Ecotoxicological assessment of p-benzoquinone oxidation byproducts in water treatment systems: oxidative stress responses and mechanisms

p-Benzoquinone analogues, a class of ubiquitous oxidation byproducts, are increasingly detected in effluents from various oxidative treatment systems. While halogenated derivatives have received substantial research attention, the ecotoxicological impacts of non-halogenated analogues—particularly in vivo—remain poorly characterized. This study systematically compared the toxicity of 2,6-dichloro-p-benzoquinone (2,6-DCBQ) and 2,6-dimethyl-p-benzoquinone (2,6-DMBQ) using zebrafish embryos. 2,6-DMBQ exhibited comparable lethality to 2,6-DCBQ (48 h EC₅₀ = 221.72 vs. 169.17 μg/L), with developmental malformations of cardiac edema and notochord malformation mainly emerging in juvenile fish at 250 μg/L. Exposure to 2,6-DCBQ of 2.5 μg/L and 2,6-DMBQ of 25 μg/L induced significant oxidative stress, elevating reactive oxygen species (ROS) levels and upregulating antioxidant enzymes of total superoxide dismutase, catalase, and glutathione peroxidase. PCR gene chip analysis showed that 23 oxidative stress-related genes were coordinately upregulated in both treatment groups. Co-exposure with N-acetylcysteine (20 μM) or Na₂SO₃ (Na₂SO₃:p-BQs = 10:1) synergistically mitigated toxicity of 2,6-DCBQ and 2,6-DMBQ at 250 μg/L, restoring morphological and biochemical endpoints to control levels. Electron paramagnetic resonance confirmed semiquinone radicals and hydroxyl radicals as key drivers of ROS amplification. These findings establish radical-mediated mechanisms as critical toxicological pathways and inform risk management strategies for quinoid byproducts in water treatment systems.