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

Abstract

Abstract The spent pulping liquor (SPL) obtained from straw processed by soda (alkaline) pulping contains dissolved non-sulfonated lignin. The lignin can be separated from SPL using acid or cationic flocculants which are potentially hazardous to the environment. In this study, the performance of the biobased protein flocculant hemoglobin (HEM) with and without added calcium chloride was compared with that of a high charge density synthetic cationic flocculant, poly (diallyldimethylammonium chlorides) (pDADMAC). Turbidity measurements gave overly broad concentration ranges for optimal lignin removal which may be related to the mechanism of flocculation which requires several major steps: Conversion of dissolved lignin to particulate lignin, formation of particulate lignin flocs, and subsequent sedimentation of these flocs. An optimal flocculant concentration or concentration range was estimated using the three methods: average Zeta potential corresponding to the lowest turbidity range; a novel method using the maximum percent pellet mass calculated using the dried masses of the pellet and supernatant after centrifugation; measurement of supernatant lignin using spectroscopy. HEM light absorption interfered with spectroscopic lignin determination, and a method for correcting the measurements was devised. This study shows that HEM is an effective flocculant for nonsulfonated lignin in SPL. The HEM-lignin complex is a potential high protein component of animal feed.