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The presence of external chloride can lead to a 47-fold increment in degradation rates of 4-chlorophenol than those in the absence of chloride in UV/peroxymonosulfate process. The other side of the same coin is an undesirable accumulation and increase in absorbable organic halogen (AOX) was observed in the presence of chloride, with formation of some more toxic tetrachlorinated byproducts.

Addition order of reagents in the Acid Orange 7 (AO7) degradation process was investigated by varying the concentration of Fe(II), the Fe(II)/peroxymonosulfate (PMS) molar ratio and stepwise addition of Fe(II) and PMS. The importance of addition order of reagents was confirmed and an order of Fe(II)–PMS to improve the oxidation efficiency was recommended.

Most of the current advanced oxidation processes (AOPs) are vulnerable to the presence of chloride in saline wastewater treatment because chloride not only affects the degradation kinetics but also probably leads to absorbable organic halogen (AOX) formation. Here we report an UV/Ce(IV) process can efficiently oxidize organic pollutants such as Acid Orange 7, even in the presence of chloride. Fluorescent probe technology suggests hydroxyl radicals were generated in UV/Ce(IV) process, but not in UV/Ce(IV)/Cl− system. In the presence of chloride, Ce(IV)–chloride complex was formed, which can directly oxidize dyes or generate reactive oxygen species by chlorine activation. Although degradation and mineralization rates of dyes were still inhibited to some extents by large amounts of chloride, but negligible AOX was generated. Therefore, UV/Ce(IV) process can be recommended as an alternative AOPs when treating acidic saline wastewater.