Acid-Base Properties of Cis-Vacant Montmorillonite Edge Surfaces: A Combined First-Principles Molecular Dynamics and
Pengyuan Gao1,2,3, Xiandong Liu4,5, Zhijun Guo1,2
1Radiochemistry Laboratory, School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China.
Environmental Science & Technology
|January 10, 2023
Summary
This study reveals that the cis-vacant structure of montmorillonite edges, unlike the assumed trans-vacant model, dictates pH-dependent surface properties. Mg2+ substitution and crystal plane orientation significantly influence acidity and charge, impacting contaminant adsorption.
Area of Science:
- Geochemistry
- Materials Science
- Environmental Science
Background:
- Montmorillonite layer edge surfaces exhibit pH-dependent properties due to the acid-base reactivity of their functional groups.
- Surface acidity, quantified by intrinsic reaction equilibrium constants (pKa), is influenced by crystal structure.
- Previous studies often assume a trans-vacant configuration, potentially misrepresenting the acid-base properties of natural cis-vacant montmorillonites.
Purpose of the Study:
- To compute the intrinsic acidity constants of surface sites in a cis-vacant montmorillonite layer.
- To evaluate the impact of Mg2+ substitution and crystallographic orientation on edge surface acidity and charge.
- To develop a surface complexation model for predicting montmorillonite surface properties.
Main Methods:
- Employed first-principles molecular dynamics with the vertical energy gap method to compute acidity constants.
- Assessed pKa values for non-substituted and Mg-substituted layers on various edge surfaces ([010], [01̅0], [110], [1̅1̅0]).
- Constructed a surface complexation model using predicted pKa values to simulate surface properties.
Main Results:
- Identified specific functional groups (e.g., ≡Si(OH), ≡Al(OH2)2) responsible for proton reactivity on cis-vacant edges.
- Observed that Mg2+ substitution increases pKa values at local sites, with effects diminishing with distance.
- Demonstrated that edge surface charge is highly dependent on crystal plane orientation, with significant variations below pH 7, while Mg2+ substitution has minimal impact.
Conclusions:
- The cis-vacant structure and crystal plane orientation are critical factors governing montmorillonite edge surface properties.
- High pKa values for structural OH sites indicate limited reactivity under common pH conditions.
- The developed model accurately predicts surface charge and properties, offering insights into contaminant adsorption by smectites.
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