Natural Attenuation of Cr(VI) Contamination in Laboratory Mesocosms

Abstract

The processes leading to the natural attenuation of hexavalent chromium (Cr(VI)) in marine systems are not well understood. To determine the rate at which Cr(VI) could be reduced and the effect of Cr(VI) on bacterial communities in marine sediments, we performed mesocosm experiments with 37.85 L aquaria containing San Diego Bay sandy sediments and seawater. Constant levels of 0, 0.25 (low), and 1.5 mM (high) Cr(VI) were maintained in the water column for 2 months. Chemical analyses of sediment cores taken from the mesocosms indicated that Cr accumulated in the upper 5 mm of the sandy sediments. In general, the distribution of total Cr did not correlate with Fe, Mn, or total organic carbon. Enrichment cultures of metal (iron and chromium)-and sulfate-reducing bacteria from the upper horizon (0-5 mm) of sediments were performed to look for the potential contributors in the detoxification/removal process. PCR of 16S rRNA genes and denaturing gradient gel electrophoresis (DGGE) was used to examine the microbial community structure in sediment depth profiles. When Cr(VI) was present, the number of DGGE bands decreased only in the upper 5 mm of sediments indicating an inhibition of certain bacterial populations and/or a selection for Cr-resistant bacteria in this region. Analysis of the DGGE bands was not especially helpful as most sequences were related to unknown, unidentified, or uncharacterized bacterial cloned sequences.

Responses of the Anaerobic Bacterial Community to Addition of Organic C in Chromium(VI)- and Iron(III)-Amended Microcosms

“…Most were similar to characterized Cr(VI)-resistant strains that have been found in Cr-polluted soils (5,20) and microcosms from marine sediments (3,4). Several of the sequences obtained in this microcosm experiment are similar (94 to 100%) to sequences of dominant DGGE bands obtained by community analysis of soils from the same location (13).…”

Inhibition of Nitrate Reduction by Chromium(VI) in Anaerobic Soil Microcosms

“…Under oxygen-limited conditions, chromium(VI) can be reduced (biologically or chemically) to insoluble and relatively nontoxic Cr(III) (22). Despite the potential interactions between biotic and chemical components, the responses of anaerobic microbial activities to Cr(VI) have not been well studied (6,7,42,43).…”

Chromium Fate in Constructed Wetlands Treating Tannery Wastewaters

“…In addition, there have been reports that showed that excess Cr 13 in soils has a significant effect on plant biodiversity, by exhibiting a strong selection pattern in two similar soils, with and without chromium present (Khan, 2001). Although there is considerable debate about the toxicity of the trivalent form, it is undoubtedly significantly less toxic than the hexavalent form (Arias, 2002;Calder, 1988). The main problem that arises with chromium, as with any metal, is that it cannot be destroyed.…”

“…The main problem that arises with chromium, as with any metal, is that it cannot be destroyed. For treatment purposes, the best option is to transform a highly mobile, bioavailable form of chromium into a less mobile, sparingly bioavailable form and control the conditions where it is ultimately disposed of, to guarantee that it will not be re-released (Arias, 2002).…”