Derivatization of Cellulose Fibres — A Useful Prospect for Recycling Mill Waste

Fauzia Sultana

Email: fauzia_j27@hotmail.com

 

Featured Article: Received 6 June  2013; received in revised form 28 August 2013; online published 14 September 2013

 

Abstract

Sludge, which is a waste emerging from paper recycling plants is typically incinerated in the mills for energy or landfilled. Sludge is a lumpy mixture of water, adhesives, ink, fillers, small cellulose fibres, etc. The cellulose fibres that are present in high content are usually lost in the waste streams. Hence, the cellulose fibres can be recovered from the waste stream and later treated to form value-added derivatives. Cellulose fibres can be recovered by oxidizing with sodium metaperiodate. The oxidation process not only extracts the cellulose fibres, but also activates the unreactive fibres for further derivatization by forming a biodegradable intermediate called dialdehyde cellulose (DAC). The DAC formed, which is highly reactive, can be further treated through crosslinking with substituent groups to form specialty cellulose-based products.

 

Introduction

Paper mill waste is a major economic and environmental problem because of the cost and space involved in its disposal. Approximately, 600 tons of waste paper is recycled in a typical paper recycling mill every day, of which 25% (around 150 tons) is papermaking waste called sludge [1], a froth like mixture of water, ink, plastics, adhesives, paper fillers and very small fibres.

 

What is Sludge?

A conventional paper recycling mill involves the classic steps of papermaking namely, repulping, screening, deinking and paper making. Sludge is a generic term for the semi-solid slurry waste that results from the repulping and deinking processes. It is a viscous, lumpy solid residue containing biosolids and ashes (the inorganic components in sludge).

Biosolids are the organic matter in plants like cellulose, hemicellulose, lignin and other plant nutrients [2]. The cellulose fibres present in deinking sludge are mostly short fibres that result because of the repetitive recycling of paper [1].

Common methods of disposal of the sludge from recycling mills are land filling, land spreading, composting or using it as alternate fuel in the recycling mill [3,4,5]. Besides, it is also used in a number of value-added applications like fertilizer mix for soil stabilization, raw material in the manufacture of construction materials like cement and bricks [6] etc.

Although the above mentioned methods of disposal are regularly applied by the industries, however these methods are likely to cause environmental issues. For instance, when recycling mill sludge is landfilled, aerobic and anaerobic decay occur, releasing carbondioxide and methane in the environment[22]. Since recycling mill sludge contains heavy metals due to the presence of ink, as a land spreading or composting material it may cause an imbalance of the carbon-nitrogen ratio in the soil, consequently harming plant growth. In addition, incineration of the sludge to generate energy at the recycling mills contributes to air, soil and water pollution by releasing harmful gases in the environment [22].

Cellulose fibres present in the recycling mill sludge amount for 50-55 % of the sludge content [1]. Thus, sludge may be treated as a source of cellulose, where the recovered cellulose fibres can be further used to produce derivatives and/or value-added products.

In order to recover the cellulose fibres from sludge and further derivatize it into value-added products, it is important that the structure, morphology and reactivity of cellulose are understood.

 

Cellulose: Structure, Morphology and Reactivity

A long chain linear polymer with repeating units of D-glucose, cellulose is the most abundant material in nature. Starting from the very basic phenomenon of photosynthesis,

CO2 + H2O + Sunlight à C6H12O6 (glucose) + O2   Eqn(1)

it is the manufactured glucose that acts as the building blocks of cellulose. The glucose units are in 6-membered rings called pyranoses that are connected to one another through single oxygen atoms by 1, 4-β glycosidic bonds [7] between the C-1 of one pyranose ring and the C-4 of the next ring, eventually forming a long chain [8,9].

Figure 1: Pyranose ring/unit in the cellulose chain [21].

The morphology of cellulose has a profound effect on its structure, chemistry and reactivity. On the chain backbone of cellulose are present numerous hydroxyl groups in equatorial (parallel to the plane) positions making them easily accessible for intermolecular and intramolecular hydrogen bonding [8]. Intramolecular hydrogen bonding occurs between adjacent anhydroglucose rings, while intermolecular hydrogen bonding occurs between hydrogen and carbon atoms.

Figure 2: Intermolecular and intramolecular hydrogen bond network in cellulose represented by ball and stick diagram [2].

These internal bonds organize themselves in a manner that results into ordered regions of high crystallinity and less ordered regions of amorphousness within the cellulose chain.

The orderliness in cellulose is highly variable and has a large effect on the physical and chemical properties of the fibers [10]. The amorphous regions are comparatively chemically more reactive than the ordered crystalline zones. The crystalline areas comprise of a dense network of chains of hydrogen bonds. On the other hand, in the amorphous regions the chains are further apart. As a result, the hydroxyl groups that are present in these amorphous zones are more accessible for reaction and react more readily than the hydroxyl groups in the more highly ordered crystalline areas [11,14].

