Interactive Fe2O3/porous SiO2 nanospheres for photocatalytic degradation of organic pollutants: Kinetic and mechanistic approach

Synthesis and characterization of Fe3O4@SiO2 -supported metal-organic framework PAEDTC@MIL-101 (Fe) for degradation of chlorpyrifos and diazinon pesticides

In this study, a new core-shell Fe3O4@SiO2/PAEDTC@MIL-101 (Fe) photocatalyst was prepared by sol-gel method and used to degrade diazinon (DZN) and chlorpyrifos (CPS) from aqueous solutions. The characteristics analyzed by various techniques indicate that the core-shell photocatalyst with a specific surface area of 992 m2/g, pore size of 1.35 nm and saturation magnetization of nanocomposite was 12 emu/g has been successfully synthesized and can be separated from the reaction solution by a magnetic field. The maximum efficiencies of DZN (98.8%) and CPS (99.9%) were provided at pH of 5, photocatalyst dosage of 0.6 g/L, pollutant concentration of 25 mg/L, radiation intensity of 15 W, and time of 60 min. The presence of anions such as sulfate, nitrate, bicarbonate, phosphate, and chloride had a negative effect on the performance of the photocatalysis system. Compared to the adsorption and photolysis systems alone, the photocatalytic process based on Fe3O4@SiO2/PAEDTC@MIL-101 (Fe) under two UV and visible light sources showed a high efficiency of 90% in the reaction time of 60 min. The BOD5/COD ratio improved after 50 min to above 0.4 with TOC and COD removal rates >80%. Scavenging tests showed that •OH radical, hole (h+), electron (e-), and O2•- anion were produced in the reaction reactor, and the •OH radical was the dominant species in the degradation of DZN and CPS. The stability tests confirmed the recyclability of the photocatalyst in 360 min of reactions, with a minimum reduction of 7%. Energy consumption for the present system during different reactions was between 15.61 and 25.06 kWh/m3 for DZN degradation and 10-22.87 kWh/m3 for CPS degradation.

Rice husk-based pyrogenic carbonaceous material efficiently promoted peroxymonosulfate activation toward the non-radical pathway for the degradation of pharmaceuticals in water

Pristine pyrogenic carbonaceous material (BRH) obtained from rice husk and modified with FeCl3 (BRH-FeCl3) were prepared and explored as carbocatalysts for the activation of peroxymonosulfate (PMS) to degrade a model pharmaceutical (acetaminophen, ACE) in water. The BRH-FeCl3/PMS system removed the pharmaceutical faster than the BRH/PMS. This is explained because in BRH-FeCl3, compared to BRH, the modification (iron played a role as a structuring agent mainly) increased the average pore diameter and the presence of functional groups such as -COO-, -Si-O-, or oxygen vacancies, which allowed to remove the pollutant through an adsorption process and significant carbocatalytic degradation. BRH-FeCl3 was reusable during four cycles and had a higher efficiency for activating PMS than another inorganic peroxide (peroxydisulfate, PDS). The effects of BRH-FeCl3 and PMS concentrations were evaluated and optimized through an experimental design, maximizing the ACE degradation. In the optimized system, a non-radical pathway (i.e., the action of singlet oxygen, from the interaction of PMS with defects and/or -COO-/-Si-O- moieties on the BRH-FeCl3) was found. The BRH-FeCl3/PMS system generated only one primary degradation product that was more susceptible to biodegradation and less active against living organisms than ACE. Also, the BRH-FeCl3/PMS system induced partial removals of chemical oxygen demand and dissolved organic carbon. Furthermore, the carbocatalytic system eliminated ACE in a wide pH range and in simulated urine, having a low-moderate electric energy consumption, indicating the feasibility of the carbocatalytic process to treat water polluted with pharmaceuticals.

