Nanoscale catalysts that can enable Fenton-like chemistry and produce reactive radicals from hydrogen peroxide activation have been extensively studied in order to overcome the limitations of homogeneous Fenton processes. Despite several advantageous features, limitation in mass transfer of short-lived radical species is an inherent drawback of the heterogeneous system. Here, we present a mechanistic foundation for the way spatial confinement of Fenton chemistry at the nanoscale can significantly enhance the kinetics of radical-mediated oxidation reactionspollutant degradation in particular. We synthesized a series of Fe 3 O 4 -functionalized nanoreactors with precise pore dimensions, based on an anodized aluminum oxide template, to enable quantitative analysis of nanoconfinement effects. Combined with computational simulation of spatial distribution of radicals, we found that hydroxyl radical concentration was strongly dependent on the distance from the surface of Fenton catalysts. This distance dependency significantly influences the gross reaction kinetics and accounts for the observed nanoconfinement effects. We further found that a length scale below 25 nm is critical to avoid the limitation of short-lived species diffusion and achieve kinetics that are orders of magnitude faster than those obtained in a batch suspension of heterogeneous catalysts. These findings suggest a new strategy to develop an innovative heterogeneous catalytic system with the most effective use of hydroxyl radicals in oxidation treatment scenarios.

Heterogeneous advanced oxidation processes (AOPs) allow for the destruction of aqueous organic pollutants via oxidation by hydroxyl radicals (•OH). However, practical treatment scenarios suffer from the low availability of short-lived •OH in aqueous bulk, due to both mass transfer limitations and quenching by water constituents, such as natural organic matter (NOM). Herein, we overcome these challenges by loading iron oxychloride catalysts within the pores of a ceramic ultrafiltration membrane, resulting in an internal heterogeneous Fenton reaction that can degrade organics in complex water matrices with pH up to 6.2. With •OH confined inside the nanopores (∼ 20 nm), this membrane reactor completely removed various organic pollutants with water fluxes of up to 100 L m–2 h–1 (equivalent to a retention time of 10 s). This membrane, with a pore size that excludes NOM (>300 kDa), selectively exposed smaller organics to •OH within the pores under confinement and showed excellent resiliency to representative water matrices (simulated surface water and sand filtration effluent samples). Moreover, the membrane exhibited sustained AOPs (>24 h) and could be regenerated for multiple cycles. Our results suggest the feasibility of exploiting ultrafiltration membrane-based AOP platforms for organic pollutant degradation in complex water scenarios.

Engineered nanoporous materials have been extensively employed in the environmental field to take advantage of increased surface area and tunable size exclusion. Beyond those benefits, recent studies have uncovered that the confinement of traditional environmental processes within several nanometer pores exerts unique nanoconfinement effects, such as enhanced adsorption capacity, reaction kinetics, and ion selectivity, compared to their analogous processes without spatial confinement. In this review, we provide a systematic discussion covering the current understanding of nanoconfinement effects reported across diverse fields using similar materials and structures as those being explored in environmental technologies. We further abstract the underlying fundamental physical and chemical principles including molecular orientation and rearrangement, reactive center creation, noncovalent binding, and partial desolvation. Finally, we establish connections between promising nanoconfinement observations and traditional environmental processes to identify challenges and opportunities for the development of innovative functional platforms for environmental applications.

Ordered nanocatalysts were produced alongside AAO channels, forming 3D-flexible nanoreactors with enhanced mass transfer and kinetics for internal catalysis.