Macrocyclic porphyrin photocatalysts without metal chelation: A novel pathway for complete degradation of tough halophenols with longwave visible LED light source

• Published in: Journal of hazardous materials 2024-10, Vol.478, p.135342, Article 135342
• Main Authors: Lau, Chun-Yin, Yeung, Chi Shun, TSE, Ho-Yin, Luk, Hoi Ling, Yu, Chung Yin, Yuen, Chun Bong, Phillips, David Lee, Leu, Shao-Yuan
• Format: Article
• Language: English
• Subjects: Fluorescence quenching; Halophenols; Metal-free porphyrin; Photodegradation; Visible light
• ISSN: 0304-3894; 1873-3336; 1873-3336
• DOI: 10.1016/j.jhazmat.2024.135342

Halophenols are toxic and persistent pollutants in water environments which poses harm to various organisms. Due to their high stability and long residence time, ultraviolet radiation, heavy metals and oxidizing agents have been largely adopted on treating these compounds. However, these treatment methods could pose toxicity or hazardous risks to the marine environment and plant operators. In this study, a water-soluble porphyrin photocatalyst was synthesized and introduced for halophenol treatment using UV-free LED white light. The porphyrin catalyst is a macrocyclic ring consisting of pyrroles linked with methine bridges, the highly conjugated ring provided the superior functionality of visible light absorption. Surprisingly, over 99 % degradation of halophenols and over 90 % dehalogenation have been achieved without metal chelation, even higher than those of transition metal porphyrins with inclusion of Fe3+, Zn2+, Cu2+, Co2+, Ni2+, and Mn2+. Ring-opening reactions were confirmed with the formation of carboxylic acids; dicarboxylic acids like acrylic acid, and malonic acid; while fumaric acid was the main product. Total organic carbon results indicated no CO2 produced during the reaction. Triplet absorbance and scavenger studies also indicated that singlet oxygen and conduction band electrons are the main radical species for halophenol degradation. The 100-fold singlet emission quenching over triplet absorption quenching indicated that the excited electrons tend to be transferred via singlet state. This concept brings along new approaches detoxifying halophenol-related wastewater without UV, metals and other additives, which is more environmentally-friendly and sheds light to the conversion of toxic materials into useful chemical precursors. [Display omitted] •A unique function of newly synthesized porphyrin photocatalyst was discovered.•Ring opening of tough trihalophenols accomplished using UV-free LED light without metal.•100 % 2,4,6-TFP degradation and > 90 % 2,4,6-TCP dehalogenation were achieved.•Singlet state photoexcited electron transfer showed significant contribution.•Valuable carboxylic acids were major products with > 80 % C4 selectivity and zero CO2