Making waves: Transforming the sulfate footprint in AOPs-based wastewater treatment from passive burden to active control.

Sulfate is a widespread and persistent byproduct in many advanced oxidation processes (AOPs) for wastewater treatment, yet its residue has been systematically overlooked. Despite the original intention and widespread adoption for degrading recalcitrant organic pollutants in industrial wastewaters, AOPs inadvertently produce substantial sulfate through catalyst/co-catalyst addition, pH adjustment, oxidant decomposition or electrolyte usage. Elevated sulfate levels affect the AOPs-treated effluent quality directly or indirectly by impairing subsequent biological treatment, posing public health and socio-economic impacts. This Making Waves critically examines the neglected sulfate footprint across mainstream AOPs and proposes forward-looking strategies for sulfate mitigation from both the source and the effluent. These include accelerating Fe(III)/Fe(II) cycling to reduce ferrous sulfate input, developing AOPs with broad pH adaptability and acid-free operation, reducing persulfate consumption and integrating post-treatment biological or bioelectrochemical systems for sulfate removal. Balancing organic pollutant degradation with sulfate management, shifting from treating sulfate as a passive burden to exercising active control, is essential for transitioning AOPs toward environmentally sustainable wastewater treatment technologies.