Enthalpy-driven directional enrichment of polycyclic aromatic hydrocarbons in gradient polycation hydrogels for high-efficiency water purification

The high-efficiency removal of harmful and persistent polycyclic aromatic hydrocarbon (PAH) pollutants from natural water is critical for safeguarding human health. A major challenge lies in the absence of effective technologies to accelerate the transfer of chemically inert and electroneutral PAHs from contaminated water to nano-adsorbents or nano-catalysts. Herein, we present a novel TiO 2 /polycation composite hydrogel membrane featuring dual gradients of positively charged groups and TiO 2 nano-catalysts for the deep and high-efficiency removal of PAH pollutants. The gradient polycation hydrogel matrix facilitates the spontaneous absorption of PAHs through cation-π interactions, while driving the directional migration of PAHs towards the high-density side via enthalpy-driven force induced by the positive charge gradient. Consequently, irradiating the high-density side of the dual-gradient composite hydrogel, enriched with PAHs and TiO 2 nano-catalysts, enables rapid and deep photocatalytic degradation of PAHs (e.g., naphthalene and phenanthrene). Furthermore, a mathematical model based on the Flory-Rehner theory is firstly developed to precisely control the distribution of PAHs within the composite hydrogels. Our findings provide a robust framework for the design of advanced composite catalysts with substantially enhanced removal efficiency for PAH pollutants.