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Milled cashmere guard hair powders: Absorption properties to heavy metal ions
Kiran Patil a, Suzanne V. Smith b, Rangam Rajkhowa a, Takuya Tsuzuki a, Xungai Wang a, Tong Lin a,⁎a Centre for Material and Fibre Innovation, Deakin University, Geelong, VIC 3217, Australiab Centre of Excellence in Antimatter Matter Studies, Australian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia
Article history:Received 24 September 2011Accepted 3 December 2011Available online 9 December 2011
Keywords:Cashmere guard hair powderHeavy metal ion absorptionRadiotracer techniqueZeta potentialIonic interactions
Cashmere guard hair was milled into fine particles using a wet milling system. The metal ion absorptionproperties of cashmere guard hair, both in the milled powder and its parent fibre forms, in comparisonwith wool and silk, were examined towards oppositely charged ionic metal species (Zn2+ and anionic speciesof Cr6+) using respective gamma emitting radioisotopes. The absorption of metal ions was found to be gov-erned by ionic interactions over different pH ranges. The absorption performance of the powder sorbents to-wards Zn2+ ions (silk powder>wool powder>cashmere guard hair powder) showed an opposite trend toCr6+ (cashmere guard hair powder>wool powder>silk power). The breakdown of cashmere guard hair cu-ticle cells and oxidation of powder surface during milling enhanced the initial rate of metal ion absorption.The absorption yield of all the studied sorbents increased with increasing the initial Cr6+ ion concentrationin the solution. However, Zn2+ ion concentration in solution showed a very little effect on absorption inthe powder sorbents studied.
Cashmere guard hair is a stiff keratinous fibre that grows on thecashmere goat throughout the year. Beneath the guard hair fibresgrow fine downy cashmere fibres during autumn and early tomid win-ter months [1]. The sheared cashmere fleece from the goat often con-tains both guard hair fibres and fine cashmere fibres. To make finestapparels, the guard hair fibres have to be removed from the fine cash-mere fibres, using a “Dehairing” process. As the guard hair fibres arenot suitable to spin into yarns for making textiles, they are often usedfor low value applications such as making brushes and interlinings.
Protein fibres possess amino and carboxylic acid groups both onthe surface and within the fibre matrix. These groups can be used asactive sites for binding of molecules targeting specific applications.For example, bound quaternary amino pyridinium salts on wool sur-face contributed to durable antimicrobial function [2]. Antibacterialsilk has been prepared by loading Co2+ or Ag+ ions onto the fibres[3]. Tin-phosphate silicate was used to improve the weight propertiesof silk [4].
Protein fibres have also been used to remove heavy and toxicmetal ions from aqueous solutions [5–9]. Recent studies on wooland silk have shown an improvement of adsorption kinetics towardstransition metal ions when the fibres are milled into fine powderform, and the improved metal ion removal efficiency is attributed tothe increased reactivity and specific surface area [10–12]. However,
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wool and silk are costly for powder making and metal ion separationapplications due to their niche position in the field of traditional tex-tiles, and an alternative option for this type of application could becashmere guard hair fibre.
Zinc(II) is stable in aqueous solution and remains at the same ionic stateover a wide pH range (pHs 1–8) [13]. In contrast, Chromium(VI) in aque-ous solution can exist in different ionic forms, such as CrO4
2−, Cr2O72− or
HCrO4− [13], and the proportion of the ionic species varies depending
on the pH condition [14]. The anionic species of Chromium(VI) and cat-ionic Zn2+ represent a valuable tool for screening the metal absorptionproperties of sorbents of interest.
In this study, we report the absorption and desorption properties ofcashmere guard hair powder towardsmetal ions Zn2+ and Cr6+. To ourknowledge this is the first study on the application of cashmere guardhair fibre for the separation and waste management of metal ions. Themetal ion absorption and desorption performance of cashmere guardhair powder is compared to its untreated parent cashmere guard hairfibre as well as wool and eri silk powders. The radiotracers 65Zn2+
(t1/2=243.66 days) and 51Cr6+ (t1/2=27.7 days) were used to moni-tor the absorption and desorption properties and the effects from pHvalue, exposure time of absorbent and initial metal ion concentration.
