The Photochemical Transformation Of Organic Phosphorus In Sediment Resuspension

Phosphorus is one of the nutrients that limiting the growth of phytoplankton in the lake,and the release of internal phosphorus is an important factor leading to eutrophication（Sondergaard et al 2001）.Most lakes in China’s Yangtze River are shallow lakes and the internal phosphorus in sediment can released into waters though resuspension under the condition of external force disturbance.This process can change the distribution of phosphorus in sediment-water interface,leads to migration and transfer of internal phosphorus.Previous studies have shown that the release of dissolved inorganic phosphorus in the resuspended sediment increased under solar irradiance.The main reason is the photolysis of organic phosphorus that release into the overlying water.However,the effects of driving force on the photolysis of organic phosphorus in the sediment and the change of combined form of organic phosphorus during the irradiation are only reported by a few papers.Hence,based on field monitoring,sampling and analysis,the distribution characteristics of phosphorus forms are obtained.Here we use the 500W Xenon lamp to simulate the sunlight to explore the release of PO₄^3-in the resuspended sediments and the influence of different factors on the degradation process are explored,such as the concentration of suspension,the grain size of sediment and the spectral irradiance.Solution³¹P-NMR was used to explore the change of the composition the fractionation of phosphorus.Based on this,the outdoor experiment was established to reveal the impact of sediments resuspension on the growth of phytoplankton.As shown in the following content is the conclusion that we get from this experiment:1.The different forms of phosphorus in the over overlying water was separated.The main forms of phosphorus in the overlying water was dissolved organic phosphorus,the concentration of particular phosphorus ix second,and the content with the least is the dissolved organic phosphorus.The result of solution ³¹P-NMR shows that the dissolved phosphorus mainly existed in the form of dissolved organic phosphorus.Ortho-P,Mono-P,Diesters-P and Pyro-P were included in the particle phosphorus.The Diesters-P is the main ingredients with the concentration of 19.25%.The content of Mono-P was followed.2.The photolysis characteristics of the resuspended sediment are investigated along with its influencing factors.The result showss that the photolysis of organic phosphorus in the filtered water has small contributed to the release of PO₄^3-because of its less content.The main reason for the release of PO₄^3-is the photolysis of the particulate phosphorus in the sediment.The release amount of PO₄^3-increased with the decrease of the sediment grain size.High concentration of suspension is more conducive to organic phosphorus photolysis compared with low concentration of suspension.But it is not to say,higher concentrations of suspension can produce more inorganic phosphate.The photolysis of the organic phosphorus that in the sediment is highly spectrum-sensitive.The generation of PO₄^3-is mainly due to the irradiation of ultraviolet during the process of photolysis.Chemical sequential extraction results shows that the concentration of labile-OP,moderately labile-OP and residual-OP all decreased in the resuspened sediment after solar irradiation for 6 h and moderately labile-P was the greatest contributor to PO₄^3-release action in sediment.Results of ³¹P-NMR shown that the Diesters-P and Pyro-P disappeared and the Mono-P is likely to occur the morphologic change.From this experiment we also get a conclusion that the ability of photolysis of organic phosphorus in sediment with high degree of eutrophication of the lake is strong than light degree.3.The influence of sediment resuspension on the growth of phytoplankton was explored.The result indicated that the influence of the coupling action of light and sediment resuspension on the growth of the phytoplankton is better than single light.Doubtless,the growth of phytoplankton has no obvious change no matter in dark group or in the group that dark and sediment resuspension combined action.By determinate the physicochemical indexes in the overlying water,the concentration of TP、PO₄^3-、NO₃^-and Chla is higher than that in the single light group,and those indexes has no signification changes in other groups.