- Open Access
Synthesis of eco-friendly ZnO-based heterophotocatalysts with enhanced properties under visible light in the degradation of organic pollutants
Environmental Systems Research volume 11, Article number: 25 (2022)
Heterogeneous photocatalysts have been widely used for the removal of various organic pollutants from wastewater. The main challenge so far resides in the sustainability of the process, with regard to the synthesis and the application under visible light. In this study the precipitated materials from the Moringa oleifera seed (MO), groundnut shells (GS) and apatite (A) agrowastes were functionalized with zinc oxide (ZnO) and silver (Ag) solution, to produce a novel bioheterophotocatalysts. Various analytical techniques such as scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), photoluminescence (PL) and X-ray diffraction (XRD) were used for the characterization of the novel photocatalysts. It was proven that agrowastes can also enhance the photocatalytic activity of a ZnO-based photocatalyst as pure metals. The combination of MO/GS/A/ZnO/Ag in a 1:1:1 ratio resulted in a lower band gap of 1.59 eV, as compared to the band gap of 2.96 eV for ZnO/Ag. These photocatalysts' efficiency was also tested on the photodegradation of polycyclic aromatic hydrocarbon (PAHs) derived from coal leaching in various water sources such as acidic mine drainage, alkaline mine drainage and sewage wastewater. From MO/GS/A/ZnO/Ag, the removal efficiency was found to be 69.59%, 61.07% and 61.68%, compared to 52.62%, 37.96 and 44.30% using ZnO/Ag in acidic mine drainage, alkaline mine drainage and sewage wastewater for 60 min under solar irradiation.
Wastewater's impact on the environment is among the main topics studied in recent years due to the continuous release of organic pollutants from various activities such as oil spills, coking processes and gasification (Gaurav et al. 2021). Most of these pollutants end up in various water sources with different physiochemical properties. The pH of these waters can deeply affect the chemistry of the water in the presence of pollutants derived from the aforementioned petroleum-related activities (Zipper et al. 2018). Several studies have been conducted using various metal oxides; however, titanium dioxide (TiO2) and zinc oxide (ZnO) have been widely used as either semiconductors or nanoparticles in various applications and especially in wastewater treatment (Nguyen et al. 2020; Rani and Shanker 2019).
ZnO has been one of the most exploited metal oxide and n-type semiconductors that have been prioritized due to its multifunctional in various applications (Thema et al. 2015). Metal oxide can further be synthesized by precipitation, hydrolysis, pyrolysis, hydrothermal, electrodeposition, chemical methods and sol–gel processes (Davar et al. 2015; Pandiyarasan et al. 2014). However, the sol–gel method has been commonly used due to its adaptability, simplicity, reliability and cost-effectiveness (Ganesan et al. 2020; Fan et al. 2020). By synthesizing metal oxides their size and morphology can be influenced by manipulating the pH, varying the annealing temperature, the rate of the reaction, and the additives in the medium to obtain a readily active metal oxide (Davar et al. 2015). The metal oxides can either be synthesised physically, chemically or biologically. However, the physical and chemical methods for synthesizing metal oxide have resulted to be costly and releasing hazards. Therefore, a greener approach has been the most promising method to mitigate these issues since less attention has been given to them (Thovhogi et al. 2015).
Moringa oleifera seed has been used for medicine (Milla et al. 2021) and its application in nanotechnology due to its enrichments in minerals such as calcium, potassium, zinc, magnesium, iron, phosphorous and copper have made it effective in synthesizing a metal oxide (El-Massry et al. 2013; Tovar et al. 2020; Jadhav et al. 2022). Its antioxidants like quercetin and chlorogenic acids such as flavonoids, phenolics, astragalin, anthocyanins, cinnamates, and carotenoids as well as the presence of terpenes, quinines, saponins, alkaloids, proteins, tannins, and vitamin C have a benefit on its uses (Gopalakrishnan et al. 2016). In addition, it has also been proven to be a good reducing agent, capping and stabilizing agent for metals (Aisida et al. 2019).
