(569d) Fabrication of Fouling-Resistant Electro-Conductive Thin Film Composite Membranes By Laminating Polyaniline (PANI) and Reduced Graphene Oxide (rGO) on Polyethersulfone

Fabrication
of Fouling-resistant Electro-conductive Thin Film Composite Membranes by Laminating
Polyaniline (PANI) and reduced Graphene Oxide (rGO) on Polyethersulfone

Amin Karkooti^a, Masoud
Rastgar^b, Mohtada Sadrzadeh^b, Neda Nazemifard^a

^aDepartment of Chemical and Material
Engineering, University of Alberta

^bDepartment of Mechanical Engineering,
University of Alberta

Abstract:

The
adsorption of organic matter onto the membrane surface is the most significant
obstacle that restrict the sustainable application of membranes for residential
and industrial wastewater treatment. Fouling on membrane decreases water flux,
shortens membrane lifespan, and consequently leads to a higher operating cost. Development of electro-conductive membranes (ECMs) can
potentially provide an effective solution to enhance fouling resistance characteristics
of membranes. The application of electrical potentials to the ECMs
offers a promising enhancement to existing membrane-based water treatment
processes. Membrane surface charge is one of the effective parameters that
influence fouling at the initial stage of filtration through electrostatic
interactions. By applying voltage to a conductive membrane, membrane’s surface
becomes more negative and it can effectively inhibit the adsorption of organic
contaminants, which are predominately negative charged. In addition, in electro-conductive
membrane, filtration process is combined with electro-chemical oxidation (EO)
technique, which is a powerful technique in removal different harmful
pollutants from water. Simultaneous to filtration, electrical potential causes
organic contaminants in water to be eliminated by either direct or indirect
oxidation mechanisms. Depending on operational time, anodic material, and the applied
electrical potential, the oxidation could be progressed entirely which clean
water and carbon dioxide would be produced through overall degradation of
organic compounds. In this work, we fabricated a novel and mechanically-stable
electro-conductive membrane by pressure-assisted laminating of reduced graphene
oxide-polyaniline (rGO-PANI) suspension on commercial polyethersulfone (PES) substrate.
PANI (50 ppm) and rGO (20 ppm) were respectively dispersed in
N-Methyl-2-pyrrolidone (NMP) and deionized water via sonication and were mixed
together in different weight ratios. Afterward, Using 20 psi air pressure, the
obtained mixtures were forced to pass through PES substrate having surface area
of 41.8 cm². In this coating layer, two neighboring rGO sheets can
create special interconnected channels that is selectively able to permeate
water, while rejecting all other solutes.  Three different membranes including
M1 (PANI (0.5 mg)-rGO (0 mg)), M2 (PANI (0.25 mg)-rGO (2 mg)), and M3 (PANI (0.5
mg)-rGO (2 mg)) were provided and used for organic fouling experiments. Prior
to laminating the thin film layer, graphene oxide (GO) nanosheets were synthesized
based on modified Hummers’ method. Next, reduced GO (rGO) nanosheets were produced
by heating GO at 1000 ˚C. The reduction procedure partially removes
functional groups, such as hydroxyl (R-OH), carboxyl (R-COOH) and carbonyl (R-O-R), 
giving rGO high electrical conductivity because of its none or  low oxidation
state. Both rGO and PANI are electrically conductive and we observed that the
obtained composite membranes were mechanically stable in aqueous environments. In
addition, It was found that rGO-PANI ECMs fabricated using sulfuric acid as the
dopant showed the best transport, electrical, and stability characteristics,
making them ideal for water treatment applications. In the next step, the
surface morphology, chemical composition and permeation properties of the
fabricated rGO-PANI membranes were characterized by transmission electron microscopy (TEM), field emission
scanning electron microscopy (FESEM), attenuated total reflectance-fourier
transform infrared (ATR-FTIR) spectroscopy, zeta potential measurements,
contact angle analysis. The filtration tests were performed by both pure water
and feed solution containing 50 ppm sodium alginate (SA) as an organic model
foulant and 5 mM NaCl as the background electrolyte. The combination of rGO
with PANI were found to have significant advantages such as increasing surface
hydrophilicity and enhancing attachment and stability of coating under anodic oxidation
and natural pH conditions. The top view FESEM images revealed that linear PANI
chains made very porous regions on top layer, while rGO deposited parts were
almost defect-free. Through cross-sectional TEM imaging, the average thickness
of top rGO-PANI layer was determined to be 120 nm. Furthermore, the water
contact angle measurements showed that by increasing the ratio of PANI to rGO,
membranes demonstrated more hydrophilic characteristics. Thus, these membrane
materials are promising candidates for electro-conductive membrane materials
suitable to participate in electro-oxidation reactions. For flux measurement, the
membranes were placed in a custom-made cross-flow non-conductive chamber in
which membrane top layers were connected to a DC power supply (anode). A
circular piece of stainless steel (Diameter = 2 cm) was used as a counter
electrode (cathode). The distance between anode and cathode was fixed in 5 mm. The
water flux and organic fouling behavior of three fabricated electro-conductive
membrane (M1, M2, and M3) containing different ratio of PANI to rGO were
compared by applying either no or 2 V anodic potential and the total flux
decline ratio (R_t) values were assessed for each membrane.  The
result is presented in Fig. 1. The fouling resistance propensity of all
membranes were enhanced by applying 2 V potential compared with no electrical
potential. The highest improvement was achieved by M3 that the total flux
decline (R_t) due to organic fouling was 33.2% less in comparison
with the same membrane without applying potential. The total flux decline ratio
in M1 membrane was 18.2% and 10.9% in M2 membrane after applying 2 V anodic
potential. As it mentioned before, electro-chemical oxidation offers a broad
range of possible reactions to control organic fouling. The SA macromolecules
could be directly oxidized over anodic electrode (membrane surface) without
involving of any other substances. This reaction is kinetically slow and may
decline catalytic performance of membrane during long-term applications. Besides,
fouling may be mitigated through indirect mechanism. In this mechanism,
super-active intermediates like hydroxyls (OH^-), chlorine (Cl^-),
hypochlorous acid (HClO) and hypochlorite (ClO^-) may be generated in
close distance to the ECMs membranes. These super active oxidizer are able to rapidly
oxidize organic materials and eliminate fouling layer in advance. Oxygen
evaluation reaction (OER) is usually driven over carbon-based electrodes in
pretty low electrical potentials. Therefore, oxygen bubbling can also help
membranes to overcome fouling in some extents. The experimental results showed
more than 30% improvement in flux recovery ratio (FRR) when 2 V DC potential is
applied on the surface of the rGO-PANI ECMs membrane compared with rGO-PANI
membrane without applying potential. Fouling experiments conducted with organic
foulant sodium alginate demonstrated the capacity of the rGO-PANI ECMs for in
situ oxidative cleaning.

Figure
1 Fouling behaviors of different fabricated
membranes.