Comparative study of the degradation of meta-chloronitrobenzene by UV/hydrogen peroxide and UV/persulfate oxidation: Degradation pathways and toxicity

Advanced oxidation processes (AOPs) offer effective means to degrade recalcitrant contaminants, yet concerns about the formation of toxic products may hinder their practical application. This study compared the degradation of meta-chloronitrobenzene (mCNB) by UV-activated hydrogen peroxide (UV/H2O2) and persulfate (UV/PDS) oxidation. Both methods demonstrated efficient mCNB removal, with 73 % and 64 % removal observed, respectively, in 30 min at an oxidant dose of 1.0 mM. However, the products formed in the UV/PDS process exhibited higher toxicity compared to those formed in the UV/H2O2 process. This difference in toxicity arose from the distinct rection behavior of hydroxyl radical (HO center dot) and sulfate radical (SO4 center dot-), the principal reactive species in UV/H2O2 and UV/PDS processes, respectively. HO center dot mainly attacked mCNB via an addition mechanism, forming hydroxylated products in UV/H2O2. In contrast, SO4 center dot- reacted with mCNB via single-electron transfer, forming a radical cation (mCNB(center dot+)). This radical cation underwent rapid dissociation or hydrolysis to 3-chrolophenol (3CP) and 2-chloro-4-nitrophenol (2C4NP), respectively. Subsequent oxidation of 2C4NP by SO4 center dot- generated a phenoxyl radical. The coupling of mCNB(center dot+) or phenoxyl radical with a nitrogen dioxide radical (NO2 center dot) derived from mCNB denitration, leading to the generation of more toxic dinitro-products. We assume these findings can be extrapolated to the treatment of other nitroaromatic compounds. SO4 center dot--based oxidation technologies tend to generate more polynitrated byproducts than HO center dot-based oxidation technologies, thus posing a greater potential risk. This consideration should guide the selection of appropriate treatment technologies for these contaminates.