Catalyst or Bystander? Disentangling True Catalytic Effects from Confounding Phenomena in Ozone-Based Wastewater Treatment

Heterogeneous catalytic ozonation has generated thousands of publications over two decades, with these publications articulating a consistent narrative: catalysts enhance ozone decomposition, generate hydroxyl radicals (or other reactive species), and improve contaminant removal beyond that achievable by use of ozone alone. Despite this narrative, performance gains demonstrated at laboratory scale are routinely inconsistent and poorly reproduced at pilot and industrial scale. This perspective argues that the field suffers from a systematic attribution problem with observed enhancements frequently misattributed to catalytic activity when, instead, they arise from gas–liquid mass transfer modification, pH effects, and/or metal leaching driving homogeneous reactions. Using quantitative modeling, we further demonstrate that faster catalyst-mediated reaction kinetics alone may not translate into meaningful process-level benefits, particularly in full-scale reactors where relatively long hydraulic retention times and constraints on O3 gas–liquid mass transfer can diminish the advantage of kinetic acceleration. A sustained catalytic advantage at scale, therefore, requires not only kinetic acceleration but also more efficient utilization of ozone, an important distinction that has received limited attention in the HCO literature. Based on these findings, we propose a framework that will assist in distinguishing true catalytic pathways from various confounders and will help establish process-relevant benefit of HCO at scale. Overall, this perspective establishes a more rigorous evidentiary basis for translating catalytic activity into reliable wastewater treatment performance.