The production of bleach-grade pulp requires the removal of lignin, a structural component of wood. To date, harsh chemistry utilizing chlorine dioxide is required to sufficiently depolymerize the recalcitrant biopolymer. Thereby, however, hazardous reaction byproducts get released into the environment. A potential strategy is to replace classical chemistry through the use of oxidizing enzymes. Oxidoreductases are promising candidates to fulfill this purpose as they catalyze a broad range of oxidation reactions on diverse substrates, including lignin-type molecules. Nevertheless, these versatile catalysts have only been explored to a limited extend. In this dissertation, the use of four different oxidoreductases is investigated for their applicability in pulp bleaching processes.
Firstly, a haloperoxidase from Curvularia verruculosa in presence of different amine catalysts were reacted with lignin model compounds. Hereby, using both DABCO (1,4-diazabicyclo[2.2.2]octane) or NH4Cl as amine source resulted in successful oxidation with NH4Cl yielding the highest reaction specificity for the
oxidation versus the undesired chlorination reaction.
Secondly, in laboratory-scale pulp bleaching experiments, delignification was achieved by using either haloperoxidase together with NH4Cl or NaCl (in presence or absence of DABCO). The presence of DABCO led to similar brightness levels as for ClO2 bleaching. Additionally, near complete removal of hexenuronic acid was achieved while large quantities of undesired adsorbable organic halides (AOX) were formed. Surprisingly, it was found that pulps bleached with NH4Cl resulted in much lower levels of AOX compared to pulp bleached with DABCO.
Thirdly, the aforementioned haloperoxidase, as well as an unspecific peroxygenase, a heme-peroxidase and a laccase were reacted with industrial wastewater from a pulp bleaching plant with the aim to remove persistent organic molecules. Haloperoxidase and heme peroxidases efficiently removed over 60% of the color and over 30% of the aromatic compounds present in the final effluent of the wastewater plant.
Hence, this dissertation contributes to the understanding and applicability of
oxidoreductases in pulp bleaching and reduction of water pollution therein.