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Int. J. Environmental Technology and Management, Vol. 12, Nos. 2/3/4, 2010 163
Copyright © 2010 Inderscience Enterprises Ltd.
Modified biopolymer adsorbent for the removal of dissolved organic pollutants
Sabrine Alila LMSE, Faculté des Sciences de Sfax, BP 802-3018 Sfax – Tunisie Fax: 216 74 274 437 E-mail: [email protected]
Alexandra Isabel Costa Laboratório de Química Orgânica, Instituto Superior de Engenharia de Lisboa, Departamento de Engenharia Química e CIEQB, R. Conselheiro Emídio Navarro 1, 1950-062 Lisboa, Portugal E-mail: [email protected]
Luís Filipe Vieira Ferreira Centro de Química-Física Molecular, IN, Complexo Interdisciplinar, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Fax: 351-21-846-44-55 E-mail: [email protected]
Sami Boufi* LMSE, Faculté des Sciences de Sfax, BP 802-3018 Sfax – Tunisie Fax: 216-74-274-437 E-mail: [email protected] *Corresponding author
Abstract: Chemically modified cellulose fibres were used as adsorbents for the removal of organic compounds and herbicides from water. The chemical modification of fibres by grafting hydrocarbon moieties enhances the adsorption capacity of cellulose substrate. The adsorption behaviour of the modified fibres towards various organic solutes and three herbicides was investigated. The viability of application of the modified cellulose fibres for the removal organic pollutant in continuous mode was confirmed by using column
164 S. Alila et al.
filled with modified fibres. Once exhausted, the column regeneration was accomplished by washing with ethanol. The regenerated column was used in several adsorption–desorption cycles without any loss of the adsorption capacity.
Keywords: cellulose; adsorption; chemical modification; organic pollutant; column.
Reference to this paper should be made as follows: Alila, S., Costa, A.I., Vieira Ferreira, L.F. and Boufi, S. (2010) ‘Modified biopolymer adsorbent for the removal of dissolved organic pollutants’, Int. J. Environmental Technology and Management, Vol. 12, Nos. 2/3/4, pp.163–191.
Biographical notes: Sabrine Alila is a PhD Student in the LMSE Laboratory at the Department of Chemistry of the University of Sfax (FSS). Her research topic is concerned with surface chemical modification of cellulose fibres to enhance their absorption capacity towards dissolved organic pollutants, including pesticides and herbicides. She has published different papers on the interaction of cellulose fibres with cationic surfactant.
Alexandra Isabel Costa Lectures at Departamento de Engenharia Química, Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa. She did her MSc (2001) in Organic Chemistry and Technology at Faculdade de Ciências e Tecnologia, New University of Lisbon. At present, she is a PhD student in Organic Synthesis and Laser Photochemistry. Her research in recent years is centred mainly in the field of nanomaterials: design, synthesis and properties evaluation of well-defined calixarene-based conjugated polymers as new materials for electrochemical switches, electronic and optoelectronic devices and chemical sensors.
Luís Filipe Vieira Ferreira is an Associate Professor with Habilitation at Instituto Superior Técnico, Technical University of Lisbon (UTL). He got his PhD in 1983 from UTL and made his Post-Doc in 1987 at the Loughborough University, UK, with Professor Frank Wilkinson in the Laser Photochemistry Group where he started his interest in Diffuse Reflectance Techniques, both transient and steady state. His research activities include cellulose and cellulosic materials, calixarenes and hydrophilic and hydrophobic zeolites as hosts for photodegradation studies of organic pollutants (namely dioxins), and dyes as guest probes. He has published more than 100 papers in international journals in the domain of surface photochemistry.
Sami Boufi is an Associate Professor in the Department of Chemistry of the University of Sfax (FSS). He has a PhD in the Sciences of Polymeric Materials from Institut National Polytechnique de Grenoble, France (1992). His research activities include chemical modification of cellulose and lignocellulosic materials, the synthesis of functional polymer for colloidal chemistry and emulsion polymerisation, and the exploitation of chemically modified cellulose fibres as reusable adsorbent for dissolved organic pollutants. He has published more than 40 papers in international journals dealing with polymer science and physical colloidal chemistry.
Modified biopolymer adsorbent for the removal 165
1 Introduction
Environmental pollution by organic compounds and pesticides or herbicides has recently raised much concern owing to the potential health hazards (harmful effects) associated with the entry of these compounds into the food chain of humans and animals, which is the consequence of surface and groundwater contamination (Philip, 1991; Adachi et al., 2001; Mottaleb et al., 2003). Their sources in the environment are wastewater discharges from chemical industries, leaching and run-off from agricultural and forest land (through the intensive use of pesticides and weed-killers), paints, solvents, discharges of industrial wastewaters (Pellizetti and Serpone, 1988). To prevent excessive water pollution, extensive legislation has been imposed in different countries to limit their threshold.
Various technologies are available for the water treatment; among them, the most employed are biological treatments (Pearce et al., 2003; McMullan et al., 2001; Fu and Viraraghavan, 2001), adsorption on various solid substrates such as activated carbon (Philip et al., 2004; Chern and Chien, 2003; Kamegawa et al., 2005), oxidation by oxygen or ozone, photochemical fragmentation (Jomaa et al., 2003) and membrane separation (Ayranci and Hoda, 2004). Each method has its advantages and drawbacks. Biological treatment, which is considered to be the cheapest alternative, is mainly used as the secondary treatment after a primary one removing the suspended matter. It takes advantage of the catabolic versatility of microorganisms to degrade/convert organic contaminants, and contributes to reduce the biochemical oxygen demand up to 90%. However, even if the major part of pollution contained in used water can be eliminated by biological treatment, a certain fraction of, particularly, some classes of dissolved organic compounds, such as organic chlorinated compounds, polyphenols, pesticides, herbicides and other contaminants, are among the most recalcitrant pollutants that need a tertiary treatment by adsorption, oxidation process or nanofiltration to remove them.
Among the various techniques cited earlier, adsorption is one of the most frequently applied methods, thanks to its efficiency, capacity and applicability on a large scale. This process has also been widely used to remove pesticides and dissolved organic pollutants and other hazardous chemicals from water. At present, there is no single process capable of totally adequate treatment, mainly owing to the complex nature of the effluents. In practice, a combination of different processes is often used to achieve the desired water quality in the most economical way.
