Removal of Ni 2+ and Zn 2+ from groundwater by adsorption onto fishbone and hydroxyapatite: Effect of salinity

Abstract

Ni²⁺- and Zn²⁺-contaminated groundwater in the coastal regions is a serious threat to water security in industrial areas. Apatite-like material is an excellent sorbent for heavy metals; however the effect of salinity on the Ni²⁺ and Zn²⁺ adsorption onto fishbone and synthesized hydroxyapatite (HAP) has not been investigated. This study investigates the effect of salinity on the single and binary adsorption of Ni²⁺ and Zn²⁺ onto apatite-like materials. The experiments were conducted in batch reactor for 24 h at 1:40 solid-to-liquid ratios (wt/wt), 25°C, and pH 5. Freundlich, Langmuir, and Dubinin–Radushkevich models fit well with the single-adsorption data. The adsorption isotherms were nonlinear (N_F = 0.350–0.710). The maximum adsorption capacities (q_mL) of the Ni²⁺ and Zn²⁺ onto HAP were higher than those onto the fishbone, attributed to the higher Brunauer–Emmett–Teller surface area (A_BET) and cation exchange capacity. In the binary adsorption, adsorption capacities of the adsorbents were less than those in the single-solute system due to the competition between Ni²⁺ and Zn²⁺. Salinity affected the single and binary adsorption by decreasing the adsorption capacities of the adsorbents. In a binary adsorption system, the selectivity of Zn²⁺ was less than that of Ni²⁺ for both fishbone and HAP at 0‰ and 30‰ salinity, respectively. Binary adsorption models, such as the Murali–Aylmore (M–A) model, competitive Langmuir model (CLM), P-factor model, and ideal-adsorbed solution theory coupled with the Freundlich (IAST-Freundlich) model were used; of these, the M–A model provided the best prediction for the binary system.