Removal Performance And Mechanism Of As(?) By Manganese Oxide Coated Schwertmannite In Water

Arsenic(As)is a high toxic metalloid element with widely presence in the environment.Arsenite(As(III))is one of the most toxic and transferable species of As which exists in acid mine drainage and groundwater,threatening human health and life safety.Schwertmannite found in the sediments of acid mine drainage is an excellent natural arsenic adsorbent.However,schwertmannite has the problems of weak adsorption performance for As(III)in acidic water and cannot weaken the toxicity of As(III)by oxidation.Moreover,Schwertmannite is likely to be reduced and recrystallized induced by Fe(II)in neutral environment,which is not conducive to the long-term removal of As(III).In view of the application defects of schwertmanite mentioned above,manganese oxide loaded schertmannite(Mn O_x@Sch)were synthesized for As(III)removal at different condition and the removal performance and mechanism were studied.The main achievements of this research were as follows:(1)Mn O_x@Sch with different Mn contents(Mn O_x@Sch-1,Mn O_x@Sch-2 and Mn O_x@Sch-3)were synthesized by liquid phase precipitation method base on schwertmannite(Sch)prepared in lab.Solid phase characterization shown that Mn O_x@Sch had higher specific surface area and lower isoelectric point.After loading,Manganese oxide shaped as a shell on the surface of schwertmannite.The crystal structure and functional groups of schwertmannite were reserved,and Mn-O functional groups emerged.(2)To evaluate As(III)removal efficiency of Mn O_x@Sch at acidic(p H=3)condition and elucidate the mechanism,we compared the As(III)removal performance of Sch and Mn O_x@Sch-2 and investigated the products both in liquid and solid phases.It shown that when the dosage of Mn O_x@Sch-2 was 0.5 g/L and the initial concentration of As(III)was 1 mg/L,the equilibrium residual concentration of As(III)was only 2.42?g/L,and the removal rate was99.8%,which was much better than that of schwertmannite(72.5%).The removal efficiency of Mn O_x@Sch increased first and then decreased with the increase of Mn content.The changes of p H change and coexisting ions had little effect on As(III)removal.The kinetic analysis shown that the reaction process was accorded with pseudo-second order kinetics and intraparticle diffusion model.During the adsorption process,neutral As(III)was oxidized to negative charged As(V)by the manganese oxide shell resulting in electrostatic adsorption.As(V)was bound to the active sites of the material through intraparticle diffusion,resulting in ligand exchange with SO₄^2-in structure of schwertmannite.(3)To evaluate the stability of Mn O_x@Sch at the condition of low oxygen,high Fe(II)and neutral(p H=6)and elucidate the mechanism,we compared the reaction products in liquid and solid phases between Fe(II)and Sch or Mn O_x@Sch-3.It shown that Fe(II)can induce the reduction and recrystallization of schwertmannite into lepidocrocite.The degree of transformation reaction increased with the increase of Fe(II)concentration and reaction time,along with the release of up to 71.2%of total SO₄^2-from the solid phase.After modification by manganese oxide,the release rate of SO₄^2-from the solid phase significantly decrease and release amount were under 40%of total SO₄^2-content.The solid phase of Mn O_x@Sch-3 didn't obviously change during the reaction.The reason was that the manganese oxide shell blocked the contact and cut off the electron transfer path between schwertmannite core and Fe(II)so that inhibited the reduction recrystallization process of the minerals.We compared the As(III)removal performance of Sch and Mn O_x@Sch-3 at the condition of low oxygen,high Fe(II)and neutral(p H=6).Little adverse impact exist on the removal of As(III)by Sch with the presence of Fe(II),but the removal efficiency of As(III)by Sch was44.7%after process of Fe(II)-induced reduction recrystallization comparing with that of the treatment of no Fe(II)(95.8%).The decreasing degree of As(III)removal efficiency corelated to the degree of recrystallization.Solid phase characterization shown that Fe(II)induced the reduction and recrystallization of schwertmannite into lepidocrocite during the process of As(III)adsorption and the released As(III)after schwertmannite dissolution could retained by coprecipitation,but the As(III)adsorption capacity of the secondary minerals decreased,which was not conducive to the continuous removal of As.However,in the group of Mn O_x@Sch-3,Fe(II)and As(III)can be oxidized by the manganese oxide shell loaded on schwertmannite.Co-precipitation between Fe(III)and As(V)improved the removal rate of As by Mn O_x@Sch-3.Since schwertmannite core reserved after the reaction with Fe(II),long term Fe(II)pretreatment had little impact on As(III)removal.As(V)was the main species of arsenic in the solid,and the mineral phase had no obvious change after reaction.In conclusion,Mn O_x@Sch can efficiently oxidize and adsorb As(III)in acidic and neutral condition,and inhibit the reduction recrystallization process induced by Fe(II)on schwertmannite in neutral condition.The results of this research could provide theoretical basis and technical support for the application of schwertmannite in arsenic polluted water purification.