Removal of heavy metal ions by nanofiltration

Removal of heavy metal ions by nanofiltration

DES-11337; No of Pages 16 Desalination xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect Desalination journal homepage: www.else...

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DES-11337; No of Pages 16 Desalination xxx (2012) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Removal of heavy metal ions by nanofiltration B.A.M. Al-Rashdi a, D.J. Johnson a, N. Hilal a, b,⁎ a b

Centre for Water Advanced Technologies and Environmental Research (CWATER), Multidisciplinary Nanotechnology Centre, Swansea University, Swansea, SA2 8PP, UK Masdar Institute of Science and Technology, P. O. Box 54224, Abu Dhabi, UAE

a r t i c l e

i n f o

Article history: Received 21 March 2012 Received in revised form 17 May 2012 Accepted 19 May 2012 Available online xxxx Keywords: Heavy metals Nanofiltration membrane Rejection Permeate flux Feed pH Membrane fouling Membrane characterisation Membrane filtration Water purification Atomic force microscopy AFM

a b s t r a c t This study describes the rejection of heavy metal ions using a commercial nanofiltration membrane (NF270). The effect of feed pH, pressure and metal concentration on the metal rejections and permeate flux and in some cases permeate pH was explored. The results showed that with all metals examined (except As (III)), when the feed pH is below the isoelectric point, the rejection increased. NF270 rejected almost 100% of copper ions at low concentrations, but decreased to 58% at the highest concentration examined. Using 1000 mg/L concentration level, pH = 1.5 ± 0.2 and 4 bar the rejection was 99%, 89% and 74% for cadmium, manganese and lead respectively. However at pH above the isoelectric point the average rejections decreased. NF270 was unable to retain As(III). The metals caused a flux decline due to membrane fouling in the order of severity: Cu2+ > Cd 2+ ≈ Mn2+ > Pb2+ ≈ As3+. The correlation between adsorbed amounts of the metals onto NF270 with the normalised flux shows that as the amount increased the normalised flux decreased, except for arsenic that had a higher deposited amount and higher flux. The RMS roughness as obtained by AFM showed that roughness was decreased by membrane fouling. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Heavy metals are a serious environmental contaminant because they are environmentally persistent, have high toxicity and have a tendency to accumulate in body tissues [1]. Because of this, environmental regulations compel industries to reduce the concentration of heavy metals in their waste water to within safe levels. In recent years, the use of nanofiltration membranes (NF) has increased rapidly in the chemical, petrochemical, biotech and desalination industries, since the NF technology overcomes operational problems that are associated with conventional techniques. Several successful studies have been reported which have used NF membranes as tools for heavy metal removal [1,2]. Most commercial NF membranes are thin-film composites made of synthetic polymers containing charged groups which can make them effective in the separation of charged metals from water. Additionally, the separation in NF occurs due to solution diffusion as well as sieving, the Donnan effect, dielectric exclusion and electromigration, which also makes it useful in the separation of both charged and uncharged organic solutes [3]. The feed pH can change the nature of the membrane surface charge and pore size, as well as that of dissolved metal species and therefore can affect the membrane separation efficiency. The effect

⁎ Corresponding author. Tel.: + 44 1792606644; fax: + 44 1792295676. E-mail address: [email protected] (N. Hilal).

of feed pH on the removal of some heavy metal and permeate flux using NF membranes has been studied and explained previously [4–7]. However, the investigation of several different heavy metals on the same membrane, under the same conditions gives much information about the membrane and its suitability at different conditions (i.e. pH, metal ions, feed pressure and concentration). The NF270 membrane has been studied before for the retention of copper [8,9] and manganese [10]. In this work retention of five metal solutions (Cd(II), As(III), Cu(II), Mn(II), Pb(II)) was investigated using the NF270 membrane. The effect of the following factors on membrane separation was studied: pressure (3–5 bar), initial feed concentration (100–2000 mg/L) and pH (1.50–5) of the solution. In this study flux reduction with these metals also was studied and explained according to the measured surface roughness (RMS) values and the amount of each metal deposited onto the membrane surface.

2. Materials and methods 2.1. Materials and apparatus All chemicals used in this research were of analytical grade and all metal solutions were prepared by dissolving the appropriate mass of each metal in high purity Milli-Q water (18.2 MΩ cm), except for arsenic which was prepared in boiled acidified or alkalised Milli-Q water (5 mL for 1 L solution). The arsenic trioxide is insoluble at

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Please cite this article as: B.A.M. Al-Rashdi, et al., Removal of heavy metal ions by nanofiltration, Desalination (2012), doi:10.1016/ j.desal.2012.05.022

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room temperature and required boiling in order to dissolve. For a multicomponent solution arsenic was dissolved first in boiling MilliQ or tap water. After cooling all other metals were added sequentially, after the previous metal had completely dissolved. Copper nitrate Cu(NO3)2 ⋅ 3H2O, lead nitrate Pb(NO3)2, arsenic trioxide As2O3, manganese nitrate Mn(NO3)2 ⋅ 4H2O, cadmium nitrate Cd (NO3)2 ⋅ 4H2O, sodium hydroxide and nitric acid (69–72%) were all obtained from Fischer Scientific, UK. The pH was measured using a calibrated pH metre (Jenway, model 3540). Concentrations of metal ion in synthetic and treated solutions were examined using a Varian AA 240FS Atomic Adsorption Spectrometer (AAS). Calibration curves of the tested metals were constructed with different concentration. The instrument was calibrated regularly and calibration was verified before each sample set. Higher concentration metal solutions were diluted to prevent blocking of the AAS sensor. Standard solutions of each metal were prepared from a stock solution of 1000 mg/L. The 1000 mg/L stock solution of each used metal was prepared following the procedures described in Ref. [11]. 2.2. Membrane filtration experiments For the membrane permeability, flux, fouling and rejection studies a cross-flow stainless steel nano-filtration unit was used (Fig. 1). This nanofiltration unit was equipped with one membrane module with an effective membrane area of 0.00076 m 2 which can be operated in the pressure range of 1 to 5 bar. The nanofiltration unit consisted of a feed tank (capacity of 5 L), a membrane module, a high-pressure pump (Tuthill Pump Co. of California) and two pressure gauges. The feed and retentate flow rates were measured at the inlet and outlet of the membrane module by two flow metres (MPB Industries, Kent, England) with minimum and maximum flow rate of 0.4 L/min and 4 L/min, respectively. In this work, the feed and retentate flow rates were maintained at 0.4 L/min. Three valves (Ham-Let, UK; H6800 for valve 1, H300SSLR1/4RS for both valve 2 and valve 3) were placed at locations (see Fig. 1) in order to control the fluid flow. Experiments on membrane filtration were carried out using commercial NF270 nanofiltration membrane (Dow membrane) at room temperature that ranged from 22 to 25 °C. The NF270 membrane is a polyamide membrane; the active layer of NF270 is a very thin semi-aromatic piperazine-based polyamide layer with a microporous polysulfone supporting layer [10,12]. NF270 pore size, pore size distribution using solute transport method and the membrane surface roughness using AFM was investigated and presented in a previous paper [13]. The surface charge at different pH and the isoelectric point of NF270 have been noted by many researchers previously [8,14–16]. The membrane surface is positively charged when solution pH is less than 3.3–4 and negatively charged when the solution pH is

