Arsenic(III) oxidation/adsorption behaviors on a new bimetal adsorbent of Mn-oxide-doped Al oxide

Abstract Arsenite [As(III)] is more difficult to remove than arsenate [As(V)] under a wide range of conditions. Several Mn-oxide-containing adsorbents could exhibit both the oxidation and the adsorption activities toward As(III). However, little information is available on the As(III) oxidation/adsorption behaviors onto the coprecipitates of Al and Mn oxides. To bridge this gap, Mn-oxide-doped Al oxide (MODAO) was synthesized by an oxidation/co-precipitation process, combining the features of manganese dioxide (MnO 2 ) and aluminum oxyhydroxide (AlOOH). MODAO was characterized using BET, EDAX, SEM and XRD analyses. The characterization results showed: (i) a rough surface exhibiting a BET area of 44.73 m 2 /g, (ii) an uneven Al/Mn distribution (Al-enriched) on the surface, (iii) an amorphous structure and the existence of MnO 2 . Additionally, a series of batch experiments were conducted using MODAO for As(III) removal. The kinetic results showed that MODAO could oxidize As(III) to As(V), and the data were best fit using a pseudo-second-order model, confirming that more than one-step adsorption process dominated the rate controlling step. The maximal adsorption capacity As(III) was calculated to be 142.19 mg/g using a Langmuir model. In addition, both As(III) oxidation and As(V) adsorption by MODAO were inhibited by a pH increase over a pH range of 4–10. Furthermore, the XPS analysis results of MODAO before and after reaction with As(III) confirmed the oxidation-sorption mechanism for As(III) uptake by MODAO.