This study deals with the preparation and characterization of
Nanofiltration cellulose acetate-based membranes and their application in
desalination of brackish waters. Cellulose acetate nanofiltration (NF) membrane
was prepared according to the phase inversion process. Pore size of the
prepared membrane was monitored by using dope solutions of different polymer
concentrations (wt-%) and annealing temperatures from 60-90°C. followed by
casting the membrane on Polytetraflouroethylene sheets (PTFE) and examining the
efficiency of the product .Characterization of the prepared membrane will be
carried out using scanning electron microscopy (SEM), and the hydraulic
permeability, water permeation as well as salt rejection percentage. Results
indicated that CA membranes reinforced with (PTFE) gave better efficiency than
normal CA membranes.

Keywards: desalination, nanofiltration, cellulose derivatives, phase inversion
process, membrane.

 

1. Introduction

            A
Nanofiltration membrane is a type of pressure driven membrane that could
perform a preferential separation for different fluids or ions. The pores of a
nanofiltration membrane are typically much larger than the membrane pores that
are used in reverse osmosis so it requires less energy to perform the
separation. Higher flux and lower operating pressure of nanofiltration makes
the membrane feasible to be applied in both water and wastewater treatment
processes.

            Needs
of drinking water with appropriate qualities are in continued expansion around
the world. At the same time, the available drinking water quantities are going
down.  Desalination of water using
reverse osmosis (RO) and nanofiltration (NF) membranes is nowadays an
appropriate choice for brackish and seawater treatment. Thus, the development
of materials assuring high fluxes and salt rejection and functioning at lower
pressures is of great interest. The first membrane structure has been obtained
by phase inversion process from Cellulose Acetate (CA) and was described by
Loeb and Sourirajan 1. This material remains an interesting polymer
with regard to its low price and to its chlorine resistance. Moreover, CA is an
environmental product as it is obtained from sustainable resources. The
elaboration of membrane material having RO performances has been reported 2-
5.

            In
the last decade, an intermediate technique nanofiltraion, using more open pores
and functioning with lower pressures, gave rise to a selective retention
between divalent to monovalent ions 6-9.

            By
comparison to other forms of water desalination, seawater desalination is by
far the most complicated and complex process. It has the lowest water recovery
ratio (30-35%). Operation is restricted to certain operation conditions.
Moreover, it tends to require extensive pretreatment, especially if the feed is
taken from an open seawater intake. The process is an energy intensive process,
and for all the above factors seawater desalination is the most expensive
desalination process. The major cause for the high expense and process
complexity is the seawater itself which is characterized by having: (1) higher
hardness degree, (2) varying degrees of turbidity, and (3) high TDS at pH 8.2.
These properties give rise to three major problems in seawater desalination
which exert severe limitation and have pronounced effects on the performance
and productivity of seawater desalination plants 10.             Nowadays, nanofiltration has been widely used in drinking
water industry 12 and wastewater treatment for removal of contaminants such
as pesticides 13, arsenic 14, soil leachate 15 and dyes 16. Recently, nanofiltration
membranes were also studied for the application in food 17 and bio-process
purposes. Interfacial polymerization (IP) technique is one of the techniques
used in preparing composite nanofiltration membranes. The membranes produced have high water permeation flux and
salt rejection. There are several advantages in making a membrane using the IP
process 17.

The
cellulose acetate (CA) membrane was the first high performance asymmetric
membrane. It has been widely used for reverse osmosis (RO), microfiltration
(MF) and gas separation. CA membranes have excellent hydrophilicity that is
very important in minimizing fouling, good resistance to chlorine and solvent.
A regenerated CA membrane that was hydrolyzed from cellulose acetate has
significantly improved solvent-resistance and thermal stability.

Commercial
CA membranes are either flat sheet or spiral-wound modules. The hollow fiber
configuration has become a favorite choice because the hollow fiber membrane
has three major advantages:

a- Hollow fiber modules have much larger
ratio of membrane area to unit volume (several thousand), and hence higher
productivity per unit volume of membrane module;

b-They are self-supporting which can be
back-washed to recover the flux; and

      c-They have good flexibility in the mode
of operation.

The aim of the present work is to
produce an advanced CA membrane using adjacent PTFE and utilizing the new
product in desalinating brackish water.

