7 research outputs found
Omniphobic Polyvinylidene Fluoride (PVDF) Membrane for Desalination of Shale Gas Produced Water by Membrane Distillation
Microporous membranes fabricated
from hydrophobic polymers such
as polyvinylidene fluoride (PVDF) have been widely used for membrane
distillation (MD). However, hydrophobic MD membranes are prone to
wetting by low surface tension substances, thereby limiting their
use in treating challenging industrial wastewaters, such as shale
gas produced water. In this study, we present a facile and scalable
approach for the fabrication of omniphobic polyvinylidene fluoride
(PVDF) membranes that repel both water and oil. Positive surface charge
was imparted to an alkaline-treated PVDF membrane by aminosilane functionalization,
which enabled irreversible binding of negatively charged silica nanoparticles
(SiNPs) to the membrane through electrostatic attraction. The membrane
with grafted SiNPs was then coated with fluoroalkylsilane (perfluorodecyltrichlorosilane)
to lower the membrane surface energy. Results from contact angle measurements
with mineral oil and surfactant solution demonstrated that overlaying
SiNPs with ultralow surface energy significantly enhanced the wetting
resistance of the membrane against low surface tension liquids. We
also evaluated desalination performance of the modified membrane in
direct contact membrane distillation with a synthetic wastewater containing
surfactant (sodium dodecyl sulfate) and mineral oil, as well as with
shale gas produced water. The omniphobic membrane exhibited a stable
MD performance, demonstrating its potential application for desalination
of challenging industrial wastewaters containing diverse low surface
tension contaminants
Membrane-Based Osmotic Heat Engine with Organic Solvent for Enhanced Power Generation from Low-Grade Heat
We
present a hybrid osmotic heat engine (OHE) system that uses
draw solutions with an organic solvent for enhanced thermal separation
efficiency. The hybrid OHE system produces sustainable energy by combining
pressure-retarded osmosis (PRO) as a power generation stage and membrane
distillation (MD) utilizing low-grade heat as a separation stage.
While previous OHE systems employed aqueous electrolyte draw solutions,
using methanol as a solvent is advantageous because methanol is highly
volatile and has a lower heat capacity and enthalpy of vaporization
than water. Hence, the thermal separation efficiency of a draw solution
with methanol would be higher than that of an aqueous draw solution.
In this study, we evaluated the performance of LiCl–methanol
as a potential draw solution for a PRO–MD hybrid OHE system.
The membrane transport properties as well as performance with LiCl–methanol
draw solution were evaluated using thin-film composite (TFC) PRO membranes
and compared to the results obtained with a LiCl–water draw
solution. Experimental PRO methanol flux and maximum projected power
density of 47.1 L m<sup>–2</sup> h<sup>–1</sup> and
72.1 W m<sup>–2</sup>, respectively, were achieved with a 3
M LiCl–methanol draw solution. The overall efficiency of the
hybrid OHE system was modeled by coupling the mass and energy flows
between the thermal separation (MD) and power generation (PRO) stages
under conditions with and without heat recovery. The modeling results
demonstrate higher OHE energy efficiency with the LiCl–methanol
draw solution compared to that with the LiCl–water draw solution
under practical operating conditions (i.e., heat recovery <90%).
We discuss the implications of the results for converting low-grade
heat to power
Bidirectional Diffusion of Ammonium and Sodium Cations in Forward Osmosis: Role of Membrane Active Layer Surface Chemistry and Charge
Systematic
fundamental understanding of mass transport in osmosis-driven
membrane processes is important for further development of this emerging
technology. In this work, we investigate the role of membrane surface
chemistry and charge on bidirectional solute diffusion in forward
osmosis (FO). In particular, bidirectional diffusion of ammonium (NH<sub>4</sub><sup>+</sup>) and sodium (Na<sup>+</sup>) is examined using
FO membranes with different materials and surface charge characteristics.