2. Experimental
2.1. Materials
High specific activity (SA) radioisotopes 65Zn2+ (t1/2=243.66 days,SA=151.63 GBq/g) and 51Cr6+ (t1/2=27.7 days, SA=20872.05 GBq/g)in the form of ZnCl2 and Na2CrO4, respectively, were purchased from
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Perkin Elmer. Zinc nitrate, potassium dichromate, Na2CO3, 2-propanoland sodium dodecyl sulphate were purchased from Aldrich. All reagentsand solvents used were of analytical grade and were used as received.Milli-Q grade water was used in all metal ion related experiments. Thebuffer solutions used were: 0.1 M KCl adjusted with 3.0 M HCl for pHs1 and 2; 0.1 M glycine/0.1 M NaCl adjusted with 3.0 M HCl for pH 3;0.1 M sodium acetate adjusted with 3.0 M HCl for pHs 4 and 5; 0.1 M so-diumphosphate dibasic adjustedwith 3.0 MHCl for pHs 6, 7 and 8; 0.1 Mglycine/0.1 M NaCl adjusted with 0.5 M NaOH for pH 9.
Cashmere guard hair fibres supplied by M/s Cashmere connectionsPty Limited, Australia, merino wool procured from M/s Australiancountry spinners, Australia and eri silk cocoons from north eastIndia were used for their respective powder preparations. Wool fibreswere used as received, while cashmere guard hair fibres and eri silkcocoons were cleaned prior to milling. Cashmere guard hair fibreswere cleaned on a Mesdan sample carding machine by three passes.The guard hair fibres were further cleaned manually to remove anyadhering traces of foreign matters. Eri silk cocoons were degummedfor 20 min in a Thies laboratory dyeing machine using laboratorygrade 2 g/L Na2CO3 and 0.6 g/L sodium dodecyl sulphate at 100 °Cwith a material-to-liquor ratio of 1:25 (kg/L). The cocoons were sub-sequently washed thoroughly with warm distilled water followed bycopious amount of cold distilled water. The degummed Eri silk co-coons were dried at 60 °C overnight.
2.2. Powder preparation
Cashmere guard hair fibres, wool fibres and eri silk cocoons werewet milled and spray dried into dry powders using a method devel-oped in our research group [15]. In brief, the fibres/cocoons were ini-tially cut into 1–2 mm snippets using a rotary blade cutter. The 200 gfibre snippets were then wet milled in an attritor (Union Process,USA) using 20 kg yttrium treated zirconium oxide grinding media(5 mm in diameter) in a Teflon-coated 9.5 L tank for 6 h. Distilledwater was added to achieve a material-to-liquor ratio of 1:10 (kg/L). Stirring speed and milling time were 280 rpm and 6 h, respective-ly. The wet milled slurry was further spraying dried (B-290 fromBuchi Labortechnik AG) into dry powder.
2.3. Characterisations
The powder morphologies were observed under a scanning elec-tron microscope (SEM, Zeiss Supra 55VP) at 5 kV accelerated voltageand 10 mm working distance. The samples were coated with about15 nm thick gold layer on a sputter coater (Bal-Tec sputter CoaterSCD 050) for SEM imaging. Laser diffraction based Malvern Instru-ments Mastersizer 2000 was used to measure powder particle size.2-propanol was used as the dispersion medium when determiningthe particle size. Refractive index of 1.553 for cashmere guard hairpowder and wool powder, 1.542 for eri silk powder and imaginary re-fractive index of 0.01 for all powders were used for necessary calcula-tions by the Malvern application software. The volume based sizedistribution of particles is used for reporting all particle size measure-ments. As there was insignificant variation between the mean particlesize (d(0.5)) values for the given powder samples due to their homo-geneous nature, no error bars were plotted in d(0.5) measurementsin this paper. The Fourier transform infrared (FTIR) spectra wereobtained using a Brucker® VERTEX 70 spectrometer under Attenuat-ed Total Reflectance (ATR) mode with a resolution of 4 cm−1 and 32scans per sample. The surface composition of the cashmere guard hairfibre and its powder was investigated using a K-Alpha X-ray photo-electron spectrometer (XPS) from Thermo Fischer Scientific usingmonochromated X-rays focused to a 400 μm spot size. Excessivecharging of the samples was minimised using a flood gun duringXPS measurements. High resolution peak scans were performed at20 eV pass energy. The peak scans were employed to obtain the
elemental composition of C, O, N, and S. The Zeta potential (usingZetasizer Nano ZS, Malvern Instruments, UK) of powder sampleswas determined after overnight incubating the powders (0.1 g) inpHs 1–9 buffer solutions (1 mL) at 25 °C. Each zeta potential valueis an average of three measurements.
