Influence of Operating Conditions on the Rejection and Flux of Cobalt and Nickel Ions in Aqueous...
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Transcript of Influence of Operating Conditions on the Rejection and Flux of Cobalt and Nickel Ions in Aqueous...
INFLUENCE OF OPERATING CONDITIONS ON THE REJECTION AND FLUX
OF COBALT AND NICKEL IONS IN AQUEOUS SOLUTIONS BY
NANOFILTRATION MEMBRANE
Abstract
Heavy metals are the major sources of environmental pollution and they are non-degradable;
therefore continue to exist in water. This paper presents the potential use of nanofiltration
membrane for the removal of cobalt and nickel ions from aqueous solution. The influences of
pH and pressure are discussed. Permeate flux was higher for both cobalt (39.804L/m²/h) and
nickel (50.525 L/m²/h) at higher pH (4) than at lower pH (pH 3) (38.37 l/m²/h and 46.507
l/m²/h) respectively. The effect of pH on cobalt and nickel rejection was investigated but the
effect of pH on % rejection of former was not significant. The average rejections of 5 mg/L
of Co2+ at pH 3 and 4 were 94.37% and 94.36% respectively. The nickel ion rejection
increased for both pH especially for pH 4. Higher operating pressure leads to higher permeate
flux for both nickel and cobalt. The rejection of both metals ions (nickel and cobalt) is high at
the three different pressures.
Keywords: acid mine drainage, nanofiltration, nickel removal, nickel rejection, cobalt
removal, cobalt rejection, fluxes, solution pH, Pressure
1. Introduction
The rate at which industrial activities is increasing has caused more environmental pollution
and the detrimental of ecosystems, especially aquatic, with the accumulation of pollutants,
such as synthetic compounds, nuclear wastes and heavy metals. The removal of heavy metals
from wastewater is of critical importance due to their high toxicity and tendency to
accumulate in living organisms [1]. Cobalt and Nickel are common pollutants found in
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various industrial effluents. They constitute a great part in the water pollution and the
removal of these metals cations from both industrial and municipal water is of extreme
importance. Both in the past decade and recently, extensive research works have focused on
the removal of cobalt and nickel ions from waste water by different techniques including
solvent extraction [2], adsorption of solids such as metal oxides [3 & 4], ion flotation [5 &
6], electrocoagulation [7 & 8], chemical precipitation [1, 9 & 10] and ion exchange [11].
Membrane separation processes, available in variety of separation capabilities have become
promising techniques for separating heavy metals from aqueous solutions. [12-14].
Nanofiltration (NF) is the most recent membrane separation process in liquid phase. It
appears as an attractive alternative technique since it allows (i) the removal of multivalent
ions without any chemical additives, (ii) continuous separation and (iii) treatment of rather
large feed water flowrates [15]. There are successful investigations using NF as tools for the
removal of heavy metal ions [15-19]. NF membranes have pores of nano-scale dimensions to
meet industrial needs in the area of small molecules and ion separations; thus solute rejection
by NF is usually influenced by membrane charge and membrane pore size.
The objective of this study was to investigate the potential of nanofiltration in the treatment
nickel and cobalt aqueous solution.
2 Experimental
2.1 Nanofiltration membrane characteristics
A composite nanofiltration membrane (Nano-Pro A 3012) was chosen for this research as
representative of a class of membranes which are acid stable in water treatment applications.
According to the manufacturer, the maximum operating pressure is 40 bar (580 psi),
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maximum operating temperature 50°C (122°F), allowable pH – Continuous Operation: 0 -
12, Recirculation Flow Rate: Minimum 90L/min (24gpm), Maximum 280L/min (74 gpm).
2.2 Analytical method
Nickel ion concentration was analysed by using inductively coupled plasma optical emission.
Measurements of solution pH and temperature were made using a pH meter (Mettler Toledo
FG20) purchased from Microsep and thermometer, respectively.
