Study On The Characteristics Of Phosphorus Pollution In Manshui River And The Effect Of Fulvic Acid On Phosphorus Release From Sediments

Phosphorus is one of the important nutrients in nature and a limiting nutrient for primary production in freshwater ecosystems,identified as a major contributor to algal growth in surface waters,and excessive phosphorus levels in natural water bodies can lead to eutrophication of surface waters and disrupt aquatic ecosystem balance.Riverbed sediments can sorb or desorb pollutants such as phosphorus from rivers,acting as both a source and a sink,acting as a buffer and balancing pollutant concentrations in surface waters.The mechanism of sediment influence on phosphorus transport is deter mined by the interaction of the sorption and desorption properties of phosphorus in riverbed sediments.Manshui River is a first-class tributary of the lower reaches of the Beijiang River in the Pearl River system.It is affected by the joint influence of far ming,livestock and poultry breeding,aquaculture,domestic sewage discharge of the surrounding residents and rainwater runoff,and the eutrophication of water bodies is relatively serious.In this study,the current status of phosphorus pollution in the overlying water was studied in the Qingyuan watershed of Manshui River,and the content of phosphorus in each form of sediment was measured by graded extraction experiments to analyze its biologically effective phosphorus content and correlation,and the effect of phosphorus transport between solid and liquid phases and the concentration of fulvic acid in the overlying water on the migration and transformation pattern of phosphorus in each form of sediment was investigated through experiments.The main conclusions are as follows:（1）Mangshui River fresh district within the watershed water quality can be roughly judged by Mangshui River Sanqing Bridge cross-section and Huangkan Bridge cross-section water quality,2020-2021 water quality testing said,Sanqing Bridge cross-section water quality average value ofⅡ-Ⅲ,part of the period did not meet the assessment requirements,Huangkan Bridge cross-section water qualityⅣ-poor V,the vast majority of the period did not meet the assessment target requirements,fixed class factor is total phosphorus,total phosphorus concentration exceeds the standard is one of the main factors of water pollution.A total of 12 samples of overlying water were collected from five agricultural irrigation canals,including XST drainage pit,JK trunk canal,JJ drainage pit,HYT drainage pit and MT drainage flood,in March 2021,and the experimental results showed that the mass concentration of TP in the water bodies was 0.87-2.83 mg/L,which exceeded the concentration limit of surface water environmental quality standard V.The mass concentration of TPP was 0.63-1.54 mg/L,which was the overlying TPP mass concentrations were 0.63-1.54 mg/L,which was the main presence form of phosphorus in the overlying water bodies.（2）The total phosphorus（TP）content of surface sediment in the watershed of Manshui River fresh area ranged from 593.35 to 2973.00 mg/kg,and the distribution of total phosphorus and each form of phosphorus in different agricultural and irrigation canals fluctuated relatively widely.Iron-bound phosphorus（Fe-P）,closed-storage phosphorus（Oc-P）,and authigenic phosphorus（Ca-P）were the main forms of phosphorus present in the sediments,among which Fe-P accounted for the highest mass ratio of TP,ranging from19.54%to 64.47%,and its content ranged from 225.07 to 1679.53 mg/kg.The distribution of biologically active phosphorus（BAP）varied widely among sampling sites,with levels ranging from 309.94 to 1,875.12 mg/kg,indicating that sediment samples from some sampling sites have a greater potential for phosphorus release.The correlation between TP content and Fe-P content was significant（R=0.938,P<0.01）,and there was also a significant positive correlation between Oc-P and detrital phosphorus（De-P）content,while the correlation between exchange state phosphorus（Ex-P）,alu minum-bound phosphorus（Al-P）,Ca-P,and organic phosphorus（Or-P）content was not significant,so the increase of Fe-P and Oc-P content would promote sediment enrichment of TP.BAP and Al-P and Fe-P were significantly and positively correlated with Oc-P,indicating that exogenous phosphorus input may be the main reason for the elevated phosphorus release capacity of the sediment.（3）Kinetic experimental results show that the quasi-secondary kinetic model can better fit the kinetics of phosphorus sorption by Manshui River sediments,indicating that chemistry plays a do minant role in this process rather than being controlled by the material transport step.The results of the intraparticle diffusion model fitting indicate that the adsorption process of phosphate by sediment is controlled by both internal and external diffusion of particles.In addition,the EPC₀calculated by the Linear model provides a theoretical basis for deter mining whether the sediment is a"source"or"sink"of phosphorus.The isothermal adsorption processes of phosphorus for the three sediment samples can be well fitted by the Freundlich model,indicating that both physical and chemical adsorption of phosphorus exist in the sediment,and the adsorption process is a surface inhomogeneous multi-molecular layer adsorption.By comparing the maximum adsorption capacity of sediment on phosphorus Q_maxand the adsorption equilibrium constant K_F,it was found that there was no significant positive correlation between the maximum adsorption capacity of sediment on phosphorus and the magnitude of the adsorption capacity of sediment on phosphorus.（4）Indoor simulations with different concentrations of fulvic acid（FA）revealed that the sediment releases phosphorus due to hydrodynamic perturbation without any exogenous pollution,and the main phosphorus component released is Fe-P.With increasing FA concentration in the reaction system,the TP content of the sediment increased from 620.80mg/kg in the control group to 2143.17 mg/kg in group 3,indicating that soil organic matter（SOM）,represented by FA,favors the presence of phosphorus in the sediment in the bound form even under the same hydrodynamic disturbance.Furthermore,the content of Ca-P in the sediment was subsequently reduced,suggesting that dissolved organic matter would have been the formation of Ca-P in the sediment.In contrast,Fe-P in the sediment increased with increasing FA concentration,and it is evident that exogenous fulvic acid promotes the dissolution of Fe OOH and the formation of Fe-P,which further increases the content and percentage of BAP,thus increasing the risk of phosphorus release from the sediment.