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International Journal of Civil Engineering and Technology (IJCIET)
Volume 8, Issue 12, December 2017, pp. 937–950, Article ID: IJCIET_08_12_102
Available online at http://http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=8&IType=12
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© IAEME Publication Scopus Indexed
QUALITY IMPROVEMENT OF WATER
RESOURCES BY REMOVAL OF MERCURY
AND LEAD CONTAMINANTS THROUGH
FORWARD OSMOSIS (FO) TECHNOLOGY
WITH VIBRATING MEMBRANE
Majid Meschi Nezami
Department of Energy, Institute of Science and High Technology and Environmental
Sciences, Graduate University of Advanced Technology, Kerman, Iran
Mohammad-Javad Khanjani
Department of Civil Engineering, Shahid Bahonar University of Kerman, Kerman, Iran
ABSTRACT
Nowadays, the forward osmosis (FO) has attracted a considerable attention due to
its numerous abilities in the field of seawater desalination and wastewater treatment,
fluid food processing and power production. This study is seeking to evaluate the
effects of various parameters before and after the membrane vibration on the function
of forward osmosis in mercury and lead desalination from water. In this study, a
laboratory pilot is prepared for these assessments, and thus the parameters, which
affect the removal of contaminants and membrane vibration, are evaluated. According
to the results of this study, the percentage of removed mercury and lead metals is
significantly increased, but the concentration of these elements is decreased in output
solution in the lack of membrane vibration by increasing the temperature. The more
the feed solution concentration has increasing trend, the more it shows the decreasing
trend in percentage of mercury and lead removal from the feed solution. The effective
osmotic pressure has a direct impact on the percentage of removed mercury and lead;
and the percentage of removal will be increased by enhancing the effective osmotic
pressure. The membrane vibration for 6000, 10000 and 14000 rpm of membrane
vibrating motor has a different impact on the removal of heavy metal at the first and
third stages respectively. The initial vibration always increases the percentage of
removal compared to the steady state. This process will be much faster in the second
vibration, and thus the percentage of removal will reach the highest level. The third
vibration shows a significant decrease on the percentage of removal which is different
depending on the rate of effective osmotic pressure. If the effective osmotic pressure is
high (about 18 atm), the percentage of removal will be significantly dropped and will
become less than before the vibration. If the effective osmotic pressure is measured
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within the average interval (about 8 atm), the percentage of removal will become
about more than the initial vibration or lack of vibration, and if the osmotic pressure
has the lowest value (about 4 atm), the third vibration has the percentage of removal
between the first and second vibration. According to the results, this rate will always
be lower than the second vibration. The results of this research indicate the
opportunity for adding the membrane vibration to a solution for fouling elimination
and increased percentage of as a novel method. According to the observed cases
about the osmotic pressure and percentage of removal in the third vibration, the
effective osmotic pressure can be considered as a parameter which affects the
selection of draw solution.
Keywords: Forward osmosis, quality improvement, Mercury, Lead, vibrating
membrane, removal efficiency, effective osmotic pressure
Cite this Article: Majid Meschi Nezami and Mohammad-Javad Khanjani, Quality
improvement of water resources by removal of mercury and lead contaminants
through forward osmosis (FO) technology with vibrating membrane, International
Journal of Civil Engineering and Technology, 8(12), 2017, pp. 937–950
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=12
1. INTRODUCTION
Water is one of the most abundant resources on the earth, but about 97% of available waters
are the salt water of seas and oceans. Only a small amount of fresh water is flowing in the
rivers and lakes and the rest of fresh water have permanent ice forms in the glaciers of the
North Pole. Therefore, the preparation of fresh water has become one of the most fundamental
human concerns. The desalination capacity has been significantly increased in the last decade
due to the increasing need for water and significant reduction in the cost of desalination
because of progressed membrane processes (El-Ghonemy, 2012).
Nowadays, most of the countries have investigated the effect of water resources and
desalination facilities on the growth and development of human societies, economic, cultural
and social systems. The adequate fresh water resources are the foundations and bases for
human societies in all dimensions from the daily life necessities to higher levels of economic
production. The absence of such resources provides the context for poverty and limits the
sustainable development options in different areas of the world. In fact, the weak governments
and economies around the world are often faced with serious water challenges, and are unable
to make effective use of innovative technologies or policies in the field of water production
and consumption (G.W.I, 2005; Mi and Elimelech, 2010).
