Removal of Lignin and Associated Impurities from Xylo-oligosaccharides by Activated Carbon Adsorption

This paper studies purification with commercial activated carbons of the xylo-oligosaccharides produced by the autohydrolysis of almond shells. Almond shells are agricultural residues with a high content of xylan that are produced abundantly in some regions with a Mediterranean climate. Adsorption equilibrium was measured in a batch system for three commercial activated carbons using a constant concentration of 20 g/L of crude xylo-oligosaccharides and loads of activated carbon from 1.5 to 50.0 mg/mL. Adsorption for ligninrelated products was higher than for xylo-oligosaccharides and the selectivity toward lignin adsorption was better when the carbon was highly microporous and had small mesopore diameters, a low volume of mesopores, a low concentration of basic surface groups to limit xylo-oligosaccharide adsorption, and acidic surface groups to favor the adsorption of the lignin-related products. Column tests were performed at a feed rate of crude xylo-oligosaccharide solution of 6.0 mL/min (35 g/L) in columns packed with 22 g of granular activated carbon and operated in up-flow mode. Average retention was around 64% for lignin products and 21% for carbohydrates for the fraction of treated solution collected during the first 2 h of operation (13.1 bed volumes circulated through the bed). Retention for lignin-derived products was limited because part of them is linked to the xylo-oligosaccharides.