There are, of course, disadvantages in using adsorption processes in wastewater treatment. They can be summarised as follows: The adsorption properties of an adsorbent depend on the different sources of raw materials. The sorption capacity depends on the chemical structure of the material, its porosity, its specific surface area, its swelling and diffusion characteristic and the abundance and the nature of the functional superficial groups. The size of adsorbent particles has also been shown to be a key parameter in the control of sorption performances. In addition, the adsorption process is influenced not only by adsorbent characteristics, but also by geometry, chemical structure, water solubility and polarity of pollutant. The extreme variability of industrial wastewater must be taken into account in the choice of the sorption substrate.
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Although, as reported by Sharma et al. (2007, 2008), activated carbon is the most widespread adsorbent, available in a variety of configurations and sizes, it suffers from some drawbacks owing to the high cost, expensive regeneration and progressive loss of the adsorption capacity with regeneration. Therefore, exploitation of new and cheap adsorbents with high selectivity and capacity, long life, and which can be more easily regenerated, currently becomes the focus of intense research (Memon et al., 2007, 2008; Akhtar et al., 2006, 2007a, 2007b).
The use of vegetal biomass as a bio-filter for remediation of waters contaminated with pesticides or metals has been widely described in the literature over the last ten years by Schneegurt et al. (2001) and Akhtar et al. (2007a, 2007b). However, the adsorption capacity and the affinity of these natural vegetable adsorbents greatly fluctuate according to their origin. The recourse to a ubiquitous available biopolymer-based adsorbent with a high adsorption property towards a large variety of organic compound should contribute to the development of these substrates. Among the different available biopolymers, cellulose is the most attractive because it is the most abundant and renewable polymer resource available worldwide. It is estimated that by photosynthesis, 1011–1012 tons of cellulose are synthesised annually in a relatively pure form. The use of cellulose as adsorption support is not recent. Previous works highlighted the ability of this natural material to adsorb a certain number of organic compounds, such as pesticides (Xilong and Baoshan, 2007; Yoshizuka et al., 2000) and organic dyes (Gupta et al., 2008; Crini, 2008). The ability of cellulose to adsorb metal ions was also shown (Reddad et al., 2003; Kim and Lim, 1999). However, the adsorption properties of native cellulose for organic pollutants and metallic contaminants remain very low (from 100 to 500 times lower) in comparison with activated carbon or zeolite. This effect is associated with the low concentration of active sites on which organic pollutant could be adsorbed.
A target chemical modification of cellulose by anchoring functional group can be carried out to achieve adequate structural durability and efficient adsorption capacity for different pollutants. This strategy was successfully used for adsorption of heavy metal ions by the introduction of chelating or metal-binding functionalities or by grafting selected monomers bearing chelating moieties to the cellulose backbone (Zhang et al., 2003; Delval et al., 2000; Lee et al., 2001). However, very few studies reported the use of similar system to remove organic pollutants.
In previous studies, our group have shown that the adsorption of cationic surfactant on bleached cellulose fibres greatly enhances the aptitude of the substrate to uptake dissolved organic compounds from aqueous media (Aloulou et al., 2004a, 2004b, 2004c; Alila et al., 2005, 2007). The improved solute adsorption was ascribed to the accumulation of the organic solutes within the aggregated domains formed by the self-assembly of surfactant monomers at the cellulose/water interface. However, the desorption of the surfactant molecules from the cellulose surface impeded the regeneration of the substrate by extraction of the trapped solute once it is exhausted. Then, a chemical grafting of hydrocarbon structures that mimics the aggregated domains generated by the adsorbed surfactant molecules was accomplished (Aloulou et al., 2006; Boufi and Belgacem, 2006). The aptitude of the ensued modified substrate to uptake dissolved organic solute in aqueous media has been investigated in a batch and in continuous operations.
Modified biopolymer adsorbent for the removal 167
In this work, we continue our research regarding the potential use of modified cellulose substrate as an adsorbent for organic compounds. Diverse types of hydrocarbon chains differing either in their length or in their terminal functionality were grafted using isocyanate (MDI) or carbodiimidazole (CDI) chemistry (Paul and Anderson, 1960; Robert et al., 2004; Rannard and Davis, 2000) that allowed us to carry on different surface modifications under mild condition.
2 Experimental section
2.1 Materials
Two types of cellulose substrates were used in this work: bleached soda pulp was from the Tunisian annual plant esparto (alfa tenassissima) and regenerated commercial cellulose film (cellophane). The fibres were highly porous and had a specific surface area in the dry state of 3 m2⋅g–1 as measured by BET. The mean fibre length and width measured by optical microscopy were 0.75 mm and 14.2 µm, respectively, thus corresponding to an aspect ratio of 52.
All the reagents, all the aliphatic amines and the different organic solutes used in this study were of analytical grade.
Table 1 presents the different products used and their abbreviations.
Table 1 Structures of tested organic solutes and herbicides, their water solubility and Kow values
Structure and abbreviation Water solubility (mg⋅L–1) Ln (Kow)
Organic solutes
0.721 6.66
2.8 3.43
0.502 6.73
0.059 7.78
0.04 9.26
Herbicides
240 2.63
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Table 1 Structures of tested organic solutes and herbicides, their water solubility and Kow values (continued)
Structure and abbreviation Water solubility (mg⋅L–1) Ln (Kow)
Herbicides
81 3.00
8400 1.75
2.2 Preparation of modified cellulose fibres
The modifications of the cellulose fibres were carried out by grafting long hydrocarbon chains in a heterogeneous environment. In our study, two methods were adopted for the chemical grafting using 4,4′-methylenebis(phenyl isocyanate) (MDI) and N, N′ Carbodimidazole (CDI) reagents. These two reagents make possible to append chemically hydrocarbon chains under mild condition and with high efficiency on cellulose surface as illustrated in Scheme 1 and 2.
Scheme 1 Mechanism of the cellulose grafting with MDI approach
Scheme 2 Mechanism of the cellulose grafting with CDI approach
2.2.1 Chemical grafting using MDI
The fibres were first swollen in water for 30 min, and then soaked in a solution of toluene/dimethylformamide (DMF) 60/40 vol. The ensuing slurry was then introduced into a three-necked flask equipped with a Dean-Stark system, and kept under reflux until all the water contained inside the fibres was removed by azeotropic distillation.
Modified biopolymer adsorbent for the removal 169
The required amount of MDI was subsequently added and the slurry was kept under magnetic stirring at 70°C and dry nitrogen atmosphere for 2 h. The reaction mixture was cooled down and the recovered fibres were rapidly washed by toluene and introduced into a three-necked flask containing dry toluene and the aliphatic amine at a concentration of 10–3 mol ⋅ L–1. The slurry was kept under magnetic stirring at 70°C and dry nitrogen atmosphere for 6 h. Finally, the reaction mixture was cooled down and the recovered product was purified by soxhlet extraction with THF/ethanol (50/50 vol.) for 48 h, and dried at 40°C for 24 h.
