Preservation of the photocatalytic activity of TiO2 by EDTA in the reductive transformation of Cr(VI). Studies by Time Resolved Microwave Conductivity

Ag-modified g-C3N4 with enhanced activity for the photocatalytic reduction of hexavalent chromium in the presence of EDTA under ultraviolet irradiation

The photocatalytic reduction of Cr6+ to Cr3+ in an aqueous solution, using 3 wt% Ag/g-C3N4 in the presence of ethylenediaminetetraacetic acid (EDTA), has been investigated here. The photocatalytic reduction of Cr6+ with pure g-C3N4 was very low. The addition of Ag and EDTA can significantly improve the photocatalytic reduction of Cr6+ using g-C3N4. In the presence of EDTA, the efficiency with Ag/g-C3N4 was better than those with Au/g-C3N4 and Cu/g-C3N4. With EDTA, the reduction rate constant increased from 0.0005 for pure g-C3N4 to 0.12 min-1 for 3 wt% Ag/g-C3N4. By increasing the concentration of EDTA from 0 to 500 mg L-1, the reduction efficiency of Cr6+ increased extremely, and the rate constant raised from 0.008 to 0.12 min-1. The optimal EDTA concentration was 500 mg L-1 for the photocatalyst Ag/g-C3N4. The Ag-EDTA complex may be reduced to metallic silver by the conduction band electrons of g-C3N4. The electron-hole recombination was significantly suppressed by the electron trapping of Ag. EDTA may act in by the formation of Cr3+-complex and the separation of Cr3+ from the g-C3N4 surface and by the valence band hole scavenger of g-C3N4. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectroscopy (DRS) and photoluminescence spectra (PL) were used to characterize g-C3N4 and Ag/g-C3N4 nanoparticles. A possible mechanism for photocatalytic Cr6+ reduction has also been demonstrated.

Application of EPR Spectroscopy in TiO2 and Nb2O5 Photocatalysis

The interaction of light with semiconducting materials becomes the center of a wide range of technologies, such as photocatalysis. This technology has recently attracted increasing attention due to its prospective uses in green energy and environmental remediation. The characterization of the electronic structure of the semiconductors is essential to a deep understanding of the photocatalytic process since they influence and govern the photocatalytic activity by the formation of reactive radical species. Electron paramagnetic resonance (EPR) spectroscopy is a unique analytical tool that can be employed to monitor the photoinduced phenomena occurring in the solid and liquid phases and provides precise insights into the dynamic and reactivity of the photocatalyst under different experimental conditions. This review focus on the application of EPR in the observation of paramagnetic centers formed upon irradiation of titanium dioxide and niobium oxide photocatalysts. TiO2 and Nb2O5 are very well-known semiconductors that have been widely used for photocatalytic applications. A large number of experimental results on both materials offer a reliable platform to illustrate the contribution of the EPR studies on heterogeneous photocatalysis, particularly in monitoring the photogenerated charge carriers, trap states, and surface charge transfer steps. A detailed overview of EPR-spin trapping techniques in mechanistic studies to follow the nature of the photogenerated species in suspension during the photocatalytic process is presented. The role of the electron donors or the electron acceptors and their effect on the photocatalytic process in the solid or the liquid phase are highlighted.

Photocatalytic Reduction of Cr (VI) over g-C3N4 Photocatalysts Synthesized by Different Precursors

Graphitic carbon nitride (g-C₃N₄) photocatalysts were synthesized via a one-step pyrolysis process using melamine, dicyandiamide, thiourea, and urea as precursors. The obtained g-C₃N₄ materials exhibited a significantly different performance for the photocatalytic reduction of Cr(VI) under white light irradiation, which is attributed to the altered structure and occupancies surface groups. The urea-derived g-C₃N₄ with nanosheet morphology, large specific surface area, and high occupancies of surface amine groups exhibited superior photocatalytic activity. The nanosheet morphology and large surface area facilitated the separation and transmission of charge, while the high occupancies of surface amine groups promoted the formation of hydrogen adsorption atomic centers which were beneficial to Cr(VI) reduction. Moreover, the possible reduction pathway of Cr(VI) to Cr(III) over the urea-derived g-C₃N₄ was proposed and the reduction process was mainly initiated by a direct reduction of photogenerated electrons.

