Adsorption of copper on rubber (Hevea brasiliensis) leaf powder: Kinetic, equilibrium and thermodynamic studies

The adsorption of Cu(II) ions from aqueous solution by rubber (Hevea brasiliensis) leaf powder (RHBL) was studied in a batch adsorption system. Characteristics of RHBL such as pH of aqueous slurry, pH of zero point charge (pHZPC), surface area and pore diameter, Fourier transform infrared (FTIR), scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS) were investigated. Factors influencing adsorption such as pH of the solution, adsorbent dosage, particle size, copper concentration and temperature have been studied. The adsorption process was relatively fast and equilibrium was achieved after about 60 min. Maximum adsorption of Cu(II) ions occurred at around pH 4–5. The kinetic data were analyzed using various kinetic models particularly pseudo-first-order, pseudo-second-order, Ritchie's-second-order and intraparticle diffusion. The pseudo-second-order kinetic model was found to agree well with the experimental data. Adsorption equilibrium data could also be described well by Langmuir, Freundlich and Dubinin–Radushkevich isotherm models. Based on the Langmuir isotherm, the monolayer adsorption capacity of Cu(II) ions was 8.92 mg g−1. Thermodynamic parameters such as enthalpy change (ΔH°), free energy change (ΔG°) and entropy change (ΔS°) were calculated and adsorption process was spontaneous and exothermic. Copper removal by RHBL involved different kinds of mechanisms such as ion-exchange, complexation and physisorption.