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del2288 del2288
del2288 del2288

SDF
Louisiana, United States


Contemporary Blues / Acoustic Blues

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A review on RO membrane technology: Developments and challenges



Reverse osmosis (RO) based desalination is one of the most important and widely recognized technologies for production of fresh water from saline water.

Since its conception and initiation, a significant development has been witnessed in this technology w.r.t. materials, synthesis techniques, modification and

modules over the last few decades. The working of a RO plant inclusive of the pretreatment and post-treatment procedures has been briefly discussed in the

article. The main objective of this review is to highlight the historical milestones achieved in RO technology in terms of membrane performance, the

developments seen over the last few years and the challenges perceived.







The material properties of the membrane dominate the performance of a RO process. The emergence of nano-technology and biomimetic
href="http://www.hzmembrane.com/ro-membrane/" target="_self">RO membranes
as the futuristic tools is capable of revolutionizing the entire RO process.

Hence the development of nano-structured membranes involving thin film nano-composite membranes, carbon-nanotube membranes and aquaporin-based membranes has

been focussed in detail. The problems associated with a RO process such as scaling, brine disposal and boron removal are briefed and the measures adopted to

address the same have been discussed.







In response to the escalating world water demand and aiming to promote equal opportunities, reverse osmosis desalination has been widely implemented.

Desalination is however constantly subjected to fouling and scaling which increase the cost of desalination by increasing the differential pressure of the

membrane and reducing the permeate flux. A bench-scale desalination equipment has been used in this research to investigate the mitigation of fouling and

scaling. This study involved the performance of membrane autopsy for fouling characterisation with special attention to flux decline due to sulphate

precipitation and biofouling. Visual inspection, scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), Fourier transform infrared

spectroscopy (FTIR) and microbiology tests (API) were performed. Results obtained showed the presence of diatoms, pseudomonas and polysaccharides as the main

foulants causing biofouling. Analysis revealed sulphate deposits as well as aluminium, calcium and silica as the main elements contributing to inorganic

scaling. Findings pointed out that the pre-treatment system of the small-scale reverse osmosis water treatment was inefficient and that selection of pre-

treatment chemicals should be based on its compatibility with the membrane structure. The importance of characterisation for the verification of fouling

mechanisms is emphasised.







This research was conducted to determine the performance of Reverse Osmosis (RO) membranes in producing pure water, pure water known as mineral-free

water or water with zero dissolved solids (TDS = 0 ppm).PDAM (Regional Drinking Water Company) Tirta Musi in Palembang, South Sumatra and water from the

Micro Filtration (MF) and Ultrafiltration (UF) processes are fed to the RO process using two feeding methods, namely a single pass and a circulation feed. In

a single pass feed, the operating pressure is set at 20 - 50 Psig, where an increase in the product rate and the rejection rate so that the flux increases.

Rejection of TDS obtained increased from 96.6% - 97.5%. Furthermore, the circulating feed system with a constant pressure of 50 Psig decreases TDS and

Conductivity. Rejection of TDS 96.1% for PDAM water feed and Rejection of TDS for feed water from MF&UF 97.3% in subsequent feedings there was a decrease

in TDS and conductivity but not significantly. The purified water produced has a TDS content of 0.16 - 0.48 ppm, a conductivity of 0.17 - 0.49 μs/cm, a pH

of 6.99 - 7.2 and a resistivity of 177 - 185 kΩ, the characteristics of this pure water are according to the standard pure water in ASTM D1193 - 99e1 and

NCCLS.



Clean water obtained by desalinating sea water or by purifying wastewater, constitutes a major technological objective in the so-called water century. In

this work, a high-performance reverse osmosis (RO) composite thin membrane using multi-walled carbon nanotubes (MWCNT) and aromatic polyamide (PA), was

successfully prepared by interfacial polymerization. The effect of MWCNT on the chlorine resistance, antifouling and desalination performances of the

nanocomposite membranes were studied. We found that a suitable amount of MWCNT in PA, 15.5 wt.%, not only improves the membrane performance in terms of flow

and antifouling, but also inhibits the chlorine degradation on these membranes. Therefore, the present results clearly establish a solid foundation towards

more efficient large-scale water desalination and other water treatment processes.