In most of the applications such as the water purification and desalination, the membrane
process effectively compete with traditional technologies (VC, MED, MSF...). However, the
membrane process often has better efficiency in energy consumption and better product
quality and its implementation is easier than such technologies. Furthermore, the membrane
process can be simply increased and decreased in terms of size and can operate without any
change or reduction of goods at ambient temperature. The membrane separation is widely
used in many issues and aspects. A number of different processes should be used in order to
achieve an assumed separation. The separation objectives are generally as follows:
concentration, purification, analysis and intermediate reactions (Mulder, 1996).
In recent decades, the costs are stably and steadily declining for all desalination
technologies. In general, the heating systems with membrane systems bear higher costs. The
water production cost by thermal systems was about US$ 0.65 to 0.90 per cubic meter in 2005
(G.W.I, 2005). The use of membranes at the industrial large scales began by desalination and
Quality Improvement of Water Resources by Removal of Mercury and Lead Contaminants through
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water treatment for industry in the early 1970s. From then on, the membranes have been
widely used as by technical and commercial methods. The forward osmosis process is one of
the new technologies which come to market for desalination.
2. FORWARD OSMOSIS (FO) PROCESS
Forward osmosis is a novel membrane process in the field of desalination and has a high
potential than the conventional membrane processes (Cath et al, 2006). This process can be
used in the field of desalination as a stand-alone process or as a preparatory phase before the
reverse osmosis (RO) process. Reverse osmosis operates based on the hydraulic pressure, but
the driving force of forward osmotic process is the osmotic pressure difference between the
feed and draw solution. High osmotic pressure of draw solution will create the water flow
from the feed solution to a semi-permeable membrane. Compared with the conventional
membrane processes such as the reverse osmosis, which operates based on the hydraulic
pressure, the forward osmosis process has numerous advantages. Low fouling and easy
cleaning, low operating cost, high water recovery and applications such as the desalination
and wastewater treatment are among the advantages of forward osmosis process (Amini et al,
2013).
The osmosis is the process of water transfer through membranes with selective
permeability from an area with higher chemical potential to an area with lower chemical
potential. Water is driven by the difference in solution concentration on both sides of the
membrane which allows the water passage, but most of the soluble molecules and ions are not
allowed. The osmotic pressure (π) refers to the pressure which its application in most of the
concentrated solutions prevents the water transfer through the membrane. Forward osmosis
(FO) utilizes the osmotic pressure difference (Δπ) in the membrane over the hydraulic
pressure difference (in reverse osmosis) as the driving force for transferring water through the
membrane. FO process concentrates the feed flow and dilutes the highly concentrated flow
(draw solution).
FO uses an osmosis phenomenon to transfer water from feed solution (high chemical
potential of water) to an draw solution (low chemical potential of water) through a semi-
permeable membrane. The driving force of this process is created by osmotic pressure
difference between the feed and draw solution. Osmotic pressure difference is the factor of
water seepage (passage) through a semi-permeable barrier, a low-concentration solution (high
chemical potential of water) to a solution with high concentration (low chemical potential of
water). The inherent energy of this natural process is known as a chemical potential or in
particular the water potential due to the difference in the concentrations of two solutions. The
driving force gradient resulting from the chemical potential difference of substances on two
sides of membrane overcomes the resistance force against the transfer from the membrane.
The FO has been recently taken into account as an alternative to other membrane
processes due to numerous advantages. The RO is compared with FO process in the use of
this process in water purification and desalination because both of them utilize the semi-
permeable membranes as a barrier to the passage of salt with the same performance.
Regardless of whether both processes have similar performance, FO has advantages over RO.
First, because the FO uses the osmotic pressure difference between the feed and draw
solutions as the driving force unlike the RO hydraulic pressure, the forward osmosis has a
lower energy consumption compared to the reverse osmosis. Secondly, because the feed
solution is not under the pressure in the FO process, it is expected that the fouling membrane
is minimal, so the forward osmosis membrane has a lower tendency towards fouling
compared with the reverse osmosis. (Cornelissen, 2008; Boo et al, 2010; Mi and Elimelech,
2010; Tang et al, 2010) It is also expected that the forward osmosis membrane has a longer
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life than the current membranes of reverse osmosis, and thus it naturally reduces the costs of
chemical cleaning and membrane replacement, so that the FO benefits reduce the operating
costs in comparison with the RO in water treatment processes.
