Accepted Manuscript
Title: Why Does pH Increase with CaCl2 as Draw Solution
during Forward Osmosis Filtration
Author: Li Shu Idowu Joshua Obagbemi Susanthi
Liyanaarachchi Dimuth Navaratna Rajarathinam
Parthasarathy Roger Ben Aim Veeriah Jegatheesan
PII:
DOI:
Reference:
S0957-5820(16)30093-3
http://dx.doi.org/doi:10.1016/j.psep.2016.06.007
PSEP 795
To appear in:
Process Safety and Environment Protection
Received date:
Revised date:
Accepted date:
4-1-2016
27-5-2016
6-6-2016
Please cite this article as: Shu, L., Obagbemi, I.J., Liyanaarachchi, S., Navaratna, D.,
Parthasarathy, R., Aim, R.B., Jegatheesan, V.,Why Does pH Increase with CaCl2 as
Draw Solution during Forward Osmosis Filtration, Process Safety and Environment
Protection (2016), http://dx.doi.org/10.1016/j.psep.2016.06.007
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The Highlight of the study is as following:
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pH of NaCl solution increases with concentration
pH of CaCl2 solution decreases with concentration
FO Process increases the pH of both NaCl feed solution and CaCl2 draw solution
Charges on salts also affect the pH of the solution
Page 1 of 24
Why Does pH Increase with CaCl2 as Draw Solution
during Forward Osmosis Filtration
Li Shua*, Idowu Joshua Obagbemib, Susanthi Liyanaarachchia, Dimuth Navaratnac,
School of Engineering, RMIT University, 402 Swanston Street, Melbourne VIC 3000
Australia (li.shu@rmit.edu.au, s3535761@student.rmit.edu.au,
cr
a
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Rajarathinam Parthasarathya, Roger Ben Aimd, Veeriah Jegatheesana
b
us
rajarathinam.parthasarathy@rmit.edu.au, jega.jegatheesan@rmit.edu.au)
School of Engineering, Deakin University, Waurn Pond VIC 3216 Australia
College of Engineering and Science, Victoria University, Melbourne VIC 8001 Australia
M
c
(dimuth.navaratna@vu.edu.au)
Institute of Filtration and Technique of Separation (IFTS), Agen, France
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(roger.ben.aim@ifts-sls.com)
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d
an
(obagbemi_121212@yahoo.com)
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* Corresponding Author: li.shu@rmit.edu.au; li.shu846@gmail.com
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Abstract
Fundamental understanding of pH dynamics in Forward Osmosis (FO) processes is integral
for further development of this emerging technology in desalination and wastewater
reclamation. In this study, the pH changes during FO membrane filtration were investigated.
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CaCl2.2H2O was used as the draw stream and NaCl as the feed stream. During the FO process,
water would flow from the feed to draw so that the feed stream would become concentrated
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while the draw stream being diluted. It was found that the pH of the feed stream increased
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with time. Assuming that the pH or the concentration of hydrogen ions to be constant in the
FO system, the pH of the draw stream should decrease. Surprisingly, the pH of the draw
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stream did not increase. To explain this unexpected phenomenon, standard curves for both
CaCl2.2H2O and NaCl were constructed. In contrast to conventional belief, the standard
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curves showed that pH of NaCl increased with the increase in concentration while pH of
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CaCl2.2H2O decreased with the increase in concentration. As such, the standard curves
explain the reason for pH increase in both feed and draw solutions. Furthermore, the curves
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showed that the pH to be dependent on salt concentration, where charges on ions have the
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ability to influence the pH of the solution.
Keywords:
Brine, Calcium Chloride, Forward Osmosis, High Salt Concentration, Saline Solution,
Sodium Chloride
Page 3 of 24
Introduction
By definition, pH is an indicator of the active concentration of hydrogen ions in a aqueous
solutions (denoted by [H+]) and expressed as pH = -log10[H+]. Almost all aquatic processes
are influenced by pH. Coagulation and flocculation to remove colloidal particles in water
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treatment, bacterial growth and their uptake of C, N and P in wastewater treatment, corrosion
in pipes which deliver seawater to a reverse osmosis (RO) plant are all affected by the pH
Hardness
Removal
Biogas
Production
Adsorption/
Desorption
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pH
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Scaling
Corrosion
Water
Reclamation
Membrane
Filtration
Sewer Gas
Generation
pH is an important parameter in Water and Wastewater Engineering
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Figure 1
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Metal
Recovery
Coagulation/
Flocculation
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(see Figure 1).
