| Ocean plays a vital role in the global mercury cycle. Especially, reduction and oxidationof mercury in surface seawater are important processes influencing the formation and evasionof dissolved gaseous element mercury (DGM). The research on the reduction and oxidationmechanisms of mercury in coastal surface seawater is helpful to understand the contribution ofmarine to atmospheric mercury. In this paper, surface seawater from Qingdao offshore wascultured to discuss the possible passways of reduction and gross redox process respectively.The influence of microbe and suspended particular matter (SPM) on the reduction and grossredox reaction under different ultra-violet irradiation was analyzed by the determination ofvariation of dissolved gaseous mercury and reactive mercury (RHg).The results show that:1) Reduction of RHg in coastal surface seawater:The formed DGM concentrations decrease gradually in the reduction reaction,accumulative DGM concentrations reach a maximum usually within5h, and then retain stablerelatively. The reduction rate constans in UVA, UVB, and dark are0.983h-1,0.306h-1, and0.174h-1respectively. The reduction process of RHg still occurs in dark, but the reaction rate issmaller compared to reduction processes under UV irradiation. The production of DGM incoastal seawater is promoted under UV irradiation, which represents that the reduction rateconstant and accumulative DGM concentration increase obviously.The influence of microbe and SPM on mercury production under different conditions isdifferent. They have a little influence on the reduction of mercury in dark. The influence ofmicrobe and SPM under UVA irradiation is significant. Photoreduction accounts for66%ofthe total DGM production rate under UVA irradiation, the reduction rate was lowered22%bymicrobe, and promoted55%by SPM. Photoreduction accounts for about63%of the totalDGM production rate under UVB irradition, the reduction rate was promoted4%by microbe,and promoted32%by SPM.Because of differencens of physichemical characteristics, concentration and species ofmercury, the mercury photoproduction in surface seawater is different between Qingdao coastal and the Yellow River estuary.2) Reduction and oxidation of mercury in coastal surface seawater:The redox reaction of mercury in costal surface seawater does not follow a simpletwo-species reversible reaction pathway. Instead, a new mercury species may present in theprocess.The influence of microbe and SPM on mercury redox reactions under different conditionsis different. In the dark, microbe promotes oxidation and inhibits reduction; otherwise SPMpromotes reduction and inhibits oxidation. Under UVA irradiation, microbe promotes thereduction and oxidation reaction, otherwise SPM inhibits the reduction and oxidation reaction.Under UVB irradiation, microbe promotes reduction and inhibits oxidation reaction; SPMinhibits the reduction and oxidation reaction.Under natural irradiation, the gross concentrations of DGM and RHg respectively vary inthe range of295pg/L-960pg/L and901pg/L-5912pg/L. The daily trends of DGM and RHg areconsistent, peak at noon (12:00-13:00) and low level in the morning and night. Amongdifferent sunlight wavelengths, the correlative coefficient between UVB irradiation andmercury redox reduction is the largest, and the correlative coefficient of UVC is the smallest. |
