Insolubilization of Inorganic Phosphate by Anoxic Lake Sediment

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Abstract
The insolubilization of soluble phosphate by anoxic lake sediments was studied at 4 to 5C under oxygen‐free conditions. After 3 to 4 weeks of reaction between added phosphate and two sediments, samples of the interstitial solution were recovered and analyzed. Sediment samples were extracted to determine exchangeable cations, CEC, and readily soluble phosphate. Insolubilization of added phosphate was accompanied by decreases in the concentrations of all major cations in the interstitial solution. Soluble cations and phosphate were lost in essentially charge‐equivalent amounts. Calcium, iron, and manganese were lost from both soluble and exchangeable forms. Soluble Mg, Na, K, and NH4 decreased also but only because these cations occupied exchange sites vacated by Ca, Fe, and Mn. The fraction of soluble and exchangeable Mn lost was greater than for any other cation. No significant changes in pH, CEC, or concentrations of indigenous anions were found at any level of added phosphate. Essentially all insolubilized phosphate was recovered as inorganic phosphate by mild extractants suggesting that inorganic chemical reactions were responsible for the losses of added phosphate. The results suggest that phosphate compounds of Ca, Fe, and Mn were precipitated.

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