Beyond pollutant removal: process metrics and synergy in high frequency sonophotocatalysis of emerging concern pollutants

Significance and Relevance High frequency sonophotocatalysis is often reported in terms of enhanced pollutant removal, but the real significance of synergistic effects is rarely evaluated beyond kinetics. Here, sonophotocatalytic processes are assessed through process-relevant metrics at the reactor level. Using emerging concern pollutants (ECPs) as model targets, the synergy is shown to be material- and regime-dependent, demonstrating that high degradation efficiency does not necessarily imply effective mineralization. The results provide a more rigorous framework to evaluate when coupled catalytic processes offer an advantage. Introduction and Motivations Emerging concern pollutants (ECPs), including pharmaceuticals and personal care products, are increasingly detected in natural and treated waters due to their continuous release and incomplete removal by conventional wastewater treatment technologies.1 Even at low concentrations, they raise significant environmental and health concerns, not only because of their persistence, but also due to the formation of transformation products whose impact is often poorly understood. Consequently, evaluating advanced treatment strategies solely in terms of pollutant removal may lead to misleading conclusions regarding their real effectiveness. Among heterogeneous advanced oxidation processes, photocatalysis and sonochemistry have attracted growing attention for ECPs abatement, although both approaches still suffer from intrinsic limitations when applied alone.2 Sonophotocatalysis has emerged as an intensification strategy combining ultrasonic cavitation and light irradiation within the same reaction environment. While enhanced degradation rates are frequently reported and described as “synergistic”, such claims are often based on apparent pollutant disappearance, without considering mineralization, by-product formation, or process relevance — an aspect particularly critical for ECPs. High frequency ultrasound (US) represents a distinctive operational regime, offering improved control over cavitation phenomena and reactive species generation compared to conventional low-frequency systems. In this framework, this study critically assesses high frequency sonophotocatalysis for ECPs by moving beyond pollutant removal and focusing on process-relevant metrics. Materials and Methods BiOCl and BiOBr were synthesized by co-precipitation, whereas commercial TiO2 P25 was the benchmark material. The physico-chemical properties of the catalysts were investigated by complementary characterization techniques. The cavitation regime generated within the high frequency ultrasonic reactor was assessed by potassium iodide dosimetry at different operating frequencies (584, 864, and 1148 kHz). Ibuprofen (IBU) abatement experiments compared sonocatalysis (584 kHz, 50% amplitude, continuous mode), photocatalysis under simulated solar light (35 W·m-2), and combined sonophotocatalysis. Results and Discussion High frequency sonophotocatalysis in the presence of BiOCl, BiOBr or TiO2 P25 was first evaluated in terms of IBU removal under photo-, sono- and sonophotocatalytic conditions at the operating frequency of 584 kHz. Figure 1a reports representative results using BiOBr as catalyst: coupling US and light irradiation led to a pronounced enhancement of IBU abatement compared to the individual processes, confirming that high frequency cavitation creates a highly oxidative environment capable of intensifying photocatalytic reactions. However, the synergy analysis (Figure 1b) revealed that enhanced IBU removal does not necessarily correspond to true synergistic behavior. Synergy was found to be strongly material-dependent: BiOBr exhibited a synergy factor significantly higher than unity, whereas BiOCl and TiO2 P25 mainly showed additive effects. This trend can be rationalized by considering the materials’ properties, as BiOBr combines a more favorable band structure with enhanced charge-carrier separation and photoresponse, enabling a more effective interaction with cavitation-generated reactive species. Moving beyond IBU disappearance, mineralization data provided further insight into process relevance. Despite high IBU removal efficiencies, TOC abatement generally remained moderate, indicating that ultrasound-assisted processes may promote molecular fragmentation rather than complete oxidation if not properly coupled with photocatalytic pathways. BiOBr-assisted sonophotocatalysis achieved the highest degree of mineralization, highlighting that synergy should be evaluated not only in kinetic terms but also through mineralization-oriented metrics (Figure 1b, inset). LC–MS/MS analysis of transformation products further supports the need to evaluate sonophotocatalytic processes beyond IBU removal. Finally, preliminary process-level considerations (Figure 1c) place the observed synergistic effects within a broader technological perspective, showing that high frequency sonophotocatalysis can offer competitive cost-of-ownership values compared to other US-based technologies.3