Adsorption of Arsenic(III) and Arsenic(V) on Activated Carbon

Abstract

Adsorption characteristic of arsenite and arsenate ions on activated carbon was studied at ambient temperature (25° C) by using five kinds of activated carbon. As(V) was determined by atomic adsorption spectrometry of molybdenum after the extraction of molybdoarsenic acid into MIBK. Total As was determined by the same method after oxydation of As(111) to As (V) with KMnO^2-. All activated carbons used were deashed with HF solution before use to prevent the interference from silicon in the determination of arsenic.
The species of As(V) adsorbed on activated carbon were considered to be 112AsO^2-- and HAs O^2-2-, since As(V) was adsorbed in the pH range from 4 to 9 on activated carbon prepared from coal, coconut shell and wood by steam activation method (Figs.1, 2, 3). The adsorption ability of activated carbon for As(V) was in the order: activated carbon from coalactivated carbon from coconut shellactivated carbon from wood (Fig.4). Influence of coexistent cations on adsorption of As(V) was small (Fig.6). The coexistence of anions interfered greatly with adsorption of As(V) because anions was adsorbed competitively on adsorption sites for H₂AsO^2-- and HAsO^2-2-. The interference form anion was large in the following order: ClO^2--SO₄2-N08-Cl- (Fig.5). It should be noted that As(V) was scarcely adsorbed on activated carbon prepared from wood by the ZnCl, activation method (Fig.3).
As(111) was not adsorbed on all the activated carbons used, since As(111) dissolved in the undissociated form, 1-13AsO^2-, below pH 8. However, As(M) was easily oxidized to As(V) with dissolved oxygen on activated carbon so that As formed was adsorbed on activated carbon. Activated carbon, therefore, having a higher activity to oxidize AWE showed a higher adsorption ability for As. The results of this study support our idea that heavy metal ions are adsorbed on activated carbon in the form of complex anion.