Study On The Adsorption Performance Of Modified Montmorillonite On Cr(Ⅵ) And Cd(Ⅱ) In Wastewater

With the rapid development of industry and agriculture as well as the continuous improvement of daily life,the anthropogenic input of pollutants containing heavy metal ions discharged into the environment poses a considerable threat to the environment,especially the water environment.Heavy metal ions enter surface water bodies through surface runoff,atmospheric deposition or direct discharge.Heavy metals are bioaccumulative,toxic,insidious,and persistent,posing a great danger to the environment and public health.Therefore,the environmental damage and ecological risks caused by heavy metal pollution have aroused widespread global concern about the fate of water bodies,and the heavy metal pollution crisis is a non-negligible problem faced by all countries in the world,especially developing countries,and the heavy metal pollution control problem can not be delayed.Chromium and cadmium metals are heavy metal pollutants widely present in the aqueous environment and are considered to be strongly teratogenic and carcinogenic.In this paper,we mainly prepare nano-zero-valent iron and montmorillonite composite adsorbent materials to study and analyze the adsorption influencing factors,adsorption effect and adsorption mechanism of Cr（VI）and Cd（II）,which are common heavy metals in water,and successfully prepared montmorillonite（MMT）and nano-zero-valent iron（NZVI）particle composites to improve the agglomeration phenomenon existing when nano-zero-valent iron is used alone,and also experimentally analyzed the composite in Cr（VI）and Cd（II）solution single system adsorption of the composite material in the adsorbent dosage,the initial p H of the solution,the initial concentration of the solution and time and other influencing factors,to explore its effect on the removal of heavy metal ions Cr（VI）and Cd（II）,and to fit the experimental data using kinetic,isothermal and thermodynamic models,and to characterize the composite material before and after the adsorption of Cr（VI）and Cd（II）,and to analyze in depth the adsorption mechanism,in addition,the adsorption tests were performed on the binary mixed system of Cr（VI）and Cd（II）solutions.The main results of the study are as follows:（1）Nano zero-valent iron（NZVI）was prepared by the liquid-phase reduction of sodium borohydride and successfully loaded on montmorillonite（MMT）to obtain nano zero-valent iron/montmorillonite（NZVI/MMT）composites,and by SEM/EDS,it was shown that NZVI was successfully loaded on MMT with spherical particles of uniform size on its surface,and most of the particles were able to exist independently without obvious The large specific surface area and larger pore capacity of NZVI/MMT provide more adsorption sites for NZVI,which is conducive to the improvement of adsorption performance;no new minerals were formed before and after the loading of NZVI on MMT,and the structure of MMT was not destroyed.（2）The best mass ratio（w/w）of montmorillonite/nano zero-valent iron（NZVI/MMT）was determined by the adsorption test as 1:1,which had the best adsorption effect on the metal anion Cr（VI）.At the initial p H=4.0,the removal rate of Cr（VI）solution with an initial concentration of 40 mg/L by NZVI/MMT was up to 92.41%.The kinetic,isothermal and thermodynamic models were used to fit the adsorption data,and the quasi-secondary kinetic model was more consistent with the adsorption process of NZVI/MMT on Cr（VI）,and the adsorption rate control step of Cr（VI）on NZVI/MMT was mainly chemisorption;the isothermal adsorption curve fitting results indicated that the removal of Cr（VI）by montmorillonite/nano zero-valent iron（NZVI/MMT）was more complex The isothermal adsorption model of Langmuir with a theoretical adsorption capacity of 61.88 mg/g for single molecular layer adsorption;the thermodynamic model indicates that the process of Cr（VI）adsorption by NZVI/MMT is a spontaneous heat absorption reaction.NZVI/MMT reduces Cr（VI）to Cr（III）through two stages of adsorption and reduction,and the Cr（OH）₃,Cr₂O₃ and Fe（III）-Cr（III）co-precipitation deposited on the adsorbent surface,thus achieving the effect of removing Cr（VI）.（3）The adsorption test showed that NZVI/MMT was the most effective for the removal of metal cation Cd（II）at p H=6.0.The removal rate reached 84.7%for the solution containing Cd（II）at an initial concentration of 50 mg/L.The adsorption data were fitted with kinetic,isothermal and thermodynamic models,and the quasi-secondary kinetic model was more consistent with the adsorption process of NZVI/MMT on Cd（II）,and the adsorption rate control step of Cd（II）on NZVI/MMT was mainly chemisorption;the isothermal adsorption curve fitting results showed that The process of Cd（II）removal by montmorillonite/nano zero-valent iron（NZVI/MMT）is more complex to the Langmuir isothermal adsorption model of single-molecule layer adsorption with a theoretical adsorption capacity of 67.07 mg/g;the thermodynamic model indicates that the process of Cd（II）adsorption by NZVI/MMT is a heat absorption reaction that proceeds spontaneously.after the adsorption of Cd（II）by NZVI/MMT,a hydroxide precipitation reaction to form Cd（OH）₂ and adsorbed on its surface as Cd（OH）₂ and Cd-Fe hydroxide co-precipitation,thus achieving the effect of Cd（II）removal.（4）NZVI/MMT can still maintain a high removal rate for Cr（VI）-Cd（II）in the binary mixed system solution.at p H=6.0,the removal rate of Cr（VI）from binary mixed solution by NZVI/MMT is 79.25%and 68.00%for Cd（II）.the adsorption competition ability of Cr（VI）on NZVI/MMT is greater than that of Cd（II）competition ability,and the presence of Cd（II）in the solution during the adsorption process can promote the removal of Cr（VI）by NZVI/MMT.