Preparation Of Zirconium-based Active Materials And Its Use In Regulating Transport And Transformation Of Phosphorus In Sediments

Eutrophication is a worldwide water pollution problem.The main cause of eutrophication is the accumulation of excess nutrient elements in water.Phosphorus?P?is one of the main nutrient elements in water eutrophication.Controlling and removing excessive phosphorus in water is the key to control water eutrophication.Adsorption method has recently been considered as a fast and efficient method to capture excess phosphate in water.The key technology of adsorption method is to find adsorbent materials with remarkable adsorption effect and good for environment.Zirconium oxide/hydroxide is a kind of rare metal which is environmentally friendly and has very high affinity for phosphate.Its adsorption capacity for phosphate in water is much higher than that of general adsorbents.Its adsorption speed is very fast,at the same time it is not susceptible to environmental changes.However,the price of pure Zirconium oxide/hydroxide is relatively high.If it is directly used to adsorb phosphate in natural water,the overall investment is large to be popularized in large areas to control eutrophication of lake water and Black and odorous waters.Therefore,it is necessary for the preparation of zirconium-based active materials by loading zirconium oxide onto the natural pore materials such as zeolite and bentonite.Firstly,the effect of zirconium-modified zeolite?ZrMZ?addition on the release and immobilization of phosphorus in heavily polluted river sediment was investigated using sediment amendment technology through microcosm incubation experiments.Results showed that the addition of ZrMZ into sediment greatly reduced the concentrations of P in pore water and overlying water,and it also reduced the releasing flux of P across the interface between the overlying water and sediment.The addition of ZrMZ into sediment resulted in the transformation of NH₄Cl extractable P?NH₄Cl-P?,Na₂S₂O₄/NaHCO₃ extractable P?BD-P?and HCl extractable P?HCl-P?into NaOH extractable P?NaOH-P?and residual P?Res-P?in sediment,thereby leading to the reduction of mobile P?sum of NH₄Cl-P and BD-P?in sediment.The contents of bioavailable P?BAP?including water soluble P?WSP?,readily desorbable P?RDP?,iron oxide paper strip extractable P?FeO-P?and anion resin extractable P?Resin-P?in sediment also were reduced by the addition of ZrMZ.The amount of sediment mobile P reduced by ZrMZ(?Q_mobile-P)could be described by the equation:W=0.124?Q_mobile-P+1.29?where W was the percentage of ZrMZ to sediment,%?,while the amount of sediment FeO-P reduced by ZrMZ could be described by the equation:W=0.228?Q_FeO-P-8.88.These two equations could be used to determine the amount of ZrMZ addition required to immobilize mobile P and BAP in sediments.The control of P release from sediment by ZrMZ could be due to the reduction of P in the pore water and the immobilization of P in sediment.Results of this work indicate that ZrMZ is a very promising amendment for controlling P release from sediments in heavily polluted rivers.Secondly,the effects of zirconium modified zeolite?ZrMZ?addition on phosphorus release from sediments of different heights in river surface sediments were further investigated by sediment amendment technology.There is generally a significant heterogeneity in the vertical distribution of mobile phosphorus?P?in sediments,but the previous studies concerning the effect of zirconium-modified zeolite?ZrMZ?addition on the mobilization of P in sediments neglected this feature.In this study,microcosm experiments were conducted to investigate the effect of ZrMZ addition on the mobilization of P in river surface sediments at different depths.A high-resolution diffusive gradients in thin films technology?DGT?was used to measure the concentration of labile P in the overlying water-sediment profiles at a submillimeter vertical resolution.Results showed that the ZrMZ amendment not only could reduce the concentration of soluble reactive P?SRP?in the overlying water,but also could decrease the concentrations of SRP in the pore water at different depths.Furthermore,the ZrMZ amendment resulted in the reduction of both the releasing flux of SRP from sediments to the overlying water and the diffusion flux of SRP from the pore water to the overlying water.After the addition of ZrMZ into the top