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It is often necessary to measure pH in saline solutions (Nir et al 2014, Wang et al 2014, Reddi
2013, Sereshti and Aliakbarzadeh 2013, Marcus 1989, Covington 1988) and for example, RO
concentrate in desalination plants and draw solutions in FO processes have very high
concentrations of salts. Although pH is a very important parameter, its investigation is limited
especially in studies related to FO processes.
Forward Osmosis (FO), an emerging separation technology, has attracted increasing attention
of both scientific research community and the process industry in the past decade (Akther et
al 2015, Altaee and Sharif 2015, Blandin et al 2015, Deshmukh et al 2015, Holloway et al
Page 4 of 24
2015, Kim et al 2015, Liyanaarachchi et al 2015a, 2015b, Qasim et al 2015, Linares et al
2014, Sagiv et al, 2014, Wang et al 2014, Xie et al, et al 2014, Boo et al 2013, Lay et al 2013,
Zhao et al 2012, Lutchmiah et al 2011, Phuntsho et al 2011, Achilli et al 2010, Philip et al
2010). FO process applies the natural osmosis phenomenon to drive water from a diluted
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stream to a concentrated stream when those streams are separated by a semi-permeable
membrane. FO has low water flux compared to pressure driven membrane filtration processes,
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such as reverse osmosis (RO). While FO could not compete with RO in terms of water
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production efficiency, it can be used to process feed waters that cannot economically be
treated by RO (Shaffer et al 2015). FO has a future which lies in sustainability, resource
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recovery and new applications of the technology and therefore research into FO is crucial. In
other words, its operation should focus on closing the loop by recovering resources from both
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draw and feed streams.
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The water flux through the FO membrane will depend on the effective osmotic pressure
difference between the surfaces of the active layer of the membrane. At the same time, the
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flux of solutes will be governed by their concentration difference at the surfaces of the active
layer. If a solute has higher concentration in the draw stream, it will be transported to the feed
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stream and vice-versa. This will be applicable to the transport of H+ as well. Furthermore, H+
should satisfy the equilibrium with OH- and H2O as well as other solutes such as CO32-,
HCO3-, HS-, H2S etc. For example, as temperature increases, dissolved CO2 in a solution will
decrease so that pH will increase and the solution will become less acidic. CO2 dissolution
also will be affected by the amount of salt present in the solution. Therefore, it is essential to
understand the changes that will occur to pH in an FO process. The following review of the
literature emphasise the importance of pH in the FO process: It is often necessary to measure
pH in saline solutions (Nir et al 2014, Wang et al 2014, Reddi 2013, Marcus 1989, Covington
1988). For example, that hydrogen and bicarbonate ions formed due to the dissolution of
Page 5 of 24
atmospheric carbon dioxide in water or saline solution exist in equal concentrations and can
be computed using the equation below (Reddi 2013):
(1)
be 5.65. The activity of H+ is reduced in the presence of saline.
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Thus the pH of water or saline solution (0.9%) exposed to atmospheric carbon dioxide would
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Nir et al. (2014) attributed Donnan effect for the increase of pH and alkalinity in the permeate
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and thus adding acidity to the brine during the reverse osmosis process. Higher rate of
diffusion of sodium cations through the membrane compared to chloride anions, inducing a
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small potential difference which drives hydroxide ions from the brine to the permeate side to
maintain electro neutrality. The importance of the interference of various ions in brine
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solutions with pH measurements had also been emphasised by Covington and Whitefield
such solutions.
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(1988) and Marcus (1989); they had recommended special pH measurement techniques for
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The pH of a solution plays an important role in osmotic pressure and therefore influences the
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effectiveness of a draw solution as flux enhancer. For example, Long and Wang (2015) found
that that osmotic pressure of sodium tetraethylenepentamine heptaacetate (STPH) to increase
from 21 to 26 bars when the pH was increased from 7 to 8. A total of 33.3% increase in
osmotic pressure was observed when the pH was increased from 7 to 10. The increase in
osmotic pressure is due to the generation of more carboxylate ions from carboxyl groups in
STPH with the increase in the pH. This in turn increased the flux from 9.31 to 11.22 L/m2.h
(when the pH was increased from 7 to 10) when 0.1 g/mL of STPH is used as draw solution.
Interestingly, reverse salt leakage was maintained at same level when pH increased from 7 to
9 as multiple negative groups in STPH which bond certain positive ions (Na+) to maintain a
Page 6 of 24
charge balance and prevented the movement of salts from the draw solution to the feed
solution.
