Assessing the Representativeness of Isolated Humic Substances to Model the Reactivity of Aquatic Organic Matter: A Proton-Binding Study

This study explores the representativeness of fulvic acid (FA) and humic acid (HA) in characterizing the seasonal and annual reactivity of riverine dissolved organic matter (DOM). Consequently, proton titrations were performed on samples from the Sorocabinha and Itapanhau Rivers (Brazil), focusing on purified FA and HA fractions, concentrated humic substances (HS), and untreated river water. The results demonstrated consistent proton-binding properties of purified FA/HA from both rivers across dry and rainy seasons and years (2014 and 2022), suggesting a potential averaging effect from purification that may obscure natural DOM variations. Significant differences in proton reactivity were observed between FA/HA and their parent HS in 2022, highlighting the necessity for separate characterization, an aspect sometimes overlooked in the literature. Notably, the proton affinity spectrum for untreated Sorocabinha River water exhibited three peaks, in contrast to the two peaks for its purified FA/HA, consistent with the presence of a hydrophilic acid fraction (HPI) also evidenced in other major rivers. Accordingly, we argue that relying solely on FA and HA reactivity parameters for modeling DOM in, for example, risk assessment, is inaccurate in the presence of HPI. We advocate for future research to improve DOM modeling by addressing the limitations of the FA/HA-based approach and formulating realistic alternatives when necessary.