Sub-level engineering strategy of nitrogen-induced Bi2O3/g-C3N4: a versatile photocatalyst for oxidation and reduction

Herein, the α-Bi₂O₃ nanocrystal decorated by nitrogen dopant and its heterojunction nanocomposite with g-C₃N₄ (N_0.1/Bi₂O₃/g-C₃N₄) is successfully fabricated for the first time, for photo-oxidation of RhB and photo-reduction of Cr(VI) to Cr(III). The resulting N_0.1/Bi₂O₃/g-C₃N₄ (3%) nanocomposite showed an optimal Cr(VI) photo-reduction and RhB photo-oxidation rates under visible-light irradiation, being 3-4 times higher than that of pure α-Bi₂O₃. The results from XPS confirmed the substitution of nitrogen with various oxidation states from N³⁺ to N^x+ (x < 5), due to the existence of different nitrogen oxides including N-O, O-N=O, and NO₃^- in the crystal structure. We investigated the reaction mechanism using catalytic tests, impedance spectroscopy, EPR technique, and density functional calculations. The DFT calculations presented the appearance of a new mid-gap hybrid of p states, comprised of N 2p, O 2p, and Bi 6P states, which enhance light absorption capacity and narrow band gap. The theoretical results were in excellent agreement with experimental UV-Vis data. The N_0.1/Bi₂O₃/g-C₃N₄ nanocomposite exhibited acceptable practical application value and recycling ability for removal of the contaminants. Such improved photocatalytic activity is originated from the modified band positions, new electron evolution pathway, introducing defects in α-Bi₂O₃ by insertion of N atoms into the Bi sites, and the enhanced charge carrier mobility between N_0.1/Bi₂O₃ and g-C₃N₄. The strategy to form nitrogen-doped bismuth-based nanocomposites may open a new opportunity to design atomic-level electronic defects by feasible methods to obtain a versatile photocatalyst material with simultaneous photo-reduction and photo-oxidation ability for removal of Cr(VI) and organic dyes from water.

Performance of EDTA modified magnetic ZnFe2O4 during photocatalytic reduction of Cr(VI) in aqueous solution under UV irradiation

The photocatalytic reduction of toxic Cr(VI) to non-pernicious Cr(III) using ZnFe₂O₄/EDTA (ethylenediaminetetraacetic acid) under UV irradiation was evaluated. The reduction of Cr(VI) with bare ZnFe₂O₄ under UV irradiation was negligible. However, the Cr(VI) in the solution was completely reduced within 3 h after the introduction of EDTA. EDTA could consume valence band holes and could be oxidized by holes into inorganic products. Therefore, photo-generated electrons could be used to reduce Cr(VI) to Cr(III). The effect of concentration of EDTA, ZnFe₂O₄ photocatalyst dosages, and initial pH on the photocatalytic reduction of Cr(VI) was investigated. The results revealed that the photocatalytic reduction of Cr(VI) accelerated by increasing EDTA concentration and ZnFe₂O₄ dosage. The present reduction process confirms the pseudo-first-order kinetic reaction. The quasi reduction rate constant increased from 3.5 x 10^-4min^-1 to 2.6 x 10^-2min^-1 with the increase in initial EDTA concentration from 0 to 1000 mg L^-1. The acidic solution is preferable for the photocatalytic reduction of Cr(VI). The entire reduction of Cr(VI) was carried out within 2 h under the optimum conditions with pH 2, 20 mg ZnFe₂O₄, and 500 mg L^-1 EDTA. The formation of [Cr-EDTA]³⁺ complex may be advantageous to accelerate the Cr(VI) reduction. A probable mechanism for the photocatalytic Cr(VI) reduction was speculated here.