Another study by Kim et al. (2012) found that boron solute flux from the feed to draw
solution decreased with the increase in pH. Boron will be mostly in the form of boric acid at
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pH less than pKa (= 9.24) in the reaction shown in equation (2) and will be in borate form
when pH is more than pKa. The larger hydrated radius of borate compared to that of boric
B(OH)4 - + H+
H2O
(2)
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B(OH)3 +
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acid results in higher rejection by the membrane.
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A study by Xie et al (2012) found several interesting factors:
(1) The contact angle of active and support layers of an HTI asymmetric FO membrane
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decreased when the pH was increased (the pH was increased from 3.5 to 7.5 and the
contact angle decreased: from 70.9 ± 3.1 to 60.2 ± 3.4 for active layer; from 79.7 ±
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1.0 to 69.7 ± 3.4 for support layer)
(2) Zeta potential of active and support layer also decreased with the increase in pH
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(3) Permeate flux increased when the pH was increased (for 1 M NaCl draw solution).
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The reason for above observations was attributed to the following: as the solution pH
increases the electrostatic repulsion between ionisable functional groups of the polymeric
matrix of the membrane increases. This causes the zeta potential to decrease and average pore
size of the membrane to increase which in turn increase the permeate flux. Further, their
study revealed that the rejection of sulfamethoxazole, a trace organic compound, present in
the feed solution was very high at higher pH and low at lower pH. This was correlated to
decreasing H+ flux from the feed to the draw solution when pH is decreased.
Wang et. al’s (2014) forward osmosis experiment results confirmed Nir et al’s (2014)
observations. In their study, Wang et al. (2014) used rain water (pH = 5.5, TDS = 4.5 mg/L
Page 7 of 24
and temperature = 15oC) as feed solution and cooling water (pH = 12.4, TDS = 2540 mg/L
and temperature = 40oC) as draw solution. They found that the pH of the feed solution to
increase from 5.6 to 11.5 after 80 minutes of operation. This suggests the movement of
hydroxide ions from the draw solution to the feed solution as stated by Nir et al. (2014).
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Thus, understanding the temporal variation in pH due to the influence of draw solutions
during FO operation is essential to predict the permeate flux, feed solute rejection and reverse
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salt leakage. This study used different feed and draw solutions to observe the changes in an
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FO process to understand the transport of H+.
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Materials and Methods
NaCl and CaCl2.H2O used in the experiments were analytical grade. Milli Q Water with a
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resistance of 18 MΩ/cm was used for constructing standard curves and conducting FO
experiments. Solution volume for the standard curve measurement was 50 mL. Solutions for
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constructing standard curves were stirred gently while both draw and feed solutions for FO
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experiments were mixed thoroughly.
The pH meter with an error ± 0.1 was calibrated daily with buffer solutions of pH 4, 7 and 10;
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pH buffer solutions were kept at room temperature before calibration so that they had the
same temperature as the solutions to be analyzed. The pH probe was kept in gel (that was
provided by the pH meter supplier) when not in use. The probe was rinsed at least three times
with Milli Q water before measurements were taken. Kimwipes (KIMTECH) tissue paper
was used to wipe pH meter probe (this is the only time that the pH probe was dry and this
period was kept to a minimum). A pH reading was taken only when it was stable for 1 minute
(sometimes it took more than 20 minutes to obtain one pH reading).
Page 8 of 24
The commercially available membrane was purchased from Hydration Technology Inc, USA.
It was composed of cellulose triacetate embedded above a polyester screen mesh. The
membranes were transported as 0.25 m × 1.00 m and were cut to size to fit into the
membrane cell.
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The membrane cell consisted of a custom made cell composed of two machined sheets of 10
mm thick Perspex which were held together using stainless steel screws and wing nuts. A 30
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mm x 250 mm channel was removed along the centre of each piece of Perspex. The cell was
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mounted onto a stainless steel stand.
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A piece of membrane with an effective area of 75 cm2 was cut to fit into the membrane cell
and was soaked in deionized water for 30 minutes. The apparatus was then assembled. The
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FO membrane filtration is operated in PRO mode where the activate layer of the membrane
was facing the draw solution side. The initial volumes of both the draw and the feed solutions
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were kept at 1 L. The pH, temperature and the conductivity were measured and recorded.
Following this, the pumps for the feed solution and the draw solution were turned on
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respectively and all air was purged from the system.
