Applied Clay Science 217 (2022) 106396 Available online 5 January 2022 0169-1317/© 2022 Elsevier B.V. All rights reserved. Research Paper Na-montmorillonite to Fe(II)-Mt using ferrous citrate/ascorbate obtained by dissolving iron powder S. Chikkamath a , J. Manjanna a, * , N. Momin a , B.G. Hegde b , G.P. Nayaka c , Aishwarya S. Kar d , B. S. Tomar d, e a Dept. of Chemistry, Rani Channamma University, Belagavi 591156, India b Dept. of Physics, Rani Channamma University, Belagavi 591156, India c Physical & Materials Chemistry Div., CSIR-National Chemical Laboratory, Pune 411008, India d Radioanalytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India e Homi Bhaba National Institute, Anushaktinagar, Mumbai 400094, India A R T I C L E INFO Keywords: Iron dissolution Citric acid Ascorbic acid Bentonite Fe(II)-montmorillonite Cr(VI) reduction ABSTRACT Bentonite containing montmorillonite (Mt) as the major clay mineral is the potential buffer material of engi- neered barrier system in the deep geological repository for high-level waste management. The corrosion products of canister/or overpack material (carbon steel) may alter the basic characteristics of bentonite through Fe/clay interaction and formation of Fe – Mt to certain extent. A convenient method is required for quantitative prep- aration of Fe(II)-Mt because the only few direct methods reported have some limitations. For this, weak ferrous complexes having good solubility to facilitate the cation-exchange with bentonite is essential. Therefore, Fe(II)- citrate, Fe(II)-ascorbate and Fe(II)-citrate/ascorbate were obtained here by dissolving the iron powder (Fe 0 ) in citric acid (CA), ascorbic acid (AA) and their equimolar mixtures from 25 mM to 300 mM at 70 ◦ C under N 2 atmosphere. The redox potential was measured to monitor the dissolution of Fe 0 in these mild organic acids. Higher dissolution of Fe 0 occurred in CA + AA mixture. The dissolved iron in the form of ferrous complex was reasonably stable in inert atmosphere, Fe 2+ /Fe total ≈ 1. The parent clay mineral (bentonite, Na–Mt) was treated with Fe(II)  cit/asc to form Fe(II)  Mt. through cation exchange process. The CEC and ferrous to ferric ratio of the Fe(II)-Mt was close to the stoichiometric amount. The XRF, XRD, ac impedance, FT-IR, TGA/DSC, XPS and FE-SEM are used for further characterization. In-situ complexation reaction of interlayer Fe 2+ ions in Fe(II)-Mt with o-phenanthroline was also observed. 1. Introduction Bentonite, a common smectite-rich material, has been accepted as a backfill or buffer material for deep geological repositories (DGR) to host high-level nuclear waste (HLW) to contain and isolate it from biosphere, until the radioactivity associated with HLW reaches to innocuous level (Pusch, 1992; Rowe et al., 1995; JNC, 2000). Bentonite consists of montmorillonite (Mt) clay mineral, a 2:1 member of smectite group. It has a high swelling capacity, low hydraulic conductivity, self-healing and adsorption capacity. Mt-rich clay has excellent sealing properties both at low and high density. These properties are pre-requisite to retard the diffusion of radio nuclides from HLW and to assure desirable per- formance of backfill or buffer material in DGR. The long-term integrity of engineered barrier system (EBS), with alterability of Mt. under repository conditions need to be evaluated for safety assessment of DGR (Juang et al., 2004). The interaction of Mt. with the canisters and/or overpack (carbon steel, Fe) in high-level radioactive waste repository must be considered in designing the EBS. Thus, the impact of corrosion- derived iron on the bentonite buffer is an important issue, still not un- derstood fully. The Fe/ clay interaction under reducing conditions may involve different processes such as sorption, redox and dissolution/ precipitation reactions. Mt. to illite transformation make changes in physical properties at higher densities while soft grouts may lead to significant loss in sealing ability (Pusch, 1992). Carlson et al. (2007) have studied the samples taken from long-term anaerobic corrosion tests of carbon steel or cast iron in compacted bentonite (Na/Ca-bentonite: Volclay MX-80, 4% Fe 2 O 3 ) in contact with a simple artificial ground- water at 30 ◦ C or 50 ◦ C. In addition to formation of Fe-rich smectite (like * Corresponding author. E-mail address: jmanjanna@rediffmail.com (J. Manjanna). Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay https://doi.org/10.1016/j.clay.2021.106396 Received 16 March 2021; Received in revised form 25 December 2021; Accepted 29 December 2021