Application of ZnO nanorods doped with Cu for enhanced sonocatalytic removal of Cr(VI) from aqueous solutions

The aim of this research was to develop a simple and inexpensive process for reduction of Cr(VI) to Cr(III). Zinc oxide nanoparticles were synthesized with an easy co-precipitation procedure, and the addition of Cu²⁺ doping agent effectively enhanced the Cr(VI) reduction in the presence of ultrasound (US). XRD, FT-IR, FE-SEM, EDX, VSM, and XPS were used to determine the structural specifications of the zinc oxide nanoparticles. Under optimal conditions such as pH 3, initial Cr(VI) content of 20 mg/L, and catalyst dosage of 0.8 g/L, the ultrasonic/Cu-ZnO process showed a higher sonocatalytic activity (96.83%) than ultrasonic/ZnO (67.36%) after 60 min. By increasing pH and Cr(VI) concentration, the removal efficacy of Cr(VI) declined. The experimental data was well described with the first-order kinetic model. When initial Cr(VI) concentration increased from 10 to 50 mg/L, the first-order rate constant declined from 0.2326 to 0.0019 min^-1 and electrical energy per order (E_EO) enhanced from 19.81 to 2425.26 kWh/m³. Also, the ultrasonic/Cu-ZnO system exhibited considerable sonocatalytic performance in Cr(VI) reduction in the presence of hydrogen peroxide and citric acid, and complete removal was achieved within 60 min. The presence of anions negatively affected Cr(VI) reduction. Complete reduction was attained when ultrasound was applied at a power of 100 W. The catalyst activity was well maintained up to six consecutive cycles. In addition, the removal efficiency was approximately 62 and 65% for field water and real electroplating wastewater samples, respectively.

The reduction of Cr(VI) to Cr(III) mediated by environmentally relevant carboxylic acids: State-of-the-art and perspectives

The detoxification process mediated by carboxylic acids (CAs) has received considerable spotlights since CAs are clean reagent and ubiquitous in the natural environments and effluents. Here, we present an exhaustive review on surface-bound/dissolved metals-catalyzed Cr(VI) reduction by CAs and CAs-mediated Cr(VI) reduction by many highly/poorly reductive reagents. The overall mechanisms of Cr(VI) reduction are mainly associated with the coordination of CAs with surface-bound/dissolved metals or Cr(VI,V,IV) species and the electron donating abilities of CAs. Additionally, the general decays of intermediate Cr(V,IV) complexes are clearly emerged in the Cr(VI) reduction processes. The performance of various reaction systems for Cr(VI) reduction that is greatly dependent on the operation parameters, including solution pH, reagent concentration, temperature, coexisting ions and gas atmosphere, are also critically commented. From the study survey presented herein, CAs-mediated Cr(VI) reduction processes exhibit good potential for remediation of various Cr(VI)-contaminated waters/sites. However, there is still a need to address the remained bottle-necks and challenges for the remediation of Cr(VI) mediated by CAs in the related natural attenuation cases and the treatment of industrial effluents. Overall, the present review offers the comprehensive understanding of the Cr(VI) reduction mediated by CAs and provide the engineering community with the guidelines for Cr(VI) remediation in the real-world applications.

S-incorporated TiO2 coatings grown by plasma electrolytic oxidation for reduction of Cr(VI)-EDTA with sunlight

