Adsorption of Arsenite from Water by Rice Husk Silica

Arsenic occurs in groundwater in two valence states, as trivalent arsenite [As(III)] and as pentavalent arsenate [As(V)]. As(III) is more toxic and more difficult to remove from water by adsorption on activated alumina. In this study the ability of rice husk silica to adsorb As(III) from water was examined. Silica was prepared by burning rice husk in a muffle furnace at 500°C. Batch adsorption tests showed that extent of As(III) adsorption depends on pH, initial As(III) concentration and the contact time. Equilibrium adsorption was attained within 180 min, with maximum adsorption occurring at a pH value of 7-8 which is in the pH range for groundwater. The adsorption of As(III) by rice husk silica followed pseudo second-order kinetics. Equilibrium adsorption data were well described by the Langmuir and Freundlich isotherm models. According to the Langmuir isotherm, As(III) adsorption capacity of rice husk silica is 1.897 mg/g which is 10.5x higher than that (0.180 mg/g) of activated alumina. Hence, rice husk silica would be an effective substitute for activated alumina in the removal of As(III) from groundwater at wells or in households.