Stability of NF membranes under extreme acidic conditions

Stability of NF membranes under extreme acidic conditions

Journal of Membrane Science 239 (2004) 91–103 Stability of NF membranes under extreme acidic conditions Samantha Platt a , Marianne Nyström a,∗ , Ald...

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Journal of Membrane Science 239 (2004) 91–103

Stability of NF membranes under extreme acidic conditions Samantha Platt a , Marianne Nyström a,∗ , Aldo Bottino b , Gustavo Capannelli b a

Department of Chemical Technology, Lappeenranta University of Technology, P.O. Box 20, 53850 Lappeenranta, Finland b Department of Chemistry and Industrial Chemistry, University of Genoa, Via Dodecaneso 31, 16146 Genoa, Italy Received 28 February 2003; received in revised form 11 September 2003; accepted 15 September 2003 Available online 12 May 2004

Abstract Two commercial nanofiltration (NF) membranes (FilmTec NF-45 and Desal-5 DK) and two new NF-1 membranes made by BPT (Bio Pure Technology) for the purpose of a European Union funded research project (RENOMEM) were tested under extreme acidic conditions. The polyethersulphone (PES) ultrafiltration (UF) supports used for casting the BPT-NF-1 membranes were also tested under similar conditions. The 006 and 015 UF supports were found to be stable in 5% nitric acid at 20 and 80 ◦ C for 4 and 3 months, respectively. Both supports (006 and 015) showed a significant reduction in flux after immersion in sulphuric acid at both temperatures. The BPT-NF-1 membranes showed excellent resistance to 20% sulphuric acid for up to 4 months at 20 ◦ C but were attacked by the nitric acid solution. The resistance of the two commercial membranes in 20% sulphuric acid at 20 ◦ C was generally lower than that of the BPT-NF-1 membranes. The NF-45 membrane was slightly more stable in 5% nitric acid at 20 ◦ C. Degradation of the membrane occurred only after 2 months while both the Desal-5 DK and BPT-NF-1 membranes degraded during the first month. At the higher temperature of 80 ◦ C in 5% nitric acid all membranes degraded in the first month. The cause of membrane degradation was attributed to oxidation of the thin NF selective skin layer in nitric acid and to acid-catalysed hydrolysis of this layer in sulphuric acid. Knowing the cause of membrane degradation is a step forward in developing a better and more stable nanofiltration membrane. © 2004 Elsevier B.V. All rights reserved. Keywords: Nanofiltration; Acid stability; Metal industry; Composite membranes

1. Introduction Industrial effluents are the purges from manufacturing processes and carry the load resulting from any process upset (harshest conditions occur) or malfunction. There has been slow progress in realising the potential of membrane processes in effluent treatment [1]. Industrial effluents are extremely variable. Usually, complete chemical analyses of the effluents are not available, particularly with respect to trace elements. As a result each industrial effluent must be assessed for the suitability of membrane processes as a treatment technique [2]. 1.1. Metal industry Recovery of acids from metal treatment wastes is one of the beneficial recycling technologies. Many metal finishing



Corresponding author. Tel.: +358-5-6212160; fax: +358-5-6212199. E-mail address: [email protected] (M. Nyström).

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2003.09.030

processes involve treating metal with acids for stripping, cleaning and pickling. Various pickling agents are used such as sulphuric acid, hydrochloric acid and nitric acid mixtures. Typical metals contained in the spent pickling agents are iron, chromium, copper, nickel and zinc. The acid solution containing high concentrations of metal ions is then discarded or free acid is recovered by the removal of metal ions [3]. 1.1.1. Applications • The removal of copper and gold from process streams produces a barren (solution chemically stripped of metal values) solution containing high amounts of heavy metals which cannot be disposed of safely. Although the filtered barren solution meets environmental regulations the filter cake is contaminated with selenium and must be disposed of using costly procedures. The use of nanofiltration for removal of selenium from the barren solutions has been investigated but the use is limited due to high concentrations of sulphuric acid [4].

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• The effluents from rinsing after metal etching operations contain two recoverable ingredients—acids and water. The recovery of acids requires that they be separated from the metal salts and concentrated. The sort of concentrations expected are 1–2 g/l HCl or H2 SO4 [5], pH 2.7 HCl or H2 SO4 [6] and pH 2.3–2.6 HCl or H2 SO4 [7]. • One of the critical pollution problems arising from the electroplating industry is the generation of large amounts of rinse water from electroplated parts. The used rinse water consists of many heavy metals and chemicals and can be detrimental to the health of biological organisms when exposed to the environment. However, the rinse water can be reused if the metals can be removed and collected. The typical pH of electroplating rinse waters is 3–7. 1.2. Mining industry Ammonium and nitrate ions from mine effluents can be removed by nanofiltration. Other ions can also be removed from mine effluents using filtration. However, there are limitations to the use of this method due to acid and caustic concentrations. Awadalla and Kumar [4] investigated the use of nanofiltration membranes for filtering acid mine drainage (pH 4–4.5) and scrubber blow-down water (pH 4.5). 1.3. Dairy industry In the food and dairy industries the majority of the membrane applications are at a pH of 5.5–6. However, cleaning the membrane materials for both industries involves the membranes being subjected to both caustic and acidic conditions. Therefore, high and low pH (pH 2–12) effluent values exist [8].

