Influence of pH on Phosphorus Retention in Oxidized Lake Sediments

Abstract

Abstract Diel pH changes in lake waters resulting from high photosynthetic activity may regulate water‐soluble P concentration (WSP) and P sorption by suspended sediments in shallow eutrophic lakes. Laboratory studies were conducted to determine the pH effect on P fractions and P sorption kinetics in oxidized sediment suspensions from two subtropical lakes (Lake Apopka and Lake Okeechobee, Florida). The P sorption rate was calculated for sediment suspensions adjusted to various pH levels: 6.5, 7.0, 8.5, 9.5, and 10.5 for Lake Apopka and 6.5, 7.0, 8.5, 9.5, and 10.5 for Lake Okeechobee. A decrease in pH increased the WSP concentrations in Lake Apopka sediment suspensions but had no effect on WSP concentrations in Lake Okeechobee sediment suspensions. Lake Apopka sediment suspensions at pH 7.0 (ambient) and below did not show net P uptake. Phosphorus uptake for Lake Apopka occurred only when pH was increased to ≥8.5 and when P treatments were increased to ≥27 mmol P kg −1 , which resulted in supersaturation with respect to octacalcium phosphate. Phosphate solubility diagrams and mineral equilibria calculations suggest that P uptake by Lake Apopka sediment suspensions at pH ≥8.5 was due to P coprecipitation with CaCO 3 and probable formation of nonapatitic Ca‐P compounds. Phosphorus sorption on Lake Okeechobee sediment suspensions followed first‐order kinetics for all pH levels studied, with rate constants ( k ) ranging from 0.003 to 0.75 h −1 . High P uptake by Lake Okeechobee sediment suspensions could be attributed to two reactive components: (i) amorphous or poorly crystalline Fe and Al oxyhydroxides at pH <7.5, and (ii) Ca/Mg carbonates and other minerals at pH ≥7.5.