higher. This trend arises in thin-film composite membranes from the dissociation of various functional groups (typically carboxyl) located on the surface of the membrane with increasing pH and pendant amino groups [17]. In all filtration experiments, the membrane was immersed in deionised water before being used in any experimental work and each membrane was pressurised at 5 bar for at least 2 hours using pure water to avoid any compression effects during measurements and also to establish leak tightness. 2.3. Permeability of NF270 Water flux measurements were carried out with high purity MilliQ water (resistivity 18.2 MΩ cm). From filtration tests at different transmembrane pressures (1.5–5 bar) at ambient temperature with membrane area of 0.00076 m 2, the pure water permeability was determined. The pressure was varied between highest pressure at 5 bar down to lowest pressure of 1.5 bar. The trans-membrane pressure and volumetric flow rate were adjusted using the concentrate outlet valve and variable speed key of the pump. Pure water flux at certain pressure can be defined as: Jv ¼

Q A

ð1Þ

where: permeate flux (L/m 2 h) effective membrane area (m 2) volume flow rate (L/h).

Jv A Q

However, in this study, the pure water flux was determined by weighing the obtained permeate during a predetermined time using an electronic balance (Precisa, Model XB3200C) connected to a computer. By plotting membrane flux (Jv) for a variation of applied pressure (ΔP), the membrane permeability (pure water permeability), Lp can be obtained from the slope of the straight line as follows: Lp ¼

Jv ΔP

ð2Þ

2.4. Heavy metal retention using NF270 In the experiments of heavy metal removal (Cu(II), Mn(II), Pb(II), Cd(II) and As(III)), an initial volume (approximately 500 mL) of the metal solution was pumped and circulated without agitation through the membrane module (Fig. 1). Permeate was continuously collected

Fig. 1. Schematic diagram of cross flow NF filtration unit.

Please cite this article as: B.A.M. Al-Rashdi, et al., Removal of heavy metal ions by nanofiltration, Desalination (2012), doi:10.1016/ j.desal.2012.05.022

B.A.M. Al-Rashdi et al. / Desalination xxx (2012) xxx–xxx

R ¼ 1−

Cp Cb

ð3Þ

where Cp is concentration of solute in permeate and Cb is an average bulk concentration of solute in the feed (Cf) and concentrate/ retentate (Cr). Cb ¼

Cf þ Cr 2

ð4Þ

2.5. Determination of membrane solute adsorption/deposition Solute adsorption experiments on NF270 membrane were carried out inside a closed bottle by measuring the metal concentration before and after 24 hours contact time with the membrane. During the experiments the bottles were shaken regularly to prevent settling. The volume and the initial concentration of metal solution were 500 mL and 1000 mg/L, respectively. The effective surface area (A) of the membrane was 25 cm 2. The effect of water evaporation and adsorption of the solute onto the recipient walls were determined through a blank experiment. The adsorption amount Q (mg/m 2) was determined using [12]: h Q¼

 i C i −C eq −ðC i −C blank Þ  V

ð5Þ

A

where Ci, Cblank and Ceq are the initial, blank and equilibrium concentrations in mg/L or mol/L, respectively. 2.6. Atomic force microscopy (AFM) All AFM measurements were performed using a Multimode AFM with Nanoscope IIIa controller (Veeco, USA) using manufacturer supplied software. All measurements were made using tapping mode in air at room temperature and were performed using TESP type cantilevers (nominal spring constant 20–80 N/m) (Bruker AXS). Roughness parameters for all images were calculated using the instrument software, with all values being the mean value of at least three images taken from different areas on the sample surface. The roughness parameters were the root mean squared roughness (RMS), roughness average (Ra) and the percentage surface difference. An explanation

Table 1 Membrane experimental conditions.

of how these parameters are calculated can be found in the following references: [13,18]. 3. Results and discussion 3.1. Effect of applied pressure 3.1.1. Membrane permeability Pure water flux measurements as a function of trans-membrane pressure (TMP) for membrane NF-was carried out. Using Eq. (2), the membrane permeability (Lp) was found to be 13.2 L/m 2 h bar (Fig. 2) which is similar with that found by others [12,15]. Moreover, Semiao and Schafer [16] found that this membrane has permeability of about 18 L/ m 2 h bar. All of these permeability results indicate that NF270 membrane is a loose NF membrane. 3.1.2. Permeate flux Fig. 3(a–e) shows the fluxes of NF270 using five different metals as a function of pressure. With all metals the permeate flux increased with increasing pressure. As pressure increases, convective transport and concentration polarisation become more important [19]. Fig. 3c for cadmium (Cd) reveals that change in the permeate flux remains linear with increasing pressure, which indicates insignificant concentration polarisation. A similar observation was made for arsenic. However with copper, lead and manganese the increase in flux is not linear with the rise of pressure, which shows a quantity of concentration polarisation. The permeate fluxes for all metal solutions were lower than the corresponding pure water flux, 60 L/m² h, 33 L/m² h and 23 L/m² h, with pressure 5, 4 and 3 bar, respectively (Table 2). This can be taken as an indication of membrane fouling and/or due to osmotic pressure build up caused by the retained salt [4,9]. A similar reduction in permeate flow using a similar range of pressure was observed by Chaabane et al. [20]. The metals ranked by the order which they caused membrane fouling is Cu 2+ > Cd 2+ ≈ Mn 2+ > Pb 2+ ≈ As 3+. This may be due to the formation of a cake layer of copper hydroxide precipitate Cu(OH)2 (s). The measured pH of the retentate was around 6, which is indeed higher than the pH at which precipitation occurs. Based on the calculation described by Ferguson [21] and using Ksp constant at 25 °C and for 1000 mg/L copper solution, copper hydroxide precipitation starts at pH = 5.24. On the other hand, the pH of precipitation of the other metals (Mn, Cd and Pb) was higher than that found in the retentate solution; thus these metals tend to diffuse through the membrane. The metal hydroxide complexes and formation of insoluble hydroxides at certain pH were also reported by 70

