Abstract
Aluminosilicate clay minerals are often a major component of soils and sediments and many of these clays contain structural Fe (e.g., smectites and illites). Structural Fe(III) in smectite clays is redox active and can be reduced to Fe(II) by biotic and abiotic processes. Fe(II)-bearing minerals such as magnetite and green rust can reduce Hg(II) to Hg(0); however, the ability of other environmentally relevant Fe(II) phases, such as structural Fe(II) in smectite clays, to reduce Hg(II) is largely undetermined. We conducted experiments examining the potential for reduction of Hg(II) by smectite clay minerals containing 0–25 wt% Fe. Fe(III) in the clays (SYn-1 synthetic mica-montmorillonite, SWy-2 montmorillonite, NAu-1 and NAu-2 nontronite, and a nontronite from Cheney, Washington (CWN)) was reduced to Fe(II) using the citrate-bicarbonate-dithionite method. Experiments were initiated by adding 500 µM Hg(II) to reduced clay suspensions (4 g clay L−1) buffered at pH 7.2 in 20 mM 3-morpholinopropane-1-sulfonic acid (MOPS). The potential for Hg(II) reduction in the presence of chloride (0–10 mM) and at pH 5–9 was examined in the presence of reduced NAu-1. Analysis of the samples by Hg LIII-edge X-ray absorption fine structure (XAFS) spectroscopy indicated little to no reduction of Hg(II) by SYn-1 (0% Fe), while reduction of Hg(II) to Hg(0) was observed in the presence of reduced SWy-2, NAu-1, NAu-2, and CWN (2.8–24.8% Fe). Hg(II) was reduced to Hg(0) by NAu-1 at all pH and chloride concentrations examined. These results suggest that Fe(II)-bearing smectite clays may contribute to Hg(II) reduction in suboxic/anoxic soils and sediments.
Highlights
Mercury (Hg) is present in the environment as a result of natural and anthropogenic processes including mineral weathering, volcanic emission, mining activity, fossil fuel burning, and industrial and consumer use [1,2]
Hg(II) was reduced to Hg(0) by NAu-1 at all pH and chloride concentrations examined. These results suggest that Fe(II)-bearing smectite clays may contribute to Hg(II) reduction in suboxic/anoxic soils and sediments
In 2003, we reported the reduction of Hg(II) to Hg(0) by green rust [43], a layered redox-reactive Fe(II)-Fe(III) hydroxide with a pyroaurite-type structure
Summary
Mercury (Hg) is present in the environment as a result of natural and anthropogenic processes including mineral weathering, volcanic emission, mining activity, fossil fuel burning, and industrial and consumer use [1,2]. Microbial reduction of ferric iron (Fe(III)) can result in the formation of a broad range of Fe(II) species including soluble and adsorbed Fe(II) and mineral phases containing structural Fe(II) (e.g., magnetite, siderite (FeCO3 ), vivianite [Fe3 (PO4 )2 ·8H2 O], green rust, chukanovite [Fe2 (OH) CO3 ], and Fe(II)-bearing clays) [17,18,19,20,21,22,23,24,25,26,27].
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