Study On The Control Efficiency And Mechanism Of Endogenous Phosphorus Release From Sediments Using Iron-based And Zirconiumbased Inactivating Materials

Eutrophication,characterized by the overgrowth of harmful algae,is a serious environmental problem all over the world,especially in the developing country.Phosphorus（P）has been found to be a key nutrient for the growth of harmful algae,and reducing the P concentration in the overlying water is beneficial to control the eutrophication of freshwater systems.Especially when the exogenous input of phosphorus is effectively controlled,the release of endogenous phosphorus in sediment becomes the main source of phosphorus in water,which has a negative impact on the treatment and recovery of eutrophication water.Therefore,it is of great significance to take effective measures to control the release of endogenous phosphorus in sediment for the prevention and mitigation of eutrophication.The in-situ sedimentary-P inactivating technology,that is using phosphorus（P）-adsorbent as P-inactivating material to be covered on the surface of sediment or added into the sediment,has been considered as an effective method to control the release of endogenous phosphorus from sediment,and the premise of the successful application of this method is to develop a low-cost and high-performance P-adsorbent.In recent years,iron oxide and zirconium oxide are both considered to be environmental-friendly and have good affinity for phosphate,which can effectively remove phosphate from water.Moreover,compared with other oxides,iron oxide and zirconium oxide are much safer.However,if iron oxide and zirconium oxide were directly used as in-situ remediation material in the environmental engineering application,the cost will be too high.Natural mineral/clay materials,such as Zeolite,Calcite and Bentonite,are widely obtained from the nature,with low price and no negative impact on the biological environment of water body,so could be used to load iron oxides and zirconium oxide.New iron-based and zirconium-based inactivating materials were prepared through using iron oxide and zirconium oxide modified zeolite,calcite or bentonite.Moreover,iron-based and zirconium-based inactivating materials are considered that have a good control effect on the endogenous phosphorus release from the sediment.However,most of the iron-based and zirconium-based inactivating materials developed with many disadvantages,including complex preparation and high cost,which are not conducive to their batch preparation in engineering.At the same time,it is not clear that the control mechanisms,advantages and disadvantages of using iron-based and zirconium-based inactivating materials to control the release of endogenous phosphorus in the sediment.In addition,the application modes of iron-based and zirconium-based inactivating materials include capping and amendment.However,it is still not clear that the impact of these two application modes on the effect of iron-based and zirconium-based inactivating materials on controlling phosphorus release in sediments.Therefore,it is necessary to study and develop new types of iron-based and zirconium-based inactivating materials,and explore the control effect,mechanism,response pattern of application modes of endogenous phosphorus release from sediments,and compare advantages and disadvantages of iron-based and zirconium-based inactivating materials.First of all,this work synthesized an iron-modified zeolite（IM-Z）by coating iron onto the surface of natural zeolite（NAT-Z）and then the as-obtained IM-Z was utilized as a P-inactivating material to block the upward mobilization of P from sediments to the overlying water.The efficiencies of IM-Z covering and amendment to prevent the liberation of P from sediments were evaluated,and the controlling mechanism was explored.Capping and amendment with IM-Z not only resulted in the tremendous reduction of the levels of soluble reactive P（SRP）in the overlying water,but also led to the decrease of the contents of SRP in the pore water.More importantly,sediment capping and amendment with IM-Z resulted in the formation of a static layer in the upper sediment directly below the sediment-water interface,with very low concentration of SRP in the pore water.In addition,IM-Z capping and addition effectively immobilized the diffusive gradients in thin films（DGT）-labile P in the overlying water and sediment.Furthermore,the decrease of the DGT-labile Fe concentrations in the overlying water as well as the top sediment were also observed after IM-Z capping and addition.Nearly 70%of P bound by IM-Z is stable and difficult to be released back into the overlying water under common p H and anoxic conditions.The immobilization of pore water SRP and DGT-labile P by IM-Z and the uptake of DGT-labile Fe on IM-Z played a significant role in the simultaneous control of SRP liberation.Compared to NAT-Z,the efficiency of IM-Z to block the liberation of sedimentary P was higher.Results of this study demonstrate that IM-Z is suitable for use in the simultaneous interception of the upward transportation of SRP from sediments into the overlying water.Then,in order to improve the control efficiency of sedimentary P using Calcite/zeolite mixture（CZ）,an iron-modified CZ（Fe-CZ）was synthesized,characterized,and employed as a capping material to simultaneously prevent P release from sediments into overlying-water.Batch and microcosm incubation experiments were performed to study the efficiency and mechanism for the control of P release from sediments by capping Fe-CZ.Results showed that sediment capping with Fe-CZ resulted in the significant reduction of soluble reactive P（SRP）in overlying-water,with reduction rates of 77.8～99.7%.Furthermore,the Fe-CZ capping layer decreased the SRP concentration in the pore water at depth of 0–30 mm.Moreover,the Fe-CZ capping layer gave rise to the great decrement of the concentration of the labile P measured by DGT（diffusive gradient in thin films）technology（DGT-labile P）in the profile of overlying-water and sediment.Additionally,the Fe-CZ capping resulted in the reduction of redox-sensitive P（BD-P）in the 0-50 mm sediment and