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This article in SSSAJ

  1. Vol. 50 No. 1, p. 45-52
     
    Received: May 9, 1985


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doi:10.2136/sssaj1986.03615995005000010009x

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

  1. Samuel J. Traina,
  2. Garrison Sposito,
  3. Dean Hesterberg and
  4. U. Kafkafi2

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 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.

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