INTRODUCTION:
Sawdust modification techniques vary from study to study, just as sawdust behavior varies from species to species. The control of the modifications undergone by the material during the chemical treatment requires a good understanding of the evolution of its microstructure according to the different stages of the chemical activation. In this context, and because of the economic and environmental importance of the valorisation of sawdust in wastewater treatment, we were interested, in the first place, in the question of the effect of chemical modification on the microstructure of sawdust Mixture of red wood and hard wood sawdust, within the framework of the formulation of adsorbent lignocellulosic materials.
It is in this spirit that our work has set itself the objective of developing a simple, efficient and environmentally friendly sorption remediation technique on a solid waste, in this case sawdust.
METHODS AND MATERIALS:
Materials:
Sawdust refers to all the residues produced by wood sawing, they are produced during machining operations performed on the raw material (wood). The sawdust used in this thesis is a mixture of hardwood and redwood. It comes from the wood unit located in CHETMA (State of Biskra-Algeria).
Methods:
a) Material pre-treatment:
The ability of sawdust to bind adsorbents can be greatly enhanced by subjecting it to chemical treatment to activate the functional adsorption sites. For this purpose: raw or untreated sawdust was washed, several times, with hot distilled water at 60°C and then oven dried at 80°C for 24 hours, in order to remove water-soluble particles adhering to the surface4.
b) Extraction of extractives with Soxhlet:
The extractives are the only fraction that can be isolated without degrading or modifying the other main constituents of the sawdust. These extractives, which may inhibit the prehydrolysis of the sawdust, are removed by Soxhlet extraction, using a continuous reflux process. The solvents solubilize the extractives contained in the plant material, which are finally concentrated in the recovery flask5. The extraction was carried out using an organic solvent mixture: ethanol/toluene 1:2 v/v of a volume v of 100 ml, according to the protocols presented in previous works5,6,7,8. The choice of solvents was made in order to extract the greatest amount of extractables.
· A mass of 30g of sawdust weighed in a cellulose cartridge and placed in the Soxhlet apparatus, was soaked in 300ml of organic solvents (ethanol and toluene).
· Extraction was performed at a rate of 1 to 4 cycles (soak/leach) per hour.
· Finally, the sawdust was dried in an oven at 80°C for 4 to 5 hours.
c) Chemical pre-treatment with sulfuric acid:
The sawdust was pre-treated with H2SO4 sulfuric acid. We opted for three types of acid pre-hydrolysis: (the first at 20%; the second at 50%, the third at 80%)9. That is to say three groups of tests, the aim being to compare the effect of acidity on hydrolysis.
· Solutions of 20%, 50% and 80% sulfuric acid are prepared, to each one of the solutions sawdust is added for 1 hour.
· After filtration, a washing with distilled water up to a pH = 7 was carried out, in order to eliminate all traces of acid and hydrolyzed sugars. The modified sawdust was dried in an oven at 100°C for 24 hours.
Methods of Characterization and Analysis:
a) Analysis by Fourier transfer spectroscopy (FTIR):
The different sawdust samples (untreated and treated) were analyzed by infrared (FTIR), using a FTIR-8400S Fourier transfer spectrometer of type SHIMADZU whose wavelength range is between 4000-400 cm-1.
b) Characterization and estimation of the cristallinity index by (XRD):
Cellulose is linear and forms intra- and intermolecular hydrogen bonds arranged in a regular and ordered system with crystal-like properties10. In order to determine the effect of chemical treatment on the crystallinity of cellulose, we opted for the determination of the crystallinity index, by the empirical method of DRX peak height11,12,13,14 that consists in examining the changes in the DRX spectra, after the chemical treatment. The crystallinity index CrI was calculated from the ratio of the height of the 002 peak (I002) and the height of the minimum value (IAM) between the 002 and the 101 peaks, using the following equation11:
This method is useful for comparing the relative differences between samples before and after chemical treatment.
The instrument is a Bruker D8 Advance type diffractometer. The incident X-ray beam comes from a copper anticathode using Kα radiation (λ=1.54056 Å) and, it is powered by a stabilized generator operating at 40KV with 40mA current. The line profiles will be measured using an automatic point-to-point counting system with a step of 0.02° during a counting time of 2 sec over an angular range of 10°- 90°.
c) Laser granulometry:
The instrument used is a Malvern-Mastersizer 2000-Hydro laser scattering granulometer. This apparatus allows the measurement of particle sizes in the range 0.02 to 2000μm as well as the specific surface. The measurements are performed in an aqueous medium (water with added sodium hexametaphosphate (dispersant) after deagglomeration of the powders by ultrasound for 15 minutes.
d) Adsorption test:
Activated carbon adsorption is the most appropriate and widely used technique for decontaminating chemically polluted water. However, its common use is still hindered by the high price of activated carbon. Many recently published works report the use of inexpensive natural or reclaimed adsorbents. These products are used as is or after chemical and/or physical transformation.
