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Effects of pH and Organic Acids on Orthophosphate Solubility in an Acidic, Montmorillonitic Soil¹

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 Al³⁺ 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.

¹ Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521 and A.R.O. Institute of Soils and Water, Volcani Center, Bet-Dagan 50250, Israel.

² Assistant Research Soil Chemist, Professor of Soil Science, and Graduate Fellow (Riverside), and Research Scientist (Bet-Dagan), respectively. Present address of the senior author is Dep. of Agronomy, Ohio State Univ. Columbus, OH 43210.

Received for publication May 9, 1985. Accepted for publication August 13, 1985.

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