Removal of organic pollutants from water by modified cellulose fibres
Introduction
Cellulose and lignocellulosic materials are among the most important organic polymers derived from the biomass with a production level approaching 1011 tons per year. In addition to their principal use in their native form as materials for cotton production, cellulose is used in papermaking and as a precursor for chemical modifications to prepare a broad variety of commercial polymers widely used in coating, cosmetic formulation and other industrial applications (Heinze and Lebert, 2001). Typical examples of these polymers are nitrocellulose, hydroxyethycellulose, carboxymethylcellulose, cellulose acetate, cellulose acetobutyrate. Though the most adopted approaches for cellulose modification call upon reactions carried out in homogeneous conditions, giving rise to a new chemical backbone, the heterogeneous modification is the most appropriate procedure to enhance the sorption properties of lignocellulosic fibres toward heavy metal ions or organic pollutants (Sciban et al., 2006).
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 10 years by Schneegurt et al. (2001) and Akhtar et al., 2007a, Akhtar et al., 2007b. However, the adsorption capacity and the affinity of these bioadsorbents greatly fluctuates according to their origin. The recourse to a ubiquitous available adsorbent biopolymer with high adsorption properties toward a large variety of organic compounds should contribute to the development of these substrates. The use of cellulose as adsorption support is not recent. Previous studies highlighted the ability of this natural material to adsorb a number of organic compounds, including 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 assessed (Reddad et al., 2003, Kim and Lim, 1999). However, the adsorption properties of native cellulose toward organic pollutants and metallic contaminants was always 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 the organic pollutant could be adsorbed.
With a target chemical modification of the surface of cellulose fibers based on anchoring functional groups displaying high affinity toward specific compounds, one can expect to boost the adsorption capacity toward pollutants dissolved in water. This strategy was successfully adopted to enhance the adsorption of heavy metal ions by grafting selected structures bearing chelating moieties onto the cellulose backbone (Zhang et al., 2003, Delval et al., 2000, Lee et al., 2001). However, to best to our knowledge, dealing with organic pollutants, no studies have applied such approach to lignocellulosic substrates.
In previous studies, our group has shown that the adsorption of cationic surfactants on bleached cellulose fibres greatly enhanced the aptitude of the substrate to uptake dissolved organic compounds from aqueous media (Aloulou et al., 2004a, Aloulou et al., 2004b, Aloulou et al., 2004c, Alila et al., 2005, Alila et al., 2007). The improved solute adsorption was ascribed to the accumulation of the organic solutes within the aggregated domains generated by the self-assembly of surfactant monomers at the cellulose/water interface. Nevertheless, the desorption of the surfactant molecules from the cellulose surface, inevitably occurring as the treated cellulose fibers were washed with water, impeded the regeneration of the substrate after striping off the adsorbed organic compound. To prevent such drawback, a chemical grafting of hydrocarbon structure likely to mimic the aggregated domains generated by the adsorbed surfactant molecules was tested successfully (Aloulou et al., 2006, Boufi and Belgacem, 2006, Alila et al., 2009a). The aptitude of the ensueing modified substrate to uptake dissolved organic solutes in water was investigated in a batch as well as in a continuous flow mode.
In the present work we continue our research regarding the potential use of modified cellulose substrates as adsorbents for organic compounds. A broad range of hydrocarbon chains, differing by either their length or their terminal functionality, were grafted t using carbodiimidazole (CDI) chemistry (Paul and Anderson, 1960, Robert et al., 2004) which made it possible to append long amino-terminated hydrocarbon moieties under mild conditions acting as hydrophobic reservoirs capable of sequestering the organic compounds (Boufi and Belgacem, 2006). This latter approach was successfully adopted by our research group to greatly enhance the adsorption capacity of cellulose toward a large variety of organic compounds, including herbicides (Aloulou et al., 2006, Alila et al., 2009a).
Section snippets
Materials
A bleached soda pulp from the Tunisian annual plant esparto (alfa tenassissima) was used in this work. 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, giving an aspect ratio of 52.
We selected some organic solutes and four herbicides as models to conduct this retention study. All the reagents and aliphatic amines used in this study
Characterization of the adsorbent
The modified cellulose fibres were characterized using different spectroscopic and surface analysis techniques like FTIR, NMR and XPS. Fig. 1 depicts the FTIR spectra of unmodified cellulose fibers, CDI activated (Cel-CDI), diaminododecane (Cel-C12) modified fibers, and (Cel-C12-MM-C12-C16), resulting from the consecutive action of diaminododecane (C12), melamine (MM), diaminododecane and hexadecylamine. At each step the ensuing fibes were reactivated with CDI before the amino derivative was
Conclusion
The experimental results showed that chemically modified cellulose fibres are effective adsorbent for the removal of organic compounds and herbicides from water. The chemical modification is based on the grafting of different organic moieties bearing hydrocarbon chains which provide hydrophobic reservoirs in which the organic compounds accumulate. The use of CDI as an activator enables a wide latitude of functionalisation procedures carried under mild condition. FTIR and NMR characterisation
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