Molecular characteristics of sedimentary organic matter in controlling mercury (Hg) and elemental mercury (Hg) distribution in tropical estuarine sediments.

Sedimentary organic matter (SOM) plays an important role in hosting and reducing Hg in marine/estuarine sediment. This study provides a better understanding on the influence of nature of SOM, in regulating sedimentary mercury (Hg) and elemental mercury (Hg) distribution, and speciation in the Zuari and Mandovi Estuaries that are representative of monsoon fed tropical estuaries, located in the central west coast of India. Salinity of the overlying water column controlled the physical and chemical characteristics of SOM in the estuarine systems. The high molecular weight (MW) SOM dominated at the mid and upstream (low salinity region) of the estuaries, whereas, the low MW SOM prevailed at the downstream (high salinity region). Sediment Hg showed more affinity towards the SOM of high MW. Increasing MW of SOM increased total sedimentary Hg in both the estuaries. SOM with low MW in the estuarine sediment displayed a negative relationship with the sediment Hg concentration. Distribution of Hg concentration in the estuarine sediment suggests that reduction of Hg in presence low MW SOM was a dominant process. It was also found that distribution and speciation of Hg in the estuarine sediment depends on the quantity, quality of the SOM, and the total sediment Hg loading. This study demonstrated that the competition between Hg-SOM complexation and Hg reduction by SOM controls Hg/Hg distribution in tropical estuarine sediment systems.