Effects of Extracellular Electron Shuttles on Microbial Iron Reduction and Heavy Metals Release from Contaminated Soils

Abstract

To test the potential effects of extracellular electron shuttles (EES) on the rate and extent of heavy metal release fromcontaminated soils during microbial iron reduction, we created anaerobic batch systems with anthraquinone-2,6-disulfonate (AQDS) as a surrogate of EES, and with contaminated soils as mixed iron (hydr)oxides and microbial sources. Two types of soils were tested: Zn-contaminated soil A and As/Pb-contaminated soil B. In soil A, the rate of ironreduction was fastest in the presence of AQDS and > 3500 mg/L of total Fe(II) was produced within 2 d. This suggeststhat indigenous microorganisms can utilize AQDS as EES to stimulate iron reduction. In the incubations with soil B, therate and extent of iron reduction did not increase in the presence of AQDS likely because of the low pH (< 5.5). Inaddition, less than 2000 mg/L of total Fe(II) was produced in soil B within 52 d suggesting that iron reduction bysubsurface microorganisms in soil B was not as effective as that in soil A. Relatively high amount of As (~500 mg/L) wasreleased to the aqueous phase during microbial iron reduction in soil B. The release of As might be due to the reduction ofAs-associated iron (hydr)oxides and/or direct enzymatic reduction of As(V) to As(III) by As-reducing microorganisms. However, given that Pb in liquid phase was < 0.3 mg/L for the entire experiment, the microbial reduction As(V) to As(III)by As-reducing microorganisms has most likely occurred in this system. This study suggests that heavy metal release fromcontaminated soils can be strongly controlled by subsurface microorganisms, soil pH, presence of EES, and/or nature ofheavy metals.