Enhanced degradation of Congo-red dye by Cr3+ doped α-Fe2O3 nano-particles under sunlight and industrial wastewater treatment

Considering the increasing amount of water pollution, nanocomposite advances for the effective elimination of hazardous pollutants are still needed. α-Fe2O3, Cr0·5Fe1·5O3 and CrFeO3 nanoparticles were synthesized via an eco-friendly material synthesis i. e hydrothermal route without using any precipitating agent and were studied to remove congo-red dye using photocatalytic properties. X-ray diffraction (XRD), Transmission electron microscopy (TEM), Field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FESEM), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS) characterizations have been performed to know about the material structure and properties of synthesized samples. High efficiency (95.2%) of degradation was achieved under sunlight using a very low amount of CrFeO3 catalyst (0.2 g/L) at a 6-pH level of dye and was confirmed using UV spectroscopy, TOC (84%), LC-HRMS. Also, the potential to degrade the CR dye was concluded from the high rate of BOD5/COD. The results showed a significant enhancement in the degradation of α-Fe2O3 from 52.3% to 95.2% in a short duration of 15 min by introducing chromium as a dopant. The doping of chromium influenced the major factors responsible for the photocatalytic activity such as the increase in range of absorbance, increased e--h+ pair separation, improvement in the charge transfer process and active site formation which significantly enhanced the process of degradation. We found that the Cr-doped α-Fe2O3 nanomaterial could effectively remove dyes, such as congo-red, from industrial water-waste.

Constructing α-Fe2O3/g-C3N4/SiO2 S-scheme-based heterostructure for photo-Fenton like degradation of rhodamine B dye in aqueous solution

This work successfully fabricated graphitic carbon nitride and magnetically recoverable α-Fe2O3/g-C3N4/SiO2 photo-Fenton catalysts using thermal polycondensation and in situ-simple precursor drying-calcination process, respectively, was examined for model synthetic rhodamine B (RhB) dye in the presence of H2O2 and acidic pH under simulated visible light irradiation. An aqueous suspension of the reaction mixture of dye-containing wastewater was fully degraded and reached 97% of photo-Fenton degradation efficiency within 120 min followed by the production of hydroxyl radical (•OH). The dominant hydroxyl radical position generated surface charge, electrostatic potential distribution, and average local ionization potential, which contributed to the complete mineralization of RhB dye, according to the density functional theory (DFT) calculations. HPLC and GCMS experiments were performed to examine the degradation fragments of RhB and draw a plausible mechanistic pathway which showed that RhB degradation generated a series of N-deethylated products, followed by a one-time ring-opening, which indicated that photosensitization induced a photocatalysis reaction mechanism.

Heterogeneous photocatalytic activation of electrogenerated chlorine for the production of reactive oxygen and chlorine species: A new approach for Bisphenol A degradation in saline wastewater

UV-E-chlorination/hematite nanoparticles (UV/E-Cl/HNs) as a heterogeneous photocatalytic activation of electrogenerated chlorine was assessed for the degradation of bisphenol A (BPA) as a new approach based on the generation of reactive chlorine and oxygen species. The prepared sample was characterized using multiple techniques, such as XRD, FTIR, FESEM, EDS, and BET-BJH. An excellent decontamination efficiency of 99.4% was achieved within 40 min of electrolysis under optimum conditions (pH of 5, HNs dosage 100 mg/L, current density of 20 mA/cm², and NaCl concentration of 50 mM). The HOCl content was reduced more swiftly in the presence of ultraviolet (UV) irradiation and hematite, resulting in the production of oxidative radicals (i.e., ^•OH, Cl^•, and Cl₂^•-). The scavenging experiments also verified the vital role of these radicals in oxidative treatment. The UV/E-Cl/HNs process is readily supplied with hydroxyl radicals through several mechanisms. Bicarbonate ions showed a noticeable inhibitory impact, whereas nitrate and sulfate anions only slightly affected BPA degradation. The HNs were a recoverable and stable catalyst for six cycles. Furthermore, the ECOSAR program predicted that the UV/E-Cl/HNs can be labeled as an environmental-friendly process. Eventually, reasonable degradation pathways were proposed based on the identified by-products through experimental and theoretical approaches.

Photocatalytic removal of organic dye using green synthesized zinc oxide coupled cadmium tungstate nanocomposite under natural solar light irradiation

