Efficiency And Mechanism Of Combined Application Of Hydrous Zirconium Oxide With Calcium-based Materials To Control Phosphorus Release From Sediments

Anthropogenic eutrophication,featured by harmful cyanobacterial blooms,can cause ecosystem degradation in aquatic ecosystems,threatening water security.Eutrophication is a global environmental problem.Phosphorus（P）is regarded as an important nutrient element that causes freshwater eutrophication.Limiting P input into freshwater bodies is an important measure to solve eutrophication problem.The external P loading from frequent human activities are effectively reduced,the liberation of P from the historical P accumulated sediment still can conduce to the elevated concentration of P in the water column.Therefore,controlling phosphorus release from sediments is an important measure to eradicate eutrophication in freshwater bodies.Several methods have been developed to manage internal P loading.These methods include sediment dredging,artificial re-oxygenation,Fe or Al salt treatment,inert material covering,sediment microbial fuel cells and in situ immobilization by dosing P-inactivating materials（also known as active capping/amendment）.Among them,in situ immobilization by dosing P-inactivating materials（PIMs）,which involves the covering of sediment with PIMs to form a covering layer onto the sediment-water interface（SWI）or the amendment of sediment with PIMs,is known as a promising method for sediment P immobilization and release control.Among them,zirconium-based PIM（Zr-PIM）attracts considerable interests,because zirconium oxide（active component of Zr-PIM）not only possesses excellent adsorption property to aqueous phosphate,but also has the characteristics of wide distribution,abundance in nature,chemical and biological stability,low water solubility,non-toxicity and environmentally friendliness.It should be noted that many natural mineral materials such as calcite,bentonite and zeolite can be used as capping materials for contaminated sediment remediation.Considering that zirconium oxide possesses high phosphate adsorption capacity,the combined utilization of calcium based natural mineral materials（e.g.,calcite,bentonite and zeolite）and Zr-PIM as a composite capping material may have application potential in cutting down the risk of sediment P liberation.Coexisting Ca²⁺can enhance phosphate(PO₄^3--P)adsorption onto hydrous zirconium oxide（HZr O₂）via the formation of phosphate-bridged ternary complex（≡Zr-P-Ca）.If Ca²⁺is released from natural mineral materials,the presence of these natural mineral materials may affect the effectiveness and mechanism of Zr-PIA to intercept P release from sediment.Thus,it is of vital importance to investigate the efficiency and mechanism for the combined use of natural materials and Zr-PIA to prevent sedimentary P release before their practical application.However,few studies have been investigated on the effectiveness and mechanism for sedimentary P release interception by Zr-PIM combined with calcite,bentonite or zeolite.Therefore,the aim of this study was to determine the effect of hydrous zirconium oxide（HZr O₂）in combination with calcium-based materials（calcite,bentonite and zeolite）on the control of endogenous phosphorus release from sediments and its mechanism.Firstly,this study investigated the effectiveness and mechanism for the removal efficiency of phosphate by hydrous zirconium oxide（HZr O₂）combined with calcite,bentonite and zeolite.The results suggested that the presence of calcite can promote the adsorptive removal of PO₄^3--P by HZr O₂.The effect of coexisting bentonite on PO₄^3--P adsorption onto HZr O₂ is related to the type of bentonite.The presence of RBT has an inhibitory effect on PO₄^3--P adsorption onto HZr O₂,while the presence of Ca BT has a promoting influence on the PO₄^3--P adsorptive removal by HZr O₂.The presence of zeolite can promote the PO₄^3--P adsorption onto HZr O₂.Phosphate can react with Ca²⁺released from calcite、Ca BT and Ca Z to form the ion pairs Ca HPO₄⁰.HZr O₂ can adsorb the formed ion pairs Ca HPO₄⁰,giving rise to the formation of the phosphate-bridged ternary complexes≡Zr（OPO₃H）Ca⁺.The removal of PO₄^3--P by HZr O₂/calcite mixture is principally attributed to the replacement of Zr-bound hydroxyl group with PO₄^3--P and thereby the formation of Zr-O-P inner-sphere complex,the surface adsorption of PO₄^3--P onto calcite,and the precipitation of PO₄^3--P with Ca²⁺dissolved from calcite.And then,the study investigated the effectiveness and mechanism for the inhibition of internal phosphorus（P）liberation from sediment by hydrous zirconium oxide（HZr O₂）combined with calcite,bentonite and zeolite.The results suggested that the amendment of sediment with HZr O₂/calcite,HZr O₂/Ca BT or HZr O₂/Ca Z mixture calciten effectively prevent the sedimentary P release,which was mainly ascribed to the immobilization of mobile P in the sediment and the uptake of DRP from the interstitial water by the amendment material.Capping sediment with HZr O₂/calcite,HZr O₂/Ca BT or HZr O₂/Ca Z mixture also calciten effectively intercept sediment P release,and the formation of P static layer attributed to the uptake of interstitial water DRP and DGT（diffusive gradient in thin-films）-unstable P in the upper sediment by the capping material was a key to the inhibition of sedimentary P migration into the overlying water by the combined capping.The great majority of P immobilized by the HZr O₂/calcite,HZr O₂/Ca BT or HZr O₂/Ca Z combined covering layer is relatively or very stable P and it has a low re-releasing risk under dissolved oxygen-deficit and common p H（5-9）condition.Furthermore,the stability of P bound by the combined covering layer was larger than that by the single HZr O₂ covering layer.HZr O₂,HZr O₂/calcite,and HZr O₂/Ca BT combined capping did not produce a large effect on the OTUs、diversity and richness of bacterial communities in the sediment.The results of this research show that the combined use of HZr O₂ and calcite,HZr O₂and Ca BT,or HZr O₂ and Ca Z as a capping material has great potential in the reduction of sediment P loading.