Abstract

The rise in the global water demand, coupled with the limited water resources readily usable on the planet, underscores the importance of developing cost-effective and modern approaches for safeguarding and reusing available water supplies. Consequently, membrane separation technologies have garnered increasing interest due to their ability to provide a sustainable, energy-efficient, and affordable solution for water treatment. Recently, ceramic membranes have been increasingly recognized in the commercial sphere due to their remarkable mechanical, thermal, and chemical stability, along with their hydrophilic properties, reduced fouling tendency, ease of cleaning, and sustained long-term flux. Nevertheless, the complex and costly manufacturing processes of these membranes limit their application to areas where polymer membranes are comparatively less efficient. Therefore, it is essential to investigate innovative membrane materials and manufacturing methods that can create cost-effective and high-performance membranes. One promising approach is the utilization of suspension plasma spray (SPS), a relatively new coating deposition technology that involves sub-micron to nano-sized feedstock materials. This single-step process holds the potential to offer a cost-effective solution for producing ceramic membranes. Additionally, the SPS process allows for the incorporation of photocatalysts like titanium dioxide (TiO2) into the feedstock, which can facilitate the degradation of organic contaminants on the membrane surface and enhance the anti-fouling properties of the membranes.
In this project, initially, the SPS process was used to produce porous TiO2 coatings with an average pore size of around 30 nm, which is in the range of ultrafiltration (UF) membranes, with pore sizes ranging from 2 to 100 nm. The coatings were then applied on the surface of the porous substrates to produce ultrafiltration membranes. The SPS membranes possess a distinct porous microstructure, where the porosity is mostly defined by the presence of unmelted feedstock particles retained within the matrix. The unmelted particles in the ultrafiltration membranes were deposited in the form of agglomerates of nanosized TiO2 particles found in the feedstock suspension. Therefore, the average pore size could be linked to the particle size of the original feedstock powder. The UF membranes exhibited a relatively high pure water flux, where the water flux decreased with increasing the thickness of the membranes. Furthermore, the SPS TiO2 membranes demonstrate photocatalytic properties under UV and visible light conditions due to the generation of oxygen vacancies in the lattice under SPS process conditions. Additionally, the feasibility of tailoring the level of porosity in the structure of SPS coatings as a potential approach to control the porosity and permeability of the membranes was investigated by employing a dual suspension injection method. Two water-based suspensions of TiO2 with different particle sizes were introduced into the plasma jet using injectors placed at two different radial distances from the plasma torch exit. The variation in the SPS parameters led to changes in the proportion of unmelted particles in the coatings originating from the injector located farther from the torch. As a result, the total porosity in the dual-injected coatings ranged from approximately 16% to about 36%. The proportion of the unmelted particles served as a reference for the fine porosity since the gaps among those particles were identified as the origin of nanosized pores. Among the spray parameters examined, it was found that the suspension feed rate and the distance between the injectors had the most notable impact on increasing the porosity levels in the coatings. Furthermore, based on previous work, microfiltration TiO2 membranes with an average pore size of around 200 nm were deposited using the SPS process. The ultrafiltration and microfiltration TiO2 membranes were characterized to assess their particle rejection ratio, where the membranes with high thickness and narrow pore size distribution showed a superior rejection efficiency. The influence of narrowing the pore size distribution of the membrane on the separation efficiency was confirmed after compacting the large pores in the membrane structure with the nanosized particles of the pristine SPS feedstock. Also, the membranes demonstrated self-cleaning properties under visible light.