Periodate oxidation of carbohydrate-enriched hydrolysis lignin and its application as coagulant for aluminum oxide suspension

Lignin is an under-utilized biopolymer, but it can be modified to broaden its application in industry. In this work, a lignin-based coagulant was prepared by periodate oxidation of hydrolysis lignin that contained carbohydrates. The results showed that water-soluble oxidized hydrolysis lignin (OHL) with an anionic charge density of 0.9 mmol/g, carboxyl group content of 1.7 mmol/g and molecular weight of 6200 g/mol was obtained under the reaction conditions of 1/1 M ratio of sodium periodate/lignin, pH 5, 50 degrees C and 2 h. The mechanism of periodate oxidation of hydrolysis lignin involved the formation of quinoid intermediate, the increase in carboxylate group in final oxidized lignin, and the release of sugars from the decomposition of lignin-carbohydrate linkage. The adsorption analysis revealed that OHL adsorbed on the surface of aluminum oxide particles via charge neutralization and hydrogen bonding following Langmuir adsorption model. When 12 mg/g OHL was added to the aluminum oxide suspension, the mean chord length of particles increased from 17 mu m to 39 mu m, while their total counts decreased by 40%. Vertical scanning analysis was also used to evaluate the coagulation/settling performance of OHL in aluminum oxide suspensions. The settling rate of particles accelerated from 22 mL/h to 600 mL/h in the presence of OHL, increasing the transparency of suspension from 15% to 78% and the compactness of sediment from 20 g/L to 84 g/L.