Evaluating the pH-dependence of DOM absorbance, fluorescence, and photochemical production of singlet oxygen

Abstract

The protonation state of dissolved organic matter (DOM) impacts its structure and function in natural and engineered environmental systems, including DOM's ability to absorb light and form photochemically produced reactive intermediates (PPRI). However, the impacts of pH on DOM optical properties and PPRI formation have largely been evaluated separately, with less information being available on their interrelationship as a function of pH for the same set of samples. It is also unclear whether the impact of pH on optical spectra and associated optical surrogates for molecular size (e.g., E2 : E3) of DOM isolates is representative of the behavior of whole water samples. To address these knowledge gaps, spectral pH titrations were performed for seven humic substance and natural organic matter isolates, three whole water samples, and three model compounds. Comparison of the fractional and differential absorption and fluorescence spectra between DOM isolates, whole water samples, and model compounds revealed similar spectral features between all samples. The results show that spectral features observed for DOM isolates also occur for whole water samples, which suggests that there is overlap in the types of chromophores present in DOM isolates and whole waters. Although results from model compounds overlapped with DOM, especially in the ultraviolet region of the spectrum, no model compound replicated DOM's pH dependence perfectly. By measuring apparent quantum yields of singlet oxygen (Φ_Δ), we show that aquatic DOM isolates exhibit a different pH-dependence (Φ_Δ ∝ pH⁻¹) than soil-derived humic acid isolates (Φ_Δ ∝ pH). For aquatic DOM isolates, Φ_Δ values measured at different pH were not correlated to apparent fluorescence quantum yields (Φ_f), suggesting that pH impacts singlet and triplet excited state DOM dynamics in different ways. In contrast, the proportional relationship between Φ_f and Φ_Δ with increasing pH for soil humic acid isolates suggests that pH impacts singlet and triplet excited DOM in these samples in a similar fashion.