Influence of Geochemical Fractionation of Fulvic Acid on its Spectral Characteristics and its Protection Against Copper Toxicity to Daphnia magna.

This article investigates how geochemical fractionation of fulvic acids (FAs) by hydrous aluminum oxides (HAOs) and hydrous iron oxides (HFOs) affects copper (Cu) bioavailability and toxicity in acid mine drainage (AMD)-impacted alpine watersheds in central Colorado, USA. Using FAs isolated from pristine, AMD, and mixed waters, along with laboratory fractionation experiments, the study found that fractionation alters FA spectral properties (e.g., specific ultraviolet absorbance at 254 nm, SUVA254) and reduces their protective effect against Cu toxicity to the aquatic invertebrate Daphnia magna, as indicated by lower median effect concentrations (EC50). Adjusting the Biotic Ligand Model (BLM) inputs by incorporating FA spectral indices, particularly SUVA254-based effective dissolved organic carbon, improved predictions of Cu toxicity, although discrepancies remained for AMD-related FAs likely due to unaccounted nitrogen- and sulfur-containing moieties. The findings highlight the importance of considering DOM quality and geochemical fractionation in modeling metal toxicity in AMD-impacted waters to enhance regulatory assessments.