A schematic illustration of the chemical reaction occurring during the two steps is presented in Scheme 1.
2.2.2 Chemical grafting using CDI
To a solution of toluene/DMF (60/40 vol.) was added regenerated cellulose film cut into small pieces, which were washed with NaOH solution (10–2 M) to remove any surface contaminant. The mixture was then introduced into a three-necked flask equipped with a Dean-Stark system, and kept under reflux until all the residual water retained by the substrate has been removed by azeotropic distillation. Then, CDI was added under nitrogen atmosphere and the system was kept under magnetic stirring at 70°C and dry nitrogen atmosphere for 3 h. Afterwards, the reaction mixture was cooled down and the recovered film was rapidly washed by dry toluene and introduced into a three-necked flask containing a solution of the aliphatic amine, melamine or diamine according to the type of modification at a concentration of 10–3 mol ⋅ L–1 and kept under magnetic stirring at 70°C and dry nitrogen atmosphere for 6 h.
In the case where further modification was carried on, the ensuing substrate was submitted to reaction with CDI followed by an aliphatic amine in the same procedure as described earlier.
Finally, the recovered product was purified by soxhlet extraction with THF/ethanol (50/50 vol.) for 48 h, and dried at 40°C for 24 h.
A schematic illustration of the chemical reaction occurring during the different steps is depicted in Scheme 2.
2.3 FTIR analysis
The FTIR spectra were obtained from KBr pellets with a Perkin–Elmer BX II spectrophotometer used in transmission mode with a resolution of 2 cm–1 in the range of 400–4000 cm–1.
2.4 CP/MAS 13C solid-state NMR
Cross polarisation/magic angle spinning CP/MAS 13C solid-state NMR experiments were performed with Bruker 300 spectrometer operating at 13C frequency of 75 MHz. The spinning speed was set at 300 Hz. The contact time for CP was 1 ms and then delay time for acquisition was 5 s. Chemical shifts were referred to tetramethylsilane (TMS).
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2.5 Adsorption isotherms
Solute adsorption experiments into batch condition were performed by adding organic solute at the desired concentration using micro-syringe in a liquid solution containing 1 wt% of cellulose-modified substrate. The suspensions were then stirred for 3–4 h at 25°C to reach the adsorption equilibrium. Fibres were then isolated by centrifugation and washed two times (20 mL for 0.1 g of fibres) by ethanol to extract the organic solute. The adsorbed amount of the organic solute was determined by UV spectroscopy on the extracted ethanol fraction. It is worth noting that by such extraction procedure, we have verified that the whole adsorbed solute was stripped off from the fibres. The herbicide solutions were stirred for 4 h to reach the adsorption equilibrium, and then, the residual concentration of the herbicide was determined by UV spectroscopy after 15 min of centrifugation at 2000 tr/min.
2.6 Column study
Continuous adsorption experiments were carried out under isothermal condition and using a packed column filled with the modified fibres. The packed column employed was a glass-jacketed column of 1 cm in diameter and 15 cm long, packed with 3 g and 1.5 g of Cel-MDI-C8 and Cel-CDI-C16, respectively. The effluent solutions were percolated through the column from the bottom to the top using a precision peristaltic pump. Effluent samples were collected at regular intervals and the concentrations were monitored using a UV spectrophotometer.
2.7 Laser-Induced Luminescence (LIL) system
Schematic diagrams of the LIL system were presented elsewhere (Vieira Ferreira and Machado, 2007). LIL experiments were performed with an N2 laser (PTI model 2000, ca. 600 ps FWHM, ∼0.7 mJ per pulse). The excitation wavelength was 337 nm. With this set-up, both fluorescence and phosphorescence spectra were easily available by the use of the variable time gate width and start delay facilities (nanosecond time range) of the intensified charge coupled device (Oriel model Instaspec V, Andor ICCD, based on the Hamamatsu S57 69-0907).
Benzophenone (BZP) adsorption on samples was performed by adsorption from water (Vieira Ferreira et al., 2007). In the case of water, the fibres were first swollen for at least 2 h, and the addition of BZP was done by adding 600 µmoles of this probe dissolved in ethanol (saturated solution so that the added amount of ethanol was minimised). The water suspensions were kept under agitation for 12 h and the modified cellulose (with the adsorbed BZP) was removed by filtration. From the initial 600 µmol⋅g–1 of BZP, about 400 µmol⋅g–1 were removed by the substrate, i.e., an increase in the retention capacity of the substrate of more than ten times, when compared with previous reported results for non-modified cellulose, was now obtained (Vieira Ferreira et al., 2007). The final solvent removal was performed for about 2 h in an acrylic chamber with an electrically heated shelf (Heto, Model FD 1.0–110) with temperature control (30 ± 1°C) and moderate vacuum at a pressure of ca. 10–3 Torr.
Modified biopolymer adsorbent for the removal 171
3 Results and discussion
3.1 Characterisation of the adsorbent
Modified cellulose fibres were characterised using different spectroscopic and surface analysis techniques like FTIR, NMR, contact angle measurement and XPS. In this work, we report only FTIR and 13C solid-state NMR, which give evidence about the chemical modification of the substrate. Figures 1 and 2 reported FTIR and 13C solid-state NMR spectra of the virgin cellulose to which Cel-CDI-C16 and Cel-MDI-C16 spectra were superimposed. Compared with the pristine substrate, a significant evolution is observed on the modified fibres, namely in the range between 1500 cm–1 and 2000 cm–1 typical of the functional groups appended on the cellulose surface. Table 2 shows the major features appearing in FTIR spectra of Cel-CDI-C16 and Cel-MDI-C16, which are assigned to the different vibrational modes, whereas the corresponding peak attribution of the 13C NMR spectra is shown in Figure 2.
