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ABSTRACT This research was focused on the development of novel in-situ enzymatic cleaning methods for reducing membrane fouling in membrane bioreactors (MBRs). This can be achieved by applying magnetic forces to attract enzymes immobilized on a superparamagnetic material, namely magnetic enzyme carriers (MECs), onto magnetic membranes (MMs), exactly where the cleaning is required.
The magnetic enzyme can be made by coupling enzymes onto magnetic Fe2O3 nanoparticles (MNPs). The MECs can thus be released from the MM surface in a later stage when for instance an additional chemical cleaning of MMs is required.
However, the idea is still in preliminary stage and this study was devoted to prove this concept. The selected enzyme and substrate in this study were Bacillus subitilis xylanase A (BsXynA) and arabinoxylan, respectively. The following steps were followed: preparation and characterization of MMs, preparation of magnetic nanoparticles (MNPs), determination of the optimum conditions of BsXynA, immobilization and optimization of the coupling parameters for immobilizing BsXynA onto MNPs and system tests to prove the concept.
The MM was prepared by the phase inversion technique after addition of MNPs to the casting solution. Characterization of a MM occurred by means of clean water permeability (CWP), scanning electron microscope (SEM) and image and Energy dispersive X-ray (EDX) analysis.
The activity of BsXynA was examined using the Colorimetric Xylazyme AX method and highperformance anion-exchange chromatography (HPAEC). BsXynA was covalently bonded onto MNPs via the EDC/NHS mediated amino coupling procedure and the immobilization was analyzed by using the acid hydrolysis method and Fourier transform infrared spectroscopy (FTIR). The performance of fouling reduction by MECs and their cleaning potential in MBRs were tested on a submerged membrane with a dead-end filtration system. MMs were successfully prepared by introducing a sonification step on the casting solution preparation.
The MMs had well distributed MNPs, had a more compact structure and had similar CWPs as membranes without MNPs. BsXynA had a maximal activity at a pH of 6-7 and at a temperature of 40°C and showed good stability over a broad pH and temperature range (pH = 3-8; temperature 20-40°C). An optimum coupling efficiency of 8.3% was achieved for MECs that were stored in MOPS buffer and by using MNPs with a 10% acid group content, BsXynA/MNP ratio of 0.1/1 and 2 hours reaction time with H2O as reaction medium in activation and reaction steps with acetic acid added to it in the activation step. The FTIR spectra of MECs showed the presence of the –CONH- bond, which is a prove for enzyme immobilization onto MNPs.
In the system tests, MECs and MNPs were successfully attracted to a superparamagnetic material. A reduction of the transmembrane pressure (TMP) of 60% was observed in a bioreactor containing arabinoxylan (1 g/l) and MECs in comparison with the control solution, containing only MNPs and arabinoxylan. The MECs also exhibited enzymatic cleaning potential if a fouled MM was introduced into an enzymatic cleaning system. A fouling reduction of 38% was achieved in a 3 hours testing period.
The tests proved that in-situ enzymatic cleaning could be effectively applied in MEC-MM systems and can be implemented for reducing fouling in MBRs.