Nanocatalysts have emerged as powerful tools in advanced oxidation processes (AOPs) for degrading refractory pollutants in water treatment. Their high surface area, tunable electronic properties, and ability to activate oxidants like persulfate or ozone make them ideal for breaking down persistent organic contaminants such as pharmaceuticals, dyes, and industrial chemicals. The integration of nanocatalysts into AOPs enhances reaction kinetics, reduces energy consumption, and improves mineralization efficiency compared to conventional methods.

Bimetallic nanoparticles, such as Fe-Co, Cu-Fe, or Pd-Ag, exhibit superior catalytic activity due to synergistic electronic effects between the metals. For example, Fe-Co nanoparticles activate peroxymonosulfate (PMS) more effectively than single-metal counterparts, generating sulfate radicals (SO4•−) and hydroxyl radicals (•OH) for pollutant degradation. The electron transfer between Fe and Co facilitates redox cycling, maintaining catalytic activity over multiple cycles. Similarly, Pd-Ag nanoparticles enhance ozonation by promoting ozone decomposition into reactive oxygen species (ROS), including •OH and atomic oxygen (O•).

Doped metal oxides, such as nitrogen-doped TiO2 or cobalt-doped CeO2, modify the band structure and surface chemistry of catalysts, improving oxidant activation. Nitrogen doping introduces mid-gap states in TiO2, enabling visible-light-driven persulfate activation. Cobalt-doped CeO2 enhances electron mobility, accelerating the conversion of persulfate to SO4•−. These doped catalysts also resist leaching and deactivation under acidic or alkaline conditions, making them suitable for industrial wastewater treatment.

The catalytic mechanisms in AOPs depend on the oxidant-nanocatalyst interaction. In persulfate activation, nanocatalysts facilitate electron transfer to break the O-O bond in persulfate, producing SO4•−. For ozonation, nanocatalysts provide surface sites for ozone adsorption and decomposition into ROS. The presence of oxygen vacancies or defects on nanocatalysts further enhances oxidant activation by serving as active sites.

Stability is a critical factor for nanocatalysts in harsh environments. Encapsulation in porous matrices (e.g., carbon shells, silica, or metal-organic frameworks) prevents aggregation and metal leaching. Magnetic nanocatalysts, such as Fe3O4@CuO, enable easy recovery using external magnets, reducing operational costs. Long-term stability tests show that some bimetallic nanocatalysts retain over 90% activity after 10 cycles in persulfate systems.

Degradation pathways of pollutants vary based on the ROS involved. SO4•− preferentially attacks electron-rich moieties in pharmaceuticals (e.g., aromatic rings or amine groups), while •OH non-selectively oxidizes organic compounds. For dyes like methylene blue, radical attack leads to demethylation, ring cleavage, and eventual mineralization to CO2 and water. Intermediate products are identified using mass spectrometry, revealing stepwise degradation mechanisms.

Reactor designs integrating nanocatalysts include fixed-bed, fluidized-bed, and membrane reactors. Fixed-bed reactors pack nanocatalysts on porous supports (e.g., alumina or activated carbon), ensuring continuous flow and high contact efficiency. Fluidized-bed reactors suspend nanocatalysts in wastewater, improving mass transfer but requiring post-treatment separation. Membrane reactors combine nanocatalysts with ultrafiltration membranes, allowing simultaneous degradation and filtration. Pilot-scale studies demonstrate that these reactors achieve over 80% removal of contaminants like sulfamethoxazole or rhodamine B within minutes.

The scalability of nanocatalyst-based AOPs depends on cost, catalyst lifetime, and energy input. While noble metal nanocatalysts (e.g., Pd or Ag) offer high activity, transition metal alternatives (e.g., Fe or Cu) are more economical for large-scale applications. Future research focuses on optimizing nanocatalyst synthesis, reactor configurations, and hybrid AOPs to maximize efficiency and minimize environmental impact.

In summary, nanocatalysts play a pivotal role in AOPs by enabling efficient, stable, and selective degradation of refractory pollutants. Their integration into advanced reactor systems paves the way for sustainable water treatment solutions.