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Abstract
The increasing worldwide contamination of freshwater system with micro-pollutants emerges as a critical environmental problem, which has driven the search for novel mitigation approaches. The use of enzymes such as laccase as biocatalyst has been recognised as a promising approach for micro-pollutants removal. However, rapid denaturation of the free enzyme and its difficulty in recycling and reuse restrict its wider application, and efficient enzyme immobilization and bioreactor design are required.
In this study, and two membrane bioreactors were proposed for carbamazepine (CBZ) degradation: the hybrid membrane system where laccase-immobilized TiO2 nanoparticles were suspended in the feed solution, and the biocatalytic membrane reactor where laccase was immobilised on TiO2 coated membrane surface. Using p-coumaric acid as a mediator, efficient CBZ removal (up to 71%) was achieved with the hybrid membrane reactor.
Functionalized TiO2 nanoparticles were further applied to immobilize crude enzyme extracts from P. ostreatus culture. The resultant biocatalytic particles had comparable performance to the immobilized purified commercial laccase and showed efficient bisphenol-A and CBZ removal in the hybrid reactor.
In addition, a cross-linked carbon nanotubes (CNTs) based membrane was prepared, which exhibited high effectiveness as support for physical adsorption of laccase. The active laccase coating on CNTs membrane can be renewed after simple cleaning and re-immobilization. The biocatalytic membrane also showed substantial improvement in micro-pollutant removal compared with the membrane having no enzyme.
At last, it is demonstrated that the intramolecular electron transfer within single enzyme molecule is an important alternative pathway which can be harnessed to generate electricity. By decoupling the redox reaction within laccase, efficient electricity production from unconventional fuels including recalcitrant pollutants and/or toxic organic was obtained in a sole-laccase based enzymatic fuel cell. The intramolecular electron-harnessing concept was also demonstrated with other enzymes, including the power generation during CO2 bioconversion to formate catalysed by formate dehydrogenase. The novel enzymatic power generation is shown to be potentially feasible utilizing wastewater as fuel as well as occurring in tandem with driving bioconversion of chemical feedstock from CO2.