Development of novel photocatalytic metal oxide foams with controlled structure for the removal of micropollutants, using 3D printing and electrochemical anodization

Abstract

In recent decades, it has become clear that current wastewater treatment practices are no longer sufficient to prevent the negative effects caused by the increasing number and complexity of discharged compounds present in water. This is particularly the case for organic micropollutants, globally widespread compounds, present in water in trace amounts, from μg/L down to ng/L. Despite the low concentration, they do bioaccumulate over time, causing significant harm to both human health and the environment. The increasing evidence of their harm has spurred the development of tertiary treatment methods to append to current wastewater treatment plant (WWTP) treatment steps, with photocatalysis having received the most attention in the academic literature, but little practical implementation, so far. Current photocatalysis systems either use photocatalyst nanoparticles dispersed in the medium to treat (slurries) or nanoparticles immobilized onto supports in contact with the contaminated solution. Slurries, while having a high surface area, require a downstream removal step to prevent the photocatalyst nanoparticles from leaching into the effluent, lest it becomes an environmental problem on its own. This poses financial and practical burdens when planning for the scale-up of this type of approach. Immobilized photocatalysts, on the other hand, reduce the chance of nanoparticle leaching by fixing them onto supports. However, these configurations suffer from lower illuminated surface area and hence are less efficient than slurries. To counter this, a so-called ‘third generation’ of photocatalyst configuration has emerged in the form of foams. These are the highly porous, self-supported structures with a higher surface area than immobilized photocatalysts but without the need for a removal step in the process as slurries do. A further advantage of foams can be realized using computer-aided design and additive manufacturing which opens possibilities for different photocatalyst designs adaptable to specific reactor setups.

This thesis aimed to explore and develop novel photocatalyst foam designs, optimized by algorithm to maximize illumination performance, and produced using additive manufacturing, or 3D printing. In addition, with further development of the design process, the hydrodynamic performance can be advanced.
In a first step towards this aim, a computational code was developed to allow the design of optimized photocatalyst shapes for subsequent additive manufacturing. Inputting dimensions, wall thickness, and cell unit length enabled the production of a three-dimensional object with the specified surface area, porosity, and channel size suitable for a flow photoreactor. Further, incorporating details about the photoreactor facilitated the calculation of an illuminated surface area metric, essential for evaluating the influence of design parameters on various shapes.

Given the current state of the art in additive manufacturing, it was not possible to directly fabricate a foam with complex shapes using a photoactive material such as titania. Therefore, an indirect approach was conceived, using a titanium alloy Ti6Al4V to print the foams, followed by the conversion of its surface to titania. Before manufacturing, an extensive study of the oxidation of Ti6Al4V with a particular focus on electrochemical anodization was conducted. A variety of electrolytes and anodization conditions were tested to identify the optimal conditions to develop a photoactive oxide layer. Such conditions were applied to 3D printed Ti6Al4V foams (via the Selective Laser Melting (SLM) technique), which were then used in the photocatalytic degradation of primidone, a frequently detected pharmaceutical in wastewater and used as an exemplary organic micropollutant, in a flow reactor. However, the produced foams eventually proved to have too low a photoactivity to be of practical use. This is presented later on, along with a hypothesis to explain the results.

The knowledge gained in the previous chapters was subsequently applied to produce photocatalytically active foams using different additive manufacturing techniques. First, a digital light processing (DLP) printing technique was applied to produce TiO2 foams, starting from a polymer resin containing a Ti-metalloorganic compound. Subsequent sintering removed the polymer and led to self-supporting, mechanically stable TiO2 foams that were photocatalytically active in the degradation of carbamazepine, another exemplary organic micropollutant of environmental concern. A second approach, in collaboration with the University of Pavia (Italy), produced foams via a combination of material extrusion (MEX) 3D printing of titania particle slurries and an ultra-fast sintering method which preserved the anatase phase in titania, leading to photocatalytically active foams which were successfully tested for the degradation of primidone.

This thesis has advanced the state-of-the-art in the field of photocatalysis by developing novel code to optimize the design of the high surface area and highly porous photocatalytically active foams produced via 3D printing using different techniques and tested in flow reactors. The results show a clear route towards the rational design of self-supporting photocatalysts which can be scaled up using existing manufacturing methods while, at the same time, addressing key limitations of existing photocatalytic technologies. These results contribute to advancing the field towards the goal of the energy-efficient removal of organic micropollutants from water, an urgent necessity given their significant adverse effects on both human health and the environment.

Date of Award	8 Oct 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Davide Mattia (Supervisor), Joseph Flynn (Supervisor) & Jannis Wenk (Supervisor)