es6b06044_si_001.pdf (1.71 MB)
Reoxidation of Chromium(III) Products Formed under Different Biogeochemical Regimes
journal contribution
posted on 2017-04-03, 00:00 authored by Charuleka Varadharajan, Harry R. Beller, Markus Bill, Eoin L. Brodie, Mark E. Conrad, Ruyang Han, Courtney Irwin, Joern T. Larsen, Hsiao-Chien Lim, Sergi Molins, Carl I. Steefel, April van Hise, Li Yang, Peter S. NicoHexavalent
chromium, Cr(VI), is a widespread and toxic groundwater
contaminant. Reductive immobilization to Cr(III) is a treatment option,
but its success depends on the long-term potential for reduced chromium
precipitates to remain immobilized under oxidizing conditions. In
this unique long-term study, aquifer sediments subjected to reductive
Cr(VI) immobilization under different biogeochemical regimes were
tested for their susceptibility to reoxidation. After reductive treatment
for 1 year, sediments were exposed to oxygenated conditions for another
2 years in flow-through, laboratory columns. Under oxidizing conditions,
immobilized chromium reduced under predominantly denitrifying conditions
was mobilized at low concentrations (≪1 μM Cr(VI); ∼
3% of Cr(III) deposited) that declined over time. A conceptual model
of a limited pool of more soluble Cr(III), and a larger pool of relatively
insoluble Cr(III), is proposed. In contrast, almost no chromium was
mobilized from columns reduced under predominantly fermentative conditions,
and where reducing conditions persisted for several months after introduction
of oxidizing conditions, presumably due to the presence of a reservoir
of reduced species generated during reductive treatment. The results
from this 3-year study demonstrate that biogeochemical conditions
present during reductive treatment, and the potential for buildup
of reducing species, will impact the long-term sustainability of the
remediation effort.