Removal of Zn(II) from aqueous solution with natural Chinese loess: Behaviors and affecting factors

The Chinese loess was proved a promising adsorbent for Zn(II) removal from aqueous solution with adsorption capacities at 70.2–83.2 mg g− 1 at 15–45 °C. Batch tests were conducted to evaluate the factors affecting the removal efficiency, of which the pH, temperature and initial Zn concentration all found in positive relevance to the increase of Zn(II) removal efficiency except for the slurry concentration. The uptake of Zn(II) on Chinese loess was considered as ion-exchange adsorption based on the calculated adsorption energy at − 12.8 to − 16.18 kJ mol− 1 by D–R isothermal adsorption model. The adsorption kinetics follows the pseudo-second-order kinetics and the equilibrating duration was found to be > 24 h. Thermodynamic investigation shows that the enthalpy and entropy changes during adsorption are in the range of 18.27–47.83 kJ mol− 1 and 52.7–129.6 J mol− 1 K− 1, respectively. The predicted Gibb's free energies were in the range of − 5.97–3.09 kJ mol− 1, indicating that the adsorption was in favor of higher temperature and lower initial Zn(II) concentration. The optimal Zn(II) removal efficiency could be obtained under the following conditions: low or intermediate Zn(II) concentration, long reaction time, high temperature and initial pH > 3.0.