Influence of Clay type on the Adsorption of Cd2+: Effect of the Interlayer Distance and the Sites Available for Complexation
Gisele Portela, Claudia Cecilia Wagner, Alejandra TironiIntroduction:
In this work, the effectiveness of three Argentine clays with a medium-grade of clay minerals – kaolinite, illite, and montmorillonite – to remove Cd2+ from aqueous solutions was evaluated. A model of their adsorption capacity as a function of the initial Cd2+ concentration and specific parameters for each clay mineral is presented to predict adsorption capacity for different clay minerals.
Methods:
The clays were characterized by X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and BET (Brunauer-Emmett-Teller) surface area measurements. Adsorption studies were carried out under different interaction times and Cd2+ concentrations.
Results:
The results showed that medium-grade clays have significant potential as adsorbents for Cd2+ from aqueous solutions. Montmorillonite clay was more efficient as an adsorbent under the conditions tested. Cd2+ adsorption could be studied with the DubininRadushkevich and moderately with the Langmuir isotherm model for three clays. The analysis of the FTIR spectrum indicated that -OH groups are involved in Cd2+ adsorption by complexation. The adsorption capacity was modeled as a linear combination of the initial Cd2+ concentration, the interlayer distance (d), and the available sites for cadmium complexation. This preliminary proof-of-concept model will allow for estimating the adsorption capacity of different medium-grade clays. Higher values in the coefficients associated with interlayer distance and the sites available for cadmium complexation indicate that the adsorption capacity of clays depends to a great extent on these factors.
Discussion:
The adsorption behavior of Cd2+ on the studied clays is governed by mineralogical characteristics, with montmorillonite showing higher performance due to its larger interlayer distance and greater availability of complexation sites. The fast adsorption kinetics and fit to pseudo-second-order models suggest that the dominant mechanism at pH 5 is chemisorption via surface complexation, as confirmed by FTIR analysis of the post-adsorption spectra. The proposed regression model highlights the central roles of the sites available for cadmium complexation and the interlayer distance in controlling adsorption capacity, providing a useful predictive tool for comparing different clay minerals under similar conditions.
Conclusion:
Medium-grade natural clays (kaolinite, illite, and montmorillonite) have great potential as adsorbents for Cd2+ from aqueous solutions. Montmorillonite clay was more efficient as an adsorbent under the conditions tested.