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The experimental set-up is shown in Figure 2. Two circulation pumps were used for better
mixing during experiments. The feed and draw solutions were continuously circulated
between the storage tanks and the membrane cell at a cross flow velocity of 0.42 m/s in
counter current flow configuration. More details could be found in Shu et al (2015) and
Neilly (2014). Laboratory used to conduct these experiments is a standard water laboratory.
Page 9 of 24
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Experimental set up
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Figure 2
Results and Discussion
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In this study, a lab scale Forward Osmosis unit was employed as shown in Figure 2. A 1.2 M
of NaCl solution was chosen as feed solution as NaCl was the main component of seawater
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and concentration of 1.2 M was similar to that of a RO reject in desalination plants. We
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choose CaCl2 as draw solution since it has higher osmotic pressure than most chemicals at the
same concentration (Cath et al 2006). The concentrations of CaCl2.2H2O chosen were 2.0, 2.5,
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3.0 and 3.5 M.
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The experiments were conducted for three hours at room temperature. The pH was measured
at the beginning of the experiment and after every hour subsequently. Figure 3a) shows the
results. At the beginning of the experiment, the feed solution contained 1.2 M of NaCl and
had a pH of 6.35. Similarly, the draw solution contained 2 M of CaCl2.2H2O and had a pH of
5.21. As time proceeded, more water was drawn from the feed to the draw solution so that the
feed solution was concentrated while the draw solution was diluted. From Figure 3a), it can
be observed that pH of feed solution increased slightly (from 6.35 to 6.88). This could be
attributed to the decrease in the dissolution of CO2 due to the increase in the salt
concentration (Zhao et al 2015, Rumpf et al 1984) which would then result in an overall
increase in the pH of the feed solution. Further, the pH of the draw solution was also
Page 10 of 24
increased from the initial value of 5.21 to 6.89 at the end of the experiment, resulting in an
overall increase of 1.7 in the pH. This outcome is surprising as the pH is determined by
hydrogen ion concentration and one would have assumed that the concentration of hydrogen
ions in a system (here the forward osmosis filtration system) should be constant unless either
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acid or base is added to it. Therefore, it was expected that the increase in pH in one stream
(NaCl feed solution) means that the hydrogen ions must have passed through the membrane
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and transported to the other stream (CaCl2. 2H2O draw solution) causing a drop in the pH of
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that stream. As can be seen in Figure 3a) the pH of both the draw and the feed solutions
increased during the forward osmosis process. Moreover, the results in Figure 3a) showed
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that the pH of both the draw and the feed solution tend to become the same over time. When
the pH is different in the feed and the draw streams, the concentration gradient of hydrogen
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ions that would exist in the system will make hydrogen ions to cross the membrane making
the concentration becomes equal on both sides. To confirm the above experimental
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observation more experiments with different concentrations of draw solution were conducted
and the results are shown in Figure 3b). Figure 3b) confirms that the pH of draw solutions
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increased after FO membrane filtration. At CaCl2.2H2O concentrations of 2.0, 2.5, 3.0 and
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3.5 M, all the final pH values of draw solutions were higher than the initial pH values.
Another point to note is that the pH of CaCl2.2H2O (used as draw solution) decreased when
the concentration was increased (see the initial pH values shown in Figure 3b) and since
CaCl2.2H2O is a strong electrolyte and its solution should be neutral). In addition to this, as
the concentration of salt increases in the solution, the dissolution of CO2 should decrease and
the pH of the solution should increase. However, the results were opposite to the expectation.
The membrane had a slight negative charge in the pH range used in this study; the zeta
potential was around - 4 mV (Xie, 2014). Therefore, the adsorption of cations can be assumed
Page 11 of 24
to be small and therefore could not have contributed to the reduction in the pH. However,
further studies should be carried out to quantify this effect.