The objective of our work is to study the adsorption on treated sawdust while comparing the adsorptive power with that of activated carbon.
Acetic acid and its vapors or aerosols are caustic and can cause chemical burns of the skin, eyes and respiratory and digestive mucosa.
In the workplace, the main routes of exposure are the respiratory and Cutaneous15.
We start by introducing a mass m = 1g of the adsorbent in Erlenmeyer flasks. Then 50ml of acetic acid solution of known concentration is added. These Erlenmeyer flasks are then closed, placed in a thermostatic bath set at a known temperature and maintained under regular stirring. The mixtures are finally filtered after a certain contact time. After filtration, and for each Erlenmeyer, a volume of 10mL is taken and titrated with a NaOH solution (0.1mol/L). In order to observe the color change at the equivalence, add a few drops of phenolphthalein. At the end of this step, the quantity of solute adsorbed per gram of adsorbent is calculated as follows:
With:
Qa(mol/g): the quantity adsorbed at equilibrium.
C0 and Ceq(mol/L): respectively the initial and equilibrium concentrations of acetic acid.
m(g): being the mass of the adsorbent.
V(mL): volume of the solution.
RESULTS:
a) Characterization of untreated and treated sawdust by FTIR spectroscopy:
The spectra obtained by Fourier transfer infrared analysis (FTIR) of untreated and treated sawdust samples (with 20% acid pretreatment, 50% acid pretreatment and 80% acid pretreatment), are shown infigure 1. The characteristic vibrational bands were assigned, mainly in agreement with literature data15,16,17, 18,19,20,21,22, and taking into consideration the main differences between the IR spectra of the material before and after treatment.
At first glance, the IR spectra of untreated and treated sawdust have the same appearance but with a decrease in absorption intensity, especially in the case of the 50% acid and 80% acid modified sawdust.
Figure 1. Evolution of the sawdust during treatment.
Figure 2. Disappearance of the C=O group of carboxylic acids and esters of xylenes in lignins and hemicellulose (zoom of figure 1).
Figure 3. Decrease in the intensity of the C-O vibrations of the methoxy groups of lignin (zoom of figure 1).
Figure 4. Decrease in the intensity of valence vibrations of C-O and C-O-C bonds of cellulose (zoom of figure 1).
Figure 5. Decrease of the deformation vibration of the N-H bond (739 cm-1) and the C-H group (850 cm-1) (zoom of figure 1).
b) Characterization of untreated and treated sawdust by XRD:
The X-ray diffraction curves of raw and modified used sawdust samples are shown in figure 6 appears that all samples show the X-ray pattern of cellulose form characteristic of native cellulose in lignocellulosic materials15.
Figure 6. XRD spectra of untreated and treated sawdust. A: Untreated SB (raw), B: SB 20%, C: SB 50%, D: SB 80%.
c) Measurement of the specific surface:
The specific surface areas as mentioned in table 1 of the sawdust were determined by adsorption using a Malvern-Mastersizer 2000- Hydro Laser Granulometer. The results show that the specific surface area of untreated sawdust is about 3.1m2/g, while that of treated sawdust is 7.11m2/g (in the case of 20% pretreatment), 35.6m2/g (in the case of 50% pretreatment) and 42.3 m2/g (in the case of 80% pretreatment). However, we note that the increase in specific surface area of the concentrated acid hydrolysed material (80%) is more significant than those of the diluted acid hydrolyzed material 20% and 50%.
Table 1. Specific surfaces of elaborated materials.
|
Material |
SD raw |
SD 20% |
SD 50% |
SD 80% |
|
Specific surface (m2/g) |
3.1 |
7.11 |
35.6 |
42.3 |
DISCUSSION:
The decrease in peak intensity in the spectra of treated sawdust may be due to the catalytic role of the dehydration reaction exerted by sulfuric acid. All the infrared spectra reveal the presence of a broad band around 3330 cm-1 that corresponds to the elongation vibrations of the O-H bond of the aromatic and aliphatic structures of phenol, lignin group and cellulose. The band that appears between 2975-2800 cm-1 corresponds to the asymmetric elongation vibration of the C-H bond of cellulose. The peak around 1725 cm-1 associated with the very intense peak at 1660 cm-1 is characteristic of the valence vibration of the (C=O) carboxylic acids and/or esters of xylans, present in lignins and hemicelluloses 18,20. This peak completely disappeared in the spectra of chemically treated sawdust (50% acid pretreatment and 80% acid pretreatment) due to the removal of most hemicelluloses (figure 2).Moreover, the vibration at 1508 cm-1, attributed to the deformation (C=C) of the aromatic rings, of the lignin and the bands observed at 1317 cm-1 and 1262 cm-1 attributed to the ν(C-O) vibration of the methoxy groups of the lignin, seems to decrease in the spectra of the treated sawdust, relatively to the intensity of the mass corresponding to the O-H vibration of the aromatic structures (cellulose and lignin) (figure 3). We can therefore assume that the lignin has been degraded and that the decrease, or even disappearance, of the carbonyl compounds corresponds to the elimination of the hemicelluloses, which is partial in the treated sawdust (20% and 40% acid pretreatment) and total in the treated sawdust (50% and 80% acid pre-treatment). This difference is significant and suggests that pre-hydrolysis would be effective in the presence of concentrated acid. The peak at 1025 cm-1 corresponds to the valence vibrations of the C-O and C-O-C bonds of cellulose16 (figure 4).
