Evaluation of behavior and fouling potential of wood extractives in ultrafiltration of pulp and paper mill process water

Journal of Membrane Science 368 (2011) 150–158

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Evaluation of behavior and fouling potential of wood extractives in ultrafiltration of pulp and paper mill process water L. Puro ∗ , M. Kallioinen 1 , M. Mänttäri 2 , M. Nyström 3 Laboratory of Membrane Technology and Technical Polymer Chemistry, Department of Chemical Technology, Lappeenranta University of Technology, P.O. Box 20, Lappeenranta, FIN-53851, Finland

a r t i c l e

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Article history: Received 30 August 2010 Received in revised form 25 October 2010 Accepted 12 November 2010 Available online 19 November 2010 Keywords: Ultrafiltration Foulant characterization Foulant identification FTIR Extractives

a b s t r a c t Colloidal extractives particles are potential foulants in pulp and paper mill applications. Characterization of them as foulants is a challenging task because of the complexity of these mill waters. In this study one regenerated cellulose (RC) membrane and two polyethersulphone (PES) membranes were used to study the fouling of extractives in ultrafiltration (UF) of two chemithermomechanical (CTMP) pulp mill process waters. The process waters originated from a pulping process using softwood and a mixture of hardwood and softwood. Extractives were analyzed by extracting them from the membranes and further analyzing them with gas chromatography and by measuring Fourier Transform Infrared (FTIR) spectra for the pure, fouled, and extracted membrane samples. In this study retention of extractives was over 90% even though their molar masses are less than 1 kg mol−1 . Both dissolved and colloidal extractives fouled the membranes. The fouling behavior was different for the process waters used and it seems that the individual extractive groups had different fouling mechanisms. Mostly fatty and resin acids fouled the membranes, but sterols contributed to the fouling of RC membrane remarkably even though their amount in the process waters used was low. © 2010 Elsevier B.V. All rights reserved.

1. Introduction In mechanical pulping different kinds of dissolved and colloidal substances are transferred into the process waters which limits the reuse of them. These substances are mainly lipophilic extractives, hemicelluloses and lignin-like substances [1–3]. They may be disturbing substances in pulp and paper mill processes but they can also be valuable compounds, which could be utilised, for instance, as raw materials in food, pharmaceutical and chemical industry as well as in biofuel manufacturing. Before that they have to be separated from the process waters. One feasible alternative for separation of them is ultrafiltration (UF) [4,5]. However, fouling still limits the adoption of UF in pulp and paper mill applications to some extent. Colloidal particles are potential foulants in pulp and paper mill applications [6–8]. Such colloidal particles are for instance lipophilic extractives, generally called wood resin. Usually dry

∗ Corresponding author. Tel.: +358 40 168 2439; fax: +358 5 621 2199. E-mail addresses: liisa.puro@lut.fi (L. Puro), mari.kallioinen@lut.fi (M. Kallioinen), mika.manttari@lut.fi (M. Mänttäri), marianne.nystrom@lut.fi (M. Nyström). 1 Tel.: +358 40 5939 881; fax: +358 5 621 2199. 2 Tel.: +358 40 734 2192; fax: +358 5 621 2199. 3 Fax: +358 5 621 2199. 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.11.032

wood contains resin less than 10% by weight [9]. Even 80% of the wood resin can be transferred into the process waters at different stages in pulp and paper production [10]. In the process waters most of the lipophilic extractives are in the form of colloidal resin particles [11–13]. Only fatty and resin acids and sterols are found in dissolved form in these waters [12]. In colloidal resin particles steryl esters and triglycerides, which are the most hydrophobic components of the extractives [14,15], form the hydrophobic core while fatty and resin acids and sterols form the thin surface layer of the particles. The carboxyl groups of fatty and resin acids are orientated toward the aqueous phase [11,14,16]. Therefore, the particles are negatively charged in the pH range 2–11 [11]. Carboxyl and hydroxyl groups on the surface of colloidal resin particles stabilize them electrostatically [16] while hemicelluloses, especially mannans, stabilize them sterically [17,18]. The particle size distribution of the colloidal resin in different pulp qualities, Kraft, sulphite and groundwood, is similar according to Allen [11]. Most of the colloidal resin particles have diameters in the range 0.1–2 ␮m [11,12,17] and the average is about 0.6 ␮m [11,17]. Moreover, these particles are spherical in shape in the process waters [11]. The resin acids have colloidal pKa values between 6 and 7 at 20 ◦ C, at 50 ◦ C they are around 6. At pH 6 resin acids are capable to dissolute from the colloidal resin particles into the water. Therefore, resin acids gradually dissolute into the water from colloidal resin

