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The characterization and chemical reactivity of powdered wool
Powder Technology 193 (2009) 200–207
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c
The characterization and chemical reactivity of powdered wool G. Wen, J.A. Rippon 1, P.R. Brady, X.G. Wang ⁎, X. Liu, P.G. Cookson Centre for Material and Fibre Innovation, Institute for Technology Research and Innovation, Deakin University, Geelong, Victoria 3217, Australia
a r t i c l e
i n f o
Article history: Received 21 November 2008 Received in revised form 5 March 2009 Accepted 14 March 2009 Available online 20 March 2009 Keywords: Wool Powder Activated charcoal Chemical reactivity Dye uptake
a b s t r a c t Wool powders with various particle sizes have been produced using different milling techniques. Scanning electron microscopy (SEM) showed gradual breakdown of the fibre as it was progressively converted into powder form. Chlorination enhanced the effectiveness of subsequent air-jet milling. X-ray photoelectron spectroscopy (XPS) revealed an increase in the surface concentrations of oxygen and nitrogen, and a decrease in carbon and sulphur on conversion of the fibres into powders, as the cortex became exposed on the powder surface. An increased surface concentration of cysteic acid was observed for the chlorinated powder. Rapid uptake of dye by wool powders was observed in situations where there was virtually no uptake by the original fibre. Hydrophobic dyes were more readily sorbed than were hydrophilic dyes. The chlorination treatment led to a decrease in the sorption of acid dyes. Confocal microscopy, used in conjunction with a fluorescent stain, showed that chemicals were able to penetrate wool particles, even at room temperature. The rate and extent of uptake of dye by the finer powders were comparable to that obtained with activated charcoal, even though the surface area of the charcoal was 100 times greater. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Natural protein fibres, such as wool and silk, are used frequently in traditional apparel items such as knitwear and woven fabrics for suits. Recently, there has been interest in developing new uses for protein fibres by converting them into powder [1–7]. The conversion of wool to a fine powder is relatively difficult because of the fibre's softness and elasticity. Modifications of the fibre prior to milling with chemicals, including sodium hypochlorite [8,9], hydrogen peroxide [10], tri-n-butylphosphine, thioglycollic acid [11], peracetic acid and sodium sulphite/sodium hydroxide mixtures [12], have been examined. Various mechanical techniques have been used to prepare wool powder. These include milling pans [8], a patented apparatus with upper and lower rotating grinding surfaces which incorporates rows of teeth [9], a three-step pulverization technology using a rotary blade, ultra-sonic crusher and nano-colliding machine [10], and an “explosive-puffing” treatment using saturated steam at elevated temperature to fracture the fibre [11]. Joko [12] reported the use of an homogenizer, ball-mill and jet-mill in the treatment of unmodified wool. Wool powder particles ranging in size from several micrometers [8,12] to less than 100 nm [10] have been prepared. Characterization of wool powder has been primarily based on the use of scanning
⁎ Corresponding author. E-mail address: [email protected] (X.G. Wang). 1 Current address. CSIRO Materials Science and Engineering, P.O. Box 21, Belmont, Victoria 3216, Australia. 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.03.021
electron microscopy, FTIR analysis, X-ray diffraction, thermogravimetry and differential scanning calorimetry [8,10,12,13]. It has been reported that the chemical structure of wool powder is not significantly changed after mechanical milling, but as the powder size is decreased, the crystallinity is reduced and the thermal stability is increased [8,10]. In common with other proteins, wool is a very reactive material. It contains three main types of reactive group: peptide bonds, side chains of amino acid residues and disulphide crosslinks. Chemicals that are known to react with wool include water, acids, bases, reducing agents, oxidising agents, alkylamines, formaldehyde, alcohols, anhydrides, acid chlorides and dyes [14]. The ability of wool to react with metal ions, including those of copper, mercury, nickel, cobalt, chromium, zinc and silver, has led to the suggestion that it could be used to remove metal ions from industrial effluents [15–18]. Investigations of the sorption of direct [19] and acid dyes [20,21] by wool has demonstrated the possibility of using the fibre for decolourising dyehouse effluents. The cuticle on the surface of untreated wool acts as a barrier to the diffusion of chemicals [22], and it is therefore expected that powdered wool would be more reactive than the fibre. Preliminary work has shown that the sorption of metal ions by powdered wool increases as the particle size is reduced [23], but there are no other significant studies on the chemical reactivity of powdered wool. In this paper, the dye-sorption capacities of wool powders with various particle sizes are investigated. The effect of dyes with different hydrophobic/ hydrophilic properties on the sorption capacity of wool powder is examined. Comparisons are made with the original fibre, as well as
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Fig. 1. Chemical structures of two related acid dyes.
with activated charcoal. Mechanistic explanations for the relative behaviours are presented. 2. Experimental 2.1. Materials Merino wool top, with a mean fibre diameter of 20.4 µm, was used as the source of wool fibre. Activated charcoal was supplied by Sigma® chemical company (untreated powder, No.C-5260). Albegal FFA (Ciba) and Aerosol OT (BDH laboratory supplies, England) were used as wetting agents to promote the removal of air from the substrates. Sodium salt of dichloroisocyanuric acid (DCCA) (BASF) and sodium metabisulphite were used to treat a wool powder. Three dyes, C.I. Acid Red 88, C.I. Acid Red 18 and Lanasol Blue CE (Ciba), were used without purification. The structures of C.I. Acid Red 88 and C.I. Acid Red 18 are shown in Fig. 1. Lanasol Blue CE is believed to be a mixture of azo and anthraquinone dyes. A fluorescent brightening agent, Uvitex CF 530% (Ciba), was used in conjunction with confocal microscopy (see below) to track chemical diffusion into the interior of the wool particles. The structure of Uvitex CF 530% is given in Fig. 2. 2.2. Production of wool powder Four wool powder samples were prepared as follows: WP-A. Wool fibres (from Merino top) were cut into small snippets using a Frisch Pulverisette 19 rotary chopper equipped with a 1.0 mm diameter sieve. WP-B. The chopped wool top (WP-A) was ground into fibrous fragments using a rotary ceramic mill between two grind stones. Grinding was carried out (approximately 30 passes) until a point
was reached where it was judged that the powder was not becoming any finer. WP-C. WP-B was then subjected to a commercial, air-jet milling process under proprietary conditions to prepare WP-C. WP-D. WP-B was pre-treated with Albegal FFA (0.1%) and Aerosol OT (0.1%) for 15 min, pH 3.5–4.5 adjusted using acetic acid (60%), liquor-to-goods ratio was 20:1. This was followed by treatment at room temperature with an aqueous solution of DCCA (4% on weight of wool), adjusted to pH 4 – 4.5 with acetic acid. After 50 min, an aqueous solution of sodium metabisulphite (2% on weight of wool) was added to remove unreacted chlorine. The chlorinated wool powder was filtered off after 30 min, rinsed three times with warm water, and dried at 105 °C. Finally, the powder was subjected to the same air-jet milling process as for WP-C. 2.3. Characterisation of wool powder 2.3.1. Scanning electron microscopy The morphology of gold-coated wool powders was investigated using a Leica S440 W-SEM, at a 5-kV acceleration voltage. 2.3.2. Particle size analysis The particle size of the wool powders was measured by means of a laser diffraction scattering type particle analyser (Malvern Mastersizer 2000 with a hydro 2000s). The dispersion medium used for wool powder was propan-2-ol (analytical reagent), and the refractive index used for wool was 1.553, and 1.376 for propan-2-ol. 2.3.3. Surface area The BET surface areas of wool powders and charcoal were determined by the BET-N2 technique using an ASAP 2020 Surface Area Porosity Analyzer (V.3.0, Micromeritics®).
