Mercury Release from Cinnabar in Water and Aqueous Solutions of Hydroquinone or Ascorbic Acid

Redox-sensitive moieties in dissolved natural organic matter have been postulated to have a mechanistic role in the dissolution of the mercuric sulfide mineral cinnabar, a reaction that may increase the bioavailability of mercury. Under controlled reducing conditions, we investigated how hydroquinone, as a function of concentration and pH, affects the dissolution of cinnabar. Dissolution of cinnabar and reduction of Hg(II) were monitored with time by measuring aqueous Hg(II) in a reaction vessel and an inline oxidizing solution that captured gaseous Hg(0) released in the reaction. The amount of mercury released and the corresponding rate of release were not affected by the presence and concentration of hydroquinone or by changes in pH. However, we observed less dissolution of Hg(II) with increasing mass of cinnabar suggesting that particle aggregation effectively reduces the reactive surface area of the solid. Furthermore, dissolution ceased after a few hours of reaction and mercury spike-recovery experiments showed that Hg(II) adsorbed to the surface. Together, the results imply that particle aggregation and surface passivation of mercuric sulfide solids are important processes that contribute to the amount of mercury available for microbial methylation reactions across the oxic/anoxic regions of aquatic systems.