Effects of pH and Organic Acids on Orthophosphate Solubility in an Acidic, Montmorillonitic Soil

Abstract

Abstract The effects of pH and organic acid addition on residual orthophosphate ( o ‐phosphate) solubility in an acidic, montmorillonitic soil separate suspended in 20 mol NaCl m −3 were investigated. In the absence of added organic acid, the solubility of residual o ‐phosphate in the soil decreased as the pH value was increased from 4 to 7. With o ‐phosphate additions that resulted in low aqueous‐solution o ‐phosphate concentrations, the soil sorbed more o ‐phosphate at pH 7 than at 5.5. The solubility data appeared consistent with the presence of an Al‐phosphate solid whose formation was initiated through the hydrolysis of exchangeable Al 3+ and subsequent reaction of the hydrolytic product with o ‐phosphate in aqueous solution. Additions of citric, tartaric, or formic acid at low concentrations (<0.1 mol m −3 ) produced a drop in residual o ‐phosphate solubility at pH 5.5 and 7.0. In the case of citric acid, o ‐phosphate solubility gradually returned to its value in the absence of organic ligand as the ligand concentration was increased to 0.6 mol m −3 , whereas no such increase occurred in the case of tartaric or formic acid. This behavior was rationalized as the outcome principally of two competing reactions: (i) the reaction of adsorbed, polymeric hydroxy‐Al with organic ligand to form an exchangeable Al species that combined with o ‐phosphate to reduce its solubility, and (ii) the complexation of adsorbed Al to form a soluble Al‐organic complex that did not subsequently combine with o ‐phosphate. The latter reaction was important only for citric acid. The former reaction, favored by a large stability constant for Al‐organic acid adducts, produced o ‐phosphate solubility decreases in the order citrate > tartrate > formate.