Dissolved black carbon mediated photo-oxidation of arsenic(III) to arsenic(V) in water: The key role of triplet states

• Published in: Chemosphere (Oxford) 2024-01, Vol.347, p.140718-140718, Article 140718
• Main Authors: Zhou, Zhicheng, Yang, Lanqing, Qu, Xiaolei, Fu, Heyun
• Format: Article
• Language: English
• Subjects: Arsenic; Dissolved black carbon; Dissolved organic matter; Photochemistry; Triplet-excited states
• ISSN: 0045-6535; 1879-1298
• DOI: 10.1016/j.chemosphere.2023.140718

Arsenic is a common contaminant found in natural waters, and has raised significant environmental concerns due to its toxicity and carcinogenicity. In this study, we investigated the mediated photo-oxidation of arsenite (As(III)) under simulated sunlight by dissolved black carbon (DBC), an important dissolved organic matter (DOM) constituent released from black carbon. Five DBC were collected from the water extracts of black carbons that were derived by pyrolyzing different biomass (i.e., bamboo, rice, peanuts, corn, and sorghum stalks), and four well-studied dissolved humic substances (DHS) were selected for benchmarking. The presence of DBC (i.e., 5 mg C−1) significantly accelerated the photo-oxidation of As(III) to arsenate (As(V)), with the observed pseudo-first-order rate constant of reaction increased by 5∼11 times. Quenching experiments of photochemically produced reactive intermediates suggested that As(III) was mainly oxidized by triplet-excited DBC (3DBC*, contribution of 48%), singlet oxygen (1O2, 18%) and superoxide anions (O2•-, 28%) in sunlight-irradiated DBC solutions. The average apparent quantum yield of As(III) photo-oxidation for DBC was found to be more than 4 times higher in comparison with DHS. Such a strong mediation efficiency of DBC was due to its smaller molecular size and higher aromaticity than DHS, which facilitated the non-charge-transfer process to produce triplet-excited states and their sensitized 1O2. Consistently, DBC exhibited a higher apparent quantum yield and a longer lifetime of triplet states as compared with DHS. The results imply that DBC may play a previously unrecognized important role in the fate of arsenic in aquatic environments. [Display omitted] •Dissolved black carbon (DBC) could mediate the photo-oxidation of As(III) to As(V).•The mediation efficiency of DBC was markedly higher than previously well-studied DOM.•DBC had a high quantum yield for triplet generation and a long triplet lifetime.