phuric acid from decontamination effluents by means of filtration is possible [14]. In general all of the spent acid solutions discussed so far have been for quite mild conditions. At these conditions there are membranes available that can be used to purify the waste streams. In most cases membranes are already being used. However, there are spent acid solutions generated in the metal and mining industries that have acid concentrations which existing membranes may not tolerate: • The effluents from steel pickling liquors contains three recoverable ingredients—acids, salts and water. The recovery of acids requires that they can be separated from the metal salts and concentrated. The sort of acid concentrations expected are 60 g/l sulphuric acid [15], 120–130 g/l nitric acid and 30 g/l hydrofluoric acid [16], and 107 g/l nitric acid and 19.3 g/l hydrofluoric acid [17]. • Visser et al. [18] investigated the nanofiltration of mining solutions in the pH range 1.8–4. They showed that commercially available NF membranes for the removal of sulphate ions from acid mine water were excellent at neutral pH. However, if the pH was decreased using sulphuric acid their performance decreased drastically. The aim of this paper is to determine if there are nanofiltration membranes available to withstand extreme acidic conditions and to also test an acid-resistant NF membrane designed for the purpose of this project. Extreme conditions in this case refers to an acid concentration of 5–20% and a temperature of 80 ◦ C. A temperature of 20 ◦ C is also used. This is not an extreme temperature but it allows for the effect of concentration and temperature to be investigated separately. Also an attempt to determine the cause of membrane degradation will be made.

2. Materials and methods 1.4. Pulp and paper industry 2.1. Materials Paper mills use huge amounts of water in their paper manufacturing processes. The effluent generated from the processes can be acidic. Typical pH values are 4.9–5.4 [9,10]. Acidic bleaching effluents can have a pH of 2 [11]. 1.5. General wastewater treatment 1.5.1. Applications • Dumpsite leachate is a direct consequence of rainfall. The average leachate production of a dumpsite in western Europe is about 5 m3 per day but during rainy periods this figure can be exceeded by a factor of 3–4. Leachate water consists of a complex mixture of components of which the pH ranges between 6.2 and 6.8 [12,13]. • Treatment of decontamination effluents is an inherent part of reactor-dismantling studies. Removal of 90% 1 M sul-

The main physical and chemical characteristics of all membranes used in this work are shown in Table 1. The BPT-NF-1 membranes were formed by depositing a thin skin layer (melamine polyamine) onto FuMA-Tech UF supports (types 006 and 015). The NF-45 and Desal-5 membranes were tested for comparison to assure that the new membranes had a better acid resistance, even though they have not been reported to be acid-resistant membranes. The acids (nitric acid and sulphuric acid) were purchased from J.T. Baker (Holland) and the purity was 96–98%. Ion-exchange water was used to measure the clean water flux and for sucrose and glucose (purchased from Fluka, Switzerland) retention experiments. The equipment was also rinsed with ion-exchange water to avoid errors in the TC (total carbon; model: SFS-ISO 8245) readings.

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Table 1 Chemical and physical characteristics of ultrafiltration and nanofiltration membranes supplied by the manufacturers Membrane property

Ultrafiltration 006

Manufacturer Support material Surface material Temperature resistance (◦ C) Retention (%) NaCl Glucose Cut-off (g/mol) pH (20 ◦ C) resistance

Nanofiltration 015

BPT-NF-1 006

015

NF-45

Desal-5 DK

FuMA-Tech (Germany) PES – 5–60

FuMA-Tech (Germany) PES – 5–60

BPT (Israel) PES MP –

BPT (Israel) PES MP –

FilmTec (USA) – PA –

Osmonics (USA) Proprietary Proprietary Tmax = 90

– – 6000 1–13

– – 15000 1–13

– 95–100 – –

– 95–100 – –

45 – – 2–11

49 93 150–300 2–11

PES: poly(ether)sulphone; PA: aromatic polyamide; MP: melamine polyamine.

2.2. Methods 2.2.1. Immersion experiments The BPT NF membranes, designated as BPT-NF-1, and UF supports were immersed in acidic solutions (5% nitric acid at 20 and 80 ◦ C; 12 and 20% sulphuric acid at 20 and 80 ◦ C) for a period ranging from 1 to 4 months depending on the membrane’s stability in these solutions. A membrane was immersed for each month, which resulted in a different membrane piece being characterised for each experiment. The results were compared with 15 virgin membranes taken from the same batch. In the case of the commercial membranes Desal-5 DK and NF-45 the same piece was used through out the experiments. At the end of each month the membrane piece was removed for characterisation and then returned to the immersion vessel for another month if its retention was not completely lost. 2.2.2. Characterisation tests 2.2.2.1. Flux and retention measurements. At the end of each immersion month a membrane piece was characterised by measuring the clean water flux. In the case of the NF membrane, sucrose and glucose retentions were measured. The equipment used for flux and retention measurements is shown in an earlier article [19]. The rig consisted of a 10 l feed vessel, two pressure gauges (0–20 bar) on the inlet and outlet of the membrane modules, three membrane modules in parallel and a flowmeter on the feed side. The membrane modules were made of stainless steel giving a surface area of 0.0216 m2 . The feed flow was tangential to the membrane, typical of crossflow filtration. The membranes were first stabilised with water at a pressure of 15 bar. The clean water flux was measured at 5, 10 and 15 bar. The water feed was then switched to 500 ppm sucrose or glucose solution. All characterisation runs were performed at 40 ± 0.5 ◦ C and lasted for 60 min. The feed velocity at the membrane surface during NF tests was 2 m/s and the pressure was 15 bar. The tests were conducted by completely recycling both the

permeate and retentate to the feed vessel. Small samples of the feed and permeate were taken every 30 min to determine the glucose or sucrose concentration. The observed retention coefficient, R, was calculated using the following equation: R=1−

