The reversibility of interlayer ion fixation in the smectite of BCV 2017 bentonite during long-term heating in air at 200 °C, a detailed mineralogical study

Abstract

In the context of studies related to the disposal of heat-generating wastes, the aim of this study was to characterize the mineralogical transformations that take place in the BCV 2017 Ca-Mg bentonite (Černý vrch bentonite, Czech Republic) following short (1-day) and long-term (47-month) heating in air at 200 °C. A laboratory procedure involving interlayer cation exchange using the Cu-trien complex with subsequent ethylene glycol solvation and full-profile diffraction pattern modelling was successfully applied for the comparison of the original bentonite and the material heated at 200 °C in air, as well as their subsequent rehydration.

In the original bentonite, two smectite-rich mixed layers minerals were modelled, consisting of 10-Å, high- and low-charge smectite layers. The 10-Å layers represented both the illite and also smectite layers with fixed Mg²⁺ and Ca²⁺ ions in the interlayer. After heating at 200 °C for 47 months, the total cation exchange capacity (CEC) decreased from 58 to 48 meq/100 g, the content of exchangeable Ca²⁺ increased from 25 to 27 meq/100 g, and the content of exchangeable Mg²⁺ decreased from 47 to 27 meq/100 g. The increase of exchangeable Ca²⁺ was caused by liberation of originally fixed Ca²⁺ in the interlayer of smectite while decrease of exchangeable Mg²⁺ was due to its fixation. The predominance of the fixation of interlayered Mg²⁺ over the liberation of Ca²⁺ led to the observed decrease of CEC. This decrease was accompanied by increase in the number of both smectite layers with interlayer-fixed Mg²⁺ (10-Å layers) from 12 to 19 % (an average value of two smectite-rich MLMs), and low-charged smectite layers from 67 to 76 % in MLMs. A simultaneous decrease in the number of high-charged smectite layers from 22 to 5 % was observed. The BCV 2017 bentonite heated for 1-day reached intermediate values between the original and 47-month heated bentonites.

The fixation of the interlayered Mg²⁺ was partially reversible, increasing from 27 to 31 meq/100 g after rehydration for 54 days under relative humidity of 97.6 %. The percentage of the 10-Å layers decreased from 19 to 16 % of MLMs and the CEC increased from 48 to 52 meq/100 g. Similar results were obtained when the heated bentonite was saturated with liquid water with 1:100 solid:liquid ratio for the same time period.