Impact of ozonation on the optical properties and photo-reactivity of dissolved organic matter

Abstract

With the increasing amount of wastewater discharges, the subsequent influence of treated effluent on the photochemistry of natural receiving waters has become more significant. Recently, the effect of treatment operations on the photo-reactivity of dissolved organic matter (DOM) has gained much attention. Meanwhile, some well-established correlations between spectroscopic measurements and photochemical behaviors become unreliable in treated DOM, thus requiring more robust relationships for better prediction. In the present research, the variations of optical and photochemical properties of DOM after ozonation were investigated. The correlations between fluorescence and reactive species photo-production were further explored through parallel factor analysis (PARAFAC). Results showed that ozonation induced the decline of aromaticity, molecular weight, and fluorescence intensities for all samples, which led to reduced photo-production of excited triplet states of DOM (3DOM*) and singlet oxygen (1O2). By contrast, the photo-formation of hydroxyl radical (•OH) was promoted. The correlations between optical parameters and quantum yields were evaluated. Furthermore, the max fluorescence intensity of PARAFAC components was used in linear regression modeling to predict the formation rate of 1O2 and •OH (R1O2 and R•OH). The contribution of each part was calculated. The good fit of R1O2 implied that ozonation would not affect the potential intrinsic relation between fluorophores and 1O2-producing chromophores. In contrast, the unsatisfactory modeling results of R•OH indicated a weak correlation between fluorescence and •OH formation. These findings suggest that ozonation influences the photo-reactivity of DOM and hint at the possibility of applying fluorescence combined with PARAFAC to explore 1O2 formation quantitatively.