چكيده به لاتين
In this study one of the most important drawbacks of membrane desalination processes has been studied. The presence of boron in the sea water and the weakness of reverse osmosis membranes in its removal can cause neurosis, headaches and, in high concentrations, cancer. Therefore, the removal of boron is important for the residents of coastal areas who use drinking water treated from desalination plants. Boron exists in nature, especially in the sea water, in the form of boric acid, and due to the small size of its non-polar molecules, its removal requires an alkaline pH of more than 9.5 so that it changes into the form of borate and its negatively charged molecules only then can be removed by the RO membrane. In this research, a membrane with a nanocomposite selective layer was made on the sublayer membrane with absorbent nanoparticles including bentonite nanoclay and layered double hydroxide (LDH), and the selective layer contains neat and modified metal-organic frameworks (MOF) of UiO- 66 in its thin film nanocomposite structure. The UiO-66 present in the selective layer has the ability to entrap the boric acid molecule and if any pollutant passes through the selective layer, it would be absorbed by the absorbent nanoparticles in the support layer. The results showed that the ultrafiltration membranes containing 2 wt.% of LDH has the ability to remove boric acid at neutral pH up to 43%. On the other hand, the membrane containing 1.5 wt.% of bentonite were able to remove 38% of boron, and in the composition of a higher percentage, due to the agglomeration of nanoparticles, the ability to absorb pollutants by the membrane decreases. In a novel attempt, UiO-66 was modified by two new modifiers using triethylamine (TEA) and polyethyleneimine (PEI). The presence of amine functional groups resulting from the modification made the pores of MOFs smaller and the ability to remove boron by nanofiltration membrane at 4 bar pressures up to 87.5% at neutral pH and up to 91% at alkaline pH where boric acid is ionized and the size of its molecules increased to 0.59 nm. Also, in order to investigate the behavior of molecules in passing through the selective layer and MOFs, molecular dynamics simulation was conducted, and the results showed that in the neutral pH, the diffusion coefficient of boric acid through the selective polyamide layer is different from that of water molecules. On the other hand, the diffusion coefficient of boric acid in MOFs modified with PEI in neutral state is 60% lower than the diffusion coefficient of water molecules, which is due to the smaller pores of modified MOFs. On the other hand, at alkaline pH, the diffusion coefficient of borate molecules is 90% lower than that of water molecules. Considering the limitations of molecular dynamics software, this simulation has been investigated for ideal conditions in which the selective layer of polyamide and MOFs perform separately.
