An economically viable method for the removal of selected divalent metal ions from aqueous solutions using activated rice husk

Biosorption of divalent metal ions, i.e. Pb(II), Cd(II), Zn(II) and Cu(II) onto rice husk activated (RHA) is investigated over pH range (1–10) via batch adsorption technique. The chemical and thermal activation of rice husk with 0.1 M HNO3 and 1 M K2CO3 at 473 K enhanced the removal efficiency of RHA (35 ± 2.1–99 ± 0.5%, 33 ± 1.2–97 ± 0.6%, 32 ± 1.3–96 ± 0.8% and 28 ± 1.8–95 ± 0.9% before and after treatment, respectively). The surface area analysis of RHA by BET (Brunauer, Emmett and Teller) nitrogen adsorption method provided pore area and average pore diameter to be 542 ± 2.3 m2 g−1 and 1076 ± 5.6 nm respectively. SEM and FTIR analyses of RHA were carried out to determine the surface morphology and functional groups involved in metal binding mechanism, respectively. The adsorption equilibrium was well described by Freundlich, Langmuir and Dubinin–Radushkevish (D–R) isotherm models by employing (4.8–48, 8.9–89, 15.3–153 and 15.7–157) × 10−5 M solution concentrations of sorbates, respectively, at equilibrium time of 20 min at pH 6 and onto 0.2 g of sorbent. The kinetics of mass transfer and intra-particle diffusion for metal ions sorption onto RHA were studied with Lagergren and Morris–Weber kinetic models. The numerical values of thermodynamic parameters indicated the exothermic nature, spontaneity and feasibility of the sorption process. The desorption study of metal components from RHA surface was carried out with 0.1 M HCl. The sorption mechanism developed illustrates the strong interactions of sorbates with the active sites of the sorbent coupled with efficient and environmentally clean exploitation of rice waste product.