Field-Flow Fractionation in a Rotating Coiled Column in the Development of Reference Samples of Natural Nanoparticles

The study of the properties and composition of natural nanoparticles is an essential problem in analytical chemistry. Currently, there are no reference samples of natural nanoparticles, which complicates the study of the role of nanoparticles in specific processes, including geochemical ones, and makes it almost impossible to compare the results obtained by different researchers. In this work, nanoparticles of kaolinite, montmorillonite, and muscovite are studied as potential reference samples of natural nanoparticles. A complex of analytical methods has been proposed for the isolation and characterization of mineral nanoparticles, including the field flow fractionation of particles in a rotating coiled column, laser diffraction, scanning electron microscopy, and inductively coupled plasma atomic emission spectrometry. According to laser diffraction data, 93–98% of the isolated particles are from 40 to 300 nm in size; 2–7% of particles have sizes of up to 830 nm. The sizes of the isolated particles were confirmed by scanning electron microscopy. Based on the aluminum content, the concentrations of mineral nanoparticles in the isolated suspensions were estimated. The aggregation stability of nanoparticles in 5 mM phosphate buffer solutions of pH 6, 7, and 8 was studied. Muscovite nanoparticles are stable at pH 7 and 8 for at least 4 weeks, while montmorillonite nanoparticles have similar stability only at pH 8. For kaolinite nanoparticles, significant aggregation was observed at all studied pH values of buffer solutions. The obtained experimental results made it possible to clarify the problems and assess the prospects for developing reference samples of natural nanoparticles.