Cp Cf

(1)

where Cp , and Cf are the solute concentrations in the permeate and feed, respectively. On completion of each run the rig was flushed with ion-exchange water (at 40 ± 0.5 ◦ C) for at least 10 min. Strictly speaking the results obtained in this way do not correspond to the true values of the observed retention. The error becomes more pronounced the higher the volume reduction is. Since the volumes taken off are negligibly small in comparison with that of the feed, the volume reduction (0.1%) can be supposed to be nearly equal to zero [20]. 2.2.2.2. Surface examination. After completion of the immersion tests, each test piece was examined visually for signs of discolouration, surface attack and cracking. The methods used for this analysis are listed below. Discolouration. The samples were checked visually for a change in colour. Microscopy analysis. The membranes were first washed in de-ionised water and then vacuum-dried at room temperature. The samples were placed in a solution of 1% OsO4 for 15 h, then washed in de-ionised water again followed by vacuum drying at room temperature. The samples were then prepared for scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis. SEM-EDS (scanning electron microscope with an EDS probe) analysis. Membrane cross-sections were prepared by cold (liquid nitrogen) fracturing. Membrane surfaces could be prepared by coating them with gold or carbon. Carbon was used since it makes osmium, which was anal-

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ysed, easier to observe using the EDS probe (Oxford, Link Pentafet) of the SEM (Leo, Stereoscan 440).

(from 1 to 4 months) in 5% nitric acid and 12–20% sulphuric acid at temperatures of 20 and 80 ◦ C. It can be seen that neither membrane was stable in 20% sulphuric acid for even 1 month at either temperature (20 or 80 ◦ C). In 12% sulphuric acid at 20 ◦ C the NF-45 membrane was still stable after 2 months of immersion while the Desal-5 DK membrane had a higher flux and a lower glucose retention after only 1 month of immersion. The NF-45 membranes immersed in 5% nitric acid at 20 ◦ C survived 3 months of immersion. However, at 80 ◦ C the membranes suffered a complete loss in retention during the first month. There was an increased flux and a loss in sucrose and glucose retention for the Desal-5 DK membranes after 1 month of immersion at both temperatures (20 or 80 ◦ C). Tables 4 and 5 show the flux and retention results for BPT-NF-1 membranes before and after immersion in 5% nitric acid and 12–20% sulphuric acid at temperatures of 20 and 80 ◦ C. It can be seen from these results that neither membrane was stable in 5% nitric acid for even 1 month at either temperature (20 or 80 ◦ C). In 20% sulphuric acid at 20 ◦ C both membranes were still stable after 4 months of immersion. Also both membranes suffered a loss in glucose and sucrose retention accompanied by a large flux increase after only 1 month in 20% sulphuric acid at 80 ◦ C. Table 6 refers to the UF supports. It can be seen from this table that the clean water fluxes for both membrane support

TEM-EDS (transmission electron microscope with an EDS probe) analysis. A direct replica of the surface was created in order to observe the thin and transparent (at 200 kV) skin layer. Thin layers of carbon, shadowed with platinum–palladium, were deposited on the skin layer surface. The membrane was then immersed in N-methylpyrrolidone (NMP) or dimethyl sulphoxide (DMSO), in order to dissolve the PES porous support, leaving a free thin (skin) layer to be directly observed under the TEM (Jeol, Jem-2010). Cross-sections were obtained by embedding the membrane into epoxy resin and cutting a thin film by using the diamond knife of the ultramicrotome, cooled by liquid nitrogen. Contact angle measurements. Contact angles were measured using the drop method, in which a water drop was placed onto the membrane, using a syringe, and the angle measured.

3. Results Tables 2 and 3 show the flux and retention results for the commercial NF membranes before and after immersion Table 2 Characterisation results for 0–4 months for NF-45 membranes Sucrose retention (%) after months

Glucose retention (%) after months

Permeability (l/h m2 bar) after months

0

1

2

4

0

1

2

4

0

80 20

81 94

0 90

89

35

78 88

0 84

83

35

5.3 5.1

13 5.3

3.9

9.1

20% H2 SO4

80 20

83 80

0 72

49

2

79 77

0 68

45

2

5.8 6.9

26 7.2

8.2

19.2

12% H2 SO4

20

86

91

86

86

89

86

10.4

10.2

H2 O

80 20

94 79

91 76

92 82

88 76

89 71

88 73

2.7 8.1

2.8 8.4

Immersion solution

5% HNO3

Temperature (◦ C)

92 78

1

10 88 75

2.5 8

2

4

2.9 8

The maximum error between readings was ±3% for retention and ±7% for permeability. Table 3 Characterisation results for 0–4 months for Desal-5 DK membranes Sucrose retention (%) after months

Glucose retention (%) after months

Permeability (l/h m2 bar) after months

0

1

2

0

1

2

0

1

2

80 20

95 91

0 71

58

70 52

0 35

33

12.1 11.3

90.3 18

28.6

80 20

89 91

0 27

28

47 56

0 16

15

11.4 11

33.1 16.2

14.9

12% H2 SO4

20

95

95

90

58

8.4

12.9

H2 O

80 20

93 95

93 96

52 56

51 56

12.2 12.5

14.7 12.6

Immersion solution

5% HNO3 20% H2 SO4

Temperature (◦ C)

92 97

4

60 95

51 58

The maximum error between readings was ±3% for retention and ±5% for permeability.

