Natural sunlight-driven aquatic toxicity enhancement of 2,6-di-tert-butylphenol toward Photobacterium phosphoreum

The tert-butylphenols (TBPs) are one group of alkylated phenolic compounds with wide applications in UV absorbers and antioxidants. They are becoming contaminants of emerging concern with residues frequently detected in natural surface water or drinking water. The direct sunlight may photolyze TBPs in waters and affect their aquatic toxicities; however, such data are very limited. In the present study, we investigate the photodegradation of 2,6-DTBP by direct sunlight in water and compare the aquatic toxicities of 2,6-DTBP with that of its product toward Photobacterium phosphoreum. 2,6-DTBP is photodegraded by 71.31 ± 2.64% under simulated sunlight following a pseudo-first-order kinetics with rate constant (k) of 0.061 h−1. Density functional theory simulations at M06–2X/def2-SVP level reveal that the photodegradation occurred sequentially through oxidation, photo-isomerization and hydrogenation. The degradation product 2,5-DTBP is toxic to P. phosphoreum (EC50 3.389 × 10−5 mol/L) whereas 2,6-DTBP is not harmful (EC50 3.917 × 10−3 mol/L) as designated by the European Union Standard, indicating the enhanced toxicities driven by the direct sunlight photodegradation. We demonstrate the enhanced toxicities of 2,6-DTBP by natural sunlight, suggesting that negligence of photodegradation of TBPs-related contaminants will underestimate the comprehensive risk of these emerging contaminant in natural waters. [Display omitted] •2,6-DTBP is photodegraded by > 70% after 24 h natural sunlight irradiation.•The photodegradation of 2,6-DTBP follows the pseudo-first-order kinetics.•2,6-DTBP is photodegraded via oxidation, photo-isomerization and hydrogenation.•2,5-DTBP is the major degradation product of 2,6-DTBP.•2,5-DTBP exhibits higher toxicity toward Photobacterium phosphoreum than 2,6-DTBP. 2,6-DTBP was photodegraded by > 70% after 24 h natural sunlight irradiation and its product 2,5-DTBP exhibits higher toxicity toward Photobacterium phosphoreum.