Surface Charges in the Natural Clays of Orenburg Oblast

The magnitude of surface charges and points of zero charge in water-clay suspensions with pH equal to 5, 7, and 9 were studied. Their changes on exposure to a microwave field and variations in the dispersity of particles were determined. Powders of two fractions of montmorillonite, polymineral, and kaolin clays of the Orenburg Oblast were chosen as the objects of investigation. The points of zero charge (PZC) of the indigenous clays were determined by means of weight titration, based on measuring the value of the limiting pH with an increase in the weight fractions of the clay. Processing of clay powders in a microwave field was conducted according to the following regime: output power 750 W, operating frequency 2.45 GHz, and exposure time 600 sec. The magnitude and sign of the charges formed at the interface between the particle surface and the electrolyte solution depend on the mineralogical composition of the particles. Finely dispersed fractions of two clays turned out to be sensitive to processing in a microwave field - montmorillonite-containing and polymineral; pH indicators of PZC shifted to the acidic region by 0.26 and 0.31, respectively (by 4%). The pH values of the PZC suspensions with kaolin clay after treatment in microwave radiation remained practically unchanged. If the pH of the solution surrounding the particle exceeds the pH of PZC, then the sorbent will be able to absorb cations; if the pH of the solution is below the pH of PZC, then anions are absorbed. The results obtained will make it possible to use microwaves as a tool for expanding pH range and thereby promoting efficient sorption of anions and cations.