Influence Of Capping Mode And Lanthanum Existing Form On The Control Of Phosphorus Release From Sediments By Lanthanum-based Materials

Surface water resources are of overwhelming importance to the circulation of substances and the development of human production and life.In recent years,China has been paying more and more attention to the health of the water environment.A lot of resources have been devoted to water pollution prevention and remediation,however,the phenomenon of water pollution still exists.Eutrophication of water bodies is a common pollution phenomenon in surface water.It is often associated with excessive amounts of nitrogen and phosphorus.Among them,phosphorus plays a dominant role.Therefore,controlling phosphorus levels in water bodies is the key to preventing eutrophication.The phosphorus in water bodies could be divided into exogenous and endogenous sources.Cutting off the input of exogenous phosphorus is the first step to remediate eutrophication in water bodies.Then the use of appropriate techniques to control the release of endogenous phosphorus from the water body is the key way.Many techniques have been developed to control the release of phosphorus from sediments in water bodies,including aeration for oxygen,sediment dredging,chemical precipitation,diversion dilution,aquatic bioremediation,and in situ mulching/modification.In recent years,in situ capping/amendment technology(C/A)has been gained more and more attentions by researchers for advantages of quick effect,long duration and low operation threshold.Capping,as a method with low operational difficulty,has been widely used in the field of controlling phosphorus release from sediments.The selection of suitable active materials plays a great important role in the application of in C/A.Many in-situ capping/amendment active materials have been developed,such as aluminum-based,iron-based,calcium-based,zirconium-based,and lanthanum-based metal materials.Lanthanum possesses great potential for application due to its strong binding ability to phosphates in solution,stable product formation and relatively low price.Lanthanum hydroxide is a prospective phosphorus passivation material in lanthanum-based materials.The addition of active material to sediments can be done in a one-time large dose or multiple small doses.Settlement of suspended particulate matter(SPM)is a common phenomenon in water bodies.SPM deposition may affect the effectiveness of in situ active capping/amendment techniques in controlling phosphorus release from sediments.Reveal the effect of the capping mode(one-time high dose and multiple small dose)on the control of sediment phosphorus release by capping lanthanum hydroxide(LH)in the absence or presence of SPM deposition is very essential for the application of LH as a capping material to control phosphorus release from sediment.However,little is known about it at home and abroad.The effect of capping mode on the controlled release of phosphorus from sediments by lanthanum hydroxide was investigated.The results show that in the presence of SPM deposition,lanthanum hydroxide can effectively control the release of phosphorus from sediments to overlying water by using one-time large dose and multiple small dose capping.The passivation of DGT(film gradient diffusion)labile phosphorus and potentially mobile phosphorus in upper sediments by lanthanum hydroxide is the key to control the release of phosphorus from sediments by multiple and one-time capping.In the absence of SPM,although the efficiency of lanthanum hydroxide controlling phosphorus release from sediments to overlying water will be negatively affected in the initial application,it will increase the stable phosphorus in the upper sediments passivated by lanthanum hydroxide in the later application while the capping mode changes from one-time large dose to multiple small dose,A.Under the condition of SPM deposition,the release of phosphorus from sediments to overlying water can be controlled effectively by one-time and multiple lanthanum hydroxide.In the presence of SPM,the efficiency of lanthanum hydroxide in controlling the release of phosphorus from the sediment to the overlying water increased in the later stages of application,although the efficiency decreased in the early stages of application when the capping mode was changed from one-time to multiple.In terms of long-term effects,multiple lanthanum hydroxide capping is a promising method for controlling phosphorus release from sediments.Currently,lanthanum-based materials have been developed in different forms,such as materials in the form of lanthanum ions,lanthanum oxide,lanthanum hydroxide,lanthanum carbonate and lanthanum oxycarbonate.Knowing their differences in the control effect and mechanism of different lanthanide materials on phosphorus release from sediments is very important for the application of in C/A of lanthanum-based materials to control sediment release.The differences in the role and mechanisms between lanthanum hydroxide and lanthanum carbonate in controlling phosphorus release from sediments are well understood nationally and internationally,however,little is known about the differences in the role and mechanisms of lanthanum oxide,lanthanum hydroxide and lanthanum oxide carbonate in controlling phosphorus release from sediments.In order to gain insight into the effect of lanthanum existing forms on the control of sediment phosphorus release from lanthanide-based materials,this study explored the effect and mechanism of lanthanum oxide,lanthanum hydroxide,and lanthanum oxycarbonate on phosphorus release from sediments,and have explored their differences.The results show that the adsorption capacity of lanthanum oxide to phosphate in water is lower than lanthanum hydroxide and lanthanum oxycarbonate.The adsorption processes of lanthanum oxide,lanthanum hydroxide and lanthanum oxycarbonate on phosphate in water belong to chemical adsorption,and the adsorption capacity is negatively correlated with the p H of solution.The concentration of dissolved active phosphorus(DRP)and dissolved total phosphorus(DTP)in overlying water can be significantly reduced by lanthanum oxide,lanthanum hydroxide and lanthanum oxycarbonate,and the release of phosphorus from the sediment to the overlying water can be effectively prevented.Lanthanum hydroxide has the best control effect.Lanthanum oxide,lanthanum hydroxide,and lanthanum oxycarbonate all have the potential to enhance the transformation of labile phosphorus(Labile-P),redox-sensitive phosphorus(BD-P)and extracted phosphorus from sodium hydroxide(Na OH-IP)to hydrochloric acid extracted phosphorus(HCl-P)from the sediments,which made the phosphorus in the sediments more stable.