The Study Of Fluoride Removal Performance And PH Buffer Mechanism Of Modified Activated Alumina

Fluoride pollution has been regarded as one of the serious pollution in the world.Among the various defluorination methods,adsorption and defluorination of activated alumina has been widely researched with the advantages of low cost and wide source,however,the low adsorption capacity hindered the large-scale application.Therefore,the modification of activated alumina remains one of the hot research areas in recent years.However,the effect of modification can only be analyzed through a large number of adsorption tests.In this work,based on the surface chemical properties,we developed a new method of exploring the adoption performance and pH buffer intensity,and evaluated the effectiveness of the modification according to pH buffer intensity for the first time.We synthesized the modified adsorbent via impregnating active alumina with metal salt solution(Fe2(SO4)3,Ce(NO3)3).The performance of fluorine removal was studied by batch adsorption experiment,and the pH buffer strength was analysised by acid-base titration experiments.Then,the mechanism of fluoride removal and pH buffer of modified adsorbent were explored on the basis of adsorption isothermal model,kinetic model fitting and characterization experiments of pHzpc,BET and EDS.The pH buffer capacity and intensity were analyzed by acid-base titration experiment,and further confirmed the relationship between adsorption performance and pH buffer intensity by analysis of adsorption data and pH buffering data.The study on unmodified active alumina showed that the adsorption isotherm experiment data fitted well with Langmuir adsorption isotherm model.The maximum adsorption amount was 9.03 mg/g,which mainly occurred by the chemical adsorption of the single molecular layer.The adsorption kinetics was more consistent with the quasi-secondary adsorption kinetics.The zero point of proton charge(pHzpc)was about 8.9,suggesting the surface has acid-base duality and a certain pH buffer capacity.The pH buffer intensity of active alumina was calculated to be 0.127 mmol/g by fitting the data.For the two kinds of modified adsorbents with the concentrations of the modified salt solution were 0.5%,1.0%,1.5%and 2.5%and 3.0%,respectively.As same with active alumina,the adsorption data fitted by isotherm model were more suitable for Langmuir adsorption isotherm model.After the kinetic equation fitting,the adsorption behavior conformed to the quasi-second-order kinetics,which all showed that the adsorption process was mainly chemisorption.The saturated adsorption capacity first increased quickly and then remained unchanged with the increase of the concentration of the modified solution.The optimum concentration of ferric sulfate and cerium nitrate modified active alumina were 3.0%and 2.0%,respectively.The maximum adsorption amounts of activated alumina modified by ferric sulfate and cerium nitrate reached 16.90 and 17.58 mg/g,respectively.The pH buffer intensity of the modified ferric sulfate adsorbent(0.5%?3.0%)were 0.225,0.318,0.357,0.374,0.399 and 0.400 mmol/g,respectively.The pH buffer intensity of cerium nitrate modified adsorbent were 0.227,0.324,0.407,0.437,0.439 and 0.433 mmol/g,respectively.With the increase of the modified concentration,the buffer strength of pH increased gradually and then tended to be constant.The pHZpc of the modified sorbent powder decreased gradually with the increase of the concentration of the modifying solution,attributing to the increased number of metal ions loaded on the surface.EDS analysis showed that the metal salts were successfully loaded on the surface of active alumina.The adsorption capacity and pH buffering ability of modified adsorbents all enhanced owing to the increasing presence of metal ions on the surface of the modified adsorbent and the increased number of acidic sites of the adsorbent,and the ion exchange ability of the adsorbent further enhanced with the exist of anions,which lead to a large adsorbed ability of F-and OH-.The data analysis found that the pH buffer intensity was positively correlated with the adsorption performance,indicating that one can predict the modification from the angle of pH buffer,then to determine the quality of the adsorption.