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Okko Ringena, Bodo Saake, Ralph Lehnen

Characterization of electrolyzed magnesium spent-sulfite liquor

Keywords: anodic oxidation, electrochemical modification, electrolysis, lignosulfonates, molar mass, molar mass distribution, size exclusion chromatography (SEC), spent sulfite liquor, Ultrafiltration

Spent sulfite liquor derived from magnesium bisulfite pulping of beech was electrolyzed in a pilot-plantelectrolysis cell. During the electrochemical treatment (125 mA cm^?2; 60°C; 180 min), desalination of the anolyte took place and the magnesium concentration was reduced to approximately 60%. In addition, the pH decreased from 5 to 1. Three differently treated liquors (SSL pH 5; SSLe pH 2.5; SSLe pH 1) were chosen for further analysis. The average molar mass of the fractions was determined by size exclusion chromatography. Extended electrolysis time increased the molar mass of the lignosulfonates (M_w: SSL pH 5, 5700 g mol^?1; SSLe pH 2.5, 6500 g mol^?1; SSLe pH 1, 7400 g mol^?1). The content of phenolic hydroxyl and sulfonic acid groups did not undergo significant changes. Nevertheless, lignosulfonates obtained after electrolysis showed higher charge densities than the unmodified product (SSL pH 5, 1590 ?eq g^?1; SSLe pH 2.5, 1760 ?eq g^?1; SSLe pH 1, 1920 ?eq g^?1). Separation of the liquors into five fractions was performed using four ultrafiltration membranes (100, 50, 10 and 1 kDa). This allowed detailed structural analysis of high- and low-molar-mass lignosulfonate fractions in terms of the modifications induced by electrolysis.

Holzforschung, Walter de Gruyter

Print ISSN: 0018-3830
Volume: 59, 11/2005
Pages: 604 - 611

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