In this work, zinc oxide coupled cadmium tungstate (ZnO-CT) was prepared as a nano-photocatalyst through a green synthesis route using lemon leaf extract and characterized based on diverse microscopic and spectroscopic techniques. To explore the applicabilties of the prepared nanocomposite (NC), its photocatalytic activity has been investigated against Congo red (CR) dye under natural solar light irradiation conditions. ZnO- CT nano-photocatalyst showcases 97% photocatalytic degradation of the CR after 90 min of natural solar light irradiation with quantum yield of 1.16 × 10^-8 molecules photon^-1. The ZnO-CT NC has shown the enhanced photocatalytic degradation performance against CR when compared to its pristine forms (e.g., ZnO (70%) or CT (44%)). According to the free radical trapping and quenching experiments, the photocatalytic activity of ZnO-CT NC appears to be driven efficiently by superoxide and hydroxyl radicals. The photocatalytic degradation kinetics for CR dye was also studied using the pseudo-first-order, diffusional, and Singh models. The high photocatalytic activity of ZnO-CT NC can be accounted for by the presence of electron-withdrawing functional groups like acids (-COOH) and aldehydes (-CHO) on its surface which helped maintain the prolonged recombination of charge carriers and enhanced stability of ZnO-CT (with moderately low leaching rate of cadmium ions (∼2-5%)).

Behavior of Silica Nanoparticles Synthesized from Rice Husk Ash by the Sol-Gel Method as a Photocatalytic and Antibacterial Agent

Silica nanoparticles (SiO₂ NPs) are one of the most well-studied inorganic nanoparticles for many applications. They offer the advantages of tunable size, biocompatibility, porous structure, and larger surface area. Thus, in this study, a high yield of SiO₂ NPs was produced via the chemical treatment of rice husk ash by the sol-gel method. Characteristics of the prepared SiO₂ NPs were validated using different characterization techniques. Accordingly, the phase, chemical composition, morphological, and spectroscopic properties of the prepared sample were studied. The average particle size of the SiO₂ NPs was found to be approximately 60-80 nm and the surface area was 78.52 m²/g. The prepared SiO₂ NPs were examined as photocatalysts for the degradation of methyl orange (MO) dye under UV irradiation. It was found that the intensity of the characteristic absorption band of MO decreased gradually with exposure time increasing, which means the successful photodegradation of MO by SiO₂ NPs. Moreover, the antibacterial activity of obtained SiO₂ NPs was investigated by counting the coliform bacteria in the surface water using the most probable number (MPN) index method. The results revealed that the MPN of coliform bacteria untreated and treated by SiO₂ NPs was estimated to be 170 CFU/100 mL and 10 CFU/100 mL, respectively, resulting in bacterial growth inhibition of 94.12%.

Simple Preparation of the CuO•Fe3O4/Silica Composite from Rice Husk for Enhancing Fenton-Like Catalytic Degradation of Tartrazine in a Wide pH Range

SiO2 was prepared from rice husk (RH) with the assistance of cetrimonium bromide (CTAB), and the CuO•Fe3O4/SiO2 composite was prepared by a simple coprecipitation method to enhance the Fenton-like degradation of dyes in a wide pH range. SiO2 was a mesoporous material with a relatively large surface area of 496.4 m2/g and a highly relative pore volume of 1.154 cm3/g. The Fe3O4 and CuO particles with the size of 20–50 nm were well dispersed in the composite, making the composite tighter and causing the disappearance of large pores in the range of 20–55 nm. The surface area and pore volume of the composite were reduced to 248.6 m2/g and 0.420 cm3/g, respectively. Fe3O4/SiO2 and Fe3O4 samples only exhibited high catalytic activity in an acidic medium, while the CuO•Fe3O4/SiO2 composite could effectively work in a wide pH range of 3–7. Besides, the effects of reaction conditions such as catalyst dosage, H2O2 concentration, and initial dye concentration on the catalytic performance of the composite were studied. The optimal conditions for the degradation of dye were tartrazine (TA) concentration of 50 mg/L, dosage catalyst of 0.5 g/L, H2O2 concentration of 120 mM, and pH 5. The CuO•Fe3O4/SiO2 composite reached the highest activity at pH 5, showing a degradation efficiency (DE) of 93.3% and a reaction rate of 0.061 min−1. The reusability of the catalyst was investigated by cyclic experiments. The DE of the 3rd reuse remained at 55.1%, equivalent to 93.5% of the first use. The catalytic mechanism for the Fenton system has also been proposed.

