| Arsenic（As）is a toxic metal element widely distributed in water,soil and other environments.Long-term exposure to arsenic pollution will cause potential harm to human health.In order to reduce health risks,the World Health Organization recommended that the limit of arsenic concentration in drinking water should be 10μg L-1.In the remediation of arsenic pollution,Fe-Mn binary complex is considered to be the most potential arsenic removal material due to its adsorption/oxidation characteristics.Therefore,the arsenic removal performance and mechanism of Fe-Mn binary oxides have been the research focus and hotspot in the field of pollution remediation.However,the interaction between iron and manganese oxides in the process of arsenic removal in Fe-Mn binary system is still unclear.At the same time,whether the application of Fe-Mn binary oxides in arsenic pollution environment can maintain efficient and stable arsenic adsorption capacity for a long time is worthy of further discussion.In this work,Fe-Mn binary oxides was taken as the research object,and the characteristics and mechanism of oxidation/adsorption of As（Ⅲ）by Fe-Mn binary oxides were discussed in depth.The main research contents and results are as follows:1.The removal of As（Ⅲ）by iron and manganese oxides in Fe-Mn binary oxides is synergistic and antagonistic.The arsenic adsorption capacities of different iron oxides and Fe-Mn binary oxides were evaluated,and the coupling effect of different iron oxides and manganese oxides on the removal of As（Ⅲ）was explored by using a multiple interface reaction device.The results showed that there was synergistic or antagonistic effect between iron oxides and manganese oxides in the arsenic removal process of Fe-Mn binary system at p H 7.8.When the iron oxide is goethite or hematite,the arsenic adsorption capacity of Fe-Mn binary system(116.3μmol g-1 and90.7μmol g-1)is greater than that of goethite(112.8μmol g-1)or hematite(51.2μmol g-1)alone,that is,there is a synergistic effect between goethite or hematite and manganese oxide.When the iron oxide is ferrihydrite,the arsenic adsorption capacity(694.4μmol g-1)of the Fe-Mn binary system is significantly reduced compared with the single ferrihydrite(871.5μmol g-1),that is,there is an obvious antagonism between ferrihydrite and manganese oxide.2.Interface coupling process and self-stability characteristics exist in Fe-Mn binary oxides except As（Ⅲ）.The kinetic process of oxidation/adsorption of As（Ⅲ）by iron oxide,manganese oxide and Fe-Mn binary oxides was investigated,and the self-stability mechanism of Fe-Mn binary system was explored by multiple interface reaction devices.The results showed that in the five adsorption/desorption experiments,the removal rate of Fe-Mn binary oxides for low concentration As（Ⅲ）decreased by 18.03%,while that for high concentration As（Ⅲ）decreased by only27.73%.The multiple interfacial reaction device experiment found that the oxidation/adsorption behavior of As（Ⅲ）was coupled with the release of low-valent Fe and Mn ions from the Fe-Mn binary system,and complex interfacial reactions（oxidation,adsorption,catalysis,etc.）occurred between Fe（Ⅱ）and Mn（Ⅱ）released into the solution and Fe and Mn oxides.XPS and other characterizations showed that the released iron and manganese ions gradually formed new iron oxides and manganese oxides on the surface of ferrihydrite and birnessite,which ensured the stability of the Fe-Mn binary system.3.Effect of manganese oxides on arsenic removal from Fe-Mn binary system.The arsenic adsorption capacities of Fe/Mn binary complexes with different Fe/Mn molar ratios were evaluated,and the influence mechanism of manganese oxides on arsenic removal in Fe/Mn binary system was revealed.The results show that with the increase of manganese content in Fe-Mn binary oxides,the arsenic adsorption capacity of Fe-Mn binary system increases first and then decreases.Among them,the maximum adsorption capacity of As（Ⅲ）is Fe/Mn molar ratio of 3:1(1494μmol g-1),while the maximum adsorption capacity of As（Ⅴ）is Fe/Mn molar ratio of 6:1(993.1μmol g-1).When the manganese content was moderate,it increased the affinity of iron oxides to arsenic species,and did not occupy too many active adsorption sites,thus greatly improving the arsenic adsorption capacity of Fe-Mn binary system.4.Adsorption differences of arsenic species by ferrihydrite.The adsorption capacity and dissolution kinetics of different arsenic species were investigated,and the mechanism of arsenic species adsorption difference was explored by combining XPS,DFT and other characterization techniques.The results showed that the adsorption capacity of As（Ⅲ）(973.2μmol g-1)was much larger than that of As（Ⅴ）(372.4μmol g-1)at p H 7,and the adsorption capacity of As（Ⅴ）gradually decreased with the increase of As（Ⅴ）content in the solution.At the same time,DFT calculation showed that the adsorption energies of As（Ⅴ）on octahedral and tetrahedral sites of ferrihydrite were-3.39 e Ⅴ and-2.75 e Ⅴ,respectively,which were lower than those of As（Ⅲ）（-1.4 e Ⅴ and 0.54 e Ⅴ）,indicating that ferrihydrite had greater affinity for As（Ⅴ）than As（Ⅲ）.However,BET fitting revealed the pore structure of ferrihydrite,and its abundant micropore structure was the main reason that the As（Ⅲ）adsorption capacity of ferrihydrite was much larger than that of As（Ⅴ）. |
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