Figure 1 FTIR spectra of original cellulose fibres and the modified Cel-CDI-C12NH, and Cel-MDI-C16
Figure 2 CP-MAS NMR spectra of the original cellulose fibres and the modified Cel-CDI-C16, and Cel-MDI-C8
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Table 2 Attribution of the main FTIR bands of the modified cellulose fibres
Cel-MDI-C16 Cel-CDI-C12NH Band Attributed group Band Attributed group 1711 Urethane 1730 Amide 1631 Urea 1712 Urethane 1592 St CH–N 1695 Urethane (linked) 1563 St CH–N 1592–1555 St CH–N 1517 δ N–H 1537 δ N–H
The 13C NMR of Cel-MDI-C16 showed a signal at 12–40 ppm assigned to methylene carbon of the anchored hydrocarbon chain, and different peaks between 64 ppm and 105 ppm related to the cellulose backbone. We also noted the apparition of peaks at 125–136 ppm assigned to aromatic rings of MDI, and a peak at 157.46 ppm related to the carbamate function. The NMR spectrum of CDI-modified fibres showed the same peaks related to aliphatic chain carbons and another one at 157.48 ppm typical of urethane linkage. Their respective assignments are depicted on the NMR spectra (Figure 2). The cellulose peaks did not undergo significant change after modification, confirming again that the heterogeneous reaction did not bring about any modification of the morphology and the crystallinity.
3.2 Adsorption isotherms
Adsorption isotherm determination is an essential tool to acquire information how pollutants interact with adsorbents and to access the thermodynamic parameters governing the interaction process. It is also helpful in establishing the most appropriate condition to remove the organic pollutant under continuous operation.
The first part of our work concerns adsorption of aromatic organic compounds on cellulose fibres modified by MDI followed by grafting hydrocarbon chains through a condensation reaction of the terminal NCO groups with amino function of the alkyl-amine reagent, according to Scheme 1. Five aromatic organic compounds differing in their structure and polarity (Table 1) were selected as a model to investigate the adsorption ability of the chemically modified fibres. The contact time and other conditions were selected on the basis of preliminary experiments that established that the equilibrium was attained within 40–50 min. Therefore, the contact period was set at 3 h in all equilibrium studies.
The adsorption isotherm of the different organic solutes on Cel-MDI-C8, shown in Figure 3, revealed that the fibre modification greatly enhanced the aptitude of the substrate to uptake dissolved organic compound from water. The adsorption capacity grows from 4 mg⋅g–1 to 8 mg⋅g–1 for the virgin fibres towards higher values ranging from 30 mg⋅g–1 up to 70 mg⋅g–1 after fibres modification (only the 2-naphthol isotherm on virgin cellulose is shown). The adsorption capacity is amplified with the grafting degree (Figure 4) and with the increase in the length of the grafted alkyl chain. Indeed, obtained results (not shown) indicated that for Cel-MDI-CX (1/0.7) using 2-naphthol, the adsorption capacity attained was 50, 69 and 72 mg⋅g–1 for alkyl chain bearing 6, 8 and 12 methylene groups, respectively. However, one can note that over a critical modification level, the adsorption capacity seems to reach a plateau at about 72 mg⋅g–1 for 2-naphthol, independently of the length of the grafted hydrocarbon chain.
Modified biopolymer adsorbent for the removal 173
Figure 3 Adsorption isotherms of the organic solutes on virgin cellulose and Cel-MDI-C8
Figure 4 The effect of grafting level on the 2-naphthol adsorption on Cel-MDI-C8
To further enhance the adsorption capacity of the substrate, other modification strategy was carried out using CDI as an activator. This method of activation opens the way to prepare a wide range of organic esters, carbamates or amides in a relatively simple way under mild condition (Paul and Anderson, 1960; Robert et al., 2004). This approach was successfully adopted to prepare a wide variety of modified cellulose substrate as illustrated in Table 3.
The interest of such a modification is the better control of the regular distribution of the hydrocarbon chains along the surface with one alkyl at least for each anhydroglucosic cycle of the surface cellulose chain. On the one substrate, referenced Cel-CDI-C16, amino-alkyl bearing 16 methyl groups were grafted, and on the second one called Cel-CDI-C12-C16, diamino alkyl chain with 12 methyl groups were first grafted followed by a condensation reaction on the ensuing terminal amino function to further extend the length of the grafted hydrocarbon chain and to generate an intermediate polar amide group capable of promoting interactions with the dissolved organic solute through polar sites or hydrogen bonding. The adsorption isotherms of five different organic solutes on Cel-CDI-C16, Cel-CDI-C12-C16 and Cel-CDI-MM-C10-C16 are shown in Figure 5(a)–(c). Compared with the Cel-MDI-C8, a significant enhancement of both the adsorption capacity and the solute affinity was noted, as confirmed by the larger initial slope of each curve. The adsorption capacity towards 2-naphthol has grown from 72 mg⋅g–1 for Cel-MDI-C8 to 125 mg⋅g–1 and 133 mg⋅g–1 for Cel-CDI-C16 and Cel-CDI-C12-C16, respectively. Moreover, the extension of the hydrocarbon chain on the surface seems to improve the adsorption capacity and affinity of the substrate.
174 S. Alila et al.
The adsorption amount for 2-naphthol, trichlorobenzene, nitrobenzene, dichlorobenzene and chlorobenzene, respectively, evolved from 125, 160, 73, 57 and 42 mg⋅g–1 in the presence of Cel-CDI-C16 to 133, 167, 98, 71 and 51 mg⋅g–1 for Cel-CDI-C16-C12.
Table 3 Schematic illustration of the surface structure of the modified cellulose fibres
Modified fibres
Structure Abbreviation
Cel-MDI-C8(1/x)*
Cel-CDI-C16
Cel-CDI-C12-C16
Cel-CDI-MM-C10-C16
Cel-CDI-MM-C12NH
*The value (1/x) corresponds to the molar ratio anhydroglucosic cycle/MDI.
According to Pavoni et al. (2006), the adsorption capacity of PK 1-3 type activated carbon for 1,2,4-trichlorobenzene was about 15.2 mg⋅g–1, which is lower than the modified cellulose substrate attaining about 160–180 mg⋅g–1. The adsorption capacity of phenolic compounds on activated carbon ranged from 50 mg⋅g–1 to 100 mg⋅g–1 (Qadeer, 2002; Taiwo and Adesina, 2005) according to its chemical structure, which is lower than the level attained for 2-naphthol on the cellulose sorbent (about 130 mg⋅g–1). In a recent study concerning 1-naphthol adsorption onto a cross-linked polymer resin, Zhang et al. (2009) reported an adsorption level of about 85 mg⋅g–1. For herbicides, it was found that the adsorption capacity of activated carbon of atrazine (Ghosh and Philip, 2005) and alachlor (ACH) (Hopman et al., 1996) was about 14 mg⋅g–1 and 100 mg⋅g–1, respectively.
When we report the maximum adsorbed amount vs. Kow (the distribution coefficient of the solute between the octanol and water phases) (Figure 6), one can note the lack of a
Modified biopolymer adsorbent for the removal 175
clear trend between these two parameters, suggesting that the hydrophobic character of the organic compounds is not the only parameter that governs the adsorption process. Other parameters such as the hydrodynamic volume, shape of the molecule, interaction potential between the adsorbent and adsorbate in relation with the presence of various functional groups on the surface of the modified fibres (such as amino or carboxylic groups, benzyl/aromatic rings) and water solubility may play an important role.
