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Kinetic Study of Citrate Effects on Orthophosphate Solubility in an Acidic, Montmorillonitic Soil¹

ABSTRACT

Previous studies by the authors on the effects of pH, organic acids, ionic strength, and Ca on residual orthophosphate solubility in an acidic montmorillonitic soil (Typic Chromoxererts) were extended to include the variation of reaction time. The objective of the experiments was to determine the time dependence of the reactions and to elucidate further the mechanism of orthophosphate solubility in a montmorillonitic soil separate containing exchangeable Al and added citric acid, and suspended in 20 mol NaCl m^–3. In the absence of added organic acids, soluble orthophosphate and Al³⁺ activities were found to be consistent with the presence of an amorphous hydroxy-Al-phosphate solid at elapsed t 24 h. Initial citrate concentrations of 10, 100, and 400 mmol m^–3 resulted in citrate sorption by the soil separate and an increase in soluble orthophosphate and Al for t 1 h. Citrate sorption continued throughout the time course of the experiments. For 1 < t < 8 h, orthophosphate solubility was not affected by the initial citrate concentration. For 8 < t < 27 h, orthophosphate solubility decreased to constant levels. Soluble Al decreased for 1 < t < 30 h and then remained constant in the 10 and 100 mmol citrate m^–3 treatments. The samples reacted with 400 mmol citrate m^–3 exhibited a steady increase in soluble Al for the same time period. The time-dependent changes in aqueous orthophosphate and Al were consistent with a previously proposed reaction describing organic ligand-induced formation of an insoluble hydroxy-Al-phosphate. The time trends in the solution concentrations of Ca and Si were not affected by the addition of organic ligands nor were they related to the changes in soluble Al or orthophosphate.

¹ Contribution from the Dep. of Agronomy, The Ohio State Univ. (OSU), Columbus, OH 43210; Dep. of Soil and Environmental Sciences, Univ. of California, Riverside (UCR), CA 92521, and Dep. of Field Crops, Hebrew Univ. of Jerusalem (HUJ), Rehovot 76-100, Israel. Salaries and research support were provided in part by State and Federal funds appropriated to OSU-OARDC. Journal Article no. 41-87.

² Assistant Professor of Soil Physical Chemistry (OSU), Professor of Soil Science (UCR), Specialist (UCR), and Professor of Soil Fertility (HUJ), respectively.

Received for publication November 17, 1986.