Cow bones that are predominant in hydroxyapatite (referred to as Apatite) have been used for wastewater treatment (Muretta et al. 2022). The adsorptive properties of bone char derived from cow bones have been tested in various wastewater treatments and proven to be effective (Muretta et al. 2022). The greener application to produce heterogeneous catalysis and its calcium, phosphate, and filler have been made to improve the conventional method (Hart and Aliu 2022). Their contribution to producing high ion exchange, good structural flexibility and also as a good potential renewable source has been suggested to be used in various applications (Iriarte-Velasco et al. 2018).
Groundnut shells have been used for wastewater treatment however, it has been reported that calcinating the groundnut shells can improve the effectiveness of the adsorption capacity (Bedane et al. 2019). Various functional groups such as alkyl, carboxyl, amide, hydroxyl, carbonyl, and alkyne are widely responsible in groundnut shells for the elimination of organic pollutants (Li et al. 2019).
Groundnut shells have been used to synthesize ZnO metal oxide to assist in the photodegradation of organic pollutants (Kamaraj et al. 2021; Dhiman and Sharma 2019; Wu et al. 2019). These agrowastes can be used to synthesize ZnO and therefore improve the ability to produce electrons from their valence bands to the conduction bands while the electrons are excited is indicative of the band gap that has made them useful in a wide range of various applications, especially in wastewater treatment (Selvaraj et al. 2022).
Figure 1 is a schematic representation of the process where the photons, with energy hv, are radiated on the material, creating a band gap that is lower than the excitation energy to ensure the delay of the electron recombination.
Conventional methods like coagulation, flocculation, sedimentation, adsorption and membrane processes have been widely used for the removal of organic pollutants from wastewater (Smol and Włodarczyk-Makuła 2017). The limitations of these conventional methods such as production costs and the release of by-products such as carbon dioxide and methane gas have made them less applicable. Photocatalysts have been proven to be the best candidates for the degradation of organic pollutants. Their performance can be further improved by introducing a metal which contributes to a delay of electron recombination holes as well as lowering the band gap to obtain a spontaneous photodegradation (Meenakshi and Sivasamy 2022; Selvaraj et al. 2022).
Since metals have been proven to delay the electron recombination hole and narrow the bandgap, Moringa oleifera seed (MO), groundnut shells (GS) and apatite (A) agrowastes have been introduced to replace and enhance the photocatalytic activity of ZnO. Biological methods for synthesizing ZnO metal oxide using MO, GS and A benefit from simplicity, eco-friendliness and extended antimicrobial activity. The presence of metabolites and phytochemicals in MO, GS and A such as terpenoids, alkaloids, flavonoids, proteins, peptides, and tannins increased the biosynthetic manufacturing of ZnO.
Material and methods
In this study, chemicals were purchased from Rochelle Chemicals & Lab Equipment, including zinc acetate dihydrate (Zn(Ac)2.2H2O, 98%) and silver nitrate (AgNO3, 99.9%). Acidic mine drainage was collected from Witbank, together with alkaline mine drainage from Middleburg, sewage wastewater from the Cape Town municipality and coal tar from Newcastle in KwaZulu-Natal, all in South Africa.
Synthesis of a zinc oxide semiconductor
A mass of 3 g Zn(Ac)2.2H2O was powdered, using a mortar and pestle and it was calcined for 4 h at 350 °C in a furnace with a heating rate of 1 °C/min.
Preparation of ZnO/Ag and MO/GS/A/ZnO/Ag heterogeneous photocatalysts
Biomasses such as Moringa oleifera seed, groundnut shells and apatite derived from cow bones were sourced at a nearby market in Cape Town, South Africa. Each biomass was grounded using a grinder to obtain particles at a size of 100 > 150 µm and was washed with deionised water more than 3 times using a centrifuge to remove any impurities. The collected materials were then dried for 24 h at a temperature of 60 °C using an oven.
