Adsorption of Mercury(II) by Variable Charge Surfaces of Quartz and Gibbsite

Abstract

The influence of pH, ionic strength, ligands (Cl, SO4, PO4), and metals (Ni and Pb) on the adsorption of Hg(II) by quartz and gibbsite was investigated to better understand the Hg(II) adsorption process and the impact of metals and ligands on this process. The triple layer model (TLM) was used to simulate Hg(II) adsorption on both surfaces. Mercury(II) adsorption from a 0.6 μM Hg(II) solution varies as a function of pH, increasing to an adsorption maximum with increasing pH before tailing off to a constant level at high pH values. The pH at which maximum Hg(II) adsorption occurs (pHmax ≈ 4.5) is comparable to the pKa (3.2) for the hydrolysis of Hg2+ to form Hg(OH)02 Further, the Hg(II) adsorption edge shifts to much higher pH values in the presence of 0.001 M and 0.01 M Cl, which also corresponds to the pH at which Hg(OH)02 is predicted to form. Only minor deviations in the degree of adsorption and the shape of the Hg(II) adsorption edge are influenced by ionic strength, suggesting the formation of inner‐sphere surface complexes. However, Hg(II) adsorption can only be successfully modeled with consideration of the formation of both an outer‐sphere surface complex [≡XO−–HgOH+] and an inner‐sphere surface complex [≡XOHg(OH)−2]. Swamping concentrations (0.01 M) of SO4 and PO4 reduced Hg(II) adsorption on quartz, a result of the predicted formation of Hg(OH)2SO2−4, Hg(OH)2H2PO−4, and Hg(OH)2–HPO2−4 aqueous species (the adsorption edge and pHmax were not influenced). The presence of SO4 also decreased Hg(II) retention by gibbsite, which was also attributed to the formation of the Hg(OH)2SO2−4 ion pair; however, the presence of PO4 increased Hg(II) retention by gibbsite, which was attributed to the formation of a phosphate bridge [≡AlOPO3Hg(OH)2−2]. Mercury(II) adsorption was decreased in the presence of 14 μM Pb and 48 μM Ni, and most noticeably in the quartz system. The adsorption of Hg(II), when in competition with Pb or Ni, could not be simulated by the TLM without the reoptimization of the Hg(II) outer‐ and inner‐sphere log Kint values. Intrinsic Hg(II) adsorption constants derived from single‐element systems could not be employed to simulate adsorption in multi‐element, competitive systems.