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

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

What does mediated electrochemistry reveal about regional differences in the redox properties of Boom Clay?

The Boom Clay is a potential host rock for geological storage of radioactive waste in the Netherlands and Belgium. The redox properties of the host rock are important in the context of safety assessment as they affect the speciation and thus the mobility of redox sensitive radionuclides. In this study, redox properties of the clay were assessed by mediated electrochemical analyses. The electron donating (EDC) and accepting (EAC) capacities and reduction potential of a suite of Boom Clay samples were determined. Boom Clay samples from various locations in the Netherlands and Belgium were investigated in unaltered form, and after size separation or chemical treatment to relate variations in redox properties to regional differences in diagenetic history or in the assemblage of allogenic minerals. In the investigated samples, the EDC can be attributed to the oxidation of pyrite, FeII in clay minerals and reduced natural organic matter (NOM) while the EAC can be ascribed to the reduction of FeIII in clay minerals and in Fe (oxyhydr)oxides. Combining Na-pyrophosphate extraction, to remove reactive NOM, with mediated electrochemical oxidation (MEO) allowed determining the individual EDC of NOM and FeII in clay minerals. Mediated electrochemical analysis showed systematic differences between samples from two locations in the Netherlands, Zeeland and Limburg. In samples from Zeeland, the reduction potential was higher, the EAC was larger, and the contribution of NOM to the EDC was smaller compared to samples from Limburg. These differences can be attributed to partial oxidation of Boom Clay in Zeeland during its diagenetic history but partial oxidation could also be a storage artefact. The electron yield obtained by pyrite oxidation in samples from Zeeland was larger compared to those from Limburg, which can be explained by a smaller particle size of pyrite in Zeeland. The size of pyrite particles, in turn, can be used as a proxy for the depositional conditions. The electrochemical activity of Fe in clay minerals did not vary systematically between the two locations in the Netherlands. In general, the fraction of electrochemically active Fe in clay minerals increased with the relative content of 2:1 clay minerals. In comparison with samples from the Netherlands, larger fractions of structural Fe in clay minerals were redox-active in samples from Belgium, which had a higher chlorite or glauconite content. This study demonstrates that mediated electrochemical analysis can reveal redox properties of Boom Clay, which might be of relevance for the migration of redox sensitive radionuclides or when assessing the impact of constructing and operating a repository for nuclear waste on the surrounding host rock

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

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

What does mediated electrochemistry reveal about regional differences in the redox properties of Boom Clay?

The Boom Clay is a potential host rock for geological storage of radioactive waste in the Netherlands and Belgium. The redox properties of the host rock are important in the context of safety assessment as they affect the speciation and thus the mobility of redox sensitive radionuclides. In this study, redox properties of the clay were assessed by mediated electrochemical analyses. The electron donating (EDC) and accepting (EAC) capacities and reduction potential of a suite of Boom Clay samples were determined. Boom Clay samples from various locations in the Netherlands and Belgium were investigated in unaltered form, and after size separation or chemical treatment to relate variations in redox properties to regional differences in diagenetic history or in the assemblage of allogenic minerals. In the investigated samples, the EDC can be attributed to the oxidation of pyrite, FeII in clay minerals and reduced natural organic matter (NOM) while the EAC can be ascribed to the reduction of FeIII in clay minerals and in Fe (oxyhydr)oxides. Combining Na-pyrophosphate extraction, to remove reactive NOM, with mediated electrochemical oxidation (MEO) allowed determining the individual EDC of NOM and FeII in clay minerals. Mediated electrochemical analysis showed systematic differences between samples from two locations in the Netherlands, Zeeland and Limburg. In samples from Zeeland, the reduction potential was higher, the EAC was larger, and the contribution of NOM to the EDC was smaller compared to samples from Limburg. These differences can be attributed to partial oxidation of Boom Clay in Zeeland during its diagenetic history but partial oxidation could also be a storage artefact. The electron yield obtained by pyrite oxidation in samples from Zeeland was larger compared to those from Limburg, which can be explained by a smaller particle size of pyrite in Zeeland. The size of pyrite particles, in turn, can be used as a proxy for the depositional conditions. The electrochemical activity of Fe in clay minerals did not vary systematically between the two locations in the Netherlands. In general, the fraction of electrochemically active Fe in clay minerals increased with the relative content of 2:1 clay minerals. In comparison with samples from the Netherlands, larger fractions of structural Fe in clay minerals were redox-active in samples from Belgium, which had a higher chlorite or glauconite content. This study demonstrates that mediated electrochemical analysis can reveal redox properties of Boom Clay, which might be of relevance for the migration of redox sensitive radionuclides or when assessing the impact of constructing and operating a repository for nuclear waste on the surrounding host rock

Kinetics of selenite interactions with boom clay: Adsorption–reduction interplay

The speciation of selenium (Se) in clay-rich host rocks is important within the framework of geological disposal of radioactive waste since it affects its migration. Removal of selenite from formation water can be caused by reduction and adsorption. Reduction could potentially be inhibited or delayed by adsorption. Here, the interplay of adsorption and reduction of selenite was investigated in batch experiments with Boom Clay and its separated size fractions. In all experiments, dissolved Se concentrations (Seaq) showed a fast initial decrease that was followed by a slower decline until removal was almost complete. X-ray absorption spectroscopy indicated that adsorption of selenite accounted for the fast removal of Seaq followed by slower selenite reduction. Eventually, almost all solid-bound SeIV became reduced to Se0 in all experiments. The progress of Seaq removal and SeIV reduction to Se0 could be described by a kinetic model involving reversible adsorption on clay minerals and reduction by pyrite. This implies that the reduction of selenite to Se0 is not significantly hindered or delayed by selenite adsorption on clay minerals. Pyrite is probably the most relevant reductant for selenite in Boom Clay, although reduction by FeII structurally bound in clay minerals might provide an additional pathway for selenite reduction in clay rocks

Kinetics of selenite interactions with boom clay : Adsorption–reduction interplay

The speciation of selenium (Se) in clay-rich host rocks is important within the framework of geological disposal of radioactive waste since it affects its migration. Removal of selenite from formation water can be caused by reduction and adsorption. Reduction could potentially be inhibited or delayed by adsorption. Here, the interplay of adsorption and reduction of selenite was investigated in batch experiments with Boom Clay and its separated size fractions. In all experiments, dissolved Se concentrations (Seaq) showed a fast initial decrease that was followed by a slower decline until removal was almost complete. X-ray absorption spectroscopy indicated that adsorption of selenite accounted for the fast removal of Seaq followed by slower selenite reduction. Eventually, almost all solid-bound SeIV became reduced to Se0 in all experiments. The progress of Seaq removal and SeIV reduction to Se0 could be described by a kinetic model involving reversible adsorption on clay minerals and reduction by pyrite. This implies that the reduction of selenite to Se0 is not significantly hindered or delayed by selenite adsorption on clay minerals. Pyrite is probably the most relevant reductant for selenite in Boom Clay, although reduction by FeII structurally bound in clay minerals might provide an additional pathway for selenite reduction in clay rocks