Using an ammonium bicarbonate (NH<sub>4</sub>HCO<sub>3</sub>) draw
solution, we observe dramatically enhanced cation fluxes with sodium
chloride feed solution compared to that with deionized water feed
solution for thin-film composite (TFC) FO membrane. However, the bidirectional
diffusion of cations does not change, regardless of the type of feed
solution, for cellulose triacetate (CTA) FO membrane. We relate this
phenomenon to the membrane fixed surface charge by employing different
feed solution pH to foster different protonation conditions for the
carboxyl groups on the TFC membrane surface. Membrane surface modification
is also carried out with the TFC membrane using ethylenediamine to
alter carboxyl groups into amine groups. The modified TFC membrane,
with less negatively charged groups, exhibits a significant decrease
in the bidirectional diffusion of cations under the same conditions
employed with the pristine TFC membrane. Based on our experimental
observations, we propose Donnan dialysis as a mechanism responsible
for enhanced bidirectional diffusion of cations in TFC membranes
Development of Omniphobic Desalination Membranes Using a Charged Electrospun Nanofiber Scaffold
In this study, we present a facile
and scalable approach to fabricate omniphobic nanofiber membranes
by constructing multilevel re-entrant structures with low surface
energy. We first prepared positively charged nanofiber mats by electrospinning
a blend polymer–surfactant solution of poly(vinylidene fluoride-<i>co</i>-hexafluoropropylene) (PVDF-HFP) and cationic surfactant
(benzyltriethylammonium). Negatively charged silica nanoparticles
(SiNPs) were grafted on the positively charged electrospun nanofibers
via dip-coating to achieve multilevel re-entrant structures. Grafted
SiNPs were then coated with fluoroalkylsilane to lower the surface
energy of the membrane. The fabricated membrane showed excellent omniphobicity,
as demonstrated by its wetting resistance to various low surface tension
liquids, including ethanol with a surface tension of 22.1 mN/m. As
a promising application, the prepared omniphobic membrane was tested
in direct contact membrane distillation to extract water from highly
saline feed solutions containing low surface tension substances, mimicking
emerging industrial wastewaters (e.g., from shale gas production).
While a control hydrophobic PVDF-HFP nanofiber membrane failed in
the desalination/separation process due to low wetting resistance,
our fabricated omniphobic membrane exhibited a stable desalination
performance for 8 h of operation, successfully demonstrating clean
water production from the low surface tension feedwater
Omniphobic Membrane for Robust Membrane Distillation
In
this work, we fabricate an omniphobic microporous membrane for
membrane distillation (MD) by modifying a hydrophilic glass fiber
membrane with silica nanoparticles followed by surface fluorination
and polymer coating. The modified glass fiber membrane exhibits an
anti-wetting property not only against water but also against low
surface tension organic solvents that easily wet a hydrophobic polytetrafluoroethylene
(PTFE) membrane that is commonly used in MD applications. By comparing
the performance of the PTFE and omniphobic membranes in direct contact
MD experiments in the presence of a surfactant (sodium dodecyl sulfate,
SDS), we show that SDS wets the hydrophobic PTFE membrane but not
the omniphobic membrane. Our results suggest that omniphobic membranes
are critical for MD applications with feed waters containing surface
active species, such as oil and gas produced water, to prevent membrane
pore wetting
High Performance Nanofiltration Membrane for Effective Removal of Perfluoroalkyl Substances at High Water Recovery
We
demonstrate the fabrication of a loose, negatively charged nanofiltration
(NF) membrane with tailored selectivity for the removal of perfluoroalkyl
substances with reduced scaling potential. A selective polyamide layer
was fabricated on top of a poly(ether sulfone) support via interfacial
polymerization of trimesoyl chloride and a mixture of piperazine and
bipiperidine. Incorporating high molecular weight bipiperidine during
the interfacial polymerization enables the formation of a loose, nanoporous
selective layer structure. The fabricated NF membrane possessed a
negative surface charge and had a pore diameter of ∼1.2 nm,
much larger than a widely used commercial NF membrane (i.e., NF270
with pore diameter of ∼0.8 nm). We evaluated the performance
of the fabricated NF membrane for the rejection of different salts
(i.e., NaCl, CaCl<sub>2</sub>, and Na<sub>2</sub>SO<sub>4</sub>) and
perfluorooctanoic acid (PFOA). The fabricated NF membrane exhibited
a high retention of PFOA (∼90%) while allowing high passage
of scale-forming cations (i.e., calcium). We further performed gypsum
scaling experiments to demonstrate lower scaling potential of the
fabricated loose porous NF membrane compared to NF membranes having
a dense selective layer under solution conditions simulating high
water recovery. Our results demonstrate that properly designed NF
membranes are a critical component of a high recovery NF system, which
provide an efficient and sustainable solution for remediation of groundwater
contaminated with perfluoroalkyl substances