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Development of an asymmetric carbon microfiltration membrane: Application to the treatment of industrial textile wastewater
Separation and Purification Technology 118 (2013) 179–187
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Development of an asymmetric carbon microfiltration membrane: Application to the treatment of industrial textile wastewater Nouha Tahri a, Ilyes Jedidi a, Sophie Cerneaux b, Marc Cretin b, Raja Ben Amar a,⇑ a b
Laboratoire Sciences des Matériaux et Environnement, Faculté des Sciences de Sfax, Route de Soukra Km 4, 3000 Sfax, Tunisia Institute Européen des Membranes, UMR 5635 (CNRS, ENSCM, UM II), 1919 Route de Mende, 34293 Montpellier Cedex 5, France
a r t i c l e
i n f o
Article history: Received 27 February 2013 Received in revised form 24 June 2013 Accepted 25 June 2013 Available online 4 July 2013 Keywords: Carbon/carbon microfiltration membrane Phenolic resin Carbon powder Carbonization
a b s t r a c t Tubular carbon microfiltration membranes have been prepared using mineral coal powder (100 lm average particle size) mixed with phenolic resin solution and organic additives. The porosity of the support can be sharply controlled by fixing the organic additives percentages, the particles size and the homogeneity of the carbon powder. A mesoporous active layer was deposited on the inner face of the carbon support by slip casting using a deflocculated slip made of mineral coal powder (1.76 lm) suspended in a phenolic resin solution. An active layer with an average pore diameter of 0.60 lm is obtained after carbonization under nitrogen atmosphere. The obtained membrane has very interesting characteristics regarding mechanical and chemical resistances. The application to the treatment of industrial textile wastewater shows good performances in term of permeate flux and efficiency (retention of COD and salinity of 50% and 30% respectively and almost a total retention of turbidity and color). Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Many efforts to achieve economical and efficient membranes for various uses have resulted in a selection of new materials, an improvement in membrane preparation techniques and an increase in the number of applications [1,2]. At present, the interest is derived toward inorganic membranes due to their superior permeability–selectivity combination and suitable performance for high temperature or corrosive environment compared to polymeric membranes [3]. Different studies on carbon membranes have shown that they can successfully compete with polymeric membranes and other porous inorganic membranes, such as silica- and zeolite-based membranes, in separation processes of industrial interest. Carbon material possess a very good chemical and mechanical resistance whatever the temperature used. Carbon membrane is generally prepared by carbonization of various carbonaceous materials [2] such as polyimide [4–6], poly-acrylonitrile [7], phenolic resins [8–10] and polyfurfuryl alcohol [11–13]. Recent studies [14] showed that coal is a good carbon material that can be used as source of carbon to prepare
Abbreviations: COD, chemical oxygen demand; MF, microfiltration; R, retention; TMP, transmembrane pressure. ⇑ Corresponding author. Address: Faculté des Sciences de Sfax, Laboratoire Sciences des Matériaux et Environnement, Route de Soukra Km 4 BP 1171, Sfax, Tunisia. Tel.: +216 74 276 400, +216 74 276 763; fax: +216 74 274 437. E-mail address: [email protected] (R. Ben Amar). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.06.042
asymmetric microfiltration carbon membranes with high porosity, controllable pore size and narrow pore size distribution. Due to their low cost and high carbon yield, phenolic resins are widely used as precursors for the preparation of carbon membranes with molecular sieving properties [15]. Phenol–formaldehyde resins are thermosetting polymers. They do not flow at high temperatures which maintain the initial shape unchanged during the heat treatment. In addition, when pyrolyzed under inert atmosphere, phenolic resins have a high carbon yield which is crucial for the mechanical properties of the final carbon material. Their use is made easier by the large variety of presentation (in solution or powdered) and of types (Novolac and Resole thermosetting respectively in the presence or absence of a cross-linking agent). Nowadays, a lot of researchers have demonstrated that extrusion is proven to be the best technique to use for preparing tubular carbon membranes. During the last decade, tubular carbon supports have been produced by extrusion using a novolac type phenolic resin [10–15]. The resin was semi-cured then ground in fine powder which was mixed (or not) with different additives and sometimes another source of carbon is added to better control the final porosity characteristics of the final material [10]. A carbonization step under nitrogen atmosphere takes care about the pores opening and the porosity is then created. Much works were done concerning the preparation of carbon molecular sieves for gas separation by carbonizing under inert atmosphere a phenolic resin layer deposited by slip casting process.
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Centeno et al. and Fang et al. [16,9] prepared carbon membrane using thermosetting phenolic resin as polymeric precursor. The membrane was formed by a thin microporous carbon layer obtained by pyrolysis of a phenolic resin film supported on the inner face of a porous alumina tube. The coating–carbonization cycle was repeated several times to obtain carbon membranes, which have good separation properties for H2/N2, H2/CH4 and O2/N2. Kishore et al. [17] used phenol–formaldehyde resin to prepare carbon membrane for nanofiltration. Little work was done to deposit carbon active layers on a carbon macroporous support. Katsaros et al. [10] prepared a carbon membrane for gas separation purpose by coating a phenolic resin solution on the outer face of a carbon macroporous support. To do so, a paste made of partially cured phenolic resin particles mixed to water and porosity agents, was extruded and then carbonized under inert atmosphere at 800 °C. Song et al. [14] prepared coal-based symmetric microfiltration carbon membrane for the treatment of oil–water mixtures. The result showed that the oil rejection coefficients of oily wastewater were up to 97%, and the oil concentrations of the permeate were less than 10 mg/L. Based on these considerations and the body of previous research, this work describes the preparation of new tubular carbon/carbon microfiltration membranes by slip-casting method using mineral coal powder and thermosetting resin. The effects of particle size, casting time, and concentration of organic additives are studied. The performances of the application of this membrane on the treatment of industrial textile wastewater are then discussed.
