Significance of pH and pCl in heavy metal ion reactions and mercury (II) adsorption by soil materials

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Virginia Polytechnic Institute and State University


A computational approach was utilized to evaluate the degree to which Zn(II), Cd(II), Hg(II), and Pb(II) hydrolyze and complex with chlorides at pH and chloride concentration levels encountered in natural waters and soil solutions. The significance of hydroxy and chloride complexes of these heavy metal ions is discussed in relation to solubilities of sparingly soluble salts, Hg(II) adsorption on clays, and Hg(II) interactions with organic functional groups.

Occurrence of and competition between Hg(II) hydroxy and chloride complexes is important at all pH and chloride concentrations encountered in natural waters. Hydrolysis of Pb(II), Zn(II), and Cd(II) increases markedly above pH 5, 7, and 8, respectively, and the chloride complexes are important above chloride concentrations of approximately 10⁻² M.

Hydrolysis and chloride complexation increases the solubilities of sparingly soluble salts of these heavy metal ions by deca-orders depending on the degree of complexation. Intrinsic solubilities of the heavy metal hydroxides are greater than the solubilities obtained from the corresponding solubility products, except for Cd(OH)₂° The effects due to complexation may in many cases overshadow salt effects.

Studies on Hg(II) adsorption by Montmorillonite (53 mg/10 ml), vermiculite (47 mg/10 ml), and kaolinite (182 mg/10 ml) showed that at Hg(II) levels of 0.1, 0.5, 2.5, and 5.0 ppm adsorption was low but increased with increase in Hg(II) concentration. Highest amount of Hg(II) adsorbed was 0.3 mmoles per 100 g clay at the 5 ppm level and in the presence of vermiculite. Lowest adsorption was with kaolinite. Equilibrium pH and pCl measurements indicate that Hg(OH)₂° was the main species present. Lower adsorption by kaolinite could not be assigned solely to clay difference since the proportion of HgCl₂° in the equilibrium solution was higher than for the other two clays. Highest percentage of added Hg(II) adsorbed was attained at 0.5 ppm level for montmorillonite and vermiculite and at 2.5 ppm level for kaolinite. When Hg(II) existed as HgCl₄ in the equilibrium solution there was a tendency towards negative adsorption (repulsion). At Hg(II) levels of (25-1000 ppm) HgCl₂° in the equilibrium solution did not exchange any significant amounts of exchangeable Ca.

Potentiometric titrations were performed first on Hg(II)-chloride solutions after an iterative procedure was developed for predicting such titrations. Titrations of Hg(II) in presence of different chloride concentrations were quantitative and followed the predicted course. The titrations of Hg(II) in presence of resin and peat at various chloride concentrations indicated that chlorides compete effectively with the organic functional groups for Hg(II). However, at the same time unexpected acidity sources were apparent at the higher chloride concentrations. Possible origin of such acidity has been postulated.

The potentiometric titrations showed that more research is required in order to obtain a full understanding of the various acidity sources especially when organic and inorganic complexes are present.