Study On Preparation Of Organic Modified Reduced-charge Montmorillonites And Its Adsorption Performance Of Anion In Water

With the rapid development of industry and agriculture,pollutants containing large amounts of inorganic anions such as Cl O₄^-,PO₄^3-and NO₃^-are discharged into water,posing a serious threat and hazard to human health and ecological environment.Montmorillonite is often applied to treat pollutants in wastewater due to its abundant resources and excellent adsorption performance,while its adsorption capacity for anions is limited by the natural electronegativity.Therefore,in this paper,lithiated montmorillonite（Li-Mt）and a series of reduced-charge montmorillonites were prepared from calcium-based montmorillonite（Ca-Mt）.Then the reduced-charge montmorillonites with high,medium and low layer charges（Li-Mt1,Li-Mt2 and Li-Mt3）were modified with polydiallyldimethylammonium chloride（PDDA）and cetyltrimethylammonium chloride（CTAC）,respectively,to synthesize organic modified reduced-charge montmorillonites.The structure change regulation of organic modified reduced-charge montmorillonites and the removal mechanisms of NO₃^-in water were investigated.The results are as follows:（1）The layer charge density of montmorillonites decreased with the increase of microwave time and power.The layer charge density of Ca-Mt,Li-Mt and reduced-charge montmorillonites increased in the following order:Ca-Mt>Li-Mt>Li-Mt1>Li-Mt2>Li-Mt3.There was no basically change in structure and surface groups of montmorillonites after lithiation and charge reducing treatment,while their layer spacing and crystallinity reduced,and their BET specific surface area increased.Li-Mt1 and Li-Mt2 exhibited stronger cation exchange capacity and dispersibility compared to Li-Mt3.（2）The Zeta potential of reduced-charge montmorillonite modified by PDDA and CTAC（PDDA-Mt and CTAC-Mt）changed from negative of reduced-charge montmorillonite to positive,and enhanced with the increase of layer charge density of montmorillonite.In addition,the specific surface area reduced with increasing layer charge density.Furthermore,the loading amount of modifiers PDDA and CTAC on PDDA-Mt and CTAC-Mt improved with the increase of the layer charge of montmorillonite,indicating that PDDA and CTAC enter the interlayer of montmorillonite mainly through ion exchange.（3）The adsorption capacity of NO₃^-by montmorillonite could be improved by lithiation,charge reduction and organic modification treatments of montmorillonite,and the adsorption effect of NO₃^-by organic modified reduced-charge montmorillonites improved with the increase of the layer charge of montmorillonite.The removal amounts of NO₃^-by lithiated,reduced-charge and organic modified reduced-charge montmorillonites increased in the following order:PDDA-Mt>CTAC-Mt>reduced-charge montmorillonite>Li-Mt>Ca-Mt.The removal effect of NO₃^-on reduced-charge montmorillonites and organic modified reduced-charge montmorillonites enhanced with the increase of contact time and the initial concentration of NO₃^-.While the adsorption amounts of NO₃^-showed a trend of first increasing and then decreasing with the increase of the initial p H of solution.In the presence of NO₃^-,SO₄^2-,PO₄^3-and Cl O₄^-at the same time,the organic modified reduced-charge montmorillonites showed strong adsorption selectivity for Cl O₄^-.The adsorption amounts of the other three anions changed little.In general,adsorption amounts of the four anions by organic modified reduced-charge montmorillonites increased as follows:Cl O₄^->PO₄^3->NO₃^->SO₄^2-.（4）The thermodynamic fitting results indicated that the increase of temperature was favorable to the spontaneous adsorption of NO₃^-by organic reduced-charge montmorillonites.The results of the adsorption isotherm study showed that the ease of NO₃^-adsorption by PDDA-Mt and CTAC-Mt increased with the increasing layer charge density.The FTIR spectra and X-ray photoelectron spectra of high level charge montmorillonite modified by PDDA（PDDA-Mt1）after anions adsorption showed that NO₃^-,Cl O₄^-,PO₄^3-and SO₄^2-were adsorbed on PDDA-Mt1.The mechanisms of NO₃^-removal were based on the ion exchange between Cl^-in the quaternary ammonium group of PDDA and NO₃^-,in addition to diffusive and electrostatic adsorption.