Heterogeneous photocatalysts were prepared following the sol–gel synthesis with modification (Mukwevho et al. 2020). A mass of 0.2 g of Zn(C2H3O2)2 2H2O and 0.1 g of Ag were mixed into a volume of 50 mL of deionised water. The solution of ZnO and Ag was stirred to ensure a homogenous mixture and 1 g each of the selected agrowaste MO, GS and A in the ratio (1:1:1) wt.% was added to the mixture. The prepared volume of 2M of NaOH was used to adjust the pH until a pH of 12 was obtained. The overall mixture was stirred at 250 rpm for 2 h. After completing this process, it was centrifuged for 20 min at 6000 rpm whereby the precipitants were obtained, and the material was dried in an oven at a temperature of 80 °C for 24 h.
Procedure for the photocatalyst degradation of PAHs in various water sources using MO/GS/A/ZnO/Ag and ZnO/Ag
A mass of 5 g of coal tar containing a PAH concentration of 2916.47 mg/L was analyzed using the Gas Chromatography Thermo Scientific (TSQ 8000) (Cape Town, South Africa) Triple Quadrupole MS technique.
The prepared MO/GS/A/ZnO/Ag and ZnO/Ag were used for the photodegradation of PAHs in a 5 g of coal tar that was leached into various water sources at a concentration of 1.8125 mg/L in sewage wastewater, 19.2553 mg/L in acidic and 1.6154 mg/L in alkaline mine drainage in a 300 mL leachate for 8 weeks respectively.
The mass of 2.5 g of MO/GS/A/ZnO/Ag or ZnO/Ag (Zhao et al. 2020) was considerable in the leachate of 300 mL under solar irradiation in 20 min, 40 min and 60 min under solar irradiation. Therefore, Eqs. 1–3 are the reactions that may occur during the photodegradation of PAHs in the sewage wastewater, acidic and alkaline mine drainage using MO/GS/ZnO/Ag and ZnO/Ag photocatalysts PAHs in water sources.
Although various scavengers are available when adding a catalyst to a reaction, hydroxyl and superoxide are widely recognized as effective oxidizing agents for organic pollutants. However, hydroxyl is produced with or without a catalyst, even in the spilling of water, and it is also highly reactive with organic pollutants (Nguyen et al. 2020). The photodegradation of PAHs is dependent on various parameters such as the physiochemistry of the water (pH, dissolved organic carbon, ionic strength, temperature (Gutierrez-Urbano et al. 2021)), the properties of the photocatalyst (narrow band-gap, delay of the electron recombination hole (Guo et al. 2022)) and solar irradiation (Kaur et al. 2020; Li et al. 2020). Also, the absorption experiment was also conducted in the absence of solar irradiation which is observed in Tables 5, 6, 7 and 8. It was found that the wavelength of 360 nm to 420 nm was responsible for the degradation of various PAHs derived from coal tar.
The photodegradation (%) of PAHs in sewage wastewater, acidic and alkaline mine drainage at room temperature by MO/GS/A/ZnO/Ag or ZnO/Ag was obtained using the Eq. 4.
The initial and residual concentrations are given a symbol Ci and Ct that is related to the contact time (t) between the pollutants and the catalyst under solar irradiation.
Results and discussions
UV–Vis absorption spectroscopy
UV–Vis absorption spectrometry (Perkin Elmer UV–Vis lambda 25 spectrometer) was used to determine the optical absorption spectra of the synthesized photocatalysts MO/GS/A/Ag/ZnO to be 376 nm, followed by 269 nm of ZnO/Ag and 339 nm of ZnO respectively The effective of the optical absorption spectrum was to provide the structure of the synthesized material.
The optical band gap value of the synthesized material can be obtained from the optical absorption spectra by extrapolating the straight line of \(\left(hv \,\alpha \right)\) the plot versus the energy \(\left(hv\right)\), based on Eqs. 5–7.
The incorporation of MO/GS/A to synthesize complex heterogeneous photocatalysts (MO/GS/A/ZnO/Ag) resulted in a narrow bandgap and enhanced the absorbance in the visible region as compared to the parent semiconductor (ZnO).
