Calcium oxide-modified activated sludge as a low-cost biomass adsorbent for Cd(II) removal in aqueous solution: biosorption behavior and mechanism

In this study, a highly effective, low-cost, and environment-friendly calcium oxide-modified activated sludge (CaO-AS) bio-adsorbent was prepared, and its potential as an adsorbent for cadmium ion (Cd(II)) was studied with respect to pH, initial concentration of Cd(II), temperature, and contact time. Further, the discussion on the mechanism for Cd(II) adsorption indicated that the adsorption equilibrium time was 90 min. A neutral medium and increasing temperature were found to be beneficial for the Cd(II) adsorption on CaO-AS. Moreover, the optimal CaO-AS dosage was 1.0 mg/L. Cd(II) adsorption by CaO-AS obeyed pseudo-second-order kinetics (R-2 > 0.9998) mainly due to chemisorption. Langmuir isotherm model was more suitable in describing the adsorption process which was endothermic in nature. The thermodynamic result shows that Cd(II) adsorption onto CaO-AS was thermodynamically feasible and spontaneous, and the adsorption process was endothermic. Intra-particle diffusion model analysis indicates that the film diffusion mechanism controlled the adsorption process. The adsorption of Cd(II) by CaO-SA was chemisorption, and the removal of Cd(II) from aqueous solutions was carried out by surface co-precipitation, ion exchange, and binding of surface functional groups. This study provides the theoretical and data support for industrial application of CaO-AS bio-adsorbent and makes it possible to reduce the preparation cost of biomass-based adsorbents and increase their application potential in the future.

Automated summary

A novel CaO-AS bio-adsorbent was prepared for Cd(II) adsorption, with optimal conditions and mechanisms identified through experiments and analysis. The study provides theoretical and data support for industrial application of CaO-AS bio-adsorbent, reducing preparation costs of biomass-based adsorbents and enhancing their future potential.