Binary adsorption of toxic nickel(II) and cadmium(II) ions from aqueous solution by acid modified chilli peppers

The acid-modified chilli peppers, a novel adsorbent, were prepared and used to absorb Ni(II) and Cd(II) ions from aqueous solution. Batch experiments with raw chilli peppers were compared with those of acid-modified chilli peppers. Batch experiments were conducted to evaluate the adsorption capacities of both sorbents under different conditions, including pH, concentration, contact time and temperature, in solutions. Bands with oxygen groups (-OH, -COOH, -C = O, -COC and -CO) on both sorbents were identified by Fourier transform infrared spectroscopy. Scanning electron micrographs of raw chilli peppers showed a morphology resembling spheres, while acid-modified chilli peppers showed a structure resembling a mesh of a mixture of different shapes, including squares and triangles. Maximum sorption capacities were achieved at pH 7, with the uptake of Ni(II) and Cd(II) ions dependent on pH.

The highest adsorption capacities for raw chilli peppers and acid-modified chilli peppers were observed at 70 °C. The sorption capacities increased with increasing temperature. The concentration effect showed that as the concentration of Ni(II) and Cd(II) ions increased on raw chilli peppers and acid-modified chilli peppers, the adsorption capacity also increased. On raw chilli peppers, the rate of removal of Ni(II) and Cd(II) ions in solution was 60 min, while on acid-modified chilli peppers it took 80 min. The adsorption of Ni(II) and Cd(II) ions was best described by the Freundlich model with heterogeneous coverage and maximum capacities of 45.33 and 39.12 mg/g, respectively. The uptake of Ni(II) and Cd(II) ions was endothermic and showed positive values (∆Hº). Gibb′s free energy (∆Gº) for Cd(II) and Ni(II) ions on the activated carbon showed that the reaction was spontaneous. The change in entropy (ΔS^o) of the reactions showed positive magnitudes for both pollutants, indicating a high degree of randomness in the solid-liquid phase. The kinetic model that best fitted the mechanism was pseudo-first order for Ni(II) and Cd(II) ions, indicating correlation numbers of R² values of 0.9923–0.9995. Therefore, the acid-modified chilli peppers can be used as a viable alternative adsorbent for effective removal of Cd(II) and Ni(II) ions from aqueous solution.