Adsorptive Isotherms and Removal of Microbial Inhibitors in a Bio-Based Hydrolysate for Xylitol Production

Abstract

Adsorption isotherms and thermodynamics of microbial inhibitors generated by the acid-catalyzed hydrolysis of hemicellulose were investigated using activated carbon as the adsorbent. Parameters of temperature (25–65°C) and pH (1.0–10.0) were employed to study the adsorption isotherms of phenol, acetic acid, and furfural. The results based on the Langmuir model indicated that a higher efficiency of adsorption could be achieved at a lower pH and lower temperature for phenol and acetic acid. Furfural removal from the aqueous solution was only dependent on temperature, with a higher performance at 25°C. Negative values obtained for the heat of adsorption (ΔH), entropy (ΔS), and free energy (ΔG) indicated that the adsorption of all three components is exothermic (physisorption) in nature, with a mechanism based on the affinity of the solute toward the adsorbent and non-spontaneous. The adsorptive removal of the main inhibitors (8.7 g/L phenols and 4.2 g/L acetic acid) from the concentrated hydrolysate of oat hull hemicellulose was dependent on the activated carbon dosage, temperature, and pH. Under operational conditions of pH 1.0 and 25°C with an incremental carbon dosage of 1.25–5% in the hydrolysate, the removal of phenols was 64–95.4% and that of acetic acid was 6–13.2%, respectively. The xylitol fermentation process indicated that oat hull hemicellulosic hydrolysate is low in toxicity to Candida guilliermondii. Meanwhile, a mild adsorptive detoxification using activated carbon resulted in over 10% increase in xylitol yield (≈ 0.8 g/g) and productivity (0.5 g/L/h).