Accurate and simple determination of sediment redox capacity by a modified chemical probe method: Implications to contaminant remediation

Abstract

Sediment redox capacity reflected by electron-accepting/donating capacity (EAC/EDC) affects biogeochemical processes and contaminant transformation in the subsurface. However, the traditional chemical probe method for determining sediment EAC/EDC has limited accuracy, and the high-precision mediated electrochemical method is unsuitable for analyzing heterogeneous sediment due to its low sample dosage. This study presents a modified chemical probe method employing two radical ions, ethyl viologen radical (•EV⁺) and 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) radical (•ABTS^-), to measure EAC and EDC, respectively. The accuracy was verified using FeSO₄ and Fe(OH)₃ standards, as well as various iron/manganese minerals and natural organic matter. The recovery for EAC and EDC analysis across 18 different sediment samples were 102.1 ± 5.9 % and 95.4 ± 5 %, respectively. Detection limits were comparable to those of the mediated electrochemical method, but 3-4 orders of magnitude lower than those of the traditional chemical probe method. The reaction time between •EV⁺/•ABTS^- and sediment was 15 min, much shorter than those reported for previous methods (2–24 h). Method applications suggest that the modified chemical probe method can accurately characterize the vertical distribution of riparian sediment redox capacity, and the initial EAC/EDC of sediments can predict the capacities for hexavalent chromium reduction, organic carbon mineralization and persulfate consumption in sediment systems.