Removal characteristics of Zn(II) from aqueous solution by alkaline Ca-bentonite

Zn(II) adsorption from aqueous solution on alkaline Ca-bentonite (ACB) was studied using batch technique and compared with the adsorption onto raw bentonite (RB). The percentage of Zn(II) removal increased with increasing adsorbent dosage, contact time and pH. The optimum initial pH for Zn(II) removal by RB and ACB was 7.0 and 4.0–7.0, respectively. The adsorption data from both RB and ACB experiments could be well-described by the pseudo-second-order kinetic model. The experimental data for Zn(II) adsorption on RB could be described by the Langmuir and Freundlich isotherms while those on ACB could be fit well to the Langmuir isotherm. Langmuir monolayer adsorption capacity of ACB (149 mg g− 1) was found to be roughly two times greater than that of RB (73.5 mg g− 1). Therefore, ACB is more effective than RB for Zn(II) adsorption from aqueous solution with respect to short equilibrium time, high efficiency and broad pH range. The equilibrium data, SEM and XRD analyses indicate that ACB removes Zn(II) from aqueous solution mainly by surface precipitation, cation exchange and fixation. In summary, the study suggests that ACB is very suitable for potential applications in Zn(II) removal from aqueous solution.

Research highlights
► Preparation of alkaline Ca-bentonite (ACB) is cheap and simple.
► To date, there has been no literature reported on Zn(II) removal by ACB.
► ACB is useful in Zn(II) removal due to large adsorption capacity and broad pH range.
► ACB removes Zn(II) mainly by surface precipitation, cation exchange and fixation.