Fig. 2. Chemical structure of Uvitex CF 530%.
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2.3.4. Zeta potential ζ-potentials for charcoal, and powders WP-C and WP-D were determined at 25 °C using a Zetasizer (Nano series, Malvern instruments) after equilibrating in a buffer (citric acid/disodium hydrogen phosphate) at pH 4.5 for 24 h.
2.3.5. X-ray photoelectron spectroscopy (XPS) The surface composition of the wool fibre and powders was investigated using a Kratos AXIS Ultra DLD X-ray Photoelectron Spectrometer in conjunction with a 165 mm hemispherical electron energy analyser. Wool fibres were attached to a sample holder with
Fig. 3. SEM images of wool powders.
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double-sided adhesive tape, and wool powders were compacted into pellets under a pressure of 7 t for 30 s. Analysis was carried out with Monochromatic Al X-rays (1486.6 eV) using an X-ray power of 150 W. Atomic concentrations were calculated using CasaXPS software and a linear baseline. Binding energy values for wool samples were calculated on the basis of the C 1s peak at 285.0 eV. 2.4. Chemical reactivity 2.4.1. Dye uptake Adsorbent (wool fibre, wool powders or activated charcoal; 2 g) was stirred for 15 min at 25 °C in a solution containing 10 ml Albegal FFA (10 g/l) and 350 ml of a pH 4.5 buffer solution (citric acid/ disodium hydrogen phosphate). Forty ml of a stock solution (10 g/l) of the dye was then added, stirring was continued at 25 °C, and small samples of dye liquor were removed and filtered through filter paper (Whatman® Glass Microfibre Filters, Cat No 1825 047) at regular time intervals from 1 min up to 120 min. The concentration of dye in the filtrate was determined using a Cary-3 UV–Visible Spectrophotometer by measuring the absorbance of the filtrate at its maximum absorbance wavelength (λmax). The λmax values for C.I. Acid Red 88, C.I. Acid Red 18 and Lanasol Blue CE were 505 nm, 507 nm and 602 nm, respectively. According to Beer's Law [24], the absorbance of a dilute solution is proportional to the concentration of the dye. The uptake of dye by the adsorbent at time t can be calculated by Eq. (1): dye uptake ðkÞ =
1−
At A0
× 100
ð1Þ
where A0 is the absorbance of initial dye solution and At is the absorbance of dye solution at different dyeing times. 2.4.2. Confocal microscopy The wool powders were treated with 0.125 g/L Uvitex CF 530% for 1 to 10 min at room temperature, pH 4.5 (acetic acid/sodium acetate buffer), liquor ratio 40:1. At designated time intervals, the stained wool powder was filtered quickly through Whatman® glass microfibre filter paper, and air-dried. The stained powders were mounted onto glass slides using DPX Mountant (Sigma-Aldrich Chemie GmbH, Switzerland) and coverslips, and then examined under a LSCM Leica SP5 confocal microscope (Leica Microsystems, USA). Images were taken at intervals of 1 μm. ‘Middle’ section images, taken through the mid-sections of the substrate, indicated the extent of penetration of chemical into the interior of the particular substrate being examined. 2.4.3. Hydrophobic/hydrophilic ratio of dye Hydrophobic/hydrophilic ratios of dyes were determined using the method similar to that of Hadfield and Lemin [25]. In the present study, dye solutions (0.05 g/l were prepared using a pH 4.5 buffer solution (citric acid/disodium hydrogen phosphate), and then equal volumes (100 ml) of dye solution and butan-1-ol (AnalaR®) were thoroughly mixed for 12 h. After separation of the two phases for 24 h, the equilibrium concentration of dye in the phase containing the most dye (A) was determined using a Cary-3 UV–Visible spectrophotometer, and the concentration of dye in the phase containing the least amount of dye (B) was calculated from Eq. (2): Concentration of B ðg = lÞ =
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3. Results and discussion 3.1. Preparation and characterization of wool powders It is very difficult to grind wool fibres into a fine powder by a single operation because the fibres are soft and extensible. A number of intermediate steps were used, therefore, in order to arrive at a final product where the particles were sufficiently fine. Wool fibres were first chopped into snippets (WP-A). Sample WP-B was then produced by milling WP-A. The effect of this grinding operation was to reduce the fibre length slightly, but more importantly it damaged the fibre surface prior to further milling. WP-B was then subjected to a commercial air-jet milling process, to produce WP-C. During air-jet milling, wool particles in the chamber collided with each other at high speed and also with the chamber wall, to reduce the particle size. The preparation route of WP-D was the same as for WP-C, except that the material was chlorinated prior to air-jet milling to increase the surface friction [26], and thus enhance the effectiveness of the subsequent airjet milling process. The intermediate ‘powders’ (WP-A and WP-B) were also examined and compared with the behaviour of the finer powders (WP-C and WP-D). Scanning electron micrographs of the various wool powders are shown in Fig. 3. WP-A shows the intact fibre snippets of average length approximately 500 µm. WP-B contains considerable fibrous material, but with shorter segments (~150 µm) than in WP-A. In this sample, the damage to the cuticle layer is evident. The majority of WPC appears to be small fibrous particles, from which the outer cuticle layer was removed and the inner cortex exposed. Although some fibrous material is present, the powders with irregular shapes in WPD are generally smaller than in WP-C. The particle sizes of the wool powders are shown in Fig. 4. The Quality Audit Standard of the Malvern instrument is a spherical glass bead material, and the model used to analyse the scattering data assumes that the sample particles are spherical. The particle size distribution of irregular particles is, therefore, expressed in terms of a spherical equivalent diameter, which is not ideal when the shape of the particles is non-spherical. Nevertheless, this technique is very useful for looking at the difference in particle size distribution between different samples. The average particle sizes measured for WP-A and WP-B are 61.0 µm and 51.4 µm, respectively. WP-C varies in size from 0.48 µm to 120.2 µm, with a mean particle size of 6.2 µm. The particle size of WP-D varies from 0.48 µm to 52.5 µm, with a mean particle size of 4.5 µm. The SEM images and the particle size measurements on WP-C and WP-D show that treatment with DCCA increased the effectiveness of air-jet milling for reducing the particle size and enhancing the uniformity of the shape of the particles.
total amount of dye − the amount of dye in A : volume of B ð2Þ
The hydrophobic/hydrophilic ratio of the dye was calculated as the ratio of dye concentration in butanol-1-ol:dye concentration in aqueous buffer.
