Enhanced simultaneous arsenite oxidation and sorption by Mn-modified biochar: Insight into the mechanisms under optimal modification condition

This study successfully prepared two types of Mn-modified biochar using chemical co-precipitation (MBC1) and impregnation pyrolysis methods (MBC2) for simultaneous oxidation and adsorption of arsenic species efficiently. Both Mn-modified biochars were able to remove significantly greater quantities of As(III) from solution than the pristine biochar. Among them, MBC1 showed better oxidation and adsorption capacity, especially at a pyrolysis temperature of 600 degrees C and a ratio of 3:10 (MnOx: BC). The As(III) adsorption on MBC1 conformed to the pseudosecond-order kinetics and Freundlich isothermal models. The maximum adsorption capacity of MBC1 for As(III) at pH 7.0 calculated from the Langmuir adsorption isotherm was 32.06 mg & BULL;g ? 1. The related mechanisms were characterized using elemental analysis, SEM, BET, FTIR, XRD, XPS and HPLC-ICP-MS techniques. Results showed that Mn-oxides significantly improved the surface physicochemical properties of biochar, with significant increase of functional groups (e.g., hydroxyl and carboxyl), the specific surface area and pore structure, which not only promoted the adsorption of arsenic species but mitigated the passivation of biochar surface and Mn-oxides by Mn(II). The oxidation of As(III) via Mn(III) and Mn(IV) in Mn-oxides on the modified biochar subsequently resulted in more efficient production of As(V). The Mn(II) and As(V) could subsequently precipitate and be readsorbed on the biochar surface. The re-adsorbed Mn(II) increased the positive charge on the biochar surface, and the strong electrostatic attraction can promote As(V) adsorption at pH < pHPZC. This study further revealed the favorable conditions and underlying mechanisms for the enhanced simultaneous arsenite oxidation and sorption by Mn-modified biochar.