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a)
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b)
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8
7
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Final
6
5
4
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pH
Initial
2
2.5
3
3.5
4
CaCl2.2H2O, mol/L
Figure 3
The pH dynamics during Forward Osmosis experiment
a) Temporal variation of pH in the feed and the draw streams
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b) Initial and final pH of CaCl2.2H2O draw solution for different initial concentrations
Experimental conditions: Feed stream: NaCl (1.2 M); Draw stream: CaCl2.2H2O (2M) or
otherwise as shown; Experimental duration - 3 hours; flow rate of the circulation pump - 1.5
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L/min; experiments were conducted at room temperature
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The above results show that CaCl2.2H2O is not a neutral salt and the dissolution of CO2 has
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only a minor effect on the pH of CaCl2.2H2O solutions. The pH difference between the final
and the initial values of CaCl2.2H2O solutions is quite large, with a pH difference of 1.7 at 2
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M of CaCl2.2H2O; 2.1 at 2.5 M; 2.3 at 3.0 M and 3.4 at 3.5 M, showing a clear trend. Initial
pH values of CaCl2.2H2O were less than 7 so that the salt is acidic. The pH of the
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CaCl2.2H2O draw solutions increased after FO membrane filtration. This is similar to the
trend observed by Nir (2014) in the RO process where the higher rate of diffusion of Na+
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through the membrane compared to Cl- drove OH- from the brine to the permeate side to
maintain electro neutrality. In this particular case, similar action could have occurred from
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the feed to the draw side to increase the pH of the draw solution. On the other hand, there is
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no additional alkalinity in the draw solution to provide electro neutrality in the feed stream
due to the movement of Ca2+. Thus, it is possible that the electro neutrality on the feed side is
maintained through the movement of H+ instead of Ca2+ from the draw to the feed (Figure 4).
Thus significant increase in pH on the draw side can be expected.
Page 13 of 24
Figure 4. Opposite directions of water flux and H+ flux during Forward Osmosis
In looking for an answer to the unexpected phenomenon of pH increase in both feed and draw
streams, two standard pH curves of NaCl and CaCl2.2H2O were constructed as shown in
Figure 5a). The pH of NaCl solution at higher salt concentration was not neutral most of the
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time and it increased as the concentration was increased, which matched the results reported
by Shu et al (2013). From the standard curve of NaCl, it can be seen that the pH of pure water
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(without NaCl) is less than 6, which is similar to that of rainwater. We may conclude that the
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pure water exposed to the air at room temperature was acidic. When NaCl was added to the
pure water and the concentration was increased from 1 M to 5 M, the pH of this solution
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increase from 6.4 to 8.4. At around 1.8 M NaCl, the pH of the solution was 7.0. Thus, the
NaCl solution was acidic at concentrations below 1.8 M and is basic at concentrations above
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1.8 M. At lower concentrations of NaCl, the pH of the solution was influenced by the pH of
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the water and at higher concentrations, the pH of the solution was determined by the pH of
the salt. This implies that NaCl is a basic salt and increases the pH of the solution at higher
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concentrations. Thus, during the FO experiments, when the NaCl concentration increased in
the feed stream, the pH of the feed stream tends to increase; however, the dissolution of CO2
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counteracted this increase which resulted in minimal increase in the pH.
Page 14 of 24
a)
8
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9
y = 0.5325x + 5.9792
R² = 0.9681
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7
pH
CaCl2.2H2O
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NaCl
6
y = -0.5275x + 7.2358
R² = 0.9693
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5
1
M
4
2
3
4
5
ed
Concentration, mol/L
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b)
14
13
pH
12
y = 0.1156x + 11.713
R² = 0.9481
11
10
9
1
2
3
4
5
Na2CO3, mol/L
Page 15 of 24
Figure 5
Salt pH at high concentrations (the standard deviation of pH measurements
were less than 0.1) a) NaCl and CaCl2. 2H2O b) Na2CO3
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As can be seen from Figure 5 a), the pH of CaCl2.2H2O decreased as the concentration
increased. The pH decrease from 6.6 to 4.5 when the concentration was increased from 1M to
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4.5 M. Thus, when CaCl2.2H2O solution was diluted during the FO process, the pH increased.
Another possible reason for the increase in pH after the FO experiments is that calcium could
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react with CO2 that is dissolved in the solution and form CaCO3. This will affect the
Ca2+ + CO32- → CaCO3(s)↓
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carbonate alkalinity,
(3)
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CaCO3 is basic and is often used to neutralize acidified soil (Haling et al 2010, Baligar 1995).
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The possible existence of CaCO3 in the draw solution would increase the final pH. The
proportion of CaCO3 which contributes to the increase in pH requires further study.
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Consideration of ionic interactions also can provide further clarifications to the above
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observations. NaCl and CaCl2.2H2O have the same anion Cl-, and the pH difference was
caused by cations, either Na+ or Ca2+. Ca2+ has 2 positive charges while Na+ has one.
