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, bur 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 limes 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 "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 x 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. (C) 1998 Elsevier Science Ltd. All rights reserved.