Alcohol adsorption on softwood lignin from aqueous solutions

Lignin prepared by acid and enzyme hydrolysis of a softwood mixture adsorbs acetone, butanol, and other alcohols while showing only a slight uptake of glucose. Adsorption of butanol is independent of temperature in the range of 30‐65°C. The Polanyi theory fits adsorption for the linear alcohols methanol through hexanol with values of AS and Δ_μ ranging from 2.6 to 26 J mol⁻¹ K⁻¹and −0.8 to −8 kJ/mol. The adsorption capacity is given by Q (g alcohol/g lignin) = KC^*. Where C^* is the equilibrium alcohol concentration (g/mL), K = ε_Wexp (Δ/R), and ε_w is the porosity of the lignin (0.23‐0.42 mL/g). The value of the adsorption capacity constant K for n‐butanol ranges from 1.3 to 2.7 mL/g on sorbent containing 26‐72% lignin, while ethanol is 0.5‐0.73, acetone is 0.62‐1.0, and glucose is 0.35. Adsorption is shown to occur through combined hydrophobic and hydrophilic interactions of the alkyl and hydroxyl groups, respectively, of the adsorbate with the lignin. Consequently, for the alcohols methanol to hexanol, we present the capacity constant K[=K(R) + K(OH)] as a sum of an alky! adsorption constant (0.1‐9.5 mL/g) and a hydrophilic (0.40‐0.50 mL/g) contribution. This approach may be applicable to organic acids. Lignin's sorbent properties have potential to moderate product inhibition in the anaerobic acetone‐butanol‐ethanol (ABE) fermentation.