Experimental study on the oxidation of sodium diatrizoate by an Fe(Ⅱ)-enhanced NaClO system

This study investigates the Fe(Ⅱ)-enhanced NaClO oxidative degradation of sodium diatrizoate (DTZ), a typical ionic iodinated X-ray contrast medium frequently detected in aquatic environments. Owing to its high stability and poor biodegradability, DTZ is difficult to eliminate through conventional wastewater treatment processes and may generate highly toxic iodinated disinfection byproducts during chlorination. Therefore, a comprehensive teaching experiment was designed by integrating advanced oxidation system construction, parameter optimization, reactive species identification, degradation pathway analysis, and toxicity evaluation. This study aims to investigate the degradation performance and mechanism of DTZ in the Fe(Ⅱ)/NaClO system while cultivating students' abilities in understanding reaction mechanisms, performance evaluation, and engineering-oriented thinking under the framework of Emerging Engineering Education.

A FeSO₄-enhanced NaClO advanced oxidation system was established for DTZ degradation. Single-factor experiments were conducted to evaluate the effects of key operational parameters, including NaClO concentration, FeSO₄ concentration, initial pH, and reaction temperature, on the degradation efficiency of DTZ. High-performance liquid chromatography was employed to monitor DTZ concentrations during the oxidation process. Radical quenching experiments using nitrobenzene, tert-butanol, and benzoic acid were performed to identify the dominant reactive species involved in DTZ degradation. Combined with a steady-state kinetic model, the contributions of hydroxyl radicals and reactive chlorine species were quantitatively evaluated. Furthermore, degradation intermediates were analyzed using ultra-high-performance liquid chromatography coupled with quadrupole Exactive high-resolution mass spectrometry, and the formation of iodinated disinfection byproducts was determined using gas chromatography. Changes in ecological toxicity before and after oxidation were assessed using the Ecological Structure Activity Relationships model.

The Fe(Ⅱ)/NaClO system exhibited excellent oxidation performance for DTZ degradation. Under optimal conditions (0.015 mmol·L⁻¹ DTZ, 0.9mmol·L⁻¹ Fe(Ⅱ), 6 mmol·L⁻¹ NaClO, pH 3.0, and 25 ℃), the DTZ degradation efficiency of the Fe(Ⅱ)/NaClO system reached 84.86% within 60 min, which was significantly higher than that achieved using NaClO alone (5.22%) or Fe(Ⅱ) alone (< 1%). Radical scavenging experiments demonstrated that hydroxyl radicals played a relatively minor role in DTZ degradation, whereas reactive chlorine species were the dominant oxidizing agents. Quantitative kinetic analysis revealed that ClO· and Cl₂·⁻ were the primary reactive radicals, accounting for 65.18% of the overall degradation contribution. The contributions of ·OH and Cl· to the overall DTZ degradation were 16.66% and 14.68%, respectively. In addition, ten degradation intermediates were identified in the Fe(Ⅱ)/NaClO system. The degradation process mainly involved deiodination, hydroxyl substitution, amide bond cleavage, decarboxylation, deacetylation, and intramolecular cyclization reactions. Compared with the NaClO system, the Fe(Ⅱ)/NaClO process generated lower concentrations of iodinated disinfection byproducts. The concentration of CHCl₂I decreased from 0.330 8 μg·L⁻¹ in the NaClO system to 0.298 8 μg·L⁻¹ in the Fe(Ⅱ)/NaClO system. Toxicity prediction results indicated that although some intermediates exhibited increased toxicity toward aquatic organisms, the oxidation process transformed DTZ into smaller molecules with improved biodegradability and reduced environmental risk.

The Fe(Ⅱ)/NaClO system can efficiently degrade DTZ through an oxidation mechanism dominated by reactive chlorine species. ClO· and Cl₂·⁻ are the key reactive species responsible for pollutant degradation. Compared with NaClO oxidation alone, the Fe(Ⅱ)/NaClO system achieves higher DTZ removal efficiency and reduces the formation potential of harmful iodinated disinfection byproducts, demonstrating improved environmental safety. The developed teaching experiment effectively integrates environmental engineering, analytical chemistry, reaction kinetics, and toxicity assessment, providing students with a comprehensive platform for developing experimental skills, integrating interdisciplinary knowledge, and innovative problem-solving abilities.