CaCl2.2H2O had a much lower pH than NaCl showing that pH was affected by charges on the
ions. Positive charges on ions tended to lower the pH of a solution. It was reasoned that the
negative charges on ions could increase the pH of a solution. Therefore, the pH of Na2CO3
was measured at different concentrations (Figure 5b)). The pH of Na2CO3 increased from
11.4 to 13.3 when the concentration was increased from 1M to 5M. The results confirmed the
assumption. NaCl and Na2CO3 have same cation Na+ and different anions; CO32- has 2
negative charges and Cl- has one. Thus, the pH of Na2CO3 with more negative charges had
Page 16 of 24
much higher values than that of NaCl. The reason for ions affecting pH might be that
negative charges have the ability to neutralize the positive charge on the hydrogen ion so that
the amount of hydrogen ions were reduced to cause an increase in pH. The higher the
concentration of negative charges the higher the augmentation of pH. Conversely, positive
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charges would react with OH- in a solution leading to a decrease in pH.
The above results suggest that pH was affected by the charge number and charge properties
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of a salt. Na2CO3 was a basic salt having the highest pH. CaCl2 was an acidic salt with the
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lowest pH and NaCl has a pH in between. The pH of the 3 salts has the following sequence:
(4)
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pHNa2CO3>pHNaCl>pHCaCl2
Table 1 shows the radius of both Cl- and Na+. Radium of Cl- (r-) is equal to 181 pm (1 pm =
M
10-12 m) and of Na+ (r+) is 102 pm (thus the ratio, r-/r+ = 1.78). The radium of Cl- is almost
double that of Na+. On the surface of NaCl the negative charges might be dominant so that
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on this area.
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NaCl showed more negatively charged and appeared to be basic. Further research is needed
Table 1 Ionic radius of some sodium salts
Symbol
Ac
ce
Name
Na+
Sodium
Ca2+
Calcium
ClChloride
CO32Carbonate
Data source: Burrows et al 2013
Cationic radius
pm
102
100
-
Anionic radius
pm
181
189
Comparing strong acid HCl and strong base NaOH, HCl was much stronger acid than CaCl2
although Ca++ has two positively charged ions and H+ has only one. Similarly, comparing
Na2CO3 with NaOH, NaOH was a much stronger base than Na2CO3 although CO3= has two
Page 17 of 24
negative charges while OH- has one. The application of this finding could be adjusting the pH
with different salts and changing the pH by either removing Ca2+ or CO32- from a solution. It
is possible that every inorganic salt as well as organic matters including natural organic
matters (NOM) have their unique pH. If they are used as draw solution pH would vary
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differently during forward osmosis membrane filtration. This is a new area with research gaps
which needs to be explored.
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The factors affecting pH of salts are: temperature; agitation/mixing/stirring speed; stirring
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duration and biological growth (American Public Health Association, American Water
Works Association, Water Environmental Federation 2015). From this study, two more
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factors could be added to the list: type of salt (charges on the salts) and the concentration of
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salt.
Conclusions
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Experiments were conducted using a lab scale FO membrane filtration unit. 1.2 M of NaCl
was used as feed stream and different concentrations of CaCl2.H2O were used as draw
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solutions. The pH dynamic was complicated in the FO system. While water flux was from
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feed to draw, [H+] in this study was transported in the opposite direction. [H+] in both sides
showed the tendency to be equalized. After the experiments, the pH values of both feed and
draw solutions increased since the pH of both salts are concentration dependent. At the end of
FO experiments, NaCl in the feed solution was concentrated and therefore the pH increased;
CaCl2.2H2O in the draw solution was diluted leading to pH augmentation. Charge types were
also found to affect solution pH. Cations were observed to lower the pH while anions
increased the pH. Ample knowledge gaps exit in this research area of pH of salts and other
materials and its relationship to surface charges and other properties, such as particle size
distribution and corrosiveness.
Page 18 of 24
Acknowledgement
The project was partially conducted at Deakin University, Australia. Authors thank Gerianne
Robles for proof reading the manuscript.
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References
for forward osmosis applications. J. Membr. Sci. 364, 233-241.
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Achilli, A., Cath, T. Y., Childress, A. E., 2010. Selection of inorganic-based draw solutions
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Akther, N., Sodiq, A., Giwa, A., Daer, S., Arafat, H. A., Hasan, S.W., 2015. Recent
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advancements in forward osmosis desalination: A review. Chem. Eng. J. 281, 502-522.
American Public Health Association, American Water Works Association, Water
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Environmental Federation, 2005. Standard Methods for the Exanimation of Water &
Wastewater, 21st ed., Port City Press, Baltimore Maryland.
ed
Altaee, A., Sharif, A., 2015. Pressure retarded osmosis: advancement in the process
pt
applications for power generation and desalination. Desalination. 356, 31-46.
Baligar, V., He, Z. L., Martens, D. C., Ritchey, K. D., Kemper, W. D., 1995. Effect of
Ac
ce
phosphate rock, coal combustion by-product, lime, and cellulose on ryegrass in an acidic soil.