The plasma electrolytic oxidation (PEO) technique was used to prepare photocatalytic S-TiO₂ coatings on Ti sheets; the incorporation of the S ions was possible from the electrolyte for modifying the structural and optics characteristics of the material. In this work, substrates of Ti (ASME SB-265 of 20 × 20 × 1 mm) were used in a PEO process in 10 min, using constant voltage pulses of 340 V with frequency of 1 kHz and duty cycles of 10% and of 30%. Solutions with H₂SO₄ (0.1 M) and CH₄N₂S (52 and 79 mM) were used as electrolytes. X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy (EDS) were utilized to analyze the surface morphology, crystalline phase, and chemical composition of the samples. According to the results, the catalyst coatings had microporous structure and contained anatase-rutile TiO₂ nanocrystalline mixture, until 73.2% rutile and 26.8% anatase in the samples grown with 30% duty cycle and the lowest concentration of CH₄N₂S. From the EDS measurements, the incorporation of sulfur ions to the coatings was 0.08 wt%. 99.5% reduction efficiency of Cr(VI)-EDTA with sunlight was observed after 2 h; it was determined by diphenyl carbazide spectrophotometric method. These coatings have potential for effective sunlight heterogeneous photoreduction of this toxic, cumulative, and non-biodegradable heavy metal that contaminates the soil and water and is a serious risk to sustainability, ecosystems, and human health.

As(III) and Cr(VI) oxyanion removal from water by advanced oxidation/reduction processes-a review

Water pollution by human activities is a global environmental problem that requires innovative solutions. Arsenic and chromium oxyanions are toxic compounds, introduced in the environment by both natural and anthropogenic activities. In this review, the speciation diagrams of arsenic and chromium oxyanions in aqueous solutions and the analytical methods used for their detection and quantification are presented. Current and potential treatment methods for As and Cr removal, such as adsorption, coagulation/flocculation, electrochemical, ion exchange, membrane separation, phyto- and bioremediation, biosorption, biofiltration, and oxidative/reductive processes, are presented with discussion of their advantages, drawbacks, and the main recent achievements. In the last years, advanced oxidation processes (AOPs) have been acquiring high relevance for the treatment of water contaminated with organic compounds. However, these processes are also able to deal with inorganic contaminants, mainly by changing metal/metalloid oxidation state, turning these compounds less toxic or soluble. An overview of advanced oxidation/reduction processes (AO/RPs) used for As and Cr removal was carried out, focusing mainly on H₂O₂/UVC, iron-based and heterogeneous photocatalytic processes. Some aspects related to AO/RP experimental conditions, comparison criteria, redox mechanisms, catalyst immobilization, and process intensification through implementation of innovative reactors designs are also discussed. Nevertheless, further research is needed to assess the effectiveness of those processes in order to improve some existing limitations. On the other hand, the validation of those treatment methods needs to be deepened, namely with the use of real wastewaters for their future full-scale application. Graphical abstract ᅟ.

Sonochemical reduction of Cr(VI) in air in the presence of organic additives: What are the involved mechanistic pathways?

The sonochemical (850 kHz) reduction of Cr(VI) (0.3 mM, pH 2, reactor open to air) was analyzed in the presence of different additives. The effects on Cr(VI) reduction efficiency of added formic acid (FA, 10 mM), citric acid (Cit, 2 mM), ethylenediaminetetraacetic acid (EDTA, 1 mM), methanol (MeOH, 0.1 M), ethanol (EtOH, 0.1 M), 2-propanol (2-PrOH, 0.1 M), tert-butanol (t-BuOH, 0.1 M), phenol (PhOH, 2 mM) and sodium lauryl sulfate (SLS, 1 mM) have been evaluated in comparison with the system in the absence of additives. Complete Cr(VI) reduction was obtained only when using EDTA (at 120 min) and Cit (at 180 min). Cr(III) complexes with these compounds or with their degradation products were detected as final products. For EDTA, Cit, t-BuOH, FA and SLS, the Cr(VI) decay could be adjusted to a zero-order kinetics; in the cases of MeOH, EtOH and 2-PrOH, there was a deviation from the zero-order kinetics. The Cr(VI) conversion increased in the order SLS (very low) < no additive ≅ MeOH ≅ EtOH ≅ 2-PrOH < FA < t-BuOH < PhOH < Cit < EDTA. The role of EDTA and Cit in stabilizing intermediate Cr(V) peroxo compounds and enhancing their direct transformation into different Cr(III) species is considered a major factor in the acceleration of Cr(VI) reduction processes. Mechanistic pathways are proposed.