2. Experimental 2.1. Material 2.1.1. Carbon material Mineral Coal powder was used as the main carbon material source. To determine the effect of particle size, carbon powders with different average particle size were prepared. A 200 lm average particles size powder was obtained by crushing–sieving of a raw mineral coal powder. This late powder was ground in a planetary crusher at 300 rpm respectively during 30 min, 2 h and 15 h to obtain three different powders having average particle size respectively equal to 100 lm, 20 lm and 1.76 lm. The particle sizing system CILAS 920 was used to determine the particle size distribution. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed with simultaneous DSC–TGA 2960 TA instrument to estimate the materials weight loss in a temperature interval varying from 20 to 1000 °C with a heating rate of 10 °C/min, under nitrogen flow of 1 ml/min. 2.1.2. Thermosetting resin Phenolic resin marketed by the company Irons Resins S.A, Spain (Sumitomo Bakelite Co.) was used as carbon precursor, binder and porosity agent. It is sold in powder form soluble in ethanol containing 7% of hexamine as curing agent. 2.1.3. Organic additives Organic additives were dry mixed to the carbon powder. Some of them, as starch powder, were used to enhance the total porosity of the final material (since it sublimate during carbonization), other additives, such as ethylene glycol, amijel and methyl-cellulose are used respectively as lubricants and plasticizers of the paste to be extruded in tubular form.
To study the influence of starch percentage on the final material structure, carbon supports differing only by starch content, were prepared (Table 1). 2.2. Preparation of carbon membrane Membrane synthesis consisted on two main steps: preparation of macroporous support and active microfiltration layer deposition. The process of microfiltration membrane preparation is described in Fig. 1. Tubular support (OD/ID = 10 mm/8 mm) were obtained by paste extrusion under a pressure of 30–40 bars. The paste was prepared following the composition described on previous work [18,19]. The extruded green tubular bodies were heat treated in a tubular electrical furnace (VTF7 VECSTAR) connected to inert gas purging during the carbonization and following the temperature–time schedule given in Fig. 2. The active microfiltration layer was prepared by slip-casting process which process consists essentially on four steps (Fig. 3): Preparation of the suspension: the elaborated suspension was made of 20% of carbon powder (1.76 lm average particle size) suspended in alcoholic solution of phenolic resin (Resin/Ethanol = 15/85 wt%). Homogenization of the suspension by magnetic stirring followed by ultrasound exposure. Slip cast the suspension on the inner face of the porous carbon support for few minutes at room temperature. In the case of the tubular membranes, the tube was closed at one end and filled with the solution. The deposited layer was treated following the same temperature program used to prepare the macroporous carbon supports. 2.3. Material physicochemical characterization The porosity and the average pore diameter of the final material were determined using Hg-porosimetry on a Micrometrics Autopore II9220 V3.05. The presence of possible defects in the membrane was checked using scanning electron microscopy (SEM). Mechanical resistance of the material was measured by the three point bending method (using LLOYD Instrument) on test bars having the following dimensions: 32 mm/10 mm/2 mm. The distance separating the two base points was 28 mm. The Shrinkage degree was evaluated by comparing the dimensions of 10 tubes (using a caliper rule) before and after carbonization step. Contact angle technique was used to evaluate the hydrophobic character of the elaborated membrane material.
Table 1 Materials used to prepare the paste: quantity and function. Component
Function
Quantity
Powder carbon Ethylene glycol Starch (RG 03408, Cerestar)a Amijel (Cplus 12076, Cerestar) Methocel (The Dow Chemical Company) Alcoholic solution of phenolic resin
Mineral charge Lubricant Porosity agent Plasticizer, binder Plasticizer
150 g 25.5 g 0%, 4%, 8%, 10%, 12%, 15% 5.75 g
Binder, porosity agent
120–130 ml
5.75 g
a The different starch quantities are calculated based on the total weight of the dry components only.
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Fig. 1. Main steps of the processing route for microfiltration membrane preparation.
60 °C) and basic (NaOH 2% at 80 °C) solutions were alternatively circulated for 20 min. The membrane was in between rinsed with deionized water until neutral pH. The efficiency of the used cleaning protocol was verified by measuring water flux after the cleaning cycle. The membrane had been applied to treat wastewater coming from Tunisian textile industry. 2.5. Effluent characterization Physico-chemical parameters of real effluent and of permeate were determined according to the standard methods suggested
Fig. 2. Temperature–time schedule.
2.4. Microfiltration treatment Single channel tubular carbon membrane was employed with 8/ 10 mm in inner/outer diameter and 150 mm in length. The crossflow mode of microfiltration was adopted in this experiment. Schematic diagram of the experimental apparatus is shown in Fig. 4. Microfiltration (MF) operations were carried out at a temperature of 25 °C, under the transmembrane pressure (TMP) between 1 and 5 bars. The transmembrane pressure was controlled by an adjustable valve at the concentrate side. Before the tests, the membrane had been conditioned by immersion in pure deionized water for at least 24 h. The water flux through the membrane was measured as a function of time at different TMP values. The regeneration of the membrane was carried out by firstly, back-flushing procedures for 15 min, then acidic (nitric acid 2% at
Fig. 3. Scheme of slip-casting process.
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Fig. 4. Scheme of the pilot plant. Fig. 5. DSC–TGA data under air of the mineral coal powder.
by American Public Health Association (Standard Methods for the Examination of Water and Wastewater 1998). Conductivity and pH determinations were done by means of a conductimeter, Tacussel model 123 and pH-meter, Metrohm 744 respectively. Turbidity was measured using a turbidity meter (Hach RATIO 2100A) in accordance with standard method 2130B; a calibration standard is needed. Color measurement was determined with standard dilution multiple method [20] and by comparing absorbance to a calibration curve [21]. Dye decolorization was determined by monitoring the decrease in the absorbance peak at the maximum wavelength for the global effluent. For the wastewater used in this study only one pick was observed at 420 nm. UV–Visible spectrophotometer (Perkin Elmer Lambda 20 UV/VIS Spectrophotometer) was used in all experiments. COD was estimated by open reflux method. The protocol present a method derived from the Standard AFNOR T90-101. The sample was refluxed in an acidic medium with a known excess of potassium dichromate and the remaining dichromate was titrated with ferrous ammonium sulphate. The COD values were obtained using a Fisher Bioblock Scientific reactor COD 10119 type COD meter. 3. Result and discussion 3.1. Thermogravimetric analyses of raw materials 3.1.1. Carbon material The DSC–TGA (from 0 to 1000 °C) under air of a coal powder sample, shows a weight loss of 8.23% at 730 °C which corresponds to fly ash (Fig. 5). This result shows that fly ash represents a minor phase in the coal rock. Knowing that the fly ash is in the molecular state and scattered through the mineral coal rock [22,23], thus, it can be considered that the membrane is made totally of coal carbon.