Overall, cellulose is chemically less reactive towards other species. In order to activate the structure, the strong network needs to be disrupted. One way this can be done is by performing oxidation on cellulose with oxidant sodium metaperiodate.

 

Activation and Intermediate formation of Cellulose  

Oxidation activates the cellulose structure by replacing the hydroxyls with active groups.

Cellulose can be oxidized with oxidants like gaseous chlorine, hydrogen peroxide, persulfates, or periodate[11]. Depending upon the oxidant used, the oxidized celluloses may contain functional groups like aldehydes, carboxylic groups or ketones. When cellulose is oxidized with sodium metaperiodate, a modified cellulose product called 2, 3-dialdehyde cellulose (DAC) forms [12]. Oxidative cleavage at the C-2 and C-3 of the pyranose units containing the hydroxyl groups occurs during the oxidation, introducing a large number of aldehyde groups in the cellulose structure.

Figure 3: Reaction for sodium metaperiodate oxidation of cellulose.

Periodate oxidation of cellulose is a complex and long process [13]. Periodate oxidation reaction begins by first attacking the accessible amorphous regions and then slowly into the rest of the material, gradually breaking the crystalline structure. Hydrolytic degradation of the cellulose consequently leads to the formation of dialdehyde cellulose.

Dialdehyde cellulose (DAC) is a biodegradable and biocompatible compound. Because it is highly reactive towards other functional groups, DAC can serve as an intermediate for cellulose derivatives.

The carbonyl groups present in DAC consist of an oxygen atom that is bonded to the carbon atom via a double bond. This implies that the structure has lone pairs of electrons on oxygen atoms, making DAC highly reactive towards various substituent groups [16]. DAC may be further modified to carboxylic acids, alcohols or imines by oxidation with acids, or amines [15]. In addition, it may also be ‘crosslinked’ [17] with substituent groups.

 

Crosslinking of Dialdehyde Cellulose

Crosslinking is the association of polymers with other functional species through a chemical bond. Carboxylic acid, hydroxyls or Schiff base with primary amines are the commonly used crosslinkers for DAC. Crosslinking may be intramelecular or intermolecular, and in most cases irreversible. Different types of crosslinking are possible: (i) covalent crosslinking, (ii) ionic bonds, and (iii) physical crosslinking.

Figure 4: (a) Intermolecular crosslinking; (b) Intramolecular crosslinking[18].

When DAC is treated with a crosslinking group, a covalent bond is formed between the aldehyde groups and the crosslinking species, resulting in the formation of cellulose derivatives.

 

Applications of Cellulose-based Derivatives

Paper recycling sludge is commonly used as alternative fuel in the paper recycling mills, in construction materials, etc. The cellulose fibres recovered from deinking sludge may be used as specialty materials, for instance as adsorbents of heavy metals, proteins or dyes, or as column packing materials. The cellulose-based derivatives may also be applied in the formation of hygroscopic materials, enzyme immobilization, or for the introduction of anionic or cationic groups [19].

 

Critical Aspects

Major factors affecting periodate oxidation of cellulose include reaction temperature and time, amount of sodium metaperiodate, presence of other salts [20]. With increased amount of oxidant and long reaction time, the aldehyde content of DAC increases. But eventually, the rate of aldehyde group formation decreases because of condensation reaction between the aldehyde and hydroxyl groups in the cellulose molecular chains. This in turn prohibits further formation of DAC and also increases the consumption of oxidant. Also, at elevated reaction temperatures and longer reaction periods, sodium metaperiodate decomposes.

 

Conclusion

Paper recycling will exist as long as the use of paper continues. Sludge, which is a major paper recycling waste, is mainly used either as an alternate fuel in the recycling mills or is landfilled. However, the high cellulose content of recycling mill sludge opens new scopes for its functions. The cellulose fibres recovered from recycling mill sludge may be applied to derive cellulose-based specialty products that can be used in various industries. The reaction route of including the extraction, intermediate formation and derivatization of cellulose is dependent upon a number of major factors. Hence, finding the optimum conditions for the process is a key aspect of the respective proposition.

 

Acknowledgement

The author would like to acknowledge to the Green Fibre Network, Dr. Theo van de Ven (McGill University) and Dr. Roger Gaudreault (Cascades Inc.).

 

References

 

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Citation: To cite this article, please use the following information (use the given format or any standard citation format): F. Sultana “Derivatization of Cellulose Fibres – A Useful Prospect for Recycling Mill Waste.” ChE Thoughts (2013), in press.

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