Figure 5 Adsorption isotherms of the different organic solutes on modified cellulose fibres by CDI grafting: (a) Cel-CDI-C16; (b) Cel-CDI-C12-C16 and (c) Cel-CDI-MM-C10-C16
(a) (b)
(c)
Figure 6 Relationship between the maximum adsorbed amount of tested organic compounds and their hydrophobicity (Ln Kow)
176 S. Alila et al.
3.3 Adsorption isotherm models
Several equilibrium models have been developed and employed to fit the experimental data with a mathematical equation. The two most commonly employed for such analysis are the Langmuir and Freundlich models. The former model was developed for well-defined localised adsorption sites having the same adsorption energy, independent of surface coverage and with no interaction between adsorbed molecules. In this model
max
1L e
eL e
Q K Cq
K C=
+
where qe is the amount of solute adsorbed at equilibrium per unit weight of adsorbent (mg⋅g–1), Ce is the equilibrium concentration of solute in the bulk solution (mmol⋅L–1), Qmax is the maximum adsorption capacity (mg⋅g–1) and KL is the constant related to the free energy of adsorption. The linearised form of the equation can be written as follows
max max
1 .e e
e L
C Cq K Q Q
= +
From the data of Ce/qe vs. Ce, KL and Qmax can be determined from the slope and intercept.
The Freundlich isotherm is an empirical one appropriate for the adsorption processes where non-uniformity of the surface of adsorbent is expected, and is given by:
1/ ne F eq K C=
where KF and n (dimensionless) are constants incorporating all factors affecting the adsorption process such as adsorption capacity and intensity, respectively. This equation can be linearised as follows:
1log log log .e F eq K Cn
= +
The applicability of the isotherm equations was compared on the basis of correlation coefficient, R2. From Table 4, it was clear that the Langmuir model yields a better fit than Freundlich model for the adsorption of the different organic solutes, except for CBZ, on Cel-CDI-C16, Cel-CDI-C12-C16, Cel-MDI-C8 but fails to describe adsorption for Cel-CDI-MM-C10-C16, which could not be fitted also by Freundlich model. However, if we observe in detail the adsorption isotherms on Cel-CDI-MM-C10-C16, one could notice that the adsorption isotherms display an initial fair narrow plateau followed by a second one that appears at higher concentrations. These two parts of the curve are probably associated with adsorption involving different sites and different interaction energies. The particular isotherm shape should be in relation with the particular surface modification characterised by the presence of two different segments of grafted hydrocarbon chains joined by a polar amido group. This structure will also affect the thermodynamic adsorption and kinetic parameters as we will show in the following sections.
Modified biopolymer adsorbent for the removal 177
Table 4 Langmuir and Freudlich constants of the different adsorption isotherms
2-N CBZ DCBZ TCBZ NTR Cel-CDI-C16
Cmax (L) (mg⋅g–1) 133 94 93 182 115
Cmax (mg⋅g–1) 126 42 57 160 73
KL (L⋅mol–1) 3.8 0.278 0.59 6.12 1
Langm.
R2 0.97 0.847 0.913 0.992 0.983 1/n 0.564 0.798 0.696 0.416 0.626
KF (mg⋅g–1) 9.89 1.72 2.17 9.42 2.72 Freun.
R2 0.912 0.958 0.920 0.834 0.96 Cel-CDI-C12-C16
Cmax(L) (mg⋅g–1) 162 94 107 179 165
Cmax (mg⋅g–1) 133 51 71 167 98
KL (L⋅mol–1) 7.36 0.57 1.1 18.54 0.792
Langm.
R2 0.968 0.823 0.887 0.997 0.942 1/n 0.547 0.76 0.693 0.379 0.673
KF (mg⋅g–1) 15.23 2.94 3.84 12.97 5.62 Freun.
R2 0.829 0.914 0.891 0.807 0.939 Cel-MDI-C8(1/0.7)
Cmax(L) (mg⋅g–1) 95 – – 113 40
Cmax (mg⋅g–1) 111 – – 82 29
KL (L⋅mol–1) 0.478 – – 0.384 0.355
Langm.
R2 0.973 – – 0.993 0.98 1/n 0.558 – – 0.589 0.606
KF (mg⋅g–1) 1.91 – – 1.65 0.783 Freun.
R2 0.992 – – 0.990 0.934 Cel-CDI-MM-C10-C16
Cmax(L) (mg⋅g–1) 158 79 100 218 156
Cmax (mg⋅g–1) 128 55 81 154 102
KL (L⋅mol–1) 4.34 1.195 3.04 1.705 1.21
Langm.
R2 0.943 0.867 0.979 0.816 0.846 1/n 0.547 0.653 0.562 0.628 0.62
KF (mg⋅g–1) 11.34 3.83 5.59 8.29 6.99 Freun.
R2 0.753 0.823 0.875 0.721 0.803
3.4 Thermodynamic parameters
To determine thermodynamic parameters, experiments were carried out at different temperatures in the range of 293–343 K using nitrobenzene and 2-naphthol adsorption as organic solute and Cel-MDI-C16(1/0.7) and Cel-CDI-MM-C10-C16 as adsorbents.
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The thermodynamic parameters such as standard Gibb’s free energy change (∆G0), enthalpy change (∆H0) and entropy change (∆S0) were evaluated. Gibb’s free energy change of the process is related to equilibrium constant by the equation:
ln .ocG RT K∆ = −
The equilibrium constant Kc was defined as
1e
ce
FK
F=
−
where Fe is the fraction of adsorbed solute at equilibrium. The thermodynamic parameters for the adsorption process, ∆H0 and ∆S0, were
evaluated using the following equation:
ln .o o
cS HKR RT
∆ ∆= −
At different temperatures (293–313 K), the corresponding Fe values for different saturation levels were calculated. The values of ∆G0 and of ∆H0 and ∆S0 calculated from the slope and intercept of linear regression of ln Kc vs. (1/T) at different coverage levels are given in Table 5.
