Beyond phase separation: New insights into chlorinated compound formation during Fe(III) coagulation with landfill leachate.

Coagulation with Fe(III) salts is a prevalent phase separation method for organic wastewater treatment. Nevertheless, our industrial applications have shown that using Fe(III) coagulants in landfill leachate treatment leads to an unexpected increase in the bio-toxicity of the effluent. This observation prompts us to identify the toxic compounds and explore their formation mechanisms. Laboratory experiments demonstrate that while Fe(III) coagulation removed approximately 92 % of the total organic matter from landfill leachate, it increased the acute bio-toxicity by about 14.5 times relative to the untreated effluent. Molecular analysis of the dissolved organic matter (DOM) in the leachate using Fourier transform ion cyclotron resonance mass spectrometry revealed a significant generation of chlorinated organic compounds during coagulation process. Compared to the Fenton system, the chlorinated compounds formed after coagulation showed a lower oxidation level and predominantly saturated and reduced state. The mechanistic studies suggest that reactive oxygen and chlorine species have a minimal role in the coagulation-induced chlorination. Instead, it is proposed that the redox reaction between Fe(III) and DOM leads to the formation of phenolic radicals, which facilitate nucleophilic reactions with Cl- (as an electron donor), thereby generating chlorinated compounds.