L. Puro et al. / Journal of Membrane Science 368 (2011) 150–158

particles at a pH around 6; the higher the pH the less resin acids are in the colloidal resin particles [19–21]. Furthermore, the most of the fatty acids have higher colloidal pKa values (at pH 8–10) than resin acids [19]. The amount of lipophilic extractives varies depending on e.g. wood species, harvest time and storage. A typical compound of each extractive group is shown in Fig. 1. The dominant fatty acids are oleic, linoleic and pinolenic acids [1]. The dominant resin acid is dehydroabietic acid but also pimaric, isopimaric, sandaracopimaric, abietic, neoabietic, levopimaric and palustric acids are found in process waters [22]. In filtration of pulp and paper mill waters extractives are mentioned many times as potential foulants [6–7] but the analysis and identification of them as foulants [23–25] is limited due to the complexity of process waters. Therefore, fouling due to extractives has been studied for instance with filtration of model substances and it is evaluated only by flux and retention measurements [26,27]. Ramamurthy et al. [6] have used tall oil to represent fatty and resin acids in characterization of fouling in filtration of pulp and paper mill process waters. They concluded from flux measurements that fatty and resin acids promote gel layer formation on the membrane surface when high molar mass solutes are present [6]. Zaidi et al. [28] used dehydroabietic acid for modelling of the behavior of resin acids and they found that the flux decline was negligible. There are rare studies in which fouling is characterized and foulants are identified with analysis methods, such as Time-ofFlight Secondary Ion Mass Spectrometry (ToF-SIMS) [29], Atennual Total Reflectance Fourier Transform Infrared Spectroscopy (ATRFTIR) [23,30], Scanning Electron Microscope (SEM) [30] and Energy Dispersive Spectrometry (EDS) [6,30], in filtration of pulp and paper mill waters. Spevack and Deslandes [29] used ToF-SIMS to analyze the adsorption of dehydroabietic acid on polyvinylidenefluoride (PVDF) membrane. They found that dehydroabietic acid adsorbs inhomogeneously on the membrane surface which suggests that specific sites are present for the adsorption. Carlsson et al. [23] used ATR-FTIR in studying membrane fouling in filtration of plug screw feeder pressate (PSFP) pulp mill effluent. They found that there were fatty rather than resin acids in the membranes according to this analysis method [23]. Kallioinen et al. [30] used ATR-FTIR to analyze organic foulants from the membrane used in filtration of groundwood mill circulation water. They found fatty and resin acids and cellulosic species from the fouling layer. Puro et al. [24] extracted organic foulants from membranes and further identified them with gas chromatography (GC). They found that especially fatty and resin acids fouled the membranes in the filtration of groundwood mill circulation water. As shown above, wood extractives are many times claimed as foulants in pulp and paper mill applications [6–8] but their contribution to the fouling is limitedly understood due to the limited characterization and identification of them as foulants. The aim of this study was thus to produce new information on fouling behavior of extractives by identifying them from both the process waters and the fouled membrane samples with various analysis methods.

2. Experimental 2.1. UF membranes used The UC030 membrane is made from regenerated cellulose (RC) and the UH030P and UH050P membranes are made from polyethersulphone (PES). According to the manufacturer (Microdyn-Nadir GmBH), the UC030 and UH030P membranes have a cut-off value of 30 kDa and the UH050P membrane has a cut-off value of 50 kDa. The recommended maximum temperature for the RC and the PES membranes is 55 ◦ C and 95 ◦ C and the recommended pH range 1–11 and

151

Table 1 Measured cut-off values with polyethyleneglycol (PEG) [31], contact angle values, surface roughness values and charges (calculated to zeta potentials) of the tested UC030, UH030P and UH050P membranes.

Measured cut-off, kg mol−1 Contact angle,◦ RMSb roughness, nm Zeta-potential along the surface at pH 6c , min/max mV Zeta-potential through the membrane at pH 6c , min/max mV

UC030

UH030P

UH050P

10 <15 1.0 ± 0.1 −4.4/−6.2

32 72 ± 4 0.76 ± 0.1 −5.4/−7.2

>35a 64 ± 5 1.8 ± 0.1 −3.5/−3.8

−0.6/−0.9

−1.2/−1.3

−1.6/−1.9

a The cut-off value was higher than the biggest PEG used in the cut-off value measurements. b Root mean square, analyzed area was 1 × 1 ␮m. c Three measurements.

1–14, respectively. Table 1 shows measured cut-off values, contact angle values, surface roughness values and charges for the clean membranes. The details of the analyses are found in our previous paper [31]. 2.2. Filtration and adsorption procedures Filtration experiments were performed as described in our previous study [31] but here briefly. The fouling experiments were made with an Amicon filter, manufactured by Millipore, USA, operating in a batch mode. The membrane area was 0.004 m2 . The membranes were pre-treated before the experiments by rinsing them three times in an ultrasonic bath with reverse osmosis purified water for 10 min to remove preservatives. A new piece of membrane was used in each filtration. The filtered RO water and the process waters were heated to 40 ◦ C before filtration because this temperature is a very common for paper mill waters. The pure water fluxes of the membranes were measured at a temperature of 40 ◦ C before and after the process water filtration for evaluation of irreversible fouling. In each filtration the feed batch was 300 g. 225 g permeate was collected during the filtration. Thus, 75 g was left in the concentrate. The experiments were performed at pressures of 1 and 2 bar. The feed was mixed continuously with a velocity of 1.8 m s−1 . Each experiment was repeated three times to verify the results. Adsorption tests were made for the tested membranes in order to test if adsorption is one reason for fouling. No pressure was used in the adsorption tests and the adsorption time was the same length as the filtrations at 1 bar. The adsorption time in softwood process water filtration was 83, 74 and 72 min and in hardwood process water filtration it was 107, 87 and 134 min for the UC030, UH030P and UH050P membranes, respectively. 2.3. Process waters used In every filtration experiment feed, permeate and concentrate samples were analyzed. The details of the water analyses are found in our previous paper [31]. The analysis of the extractives is explained in detail in Sections 2.5 and 2.6. Two different process waters from pulp mill were used in the fouling experiments. The process water samples for the filtration experiments were collected at a CTMP mill. One water originated from a pulping process using softwood and the other was from a pulping process using hardwood. These kinds of process waters generally contain extractives, dissolved hemicelluloses, pectic acids, lignin, lignans, and low-molar mass carboxylic acids, mainly acetic acid. As a result of the differences in the extractive and hemicellulose content of softwood and hardwood, the contents of the process waters may differ from each other; only softwood con-

