Adsorption of antimony(V) onto Mn(II)-enriched surfaces of manganese-oxide and Fe[sbnd]Mn binary oxide

Abstract

Manganese(IV) oxide [Mn(IV)] potentially oxidizes antimony(III) [Sb(III)] to antimony(V) [Sb(V)] and improves Sb removal by FeMn binary oxide (FMBO) through an oxidation-adsorption mechanism. This study focused on the effect of Mn(IV) reductive dissolution by potassium sulfite (K2SO3) on Sb(V) adsorption onto manganese oxide (Mn-oxide) and FMBO. The maximum Sb(V) adsorption (Qmax,Sb(V)) increased from 1.0 to 1.1mmolg−1 for FMBO and from 0.4 to 0.6mmolg−1 for Mn-oxide after pretreatment with 10mmolL−1 K2SO3. The addition of 2.5mmolL−1 Mn2+ also significantly improved Sb(V) adsorption, and the observed Qmax,Sb(V) increased to 1.4 and 1.0mmolg−1 for FMBO and Mn-oxide, respectively, with pre-adsorbed Mn2+. Neither K2SO3 nor Mn2+ addition had any effect on Sb(V) adsorption onto iron oxide (Fe-oxide). Mn2+ introduced by either Mn(IV) dissolution or addition tended to form outer-sphere surface complexes with hydroxyl groups on Mn-oxide surfaces (MnOOH). Mn2+ at 2.5mmolL−1 shifted the isoelectric point (pHiep) from 7.5 to 10.2 for FMBO and from 4.8 to 9.2 for Mn-oxide and hence benefited Sb(V) adsorption. The adsorption of Sb(V) onto Mn2+-enriched surfaces contributed to the release of Mn2+, and the X-ray photoelectron spectra also indicated increased binding energy of Mn 2p3/2 after the adsorption of Sb(V) onto K2SO3-pretreated FMBO and Mn-oxide. Sb(V) adsorption involved the formation of inner-sphere complexes and contributed to the release of Mn2+. In the removal of Sb(III) by Mn-based oxides, the oxidation of Sb(III) to Sb(V) by Mn(IV) oxides had an effect; however, Mn(IV) dissolution and Mn2+-enrichment also played an important role.