reverse osmosis process, separation is occurred in pressurised system which
leads to the attachment of foreign particle to the membrane surface, fouling of
membrane, which would decrease the water flux or increases the pressure
difference across the membrane. This fouling has relation with membrane
properties like surface roughness, hydrophilicity, surface charge etc.
Therefore many researches have been done to improve the antifouling property of
membrane by modifying the active polyamide layer. To reduce membrane fouling, researchers
tried to modify membrane surface which can be done through surface coating or
surface grafting 28. In physical modification, the materials interact with
polyamide layer of RO membranes and attach it by van der Waals attraction,
electrostatic interaction or hydrogen bonding, which may not be stable in
long-term operation. In contrast, in chemical modification, the materials
connected to the surface of RO membranes by covalent bonds and have better
chemical and structural stabilities 23.



Physical surface modification:

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surface modification of TFC RO membrane has two sub category which is surface
adsorption and surface coating. In Table 1, different physical surface
modification is summarised in order from past to present.

have carried out surface modification of RO membranes by adsorption of compounds
such as surfactants 29 and charged polyelectrolytes 31.

year 1998, Wilbert et al. 29 applied a homologous series of polyethyleneoxide
(PEO) surfactants with either octylphenol or polypropylene oxide head groups
(T-X series and P series) to modify the surface of commercial polyamide TFC RO
membranes. The strategy in that study combined the benefits of increased
hydrophilicity with steric hindrance by adsorbing surfactants. The results
showed that surfactants decreased the roughness of membranes without a large
change in zeta potential and the membranes exhibited improved antifouling
property in a vegetable broth solution compared to unmodified membrane.

2006, Louie et al. 30 performed physical coating study of commercial
polyamide RO membranes with PEBAX-1657, which was a very hydrophilic block
copolymer of nylon-6 and PEG. The coating greatly reduced surface roughness
without significant change in contact angle. During a long-term (106 day)
fouling test with an oil/surfactant/water emulsion, the rate of flux decline
was slower for coated than for uncoated membranes that showed the coated
membranes had enhanced fouling resistance. However, the coating resulted in
large water flux reduction, especially for high-flux RO membranes 30.

2009, Yong Zhou et al. 31 used surface adsorption method by using charged
polyelectrolytes adsorption for surface modification of RO membrane. The
modification was done by electrostatic self deposition of polyethyleneimine
(PEI) (a branched hydrophilic cationic polyelectrolyte) on the membrane
surface, and the modified membrane showed significantly improved antifouling
properties. The charge reversal on the membrane surface due to the application
of the polyethyleneimine layer was shown to increase the fouling resistance of
the membrane to cationic foulants because of the enhanced electrostatic
repulsion, and the increased surface hydrophilicity would help minimize the flux
reduction. In same year, Sagle et al. 32 investigated the effect of coating
of a series of cross-linked PEG-based hydrogels on the RO membranes. NaCl
rejection for both uncoated and coated membranes was 99.0% or greater while the
water flux was decreased for coated membranes than that of uncoated membranes. Surfactant
fouling of DTAB and SDS has been carried out which showed that lower salt
rejection for DTAB fouled membranes and SDS-fouled membranes had higher salt
rejection than membranes not exposed to surfactants. In both fouling experiments,
coated membranes had less flux decline than uncoated AG RO membranes.

2010, Y. Kwon et al. 33 the homopolymer poly(ethylene glycol) acrylate (PEGA)
was used as surface coating to enhance antifouling property of RO membranes.
PEGA coated RO membranes were cross-linked by the glutaraldehyde (GA) solution
to enhance the durability of the coating layer. After the surface modification,
the RO membranes showed a lower surface roughness, more hydrophilicity, and
better performance compared to the unmodified RO membranes. The modified RO
membranes exhibited better antifouling properties and recovered almost 100% of
their initial water flux after physical cleaning. Dihua Wu et al. 34 used thermo-responsive
copolymers poly(Nisopropylacrylamide-co-acrylamide) (P(NIPAM-co-Am)) for
surface modification to improve membrane properties. McCloskey et al. 35 experimented
polydopamine (PDOPA) and poly(ethylene glycol) (PEG) grafting on polysulfone
ultrafiltration membranes, a poly(vinylidene fluoride) microfiltration
membrane, and a polyamide reverse osmosis membrane to improve pure water flux
and bovine serum albumin (BSA) adhesion resistance. Also used PDOPA which can reduce
protein adhesion. Abhijit Sarkar et al. 36 used the Polyamidoamine
(PAMAM) and PAMAM– polyethylene glycol (PAMAM–PEG) to reduce contact angle and to
increase biofoulants and organic pollutants resistance.

