Adsorptions of Cd(II) and Pb(II) in aqueous solution by rice-straw char

Abstract

In this study, a low-cost adsorbent was prepared from rice straw without any activation process and studied for the adsorption capacities and mechanisms of Cd and Pb ions in water. The adsorption of these metal ions on rice-straw char (RSC) was fast and reached equilibrium within 24 h. The adsorption isotherms exhibited the L-type and the adsorption capacities of Pb(II) and Cd(II) at pH 4 were determined to be 68 and 26 μmol g−1, respectively. The difference between the adsorption capacities of these two metal ions implied different adsorption mechanisms. The adsorptions of Pb(II) and Cd(II) both increased with increasing pH. The adsorption of Pb(II) was insensitive to ionic strength, and the adsorption edge was significantly lower than the PZC of RSC. On the contrary, the adsorption of Cd(II) was affected by changing ionic strength, and the adsorption edges was close to the PZC of RSC. Based on these results, the adsorption mechanism of Pb(II) was determined to be inner-sphere complexation, whereas Cd(II) interacted with the RSC surface through electrostatic attraction and formed outer-sphere complexes on RSC.