Real-time monitoring of aqueous Hg reduction dynamics by magnetite/iron metal composite powders synthesized hydrothermally.

An iron-based powder material composed of zerovalent iron (8 wt.%) and magnetite (92 wt.%), has been synthesized hydrothermally at 200 °C from zero-valent iron. Its effect on the reduction of aqueous Hg into gaseous Hg has been investigated at ambient conditions for pH comprised between 4 and 8.5. The production of Hg was monitored with an online mercury vapor analyzer at the picogram level for concentrations of iron-based composite of a few tenths of mg L. Starting from a solution having an Hg concentration of 25 ng L at pH = 4, a succession of two Hg production events was recorded. The first event is related to the Hg reduction by ZVI which fully dissolved within the first hours. Upon ZVI consumption, pH drifted towards the pH window where magnetite can efficiently reduce Hg at the hour timescale, resulting in a second Hg production peak. The combined use of ZVI and magnetite to remove aqueous Hg by formation of Hg (volatile) under mild acidic pH allows (1) to maximize the Hg reduction rate and (2) to take benefit of the longer lifetime of magnetite compared to ZVI.