sediment,the static layer with low DGT-liable P?DGT-P?concentration was observed in the upper sediment.The addition of ZrMZ into the upper sediment resulted in the reduction of mobile P?P_m?in the upper and lower sediments via the transformation of Pm to more stable NaOH-extractable P?NaOH-rP?and residual P?Res-P?.In addition,the contents of bioavailable P?BAP?including water-soluble P?WSP?,readily desorbable P?RDP?and iron oxide paper extractable P?FeO-P?in the upper sediment were greatly reduced by the ZrMZ addition.Results of this study show that the immobilization of pore water SRP,DGT-P,sediment Pm and sediment BAP by ZrMZ played a very important role in the control of P release from sediments to the overlying water by the ZrMZ amendment.Then,a zirconium/magnesium-modified bentonite?ZrMgBT?was prepared and characterized by SEM,EDS,XRD and pH_PZC.The performance and mechanism of phosphate adsorption onto ZrMgBT was evaluated in detail using batch experiments and ³¹P NMR.Results showed the adsorption isotherm data were well described by the Langmuir,Freundlich and Dubinin-Radushkevich models,and the kinetic data fitted better to the pseudo-second-order kinetic model than the pseudo-first-order kinetic model.The phosphate adsorption capacity of ZrMgBT was slightly affected by the presence of Na⁺,K⁺,Cl^-,SO₄^2-and NO^3-,but it was enhanced by coexisting Mg²⁺and HCO^3-.The mechanism for phosphate adsorption onto ZrMgBT at pH 7 was mainly the complexation reaction between phosphate and zirconium.In addition,ZrMgBT exhibited more excellent adherence to phosphate than zirconium-modified bentonite?ZrBT?.Especially,the maximum monolayer phosphate adsorption capacity for ZrMgBT at pH 7 and 0.5 g/L of adsorbent dosage calculated based on the Langmuir isotherm model?13.0 mg P/g?was 67.5%higher than that for ZrBT.The higher phosphate adsorption capacity for ZrMgBT than ZrBT could be attributed to the higher specific surface area as well as higher Mg²⁺releasing ability of the former.The enhancement of phosphate adsorption by the release of Mg²⁺from ZrMgBT could be mainly due to the formation of MgHPO₄⁰ in the solution firstly and then the adsorption of MgHPO₄⁰ on ZrMgBT forming?Zr?OPO₃H?Mg on the ZrMgBT surface.In general,we conclude that ZrMgBT is a more promising adsorbent for phosphate removal from aqueous solution than ZrBT.Last,Three different types of zirconium-modified bentonites?ZrMBs?including zirconium-modified original bentonite?ZrMOB?,zirconium-modified magnesium-pretreated bentonite?ZrMMgB?and zirconium-modified calcium-pretreated bentonite?ZrMCaB?were synthesized and used as active covering materials to suppress the release of phosphorus?P?from sediments.To assess the covering efficiency of ZrMBs to inhibit P release from sediments,we examined the impact of ZrMBs covering layer on P mobilization in sediments at different depths as well as the release of P through the interface between sediment and overlying water?SWI?by use of simulating P release control experiments and diffusive gradients in thin films?DGT?technology.The results showed that the amount of soluble reactive P?SRP?in the overlying water greatly decreased after covering with ZrMBs.Moreover,both pore water SRP and DGT-liable P?DGT-P?in the top sediments decreased after capping with ZrMBs.An obvious stratification of DGT-P was observed along the vertical direction after covering with ZrMBs,and static and active layers were found in the top sediment and in the lower sediment directly below the static layer,respectively.Furthermore,ZrMBs covering led to the change of P species from easily released P to relatively or very stable P,making P in the top sediment more stable compared to that without ZrMBs covering.Besides,overwhelming majority of P immobilized by ZrMBs is hard to be re-released into the water column in common environment.Overall,the above results demonstrate that sediment covering with ZrMBs could effectively prevent the transport of SRP from sediments into the overlying water through the SWI,and the control of P transport into the overlying water by ZrMBs covering could be mostly due to the immobilization of pore water SRP,DGT-P and mobile P in the top sediment by ZrMBs.Generally,zirconium modified zeolite and zirconium modified bentonite are two zirconium-based active materials suitable for sediment amendment and covering materials to control the release of phosphorus in water.