Phosphate adsorption by fresh and aged aluminum hydroxide. Consequences for lake restoration

Lake restoration by in-lake precipitation of PO4(3-) with Al(OH)3 is commonly used but there is currently no good guidelines for calculating doses (amounts and application) that can ensure long lasting effects. We studied the effect of aging of Al(OH)3 on PO4(3-) adsorption and desorption properties with and without PO4(3-) in solution and found that Al(OH)3 aged without PO4(3-) lost 75% of the maximum adsorption capacity in 90 days after which no further changes occurred. Al(OH)3 aged in presence of PO4(3-) maintained the adsorption capacity through 6 months and even increased it for PO4(3-) concentrations < 150 microM. On this basis, we suggest that repeated dosing of smaller Al-aliquots may be more efficient than adding a single big dose. Also, Al should be added at the time when PO4(3-) availability in the lake is highest. At laboratory conditions we obtained molar P:Al binding ratios of 0.12-0.19 at PO4(3-) concentrations similar to those in eutrophic lake sediments, but when examining Al(OH)3 aged in situ in two lake sediments lower ratios (approximately 0.1) were found. We suggest that total Al-dosage should be calculated relative to the pool of potential mobile P in the lake with a molar ratio not less than 10 Al: 1 P.