Fig. 6. DSC–TGA data under nitrogen atmosphere of paste prepared with Novolac Resin and carbon powder.
The pore structure is formed after all volatile components present in the green body (the additives part and the noncarbonaceous part of the resin) are sublimated.
3.1.3. Starch Starch is a complex Glucide composed of molecular chains of DGlucose having a brute formula as: (C6H10O5)n. The result of DSC–TGA analyzes of a sample of starch (Fig. 7) shows that the mass loss took place in the temperature range of 250–550 °C and reached the maximum of 61%,37% at 300 °C. It can be clearly seen that when the final temperature of 750 °C was reached, all starch quantity was sublimated and was then responsible in part of the material porosity.
3.2. Optimization of carbon support composition 3.1.2. Thermosetting resin The weight loss during the carbonization of a paste prepared from resin and carbon powder without any additives was evaluated by thermogravimetric analysis. Fig. 6 shows a maximum weight loss of about 25% reached at 700 °C. Since the carbon content in the resin represents 25% of its total molecular weight, it can be concluded that coal carbon material does not participate in the total porosity of the final material. Porosity results principally from resin weight loss. Therefore, besides its role as binder and source of carbon, resin acts also as porosity agent. In addition, the sublimation of the other organic additives (especially starch) contributes also to the final porosity of the coal-based carbon membrane.
As known, powder particle size and organic porous agent percentages (starch in our case), in addition to sintering temperature, are the most important parameters affecting the structure and the porosity of porous materials [19]. In the case of carbon membranes prepared following the process described in this paper, the consolidation of the material is due to the phenolic resin crosslinking. In addition, resin participate with a constant value to the final porosity, but the main parameters to control both the pore size and the porous volume of the final material are respectively coal particle size and starch percentage the powder particle size and the presence of starch should also contribute to the structure and porosity of the membrane.
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Fig. 7. The DSC–TGA data under nitrogen atmosphere of the starch.
3.2.1. Effect of carbon particle size Different tubular supports were prepared using four carbon powders with different average particle sizes: 200 lm, 100 lm, 20 lm and 1.76 lm. Fig. 8 shows that total porous volume remains unchanged at a value of 38% for all prepared supports. However, average pore size increases when particle size increases. Average pore diameters were equal to 0,066 lm, 0.1 lm, 8 lm and 12 lm respectively for powder particle size of 1.76 lm, 20 lm, 100 lm and 200 lm. Similar results were obtained by Song et al. [14] whose found that bigger is the coal particles, higher is the average pore sizes. 3.2.2. Effect of starch percentage In order to determine the effect of starch percentage on the physical characteristics of the final material, carbon supports were prepared using various starch percentages: 0%, 2%, 4%, 8%, 10%, 12% and 15%. Fig. 9 shows that all samples have very good mechanical strength since they resisted to a mean maximum force before break varying from 29N for 0% starch content to 18N for 15% starch content. The material’s mechanical resistance decreased with the increase of starch content due to the porous volume increase. On the other hand, the porous volume increased with starch content from 25% (0% starch) to 38% (12% starch). Beyond 12% starch content, the porous volume remained constant. 25% of porosity obtained at 0% is due to resin weight loss. The increase
Fig. 9. Evolution of porous volume and mechanical resistance with the starch content.
of the porosity until 12% added starch (limit value) can be attributed to the starch sublimation. SEM pictures (Fig. 10) shows a drastic change in the final material texture with the added percentage of starch. 12% starch content show the greatest porosity. As we can see, the porous texture of the obtained tubular support is not the result of a sintering phenomenon as observed during the formation of conventional ceramic porous materials [19,24,25]. In fact, the elaborated paste is no more than a composite polymer in which the resin represents the polymer matrix and the carbon particles are the mineral charge. During the carbonization step organic additives sublimate and the resin lose weight, thus, only the carbon skeleton of the resin and the carbon coal particles remained (Fig. 11). 12% of starch content was found to be the most appropriate to obtain a material with a good mechanical strength, porosity and texture. 3.3. Active layer characterization
Fig. 8. Variation of pore size distribution with particle size of carbon powder.
A microfiltration active layer was deposited on the optimized support by slip casting. The same suspension (described earlier) was used to cast the layer. Nevertheless, different casting times (2, 4, 6 and 8 min) were tested. Fig. 12 presents the evolution pore
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Fig. 10. SEM images of two samples of carbon supports using 0% starch content and 12% starch content.
Fig. 11. SEM pictures of the final carbon support.
size distribution of the carbon membranes with casting time. It can be observed that the average pore diameter of the active layer (0.60 lm) remained unchanged for all casting times. Surface and cross-section morphologies for the different samples obtained with the different casting times were characterized by SEM micrographs. All samples showed a typical asymmetric structure but the thicknesses of the active layer increased with
casting time respectively from 6 lm for 2 min, to 30 lm for 8 min casting time (Fig. 13). When analyzing the surface texture of the different samples, it can be concluded that the sample obtained with 6 min casting time do not contain defects (cracks and pinholes) at the opposite of all other samples. So the best layer is obtained with 6 min casting time with thickness around 22 lm which is a good thickness for a microfiltration layer [18,19,24,26,27]. 3.4. Determination of microfiltration membrane permeability
Fig. 12. Mercury porosimetry analysis of the microfiltration membrane for the different casting time.
