Catalytic hydrodehalogenation activity and selectivity of polyiodinated phenolic disinfection byproducts at ambient...

Iodo-phenolic disinfection byproducts (DBPs) widely occur in disinfected water, posing potential risks to human health and the ecosystem as they possess higher toxicity than the bromo- and chloro-analogs. Herein, we elucidated the catalytic hydrodehalogenation (HDH) activity and selectivity of polyiodinated phenolic DBPs on supported noble metal catalysts at ambient conditions. Both 2,4,6-triiodophenol and 4-chloro-2,6-diiodophenol can be efficiently eliminated on Pd/TiO₂ and Rh/TiO₂ within 20 min, with Pd/TiO₂ exhibiting higher turnover frequency. The HDH reactions proceeded in both stepwise and concerted pathways on Pd/TiO₂, while they were dominantly stepwise on Rh/TiO₂. Experimental results and theoretical calculations revealed that the HDH selectivity depends on the position and the bond energy of halo-substitutions. For the HDH of 2,4,6-triiodophenol, the para-substituted iodine was more favorable to be dehalogenated than the ortho-substituted ones due to the steric hindrance of the hydroxyl group. For the HDH of 4-chloro-2,6-diiodophenol, the ortho-substituted iodine was removed before the para-substituted chlorine as CI bond had higher reactivity than CCl bond. Significant catalyst deactivation was observed for the HDH of 4-chloro-2,6-diiodophenol on Pd/TiO₂ due to iodine poisoning, resulting in 4-chlorophenol as the dominant product. In contrast, Rh/TiO₂ can completely hydrodehalogenate 4-chloro-2,6-diiodophenol into cyclohexanone with little iodine poisoning. Our results suggest that HDH is an efficient process for abating iodo-phenolic DBPs. Rh/TiO₂ is a more promising HDH catalyst for iodinated DBP removal than Pd/TiO₂ with excellent resistance to iodine poisoning.