The osmotic pressure may be prevented by increasing the pressure (ΔP) in an area with
high concentration compared to the area with low concentration in order to reverse the water
movement; and this pressure should be at least equal to or greater than the osmotic pressure of
solution. The gradient or difference in osmotic pressure (Δπ) is a criterion and standard of
driving force of water transfer from solution with a low concentration through the membrane
to the solution with high concentration. Therefore, the determination of required driving force
for reverse osmosis process becomes possible by calculation of Δπ. The direction of water
flow through FO and RO is shown in Figure 1. The direction of water passage and driving
forces in all three processes were found by Lee et al in 1980. ΔP is zero in FO and much
water penetrates the other side of membrane, but water has low diffusion in RO due to the
hydraulic pressure towards the brine (Lee et al., 2010).
Figure 1 Solvent flow in FO and RO
2.1. Forward osmosis membrane
Before the development of membranes, which are specifically used for forward osmosis, the
reverse osmosis membranes were used in all studies on the forward and reverse osmosis from
the 1970s to 1980s. According to a similar result of all these studies, the water flow of FO and
PRO processes through the reverse osmosis membranes is generally low. Obviously, the basic
requirement of forward osmosis research includes the development of a new membrane with
high water flow and appropriate salt rejection. The osmosis phenomenon is seriously
dependent on separation of solutions on two sides of membrane and also the flow; and there is
only a kind of commercial flat sheet membrane of forward osmosis. This type of membrane
was first built by Company Hydration Technology (HTI) in the 1990s.
Figure 2 Forward osmosis membrane (Cornelissen et al, 2008)
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Forward osmosis membranes have higher water flow due to the reduced ICP effects as the
result of reduced thickness of support layer. Based on the most of the results, most of the
researchers have found that the forward osmosis membrane needs the review, so that the
internal concentration polarization (ICP) is decreased resulting in maximum driving force of
draw solution, and thus numerous studies have been conducted on forward osmosis
membranes in recent years; and the membrane performance has been strengthened compared
with the HTI membrane.
Forward osmosis membrane for mechanical maintenance has a Polyester network (mesh)
located between two layers of Cellulose triacetate (CTA) substances. This network is unlike
the thick support layers in the reverse osmosis membrane. With a thickness of about 50
micrometers, this network reduces the effects of internal concentration polarization (ICP)
caused by the thick support layers of reverse osmosis membranes. This membrane is
introduced as the advanced generation of forward osmosis membranes in the HTI hydration
chambers (HTI, 2013).
Construction of forward osmosis membrane with proper function is one of the research
priorities on the forward osmosis. The performance of existing commercial membranes
(cellulose triacetate asymmetric membrane made by HTI Company) is generally limited by
low permeability and salt rejection. The thin-film composite membrane with high
performance is composed of a thin polyamide layer (salt rejection layer) and a porous sub-
layer (Mi and Elimelech, 2008). In general, the membranes with high separation power should
be as the flat sheets with hollow fiber configuration. The hollow fiber membranes have higher
potential than the flat sheet membranes due to the larger surface to volume ratio as well as the
self-support ability.
Since the sheet membranes made by the HTI is the only available forward osmosis
membrane in the commercial markets, the membrane module configuration will be in the
forms of frame and plate or spiral screw. Based on the literature on the commercialization of
forward osmosis membrane with hollow fiber for application in forward osmosis process, the
corresponding membrane module should also be consistent with this type of membrane
(Wang et al, 2010; Wang et al, 2009; Shi et al, 2010).
2.1.1. Membrane fouling
The average diameter of PBI membrane pores is modified in order to reach the dimension of
0.29 mm with Molecular weight cut-off (MWCO) of 354 Dalton (DA) in order to improve
the separation of electrolytes (usually the bivalent electrolytes such as Na2SO4 and MgCl2,
MgSO4) and solvents with low molecular weight. The surface modification of this membrane
is simply achievable by wetting (immersing) the membrane made in Poly-Xylene Di-hydrate
for a certain time. Based on the obtained results and depending on the degree of surface
modification, the removal efficiency is achievable over 95% for MgCl2 (1 molar feed
solution) compared to 75% for unmodified PBI membrane. However, due to the tighter pores
in the membrane, the water flow is reduced, and a third of water flow efficiency is not
modified compared with PBI membrane. Despite the fact that the electrolytic removal
efficiency is improved, there is a need for more work to reduce ICP. This study seeks to
introduce the membrane vibration as a way to reduce fouling and increase the percentage of
heavy metals; hence, we compare the vibration and non-vibration of membrane vibrator and
different numbers of vibration and parameters which affect the decreasing and increasing
percentage of heavy metal removal from the membrane.
