Factors affecting phosphate adsorption to aluminum in lake water: Implications for lake restoration

Treatment of lake inlets or lake sediments with aluminum (Al) is being increasingly used for lake restoration but only few studies exist concerning competitive substances that might influence phosphate (PO43−) removal from lake water. Therefore, chemical interferences of several ions (magnesium, silicate, chloride and humic acid) on PO43− adsorption to Al(OH)3 were studied. Interference of each ion was studied in artificial lake water, and the complex interactions occurring in natural water were studied in water from 30 Danish lakes at pH 7 in both cases. In the artificial lake water Al:P ratio was high as sediment P-pools were the targets while in the natural lake water Al addition was generally lower as only P present in the water was targeted (i.e. inlet water). The single-ion experiments evidenced that silicate (> 200 μM) and humic acids significantly decreased the effectiveness of PO43− adsorption to Al(OH)3 by 10–13% at 450 μM Si and 17% at 1 mM C, respectively. NaCl did not influence adsorption of PO43− to Al(OH)3, however, PO43− removal was slightly reduced in seawater, mainly due to the presence of Mg2+. The studies on interferences in natural lake water showed that as long as the PO43− concentration was low (< 5 μM), silicate competed with PO43− for adsorption sites on Al(OH)3 but at higher PO43− concentrations, color and DOC (as indicators of HA) were the main variables decreasing PO43− removal from lake water. Inhibition of PO43− precipitation in natural lake water appeared complex and did not allow for a simple calculation of Al dose from the concentration of potentially competitive ions. Recommendation for lake management is therefore still that precipitation assays should be carried out for any type of inlet or lake water prior to Al application.