Introduction



The availability of clean water has become a global problem because of the continuously increasing costs of energy and increasing scarcity of water

resources1. This problem has been exacerbated in recent years in the so-called century of water. By far, the domestic ro membrane process persists as the most reliable and cost-effective water desalination technique

and numerous large-scale RO plants have been constructed around the world2,3. A wide range of polymers have shown potential for fabricating desalination

membranes to be used in RO4. However, PA-based membranes tend to exhibit the best performance in terms of selectivity, flow, chemical stability and ease of

large-scale fabrication. PA membrane technology was developed in the mid-70 s and has become the commercial benchmark in RO membranes5. In order to improve

the membrane performances, the recent trend in polymer-based membrane research has been to investigate various types of nanocomposite films as an active

layer of RO membrane, so-called nanocomposite membranes, in which these films are fabricated using a nanosized filler such as MWCNT, graphene, graphene

oxide, silica, or zeolite6. In this regard, MWCNT·PA-based membranes have been prepared by several groups and in general, these membranes have exhibited

some level of improved performance7,8,9,10,11,12. The advantages claimed for these membranes range from increased salt rejection, large fluxes, greater

durability and even antimicrobial properties.







MWCNT synthesized by catalytic chemical vapour deposition13,14 have been widely studied due to their fascinating chemical and physical properties and

among all nanocarbon materials, they can be mass-produced for commercially available applications, such as the electrode additives in high performance

lithium ion batteries15. Interestingly, while the structure of the fully aromatic PA-based commercial ro membrane derived from m-phenylendiamine (MPD)-trimesoyl chloride (TMC) is constrained due to its stoichiometry;

the addition of MWCNT can significantly vary their performance due to their unique features such as dispersability diameter, length, straightness and

chemical functionalities, among many others. Therefore, although these past reports acknowledge the key role of MWCNT in aromatic PA nanocomposite membranes,

still little attention has been devoted to the mechanisms related to the improvement of flow rate, selectivity and chlorine tolerance2. Carbon nanotubes

inducing chlorine tolerance are particularly interesting because chlorine sensitivity has been recognized as a major drawback of PA-based RO membranes16,17.

During long-term operation, chlorine is often added as a pre-treatment to reduce algae biofouling18 and is particularly needed for drinking water

purification. Moreover, high-concentration short-term exposure to chlorine is also common during domestic nf membrane backwashing. For these reasons, several studies have been carried out and the degradation mechanism of

aromatic PA membranes during chlorine exposure is relatively well-known19,20. Recently, our group demonstrated that the addition of MWCNT to rubber can

considerably reduce the chlorine-induced degradation of the polymer matrix21. Although the degradation mechanism of rubber by chlorine is different from that

of PA, particularly due to the lack of hydrolysis, covalent chlorination is a common problem for both polyamide and rubber. For rubber, we found that MWCNT

effectively restricted the adsorption of chlorine within the polymer matrix, thus resulting in a limited exposure of the polymer to this reactive reagent and

thereby decreasing the oxidative degradation. For these reasons, we believe MWCNT are not only promising composite fillers with chlorine protective

properties, but might also help to provide mechanical robustness to PA-based RO membranes.







Results and Discussion



We prepared aromatic PA membranes using a support consisting of a porous polysulfone layer deposited on a polypropylene nonwoven. These support membranes

were soaked sequentially in MPD and TMC solutions, to synthesize the aromatic PA membrane by interfacial polymerization. In order to incorporate MWCNT into

the PA membrane, an anionically stabilized dispersion of MWCNT (Supplementary Fig. S1) was mixed with the MPD solution and the synthesis was conducted

similarly. Figure 1a shows an image of the resulting membranes, with and without MWCNT. The black color developed in the membrane prepared using surfactant

dispersed MWCNT is characteristic of the high carbon nanotube content of the present membrane (Fig. 1a). Thermogravimetry of the active layer (Supplementary

Fig. S2) of the black color membrane indicates that it contains ca. 15.5 wt. % of MWCNT, which is at least 150 times higher than previously reported MWCNT-

filled RO PA membranes7,8,12. The SEM image showing the surface morphology of the membrane is typical for the interfacial PA polymerization22, consisting of

the multi-layered ridge-and-valley (Fig. 1b); the morphology of this membrane clearly changed after the addition of MWCNT (Fig. 1c). The thickness of the

membranes was measured using SEM (Supplementary Fig. S3). The addition of MWCNT did not modify the thickness of the active layer and both samples were

approximately 100 nm thick. However, water contact angle measurements showed a slight increase in wettability upon addition of MWCNT to the PA membrane

