Adsorption Characteristics And Mechanism Of Phosphate From Aqueous By Hydrous Zirconium Oxide

Phosphorus?P?is generally considered to be a limiting nutrient for the growth of algae,and excessive concentrations of phosphorus present in the overlying water in the form of phosphate can cause eutrophication of surface water bodies such as rivers,lakes,reservoirs,and streams.Therefore,in order to prevent the eutrophication of surface water bodies,it is very important to remove the phosphate from the waste water before discharging the waste water.Different technologies have been developed,including physical,chemical and biological methods,for the removal of phosphate from wastewater.In the existing technology,adsorption method is considered as a very promising method to remove phosphate in wastewater.Because of its high phosphate removal efficiency,high cost efficiency,simple design and convenient operation,phosphate can be recovered from waste water.In recent years,zirconium?Zr?based materials are used as adsorbent to remove phosphate from aqueous solution,which has attracted considerable attention in the field of research.Among these metal oxides,zirconium oxides/hydroxides have been found to be an attractive adsorbent choice for phosphate removal due to their high binding ability for phosphate through formation of inner-sphere complexes via Lewis acid-base interactions,easy regeneration with alkaline solution,non-toxicity,environmental friendliness,low solubility in water and good resistance against attacks by oxidant agents and acid/base.It should be pointed out that there are two types of zirconium oxide?ZrO₂?.One is the amorphous hydrous ZrO₂,and the other is the crystalline ZrO₂.Compared to the crystalline ZrO₂,the amorphous hydrous ZrO₂generally has a higher amount of surface hydroxyl group and thus it is thought to be more effective for phosphate removal from aqueous solution.Hydrated zirconia can usually be prepared by solution precipitation.The surface properties of the hydrated zirconia prepared by different precipitation pH values may be different,so the adsorption characteristics of phosphate in water may be different.In addition,Ca²⁺and Mg²⁺and other cations are usually contained in the wastewater,which may affect the adsorption and removal of phosphate by hydrated zirconia,and the effects of Ca²⁺and Mg²⁺ions coexisting in water on the phosphorus absorption of hydrated zirconia prepared under different precipitation pH conditions can be different.However,there are few reports on this aspect at home and abroad.In addition,the phosphorus absorption capacity of hydrated zirconia prepared at the stage is still not high enough and needs further improvement.Previous studies have found that Mg?OH?₂ has a certain adsorption capacity for phosphate in water,so if Mg?OH?₂ is combined with ZrO₂,it is possible to prepare a desulfurization adsorbent with better adsorption effect.At present,domestic and foreign reports on this aspect are relatively rare.First of all,in this study,hydrous zirconium oxide?HZO?samples precipitated at different pH values were prepared,characterized and used as adsorbents to remove phosphate from aqueous solution.The adsorption characteristics and mechanisms of phosphate on these HZO samples were investigated.Results showed that the presence of Na⁺slightly enhanced the adsorption of phosphate on HZO samples prepared at precipitated pH 4.8 and 8.0,but it greatly enhanced the adsorption of phosphate on HZO prepared at precipitated pH 10.6.The presence of Ca²⁺slightly enhanced the adsorption of phosphate on HZO prepared at precipitated pH 4.8,but it significantly enhanced the adsorption of phosphate on HZO samples prepared at precipitated pH8.0 and 10.6.The presence of HCO₃^-or SO₄^2-inhibited phosphate adsorption onto HZO,and the inhibiting effect of these anions on phosphateadsorption onto HZO precipitated at pH 4.8 was much higher than that on phosphate adsorption onto HZO samples precipitated at pH 8.0 and 10.6.The phosphate adsorption was dependent upon solution pH,and it decreased with increasing solution pH.The Langmuir,Freundlich and Dubinin-Redushckevich?D-R?isotherm models fitted well to the adsorption equilibrium data of phosphate on HZO samples precipitated at pH 4.8,8.0and 10.6.In the presence of Na⁺but in the absence of Ca²⁺,there was no significant difference of the maximum phosphate monolayer adsorption capacity derived