Oxidation of Arsenic(III) with Air Catalysed by Activated Carbon

Abstract

It was found that arsenite ion is easily oxidized to arsenate ion by dissolved oxygen in the presence of activated carbon. The catalytic oxidation of As(III) has been investigated at 25° C by using five species of activated carbon.
The activated carbon shows high catalytic activity for the oxidation in acidic or alkaline solution, especially in alkaline solution (Figs.1, 4). The catalytic activity depends on the raw materials from which an activated carbon is obtained. The order of the activity in alkaline solution is as follows: activated carbon from coal>activated carbon from coconut shell activa- ted carbon from wood (Fig.4). This order is reversed in acidic solution (Fig.3). The activated carbon which adsorbs more HCl or NaOH shows the higher catalytic activity in alkaline or acidic solution, respectively (Fig.5). Therefore, it seemed that the catalytic activity depends on the amount of basic and acidic surface oxides on activated carbon. The oxidation rate of As(l11) to As(V) V was found to be proportional to the concentration of As(111) and the amount of activated carbon added (Figs.7, 9). From these results, oxidation of AWE on activated carbon was considered to proceed as follows: As(11) is oxidized to As(V) V by such active surface oxides on activated carbon as quinone or hydroperoxide type. Subsequently, the resulted such inactivated surface oxides as hydroquinone type are regenerated by oxidation with dissolved oxygen in the solution. As an overall reaction, therefore, the dissolved oxygen oxidizes As (la) to As(V) V and the active surface oxide acts as a catalyst of this reaction.