Comparative analysis of the photocatalytic reduction of drinking water oxoanions using titanium dioxide

Regulated oxidized pollutants in drinking water can have significant health effects, resulting in the need for ancillary treatment processes. Oxoanions (e.g., nitrate) are one important class of oxidized inorganic ions. Ion exchange and reverse osmosis are often used treatment processes for oxoanions, but these separation processes leave behind a concentrated waste product that still requires treatment or disposal. Photocatalysis has emerged as a sustainable treatment technology capable of catalytically reducing oxoanions directly to innocuous byproducts. Compared with the large volume of knowledge available for photocatalytic oxidation, very little knowledge exists regarding photocatalytic reduction of oxoanion pollutants. This study investigates the reduction of various oxoanions of concern in drinking water (nitrate, nitrite, bromate, perchlorate, chlorate, chlorite, chromate) using a commercial titanium dioxide photocatalyst and a polychromatic light source. Results showed that oxoanions were readily reduced under acidic conditions in the presence of formate, which served as a hole scavenger, with the first-order rate decreasing as follows: bromate > nitrite > chlorate > nitrate > dichromate > perchlorate, corresponding to rate constants of 0.33, 0.080, 0.052, 0.0074, 0.0041, and 0 cm²/photons × 10¹⁸, respectively. Only bromate and nitrite were reduced at neutral pH, with substantially lower rate constants of 0.034 and 0.0021 cm²/photons × 10¹⁸, respectively. No direct relationship between oxoanion physicochemical properties, including electronegativity of central atom, internal bond strength, and polarizability was discovered. However, observations presented herein suggest the presence of kinetic barriers unique to each oxoanion and provides a framework for investigating photocatalytic reduction mechanisms of oxoanions in order to design better photocatalysts and optimize treatment.

Hydrothermal derived nitrogen doped SrTiO3 for efficient visible light driven photocatalytic reduction of chromium(VI)

In this work, we report on the synthesis of nitrogen doped SrTiO₃ nanoparticles with efficient visible light driven photocatalytic activity toward Cr(VI) by the solvothermal method. The samples are carefully characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV–Vis diffuse reflectance spectroscopy and photocatalytic test. It is found that nitrogen doping in SrTiO₃ lattice led to an apparent lattice expansion, particle size reduction as well as subsequent increase of Brunner–Emmet–Teller surface area. The visible light absorption edge and intensity can be modulated by nitrogen doping content, which absorption edge extends to about 600 nm. Moreover, nitrogen doping can not only modulate the visible light absorption feature, but also have consequence on the enhancement of charge separation efficiency, which can promote the photocatalytic activity. With well controlled particle size, Brunner–Emmet–Teller surface area, and electronic structure via nitrogen doping, the photocatalytic performance toward Cr(VI) reduction of nitrogen doped SrTiO₃ was optimized at initial hexamethylenetetramine content of 2.

Influence of Nitrogen Doping on Device Operation for TiO₂-Based Solid-State Dye-Sensitized Solar Cells: Photo-Physics from Materials to Devices

Solid-state dye-sensitized solar cells (ssDSSC) constitute a major approach to photovoltaic energy conversion with efficiencies over 8% reported thanks to the rational design of efficient porous metal oxide electrodes, organic chromophores, and hole transporters. Among the various strategies used to push the performance ahead, doping of the nanocrystalline titanium dioxide (TiO₂) electrode is regularly proposed to extend the photo-activity of the materials into the visible range. However, although various beneficial effects for device performance have been observed in the literature, they remain strongly dependent on the method used for the production of the metal oxide, and the influence of nitrogen atoms on charge kinetics remains unclear. To shed light on this open question, we synthesized a set of N-doped TiO₂ nanopowders with various nitrogen contents, and exploited them for the fabrication of ssDSSC. Particularly, we carefully analyzed the localization of the dopants using X-ray photo-electron spectroscopy (XPS) and monitored their influence on the photo-induced charge kinetics probed both at the material and device levels. We demonstrate a strong correlation between the kinetics of photo-induced charge carriers probed both at the level of the nanopowders and at the level of working solar cells, illustrating a direct transposition of the photo-physic properties from materials to devices.

