Study On The Adsorption Of As(?) And As(?) By Mg/Mn/Fe Layered Double Hydroxide And Its Oxides

The situation of arsenic pollution is becoming more and more serious nowadays.A large number of arsenic-containing pollutants are discharged into the environment,causing serious pollution of water resources.Therefore,the treatment of arsenic pollution in water has become one of the research hotspots in countries around the world.In this paper,co-precipitation method was used to synthesize the Mg₃Fe/Mg₃Mn/Mg₃MnFe layered double hydroxide and their oxides roasting at 400? using magnesium nitrate,manganese nitrate and ferric nitrate as the main raw materials.All samples were characterized by XRD,SEM,Raman,ATR-FTIR.XRD analysis showed that the three kinds of synthesized hydrotalcites all had typical hydrotalcite characteristic diffraction peaks?003?,?006?,?009?,?110?,and?113?at the diffraction angle 2?,and peaks were sharp,with a stable baseline and no extra peaks,showing good crystallinity and regularity.After roasting at 400?,the characteristic peaks?003?and?006?disappeared,?009?became weak,and the structure of hydrotalcite was destroyed.Observed from the SEM picture,the three hydrotalcite samples all showed the typical layered structure of hydrotalcite,the grain sizes were relatively small;after roasting at 400?,they still had the layered structure of hydrotalcite,but some parts appeared concentrated phenomenon.From the ATR-IR spectrum and Raman spectrum,it could be seen that the three hydrotalcite samples had a relatively complete crystal structure.After roasting at 400?,the layered structure of the hydrotalcites were destroyed,the part of interlayer water,OH^-and CO₃^2-ions was removed,the crystal structure were changed.According to experimental datas of the adsorption of As?V?by Mg₃Fe/Mg₃ Mn layered double hydroxide and their oxides,it could be concluded that the acidic condition was favorable for the removal of As?V?,and the adsorption effect of Mg₃Fe-LDO was the best.When the p H value was 3.0,the highest adsorption capacity was 40.94 mg/g,and when the p H was 9.0,the lowest adsorption capacity was 36.27 mg/g.It could be seen from the fitting of the experimental datas that the adsorption processes of As?V?by the four adsorbents all conformed to the quasi-second-order kinetic equation and the Langmuir adsorption model,which belonged to the surface single-layer adsorption.The maximum adsorption capacity obtained by fitting was Mg₃Mn-CO₃-LDHs: 57.70 mg/g,Mg₃Mn-LDO: 53.62 mg/g,Mg₃Fe-CO₃-LDHs: 64.39 mg/g,Mg₃Fe-LDO: 105.37 mg/g respectively.The effects of competitive ions on the adsorption of As?V?by the four adsorbents was PO₄^3-> C₆H_7O8-> CO₃^2-> SO₄^2-.Comparing the results of adsorption of As???and As?V?by Mg₃Fe/Mg₃Mn/Mg₃MnFe layered double hydroxide and their oxides,it could be concluded that the adsorption effect of Mg₃MnFe-LDO was better for As???and As?V?and the removal rate were 95% and 80% respectively.When the pH was 3.0,the removal rates of As???and As?V?by Mg₃MnFe-LDO were up to 97% and 85%.When the p H was 12.0,the removal rates drop to 65% and 74%.The adsorption processes of Mg₃MnFe-LDO to As???and As?V?conformed to the Langmuir adsorption model and quasi-second-order kinetic equation.The adsorption of As???was a process of oxidation and adsorption,and new adsorption sites were created during the reaction.Different concentrations of humic acid?HA?had certain inhibitory effects on the adsorption of As???and As?V?by Mg₃MnFe-LDO.But the inhibitory effects on adsorption of As?V?was greater than As???.Characterization of samples before and after adsorption of arsenic?XRD,ATR-IR,Raman,XPS?could be concluded that in the process of adsorption of As???by Mg₃MnFe-LDO,As???could be oxidized to As?V?.The adsorption mechanism of all samples included the surface complexation of hydrotalcite and the reconstruction of hydrotalcite.