Distribution Characteristics And Formation Process Of Phosphorus Colloid In Weining Caohai Lak

Weining Caohai,a typical karst natural freshwater lake,is an important reservoir of water resources and a habitat for protected birds such as the Black-necked crane.Over the past decade,urban sewage discharge and agricultural non-point sources have caused the deterioration of Caohai water bodies and the eutrophication of water bodies,which has become more and more serious in recent years.At present,the deterioration of Caohai water quality has led to the extinction of aquatic plants and the degradation of ecosystem.At present,Caohai lake is in the process of transforming from clear water grass type lake to turbidized algal type lake,which is specifically manifested as turbidity of water,algae blooms,and total phosphorus（TP）content increasing year by year,which is far beyond eutrophication conditions.Phosphorus（P）is the main limiting factor of lake eutrophication.The chemical composition and occurrence form of P in lake water determines the behavior of phosphorus fixation,release,migration and transformation,and is the key factor affecting the geochemical cycle of phosphorus and its ecological environment effect.In addition to the dissolved state and granular state,there is also a colloidal state of P.Colloid plays an important role in the migration and transformation behavior of nutrients such as P due to its large surface area and high chemical activity.However,previous studies mainly focused on the systematic study of dissolved active P,and lack of in-depth research on the behavior characteristics and formation process of lake colloid P.This study relies on Caohai,as the research site,and makes comprehensive use of field investigation and indoor microcosmic simulation,through the comprehensive application of various methods.Firstly,the spatio-temporal variation characteristics of colloid P in Caohai lake and pore water samples are investigated.Secondly,the binding process of P colloid with iron（Fe）colloid and organic matter（NOM）and the stability of phosphorus colloid are studied.Finally,the kinetic process of phosphorus colloid formation was discussed.The main conclusions as follows:（1）Spatial and temporal distribution characteristics of colloid P in Caohai water and pore water.The pH of Caohai water is between 8 and 9,and the dissolved oxygen is between 5.45 and 10.60 mg/L,Pore water is strictly anaerobic environment.The main ionic compositions in water are Ca²⁺,Mg²⁺,SO₄^2-,etc.The distribution of colloidal P showed significant time anisotropy.In spring,the content of colloidal P in lake water can account for 60%of total phosphorus.In summer,the total phosphorus in lake water mainly exists in the form of dissolved organic P.Compared with lake water,the contents of total phosphorus（0.130-7.37 mg/L）and colloidal P（0.005-5.00 mg/L）in pore water of sediments in different seasons were significantly higher than those in lake water.Colloidal P mainly exists in the form of inorganic P in pore water in different seasons.The average content of total phosphorus in pore water was the highest in spring,and the inorganic P colloid content accounted for 53%～96%of total phosphorus in TP.（2）Composition and stability of lake P colloid.The results of chemical particle size classification and Cs-STEM showed that the main chemical compositions of colloid P in Caohai water and pore water were P,Fe and organic matter.The colloidal Fe in pore water accounted for 3%-94%of the total iron,in which the proportion of bivalent iron colloid was 2%-92%,and the proportion of trivalent iron colloid was 11%-97%.NOM colloid concentration in pore water was less than 38%.The results of dynamic light scattering measurements show that the particle size of colloidal particles is less than 450 nm,which is consistent with the results of Cs-STEM and particle size classification.The results of colloidal deposition kinetics showed that the colloidal concentration and particle size did not change significantly within seven days,indicating that colloidal P had high colloidal stability.The high stability of colloid may be related to the complex mechanism of organic matter.The high colloid stability indicates that P can be suspended in water environment for a long time,and can migrate from pore water to lake water,causing the deterioration of water environment quality.（3）Formation process of phosphorus colloids.The results of laboratory microcosmic experiments show that the rate of P released from sediments to pore water is very fast,and the P released mainly exists in the dissolved state and colloidal state.The concentrations of colloid P and dissolved P change dynamically with reaction time.At many times,the concentration of dissolved P and colloid P is negatively correlated,but the concentration of total water phase P remains constant,indicating the presence of colloid pump effect.P colloid formation and release may be closely related to the biogeochemical reaction kinetics of iron and sulfur in sediments.In conclusion,this study investigated the spatial and temporal distribution characteristics of P colloid in lakes through field studies,and analyzed the chemical composition and formation mechanism of P colloid through laboratory characterization and microcosmic simulation experiments.The research results deepen the understanding of the biogeochemical cycle of P,provide data basis for the migration and release of colloid P in lake environment,and provide certain scientific guidance for lake eutrophication prevention and control and water quality safety management.