Redox properties of clay-rich sediments as assessed by mediated electrochemical analysis: Separating pyrite, siderite and structural Fe in clay minerals

Abstract

Redox reactions with Fe-containing minerals in clay-rich sediments largely affect the speciation, mobility, and (bio-) availability of redox-sensitive contaminants. Here, we use mediated electrochemical oxidation (MEO) and reduction (MER), to quantify the electron accepting and donating capacities (EAC and EDC) of Boom Clay, a potential host formation for radioactive waste disposal. The relevant redox-active minerals pyrite, siderite, smectite and illite were first studied separately. MEO and MER of smectites and illites resulted in sharp current peak responses, reflecting fast electron transfer kinetics. Conversely, broad current peaks were obtained from MEO of pyrite. The current response to MEO of siderite was very small. Under the applied electrochemical conditions in MEO, pyrite was not completely oxidized and only a marginal fraction of siderite was oxidized. All structural Fe (Festruct) in smectites SWa-1 and SWy-1 was redox-active in MER and MEO, whereas in Fithian Illite and IMt-1 only 12-22% of the total Festruct was available. An empirical equation was used to describe the current curves of the tested minerals. This equation allowed to delineate the relative contributions of these minerals to MEO of their mixtures. The EDC of Boom Clay determined by MEO was 0.2±0.05mmol e-/g and predominantly consisted of contributions of pyrite, Festruct in clays and natural organic matter (NOM). Applying the empirical equation allowed to separate the oxidative current response into the contribution of pyrite with slower oxidation kinetics and the combined contribution of faster reacting Festruct and natural organic matter (NOM). Due to the absence of NOM isolates from Boom Clay, the EDC of NOM was estimated based on MEO measurements of dissolved organic matter in Boom Clay pore water and the organic carbon content of Boom Clay. The EDC of Festruct in clays was then obtained by subtracting the contributions of NOM and pyrite from the measured EDC. About 14% of the measured EDC can be attributed to Festruct which implies that about 50% of the structural FeII in Boom Clay is redox-active. In contrast, EAC measurements indicate that FeIII struct in Boom Clay is electrochemically inactive.