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The Simultaneous Effect of pH and Chloride Concentrations upon Mercury(II) as a Pollutant¹

ABSTRACT

The mercuric ion, compared to other heavy metal pollutants, hydrolyzes at low pH values and forms soluble chloride complexes at low chloride concentrations. To evaluate the possible implications of such behavior in natural aquatic systems, Hg(II) hydroxy and chloride species distributions were calculated using pH and chloride ion concentrations as variables simultaneously. Range in pH values selected was from 2 to 9 to cover conditions encountered in acid mine drainages, rivers, all types of soil solution extracts, and sea-water. In addition, a study was performed in which montmorillonite, illite, and kaolinite were equilibrated with different HgCl₂ solutions. The range of HgCl₂ concentrations in equilibrium solutions was 1.25 x 10^-4M to 0.005M.

Results of the calculations show that chloride complexes compete effectively with the hydroxy complexes of Hg(II). The chloride and hydroxy complexes may contribute appreciably to the mobilization of Hg(II). The mobility of Hg(II)-hydroxide is limited by the intrinsic solubility of Hg(OH)₂. At pH 4 and 5, chloride ion concentrations of 3.5 and 14 ppm, respectively, are sufficient for all Hg(II) to be in the HgCl₂° form. At higher pH values, partial to complete mobilization is possible depending on the prevailing chloride and Hg(II) concentrations. The equilibria study with the clay minerals revealed that little or no Hg(II) was adsorbed. The chloride ion concentrations of the equilibrium solutions varied between 1.4 and 14 ppm and pH values ranged from 3.68 to 4.90.

¹ Contribution from the Agronomy Dep., Virginia Polytechnic Institute and State University, Blacksburg, Va. 24061.

² Graduate Research Assistant, now Research Chemist, Soils Research Institute, Pretoria, Republic of South Africa; and Professor of Agronomy, respectively.

Received for publication November 27, 1972. Accepted for publication July 2, 1973.