.

View larger version (47K): [in a new window]   Fig. 4. Profiles of Cr in soils contaminated with 1000 mg L-1 Cr(VI) solutions, then exposed to 4000 mg L-1 organic carbon (OC) solutions (Days 5 [circles], 84 [solid triangles], and 205 [squares], relative to addition of OC). (A) Measured profiles of total Cr concentrations. (B) Measured profiles of Cr(VI) to total Cr concentration ratios, with the predicted profile at the initial addition of Cr(VI) shown as a dashed line.

View larger version (38K): [in a new window]   Fig. 5. Comparisons between measured (points) and modeled (curves) Cr(VI) reduction trends for columns initially exposed to (A) 1000 mg L-1 Cr(VI) and (B) 10 000 mg L-1 Cr(VI). Data from nonsterile soils were fit for first-order Cr(VI) reduction (finer lines, using Eq. [1]) with effective rate constants shown. The sterile soil data (Day 84 only) were fit with Eq. [5] and [6] (heavier lines, with k''' values shown). Extrapolations of these latter fits beyond Day 84 are shown as fine lines.

View larger version (23K): [in a new window]   Fig. 6. (A) Plots of aqueous Fe(II) concentrations for equilibrium with ferric hydroxide versus p. Lines are for pH 7.5 (characteristic of the experiment), and range bars indicate shifts in Fe(II) concentrations for pH 7 and 8. (B) Effective first-order rate constants for Cr(VI) reduction by Fe2+ as functions of the negative logarithm of the electron activity (p), for equilibrium with ferric hydroxide. Lines are for pH 7.5, and range bars indicate shifts in rate constants for pH 7 and 8. Also shown are ranges of measured k and estimated p from the nonsterile soils.

View larger version (46K): [in a new window]   Fig. 7. Comparison of Cr(VI) reduction at Day 84 for sterile and nonsterile soils. Values for unreduced Cr(VI) fractions at Day 84 for nonsterile soils were obtained with first-order fits (see Fig. 5). Alpha values from t tests of significance of further Cr(VI) reduction in nonsterile soils relative to their sterile counterparts are indicated in italic type. NS, not significant.

View larger version (31K): [in a new window]   Fig. 8. Estimating the concentration of the native soil organic carbon (OC) that has an equivalent reactivity to Cr(VI) as tryptic soy broth (TSB). (A) Sterile soils, based on Eq. [5] and [6]. This graph indicates that the native soil contains 339 ± 88 mg kg-1 of OC having the equivalent reactivity of TSB. (B) Nonsterile soils, based on Eq. [1] and [2]. This graph indicates that the native soil contains 332 ± 55 mg kg-1 of OC having the equivalent reactivity of TSB.