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2.2. Draw solution
The concentrated solution on the membrane leakage side is the driving force in the FO
process. The studies have considered different names for this solution including the draw
solution, osmotic agent, osmotic intermediate, stimuli solution, osmotic motor, sample
solution or the brine. When the draw solution is selected, the most important criterion is that
this material has higher osmotic pressure than the feed solution. In recent years, the selection
of draw solution has been on the basis of substances which are capable of creating high flow
and low diffusion in the reverse direction in the membranes. This is very important because
the inverse transmission of salt into the feed water effectively increases the operating costs of
system.
Hancock and Cath (2009) conducted a comprehensive study on the impact of operating
conditions on the feed and draw solutions of forward osmosis. This study indicates that the
polyvalent soluble substances have lower diffusion than the monovalent or unloaded soluble
substances. In a study by Achili et al. (2010) on electronic draw solutions, a protocol is
prepared for selection of suitable inorganic draw solution. 14 draw solutions are listed by
protocol, and seven suitable draw solutions are identified based on modeling and laboratory
tests. Different chemicals called the substances dissolved in draw solution are proposed and
tested. Batchelder proposed the use of sulfur dioxide solution as the draw solution in FO for
seawater desalination (Batchelder, 1965). Glew gave the idea of using a mixture of water and
gas (such as sulfur dioxide) or liquid (such as fatty alcohols) as the draw solution offered in
FO (Glew, 1965). He was also the first researcher who proposed the recovery of draw solution
combined with FO process. Frank used ammonium sulfate solution (Frank, 1972), Kravath
and Davis used glucose solution (Kravath and Davis, 1975), Stache applied the concentrated
fructose solution (Stache, 1989), and Kessler and moody used a mixture of glucose and
fructose (Kessler and moody, 1976) in the FO process for seawater desalination.
McGinnis provided the two-stage FO process by using the substance solubility at different
temperatures and also proposed KNO3 (potassium nitrate) and SO2 (sulfur dioxide) solutions
as the draw solutions in seawater desalination (McGinnis, 2002). According to the new FO
applications applied by McGinnis et al, the combination of ammonia and carbon dioxide
gases in certain proportions for heat removal of ammonium salts produces high concentrations
of draw solution. This method created the draw solution with osmotic pressure over 250 atm
for FO process and it allowed the unprecedented recovery of drinking water from
concentrated salty feed (Aaberg, 2003; Loeb, 2001).
3. RESEARCH METHOD
3.1. Forward osmosis membrane
In this research, the tests apply the forward osmosis membrane of cellulose triacetate (CTA)
made by HTI Company. This type of membrane is very unique compared with the other semi-
permeable membranes (RO membranes) and it is the best membrane in the field of forward
osmosis membrane process (Mccutcheon et al., 2006; Cath et al., 2006).
3.2. Feed and draw solution
All measured samples are sent to the laboratory for determining the percentage of removal,
and thus the final approval. Magnesium chloride (25 g/Lit), potassium chloride (15 g/Lit) and
ammonium bicarbonate (8 g/Lit) are used for draw solution. The osmotic pressure is
calculated for feed and draw solutions at temperatures of 14 and 24 degrees Celsius through
OLI software.
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HgCl2 (Mercury (II) chloride) +deionized water is used in a test for removal of mercury.
The concentration of mercury in the feed solution is evaluated in three steps (0.5, 1 and 2
mg/Lit). Pb (No3)2 (Lead (II) nitrate) + deionized water is used to test the removal of lead.
The concentration of lead in the feed solution is evaluated in three steps (1, 2 and 4 mg/Lit).
3.3. The experimental pilot characteristics of forward osmosis and the operating
conditions
The pilot should be designed and built in a way that the experimental conditions are
consistent with operating conditions in order to achieve the accurate and reliable results.
Given the numerous restrictions in terms of preparation and use of membrane and draw
materials, this research is seeking to design and examine the target pilots by the best method.
