Montmorillonite clay particles that had been prepared with an alklyl-ammonium surfactant were used to modify the moisture-sensitivity of bleached softwood kraft fibers through solvent exchange and adsorption methods. Moisture absorption and water uptake of the wood pulp fibers were significantly lower after the organo-nanoclay treatment. Thermal stability, surface energy, and surface morphology of the treated fibers were characterized using Thermogravimetric Analysis (TGA), Inverse Gas Chromatography (IGC), Scanning Electron Microscopy-Energy Dispersive X-ray Analysis (SEM-EDX), and Transmission Electron Microscopy (TEM) imaging. The Fourier Transform Infrared (FT-IR) spectral characteristics of the treated fibers were obtained to better understand the modified surface functional groups of the treated fibers. The treated bio-fibers had nano-scale surface roughness and a much reduced surface energy. The contact angle of water on the treated fiber mat was found to be higher than 160º. The thermal stability of the treated fibers was not affected by the modification.
Adsorption of TOLUENE ONTO bleached eucalyptus pulp treated with Ultrasound
Iñaki Urruzola, Maria Angeles Andrés, Luis Serrano, and Jalel Labidi*
Bleached kraft semichemical eucalyptus pulp was used as raw material to adsorb an organic compound, toluene, from aqueous solution. The pulp was sonicated with different powers and different times to obtain smaller cellulose fibers. The adsorption capacity for toluene of sonicated fibers and bleached eucalyptus pulp was measured by ultraviolet spectroscopy. The absorption capacity for toluene was increased considerably when cellulose nanofibres were obtained. The adsorption capacity of bleached eucalyptus pulp was 36 μmol/g, while sonicated fibres at 30 W and 20 hours increased the adsorption by 47% and at 50 W and 20 h increased it by 67% compared with untreated fibres. Visual examination and optical microscopy were used to observe the reduction of fibers width and the dispersion increase. Contact angle measurements were used to analyze the variation of hydrophilic character of cellulose. Fourier transform infrared spectroscopy was used to study variations introduced by the ultrasound treatments on the chemical structure of the samples. The adsorption capacity studies showed that the treatment with ultrasound improved the retention capacity of the fibres, increasing considerably the adsorption capacity when the fiber width approached the nanoscale.
Keywords: Cellulose; Ultrasound; Adsorption; Toluene; Nanofibers
Contact information: Chemical and Environmental Engineering Department, University of the Basque Country, Plaza de Europa 1, 20018, Donostia-San Sebastián, Spain.
* Corresponding author: firstname.lastname@example.org
Cellulose is one of the most important biopolymers that can be found in nature due to its natural character, biocompatibility, biodegradability, and low price. It is a polysaccharide composed exclusively of glucose molecules, is rigid, insoluble in water, and contains several hundred to several thousand units of β-glucose. It is the most abundant organic biomolecule, as it forms the bulk of terrestrial biomass (Fratzl 2003; Vincent 1999; Bidlack et al. 1992).
In recent years, the study of cellulose has been increased due to its high avail-ability; annually about 100 million tons of cellulose are produced by living plants around the world (Samir et al. 2005).
Cellulose has various applications in industry. One important aspect is the conversion of cellulose into ethanol for the production of alternative biofuels, which offers the potential for low environmental impacts (Andren et al. 1976). It is also used in the manufacture of explosives – the most famous of which is nitrocellulose, artificial silk, varnish, as well as thermal and acoustic insulation. The principal application of cellulose is the production of pulp and paper, using different bleached wood pulps obtained from different raw materials. Industrially, bleached cellulose pulp is obtained through two process stages, the pulping and bleaching. The objective of pulping is to remove the lignin to release cellulose fibers, thus separating the cellulose from the other components of wood by using mechanical or chemical processes. Moreover, lignin produces a brown discoloration in the final paper. The bleaching is carried out using different stages. Some of the main chemical reagents used are elemental chlorine (Cl2), chlorine dioxide (ClO2), and hydrogen peroxide (H2O2).
