Although unmodified granulated activated carbon has a rich pore structure, its surface functional groups are of a single type, and its binding ability with heavy metal ions is relatively weak. The adsorption capacity is often limited by physical adsorption. In recent years, through surface modification techniques such as chemical oxidation, loading metal oxides, and grafting organic ligands, the surface chemical properties of granular activated carbon can be significantly optimized, providing an effective way to improve the adsorption performance for heavy metals.
Chemical oxidation modification is one of the most commonly used methods. By treating granulated activated carbon with oxidants such as nitric acid and hydrogen peroxide, a large number of oxygen-containing functional groups can be introduced. These groups combine with heavy metal ions through electrostatic attraction and coordination reactions. For example, after 5 mol/L nitric acid oxidation of coconut shell activated carbon, the surface carboxyl content increased from 0.5 mmol/g to 2.3 mmol/g. The adsorption process shifted from being dominated by physical adsorption to being dominated by chemical adsorption, and the adsorption equilibrium time was shortened by 30%. Moreover, oxidation modification can also increase the surface negative charge density, and through ion exchange, enhance the selective adsorption of cations.
Loading metal oxides modifies the material by introducing metal active sites with high affinity onto the surface of the granulated activated carbon, thereby constructing a "porous structure - metal site" synergistic adsorption system. Studies have shown that the surface groups of the loaded magnetic granular activated carbon can form inner complexes with it, with an adsorption capacity of 126 mg/g, which is 4.2 times that of the unmodified sample. Moreover, it can be quickly separated and recovered under an external magnetic field. Similarly, the adsorption capacity of the loaded granular activated carbon is increased to 98 mg/g. It not only provides redox sites but also enhances the chemical adsorption effect through hydroxyl groups.
Grafting organic ligands for modification involves fixing organic molecules with specific chelating functions onto the surface of granular activated carbon through covalent bonds, thereby achieving targeted adsorption of heavy metal ions. For instance, granular activated carbon grafted with disulfiramate has sulfur functional groups on its surface that can form stable chelates with it, with an adsorption capacity of up to 210 mg/g, and maintaining stable adsorption efficiency within the range. This solves the problem of a significant drop in adsorption capacity of traditional adsorption materials under strong acidic conditions. Moreover, the spatial steric hindrance effect of organic ligands can reduce non-specific binding at the adsorption sites, improving the selectivity for the target heavy metals.
However, surface modification also faces challenges: excessive oxidation may cause the collapse of the pore structure, resulting in a decrease in the specific surface area; excessive loading of metal oxides can lead to agglomeration, reducing the effective active sites; the grafting rate of organic ligands is limited by the number of surface hydroxyl groups, and it may also dissolve during long-term use. Future research should focus on optimizing the modification process parameters, developing "multifunctional synergistic modification" strategies, and combining density functional theory calculations to reveal the interaction mechanism between surface functional groups and heavy metal ions, providing theoretical support for the design of efficient heavy metal adsorption materials.