The permeability of the support and MF membrane was determined using pure distilled water. Flux values of distilled water at different operating pressures were measured and were plotted against transmembrane pressure. Samples were conditioned in the pure water during the 24 h which precede the filtration, this allows obtaining a fast stabilization of the permeate flux. Permeability was obtained according to the Darcy’s law (J = DP/l.TMP) (Fig. 14). The average values for the elaborated support and MF membrane permeability were respectively 280 L/h m2 bar and 100 L/ h m2 bar. Contact angle technique is used to estimate the hydrophobicity character of the material. The measure was repeated 4 times using a water drop volume varying between 10 and 15 lL. The measures
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Fig. 13. SEM micrographs for carbon microfiltration membrane obtained with different casting times. Surface texture is shown in the inserts.
1000
cal substances such as dyes, detergents, salts, auxiliaries (e.g. surfactants, emulsifiers) and caustic soda are used. Generally, the wastewater was coming from the dyeing, washing and bleaching baths (Table 2).
900 800
600 500 400 300 200 100
MF
Support
0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
TMP (Bar) Fig. 14. Determination of the support and MF membrane permeability.
of the angle were taken after 12 s. Macroporous supports contact angle value was equal to 80° showing a slight hydrophobic character while it was found 96.1° for the microfiltration layer corresponding to a stronger hydrophobic character. This difference in wettability, even if both support and layer had a similar chemical composition, can be related to the difference in pore size [28,29]. When using membranes, a cleaning cycle is needed for regeneration according to a protocol using an alternating rinsing of basic and acid solutions. Tests of corrosion resistance were performed on carbon supports by immersion in basic and acid solutions. For this, membrane sample was soaked at 80 °C for 30 days in bath solution at 2 wt% nitric acid (pH not exceeding 2) and NaOH (pH = 14). Every 24 h, a sample was removed, washed, dried and weighed to estimate the weight loss. It was concluded that no weight loss was recorded throughout the testing period. Therefore, it appears that the elaborated carbon membrane has a very high chemical resistance.
3.5.2. Microfiltration treatment The filtration performances were determined at a TMP of 3 bar, a velocity of 5.6 m/s and a temperature of 25 °C. The evolution of the flux with time is given by Fig. 15. The flux decreased during the first 10 min from 42 L/h m2 to 35 L/h m2 and then stabilized at 32 L/h m2. The decline of permeate flux with time is a typical behavior of membrane processes, and can be interpreted mostly in terms of concentration polarization and fouling due to the interaction between membrane material and solution [30].
Table 2 Principle physico-chemical characteristics of textile wastewater.
b
Parameters
pH
Salinity (g/L)
COD (mg/L)
Turbidity (NTU)
Colorb
Values
8.5
22.8
960
210
52
Integral of the absorbance curve in the whole visible range (400–800 nm).
50 45 40 35
J (L/h.m²)
J (L/h.m²)
700
30 25 20 15 10
3.5. Application to textile wastewater treatment
5 0
3.5.1. Wastewater characterization The study was conducted with a textile wastewater sample supplied from a Tunisian textile factory in which, different chemi-
0
10
20
30
40
50
Time (min) Fig. 15. Evolution of the permeate flux with time.
60
70
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100%
R (%)
80% 60% 40% 20%
Carbon Membrane
0%
Salinity
Alumina Membrane
COD Turbidity Color
Fig. 16. Evolution of retention of the different parameters by carbon and alumina microfiltration membrane.
carbon precursor using a new process. The porosity and the texture of the final material were controlled by adjusting coal particle size and percentage of organic additives. Tubular support (OD/ ID = 10 mm/8 mm) elaborated by paste extrusion had a porous volume of 38% and a good mechanical and chemical resistances. The active microfiltration layer was prepared by slip-casting process. The casting time was optimized to 6 min which produced a layer with thickness around 22 lm. Performances of this membrane toward the treatment of industrial textile wastewater have been studied in term of permeate flux and efficiency. The retention of polluting substances regarding COD, Turbidity, salinity and color was respectively 57%, 90%, 30% and 80%. By comparison with alumina membranes of similar porous characteristics, carbon microfiltration membrane showed the best performances.
Acknowledgements In order to explain permeate flux decline, the resistance series model can be used:
RT ¼ Rm þ Rrev þ Rirrev RT is the total filtration resistance which represents the distribution of the different resistances. Rm is the inherent hydraulic resistance of clean membrane caused by the membrane itself, it was given by the determination of pure water permeability. The Rrev resistance is due to concentration polarization and solids (cake layer) on the membrane surface, which can be removed by rinsing with water after the filtration experiment. At the contrary, the Rirrev resistance is due to pore blocking and absorption of materials onto the membrane surface and pores, which cannot be removed by a simple water rinsing but needs a chemical cleaning. After each run, the membrane was cleaned with pure water and then the permeate water flux was determined, given the Rirrev. The Rrev value was calculated following:
Rrev ¼ RT ðRm þ Rirrev Þ In our case, the different resistance values are: RT = 3.37 1011 m1, Rirrev = 2 1011 m1 et Rrev = 1.29 1011 m1. The fouling is rather irreversible (Rrev > Rirrev). Regarding the permeate quality; a total retention of color and turbidity was achieved. By an other hand the retention of polluting substances was higher than 50% except for salinity which was of only 30%. Indeed, the main mechanism of microfiltration is usually sieving but adsorption of organic and mineral pollution towards membrane surface can also occurred due to ionic and hydrophobic interaction. So, at high concentration of salt, surface charge effect can occurred which explains the retention of 30% of salt with initial concentration of 22 g/L. The difference of charge between the membrane surface (negative since pH is of 6.9) and the solution having a pH of 8.5 contributed to this retention. By comparison with alumina microfiltration membrane, carbon membrane shows similar retention of turbidity and COD and higher removal of color (Fig. 16). Removal of color is due mainly to the adsorption of dyes onto the membrane material. Retention of salinity by carbon membrane is almost twice higher than that obtained by alumina membrane. Therefore, the carbon microfiltration membrane seems to be suitable to textile effluent treatment. 4. Conclusion New carbon/carbon asymmetric microfiltration membranes have been prepared based on mineral coal and phenolic resin as