 3.
Results and discussion

3.1
Characteristics of feeding water

The characteristics of feeding water listed in Table
(1), The ratio of cations and anions was not balanced exactly. This may be
attributed to some of minor ions were not analyzed. Table (1) shows the
hardness of the influent water (the sum of Mg2+ and Ca2+
ion concentrations) was 340 ppm. Also, the sum of the concentration of chloride
and sodium ions was about 950 ppm. Since the main activity of the area of
study, the intensive use of chemical fertilizers increases the concentration of
nitrate ions (170 ppm).3.2. Characterization of the CA membranes (CA-x-y) The most important
factor affecting the final structure of the asymmetric membranes are polymer
concentration of the dope solution and the heat post-treatment temperature of
films. It is well known that the higher is the
polymer concentration, the more compact is the top layer and the smaller is the
pore size distribution 12,13. On the other
hand, by increasing the annealing temperature an increase of the ratio of
crystalline domains in the polymer structure will take place which yields the
skin layer denser 14. Both these parameters
allow designing the membrane material by acting on the thickness of the dense
skin layer and on the pore size distribution. Asymmetric
CA membranes were prepared by the immersion method from 20 and 22 wt. % polymer
concentrations. These concentration values were chosen because it had been
found in a certain study elsewhere that they can give better results for
desalination of synthetic NaCl solutions using PTFE-CA composite membranes
11. The annealing temperature was applied
between 60 and 90°C. The dry films thickness
were determined by SEM about 60-80 µm depending on the starting polymer concentration
in the dope solution. The cross sectional
micrographs (Fig. 1-3) show the asymmetric structure of the obtained materials
consisting of a top skin layer supported by an open sponge like structure.By changing of the two above-mentioned
parameters involved strong variations of the permeability coefficient of the
materials. As expected the permeability increased with decreasing
of the polymer concentration and/or of the annealing temperature revealing the
loosening of the structure formed and the shifting of the pore size
distribution from RO to NF materials. At the same time, the mechanical strength
decreased so that the maximum operating pressure for CA-20-60 was 0.8 Mb. whereas
the other samples were tested up to 1.5 Mb. In order to find a compromise
between high flux and mechanical strength, we limited our study to materials
annealed between 70 and 80°C.

 3.3. Characterization of the PTFE-CA membranes (PCA-x-y)

 The composite PTFE-CA membranes were prepared by
coating a very thin layer of PTFE onto the surface of CA supports as displayed
by the SEM micrograph in Fig. lb-c for CA-22-80 and PCA-22-80. The modification
leads to more regular and smoother surfaces. However, the SEM analysis seems
also to indicate a partial pore obstruction of the top layer morphology that
could be originated in a more or less penetration of PTFE within the CA support
structure.

 The comparison between water contact angle values
obtained for unmodified and composite membranes shown in Table 3 evidences the
surface modification by the PTFE coating. The higher values found for the PCA
samples agree with the hydrophobic nature of PTFE. Besides the results refers
to a lower contact angle for materials prepared from higher polymer
concentrations. This trend is probably related to the denser structure of the
top layer surface. On the other hand no obvious variation was observed with the
annealing temperature. This indicates that this parameter acted more on the
film morphology than on the surface structure. 3. 4. Desalination performancesThe comparison between the CA supports (CA-x-y) and
the composite membrane (PCA-x-y) for the desalination of the chosen brackish
water is displayed in Table 4. The permeation rate when going from pure to
brackish water declined of 10-20% for the CA supports and composite membranes,
respectively. It should be noted that the decrease is even much higher for the
membranes having the tighter pore diameter (CA-22-80 and PCA-22-80).

 The retention
data were expressed as the TDS % rejection. As expected, the preparation
conditions strongly influenced the desalting Performances. Both increasing the
polymer concentration and annealing temperature enhanced the salt retention
value from 66 to 90% for CA membranes. The surface modification with PTFE
raised the retention (PCA
membranes). The higher increase was observed for the membranes prepared at 70°C
whereas almost no change was seen for materials prepared at 80°C. It was
assumed that the selectivity resulted from the CA support in the case of the
tighter pore structure.

 

4. Conclusion

CA-membranes
reinforced with PTFE sheet were utilized as brackish water desalination
membranes, different parameters were used to obtain the higher performance
membrane such as dope solution concentration as well as the annealing
temperature. The results indicated that the higher performance was obtained
with composite membrane prepared with 20% and 90°C.  The study assured that using reinforced PTFE
increased the composed membrane performance 

 

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