Catalyst Recovery, Regeneration and Reuse during Large-Scale Disinfection of Water Using Photocatalysis

Abstract

The deployment of photocatalysis for remediation of water has not yet been realized, although laboratory-scale studies have demonstrated promise. Accomplishing this requires the development of photocatalysis as a process, including studying its efficiencies in remedying water when high volumes of water are processed, and addressing the recovery, possible regeneration and reuse of the photocatalysts. To that end, this work is aimed at demonstrating the use of a custom-built mobile platform for disinfecting large quantities of water. The benchtop platform built is capable of processing 15.14 L (4 gallons) per minute of water, with possibility for further scale-up. Preliminary studies on the catalyst recovery, regeneration and reuse via gravity-assisted settling, centrifugation and air plasma treatment indicated that 77% of Aeroxide® P25 titania (TiO2) nanoparticle and 57% of porous TiO2 nanowire photocatalysts could be recovered and regenerated for further use. Overall, this study indicated that process improvements, including increasing the kinetics of the photocatalysis, and optimization of the efficacies of the catalyst recovery and regeneration processes will make it useful for water remediation on any scale. More importantly, the portable and flexible nature of the benchtop photocatalysis system makes it amenable for use in conjunction with existing technologies for remedying large quantities of water.

Real-Time Deduction of Mechanisms and Kinetics Underlying Photocatalytic Water Disinfection: Cell Motility and Particle Tracking

“…coli cells to UV-A light under constant magnetic stirring. This procedure was previously reported in detail by our group. , The resultant kinetics of E. coli inactivation are indicated in Figure b (for comparison, the kinetics of E.…”

“…Advanced oxidation processes (AOPs), such as heterogeneous and homogeneous photocatalysis, have the potential to complement the current methods employed for treating water in the removal of these emerging contaminants and alleviate some of the water stresses. , Heterogeneous photocatalysis typically involves the ultraviolet (UV) and/or visible-light excitation of solid semiconductor photocatalysts added to the contaminated water that needs to be purified. , The excitation of the photocatalysts leads to the creation of reactive oxygen species (ROS) that oxidize the contaminants . The ROS generated during heterogeneous photocatalysis include hydroxyl radicals (OH), superoxide radical ions (O 2 • – ), and hydrogen peroxide (H 2 O 2 ). ,− The supply of photocatalysts for removing contaminants from water during heterogeneous photocatalysis is accomplished in many ways, including deployment in the form of slurries or suspensions, or by immobilizing them on reactor walls or other neutral catalyst supports. , The primary advantage of heterogeneous photocatalysis over homogeneous photocatalysis is the potential to recover, regenerate, and reuse the photocatalyst …”

“…The literature on heterogeneous photocatalysis primarily focuses on the development and optimization of the photocatalyst material chemistry, with only a minor focus on photocatalysis process development. In particular, a wide variety of heterogeneous photocatalysts has been employed for disinfecting water and inactivating bacteria, fungi, and viral pathogens present in water. ,, Numerous studies have also elucidated the factors controlling the kinetics of photocatalytic disinfection in bacterial cultures, both at laboratory scales (comprising of systems of tens of milliliters) − and at pilot-scale deployments (of up to 10 L). , A noteworthy conclusion from these studies is that the disinfection kinetics during heterogeneous photocatalysis are relatively slow when compared to homogeneous AOPs, with disinfection requiring times on the order of a few hours or more. ,, …”

“…In addition to the size, shape, and chemistry of the photocatalyst, the method of deployment of the photocatalyst and the mode of interaction between the photocatalysts and the microorganism play a major role in the determination of the disinfection kinetics. For example, TiO 2 nanoparticles suspended in water (e.g., Aeroxide P25, diameter: ∼25 nm) likely inactivate bacterial cells faster compared to TiO 2 nanowire suspensions (e.g., anatase nanowires, diameters: ∼100 nm and lengths: ∼2–10 μm, Afreen et al) owing to the difference in their sizes and shapes. , It is generally postulated that multiple TiO 2 nanoparticles adsorb on the microorganism (e.g., Escherichia coli) surfaces. ,, This constant contact mode of interaction between the photocatalyst and the microorganisms may lead to disinfection kinetics that are relatively faster than those observed with TiO 2 nanowire photocatalysts. This is unlike the case of TiO 2 nanowires, whose lengths may be similar to (or larger than) the dimensions of the microorganisms .…”

“…For this study, E. coli served as the model bacteria, with TiO 2 nanowires (produced using the solvo-plasma method ,,, ) serving as the photocatalyst and ultraviolet-A (UV-A) lamps providing for the photoactivation of the nanowires. The micron-scale lengths of the TiO 2 nanowires employed as photocatalysts allowed for observing both E.…”