As the need to use green chemistry routes increases, environmentally friendly catalytic processes are a demand. One of the most important and abundant naturally occurring catalysts is chlorophyll. Chlorophyll is the first recognized catalyst; it is a reducing agent due to its electron-rich structure. The effects of spinach on the preparation of zinc oxide nanoparticles and the photocatalytic degradation of methyl orange and paraquat in sunlight and under a UV lamp and photocatalytic degradation in sunlight were studied. Different parameters of the catalytic preparation process and photocatalytic degradation process were studied. Characterization of differently prepared samples was carried out using different analytical techniques such as XRD, SEM, and EDX and finally the photocatalytic activity towards decomposition of methyl orange and paraquat.
Nanostructure materials have attracted increasing attention during the past few decades due to their marvelous properties and a wide range of applications such as catalysis, electronics, optics, and environmental and biotechnology applications [
In general, many types of research discuss the synthesis of nanostructured materials and zinc oxide nanoparticles especially that these methods include chemical precipitation, sol-gel synthesis, hydrothermal reaction, and microwave. Many other chemical and physical attempts were carried out to improve and control the synthesis of metal oxide nanoparticles [
Green route synthesis is one of the most promising techniques for the synthesis of nanostructured materials being simple, environmentally safe (mild reaction conditions and no need for toxic chemicals) and inexpensive and can be produced in a large-scale process [
Zinc powder (Ranbaxy Chemicals, >5
Spinach (100 g) bought from a local market was washed with deionized water and patted dry with wipes. The fresh spinach leaves were heated using 200 ml of deionized water and filtered using a filter paper. The filtrate was then used to prepare zinc oxide nanoparticles.
Zinc metal (powder) (0.1 g) was added to 40 ml deionized (DI) water, transferred into a stainless steel Teflon-lined metallic bomb of 100 ml capacity, and sealed under inert conditions. Then, it was placed in the microwave (Milestone company (high-performance microwave digestion system) ETHOS One, SN: 1301 0243) at 800 W/180°C for 30 minutes. The furnace allowed to cool after the desired time, and the resulting suspension was centrifuged to retrieve the product (S1), washed, and then finally vacuum dried for few hours. Zinc metal (powder) (0.2 g) was added to 80 ml aqueous fresh spinach extract transferred into a stainless steel Teflon-lined metallic bomb of 100 ml capacity and sealed under inert conditions. The closed chamber was then placed in a preheated box furnace, and the mixture was heated slowly (2°C/min) to 180°C and maintained at this temperature for 72 hours. The furnace allowed to cool after the desired time, and the resulting suspension was centrifuged to retrieve the product (SA1), washed, and then finally vacuum dried for few hours.
Zinc metal (0.2 gram) was added to 40 ml aqueous fresh spinach extract transferred into a stainless steel Teflon-lined metallic bomb of 100 ml capacity and sealed under inert conditions. Then, it was placed in the microwave (Milestone company (high-performance microwave digestion system) ETHOS One, SN: 1301 0243) at 800 W/180°C for 30 minutes (SA2), 60 minutes (SA3), 90 minutes (SA4), and 120 minutes (SA5). The furnace was allowed to cool after the desired time, and the resulting suspension was centrifuged to retrieve the product, washed, and then finally vacuum dried for few hours Scheme
Preparation equipment of ZnO catalyst both with conventional heating technique and microwave-assisted technique.
The photocatalytic degradation of methyl orange (MO) was performed in a Pyrex beaker using as-synthesized ZnO nanoparticles as a photocatalyst under UV illumination for various time intervals and sunlight at 48°C. MO dye (10 ppm) solution was prepared in 100 ml DI water and mixed with 0.3 g of different kinds of synthesized ZnO, in which nanoparticles powder was added to it. The resulting suspension was equilibrated by stirring for 30 min to stabilize the absorption of MO dye over the surface of the photocatalyst, that is, ZnO nanoparticles, before exposing to the light. The photocatalytic decomposition of MO was examined by measuring the absorbance at regular time intervals by using the ultraviolet and visible (UV-Vis) spectrophotometer wavelength at 465 nm. Analytical samples were taken from the reaction suspension at regular time intervals for 10 minutes and were then analyzed for their absorption using UV-Vis spectrophotometer.
