Adsorption of Cu<sup>2+</sup>, Ni<sup>2+</sup>, and\nCr(VI) Heavy Metal Ions on Sodium Hexatitanate Nanorods from Industrial\nElectroplating Wastewater Assisted by Calcium Hypochlorite Oxidation

The\nremoval of Cu<sup>2+</sup>, Ni<sup>2+</sup>, and Cr(VI) heavy\nmetal ions from industrial electroplating wastewaters at pH values\nof 8.5–8.8 and 20–60 °C was conducted by the in\nsitu oxidative adsorption technique using calcium hypochlorite as\nan oxidant and sodium hexatitanate nanorods (64 × 484 nm) as\nadsorbents. The in situ oxidative adsorption technique enhances the\nadsorption of Cu<sup>2+</sup>, Ni<sup>2+</sup>, and Cr(VI) heavy metal\nions on sodium hexatitanate nanorods from actual electroplating wastewater\nbecause the complexing interactions between organic ligands and heavy\nmetal ions were broken and weakened by the oxidation with calcium\nhypochlorite. When the oxidative adsorption was conducted at 25 g·L<sup>–1</sup> calcium hypochlorite, 125 mg·L<sup>–1</sup> hexatitanate nanorods, and 60 °C for 5 h, the concentrations\nof Cu<sup>2+</sup> and Ni<sup>2+</sup> ions could be efficiently reduced\nfrom 3.24 to 0.167 and from 0.248 to 0.0936 mg·L<sup>–1</sup>, respectively. The residual concentrations are lower than the discharge\nthresholds authorized by the Chinese Ministry of Ecology and Environment.\nThe removal percentages of Cr(VI) ions were around 21%, while the\ninitial Cr(VI) concentrations were in a range of 3.0–6.0 mg·L<sup>–1</sup>. The oxidative adsorption processes of Cu<sup>2+</sup>, Ni<sup>2+</sup>, and Cr(VI) on sodium hexatitanate nanorods from\nindustrial electroplating wastewater are well simulated by both pseudo-second-order\nadsorption kinetics and Langmuir adsorption isotherm. The thermodynamic\nanalyses show that the oxidative adsorptions of Cu<sup>2+</sup>, Ni<sup>2+</sup>, and Cr(VI) are spontaneous and endothermic. The value of\nΔ<i>G</i><sub>0</sub> decreases, whereas the values\nof Δ<i>H</i><sub>0</sub> and Δ<i>S</i><sub>0</sub> increase with increasing the oxidation extent of industrial\nelectroplating wastewater.