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.