2.4. Metal ion absorption
The metal ion absorption property towards Zn2+ and Cr6+ wastested over the pH range of 3 to 9 and 1 to 8, respectively. A typicalmetal ion absorption study involved accurately weighing approxi-mately 10 mg in quadruplicate of each sorbent (powders/fibres) fol-lowed by the addition of buffer solution (1.98 mL) at 21 °C. Thesorbents were allowed to incubate in buffer solution for 30 min.Then a stock solution of Zn2+ or Cr6+ (as zinc nitrate or potassiumdichromate) doped with their respective radioisotope (i.e. 65Zn2+
or 51Cr6+) was prepared for each buffer condition. An aliquot of thestock solution was added to the suspension so that the final concen-tration of metal ions was 10−4 M in a total reaction mixture of2 mL. Each reaction mixture contained approximately 100,000 countsper minute (CPM) of 65Zn2+ or 51Cr6+ as appropriate. The reactionmixtures were vortexed (SLRM-2M Intelli Mixer rotomix) and thenleft to rotate at 30 rpm for up to 24 h. At set time intervals (Zn2+:t=5, 15, 30, 60 min and 24 h; Cr6+: t=15 min, 2, 6 and 24 h), themixtures were centrifuged at 5000 rpm for 5 min (Eppendorf 5804R)and supernatant sampled (3×20 μL). The radioactivity associated witheach aliquot was determined using a gamma counter (Wallac Wizard1480). The gamma emissions used to monitor 65Zn2+ and 51Cr6+
radioisotopes were 1115.55 keV and 320.08 keV, respectively.The concentration of the natural metal ion, C, has a linear relation-
ship with the radioactivity count, A:
C ¼ A� a
where a is a coefficient related to the type of metal ions, the ratio be-tween the natural metal ion and its radioisotope in the solution. Thepercentage of the metal ions removed by the sorbent, i.e. absorptionyield, at the given timepoint twas calculated by the following equation:
%AbsorptionYield ¼ A0−At
A0� 100
where, A0 is the total amount of radioactivity added to the reactionmixture and At is the radioactivity in the buffer solution at time tafter exposure to sorbent.
The effect of metal ion concentration on the absorption yield wasexamined by following the same procedure with an initial metal ionconcentration in the range from 1×10−6 M to 1×10−4 M. The mea-surements were conducted at the optimum pH conditions, i.e. pHs8 and 2 for Zn2+ and Cr6+, respectively, which were determined inthe absorption study.
2.5. Desorption
To examine the desorption properties, each sorbent was pre-loaded with metal ion (doped with their respective radioisotope) byincubating for 24 h at the optimum pH condition. The samples afterloading with metal ions were then separated from the buffer solutionby centrifugation and then re-dispersed in a fresh buffer solution at apH known to have poor metal absorption (pHs 4 and 8 for Zn2+ andCr6+, respectively, as determined in the absorption study). The con-centration of the metal ions was determined by measuring the radio-activity of the solution after desorption. The desorption yield wascalculated by the following equation:
%DesorptionYield ¼ Ar;t
A0−At¼24ð Þ � 100:
Fig. 2. Particle size distribution of cashmere guard hair powder measured, a) from SEMimage, and b) by the optical diffraction method.
Fig. 3. Infrared spectra's of cashmere guard hair fibre and its powder.
Fig. 1. SEM images of a) cashmere guard hair fibre, and b) cashmere guard hair powder.
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A0 is the initial radioactivity of the solution for metal ion preload-ing onto sorbent at the optimum pH. At=24 is the radioactivityremaining in the solution after 24 h of preloading. Ar,t is the radioac-tivity released at time t after exposure to the buffer solution.
