ZnO nanostructure integrated microfluidic system for photocatalytic water purification

The impact on environment and on health of different pollutants, especially chemical pollutants, is becoming critical due to their drastic consequences on our main vital resource: water. In recent years, extensive efforts of fundamental research and developing practical processes have been devoted to the polluted water treatment. The semiconductor-based photocatalytic process has shown a great potential as an environmental-friendly and sustainable treatment technology due to its low-cost, and its ability to decompose the wide spectrum of contaminants in wastewater at room temperature without residual deposits requiring further post-treatment [1,2]. With high surface/volume ratio, the nanostructured semiconductor shows enhanced photocatalytic efficiency leading to very promising advances in drinking water and wastewater treatment [3,4].

Among the photocatalytic materials, nanostructured ZnO is a promising candidate for its easy-controllable synthesis, its chemical and thermal stability. On the other hand, microfluidic systems can overcome the main limit of the mass transfer during the photo-degradation process of polluted water due to the shorter diffusion lengths within microscale chambers [5]. In this work, we present a high efficiency microfluidic system decorated inside by ZnO nanostructure as a micro-reactor for threes dyes (MB, MO & AR14) as well as for VOCs-polluted water purification.

ZnO nanowire array (NWA) samples have been prepared using two-step hydrothermal method as descripted in our previous work [6]. The SEM images show a quite homogenous NWA with the c-axis preferential growth direction of Wurtzite structure according to the X-ray diffractogram (Fig.1). Fig.2 shows the photodegradation effect of three dyes, it is worth noting that the degradation time is quite long in static mode: after ~3h UV irradiation, the degradation rate (X = (A₀-A/A₀) *100%) reach 86%, 49% and 93% for MB, MO & AR14 respectively. However, by using the micro-reactor with integrated ZnO NWA (Fig. 3), the same degradation efficiency was reached after only 6-7 min photocatalysis process (Fig. 4).

In order to confirm the photocatalysis efficiency of the ZnO-based microfluidics system, an even smaller micro-reactor including a micro-pillar array has been realized. This micro-reactor also co-integrates in-situ grown ZnO nanostructures. The same initial concentration dye-polluted water needs only one-pass (with 250 µL/min flow-rate) to reach quasi total degradation (not shown in the figure). This microfluidic system has also been used to test the VOCs-polluted water purification efficiency. The contaminated water sample contains mixture of six VOC pollutants: Benzene, Toluene, Ethylbenzene and m-p-o Xylenes (BTEX) diluted in water at 10 ppm concentration of each. Fig 6 shows superimposition of two chromatograms before and after one-pass degradation (with 50 µL/min flow-rate). This enables in a selective manner to prove that all VOCs have been dropped below the threshold concentration level of 1 ppm of the maximum allowed contamination level.