The supporting plastic meshes are used on both sides to protect and maintain the test
conditions with and without vibration. Each of the feed and draw solutions are flowed in
distinct channels on both sides of membrane. The flow inside in the channels is retained
inside the path by the height adjustment conditions, the solenoid valve and flow meters in the
form of steady flow on both sides.
The reservoirs of feed and draw flows are constant at temperatures of 14 and 24°C; and an
automatic system is applied to retain the conditions for maintenance of osmotic pressure of
draw solution; and it retains this osmotic pressure at a constant level as soon as applying the
flow and osmotic pressure drop by injection of draw solution to a necessary extent to draw
solution reservoir. Figure 3 shows a schematic image of applied pilot in forward osmosis
system with the following components:
Draw solution reservoir containing Magnesium chloride (Mgcl2), potassium chloride
(Kcl), and ammonium bicarbonate (NH4HCO3)
Draw solution reservoir containing mercury II chloride and Lead(II) nitrate+ deionized
water
The Cellulose triacetate (CTA) membrane made by HTI company
Pneumatic vibrator of membrane with revolutions of 6000, 10000 and 14000 for each
stage of vibration 1, 2, 3
PH meter of HANNA pen model
Digital thermometer of Thermo-TA: 288KTJ
A&D laboratory scale model GH202 with precision of 0.00001 g
Control panel for controlling the solenoids and membrane vibration motor
The chemicals by Merck Company of Germany are utilized for FO tests.
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Figure 3 Schematics of forward osmosis setup
3.4. Forward osmosis test procedure
All tests about forward osmosis are performed with at least two repeats verification. The
output concentration and subsequently the percentage of removal are applied for reflecting the
results of feed solution. The percentage of removal can be calculated according to the
following equation:
Magnesium chloride, potassium chloride and ammonium bicarbonate solutions are used
for mercury and lead removal test with concentrations of 25, 15 and 8 grams per liter and
concentration of mercury in the feed solution in three steps (0.5, 1 and 2 mg/Lit) and lead
concentration in the feed solution in three steps (1, 2 and 4 mg/Lit). The vibration test is done
for each revolution of vibration motor cycles with the same conditions. In some kinds of
fouling, the membrane is excluded in non-cleaned state for evaluation, and microscopic tests
and imaging.
3.5. Evaluation of flux mass transfer in forward osmosis
The membranes are tested in both directions of AL-FS (active membrane surface next to the
feed solution) and AL-DS (active membrane surface next to the draw solution). Water flux
leakage by changing the volume of draw solution is calculated as follows.
(1)
In equation (1), is the water flux leakage (in Litr/m2h); ΔV is the change of draw
solution volume (Litr); Δt is the time duration based on the hours (h); and is the
membrane cross section (m2). The diffusion of returned salt from the draw solution to the feed
solution is measured by changing the amount of salt in the feed solution.
(2)
In the equation (2), is the amount of returned salt diffusion; and are respectively
the final and initial volumes of feed solution (Litr), and finally and are the initial and
final concentrations of feed solution (mol/Litr).
4. RESEARCH RESULTS
4.1. Evaluation of draw solution effect on removal of contaminants at the lowest
and highest levels of input concentration
The type of draw solution at the lowest concentration of contaminants can be evaluated as an
effective parameter in selection of draw solutions type. For removal of mercury contaminant,
the Ammonium bicarbonate solution is put at the first rank among three vibrations before the
vibration. Magnesium chloride is put in the second rank and it has the removal percentage
higher than Potassium chloride from the pre-vibration to the second vibration. All three
solutions have the same removal percentage in the second vibration, but in the third vibration,
Potassium chloride has higher removal percentage in tests due to its declining procedure until
the third vibration. The lead contaminant removal is different from mercury. According to the
previous studies, the draw solution of Magnesium chloride has the highest efficiency before
the vibration and in the first and second vibration, but it has the severe decline in the third
vibration, and thus it is put in the lowest level of contaminant removal. Ammonium
bicarbonate is put in the second position at the stages without the vibration and during the first
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and second vibration. In the third vibration, it is put in the first rank with a more gentle slope
of reduction than the other solutions at the third stage. Potassium chloride has the lowest
ability to remove the contaminants at three stages and before the vibration. The removal of
contaminant removal with very high slope in Magnesium chloride is due to its higher osmotic
pressure than the other solutions. The lower slope for reduced ability to remove the
contaminants can be understood according to the same reason. These results are presented in
Figure 4.
