On the Oxidation of Kraft Black Liquor for Lignin Recovery: A Voltammetric Study

The present study focuses on the electrolysis of kraft black liquor (BL) for energy and lignin recovery. The concept has economic and environmental advantages, as it simultaneously generates a clean fuel gas (hydrogen) at the cathode and a solid with economic value (lignin) at the anode. Platinum (Pt), nickel (Ni), and AISI 304 stainless steel (SS) are assessed as potential anodes and cathodes for the BL electrolysis. Voltammetric methods are used to study the lignin oxidation in the BL, allowing the calculation of the charge transfer coefficient and the number of exchanged electrons. The hydrogen evolution reaction (HER) in BL is also evaluated in the same electrodes, with the Tafel slopes, charge transfer coefficients and exchange current densities being determined. Pt leads to the best results for both HER and lignin oxidation, followed by Ni, whereas AISI 304 SS is not appropriate. A small-scale laboratory BL electrolyzer using Ni plate electrodes is assembled and tested. The lignin electrodeposited at the anode is characterized by Fourier Transform Mid Infrared Spectroscopy and compared with lyophilized lignin and Klason lignin. The higher purity of the lignin obtained by BL electrolysis suggests further work on the development of this new technology. (C) The Author(s) 2019. Published by ECS.