4

28 57

17.4 12.9

4

23.6 11.7

Table 4 Characterisation results for 0–4 months for BPT-NF-1 (type 015) membranes Temperature (◦ C)

5% HNO3

80 20

88–97 88–97

3.5–10 74.4–76

80 20

88–97 88–97

8.1–16.2 90.8–92.9

80 20

90 91

20% H2 SO4 H2 O

Sucrose retention (%) after months 0

1

91 90

2

3

4

0

59.3 71.8

54–58.8

41–43

78–93 78–93

1.7–3.5 50.8–51

87.3–88

82.2–83

78–93 78–93

6.2–9.8 82.7–87.5

90 92

90 91

85 89

6.1–9.9 91.8–92 90 92

Permeability (l/h m2 bar) after months

Glucose retention (%) after months 1

2

84 88

42.5–53

3

4

0

1

2

3

4

37.8–39

19.7–20

1.8–4.9 1.8–4.9

120–421 16.3–19.9

17–23.5

26.7–27.7

29–30

41.3–54.6 4.7–4.8

85.8–129 4.8–4.7

5.4–5.3

5.3–5.4

2.8 2.2

2.8 2.2

2.9 2.2

2.8 2.2

1.6–5.9 85.4

78.9

70.2

1.8–4.9 1.8–4.9

85 89

85 89

84 89

2.2 2.2

For 5% HNO3 and 20% H2 SO4 two different membranes were characterised each month and 15 different untreated membranes were characterised to obtain results for month 0. For the 15 membranes characterised 28.6% had a permeability between 1.8 and 2.9 l/h m2 bar, 14.3% had a permeability between 3.5 and 3.9 l/h m2 bar and 57.1% had a permeability between 4 and 4.9 l/h m2 bar. In the case of water the same membrane was characterised each month including month 0. The values recorded are the average of three experiments and the calculated error is ±3% for retention and ±5% for permeability.

Table 5 Characterisation results for 0–4 months for BPT-NF-1 (type 006) membranes Immersion solution

Temperature (◦ C)

Sucrose retention (%) after months 0

5% HNO3

1

2

Permeability (l/h m2 bar)

Glucose retention (%) after months 3

4

0

1

2

3

4

0

1

2

3

4

80 20

83–91.5 83–91.5

4.3–7.3 78.5–82.5

76.7–87.2

53.5–65.8

39.2–40.2

75–88 75–88

1.5–3.4 71.1–76

65.6–79.4

43–54.9

31.3–33

1.7–3.9 1.7–3.9

87.1–120 3.3–3.8

3.2–6.2

7.4–12.5

12.4–13.1

80 20

83–91.5 83–91.5

28.5–30.4 81.5–82

8.6–12.9 83.9–84.1

83.7–84.2

85.2–86

75–88 75–88

14.9–19.8 77.7–82

8–8.9 81–82.1

81–82.2

82–82.8

1.7–3.9 1.7–3.9

23.2–27.3 3.0–3.2

60.8–65 3.0–3.1

3.0–3.2

3.0–3.1

12% H2 SO4

20

96

95

95

90

2.2

2.3

2.3

4.3

H2 O

80 20

90 92

90 91

90 90

91 91

2.5 2.5

2.5 2.5

2.5 2.4

2.5 2.5

20% H2 SO4

90 92

92

90

90

82

85 88

85 88

84 87

85 86

83 87

S. Platt et al. / Journal of Membrane Science 239 (2004) 91–103

Immersion solution

2.5 2.5

For 5% HNO3 and 20% H2 SO4 two different membranes were characterised each month and 15 different untreated membranes were characterised to obtain results for month 0. In the case of water and 12% H2 SO4 the same membrane was characterised each month including month 0. The values recorded are the average of three experiments and the calculated error is ±3% for retention and ±5% for permeability.

