Preparation Of Organic Modified Metakaolin-based Geopolymers And Their Simultaneous Adsorption Properties For Cu(Ⅱ) And Cr(Ⅵ)

With the rapid development of industrialization in the country,pollutions caused by heavy metals has increasingly become one of the key environmental issues for human survival and development.These pollutions are produced during the production and use of many industrial products（such as metal products,batteries,pesticides,etc.）,and generally enter the environment in the form of wastewater.After enrichment and transmission through the food chain（web）,heavy metals,cannot be biodegraded by the natural environment,which exist in the organism and threaten the survival of the organism and human health.There are many treatment methods for heavy metals in wastewater,including membrane separation method,precipitation method and adsorption method.The adsorption method has become the research object of scholars because of its simple operation,abundant material sources and low cost.In this paper,metakaolin is used as a synthetic raw material,a certain proportion of sodium hydroxide,sodium meta-aluminate and sodium silicate are added as activators,and a certain proportion of CTAB is added as an organic modifier to synthesize geopolymers.The effects of different activator concentrations,ratios,solid-liquid ratio and different modifier concentrations on the adsorption properties of geopolymers were explored,and the optimal conditions were selected.Furthermore,explore the simultaneous adsorption process of this product to Cu（Ⅱ）and Cr（Ⅵ） at different p H,reaction time and adsorbent dosage,relevant data were fitted using various adsorption kinetics and adsorption isotherm models,combined with material characterization results to briefly explain the adsorption mechanism,and the following conclusions were obtained:（1）Using metakaolin obtained by heating kaolin as raw material,sodium hydroxide,sodium metaaluminate and sodium silicate as activators were used to prepare geopolymers,and the ratio was optimized,the best activators ratio is n（Na OH）:n（NaAlO2）:n（Na2SiO3）=1:1.3:2.8;at the same time,using CTAB as the organic modifier,the ratio of the weight of metakaolin,the volume of the activator and the mass of the organic modifier is 1.5 g:50 m L:0.5 g.The environmental conditions for simultaneous activation and modification were 30℃and 180 rpm shaking for 24 hours.The cation adsorption performance of the geopolymer（CTAB-GP）generated after activation and modification is slightly lower than that of the original geopolymer,at the same time,the anion adsorption performance that the original geopolymer does not have is obtained.（2）The static simultaneous adsorption of Cu（Ⅱ） and Cr（Ⅵ） by CTAB-GP was studied.The optimal p H for simultaneous adsorption of Cu（Ⅱ）/Cr（Ⅵ） is 5.The adsorption of geopolymers to Cu（Ⅱ）/Cr（Ⅵ） has gone through two processes:In the early stage of adsorption（first 60 min）,the speed of heavy metal adsorption increases almost linearly with time;2.In the later stage of adsorption（60～1440min）,the growth of adsorption capacity tends to be flat to remain unchanged.In a short time of 0-5 min,the adsorption capacity of Cu（Ⅱ）/Cr（Ⅵ） by CTAB-GP first decreased briefly and then gradually increased.（3）In Cu（Ⅱ）/Cr（Ⅵ） binary system,the adsorption of Cu（Ⅱ） is slightly reduced by the influence of interfering ions,and the adsorption of Cr（Ⅵ） is promoted by the electrostatic shielding caused by the unadsorbed Cu（Ⅱ） in the solution.With the increase of the dosage of adsorbent,the removal rate of Cu（Ⅱ） by CTAB-GP showed a rapid increase trend,and it was almost completely adsorbed by dosage 1g/L.The removal rate of Cr（Ⅵ） first increased to 25.6%（dosage 1g/L）,then the removal rate decreased and stabilized at about 14%.The removal efficiency of Cr（Ⅵ） is first increased and then decreased:since Cu（Ⅱ） is wholly adsorbed,there is no Cu（Ⅱ） to promote the adsorption of Cr（Ⅵ） in solution;as interfering ions,SO₄^2-exist in a binary system;the p H of the solution rises during adsorption.（4）The Langmuir maximum adsorption capacity of CTAB-GP for Cu（Ⅱ） and Cr（Ⅵ）was 108.2 mg/g and 95.3 mg/g in the binary system,respectively.The adsorption of CTAB-GP is mainly electrostatic and ion exchange adsorption between the adsorbate molecules and the adsorbent.Compared with other literatures,it has a higher adsorption capacity and the advantage of simultaneous adsorption.XRD,FTIR,SEM and BET characterization analysis results show that the prepared product after organic modification still belongs to geopolymer,which is different from conventional geopolymers in that quaternary ammonium salt cations are attached to the surface.The simultaneous adsorption of CTAB-GP for heavy metal anions/cations is coexistence of physical adsorption and chemical adsorption.During the adsorption process,Cu（Ⅱ） exchanges with Na⁺that is in the CTAB-GP tetrahedral structure,and Na⁺is released into the solution;Cr（Ⅵ） is pulled and combined by the electrostatic force of CTAB-GP’s CTA⁺ functional group.