INTERACTIVE MECHANISMS OF ANION ADSORPTION WITH CALCIUM LEACHING AND EXCHANGE

Abstract

Anions can be adsorbed to soil colloids by specific (inner-sphere complexation) and nonspecific adsorption (outer-sphere complexation and diffuse ion association). The mechanism for anion adsorption determines whether and by what reaction anions in soil solution will enhance metal adsorption. The objectives of this study were (i) to determine the influence of the accompanying anion on Ca leaching and cation exchange in a sandy, kaolinitic Ultisol, (ii) to describe possible mechanisms for anion effects on Ca adsorption based on diffuse double-layer theory, and (iii) to discuss the practical implications of these mechanisms on soil fertility management. The Ca forms used in this study were calcium chloride, calcium nitrate, calcium sulfate, calcium phosphate, and calcium carbonate. Undisturbed soil cores from a Plinthic Kandiudult were collected, and a Ca solution (5, 500, and 1500 mg L−1) was applied to soil columns. The effluent was collected from each column, and cations (Ca2+, Mg2+, K+) in solution were measured. Exchangeable bases and effective cation exchange capacity (CEC) were measured in soil from soil cores after effluent collection. Sulfate and phosphate retarded Ca break-through, compared with chloride and nitrate. Specific adsorption of sulfate and phosphate in the inner-sphere increased the net negative surface charge of the soil and, thus, increased Ca adsorption. Higher influent concentration increased the extent of ion pairing (CaSO4°), and consequently, enhanced Ca adsorption directly. Calcium breakthrough was most gradual in the carbonate effluent as a result of ion pairing and increased pH dependent charge leading to both direct and indirect enhancement of Ca adsorption. The Mg peaks for phosphate and carbonate were broader and shorter than those peaks for nitrate, chloride, and sulfate. The choice of the accompanying anion in Ca fertilizer materials could be critical in controlling the leaching of Ca from the soil surface and optimizing the exchange and subsequent leaching of other cations.