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Fig. 1. Structures of typical compounds of each extractive group.

tains resin acids. Their composition of phenolic extractives is also different [32]. The process waters were centrifuged twice with a force of 500 g in order to remove the fines and fibres; first before the filtration experiment and then before the analysis. The analysis results of the process waters from the pulping of softwood and hardwood are shown in Table 2. The pH of the process waters is similar but the total carbon amount and turbidity are twofold greater in the hardwood process water than in the softwood process water. The higher turbidity indicates that the hardwood process water contains more colloidal substances or fines than the softwood process water. In addition, the cationic demand of the hardwood process water is threefold that of the softwood process water. Fatty and resin acids are negatively charged but the difference between the

process waters in cationic demand cannot be explained by the amount of fatty and resin acids, because hardwood contains less fatty and resin acids and its cationic demand is higher. Therefore, the difference in cationic demand may be due to the hemicelluloses. In this study extractives were divided into six different groups: fatty and resin acids, lignans, sterols, steryl esters and triglycerides according to the analysis method used [33]. All but lignans are lipophilic extractives. Analysis of extractives composition of the process waters used revealed that the hardwood process water contains resin acids. Therefore, the hardwood process water used is originated from the pulping process using mixture of hardwood and softwood because hardwood lacks resin acids. The total amount of lipophilic extractives is about 1650 mg L−1 for both process waters used. However, the extractives composition varies; the softwood

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153

Table 2 Characteristics of the filtered softwood and hardwood + softwood waters (results of 6 samples). Parameter

pH, – Conductivity, mS cm−1 Turbidity, NTU Total carbon (TC), mg L−1 Carbohydrates, mg L−1 Cationic demand, ␮eq g−1 Fatty acids, mg L−1 Resin acids, mg L−1 Lignans, mg L−1 Sterols, mg L−1 Steryl esters, mg L−1 Triglycerides, mg L−1

Softwood

Hardwood + softwood

Average

Range

Average

Range

6.1 5.2 ± 0.1 5800 ± 2100 4300 ± 400 970 ± 130 3.4 ± 0.1 300 ± 150 560 ± 180 61 ± 19 17 ± 8 270 ± 35 500 ± 47

6.09–6.12 5.1–5.3 4400–9700 3800–4700 820–1100 3.2–3.5 110–420 330–770 45–90 11–28 230–320 450–540

6.4 14 ± 5 9200 ± 2000 8300 ± 400 1200 ± 200 11 ± 0.6 130 ± 60 410 ± 140 60 ± 30 20 ± 7 620 ± 130 500 ± 100

6.38–6.42 12–21 6900–11,500 8000–8900 1000–1400 10–11 60–210 270–570 28–110 11–30 460–770 410–630

process water contains more fatty and resin acids while the hardwood + softwood process water contains more steryl esters. Thus, the extractives composition of the process waters used is different. In addition hardwood + softwood process water contains more fines because the amount of extractives is the same but its turbidity is higher than the turbidity of the softwood process water. 2.4. Isolation and characterization of extractives from the membranes The extractives were isolated from the fouled membranes by extraction and further analyzed by gas chromatography (GC). A Soxtec System HT6 1433 Extraction unit (Tecator, Höganäs, Sweden) was used in the extractions. The membrane sample of 0.0034 m2 was placed in a 26 mm × 60 mm extraction thimble (supplied by the manufacturer), and extracted with 30 mL acetone–water solution (1:9, v/v) in boiling solvent for 30 min. Thereafter the thimble was raised to the rinse position for another 30 min. After extraction all the solvent was collected to the extraction cups and the obtained solution was weighed to be able to calculate the extractives amount in the extracted membrane area. From the whole extract 1 mL of the samples were taken, which was weighted. After that the samples were dried in a nitrogen stream. The samples may contain polymeric material from the membranes and it may harm the gas chromatograph (GC) column. Therefore, the samples were diluted in 1 mL of reverse osmosis (RO) water and they were extracted a second time with liquid–liquid extraction as described in Section 2.6, in order to get rid of the polymeric membrane material. Isolation and identification of extractives with GC after extraction is time-consuming. To find a faster way to characterize extractives as foulants the chemical structure of the pure and fouled membrane samples were analyzed with ATR-FTIR. In this analysis a Perkin-Elmer 2000 FTIR spectrometer with the wire coil operating at 1350 K as a radiation source, triglycine sulphate (TGS) as a detector and optical KBr as a beamsplitter was used. A KRS-5 crystal (thallous bromide iodide) was used as an internal reflection element. The effective incident angle of the IR radiation was 54◦ . The measurement was repeated after extraction for the membrane samples, which were dried at room temperature. The missing or changed bands can implicate two things; they are the bands for the foulants such as extractives or the membrane structure has changed during the extraction procedure. 2.5. Liquid–liquid extraction and silylation The extractives from the process water samples and the membrane extracts were separated by the liquid–liquid extraction as Örså and Holmbom have described [33]. First a drop of 1 M sulphuric acid was added to adjust the pH to 3–3.5. Then

