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As a classic approach to water pollution control, adsorption is implemented in engineered systems predominantly as fixed-bed operation, in which wastewater flows through a packed bed of adsorbent to achieve pollutant removal. This mode is operationally straightforward and readily scalable, and it has been widely used for the advanced removal of inorganic pollutants such as heavy metals, arsenate/arsenite, phosphate, and fluoride from water. In contrast to the static adsorption protocols commonly used in laboratory studies, fixed-bed adsorption generally operates under thermodynamic non-equilibrium conditions, and its practical treatment performance is governed jointly by adsorption thermodynamics and kinetics. Surface complexes with thermodynamically stable configurations (e.g., multidentate complexes) are widely observed on existing adsorbents for the selective removal of inorganic pollutants, and their formation is generally regarded as the origin of adsorbent selectivity. Because the intraparticle diffusion of pollutants is mainly mediated by a “tumbling” process involving adsorption–desorption–readsorption, such thermodynamically stable configurations entail higher desorption energy barriers and lower surface diffusion rates, thereby constraining the practical working performance of fixed-bed adsorption. Synergistic modulation of the thermodynamic and kinetic properties of adsorbents is therefore of considerable importance for developing high-performance fixed-bed adsorption technologies. However, current research on adsorbent materials still focuses largely on thermodynamic properties, and studies on the synergistic modulation of both thermodynamic and kinetic properties for fixed-bed applications remain scarce.
Professor Bingcai Pan and Professor Xiaolin Zhang’s team has long been engaged in the design, preparation, mechanistic investigation, and application of millimeter-scale nanocomposites. Focusing on the advanced removal of characteristic pollutants in water, including heavy metal ions, oxyanions (arsenate/arsenite, phosphate, and antimonate), and fluoride, the research team has pursued a sustained series of fundamental investigations, technological developments, and engineering applications, effectively advancing nanomaterial-based fixed-bed adsorption technology from laboratory research to engineering practice (Environ. Sci. Technol. 2017, 51, 13363; Adv. Funct. Mater. 2020, 30, 1909014; Engineering 2023, 23, 149; ZL201910027161.2; ZL202110374731.2; ZL202310700790.3). Related outcomes have been demonstrated and deployed in the advanced treatment of industrial wastewater from the photovoltaic, semiconductor, electroplating, mining, and metallurgical sectors, as well as from municipal wastewater. Recently, the team developed a built-in electric field (BEF)-based strategy for the synergistic enhancement of fixed-bed performance. Through electrostatic attraction between the BEF formed by the millimeter-scale support and the negatively charged centers of pollutants, the adsorption configuration is locked into a thermodynamically metastable state, thereby achieving synergistic enhancement of adsorption thermodynamics and kinetics. Using the adsorption of arsenite [As(III)] by nanosized hydrous zirconium oxide (HZO) as a model system, the researchers loaded HZO particles into quaternized polystyrene resin (PS+) to prepare HZO@PS+, a nanocomposite with a built-in positive electric field. The BEF electrostatically attracts the negatively charged oxygen atoms of As(III), stabilizing a monodentate mononuclear (MM) configuration on the HZO@PS+ surface and effectively inhibiting the transformation of the metastable MM configuration to the stable bidentate binuclear (BB) configuration. Compared with the BB configuration (ΔG = −56.79 kJ/mol), the MM configuration (ΔG = −28.14 kJ/mol) exhibits a moderate adsorption free energy, thereby enhancing adsorption site utilization and equilibrium adsorption capacity (Qe) while maintaining high pollutant selectivity; it also increases the surface diffusion coefficient (Ds) of As(III) by accelerating desorption. The synergistic gains in adsorption thermodynamics (Qe) and kinetics (Ds) enabled HZO@PS+ to achieve an order-of-magnitude increase in effective working capacity in fixed-bed operation relative to the BEF-free material. This strategy was further extended to adsorption systems in which various metal oxides adsorb other oxyanions, such as As(V) and P(V), confirming the generality of this BEF modulation approach. This work provides a new route to the rational design of high-performance adsorbents for fixed-bed water treatment.
The study, entitled “Built-in electric field leverages synergistic thermodynamic–kinetic enhancement for superior fixed-bed oxyanion adsorption”, was published in the Proceedings of the National Academy of Sciences of the United States of America (article link: https://www.pnas.org/doi/10.1073/pnas.2613408123). Professor Bingcai Pan and Professor Xiaolin Zhang are the corresponding authors. Pengfei Shen, a doctoral student, is the first author, with Yidong Zhang and Huai Wang, master’s students, serving as co-authors. The research was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, and the Fundamental Research Funds for the Central Universities. The BL14W1 beamline at the Shanghai Synchrotron Radiation Facility provided support for X-ray absorption fine structure (XAFS) measurements.
Figure 1. (A) Photograph of the fixed-bed column adsorption apparatus, (B) fixed-bed As(III) adsorption capacity (NBV) for the nanocomposite with BEF (HZO@PS+) and without BEF (HZO@PS0) at 288–308 K, (C) NBV of HZO@PS+ over five adsorption–desorption cycles, (D) comparison between experimental (symbols) and model-predicted (lines) NBV values, derived from thermodynamic (Qe) and kinetic (βk) factors, and (E) schematic cascade illustrating how BEF-induced stabilization of metastable complexes leads to synergistic enhancement of oxyanion adsorption thermodynamics and kinetics, and ultimately improves fixed-bed working capacity.
