Separation of lignin from beech wood hydrolysate using polymeric resins and zeolites - Determination and application of adsorption isotherms

Separation of lignin from hemicellulose sugar out of beech wood hydrolysate (BWH) by four polymeric and one zeolitic adsorbents was investigated. First, by means of single and binary model solutions (MS), consisting of 3.1 g/L phenol and/or 4.2 g/L xylose, the two most suitable adsorbents were chosen and single- and multi-component isotherms were determined. Second, the isotherm model parameters from the MS were applied to the equilibrium measurements of BWH and statistically evaluated regarding their adaptability. All experiments were carried out in batch mode. Of the five tested adsorbents, Amberlite XAD7HP and SEPABEADS SP700 were found to be the most efficient. Phenol and lignin removal was 100% and over 93% respectively and xylose and hemicellulose sugar recovery was greater 92% at an adsorbent-to-solution ratio of 1:5 w/v (g/mL). Phenol adsorption agreed best to Freundlich equation and xylose adsorption to Langmuir equation for XAD7HP and SP700 in the single-component experiments. In a multi-component system, adsorption behavior of both phenol and xylose for the two adsorbents can be fitted best to an extended Freundlich isotherm. For practical applications, it is reasonable to use Freundlich and extended Freundlich isotherms for xylose adsorption onto XAD7HP and SP700. Application and evaluation of the isotherm model parameters determined by the MS to the equilibrium data from adsorption experiments of BWH onto XAD7HP and SP700 showed that the valid isotherm type and thus the predominant adsorption mechanisms can be predicted accurately. However, the isotherm model parameters of the MS do not adequately describe the specific adsorption process of the BWH onto the resins. Hence, they have to be determined individually. More than 90% of adsorbed lignin and 95% of adsorbed hemicellulose sugar were desorbed using 50 wt% ethanol solution.