The adsorption capacity of cellulose is an aspect that has attracted considerable interest in recent years (Brochier et al. 2005; Bel-Hassen et al. 2008; Alila et al. 2005). This is because of the possibility of removal of organic matter by adsorption onto waste materials at low cost (Xu et al. 2007; Aloulou et al. 2004a-c; Bras et al. 1999). Several studies have been performed to evaluate the toluene retention capacity on cellulose, which demonstrates the ability of cellulose to adsorb aromatic compounds (Voronova and Zakharov 2009; Xing et al. 1994; da Silva Perez et al. 2004). Also, the ability of cellulose to fix metal ions by adsorption has been demonstrated (Chakravarty et al. 2008; Al-Ghouti et al. 2010; Gerente et al. 2000; Marshall and Campagne 1995). However, the characteristics of adsorption of native cellulose are not constant and vary depending on the origin of the cellulose and the preliminary treatments. Native cellulose has a relatively low adsorption capacity that can be increased by chemical functionalization of the fibres, where the adsorption capacity of modified cellulose fibres may be increased by as much as 10 times in comparison to the same fibres without any treatments. This can be achieved by the special structure of cellulose, introducing chemical groups that exhibit a high affinity for chemical species in aqueous solution such as acrylamide and acrylic acid to adsorb water, chitin, to adsorb heavy metals, or sorption of Cu2+ ions by cellulose graft copolymers (Zhou et al. 2004; Chauhan et al. 2000; Chauhan and Lal 2003). Numerous studies on this subject (Alila et al. 2011; Aloulou et al. 2006; Boufi and Belgacem 2006) have shown that the retention capacity can reach 300 to 600 mol/g substrate.
Different studies have shown that there are several factors that influence the adsorption capacity of cellulose. Two important factors are the hydrophobicity and their water solubilty of organic solute. In addition, the hydrodynamic volume, the shape of the molecule, and the interaction potential between the adsorbent and adsorbate are likely to play an important role (Alila and Boufi 2009).
The individualization of cellulose nanofibres from renewable sources has gained more attention in recent years because of their exceptional mechanical properties (high specific strength and modulus), large specific surface area, low coefficient of thermal expansion, high aspect ratio, environmental benefits, and low cost (Nishino et al. 2004). Suitable applications of cellulose nanofibres, such as reinforcement components in flexible display panels (Iwamoto et al. 2007), and oxygen-barrier layers (Fukuzum et al. 2009) have been proposed as well. Different steps of chemical treatments, such as mercerization, acetylating, hydrolysis, or organosolv pulping have been used for the elimination of non-cellulosic components and then mechanical treatments to obtain cellulose nanofibres.
Mechanical treatments such as high pressure homogenization and ultrasound techniques have been used to reduce the size of the cellulose fibers to the nano size scale, where the properties of the fibers vary considerably. Recently, the ultrasonic technique has been sufficiently described as an emerging method to reduce cellulose fibre size (Cheng et al. 2007, 2009, 2010). Ultrasound energy is transferred to cellulose chains through a process called cavitation, which refers to the formation, growth, and violent collapse of cavities in water. The energy provided by cavitation in this so-called sonochemistry is approximately 10 to 100 kJ/mol, which is within the hydrogen bond energy scale. Thus, the ultrasonic impact can gradually disintegrate the micron-sized cellulose fibres.
In this work, bleached eucalyptus pulp has been used as a raw material for the adsorption of toluene. The pulp has been submitted to a sonication process at different times and powers with the aim to increase the adsorption capacity of bleached pulp.
Semi-chemical kraft bleached eucalyptus pulp, with a fiber length about 1 mm and fiber width about 20 μm, used in this work was kindly supplied by Papelera Guipuzcoana de Zicuñaga, S.A. from Hernani (Spain). Table 1 shows the composition of these fibres, which was determined according to standard methods (TAPPI Standards 2007) and published procedures (Rowell 1983). This pulp presented a low lignin content (0.2 %) and high holocellulose content (93.9%).
Table 1. Composition of the Raw Material
Sonication was carried out with a Bandelin Sonopuls ultrasonic homogeniser with the purpose of reducing the size of the fibers to nanoscale. Then 0.2 g of bleached eucalyptus pulp in 100 mL of distilled water were used with two different powers: 30 W and 50 W (20 to 25 KHz of frequency) and three time durations (5 h, 10 h, and 20 h) to study the effect of the treatment on the fibre size.Sonication of Bleached Eucalyptus Pulp
The fixed frequency was due to the ultrasonic homogeniser limitations, whereas the powers and times were selected in order to obtain the minimum fibre size. An optimized study will be carried out in future works to measure the energy consumption.