3. Results and discussion
3.1. Powder morphology and particle size
Fig. 1 shows the morphology of cashmere guard hair fibres beforeand after the milling treatment. Cashmere guard hair fibres appearedsimilar to other animal hair fibres with a cylindrical shape and scaleson its surface (Fig. 1a). The cashmere guard hair fibres appeared to bedistinctly larger in diameter (approx. 80 μm), compared to fine wooland silk fibres which were typically around 20 μm. After milling andspray-drying, cashmere guard hair powder showed a globular mush-room like morphology (Fig. 1b). The mean powder particle size basedon SEM image was 3.86 μm (Fig. 2a), which was slightly smaller thanthat measured by the optical diffraction method (Fig. 2b). The d(0.5)values for the cashmere guard hair powder, wool powder and eri silkpowder were 4.63 μm, 6.05 μm and 5.85 μm, respectively. Wool anderi silk powders had a similar morphology to the cashmere guardhair powder.
3.2. FTIR analysis
The chemical components of the cashmere guard hair powder werestudied by Fourier transform Infrared (FTIR) spectroscopy. As shown inFig. 3a, both cashmere guard hair fibre and milled powder exhibitedthe absorption bands at 3288 cm−1, 1649 cm−1, 1539 cm−1, and1240 cm−1, which were assigned to the N\H stretching, C_O stretch-ing (Amide I), N\H in-plane bending (Amide II), and C\N stretchingvibrations, respectively [16]. The distinctly increased absorption at1396 cm−1 after milling treatment, which corresponded to the C_Ostretching vibration of COO− [17], indicated the oxidation of cashmereguard hair fibre during the milling treatment.
Fig. 3b shows more significant difference in the FTIR spectra be-tween the cashmere guard hair fibre and powder. After milling,strong vibration peaks at 2920 cm−1 and 2850 cm−1, which were
assigned to the C\H asymmetric and symmetric stretching vibrationsof alkyl [17–18], disappeared. Since these peaks are stemmed fromthe outermost membrane of long hydrocarbon lipid chains of haircuticle cells [19], their disappearance indicates the breakdown ofthe cuticle cells and the exposure of the inner cortical cells as a resultof milling.
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3.3. XPS analysis
The chemical components of the cashmere guard hair fibre and itspowder surface were further characterised by X-ray photoelectronspectroscopy (XPS). The cashmere guard hair fibres consist of carbon,oxygen, nitrogen and sulphur elements. The atomic contents of theseelements on the surface of cashmere guard hair fibre and its powderare listed in Table 1. Compared with the cashmere guard hair fibre,the milled powder showed reduced C and S contents, but increasedN and O contents. A similar result was found on the milled wool pow-der and the change was attributed to the exposure of the fibre cortexdue to milling [20].
Fig. 4 shows the high resolution XPS C1s and S2p spectra of cash-mere guard hair fibre and its powder. The curve-fitted C1s spectra ofcashmere guard hair fibre showed two peaks with the binding ener-gies at 284.60 and 288.01 eV. The peak at 284.60 eV has beenassigned to C\C, C\H and C\S species, corresponding to the hydro-carbon backbone of the covalently bound fatty acids and side groupsof the amino acids [21], while the peak at 288.01 eV represents theamide bond (\NH\CO\) [22]. In case of cashmere guard hair pow-der, the C1s spectra revealed three more peaks at 285.91, 287.50 and288.31 eV, assigned to oxidised carbon species (C\O, O\C\O, C_Oand O\C_O) [23] along with the peak at 284.60 eV. For the S2p spec-tra, the major peak for the cashmere guard hair fibre was at ~164 eV,corresponding to disulfide bonds (S\S) [24]. Along with this peak, anew peak at 168.05 eV, which represented the formation of sulfonate(RSO3
−) species [25], emerged on the cashmere guard hair powder.This confirms the occurrence of oxidation reaction during the millingtreatment of cashmere guard hair fibres. This could be caused by thelocalised heating on powder during the milling process. The millingprocess could lead to the dissolution of certain fibre material intothe milling solution, which could be another reason for the reduced
Fig. 4. Curve-fitted high resolutio
C and S contents in the powder. In contrast, the N1s and O1s spectrashowed little change after the milling treatment.
3.4. Metal ion absorption
Different techniques have been adopted in the past to measure themetal ion concentration in aqueous solutions. Most commonlyemployed techniques include atomic absorption spectroscopy [8–9],chelate titration [26], proton induced X-ray emission [27], inductivelycoupled plasma mass spectroscopy, inductively coupled plasma-atomic emission spectroscopy [27] and X-ray photoelectron spectros-copy [28]. In this study, a radiotracer technique in which a solution ofthe desired metal ion (at a defined concentration) was doped with itscarrier-free radioisotope was used and the concentration of metal ionwas measured based on the gamma emission. The radioisotope heredoes not significantly change the total concentration of the metal so-lution and therefore acts as an indicator to the metal ion of interest.This technique is advantageous in simple and fast detection of metalions in liquid with a broad detection range.
