Careful attention to the eco-toxicity and secondary water pollution of the degradation by-products.

Most of studies have mainly paid attention to the removal efficiency of a water treatment technology, while often overlooking the environmental and ecological toxicity. However, are water treatment technologies really environmental-/eco-friendly? If the as-generated chemicals are more toxic, it may lead to more severe secondary pollution. In this work, two novel integrated systems of Vis/Cu2O/H2O2 (visible light-excited H2O2 activation) and Vis/Cu2O/PS (visible light-excited persulfate (PS, Na2S2O8 activation) were designed and investigated fully, in which rhodamine B (RhB) was selected as the representative organic wastewater pollutant to assure the technology feasibility, although it has been studied widely. It is found that H2O2 and PS significantly enhance the degradation efficiency of RhB, and the degradation efficiencies of RhB for the Vis/O-Cu2O/H2O2 and Vis/O-Cu2O/PS systems have increased by 43.2 and 98.7 times than that of conventional Vis/O-Cu2O system (O-Cu2O, octahedral Cu2O). Furthermore, considering the incomplete mineralization, the degradation by-products are analyzed and their toxicity to grass carp and bacteria is evaluated. It is found that due to the production of the nitro- and chlorine-containing chemicals, the degradation by-products showed a higher toxicity to bacteria and grass carp, compared to RhB. This work warns us that it is crucial to completely mineralize the organic pollutants for all the present clean technologies, otherwise secondary pollution will be more dangerous to ecosystem.