Mechanistic Insights into Enhanced Haloacetaldehyde Formation from Algal Organic Matters after Pre-ozonation: A Combined Computational and Experimental Study

Abstract

Pre-ozonation is widely used in the treatment of algae-containing water. Recent studies suggest that pre-ozonation could enhance the formation of haloacetaldehydes (HALs) but the mechanisms have not been clearly clarified. The study assessed the formation of dichloroacetaldehyde (DCAL) and trichloroacetaldehyde (TCAL) from model algal organic matters (Gly-Ala and Leu-Ala), and clarified the transformation mechanisms from the insight of quantum chemical computations. Results demonstrated a significant increase in HALs formation potential after pre-ozonation. When the dosage of O₃ was 0.5 mg L^-1, the formation of DCAL and TCAL increased 100% and 113% after chlorination, respectively. The initial pH of pre-ozonation was observed to be an important factor that influenced the HALs formation potential. The alkaline conditions promoted the formation of HALs, while acidic conditions have an inhibitory effect. Furthermore, the ultra-performance liquid chromatography high-resolution mass spectrometry (UPLC-HRMS) was applied to identify the transformation products of the pre-ozonation-chlorination process. Combined with quantum chemical computations, the transformation pathways of HALs were proposed. It was found that pre-ozonation promotes the formation of acetaldehyde, an intermediate during the formation of HALs. The initial pH plays an important role in the production of HALs, which influenced the reaction pathways and affected OH• formation. This study provides an in-depth analysis of the reaction mechanism and offers new theoretical insights for mitigating, controlling, and managing the risks associated with HALs.