UVC/Sulfite coupled to persulfate for removing chlorinated compounds under oxic conditions: effects of process...

Chlorinated phenols are persistent pollutants frequently detected in industrial effluents and contaminated surface waters, posing risks to aquatic ecosystems. Herein, the best initial sulfite concentration [SO₃^2-]₀ and initial pH for the degradation of 2,4-dichlorophenol (2,4-DCP) in the presence of sulfite under UVC radiation and oxic conditions are reported. Low sulfite dosages (60-150 mg L^-1) were explored to use UVC/sulfite as a pretreatment for persulfate (PS)-based processes. The UVC/persulfate (UVC/PS), UVC/sulfite, and combined UVC/sulfite + PS processes were used for the degradation of 2,4-DCP and 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP). Results have shown that increasing [SO₃^2-]₀ up to 60 mg L^-1 (pH₀ ∼ 8.5) has led to higher 2,4-DCP degradations through UVC/sulfite. In contrast, at pH₀ values around 3, 5, and 10, direct photolysis of 2,4-DCP and 2,3,4,6-TeCP occurred. Removals of 2,4-DCP and 2,3,4,6-TeCP were improved by the addition of PS after the UVC/sulfite pretreatment, at optimum conditions. Hydroxylation was predominant in the UVC/sulfite + PS system, by improved generation of hydroxyl radicals, in addition to a slightly higher contribution of sulfate radicals. Low molecular weight acids were observed as transformation products for the UVC/sulfite + PS process, due to dechlorination and cleavage of the benzene ring. Finally, according to predictions from the ECOSAR (Ecological Structure-Activity Relationships) program, fewer toxic products were identified for different organisms (fish, daphnids and green algae) in the UVC/sulfite + PS process. This result is important and reinforces that the UVC/sulfite + PS process in the presence of oxygen can lead to lower toxicity of the treated water.