In this study, various agrowastes were synthesized to produce various heterogeneous photocatalysts shown in Table 1.
The study found that the application of a biological method in combination with various agrowastes for the synthesis of heterogeneous photocatalysts contributed to narrowing the band gap in agreement with literature studies (Yu et al. 2016; Kumar and Luxmi 2021).
By incorporating MO/GS/A agrowastes and Ag metal into ZnO, the absorption spectra of the parental ZnO (376 nm) reduces to MO/GS/A/ZnO/Ag (269 nm) and further to ZnO/Ag (339 nm).
The addition of these agrowastes MO, GS, A and metal such as Ag into ZnO have resulted to reduce the bandgap of the parental ZnO (3.35 eV) to ZnO/MO (2.74 eV), ZnO/GS (2.51 eV), ZnO/A (2.15 eV) and ZnO/Ag (2.96 eV). From these agrowastes it occurs that A agrowastes resulted to narrow the bandgap of ZnO. Beside these agrowastes influence to reduce the bandgap, their impurities also contribute to the narrower of the bandgap (Crowley et al. 2016).
Energy-dispersive X-ray spectroscopy
Energy-dispersive X-ray spectroscopy (EDS) (Nova NanoSEM using an Oxford X-max 20mm2 detector and INCA software) was used to determine the elemental composition of MO, GS and A presented in Fig. 2. The presence of the oxygen element in these materials and the synthesized ZnO from MO, GS, A and Ag will therefore best explain the delay of the electron recombination holes (Moussawi and Patra 2016). Figure 2e MO/GS/A/ZnO/Ag shows a higher oxygen concentration as compared to ZnO/Ag. In the raw material, the sharp peak in Fig. 2a MO is shown to have a high oxygen content, followed by Fig. 2c A and Fig. 2b GS.
The oxygen content in the raw material and the synthesized material can best be explained as obtaining a delay in the electrons recombination hole and that can be observed in the photoluminescence (PL).
Photoluminescence was used and recorded on a PerkinElmer LS55 Luminescence Spectrometer. The excitation wavelength (ʎ) of 310 nm for the raw materials (MO, GS and A) and 300 nm for the synthesized materials MO/GS/A/ZnO/Ag, MO/ZnO/Ag, GS/ZnO/Ag, A/ZnO/Ag and (ZnO/Ag) were observed. Figure 4 shows the PL emission spectra for both the raw materials and the synthesized materials at room temperature.
As observed in Fig. 3a MO provided a broader shoulder sharp peak with a higher PL intensity followed by GS and A. The following synthesized materials MO/ZnO/Ag, GS/ZnO/Ag and A/ZnO/Ag indicated that the presence of A provided a lower PL intensity, followed by GS and MO which resulted in a higher photoactivity. According to Ji et al. (2020), the doping of metals was found to reduce the PL intensity and increase the photocatalytic activity, as shown in Fig. 4: MO/GS/A/ZnO/Ag as compared to ZnO/Ag. The incorporation of agrowastes combined resulted in a decrease in PL intensity (Komaraiah et al. 2019).
It also shows that a higher oxygen content in MO/GS/A/ZnO/Ag is also responsible to influence the electron recombination. This also causes a delay in the recombination of the electrons (Park et al. 2003). This mostly occurs when the electrons are in contact with the material and the oxygen in that material happens to trap the produced electrons from the visible light (solar irradiation) (He et al. 2019).
Scanning electron microscope
A scanning electron microscope NovaNano SEM and the Tescan MIRA3 RISE SEM was used to determine the structure and the morphology of the material presented in Fig. 5. MO showed the plant cells that constituted fairy pores and GS showed the fibres (stems), whereas A shows a smooth structure derived from lipid compounds known as phospholipids. The image of the heterogeneous photocatalyst MO/GS/A/ZnO/Ag shows tissue cells (stem plant) and ZnO/Ag shows nanocrystals.
