Lignocellulose-clay nanocomposites were synthesized using an in situ intercalative polymerization method at 60°C and a pressure of 1 atm. The ratio of the montmorillonite clay to the lignocellulose ranged from 1 : 9 to 1 : 1 (MMT clay to lignocelluloses, wt%). The adsorbent materials were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and X-ray powder diffraction (XRD). FTIR results showed that the polymers were covalently attached to the nanoclay and the lignocellulose in the nanocomposites. Both TEM and XRD analysis showed that the morphology of the materials ranged from phase-separated to intercalated nanocomposite adsorbents. Improved thermal stability, attributable to the presence of nanoclay, was observed for all the nanocomposites. The nanocomposite materials prepared can potentially be used as adsorbents for the removal of pollutants in water treatment and purification.
Nanotechnology based methods for water purification, if well developed, have the potential to produce highly purified water at low cost. Nanoadsorbents such as carbon nanotubes, zeolites, carbon nanotubes-supported cerium oxide, activated carbon fibres, alginate/carbon nanotubes composites, and layered double hydroxides have been investigated for the removal of pollutants ranging from heavy metals, dyes, and organics from aqueous systems [
Use has been made of different types of nanocomposites for the removal of a wide array of pollutants from contaminated water. Such pollutants include nitrophenols [
Nanocomposites thus have the potential to give highly treated and pure waste water effluents. The major hindrance in the widespread application of nanotechnology, however, is the unavailability of a large quantity of nanomaterials at economically viable prices [
Lignocellulose and montmorillonite clay have been identified as low-cost and potentially effective adsorbent materials. Lignocellulose-montmorillonite clay nanocomposites have been widely studied for application in structural materials especially in the furniture and in the packaging industry. The relatively low-cost lignocellulose-clay nanocomposites can also be used as adsorbent materials for the removal of inorganic and organic pollutants from aqueous solutions. Being derived from biomass, the advantages include low density, low equipment abrasiveness, relatively low cost, and biodegradability. Montmorillonite (MMT) clay is a naturally occurring 2 : 1 phyllosilicate mineral which comes in powder form and when completely delaminated, approximately 1 nm thick platelets or sheets are obtained with surface areas of about 750 m2/g and an aspect ratio >50 in comparison to conventional microsized fillers [
Most lignocellulose-clay nanocomposites have been prepared using the melt intercalation method. Wang et al., 2003 [
In order to achieve a more porous material and unlock more surface area in the product, an in situ intercalative polymerization method was employed in the present study. It was also necessary to achieve covalent bonding of the polymer to both the lignocellulose and the clay to avoid leaching of the polymer (especially the water soluble polymers like poly(methacrylic acid)) into water to avoid secondary pollution problems. As such, the polymers served as swelling agents to trigger clay exfoliation and coupling agents, which served as bridges between the lignocellulose and the montmorillonite clay. The in situ intercalative polymerization method has not been widely applied in the preparation of wood-clay nanocomposites. The prepared nanocomposites were soxhlet-extracted with different organic solvents to remove any unreacted monomers. It is postulated that sodium montmorillonite (NaMMT) clay particles get covalently attached to lignocellulose to create some kind of a brush-like structure with the NaMMT particles sticking out for adsorption of pollutants from aqueous solution.
Methyl methacrylate (MAA, 99%) purchased from SAARCHEM (Pty) Ltd was distilled before use and methyl orange was used as received. Methacrylic acid and dibutyl tin dilaurate (DBTDL, 95%) from Sigma-Aldrich were used as received. Methacryloxypropyltrimethoxysilane (MPS, 99%), ammonium persulphate (AMPS, 98%), and aluminium chloride hexahydrate (99%) were from Associated Chemical Enterprises. Dodecylbenzylsulphonic acid (DBS) and sodium metabisulphite (SMBS, 95%) were purchased from BDH Chemicals and sodium chloride salt (99.5%) was from MET-U-ED CC.