y=13.201x-13.557 R²= 0.9402

60 50

Flux (L/m².h.bar)

in a beaker until 50 g of the permeate had been acquired, at which point the experiment was stopped. The permeate flux was determined by weighing the obtained permeate during the experiment as mentioned in Section 2.3. Final permeate flux (i.e. when the permeate mass = 50 g) was used in discussion of the results unless otherwise stated. The experimental conditions used to study the effect of pressure, metal concentration (Cmetal), and pH (adjusted using 1 M NaOH or 1 M HNO3) of metal solutions are summarised in Table 1. A fresh piece of the NF270 membrane was used in each experiment. Solute separation or removal can be represented by rejection factor (R):

3

40 30 20

Exp.

Cmetal (mg/L)

pHmetal

Pressure (bar)

1 2 3 4 5 6

1000 1000 1000 100 500 2000

5 3.3 1.5 5 5 5

5, 4, 3 4 4 4 4 4

10 0 0

1

2

3

4

5

6

Pressure (bar) Fig. 2. Water flux as a function of transmembrane pressure (membrane NF270).

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30

25

25

20

Flux (L/m².h)

Flux(L/m².h)

30

(a)

35

20 15

(b)

15 10

10 5

5

Exp.1

EXP.2

EXP.1

Average

0

EXP.2

Average

0 2

3

4

5

6

2

3

Pressure (bar) 40

(c)

30

4

5

6

Pressure (bar)

(d)

35 25

Flux (L/m².h)

Flux (L/m².h)

30 20

15

10

25 20 15 10

5 Exp.1

EXP.2

EXP.1

5

Average

0

EXP.2

Average

0 2

3

4

5

2

6

3

Pressure(bar)

4

5

6

Pressure(bar)

40

(e)

35

Flux (L/m².h)

30 25 20 15 10 5

Exp.1

EXP.2

Average

0 2

3

4

5

6

Pressure(bar) Fig. 3. Effect of pressure on final permeate flux of NF270 membrane with (a) Cu(II), (b) Mn(II), (c) Cd(II), (d) Pb(II) and (e) As(III) (experimental conditions: 1000 mg/L, pH = 5, collected permeate = 50 g).

Ozaki et al. [22]. Moreover Pb 2+ has a higher mobility and diffusion coefficient than that of with Cd 2+ and Mn 2+ [23]. Brandhuber and Amy [24] confirmed that small and uncharged As(III) was easily passed through a loose NF membrane with a similar flux as observed for water. This can explain the higher permeate flux and the lower difference between pure water flux and the flux when arsenic solution was used in this study (Table 2).

It is evident from Fig. 3 that sometimes there is a clear difference between experiment 1 and experiment 2 (even though the conditions were the identical). However, with each experiment a different section of the NF270 membrane was used. Thus the differences in the results may indicate heterogeneity of the membrane surface. The use of different samples of membrane in each experiment may represent nearly the actual permeate flux or rejections of the membrane.

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B.A.M. Al-Rashdi et al. / Desalination xxx (2012) xxx–xxx

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Table 2 Final permeate flux and percentage flux reduction of NF270 membrane for some metals with different parameters. Metals

Cu(II)

Parameters

Flux (L/m² h)

Flux Red.%

Flux (L/m² h)

Flux Red.%

Flux (L/m² h)

Flux Red.%

Flux (L/m² h)

Flux Red.%

Flux (L/m² h)

Flux Red.%

9.0 22.0 25.3 15.7 14.9 22.0 – 29.8 23.4 22.0 11.1

61 33 58 52 55 33 – 10 29 33 66

12.9 21.7 26.3 14.5 23.8 21.7 – 29.6 26.4 21.7 21.6

44 34 56 56 28 34 – 10 20 34 35

12.7 21.0 29.0 21.0 22.6 21.0 – 24.5 25.5 21.0 18.2

45 36 52 36 32 36 – 9 23 36 45

18.1 21.8 33.8 24.8 21.8 21.8 – 29.6 27.0 21.8 20.3

21 34 44 25 34 34 – 10 18 34 38

17.9 25.4 33.6 32.6 20.8 25.4 32.8 31.5 – 25.4 20.6

22 23 44 1.1 37 23 0.53 4.4 – 23 38

Pressure (bar)

Feed pH

Concentration (mg/L)

3 4 5 1.5 3.3 5 9.3 100 500 1000 2000

Mn(II)

Cd(II)

Fig. 4 presents the permeate flux for the NF270 membrane plotted against time for different pressure values during the treatment of 1000 mg/L copper solution. When the pressure was 3 or 4 bar the  permeate flux (Jv) reduction in (ðJ0 −J v Þ J  100) compared with that 0 of pure water (J0) (Fig. 5) was 61% for 3 bar pressure and 33% for 4 bar pressure. When 5 bar pressure was applied, the reduction was higher, 58%. In the case of 3 bar pressure, the drop in the flux starts from the beginning (t = 1 min) whereas with 4 and 5 bar pressures, this drop occurred after 1 minute and decreased to an almost constant value in the later stage. However the initial flux (t = 1 min) is not the same as that of pure water at 5 bar pressure. Obviously with 3 bar pressure the permeate flux was less and thus more time was needed to collect 50 g permeate (about 8 hours). In order to moderate the effects of fouling and the time, pressure of 4 bar was kept in the latter experiments. 3.1.3. Rejection Concentration polarisation, which increased with increasing pressure, results in a decrease in retention. However, convective transport causes an increase in retention. The counteracting contributions of increased convective transport and increased concentration polarisation result in a nearly constant retention value [19]. Fig. 6a–d reveals that all used metals have an effect on both concentration polarisation and convective transport but with some variations. For example, the rejection of copper at pressure = 3 bar was 96% (average of Exp.1 and Exp.2) and then decreased to 90% at both pressures 4 bar and 5 bar which implies that concentration polarisation followed by both convective transport and concentration polarisation had occurred. When the applied pressure was increased, the manganese rejection