Fig. 5 compares the absorption of Zn2+ and Cr6+ to cashmere guardhair powder, wool powder and eri silk powder after 24 h of incubationat various pH values. All the powder samples showed similar binding pro-files for bothmetal ions over the pH ranges studied. The Zn2+had a sharpabsorption peak at pH8 for all the curves. However, a shoulder at pHs 5–6was found only in the case of cashmere guard hair powder. The Zn2+
absorption properties of the cashmere guard hair powder was similar tothat of wool powder [12], where the pH condition in achieving optimumbindingwas 6–8 for Cu2+ and Cd2+ and 8 for Co2+. In contrast, the Cr6+
absorption showed a peak at pH 2 for all the powder sorbents studied. Asimilar Cr6+ absorption behaviour on wool fibres was also reported byDakiky et al. [7]. For cashmere guard hair powder, the Cr6+ absorptioncurve had an additional peak at pH 5.
Interestingly the absorption affinity of the powder sorbents forZn2+ is greatest for eri silk powder and decreases for wool powderfollowed by cashmere guard hair powder. The reverse behaviourwas observed for Cr6+, where cashmere guard hair powder absorbedCr6+ to the highest degree followed by wool and then silk powders.These results may reflect the amino acid composition of the respec-tive fibres. Based on the calculations on reported amino acid
Fig. 5. Absorption yield of powder sorbents to metal ions at different pH, a) Zn2+ andb) Cr6+ (initial concentration of metal ions=10−4 M, temperature=21 °C, exposuretime=24 h).
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composition data [29–30], cashmere guard hair fibre and wool fibrehave cumulative 15 mol% and 13.52 mol% of positively charged basicamino acids (i.e. Arginine, histidine and lysine), respectively, com-pared to only 5.1 mol% in eri silk [31]. The higher molar percentageof positively charged basic amino acids in cashmere guard hair fibreis likely to be responsible for higher absorption of Cr6+ which ismostly in its anionic form at pH 2 i.e. HCrO4
−. However the absorptionbehaviour for Zn2+ was not readily explained by only the amino acidcomposition of respective fibre types. The types of donor and quater-nary structure of protein are most likely to play a role in Zn2+ absorp-tion to the studied powder sorbents.
The Zeta potential of the powders was measured at different pHvalues. The data reveals the ionic interactions of metal ions with thestudied powder sorbents. As shown in Fig. 6, the Zeta potential was pos-itive when pH was below 5. This positive charge explains the high ab-sorption of Cr6+ (in the form of HCrO4
− anion) at a low pH condition.At higher pH value, the dominant Cr6+ species were CrO4
2− and there-fore it was consistent that the cashmere powder would not absorb wellat a high pH condition, because the surface was negatively charged atthis pH condition also. With similar reasons, the weak absorption of
Fig. 6. Zeta potential of powder sorbents at different pH.
Zn2+ is not surprising at low pH conditions due to its cationic nature.As the increase in pH value, the absorption of Zn2+ increased. Thepoor absorption of Zn2+ at pH 9 may be explained by the possiblestronger interaction of Zn2+ ions with buffer ligand (glycine/NaCl)than with the sorbents.
Fig. 7 shows the change of absorption yield with time for cash-mere guard hair fibre and its powder towards Zn2+ and Cr6+ atpHs 8 and 2, respectively, with the initial concentration of 10−4 M.For Zn2+, it was clear that the equilibrium was established quickly(b10 min) for cashmere guard hair powder, while it took consider-ably a longer time for the cashmere guard hair fibre (approximately80 min). For Cr6+, the absorption profile was different with equilibri-um established only after 24 h for both fibre and powder forms of thecashmere guard hair. The initial rapid absorption of Zn2+ and Cr6+ tocashmere guard hair powder is due to its high surface specificity ofthe powder with regard to its parent fibres. However the equilibriumabsorption of Zn2+ as well as Cr6+ to the cashmere guard hair fibrewas significantly higher than that of the powder with the sameweight, indicating a definitive role of cuticle cells in metal ion absorp-tion. The gaps between the fibre cuticle cells provide diffusion path-ways for the absorption of reactive species [32]. The breakdown ofcashmere guard hair cuticle cells during milling, as represented byFTIR studies, is likely to retard the metal ion diffusion process, result-ing in no or very less time dependent metal ion absorption by cash-mere guard hair powders. Overall, the absorption of the cationspecies (Zn2+) is considerably higher than that of the anion(HCrO4
−), 100–80% versus 80 to 50% at 24 h, suggesting that the ab-sorption of different types of metal ions to the cashmere guard hairfibre and powder follows different mechanisms.