Authors would like to thank the Ministry of higher education and scientific research, Tunisia for funding this work. Authors are also very grateful to Sumitomo Bakelite Europe (Barcelona) for their help by given samples of resin.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2013.06.042.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Development of an asymmetric carbon microfiltration membrane: Application to the treatment of industrial textile wastewater Nouha Tahri a, Ilyes Jedidi a, Sophie Cerneaux b, Marc Cretin b, Raja Ben Amar a,⇑ a b
Laboratoire Sciences des Matériaux et Environnement, Faculté des Sciences de Sfax, Route de Soukra Km 4, 3000 Sfax, Tunisia Institute Européen des Membranes, UMR 5635 (CNRS, ENSCM, UM II), 1919 Route de Mende, 34293 Montpellier Cedex 5, France
a r t i c l e
i n f o
Article history: Received 27 February 2013 Received in revised form 24 June 2013 Accepted 25 June 2013 Available online 4 July 2013 Keywords: Carbon/carbon microfiltration membrane Phenolic resin Carbon powder Carbonization
a b s t r a c t Tubular carbon microfiltration membranes have been prepared using mineral coal powder (100 lm average particle size) mixed with phenolic resin solution and organic additives. The porosity of the support can be sharply controlled by fixing the organic additives percentages, the particles size and the homogeneity of the carbon powder. A mesoporous active layer was deposited on the inner face of the carbon support by slip casting using a deflocculated slip made of mineral coal powder (1.76 lm) suspended in a phenolic resin solution. An active layer with an average pore diameter of 0.60 lm is obtained after carbonization under nitrogen atmosphere. The obtained membrane has very interesting characteristics regarding mechanical and chemical resistances. The application to the treatment of industrial textile wastewater shows good performances in term of permeate flux and efficiency (retention of COD and salinity of 50% and 30% respectively and almost a total retention of turbidity and color). Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Many efforts to achieve economical and efficient membranes for various uses have resulted in a selection of new materials, an improvement in membrane preparation techniques and an increase in the number of applications [1,2]. At present, the interest is derived toward inorganic membranes due to their superior permeability–selectivity combination and suitable performance for high temperature or corrosive environment compared to polymeric membranes [3]. Different studies on carbon membranes have shown that they can successfully compete with polymeric membranes and other porous inorganic membranes, such as silica- and zeolite-based membranes, in separation processes of industrial interest. Carbon material possess a very good chemical and mechanical resistance whatever the temperature used. Carbon membrane is generally prepared by carbonization of various carbonaceous materials [2] such as polyimide [4–6], poly-acrylonitrile [7], phenolic resins [8–10] and polyfurfuryl alcohol [11–13]. Recent studies [14] showed that coal is a good carbon material that can be used as source of carbon to prepare
Abbreviations: COD, chemical oxygen demand; MF, microfiltration; R, retention; TMP, transmembrane pressure. ⇑ Corresponding author. Address: Faculté des Sciences de Sfax, Laboratoire Sciences des Matériaux et Environnement, Route de Soukra Km 4 BP 1171, Sfax, Tunisia. Tel.: +216 74 276 400, +216 74 276 763; fax: +216 74 274 437. E-mail address: [email protected] (R. Ben Amar). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.06.042
asymmetric microfiltration carbon membranes with high porosity, controllable pore size and narrow pore size distribution. Due to their low cost and high carbon yield, phenolic resins are widely used as precursors for the preparation of carbon membranes with molecular sieving properties [15]. Phenol–formaldehyde resins are thermosetting polymers. They do not flow at high temperatures which maintain the initial shape unchanged during the heat treatment. In addition, when pyrolyzed under inert atmosphere, phenolic resins have a high carbon yield which is crucial for the mechanical properties of the final carbon material. Their use is made easier by the large variety of presentation (in solution or powdered) and of types (Novolac and Resole thermosetting respectively in the presence or absence of a cross-linking agent). Nowadays, a lot of researchers have demonstrated that extrusion is proven to be the best technique to use for preparing tubular carbon membranes. During the last decade, tubular carbon supports have been produced by extrusion using a novolac type phenolic resin [10–15]. The resin was semi-cured then ground in fine powder which was mixed (or not) with different additives and sometimes another source of carbon is added to better control the final porosity characteristics of the final material [10]. A carbonization step under nitrogen atmosphere takes care about the pores opening and the porosity is then created. Much works were done concerning the preparation of carbon molecular sieves for gas separation by carbonizing under inert atmosphere a phenolic resin layer deposited by slip casting process.
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Centeno et al. and Fang et al. [16,9] prepared carbon membrane using thermosetting phenolic resin as polymeric precursor. The membrane was formed by a thin microporous carbon layer obtained by pyrolysis of a phenolic resin film supported on the inner face of a porous alumina tube. The coating–carbonization cycle was repeated several times to obtain carbon membranes, which have good separation properties for H2/N2, H2/CH4 and O2/N2. Kishore et al. [17] used phenol–formaldehyde resin to prepare carbon membrane for nanofiltration. Little work was done to deposit carbon active layers on a carbon macroporous support. Katsaros et al. [10] prepared a carbon membrane for gas separation purpose by coating a phenolic resin solution on the outer face of a carbon macroporous support. To do so, a paste made of partially cured phenolic resin particles mixed to water and porosity agents, was extruded and then carbonized under inert atmosphere at 800 °C. Song et al. [14] prepared coal-based symmetric microfiltration carbon membrane for the treatment of oil–water mixtures. The result showed that the oil rejection coefficients of oily wastewater were up to 97%, and the oil concentrations of the permeate were less than 10 mg/L. Based on these considerations and the body of previous research, this work describes the preparation of new tubular carbon/carbon microfiltration membranes by slip-casting method using mineral coal powder and thermosetting resin. The effects of particle size, casting time, and concentration of organic additives are studied. The performances of the application of this membrane on the treatment of industrial textile wastewater are then discussed.
