Sequential oxidation procedures with KMnO4: Component characteristics of labile reducing capacity fractions in anaerobic sediments.

Abstract

Reducing substances are a mixture of different forms and types and play extremely important roles in manipulating the redox status of sediments, benthic habitats, and substance exchanges at the sediment-water interface in aquatic ecosystems. However, little is known about their abundance, forms, and reducibility in sediments. In this study, the procedures were developed to sequentially fractionate sediment reducing capacity (RC) fractions with the pH dependence of KMnO₄ oxidability. The procedures were then applied to 60 sediments from 2 lakes and 3 reservoirs, generating an RC_pH7.0 fraction (oxidized at ~0.48 V [reference: SHE]) and an RC_pH2.0 fraction (oxidized at ~0.95 V [reference: SHE]), and the component of each fraction was characterized. The RC_pH7.0 fraction amounted to 45.4 ± 25.9 cmol e^-·kg^-1 DW, and the RC_pH2.0 fraction amounted to 42.8 ± 22.9 cmol e^-·kg^-1 DW; fraction sizes depended greatly on sediment origin. Reducing organic substances (ROS) were the main contributors to the RC fractions, with mean value of 30.0 ± 24.1 and 38.5 ± 22.2 cmol e^-·kg^-1 DW in RC_pH7.0 (% contribution: 68.0 ± 5.3 % of RC_pH7.0) and RC_pH2.0 (90.0 ± 1.5 % of RC_pH2.0), respectively. The next contributor was Fe(II), with mean value of 13.5 ± 8.2 and 3.8 ± 3.7 cmol e^-·kg^-1 DW in RC_pH7.0 (28.3 ± 5.2 %) and in RC_pH2.0 (9.9 ± 8.6 %), respectively. The smallest component was sulfide (Sⁿ), which had a mean of 2.0 ± 3.1 cmol e^-·kg^-1 DW in RC_pH7.0 and was essentially negligible in RC_pH2.0. The number of electrons lost per mole of reducing substances (N_i) differed between the two RC fractions and among sediments of different origins. N_ROS was lower in the RC_pH7.0 fraction (0.22 ± 0.09) compared to the RC_pH2.0 fraction (0.31 ± 0.12) and significantly related to levels of active Fe(III) and sulfides (Sⁿ) (p < 0.05). The opposite pattern was seen for N_Fe(II) and N_Sⁿ. Based on the compositive reducing capacity (CRC) for the RC_pH7.0 fraction, sediment redox status could be classified as ROS-Fe(II) (3.8 ± 1.7 cmol e^-·kg^-1 DW) or ROS-Sⁿ (10.1 ± 4.8 cmol e^-·kg^-1 DW) (weaker vs. stronger, respectively; p < 0.01). The RC-based index provides a more comprehensive perspective on characterizing sediment redox status compared to the Eh.