Zn(II) adsorption from synthetic solution and kaolin wastewater onto vermicompost

The adsorption of Zn(II) from both synthetic solution and kaolin industry wastewater by cattle manure vermicompost was studied. The adsorption process was dependent on the various operating variables, viz., solution pH, particle size of the vermicompost, mass of vermicompost/volume of the Zn(II) solution ratio, contact time and temperature. The optimum conditions for Zn adsorption were pH 6.0, particle size of ≤250 μm, 1 g per 10 mL adsorbent dose, contact time of 4 h and temperature of 25 °C. Langmuir and Freundlich adsorption isotherms fit well in the experimental data and their constants were evaluated, with R2 values from 0.95 to 0.99. In synthetic solution, the maximum adsorption capacity of the vermicompost for Zn2+ ions was 20.48 mg g−1 at 25 °C when the vermicompost dose was 1 g 10 mL−1 and the initial adjusted pH was 2. The batch adsorption studies of Zn(II) on vermicompost using kaolin wastewater have shown the maximum adsorption capacity was 2.49 mg g−1 at pH 2 (natural pH of the wastewater). The small values of the constant related to the energy of adsorption (from 0.07 to 0.163 L mg−1) indicated that Zn2+ ions were binded strongly to vermicompost. The values of the separation factor, RL, which has been used to predict affinity between adsorbate and adsorbent were between 0 and 1, indicating that sorption was very favorable for Zn(II) in synthetic solution and kaolin wastewater. The thermodynamic parameter, the Gibbs free energy, was calculated for each system and the negative values obtained confirm that the adsorption processes are spontaneous. The ΔG° values were −19.656 kJ mol−1 and −16.849 kJ mol−1 for Zn(II) adsorption on vermicompost in synthetic solution at pH 6 and 2, respectively, and −13.275 kJ mol−1 in kaolin wastewater at pH 2.