X-ray diffraction (XRD) Powder XRD measurements were recorded on an Analytical XPERT-PRO diffractometer, using Ni filtered CuKα radiation (λ = 1.5406 Å) at 45 kV/40 mA which was used to determine the crystal structure of the materials. The diffraction measurements in Fig. 6 were collected at room temperature in a Bragg–Brentano geometry. The XRD of MO/GS/A/ZnO/Ag and ZnO/Ag did not completely corroborate with the hexagonal structure of the raw reference pattern: ZnO, 00-036-1451 and Ag, 00-004-0783. Since the studied ZnO and Ag were used in combination with the biomasses, the impurities could have interfered with or influenced the diffraction peak. The sharp diffraction peaks were observed at 2θ values 32, 33, 36, 48, 56, 57, 63 and 68 degrees. These peaks are indexed as (100), (002), (101), (102), (110), (103) and (112), and diffraction lattice planes respectively which confirm the hexagonal wurtzite structure for the synthesized materials (MO/GS/A/ZnO/A and ZnO/Ag).
The elemental composition of the various biomasses used in the synthesis of photocatalysts using ICP-MS
In this study, the elementary analysis of the metal content in MO, GS and A using Thermo ICap6300 ICP-AES is represented in Table 2.
This study agrees with Yu et al. (2016) have reported, the doping of various elements narrowed the band gap. Fu et al. (2018) have reported, earth metals are effective as compared to noble metals to enhance photocatalytic activity. Various studies (Selvaraj et al. 2022; Boruah et al. 2019; Uma et al. 2022; Naciri et al. 2020) have proven that the dopant of strontium (Sr) which is a rare earth metal, happens to have a high photocatalytic activity which can be determined under the visible spectrum (Márquez-Herrera et al. 2016). In this study, Sr was highly concentrated in apatite followed by groundnut shells and Moringa oleifera. Due to the presence of Sr in apatite, it can confirmed that apatite can provide a higher photocatalytic as compared to MO and GS. In addition, the remaining metal content in these agrowastes also contributed to enhancing the photocatalytic activity of the parental ZnO (Rani and Shanker 2019; Jana et al. 2015) (Table 3).
Photodegradation of PAHs in various water sources
The decomposition of PAHs in coal tar using Gas Chromatography–Mass Spectrometry (GC–MS) with a Thermo Scientific (TSQ 8000) (Cape Town, South Africa) Triple Quadrupole MS is illustrated in Table 4. Naphthalene, phenanthrene, fluoranthene, acenaphthylene and fluorene are the most concentrated PAHs in coal tar at 788 mg/L, 632 mg/L, 395 mg/L and 356 mg/L. From Tables 5, 6 and 7, the photodegradation of PAHs in acidic mine drainage, alkaline mine drainage and sewage wastewater are presented and their removal efficiency are illustrated in Table 8.
From Tables 5, 6 and 7, the photodegradation of PAHs was most favoured in acidic mine drainage followed by sewage wastewater and alkaline mine drainage under solar irradiation over time. This was succeeded according to the physiochemistry of sewage wastewater and alkaline mine drainage that influenced the photodegradation of PAHs (Batchamen Mougnol et al. 2022). The molecular weight PAH of 2 to 3 rings such as naphthalene, anthracene and phenanthrene degraded rapidly from their normal regions over time due to their volatility which able them to be destabilized.
The introduction of MO, GS and A were due to their abundance, availability and eco-friendly. Their ability to narrow the band gap and delay the electron recombination charger carrier highly contributes successfully to photocatalytic degradation. The higher oxygen content in the material happen to traps the electrons and prevents them from recombining (Biroju and Giri 2017). The presence of O2 in the material in water with an OH- happens to form Hydrogen peroxide (H2O2). Therefore, the radical that was responsible for degrading the PAHs derived from coal tar was H2O2.
