Spectrophotometric Analysis of Nano Ca-Bentonite Tactoid Breakdown by Exchangeable Na and K

The montmorillonite mineral, commonly found in soil clay fractions, forms Ca-tactoids, composed of stacked platelets. Interactions between cations in the soil solution, such as Na+ and K+, and Ca-tactoids through ion-exchange processes can lead to the breakdown of these structures into single platelets, thereby influencing soil properties. Previous studies have employed spectrophotometry to measure properties of montmorillonite suspensions, investigating phenomena such as light transmission and scattering. However, challenges exist in applying these methods accurately, particularly concerning variations in adsorbed ion composition and particle-size range. This study aims to address these challenges and advance understanding by (i) developing a theoretical framework for spectrophotometry that enhances sensitivity to particle dimensions, (ii) analyzing logistic function parameters to gain insights into platelet growth, and (iii) investigating the effects of suspension ageing on tactoid aggregation. XRD and TEM results indicated that a nano Na-bentonite after removal of impurities held four clay minerals: smectite (most probably montmorillonite), kaolinite, illite, and palygoreskite. This nano Na-bentonite used to prepare nano NaB-CaB salt free suspensions having charge fractions of exchangeable sodium, NNa equal 0, 0.1, 0.2,…, 1.0. A similar set was prepared for nano KB-CaB suspensions. Results demonstrated that smectite tactoids saturated with Ca2+ undergo partial breakdown by exchangeable Na+ up to a charge fraction of 0.5, resulting in a steeper increase in light transmittance and decline in absorbance at 800 nm wavelength compared to 700 and 550 nm. Complete breakdown of Ca tactoids occurred at charge fractions above 0.8. Conversely, exchangeable K+ acts as a staking agent up to a charge fraction of 0.5, leading to slower platelet growth compared to Na+. Monte Carlo simulations revealed distinct binding behavior of Na+ and K+ ions on the bentonite surface. Furthermore, the study highlighted the higher efficacy of K+ over Na+ in replacing exchangeable Ca2+, attributed to differences in hydrated ion radius and standard free energy values. Overall, these findings contributed to a deeper understanding of the ion-induced delamination dynamics in bentonite. Furthermore, the study examined the impact of ageing on tactoid dynamics in nano NaB-CaB and KB-CaB suspensions over 7, 61, and 93 days. Spectrophotometry absorbance data, measured at 800 nm wavelength, revealed distinct differences in the single platelet growth rates (parameter c of the logistic function) between NaB-CaB and KB-CaB suspensions, with NaB-CaB exhibiting higher c values across all ageing periods. Specifically, exchangeable Na+ was 34% more effective than exchangeable K+ in breaking down Ca-tactoids. The present study demonstrates that spectrophotometry with sound theoretical bases, carefully prepared suspensions, and logistic modelling, was a useful technique to characterize clay dispersion, particularly in aggregated systems.