UVC/Sulfite coupled to persulfate for removing chlorinated compounds under oxic conditions: effects of process variables, transformation products and toxicity

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 [SO₃²⁻]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 [SO₃²⁻]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.