Abstract
Most methods currently employed for arsenic removal from water are based onoxidation followed by adsorption or co-precipitation of As(V) with iron and other metal oxides. An extensive electrochemical investigation of arsenic in aqueous solutions is reported here, in order to assess the possibility of removing it by reduction to its elemental form.
Cyclic voltammetry and rotating disk electrode studies on Pt pointed to the
irreversibility and mixed-control mechanism of reactions among arsenic species. The oxidation of As(III) to As(V) shows a well defined peak at about 1 VSCE, with high overpotential (> 0.5 V). The corresponding reduction of As(V) cannot be observed in the potential range under study (-0.6 to +1.2 VSCE). With As(III) in solution, hydrogen evolution on Pt is shifted towards more negative potentials, and it takes place simultaneously with arsenic deposition and arsine formation; special attention should be devoted to avoid arsine evolution when studying removal of arsenic by reduction.
Arsenic species are more reactive at pH =1 rather than 4. Iron gave the best results as a cathode for electrochemical deposition of As(III) when
compared to Pt and Cu, with the highest amount of arsenic in the deposit, negligible arsine evolution and less negative applied potential (-0.56 VSCE). It also proved a good candidate for cementation, but with a slow reaction rate. Arsine evolution was significant at potentials below -0.650 Vsce on Cu, and below -0.728 Vsce on Pt. As(V) can also be removed on Cu, generating more arsine.
As(III) cementation on Zn produced large amounts of arsine. The addition of Cu++ in solution greatly improved it by nearly eliminating arsine evolution and yielding arsenic deposition, suggesting the formation of a Cu-As alloy. Removal of As(V) by cementation on Zn with Cu++ occurs with minimum production of arsine, but less efficiently. To assess a possible practical application, cementation tests were performed on Zn powder and the concentration of arsenic went down from 500 to 10 ppb, a level within the limits imposed by the World Health Organization for arsenic content in drinking water.
| Date of Award | 2007 |
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| Original language | English |
| Awarding Institution |
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