Cd (II) removal from aqueous solution by Eleocharis acicularis biomass, equilibrium and kinetic studies

Batch experiments were carried out to determine the capacity of Eleocharis acicularis biomass to adsorb Cd2+ ions from contaminated solutions with respect to pH, initial Cd2+ concentration, contact time, solution ionic strength and biomass dose. The experimental data were modeled by Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherm models. Freundlich and D–R models resulted in the best fit of the adsorption data. The maximum adsorption capacity for Cd2+ was 0.299 mmol g−1 (33.71 mg g−1) with efficiency higher than 80% (pH 6.0 and 5 g L−1 biomass dose). The mean adsorption free energy value derived from the D–R model (8.058 kJ mol−1) indicated that adsorption was governed by an ionic exchange process. The pseudo-first order, pseudo-second order, Elovich kinetic models and the intra-particle diffusion models were used to describe the kinetic data and to evaluate rate constants. The best correlation was provided by the second-order kinetic model, implying that chemical sorption was the rate-limiting step, although intra-particle diffusion could not be ignored. The practical implication of this study is the development of an effective and economic technology for Cd2+ removal from contaminated waters. The macrophyte biomass used in this study did not undergo any chemical or physical pre-treatment, which added to macrophyte abundance and its low cost makes it a good option for Cd2+ removal from waste water.