Fig. 4. Particle size distributions of WP-A (-•-), WP-B (••••), WP-C (-°-) and WP-D (—).
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Table 1 Surface composition (atom %) for the wool substrates. Sample
C
O
N
S
Wool fibre WP-A WP-B WP-C WP-D
77.37 76.08 73.20 66.10 65.74
10.97 12.23 13.51 17.59 17.98
8.36 8.77 10.81 14.64 14.45
3.30 2.92 2.49 1.67 1.77
3.2. XPS analysis The major elements analysed on the surface of both wool fibres and powders were carbon, oxygen, nitrogen and sulphur due to the proteinaceous nature of wool [22]. The percentages of these elements on the surfaces of the various wool substrates are listed in Table 1. As the wool was converted from fibrous form into a progressively finer powder, the proportions of C and S were reduced from 77.4% and 3.3% for wool fibre to ~66% and 1.7% for the finer wool powders. The concentrations of O and N were increased, however, from 11.0% and 8.4% for the fibre to ~ 18% and ~ 14.5% for the powders. These results can be related to progressive breakdown of cuticle cells from the wool and subsequent exposure of the cortex as the fibres were converted into powder. Wool consists mainly of two types of cell: an outer layer of overlapping cuticle cells, which constitute approximately 10% of the fibre, with the remainder being mainly the inner cortical cells [22]. A lipid layer, covalently-binding to the outermost membrane of the cuticle cells, consists mainly of C14–C18 and C21 fatty acids [22,27]. The SEM images of the wool powders in Fig. 3 show that the cuticle cells of WP-A were not obviously damaged, whereas the surface structure of WP-B was more severely disrupted with some exposure of the inner cortex. For WP-C and WP-D, however, the outer cuticle cells were completely disrupted, resulting
Fig. 6. Uptake of C.I. Acid Red 88 (1.0 g/l) by wool fibre (■), WP-A (●), WP-B (▲), WP-C (★), WP-D (▼) and activated charcoal (♦) at 25 °C and pH 4.5.
in complete exposure of the cortex. The breakdown of the cuticle cells and exposure of the cortex in wool powders (WP-C and WP-D) are responsible for the differences in their surface chemical composition data (Table 1), compared with the other wool samples which are more fibrous in nature. The elemental composition for whole wool fibres in Table 1 is consistent with previous studies [28]. The relatively high values for carbon and sulphur, and low values for oxygen and nitrogen, are expected on the basis of the C21 fatty acid layer bound to the surface of the epicuticle. For the finest powder, WP-D, decreased amounts of carbon and sulphur and increased amounts of oxygen and nitrogen, compared with whole fibres, are consistent with exposure of the cortex in the degraded substrate. Essentially, what was inside the fibre now forms part of the outer surface of the powder. In terms of surface composition, there is a relatively consistent trend as the level of fibre degradation increases. The high-resolution S2p spectra obtained from wool fibres and the powders are shown in Fig. 5. The peaks at ~164 eV correspond to disulfide crosslinks (\S\S\). The peaks at ~ 168 eV for WP-C (~ 33% concentration) and WP-D (~46% concentration) correspond to cysteic acid residues [28]. Both WP-C and WP-D were prepared by air-jet milling, and it is possible that cysteic acid was formed on the powder surfaces by localised heating (leading to degradation) caused by the mechanical action. The chemical oxidation process used in the production of WP-D is the most likely explanation for the higher cysteic acid content of this powder. 3.3. Dye uptake by wool powders and activated charcoal
Fig. 5. S2p spectra of wool samples.
The dyes used in this study are typical of those used for dyeing wool. They contain sulphonic acid groups, which make them anionic and water soluble. The number of sulphonic acid groups per molecule varies between the dyes, and this is largely responsible for differences in their hydrophobic/hydrophilic characteristics. C.I. Acid Red 88 (Fig. 1) is a monosulphonated, azo dye. This is a relatively hydrophobic dye, with a measured hydrophobic/hydrophilic ratio of 58.4. This ratio measures the extent to which the dye, originally dissolved in water, transfers to an organic phase (butan-1-ol); the higher the value, the more hydrophobic is the dye. C.I. Acid Red 18 (Fig. 1) is a trisulphonated, azo dye with a structure very similar to that of C.I. Acid Red 88. The hydrophobic/hydrophilic ratio is 0.04, revealing that the dye is relatively hydrophilic. Lanasol Blue CE is a ‘reactive’ dye, containing an alphabromoacrylamido group that forms a covalent linkage with wool, as well as sulphonic acid groups [29]. The hydrophobic/hydrophilic ratio is 0.41.
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The measured zeta potentials (at pH 4.5) for charcoal, WP-C and WP-D are −19.8 mV, −1.02 mV and −2.6 mV, respectively. The low values for the wool powders are consistent with the known isoelectric region for wool at pH 4.5–5, where the numbers of protonated amino groups and deprotonated carboxylic acid groups approximately equal [22]. Fig. 6 shows the variation of uptake (with time) of C.I. Acid Red 88 by the different wool substrates, and also by activated charcoal at 25 °C. As expected, there was minimal uptake by the fibre, even after 2 h; wool is usually dyed commercially at temperatures close to the boil to ensure a uniform and high uptake of dye [30]. The fibre snippets (WP-A), on the other hand, absorbed about 60% of the dye after 1 h, and WP-B absorbed about 80%. For the two finest powders (WP-C and WP-D), the rate of dye uptake was very rapid with almost 100% uptake being achieved after about 30 min, which was faster than on activated charcoal. The dye sorption capacity of wool powders was somewhat surprising, given that the surface area of the charcoal (858 ± 7 m2/g) was more than 100 times greater than that of the finest wool powders WP-C (5.9 ± 0.2 m2/g) and WP-D (6.1 ± 0.2 m2/g). Dyes are readily sorbed onto the surface of activated charcoal [31]. In order to examine the staining pattern of dye on wool powder, wool samples were treated, as described in Fig. 7, with an aqueous solution of a fluorescent brightening agent, Uvites CF 530%. Confocal microscopy was used to track the uptake of the chemical by the substrates. Although the behaviour observed is specific to the chemical used, the sulphonated structure of Uvitex CF 530% (Fig. 2) is similar to that of a wool dye, and so the results obtained are relevant to the present study. Confocal microscopy images from the mid-sections of wool powders are shown in Fig. 7. For WP-A, after a treatment time of 10 min, the penetration of the stain into the wool via the exposed fibre end is evident, but very little penetration occurred through the undamaged cuticle of the fibre. WP-B, after a 5-minute treatment time, shows the penetration of the chemical through both the fibre ends and the parts of the fibres where the cuticle had been damaged.
