Activated Carbon Adsorption of Trihalomethanes in Drinking Water: Effects of Pore Structure, Surface Functional Groups, and Hydration Structure—A Review

This review examines the adsorption of trihalomethanes (THMs) by activated carbon by treating surface hydration and internal pore structure as a continuous and interconnected sequence of phenomena. At pore entrances, an initial hydration layer is first established, and the superposition of hydration retained by surface acidic functional groups on this water distribution may influence the accessibility of THM molecules to pore walls. Within micropores, particularly in the primary adsorption region corresponding to pore widths of 0.6–0.9 nm (6–9 Å), dispersion interactions dominate. In this region, the interfacial environment is affected by stepwise changes in the state of water, proceeding from isolated molecules to critical density and subsequent clustering. In addition, the connectivity of pores from access pores to the primary adsorption region may affect molecular mobility during penetration into the internal pore network and plays a crucial role in determining reachability under flowing water conditions. These phenomena do not act independently. Rather, five factors—(i) initial hydration at pore entrances, (ii) hydration environments retained around surface acidic functional groups, (iii) stepwise changes in water structure within the primary adsorption region, (iv) pore connectivity from access pores to the primary adsorption region, and (v) the characteristics of the primary adsorption region governing equilibrium capacity—interact at different spatial locations to collectively shape THM adsorption behavior. When an appropriate balance among these factors is achieved, both the accessibility and reachability of THMs to activated carbon surfaces are effectively expressed under practical operating conditions. This integrated understanding may contribute to improved drinking water safety by supporting the development of activated carbons with enhanced THM removal performance through the integrated control of pore architecture, surface chemistry, and hydration environments.