Treatment of enzyme-initiated delignification reaction mixtures with ceramic ultrafiltration membranes: Experimental investigations and modeling approach

Abstract

Membrane reactors offer a promising configuration for enzymatic delignification processes (lignin modification, removal and utilization). However, membrane fouling reduces the efficiency of filtration and of the entire bioprocess. The flux and retention characteristics of protein-ligninsulfonate model mixtures were investigated. A 5-kDa tubular ceramic membrane achieved a sufficient and constant ligninsulphonate retention of 80â90%. The retention of phenolic monomers (e.g. guaiacol) increased with the ligninsulphonate concentration. Mass transfer-controlled regions were observed in which permeate fluxes could be predicted by the limiting flux model. Finally, the underlying fouling mechanisms were evaluated, revealing a predominance of complete pore blocking and cake filtration

Lignin Degradation Processes and the Purification of Valuable Products

“…Accordingly, the filtration performance of membranes used in delignification reactions must be robustly tested, including their fouling and separation characteristics. Such tests were recently reported for a ceramic ultrafiltration membrane exposed to a model reaction mixture containing a commercial polydisperse lignosulfonate substrate, guaiacol as a model degradation product, and ovalbumin as a model biocatalyst in place of the genuine versatile peroxidase [92]. As anticipated, the flux declined significantly during filtration but reached steady-state levels within 1 h, and the flux could be improved by 50% by increasing the crossflow velocity from 0.2 to 1 m/s.…”

“…The retention of lignosulfonates using a 5 kDa membrane remained stable at 80-90% under all conditions, whereas the retention of the model product guaiacol increased with the lignosulfonate load. The authors noted that the retention of degradation products should be avoided because this can lead to undesirable enzyme inactivation or polymerization reactions if real enzymes are used instead of the model ovalbumin [92]. Based on the results summarized above [92,95] …”

“…Importantly, the performance and productivity of membrane filtration systems are reduced by fouling, which increases membrane resistance and reduces flux due to the formation of unwanted deposits on the membrane surface and/or in the membrane pores [97,98]. Extensive fouling may be caused, among others, by proteins, polysaccharides and polydisperse technical lignin, all of which are present during the enzymatic degradation of lignocellulosic biomass [92,97]. Accordingly, the filtration performance of membranes used in delignification reactions must be robustly tested, including their fouling and separation characteristics.…”

“…An EMRS also allows the continuous separation of low-molecular weight products from high-molecular weight substrate and catalyst molecules, thus preventing the re-polymerization of target products. The biocatalyst is simple to replace (according to enzyme inactivation rates), which achieves stable substrate conversion rates [92]. EMRS processes have also been tested for the continuous removal of endocrine disrupting compounds from wastewater, using versatile peroxidases and polyethersulfone (PES) ultrafiltration membranes [93,94].…”

“…Filtration performance: an appropriate membrane is crucial for the design and productivity of EMRS processes because the filtration performance influences process parameters such as the hydraulic retention time (τ), the input of substrate and oxidizing agent (H 2 O 2 for peroxidases), and therefore the enzyme reaction kinetics and substrate conversion rates [92,96]. Importantly, the performance and productivity of membrane filtration systems are reduced by fouling, which increases membrane resistance and reduces flux due to the formation of unwanted deposits on the membrane surface and/or in the membrane pores [97,98].…”