2. Experimental 2.1. Material 2.1.1. Carbon material Mineral Coal powder was used as the main carbon material source. To determine the effect of particle size, carbon powders with different average particle size were prepared. A 200 lm average particles size powder was obtained by crushing–sieving of a raw mineral coal powder. This late powder was ground in a planetary crusher at 300 rpm respectively during 30 min, 2 h and 15 h to obtain three different powders having average particle size respectively equal to 100 lm, 20 lm and 1.76 lm. The particle sizing system CILAS 920 was used to determine the particle size distribution. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed with simultaneous DSC–TGA 2960 TA instrument to estimate the materials weight loss in a temperature interval varying from 20 to 1000 °C with a heating rate of 10 °C/min, under nitrogen flow of 1 ml/min. 2.1.2. Thermosetting resin Phenolic resin marketed by the company Irons Resins S.A, Spain (Sumitomo Bakelite Co.) was used as carbon precursor, binder and porosity agent. It is sold in powder form soluble in ethanol containing 7% of hexamine as curing agent. 2.1.3. Organic additives Organic additives were dry mixed to the carbon powder. Some of them, as starch powder, were used to enhance the total porosity of the final material (since it sublimate during carbonization), other additives, such as ethylene glycol, amijel and methyl-cellulose are used respectively as lubricants and plasticizers of the paste to be extruded in tubular form.
To study the influence of starch percentage on the final material structure, carbon supports differing only by starch content, were prepared (Table 1). 2.2. Preparation of carbon membrane Membrane synthesis consisted on two main steps: preparation of macroporous support and active microfiltration layer deposition. The process of microfiltration membrane preparation is described in Fig. 1. Tubular support (OD/ID = 10 mm/8 mm) were obtained by paste extrusion under a pressure of 30–40 bars. The paste was prepared following the composition described on previous work [18,19]. The extruded green tubular bodies were heat treated in a tubular electrical furnace (VTF7 VECSTAR) connected to inert gas purging during the carbonization and following the temperature–time schedule given in Fig. 2. The active microfiltration layer was prepared by slip-casting process which process consists essentially on four steps (Fig. 3): Preparation of the suspension: the elaborated suspension was made of 20% of carbon powder (1.76 lm average particle size) suspended in alcoholic solution of phenolic resin (Resin/Ethanol = 15/85 wt%). Homogenization of the suspension by magnetic stirring followed by ultrasound exposure. Slip cast the suspension on the inner face of the porous carbon support for few minutes at room temperature. In the case of the tubular membranes, the tube was closed at one end and filled with the solution. The deposited layer was treated following the same temperature program used to prepare the macroporous carbon supports. 2.3. Material physicochemical characterization The porosity and the average pore diameter of the final material were determined using Hg-porosimetry on a Micrometrics Autopore II9220 V3.05. The presence of possible defects in the membrane was checked using scanning electron microscopy (SEM). Mechanical resistance of the material was measured by the three point bending method (using LLOYD Instrument) on test bars having the following dimensions: 32 mm/10 mm/2 mm. The distance separating the two base points was 28 mm. The Shrinkage degree was evaluated by comparing the dimensions of 10 tubes (using a caliper rule) before and after carbonization step. Contact angle technique was used to evaluate the hydrophobic character of the elaborated membrane material.
Table 1 Materials used to prepare the paste: quantity and function. Component
Function
Quantity
Powder carbon Ethylene glycol Starch (RG 03408, Cerestar)a Amijel (Cplus 12076, Cerestar) Methocel (The Dow Chemical Company) Alcoholic solution of phenolic resin
Mineral charge Lubricant Porosity agent Plasticizer, binder Plasticizer
150 g 25.5 g 0%, 4%, 8%, 10%, 12%, 15% 5.75 g
Binder, porosity agent
120–130 ml
5.75 g
a The different starch quantities are calculated based on the total weight of the dry components only.
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Fig. 1. Main steps of the processing route for microfiltration membrane preparation.
60 °C) and basic (NaOH 2% at 80 °C) solutions were alternatively circulated for 20 min. The membrane was in between rinsed with deionized water until neutral pH. The efficiency of the used cleaning protocol was verified by measuring water flux after the cleaning cycle. The membrane had been applied to treat wastewater coming from Tunisian textile industry. 2.5. Effluent characterization Physico-chemical parameters of real effluent and of permeate were determined according to the standard methods suggested
Fig. 2. Temperature–time schedule.
2.4. Microfiltration treatment Single channel tubular carbon membrane was employed with 8/ 10 mm in inner/outer diameter and 150 mm in length. The crossflow mode of microfiltration was adopted in this experiment. Schematic diagram of the experimental apparatus is shown in Fig. 4. Microfiltration (MF) operations were carried out at a temperature of 25 °C, under the transmembrane pressure (TMP) between 1 and 5 bars. The transmembrane pressure was controlled by an adjustable valve at the concentrate side. Before the tests, the membrane had been conditioned by immersion in pure deionized water for at least 24 h. The water flux through the membrane was measured as a function of time at different TMP values. The regeneration of the membrane was carried out by firstly, back-flushing procedures for 15 min, then acidic (nitric acid 2% at
Fig. 3. Scheme of slip-casting process.
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Fig. 4. Scheme of the pilot plant. Fig. 5. DSC–TGA data under air of the mineral coal powder.
by American Public Health Association (Standard Methods for the Examination of Water and Wastewater 1998). Conductivity and pH determinations were done by means of a conductimeter, Tacussel model 123 and pH-meter, Metrohm 744 respectively. Turbidity was measured using a turbidity meter (Hach RATIO 2100A) in accordance with standard method 2130B; a calibration standard is needed. Color measurement was determined with standard dilution multiple method [20] and by comparing absorbance to a calibration curve [21]. Dye decolorization was determined by monitoring the decrease in the absorbance peak at the maximum wavelength for the global effluent. For the wastewater used in this study only one pick was observed at 420 nm. UV–Visible spectrophotometer (Perkin Elmer Lambda 20 UV/VIS Spectrophotometer) was used in all experiments. COD was estimated by open reflux method. The protocol present a method derived from the Standard AFNOR T90-101. The sample was refluxed in an acidic medium with a known excess of potassium dichromate and the remaining dichromate was titrated with ferrous ammonium sulphate. The COD values were obtained using a Fisher Bioblock Scientific reactor COD 10119 type COD meter. 3. Result and discussion 3.1. Thermogravimetric analyses of raw materials 3.1.1. Carbon material The DSC–TGA (from 0 to 1000 °C) under air of a coal powder sample, shows a weight loss of 8.23% at 730 °C which corresponds to fly ash (Fig. 5). This result shows that fly ash represents a minor phase in the coal rock. Knowing that the fly ash is in the molecular state and scattered through the mineral coal rock [22,23], thus, it can be considered that the membrane is made totally of coal carbon.