SMBS (4.3 × 10−5 mol) and DBS (4.5 × 10−5 mol) were dissolved in deionized water in a three-neck round bottom flask. Soxhlet-extracted lignocellulose was then added slowly to the mixture under high speed magnetic stirring, which was then followed by the slow addition of NaMMT. The mixture was stirred for 1 h to completely disperse the solids followed by the addition of MMA to form an emulsion. N2 gas was bubbled into the system to purge oxygen and was maintained till the end of the reaction. After mixing for a further 30 min, AMPS (dissolved in 10 mL of deionized water) was added to the mixture and stirred for 30 min before refluxing the mixture at 60°C under N2 for 4 h. The mixture was cooled to room temperature, and the product (PMMAgLig-NaMMT) was isolated by filtration, followed by purification by solvent extraction with THF (20 mL) to remove PMMA homopolymer, and dried at 50°C for 24 h. The materials were prepared at different compositions of montmorillonite ranging from 10 to 50% w/w for all nanocomposite adsorbents. However, the results reported are only for the sample prepared at 40% w/w.
Soxhlet-extracted lignocellulose (3.0 g) and NaMMT (3.0 g) were dispersed in a 1 : 1 v/v EtOH : H2O solvent (150 mL) mixture under high speed magnetic stirring for 1 h. MPS (1.4 × 10−2 mol) was added to the lignocellulose-NaMMT dispersion and mixed for 10 min. A catalyst, dibutyl tin dilaurate (2.1 × 10−4 mol), was added and the mixture stirred for 10 min. The mixture was then heated to 70°C and left to react at this temperature for 24 h.
After 24 h, the mixture was cooled to room temperature and the MPS-coupled lignocellulose-montmorillonite (PMPSgLig-NaMMT) nanocomposite isolated by filtration. THF was used as solvent in soxhlet extraction for 24 h to remove the free MPS monomer, the homopolymerized silane, and the catalyst. The same NaMMT clay loading to lignocellulose of 10 to 50% w/w was used. The PMPSgLig-NaMMT nanocomposite was dried at 50°C for 24 h. The sample was characterized by FTIR, TGA, XRD, and TEM and the results at 40% w/w NaMMT loading were reported.
PMAA-grafted lignocellulose-montmorillonite (PMAAgLig-NaMMT) nanocomposites were prepared at different montmorillonite clay loadings. The required amount of MPS-grafted lignocellulose was slowly dispersed in deionized water (150 mL) to which ethanol (5 mL) had been added. The ethanol was added to improve the dispersion of both the MPS-grafted lignocellulose and MPSgMMT in the water. After all the solids had been homogeneously dispersed in the water (about 1 h), MAA was added and mixed for 30 min, followed by the addition of AMPS. The AMPS was first dissolved in deionized water (10 mL) before being added to the mixture. The system was heated to 70°C under N2 atmosphere to initiate the graft polymerization reaction, and the temperature was maintained for 24 h, after which the mixture was cooled to room temperature and the product (PMAAgLig-NaMMT) isolated by filtration. The NaMMT clay loading to lignocellulose of 10 to 50% w/w was used. The product was washed with 0.01 M NaOH solution and then deionized water until the filtrate was at pH 5. The product was further soxhlet-extracted with THF for 24 h in order to remove all PMAA homopolymer and then dried at 50°C for 24 h. The nanocomposites were characterized by FTIR, TGA, XRD, and TEM and results for the 40% w/w NaMMT loading were reported.
FTIR analysis was performed (PerkinElmer, 2000) using the procedure described by Rajendran et al., 2001 [
TEM images were recorded on a JEM 200CX transmission electron microscope (JEOL, Tokyo, Japan) at an accelerating voltage of 120 kV. Prior to analysis, the nanocomposites were stained with OsO4, then embedded in epoxy resin, and cured for 24 h at 60°C. The embedded samples were then ultramicrotomed with a diamond knife on a Reichert Ultracut S ultramicrotome at room temperature to give sections about 100 nm thick. The sections were transferred from water at room temperature to 300-mesh copper grids, which were then transferred to the TEM instrument. The results reported are for the nanocomposites prepared at a NaMMT clay loading of 40%.
The presence of the peak at 1734 cm−1 (C=O of the ester linkage of PMMA) (Figure
(a) FTIR spectra of raw lignocellulose, NaMMT, and PMMAgLig-NaMMT nanocomposite. (b) TGA thermograms of NaMMT, raw lignocellulose, and PMMAgLig-NaMMT. (c) X-ray diffractograms of NaMMT and PMMAgLig-NaMMT. (d) Transmission electron micrograph of PMMAgLig-NaMMT nanocomposite.
