Time-domain NMR analysis of mixtures of calcium- and sodium-bentonites: A pilot study with chemometric approach

Clay materials, such as bentonites, play an important role in oil drilling, as well as environmental, food and cosmetic industries. There are two main forms of bentonite, which differ by their charge-compensating ions: sodium and calcium. The most important property of bentonites is they interaction with liquids resulting in swelling. The degree of swelling is much greater if sodium ions compensate the charge of the layers; therefore, it is very important to determine and regulate the sodium to calcium ratio (Na/Ca). In our preliminary study we investigated the 1 H NMR relaxation behaviour of water contained in bentonites with different sodium content and found that low-field NMR relaxometry (LF-NMR) can be applied to determine the sodium to calcium ratio. LF-NMR is a non-invasive, relatively cheap, sensitive, and simple analytical tool for studying the dynamic properties of swelling water, requiring no complicated sample preparation. We found that LF-NMR can be used to determine the Na-bentonite to Ca-bentonite ratio using multivariate chemometric analysis, provided the sodium content is not lower than approximately 25% w/w. Industrial on-site analysis is possible using a single-sided magnet, which is also of some relevance for natural environmental applications.