Production of higher toxic intermediates of organic pollutants during chemical oxidation processes: A review

Incomplete degradation of organic pollutants might lead to the increased environmental risk because of the potentially toxic intermediates, especially during chemical treatment processes. However, toxicity changes of common pollutants during chemical oxidation are still unclear because the toxicity results were easily influenced by oxidation means, reaction condition, toxicity assessment method, etc. This review summarized and evaluated the toxicity test and prediction methods commonly used in oxidation process, and then the potential toxic intermediates and their possible producing pathways was proposed as for polycyclic aromatic hydrocarbon, phenols, azo dyes, organophosphorus, organonitrogens, organochlorine pesticides, pharmaceuticals of different classes. Various intermediates, especially the oxygen-containing ones, such as hydroquinone-, benzoquinone-, catechol-like products and halogenated ones are prone to have higher toxicity while some of the dimers and polymers are highly toxic and refractory. In addition, environmental elements such as DOM and halogen ion have the potential to enhance the toxicity. The strategies to reduce the residual toxicity were usually to strengthen the reaction, for example, to increase the oxidant concentration or prolong reaction time. Coupling of multiple technologies is also an effective strategy.