Adsorption of Divalent Copper Ions from Synthetic Wastewater Using Layered Double Hydroxides (NiZnFe) and Its Composites with Banana Biochar and Carbon Nanotubes

The adsorption potential of layered double hydroxides (LDH) of nickle–zinc–iron (NiZnFe) and its composites with single-wall carbon nanotubes (CNTs) and banana biochar (Bb) was investigated for divalent copper (Cu²⁺) removal in a batch system. Field emission scanning electron microscopy and FT-IR spectra confirmed the adsorption of Cu²⁺ onto LDH (NiZnFe) and its composites with Bb (LDH/Bb) and CNTs (LDH/cnt). The optimum equilibrium contact time was determined to be ~ 30 min, with LDH/Bb displaying the maximum uptake and removal efficiency (95%) for an initial Cu²⁺ concentration of 20 mg L⁻¹. Pseudo-second-order kinetic models presented high R² values (1.0) for all adsorbents, indicating good agreement between the theoretical adsorption capacities with experimental values. Multistep adsorption with both the surface and pore diffusion mechanism was suggested as well based on the intraparticle diffusion kinetic model. An optimum pH of 5.0 was considered with an increase in the uptake of Cu²⁺ and its removal efficiency, wherein LDH/Bb presented a greater removal efficiency and higher Cu²⁺ uptake compared with that of LDH (NiZnFe) and LDH/cnt. A gradual increase in Cu²⁺ uptake was observed in association with an increase in adsorbent dose from 0.2 to 0.5 g, with insignificant changes upon further increasing the dose of the adsorbent from 0.5 to 0.9 g. An increase in the initial Cu²⁺ concentrations from 10 to 100 mg L⁻¹ resulted in a decrease in the removal efficiency, whereas Cu²⁺ uptake increased almost linearly in the Cu²⁺ concentration range of 10–60 mg L⁻¹. Results of experimental data fitting using the Langmuir and Freundlich isotherm models suggest a dominance of monolayer adsorption, although multilayer adsorption appears to occur onto adsorbents with heterogeneous surfaces. Notably, chemisorption was also proposed to occur owing to the values of mean free energy of adsorption falling in the 8–16-kJ mol⁻¹ range, as calculated using the Dubinin–Radushkevich isotherm model. Importantly, the use of the SIP isotherm model indicated LDH/Bb to exhibit higher energy of adsorption and degree of heterogeneity than other two adsorbents. Thus, biochar or CNTs composited NiZnFe-LDH could serve as efficient adsorbents for Cu²⁺ removal from wastewater streams.