Preparation And Interfacial Electrochemical Properties Of Mg-Al-Ti LDHs And Their Adsorption For Phosphates

Layered double hydroxides(LDHs)are a kind of inorganic layered materials with positive structural charges.They have attracted much attention because of their special structure and wide potential applications.Previous studies mainly focused on the LDHs formed by divalent and trivalent metal cations(MⅡ-MⅢ),and rarely involved those containing tetravalent metal ions(MⅣ,such as Ti4+).In this work,the preparation,interface electrochemical properties,and phosphate adsorption behavior of Mg-Al-Ti LDHs were investigated,and the "general" surface-charging(1-pK and 2-pK)models,"the point of zero net charge model",and the "surface component activity"(SCA)model previously proposed by our group were verified.It is expected to deepen the understanding of the properties of Ti4+-containing LDHs and provide a basis for its green preparation and application in sewage treatment.Main research contents and conclusions:(1)Preparation and characterization of Mg-Al-Ti LDHsMg-Al-Ti LDHs were prepared by hydrothermal method,coprecipitation method,and mechano-hydrothermal method with constant Mg2+/(Al3++Ti4+)molar ratio of 3.00 and varying Ti4+/(Al3+/Ti4+)molar ratio(RTi).The influence of preparation conditions on the formation and properties of LDHs was investigated.Mg-Al-Ti LDHs can be prepared by hydrothermal method(120 ℃ and 24 h)in the RTi range of 0-0.4(here R Ti=0 corresponding to Mg-Al LDHs),but the samples thus obtained are not pure and contain impurities such as MgCO3 and TiO2.Their particles are relatively regular hexagonal flakes.The coprecipitation and mechano-hydrothermal methods both can produce pure Mg-Al-Ti LDHs in the RTi range of 0-0.4,with irregular flake.No pure samples can be obtained when RTi>0.4.With an increase in RTi,the cell parameters(a,b,and c)and d-spacing(ds)of the LDHs all increase,while their lateral size and crystallinity decreases;the specific surface area(As,BET)of the samples obtained by the hydrothermal and mechano-hydrothermal methods increase,while those by the coprecipitation decrease.The As,BET of samples prepared by the mechanohydrothermal method(107.7-161.0 m2/g)is obviously higher than those by the hydrothermal method(21.3-53.1 m2/g)and the coprecipitation method(31.1-58.1 m2/g).Compared with the hydrothermal method,the mechano-hydrothermal method can obviously reduce the hydrothermal temperature and time,and compared with the coprecipitation method,it can obviously reduce the amount of wastewater,so it has a practical potential.(2)Interface electrochemical properties of Mg-Al-Ti LDHsTaking the samples prepared by the coprecipitation method as the model,the effective structural charge density(σst,eff),point of zero net charge(pHPZNC),point of zero net proton charge(pHPZNPC),and isoelectric point(pHIEP)of Mg-Al-Ti LDHs were determined by potentiometric titration,salt titration,potentiometric mass titration,and micro-electrophoresis.Based on the general 1-pK and 2-pK models,the intrinsic equilibrium constants of the surface hydroxyl acid-base reactions,including pK,pKa1int,and pKa2int,were estimated.Special emphasis was placed on the effect of RTi on the interface electrochemical properties of LDHs.With the increase of RTi,the parameters σst,eff,pHPZNC,pHPZNPC,pK,pKa1int,and pKa2int of the LDHs increase,indicating that the deprotonation tendency of surface hydroxyl groups decreases.The pHPZNPC of each LDHs sample is lower than its pHPZNC,and increases with the increase of electrolyte(NaNO3)concentration,arising from the effect of positive structural charges.In addition,the potential titration results and the experimental pHPZNC values are consistent with the predictions of the general surface-charging models and point of zero net charge model,which provides an example for the universality of the models.(3)Adsorption behavior of phosphate ions on Mg-Al-Ti LDHsThe adsorption of phosphates on Mg-Al-CO3 and Mg-Al-Ti-CO3(RTi=0.2)LDHs samples(MH0 and MH0.2)synthesized by the mechano-hydrothermal method was investigated,and the adsorption mechanism was discussed.The adsorption kinetics and isotherms of MH0 and MH0.2 for phosphates can be described using the pseudo-second-order model and the Langmuir model,respectively.With the increase of pH from 6 to 12,the adsorption capacity decreases,but the adsorption can obviously occur at pH>pHPZNC.In particular,an obvious "solid-effect" was observed for the adsorption,namely,the saturated adsorption capacity(Γm)decreases with the increase of adsorbent dosage.The solid-effect data can be well described using the SCA model,also confirming the universality of the model.The Γm(mmol-P/g)of MH0.2 for phosphates is larger than that of MH0,indicating the partial replacement of Ti4+for Al3+can enhance the adsorption capacity of LDHs.The "apparent"affinity of MHo for phosphates is stronger than MH0.2,arising from the formation of the AIPO4 phase between MHo and phosphates.The phosphates are adsorbed on the exterior surfaces of LDHs,and no obvious anion exchange(or intercalation)adsorption occurs.The adsorption driving forces are mainly the electrostatic interaction,the van der Waals force,and the hydrogen-bonding interaction.