Nickel ion removal from aqueous media using polyaniline–macadamia nutshells and its reuse for photodegradation of orange dye

The present study describes the adsorption of nickel(II) ion onto an agrowaste-polymeric nanocomposite, polyaniline–macadamia nutshells. The pre-synthesized nanocomposite was characterized via scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffractometry and Fourier transform infrared spectroscopy analyses. The adsorption process was studied as a function of the initial pH (2–8) of simulated nickel ion solution, initial nickel ion concentration (25–100 mg/L), contact time (5–300 min), adsorbent dosage (0.5–4 g/L), temperature (25–45 °C) and agitation speed (180 rpm). The optimum pH was found to be pH 7 and the ideal adsorbent dosage was 2 g/L. The best fit for the kinetics data (R² = 0.99) was recorded using the pseudo-second order. The maximum adsorption capacity (238.1 mg/g) was obtained via the Langmuir isotherm model at 45 °C. The values recorded for the changes in Gibbs free energy (− 2.30, − 2.94 and − 3.44 kJ/mol) were found to be below 20 kJ/mol. Similarly, the value recorded for the enthalpy change (14.52 kJ/mol) was also below 20 kJ/mol. Both instances are indicative of the existence of physisorption. Surface complexation and cation–π interaction are possibly involved in the adsorption process. In order to elucidate on the reusability of the nanocomposite, the spent adsorbent was reused for the photodegradation of another contaminant, orange 2 dye. The complete degradation (99.7%) of the dye was achieved within 25 min. Overall, the results show that polyaniline–macadamia nutshells have the potential to remove nickel ions from aqueous media. Graphical abstract: [Figure not available: see fulltext.]