50

P= 3 bar

P= 4 bar

As(III)

increased slightly (29% to 36%). Cadmium and lead behaved differently: when the pressure was increased from 3 bar to 4 bar, the rejections increased, but when the pressure increased further to 5 bar then the rejection decreased. However, all these changes in rejection were small; thus they might be considered nearly constant. With increased pressure, rejection increased or decreased slightly. The small differences between Cu, Cd, Pb and Mn rejection as the pressure increased can be attributed to the water permeation rate becoming greater at higher pressure, or to the solute diffusion rate which would not be expected to be affected significantly by higher pressure because it is mainly controlled by the solute concentration [4]. The average rejections of Cd, Cu, Pb and Mn were 63, 92, 54 and 33%, respectively. The reason for higher copper removal and lower manganese removal might be credited to the formation of metal hydroxide precipitate as discussed in Section 3.1.2. Other explanations for the experimentally determined rejection sequence are: i) The diffusion coefficients of cations Pb 2+, Cd 2+, Cu 2+ and Mn 2+ in water at 25 °C are 1.89 × 10 − 5, 1.438 × 10 − 5, 1.428 × 10 − 5 and 1.424 × 10 − 5 cm 2/s, respectively [23]. The order of diffusion coefficients is inversely reflected in the rejection sequence except for manganese which might oxidise to Mn 3+. Similar trends with NF were observed in the literature for different ions [5] ii) The ionic radius of cations Pb 2+, Cd 2+, Cu 2+ and Mn 2+ are 133, 97, 72 and 80 pm [25], respectively. Tansel et al. [26] reported that ions with lower ionic radius tend to hold their hydration shell (i.e. are strongly attached to water molecules) thus would be more removed by membrane. Copper has a comparatively

P= 5 bar

P= 3 bar

70

P= 4 bar

P= 5 bar

60

40

50

Flux (L/m².h)

Flux(L/m².h)

Pb(II)

30

20

40 30 20

10 10 0

0 1

101

201

301

401

Time(min) Fig. 4. Permeate flux with time at different pressure, experimental conditions: metal concentration = 1000 mg/L copper, pH = 5 ± 0.2, collected permeate = 50 g.

1

101

201

301

401

Time (min) Fig. 5. Change in pure water flux observed over time for several different pressure values.

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Fig. 6. Effect of pressure on rejection (a) Cu(II), (b) Mn(II), (c) Cd(II), (d) Pb(II) using NF270 membrane. Experimental conditions: metal concentration = 1000 mg/L, pH = 5 ± 0.2, collected permeate = 50 g.

small ionic radius and hence will be more easily rejected. Pb will be easily permeated because of the relatively loose hydration shell. However with loose and charged NF membranes size exclusion is not the main mechanism to explain rejection. Charge exclusion and metal deposition play very important roles which cannot be ignored. Rejection of As(III) by NF270 was very small and occasionally the rejection was below the level of detection. The rejection of As(III) at different pressures ranged from 0 to 11%. However, this result was expected because NF270 is a loose nanofiltration membrane. Many researchers [6,24,27] found that rejection of As(III) by loose NF membrane was very small compared with As(V); thus they recommended oxidation of As(III) to As(V) prior to use of loose NF membrane to remove arsenic from water. 3.1.4. Permeate flux and metal rejection The relation between permeate flux and metal rejection of Cu 2+, Cd 2+, Mn 2+, and Pb 2+ for NF270 in pressure range 3–5 bar is shown in Fig. 7. It is interesting to note that the highest flux was obtained for lead. For copper the rejection fell off with increasing

permeate flux. An almost linear relationship (R 2 = 0.963) was found to relate the permeate flux to Cu(II) rejection. On the contrary, for manganese as the permeate flux increased the rejection was found to increase with a linear relationship (R 2 = 0.99). For both cadmium and lead, at higher and lower fluxes the metal rejections were similar. This result suggests that these metals are rejected by different mechanisms. 3.2. Effect of feed pH on NF270 performance 3.2.1. Permeate flux Fig. 8(a–e) shows the flux of NF270 as a function of feed pH for 1000 mg/L cadmium, arsenic, copper, manganese and lead solutions. It can be seen that the flux varied over the used pH range depending on the metal tested. In general, the change of flux with feed pH was small (average flux for all used metals ranged from 14 L/m 2 h to 25 L/m 2 h). With the manganese solution the maximum flux was at pH = 3.3 ± 0.2 (at or close to the iso-electric point of the membrane). Childress and Elimelech [28] and Qin et al. [29] found a similar result; maximum permeate flux was found to be at the isoelectric point. However, with the copper solution (Fig. 8a) the maximum flux occurs

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B.A.M. Al-Rashdi et al. / Desalination xxx (2012) xxx–xxx

Cu(II)

Mn(II)

Cd(II)

Pb(II)

120

y = -0.3927x+99.367 R² = 0.9627

100

%Rejection

80

60

40

y = 0.5111x + 22.281 R² = 0.9903

20

0 0

10

20 Flux(L/m².h)

30

40

Fig. 7. Relation between permeate flux and Cu(II), Mn(II), Cd(II) and Pb(II) rejection. Experimental conditions: 1000 mg/L metal solution, pressure 3–5 bar, pH = 5 ± 0.2.