Fig. 7. Confocal microscopy images (middle slices) of wool powders treated with Uvitex CF 530% at room temperature and pH 4.5; treatment times indicated.
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Fig. 8. Uptake of Acid Red 18 (1.0 g/l) by WP-C (★), WP-D (▼) and activated charcoal (♦) at 25 °C and pH 4.5.
WP-C, which consists mainly of small particles from which the cuticle cells had been completely disrupted, shows complete penetration after a treatment time of 1 min, while there was a significant penetration of the stain into WP-D after only a 2 min treatment time. The cuticle cells of wool form a resistant barrier to chemical diffusion [32,33]. In normal wool dyeing carried out close to the boil, dye penetrates into the interior of the fibre through the gaps between the cuticle cells [33]. At room temperature, however, penetration does not occur to a significant extent. The results presented in Figs. 6 and 7 are consistent with the idea that as the fibre is increasingly broken down into finer particles; both the external and internal barriers to diffusion are increasingly removed, allowing penetration of dye molecules at room temperature. For the chopped fibre, WP-A, enhanced dye uptake occurred by dye diffusion through the exposed fibres ends. The ability of the wool powders–especially WP-C and WP-D–to take up dye quickly is related more to dye penetration than to their increased surface area. Uptakes of C.I. Acid Red 18 by wool powders WP-C and WP-D and charcoal are shown in Fig. 8. In this case, the dye uptake rates and extents are significantly lower than for C.I. Acid Red 88. This difference reflects the effect of sulphonation on the substantivity of the two dyes for wool. The equilibrium dye uptake of C.I. Acid Red 18 after 2 h on WP-D was 35%, and 55% on both WP-C and charcoal. Dye sorption by wool involves a range of interactions between the wool and dye molecules. These include ionic interactions, hydrophobic interactions and hydrogen bonding. When dye uptake by the wool powders was measured at pH 4.5, the electrostatic interactions between the slightly negatively charged wool powders (WP-C and WP-D) and anionic acid dye molecules would be expected to be relatively small. In this case, the interactions between hydrophobic parts of the dye molecules and certain hydrophobic proteins in specific regions of wool are mainly responsible for the affinity of a dye for wool. The lower uptake of C.I. Acid Red 18, compared to that of C.I. Acid Red 88, can be attributed to the relatively hydrophilic properties of C.I. Acid Red 18. The difference in behaviour between WP-C and WP-D may be related to the more negative zeta potential of the latter powder. This is consistent with its higher surface concentration of sulphonate-containing cysteic acid residues, as determined by XPS. Activated charcoal is widely used as an efficient sorbent to remove inorganic and organic materials from industrial effluents. The sorption capacity of activated charcoal depends mainly on its surface area, degree of surface reactivity, microporous structure and the presence of acid or basic groups on the surface [34]. The sorption behaviour of dyes on activated charcoal has been explained in terms of electrostatic and dispersion forces between the charged charcoal surface and ionized dyes [35]. When acid dyes were sorbed on charcoal with a ζpotential of − 19.8 mV at pH 4.5, electrostatic repulsive forces
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more hydrophilic dyes were used. Although chlorination prior to airjet milling produced the finest wool powder, it also reduced the substantivity of hydrophilic dyes. The oxidative chlorination treatment increases the concentration of cysteic acid residues on the surface of the particles, and it is likely that the increased negative charge decreases the substantivity of the wool powder for hydrophilic dyes. In situations where wool fibres have been found to be useful for the removal of metal ions and dyes from industrial effluents, wool powders offer an alternative for removing greater amounts of chemicals more quickly. Acknowledgement
Fig. 9. Uptake of Lanasol Blue CE (1.0 g/l) by WP-C (★), WP-D (▼) and activated charcoal (♦) at 25 °C and pH 4.5.
We are grateful to Australian Wool Innovation for supporting this research through the China Australia Wool Innovation Network (CAWIN) project. References
between the negatively charged charcoal surface and the anionic acid dyes hinder dye sorption. On the other hand, the relatively short range dispersion forces between the dyes and the charcoal surface favour the sorption of dyes. The percentage uptake of dye at equilibrium is governed by a combination of these two forces. Thus, for the dyes used in this study, the higher uptake at equilibrium on charcoal of C.I. Acid Red 88, compared with C.I. Acid Red 18, can be attributed to its more hydrophobic character, which in turn favours stronger interaction via dispersion forces. Uptake of the acid dyes by the wool powders are comparable to that of activated charcoal under the particular conditions used (pH 4.5). This occurs despite the lower surface area of the wool powders compared with charcoal. For the wool powders, however, both surface adsorption and penetration of dye into the particles can occur. It is suggested that this factor is responsible for the similar uptake of dyes by both powdered wool and charcoal. Uptakes of Lanasol Blue CE by WP-C, WP-D and activated charcoal are shown in Fig. 9. The partition coefficient for this dye suggests that its hydrophilic/hydrophobic character is between that of C.I. Acid Red 88 and C.I. Acid Red 18. This is consistent with the dye uptake results for Lanasol Blue CE in Fig. 9, which are also intermediate between those for the two acid dyes. 4. Conclusions When wool fibres were subjected to mechanical action in the form of milling, the outer cuticle cells were disrupted, and the inner cortex exposed. With the finest wool powders, more than 90% of a typical acid dye (C.I. Acid Red 88) was taken up after less than 5 min, where there was no significant dye sorption by unmodified wool fibres. Under the same conditions, the rate and extent of dye uptake by activated charcoal was slightly lower, but still comparable to that of the powdered wool. Although the surface area of the charcoal was more than two orders of magnitude greater than that of the wool powders, the zeta potentials of the wool powders were less negative than that of charcoal, which would be expected to favour uptake by the wool. Use of a fluorescent stain to visualise the sorption behaviour of wool showed that penetration of the wool particles occurs, even at room temperature. This ability to absorb (as well as adsorb) dyes increases the effective surface area on and inside the wool particles where dyes can bind to the wool protein. C.I. Acid Red 88 is a relatively hydrophobic dye. When dyes of lesser hydrophobic character were used, the rates and extents of dye uptake were reduced, and this is consistent with the higher substantivity of hydrophobic dyes on wool. In spite of this, the behaviour of charcoal was still comparable with that of the most reactive wool powder when
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c
The characterization and chemical reactivity of powdered wool G. Wen, J.A. Rippon 1, P.R. Brady, X.G. Wang ⁎, X. Liu, P.G. Cookson Centre for Material and Fibre Innovation, Institute for Technology Research and Innovation, Deakin University, Geelong, Victoria 3217, Australia
a r t i c l e
i n f o
Article history: Received 21 November 2008 Received in revised form 5 March 2009 Accepted 14 March 2009 Available online 20 March 2009 Keywords: Wool Powder Activated charcoal Chemical reactivity Dye uptake
