5 research outputs found
Superhydrophilic Thin-Film Composite Forward Osmosis Membranes for Organic Fouling Control: Fouling Behavior and Antifouling Mechanisms
This study investigates the fouling behavior and fouling
resistance
of superhydrophilic thin-film composite forward osmosis membranes
functionalized with surface-tailored nanoparticles. Fouling experiments
in both forward osmosis and reverse osmosis modes are performed with
three model organic foulants: alginate, bovine serum albumin, and
Suwannee river natural organic matter. A solution comprising monovalent
and divalent salts is employed to simulate the solution chemistry
of typical wastewater effluents. Reduced fouling is consistently observed
for the superhydrophilic membranes compared to control thin-film composite
polyamide membranes, in both reverse and forward osmosis modes. The
fouling resistance and cleaning efficiency of the functionalized membranes
is particularly outstanding in forward osmosis mode where the driving
force for water flux is an osmotic pressure difference. To understand
the mechanism of fouling, the intermolecular interactions between
the foulants and the membrane surface are analyzed by direct force
measurement using atomic force microscopy. Lower adhesion forces are
observed for the superhydrophilic membranes compared to the control
thin-film composite polyamide membranes. The magnitude and distribution
of adhesion forces for the different membrane surfaces suggest that
the antifouling properties of the superhydrophilic membranes originate
from the barrier provided by the tightly bound hydration layer at
their surface, as well as from the neutralization of the native carboxyl
groups of thin-film composite polyamide membranes
Mechanism of Chitosan Adsorption on Silica from Aqueous Solutions
We present a study of the adsorption
of chitosan on silica. The
adsorption behavior and the resulting layer properties are investigated
by combining optical reflectometry and the quartz crystal microbalance.
Exactly the same surfaces are used to measure the amount of adsorbed
chitosan with both techniques, allowing the systematic combination
of the respective experimental results. This experimental protocol
makes it possible to accurately determine the thickness of the layers
and their water content for chitosan adsorbed on silica from aqueous
solutions of varying composition. In particular, we study the effect
of pH in 10 mM NaCl, and we focus on the influence of electrolyte
type and concentration for two representative pH conditions. Adsorbed
layers are stable, and their properties are directly dependent on
the behavior of chitosan in solution. In mildly acidic solutions,
chitosan behaves like a weakly charged polyelectrolyte, whereby electrostatic
attraction is the main driving force for adsorption. Under these conditions,
chitosan forms rigid and thin adsorption monolayers with an average
thickness of approximately 0.5 nm and a water content of roughly 60%.
In neutral solutions, on the other hand, chitosan forms large aggregates,
and thus adsorption layers are significantly thicker (∼10 nm)
as well as dissipative, resulting in a large maximum of adsorbed mass
around the p<i>K</i> of chitosan. These films are also characterized
by a substantial amount of water, up to 95% of their total mass. Our
results imply the possibility to produce adsorption layers with tailored
properties simply by adjusting the solution chemistry during adsorption
Highly Hydrophilic Thin-Film Composite Forward Osmosis Membranes Functionalized with Surface-Tailored Nanoparticles
Thin-film composite polyamide membranes are state-of-the-art
materials
for membrane-based water purification and desalination processes,
which require both high rejection of contaminants and high water permeabilities.
However, these membranes are prone to fouling when processing natural
waters and wastewaters, because of the inherent surface physicochemical
properties of polyamides. The present work demonstrates the fabrication
of forward osmosis polyamide membranes with optimized surface properties
via facile and scalable functionalization with fine-tuned nanoparticles.
Silica nanoparticles are coated with superhydrophilic ligands possessing
functional groups that impart stability to the nanoparticles and bind
irreversibly to the native carboxyl moieties on the membrane selective
layer. The tightly tethered layer of nanoparticles tailors the surface
chemistry of the novel composite membrane without altering the morphology
or water/solute permeabilities of the membrane selective layer. Surface
characterization and interfacial energy analysis confirm that highly
hydrophilic and wettable membrane surfaces are successfully attained.
Lower intermolecular adhesion forces are measured between the new
membrane materials and model organic foulants, indicating the presence
of a bound hydration layer at the polyamide membrane surface that
creates a barrier for foulant adhesion
Improved Antifouling Properties of Polyamide Nanofiltration Membranes by Reducing the Density of Surface Carboxyl Groups
Carboxyls are inherent functional groups of thin-film
composite
polyamide nanofiltration (NF) membranes, which may play a role in
membrane performance and fouling. Their surface presence is attributed
to incomplete reaction of acyl chloride monomers during the membrane
active layer synthesis by interfacial polymerization. In order to
unravel the effect of carboxyl group density on organic fouling, NF
membranes were fabricated by reacting piperazine (PIP) with either
isophthaloyl chloride (IPC) or the more commonly used trimesoyl chloride
(TMC). Fouling experiments were conducted with alginate as a model
hydrophilic organic foulant in a solution, simulating the composition
of municipal secondary effluent. Improved antifouling properties were
observed for the IPC membrane, which exhibited lower flux decline
(40%) and significantly greater fouling reversibility or cleaning
efficiency (74%) than the TMC membrane (51% flux decline and 40% cleaning
efficiency). Surface characterization revealed that there was a substantial
difference in the density of surface carboxyl groups between the IPC
and TMC membranes, while other surface properties were comparable.
The role of carboxyl groups was elucidated by measurements of foulant-surface
intermolecular forces by atomic force microscopy, which showed lower
adhesion forces and rupture distances for the IPC membrane compared
to TMC membranes in the presence of calcium ions in solution. Our
results demonstrated that a decrease in surface carboxyl group density
of polyamide membranes fabricated with IPC monomers can prevent calcium
bridging with alginate and, thus, improve membrane antifouling properties
Highly Hydrophilic Polyvinylidene Fluoride (PVDF) Ultrafiltration Membranes via Postfabrication Grafting of Surface-Tailored Silica Nanoparticles
Polyvinylidene
fluoride (PVDF) has drawn much attention as a predominant ultrafiltration
(UF) membrane material due to its outstanding mechanical and physicochemical
properties. However, current applications suffer from the low fouling
resistance of the PVDF membrane due to the intrinsic hydrophobic property
of the membrane. The present study demonstrates a novel approach for
the fabrication of a highly hydrophilic PVDF UF membrane via postfabrication
tethering of superhydrophilic silica nanoparticles (NPs) to the membrane
surface. The pristine PVDF membrane was grafted with poly(methacrylic
acid) (PMAA) by plasma induced graft copolymerization, providing sufficient
carboxyl groups as anchor sites for the binding of silica NPs, which
were surface-tailored with amine-terminated cationic ligands. The
NP binding was achieved through a remarkably simple and effective
dip-coating technique in the presence or absence of the <i>N</i>-(3-dimethylaminopropyl)-<i>N</i>′-ethylcarbodiimide
hydrochloride (EDC)/<i>N</i>-hydroxysuccinimide (NHS) cross-linking
process. The properties of the membrane prepared from the modification
without EDC/NHS cross-linking were comparable to those for the membrane
prepared with the EDC/NHS cross-linking. Both modifications almost
doubled the surface energy of the functionalized membranes, which
significantly improved the wettability of the membrane and converted
the membrane surface from hydrophobic to highly hydrophilic. The irreversibly
bound layer of superhydrophilic silica NPs endowed the membranes with
strong antifouling performance as demonstrated by three sequential
fouling filtration runs using bovine serum albumin (BSA) as a model
organic foulant. The results suggest promising applications of the
postfabrication surface modification technique in various membrane
separation areas