Hg(II)-Dissolved Organic Matter (DOM) Interactions in Freshwater and their Removal in Conventional Water Treatment

In aquatic environments, Hg(II) is strongly reactive with the dissolved organic matter (DOM) and form complexes with acidic functional groups like carboxylic acids, phenols, ammonia, alcohol, and reduced sulfur ligands (S_red). It has been suggested, however, that both the concentration of Hg(II) and the composition of the DOM influence the potential for the removal of Hg(II) in water treatment.

Jar test experiments were performed at optimum coagulation conditions of pH and alum dose, using unfractionated DOM, humic substances (humic acids and fulvic acids) and different Hg(II) concentrations. Samples used in this work were obtained from the International Humic Substances Society (IHSS) and were selected based on the differences in aromatic carbon, reduced sulfur content, acidic functional groups concentration, geographical location, and commercial availability.

Results showed that good removal of Hg(II) can be achieved by alum coagulation under two circumstances: (1) when the DOM is low in S_red ligands but rich in aromatic content and (2) when the DOM is rich in Sred ligands and low in aromaticity. At low Hg/DOM ratio (0.05 μg Hg/mg DOM), Hg(II) removal can be as high as 95%, while at high Hg/DOM ratio (1.0 μg Hg/mg DOM) Hg removal can be as high as 73%. The aromaticity of the DOM and the presence of Hg(II)-binding ligands in the fraction of carbon that adsorbs to aluminum hydroxide flocs were the key variables that control the removal of mercury. All the same, the character of DOM determines the extent of the removal of carbon and the minimum amount of Hg(II) that can be removed from solution.