The Release Mechanism And In-Situ Control Technology Of Internal Phosphorus Pollution In Sediment Of Lake Caohai In Guizhou Province

The eutrophication of lakes caused by excessive enrichment of nutrients such as nitrogen and phosphorus（P）is a global water environment problem.Among all the elements that affect the growth of aquatic plants,P is considered to be the limiting nutrient that determines primary productivity.Sediments are considered to be an important source and sink of P in lakes,which play a crucial role in the transfer and transformation of nutrients in aquatic ecosystems.However,in macrophytes-dominated Lake,the transition between clear conditions dominated by macrophytes and turbid conditions dominated by phytoplankton is abrupt and difficult to reverse.Simply reducing nutrients may have little effect on the turbidity of the water but the disturbance and utilization of the ecosystem can make the water reach a stable state of clarity.Lakes with high cover densities of aquatic plants tend to have higher transparency than lakes with the same nutrient status and sparse or no macrophytes.Therefore,the development of an efficient and environmentally friendly in-situ passivator and macrophyte combination technology suitable for lakes dominated by macrophytes is of great significance for the remediation of internal pollution in lakes and the restoration of water ecosystems.This study selects Lake Caohai,a typical macrophytes-dominated lake in Guizhou,as the research object.Through systematic sampling and observation on the overlying water,pore water,and sediments of the Lake Caohai,the release mechanism of internal P in sediments under carbonate geological background was studied.Combined with indoor and field simulation experiments,the techniques of in-situ passivation and macrophytes were established suitable for plateau karst lakes.The main conclusions are as follows:（1）The P concentration between the overlying water and sediment pore water in macrophytes-dominated Lake is relatively high,and some areas are inferior to V class water.The pore water P in sediments has a high gradient diffusion risk to overlying water.（2）The internal P loading in the Lake Caohai was relatively high,and the P release flux in sediment was about 0.37 mg·m^-2·d^-1.The main forms of P in sediments are BD-P,Na OH-Pi and Na OH-Po.The phosphate ester organophosphorus components（i.e.,Mono-P and Diester-P）continuously deposited and degraded in the sediments maintained the high productivity of the lake.（3）The Lanthanum-modified passivation and macrophytes have synergistic effects on water quality restoration of macrophytes-dominated eutrophic lakes,and lanthanum-modified passivation agents have lower negative effects on the growth of these plants.Among them,the combination of Lanthanum-modified attapulgite and macrophytes had the best effect in Macrophyte-dominant shallow Lake(0.28 mg·m^-2·d^-1).（4）The analysis of ultra-high-resolution SEM-EDS images and XRD spectra showed that the PO₄^3-in the water column mainly complexed with La³⁺in the lanthanum modified passivator to form lanthanum phosphate compounds（La PO₄·n H₂O）and precipitated.At the same time,it will reduce the mass fraction of pyrite and Ca（PO₃）₂ in the sediments.