Real-time monitoring of aqueous Hg2+ 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 degrees C from zero-valent iron. Its effect on the reduction of aqueous Hg2+ into gaseous Hg-0 has been investigated at ambient conditions for pH comprised between 4 and 8.5. The production of Hg-0 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-1. Starting from a solution having an Hg2+ concentration of 25 ng L-1 at pH = 4, a succession of two Hg-0 production events was recorded. The first event is related to the Hg2+ reduction by ZVI which fully dissolved within the first hours. Upon ZVI consumption, pH drifted towards the pH window where magnetite can efficiently reduce Hg2+ at the hour timescale, resulting in a second Hg-0 production peak. The combined use of ZVI and magnetite to remove aqueous Hg2+ by formation of Hg-0 (volatile) under mild acidic pH allows (1) to maximize the Hg2+ reduction rate and (2) to take benefit of the longer lifetime of magnetite compared to ZVI.