95

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types (006 and 015) were stable in 5% nitric acid at both temperatures for at least 4 months (the test period investigated). In the case of 20% sulphuric acid there appeared a significant decrease in flux for both membrane support types (006 and 015) at 20 ◦ C. At the higher temperature of 80 ◦ C a large flux decrease was observed for the 015 support and only a slight decrease for the 006 support. It is important to mention that the changes in flux for each month were compared with the initial clean water fluxes of a set of totally different membranes and therefore, small changes in flux could be due to this and not because of any membrane degradation. Fig. 1 shows the images of the BPT-NF-1 membranes and the UF supports after Osmium treatment. Generally, the following was observed: • The osmium did not adhere to any of the UF supports (006 and 015) and therefore no colour change after osmium treatment for either immersed membranes or virgin membranes was observed. • The osmium did adhere to some of the NF membranes (prepared on the UF supports) and this was indicated by a colour change. ◦ Generally, the colour change was stronger for the BPT-NF-1 membrane type 015. ◦ The virgin NF membranes always produced a colour change after osmium treatment. This showed that the NF modification made to the UF supports was what the osmium adhered to. ◦ The NF membranes always showed a colour change after osmium treatment if the membranes had been im-

mersed in 20% sulphuric acid at either temperature. However, the colour change was stronger for the membrane immersed at the lower temperature. Taking the results for the virgin membrane into consideration this indicated that the NF skin layer modification was thinner on the membranes as the temperature of the acid increased. ◦ No colour change occurred after osmium treatment of the NF membranes immersed in 5% nitric acid at either temperature. This showed that the NF skin layer no longer existed (since the osmium did not adhere to the UF support). Tables 7–9 show the results obtained from the visual inspection of the membranes. The SEM (Fig. 2) and TEM images (not reported here) for the UF supports (Table 7) showed no signs of chemical attack, no change in flux and no change in strength. The only change observed was that the membranes were slightly yellow (instead of white) after immersion in 5% nitric acid at both temperatures (20 and 80 ◦ C). SEM and TEM observations of the osmium-treated BPT-NF-1 membranes confirmed the initial findings shown in Fig. 1. The TEM images (Fig. 3) reveal that the thick osmium layer present on the virgin membranes was thinner on the membranes immersed in 20% sulphuric acid and disappeared for the membranes immersed in 5% nitric acid. It was considered that these changes in the skin layer were responsible for the increase in the flux and the decrease in glucose and sucrose retention, especially at higher acid concentrations and temperatures. However, these changes did not affect the mechanical strength of the membrane. A colour

Fig. 1. Photos ofBPT-NF-1 membranes and FuMA-Tech UF supports (006 and 015 series) after Osmium treatment.

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Table 6 Characterisation results for 0–4 months for ultrafiltration supports (types 006 and 015) used in the BPT-NF-1 membranes Immersion solution

5% HNO3 20% H2 SO4

Permeability (l/h m2 bar) support 006 after months

Temperature (◦ C)

Permeability (l/h m2 bar) support 015 after months

0

3

4

0

3

4

80 20

151–188 151–188

152–153

168–173 159–161

203–285 203–285

270–280 267–280

280–315

80 20

151–188 151–188

146–150 135–137

137–140 128–130

203–285 203–285

130–134 195–197

137–139 193–194

Table 7 Visible changes and possible causes for change in the membrane physical and chemical properties after immersion in 20% sulphuric acid and 5% nitric acid at 20 and 80 ◦ C Membrane type

Solution concentration

Temperature (◦ C)

Colour change Flux after immersion changes

Colour change after Os treatment

Strength test (visual results)

SEM and TEM

Possible causes

UF Fuma-Tech 006

5% HNO3

20

Slight yellow

NC

NC

NC

80

Slight yellow

NC

NC

NC

No signs chemical No signs chemical

of attack of attack

Yellowing due to aromatic structure Yellowing due to aromatic structure

20

NC



NC

NC

NC



NC

NC

of attack of attack

Cross-linking/NC

80

No signs chemical No signs chemical

20

Slight yellow

NC

NC

NC

80

Slight yellow

NC

NC

NC

No signs chemical No signs chemical

of attack of attack

Yellowing due to aromatic structure Yellowing due to aromatic structure

20

NC



NC

NC

NC



NC

NC

of attack of attack

Cross-linking/NC

80

No signs chemical No signs chemical

20% H2 SO4

UF Fuma-Tech 015

5% HNO3

20% H2 SO4

Cross-linking/NC

Cross-linking/NC

NC, no change.

Fig. 2. SEM images of FuMA-Tech supports (015 series) before (a) and after immersion in sulphuric acid (b) or nitric acid (c) solutions.

Fig. 3. TEM images of BPT-NF-1 membrane (015 series) before (a) and after immersion in sulphuric acid (b) or nitric acid (c) solutions.

20% H2 SO4

5% HNO3

12% H2 SO4

NC

NC

80

Yes

80

20

Yes

20

NC

NC

80

20

NC

Yes

80

20

Yes

Colour change after immersion

20

Temperature (◦ C)

−−−

−−−

+++

+++

−−

++

NC



+

NC

−−−

+++

−−−

+++

NC

−−

++

NC

Retention

Flux

Change in

NC

NC

Yes

Yes

NC

NC

NC

Yes

Yes

Colour change after Os treatment

+, slight increase; ++, increase; +++, large increase; −, slight decrease; −−, decrease; −−−, large decrease; NC, no change.