Fig. 6. DSC–TGA data under nitrogen atmosphere of paste prepared with Novolac Resin and carbon powder.
The pore structure is formed after all volatile components present in the green body (the additives part and the noncarbonaceous part of the resin) are sublimated.
3.1.3. Starch Starch is a complex Glucide composed of molecular chains of DGlucose having a brute formula as: (C6H10O5)n. The result of DSC–TGA analyzes of a sample of starch (Fig. 7) shows that the mass loss took place in the temperature range of 250–550 °C and reached the maximum of 61%,37% at 300 °C. It can be clearly seen that when the final temperature of 750 °C was reached, all starch quantity was sublimated and was then responsible in part of the material porosity.
3.2. Optimization of carbon support composition 3.1.2. Thermosetting resin The weight loss during the carbonization of a paste prepared from resin and carbon powder without any additives was evaluated by thermogravimetric analysis. Fig. 6 shows a maximum weight loss of about 25% reached at 700 °C. Since the carbon content in the resin represents 25% of its total molecular weight, it can be concluded that coal carbon material does not participate in the total porosity of the final material. Porosity results principally from resin weight loss. Therefore, besides its role as binder and source of carbon, resin acts also as porosity agent. In addition, the sublimation of the other organic additives (especially starch) contributes also to the final porosity of the coal-based carbon membrane.
As known, powder particle size and organic porous agent percentages (starch in our case), in addition to sintering temperature, are the most important parameters affecting the structure and the porosity of porous materials [19]. In the case of carbon membranes prepared following the process described in this paper, the consolidation of the material is due to the phenolic resin crosslinking. In addition, resin participate with a constant value to the final porosity, but the main parameters to control both the pore size and the porous volume of the final material are respectively coal particle size and starch percentage the powder particle size and the presence of starch should also contribute to the structure and porosity of the membrane.
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Fig. 7. The DSC–TGA data under nitrogen atmosphere of the starch.
3.2.1. Effect of carbon particle size Different tubular supports were prepared using four carbon powders with different average particle sizes: 200 lm, 100 lm, 20 lm and 1.76 lm. Fig. 8 shows that total porous volume remains unchanged at a value of 38% for all prepared supports. However, average pore size increases when particle size increases. Average pore diameters were equal to 0,066 lm, 0.1 lm, 8 lm and 12 lm respectively for powder particle size of 1.76 lm, 20 lm, 100 lm and 200 lm. Similar results were obtained by Song et al. [14] whose found that bigger is the coal particles, higher is the average pore sizes. 3.2.2. Effect of starch percentage In order to determine the effect of starch percentage on the physical characteristics of the final material, carbon supports were prepared using various starch percentages: 0%, 2%, 4%, 8%, 10%, 12% and 15%. Fig. 9 shows that all samples have very good mechanical strength since they resisted to a mean maximum force before break varying from 29N for 0% starch content to 18N for 15% starch content. The material’s mechanical resistance decreased with the increase of starch content due to the porous volume increase. On the other hand, the porous volume increased with starch content from 25% (0% starch) to 38% (12% starch). Beyond 12% starch content, the porous volume remained constant. 25% of porosity obtained at 0% is due to resin weight loss. The increase
Fig. 9. Evolution of porous volume and mechanical resistance with the starch content.
of the porosity until 12% added starch (limit value) can be attributed to the starch sublimation. SEM pictures (Fig. 10) shows a drastic change in the final material texture with the added percentage of starch. 12% starch content show the greatest porosity. As we can see, the porous texture of the obtained tubular support is not the result of a sintering phenomenon as observed during the formation of conventional ceramic porous materials [19,24,25]. In fact, the elaborated paste is no more than a composite polymer in which the resin represents the polymer matrix and the carbon particles are the mineral charge. During the carbonization step organic additives sublimate and the resin lose weight, thus, only the carbon skeleton of the resin and the carbon coal particles remained (Fig. 11). 12% of starch content was found to be the most appropriate to obtain a material with a good mechanical strength, porosity and texture. 3.3. Active layer characterization
Fig. 8. Variation of pore size distribution with particle size of carbon powder.
A microfiltration active layer was deposited on the optimized support by slip casting. The same suspension (described earlier) was used to cast the layer. Nevertheless, different casting times (2, 4, 6 and 8 min) were tested. Fig. 12 presents the evolution pore
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Fig. 10. SEM images of two samples of carbon supports using 0% starch content and 12% starch content.
Fig. 11. SEM pictures of the final carbon support.
size distribution of the carbon membranes with casting time. It can be observed that the average pore diameter of the active layer (0.60 lm) remained unchanged for all casting times. Surface and cross-section morphologies for the different samples obtained with the different casting times were characterized by SEM micrographs. All samples showed a typical asymmetric structure but the thicknesses of the active layer increased with
casting time respectively from 6 lm for 2 min, to 30 lm for 8 min casting time (Fig. 13). When analyzing the surface texture of the different samples, it can be concluded that the sample obtained with 6 min casting time do not contain defects (cracks and pinholes) at the opposite of all other samples. So the best layer is obtained with 6 min casting time with thickness around 22 lm which is a good thickness for a microfiltration layer [18,19,24,26,27]. 3.4. Determination of microfiltration membrane permeability
Fig. 12. Mercury porosimetry analysis of the microfiltration membrane for the different casting time.