(Supplementary Fig. S4). Notably, no MWCNT were visible on the surface, thus indicating that they were perfectly embedded within the PA matrix, a key factor

needed for avoiding MWCNT leakage during operation. Flow permeation rates, as indicated below and SEM images confirmed that the membranes can be produced

pinhole-free in a reproducible way. After the membrane was dried for SEM studies, cracks were generated by manual deformation of the membrane (Fig. 1d) and

MWCNT embedded, parallel along the membrane surface, were observed bridging the fracture within the polymer matrix. The apparent diameter of these nanotubes

are ca. 20 nm, which is about two times larger than the pristine nanotubes (Fig. S1a). These facts suggest that the nanotubes must be coated with polymer to

achieve a good matrix-nanotube adhesion. In order to support our proposed structure consisting of a polymeric network with aromatic moieties in parallel

arrangement to the MWCNT walls, we performed theoretical simulations of the monomer molecules orientation in the vicinity of a carbon nanotube surface, see

Supplementary Fig. S5. Here, four different cases, consisting of two geometrical configurations, are demonstrated: horizontal and vertical alignments with

respect to the MWCNT surface (modelled as a graphene surface), for both monomers (MPD and TMC). The results indicate a clear energetic preference for the

horizontal arrangements of these molecules interacting with sp2 hybridized carbon networks; these preferences are related to π-π stacking and are known to

be common for aromatic compounds on sp2 hybridized carbon surfaces. Similarly, Fig. S5b shows a simulation of 50 MPD molecules absorbed on a graphene surface

and it can be seen that the molecules adopt a similar geometrical orientation after relaxation (Fig. S5c). In order to rule out curvature effects, we carried

the simulations using a (10,10) single-walled carbon nanotube (Fig. S5d), which evidently has a higher curvature than the 10 nm diameter MWCNT experimentally

used in the membrane fabrication. It can be seen on Fig. S5e that after relaxation, the aromatic ring of the MPD molecules lies parallel to the carbon

nanotube surface. We confirmed the strong affinity of MPD with MWCNT by filtering the solution and carrying out UV-Vis spectroscopy. We found that 16.7% of

the MPD monomer remained attached to the MWCNT. These MPD functionalized MWCNT were polymerized in TMC solution. Supplementary Fig. S6a shows a homogeneous

PA coating on the MWCNT. Supplementary Fig. S6b depicts a higher resolution image showing a coating of about 5 nm thick on the MWCNT surface. We used fast

Fourier transformation (FFT) of the HRTEM images to analyze the orientation of the PA network and it is clear that PA regions that do not contain MWCNT, show

an anisotropic molecular arranged structure (Supplementary Fig. S6c), whereas the PA coating the nanotubes show a preferential orientation of PA molecules

along the MWCNT surface (Supplementary Fig. S6d). These experiments strongly support a templating effect caused by MWCNT. To assess the distribution of the

MWCNT within the membrane, a Raman mapping of the characteristic D- and G- bands of MWCNT was conducted (see Fig. 1e,f). Through all the studied areas only

the D- and G- peaks could be observed, indicating a homogenous mixture and a high content of MWCNT, which is not common in these type of nanocomposites,

because the MWCNT are prone to aggregation even when loading at low concentrations. Commercial
target="_self">NF membrane
exhibited a lower contact angle; however in this case, the presence of wetting additives or a surface treatment is likely

responsible for this phenomena. The method used to synthesize the MWCNT·PA nanocomposite relies on the transport of the MWCNT to the organic/aqueous

interface during polymerization23. Indeed, the presence of a limited amount of anionic surfactant has been recently reported to improve PA membrane

formation, resulting in better performance24. This is most likely due to a reduction of the oil/water interfacial tension, a process that in our case is also

promoted by the small amount of surfactant that provides amphiphilicity to the nanotubes It is important to emphasize that we did not used covalent

functionalization of MWCNT, in contrast to some previous reports8,11.





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