at pH = 5 ± 0.2 (22.0 ± 4 L/m² h) and the minimum flux occurred at both pH = 1.5 ± 0.2 (16 ± 0.83 L/m² h) and 3.3 ± 0.2 (15.0 ± 2.3 L/m² h). With As(III) solution, the minimum flux occurs at pH= 3.3 ±0.2 (21 ± 4 L/m²h) (Fig. 8e). Wang et al. [4] reported that the permeate flux was minimal around the isoelectric point of the membrane. With the lead solution, as well as the cadmium solution, the permeate flux was nearly constant as the pH increased. Childress and Elimelech [28] proposed mechanisms to explain the flux at the isoelectric point: the pore size changes with pH due to i) the expanding or shrinking of the cross linked-membrane polymer network, ii) the lowest electroviscous effect and iii) the highest net driving force due to lowest osmotic pressure at the membrane surface. Wang et al. [4] explained the minimum permeate flux in terms of concentration polarisation and membrane fouling. In addition, Ballet et al. [7] explained the effect of decreasing permeate flux with changes in pH due to shrinking of the skin layer which result from the differences of the hydration of the ionised groups of the membrane. 3.2.2. Rejection Fig. 9(a–d) presents the effects of feed pH on the Cu, Mn, Cd and Pb rejections. Generally, as the pH increased from pH = 1.50 to pH = 5 ± 0.2, the rejections of these metals decreased. For instance, the average rejection of cadmium was 99% and 68% at pH = 1.50 ± 0.2 and 5 ± 0.2 respectively. The decrease in removal as the pH increased can be caused by the membrane charge. As mentioned previously, the isoelectric point (IEP) of NF270 is in the range 3.3–4 and it is negatively charged in solution above pH = 4 and positively charged in solution less than pH = 4. At pH = 1.50 the membrane is positive; thus the retention of these cations would be higher because of charge repulsion. Moving to pH = 5 the membrane becomes negative; thus the cations would more easily permeate the membrane and thus retention would be less. For example, retention of manganese ion at pH = 1.50 was 89% while at pH = 5 the retention decreased to 33% only (Fig. 9b) which can be considered as a big change, proving that pH plays an important role in manganese removal using NF membrane. A similar result for manganese removal in acidic medium has previously been reported [10]. Similar trends of high at pH values below the isoelectric point and low rejection at pH values above the IEP for NF membrane was reported by Wang et al. and Ballet et al.

7

[4,7]. As discussed in Section 3.2.1, the permeate flux at the IEP could be the minimum or the maximum; this discussion can also be used to explain the rejection. For example Wang et al. [4] explained the higher rejection at IEP as at this point the electrostatic interaction between the membrane and the ions was zero. Therefore the ions did not easily permeate the membrane. In this study this occurs with copper solution where the rejection at pH = 1.50 ± 0.2 and pH = 3.3 ± 0.2 was similar at 100%. However, shrinking of the skin layer of the membrane as explained by [7,28] causes a higher permeate flux and lower rejection. This happened with lead and manganese, where the rejection at the IEP was 59% and 26%, respectively. At all pH values studied, the rejection of copper was higher than for the other metals. The order of metal rejections with NF270 was Cu > Cd > Mn > Pb at pH = 1.50 ± 0.2, Cu > Cd > Pb > Mn at pH = 3.3 ± 0.2 and pH = 5 ± 0.2. Higher rejection of copper by the same membrane, NF270, was also reported by Tanninen et al. [9] and Ortega et al. [10]. With As(III) the feed pH appears to have very little effect at pH 5 and below. At pH = 1.50, 3.3, 5 and 9.30 the rejection was 12%, 12%, 13% and 5%, respectively. However, Urase et al. [6] found that the rejection of arsenite As(III) increased with the increase of pH from 50% at pH 3 to 89% at pH 10. At pH 10, most of the arsenite is in monovalent anion form (H2AsO3−) while at lower pH values most of the arsenite is in neutral solute H3AsO3 because the pKa of arsenite is 9.1. The decrease of As(III) at pH = 9.30 in this work might be attributed to the decrease of the arsenic deposition on membrane surface due to similar charge, as will be discussed later. 3.2.3. Permeate pH Differences in permeate pH relative to feed pH can give information about the passage of protons through the membrane. During the filtration of copper and cadmium solution (pH = 1.5), the permeate pH decreased to about 1.45 and 1.47 respectively, which indicates a greater passage of protons compared with Cu and Cd ions. This negative proton retention when the pH of the feed solution is lower than the isoelectric point has been studied by others [10,28]. This was explained in terms of mobility and ionic size; hydrogen ions have a higher ionic mobility and smaller size. With the other metals, Mn, Pb and As(III), and at the same feed pH, positive rejections of protons occurred (permeate pH was 1.60, 1.56 and 1.75 for Mn, Pb and As respectively). Qin et al. [29] reported the same outcome; positively charged membrane resulted in higher permeate pH. Protons did not easily pass through the NF membrane for feed pH values below the isoelectric point due to the electrostatic repulsion between the protons and the positively charged membrane surface. The results showed that the effect of pH depends mainly on the nature of the surface ionic groups of the membrane and changes of the speciation in solution which is in agreement with data reported in the literature [7]. When the feed solution has a pH of around 5, the permeate pH decreased for all metals in the range 4.50 to 4.80 except for As(III) where the permeate pH increased to 6.50. 3.3. Effect of metal concentration on NF270 performance 3.3.1. Permeate flux Fig. 10(a–e) illustrates the effect of metal concentration (100 to 1000 mg/L) on permeate flux of NF270. It is clear from this figure that the permeate flux declines with increasing concentration for all investigated metals. The explanation for this is that it might be due to: (i) decrease of the effective membrane pore size due to adsorption or deposition of solute on the membrane surface [4,30] and (ii) increase in osmotic pressure [4]. Using 2000 mg/L of each metal results a flux of 11.1 ± 4, 21.6 ± 3, 18.2 ± 3, 20.3± 0.86 and 20.6 ± 0.31 L/m 2 h for Cu(II), Mn(II), Cd(II), Pb(II) and As(III), respectively. Thus the copper was the metal that most blocked or fouled the membrane compared

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Please cite this article as: B.A.M. Al-Rashdi, et al., Removal of heavy metal ions by nanofiltration, Desalination (2012), doi:10.1016/ j.desal.2012.05.022

B.A.M. Al-Rashdi et al. / Desalination xxx (2012) xxx–xxx

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Fig. 9. Effect of feed pH on rejection of (a) Cu (II), (b) Mn (II), (c) Cd (II) and (d) Pb (II) using NF270 membrane. Experimental conditions metal concentration = 1000 mg/L, pressure = 4 bar, collected permeate = 50 g.

with other used metals. The relationship of copper concentration levels to permeate flux was almost linear. 3.3.2. Rejection The rejection of cadmium, copper, lead, manganese and arsenic at different feed concentrations (100–2000 mg/L) is shown in Fig. 11(a–e). For cadmium, copper and lead, at low concentration the rejection was higher and as the concentration of the metal solution increased the rejection decreased. NF270 can reject Cu completely at 100 and 500 mg/L and only 58% at a concentration of 2000 mg/L at 4 bar. Nevertheless the rejections at higher concentration of 1000 and 2000 mg/L have higher standard deviation 14 and 20, respectively. For cadmium and lead ions, the rejection at 100 mg/L was 83 ± 3.5% and 79 ± 4.5% and at 2000 mg/L the rejection was 57 ± 2.8% and 58 ±3.5%, respectively. The trend of decreasing rejection as metal concentration increased was also reported in the literature for the same membrane NF270 but with different metals [31]. Nevertheless, Tanninen et al. [8] found that the rejection of 25 g/L Cu using NF270 was 99.5%. In this work the rejection even for 2000 mg/L was small. This might be attributed to different experimental condition of pressure and pH. Using Mn solution, the rejection was smaller than that of Cu(II), Cd(II) and Pb(II). Approximately 100 mg/L manganese solution results only in a 45 ± 2.8% rejection. At a concentration level of 500 mg/L, the rejection decreased to 28 ± 0.71%. However from 500 to 2000 mg/L, the rejection then increased very slightly (Fig. 11b).

