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

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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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