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

The adsorption of PO43- 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 PO43- 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 PO43- adsorption by sediments. This study examined the effects of LFOM removal on PO43- adsorption by lake sediments with different trophic status. Results showed that the removal of LFOM did not significantly affect PO43- adsorption kinetics, which was greatest during the first 0.5 h and reached an equilibrium state thereafter. The amount of PO43- adsorbed decreased due to LFOM removal from the sediments, especially in the first 0.5 h. The PO43- 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 PO43- release from sediments to the overlying water, and PO43- 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 PO43- 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).

Research highlights
► The LFOM removal decreased the PO43- adsorbed, especially in the first thirty minutes.
► The replacing ion and destroying colloid were the reasons of decrease PO43- adsorbed.
► The use of heavy liquid NaI isolated LFOM was the physical protection of organic matter.
► The LFOM removal demolished the colloid material, and exposure mineral and oxide surfaces.