The modification of the material MO/GS/A/ZnO/Ag was found to provide a low PL intensity which contributed to poorer recombination rates of photo-induced charge carriers as compared to ZnO/Ag (Nagajyothi et al. 2020; Mukwevho et al. 2020; Park et al. 2003). The removal efficiency was better in acidic mine drainage, followed by alkaline mine drainage and sewage wastewater over time under solar irradiation.
Various studies (Sescu et al. 2020; Wang et al. 2021; Haruna et al. 2020; Sanakousar et al. 2022) have confirmed that, doping of metals can potentially enhance the photocatalytic process by achieving a spontaneous photodegradation outcome. Therefore, the content of the metal in these agrowastes was proven to contribute to the photodegradation of PAHs in various water sources.
In this study, the introduction of these MO, GS, and A indicate their ability to produce electrons and trapped them from their valance bands to their conduction bands which will further favour the photodegradation.
The effectiveness of adsorption in a dark was also proven to be effective however the removal efficiency was lesser as compared to using solar irradiation. It was observed that the higher removal efficiency in 60 min was obtained on the sewage wastewater, acidic and alkaline mine drainage using MO/GS/A/ZnOAg while using ZnO/Ag, the higher removal efficiency was obtained in sewage, alkaline and acidic mine drainage.
A study conducted by Tong et al. (2015) reported that the narrow band gap of a photocatalyst resulted in the rapid recombination of electrons and holes which restricted the photocatalytic efficiency. However, this study had contrary findings, in agreement with Guo et al. (2022), that MO/GS/A/ZnO/ZnO provided a delay in the recombination of electron hole-pairs and narrow band gap. Its application in the photodegradation of PAHs in different water sources under solar irradiation drastically improved over time. The application of this heterogeneous photocatalyst generated high photons, also by being able to produce reactive scavengers in comparison to ZnO/Ag. In this study, it was found that agrowastes that contain various metals (including Sr which is reported to have high photocatalytic activity and can be naturally found in a few agrowastes) obtained a higher removal efficiency than metals without agrowastes.
The application of agrowastes was found to be a better candidate as compared to metals in obtaining a higher removal efficiency. The synthesis of complex heterogeneous photocatalysts resulted in narrowing the band gap and its oxygen content resulted in retarding the electron recombination. Free energy in the form of solar irradiation was effective in destabilizing the aromatic ring in PAHs in water sources. In the study hydrogen peroxide was the radical responsible for destabilizing the aromatic rings from the PAHs. Since PAHs were highly concentrated in acidic mine drainage depending on the water physiochemistry like the pH and total organic carbon. Time was also observed to be an important factor in obtaining acceptable photodegradation in various water sources. The heterogeneous photocatalyst MO/GS/A/ZnO/Ag was found to be effective as compared to ZnO/Ag. Their application in wastewater treatment was tested and proven that MO/GS/A/ZnO/Ag was highly effective under solar irradiation, with a removal efficiency of 69.59%, 61.07% and 61.68% as compared to ZnO/Ag with the removal efficiency of 52.62%, 37.96%, and 44.30%. The adsorption of PAHs was also tested in a dark chamber and again MO/GS/A/ZnO/Ag happen to be favoured in comparison to ZnO/Ag. The metals content in MO, GS and A were proven to have a significant outcome on the photocatalytic activity and photodegradation. The abundance, availability, cost-effectiveness, oxygen and metal contents of these agrowastes have made them to be favoured in the study.
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The authors are thankful to the sponsors, the Water Research Commission (WRC, Project 2974) and the North-West University in South Africa.
Open access funding provided by North West University. Water Research Commission (WRC, Project 2974) and the North-West University in South Africa.
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Batchamen Mougnol, J.B., Waanders, F., Ntwampe, S.K.O. et al. Synthesis of eco-friendly ZnO-based heterophotocatalysts with enhanced properties under visible light in the degradation of organic pollutants. Environ Syst Res 11, 25 (2022). https://doi.org/10.1186/s40068-022-00271-7
- Heterogeneous photocatalyst
- Band gap
- Acidic mine drainage
- Alkaline mine drainage