Selective oxidation of nitrogenous heterocyclic compounds by heat/peroxymonosulfate in phenol-rich wastewater

In phenol-rich wastewater, such as coking wastewater, due to the high reactivity of phenol to various reactive oxygen species, it is difficult to selectively oxidize pollutants having lower biodegradability and higher toxicity than phenol. As one kind of such pollutants in coking wastewater, some nitrogenous heterocyclic compounds (NHCs) are more difficult to be removed by SO₄^•- or HO• than phenol, but this study found that NHCs (quinoline, isoquinoline, and pyridine) can be selectively removed by peroxymonosulfate (PMS) direct oxidation in the presence of 10 mM phenol under thermal condition. The selective oxidation of NHCs needs a suitable pH range (4 < constant pH < 9) because protonated state of NHCs (pH < 4) is unfavorable to their oxidation and high pH would improve the extra PMS consumption by phenol. Under the conditions benefiting the removal of NHCs in heat/PMS system, there was no generation of SO₄^•- and HO•. Being treated by 60 °C/PMS for 60 min, the biodegradability (BOD₅/COD) of real coking wastewater (RCW) was improved from 0.21 to 0.44 with low removal rate of phenols (about 10%). Quinoline and indole, as the two typical NHCs in the studied RCW, their removal rates can be up to 45% and 85%, respectively. Thus, heat/PMS pretreatment is a potential good way to selectively remove high toxic pollutants in phenol-rich wastewater.