BPT-NF-1 015

5% HNO3

BPT-NF-1 006

20% H2 SO4

Solution concentration

Membrane type

NC

NC

NC

NC

NC

NC

NC

NC

NC

Strength test (visual results)

No signs of chemical attack No signs of chemical attack

Surface layer missing but no signs of chemical attack Surface layer missing no signs of chemical attack

No signs of chemical attack

No signs of chemical attack No signs of chemical attack

Surface layer missing but no signs of chemical attack Surface layer missing but no signs of chemical attack

SEM and TEM

Acid-catalysed hydrolysis

NC

Oxidation

Oxidation

NC/acid-catalysed hydrolysis

Acid-catalysed hydrolysis

NC

Oxidation

Oxidation

Possible causes

Table 8 Visible changes and possible causes for change in BPT-NF-1 membrane physical and chemical properties after immersion in 5% nitric acid and 12 and 20% sulphuric acid at 20 and 80 ◦ C

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Table 9 Visible changes and possible causes for change in commercial NF membranes physical and chemical properties after immersion in 5% nitric acid and 12 and 20% sulphuric acid at 20 and 80 ◦ C Membrane type Solution concentration

Desal-5 DK

Change in Flux

Retention −−

Strength test (visual results)

SEM and TEM

Possible causes

NC

No signs of chemical Oxidation attack No signs of chemical Oxidation attack

20

Yes

++

80

Yes

+++ −−−

NC

20

NC

++

NC

80

NC

+++ −−−

NC

12% H2 SO4

20

NC

+



NC

No signs of chemical Acid-catalysed hydrolysis attack

5% HNO3

20

Yes

++

−−

NC

80

Yes

+++ −−−

NC

No signs of chemical Oxidation attack No signs of chemical Oxidation attack

20

NC

+++ −−−

NC

80

NC

+++ −−−

NC

20

NC

NC

NC

5% HNO3

20% H2 SO4

NF-45

Temperature Colour change (◦ C) after immersion

20% H2 SO4

12% H2 SO4

−−

NC

No signs of chemical Acid-catalysed hydrolysis attack No signs of chemical Acid-catalysed hydrolysis attack

No signs of chemical Acid-catalysed hydrolysis attack No signs of chemical Acid-catalysed hydrolysis attack No signs of chemical NC attack

+, slight increase; ++, increase; +++, large increase; −, slight decrease; −−, decrease; −−−, large decrease; NC, no change.

Table 10 Membrane weight, thickness and contact angle change results after immersion in 20% sulphuric acid at 20 and 60 ◦ C for different membrane types Membrane

Desal-5 DK NF-45 BPT-NF-1 (006)

Contact angle after weeks

Weight (g) after weeks

0

4

7

0

4

7

0

4

7

17 28 33

27 33 31

33 39 37

0.457 0.337 0.594

0.460 0.339 0.635

0.457 0.338 0.638

0.187 0.125 0.325

0.184 0.126 0.324

0.185 0.125 0.322

change was observed for all the membranes immersed in nitric acid at both temperatures (20 and 80 ◦ C). The SEM and TEM images for the commercial NF membranes (Table 9) showed no signs of chemical attack, there was a large change in flux and a decrease in sucrose and glucose retention, and the strength tests showed no change. Also, the membranes were yellow after immersion in 5% nitric acid at both temperatures (20 and 80 ◦ C). Table 10 compares the contact angle, weight and thickness values for all types of NF membrane before and after consecutive immersion (4 and 7 weeks) in 20% sulphuric acid at 20 ◦ C. For all membrane types neither weight change nor membrane thickness change was observed over a period of 7 weeks. With regards to the contact angle measurements it is apparent that the Desal-5 DK and the NF-45 membranes became progressively more hydrophobic over the 7 weeks immersion period. However, the BPT-NF-1 membranes showed only a slight change in contact angle. The change was small enough to be considered an experimental error.

Thickness (mm) after weeks

4. Discussion Aggressive chemicals can irreversibly degrade the membranes by processes such as oxidation, nitration, hydrolysis, acid-catalysed hydrolysis, etc. These processes ultimately lead to chain scission, cross-linking and chemical modification. The extent of attack is dependent on many factors such as the concentration of the chemical agent, duration of contact and temperature as well as the chemical structure and morphology of the membrane. The following features are key indicators of chemical degradation [21]: • • • • • • •

embrittlement; surface cracking; blistering; pock-marks on the surface (due to etching); swelling/distortion; discolouration (due to oxidation); voids/holes (caused by selective dissolution).

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4.1. UF supports Even though PES is believed to be a polymer with a strong resistance to chemical attack from acids, it is likely that at high concentrations and temperatures these chemicals do attack and degrade the membranes [21]. Table 6 shows no change in flux, no cracks, no blistering after immersion in 5% nitric acid even after 3 months at either 20 or 80 ◦ C, for the UF supports. The only change observed was a slight yellowing of the membrane after immersion. It is considered that this was due to membrane yellowing, and not osmium adhesion. Yellowing is usually associated with a degradation process but this is not always the case since PES UF supports are prone to yellowing due to the highly aromatic structure of the trunk polymer [21]. After immersion in 20% sulphuric acid at both temperatures 20 or 80 ◦ C the clean water flux was lower but no other changes were observed. The cause of the flux decline could be cross-linking, which leads to the formation of a denser membrane of increased molar mass and diminished porosity [21]. Also, since different membrane pieces were being characterised each month it is possible that the flux changes are just the result of a tighter membrane piece and not the result of membrane degradation. Na et al. [22] investigated the development of PVA composite membranes and showed that membrane modification using cross-linking agents caused a lowering in the flux attributed to the formation of a denser coating. Burczak et al. [23] reported that the cross-linking agent formed a denser cross-linked PVA hydrogel network. They also observed that after cross-linking the retention increased and the water fluxes decreased. 4.2. NF membranes 4.2.1. Nitric acid Immersion in 5% nitric acid at 20 and 80 ◦ C for all membrane types (Tables 2–5, 8 and 9) caused a large increase in clean water flux and a large decrease in sucrose and glucose retention. There was also discolouration of all the membranes at both temperatures. The Osmium treatment also revealed that the BPT-NF-1 membranes immersed in nitric acid no longer gave a black surface. The black colour change was observed for membranes with the nanofiltration modification surface still intact. If this surface no longer existed then no colour change would be observed. Taking the above into consideration it was considered that the membranes had suffered oxidative degradation. The degradation was greatest at the higher temperature (80 ◦ C) for all membrane types. The membranes Desal-5 DK and NF-45 at this temperature had zero retention to sucrose and glucose after just 1 month (Tables 2 and 3). The BPT-NF-1 membranes were not a lot better, after just 1 month they also had very low retentions to sucrose and glucose. However,

