EXAFS study of mercury(II) sorption to Fe- and Al-(hydr)oxides. II. Effects of chloride and sulfate

Common complexing ligands such as chloride and sulfate can significantly impact the sorption of Hg(II) to particle surfaces in aqueous environmental systems. To examine the effects of these ligands on Hg(II) sorption to mineral sorbents, macroscopic Hg(II) uptake measurements were conducted at pH 6 and [Hg](i)=0.5 mM on goethite (alpha-FeOOH), gamma-alumina (gamma-Al(2)O(3)), and bayerite (beta-Al(OH)(3)) in the presence of chloride or sulfate, and the sorption products were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. The presence of chloride resulted in reduced uptake of Hg(II) on all three substrates over the Cl(-) concentration ([Cl(-)]) range 10(-5) to 10(-2) M, lowering Hg surface coverages on goethite, gamma-alumina, and bayerite from 0.42 to 0.07 micromol/m(2), 0.06 to 0.006 micromol/m(2), and 0.55 to 0.39 micromol/m(2) ([Cl(-)]=10(-5) to 10(-3) M only), respectively. This reduction in Hg(II) uptake is primarily a result of the formation of stable, nonsorbing aqueous HgCl(2) complexes in solution, limiting the amount of free Hg(II) available to sorb. At higher [Cl(-)] beam reduction of Hg(II) to Hg(I) was observed, resulting in the possible formation of aqueous Hg(2)Cl(2) species and the precipitation of calomel, Hg(2)Cl(2(s)). The presence of sulfate caused enhanced Hg(II) uptake over the sulfate concentration ([SO(4)(2-)]) range 10(-5) to 0.9 M, increasing Hg surface coverages on goethite, gamma-alumina, and bayerite from 0.39 to 0.45 micromol/m(2), 0.11 to 0.38 micromol/m(2), and 0.36 to 3.33 micromol/m(2), respectively. This effect is likely due to the direct sorption or accumulation of sulfate ions at the substrate interface, effectively reducing the positive surface charge that electrostatically inhibits Hg(II) sorption. Spectroscopic evidence for ternary surface complexation was observed in isolated cases, specifically in the Hg-goethite-sulfate system at high [SO(4)(2-)] and in the Hg-goethite-chloride system.