Humic acids as reducing agents: the involvement of quinoid moieties in arsenate reduction

Purpose Dissolved humic acids abiotically reduced inorganic arsenic to varying degrees, depending on solution pH, ionic strength, and type of humate used. The functionalities of dissolved organic matter responsible for these redox reactions remained in question, but quinoid moieties undoubtedly played an important role. It is not fully understood whether the quinoids are solely responsible for arsenate reduction, and what the kinetics of the relevant processes are. Methods Electron spin resonance (ESR) spectroscopy was used to monitor the radical content of the humates, both as bulk material and as size fractions. Information on the redox status of the humates was obtained from fluorescence excitation-emission matrices and correlated with the observed spin count. Size data were obtained from fractionation and UV-Vis spectrometry. Arsenic speciation was carried out by ion chromatography. Results ESR spectroscopy showed a free radical content of 3.4 × 1,017-20 × 1,017 spins/g for bulk and fractionated aqueous humic acids. The number of electrons corresponding to these counts could not account for the entire charge transferred to arsenate during abiotic reduction. The rate constants of the reactions were found to be independent of the humic concentration. Leonardite humic acid separated on a XAD-8 resin yielded fractions that on the short time frame (0-5 h) had rate constants of 0.035 h⁻¹ for the hydrophobic fraction compared to 0.0052 h⁻¹ for the hydrophilic fraction. The rate constants for the hydrophobic and hydrophilic fractions over the longer time frame (100-200 h) were similar—7.3 × 10⁻⁴ and 7.2 × 10⁻⁴ h⁻¹, respectively. Fluorescence excitation-emission matrices of humates involved in arsenate reduction exhibited shifts typical of quinoid components undergoing redox transformations. These gradual shifts took place during the first 24 h of reduction process, after which the spectra no longer changed. The reduction of arsenate, however, continued after this period, indicating that species other than quinoids were involved. This was consistent with the fact that the rate constants for the later processes were smaller. Conclusions The existence of different redox pools within the humate was confirmed, with the quinoid-centered redox entities showing the fastest kinetics. The results pertained to all size and polarity fractions.