caused the transformation of BD-P to calcium-bound P（HCl-P）and residual P（Res-P）in the 0-10 mm sediment as well as to Res-P in the 10-20 mm sediment.Results of this work indicate that the Fe-CZ capping has a high potential for the simultaneous management of P release from sediments,and the decrease of the contents of sediment DGT-labile P,sediment BD-P,pore-water SRP as well as the conversion of Mobile-P to more stable P in the top sediment should have a significant role in the simultaneous interception of sedimentary P liberation into overlying-water by the Fe-CZ capping.Additionally,this research evaluated the effect of the addition of zirconium-modified zeolite（Zr MZ）on the migration and transformation of phosphorus（P）in river sediments under static and hydrodynamic disturbance conditions was studied using sediment core incubation experiments.Results showed that,whether under static or hydrodynamic disturbance condition,the Zr MZ amendment suppressed the release of SRP from sediments to the overlying water.Furthermore,the addition of Zr MZ into the upper sediment（0-10 mm）not only resulted in the decrease of the soluble reactive P（SRP）concentration in the overlying water at a depth of 0-30 mm,but also led to the decrease of the diffusion flux of SRP from the pore water to the overlying water across the sediment-water interface（SWI）.In addition,the Zr MZ amendment induced the transformation of the redox-sensitive P（BD-P）and HCl extractable P（HCl-P）into the metal oxide-bound P（Na OH-r P）and residual P（Res-P）,thus resulting in the reduction of Mobile-P（sum of NH₄Cl extractable P and BD-P）in the top 10 mm sediment.In addition,the addition of Zr MZ into the top 10 mm sediment resulted in the reduction of the content of mobile P in the 10-20 mm sediment.Furthermore,the effect of Zr MZ addition on the migration and transformation of P in sediments under hydrodynamic disturbance condition had a certain difference to that under static condition.The presence of hydrodynamic disturbance enhanced the immobilization efficiency of SRP in the pore water at a depth of 0-20 mm by the Zr MZ amendment,and also increased the reduction efficiency of the SRP diffusion flux from the pore water to the overlying water across the SWI by the Zr MZ amendment.However,the efficiency of the control of SRP release from sediments to the overlying water by the Zr MZ amendment was slightly reduced by the hydrodynamic disturbance.The reductions of mobile P in the top sediment,SRP in the pore water as well as the diffusion flux of SRP from the pore water to the overlying water across the SWI played a key role in the control of SRP release from sediments to the overlying water by the Zr MZ amendment.Results of this work indicate that Zr MZ is a very promising amendment for the control of SRP release from river sediments under static and hydrodynamic disturbance conditions.Moreover,the influence of zirconium-modified bentonite（ZMBT）addition,capping,and addition/capping on the transport and transformation of phosphorus（P）in sediments were comparatively investigated using incubation experiments to determine the effect of ZMBT application mode on the controlling efficiency.Results showed that the release of soluble reactive P（SRP）from sediment to the overlying water was effectively intercepted by all the ZMBT treatments.The inactivation of pore-water SRP,diffusive gradients in thin films-labile P（DGT-labile P）and Mobile P in sediment played a pivotal role in the regulation of SRP liberation from the sediment to the overlying water by ZMBT.An application mode change from capping and addition/capping to addition resulted in a decline of the reduction efficiency of overlying water SRP by the ZMBT treatment to some extent.The variation in the reduction efficiency of pore-water SRP and DGT-labile P in the uppermost sediment were responsible for the change of the reduction efficiency of overlying water SRP by the ZMBT treatment.A change in application mode from capping to addition/capping and addition caused an obvious increase in the immobilization efficiency of pore-water SRP,DGT-labile P and Mobile-P in the lower sediment by the ZMBT treatment.Results of this work indicate that,when the ZMBT capping layer on the top of sediment was completely mixed with the sediment,although the stability of P in the lower sediment obviously increases,the controlling efficiency of SRP liberating from the sediment to the overlying water decreases to some extent.Thus,the repeated addition of ZMBT to form a covering layer on the ZMBT-amended sediment is very necessary for the effective control of sediment-P release to the overlying water.Finally,this study compared and analyzed the differences between iron-based and zirconium-based inactivating materials,including preparation process,preparation cost,phosphorus removal efficiency,and application prospects.The results show that,in terms of the laboratory preparation time and the simplicity of the scheme,iron-modified calcite/zeolite mixture and zirconium-modified bentonite are more conducive to batch preparation in the project,while iron-modified granular zeolite and zirconium-modified zeolite need further optimize its preparation method;in terms of raw material costs required for laboratory preparation,the cost of preparing zirconium-based inactivating materials are higher than that of iron-based inactivating materials,which is about 3 to 7 times the cost of iron-based inactivating materials.The zirconium-based inactivating materials are better than iron-based inactivating in reducing the SRP concentration in the overlying water,and their control effect are better than that of iron-based inactivating materials.Based on the above studies,iron-based and zirconium based inactivating materials are environmental-friendly,efficient and safe,and can effectively control the release of endogenous phosphorus from sediment.The cost performance and acceptability of iron-based inactivating materials are higher than that of iron-based inactivating materials,and the control effect and stability of zirconium based inactivating materials for phosphorus in sediment are better than that of iron-based inactivating materials.