Synthesis of Clay-Based Adsorptive Microfiltration Membranes

Chromium is one of the most hazardous inorganic water pollutants which is constantly released into water resources by natural and industrial processes. Microfiltration membranes (with pore sizes between 0.1–10 μm) cannot separate chromium ions and hence nanofiltration membranes (with pore sizes between 0.5–2 nm) are necessary which need high pressure pumps. Using adsorptive membranes, i.e. membranes which can adsorb impurities without using any extra adsorptive particles, is a new and developing method for water treatment which can be considered as a combination of adsorption and membrane technology. In this paper, clay-based adsorptive microfiltration membranes were successfully synthesized for chromium removal from water. 80 wt % of bentonite and 20 wt % of carbonates (calcium, magnesium and their mixture) were mixed, uniaxially pressed, dried, and fired at 1100°C for 3 h. Then, phase analyses of the samples, their physical and mechanical properties, microstructure, mean pore size and also their ability for chromium removal from water were studied. Results showed that the addition of carbonates lead the porosity to increase while contrary to organic pore formers like starch, due to the formation of phases like wollastonite, the mechanical strength not only didn’t collapse but also improved. It was seen that Cr 3+ ions were removed from water up to 95% and regarding that the mean pore sizes of the microfiltration membranes used in this work (0.6–2.5 μm) were 10 000 times bigger than the size of Cr 3+ ions (0.615 A), it was deduced that Cr 3+ ions were removed through adsorption mechanism and the microfiltration membrane prepared the media for adsorption. By analyzing the filtered water and observation of Ca 2+ ions in it, it was concluded that ion exchange was the main mechanism. Hence, a combination of membrane filtration and adsorption was achieved for water treatment which made microfiltration membranes act as nanofiltration ones and considering that the concentration of Cr 3+ ions in real drinking water resources is less than 5 ppm (which is regarded in this research), it can be said that these low-cost adsorptive microfiltration membranes can be used to gain high quality drinking water.