Enhanced biodegradation resistance of biomodified jute fibers
Highlights
► A whole-cell catalyzed bio-process is developed to transesterify LCFs fibers. ► Treated fibers were stronger, more hydrophobic and biodgradation resistant. ► The process is inexpensive compared to chemical and enzymatic alternatives.
Introduction
Because of their hydrophilicity, lignocellulosic fibers (LCFs) usually absorb water amounting to several times their dry weight when they are exposed to moist environments. The swelling that results from water absorption exposes the cellulose chains, an essential constituent of the fibers, to cellulase producing microorganisms. This, in turn, leads to a rapid and significant strength loss making LCFs unsuitable in many engineering applications, e.g., in manufacturing geotextiles and composites. The hydroxyl groups present in the cellulose chains are a major contributor to the water affinity of LCFs. Chemical modification of these hydroxyl groups therefore has been used as a strategy for LCFs less hydrophilic and consequently less biodegradable. LCFs modified by acetylation, esterification, grafting, and transesterification, for instance, were found to lead to greater tensile strength, hydrophobicity, and resistance against biological, chemical and physical degradation (Mohanty, Khan, & Hinrichsen, 2000Rowell, 2005, Saha et al., 2012, Sun et al., 2001, Zbidi et al., 2009). These chemical processes often use toxic or hazardous reagents, require elevated pressure and temperature, or generate potentially hazardous by-products. A biomediated process could therefore be a greener alternative.
Purified enzymes such as pectinase, polygalacturonase, laccase, cellulase, and hemicellulase have been used for modification of natural fibers to enhance hydrophobicity and strength properties by removing amorphous hydrophilic substances from the matrix and enhancing fibril separation (Gubitz and Paulo, 2003, Zhang and Fan, 2010). Purified lipase has been used to graft, transesterify and esterify cellulosic fibers to make them less hydrophilic (Li et al., 1999, Sereti et al., 2001Xie and Hsieh, 2001, Yang et al., 2004; Zhang & Fan, 2010). These enzyme-catalyzed processes use purified enzymes and need an organic solvent based medium. As a result, they are expensive. Further, some of these processes also use hazardous chemicals such as ɛ-caprolactone, vinyl acrylate, vinyl propionate, vinyl neodecanoate, and acrylonitrile (Li et al., 1999, Sereti et al., 2001, Xie and Hsieh, 2001).
Although pure cellulose has been transesterified using purified lipase, hazardous chemicals has been used in the process and recovery of the enzyme after treatment is not easy. In this study lipase producing bacteria were used directly to develop an inexpensive process for treating jute fibers to make them stronger and more degradation resistant. The reagents used in the process are all non hazardous. The strain of bacteria isolated from a riparian environment was found to produce lipase as well as cellulase. The lipase produced by the strain was found to transesterify the fibers.
Two challenges had to be addressed in the development. Since the bacteria produced cellulase along with lipase, the bio-catalyzed process had to be optimized considering the influence of lipase and cellulase on fiber chemistry. Second, the aqueous environment required for the sustenance of microbes also was expected to hydrolyze the newly formed ester bonds. To circumvent this difficulty, the microbes were grown via solid state fermentation using minimal water content. Additionally, potassium carbonate catalyst in phosphate buffer was used to minimize hydrolysis of ester bonds (Schuchardt, Sercheli, & Vargas, 1998).
The objective of the present study is the use of lipase producing microbes to transesterify the jute fibers for improvement of the tensile strength, hydrophobicity and degradation resistance against microbial attack. No previous report on such an approach for transesterification of lignocellulosic fibers using lipase producing microbes in a controlled aqueous environment was found in the literature. Further, as shown later, the process is expected to be extremely inexpensive compared to similar chemical and enzyme-catalyzed processes found in the literature. The process could be useful in manufacturing degradation resistant technical textiles, composites, cellulose ester films and plastics.
Section snippets
Bacteria
The bacterium, identified as Bacillus megaterium RB-05 [GenBank Accession Number HM371417] (Chowdhury, Manna, Saha, Basak, Roy, Sen, & Adhikari, 2011), used in this study was isolated from the sediments of river Rupnarayan near Kolaghat, West Bengal, India. Reports on lipolytic and cellulolytic activities of B. megaterium can be found in the literature (Sekhon, Dahiya, Tewari, & Hoondal, 2006). Another lipolytic bacteria Variovorax paradoxus MTCC1193 obtained from Microbial Type culture
Lipase
Lipolytic living cells are known to produce lipase during lipid digestion (Voet & Voet, 2010). Sekhon et al. (2006) found B. megaterium (AKG-1) to produce lipase while decomposing the ester bonds of edible vegetable oils. The strain of B. megaterium (RB-05) used in this study was also found to produce lipase while breaking neem oil ester bonds. The lipase production by RB05 was confirmed by the plate grooving method, which showed development of clear zones around all the grooves except for the
Conclusions
A whole-cell-catalyzed bio-process has been developed for transesterifying jute fibers for enhancing tensile strength and biodegradation resistance. The short-term tensile strength of the treated fibers was twice as those of untreated fibers and the tensile strength retained by treated fibers after 21-d biodegradation was 4-times as much as that of untreated fibers. The performance of the proposed process appears to be comparable with similar chemical processes and better than those catalyzed
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