2.3 Flux decline experiments
The experiments were carried out with one litre of single solution of (CoSO4.7H2O and
NiSO4.6H2O) in varied concentration of 5mgL, 10mg/L while varying solution pH from 3 to
4 and pressure (30bar, 20bar and 10bar). The solution pH was varied from 3 and 4 at constant
pressure of 30bar. Flux decline experiments were conducted by using a 1 000-ml dead-end
membrane filtration apparatus (Memcon South Africa) with magnetic stirrer. A membrane
sheet was fitted to the cell. The membrane active area is about 0.01075m2. The operating
pressure was employed via high-pressure regulator and a nitrogen gas cylinder. The permeate
flux was collected in a beaker on the electrical balance and the permeate mass was
determined.
2.4 Filtration Experiments
Membrane sheet stored in 0.7% w/w benzalkonium chloride at 2-30°C was used for the
study. The membrane sheet was initially rinsed in clean distilled water and was used to
measure the clean water flux (CWF) using distilled water before each nickel solution was
used with the system. The clean water flux experiments were done to see if membrane did not
foul. The clean water flux was done at stirring velocity rate of 500 rpm and a pressure of 30 3
bar. Feed single nickel solutions and cobalt solutions were prepared for each test condition.
The pH of the feed was controlled by addition of NaOH. 0.0208g of NaOH was used to raise
the pH of cobalt sulphate solution to 4 and 0.082g of NaOH was used to raise the pH of
nickel sulphate solution to 4. After filtration was terminated, the membrane was cleaned with
deionized water, followed by a clean water flux measurement. The water fluxes at different
operating conditions were measured to determine water flux recovery.
2.5 Laboratory Dead-End Test Cell
The investigation was done using a Memcon Laboratory Stirring Cell as shown in fig. 1. The
membrane tested was placed in the cell. A litre of sample was then placed in the cell at the
product inlet. Pressure was then applied with nitrogen gas and the permeate collected and its
mass determined.
2.6 Analysis of Results
The permeate flux and rejection were investigated as a function of working parameters such
as operating time and water recovery. The permeate flux Jv (l/m2/h) was determined by
measuring the volume of permeate collected in a given time interval divided with membrane
area by the relation:
(1)
Where, Q and A represents flow rate of permeate and the membrane area, respectively.
The observed rejection which is the measure of how well a membrane retains a solute was
calculated by the following relation:
(2)
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Where Cp and Ci are the solution concentrations in the permeate and in the initial feed
solution, respectively.
Figure 1: Schematic diagram of laboratory dead-end filtration system.
3. Results and Discussions
3.1 Clean water flux as a function of pressure
Clean water flux as a function of pressure was done for three different pressures (30, 20, and
10 bar) before nickel was added to the feed solutions to establish initial conditions and to
determine the effect of pressure on flux. The fluxes as a function of time and water recovery
are shown in figure 2. The feed pressure had a significant effect on nanofiltration membrane
performance. A relatively high flux (46.94 l/m²/h) was obtained at 30 bar and the flux
decreased significantly at 20 bar (28.10 l/m²/h) and 10 bar (16.29 l/m²/h). It is also intresting
to note that the flux declined a little bit with increasing water recovery as a result of the
higher osmotic pressure of the feed solution. However, the decline on the flux was not very
much. These fluxes are low for a nanofiltration membrane and it was decided to conduct all
subsequent runs at a pressure of 30 bar.
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Compressed Nitrogen Gas
Membrane
Stirring Rod
Scale
Magnetic Stirrer
Feed Reservoir
(a) (b)
Figure 2: Flux of deionized water as function of time and water recovery (30 bar).
