Formation of sandwich structure through ion adsorption at the mineral and humic interfaces: A combined experimental computational study

• Sandwich structure is prepared by ion adsorption at humic and mineral interfaces.
• Structure is unraveled by FT-IR spectroscopy and DFT calculations.
• Three potential formation routes are explored.
• Frequent conformational transitions for humic acids are detected.
• Role of metal ions during the sandwich structure formation is determined.

Although ion adsorption at interfaces governs a variety of chemical processes, the underlying mechanisms remain controversial. We present a combination of dynamic light scattering, FT-IR spectroscopy and DFT calculations to probe the ion adsorption behaviors on montmorillonite and its mixture with humic acid (HA) as well as to unravel the composite structure and formation mechanism. Interaction structures of Ca2+ with HA, montmorillonite and their mixture are calculated, and computational frequencies show fine agreement with FT-IR results. The low ion affinities for HA explain that the aggregation kinetics of montmorillonite is significantly inhibited by forming composite with HA. Adsorption of Ca2+ on the mixture of HA and montmorillonite facilely obtains sandwich-structured composite, where Ca2+ is situated at the midst and exhibits an essential stabilization effect. Furthermore, conformational transitions occur frequently for HA carboxylic groups during composite formation.