Influence of sodium pretreatments on the preparation of Ag-doped polycationic bentonite for antibacterial purposes

Sodium bentonites have excellent cation exchange capacity (CEC) giving them great power of silver adsorption, which enables their use as precursors of antimicrobial materials. Countries like Brazil, however, do not have natural sources of sodium bentonite, only calcium or polycationic clays that do not show the same adsorption and ion exchange potential. In this study, the adsorption of silver in a polycationic bentonite from Quatro Barras, Brazil, previously subjected to a sodium treatment is evaluated. Samples were first modified with Na2CO3 or NaOH and then, subjected to silver impregnation in a batch system under controlled ambient conditions. Antibacterial properties of silver-exchanged clays were evaluated by the disk susceptibility and the minimum inhibitory concentration tests on Escherichia coli and Staphylococcus aureus bacteria. Results show that the sodium treatment with Na2CO3 allowed higher concentrations within a shorter time, increasing the pH without compromising the montmorillonite structure, which resulted on greater CEC and swelling values. Such better performance of the samples previously treated with sodium carbonate also caused an enhanced silver adsorption, resulting on a material of greater antibacterial potential. Experimental adsorption data fitted well to Freundlich isotherm.