Novel β-In2.77S4 nanosheet-assembled hierarchical microspheres: synthesis and high performance for photocatalytic reduction of Cr(vi)

A novel nanosheet-assembled hierarchical sulfur deficient β-In_2.77S₄ microsphere photocatalyst was prepared, and the β-In_2.77S₄ microsphere shown high visible light photocatalytic activity in the reduction of aqueous Cr(vi).

Role of Cr(iii) deposition during the photocatalytic transformation of hexavalent chromium and citric acid over commercial TiO2 samples

A larger specific area makes UV100 more efficient than P25 for Cr(vi) and citric acid removal. Citric and 3-oxoglutaric acids prevent surface deposition of Cr(iii) recombining centers.

Application of the Stopped Flow Technique to the TiO₂-Heterogeneous Photocatalysis of Hexavalent Chromium in Aqueous Suspensions: Comparison with O₂ and H₂O₂ as Electron Acceptors

The dynamics of the transfer of electrons stored in TiO2 nanoparticles to Cr(VI) in aqueous solution have been investigated using the stopped flow technique. TiO2 nanoparticles were previously irradiated under UV light in the presence of formic acid, and trapped electrons (e(trap)(-)) were made to react with Cr(VI) as acceptor species; other common acceptor species such as O2 and H2O2 were also tested. The temporal evolution of the number of trapped electrons was followed by the decrease in the absorbance at 600 nm, and the kinetics of the electron-transfer reaction was modeled. Additionally, the rate of formation of the surface complex between Cr(VI) and TiO2 was determined with the stopped flow technique by following the evolution of the absorbance at 400 nm of suspensions of nonirradiated TiO2 nanoparticles and Cr(VI) at different concentrations. An approximately quadratic relationship was observed between the maximum absorbance of the surface complex and the concentration of Cr(VI), suggesting that Cr(VI) adsorbs onto the TiO2 surface as dichromate. The kinetic analyses indicate that the electron transfer from TiO2 to Cr(VI) does not require the previous formation of the Cr(VI)-TiO2 surface complex, at least the complex detected here through the stopped flow experiments. When previously irradiated TiO2 was used to follow the evolution of the Cr(VI)-TiO2 complex, an inhibition of the formation of the complex was observed, which can be related to the TiO2 deactivation caused by Cr(III) deposition.

K4Nb6O17·4.5H2O: a novel dual functional material with quick photoreduction of Cr(VI) and high adsorptive capacity of Cr(III)

A series of orthorhombic phase K4Nb6O17·4.5H2O was synthesized via a hydrothermal approach. When presented in an acidic pH range, K4Nb6O17·4.5H2O showed a strong ability in quick reduction from Cr(VI) to Cr(III). The resulted Cr(III) ions were removed by an effective adsorption through simply adjusting the solution pH from strong acidity to near neutrality, owing to the sample's unique nano-sheet structure with a wide layer spacing. The Cr(III) ions adsorbed onto samples were released again for reusing by eluting with 1molL(-1) HCl solution, and K4Nb6O17·4.5H2O regenerated by immersing in a KOH solution. The reduction efficiency of Cr(VI) was still up to 98% after irradiation for 60min, and the removal efficiency of Cr(III) ions was as high as 83% even after five cycles. Therefore, K4Nb6O17·4.5H2O is clearly demonstrated to be an excellent dual functional material with quick photoreduction of Cr(VI) and high adsorptive capacity of Cr(III). The relevant materials reported herein might be found various environment-related applications.