2 mL of tert-butylmethylether (MTBE) and 200 ␮L of standard solution were added. The internal standard solution contained heneicosanoic acid, betulinol, cholesteryl heptadecanoate and 1,3dipalmitoyl-2-oleoyl glycerol, each 35 mg L−1 in MTBE. Then the samples were vigorously shaken by a shaker machine for 5 min and after that they were centrifuged at 500 g for 5 min. Finally the clear MTBE layer was carefully pipetted off. The obtained MTBE solution was evaporated in a nitrogen stream and was further freeze-dried. For GC analysis the samples were silylated with 100 ␮L BSTFA [bis-(trimethylsilyl)-trifluoroacetamide] and 50 ␮L TMCS (trimethylchlorosilane). The solution was kept in an oven at 70 ◦ C for 40 min and after that it was ready for analysis with GC. 2.6. Gas chromatograph analysis The samples were injected with an autoinjector directly in the column. The column was a 6 m/0.53 mm i.d. wide-bore capillary column with a nonpolar phase (MTX-1, Restek), film thickness 0.15 ␮m. During the injection the injector temperature was 85 ◦ C. The column temperature was 85 ◦ C–12 ◦ C/min–345 ◦ C, 5.5 min, 100 ◦ C/min–365 ◦ C. The flame ionization detector (FID) temperature was 370 ◦ C. Hydrogen was used as a carrier gas. The total analysis time of each sample was about 35 min. The fatty and resin acids were determined relative to the peak area of the heneicosanoic acid standard, lignans and sterols relative to betulinol, steryl esters relative to cholesteryl heptadecanoate and triglycerides relative to 1,3-dipalmitoyl-2-oleoyl glycerol. The amount in mg/area was calculated from the following equation: Extractives, mg Area, m2 =

(Sample peak area) (Standard amount, mg) (Total extract amount, g) (Standard peak area) (Membrane area, m2 ) (Extract sample amount, g)

3. Results and discussion 3.1. Adsorptive fouling caused by extractives The adsorptive type of fouling due to the extractives was remarkable with both process waters tested because extractives were found in all the membranes after the adsorption tests (Fig. 2). However, triglycerides could not be found from the membranes after the adsorption test with the softwood process water, but they were found from the membranes after the adsorption test with hardwood + softwood process water. Thus, it seems that the fouling behavior of triglycerides was different with different process waters. One reason for this could be the different state of

300

200

100

0 Soft

Hard + Soft

s er l yc

Tr ig

St er

yl

es

ter

id e

s

ols St er

s

an s Lig n

Fa tt

3.2. Fatty and resin acids In the filtrations the fatty and resin acid retentions were over 97% and 98%, respectively, for both process waters and for all the membranes and pressures used, even though fatty and resin acids have a typical molar mass of about 300 g mol−1 . Similar results were obtained by Dal-Cin et al. [8], Zaidi et al. [28] and Elefsiniotis et al. [34]. Dal-Cin et al. [8] found that the retention of fatty and resin acids was 70–90% with membrane which had a cut-off value of 100 kDa. Zaidi et al. [28] filtered dehydroabietic acid to represent resin acids and even though this model substance had lower molar mass than the cut-off value of the membranes used (about 2–60 kDa) the retentions were 40–90%. They concluded also that generally the retention of the model substances used was lower for the more hydrophilic membranes. In a research of Elefsiniotis et al. [34] the retention of fatty acids was over 90% while the resin acid retention was 25–45% with PES membranes with cut-off values of 10 and 100 kDa in filtration of simulated white water. In all the above filtrations retention values vary that maybe due to the different waters used in the filtrations. Based on all above results fatty and resin acids are in colloidal form in the process waters but it is also possible that they have decreased the pore size of the membranes by pore blocking and thus, the retentions are high. Fatty and resin acids were found in the membranes fouled at all the pressures used as was expected because in an earlier study [24] dissolved fatty and resin acids fouled the membranes in the filtration of groundwood mill circulation water. Resin acids were found more than fatty acids (Fig. 3) and they were found more from the more hydrophobic PES membranes than from the hydrophilic RC membrane (Table 1). The result is consistent with our earlier study [24]. The more hydrophobic was the membrane the more fatty and resin acids were found from it. Thus, the UH030P membrane contained most of them while the UC030 membrane contained the

Amount in membrane, mg/m2

Amount in membrane, mg/m2

Fatty acids

0

) membranes at 40 ◦ C after adsorption test of softwood and hardwood + softwood

Fig. 2. Average amount of extractives found in the UC030 (), UH030P ( ) and UH050P ( process waters at 0 bar pressure. There are three replicates of each membrane.

triglycerides in water. In colloidal resin particles steryl esters and triglycerides form the hydrophobic core while fatty and resin acids and sterols form the thin surface layer of the particle. The carboxyl groups of fatty and resin acids stabilize particles electrostatically [16] while hemicelluloses, especially mannans, stabilize these particles sterically [17,18]. The composition of fatty and resin acids is different for the process waters used and thus, the colloidal particles could have different electrostatic stabilization. Furthermore, mannans are found more from softwood because the principal hemicellulose in it is galactoglucomannan (about 20%). On the other hand, hardwood contains only 2–5% glucomannan because the major hemicellulose in it is glucuronoxylan (15–30%) [9]. Thus, the sterical stabilization of the colloidal particles in the process waters used could be different due to the different composition of the hemicelluloses. This difference in stabilization of colloidal particles can explain the difference in triglycerides found in the membrane. In the softwood process water they are in these stabilized droplets and, therefore they cannot adsorb on the membrane surface. The amount of sterols was less than 30 mg L−1 in the process waters used (Table 2) but they were found significantly from the RC membrane after the adsorption tests. About 1–2% of sterols of the process waters used were adsorbed on the RC membrane while less than 1% of the other extractives of the process waters used were adsorbed on the RC membrane. The influence of electric repulsion on adsorptive fouling was also negligible because the adsorptive fouling was similar for both process waters. If the electric repulsion between the process water and the membrane would play a significant role in the fouling it should have been more severe in UF of the softwood process water because the cationic demand of it is a third of the cationic demand of the hardwood + softwood process water (Table 2). However, this was not the case.