year 2011, J.S. Louie et al. 37 experimented on polyether-polyamide block
copolymer (PEBAX): 1 wt% in n-butanol or 1:1 ethanol: water (mass ratio) to enhance water flux. L. Zou et al. 38
used Triethylene glycol dimethyl ether (Triglyme) to
reduce RO membrane’s organic fouling tendency. J. Xu et al. 39 treated membrane by chlorine and followed by chitosan
deposition on membrane to increase water flux and salt rejection, in addition,
divalent salt rejection having good rejection. M. Liu et al. 40 used coating
agent N-isopropylacrylamide – co – acrylamide (P(NIPAM-co-Am)) to to improve
acid stability and chlorine resistance. Modified membrane showed 3000 ppm h
Sodium hypochlorite resistance and 0.5 mol/l HCl for 2 months resistance
without much change in membrane performance. S. Yu et al. 41 coated N-isopropylacrylamide-co-acrylic
acid copolymers (P(NIPAm-co-AAc)) to increase NaCl and Na2SO4
rejection and fouling resistance. S. Yu et al. 42 used a thermo-responsive polymer
poly(N-isopropylacrylamide – co – acrylamide) (P(NIPAM-co-Am)) with low critical
solution temperature (LCST) to improve fouling resistance and cleaning
efficiency. A. Matin et al. 43 used hydroxyethylmethacrylate (HEMA) monomer
with the hydrophobic per fluorodecyl acrylate (PFA) monomer for antifouling

year 2012, various coating agents like Methyl methacrylate- hydroxyl poly
(oxyethylene) methacrylate (MMA-HPOEM), DMAP followed by SPGE and glycerol
coating, PDA deposition followed by PEG-NH2 grafting, 3-(3,4-dihydroxyphenyl)-
L alanine (L-DOPA) and PEG acrylate multilayers (containing PAA-Alk, PEG-Az or
PEG-Alk) used to improve fouling, chlorine resistance, bioinspired fouling and flux
of the membrane 44-48.

year 2013, many surface coating materials are used lik sericin followed by GA
crosslinking, Polyvinylalcohol (PVA) polyhexamethylene guanidine hydrochloride
(PHMG), Polydopamine (PDA), Polydopamine (PDA) and polydopamine-g-PEG, Hydroxyethyl
methacrylate and perfluoro decylacrylate (PFA) used to reduce attachment of
bacterial cells, to improve fouling resistance and membrane performance 49-53.

year 2014, various chemical like P(MDBAC-r-Am-r-HEMA) coating followed by GA, poly2-methacryloyloxyethyl
phosphorylcholine (MPC)-co-2-amino ethylmethacrylate (AEMA) (p(MPC-co-AEMA)), polydopamine
(PDA), PDA followed by covalently immobilization (MPC-co-AEMA), P(4-VP-co-DVB),
Hydroxyethyl methacrylate (HEMA) and the hydrophobic perfluoro decylacrylate
(PFDA), Hydroxyethyl methacrylate (HEMA) and the hydrophobic perfluoro
decylacrylate (PFDA), trimethylaluminium (AlMe3) – (ALD-Al2O3),
BaSO4 is used to increase membrane performance and antifouling

year 2015, different advanced chemicals and zwitterionic material like P(4-VP-co-EGDA),
HPOEM or PEI coating followed by GA, p(4-VP-co-EGDA) was coated to control
fouling particularly biofouling63-65.

In year 2016, 2-
acrylamido-2-methyl propanesulfonic acid, acrylamide and HEMA-co-PFDA were used
to improve chlorine resistance and to reduce sodium alginate fouling66,67.

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