Aluminum dose required to inactivate phosphate in lake sediments

Surface sediments from two shallow, eutrophic Swedish lakes with different sediment BD–P (Fe–P) content, but similar NH4Cl–P (e.g. pore water P) concentration were treated with different amounts of aluminum sulphate at pH 6 in the laboratory to simulate the recommended dose for in-lake alum treatments. NH4Cl–P was depleted with less than half the recommended dose of Al in Lake Vallentuna sediments that contained a low concentration (0.1 mg P g−1 DW) of BD–P. Lake Finja, on the other hand, with high concentration of BD–P (2.3 mg P g−1 DW), required three times the recommended Al dose to deplete NH4Cl–P, indicating that BD–P correlated with NH4Cl–P. Six times the recommended Al-dose converted all BD–P (Fe–P) to NaOH–rP (Al–P) under anoxic conditions in the BD–P rich sediment, while one dose was enough in Lake Vallentuna. Other P-pools, defined by P-fractionations, remained constant. In order to simulate a summer resuspension event, sediments were placed in a “resuspension chamber” through which oxygen-saturated water with pH 9.5 passed at 100 cm day−1. The iron-rich sediment released 1 mg BD–P g−1 DW of an initial 2.3 mg BD–P g−1 DW after one day. With addition of 90 mg Al g−1 DW (6×recommended dose) the subsequent release when resuspended was only 0.25 mg P g−1 DW after one day, showing a general increase in P retention capacity with large doses of Al. It is concluded that by transforming all Fe–P to Al–P in surface sediments, this should drastically reduce internal loading of P caused by anoxic hypolimnetic water. Thus, the burial rate of potentially mobile P in accumulation sediments would increase.