The fate of Pb during dissimilatory iron reduction

In surface and subsurface environments native microbial communities influence the mobility and distribution of trace elements. Understanding how microbial metabolic activities and cell surfaces influnce the behavior of trace elements is integral to successful remediation programs in modern systems and the correct interpretation of diagenetic signatures preserved in sedimentary rocks. Ferric iron oxides and oxyhydroxides are ubiquitous in surficial environments where they often control the cycling of heavy metals. It is well established that the use of Fe(III) as a terminal electron acceptor by dissimilatory iron reducing bacteria (DIRB), causes the reductive dissolution of iron oxide minerals and leads to the release of Fe2+ and the oxidation of organic matter. In this study we examine the fate of Pb associated with Fe oxyhydroxides during DIR. We synthesized Pb-bearing and Pb-free Fe oxyhydroxides and incubated them anaerobically with Shewanella putrefaciens 200R and lactate as the sole electron donor. The extent of Fe reduction was determined using 0.5 mol L-1 HCl extractions and was equivalent in incubations containing either Pb-bearing or Pb-free Fe oxyhydroxides. During the 2 month experiment aqueous Pb concentrations increased with progressive Fe reduction from 1 nmol L-1 to >150 nmol L-1. Aqueous Fe(II) concentrations in incubations with Pb-bearing Fe oxyhydroxides were significantly higher than those in reactors containing Pb-free Fe oxyhydroxides . This suggests that the secondary mineralization is different in Pb-bearing systems and results in less incorporation of Fe(II) into the solid phase. This was confirmed using x- ray diffraction analysis and a suite of sequential extractions. Intriguingly our sequential extraction data reveal a highly refractory phase (extractable only by 6N HCl), which formed during the DIR in the incubations with Pb- bearing Fe oxyhydroxides. This refractory phase is responsible for the sequestration of approximately 80% of the total solid Fe and between 20 and 30% of the solid Pb. Although bacterial Fe reduction induces the remobilization of some Pb associated with Fe oxyhydroxides, more Pb is sequestered into an unknown refractory, solid phase. The identity of this phase remains unknown and should be elucidated in future studies.