Removal of straw lignin from spent pulping liquor using synthetic cationic and biobased flocculants

Authors: Piazza, George; LORA, J.H. - Green Value; Wayman, Luke; Garcia, Rafael

Submitted to: Separation and Purification Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/13/2017
Publication Date: 7/14/2017
Publication URL: https://handle.nal.usda.gov/10113/5801860
Citation: Piazza, G.J., Lora, J., Wayman, L.I., Garcia, R.A. 2017. Removal of straw lignin from spent pulping liquor using synthetic cationic and biobased flocculants. Separation and Purification Technology. doi:10.1016/j.seppur.2017.07.042.

Interpretive Summary: Biomass is a term which denotes organic material derived from agriculture such as crop wastes or wood. The two main components of biomass are carbohydrate and lignin. Lignin is usually removed from carbohydrate in processing biomass to produce fuel ethanol. There are at least two reasons why lignin removal is important: The presence of lignin inhibits the rate of conversion of carbohydrate to fuel ethanol, and lignin is a valuable material, whose return enhances the profitability of the biomass facility. In current process, strong acid is used to sediment lignin. The strong acid must be removed from the lignin and neutralized to reduce environmental pollution. Materials called flocculants may be able to selectively remove lignin through a process called flocculation which aids the sedimentation of lignin, thus removing the need for strong acid. To test this idea, a material called spent pulping liquor (SPL) was obtained from a wheat straw processing facility. A common commercial synthetic flocculant and a recently discovered biobased flocculant, obtained from a byproduct of animal processing, were tested for their abilities to remove lignin from SPL. Both types of flocculants were able to promote the sedimentation of lignin from SPL. Neither flocculant aided the sedimentation of hemicellulose, the main carbohydrate in wheat straw. Future use of flocculants in biomass processing can displace the use of strong acid for lignin sedimentation.

Technical Abstract: Aqueous alkaline delignification of wheat straw produces hemicellulose for bioenergy and other applications. After removal of the hemicellulose, spent pulping liquor (SPL) remains. The spent pulping liquor is approximately 28% water, 40% ash, 3% hemicellulose, 25% lignin, 5% protein, and less than 1% phosphate. The purpose of this research was to determine lignin removal from the SPL using flocculant. SPL was treated with different concentrations of the synthetic flocculant, poly (diallyldimethylammonium chloride) (pDADMAC), and the biobased flocculant, hemoglobin (HEM), and a mixture of HEM and CaCl2 (HEM-CaCl2). The turbidities of flocculant-SPL mixtures were followed over time, and the Zeta potentials were measured. The dry mass of the pellets and supernatants were determined after centrifugation. Lignin removal from the supernatant was followed by ultraviolet (UV) absorbance. The flocculant level which gave the greatest lignin removal from the supernatant also gave the highest percent pellet mass. The Zeta potentials that corresponded to the greatest lignin removal were -6.4 +/- 0.7, -5.7+/-0.4, and the range -8.7 +/- 0.4 to -2.6 +/- 0.2 mV for pDADMAC, HEM, and HEM-CaCl2, respectively. The highest amounts of lignin removed by pDADMAC, HEM, HEM-CaCl2, and acid (pH 2.2) were 63 +/-5%, 51 +/- 7%, 59 +/- 3%, and 67 +/- 8%, respectively. In contrast to lignin, the level of hemicellulose in supernatant was hardly influenced by the flocculant level. This is the first report of the action of flocculants on nonsulfonated lignin from SPL.