50

id

id es

er s st le ry

St e

Tr igl yc er

ro ls St e

Lig na ns

ids ac

Re si n

Fa t

ty a

cid

s

0

100

ac

50

150

in

100

200

s

150

Hardwood + Softwood

250

Re s

200

ya c id

Softwood

250

Amount in membrane, mg/m2

L. Puro et al. / Journal of Membrane Science 368 (2011) 150–158

Amount in membrane, mg/m2

154

Resin acids 300

200

100

0

Fig. 3. Average amount of fatty and resin acids found in the UC030 (), UH030P ( ) and UH050P ( hardwood + softwood process water. There are three replicates of each experiment.

Soft

Hard + Soft

) membranes at 40 ◦ C and at 1 bar pressure after UF of softwood and

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155

Fig. 4. Retention of lignans for the UC030, UH030P and UH050P membranes at 40 ◦ C and at pressures of 1 and 2 bar for softwood and hardwood + softwood process waters. There are three replicates of each experiment.

UC

0

UH

03

0P

0P

05

UH

0 0P

30

5

05

0

10

H

5

15

U

10

Only one sample

20

0P

15

25

03

20

H

25

Hardwood + Softwood 30

U

Softwood

30

0

Amount in membrane, mg/m2

The low molar mass of sterols would suggest (about 400 g mol−1 ) that their retention should be low; however, the situation was the opposite. The sterols retention was over 90% in almost all filtrations. Sterols contributed about 7% and 8% of extractives fouling of RC membrane in the filtration of softwood and hardwood + softwood process waters, respectively, even though their

03

3.4. Sterols

The retention of steryl esters and triglycerides was almost 100% in all the filtrations for both process waters. However, the fouling behavior of them was different for the process waters used. It seems that the membrane material or hydrophilicity significantly influenced the fouling behavior of them in UF of the softwood process water, because the RC membrane was fouled the most (Fig. 6). The dominant extractive group in hardwood + softwood process water is steryl esters but their contribution to fouling was minor (less than 11%) compared to fatty and resin acids (an average 80%). In UF of hardwood + softwood process water the fouling behavior was different for steryl esters and triglycerides. The influence of the membrane material or hydrophilicity was negligible in the fouling of steryl esters because only one of the PES membranes was fouled significantly when pressure was applied. The zeta potential along the surface and/or, roughness could be the reasons for fouling by steryl esters, because the UH050P had the least negative zeta potential and it was the roughest of the membranes used (Table 1). However, strong conclusions cannot be drawn from membrane charge because the differences in zeta potential are negligible. Triglycerides were found mainly in the fouled UC030 and UH050P membranes in UF of hardwood + softwood process water. Thus, it seems that the membrane material or the hydrophilicity were unimportant in the fouling behavior of triglycerides because the fouled membranes are made of different materials (RC and PES)

C

The lignans retention depended on the membrane and the process water used (Fig. 4). It was similar for all the membranes in UF of the hardwood + softwood process water. However, in UF of the softwood process water the retention was the lowest with the UC030 and UH050P membranes, which had the lowest and highest cut-off of the used membranes, respectively. The composition of lignans is different in softwood and hardwood [35]. This difference in composition of lignans and/or molar mass may explain the lower retentions in UF of softwood. The amount of lignans found on the fouled membranes was less than 20 mg m−2 and their amount in all the membranes was almost the same.

3.5. Steryl esters and triglycerides

U

3.3. Lignans

amount in the feed was only 1% of the analyzed extractives. Sterols were the worst foulants for the RC membrane in UF of both process waters, if the fouling behavior is evaluated based on which amount of extractives in the feed is found in the fouled membranes. The reason for this could be the fact that sterols are the most hydrophilic compounds of the extractives [14]. The fouling behavior of sterols was similar in both process water filtrations (Fig. 5). Probably the membrane material or hydrophilicity had a great effect on the fouling potential of sterols because they were found especially from the UC030 membrane, which was the most hydrophilic of the membranes used (Table 1).

Amount in membrane, mg/m2

least amount of fatty and resin acids. Because both PES membranes used were made by the same manufacturer and FTIR data revealed that their material was the same, the difference in contact angle (72◦ and 64◦ for UC030P and UC050P, respectively) could be due to the different morphology of the membranes. Based on these results, it seems that the hydrophilicity was important factor affecting the fouling behavior of the fatty and resin acids. Fouling of fatty and resin acids was quite similar with both process waters even though their amount in the waters was different; the softwood process water contained more fatty and resin acids than the hardwood + softwood process water (Table 2). It is remarkable that the fatty and resin acids contributed on an average about 80% of the extractive fouling in the membranes even though their part of the extractives in the feed was 50% and 30% in the softwood and hardwood + softwood process water, respectively.