a b s t r a c t Wool powders with various particle sizes have been produced using different milling techniques. Scanning electron microscopy (SEM) showed gradual breakdown of the fibre as it was progressively converted into powder form. Chlorination enhanced the effectiveness of subsequent air-jet milling. X-ray photoelectron spectroscopy (XPS) revealed an increase in the surface concentrations of oxygen and nitrogen, and a decrease in carbon and sulphur on conversion of the fibres into powders, as the cortex became exposed on the powder surface. An increased surface concentration of cysteic acid was observed for the chlorinated powder. Rapid uptake of dye by wool powders was observed in situations where there was virtually no uptake by the original fibre. Hydrophobic dyes were more readily sorbed than were hydrophilic dyes. The chlorination treatment led to a decrease in the sorption of acid dyes. Confocal microscopy, used in conjunction with a fluorescent stain, showed that chemicals were able to penetrate wool particles, even at room temperature. The rate and extent of uptake of dye by the finer powders were comparable to that obtained with activated charcoal, even though the surface area of the charcoal was 100 times greater. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Natural protein fibres, such as wool and silk, are used frequently in traditional apparel items such as knitwear and woven fabrics for suits. Recently, there has been interest in developing new uses for protein fibres by converting them into powder [1–7]. The conversion of wool to a fine powder is relatively difficult because of the fibre's softness and elasticity. Modifications of the fibre prior to milling with chemicals, including sodium hypochlorite [8,9], hydrogen peroxide [10], tri-n-butylphosphine, thioglycollic acid [11], peracetic acid and sodium sulphite/sodium hydroxide mixtures [12], have been examined. Various mechanical techniques have been used to prepare wool powder. These include milling pans [8], a patented apparatus with upper and lower rotating grinding surfaces which incorporates rows of teeth [9], a three-step pulverization technology using a rotary blade, ultra-sonic crusher and nano-colliding machine [10], and an “explosive-puffing” treatment using saturated steam at elevated temperature to fracture the fibre [11]. Joko [12] reported the use of an homogenizer, ball-mill and jet-mill in the treatment of unmodified wool. Wool powder particles ranging in size from several micrometers [8,12] to less than 100 nm [10] have been prepared. Characterization of wool powder has been primarily based on the use of scanning
⁎ Corresponding author. E-mail address: [email protected] (X.G. Wang). 1 Current address. CSIRO Materials Science and Engineering, P.O. Box 21, Belmont, Victoria 3216, Australia. 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.03.021
electron microscopy, FTIR analysis, X-ray diffraction, thermogravimetry and differential scanning calorimetry [8,10,12,13]. It has been reported that the chemical structure of wool powder is not significantly changed after mechanical milling, but as the powder size is decreased, the crystallinity is reduced and the thermal stability is increased [8,10]. In common with other proteins, wool is a very reactive material. It contains three main types of reactive group: peptide bonds, side chains of amino acid residues and disulphide crosslinks. Chemicals that are known to react with wool include water, acids, bases, reducing agents, oxidising agents, alkylamines, formaldehyde, alcohols, anhydrides, acid chlorides and dyes [14]. The ability of wool to react with metal ions, including those of copper, mercury, nickel, cobalt, chromium, zinc and silver, has led to the suggestion that it could be used to remove metal ions from industrial effluents [15–18]. Investigations of the sorption of direct [19] and acid dyes [20,21] by wool has demonstrated the possibility of using the fibre for decolourising dyehouse effluents. The cuticle on the surface of untreated wool acts as a barrier to the diffusion of chemicals [22], and it is therefore expected that powdered wool would be more reactive than the fibre. Preliminary work has shown that the sorption of metal ions by powdered wool increases as the particle size is reduced [23], but there are no other significant studies on the chemical reactivity of powdered wool. In this paper, the dye-sorption capacities of wool powders with various particle sizes are investigated. The effect of dyes with different hydrophobic/ hydrophilic properties on the sorption capacity of wool powder is examined. Comparisons are made with the original fibre, as well as
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201
Fig. 1. Chemical structures of two related acid dyes.
with activated charcoal. Mechanistic explanations for the relative behaviours are presented. 2. Experimental 2.1. Materials Merino wool top, with a mean fibre diameter of 20.4 µm, was used as the source of wool fibre. Activated charcoal was supplied by Sigma® chemical company (untreated powder, No.C-5260). Albegal FFA (Ciba) and Aerosol OT (BDH laboratory supplies, England) were used as wetting agents to promote the removal of air from the substrates. Sodium salt of dichloroisocyanuric acid (DCCA) (BASF) and sodium metabisulphite were used to treat a wool powder. Three dyes, C.I. Acid Red 88, C.I. Acid Red 18 and Lanasol Blue CE (Ciba), were used without purification. The structures of C.I. Acid Red 88 and C.I. Acid Red 18 are shown in Fig. 1. Lanasol Blue CE is believed to be a mixture of azo and anthraquinone dyes. A fluorescent brightening agent, Uvitex CF 530% (Ciba), was used in conjunction with confocal microscopy (see below) to track chemical diffusion into the interior of the wool particles. The structure of Uvitex CF 530% is given in Fig. 2. 2.2. Production of wool powder Four wool powder samples were prepared as follows: WP-A. Wool fibres (from Merino top) were cut into small snippets using a Frisch Pulverisette 19 rotary chopper equipped with a 1.0 mm diameter sieve. WP-B. The chopped wool top (WP-A) was ground into fibrous fragments using a rotary ceramic mill between two grind stones. Grinding was carried out (approximately 30 passes) until a point
was reached where it was judged that the powder was not becoming any finer. WP-C. WP-B was then subjected to a commercial, air-jet milling process under proprietary conditions to prepare WP-C. WP-D. WP-B was pre-treated with Albegal FFA (0.1%) and Aerosol OT (0.1%) for 15 min, pH 3.5–4.5 adjusted using acetic acid (60%), liquor-to-goods ratio was 20:1. This was followed by treatment at room temperature with an aqueous solution of DCCA (4% on weight of wool), adjusted to pH 4 – 4.5 with acetic acid. After 50 min, an aqueous solution of sodium metabisulphite (2% on weight of wool) was added to remove unreacted chlorine. The chlorinated wool powder was filtered off after 30 min, rinsed three times with warm water, and dried at 105 °C. Finally, the powder was subjected to the same air-jet milling process as for WP-C. 2.3. Characterisation of wool powder 2.3.1. Scanning electron microscopy The morphology of gold-coated wool powders was investigated using a Leica S440 W-SEM, at a 5-kV acceleration voltage. 2.3.2. Particle size analysis The particle size of the wool powders was measured by means of a laser diffraction scattering type particle analyser (Malvern Mastersizer 2000 with a hydro 2000s). The dispersion medium used for wool powder was propan-2-ol (analytical reagent), and the refractive index used for wool was 1.553, and 1.376 for propan-2-ol. 2.3.3. Surface area The BET surface areas of wool powders and charcoal were determined by the BET-N2 technique using an ASAP 2020 Surface Area Porosity Analyzer (V.3.0, Micromeritics®).
