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TECHNICAL REPORTS

Bioremediation and Biodegradation

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

^* Corresponding author (tktokunaga{at}lbl.gov).

Received for publication August 9, 2002. Chromium has become an important soil contaminant at many sites, and facilitating in situ reduction of toxic Cr(VI) to nontoxic Cr(III) is becoming an attractive remediation strategy. Acceleration of Cr(VI) reduction in soils by addition of organic carbon was tested in columns pretreated with solutions containing 1000 and 10 000 mg L^-1 Cr(VI) to evaluate potential in situ remediation of highly contaminated soils. Solutions containing 0, 800, or 4000 mg L^-1 organic carbon in the form of tryptic soy broth or lactate were diffused into the Cr(VI)-contaminated soils. Changes in Cr oxidation state were monitored through periodic micro-XANES analyses of soil columns. Effective first-order reduction rate constants ranged from 1.4 x 10^-8 to 1.5 x 10^-7 s^-1, with higher values obtained for lower levels of initial Cr(VI) and higher levels of organic carbon. Comparisons with sterile soils showed that microbially dependent processes were largely responsible for Cr(VI) reduction, except in the soils initially exposed to 10 000 mg L^-1 Cr(VI) solutions that receive little (800 mg L^-1) or no organic carbon. However, the microbial populations (2.1 x 10⁵ g^-1) in the viable soils are probably too low for direct enzymatic Cr(VI) reduction to be important. Thus, synergistic effects sustained in whole soil systems may have accounted for most of the observed reduction. These results show that acceleration of in situ Cr(VI) reduction with addition of organic carbon is possible in even heavily contaminated soils and suggest that microbially dependent reduction pathways can be dominant.

Abbreviations: k''', Wittbrodt–Palmer rate constant • k_oc, effective first-order rate constant, dependent on organic carbon • K_s, Monod half-velocity constant • OC, organic carbon • OC, organic carbon added per mass of soil • SHA, soil humic acid • TSB, tryptic soy broth • X, biomass concentration • X_e, fraction of humic acid oxidized • XANES, X-ray absorption near-edge structure • µ_Max, Monod maximum specific reduction rate constant • Cr, total chromium

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This Issue in Journal of Environmental Quality

JEQ 2003 32: 1577-1582. [Full Text]