An insight into mercury reduction process by humic substances in aqueous medium under dark condition

Mercury (Hg) reduction by humic substances (HS) in the aquatic medium under the dark condition is a poorly understood but important process in Hg biogeochemical cycling. In this study, an effort was made to provide a better understanding of Hg(II) reduction by well-characterized humic substances under dark condition. Reduction of Hg(II) by dissolved HS in aquatic systems increases with increasing Hg loading. However, Hg(II) reduction gradually decreases with the increasing total S content and oxygen containing functional groups in the dissolved HS under dark condition. Increasing major cation concentration decreases the rate of Hg(II) reduction in aquatic systems. High concentration of Ca2+ ion slows down the intermolecular electron transfer from HS to Hg(II) and inhibits the formation of Hg0 in absence of light. This study indicates that complexation of Hg(II) and HS is essential for Hg reduction under dark condition.