Fig. 5. Average amount of sterols found in the UC030, UH030P and UH050P membranes at 40 ◦ C and at pressures of 0 (), 1 ( hardwood + softwood process water. There are three replicates of each.

) and 2 (

) bar in UF of softwood and

Softwood

100 80 60 40 20 0

0 03 UC

0P 03 UH

0P 05 UH

Amount in membrane, mg/m2

L. Puro et al. / Journal of Membrane Science 368 (2011) 150–158

Amount in membrane, mg/m2

156

Hardwood + Softwood 100 80 60 40

Only one sample

20 0

0 03 UC

0P 03 UH

0P 05 UH

Fig. 6. Average amount of steryl esters found in the UC030, UH030P and UH050P membranes at 40 ◦ C and at pressures of 0 (), 1 ( hardwood + softwood process water. There are three replicates of each experiment.

UC

0 UH

03

0P UH

05

0P

0

0P

03

20

05

0

Only one sample

40

U H

20

60

0P

40

80

03

60

100

U H

80

0

100

Hardwood + Softwood 120

03

Softwood 120

A rapid and reliable test method for the identification of extractives, such as FTIR, from the membranes is needed, because analysis of them by extraction and GC is difficult, expensive and timeconsuming. However, the results of this study demonstrated well that it is very challenging to use FTIR, especially in the characterization of real pulp and paper mill effluents fouling, due to their chemical complexity. The main lipophilic extractives, fatty and resin acids, steryl esters and triglycerides all contain C O groups in their structures. The C O group gives a strong band in the range 1690–1750 cm−1 depending on the structure, in which the C O group is situated. Thus, this band in the spectra of the fouled membrane samples is significant when the fouling of extractives is analyzed. Extractives fouling was possible to evaluate from the PES membranes UH030P and UH050P with FTIR by using the band for the C O group because there was no interfering overlapping of this band and the bands of the membrane material. In Fig. 8 is seen the C O stretching band at 1731 cm−1 for the UH050P membrane and the amount of the compounds containing C O groups found in the membranes. It is assumed that mostly fatty and resin acids contribute to the C O stretching band at 1731 cm−1 because their amount found in the membranes was the highest. A clear correlation between the amount of extractives and the intensity of the band at 1731 cm−1 was not found. The absence of correlation can be due to the fact that the extractives can be inhomogenously distributed on the membrane matrix and surface. Spevack and Deslandes [29] found in their study that dehydroabietic acid was inhomogenously distributed on the membrane surface.

C

Amount in membrane, mg/m2

In previous research [31] we noticed that the contact angles of the fouled UC030 membranes increased significantly due to fouling in UF of CTMP process waters. The contact angle of the virgin membrane was <15◦ while contact angles of the fouled UC030 membranes ranged between 65◦ and 97◦ . It was assumed that the reason for this was the adsorption of e.g. fatty and resin acids on the membrane surface in such a way that their hydrophobic tails were orientated toward the feed solution. In this research triglycerides were found from the UC030 membrane and they are the most hydrophobic of the extractives. It is possible thus, that triglycerides increase the contact angles also. To evaluate wood extractive fouling faster in the future the correlation between the contact angles and the amount of fatty and resin acids or other extractives was tried to find for the UC030 membrane. However, no clear correlation between the contact angles

3.7. FTIR analysis

U

3.6. Contact angle and extractives

) bar in UF of softwood and

of the used membranes and the amount of fatty and resin acids or other extractives found in the membranes was noticed for the UC030 membrane for any of the filtration pressures used. This can be due to the fact that fouling layer on the membrane surface may contain other foulants, which influence on the contact angle.

Amount in membrane, mg/m2

and their hydrophilicity is different (Table 1). Moreover, there were only few triglycerides in the other PES membrane UH030P although its material and hydrophobicity are similar to the UH050P membrane. It seems that the reason for the fouling was the membrane charge and/or the roughness; the UC030 and UH050 membranes had the least negative zeta potential values along the surface, and the highest roughness values of the membranes used. Steryl esters and triglycerides were expected to have similar fouling behavior in filtrations because they form the hydrophobic core of the colloidal particle [14,15] and they are not in dissolved form in the pulp and paper mill process waters [12]. However, this was not the case (Figs. 6 and 7). Moreover, in earlier study [24] dissolved fatty and resin acids not the colloidal ones fouled the membranes in the filtration of groundwood mill circulation water. Thus, the obtained results in this research are different from the earlier ones and they indicate that not only dissolved compounds but also colloidal resin particles fouled the membranes used.

) and 2 (

Fig. 7. Average amount of triglycerides found in the UC030, UH030P and UH050P membranes at 40 ◦ C and at pressures of 0 (), 1 ( of each experiment.

) and 2 (

) bar. There are three replicates

L. Puro et al. / Journal of Membrane Science 368 (2011) 150–158 UH050P sample 1

UH050P sample 2

UH050P sample 3

500

Amount in membrane, mg/m2

UH050P reference

1731 cm-1

157

400

Triglycerides Steryl esters Resin acids Fatty acids

300

200

100

0 1800

1700

1600

1500

1400

1300

1200

1100

1000

1 2 3 Membrane sample

Fig. 8. IR-spectra of virgin and fouled UH050P membranes and the amount of fatty and resin acids, steryl esters and triglycerides found in the membrane at 40 ◦ C and 2 bar in UF of hardwood + softwood process water.

