Removal of arsenate, chromate and ferricyanide by cationic surfactant modified powdered activated carbon

In this study, surfactant-modified powdered activated carbon (PAC) was used to remove anionic contaminants from water. The conventional modification method of PAC with surfactants produced large quantity of unbound surfactant in aqueous phase (3.0-3.5 mM). However, the revised modification method - filtration followed by centrifugation - released negligible amount of residual surfactant (<0.01 mM). Adsorption characteristics were investigated with arsenate (H₂AsO₄^-), chromate (CrO₄^2-) and ferricyanide (Fe(CN)₆^3-). The adsorption isotherm followed Langmuir model, and it suggested that the anions bind on the monolayer of the modified PAC by the electrostatic attraction. The order of maximum adsorbed amount was chromate (0.234 mmol/g) > arsenate (0.087 mmol/g) ≈ ferricyanide (0.085 mmol/g). On the other hand, the order changed to ferricyanide > chromate > arsenate in multi-contaminant system. Anions competed with each other for limited binding sites in the multicontaminant system, hence the order of maximum adsorbed amount followed the valence of each anion. With 2 g/L of modified PAC, it was possible to treat 0.001 mM of arsenate, 0.5 mM of chromate and 0.2 mM of ferricyanide completely.