Advances in biocatalytic membrane reactors and strategies to implement system efficiency for industrial sustainable production

Short Introduction Analyzing the use of membrane bioreactor (MBR) technology in patents development as well as their industrial application, it seems that the major development has been obtained by MBR for water treatment, whilst lower efforts have been devoted to MBR for pharmaceutical, food, cosmetics, etc. This is mainly due to more strict regulations that govern the discharge of waste water into the environment. However, the need to promote more sustainable processes for industrial production will force the development of MBR also in other fields, including biorefinery for bioderived chemicals. Furthermore, biosensing and decontamination of harmful substances in air, water, and food is becoming also very crucial in modern society. Biocatalytic membranes using immobilized enzymes will certainly play a key role. Despite the vast literature about immobilized enzyme, fundamental understanding on how to govern enzyme immobilization to keep native specific activity while increasing catalytic activity as well as how to avoid enzyme deactivation during membrane cleaning and maintenance still struggle scientists and technologists. Long term studies that proof the technology robustness, when existing, are proprietary. In this work, parameters that strongly affect immobilized enzyme performance will be discussed. Strategies to reversibly immobilize enzymes on membranes, in order to remove it during membrane cleaning will be presented. The development of biocatalytic membrane reactors (BMRs) lab prototypes for bioderived molecules (such as oleuropein aglycon) and for organophosphate decontamination will be presented as case studies. Material and Methods The biomolecules used were: ?-glucosidase from almond, phosphotriesterase, protein G, lipase and Bovine serum albumin. Commercial magnetic nanoparticles are used (MNP). Both hydrophilic (regenerated cellulose, home-made alumina membranes) and hydrophobic (PVDF) membrane were used. Immobilization methods were based on covalent bond and by magnetic force. Results and Discussion The amount of immobilized enzyme and its spatial distribution within the membrane support plays a crucial role, as it influences crowding phenomena that reduces specific activity and mass trasport. The final immobilized enzyme characteristics and its possible molecular aggregation at the membrane level is mainly affected by properties of enzyme molecules in starting solution used to carry out the immobilization procedure. Therefore, a careful characterization, of free enzyme to determine its dynamic diameter, aggregation state and charge is very useful, since it can predict the way in which it rearranges on the membrane surface during immobilization. The enzyme loading capacity for a given membrane must be matched with the enzyme-loaded membrane productivity (or immobilized enzyme catalytic activity, i.e moles/time) and with the enzyme specific activity (moles/time·mass of immobilized enzyme). This permits to work at the highest efficiency with proper enzyme amount, thus avoiding waste of enzyme and further resistance to mass transport (Fig. 1). Taking into account these aspects, continuous biocatalytic membrane systems were developed which demonstrated good performance. In one case an intensified multiphase process, which integrates a biocatalytic membrane reactor and a membrane emulsification process was developed to produce oleuropein aglycon, an antioxidant molecule difficult to stabilize and not yet available commercially in a pure form. The system was able to sequestrate the poor water stable intermediate reaction product in the organic phase, preventing its rearrangement in less valuable molecules. In another case study, a very long stability of phosphotriesterase was obtained in a continuous biocatalytic membrane reactor for organophosphate degradation, demonstrating a constant specific activity for one year of observation. On the contrary the free enzyme lost completely its activity within two months. We have also demonstrated that magnetic nanoparticles (MNP) containing immobilized enzyme and deposited on the membrane by magnetic field were able to control membrane fouling during pectins hydrolysis. Furthermore it was possible to remove/reload enzyme-loaded MNP as necessary. The cake layer thickness was much lower (67% less) when using this strategy compared to bare membrane. Figure 1. Performance of biocatalytic membrane reactor as a function of the increase of immobilized enzyme (crude extract of lipase covalently immobilized on home-made alumina membrane) Conclusions BMR will become a key technology to implement precise selective and clean process in industrial application in the logic of green and intensified processes. The work carried out contributed to elucidate basic properties that affect biocatalyst and overall process performance.