at the lower temperature of 20 ◦ C the reduction in retention was at a slower rate for all membrane types. This was to be expected since temperature increases the rate of oxidation, meaning the reaction rate is doubled for every 10 ◦ C increase in temperature [18]. 4.2.2. Sulphuric acid Immersion in 20% sulphuric acid at 80 ◦ C, for all membrane types (Tables 2–5, 8 and 9), resulted in a large increase in clean water flux and a large decrease in sucrose and glucose retentions. The NF-45 and Desal-5 DK membranes had zero retention to sucrose and glucose after just 1 month of immersion. At 20 ◦ C the reduction in sucrose and glucose retention was at a slower rate than at 80 ◦ C but even after 1 month the retention had decreased to an unacceptable level. The BPT-NF-1 membranes were not much better at 80 ◦ C but at 20 ◦ C (Tables 4 and 5) the membranes survived for 4 months. It was considered that catalysed hydrolysis was the cause of membrane degradation (Tables 8 and 9). Degradation by catalysed hydrolysis usually manifests itself in the following form: • • • • • •

layer of degraded material; blistering and flaking; cracking; embrittlement (reflecting loss in molar mass); swelling (due to water uptake); change in the glass transition temperature Tg (indication of degradation or plasticity); • changes in hydrophobicity; • change in thickness; • weight loss (due to migration of the degradation products out of the sample). The sort of materials susceptible to hydrolysis are condensation polymers (amino resins, polyacetals, polyamides, polyamines, polyarylates, polyesters and polyimides). Most of the expected forms of degradation (cracking, embrittlement, swelling) were not observed or it was not possible to measure them. Taking this into consideration the conclusion that catalysed hydrolysis occurred is still maintained and can be explained by the following. 4.2.3. Weight change Membranes were tested to see if there was any water uptake (swelling) or any loss in sample but it was found that there was no weight change (Table 10). However, it is important to consider that the membranes were formed by a very thin skin layer deposited onto the surface of a much thicker PES support. To try to measure a weight change when only the surface layer, of a few nanometers, is affected is impossible. No water uptake was expected from the support since PES contains no hydrolysable bridging groups and is therefore resistant to hydrolysis [21]. The images (Fig. 1) after osmium treatment (only carried out on the BPT-NF-1 membranes and the ultrafiltration supports) revealed a colour change only for the membranes

S. Platt et al. / Journal of Membrane Science 239 (2004) 91–103

with the NF skin layer still intact. After immersion in sulphuric acid it was revealed that the colour change was not so intense at 80 ◦ C. Also TEM images (Fig. 2) revealed the presence of a much thinner layer of osmium after immersion in sulphuric acid. These results indicate that some of the surface layer is lost. However, it is an amount that is not possible to measure at this stage. If the membrane had been left immersed long enough it may have been possible to observe the complete disappearance of the surface layer (same as for nitric acid). Serpe et al. [24] investigated the ageing of polyamide 11 in acid solutions (pH 0, 2, 4 and 7) at various temperatures (53–120 ◦ C). They reported that in a temperature range of less than 100 ◦ C and for all acid concentrations (pH 0–7) there was an increase in weight and this was attributed to water uptake. It was also shown that the degradation products formed during hydrolysis did not migrate out of the sample. At temperatures above 100 ◦ C there was initially observed a weight gain (water uptake) but this was followed by large weight losses due to the degradation products migrating out. 4.2.4. Change in thickness Again no change was observed for any of the immersed membranes (Table 10). This is not surprising since only the NF skin layer is expected to change. Therefore, a small change in thickness of a layer that is only a few nanometers (50–60) is difficult to measure. Again the osmium treatment revealed that there was a change in thickness. 4.2.5. Cracking, embrittlement and plastisation There was no cracking or embrittlement observed after immersion in sulphuric acid at either 20 or 80 ◦ C (Tables 8 and 9). A very important consequence of extreme hydrolysis is embrittlement as revealed by the appearance of spontaneous cracks. Most hydrolysable industrial polymers (polycarbonates, polyethylene and unsaturated esters) exhibit large cracks under these conditions. However, Serpe et al. in the above mentioned paper [24] also reported that hydrolysis had taken place and that it was accelerated by a decrease in pH, an increase in temperature and a decrease in sample thickness. However, at temperatures below 100 ◦ C no cracks or embrittlement were observed. Even at temperatures above 100 ◦ C cracks were only observed after a long exposure time. They attributed the lack of cracks to the low molar mass compounds not displaying significant plasticising effects. Plastic deformations are only possible when the chains are entangled in the amorphous phase of the polymer. 4.2.6. Change in hydrophobicity The contact angle results showed (Table 10) that there was an increase in hydrophobicity especially in the case of the Desal-5 DK and NF-45 membranes. In the case of the BPT-NF-1 membranes there was no change. It would

