Electrospun nanofiber membranes as supports for enzyme immobilization and its application in wastewater treatment

Life on earth depends on water, which covers 71% of the planet's surface. Despite this, the availability of freshwater is endangered by factors such as climate change and excessive extraction, unable to keep up with the rising global demand. Moreover, the discharge of vast quantities of wastewater by industry, municipalities, and agriculture worsens the shortage of water resources and poses a threat to human health due to pollution. Water treatment, thus, is crucial for ensuring a clean water supply in the future.

Presently, wastewater treatment plants (WWTPs) face a challenge in monitoring and regulating contaminants of emerging concerns (CECs) due to their persistence and trace concentrations. The discharge of such compounds in the effluent can potentially have adverse effects on aquatic ecosystems and wildlife. To mitigate this issue, there is an increasing emphasis on employing advanced treatment technologies to reduce the discharge of CECs from WWTPs. However, due to their low concentrations, traditional treatment technologies like active sludge and moving bed biofilms have limited efficacy in removing CECs. In contrast, membrane technology has demonstrated high-quality removal of CECs, which may result in concentrated pollutants. One potential solution is to incorporate enzymes into membrane technology, taking advantage of the eco-friendly biotransformation of CECs and regulating water quality.

This thesis aims to investigate the potential of using three different types of membranes to immobilize enzymes, with a particular focus on adjusting the membranes' structure and functional groups. The initial phase of the research indicates the possibility and suitability of a polymer-based membrane system for enzyme immobilization. Specifically, the study used laccase, which was attached to nanofibers made of polyacrylonitrile (PAN), as well as a lab-created nanofibrous membrane with a selective layer. Results suggested that introducing β-cyclodextrin into the fibers and incorporating –OH and –NH₂ functional groups into the membrane surface could enhance the enzymatic activity of the membranes. The immobilized enzyme also demonstrated greater thermal stability and a wider range of pH tolerance compared to free enzymes.

The second part of the thesis focuses on the development of biodegradable materials and their potential for enzyme immobilization. Building on earlier research, a new process was utilized to produce biodegradable fibrous membranes, utilizing β-cyclodextrin as the primary material and citric acid as a crosslinker. The study examined the ability to immobilize laccase in the molecular sieving structure of β-cyclodextrin, finding that the immobilized enzymes retained their catalytic activity. Implementing this biodegradable membrane system offers a solution to the plastic waste commonly associated with the disposal of polymeric membranes.

Despite the potential benefits of biodegradable membranes as carriers for enzymes, concerns persist regarding their reusability and durability due to inconsistent material properties and limited shelf life. To address these issues, the third segment of the study focuses on developing ceramic membranes as enzyme carriers that are mechanically stable, recyclable, and flexible. The findings demonstrated that the flexible ceramic membranes achieve a high immobilization yield of 57.9 ± 0.5% with a specific activity of 0.53 ± 0.09 U mg^-1. Furthermore, the membranes exhibited efficient removal of up to 95% of five emerging pollutants, providing the first proof of concept that laccase-immobilized ceramic nanofiltration membranes could effectively transform emerging pollutants while being easy to retrieve, making them a promising option for environmental bioremediation processes.