When arsenic solution was used the rejection was lower than all other metals studied: 7 ± 1.3% for 100 mg/L and 30 ± 3% for 2000 mg/L. As the As(III) feed concentration increased the rejection increased. The poor retention of As(III) by NF270 might be ascribed to the neutral molecules of As(III) in aqueous solution at the used pH (pH = 5). Increasing of As(III) retention as As(III) feed concentration increased was also found by Ref. [32]. Refs. [4] and [33] explained the increasing of rejection as feed concentration increased by the adsorption of the solute on the membrane surface. 3.4. Roughness of NF270 with different metals AFM images at scan area 1 μm × 1 μm for NF270 membrane filtered 1000 mg/L metals Cu(II), Cd(II), Pb(II), Mn(II), As(III) at pH = 5 and Cu(II) at pH = 1.50 are shown in Fig. 12. NF270 with different filtered metals and with different pH (copper) shows different topographies. NF270 membranes fouled with different metals (Cu, Cd, Mn, Pb and As) were scanned using AFM tapping mode in air, in order to compare their roughness with that of unused NF270 and with each other. Measurements were made at scan sizes of 1, 5, 20 and 50 μm. In order to see the effect of pH, Cu(II) was filtered at two pH values; 1.50 and 5. For NF270 fouled membranes, the data for average roughness (Ra), root mean square roughness (RMS) and surface area difference (%) are presented in Table 3. Due to the effect of image size on various roughness parameters the surface roughness, RMS and Ra should be compared for identical scan sizes. As shown in Table 3 and Fig. 13

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Cocentration (mg/L) Please cite this article as: B.A.M. Al-Rashdi, et al., Removal of heavy metal ions by nanofiltration, Desalination (2012), doi:10.1016/ j.desal.2012.05.022

B.A.M. Al-Rashdi et al. / Desalination xxx (2012) xxx–xxx

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Fig. 10. Effect of initial concentration on permeate flux of NF270 membrane with (a) Cu (II), (b) Mn(II), (c) Cd(II), (d) Pb(II) and (e) As(III). Experimental conditions: pH = 5 ± 0.2, pressure = 4 bar, 50 g permeate.

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B.A.M. Al-Rashdi et al. / Desalination xxx (2012) xxx–xxx

Fig. 12. AFM images after filtering sample 1) Pb(II), 2) As(III), 3) Cu(II), 4) Cd(II), 5) Cu(II) and 6) Mn(II), about 1000 mg/L at pH = 5 ± 0.2 except sample 3 at pH = 1.5 ± 0.2.

(RMS), at larger scan sizes (20 μm and 50 μm) the variation between images taken on the same sample was very high for some metals and the virgin membrane. Thus 5 μm × 5 μm was chosen for comparison. Table 3 also shows that as the scan size was increased the Ra and RMS values increased except for Pb(II) (which might be due to the large deviation of 26%). Some studies [34] found that the surface area increases as the roughness increases; however this trend was not observed in this study. The higher roughness parameter of virgin NF270 (Fig. 13) indicates that this membrane is expected to have relatively high fouling on its surface because foulant particles could accumulate in the valleys on the membrane surface due to higher local flux over valley regions [35–37]. Virgin NF270 membrane with image scans size 5 μm (except for As(III)), 20 μm and 50 μm has higher roughness values compared with fouled membrane. NF270 membrane fouled with Pb and Mn has the lowest observed RMS roughness. The permeate flux reduction (Table 2) at pH = 5, ranged from 23 to 36%. In contrast the membrane used to filter the 5 component mixed solution had a higher RMS value 10 ± 5 nm. Song et al. [38] and Zahrim et al. [39] reported that fouled membrane exhibits high surface roughness compared with the virgin membrane. However, it is not the case with the metals carried out in this work. In this study some surfaces fouled with metals such as copper, as shown in Tables 2 and 3, have lower surface roughness,

although their permeate flux reduction in some condition such as pH = 5 and pressure = 3 bar was high (61%). Hobbs et al. [17] found similar results; some fouled membrane shows lower average roughness although it suffered more flux decline (fouling); thus, surface area difference was used instead of RMS as a measure of surface roughness. At 5 μm, RMS value for NF270 membrane used to filter copper at pH = 1.50 and that for copper at pH = 5, the difference was insignificant. While at 20 μm, NF270 membrane used to filter copper at pH = 1.50 has higher RMS roughness (27.15 ± 8 nm) than that for copper at pH = 5 (6.79 ± 0.18 nm). This means that the pH influences the surface roughness of the membrane. At lower pH, the membrane pores might expand [28] so more metal ions may be retained within the membrane pores, thus higher RMS and permeate flux reduction (52%). 3.5. Depositions of metals on NF270 membrane The high flux nanofiltration membrane (NF270) has been found to adsorb or deposit high quantities of micropollutants, such as hormones [16] and organic compounds [30]; thus the deposition or adsorption of the used metals in this study on the surface of nanofiltration membrane (NF270) has been investigated in this work.

Please cite this article as: B.A.M. Al-Rashdi, et al., Removal of heavy metal ions by nanofiltration, Desalination (2012), doi:10.1016/ j.desal.2012.05.022

B.A.M. Al-Rashdi et al. / Desalination xxx (2012) xxx–xxx Table 3 Some surface characteristics of fouled NF270 membrane.