Fig. 2. Chemical structure of Uvitex CF 530%.
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2.3.4. Zeta potential ζ-potentials for charcoal, and powders WP-C and WP-D were determined at 25 °C using a Zetasizer (Nano series, Malvern instruments) after equilibrating in a buffer (citric acid/disodium hydrogen phosphate) at pH 4.5 for 24 h.
2.3.5. X-ray photoelectron spectroscopy (XPS) The surface composition of the wool fibre and powders was investigated using a Kratos AXIS Ultra DLD X-ray Photoelectron Spectrometer in conjunction with a 165 mm hemispherical electron energy analyser. Wool fibres were attached to a sample holder with
Fig. 3. SEM images of wool powders.
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double-sided adhesive tape, and wool powders were compacted into pellets under a pressure of 7 t for 30 s. Analysis was carried out with Monochromatic Al X-rays (1486.6 eV) using an X-ray power of 150 W. Atomic concentrations were calculated using CasaXPS software and a linear baseline. Binding energy values for wool samples were calculated on the basis of the C 1s peak at 285.0 eV. 2.4. Chemical reactivity 2.4.1. Dye uptake Adsorbent (wool fibre, wool powders or activated charcoal; 2 g) was stirred for 15 min at 25 °C in a solution containing 10 ml Albegal FFA (10 g/l) and 350 ml of a pH 4.5 buffer solution (citric acid/ disodium hydrogen phosphate). Forty ml of a stock solution (10 g/l) of the dye was then added, stirring was continued at 25 °C, and small samples of dye liquor were removed and filtered through filter paper (Whatman® Glass Microfibre Filters, Cat No 1825 047) at regular time intervals from 1 min up to 120 min. The concentration of dye in the filtrate was determined using a Cary-3 UV–Visible Spectrophotometer by measuring the absorbance of the filtrate at its maximum absorbance wavelength (λmax). The λmax values for C.I. Acid Red 88, C.I. Acid Red 18 and Lanasol Blue CE were 505 nm, 507 nm and 602 nm, respectively. According to Beer's Law [24], the absorbance of a dilute solution is proportional to the concentration of the dye. The uptake of dye by the adsorbent at time t can be calculated by Eq. (1): dye uptake ðkÞ =
1−
At A0
× 100
ð1Þ
where A0 is the absorbance of initial dye solution and At is the absorbance of dye solution at different dyeing times. 2.4.2. Confocal microscopy The wool powders were treated with 0.125 g/L Uvitex CF 530% for 1 to 10 min at room temperature, pH 4.5 (acetic acid/sodium acetate buffer), liquor ratio 40:1. At designated time intervals, the stained wool powder was filtered quickly through Whatman® glass microfibre filter paper, and air-dried. The stained powders were mounted onto glass slides using DPX Mountant (Sigma-Aldrich Chemie GmbH, Switzerland) and coverslips, and then examined under a LSCM Leica SP5 confocal microscope (Leica Microsystems, USA). Images were taken at intervals of 1 μm. ‘Middle’ section images, taken through the mid-sections of the substrate, indicated the extent of penetration of chemical into the interior of the particular substrate being examined. 2.4.3. Hydrophobic/hydrophilic ratio of dye Hydrophobic/hydrophilic ratios of dyes were determined using the method similar to that of Hadfield and Lemin [25]. In the present study, dye solutions (0.05 g/l were prepared using a pH 4.5 buffer solution (citric acid/disodium hydrogen phosphate), and then equal volumes (100 ml) of dye solution and butan-1-ol (AnalaR®) were thoroughly mixed for 12 h. After separation of the two phases for 24 h, the equilibrium concentration of dye in the phase containing the most dye (A) was determined using a Cary-3 UV–Visible spectrophotometer, and the concentration of dye in the phase containing the least amount of dye (B) was calculated from Eq. (2): Concentration of B ðg = lÞ =
203
3. Results and discussion 3.1. Preparation and characterization of wool powders It is very difficult to grind wool fibres into a fine powder by a single operation because the fibres are soft and extensible. A number of intermediate steps were used, therefore, in order to arrive at a final product where the particles were sufficiently fine. Wool fibres were first chopped into snippets (WP-A). Sample WP-B was then produced by milling WP-A. The effect of this grinding operation was to reduce the fibre length slightly, but more importantly it damaged the fibre surface prior to further milling. WP-B was then subjected to a commercial air-jet milling process, to produce WP-C. During air-jet milling, wool particles in the chamber collided with each other at high speed and also with the chamber wall, to reduce the particle size. The preparation route of WP-D was the same as for WP-C, except that the material was chlorinated prior to air-jet milling to increase the surface friction [26], and thus enhance the effectiveness of the subsequent airjet milling process. The intermediate ‘powders’ (WP-A and WP-B) were also examined and compared with the behaviour of the finer powders (WP-C and WP-D). Scanning electron micrographs of the various wool powders are shown in Fig. 3. WP-A shows the intact fibre snippets of average length approximately 500 µm. WP-B contains considerable fibrous material, but with shorter segments (~150 µm) than in WP-A. In this sample, the damage to the cuticle layer is evident. The majority of WPC appears to be small fibrous particles, from which the outer cuticle layer was removed and the inner cortex exposed. Although some fibrous material is present, the powders with irregular shapes in WPD are generally smaller than in WP-C. The particle sizes of the wool powders are shown in Fig. 4. The Quality Audit Standard of the Malvern instrument is a spherical glass bead material, and the model used to analyse the scattering data assumes that the sample particles are spherical. The particle size distribution of irregular particles is, therefore, expressed in terms of a spherical equivalent diameter, which is not ideal when the shape of the particles is non-spherical. Nevertheless, this technique is very useful for looking at the difference in particle size distribution between different samples. The average particle sizes measured for WP-A and WP-B are 61.0 µm and 51.4 µm, respectively. WP-C varies in size from 0.48 µm to 120.2 µm, with a mean particle size of 6.2 µm. The particle size of WP-D varies from 0.48 µm to 52.5 µm, with a mean particle size of 4.5 µm. The SEM images and the particle size measurements on WP-C and WP-D show that treatment with DCCA increased the effectiveness of air-jet milling for reducing the particle size and enhancing the uniformity of the shape of the particles.
total amount of dye − the amount of dye in A : volume of B ð2Þ
The hydrophobic/hydrophilic ratio of the dye was calculated as the ratio of dye concentration in butanol-1-ol:dye concentration in aqueous buffer.
