Contrasting variations in solid acidity of smectite and kaolinite in acid-base environments and implications for organic matter sequestration

The nanoscale interactions between clay mineral active sites and organic matter (OM) determine OM sequestration and transformation. Smectite and kaolinite, prominent OM adsorbing minerals, are sensitive to acid-base fluctuations, but dynamics of their active sites remain unexplored, limiting our knowledge of the mineral matrix-related fate of OM. Solid acidity (Brønsted and Lewis acid sites) is a tangible indicator to quantify the active sites. This study examined the dynamics of solid acidity in smectite and kaolinite subjected to acid-base environmental variability, exploring implications for OM sequestration. Results revealed that both acid and alkaline environments increased the number of total acid sites (Q_T) for both smectite and kaolinite, with higher Brønsted acid site fraction (f_B) as pH decreased, but smectite and kaolinite exhibited contrasting behaviors. Smectite underwent more prominent structural rearrangements under acid environments, with interlayer cations substitution and preferential dissolution of octahedron. This process exposed Al³⁺ and/or facilitated re-adsorption of Al³⁺/Fe³⁺, contributing to Lewis acid sites, while exposed silanols and enhanced interlayer water polarization contributed more to Brønsted acid sites, yielding substantial Q_T and f_B increases. Acid environments exposed silanols in kaolinite, but it responded more strongly to alkaline environments, where disrupted Si-O-Al bonds at edges exposed incompletely coordinated Al³⁺, contributing to Q_T increase and f_B decrease. Increased Q_T enhanced the adsorbed lysine content; therefore, the mineral- and environment-specific dynamics of solid acidity have the potential to shape OM sequestration capacity of clay minerals, and their catalytic efficiency and mechanisms in driving the diagenetic fate of OM.