Determination of the Optimal Conditions for the Purification of Water from Iron and Manganese by Microfiltration Ceramic Membranes, Based on Theoretical Calculations

Abstract

To substantiate the operating parameters of the extraction of manganese and iron ions from water using microfiltration ceramic membranes, an essential condition is their complete precipitation in the form of hydroxides, since the main factor affecting the impermeability of such membranes is steric. The formation of insoluble hydroxide particles is directly dependent on the products of their solubility, as well as the initial concentrations of metal ions and pH of water: the higher the concentration of metal ions, the lower the pH values at which the formation of an insoluble precipitate begins. At pH 4.93 and 10.83, there is quantitative precipitation of Fe(III) and Mn(II) hydroxides, respectively. It is at the pH values indicated above that the minimum concentration of ionic (soluble) forms of these metals is observed. Therefore, at pH 7.5, typical for groundwater, there are practically no soluble forms of Fe(III), regardless of its input concentration (Fe(III) hydroxides are retained by the membrane), and the residual concentration of iron in water is significantly lower than the maximum permissible concentration for drinking water (0.2 mg/dm3). For Mn(II), at its concentrations typical for natural groundwater, the precipitation of its hydroxides, according to calculations based on the product of solubility, does not occur. However, experimental studies have shown that the concentration of Mn(II) in the input water is much higher than in the purified one. The results indicate a possible mechanism for the removal of Mn(II) from water, both due to the complexation of its ionic forms with the ferrinol groups of the dynamic membrane formed by Fe(III) hydroxides on the ceramic membrane and partly due to the steric factor, that is, a decrease in the pore size of the membrane itself. The calculations have shown the expediency of the conditions for the selection of quantitative precipitation of metal hydroxides based on the solubility products of low-dissociated compounds, as well as the possibility of assessing the mechanism of the removal of toxic metals when used as precipitation (coagulation), adsorption, membrane, and other methods. Based on the determination of the concentration of toxic metals in the input water, it is possible to estimate their equilibrium concentration in purified water.