Fig. 4. Particle size distributions of WP-A (-•-), WP-B (••••), WP-C (-°-) and WP-D (—).
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Table 1 Surface composition (atom %) for the wool substrates. Sample
C
O
N
S
Wool fibre WP-A WP-B WP-C WP-D
77.37 76.08 73.20 66.10 65.74
10.97 12.23 13.51 17.59 17.98
8.36 8.77 10.81 14.64 14.45
3.30 2.92 2.49 1.67 1.77
3.2. XPS analysis The major elements analysed on the surface of both wool fibres and powders were carbon, oxygen, nitrogen and sulphur due to the proteinaceous nature of wool [22]. The percentages of these elements on the surfaces of the various wool substrates are listed in Table 1. As the wool was converted from fibrous form into a progressively finer powder, the proportions of C and S were reduced from 77.4% and 3.3% for wool fibre to ~66% and 1.7% for the finer wool powders. The concentrations of O and N were increased, however, from 11.0% and 8.4% for the fibre to ~ 18% and ~ 14.5% for the powders. These results can be related to progressive breakdown of cuticle cells from the wool and subsequent exposure of the cortex as the fibres were converted into powder. Wool consists mainly of two types of cell: an outer layer of overlapping cuticle cells, which constitute approximately 10% of the fibre, with the remainder being mainly the inner cortical cells [22]. A lipid layer, covalently-binding to the outermost membrane of the cuticle cells, consists mainly of C14–C18 and C21 fatty acids [22,27]. The SEM images of the wool powders in Fig. 3 show that the cuticle cells of WP-A were not obviously damaged, whereas the surface structure of WP-B was more severely disrupted with some exposure of the inner cortex. For WP-C and WP-D, however, the outer cuticle cells were completely disrupted, resulting
Fig. 6. Uptake of C.I. Acid Red 88 (1.0 g/l) by wool fibre (■), WP-A (●), WP-B (▲), WP-C (★), WP-D (▼) and activated charcoal (♦) at 25 °C and pH 4.5.
in complete exposure of the cortex. The breakdown of the cuticle cells and exposure of the cortex in wool powders (WP-C and WP-D) are responsible for the differences in their surface chemical composition data (Table 1), compared with the other wool samples which are more fibrous in nature. The elemental composition for whole wool fibres in Table 1 is consistent with previous studies [28]. The relatively high values for carbon and sulphur, and low values for oxygen and nitrogen, are expected on the basis of the C21 fatty acid layer bound to the surface of the epicuticle. For the finest powder, WP-D, decreased amounts of carbon and sulphur and increased amounts of oxygen and nitrogen, compared with whole fibres, are consistent with exposure of the cortex in the degraded substrate. Essentially, what was inside the fibre now forms part of the outer surface of the powder. In terms of surface composition, there is a relatively consistent trend as the level of fibre degradation increases. The high-resolution S2p spectra obtained from wool fibres and the powders are shown in Fig. 5. The peaks at ~164 eV correspond to disulfide crosslinks (\S\S\). The peaks at ~ 168 eV for WP-C (~ 33% concentration) and WP-D (~46% concentration) correspond to cysteic acid residues [28]. Both WP-C and WP-D were prepared by air-jet milling, and it is possible that cysteic acid was formed on the powder surfaces by localised heating (leading to degradation) caused by the mechanical action. The chemical oxidation process used in the production of WP-D is the most likely explanation for the higher cysteic acid content of this powder. 3.3. Dye uptake by wool powders and activated charcoal
Fig. 5. S2p spectra of wool samples.
The dyes used in this study are typical of those used for dyeing wool. They contain sulphonic acid groups, which make them anionic and water soluble. The number of sulphonic acid groups per molecule varies between the dyes, and this is largely responsible for differences in their hydrophobic/hydrophilic characteristics. C.I. Acid Red 88 (Fig. 1) is a monosulphonated, azo dye. This is a relatively hydrophobic dye, with a measured hydrophobic/hydrophilic ratio of 58.4. This ratio measures the extent to which the dye, originally dissolved in water, transfers to an organic phase (butan-1-ol); the higher the value, the more hydrophobic is the dye. C.I. Acid Red 18 (Fig. 1) is a trisulphonated, azo dye with a structure very similar to that of C.I. Acid Red 88. The hydrophobic/hydrophilic ratio is 0.04, revealing that the dye is relatively hydrophilic. Lanasol Blue CE is a ‘reactive’ dye, containing an alphabromoacrylamido group that forms a covalent linkage with wool, as well as sulphonic acid groups [29]. The hydrophobic/hydrophilic ratio is 0.41.
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The measured zeta potentials (at pH 4.5) for charcoal, WP-C and WP-D are −19.8 mV, −1.02 mV and −2.6 mV, respectively. The low values for the wool powders are consistent with the known isoelectric region for wool at pH 4.5–5, where the numbers of protonated amino groups and deprotonated carboxylic acid groups approximately equal [22]. Fig. 6 shows the variation of uptake (with time) of C.I. Acid Red 88 by the different wool substrates, and also by activated charcoal at 25 °C. As expected, there was minimal uptake by the fibre, even after 2 h; wool is usually dyed commercially at temperatures close to the boil to ensure a uniform and high uptake of dye [30]. The fibre snippets (WP-A), on the other hand, absorbed about 60% of the dye after 1 h, and WP-B absorbed about 80%. For the two finest powders (WP-C and WP-D), the rate of dye uptake was very rapid with almost 100% uptake being achieved after about 30 min, which was faster than on activated charcoal. The dye sorption capacity of wool powders was somewhat surprising, given that the surface area of the charcoal (858 ± 7 m2/g) was more than 100 times greater than that of the finest wool powders WP-C (5.9 ± 0.2 m2/g) and WP-D (6.1 ± 0.2 m2/g). Dyes are readily sorbed onto the surface of activated charcoal [31]. In order to examine the staining pattern of dye on wool powder, wool samples were treated, as described in Fig. 7, with an aqueous solution of a fluorescent brightening agent, Uvites CF 530%. Confocal microscopy was used to track the uptake of the chemical by the substrates. Although the behaviour observed is specific to the chemical used, the sulphonated structure of Uvitex CF 530% (Fig. 2) is similar to that of a wool dye, and so the results obtained are relevant to the present study. Confocal microscopy images from the mid-sections of wool powders are shown in Fig. 7. For WP-A, after a treatment time of 10 min, the penetration of the stain into the wool via the exposed fibre end is evident, but very little penetration occurred through the undamaged cuticle of the fibre. WP-B, after a 5-minute treatment time, shows the penetration of the chemical through both the fibre ends and the parts of the fibres where the cuticle had been damaged.
