Adsorption of Phosphorus by Lake Sediment

The sediment of an eutrophic lake is capable of adsorbing a large amount of phosphorus from the water. To study the sorption mechanism, between 0 and 2.2 mg P was added to 0.1‐g sediment samples (1:50 sediment‐solution ratio). After equilibrium had been established, P remaining in solution was determined, and adsorbed P was extracted with 0.5 N NH 4 F and 0.1 N NaOH. Whereas all P adsorbed into the NaOH‐extractable fraction appeared to occur as an iron phosphate, NH 4 F apparently extracted P bonded by two different mechanisms. When less than about 0.1 mg P was added, NH 4 F extracted a tightly bonded form of P, probably occurring as an aluminum phosphate. When more than 0.1 mg P was added, additional P in a more loosely bonded form was adsorbed into the NH 4 F‐extractable fraction. The loosely bonded P appeared to be independent of Al content of the sediment, and could be removed by successive water extraction. The capability of the sediment to adsorb considerable loosely bonded P means that large influxes of P into the lake may be held temporarily and subsequently released to growing plants and algae.