*3.1.1 Metal organic frameworks*

MOFs consist of metal ions or clusters coordinated to organic ligands to form 1D, 2D or 3D structures. High crystallinity, porosity (up to 90%), internal surface areas (over 6000 m<sup>2</sup> /g), and stability make MOF ideal candidate for the enhancement of the membrane performance. The possibility of synthesizing different structures having various sizes and functionalities for a specific application is another advantageous of the MOFs [61]. This is important, since the main problem encountered during incorporation of inorganic nanomaterials into polymeric matrix is their incompatibilities [62]. Filler-polymer compatibility can be improved by the organic constituents of MOFs.

Ze-Xian Low studied the effect of 2D ZIF-L nanoflakes on the performance of the PES UF membrane [63]. Incorporation of ZIF-L was significantly improved water flux without greatly altering the MWCO of the modified UF membrane. Similarly, the combined effect of lower surface roughness, zeta potential, and higher hydrophilicity, caused to a lower bovine serum albumin (BSA) attachment onto the surface of the composite membrane. With those outstanding features, fouling resistance of the so-called membrane against BSA enhanced almost twice with more than 80% flux recovery.

In literature, MOF type materials have been extensively studied in heavy metals adsorption as they provide tunable pores and high specific area [64, 65]. The adsorptive characteristics of UiO-66-NH2 MOF has been tested by incorporating into PAN/CHI dope solution to make composite nanofiber [66]. The adsorptive membrane incorporated with 10 w% of UiO-66-NH2 MOF showed maximum monolayer coverage of 441.2, 415.6, and 372.6 mg/g for Pb2+, Cd2+, and Cr6+, respectively, in static condition. In crossflow filtration, carried out 20 mg/L initial metal concentration at 1 bar TMP, the permeation flux and metal removal were observed as for 452, 463, and 479 L/m<sup>2</sup> .h., and 94, 89, and 86%, respectively for Pb2+, Cd2+, and Cr6+. During long term filtration, slightly reduced permeation flux and rejection were obtained up to 18 h., beyond this point, a significant reduction in both flux and rejection revealed that the nanofibrous membrane pores were saturated.

### *3.1.2 Zeolite NPs*

Zeolite nanoparticles with their unique properties such as high ion exchange capacity and fast adsorption rate make it excellent choice for the separation of heavy metals in wastewater treatment [67] and desalination process [68]. In the study of Yurekli, variations in the morphologies and the filtration performances of the zeolite NPs blended PSf hybrid membranes have been investigated with respect to the loading amount of zeolite NPs [67]. **Figure 1** indicates surface and cross-sectional SEM microphotographs of the native and zeolite NPs incorporated PSf membranes. Formation of the new pores with larger diameters in **Figure 4** has been attributed to the phase separation occurred quickly and to the aggregation of the NPs. Compared to the native PSf membrane, zeolite loaded PSf membrane has more uniformly distributed finger-like pores which are extended through the thickness of the membrane that shortens the pathway (tortuosity) of the solutes, hence improve the hydraulic permeability. An increase in the hydraulic permeability value of 94% for the PSf10–30 membrane has been attained compared to the pristine PSf (23.2 L/m2.h.bar). It was observed that the retention of heavy metals through the PSf10–30 membrane was more pronounced at lower transmembrane pressures and heavy metals concentrations.

*Principles of Membrane Surface Modification for Water Applications DOI: http://dx.doi.org/10.5772/intechopen.96366*

#### **Figure 4.**

*SEM microphotographs of the native and zeolite incorporated PSf membranes; (a, c) top surface and (b, d) cross-sectional images of the native and 10% zeolite added PSf membranes, (e) hydraulic permeabilities of the PSf membranes prepared with different amount of zeolite NPs, (f) Pb2+ and (g) Ni2+ concentrations in permeate during filtrations of Pb2+ or Ni2+ aqueous solutions, respectively in different initial concentrations through PSf 10–30 membrane at 1 bar. [67].*

