MICROBIAL MOBILIZATION AND IMMOBILIZATION OF CHROMIUM IN HETEROGENEOUS SUBSURFACE SOILS

Chromium (VI) is a pollutant that may exist in subsurface systems contaminated with radioactive waste, present in iron-based metal/alloys. Microbial reduction of Cr(VI) to Cr(III) offers a viable option for the treatment of subsurface systems contaminated with Cr(VI). However, a number of recent studies shows that Cr(VI) reduction by some bacteria strains (e.g., P. putida) can produce highly soluble organo-Cr(III) end products that have a pronounced impact on solubility, sorption and transport behavior of Cr(III) in subsurface environment. The formation of such soluble Cr products is mainly caused by complexation reactions between Cr(III) and exopolymeric substances (EPS) released by bacteria during Cr(VI) reduction since chromium (VI) is a toxic substance that may stimulate the production of EPS by soil bacteria.

In this study, laboratory sorption, ion exchange, microbial reduction and column experiments were performed to investigate the influence of exopolymeric substances (EPS) extracted from Pseudmonas bacteria (Pseudomonas aeruginosa P16, Pseudomonas putida P18 and Pseudomonas stutzeri P40) on microbial Cr(VI) reduction and stability in heterogeneous subsurface soils. The results from laboratory batch experiments indicate that microbial EPS led to a significant increase in microbial Cr(VI) reduction rates and Cr(III) solubility relative to non-EPS containing systems. Column experiments provided further evidence on the enhanced mobilization of Cr(III) species in the presence of EPS, especially under alkaline pH conditions indicating that Cr(III) that forms after microbial Cr(VI) reduction is not stable in subsurface systems despite the fact that Cr(III) is very low toxicity, and is immobile under slightly acidic to alkaline pH conditions due to its low solubility and highly sorptive characteristics. Our ion exchange experiments suggest that the increase in Cr(III) mobility occurs due to the formation of strong Cr-EPS complexes.