Kinetic and isotherm modeling of lead(II) sorption onto some waste plant materials

Peepul leaves were identified as the most potent Pb(II) sorbent (qmax = 127.34 mg g−1), followed in decreasing order by banana peels (72.79 mg g−1), peanut hulls (69.75 mg g−1), coir fibers (52.03 mg g−1), rice stem (49.57 mg g−1), teak saw dust (40.70 mg g−1), discarded tea leaves (35.89 mg g−1), mango leaves (31.54 mg g−1), rice husk (31.13 mg g−1) and grass clippings (29.05 mg g−1). The tested plant materials sorbed Pb(II) optimally at pH 4 or 5, but peepul leaves showed substantial sorption even at lower pH values. Kinetics of Pb(II) sorption by the selected biomass types, excepting peepul leaves, was very fast with >90% sorption occurring within 10 min. The kinetic data of Pb(II) sorption are in good agreement with both pseudo-first-order and -second-order reaction models. However, external diffusion, rather than intra-particle diffusion, seems to be the major mechanism of Pb(II) sorption by the tested plant materials. The isotherm data of Pb(II) biosorption more significantly fitted to Langmuir (r2 = 0.96–0.99) than to Freundlich (r2 = 0.88–0.97) model.