Characterization of mesoporous rice husk ash (RHA) and adsorption kinetics of metal ions from aqueous solution onto RHA

The present study deals with the characterization of low-cost rice husk ash (RHA) for its various physico-chemical properties and adsorption characteristics of metal ions. The average particle size of RHA was 150.47 μm. Proximate analysis showed the presence of high amount of ash in RHA. Bulk density and the heating value of RHA were 104.9 kg/m3 and 9.68 MJ/kg, respectively. The pore size distribution results showed that the RHA was predominantly mesoporous. The BET surface area was 36.44 m2/g. The average pore diameter by BET was 42.603 Å. The BJH pore area showed 80% of the pore area due to the mesopores. The polar groups present on the RHA surface imparted considerable cation exchange capacity to it. RHA was found to be an effective adsorbent for the removal of cadmium (Cd(II)), nickel (Ni(II)) and zinc (Zn(II)) metal ions from aqueous solutions. The pH0 ≈6.0 is found to be the optimum for the removal of individual cations from the aqueous solutions by RHA at an optimum dose of 10 kg/m3. The kinetics of adsorption showed that the metal ions adsorption on RHA is a gradual process with quasi-equilibrium being attained in 5 h. The pseudo-second-order kinetics represents the equilibrium data well. The effective diffusion coefficient of the cations onto the RHA is of the order of 10−13 m2/s.