Membrane concentration of hydrothermal SiO2 nanoparticles

Abstract

The paper experiments with membrane concentration of hydrothermal SiO2 nanoparticles. The nanoparticles with average diameters of 10–100 nm are formed in hydrothermal solutions as a result of polycondensation of orthosilic acid (OSA) molecules. Comparative experiments were performed using membranes with different pore diameters: microfiltration - average pore diameter of 70 nm, ultrafiltration - MWCO (minimum weight cut off) parameter −10–150 kD, and reverse osmosis membranes - pore diameters in the range 0.1–1.0 nm. The dependences of membrane permeability and selectivity for SiO2 nanoparticles and ions of dissolved electrolytes on time, SiO2 content, and transmembrane pressure were determined. During the process of concentration of SiO2 nanoparticles using microfiltration and ultrafiltration membranes, gel deposits formed on the membrane layer. Samples of gel deposits were studied by X-ray diffraction, scanning electron microscopy, X-ray fluorescence spectroscopy, and low-temperature nitrogen adsorption. The increased sorption ability of the gel layer with respect to the cations Na+, K+, Ca2+, Mg2+, Al3+ was revealed. Microfiltration membranes showed low selectivity for SiO2 nanoparticles, to increase which the addition of Al3+ coagulant cations was required. Ultrafiltration membranes had high selectivity for SiO2 nanoparticles and low selectivity for ions of dissolved electrolytes. This allowed ultrafiltration to obtain a stable aqueous concentrate with a high SiO2 content (up to 62.5 wt% = 940 g/dm3), a low relative concentration of impurity ions and a zeta potential of SiO2 nanoparticles in the range from −56 to −25 mV, which ensured their stability. Reverse osmosis membranes had simultaneously high selectivity for SiO2 nanoparticles, OSA molecules, and ions of dissolved electrolytes; therefore, aqueous SiO2 concentrates were unstable. Ultrafiltration membranes have the advantage to solve the problem of cleaning the water heat carrier of geothermal power plants and to obtain an additional product in the form of concentrated water sol SiO2, which has promising industrial applications. The combination of ultrafiltration and reverse osmosis opens up the possibility of co-extraction of compounds Li, Rb, Cs.