3.2 Effect of solution pH on flux
In several studies on pH effects, it was found that pH of the feed solutions has a significant
effect on solute flux and rejection. The effect of solution pH on flux is shown in figures 3a
and 3b for a solution pH of 3 and 4 at the concentration of 10mg/l nickel and 5mg/L cobalt. A
higher permeate flux was experienced at the higher pH (pH 4) for both nickel and cobalt
(50.525 l/m²/h and 39.804 l/m²/h) respectively than at lower pH (pH 3) (46.507 l/m²/h and
38.37 l/m²/h) respectively. The solution flux increased at higher pH (pH 4), as a result of
increases in membrane permeability. Such an increase is probably consistent with increased
fixed charge, increased electrical double layer thickness within membrane pores, or both [20].
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(a) (b)
Figure: 3 Effect of pH on flux (a) as function of time (b) as function of water recovery at two
different pH for Ni2+ and Co2+
3.3 Effect of solution pH on ions rejection
The effect of solution pH on nickel and cobalt ion rejections is shown in figures 4a and 4b of
10mg/l nickel and 5mg/L cobalt. It is well known that pH plays a major role in the
performance of nanofiltration membrane but the effect of pH on % rejection of cobalt was not
significant. Never the less it must be noticed that the pH effect is different for different salts.
The average rejections of 5 mg/L of Co2+ at pH 3 and 4 were 94.37% and 94.36%
respectively. The nickel ion rejection increased for both pH especially for pH 4. The result
could be the effect of increase in the negative charge of the membrane. Increased ion
concentration could cause a reduction in the electrical double layer thickness in membrane
pores and thus increase the solute partition coefficient, causing a reduction in the rejection of
ionic species.
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(a) (b)
Figure 4: Effect of pH on the ion rejection (a) as function of time (b) as function of water
recovery at two different pH for Ni2+ and Co2+
3.4 Effect of pressure on flux
The results of flux of water recovery and time are shown in figures 5a and 5b. Pressure
difference is the driving force responsible for a nanofiltration process. At increase effective
feed pressure, the permeate flux increased for the both metal ions (nickel and cobalt). At
higher pressure, the compressing effect of the pressure on the membrane is little thus more
water passes through the membrane and thus leads to higher flux and increase in water
recovery. At lower pressure, the average pore size of the separation layer of the membrane
reduced because the compressing effect of the pressure on the membrane is high. At pressure
of 10bar, the water recovery for nickel and cobalt was 38% and 23% respectively; that is the
lower the pressure the lower water recovery for the period of filtration.
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(a) (b)
Figure 5: Effect of pressure on flux decline (a) as time (b) as function of function of water
recovery at two different pH for Ni2+ and Co2+
3.5 Effect of pressure on ion rejection
The results of ion rejection as a function of water recovery and time are shown in figures 6a
and 6b. It is clear that the rejection of both metals (nickel and cobalt) is high at the three
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different pressures.. From the result below, it can be concluded that the three different
pressures will obtain the best selectivity during nanofiltration.
Figure 6: Effect of pressure on ion rejection (a) as function of water recovery (b) as function
of time at two different pH for Ni2+ and Co2+
Conclusion
The performance of a nanofiltration membrane for the removal of nickel and cobalt ions from
an aqueous solution was investigated using a dead-end test cell. The following conclusions
can be made as a result of the investigation:
The pH has a significant effect on permeate flux for both cobalt and nickel. The flux at
pH 4 is higher the flux at pH 3 for both nickel and cobalt.
The result shows that the pH effect is different for different salts. The effect of pH on %
rejection of cobalt was not much. The average rejections of 5 mg/L of Co2+ at pH 3 and 4
were 94.37% and 94.36% respectively. The nickel ion rejection increased for both pH (3
10
and 4) especially for pH 4. The average rejection for pH 3 and pH 4 are 83.063% and
86.155% respectively.
Operating pressure is an important parameter for the running of the membrane equipment.
High operating pressure (30bar) led to high permeation flux for both nickel and cobalt.
But high pressure means high consumption, and therefore high power equipment should
be used.
Higher nickel ion and cobalt ion rejection were experienced at the three different
pressures.
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