Photocatalytic degradation experiments were carried in a reactor system that has been already described. It has a Pyrex glass tube reactor (200 ml) that can be irradiated with sunlight at 48°C temperature. Since the concentration of organic pollutants is a very important parameter in the wastewater treatment processes, we have studied the effect of paraquat initial concentration on the reaction rate to develop a kinetic model for the photocatalytic degradation of paraquat. Several experiments were carried out with paraquat aqueous solutions with different initial concentration (10 and 50–100 ppm), and different kinds of synthesized ZnO nanoparticles powder was added to it. For each of the experiments, 100 ml of a paraquat solution was placed inside the glass reactor and mixed with 0.3 g of ZnO. This slurry was agitated with a magnetic stirrer. Samples for analysis were taken at different times to monitor the reaction. Analytical samples were taken from the reaction suspension at regular time intervals for 60 min and measuring the absorbance at regular time intervals by using the UV-Vis spectrophotometer wavelength at 230 nm wavelength [
Spinach is considered as an amazing green chemistry candidate, which indeed contains chlorophyll. Chlorophyll is considered as the most important naturally occurring photocatalyst on earth. The supposed mechanism of the preparation of zinc oxide could be started with the reducing effect of chlorophyll a (Chl a) (Figure
Chlorophyll a (Chl a).
A mechanism proposed for the catalytic preparation of ZnO catalyst.
The SEM images were shown in Figure
Scanning electron microscope spectroscopy images (S1: zero sample, SA1: conventional heating; SA2, SA3, SA4, and SA5 samples with assisted microwave technique).
XRD patterns of ZnO nanoparticles.
Study of standard data JCPDS 76-0704 confirmed that the synthesized materials are hexagonal ZnO phase (wurtzite structure). The pattern was indexed with hexagonal unit cell structure with P63mc, and the lattice parameters are given in Table
The variation of lattice parameters with a variation of experimental conditions.
Sample | Volume (A°)3 | a (A°) | b (A°) | c (A°) | c/a (A°) | Crystalline size (A°) |
---|---|---|---|---|---|---|
S1 | 47.65 | 3.2494 (14) | 3.2494 (14) | 5.211 (3) | 1.4914 | 270 (8) |
SA1 | 47.68 | 3.2500 (5) | 3.2500 (5) | 5.2056 (9) | 1.6017 | 488 (12) |
SA2 | 47.73 | 3.2517 (9) | 3.2517 (9) | 5.2125 (18) | 1.6030 | 221.1 (7) |
SA3 | 47.63 | 3.2494 (17) | 3.2494 (17) | 5.208 (3) | 1.6028 | 224.97 (6) |
SA4 | 47.51 | 3.2473 (14) | 3.2473 (14) | 5.202 (2) | 1.6020 | 600 (7) |
SA5 | 47.57 | 3.2481 (14) | 3.2481 (14) | 5.207 (2) | 1.6030 | 234 (2) |
Variation of % of photodegradation versus time interval for the photodegradation of MO with a different dosage.
Variation of % of photodegradation versus time for various different ZnO catalyst (0.3 g) under sunlight.
Variation of % of photodegradation versus time for various catalyst of ZnO (0.3 g) under a UV lamp.
Variation of % of photodegradation versus for time of sample SA1 (0.3 g) under sunlight.
TOC values of SA1 versus time comparison photodegradation of paraquat (100 ppm) under sunlight.
The preparation of zinc oxide nanocatalyst using a green chemistry route as a catalyst (spinach extract) and also the catalytic activity evaluation of the prepared samples was studied. It is clearly concluded that the obtained ZnO nanocatalysts represent an excellent candidate with the commercial ZnO catalyst with high catalytic activity in the degradation of the methyl orange dye (MO) as well as paraquat herbicide. The produced ZnO catalyst samples were found to depend on the preparation parameters in their shape, size, and consequently catalytic activity. Among the obtained samples prepared with different heating techniques (traditional and microwave-assisted), the most promising sample is the one prepared under traditional heating technique (SA1). This behavior could be devoted to that the fact that the rate of heating in the traditional way is much slower than that in a microwave-assisted way. The slower rate of heating, in this case, provides the needed time for the produced ZnO catalyst to get the preferred crystalline orientation and in this case for the rod structure to be elongated with small diameter (150 nm); this increases the surface area and consequently increases the catalytic activity. All the used characterization and evaluation tools support the conclusion that chlorophyll is an excellent catalyst which acts under mild conditions and is favored economically.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was funded by the King Abdulaziz City for Science and Technology (KACST), under Grant no. 177-34. The authors, therefore, acknowledge with thanks the KASCT for technical and financial support.