Herbal medicinal waste black cardamom (Amomum subulatum) as a novel adsorbent for removing Cd (II) from water

A novel herbal medicinal waste-derived black cardamom (Amomum subulatum) adsorbent was investigated for cadmium (II) removal from water. The biosorbent was characterized by SEM, EDX, XRD, FTIR, and pH_ZPC. The equilibrium uptake of cadmium (II) from water was studied under the influence of solution pH, contact time, operating temperature, adsorbent dose, cadmium (II) initial concentration, and adsorbent size. The maximum removal of cadmium (II) (96%) through biosorbent was observed at pH of 4, 150–180 micron particle size, initial concentration of 50 mg/L in 50 ml solution having 0.5 g of dose. The cadmium (II) uptake by biosorbent was well fitted by Freundlich isotherm, suggesting multilayer adsorption as Chi-squared and sum of square errors values for the Freundlich isotherm model also lay in narrow range, i.e., 0.009–0.035 and 0.016–0.059, respectively. The higher coefficient of determination values of 0.972–0.997 also indicated a strong fit of the Freundlich model to the experimental data. Kinetic studies suggested appropriateness of the pseudo-second-order model with coefficient of determination values close to 1. Likewise, Chi-squared and normal standard deviation values also fell within smaller ranges of 0.002–0.007 and 0.006–0.016, respectively, for pseudo-second-order model. The thermodynamic evaluation revealed the adsorption of cadmium (II) to be spontaneous, random, and exothermic. The pore volume and surface diffusion model simulated in gPROMS software was used to analyze cadmium adsorption. The simulation results closely fitted with the experimental data. Thus, this novel adsorbent could be a low-cost and efficient alternative for removing cadmium (II) from polluted waters. Graphical abstract: [Figure not available: see fulltext.].