Fig. 7. Confocal microscopy images (middle slices) of wool powders treated with Uvitex CF 530% at room temperature and pH 4.5; treatment times indicated.
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Fig. 8. Uptake of Acid Red 18 (1.0 g/l) by WP-C (★), WP-D (▼) and activated charcoal (♦) at 25 °C and pH 4.5.
WP-C, which consists mainly of small particles from which the cuticle cells had been completely disrupted, shows complete penetration after a treatment time of 1 min, while there was a significant penetration of the stain into WP-D after only a 2 min treatment time. The cuticle cells of wool form a resistant barrier to chemical diffusion [32,33]. In normal wool dyeing carried out close to the boil, dye penetrates into the interior of the fibre through the gaps between the cuticle cells [33]. At room temperature, however, penetration does not occur to a significant extent. The results presented in Figs. 6 and 7 are consistent with the idea that as the fibre is increasingly broken down into finer particles; both the external and internal barriers to diffusion are increasingly removed, allowing penetration of dye molecules at room temperature. For the chopped fibre, WP-A, enhanced dye uptake occurred by dye diffusion through the exposed fibres ends. The ability of the wool powders–especially WP-C and WP-D–to take up dye quickly is related more to dye penetration than to their increased surface area. Uptakes of C.I. Acid Red 18 by wool powders WP-C and WP-D and charcoal are shown in Fig. 8. In this case, the dye uptake rates and extents are significantly lower than for C.I. Acid Red 88. This difference reflects the effect of sulphonation on the substantivity of the two dyes for wool. The equilibrium dye uptake of C.I. Acid Red 18 after 2 h on WP-D was 35%, and 55% on both WP-C and charcoal. Dye sorption by wool involves a range of interactions between the wool and dye molecules. These include ionic interactions, hydrophobic interactions and hydrogen bonding. When dye uptake by the wool powders was measured at pH 4.5, the electrostatic interactions between the slightly negatively charged wool powders (WP-C and WP-D) and anionic acid dye molecules would be expected to be relatively small. In this case, the interactions between hydrophobic parts of the dye molecules and certain hydrophobic proteins in specific regions of wool are mainly responsible for the affinity of a dye for wool. The lower uptake of C.I. Acid Red 18, compared to that of C.I. Acid Red 88, can be attributed to the relatively hydrophilic properties of C.I. Acid Red 18. The difference in behaviour between WP-C and WP-D may be related to the more negative zeta potential of the latter powder. This is consistent with its higher surface concentration of sulphonate-containing cysteic acid residues, as determined by XPS. Activated charcoal is widely used as an efficient sorbent to remove inorganic and organic materials from industrial effluents. The sorption capacity of activated charcoal depends mainly on its surface area, degree of surface reactivity, microporous structure and the presence of acid or basic groups on the surface [34]. The sorption behaviour of dyes on activated charcoal has been explained in terms of electrostatic and dispersion forces between the charged charcoal surface and ionized dyes [35]. When acid dyes were sorbed on charcoal with a ζpotential of − 19.8 mV at pH 4.5, electrostatic repulsive forces
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more hydrophilic dyes were used. Although chlorination prior to airjet milling produced the finest wool powder, it also reduced the substantivity of hydrophilic dyes. The oxidative chlorination treatment increases the concentration of cysteic acid residues on the surface of the particles, and it is likely that the increased negative charge decreases the substantivity of the wool powder for hydrophilic dyes. In situations where wool fibres have been found to be useful for the removal of metal ions and dyes from industrial effluents, wool powders offer an alternative for removing greater amounts of chemicals more quickly. Acknowledgement
Fig. 9. Uptake of Lanasol Blue CE (1.0 g/l) by WP-C (★), WP-D (▼) and activated charcoal (♦) at 25 °C and pH 4.5.
We are grateful to Australian Wool Innovation for supporting this research through the China Australia Wool Innovation Network (CAWIN) project. References
between the negatively charged charcoal surface and the anionic acid dyes hinder dye sorption. On the other hand, the relatively short range dispersion forces between the dyes and the charcoal surface favour the sorption of dyes. The percentage uptake of dye at equilibrium is governed by a combination of these two forces. Thus, for the dyes used in this study, the higher uptake at equilibrium on charcoal of C.I. Acid Red 88, compared with C.I. Acid Red 18, can be attributed to its more hydrophobic character, which in turn favours stronger interaction via dispersion forces. Uptake of the acid dyes by the wool powders are comparable to that of activated charcoal under the particular conditions used (pH 4.5). This occurs despite the lower surface area of the wool powders compared with charcoal. For the wool powders, however, both surface adsorption and penetration of dye into the particles can occur. It is suggested that this factor is responsible for the similar uptake of dyes by both powdered wool and charcoal. Uptakes of Lanasol Blue CE by WP-C, WP-D and activated charcoal are shown in Fig. 9. The partition coefficient for this dye suggests that its hydrophilic/hydrophobic character is between that of C.I. Acid Red 88 and C.I. Acid Red 18. This is consistent with the dye uptake results for Lanasol Blue CE in Fig. 9, which are also intermediate between those for the two acid dyes. 4. Conclusions When wool fibres were subjected to mechanical action in the form of milling, the outer cuticle cells were disrupted, and the inner cortex exposed. With the finest wool powders, more than 90% of a typical acid dye (C.I. Acid Red 88) was taken up after less than 5 min, where there was no significant dye sorption by unmodified wool fibres. Under the same conditions, the rate and extent of dye uptake by activated charcoal was slightly lower, but still comparable to that of the powdered wool. Although the surface area of the charcoal was more than two orders of magnitude greater than that of the wool powders, the zeta potentials of the wool powders were less negative than that of charcoal, which would be expected to favour uptake by the wool. Use of a fluorescent stain to visualise the sorption behaviour of wool showed that penetration of the wool particles occurs, even at room temperature. This ability to absorb (as well as adsorb) dyes increases the effective surface area on and inside the wool particles where dyes can bind to the wool protein. C.I. Acid Red 88 is a relatively hydrophobic dye. When dyes of lesser hydrophobic character were used, the rates and extents of dye uptake were reduced, and this is consistent with the higher substantivity of hydrophobic dyes on wool. In spite of this, the behaviour of charcoal was still comparable with that of the most reactive wool powder when
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