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3.7.1. Concentration of monomer AMPS and MgSiO3 NPs.

Figures (12& 13), show water flux and the salt rejection of prepared membranes with different concentrations of AMPS monomer,& MgSiO3 NPs respectively. It is observed that both water flux and salt rejection increases with nanomaterial concentration increasing. Then, for higher concentrations, it decrease, this results can be clarified as the increasing of coating layers on the membrane could reduce the permeation flux. Also, the reduction in the salt rejection may be due to some coating layers could minimize the surface charge due to the concealment, which then minimizes the salt rejection due to the Donnan effect phenomena [33]. The results reflect the existence of an optimum concentration of AMPS monomer& MgSiO3 NPs.

3.7.2. Grafting time and temperature.

By over increasing in grafting time or curing temperature as in Fig. 14&15, membranes water flux, and salt rejection have been decreased, this may be due to The decreasing of interchain hydrogen bonds as large amount of AMPS molecules were incorporated into aromatic polyamide chains, which may increase the polymer chain mobility,causing conformational alteration of aromatic polyamide chains.It may also cause local collapse or compaction in the modified membrane surface and finally result in increased passage of both water and salt [34].

3.8. Application of the resulting MgSiO3 NPs modified AMPS-g-TFC membrane on sea water and different ground water salinity.

A seawater sample from a beach well located in Cleopatra region, Marsa Matrouh, and a groundwater samples collected from different sites on the northwestern coast in Egypt were used as a feed source in the RO pilot scale unit using the prepared sheets of MgSiO3 NPs modified AMPS-g-TFC membrane.

The different kind of both feed water and the corresponding product water through the Lab. RO unit was tested for major ion constituents to determine the efficiency of the membranes under different applied pressure, at a temperature (25°C), and flow rate (5 L/min), a brief result of feed and product water analysis results are observed in Table (2). From results the salt rejections of bivalent ions (Mg2+ and SO42-) are higher than that of monovalent ions (Na+ and Cl-) for cations and anions, where the retention for the bivalent anions is fewer than cations. This can be demonstrated by the mass transfer coefficients for divalent ions are lower than those for themonovalent ions and hence higher values for solute separations with respect to divalent ions [35]. Additionally, the degree of hydration which is a function of both size and charge/ being higher for small ions with a large charge, since there is a strong interaction of the solute ions with water molecules (ion-dipole influence) [36]. So that, the rejection of different ions is in the order:

R HCO3- > R SO42-> R Cl- and R Mg2+> R Ca2+> R Na+.

By applying, different pressure on the prepared composite membranes of MgSiO3 NPs modified AMPS-g-TFC membrane with different water types as in Table (3) and Figs. (16, 17) the results of both water flux and salt rejection increase gradually as an operation pressure increases, and by the further increasing of the operation pressure water flux increases dramatically comparing with increasing in salt rejection. It can be clarified by the way that, permeate is directly corresponding to the net wrking pressure and the solute diffusion across the membrane [37, 38]. By analyzing the metal concentration in the permeate flux of the applicant water which applied on MgSiO3 NPs modified AMPS-g-TFC membrane, we observed that, there is no release of both magnesium and silicon which confirms that, MgSiO3 NPs attached to thin film composite membrane via a chemical bond. Application results demonstrate that MgSiO3 NPs modified AMPS-g-TFC membrane show high desalination performance under different operating conditions and can be use as an applicable good modification for PA TFC membrane with modified properties.  

4. Conclusions

MgSiO3 NPs were attached to the surface of TFC membrane with AMPS monomer as a bridging agent via free radical grafting technique. MgSiO3 NPs modified AMPS-g-TFC membrane showed good mechanical and thermal stability while was high hydrophilic, with an increase in water contact angle (~45.9°). The modified  composite membrane and  its morphology were also examined using FT-IR, XRD, and SEM. The performance of  the modified membranes was evaluated with respect  to water flux, salt rejection. The new membrane presented a water flux of 28.2 L/m2•h and a salt rejection of ≥95.5%were obtained for a saline water (2000 ppm of NaCl) at a modified membrane pressure 15 bar with a 32% increase in water flux was fulfill when compared to the pristine TFC membrane. This study demonstrates that the MgSiO3 NPs modified AMPS-g-TFC membrane can significantly enhance selectivity, permeability and hydrophilic properties of the surface of membranes for water desalination.

Acknowledgments

It is a pleasure to acknowledge the financial support provided by the Science and Technological Development Fund (STDF) in Egypt through Grant 5240 (Egyptian Desalination Research Center of Excellence, EDRC)

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