Table 5 Thermodynamic parameters for the adsorption of 2-N and NTR on Cel-MDI-C16 (1/0.7) and Cel-CDI-MM-C12-C16
∆H°
(kJ⋅mol–1) ∆S°
(J⋅K–1⋅mol–1) ∆G°(298)
(kJ⋅mol–1) ∆G°(308)
(kJ⋅mol–1) ∆G°(318)
(kJ⋅mol–1)
Cel-CDI-MM-C12-C16 (NTR)
1st level –30 –24 –22.96 –22.74 –22.5 2nd level –38 –51 –23.4 –22.5 –22.38 3rd level –58 –117 –23.97 –22.04 –21.67
Cel-MDI-C16 (1/0.7) (2-N) 1st level –35 –92 –5.9 –7.15 –8.7 2nd level –28 –68 –6.3 –7 –8.6 3rd level –28 –66 –6.3 –7.15 –8.6
Cel-MDI-C16 (1/0.7) (NTR)
–27 –67 –6 –7 –9.3
The negative value of ∆G0 at all temperatures indicates that organic solute adsorption onto modified cellulose substrate is spontaneous. However, one can note a higher absolute value of ∆G0 for Cel-CDI-MM-C10-C16 adsorbent compared with Cel-MDI-C16, indicating that adsorption is more thermodynamic and more favoured on the former cellulose substrate.
Modified biopolymer adsorbent for the removal 179
For all the studied solutes, the negative value of ∆H0 is indicative of an exothermic adsorption process. The value of ∆H0 being in the range from –30 kJ⋅mol–1 to –55 kJ⋅mol–1 suggests that adsorption is driven by dispersive and polar van der Waals interactions between the grafted chain moiety and the solute molecules. Furthermore, the negative value of ∆S0 indicates that the motion of the molecules becomes restricted and their randomness degree is reduced after adsorption.
One can also note a different value of ∆H0 and ∆G0 according to the coverage degree of the surface in the case of Cel-CDI-MM-C10-C16. The difference is notable if we consider the two different parts of the adsorption isotherm, indicating that adsorption involves different interaction energies and became more and more favoured as the solute is entrapped on the surface. This result is in line with the above-reported hypothesis regarding the particular behaviour of Cel-CDI-MM-C10-C16 adsorbent. On the other hand, we could observe that the energetic parameters are roughly the same independently of the coverage level of the surface for Cel-MDI-C16 suggesting that adsorption proceeds in the same manner on the surface.
3.5 Adsorption kinetic modelling
To investigate the controlling mechanism of the adsorption process, the pseudo-first-order and pseudo-second-order equations are applied to model the kinetics of organic solute adsorption onto modified cellulose substrate.
The first-order rate expression of Lagergren (Lagergren, 1898) is given as:
1log( ) log2.303e t e
kq q q t− = −
where qe and q (mg⋅g–1) are the amounts adsorbed at equilibrium and at time t, respectively, and k1 (min–1) is the rate constant of first-order adsorption.
The second-order kinetic model (Ho and McKay, 1998) is expressed as:
22
1( )t ee
t tq qk q
= +
where k2 (mg µmol–1 min–1) is the rate constant of second-order adsorption. For the different studied substrates, the plot of log (qe –qt) vs. t does not display a
linear behaviour over the whole range of contact time (plot not shown) indicating that the first-order model failed to match the experimental data.
The slopes and intercepts of plots of t/q vs. t were used to determine the pseudo-second-order rate constant k2 and equilibrium adsorption amount qe. The relatively high correlation coefficients for the pseudo-second-order kinetic model (above 0.98) and the agreement between the calculated and the experimental qe values suggest that the adsorption kinetics of organic compound on modified cellulose followed the pseudo-second-order kinetic model. Table 6 lists the fitting and the experimental results and a comparison of results with the corresponding correlation coefficients (R2) for the different modified substrates.
180 S. Alila et al.
Table 6 Kinetic parameters for the different studied systems
Cel-CDI-C12-C16 Cel-CDI-C16 Cel-MDI-C8* Cel-CDI-C12NH
2-N NTR CBZ 2-N 2-N ACH LNR
C0 (mg⋅g–1) 144.17 123 112.5 144.17 144.17 68.18 47.4
qexp (mg⋅g–1) 110 90 47 100 61 12.5 22.5
Pseudo-first order
qcal (mg⋅g–1) 56.5 132 113 30 41 57.66 48.24
K1 (g⋅mg–1⋅min–1) –0.076 –0.165 –0.234 –0.045 –0.085 –0.004 –0.007
R2 0.832 0.916 0.928 0.873 0.817 0.907 0.976
SSE (%) 7.31 6.53 8.12 8.37 4.47 6.72 5.07
Pseudo-second order
qcal (mg⋅g–1) 128 114 59 101 72.5 13.87 23.53
K2 (g⋅mg–1⋅min–1) 5.45 × 10–4 3.52 × 10–4 7.3 × 10–4 68.55 × 10–4 12 × 10–4 0.012 0.015
R2 0.951 0.864 0.9 0.999 0.98 0.998 0.994
SSE (%) 4.243 4.89 3.46 1 2.9 1.17 1.015
Intraparticle diffusion *idK (mg⋅g–1⋅mn–1/2) 12.42/38.2 7.1/34.12 4.6/21.19 26.79 10.64 2.35 0.75/1.6
R2* 0.98/0.986 0.99/0.994 0.97/0.91 0.962 0.9588 0.92 0.93/0.95
*Cel-MDI-C8(1/1). *idK and R2*: for C1000(2), the values are attributed to the first and the second segments
of the plot.
Owing to the particular structure of the substrate with a surface bearing a relatively high density of hydrocarbon chains protruding towards the continuous medium, the diffusion of sorbate inside these chains cannot be ignored, which may constitute the rate controlling step. This diffusion process could be described by Weber and Morris (1963), which is based on Fick’s second law (Furusawa and Smith, 1973).
t Dq k t=
where qt is the sorbed concentration (mg⋅g–1) at time t, and kD is the internal diffusion parameter for intraparticle transport. Result of this analysis carried on three different organic solutes and one herbicide on two different modified substrates is illustrated in Figure 7 as plots of qt vs. t0.5.
In the presence of cellulose substrate modified by grafting only one type of alkyl chain, this analysis indicated that after five minutes of the experiment, where external mass transfer involving the movement of adsorbate molecules from the bulk of the solution towards the external surface of the adsorbent is the dominant process, the evolution of qt vs. t0.5 could be fitted with two linear portions.
Modified biopolymer adsorbent for the removal 181
Figure 7 (a) Pseudo-second-kinetic order; (b) pseudo-first-order; and intraparticle diffusion kinetic plots for; (c) organic solutes and (d) herbicides at 25°C
(a) (b)
(c) (d)
On the other hand, when two alkyl chains are connected through a polar amido linkage, the plot could be linearised with three linear portions. The internal diffusion parameters for each region and for the three cellulose substrates are reported in Table 6. The presence of different regions is consistent with an adsorption involving multistep process.