The region of bands between 400 and 894 cm-1 characteristic of the C-H and N-H grouping in cellulose 22,23 show a clear decrease (figure 5). They are observed both in the spectrum of untreated sawdust and in those of treated sawdust. Similarly, the clear decrease in intensity of the absorption band at 889 cm-1 in the spectrum of chemically treated sawdust (50% acid pretreatment and 80% acid pretreatment) shows that our modified sawdust is less rich in acetyl groups or carboxylic acids, and no longer contains hemicelluloses.
Thes results reveal an increase in the crystallinity index following the chemical treatment applied to sawdust, which results in an improvement in the crystallite order. The degree of crystallinity of cellulose is higher in the treated samples than in the raw sawdust, due to the reduction of hemicelluloses during the treatment. These observations are consistent with those of Alemdar A and al23, whose work showed that the crystallinity index increases after chemical treatments. However, hydrolysis at 50% and 80% has an effect on the increase of the crystallinity of the material (62.15% for sawdust treated at 80%; 61.06% for sawdust treated at 50%) higher than that of hydrolysis at 20% which has a crystallinity index equal to 49.35% .This increase is reflected by the peak of 21 to 22 characteristics of cellulose. These results are in perfect agreement with the results of S. Benyoucef and al8.
Figure 7 shows a rearrangement of the longitudinal tracheids in a uniform and staggered structure, which is only the effect of the acid hydrolysis suffered by the material. The porous structure is apparent on the surface and testifies to the degradation of the hemicelluloses and the elimination of the extractables, following the chemical modification of the cellulosic material.
Adsorption tests: comparative study with activated carbon:
Measuring the equilibrium time:
The study of the kinetics of lead elimination by sawdust in aqueous medium showed that equilibrium is reached after 3 minutes (figure 7). Beyond this time, the residual acetic acid concentration remains constant23-26.
In both cases, the rapid adsorption of acetic acid molecules is attributed to the existence of free sites during the first few minutes, but as time goes on the sites become saturated and the number of free sites is reduced. The amounts of acetic acid extracted are much greater for activated carbon than for sawdust.
Figure 7. Residual concentration of acetic acid as a function of time for sawdust and Activated carbon
Effect of initial concentration on adsorption
It is observed in figure 8 that the retention of acetic acid increases with increasing initial concentration for both adsorbents.
The curve presented in figure 8 has two essential aspects. It is of type 1, it is linear and ends with a saturation level from 0.5mol/l (initial solution), where the quantity of acid eliminated by the sawdust becomes constant (Qe = 3.12mmol/g) (Qe = 5.6mmol/g for activated carbon).
These results allow us to conclude that these values represent the maximum quantities of lead that can be fixed on one gram of sawdust and one gram of activated carbon under our operating conditions27, 28, 29.
Figure 8. Quantity of acetic acid adsorbed Qe as a function of the initial concentration Ce (Vstirring: 160 rpm; pHmedium: 4-4,4; Tmedium: 20 °C; Msawdust: 1 g; Mactivated carbon: 1 g)
CONCLUSION:
The chemical modification of the cellulose material based on sawdust (mixture of red and hard wood), allowed the removal of hemicelluloses and the improvement of the crystallite order in the microstructure of the material, which is reflected in the rearrangement of the longitudinal tracheids in a uniform porous structure and in a staggered arrangement, which favored an increase of the crystallinity. This effect on the microstructure of the modified material is higher when the concentration of the acid used in the pre-hydrolysis highest. We can conclude that the sawdust thus modified, can be valorized and find a potential application of adsorption in the field of the depollution of wastewater.
This study has shown the efficiency of the studied sawdust in the retention of acetic acid. Its greater potential of adsorption makes it suitable to be used to eliminate other families of harmful organic acids.
Its natural abundance with negligible prices reduces the costs of the disposal process and offers the advantage of this material being tested on an industrial scale for the treatment of persistent and toxic organic pollutants from industrial waste.
ACKNOWLEDGMENT:
We would like to thank the chemistry laboratory of Mohamed Khider University of Biskra- Algeria for providing us a suitable working environment to fulfill our research and being a great aid when necessity called.
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Received on 02.04.2022 Modified on 24.07.2022
Accepted on 19.09.2022 ©AJRC All right reserved
Asian J. Research Chem. 2022; 15(6):393-398.