Phase speciation of mercury (Hg) in coastal water of the Yellow Sea, China

The objective of this study is to investigate the phase distribution of mercury (Hg) in water and the relative importance of processes affecting its distribution and behavior at a coastal site of the Yellow Sea with high tidal current speeds in the eastern continental margin of China. Total Hg (THg) and reactive Hg (RHg) were measured in unfiltered and filtered (0.45 μm) water in four different seasons at 1–3 h intervals using ultra-trace Hg techniques. The Hg in this coastal water showed typical Hg-unpolluted coastal region levels. Mean (± SD) concentrations of THg in unfiltered water were 13.4 ± 3.9 pM with a range of 6.7 to 27.5 pM and a considerable RHg fraction (%RHg/THg = 36.9%). Particulate Hg (PHg) was the dominant phase in unfiltered water samples (%PHg/THg = 70 ± 11%), and the distribution coefficient (Log Kd) of Hg in particulate and dissolved (0.45 μm) phases averaged 4.96 ± 0.20 with a range of 4.26 to 5.38. Enhanced suspended particulate matter (SPM) in water induced by wind and tidal mixing probably explained most of the variation in the unfiltered THg. In the dissolved phase, RHg accounted for a major fraction of THg (%RHg/THg = 86 ± 16%). The weak correlation between dissolved organic carbon (DOC) and THg in the dissolved phase indicated that Hg in this study region might primarily originate from atmospheric deposition and also reflected the importance of other competing ligands (e.g., Cl−) in the water column. Interestingly, measurements covering four different seasons showed no seasonality in these Hg species. Although there was some uncertainty in the measurements, using the difference of RHg between unfiltered and dissolved samples revealed that a considerable fraction (28 ± 19%) of RHg could bind to SPM.

Research highlights
► Hg in coastal water with high current speeds influenced by wind and subsequent resuspended sediment.
► RHg accounted for a considerable fraction of THg in unfiltered water.
► RHg accounted for a dominant fraction of THg in filtered water.
► Weak correlation between DOC and RHg reflected the importance of other competing ligands and atmospheric Hg deposition.