The presence of NaMMT in the lignocellulose matrix resulted in an increased thermal stability of the nanocomposite (Figure
A slight shift of the basal reflection peak to lower 2
The micrograph showed that the NaMMT was dispersed in the lignocellulose matrix at nanoscale (Figure
The FTIR spectrum for PMPSg-LigNaMMT prepared at a NaMMT loading of 40% is shown in Figure
(a) FTIR spectra of PMPS and PMPS-grafted lignocellulose-MMT nanocomposite. (b) Thermograms of raw lignocellulose, NaMMT, and PMPS-grafted lignocellulose-montmorillonite nanocomposite. (c) XRD patterns of NaMMT and PMPS-grafted lignocellulose-NaMMT nanocomposite. (d) Transmission electron micrograph of PMPSgLig-NaMMT nanocomposite.
The nanocomposite was more thermally stable compared with the raw lignocellulose, but less stable relative to the NaMMT (Figure
There was a shift in the montmorillonite basal reflection peak towards lower 2
The micrograph of PMPSgLig-NaMMT (Figure
The PMAA-grafted nanocomposite (PMAAgLig-NaMMT) sample was prepared at a NaMMT loading of 40%. The PMAAgLig-NaMMT spectrum showed an additional peak at 1268 cm−1 (Figure
(a) FTIR spectra of PMAA, MPS-grafted lignocellulose, and PMAA-coupled lignocellulose-NaMMT nanocomposite. (b) TGA thermograms of MPSgMMT, MPSgLig, and PMAA-grafted lignocellulose-MMT nanocomposite. (c) XRD patterns of NaMMT and PMAAgLig-NaMMT. (d) Transmission electron micrograph of PMAAgLig-NaMMT nanocomposite.
The structural OH stretching vibration (3646 cm−1) of the MMT was not very visible in the nanocomposite possibly because of the presence of hydrogen bonded OH groups from the lignocellulose.
The PMAAgLig-NaMMT sample prepared at a NaMMT loading of 40% showed three decomposition steps at temperatures 30–120°C, 220–420°C, and 420–650°C (Figure
Summary of thermal properties of synthesized materials compared to the precursors.
Material | Onset temperature (°C) | Degradation temperature |
Remarks |
---|---|---|---|
PMPSgLig-NaMMT | 220 | 220–650 | Loss of water at 30–120°C; gradual degradation, showing decomposition of lignocellulose at 400°C |
PMPSgLig-NaMMT | 320 | 425–900 | Loss of water at 30–120°C; gradual degradation |
PMAAgLi-NaMMT | 240 | 240–600 | Loss of water at 30–120°C; decomposition of lignocelluloses at 220–420°C, decomposition of grafted polymer at 420–650°C; gradual degradation |
NaMMT | 220 | — | Thermally stable within the analytical temperature range used; only lost moisture around 100–120°C and water of crystallization |
Lignocellulose | 220 | 340–600 | Loss of water at 30–120°C; fairly rapid degradation |
The X-ray diffractograms of NaMMT and PMAAgLig-NaMMT nanocomposite showed a shift of the basal reflection peak to lower 2
TEM showed the NaMMT still fairly ordered within the lignocellulose matrix (Figure
Novel adsorbent materials (PMMAgLig-MMT nanocomposite, PMAAgMMT, PMAAgLig, PMPSgLig-MMT, and PMAAgLig-MMT nanocomposite) were prepared through condensation as well as free-radical graft polymerization reactions. The intended modification of the lignocellulose and MMT separately was achieved as confirmed by FTIR, TGA, and XRD. XRD analysis showed some partial intercalation of MMA into the interlayer space of the clay sheets. The nanocomposite adsorbents also showed a slight increase in the basal spacing of the MMT clay sheets. Intercalated nanocomposite adsorbent materials were successfully prepared as shown by FTIR, TGA, and XRD. Modification of lignocellulose with MPS resulted in improved thermal stability. All the nanocomposites prepared also showed an increase in thermal stability, which was attributed to the presence of clay in the lignocellulose matrix. However, modification of lignocellulose with PMMA resulted in a reduction in thermal stability. The nanocomposite materials prepared can potentially be used as adsorbents for the removal of pollutants in water treatment and purification.
Now Tavengwa Bunhu’s affiliation is Chinhoyi University of Technology, Private Bag 7724, Chinhoyi, Zimbabwe, and Lilian Tichagwa’s affiliation is Harare Institute of Technology, P.O. Box BE277, Belvedere, Harare, Zimbabwe.
The authors declare that they have no competing interests.