Reduction of Hg(II) by Fe(II)-Bearing Smectite Clay Minerals

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.

Comparison of production of gamma‐decalactone via microbial biotransformation based on different adsorption‐embedding systems

“…The main component of halloysite is Al 2 Si 2 O 5 (OH) 4 . 26 And clay is a silicate mineral combined with aluminum oxide. 27 The characteristic peaks of halloysite AEC in Figure 3A shows that the stretching vibration of –OH inside and outside aluminum layer appeared at 3619.89 and 3693.17, and 3455.00 cm −1 (clay alike in Figure 3B).…”

Demystifying mercury geochemistry in contaminated soil–groundwater systems with complementary mercury stable isotope, concentration, and speciation analyses

“…Abiotic reduction of Hg 2+ can be facilitated by electron transfer from organic matter or most other less precious metals. The oxidation of Fe 2+ or mixed valence (Fe 2+ /Fe 3+ ) iron oxide minerals such as magnetite, 102,106 siderite, 107 vivianite, 108 green rust, 109 and clay minerals 110 are likely to drive much of this reduction due to their relatively high abundance. This process can be expedited under anoxic or suboxic conditions within which such ferrous iron containing minerals become important electron donors.…”

Anaerobic Microscopic Analysis of Ferrous Saponite and Its Sensitivity to Oxidation by Earth’s Air: Lessons Learned for Analysis of Returned Samples from Mars and Carbonaceous Asteroids

“…The reducing ability of ferrous iron-bearing minerals, such as magnetite and green rust, on early Martian environments has previously been discussed (e.g., [60]). However, knowledge of the reducing ability of ferrous smectite is limited [61]. A continuous supply of reductants is required for chemoautotrophic life on Earth [62] and beyond [63,64].…”