Pb

As

Cu, PH = 1.5 ± 0.02

Cd

Cu-pH = 5 ± 0.02

Mn

Mixed

800

Image size

RMS*

Ra*

(μm)

(nm)

SD*

(nm)

SD*

(%)

SD*

1 5 20 50 1 5 20 50 1 5 20 50 1 5 20 50 1 5 20 50 1 5 20 50 1 5

2.47 4.52 22.39 15.02 2.78 20.68 38.04 54.89 2.51 4.71 27.15 63.38 2.94 4.81 9.60 38.75 2.83 4.76 6.79 33.80 4.94 5.97 8.37 20.01 2.54 10.18

0.10 0.16 25.88 7.22 0.23 14.30 30.31 26.61 0.14 0.80 8.25 28.31 0.08 0.95 6.83 24.80 0.45 0.17 0.18 34.68 2.41 1.74 1.79 7.87 0.30 5.43

1.91 3.30 11.01 7.65 2.18 17.56 29.46 41.48 1.97 3.68 20.05 47.09 2.29 3.81 7.10 17.42 2.23 3.73 5.29 15.85 3.55 4.15 6.01 13.14 2.01 4.53

0.03 0.15 9.76 2.97 0.16 12.37 25.78 23.09 0.13 0.55 7.08 28.05 0.07 0.77 4.73 7.83 0.36 0.15 0.09 13.90 1.70 0.45 0.88 5.17 0.08 1.10

2.67 3.44 1.42 0.53 5.77 3.12 2.10 0.47 3.85 1.57 1.47 0.34 4.09 1.97 1.01 0.33 5.10 6.38 2.11 0.37 3.86 5.39 2.06 0.61 1.06 2.64

0.23 1.07 0.87 0.34 1.17 1.80 1.41 0.13 0.39 0.23 0.01 0.08 0.24 0.42 0.25 0.13 0.86 1.33 0.22 0.09 0.55 0.88 1.07 0.09 0.39 1.79

Surface area diff. %*

700

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Fouled membrane

5µm×5µm

300 200

Cu,pH=5 Cu,pH=1.5 Cd,pH=5

Pb,pH=5 Mn,pH=5

Fig. 14. The deposited amount of As(III), Cu(II), Cd(II), Pb(II), Mn(II) onto NF270 membrane surface at pH=5±0.02 and pH=1.50±0.02. Experiment condition: metal concentration=1000 mg/L, 500 mL solution and 25 cm² membrane surface area.

associated error bars for As(III) (215.5 ± 9.00 mg/m²) make it doubtful whether this could be considered a significant result. Representing this data without arsenic gives a correlation R 2 = 0.70. Comparable correlation was also demonstrated by Van der Bruggen et al. [40]. Fig. 14 also shows that the adsorption or deposition of copper onto the NF270 surface at pH = 1.50 was higher than at pH = 5, despite the membrane being positive at pH = 1.50 which suggests that repulsion will occur between copper cations and the membrane surface. According to the effect of ionic strength, as explained by Li et al. [41], the presence of nitrate groups from the acid HNO3, NO3−, reduces the repulsion between the Cu 2+ ions and the membrane surface and between the copper cations leading to an accelerated accumulation of copper on the membrane surfaces. Another suggested explanation is that at acidic conditions the membrane shows greater stretching; this has been speculated to cause greater “pore” opening and easier penetration of metal aggregates [42]. The relation between metal rejections and the adsorbed or deposited amount of the metals onto the NF270 membrane surface is shown in Fig. 16. It appears that as the metal deposition within or onto the membrane surface increased, the rejection increased. This

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Fig. 14 presents the adsorbed amount, Q (mmol/m²) of metals Cu(II), Mn(II), Pb(II), As(III) and Cd(II) at pH = 5 ± 0.2 and at pH = 1.50 ± 0.2 for Cu(II). The adsorption order of the used metals at pH = 5 onto NF270 membrane is Cu 2+ > Cd 2+ > As 3+ > Mn 2+ > Pb 2+. Investigation of the relation between the adsorbed amount and atomic mass of the used metals shows no correlation between them. Similar results were also reported by Van der Bruggen et al. [40]. However, lead (Pb), with a larger atomic mass, has a lower adsorption and less fouling. On the other hand correlation between the adsorbed amount of the metals onto NF270 with the normalised flux (i.e. the permeate flux (Jv) divided by the pure water flux (J0)), is given in Fig. 15. The correlation shows that as the adsorption increased the normalised flux decreased, except for arsenic which has a higher deposited amount and higher flux. However the large

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14

B.A.M. Al-Rashdi et al. / Desalination xxx (2012) xxx–xxx

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relation can be correlated to a linear function with R 2 = 0.86. Nghiem and Schäfer [43] stated that if the pore size of the membrane is larger than solute molecules, breakthrough of the rejection can be observed when the membrane adsorption sites are saturated. Semiao and Schafer [16] found that adsorption onto NF membrane caused lower retention when steric interactions take place. However, adsorption or deposition of the materials onto the membrane is highly dependent on the membrane material used and the filtered material properties such as size, hydrophobicity, acid dissociation constant (pKa), ability for hydrogen bonding and other possible interaction mechanisms [16]. For arsenic (III) the uncharged and small arsenic(III) easily pass through the loose NF270. Thus sieving or size exclusion and charge exclusion mechanism with this metal was unexpected. However some of the As(III) may be retained within the membrane valleys which might explain the rejection mechanism of NF270 for As(III) and which is indeed very low. Calculating the removal of arsenic by adsorption onto NF270 gives 10% and that actually the range of the arsenic(III) rejections that was found in this study. Evaluation of the results of deposited or adsorbed amount of each metal onto NF270 surface with the results of the surface roughness (Table 3) demonstrates that membrane that fouled with manganese and lead and which has lower surface roughness, has lower adsorbed amounts of metal. The metals that were highly adsorbed by NF270 may lead to a higher measured surface roughness.

Fig. 16. The effect of adsorbed/deposited amount of some metals onto NF270 membrane on their rejections.

