Investigation On As(Ⅲ) Adsorption/Oxidation Mechanism On The Surface Of Fe-Mn Oxides And Their Modified Materials

Metalloid arsenic（As）has high toxicity and carcinogenicity properties.Moreover,iron-manganese（Fe-Mn）oxides have the oxidizing characteristics of Mn oxides and the adsorption capability of Fe oxides,which synergistically contribute to a higher removal efficiency of arsenite（As（Ⅲ））than single Mn oxides or Fe oxides.Therefore,Fe-Mn oxides are widely employed in the purification of As-containing systems.However,the oxidation and adsorption processes of As（Ⅲ）on the surface of Fe-Mn oxides are still unclear,and the potential catalytic oxidation process of As（Ⅲ）by Fe-Mn oxides in the presence of oxygen needs to be further studied.The naturally existing adsorbents in the environment play an essential role in the migration and transformation of As.This work has investigated the interaction mechanisms of Fe-Mn nodules and As（Ⅲ）under different atmospheres.Then,the diatomite（DE）-loaded Fe-Mn binary oxide（FMD）and bamboo biochar（BC）-loaded Fe-Mn oxide（FMBC）were prepared for As（Ⅲ）oxidation and adsorption from aquatic systems.Furthermore,the physicochemical and structural characteristics of the adsorbents were also investigated.The influence of initial concentrations of As,initial p H and the contribution of Fe and Mn oxides in these materials were also evaluated.The main results of these experiments are summarized as follows:（1）During redox and adsorption processes,Fe-Mn nodules influence As speciation,transformation,and migration.Mn oxides and Fe oxides in nodules respectively contribute to As（Ⅲ）oxidation and As（Ⅲ,V）adsorption.Under aerobic environments,Fe-Mn nodules worked as a catalyst to accelerate the oxidation of Mn（II）to Mn（Ⅲ,V）oxides,which considerably improved As（Ⅲ）oxidation.In the process that incorporates 10 mg L^-1 As（Ⅲ）and 1.0 g L^-1 Fe-Mn nodules,the maximum oxidation capacity of As（Ⅲ）reached 3.22,3.48and 3.71 mg g^-1,and the corresponding As（Ⅲ,V）adsorption capacity reached 2.49,2.40,and 2.39 mg g^-1 in nitrogen,air and oxygen atmosphere,respectively.As（Ⅲ）oxidation activity rose and declined as dissolved oxygen（DO）concentration and p H increased,respectively.These findings contribute to a better knowledge of the behaviours and destiny of As in the ecosystem by clarifying the process of As（Ⅲ）oxidation on Fe-Mn nodules under different conditions.（2）As adsorption capacity by DE and FMD improved with increasing initial As（Ⅲ）concentration.In the reaction system containing 10 mg L^-1 As（Ⅲ）and 1.0 g L^-1 DE at p H7.0,the highest adsorption capacity of DE was 1.77 mg g^–1.At the same time,the FMD revealed a higher As adsorption capacity of 4.45 mg g^–1,which was more than 2.5 times that of DE.The increase in the surface area and changes in the porous structure and functional groups on the surface of FMD significantly improved its adsorption efficiency.After 1440min,the oxidation capacities of FMD were 1.82 and 2.01 mg g^-1 in nitrogen and air atmospheres,respectively.No obvious oxidation of As（Ⅲ）by DE was detected in nitrogen and air atmospheres,indicating that Mn oxides played a primary role in As（Ⅲ）oxidation.FMD had better As（Ⅲ）adsorption performance with higher binding force and stability.As（Ⅲ）oxidation capacity increased and decreased with increasing DO concentration and p H,respectively.FMD was considered an efficient surface sorbent that could immobilize As,especially under acidic and oxic conditions.（3）As（Ⅲ）adsorption on modified BCs（MBC,FBC,and FMBC）was biphasic,with a rapid initial adsorption phase,followed by a later slower phase.The As adsorption capacities of MBC,FBC,and FMBC were 4.00,4.98,and 5.89 mg g^-1,respectively,which were much higher than those of BC(2.68 mg g^-1).The maximum adsorption capacity of FMBC for As was about 2.18 times greater than that of BC.The Langmuir and Langmuir-Freundlich models fit the data better than the Freundlich model.In the system containing 15 mg L^-1As（Ⅲ）and 1.0 g L^-1 of MBC,FBC,and FMBC,the As（Ⅲ）adsorption capacities were 2.87,3.59,and 4.39 mg g^-1 in nitrogen,and 2.99,3.78,and 4.60 mg g^-1 in air atmospheres,respectively,indicating that As（Ⅲ）oxidation enhanced As adsorption under oxic conditions.As adsorption by modified BCs increased with DO concentration and p H decreasing.These results showed that all modified BCs could facilitate As（Ⅲ）adsorption and oxidation in the presence of DO.The FMBC demonstrated better performance compared to other modified BCs.All the natural adsorbents showed promising results in reducing As mobility and improving its adsorption capacity from an aqueous solution.FMBC showed promising results compared with all adsorbents with no released ions in the solution at the end of the reaction.Furthermore,this work clarified the mechanism of As（Ⅲ）adsorption and oxidation by Fe-Mn oxides in various systems and recommended natural adsorbents for As（Ⅲ）removal from aqueous solutions.