English abstract
Textile industries consume large amounts of water and generate large quantities of
wastewater daily. Globally, around 700,000 tons of dyes are synthesized, and about
280,000 tons of textile dyes are discharged into water bodies annually. Textile
wastewater contains several complex and hazardous materials. Therefore, new, cost effective, and environmentally friendly technologies are needed to treat textile
wastewater effluents. This study explores using zinc oxide nanorods (ZnO NRs) as a
supported catalyst in the photocatalytic degradation of methylene blue dye as a model
contaminant in textile wastewater under natural sunlight irradiation. ZnO NRs catalysts
were fabricated on glass substrates using the microwave-assisted hydrothermal method.
The characteristics of the synthesized ZnO NRs were investigated by using X-ray
diffraction (XRD), photoluminescence (PL), scanning electron microscopy (SEM), and
UV-visible spectroscopy. The XRD and SEM results confirmed that the synthesized
ZnO NRs have a hexagonal Wurtzite crystal structure.
Moreover, the water contact angle of the ZnO NRs surface was conducted to determine
the surface wettability. The highest surface wettability of ZnO NRs and physical
adsorption was achieved at pH 8. Photocatalytic degradation was conducted with
methylene blue (MB) as a contaminant under natural sunlight in a batch system followed
by a continuous flow reactor. In the batch reactor, three different concentrations of MB
were examined under simulated and natural sunlight. MB degradation reached 100%,
48%, and 52% of MB under simulated sunlight and 100 %, 83 %, and 62 % under
natural sunlight for 10, 50, and 100 ppm, respectively, after 3 hours of irradiation. The
continuous flow reactor showed higher degradation efficiency, especially at a low flow
rate. This is because of the high residence that leads to long time of interaction between
MB molecules and the surface of ZnO NRs catalyst. Methylene blue degradation was 80
%, 67 %, 58 %, and 46 % at flow rates of 1.5, 2.5, 4.5, and 6 ml/min, respectively. MB's
photocatalytic degradation followed a pseudo-first-order Langmuir-Hinshelwood kinetic
model. Furthermore, five single-channel continuous flow reactors were connected in
series to emulate the properties of a multi-channel flow reactor, and the photocatalytic
efficiency reached 26 %, 44 %, 63 %, 85 %, and 90 % after passing through the first,
second, third, fourth and fifth flow channel, respectively. The concentration of the final
test solution was reduced from 100 ppm to 11 ppm after passing the five single-channel
serpentine flow reactors. Regarding the reusability test of the used ZnO NRs, the
degradation efficiency of MB decreased from 80.1% in the first cycle to 45 % after five
consecutive cycles due to the accumulation of the MB molecules in the surface of the
ZnO NRs.
The used ZnO NRs were cleaned by dipping them in DI for 30 minutes under natural
sunlight to improve the reusability of ZnO NRs for MB removal. The cleaning of used
ZnO NRs for 30 minutes achieved a stable catalyst surface with good experimental
repeatability. SEM and UV-visible characterization showed that the ZnO NRs structures
and the optical absorption were almost the same as the unused ZnO NRs. A single channel photocatalytic reactor was developed and utilized to degrade natural textile
wastewater (TWW). The removal efficiency of color in the sample was measured using
UV-visible spectroscopy. Discoloration (color degradation) reached 96 % after 240
minutes of the photocatalytic process. The total organic carbon (TOC) was used to
investigate the mineralization of TWW. It was found that the total organic content in
TWW decreased to 49 % after 7 hours of sunlight irradiation using ZnO NRs
