(580d) Development and Performance of a Silver Nanoparticle Impregnated Reusable Anti-Biofouling Membrane in a Continuous Cross Flow Membrane Module

It is well-known that biofouling decreases membrane-life significantly with simultaneous decrease in permeate flux. To resolve this issue, silver nanoparticle (Ag-NP) - as a biocidal material - has been impregnated in a polyethersulfone (PES) membrane; the latter being typically used in water treatment applications. The main challenges in using such a membrane would be to: (i) prevent membrane biofouling, (ii) achieve zero bacterial cell (E. coli, MTCC 1302) count in the permeate water, (iii) maintain silver (Ag) concentration in the permeate stream within the permissible limit of drinking water and (iv) obtain a high tensile strength of the membrane for continuous performance. Addressing these factors would ensure a long and productive service-life of the membrane.

Therefore, the goal of the present research was to develop and test the anti-biofouling efficacy of an Ag-NP incorporated, sulfonated, polyethersulfone (SPES) membrane and reuse it over multiple cycles. We call it as the Ag-SPES membrane and tested it in a laboratory-scale cross-flow membrane module. To this end, the PES membrane having an overall thickness of 150 µm was first sulfonated and then Ag-NPs were selectively impregnated only on the upper, external surface (having a thickness of 1.74 µm) of the membrane by UV-reduction. This leaves the bulk of the membrane and its lower surface free of any Ag-NP. Cross-section FEG-SEM images of the Ag-SPES membrane confirm the selective presence of Ag-NPs only on the upper surface of the otherwise thicker (150 µm) membrane. This ensures better anti-biofouling performance with lower Ag loading, since most of the Ag on the upper surface can come into contact with the E. coli in the inlet, contaminated water; whereas there is no extra Ag in the bulk or lower surface of the membrane, which otherwise does not come into contact with E. coli, and is hence of no consequence. Such selective Ag-NP impregnation as a thin layer in the post-membrane formation stage, ensures superior anti-biofouling performance with a much lower amount of Ag itself.

Now, in actual operation, contaminated, inlet water contains a low concentration of microorganisms (like E. coli), which overt time attaches to the membrane surface and slowly leads to biofouling after a somewhat long duration. However in current experiments, a very high E. coli cell concentration (10⁴ CFU/ml of E. coli) was purposefully used for reducing the timescale of biofouling. This facilitates an easy check on anti-biofouling efficacy of the Ag-SPES membrane.

To this end, E. coli cell suspension at 10⁴ CFU/ml of E. coli concentration was dispersed in PBS (phosphate saline buffer) and was passed through both PES (as control) and Ag-SPES (as test) membranes. However, E. coli cells having diameter 0.8-1 µm and length 1.5-2.5 µm are much larger than the 200 nm pore diameter of the membrane. Therefore, cells do not pass through the pores and instead attaches on the membrane surface. This was confirmed by both confocal microscopy and cryo-FEG-SEM images of PES membrane used in the membrane module, showing presence of only live bacterial cells on the outer surface of the membrane. This leads to biofouling and hence as expected, PES membrane showed a continuous and significant decrease in permeate water flow rate, from 64.5 to 12.4 ml/min, after the passage of 240 L of water (in 4 h). In contrast, due to the presence of Ag-NPs on the upper surface of Ag-SPES membranes, cells attached on this membrane are completely killed, showing only a marginal decrease in permeate flow rate (from 58.3 to 50.2 ml/min in 4 h).

In addition, a very low Ag concentration (0.01 mg/L) was also maintained in the permeate stream, which is well below the recommended limit of 0.1 mg/L (WHO) for use as drinking water. Finally, due to sulfonation and impregnation of Ag-NPs, the PES membrane showed an increase in maximum tensile strength from 2.78 MPa to 3.92 MPa, which is beneficial from a mechanical standpoint of membrane integrity in a water treatment process.

Therefore, we could achieve all the four targets set out by us in the beginning. Furthermore, we also found that the Ag-SPES membrane can be synthesized with different, controlled amounts of Ag loading (e. g. 4, 8.8 and 15 wt.% Ag), by tuning the degree of sulfonation of the Ag-SPES membrane. On capturing time-dependent confocal microscopy (using SYTO-9 and propidium iodide) images it turns out that the Ag-SPES membrane with 8.8 wt.% Ag loading (used in the previous results of this study) shows maximum permeability and best anti-biofouling performance. In contrast, the membrane with 4 wt.% Ag is sub-optimum and hence takes a longer time to kill the attached E. coli cells; whereas the membrane with 15 wt.% Ag shows a significant decrease in permeability due to pore blockage from excess Ag.

During long usage over repeated cycles, eventually the Ag content in the membrane may decrease, which might affect the anti-biofouling performance of the membrane. To simulate this condition, the Ag-NPs were purposefully and completely removed from the membrane by acid wash. Subsequently, the membranes were again reloaded with the same amount of Ag loading. It was observed that this does not affect the membrane performance. For example, Ag-SPES membrane reloaded with the optimum 8.8 wt.% Ag (after 4 cycles of operation and reloading), shows an almost identical permeate flow rate of 49.71 ml/min after 4 h, compared to 50.2 ml/min (after 4 h) obtained for the fresh, new membrane.

Thus this work demonstrates the viability of the reusable, anti-biofouling, Ag-SPES membrane, to produce decontaminated water over a long period. This was achieved without compromising the mechanical strength of the membrane and maintaining a permissible Ag concentration in the treated water, which is safe for potable purposes. Additionally, after certain periods of operation, Ag-NP could be selectively impregnated as a thin layer on the upper surface of the membrane repeatedly, gaining back the same degree of performance in continued operation.