Study On The Adsorption Characteristics And Mechanism Of Zirconium Modified Bentonite For Phosphate In Water

Nowadays,the eutrophication of water body in China is becoming more and more serious,and the treatment of eutrophic water is imminently.In the cause of eutrophication,the high phosphorus concentration is a main factor,and the source of phosphorus in water is mainly including exogenous input and endogenous release.Now,government has strictly managed the exogenous phosphorus emissions.So,it is very important to adsorb the phosphorus in the upper layer of the surface and to inhibit the release of the endogenous phosphorus from the sediment.Adsorption is a very effective method to reduce phosphate in the overlying water as well as to control the release of P from sediment,and how to find suitable and efficient adsorbent material is the key factor for the successful application of adsorption method.In recent years,ZrO₂ has been used in the preparation of phosphate adsorbent in water,and compared with other oxides,it has the characteristics of non-toxicity,stable property,low solubility and high binding ability with phosphate in water.But the pure ZrO₂ is too expensive to be the adsorbents for phosphate.Therefore,the selection of porous clay minerals,such as bentonite,which is abundant,inexpensive and harmless to the environment,is used to load ZrO₂,and made the Phosphate adsorbent--ZrBTs.it would reduce the material's cost of preparation without affecting the adsorption properties of the material,thereby increasing the feasibility of the material being used in actual water bodies.In this study,bentonite was used as the clay loading ZrO₂ and ZrBTs was prepared.Then,ZrBTs was studied in the adsorption capacity and adsorption mechanism of phosphate in water,so as to provide theoretical basis and technical support for the application of zirconium modified bentonite to the actual water body.Firstly,zirconium-modified bentonites?ZrBTs?with different zirconium loading levels were prepared,and the adsorption behaviors of phosphate on these ZrBTs were comparatively investigated using batch experiments.Results showed that the kinetic process of phosphate on ZrBTs well followed the pseudo-second-second kinetic model.The kinetic process was divided into three stages,including a rapid external surface adsorption stage,a gradual adsorption where both the intra-particle diffusion and film diffusion were rate-controlled,and a final equilibrium adsorption stage.The equilibrium adsorption data of phosphate on ZrBTs could be well described by the Langmuir,Freundlich,Sips and Dubinin-Radushkevich isotherm models.Phosphate adsorption onto ZrBTs was more favorable under strongly acidic condition than that under weakly acidic or neutral condition,while phosphate adsorption onto ZrBTs under weakly acidic or neutral condition was more favorable than that under alkaline condition.Coexisting Na⁺and K⁺slightly enhanced phosphate adsorption onto ZrBTs,while coexisting Ca²⁺greatly enhanced the phosphate adsorption.The presence of HCO₃^-or SO₄^2-inhibited the adsorption of phosphate on ZrBTs.The mechanism for phosphate adsorption onto ZrBTs followed the ligand exchange and inner-sphere complexing mechanism.The phosphate adsorption capacity for ZrBT increased with increasing its loading level of zirconium,while the amount of phosphate adsorbed on unit mass of ZrO₂ in ZrBT decreased with increasing the loading amount of zirconium in ZrBT.When the loading amount of ZrO₂ in ZrBT increased from 3.61%to 13.15%,the maximum phosphate adsorption capacity?MPAC?for ZrBT increased from 3.83 to 9.03 mg P/g,but a further increase in the ZrO₂ loading amount to 19.63%resulted in a slight increase of MPAC to9.66 mg P/g.However,an increase in the loading amount of ZrO₂ in ZrBT increased from 3.61%to 19.63%caused a decrease of the MPAC for the ZrO₂ located in ZrBT from 106 to 49.2 mg P/g?ZrO₂?.Considering both cost and adsorption capacity of adsorbent,the ZrBT with 13.15%of zirconium loading amount could be more suitably used as an adsorbent to remove phosphate from aqueous solution than the other ZrBTs.Secondly,to understand the influence of the pre-treatment of bentonite with Na⁺and Ca²⁺on the adsorption of phospahte on zirconium-modified bentonite?ZrBT?,three kinds of ZrBTs including zirconium-modified raw,Na⁺-pretreated and Ca²⁺-pretreated bentonites were synthesized and characterized firstly,and afterward their adsorption performance and mechanism of phosphate from aquous solution were studied comparatively.The pre-treatment of bentonite with Na⁺increased the amount of exchangeable Na⁺in ZrBT but decrease the amount of exchangeable Ca²⁺in ZrBT,while the pre-treatment of bentonite with Ca²⁺increased the amount of exchangeable Ca²⁺in ZrBT but decreased the amount of exchangeable Na⁺in ZrBT.The pre-treatment of bentonite with Na⁺decreased the specific surface area of ZrBT,but that with Ca²⁺did not result in the change of the specific surface area.The phosphate adsorption ability for ZrBT decreased after the pre-treatment of bentonite with Na⁺,but it increased after the pre-treatment of bentonite with Ca²⁺.The maximum phosphate adsorption capacity calculated from the Langmuir isotherm model for zirconium-modified Ca²⁺-pretreated bentonites?13.4 mg P/g?was much higher than that for the zirconium-modified raw bentonite?9.06 mg P/g?.The pre-treatment of bentonite with Na⁺and Ca²⁺did not change the interaction type between ZrBT and phosphate,i.e.,the coordination of phosphate to zirconium.The decreased phosphate adsorption capacity for ZrBT induced by the Na⁺pre-treatment could be mainly attributed to the decrease of the specific surface area and the content of exchangeable Ca²⁺.The increased phosphate adsorption capacity for ZrBT induced by the Ca²⁺pre-treatment could be mainly due to the increase in the amount of exchangeable Ca²⁺.Resutls of this work suggest that the zirconium-modified Ca²⁺pre-treated bentonite is more suitably used as an adsorbent for the removal of phosphate from wastewater than the zirconium-modified raw and Na⁺pre-treated bentonites.Then,the study prepare the zirconium-modified Ca²⁺per-treatment bentonites?ZrCaBTs?with different zirconium loading levels.Results showed that the equilibrium adsorption data of phosphate on ZrBTs could be well described by the Langmuir,Freundlich and Dubinin-Radushkevich isotherm models.the kinetic process of phosphate on ZrCaBTs well followed the pseudo-second-second kinetic model.The kinetic process was divided into three stages,including a rapid external surface adsorption stage,a gradual adsorption where both the intra-particle diffusion and film diffusion were rate-controlled.Phosphate adsorption onto ZrBTs was more favorable under acidic condition than that under neutral condition or under alkaline condition.Coexisting Na⁺and K⁺slightly enhanced phosphate adsorption onto ZrCaBTs,while coexisting Mg²⁺and Ca²⁺would greatly enhanced the phosphate adsorption.The presence of HCO₃^-,SO₄^2-orNO₃^-inhibited the adsorption of phosphate on ZrBTs.The mechanism for phosphate adsorption onto ZrBTs followed the ligand exchange and inner-sphere complexing mechanism.Finally,the study made the magnetic zirconium-modified Ca²⁺per-treatment bentonites?ZrFeCaBT?,in compare with magnetic zirconium-modified bentonites?ZrFeBT?.The result shows that the per-treatment of CaCl₂ would strongly enhance the adsorption capacity of ZrBTs.Whatsmore the mechanism for phosphate adsorption onto ZrBTs followed the ligand exchange and inner-sphere complexing mechanism.besides,Phosphate adsorption onto ZrBTs was more favorable under acidic condition than that under neutral condition or under alkaline condition.In summary,zirconium-modified bentonites would be an efficient phosphate adsorbent used in water.and it has a good application prospect.