Study On The Adsorption Of Phosphate From Aqueous Solution And The Control Of Phosphorus Release From Sediments By Using Zirconium-based Inactivation Agents

Eutrophication has become one of the most severe threat to the health of aquatic ecosystems in freshwater bodies.Phosphorus?P?is a vital element to freshwater aquatic ecosystems,and excessive P concentration of overlying waters can cause eutrophication of freshwater bodies.The main sources of P in overlying waters include external loading and internal loading.In order to restore the eutrophic freshwater bodies,it is essential for the reduction of the external P loading.However,after a reduction in P external loading,the restoration of eutrophic freshwater bodies is usually delayed by the internal P loading,i.e.,the release of P from sediments into overlying waters.Therefore,in order to control eutrophication,it is recommended to apply in-situ P immobilization technique besides reducing P external loading.One of the key points of successful application of in-situ P immobilization technique is to select the suitable inactivation agents.Zirconium oxide is a widely used inorganic material,and it is non-toxic,stable physical and chemical properties,low water solubility.Zirconium oxide is found to have a strong ability in adsorbtion of phosphate in water and wastewater.Porous minerals such as kaolin and zeolite,have a large surface area,high cation exchange capacity,stable physical and chemical properties,low cost,extensive sources and have no pollution to the environment.It is hope to form a composite with zirconium oxide.Zirconium modified kaolin,zirconium modified zeolite will be good adsorption,having low cost solid phosphorus adsorption materials.In this study,microcosm incubation experiments were conducted to investigate the effect of zirconium modified kaolin?ZrMK?-based cap on the migration and transformation of phosphorus?P?between sediments collected from a heavily polluted river and overlying waters under anaerobic conditions.The results showed that a large amount of P was released from the sediments into the overlying water column under anaerobic conditions,and the overwhelming majority of P in the overlying water existed in the form of phosphate.The flux of P from the anaerobic sediments was slightly reduced by the kaolin-based cap,but was significantly reduced by the ZrMK-based cap.Sequential extraction of P from the kaolin-based cap at the end of incubation experiments suggested that 29% of P adsorbed by kaolin existed as the bicarbonate-dithionite extracted P?BD-P?,and 63% of adsorbed P existed as the residual P?Res-P?.Sequential extraction of P from the ZrMK-based cap at the end of incubation experiments suggested that 90% of P adsorbed by ZrMK existed as the NaOH extractable P?Na OH-P?and Res-P,which were unlikely to be released under anaerobic conditions.Compared with no capping,sediments capping with ZrMK not only did not promote BD-P release from the sediments under anaerobic conditions,but also promoted the formation of NaOH-P in the sediments.X-ray photoelectron spectroscopy?XPS?and solid state 31 P nuclear magnetic resonance?NMR?analysises of ZrMK-based caps before and after sediment incubation experiments indicated that the adsorption of P by the ZrMK-based caps followed the ligand exchange and inner-sphere complexing mechanism.Results of this work indicate that ZrMK is a promising active cappingA zirconium-modified zeolite?ZrMZ?was prepared and then the humic?HA?was immobilized on the ZrMZ surface to prepare HA-loaded ZrMZ?HA-ZrMZ?.The obtained ZrMZ and HA-ZrMZ were characterized by EDX,elemental analyzer,N2 adsorption/desorption isotherms,pHPZC and XPS.The adsorption characteristics of phosphate on ZrMZ and HA-ZrMZ were comparatively investigated in batch mode.The adsorption mechanism of phosphate on ZrMZ and HA-ZrMZ was investigated by ionic strength effect and 31 P NMR.The mechanism for phosphate adsorption onto ZrMZ was the formation of inner-sphere phosphate complexes at the solid/solution interface.The preloading of HA on Zr MZ reduced the phosphate adsorption capacity,and the more the HA loading amount,the lower the phosphate adsorption capacity.However,the preloading of HA on ZrMZ did not change the phosphate adsorption mechanism,i.e.,the formation of inner-sphere phosphate surface complexes was still responsible for the adsorption of phosphate on HA-ZrMZ.The decreased phosphate adsorption capacity for ZrMZ after HA coating could be attributed to the fact that the coating of HA on ZrMZ reduced the amount of binding active sites available for phosphate adsorption,changed the adsorbent surface charges,as well as reduced the specific surface areas and pore volumes of ZrMZ.A zirconium-modified zeolite?ZrMZ?was prepared and then inorganic salt?NaCl and CaCl2?was immobilized on the ZrMZ surface to prepare inorganic salt-modified zeolite,NaCl-modified zeolite?Na-ZrMZ?and CaCl2-modified zeolite?Ca-ZrMZ?.Batch incubation experiments were conducted to investigate the effect of inorganic salt modification on phosphate adsorption on ZrMZ whether in the presence of ammonium or not.The results showed that In the presence of ammonium,Na+ and Ca²⁺ could be released from the raw,NaCl-treated and CaCl2-treated ZrMZs through Na+/NH4+ and Ca²⁺/NH4+ exchange mechanisms.Coexisting Na+ and Ca²⁺ both enhanced the adsorption of phosphate on ZrMZ,resulting in that the presence of ammonium enhanced the adsorption of phosphate on the raw,NaCl-treated and CaCl2-treated ZrMZs.The treatment of ZrMZ with CaCl2 solution increased the phosphate adsorption,while the treatment of ZrMZ with NaCl solution decreased the phosphate adsorption whether in the presence of ammonium or not.The ZrMZ with more Ca²⁺ and less Na+ had a higher adsorption capacity for phosphate ions in the presence of a certain amount of ammonium ions.The ligand exchange of hydroxyl groups by phosphate was the main mechanism for phosphate adsorption onto ZrMZ?Na-ZrMZ and Ca-ZrMZ.In this study,microcosm incubation experiments were conducted to investigate the effect of zirconium modified zeolite?ZrMZ?-based cap on the migration and transformation of phosphorus?P?between sediments collected from a heavily polluted river and overlying waters under anaerobic conditions.The results showed that,the efficiency of sediment capping with an active barrier using ZrMZ to control phosphorus from river sediments was better than original zeolite.The flux of P from the anaerobic sediments was significantly reduced by ZrMZ.Besides,ZrMZ-based cap has a long time of maintaining effective adsorption.The adsorption capacity of ZrMZ on phosphate mainly depends on zirconium oxides.Sequential extraction of P from the ZrMZ-based cap at the end of incubation experiments suggested that 91.37% of P dsorbed by ZrMZ existed as the NaOH extractable P?NaOH-P?and Res-P,which were unlikely to be released under anaerobic conditions.Compared with no capping,sediments capping with ZrMZ not only did not promote BD-P release from the sediments under anaerobic conditions,but also promoted the formation of NaOH-P in the sediments.Results of this work indicate that ZrMZ is a promising zirconium-based inactivation agent for controlling P release from sediments in heavily polluted rivers.The above conclusions revealed that zirconium-based inactivation agents are expected to be effective in controlling the release of phosphorus from sediments.