Mercury Reduction by Nanoparticulate Vivianite

Mercury (Hg) is a toxic trace element of global environmental concern which has been increasingly dispersed into the environment since the industrial revolution. In aquatic and terrestrial systems, Hg can be reduced to elemental Hg (Hg⁰) and escape to the atmosphere or converted to methylmercury (MeHg), a potent neurotoxin that accumulates in food webs. Fe^II-bearing minerals such as magnetite, green rusts, siderite, and mackinawite are recognized Hg^II reducers. Another potentially Hg-reducing mineral, which commonly occurs in Fe- and organic/P-rich sediments and soils, is the ferrous iron phosphate mineral vivianite (Fe^II₃(PO₄)₂·8H₂O), but its reaction with Hg^II has not been studied to date. Here, nanoparticulate vivianite (particle size ∼ 50 nm; Fe^II content > 98%) was chemically synthesized and characterized by a combination of chemical, spectroscopic, and microscopic analyses. Its ability to reduce Hg^II was investigated at circumneutral pH under anoxic conditions over a range of Fe^II/Hg^II ratios (0.1-1000). For Fe^II/Hg^II ratios ≥1, which are representative of natural environments, Hg^II was very quickly and efficiently reduced to Hg⁰. The ability of vivianite to reduce Hg^II was found to be similar to those of carbonate green rust and siderite, two of the most effective Hg-reducing minerals. Our results suggest that vivianite may be involved in abiotic Hg^II reduction in Fe and organic/P-rich soils and sediments, potentially contributing to Hg evasion while also limiting MeHg formation in these ecosystems.