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Distribution of Chromium Contamination and Microbial Activity in Soil Aggregates

^a Lawrence Berkeley National Laboratory, Berkeley, CA, 94720
^b Univ. of California, Berkeley, CA 94720
^c Univ. of Chicago, Chicago, IL 60637
^d Savannah River Ecology Laboratory, Univ. of Georgia, Aiken, SC 29802

View larger version (138K): [in a new window]   Fig. 1. Conceptual model of flow and transport in structured subsurface environments. Most of the flow occurs within the advective domain, which is often a small fraction of the system volume. The remaining larger fraction of the subsurface exchanges chemical species primarily through diffusion. Microbial activity can cause large gradients in redox potentials within these diffusion-limited domains, spatial stratification of redox processes, and localized precipitation of redox-sensitive contaminants.

View larger version (24K): [in a new window]   Fig. 2. Chromium K-edge X-ray absorption near-edge structure (XANES) of Cr(III) and Cr(VI). The X-ray fluorescence intensity at the pre-edge peak energy (at 0 eV on the relative energy scale) is normalized to the edge step height (average fluorescence in the relative energy range of +150 to +180 eV).

View larger version (29K): [in a new window]   Fig. 3. Time trends for redox potentials measured at various depths in a synthetic soil aggregate exposed to 260 mg L-1 Cr(VI) on Day 0. Data points are averages of readings from duplicate electrodes, and range bars indicate their individual values.

View larger version (59K): [in a new window]   Fig. 4. (a) Synthetic soil aggregate column design. (1) Homogeneous soil pack, (2) plastic column, (3) piston for soil extrusion, (4) boundary reservoir for Cr(VI) solution, (5) Pt redox electrodes, and (6) plug. (b) X-ray microprobe map (3.0 by 9.5 mm) of total Cr in a synthetic soil aggregate, pre-incubated with 80 mg L-1 organic carbon solution, and exposed to a pool of 260 mg L-1 Cr(VI) for 2.5 d. Greater than 95% of the contaminant was reduced to Cr(III) within 2 mm of the exposure surface. The native soil Cr is at least 95% in the Cr(III) oxidation state, with an average concentration of 150 mg kg-1, but heterogeneously distributed (note Cr map at depths greater than about 2 mm). A characteristic redox potential profile is plotted over the Cr map. (c) Principal component plot of bacterial terminal restriction fragment length polymorphism (TRFLP) patterns from the synthetic soil aggregate showing changes in bacterial community composition occurring in domains defined by [Cr] and redox potential.

View larger version (29K): [in a new window]   Fig. 5. X-ray microprobe profiles of Ti, Cr, and Br, obtained on an aggregate incubated with 800 mg L-1 organic carbon, then placed in diffusive contact with a reservoir containing 1000 mg L-1 (19.2 mM) Cr(VI) and 320 mg L-1 (4.0 mM) Br- for 3 d. After removal from the reservoir, the aggregate was immediately frozen, freeze-dried, and resin-fixed.

View larger version (40K): [in a new window]   Fig. 6. Reduction of Cr(VI) to Cr(III) after resin-preserving CrO2-4–spiked Altamont soil. Percentages shown on bars indicate the relative amounts of the initial spikes that were reduced on resin-treating. Error bars indicate standard deviations (36 measurement spots per sample).

View larger version (72K): [in a new window]   Fig. 7. (a) Diffusion of Cr(VI) into natural soil aggregates. (b) Sectioning of aggregates for X-ray mapping. (c) Maps of total Cr distributions from aggregate sections (different levels of organic carbon [OC], Days 3 and 31). (d) Dehydrogenase activities (7-d incubations) at different levels of OC addition and different depths (nominally 0- to 10-, 20- to 30-, and 40- to 50-mm depths), averaged over 31 d.

View larger version (37K): [in a new window]   Fig. 8. Depth profiles of total Cr and Cr(VI) concentrations from Days 3 (a) and 31 (b) core plug sampling of soil aggregates preincubated with different levels of organic carbon (OC). Plots are from triplicate cores of the soils, with variability indicated by range bars. The micro–X-ray absorption near-edge structure (XANES) measurements were made on samples refrigerated for 41 d after coring, without freeze-drying or resin-fixing.

View larger version (70K): [in a new window]   Fig. 9. Predicted dispersed or localized reduction of Cr(VI) within soil aggregates, depending on aggregate-size and effective first-order reduction rate constant. Approximate boundaries for reaction rate–limited ( = 0.3) versus diffusion-limited ( = 3) conditions are based on Thiele's (1939) analysis, using an effective diffusivity of 4 x 10-4 mm2 s-1.