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

Dissolved copper (Cu) can contribute to toxicity in aquatic systems impacted by acid mine drainage (AMD), and its bioavailability is influenced by aqueous complexation with organic ligands that predominantly include fulvic acids (FAs). Because geochemical fractionation of FA that accompanies sorption to hydrous aluminum oxides (HAO) and hydrous iron oxides (HFO) can alter Cu complexation with FA, we investigated FAs isolated from 3 categories of water (Pristine, AMD, and in-situ-fractionated mixtures of Pristine and AMD collected at stream confluences) in 3 mining-impacted alpine watersheds in central Colorado, USA. We also conducted geochemical fractionation of field-collected FA and Suwannee River FA by precipitating HAO and HFO in the laboratory. Spectral properties of the FAs (e.g., UV-VIS absorbance) were altered by geochemical fractionation; and in acute toxicity tests with an aquatic invertebrate (Daphnia magna), Cu was more toxic in the presence of in-situ- and laboratory-fractionated FAs [median lethal concentration (EC50): 19-50 µg Cu L -1 ] than in the presence of non-fractionated FAs (EC50: 48-146 µg Cu L -1 ). After adjusting for the strain-specific sensitivity of our D. magna, we improved the accuracy of Biotic Ligand Model predictions of Cu EC50 values for AMD-related FAs by using an "effective DOC" based on spectral properties that account for among-FA differences in protectiveness against Cu toxicity. However, some differences remained between predicted and measured EC50 values, especially for FAs from AMD-related waters that might contain important metal-binding moieties not accounted for by our measured spectral indices. This article is protected by copyright. All rights reserved. Environ Toxicol Chem 2022;00:0-0. © 2022 SETAC.