Figure 4 Evaluated impact of different types of draw solutions on the removal of mercury and lead at
the lowest concentration of contaminants and temperature of 14 °C
Unlike the lowest concentration of Mercury contaminant removal in Ammonium
bicarbonate, it is put in the last rank before the vibration and among 2 vibrations, but its
declining procedure has a gentle slope like the osmotic process; hence, the third vibration
reaches the highest removal rate. However, magnesium chloride has the highest percentage of
removal before to the second vibration, and it has the higher ability of reduction due to the
higher osmotic pressure. Potassium chloride is in the middle and has higher removal
percentage than Ammonium bicarbonate in high concentrations and without vibrations and
with vibrations with a few revolutions; and it is an appropriate draw solution. The removal of
Lead contaminant is totally different from Mercury. Ammonium bicarbonate has the highest
efficiency at all stages of test and it has a very limited decline in the third vibration, so that it
has the highest ability to remove the contaminants. Magnesium chloride is put in the second
rank in the state without vibration and with first and second vibration. In the third vibration, it
is put in the last rank in terms of slope for reduction of removal compared to the other
solutions. Potassium chloride is put in the lowest rank of ability to remove the contaminants
almost at three stages and also before the vibration.
The different performance of Ammonium bicarbonate in low and high concentrations and
also different contaminants can be evaluated according to the molecular structure and mass
transfer. The impact of draw solution type on the percentage of concentration removal is
evaluated in the lowest and highest concentrations of contaminants in input feed solution of
tests at the temperature of 24°C. The results do not indicate a significant change in the type of
draw solution. The growth and decline rates for the ability to remove the Lead and Mercury
contaminants in the highest concentrations are shown in Figure 6.
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Figure 5 Graphs for study on the impact of different types of draw solution on the removal of mercury
and lead in the highest concentrations and temperature of 14°C
4.2. Assessment of solution temperatures in removal of contaminants
According to the diagrams above and since Magnesium chloride has the less unbalance in
behavior and higher stable output, it is considered as an appropriate draw solution for
temperature assessment. The tests are done for determining the temperature effects on the
ability to remove the contaminants in three different concentrations of contaminants in feed
solution and two temperatures of 14 and 24 °C in both solutions. The output diagrams are
shown in Figure 6.
According to the diagrams and output results, the removal of mercury contaminant is
clearly obvious. The significant difference indicates the higher ability to remove the
contaminants at higher temperatures. At 24°C, all three different concentrations removed the
contaminant at the level higher than the temperature of 14 °C. On the other hand, the removal
ability is reduced by increasing the concentration of contaminants in the feed solution. The
same process is seen in removal of Lead contaminant.
Figure 6 Graphs for study on the impact of temperature changes on the ability to remove mercury and
lead contaminants at temperatures of 14 and 24°C
4.3. Evaluated impact of contaminant concentrations in the feed solution
Like the temperature and type of solution, the concentration of input feed solution can be
considered as one of the main factors in determining the efficiency of method and percentage
of removal. The impact of Molarity in draw solutions on the removal ability can be assessed
according to the constant temperature at 14°C and the lowest instability in draw solution with
Quality Improvement of Water Resources by Removal of Mercury and Lead Contaminants through
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potassium chloride and magnesium chloride. In the field of removed mercury contaminant,
magnesium chloride has the highest ability to remove mercury in the concentration of 0.5
Mg/Lit, and then the removal ability is reduced to 1 Mg/Lit and 2 Mg/Lit due to the increased
concentration of mercury in the feed solution.
It should not be noted that Magnesium chloride has higher ability than potassium chloride
in removal of mercury as shown in the figure. However, the impact of changed input
contaminant concentration on the removal percentage is higher than the draw solution. The
draw solution of Potassium chloride in Mercury concentration of 0.5 Mg/Lit has higher ability
than Magnesium chloride with Mercury concentration of 1 Mg/Lit, and the same process is
seen in change of concentration. The changes in removal of lead contaminants follow the
same process. Figure 7 shows the diagrams for impact of feed solution contaminant
concentrations on the ability to remove mercury and lead.
Figure 7 Study on the impact of concentration change in feed solution on the ability to remove
mercury and lead contaminants at temperature of 14 °C
4.4. Relationship between the initial flux and concentration of draw solution
Previous studied on the forward osmosis process had led to limited achievements in water
flux in lower feed solution and also feed solution with low concentration (Ng et al., 2006).