Fig. 8 shows the influence of initial metal ion concentration(10−6 M–10−4 M) on the absorption, at pHs 8 and 2 for Zn2+ andCr6+ respectively. A very little effect of the initial concentration onthe Zn2+ absorption was found in case of the studied powder sorbents(Fig. 8a). However, the Zn2+ absorption by cashmere guard hair fibrewas substantially poor at low metal ion concentration. Therefore cash-mere guard hair powder could be an effective adsorbent for Zn2+ incomparison with its parent fibre. In the case of Cr6+, the absorptionyield of all the studied sorbents increased with increasing the initialCr6+ concentration (Fig. 8b).
3.5. Desorption properties
The release of metal ions was examined by re-dispersing the sor-bents that were pre-loaded with metal ions in a fresh buffer solutionat a pH condition known to have poor respective metal ion absorp-tion, i.e. pH 4 and pH 8 for Zn2+ and Cr6+, respectively. The Zn2+
was released quickly (within 30 min) and completely (100%) for allthe studied sorbents after exposing to pH 4 buffer solution. However,the behaviour of Cr6+ was not the same (Fig. 9), with 45% release atpH 8 by cashmere guard hair powder but only 17% by its parent fibre
Fig. 7. Absorption yield of Zn2+ and Cr6+ at pHs 8 and 2, respectively (initial metal ionconcentration=10−4 M, temperature=21 °C).
Fig. 8. A comparison of metal ion absorption of powder sorbents cashmere guard hairfibre and its powder with wool and silk powders, a) Zn2+ at pH 8 and b) Cr6+ at pH 2(temperature=21 °C).
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after 24 h, suggesting strong binding of Cr6+ to the fibre. The desorp-tion of Cr6+ from the wool powder and eri silk powder was similar tothat of the cashmere guard hair powder, with only approximately 45%release after 24 h.
4. Conclusion
We have examined the absorption behaviour of cashmere guardhair fibre and its powder towards oppositely charged metal ion spe-cies and compared with other protein fibre powder sorbents i.e.wool and silk. The absorption of Zn2+ and Cr6+ from aqueous solu-tions can be achieved using cashmere guard hair powders as a sor-bent. The Zn2+ and Cr6+ absorption is pH value dependent and ismainly governed by ionic interactions. When the solution pH valueis at 8 and 2, the cashmere guard hair, wool and silk powders showlargest absorption yield to Zn2+ and Cr6+, respectively. The cash-mere guard hair powder is more efficient in the absorption of Cr6+
compared to wool and silk powders. However, the absorptiontowards Zn2+ is appeared to be competent enough although cash-mere guard hair lags wool and silk powders. Zn2+ can be effectivelyabsorbed even at micro-molar concentrations by using cashmere
Fig. 9. Cr6+ desorption from cashmere guard hair fibre and different powder samplesat pH=8, exposure time=30 min and 24 h. (The samples were pre-treated by absorb-ing the ions from 10−4 M solution at pH=2 for 24 h at ambient conditions).
guard hair powder which is not possible with its parent cashmereguard hair fibre. The absorbed Zn2+ can be quickly and completelydesorbed from cashmere guard hair fibre and its powder at pH 4.The Cr6+ desorption from cashmere guard hair fibre in a pH 8 buffersolution has lower yield in comparison with its powder. In short,cashmere guard hair powder may serve as an economical alternativeto wool and silk powders for removal of heavy toxic metal ions fromwater for environment protection applications.
Acknowledgements
Funding support from the Australian Research Council (ARC)under its discovery project and Australian Institute of Nuclear Scienceand Engineering is acknowledged. The authors also acknowledgeRMIT University for its XPS facility and Dr. Xiujuan J. Dai from DeakinUniversity for her technical assistance in XPS measurements andanalysis.
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