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be expected that if there was hydrolysis the resulting low molar mass compounds would migrate out of the membrane making it more hydrophobic. However, these changes were not observed to a large extent by any of the membranes and therefore it was considered that at least in the case of the NF-45 and the Desal-5 DK membranes a large proportion of the degradation products did not migrate out of the samples. In the case of the BPT-NF-1 membranes the absence of any contact angle change was to be expected since these membranes were stable in 20% sulphuric acid at 20 ◦ C for 4 months (Tables 4, 5 and 8). Serpe et al. [24] also observed that hydrolysis does not always induce significant variations in the polyamide 11 hydrophilicity. They found that if the low molar mass compounds (monomer/oligomer) resulting from the degradation did not migrate out of the samples then there was a build up of acids and amines (monomers) which compensated for the disappearance of amides which had the same contribution to water solubility. It was found that at temperatures below 100 ◦ C the low molar mass products (confirmed by gravimetric and steric exclusion chromatography data) did not migrate out of the samples. 4.2.7. Water Almost all the membranes immersed in water at both temperatures 20 and 80 ◦ C were stable even after 4 months (Tables 2–5). However, the Desal-5 DK membranes (Table 3) at the higher temperature of 80 ◦ C showed a decrease in sucrose and glucose retention and a large increase in flux after 4 months. It was considered that this was the start of membrane hydrolysis. Hydrolysis of polyamines and polyamides can occur when placed in contact with water. This process is accelerated at higher temperatures or when catalysed by acids or bases. 4.2.8. Mechanical changes Mechanical changes were not measured but it is important to mention here that even if they existed and changes were recorded this does not mean that there are no chemical changes. Mechanical changes can be an indication of a chemical change. Yoon et al. [25] reported that as a sample hydrolyses the mechanical strength of the degraded sample also decreases and many small fragments are separated from the sample resulting in weight loss. Stirling et al. [26] reported for the chemical attack of poly(vinylidene fluoride) in sulphuric acid that there were no chemical changes, just mechanical changes. The chemical changes they investigated were weight changes and surface examination (by SEM). The mechanical change which was measured was the tensile strength and this was found to be considerably reduced. It is possible that this change was an indication of a chemical change and their conclusion that there was significant loss of mechanical properties but no evidence of chemical attack was incorrect, since the me-

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chanical changes could have been the evidence of chemical attack.

5. Conclusions • The UF supports (types 006 and 015) showed no change after immersion in 5% nitric acid at either 20 or 80 ◦ C for 3 months. • The BPT-NF-1 membranes which were formed by a thin skin layer deposited onto UF supports (types 006 and 015) were found to be unstable in nitric acid after only 1 month of immersion. This was attributed to the oxidation of the NF skin layer. The disappearance of this layer was proved from TEM analysis. • All of the NF membranes were unstable in 5% nitric acid at 80 ◦ C, and at 20 ◦ C the NF-45 membrane proved to be slightly more stable than the BPT-NF-1 membranes and the Desal-5 DK membrane. • All of the NF membranes were unstable in 20% sulphuric acid at 80 ◦ C, and at 20 ◦ C the NF-45 and Desal-5 DK membranes were unstable after 1 month while the BPT-NF-1 membranes at 20 ◦ C were resistant for the entire duration of the experiment (4 months). • It was considered that acid-catalysed hydrolysis was the cause of membrane degradation in the case of the membranes immersed in sulphuric acid. • Immersion in water caused no degradation after 4 months for any of the membranes except for the Desal-5 DK. At 80 ◦ C this membrane degraded after 3 months and this was attributed to hydrolysis. Finally, it can be concluded that the BPT-NF-1 membranes were by far the most stable in 20% sulphuric acid at 20 ◦ C. In the case of nitric acid all of the nanofiltration membranes were unstable long-term. Concerning the 006 and 015 ultrafiltration supports these membranes were stable in 5% nitric acid at all temperatures for 4 and 3 months, respectively. In 20% sulphuric acid at 20 and 80 ◦ C there was a significant decrease in flux observed for both supports. It was not clear if this decrease was due to membrane degradation or due to a tighter membrane being characterised. This is further supported by the fact that a higher flux decrease was observed for the 006 support at 80 ◦ C than at 20 ◦ C.

Acknowledgements Financial support from the RENOMEM (Recycling chemicals, energy and water from aggressive waste streams with novel modified nanofiltration, NF, membranes) project (contract number: EVK1-2000-00067) is acknowledged including the supply of membranes from FuMA-Tech (Germany) and BPT (Israel) partners in the project. Jussi Vittaniemi (graduate student) and Marko Liikanen (M.Sc.) are acknowledged for their help with immersion experiments.

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