We infer that the first portion corresponds to the external surface adsorption or instantaneous adsorption stage. The second portion involves the gradual adsorption stage, where the diffusion within the grafted hydrocarbon chains was rate-controlled. The third portion was the final equilibrium stage (Rua et al., 2007).
3.6 Adsorption of herbicides
To test the ability of modified cellulose substrate to adsorb herbicides, adsorption tests of ACH, metalaxyl (MTX) and linuron (LNR) on various substrates at a constant initial concentration were carried on. Results reported in Table 7 revealed that adsorption of the pesticides occurred on the modified substrate. However, the adsorption efficiency depends on the chemical structure of pesticides as well as the type of the cellulose modification. LNR, which is the least soluble in water, displays the highest adsorption level on the different substrates, whereas ACH and MTX are better adsorbed on Cel-CDI-MM-C12NH. We ascribe this behaviour to the difference in the size of the herbicide and to the mechanism of interaction during adsorption. Indeed, if we assume that the driving force during adsorption is principally the van der Waals interaction
182 S. Alila et al.
between the grafted chains and the pesticide molecules, then any factor privileging such interaction will contribute to enhance the adsorption capacity. The planar structure of LNR favours more possible diffusion and intercalation of the molecules inside the domain formed by the grafted chains. On the other hand, the more globular structure of MTX and ACH (as shown in Figure 8) and their relatively high size compared with the available volume between the grafted chains will hamper their accumulation inside the hydrophobic domains. Likewise, the presence of a terminal amino function on Cel-CDI-MM-C12NH allows hydrogen bond formation to occur between the C=O or amino groups of pesticide, which enhances the surface adsorption capacity and may explain the higher adsorption level in the presence of this substrate. A schematic illustration of the adsorption mechanism of herbicide onto modified cellulose substrate is depicted in Figure 8.
Table 7 Adsorbed amounts (%) of tested herbicides on some modified cellulose fibres
Virgin cellulose
fibres Cel-MDI-C16(1/0.7)
Cel-CDI-C16
Cel-CDI-C12-C16
Cel-CDI-C12NH
Cel-CDI-MM-C12NH
ACH 32 34 27 27 20 50 LNR 5 68 35 45 48 43 MTX 0 16 10 32 18 36
Figure 8 3d structure of the tested herbicides: (a) ACH; (b) LNR; (c) MTX and (d) adsorption mode of tested herbicides on Cel-CDI-C12NH
(a) (b) (c)
(d)
Given the low solubility of ACH and LNR, the adsorption isotherm on Cel-CDI-MM-C12NH was carried on by maintaining a constant amount of adsorbent (50 mg) and
Modified biopolymer adsorbent for the removal 183
varying the volume of the mother solution so that different initial concentrations of herbicide could be used. As shown in Figure 9, the higher adsorption capacity is observed with LNR followed by ACH and MTX. Moreover, the significant enhancement of the adsorption capacity and affinity noted for the substrate Cel-CDI-MM-C12NH bearing two terminal NH2 functional groups is in line with the high contribution of hydrogen bonding in the adsorption of herbicide by modified cellulose substrate.
Figure 9 Adsorption isotherms of tested herbicides on Cel-CDI-C12NH and ACH on Cel-MM-C12NH
The adsorption equilibrium kinetic of herbicide is relatively rapid and is achieved within 15–20 min. As observed for aromatic organic compounds, the adsorption kinetic follows a pseudo-second-order model (Table 6) involving multistep diffusional process as shown in Figure 7(d).
The effect of pH on the adsorption of ACH was studied at pH values 3–9.5 using Cel-CDI-MM-C12NH as an adsorbent. Results indicated that higher adsorption level is attained at lower pH where the terminal amino function of the grafted chain is in the form of –NH3
+ that favours stronger interaction with polar group of the adsorbate molecules.
3.7 Column studies
Batch isotherms have shown that the modified cellulose fibres are efficient substrates for the uptake of dissolved organic compounds in water. To explore the adsorption capacity during continuous operation, a laboratory column filled with modified fibres was designed. The ratio (Cs/C0) of the effluent concentration Cs to the input concentration C0 was plotted against time to obtain the breakthrough curve at a constant flow rate. For all performed experiments, breakthrough time is defined as the point when the concentration of the effluent reaches about 5% of the input concentration.
The influence of the sample flow rate on sorption of 2-naphthol on 3 g of Cel-MDI-C16(1/0.7) was examined in the flow rates range of 10–20 mL⋅min–1 and a bed length of 15 cm (shown in Figure 10). With the increase in the input flow rate at a constant feed concentration, the breakthrough curves were steeper and occur at a shorter time (Figure 10(a)). At an inlet concentration of 10–3 mol⋅L–1, the breakthrough time is reached at 150, 90 and 60 min for a flow rate of 10, 15 and 20 mL⋅min–1, respectively. This behaviour may be due to insufficient residence time of the solute in the column and
184 S. Alila et al.
the diffusion limitations of the solute into the pores of the adsorbent, namely if we take into account the relatively high time needed to reach adsorption equilibrium with modified cellulose fibres. Figure 10(b) reports the total adsorbed amount of solute vs. time. The curves display a linear part corresponding to an increase in the total solute uptake, followed by plateau attributed to the exhaustion of the column, which decreases with the increase in the flow rate and growingly diverges from the maximum adsorbed amount at equilibrium conditions.
Figure 10 Effect of the feed flow rate on the breakthrough curves (a) and on the corresponding adsorbed amount vs. time (b) of 2-naphthol uptake under continuous flow: (substrate: Cel-MDI-C16(1/0.7), temperature: 20°C)
(a) (b)
For the different tested solutes, Figure 11 shows that a similar trend on the breakthrough is obtained with different breakthrough time according to adsorption capacity towards the organic solutes.
Figure 11 Breakthrough curves of 2-N, NTR and TCBZ adsorbed on Cel-CDI-C16 column; flow rate= 15 ml ⋅ min–1, C0 = 10–3 M
3.8 Column regeneration
The regeneration ability is one of the most important parameters that determine, from an economical point of view, the potential exploitation of a material as an adsorbent. It is worth noting that in most cases, the regeneration of the saturated adsorbent like activated carbon or zeolite by the elimination of the loaded pollutant is time- and
Modified biopolymer adsorbent for the removal 185
energy-consuming (Sharma et al., 2007, 2008). Likewise the cost involved in such a treatment is too high to reconsider the regeneration of the adsorbent.
