Synergistic utilization of inherent halides and alcohols in hydraulic fracturing wastewater for radical-based treatment: A case study of di-(2-ethylhexyl) phthalate removal

The degradation of di-(2-ethylhexyl) phthalate (DEHP) was examined as an example to capitalize on the potential interactions of peroxydisulfate (PS) and ferrous iron (Fe²⁺) in the model Day-1/Day-90 and on-site hydraulic fracturing wastewater (FWW). The primary oxidative radicals in the Fe²⁺/PS system (i.e., SO₄^[rad]− and ^[rad]OH) were less effective for the degradation of DEHP (6.45%) in ultrapure water. Both chloride (Cl⁻) and bromide (Br⁻) at equivalent molar ratio with PS enhanced DEHP degradation (15.6% and 45.5%, respectively) via the generation of Cl^[rad] and Br^[rad] radicals, whereas the degradation rate was inhibited by the excessive amount of Cl⁻ or Br⁻ in the Day-1/Day-90 FWW. However, the co-presence of ethylene glycol (C₂H₄(OH)₂, 0.043% v/v in the FWW) and halide ions (Cl⁻ or Br⁻, 0.05 mM) resulted in the highest removal efficiency of 82.6 − 88.5% within 10 min by Fe²⁺/PS. Further investigation revealed that the formation of reductive alcohol radicals (^[rad]C₂H₃(OH)₂) slowed down or replenished the Fe²⁺ exhaustion. This study demonstrated that the Fe²⁺/PS-based advanced oxidation may show a synergistic interplay with Cl⁻/Br⁻ and C₂H₄(OH)₂ in the FWW, which depends on their relative concentrations. Thus, the inherent constituents in the fracturing wastewater can be utilized for the catalytic degradation of co-existing organic contaminants.