Adsorption and Desorption of Mercury by Bangpakong River Sediments as Influenced by Salinities

Abstract

Mercury adsorption and desorption by Bangpakong River sediments (pH range 6.8-7.8) of Thailand were investigated at salinity levels of 0, 5, 15 and 30@1000 using a batch equilibration techniques. Using Freundlich isotherm adsorption and desorption data collected represented a nonlinear form. A significant portion (90-99%) of added Hg (2-10 mg L-1) was sorbed by the sediment. Mercury adsorption isotherms or percentage adsorbed were similar among the four salinities tested. In the absence of salinity, Hg adsorption was highly influenced by pH. Whereas in the presence of salinity gradients, Hg(II)-organic complexes predominated over Hg(II)-Cl complexes, neutralizing effect of chloride concentrations and pH on the adsorption phenomena. Organic matter and clay contents of sediment were responsible for the Hg adsorption. Fe and Mn oxides showed negative correlation with the Hg adsorption capacity (r = -0.549 and -0.594 respectively). Increase in Hg desorbed from the sediments occurred only at the 30@1000 salinity level. The Hg adsorption-desorption characteristics of the sediments studied exhibited a very strong irreversible sorption of added inorganic Hg. River sediment such as sediment as shown in this study with significant levels of organic matter and clay have the potential to retain Hg making it less bioavailable and mobile thus reducing potential toxicity to aquatic organisms in the environment.