Generation of acidity in bentonites at elevated temperatures

Abstract

Bentonites from different sources differ in their chemical and physical properties, challenging a reliable assessment of the bentonite’s performance as a sealing material in repositories. Still open questions exist for fast acidification reactions, where iron is the primary redox-active element. Upon oxidation to Fe(III) acidification can be triggered. Whereas in bentonite deposits anoxic conditions prevail stabilizing Fe(II), the exposure to free O2 during mining and drying induces oxidation of Fe(II), whereby the progress of this reaction is not easy to get. Moreover secondary reactions of H+ release such as carbonate dissolution, degassing and neoformation of Fe oxides are of interest. In the investigations five bentonites, partly showing degassing reactions, were included. Oxidation reactions in bentonites were determined in batch experiments up to 1500 h in 20 and 60°C treatments by measuring pH, electrical conductivity, soluble ions and Fe(II) concentration. For determination of mineral contributions, the light and heavy mineral factions were separated by density fractionation. Fe oxides formed were quantified by an extraction. Micromorphology of degassing in bentonites was determined by in situ microtomography. It was found out that Fe(II) oxidation in bentonites going along with acidification and degassing is in presence of free oxygen, water and elevated temperatures (60°C, 110°C) a fast process within minutes to hours. Despite extensive proton buffering on bentonites by carbonate dissolution, ion exchange, and protolysis of silicates a distinct decrease in pH was obtained. The light as well as the heavy mineral fraction of the bentonites contributed to acidification. Turnover could be described by the Fe(II) concentration and content of Fe oxides. For the bentonite samples under investigation it appears that marked shares of Fe(II) and other redox-sensitive atoms deriving from the reduced state in the bentonite deposit were preserved despite intense processing of bentonites with drying and grinding. For avoiding undesired acidification and degassing reactions, the bentonites have to be brought into function. Based on the high susceptibility of Fe(II) for oxidation simple measures exist here, e.g. by creating an environment with presence of water and at elevated temperature (60°C), where Fe(II) readily loses electrons upon introduction of oxygen.