The changes in dispersion of the bleached pulp suspension after ultrasonic treatment were observed trough visual examination of the pulp-water flask. For this purpose, 0.02 g of fibre were introduced in 10 mL distilled water.
In order to observe the variation of fibre size of the eucalyptus bleached pulp microscopically, the samples were prepared by adding 0.05 grams of bleached eucalyptus pulp in 25 mL of distilled water to obtain a good dispersion of fibers in solution. Optical images of the bleached eucalyptus pulp fibers were taken, before and after sonication treatments, using an optical microscope (Nikon Eclipse E600) at appropriate magnifications (200 x, and 500 x).
Contact Angle Measurements
Contact angle measurements were also carried out with water in order to determine changes in the hydrophilic character of each sample, before and after the ultrasound treatment. These measurements were performed using a Dataphysics Contact-angle system OCA 20. Uniform bleached pulp pellets were used for this propose.
Fourier Transforms Infrared Spectroscopy (FT-IR)
The FTIR spectra were recorded on a Perkin-Elmer 16PC instrument, by direct transmittance with an MKII Golden Gate SPEACAC accessory in the range of 400 to 4000 cm−1 with a resolution of 8 cm−1 and 20 scans.
Batch Adsorption Studies
Solute adsorption experiments in batch mode were performed by adding toluene at 1×10-4 mol toluene/L concentration using a micro-syringe to a water solution containing 0.05 mg of bleached eucalyptus pulp before and after ultrasonic treatments. This low concentration permits the partial dissolution of toluene in water (solubility of toluene in water 0.052% at 25 ºC).
Toluene adsorption experiments were performed in a 50 mL flask with a contact time of 12 hours. Toluene concentration was determined at several times using a UV-visible spectrophotometer Jasco V-630 and wavelength of 208 nm.
RESULTS AND DISCUSSION
Aqueous suspensions of the original fibre, before and after sonication treatment, at 30 W and 50 W during 5, 10, and 20 hours were placed in a flask to obtain pictures, such as that shown in Fig. 1. Bleached eucalyptus pulp without treatment was precipitated at the bottom of the glass bottle. A substantial increase in the dispersion of the fibre suspensions was observed after sonication treatment. The dispersion of the fibres increased substantially when the treatment time was increased. This dispersion was not very significant at 5 hours, but when the treatment increased to 10 hours, the dispersion increased and the dispersion was considerably higher at 20 hours. The spread with the different powers also had a significant effect, because increasing the power to 50 W resulted in a greater dispersion of the fibres. Treatment at this power level for the time period 20 hours resulted in the highest dispersion, and the material was converted into a highly viscous suspensions.
This study indicated that there was an improvement in the degree of fibrillation on the fibres, and more surface area on the fibres was exposed as the sonication output power increased.
Fig. 1. Dispersion state of the a) bleached eucalyptus pulp, b) 30 W 5 h, c) 30 W 10 h, d)30 W 20 h, e) 50 W 5 h, f) 50 W 10 h, and g) 50 W 20 h
Images of fibres were obtained by light microscopy to analyze the variation of fibres size with a concrete power as a function of time.
Bleached eucalyptus pulp fibres before the ultrasound treatment had an average width of 10 to 12 microns, as shown in Fig. 2.
Fig. 2. Optical images of the bleached eucalyptus pulp fibres before sonication treatments
When ultrasound treatment is applied to bleached eucalyptus pulp, the reduction in size of the fibres can be attributed to the effect of acoustic cavitation of high frequency (20 to 25 kHz) ultrasound in the formation, expansion, and implosion of microbubbles in the aqueous solution. The violent collapse that occurs causes microjets and shock waves on the surface of cellulose fibres, causing erosion of the surface of the fibres and splitting along the axial direction. The impact of sonication can break relatively weak hydrogen bonds of the fibres. Thus, the ultrasonic treatment gradually disintegrates the cellulosic fibres, reducing their width to a few microns and even to nano scale (Tischer et al. 2010; Wang and Cheng 2009; Zhao et al.2007).
Figures 3 and 4 show the evolution of the fibres dimensions for the different treatments. It can be observed as the increase of time reduces the size of the fibres.