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 . 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 . 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.