Solubility of inorganic phosphorus in stream water as influenced by pH and calcium concentration

Abstract

Solubility of inorganic phosphorus (P) in aquatic systems is regulated by physico-chemical characteristics of the water column. Laboratory studies were conducted to determine the influence of pH and calcium (Ca) on inorganic P precipitation and solubility relationships in the water column. Water samples from selected streams and canals in south Florida were incubated at 25°C in 2.8 L. continuouslystirred reactors under different PCO 2 levels. Precipitation of soluble P wwas influenced by the initial Ca concentration of the water. Water samples from a drainage canal of the Everglades agricultural area (EAA)(115 mg Cal −1) showed the highest P precipitation rate and Otter Creek water (20 mg Ca l −1) the lowest. Precipitation of inorganic P increased at Ca concentrations above 100 mg l −1 and at pH > 9.0. Solubility of inorganic P in the EAA water decreased above pH 8.5. At pH 9.0 more than 60% of the soluble P at all Ca levels was precipitated. In contrast, P solubility at low Ca levels (20 and 50 mg Ca l −1) was not affected at pH< 9.0. The decrease in P solubility as pH and Ca levels were increased suggests that calcium phosphate mineral formation occurred in the stream and canal water. Calcium phosphate formed in Otter Creek and Dry Lake waters rapidly dissolved as the pH decreased. Most of the soluble P in the Otter Creek (>90%) and Dry Lake (>75%) waters came back into solution; however, about 50% of the soluble P from the EAA canal water remained precipitated. Ion activity product calculations indicated that Otter Creek and Dry Lake waters were at equilibrium with octacalcium phosphate. In contrast, the EAA canal water was supersaturated with respect to beta tricalcium phosphate or with a similar mineral such as whitlockite. High Mg concentrations in the EAA water were apparently responsible for a different P behavior than for the Dry Lake and Otter Creek waters.