Adsorption of Mercury onto Kaolinite in the Presence and Absence of Dissolved Organic Matter

Adsorption of Hg onto high (KGa-2) and low (KGa-1b) defect kaolinite in the presence and absence of two dissolved organic matter (DOM) isolates was investigated as a function of solution pH, Hg concentration, and sulfur and aromatic content of the DOM. Hg adsorption from a 5 x 10-7 M Hg, 0.01 M Na2CO3, and 0.0003 M NaCl solution increased sharply between pH 6 and 8, with maximum adsorption (66% of initial Hg on Kga-1b and 80% of initial Hg on KGa-2) at pH 8. Mercury adsorption normalized to edge surface area was similar for both kaolinites at low concentrations of Hg. However, at the same equilibrium Hg concentration, KGa-2 adsorbed more Hg than KGa-1b. The difference increased with increasing Hg concentration likely because KGa-2 has more strong binding sites. Mercury desorption experiments confirmed that Hg was more strongly bound to KGa-2 than to KGa-1b. The presence of 0.03 g/L DOM enhanced Hg adsorption at pH < 6 and inhibited Hg adsorption at pH > 6. At pH < 6 more Hg was adsorbed in the presence of the more aromatic isolate (F1HpoA) whereas at pH > 6 more Hg was adsorbed in the presence of the less aromatic isolate (WLHpoA). Also, more DOM was adsorbed at pH 3 than at pH 8, with the more aromatic isolate adsorbing to a greater extent at both pHs. At pH < 6 the adsorption of Hg in the presence of both DOM isolates was a function of the total edge surface area of kaolinite indicating that adsorption was dominated by ligand exchange. At pH > 6 interactions between the hydrophobic basal surface of kaolinite and the hydrophobic regions of the DOM isolates was the dominant DOM adsorption mechanism. Here the higher sulfur content of F1HpoA maintained more of the Hg in solution than at pH < 6 whereas the lower sulfur content of WLHpoA may have allowed some Hg to adsorb directly to the edge surfaces of kaolinite.