Plant Soil. 195, 129-136.
Blandin, G., Verliefde, A. R. D., Tang, C.Y., Le-Clech, P., 2015. Opportunities to reach
economic sustainability in forward osmosis-reverse osmosis hybrids for seawater desalination.
Desalination. 363, 26-36.
Boo, C., Elimelech, M., Hong, S., 2013. Fouling control in forward osmosis process
integrating seawater desalination and water reclamation. J. of Membr. Sci. 444, 148-156.
Page 19 of 24
Burrows, A., Holman, J., Parsons, A., Pilling, G., Price, G., 2013. Chemistry - Introducing
Inorganic, Organic and Physical Chemistry, second ed., Oxford University Press.
Cath, T.Y., Childress, A.E., Elimelech, M., 2006. Forward osmosis: Principles, applications,
ip
t
and recent developments. J. Membr. Sci. 281, 70-87.
Covington, A. K., Whitefield, M., 1988. Recommendations for the determination of pH in sea
cr
water and estuarine waters. Pure & Appl. Chem. 60, 865-870.
us
Deshmukh, A., Yip, N. Y., Lin, S., Elimelech, M., 2015. Desalination by forward osmosis:
Identifying performance limiting parameters through module-scale modelling. J. Membr. Sci.
an
491, 159-167.
Haling, R. E., Simpson, R. J., Delhaize, E., Hocking, P. J., Richardson, A. E., 2010. Effect of
M
lime on root growth, morphology and the rhizosheath of cereal seedings growing in an acid
ed
soil. Plant Soil, 327, 199-212.
Holloway, R. W., Maltos, R., Vanneste, J., Cath, T. Y., 2015. Mixed draw solutions for
pt
improved forward osmosis performance. J. Membr. Sci. 491, 121-131.
Ac
ce
Kim, C., Lee, S., Shon, H.K., Elimelech, M., Hong, S., 2012. Boron transport in forward
osmosis: Measurements, mechanisms, and comparison with reverse osmosis, J. Membr. Sci.
419-420, 42-48.
Kim, D. I., Kim, J., Shon, H. K., Hong, S., 2015. Pressure retarded osmosis (PRO) for
integrating seawater desalination and wastewater reclamation: energy consumption and
fouling. J. Membr. Sci. 483, 34-41.
Lay, W. C. L., Zhang, J., Tang, C., Wang, R., Liu, Y., Fane, A. G., 2012. Factors affecting
flux performance of forward osmosis systems. J. Membr. Sci. 394-395, 151-168.
Page 20 of 24
Linares, R. V., Li, Z., Sarp, S., Bucs, S. S., Amy, G., Vrouwenvelder, J.S., 2014. Forward
osmosis niches in seawater desalination and wastewater reuse. Water Res. 66, 122-139.
Liyanaarachchi, S., Shu. L., Muthukumaran, S., Jegatheesan, V., Baskaran, K., 2014(a).
Problems in seawater industrial desalination processes and potential sustainable solutions: a
ip
t
review. Reviews in Env. Sci. Bio/Tech. 13, 203-214.
cr
Liyanaarachchi, S., Jegatheesan, V., Shu, L., Muthukumaran, S., Baskaran, K., 2014(b). A
preliminary study on the volume reduction of pre-treatment sludge in seawater desalination
us
for forward osmosis. Desalination Water Treatment. 52, 556-563.
an
Long, Q. W., Wang, Y., 2015. Sodium Tetraethylenepentamine Heptaacetate as novel draw
solute for forward osmosis—synthesis, application and recovery, Energies. 8, 12917-12928.
M
Lutchmiah, K., Cornelissen, E. R., Harmsen, D. J. H., Post, J. W., Lampi, K., Ramaekers, H.,
Sci. Tech. 64 (7), 1443-1449.
ed
Rietveld, L. C., Roest, K., 2011. Water recovery from sewage using forward osmosis. Water
Ac
ce
1133-1138.
pt
Marcus, Y., 1989. Determination of pH in highly saline waters, Pure & Appl. Chem. 61,
Neilly, A., 2014. Forward Osmosis: The relationship between draw solution, membrane
orientation and water flux, Master’s Thesis, School of Engineering and Physical Science,
James Cook University, Australia.
Nir, O., Marvin, E., Lahav, O., 2014. Accurate and self-consistent procedure for determine
pH in seawater desalination brines and its manifestation in reverse osmosis modeling. Water
Res. 64, 187-195.
