Performance evaluation and mechanistic insights into the adsorption of heavy metal ions in water by various carbon materials

To develop effective adsorbents for removing trace heavy metal ions from aqueous solutions, four representative carbon materials (activated carbon, N-modified activated carbon, hydroxyl-functionalized carbon nanotubes, and whisker carbon nanotubes) were systematically investigated for their cobalt ion (Co ²⁺ ) adsorption capabilities. This study comprehensively examines the influence of morphological structure, surface functional groups, and charge properties on adsorption performance. The predominant adsorption mechanisms were elucidated through adsorption models and thermodynamic analyses. Results demonstrate that activated carbon and its N-modified counterpart exhibit superior adsorption capacities for trace Co ²⁺ , with N-modified activated carbon achieving the highest removal efficiency of approximately 80%. The adsorption process follows the Langmuir isotherm model, indicating monolayer adsorption behavior with exothermic characteristics. Physical adsorption dominates Co ²⁺ removal by activated carbon and whisker carbon nanotubes, where adsorption capacity correlates with specific surface area and pore volume. In contrast, both physical and chemical adsorption mechanisms operate in N-modified activated carbon and hydroxyl-functionalized carbon nanotubes, with adsorption performance being governed by surface hydroxyl group density. All materials maintained stable adsorption efficiencies (>95% of initial capacity) through three consecutive adsorption-desorption cycles. Notably, N-modified activated carbon demonstrates exceptional versatility in removing other heavy metal ions (Pb ²⁺ and Cr ⁶⁺ ), highlighting its significant potential for practical wastewater treatment applications.