Adsorption of Cu²⁺, Ni²⁺, and Cr(VI) Heavy Metal Ions on Sodium Hexatitanate Nanorods from Industrial Electroplating Wastewater Assisted by Calcium Hypochlorite Oxidation

The removal of Cu2+, Ni2+, and Cr(VI) heavy metal ions from industrial electroplating wastewaters at pH values of 8.5–8.8 and 20–60 °C was conducted by the in situ oxidative adsorption technique using calcium hypochlorite as an oxidant and sodium hexatitanate nanorods (64 × 484 nm) as adsorbents. The in situ oxidative adsorption technique enhances the adsorption of Cu2+, Ni2+, and Cr(VI) heavy metal ions on sodium hexatitanate nanorods from actual electroplating wastewater because the complexing interactions between organic ligands and heavy metal ions were broken and weakened by the oxidation with calcium hypochlorite. When the oxidative adsorption was conducted at 25 g·L–1 calcium hypochlorite, 125 mg·L–1 hexatitanate nanorods, and 60 °C for 5 h, the concentrations of Cu2+ and Ni2+ ions could be efficiently reduced from 3.24 to 0.167 and from 0.248 to 0.0936 mg·L–1, respectively. The residual concentrations are lower than the discharge thresholds authorized by the Chinese Ministry of Ecology and Environment. The removal percentages of Cr(VI) ions were around 21%, while the initial Cr(VI) concentrations were in a range of 3.0–6.0 mg·L–1. The oxidative adsorption processes of Cu2+, Ni2+, and Cr(VI) on sodium hexatitanate nanorods from industrial electroplating wastewater are well simulated by both pseudo-second-order adsorption kinetics and Langmuir adsorption isotherm. The thermodynamic analyses show that the oxidative adsorptions of Cu2+, Ni2+, and Cr(VI) are spontaneous and endothermic. The value of ΔG0 decreases, whereas the values of ΔH0 and ΔS0 increase with increasing the oxidation extent of industrial electroplating wastewater.