Migration of manganese and iron during the adsorptionregeneration cycles for arsenic removal

Fe-Mn binary oxide incorporated into porous diatomite(FMBO-diatomite)was prepared in situ and regenerated in a fixed-bed column for arsenite[As(III)]and arsenate[As(V)]removal.Four consecutive adsorption cycles were operated under the following conditions:Initial arsenic concentration of 0.1 mg·L^(-1),empty bed contact time of 5 min,and pH 7.0.About 3000,3300,3800,and 4500 bed volumes of eligible effluent(arsenic concentration≤0.01 mg·L^(-1))were obtained in four As(III)adsorption cycles;while about 2000,2300,2500,and 3100 bed volumes of eligible effluent were obtained in four As(V)adsorption cycles.The dissection results of FMBOdiatomite fixed-bed exhibited that small amounts of manganese and iron were transferred from the top of the fixed-bed to the bottom of the fixed-bed during As(III)removal process.Compared to the extremely low concentration of iron(<0.01 mg·L^(-1)),the fluctuation concentration of Mn2+in effluent of the As(III)removal column was in a range of 0.01-0.08 mg·L^(-1).The release of manganese suggested that manganese oxides played an important role in As(III)oxidation.Determined with the US EPA toxicity characteristic leaching procedure(TCLP),the leaching risk of As(III)on exhausted FMBO-diatomite was lower than that of As(V).