Development of a new technology for delignification process

Cellulose is a natural polysaccharide made of glucose subunits. It is the main component of most plants and coexists with hemicellulose and lignin. Hemicellulose is a polysaccharide like cellulose but it is branched and made of not only glucose, but also several subunits including xylose, mannose, galactose, arabinose, glucuronic acid, methyl-glucuronic acid, and galacturonic acid. Lignin has much different structure and consists of phenolic subunits. Lignin is hydrophobic and connected to cellulose by chemical bonds. Therefore, removal of lignin from cellulose (delignification) requires breakdown of the chemical bonds by treating biomaterials with concentrated chemicals at high temperature and pressure in cooking reactors specially designed for these harsh conditions. Hemicellulose is connected to cellulose by hydrogen bonds, so hemicellulose removal is much easier than that of lignin, so most of delignification processes also remove hemicellulose. Therefore, delignification process is the most difficult step of cellulose (pulp) production and has a great industrial importance. Heap leaching and heap bioleaching technologies are metallurgical methods. They are based on irrigation of heaped mine or mine slags on the top, dissollution of precious metals by irrigated solution, and recirculation of the solution after several adjustments like pH setting. Heap leaching technology is used mostly for copper and gold recoveries. Heap bioleaching technology adds bacteria into heaps in order to increase rate and recovery of precious metal. Without a need for complicated reactors and factory building, capital and operating costs of the heap leaching and heap bioleaching technologies are lower than those of other metallurgical methods. Therefore, these methods are used for low content mine and mine slags that are not feasible to process via other metallurgical methods. In this study, it was aimed to develop a new technology for delignification process by adapting heap leaching technologies to delignification. For this purpose, it was targeted to design and install a laboratory scale "heap delignification" system and process fall leaves, fir and beech sawdust as wasted biomaterials by using the new design. Also, it was purposed to further process delignified celluloses to antibacterial cellulose for industrial use by employing the same system. Laboratory scale (0.05-5L) heap delignification systems were constructed from common materials. The main system consisted of a plastic or glass container having heaped biomaterials (heap column), and a plastic or glass container (irrigation pool) having an aquarium circulation pump and reactive agent solution below the heap column. The main system was operated by irrigation of the reactive agent solution from the irrigation pool to top of the heap column and recirculation of the reactive agent solution leached to the irrigation pool until desired time. The reactive agent solution was controlled and adjusted in the irrigation pool. The system had a modular structure. A plastic lime column could be inserted to the system between the heap column and the irrigation pool. Also, the system could be aerated using an aquarium aeration pump. Moreover, the system could be operated at acidic or alkaline conditions like heap leaching technology of metallurgy. Fall leaves and fir and beech sawdust were selected as wasted biomaterials to be delignified by the new heap delignification system for cellulose production. In order to test heap delignification system and compare results, stirring reactions with the same chemical concentrations were performed. Optimizations of heap delignification for fall leaves were performed by investigating the effects of several components such as H2O2, pH, lime, aeration and NaCl. Two systems were noteworthy among the heap delignification optimizations for fall leaves, • The highest delignification (83.9%) was achieved when solid NaOH additions were performed as pH decreases to 13 and the end filtrate of previous system was used to initiate the system. NaOH consumption was 0.3g per g of initial fall leaves (0.3g/gFL) at this system. Stirring reactions with the same NaOH achieved much lower (56.1%) delignification. Moreover, filtering of the end products was the most difficult step of stirring reactions because of clogging of filters by ground fall leaves. This problem did not occur at heap delignification method because of self-filtration. Success of heap delignification system was still higher than 10 times more NaOH consumed (3g/gFL) stirring reactions. Stirring reactions needed 10 times more NaOH and additional H2O2 to achieve close delignification success. • The another noteworthy results were obtained when aeration was performed, irrigition solution contained 3% NaCl (like seawater) and irrigation solution was passed through a lime column. 33.8% delignification was achieved by that system without use of chemicals like NaOH and H2O2. When applied to industry, that system may allow use of seawater which may increase water availability and decrease cost of water significantly. Moreover, lime was inserted into a separate column instead of mixing with biomaterials. This modular design of heap delignification system allows removal of lime column from the system for recycling. Heap delignification experiments for fir and beech sawdust were performed using the same conditions which were performed for fall leaves. Stirring reactions were performed in order to compare the results with those of heap delignification experiments. However, the id€al conditions for fall leaves did not work for sawdust at both heap delignification and stirring reactions. Therefore, acidic conditions were investigated and persulfuric acid achieved both delignification and whitening of fir and beech sawdust. 92.4% delignification of fir sawdust was performed by heap delignification when 3ml persulfuric acid was used per g of sawdust. Stirring reactions with the same persulfuric acid usage achieved only 75.1% delignification. The new (heap delignification) system was also used to further process the delignified product. Antibacterial cellulose production was chosen as a sample process. Cellulose produced by heap delignification of fir sawdust was processed to antibacterial cellulose using the same system configuration. Metallic silver or benzalkonium chloride were used to activate cellulose and effects of the activated celluloses against several bacteria causing hospital acquired infections were analyzed. • Silver activated antibacterial celluloses were produced by 2 steps. First, the cellulose produced via heap delignification system was irrigated with silver nitrate while it is still in the heap column. Then, metallic silver synthesis was performed by adding ascorbic acid into the irrigation solution. Silver activated celluloses showed antibacterial effects against Staphylococcus aureus and Staphylococcus epidermidis depending on the dose. Both wild type (PAO1) and mutant (PAOtatC) strains of Pseudomonas aeruginosa showed resistance to the silver activated celluloses. • Benzalkonium chloride activated celluloses were produced via irrigation of cellulose produced by heap delignification system with benzalkonium chloride. Benzalkonium chloride activated celluloses showed antibacterial activities against Staphylococcus aureus and Staphylococcus epidermidis. The antimicrobial effects of benzalkonium chloride activated celluloses were higher than those of silver activated celluloses. Wild type Pseudomonas aeruginosa (PAO1), capable of pyoverdine synthesis, showed resistance to benzalkonium chloride activated celluloses while inhibition of pyoverdine synthesis by tatC mutation broke the resistance and made Pseudomonas aeruginosa sensitive. In other words, benzalkonium chloride activated celluloses showed antibacterial activities against tatC deletion mutant strain of Pseudomonas aeruginosa (PAOtatC) but not against wild type strain of Pseudomonas aeruginosa (PAO1). In this study, "heap delignification" method is presented as a new method for delignification process that promises many opportunities for industrial use. In contrast with the present delignification systems, simplicity and mild conditions of heap delignification method will probably decrease capital and operation costs when applied in industry. Outstanding features of the system may also decrease safety risks, make possible to elongate operation time feasibly, facilitate system control and increase product quality. Moreover, it may be possible to process heap delignified products further by using the same system while the products are still at heap area. These advantages may also allow processing of cellulose bearing organic wastes which are not feasible to process using present technologies. Therefore, heap delignification method should be applied at larger scales and optimized for industrial use.