Recent developments in photocatalysis of industrial effluents ։ A review and example of phenolic compounds degradation

Industrial expansion and increased water consumption have created water scarcity concerns. Meanwhile, conventional wastewater purification methods have failed to degrade recalcitrant pollutants efficiently. The present review paper discusses the recent advances and challenges in photocatalytic processes applied for industrial effluents treatment, with respect to phenolic compounds degradation. Key operational parameters including the catalyst loading, light intensity, initial pollutants concentration, pH, and type and concentrations of oxidants are evaluated and discussed. Compared to the other examined controlling parameters, pH has the highest effect on the photo-oxidation of contaminants by means of the photocatalyst ionization degree and surface charge. Furthermore, major phenolic compounds derived from industrial sources are comprehensively presented and the applicability of photocatalytic processes and the barriers in practical applications, including high energy demand, technical challenges, photocatalyst stability, and recyclability have been explored. The importance of energy consumption and operational costs for realistic large-scale processes are also discussed. Finally, research gaps in this area and the suggested direction for improving degradation efficiencies in industrial applications are presented. In the light of these premises, selective degradation processes in real water matrices such as untreated sewage are proposed.

Application of Functional Modification of Iron-Based Materials in Advanced Oxidation Processes (AOPs)

Advanced oxidation processes (AOPs) have become a favored approach in wastewater treatment due to the high efficiency and diverse catalyzed ways. Iron-based materials were the commonly used catalyst due to their environmental friendliness and sustainability in the environment. We collected the published papers relative to the application of the modified iron-based materials in AOPs between 1999 and 2020 to comprehensively understand the related mechanism of modified materials to improve the catalytic performance of iron-based materials in AOPs. Related data of iron-based materials, modification types, target pollutants, final removal efficiencies, and rate constants were extracted to reveal the critical process of improving the catalytic efficiency of iron-based materials in AOPs. Our results indicated that the modified materials through various mechanisms to enhance the catalytic performance of iron-based materials. The principal aim of iron-based materials modification in AOPs is to increase the content of available Fe2+ and enhance the stability of Fe2+ in the system. The available Fe2+ is elevated by the following mechanisms: (1) modified materials accelerate the electron transfer to promote the Fe3+/Fe2+ reaction cycle in the system; (2) modified materials form chelates with iron ions and bond with iron ions to avoid Fe3+ precipitation. We further analyzed the effect of different modifying materials in improving these two mechanisms. Combining the advantages of different modified materials to develop iron-based materials with composite modification methods can enhance the catalytic performance of iron-based materials in AOPs for further application in wastewater treatment.

Synthesis of porous iron hydroxy phosphate from phosphate residue and its application as a Fenton-like catalyst for dye degradation

Phosphate residue is a kind of hazardous solid waste and if not properly disposed of, could cause serious environmental contaminations. The abundant iron salt available in phosphate residue can be used to prepare photo-Fenton catalytic reagent for wastewater treatment. In this study, the phosphate residue was effectively purified by a hydrothermal recrystallization method, reaching an iron phosphate purity of 94.2%. The particles of iron phosphate were further processed with ball milling with their average size reduced from 19.4 to 1.6 μm. By hydrothermal crystallization of iron phosphate and thermal decomposition of oxalate precursor, porous iron hydroxy phosphate was prepared. The modified porous iron hydroxy phosphate (m-PIHP) of higher surface area with iron oxalate on its surface can degrade 98.87% of Rhodamine B in 15 min. Cyclic experiment showed that the catalyst still had a good catalytic activity after six cycles (>40%). The X-ray photoelectron spectroscopy results showed that the iron oxalate complex on the catalyst surface decomposed to produce ferrous ions and accelerated the rate of •OH production. The current work demonstrated that the m-PIHP synthesized from phosphate residue and modified with iron oxalate can be used as an effective dye wastewater treatment agent.

CuO-ZnO-CdWO4: a sustainable and environmentally benign photocatalytic system for water cleansing

Currently, there is a major problem of water contaminations, especially of dyes, all over the world. A new technique is being developed daily for the treatment of contaminated water. In many ways, a photocatalytic degradation of a dye by a mixed metal oxide photocatalyst is counted as the best technique for water treatment. This paper also addresses the preparation and photocatalytic application of newly developed mixed metal oxide nanocomposite, CuO-ZnO-CdWO₄. A novel mixed metal oxide CuO-ZnO-CdWO₄ nanocomposite has been synthesized by a green route using Brassica Rapa leaves extract. The application of CuO-ZnO-CdWO₄ as a photocatalyst in wastewater treatment has been thoroughly discussed. Several spectroscopic and microscopic techniques were used to characterize the prepared nanocomposite. The photocatalytic activity of CuO-ZnO-CdWO₄ nanocomposite with a band gap of 3.13 eV was observed under the artificial visible light and sunlight for the degradation of Congo red dye. The results under sunlight show the 1.45 times greater removal efficiency than under the artificial visible light. Pseudo-first-order, diffusion, and Singh kinetics models were used to describe the kinetics of dye degradation. Pseudo-first-order model was found to be best fitted model for present study. The performance of CuO-ZnO-CdWO₄ was estimated by significant parameters such as quantum yield, figure of merit, turnover number, and mean turnover frequency. The value these parameters were calculated as 1.70 × 10^-8 molecules photon^-1, 1.77 × 10^-4, 2.98 × 10⁸ s^-1, and 3.31 × 10^-4 s^-1, respectively. These parameters revealed high potential of CuO-ZnO-CdWO₄ for Congo red dye degradation.