In a previous work using modified cellulose fibres prepared by grafting fatty acid chains (Aloulou et al., 2006; Boufi and Belgacem, 2006), we have shown that the exhausted adsorbent could be regenerated by washing with solvent on which the adsorbed organic compound is highly soluble.
The column is regenerated with ethanol as elutant, after it was fully loaded with organic solute, by passing the elutant at a fixed flow rate. The evolution of the desorbed fraction carried on Cel-CDI-C16 showed that 2-naphthol was totally stripped off with 25 bed volumes of ethanol (Figure 12). The regenerated column was used in several adsorption–desorption cycles without any loss of the adsorption capacity. The satisfactory adsorption and regeneration behaviour of modified cellulose adsorbent supported our idea of considering it as a potential candidate for treatment of chemical wastewater containing dissolved organic pollutants.
Figure 12 Breakthrough curves of removed 2-naphthol by washing with ethanol and the corresponding removed amount vs. time (solvent flow: 15 mL⋅min–1, substrate: Cel-CDI-C16, temperature: 20°C)
3.9 Investigation of adsorption mechanisms by Laser-Induced Luminescence
Benzophenone is an extremely useful molecule for probing new hosts. The n → π*
absorption transition was found to be very sensitive to the environment characteristics and also exhibits a photochemistry, which depends on the host properties (Vieira Ferreira and Machado, 2007; Vieira Ferreira et al., 1995, 2003). In a very recent paper (Vieira et al., 2007), we reported the use of BZP as a probe for the study of a new host, i.e., a modified cellulose. The modification consisted in grafting with alkyl chains, bearing 12 carbon atoms, by surface esterification, therefore transforming the polar surface of the normal cellulose into a surface with a certain degree of non-polar character. A comparison of the photochemical behaviour of BZP adsorbed onto ‘normal’ microcrystalline cellulose was made. We conclude that alkylation boosted the capacity of this host towards uptaking the organic solute from aqueous solutions as a consequence of the creation of new organic domains where BZP molecules could be accumulated. In this work, the same technique was used to investigate the adsorption mechanism and gives evidence to some hypothesis regarding the role of the grafted chains in the adsorption process.
186 S. Alila et al.
Figure 13 shows the room temperature phosphorescence spectra of BZP onto the surfaces of the Cel-CDI-MM-C16 modified cellulose in a long time scale whereas Figure 14 refers to BZP luminescence of the same sample in a nanosecond time scale.
Figure 13 Laser-induced phosphorescence spectra of air-equilibrated samples of benzophenone adsorbed onto Cel-CDI-MM-C16 modified cellulose from water. Curves were recorded 25 µs, 275 µs, 775 µs, 1275 µs, 1775 µs and 2275 µs (from top to bottom) after the laser pulse. The excitation wavelength was 337 nm
Figure 14 Laser-Induced Luminescence spectra for air equilibrated samples of benzophenone adsorbed onto Cel-CDI-MM-C16 modified cellulose from water. Curves were recorded immediately after laser pulse and then 2, 5, 10, 15, 20, 30 and 45 ns after the laser pulse. The excitation wavelength was 337 nm
Those time-resolved spectra were obtained with air-equilibrated conditions and were identical to the ones obtained with argon-purged samples within experimental error. Half-lives of about 1 ms can be obtained from time-resolved spectra shown in Figure 13. For comparison purposes, lifetimes of about 80 microseconds were determined for the calixarene inclusion (Vieira Ferreira et al., 2002, 2003) and 3.1 ms for inclusion into the narrower channels of silicalite (Vieira Ferreira et al., 2002; Branco et al., 2005),
when compared with about 40 microseconds for BZP microcrystals, all determined at the maximum emission wavelength (about 448 nm) (Vieira Ferreira et al., 2006). Therefore, we conclude that BZP is well entrapped within the modified cellulose polymer chains, which provide a rigid environment to the guest molecule. Figure 13 also shows emission bands with reduced vibrational structure, therefore the location of BZP is certainly in
Modified biopolymer adsorbent for the removal 187
proximity of polar groups, either the hydroxyl groups of the main cellulose polymer chains, the nitrogen atoms of the triazine group or the adjacent amide.
Another important piece of information comes from the LIL spectra of the same sample, but now in a nanosecond time scale, as shown in Figure 14.
The short time scale luminescence time-resolved emission spectra of BZP onto the surfaces of the Cel-CDI-MM-C16 modified cellulose clearly show the existence of two emissive species, BZP hydrogen bonded, which we will abbreviate as *BZP … H with a maximum at about 430 nm and a longer lived emission centred at about 470 nm, which evidences the emission of protonated forms of BZP, which we will name *BZPH+. This short time scale emission spectrum clearly proves that a significant fraction of the adsorbed BZP molecules is in close amide groups of the alkyl chains, which obviously play an important role in the adsorption process.
We conclude that data shown in Figures 13 and 14 provide important information on the location of BZP as a guest on Cel-CDI-MM-C16 modified cellulose.
4 Conclusion
Chemical modification of cellulose substrate through grafting hydrocarbon moieties and various polar derivatives greatly enhances the adsorption capacity of cellulose towards a large variety of organic compounds including herbicides. The maximum adsorption amount ranged from 30 mg⋅g–1 up to 160 mg⋅g–1 and depends on the structure of the organic compound and on the type of modification carried on. The sorption data have been fitted to linear form of Langmuir and Fruendlich model from which various adsorption parameters have been calculated.
The pseudo-second-order kinetic model was successfully applied to describe the progress of the adsorption, and the Weber–Morris approach revealed the presence of different regions consistent with a multistep adsorption process. The evolution of sorption with temperature has been used to estimate ∆H0, ∆S0 and ∆G0 thermodynamic parameters. The negative values of ∆G0 and ∆H0 indicate that sorption is spontaneous and exothermic.
The results from the column test indicated that efficient uptake of dissolved organic compounds can be achieved using fixed bed column with adequate operating parameters. Once exhausted, the modified cellulose adsorbent can be regenerated easily and reused for multiple cycles of treatment without any reduction of its adsorption capacity. Evidence of the fundamental role of the grafted chains on the adsorption process was provided using laser-induced room temperature luminescence and BZP as a probe. It was shown that adsorption is driven not only by Wan der Waals interaction but also by hydrogen bonding between the NH and CO sites on the appended structure on the modified cellulose fibres and the polar moiety of the organic solutes. The satisfactory adsorption and regeneration behaviour of cellulose-based adsorbent promoted us to believe it is a potential candidate for treatment of chemical wastewater containing dissolved organic pollutants.
188 S. Alila et al.
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