Removal of Ni2+ and Zn2+ from groundwater by adsorption onto fishbone and hydroxyapatite: Effect of salinity

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 (NF = 0.350–0.710). The maximum adsorption capacities (qmL) of the Ni2+ and Zn2+ onto HAP were higher than those onto the fishbone, attributed to the higher Brunauer–Emmett–Teller surface area (ABET) 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.