Adsorption of phosphate on variable charge minerals and soils as affected by organic and inorganic ligands

Abstract

Metal oxides, noncrystalline or short-range ordered iron and aluminum hydroxides, poorly crystalline aluminosilicates, which are found within a wide range of soil orders, as well as organo-mineral complexes, are responsible for phosphate retention in soil environments. Strongly chelating organic acids produced by microorganisms or by plants (i.e., root exudates), as well as humic and fulvic acids, may strongly influence the adsorption of phosphate and its availability for plants. Maximum reduction in phosphate adsorption occurs when organic ligands are previously adsorbed on variable charge minerals or soils. The competitive adsorption of phosphate and organic ligands (e.g., oxalate, tartrate, malate, citrate) is influenced by pH, the nature of the ligands, and the nature of the surfaces of clay minerals and soils. Organic ligands may coprecipitate with OH-Al or OH-Fe species, forming organo-mineral complexes, which differ in chemical composition, surface properties, and reactivity toward phosphate. Nutrients and pollutants also compete with phosphate for the sorption sites of soil constituents. Sulfate inhibits phosphate adsorption or is not completely removed from the surfaces on which it is previously adsorbed only at low pH values. However, sulfate present in hydroxy-Al sulfate complexes is only partially removed even by large amounts of phosphate. Arsenate strongly competes with phosphate, but its efficiency in inhibiting phosphate adsorption is influenced by pH, concentration, order of anion addition, and nature of the surface of clay minerals and soils.