No abstract

Recycled paper mills produce large quantities of fibrous rejects and fines which are usually sent to landfills as solid waste. These cellulosic materials can be enzymatically hydrolyzed into sugars for the production of biofuels and biomaterials. Paper mill wastes also contain large amounts of calcium carbonate which inhibits cellulase activity. The calcium carbonate (30%, w/w) decreased 40-60% of sugar yield of unbleached softwood kraft pulp. The prime mechanisms for this are by pH variation, competitive and non-productive binding, and aggregation effect. Addition of acetic acid (pH adjustment) increased the sugar production from 19 to 22 g/L of paper mill waste fibers. Strong affinity of enzyme-calcium carbonate decreased free enzyme in solution and hindered sugar production. Electrostatic and hydrogen bond interactions are mainly possible mechanism of enzyme-calcium carbonate adsorption. The application of the nonionic surfactant Tween 80 alleviated the non-productive binding of enzyme with the higher affinity on calcium carbonate. Dissociated calcium ion also inhibited the hydrolysis by aggregation of enzyme.

Background: A significant fraction of short fibers commonly called "reject fines" is produced while recycling wastepaper at paper mills producing linerboard. These fines are usually rejected into the solid waste stream that further requires land filling and poses environmental problems. The major component of these rejects is cellulose that can be a potential source of fermentable sugars for biofuels, bioplastics or other products. Therefore, a feasible process for converting these reject fines into sugars can profit the paper mills by producing value for their waste products while simultaneously mitigating their adverse environmental impact by avoided solid waste. Additionally, the sugar feedstocks can be used to reduce fossil carbon contributing to the sustainability of the industry.Results: Enzymatic conversion of rejects fines from paper mills was achieved using commercial cellulases from Trichoderma reesei. The presence of mineral particles along with the cellulosic fines was found to have potent inhibitory effects on enzyme hydrolysis. The mineral particles are kaolin and calcium carbonate and originate from the fillers used in the wastepaper. The adsorption of the cellulase onto these mineral components was measured and quantified by the slope of the adsorption isotherm. The application of a nonionic surfactant Tween-80, decreased the adsorption of cellulase and this improved the hydrolysis yield of sugars. Conclusions:Enzymatic hydrolysis of rejects from recycled paper mills is feasible and provides a source of sugars for biofuels and bioplastics. However, the presence of mineral particles can be detrimental to this bioconversion. Calcium carbonate which occurs as a filler in waste paper shows high adsorption affinity to the cellulase enzymes and thus reduces the available enzyme for cellulolysis. This can be remedied by the application of surfactants which preferentially occlude to the mineral surfaces and thus increase enzyme availability in solution. The non-ionic surfactant, Tween-80, shows the best hydrolysis enhancement at a dosage of 3 % based on the dry weight of the biomass.