A correlation between the intensity of the band corresponding to the C O stretching at 1731 cm−1 and the amount of the compounds containing C O groups could not be found for the UC030 membrane either. Earlier studies have shown that the UC030 membrane contains cellulose acetate [36], in which the C O group gives a strong band at 1735 cm−1 . This band is overlapping with the band at 1731 cm−1 , which interferes the analysis. Thus, to be able to analyze extractives as foulants on the UC030 membranes the samples should be derivatized somehow to change the place of the extractive band in the spectra. FTIR was also used to verify the effectiveness of the extractives removal from the membranes. Acetone, which contains the C O group having a strong band at 1731 cm−1 , was used in the extraction of the membrane samples. Due to the overlapping of the bands of acetone and extractives the verification of extractives removal efficiency was difficult. The chemical structure of the membranes was also broken in extraction which changed the IR spectra significantly and made the evaluation harder. Thus, based on this study it is very challenging to see from FTIR spectra if all the foulants are extracted from the membranes.

groups had different fouling behavior. Therefore, to understand fouling more profoundly the foulants in the membranes have to be identified and their fouling has to be analyzed individually as in this research. FTIR can be used in fouling detection in the membranes, but alone and without derivatization of examined compounds it is an insufficient analysis method for the identification of extractives as foulants of membranes. However, it is a good supporting method together with other analysis methods. In the future derivatization of the samples for FTIR analysis will be tried to indicate the extractives as foulants because the easier, a cheaper and quicker method than extraction is needed for analyses of foulants.

4. Conclusions

References

In this study the fouling behavior of extractives was evaluated with RC and PES membranes in filtration of two pulp mill process waters. One process water came from a pulping process using softwood and the other from a pulping process using hardwood and softwood. Based on the results both dissolved and colloidal extractives fouled the membranes, which is contradictory to an earlier study [24]. The fouling behavior of extractives was different for the used process waters. This can be due to the extractives composition that was different in the softwood and hardwood + softwood process waters even though their total amount was the same. Mostly fatty and resin acids were found on the fouled membranes. Sterols contributed to the fouling of RC membrane remarkably even though their amount in the process waters used was low. Adsorption was at least one of the fouling mechanisms because there were extractives in all the membranes after an adsorption test. In UF of softwood process water the membrane hydrophilicity played an important role in fouling for all extractives. On the other hand, in UF of hardwood + softwood process water the hydrophilicity affected the fouling behavior of sterols and fatty and resin acids. It seems that the individual extractive

Acknowledgements The authors wish to thank Ms. Elsi Koivula and Mrs. Helvi Turkia for their valuable contribution to this study. The authors are also grateful to the Academy of Finland (project SA/206064 and project SA/122181) and The Finnish Foundation for Economic and Technology Sciences for financial support.

[1] R. Ekman, B. Holmbom, Analysis by gas chromatography of the wood extractives in pulp and water samples from mechanical pulping of spruce, Nord. Pulp Pap. Res. J. 4 (1989) 16–24. [2] J. Sjöström, Fractionation and characterization of organic substances dissolved in water during groundwood pulping of spruce, Nord. Pulp Pap. Res. J. 5 (1990) 9–15. [3] J. Thornton, R. Ekman, B. Holmbom, F. Örså, Polysaccharides dissolved from Norway spruce in thermomechanical pulping and peroxide bleaching, J. Wood Chem. Technol. 14 (1994) 159–175. [4] S. Willför, K. Sundberg, M. Tenkanen, B. Holmbom, Spruce-derived mannans—a potential raw material for hydrocolloids and novel advanced natural materials, Carbohydr. Polym. 72 (2008) 197–210. [5] T. Persson, A. Nordin, G. Zacchi, A. Jönssön, Economic evaluation of isolation of hemicelluloses from process streams from thermomechanical pulping of spruce, Appl. Biochem. Biotechnol. 137–140 (2007) 741–752. [6] P. Ramamurthy, R. Poole, J.G. Dorica, Fouling of ultrafiltration membranes during treatment of CTMP screw press filtrates, J. Pulp Pap. Sci. 21 (1995) J50–54. [7] C.S.F. Ragona, E.R. Hall, Parallel operation of ultrafiltration and aerobic membrane bioreactor treatment systems for mechanical newsprint mill white water at 55 ◦ C, Water Sci. Technol. 38 (1998) 307–314. [8] M. Dal-Cin, C.N. Striez, T.A. Tweddle, F. McLellan, P. Ramamurthy, Membrane performance with plug screw feeder pressate: operating conditions and membrane properties, Desalination 105 (1996) 229–244. [9] E. Sjöström, Wood Chemistry: Fundamentals and Applications, 2nd ed., Academic Press, San Diego, 1993. [10] J. Käyhkö, The influence of process conditions on the deresination efficiency in mechanical pulp washing, Ph.D. Thesis, Lappeenranta University of Technology, Lappeenranta, Finland, 2002.