Visible light-conducting polymer nanocomposites as efficient photocatalysts for the treatment of organic pollutants in wastewater

This review compiles recent advances and challenges on photocatalytic treatment of wastewater using nanoparticles, nanocomposites, and polymer nanocomposites as photocatalyst. The review provides an overview of the fundamental principles of photocatalytic treatment along the recent advances on photocatalytic treatment, especially on the modification strategies and operational conditions to enhance treatment efficiency and removal of recalcitrant organic contaminants. The different types of photocatalysts along the key factors influencing their performance are also critically discussed and recommendations for future research are provided.

New insights on the enhanced non-hydroxyl radical contribution under copper promoted TiO2/GO for the photodegradation of tetracycline hydrochloride

TiO₂/graphene oxide (GO) as photocatalyst in the photo-degradation of multitudinous pollutants has been extensively studied. But its low photocatalytic efficiency is attributed to the high band gap energy which lead to low light utilization. Cu-TiO₂/GO was synthesized via the impregnation methods to enhance the catalytic performance. The Cu-TiO₂/GO reaction rate constant for photo-degradation of pollutants (tetracycline hydrochloride, TC) was about 1.4 times that of TiO₂/GO. In 90 min, the removal ratio of Cu-TiO₂/GO for TC was 98%, and the maximum degradation ratio occurred at pH 5. After five cycles, the removal ratio of Cu-TiO₂/GO still exceeded 98%. UV-visible adsorption spectra of Cu-TiO₂/GO showed that its band gap was narrower than TiO₂/GO. Electron paramagnetic resonance (EPR) spectra test illustrated the generation rate of •O₂^- and •OH was higher in Cu-TiO₂/GO system than TiO₂/GO and TiO₂ system. The contribution sequence of oxidative species was •O₂^- > holes (h⁺) > •OH in both TiO₂/GO and Cu-TiO₂/GO system. Interestingly, the contribution of •OH in Cu-TiO₂/GO was less than that in TiO₂/GO during the photo-degradation process. This phenomenon was attributed to the better adsorption performance of Cu-TiO₂/GO which could reduce the accessibility of TC to •OH in liquid. The enhanced non‑hydroxyl radical contribution could be attributed to that the more other active species or sites on (nearby) the surface of Cu-TiO₂/GO generated after doping Cu. These results provide a new perspective for the tradition metal-doped conventional catalysts to enhance the removal of organic pollutants in the environment.

Efficient removal of toxic organic dyes and photoelectrochemical properties of iron-doped zirconia nanoparticles

Iron (Fe)-doped ZrO₂ tetragonal nanoparticles were synthesized by a facile and inexpensive hydrothermal technique, that were doped with Fe³⁺ ions (0.1, 0.3, and 0.5 mol%) into the host lattice without altering the morphology and crystal structure of the nanoparticles. SEM and TEM investigations indicated that the morphology of ZrO₂ nanoparticles did not change even after incorporation of Fe, while the band gap of semiconducting ZrO₂ nanoparticles was reduced from 4.97 to 1.77 eV. Such a in band gap was responsible to harvest more photons to stimulate the generation of more electrons in the valence band, thereby enhancing the photoelectrochemical (PEC) water splitting as well as photocatalytic and photoelectrocatalytic activities in the photodegradation of Rhodamine B. The 0.3 mol%-doped ZrO₂ electrode showed enhanced photocurrent density (0.07 × 10^-3 A/cm²), that was 45-times greater than the pure sample. The electrochemical impedance spectroscopy (EIS) confirmed that 0.3 mol%-doped ZrO₂ exhibited the best charge transfer characteristics, which increased with PEC water splitting activity. The maximum photocurrent density and long-term photo-stability were achieved in the light on-off states.