from the Langmuir isotherm model among HZO samples prepared at precipitated pH 4.8,8.0 and 10.6.In the presence of Ca²⁺,the maximum phosphate monolayer adsorption capacity derived from the Langmuir isotherm model for HZO precipitated at pH 8.0or 10.6 was much higher than that for HZO precipitated at pH 4.8.The mechanism for phosphateadsorption onto HZO mainly obeyed the inner-sphere complexing mechanism.The surface chloride and hydroxyl groups played the key role in the adsorption of phosphate on HZO precipitated at pH 4.8 or 8.0,while only the surface hydroxyl groups played the key role in the adsorption of phosphate on HZO precipitated at pH 10.6.Results of this work demonstrated that the HZO precipitated at pH 8.0 or 10.6 was a more promising adsorbent for removing phosphate from wastewater than the HZO precipitated at pH 4.8.Second,to understandthe effect of precipitated pH and coexisting Mg²⁺on phosphate adsorption onto zirconium oxide?ZrO₂?,ZrO₂ particles precipitated at pH5.3,7.1 and 10.5,i.e.,ZrO₂?5.3?,ZrO₂?7.1?and ZrO₂?10.5?,respectively were prepared and characterized firstly,and then their adsorption performance and mechanism in the absence and presence of Mg²⁺were comparatively investigated in this study.The results showed that the Elovich,pseudo-second-order and Langmuir isotherm models correlated with the experimental data well.The adsorption mechanism was the complexation between phosphate and zirconium.The presence of Mg²⁺slightly inhibited the adsorption of phosphate on ZrO₂?5.3?including the adsorption capacity and rate,but coexisting Mg²⁺greatly increased the adsorption capacity and rate for ZrO₂?7.1?and ZrO₂?10.5?.The enhanced adsorption of phosphate on ZrO₂?7.1?and ZrO₂?10.5?by the presence of Mg²⁺was mainly due to the formation of MgHPO₄⁰ in the solution and then the adsorption of MgHPO₄⁰ on the adsorbent surface forming phosphate-bridgedternary complexZr?OPO₃H?Mg.In the absence of Mg²⁺,the maximum phosphate monolayer adsorption capacity at pH 7calculated from the Langmuir isotherm model decreased in the order of ZrO₂?7.1??67.3 mg/g?>ZrO₂?5.3??53.6 mg/g??ZrO₂?10.5??53.1 mg/g?,but it followed the order of ZrO₂?7.1??97.0 mg/g?>ZrO₂?10.5??79.7 mg/g?>ZrO₂?5.3??51.3 mg/g?in the presence of Mg²⁺.Results of this work suggest that ZrO₂?7.1?is more suitably used as an adsorbent for the effective removal of phosphate from municipal wastewater than ZrO₂?5.3?and ZrO₂?10.5?because Mg²⁺is generally present in this wastewater.Then,in this work,a certain amount of Mg?OH?₂ was incorporated into ZrO₂ to obtain a novel Mg?OH?₂/ZrO₂ composite?MZ?by a simple co-precipitation method for phosphate adsorption.For comparison purpose,pure ZrO₂ and Mg?OH?₂ were also fabricated.The as-prepared MZ,ZrO₂ and Mg?OH?₂ were characterized by SEM,EDS,XRF and N₂ adsorption/desorption,and their phosphate adsorption properties were comparatively investigated in batch mode.The MZ samples before and after phosphate adsorption were also characterized by XPS and ³¹P NMR.The results showed that MZ was mainly composed of ZrO₂ and Mg?OH?₂,and its Mg/Zr molar ratio was 0.186.In addition,MZ had a larger specific surface area than single-component ZrO₂ and Mg?OH?₂.MZ exhibited a much higher affinity towards aqueous phosphate than pure ZrO₂ and Mg?OH?₂.The Langmuir maximum phosphate adsorption capacity of MZ was 93.0 mg PO₄/g at initial pH 7,which was 74.8%and4.60 times higher than those of single-component ZrO₂ and Mg?OH?₂,respectively.Phosphate was adsorbed onto the MZ surface via a ligand exchange between the ZrO₂phase and phosphate forming inner-sphere phosphate complexes as well as via a chemical reaction between the Mg?OH?₂ phase and phosphate forming MgHPO₄ and Mg₃?PO₄?₂.The enhanced phosphate adsorption by the incorporation of a small quantity ofMg?OH?₂ into ZrO₂ could be attributed to:?i?the increase in the specific surface area of the adsorbent material;and?ii?the enhanced adsorption of phosphate on the ZrO₂ component by the presence of Mg²⁺that was dissolved from the Mg?OH?₂ component.In summary,considering that Ca²⁺and Mg²⁺are usually present in natural water bodies,the hydrated zirconia prepared under the condition of a precipitation pH of 7.1is a promising phosphorus removal sorbent.The new complex Mg?OH?₂/ZrO₂composite prepared by adding a small amount of Mg?OH?₂ to ZrO₂ is more desirable as a sorbent material than pure ZrO₂ to remove the phosphate in natural water bodies.