158

L. Puro et al. / Journal of Membrane Science 368 (2011) 150–158

[11] L.H. Allen, Characterization of colloidal wood resin in newsprint pulps, Pulp Paper Can. 76 (1975) T139–T146. [12] M. Rundlöf, A. Sjölund, H. Ström, I. Åsell, L. Wågberg, Effect of dissolved and colloidal substances released from TMP on the properties of TMP fines, Nord. Pulp Pap. Res. J. 15 (2000) 256–265. [13] A. Sundberg, R. Ekman, B. Holmbom, K. Sundberg, J. Thornton, Interactions between dissolved and colloidal substances and a cationic fixing agent in mechanical pulp suspensions, Nord. Pulp Pap. Res. J. 8 (1993) 226–231. [14] M. Qin, T. Hannuksela, B. Holmbom, Physico-chemical characterisation of TMP resin and related model mixtures, Colloids Surf. A 221 (2003) 243–254. [15] M. Qin, B. Holmbom, Effect of hydrophilic substances in spruce TMP resin on its physico-chemical characterization and deposition tendency, Colloids Surf. A 312 (2008) 226–230. [16] J. Nylund, K. Sundberg, Q. Shen, J.B. Rosenholm, Determination of surface energy and wettability of wood resins, Colloids Surf. A 133 (1998) 261–268. [17] B. Holmbom, A. Sundberg, Dissolved and colloidal substances accumulating in papermaking process waters, Wochenbl. Papierfabr. 131 (2003) 1305–1311. [18] K. Sundberg, J. Thornton, C. Pettersson, B. Holmbom, R. Ekman, Calciuminduced aggregation of dissolved and colloidal substances in mechanical pulp suspensions, J. Pulp Paper Sci. 20 (1994) J317–J322. [19] D.S. McLean, D. Vercoe, K.R. Stack, D.E. Richardson, The colloidal pKa of lipophilic extractives commonly found in Pinus radiata, Appita J. 58 (5) (2005) 362–366. [20] K. Sundberg, C. Pettersson, C. Eckerman, B. Holmbom, Preparation and properties of a model dispersion of colloidal wood resin from Norway spruce, J. Pulp Paper Sci. 22 (1996) J248–J252. [21] V. Saarimaa, L. Vähäsalo, A. Sundberg, A. Pranovich, B. Holmbom, M. Svedman, Influence of pectic acids on aggregation and deposition of colloidal pitch, Nord. Pulp Pap. Res. J. 21 (2006) 613–619. [22] H.-W. Liu, S.-N. Lo, H.-C. Lavallee, Mechanisms of removing resin and fatty acids in CTMP effluent during aerobic biological treatment, Tappi J. 79 (1996) 145–154. [23] D.J. Carlsson, M. Dal-Cin, P. Black, C.N. Lick, A surface spectroscopic study of membranes fouled by pulp mill effluent, J. Membr. Sci. 142 (1998) 1–11. [24] L. Puro, J. Tanninen, M. Nyström, Analyses of organic foulants in membranes fouled by pulp and paper mill effluent using solid–liquid extraction, Desalination 143 (2002) 1–9.

[25] A. Weis, M.R. Bird, M. Nyström, The chemical cleaning of polymeric UF membranes fouled with spent sulphite liquor over multiple operational cycles, J. Membr. Sci. 216 (2003) 67–79. [26] C. Jönsson, A. Jönsson, Influence of the membrane material on the adsorptive fouling of ultrafiltration membranes, J. Membr. Sci. 108 (1995) 79–87. [27] A.-S. Jönsson, J. Lindau, R. Wimmerstedt, J. Brinck, B. Jönsson, Influence of the concentration of a low-molecular organic solute on the flux reduction of a polyethersulphone ultrafiltration membrane, J. Membr. Sci. 135 (1997) 117–128. [28] A. Zaidi, H. Buisson, S. Sourirajan, Ultrafiltration in the concentration of toxic organics from selected pulp and paper effluents, in: Proc. Environ. Conf., TAPPI, San Antonio, TX, USA, 1991, 453–468. [29] P. Spevack, Y. Deslandes, ToF-SIMS analysis of adsorbate/membrane interactions I. Adsorption of dehydroabietic acid on poly(vinylidene fluoride), Appl. Surf. Sci. 99 (1996) 41–50. [30] M. Kallioinen, S.-P. Reinikainen, J. Nuortila-Jokinen, Membrane foulant characterization in pulp and paper applications, in: 5th International Membrane Science and Technology Conference (IMSTEC’03), 10–14 November 2003, Sydney, Australia, 2003. [31] L. Puro, M. Kallioinen, M. Mänttäri, G. Natarajan, D.C. Cameron, M. Nyström, Performance of RC and PES membranes in filtration of pulp mill process waters, Desalination 264 (2010) 249–255. [32] E. Sjöström, R. Aleˇın (Eds.), Analytical Methods in Wood Chemistry, Pulping, and Papermaking, Springer, Berlin, 1999. [33] F. Örså, B. Holmbom, Convenient method for the determination of wood extractives in papermaking process waters and effluents, J. Pulp Paper Sci. 20 (1994) 361–366. [34] P. Elefsiniotis, E.R. Hall, R.M. Johnson, Removal of contaminants from recirculated white water by ultrafiltration and/or biological treatment, Can. J. Chem. Eng. 75 (1997) 88–94. [35] B. Holmbom, C. Eckerman, P. Eklund, J. Hemming, L. Nisula, M. Reunanen, Knots in trees—a new rich source of lignans, Phytochem. Rev. 2 (2003) 331–340. [36] M. Kallioinen, M. Mänttäri, M. Nyström, J. Nuortila-Jokinen, Effect of high filtration temperature on regenerated cellulose ultrafiltration membranes, Sep. Sci. Tech. 42 (2007) 2863–2879.