#### *3.1.3 GO NPs*

Similarly, Mukherjee et al. studied the impact of GO NPs on the removal of heavy metals using mixed matrix membrane (MMM) [69]. GO NPs were synthesized based on the modified Hummer method, blended in different fractions with PSf dope solution and prepared the MMM by phase inversion method. The main results extracted from the study of Mukherjee is depicted in **Figure 5**. From **Figure 5(a)-(c)** addition of GO NPs into PSf matrix increased MWCO, porosity, negative charge density, and permeability of the MMM. Based on the preliminary cross-flow tests 414 kPa was selected as optimum TMP considering both rejection and permeabilities of the membrane. In order to investigate reusability of the MMM, the fouled membrane with 50 mg/L chromium aqueous solution

#### **Figure 5.**

*Effect of GO concentration on (a) MWCO and porosity; (b) zeta potential at neutral pH (c) contact angle and permeability. Effect of regeneration on (d) permeate flux and (e) rejection by GO 0.2 membrane, with 414 kPa TMP, 40 l/h CFR and operated at pH 10. (f) Long duration filtration performance of GO 0.2 membrane in case of filtration of mixed metal solution at 414 kPa TMP and 40 L/h CFR. [69].*

in cross-flow mode of 414 kPa and 20 L/h of retentate flow rate for 1 hour was washed first with water for 10 min and then with acidic solution at pH 5.5 for 30 min and again with water to remove all the acidic residuals. All the steps in the regeneration cycle were carried out in dynamic conditions similar to the case in fouling. The reduction in solution flux at each cycle was explained as the accumulation of chromium ion at the GO surface, which was also observed by the reduction in rejection values in the consecutive cycles (**Figure 5(a)** and **(b)**). The results from long term filtration performance of the MMM with respect to the mixed metal solution (Cr, Cu, Cd, and Pb) each having concentrations of 50 mg/L, as illustrated in **Figure 5(f )** demonstrated the permeate flux decreased continuously. The authors concluded that the simultaneous adsorption of different heavy metals, on the membrane increased the resistance of adsorption, which resulted in permeate flux reduction rapidly with time. Finally, the concentrations of all heavy metals in permeate remained almost constant up to breakthrough time (8 h) and increased thereafter till feed concentration.

### *3.1.4 TNT nanotubes*

Subramaniam et al. fabricated PVDF hollow fiber UF membrane incorporated with titanate nanotubes (TNTs) for decolourization of aerobically-treated palm oil mill effluent [70]. TNTs which were synthesized based on the alkaline hydrothermal process were dispersed in NMP under sonication for 30 min. Hollow fiber membranes were fabricated by means of a dry-jet wet spinning method by changing the amount of TNTs in the bore solution of TNT/PVP/PVDF in between 0–1%. The spherical TiO2 nanoparticles were reported to be converted into completely TNTs with an average diameter of 24 nm at the end of the hydrothermal process. The variation in the pore size was more pronounced by the addition of pore former than the addition of TNT into the dope solution meaning that the addition of TNT had no considerable effect on the pore size and finger-like structure of the membrane. The result was evidenced by the similar porosity values obtained for all the membrane formulations. Addition of TNT increased the membrane roughness, BSA rejection, color rejection and water flux simultaneously (**Figure 6a**). However, beyond a certain amount of TNT loading, a reduction in water flux has been observed, which was ascribed to the aggregation of NPs. The authors finally investigated the flux recovery and antifouling performances of the resultant membranes during 5 regeneration cycles as depicted in **Figure 6**. Regeneration was accomplished by first fouled the membrane with AT-POME at 1 bar for 240 min using cross-flow filtration, then the fouled membrane was washed with water for 30 min. In **Figure 6**, the fluxes in all membrane configurations decreased over time but the flux recoveries for all the membranes after 5 cycles were observed above 80% except the pristine PVDF, which exhibited gradual declination of flux throughout the test. Similarly, all the membranes comprised of TNT regardless of loading were able to recover rejection performance after water washing

#### **Figure 6.**

*(a) Pure water fluxes, AT-POME flux, BSA and color removal (b) AT-POME flux and (c) AT-POME color removal for five cycles of AT-POME filtration [70].*

but pristine PVDF showed declining rejection through 5 cycles. The results has been related to the variation of the hydrophilic natures of the membranes.
