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Effect of calcium on micropollutant removal with nanofiltration
In natural water sources, Ca2+ is present at different concentrations, potentially affecting the removal of micropollutants (MPs). Hence, this work focuses on the effect of Ca2+ concentration on MP removal with nanofiltration (NF), for which several mechanisms have been outlined in literature. Experimentally and theoretically, we evaluated three phenomena associated with the presence of Ca2+ in solution: Ca2+ and MP complexation, membrane charge screening or reversal, and pore narrowing. Experimentally, we determined the relative importance of each of these phenomena.
Furthermore, we developed a transport model, which is an extended version of the Donnan steric pore model (DSPM) that accounts for the formation of Ca-MP complexes and the effect of Ca2+ on the membrane charge density. Chemical equilibrium is assumed to study Ca-MP complexation, and a Langmuir isotherm is used to determine the effect of calcium on membrane charge density. With the given model, we could fit experimental data for MP removal and reinforce the main conclusion that the effect of calcium on the membrane charge density is the dominant phenomenon affecting MP removal.
Model description
To model OMP removal, we used an extended version of the Donnan steric pore model. Within this model, we included the effect of Ca-MP complexation and Ca-membrane interaction. We assumed chemical equilibrium between Ca2+ and the MP, and the membrane charge density is calculated as a function of [Ca2+]. We do not consider the diffusion boundary layer or concentration polarization.
Results
The interaction of Ca2+ with the carboxylic groups can affect membrane charge. Therefore, the effect of Ca2+ on MP removal would depend on the charge and size of the MPs. We tested 3 neutral MPs (paracetamol, metronidazole, and atrazine) and 3 positively charged MPs (sotalol, metformin, and guanyl area) using NaCl and CaCl2 as background salts. The results show that the effect Ca2+ on MP removal is indeed dependent on the charge and size of the MPs. The removal of neutral MPs is not affected by the presence of Ca2+ in solution, which indica. However, theoretical calculations and experimental data indicated that Ca2+ can increase the passage of small positively charged MPs, Mw=100 g/mol. On the other hand, the removal of large positively charged MPs, i.e., Mw> 200g/mol, will increase due to the binding of Ca2+ to the carboxylic groups of the membrane and enhanced Donnan exclusion.