Abstract

A simplistic equilibrium, computer model was devised to simulate the removal of orthophosphates from wastewater. Data from laboratory experiments were tested in the model to determine the apparent solubility constants (activity products) of calcite, hydroxyapatite, tricalcium phosphate and brucite. It was found that the degree of supersaturation of hydroxyapatite was between 15 to 20 orders of magnitude dependent on the aqueous species included in the calculations. The pK values for different sets of data showed the mean activity products ranging from 95 to 102.5, standard deviations 2.0 to 5.0. The inclusion of aqueous ion complexes Ca2.HPO4.Co30 and Ca2.PO4.CO3 when calculating the apparent activity products results in a pK value of 102.5 which does not vary with pH or the presence of magnesium. The pK mean of beta tricalcium phosphate was 26.8 which compared favourably with the pK of 27.0 quoted in the literature. When hydroxyapatite precipitated, there was a minimum residual phosphate between pH 8.5-9.0, followed by an increase of phosphates in solution due to calcium being removed by the precipitation of calcite. Beyond pH 10, the phosphate concentration in solution decreased as there was an increase in the precipitation of hydroxyapatite. The use of apparent activity products compared to literature solubility products results in 2 to 3 orders of magnitude greater phosphate residuals in solution.

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