Removal of a Model Chlorophenol From Water Using Rice-Straw Ashes.

The removal of a model chlorophenol, 2-chlorophenol (2-CP), from water using ashes derived from burning rice straw at 400¢J and 700¢J was evaluated in this study. The results showed that burning rice straw can provide a better adsorbent for 2-CP. The adsorption capacities of the ashes obtained at 400¢J and 700¢J were 37 and 52 mg g^-1, respectively, at pH 4. Their adsorption capacities decreased to 9.0 and 40 mg g^-1, respectively, at pH 10. Adsorptions of either neutral or anionic 2-CP by the ashes were characterized by L-shaped nonlinear isotherms, suggesting surface adsorption rather than partitioning was occurring. A higher burning temperature increased the surface area, porosity, point of zero charge (PZC) and aromaticity of the resultant ash, but decreased the oxygen content and surface acidity. The combined effects of these changes resulted in a higher 2-CP adsorption of the ash obtained at 700¢J. The adsorption of 2-CP was correlated to the relative content of its anionic species depending on solution pH; the adsorption was lower at higher pH because of the stronger electrostatic repulsion due to both the deprotonation of surface functional groups of the ashes and the dissociation of 2-CP. For the ash obtained at 700¢J, the smaller effect of pH on the adsorption of 2-CP was due to the lower content of oxygen-containing functional groups. Oxygen-containing functional groups contribute to surface acidity and H-bonding sites for adsorbed water, which compromises the interaction between 2-CP and the adsorbents. These results suggest that ashes from burning rice straw may be an effective low-cost substitute for activated carbon for the removal of chlorophenols from water.