This research performs the tests on the behavior of initial water flux in diversity of draw
solution and correspondingly the diversity of contaminant concentration in the feed solution.
The results are presented as follows. As shown, the diffusion of initial water flux is
considered as a function of osmotic pressure; and Magnesium chloride has the highest level of
initial flux due to the higher osmotic pressure in different concentrations. Accordingly,
Potassium chloride has the medium initial flux since the osmotic pressure of its draw solution
is at medium level; and Ammonium bicarbonate is put in the last rank among three solutions.
These procedures are exactly similar to what is predicted.
The impact of input contaminant concentration in the feed solution on the amount of
initial flux is another procedure in this field. According to the figures, the initial flux is
reduced by increasing the input contaminant concentration of feed solution. This reduction is
limited and very low, so that it will not lead to a significant difference in values according to
the type of draw solution. These procedures are observable in the same ratios for removal of
Lead contaminants from water. These procedures and values are shown in Figure 8.
This test is also done for the impact of contaminant concentration in input feed solution on
the initial flux at the temperature of 24°C, but it has not led to results different from 14°C.
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Figure 8 Study on the impact of contaminant concentration and the type of draw solution on the initial
flux at the temperature of 14 °C
4.5. Relationship between the osmotic pressure, type of draw solution and
concentration of contaminant in the feed solution
The solution-diffusion model (Wijmans and Baker, 1995) has made predictions about the
reduction of osmotic pressure driving force and flow flux by considering the concentration of
feed and draw solution concentrations. According to the results of our test, Figure 9 shows the
differences in amount of flux for osmotic pressures, so that the higher feed solution
concentration will lead to the lower input flow flux at the longer time interval. However, the
osmotic pressure will not be changed by increasing the contaminant concentration, and thus
the osmotic pressure, which is a key parameter in driving force of membrane, is only
dependent on the type of draw solution and it remains constant in different concentrations of
input feed solution contaminant. Magnesium chloride with osmotic pressure of 17.84 atm and
Ammonium bicarbonate with 4.33 atm are put in the highest and lowest ranks of driving
force. On this basis, the osmotic pressure is dependent on the type of draw solution and
independent of feed solution contaminant concentration. The diagrams are shown in Figure 9.
Figure 9 Assessment of feed solution contaminant concentration on the osmotic pressure in removing
the mercury and lead contaminants
5. SUMMARY AND CONCLUSION
Forward osmosis technology development is one of the ways to reduce the operating costs of
desalination process and other processes. This technology is faced with challenges for its
development in spite of numerous benefits. Therefore, a lot of studies are now conducting on
this field around the world. Given the low cost of forward osmosis process and also the
Quality Improvement of Water Resources by Removal of Mercury and Lead Contaminants through
forward Osmosis (Fo) Technology with Vibrating Membrane
http://www.iaeme.com/IJCIET/index.asp 949 editor@iaeme.com
minimum energy consumption compared with other desalination processes, the consumers of
underground salty and brackish water will be faced with bright future.
This research evaluates the impact of different parameters namely the temperature, type of
draw solution, feed solution contaminant concentration, and effective osmotic pressure on the
efficiency of mercury and lead contaminant removal. The increase in the temperature and
effective osmotic pressure directly increases the percentage of removal and efficiency of Lead
and Mercury contaminants from feed solution, but the increase in the contaminant
concentration of feed solution inversely reduces the efficiency of removal.
The membrane fouling phenomenon will reduce the membrane efficiency and removal
performance. This research tests and evaluates the efficiency of membrane vibration process
for removing fouling and increasing the efficiency of removal at three stages with revolutions
of 6000, 10000, and 14000 per minute under the same conditions for feed and draw solution
compared to the state without the vibration. According to the results, the efficiency and
removal percentage are increased in revolutions of 6000 and 10000, which represent the
vibration of first and second stages; and the decline of efficiency and increase in percentage of
contaminant in output solution are seen at the third stage with revolution of 14000, and this
reduction is different according to different amounts of osmotic pressure in draw solution, and
it can be analyzed and evaluated as an effective parameter in selecting the optimal draw
solution and feed solution concentration.
According to the challenges for water in our country, there is a need for development of
more comprehensive studies on the conservation of water resources and new and competing
technologies of desalination such as the forward osmosis.
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