Effects of light fraction organic matter removal on phosphate adsorption by lake sediments

The adsorption of PO 4 3 - on lake sediments has an important influence on the transport, degradation, and ultimate fate of P in lake ecosystems. Organic matter in sediment strongly affects PO 4 3 - adsorption, with light fraction organic matter (LFOM), the labile fractions representing only a small proportion of the total organic matter, playing a key role in nutrient cycling in lakes. However, little is known about the effect of LFOM on PO 4 3 - adsorption by sediments. This study examined the effects of LFOM removal on PO 4 3 - adsorption by lake sediments with different trophic status. Results showed that the removal of LFOM did not significantly affect PO 4 3 - adsorption kinetics, which was greatest during the first 0.5 h and reached an equilibrium state thereafter. The amount of PO 4 3 - adsorbed decreased due to LFOM removal from the sediments, especially in the first 0.5 h. The PO 4 3 - adsorption rates for the sediments from Gonghu Lake, Wuli Lake and Yuehu lake decreased from 302.44 mg (kg h)−1 to 138.93 mg (kg h)−1, 322.88 mg (kg h)−1 to 149.21 mg (kg h)−1 and 415.36 mg (kg h)−1 to 46.77 mg (kg h)−1, respectively. The removal of LFOM accelerated PO 4 3 - release from sediments to the overlying water, and PO 4 3 - adsorption efficiency decreased by 83.75%, 70.90% and 66.75% for Gonghu Lake, Wuli Lake and Yuehu Lake, respectively. Two plausible explanations contributed to the decrease, one was the replacement of PO 4 3 - which was in the form of Fe/Al-bound P in the sediment by OH due to the increased pH, and the other is the destruction of LFOM colloids by the carboxyl group (COOH) and hydroxyl (–OH).