Control Mechanism Of Lanthanum-calcium-aluminum Composite Passivator On Phosphorus Release From Eutrophic Lake Sediments

Lake eutrophication has become one of the major water environmental problems faced by our country.Controlling the endogenous phosphorus load has become the top priority of eutrophication management in many lakes.In situ chemical passivators have been recognized as an effective method to remove phosphorus rapidly and efficiently from water and control the release of endogenous phosphorus in recent years.Although iron-based,aluminum-based,lanthanum-based and calcium-based passivators are widely used for the passivation of phosphorus in lake sediments,they all have specific application conditions,and it is difficult to cope with the changes in physical and chemical conditions of frequent transformation of eutrophic lakes.In this study,the advantages and disadvantages of aluminum-based,lanthanum-based and calcium-based passivators were combined to prepare calcium-aluminum composite（CA）and lanthanum-calcium-aluminum composite（LCA）by combining calcium,aluminum and lanthanum.The effect of controlling the release of endogenous phosphorus in sediments under different pH and DO conditions was investigated,and the control mechanism of endogenous phosphorus release was analyzed by high-resolution thin-films diffusive gradients technique and changes in sediment phosphorus form.The main results obtained are as follows:（1）Both CA and LCA have high phosphorus adsorption effect in the pH range of 4-11,and have a certain correction effect on the pH value of water.The adsorption effect of CA and LCA on phosphorus was better than that of acidity under alkaline conditions,and the addition of lanthanum was beneficial to increase the adsorption capacity of calcium-aluminum composite under acidic conditions.When CA and LCA were added to the sediments of East Lake,the maximum phosphorus adsorption(Q_max)increased by 82.57%and 102.75%,respectively,and the equilibrium phosphorus concentration（EPC₀）decreased by 75.70%and88.49%,respectively,which effectively reduced the risk of phosphorus release from sediments.（2）Under the conditions of pH 5.5,7.5 and 9.5,the addition of CA and LCA can effectively control the release of phosphorus in the sediments of East Lake.The conversion of Fe-P to Al-P,Res-P and Others-P was promoted under acidic conditions,and the conversion of Fe-P to Res-P was promoted under alkaline conditions.The addition of LCA promoted the conversion of Fe-P to other P form more than CA under acidic conditions,while CA has a stronger effect on promoting the conversion of Fe-P in sediments under alkaline conditions.The addition of CA and LCA can effectively inhibit the release of sediment phosphorus under both anaerobic and aerobic conditions.LCA mainly promoted the conversion of Fe-P to Ca-P and Res-P in East Lake sediments,while CA mainly promoted the conversion of Labile-P and Fe-P to Res-P in East Lake sediments.（3）The phosphorus adsorption and phosphorus passivation performance of South Lake sediments were compared between washed CA（WCA）and unwashed CA（UCA）.The phosphorus adsorption capacity of WCA was stronger in the acidic and neutral ranges,and the phosphorus adsorption capacity of UCA was stronger in the alkaline range.Compared with the original South Lake sediments,the Q_max of the sediments after UCA amended and WCA amended increased by 46.45%and 36.02%,respectively,and the EPC₀ decreased by 37.98%and 46.13%,respectively.（4）During a 135-day static culture of continuous aerobic-anaerobic-aerobic culture,covering with UCA and WCA can effectively control the release of phosphorus in the sediments of South Lake and maintain a lower DIP concentration in the overlying water.The results of interface phosphorus kinetics and sediment phosphorus form analysis confirmed that the sediments of South Lake were mainly iron redox-driven phosphorus release mechanism.However,the passivation of phosphorus in the sediments of South Lake by UCA and WCA was not affected by redox conditions.In addition,UCA amended and WCA amended sediments would strongly adsorb the phosphorus released by the extinction of bloom cyanobacteria and convert it to stable Ca-P and Res-P.（5）The sediments of South Lake after amended by UCA and WCA did not significantly affect the pH value of the overlying water.The leached metal ions are all below the safe concentration.In addition,the addition of UCA and WCA did not significantly adversely affect the microbial community in the sediment,whereas the addition of UCA increased its community diversity and richness.