Mechanisms of the photochemical release of phosphate from resuspended sediments under solar irradiation

Abstract

In previous studies, resuspended sediments that were exposed to simulated solar irradiation could release dissolved phosphate (PO43−). However, the mechanisms of phosphate release remain unclear. In this research, a battery of experiments was performed to reveal the mechanisms of the photochemical release of phosphate from resuspended sediments of a shallow eutrophic lake under solar irradiation. The results show that the PO43− released in resuspended sediments was significantly higher than that in the dark control or in water alone after treatment with solar irradiation for 6h. The results of sequential chemical extractions showed that the concentrations of labile organic, moderately labile organic and residual organic phosphorus decreased in the resuspended sediment after 6h of solar irradiation; of these, moderately labile organic phosphorus was the greatest contributor to the release of dissolved phosphate in resuspended sediment. Orthophosphate, phosphate monoesters, phosphate diesters and pyrophosphate were detected with 31P NMR. It is worth mentioning that the diester-P and pyro-P species disappeared after 6h of irradiation. In addition, enzyme activity and radical trapping experiments were applied to identify the roles of biomineralization and photochemical degradation during phosphate release from resuspended sediments under solar irradiation. The amount of PO43− released in fresh sediment was greater than that in the autoclaved sediment, which should be attributed to the higher alkaline phosphatase activity in the fresh sediment. However, the PO43− released from the photochemical degradation of organic phosphorus is the primary phosphate source during sediment resuspension under 6h of solar irradiation. The phosphate photorelease was inhibited when methanol was added to the suspension and decreased significantly when the concentration of methanol was increased from 0.5M to 2.0M. All of these results suggest that photochemical processes may lead to PO43− release from sediment in aquatic environments.