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Effect of pH and Complex Formation on Mercury (II) Adsorption by Bentonite¹

ABSTRACT

Reactions of Hg with bentonite clay were studied to determine behavior of the metal in aquatic ecosystems. Mercury (II) adsorption by bentonite as a function of pH and complex formation was investigated using a radioisotopic technique. Maximum Hg(II) adsorption in 0.01M Ca(NO₃)₂ systems occurred in the pH range 4.5–5.5. Varying the Ca(NO₃)₂ concentration only slightly influenced adsorption or the pH of maximum adsorption.

Chloride ions sharply reduced Hg(II) adsorption, especially at low pH's. At pH 6 or lower, increasing CaCl₂ concentration from 10^–5 to 10^–4M depressed adsorption; higher CaCl₂ levels were required to decrease adsorption at pH 7. At a given Cl^– concentration, maximum observed Hg(II) adsorption occurred near the calculated pH where HgClOH occurred as a transition complex between HgCl₂ and Hg(OH)₂.

Chloride salts (0.01M CaCl₂, NaCl, and KCl) were more effective desorbers of Hg(II) than was 0.01M Ca(NO₃)₂ or various 0.01N acids. HCl (0.01N) removed the most adsorbed Hg(II).

Key Words: heavy metals • clays • HgClOH • desorption • salt effects • chloride

¹ Contribution No. 1391, Dep. of Agronomy, Kansas Agric. Exp. Stn. Contribution No. 128, Kansas Water Resour. Res. Inst., Manhattan, Kans. The U. S. Department of Interior, Office of Water Resources Research provided partial support (B-027-KAN).

² Postdoctoral Fellow, Research Soil Chemist, and Research Crop Physiologist, respectively, Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506.

Received for publication February 24, 1976.

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