Page 21 of 24
Philip, W. A., Yong, J. S., Elimelech, M., 2010. Reverse draw solute permeation in forward
osmosis: Modeling and experiments. Env. Sci. Tech. 44, 5170-5176.
Phuntscho, S., Shon, H. K., Hong, S., Lee, S., Vigneswaran, S., 2011. A novel low energy
performance of fertilizer draw solutions. J. Membr. Sci. 375, 172-181.
ip
t
fertilizer driven forward osmosis desalination for direct fertigation: Evaluation the
cr
Qasim, M., Darwish, N. A., Sarp, S., Hilal, N., 2015. Water desalination by forward (direct)
us
osmosis phenomenon: A comprehensive review. Desalination. 374, 47-69.
Reddi, B. A. J., 2013. Why is saline so acidic (and does it really matter?). Int. J. Med. Sci. 10,
an
747-750.
Rumpf, B., Nicolaisen, H., Öcal, C., Maurer, G., 1994. Solubility of carbon dioxide in
M
aqueous solutions of sodium chloride: Experimental results and correlation. J. Sol. Chem. 23,
ed
431-448.
Sagiv, A., Zhu, A., Christofides, P. D., Cohen, Y., Semiat, R., 2014. Analysis of forward
pt
osmosis desalination via two-dimensional FEM model. J. Membr. Sci. 464, 161-172.
Ac
ce
Sereshti, H., Aliakbarzadeh, G., 2013. Response surface methodology optimized dispersive
liquid-liquid microextraction coupled with UV-vis spectrophotometry for determination of
quinine. Anal. Methods. 5, 5253-5259.
Shaffer, D. L., Werber, J. R., Jaramillo, H., Lin, S., Elimelech, M., 2015. Forward osmosis:
Where are we now? Desalination. 356, 271-284.
Shu, L., Obagbemi, I. J., Jegatheesan, V., Liyanaarachchi, S., Baskaran, K., 2015. Effect of
multiple cations in the feed solution on the performance of forward osmosis. Desalination
Water Treatment. 54, 845-852.
Page 22 of 24
Shu, L., Wu, S., Jegatheesan, V., 2013. Directly observe sodium chloride aggregates waltzing
through dilute solutions, in: L. Shu, V. Jegatheesan, A. Pandey, J. Virkutyte, H. Djati Utomo
(Eds), Solutions to environmental challenges through innovations in research. Asiatech
Publishers Inc., New Delhi, 213-223.
ip
t
Wang, W., Zhang, Y., Esparra-Alvarado, M., Wang, X., Yang, H., Xie, Y., 2014. Effect of
pH and temperature on forward osmosis membrane flux using rainwater as the makeup for
cr
cooling water dilution. Desalination. 351, 70-76.
us
Xie, M., Lee, J., Nghiem, L. D., Elimelech, M., 2015. Role of pressure in organic fouling in
an
forward osmosis and reverse osmosis. J. Membr. Sci. 493, 748-754.
Xie, M., Lee, J., Nghiem, L. D., Price, W. E., Elimelech, M., 2014. Toward resource recovery
M
from wastewater: phosphorus extraction from digested sludge using hybrid forward osmosis
ed
– membrane distillation process. Env. Sci. Tech. Letters. 1, 191-195.
Xie, M., 2014. Forward osmosis for wastewater treatment: Advancing trace organic
pt
contaminant removal and nutrient recovery. PhD thesis, University of Wollongong, Australia.
Ac
ce
Xie, M., Price, W. E., Nghiem, L.D., 2012. Rejection of pharmaceutically active compounds
by forward osmosis: Role of solution pH and membrane orientation, Sep. Pur. Technol. 93,
107-114.
Zhao, H., Fedkin, M. V., Dilmore, R. M., Lvov, S. N., 2015. Carbon dioxide solubility in
aqueous solutions of sodium chloride at geological conditions: Experimental results of at
323.15, 373.15 and 423.15 and 150 bar and modeling up to 573.15 K and 200 bar. Geochim.
Cosmochim. Acta. 149, 165-189.
Zhao, S., Zou, L., Tang, C. Y., Mulcahy, D., 2012. Recent development in forward osmosis:
opportunities and challenges. J. Membr. Sci. 396, 1-21.
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Graphical Abstract (for review)
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ed
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an
Increase the pH
on the feed side
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i
Increase the pH
on the draw side
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ce
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Increase the pH
on the draw side
draw
Ca2+
feed
OH-
H+
Cl-
Na+
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