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

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 (MnO2) 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 m2/g, (ii) an uneven Al/Mn distribution (Al-enriched) on the surface, (iii) an amorphous structure and the existence of MnO2. 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.

Oxidation and sorption mechanism could be responsible for the higher As(III) uptake by MODAO than by Al2O3 and MnO2. Based on this work, it could be concluded that the As(III) removal by MODAO was completed through the oxidation–adsorption mechanism. The main pathways are proposed in this figure.Figure optionsDownload as PowerPoint slideHighlights
► A new bimetal adsorbent of MODAO was synthesized for As(III) removal.
► MODAO exhibited a remarkable adsorption capacity for As(III).
► As(III) oxidation and As(V) adsorption were involved in As(III) uptake by MODAO.
► Increasing pH inhibited both As(III) oxidation and As(V) adsorption by MODAO.