The permeability of the support and MF membrane was determined using pure distilled water. Flux values of distilled water at different operating pressures were measured and were plotted against transmembrane pressure. Samples were conditioned in the pure water during the 24 h which precede the filtration, this allows obtaining a fast stabilization of the permeate flux. Permeability was obtained according to the Darcy’s law (J = DP/l.TMP) (Fig. 14). The average values for the elaborated support and MF membrane permeability were respectively 280 L/h m2 bar and 100 L/ h m2 bar. Contact angle technique is used to estimate the hydrophobicity character of the material. The measure was repeated 4 times using a water drop volume varying between 10 and 15 lL. The measures
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Fig. 13. SEM micrographs for carbon microfiltration membrane obtained with different casting times. Surface texture is shown in the inserts.
1000
cal substances such as dyes, detergents, salts, auxiliaries (e.g. surfactants, emulsifiers) and caustic soda are used. Generally, the wastewater was coming from the dyeing, washing and bleaching baths (Table 2).
900 800
600 500 400 300 200 100
MF
Support
0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
TMP (Bar) Fig. 14. Determination of the support and MF membrane permeability.
of the angle were taken after 12 s. Macroporous supports contact angle value was equal to 80° showing a slight hydrophobic character while it was found 96.1° for the microfiltration layer corresponding to a stronger hydrophobic character. This difference in wettability, even if both support and layer had a similar chemical composition, can be related to the difference in pore size [28,29]. When using membranes, a cleaning cycle is needed for regeneration according to a protocol using an alternating rinsing of basic and acid solutions. Tests of corrosion resistance were performed on carbon supports by immersion in basic and acid solutions. For this, membrane sample was soaked at 80 °C for 30 days in bath solution at 2 wt% nitric acid (pH not exceeding 2) and NaOH (pH = 14). Every 24 h, a sample was removed, washed, dried and weighed to estimate the weight loss. It was concluded that no weight loss was recorded throughout the testing period. Therefore, it appears that the elaborated carbon membrane has a very high chemical resistance.
3.5.2. Microfiltration treatment The filtration performances were determined at a TMP of 3 bar, a velocity of 5.6 m/s and a temperature of 25 °C. The evolution of the flux with time is given by Fig. 15. The flux decreased during the first 10 min from 42 L/h m2 to 35 L/h m2 and then stabilized at 32 L/h m2. The decline of permeate flux with time is a typical behavior of membrane processes, and can be interpreted mostly in terms of concentration polarization and fouling due to the interaction between membrane material and solution [30].
Table 2 Principle physico-chemical characteristics of textile wastewater.
b
Parameters
pH
Salinity (g/L)
COD (mg/L)
Turbidity (NTU)
Colorb
Values
8.5
22.8
960
210
52
Integral of the absorbance curve in the whole visible range (400–800 nm).
50 45 40 35
J (L/h.m²)
J (L/h.m²)
700
30 25 20 15 10
3.5. Application to textile wastewater treatment
5 0
3.5.1. Wastewater characterization The study was conducted with a textile wastewater sample supplied from a Tunisian textile factory in which, different chemi-
0
10
20
30
40
50
Time (min) Fig. 15. Evolution of the permeate flux with time.
60
70
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100%
R (%)
80% 60% 40% 20%
Carbon Membrane
0%
Salinity
Alumina Membrane
COD Turbidity Color
Fig. 16. Evolution of retention of the different parameters by carbon and alumina microfiltration membrane.
carbon precursor using a new process. The porosity and the texture of the final material were controlled by adjusting coal particle size and percentage of organic additives. Tubular support (OD/ ID = 10 mm/8 mm) elaborated by paste extrusion had a porous volume of 38% and a good mechanical and chemical resistances. The active microfiltration layer was prepared by slip-casting process. The casting time was optimized to 6 min which produced a layer with thickness around 22 lm. Performances of this membrane toward the treatment of industrial textile wastewater have been studied in term of permeate flux and efficiency. The retention of polluting substances regarding COD, Turbidity, salinity and color was respectively 57%, 90%, 30% and 80%. By comparison with alumina membranes of similar porous characteristics, carbon microfiltration membrane showed the best performances.
Acknowledgements In order to explain permeate flux decline, the resistance series model can be used:
RT ¼ Rm þ Rrev þ Rirrev RT is the total filtration resistance which represents the distribution of the different resistances. Rm is the inherent hydraulic resistance of clean membrane caused by the membrane itself, it was given by the determination of pure water permeability. The Rrev resistance is due to concentration polarization and solids (cake layer) on the membrane surface, which can be removed by rinsing with water after the filtration experiment. At the contrary, the Rirrev resistance is due to pore blocking and absorption of materials onto the membrane surface and pores, which cannot be removed by a simple water rinsing but needs a chemical cleaning. After each run, the membrane was cleaned with pure water and then the permeate water flux was determined, given the Rirrev. The Rrev value was calculated following:
Rrev ¼ RT ðRm þ Rirrev Þ In our case, the different resistance values are: RT = 3.37 1011 m1, Rirrev = 2 1011 m1 et Rrev = 1.29 1011 m1. The fouling is rather irreversible (Rrev > Rirrev). Regarding the permeate quality; a total retention of color and turbidity was achieved. By an other hand the retention of polluting substances was higher than 50% except for salinity which was of only 30%. Indeed, the main mechanism of microfiltration is usually sieving but adsorption of organic and mineral pollution towards membrane surface can also occurred due to ionic and hydrophobic interaction. So, at high concentration of salt, surface charge effect can occurred which explains the retention of 30% of salt with initial concentration of 22 g/L. The difference of charge between the membrane surface (negative since pH is of 6.9) and the solution having a pH of 8.5 contributed to this retention. By comparison with alumina microfiltration membrane, carbon membrane shows similar retention of turbidity and COD and higher removal of color (Fig. 16). Removal of color is due mainly to the adsorption of dyes onto the membrane material. Retention of salinity by carbon membrane is almost twice higher than that obtained by alumina membrane. Therefore, the carbon microfiltration membrane seems to be suitable to textile effluent treatment. 4. Conclusion New carbon/carbon asymmetric microfiltration membranes have been prepared based on mineral coal and phenolic resin as
Authors would like to thank the Ministry of higher education and scientific research, Tunisia for funding this work. Authors are also very grateful to Sumitomo Bakelite Europe (Barcelona) for their help by given samples of resin.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2013.06.042.
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