Zahrim et al. [44] reported that membrane fouling can be classified as reversible and irreversible. Reversible fouling can give a flux reduction of up to 18%, whereas irreversible fouling can produce permeate flux reduction of 26–46%. Concentration polarisation, gel layer formation and osmotic pressure are the expected mechanisms for reversible fouling while cake layer formation, adsorption and pore blocking are the known mechanisms for irreversible fouling. Based on this criteria, the permeate flux reduction of the used metals as represented in Table 2 shows that both types of reversible (permeate flux reduction ≤ 18%) and irreversible fouling (permeate flux reduction = 26–46%) were present. Also, based on the mechanisms of irreversible fouling described by Zahrim et al. [44], the deposition results of the used metals, such as copper and cadmium onto NF270 as described above gives indication of irreversible fouling. Moreover, the membrane properties, the operating parameters and the type of wastewater can be the main factors which affect membrane fouling [44]. As well as cake filtration a study by Wang and Tarabara [45] showed that 53% of flux declines during the first stage of filtration can be attributed either to intermediate blocking of relatively more permeable areas of the membrane skin, or to cake formation or a combination of both mechanisms. The membrane used in this study, NF270, was previously characterised [13] as having a membrane pore size of 0.40 nm, which might be blocked to some extent depending on the ions or the particles found within the used metal solution.

3.6. Multi-component solutions 3.6.1. Permeate flux For both multi-component solution (Cu 2+, Mn 2+, Cd 2+, Pb 2+ and 3+ As ) prepared using tap water or Milli-Q water, the flux was slow at less than 4 L/m 2 h. Flux reduction reached 90% as compared with water alone. This observed flux reduction is also very high compared with that for each single metal solution (Table 2). This pronounced decrease in flux might be attributed to cake layer formation and adsorption of the metal hydroxide species on the membrane surface [46]. In addition Van Der Bruggen et al. [47] and Ozaki et al. [22] found that the flux reduction can be ascribed to osmotic pressure: high ionic concentrations have a strong influence on osmotic pressure, since a large part of the ions are retained by the membrane. The concentration difference on both sides of the membranes gives rise to an osmotic pressure.

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B.A.M. Al-Rashdi et al. / Desalination xxx (2012) xxx–xxx

3.6.2. Retention of the metals The effect of 5 component (metal) concentrated mixtures was investigated. In order to mimic the real solution tap water was used to prepare a mixture solution. Fig. 17 illustrates the removal of Cu, Cd, Mn, Pb and As from their mixtures prepared using tap water and Milli-Q water with approximately 1000 mg/L of each metal. The measured pH of both solutions was about 2.35. A difference in retention between the single solution of each metal and their mixture was evident. The retention of copper and lead in single solution was 100% and 69% respectively, whereas with the 5 metal solution the retention decreased to 40% and 5.8%, respectively; the difference was about 60%. Also the retention of Cd decreased but the difference was about 19%. On the other hand, the retention of Mn in both single and mixed solutions was similar. The different rejection observed between multicomponent solution and single solution was also reported by other authors [26,29]. On the other hand, Qdais and Moussa [48] found that the efficiency of NF membrane was similar for both single solution of copper (200 mg/L) and cadmium (200 mg/L) and a mixture of both. However the total concentration of both metals in the mixture was 500 mg/L. To the best of the authors' knowledge, no other study of high concentrated multicomponent solutions using NF membrane has been reported. Fig. 17 also shows that retention of the metals was slightly dependent on the water used. Copper, manganese, cadmium and lead retention differences were only 4%, 8%, 5%, and 6% respectively. For copper and lead their rejections using tap water were higher than using Milli-Q water. The reverse was observed for manganese and cadmium. Ozaki et al. [22] found that the rejection of heavy metals tends to decrease as the feed concentration of other ions increased. They explained this decrease in rejection in terms of the Donnan effect; negative anions present in the feed solution can easily pass through the membrane and counter ions are also forced to pass through the membrane to maintain electro-neutrality around the membrane, and in terms of osmotic pressure that increased due to the presence of other co-ions. This explanation leads to the suggestion that the concentration of the co-ions present in tap water was small or/and their mobility was high; thus their effects were small. Contrarily Sato et al. [49] found that the removals of arsenic by NF membranes were not affected by the source of water composition.

80

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4. Conclusions In this study and with all used metals (except arsenic), when the feed pH was lower than the isoelectric point of the NF membrane the rejection was higher due to electrostatic repulsion and/or adsorption/deposition of metal onto the membrane. These indicate that Donnan exclusion and/or adsorption mechanisms play an important role. NF270 membrane rejected almost 100% of copper ions in 1000 mg/L copper solution with all used pressures (3–5 bar) and pHs (1.50–5) which demonstrates the suitability of NF270 for copper rejection. However NF270 membrane rejection effectiveness decreased as the concentration of copper concentration reached 2000 mg/L. Using 1000 mg/L level concentration at pH = 1.50 and 4 bar pressure, NF270 removed about 99%, 89% and 74% of cadmium, manganese and lead, respectively. NF270 is a loose membrane; thus it failed to reject As(III). With all metals used the permeate flux increased with the rise of pressure. With Cd(II) and As(III) the permeate flux remains linear with increasing pressure, which indicates insignificant concentration polarisation. With copper, lead and manganese metals the increase in flux was not linear with the rise of pressure, which shows that a quantity of concentration polarisation may have occurred. With increasing the pressure, results increased or decreased metals rejections. However, all these changes in rejection were small. The reduction of permeate flux might be attributed to a number of causes, such as adsorption of soluble metal hydroxide species on the membrane surface, formation of a cake layer of metal hydroxide precipitate, concentration polarisation, and osmotic pressure. The order of the metals which caused a flux decline due to NF270 fouling was Cu 2+ > Cd 2+ ≈ Mn 2+ > Pb 2+ ≈ As 3+. The correlation between adsorbed amounts of the metals onto NF270 with the normalised flux shows that as the amount increased the normalised flux decreased, except for arsenic that has higher deposited amount and higher flux. The difference in NF270 retention between the single solution and multicomponent solution was about 60% for both copper and lead, 19% for cadmium and similar for manganese. The type of water used to prepare the 5 components solution had a slight effect on the metal retentions. Difference in retention for copper, manganese, cadmium and lead was only 4%, 8%, 5%, and 6% respectively.

Acknowledgments

75 72 67

70

67 Tap Water

60

B.A.M. Al-Rashdi would like to thank the Ministry of High Education in Sultanate of Oman for funding her PhD.

MilliQ Water

% Retention

References

50

44 40

40 30 20 10

5.8 0.01

0.99 0.62

0 Cu(II)

Cd(II)

Mn(II)

Pb(II)

As(III)

Fig. 17. Retention of Cu, Cd, Mn, Pb and As from their mixtures prepared using tap water and Milli-Q water, 1000 mg/L of each metal, pH = 2.35 ± 0.2.

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