Cr(VI) remediation from aqueous environment through modified-TiO2-mediated photocatalytic reduction

Impact of Anchoring Groups on the Photocatalytic Performance of Iridium(III) Complexes and Their Toxicological Analysis

Three different iridium(III) complexes, labelled as Ir1-Ir3, each bearing a unique anchoring moiety (diethyl [2,2'-bipyridine]-4,4'-dicarboxylate, tetraethyl [2,2'-bipyridine]-4,4'-diylbis(phosphonate), or [2,2'-biquinoline]-4,4'-dicarboxylic acid), were synthesized to serve as photosensitizers. Their electrochemical and photophysical characteristics were systematically investigated. ERP measurements were employed to elucidate the impact of the anchoring groups on the photocatalytic hydrogen generation performance of the complexes. The novel iridium(III) complexes were integrated with platinized TiO2 (Pt-TiO2) nanoparticles and tested for their ability to catalyze hydrogen production under visible light. A H2 turnover number (TON) of up to 3670 was obtained upon irradiation for 120 h. The complexes with tetraethyl [2,2'-bipyridine]-4,4'-diylbis(phosphonate) anchoring groups were found to outperform those bearing other moieties, which may be one of the important steps in the development of high-efficiency iridium(III) photosensitizers for hydrogen generation by water splitting. Additionally, toxicological analyses found no significant difference in the toxicity to luminescent bacteria of any of the present iridium(III) complexes compared with that of TiO2, which implies that the complexes investigated in this study do not pose a high risk to the aquatic environment compared to TiO2.

High efficient COVID-19 waste co-pyrolysis char/TiO2 nanocomposite for photocatalytic reduction of Cr(VI) under visible light

Titanium dioxide (Titania) nanoparticle-coated biochar derived through co-pyrolysis of COVID-19 waste face mask (WFM) and Moringa oleifera seed cake (MO) provides an effective way to alleviate toxic metal in wastewater. This study investigates the effects of Biochar/titania photocatalyst preparation, characterization, and its photoreduction of Cr(VI). The morphological and functional modifications in the catalyst were identified using X-Ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, ultraviolet spectrophotometer, surface area analysis, and Raman spectrophotometer, respectively. The influencing parameters, namely, pH, photocatalyst dosage, initial pollutant concentration, and visible light irradiation time, have been investigated. The findings reveal that the Cr(VI) reduction by the photocatalyst was highly facilitated by photocatalytic process. The prepared photocatalyst shows higher and faster reduction rate of Cr(VI) and also improves the catalyst stability. The photoreduction of Cr(VI) ensembles well with pseudo-first order kinetics. At 180 min of reaction time, maximum Cr(VI) reduction of 98.65% was achieved at pH 2, 0.3 g/L catalyst dosage, and 10 ppm initial concentration, respectively. The synthesized photocatalyst shows excellent recycling performance up to 7 times, and these studies proved that the prepared catalyst is cost-effective and efficiently employed for removing pollutants.

Solar photocatalytic disinfection of real municipal wastewater using highly durable TiO2-coated composite in a pilot scale once through reactor

Pollution of water sources by pathogens is a significant concern worldwide. In the present study, a pilot-scale once-through reactor was fabricated to investigate bacteria's inactivation and the degradation of organic matter present in municipal wastewater using an iron-mediated TiO₂ catalyst in fixed mode. The catalyst was fabricated (in a spherical shape) using waste material such as foundry sand and fly ash and coated with TiO₂ for a combined hybrid effect. The influence of H₂O₂ concentration and the flow rate of the reactor were examined. 4.1 log reductions of bacteria with 52% and 39% of BOD and COD reductions in 45 min of treatment were observed. The catalyst was also found to be highly durable, with only a 12.5% of reduction in catalyst activity observed after 200 recycles. Therefore, this pilot-scale research indicates the ability of waste materials to be employed as a practical approach for water disinfection applications.

Conversion of novel tannery sludge-derived biochar/TiO2 nanocomposite for efficient removal of Cr (VI) under UV light: photocatalytic performance and mechanism insight

An investigation on the reduction of Cr (VI) pollutant from tannery effluents using TiO₂, SB/TiO₂, and c-SB/TiO₂ nano photocatalysts was presented in this study. For the preparation of Biochar-based TiO₂ photocatalyst (SB/TiO₂), tannery sludge was utilized as a precursor. Hydrothermal pre-treatment was adopted to prepare chemically activated SB/TiO₂ and SB/TiO₂ nanocomposites. The morphology, crystal structure, optical properties, and elemental composition of the prepared catalysts were analyzed by XRD, FT-IR, SEM-EDX, BET analysis, ZPC, PL, TGA, and Raman spectroscopy. The band gap analysis of Photocatalyst was measured using a DRS instrument, and band gap energy of 3.39 eV was obtained for c-SB/TiO₂ photocatalyst. The developed c-SB/TiO₂ catalyst exhibits a larger specific surface area of 646.85 m²/g than TiO₂ and SB/TiO₂ (74.58 m²/g and 573.74 m²/g), respectively. The enhanced photocatalytic activity for the pollutant removal was achieved by the photocatalyst due to their wide band gap and effective charge separation. The kinetic rate constant was achieved in the pseudo-first-order model, which fits well for the reduction of Cr (VI). Furthermore, at the optimal conditions of 10 mg/L contaminant concentration, pH 2, and 0.5 g/L catalyst dosage, 98.56% reduction was observed after 180 min of reaction. The OH acts as a major removal pathway for Cr (VI) contaminants with more than 50% reduction in COD. This study proves that c-SB/TiO₂ photocatalysts can remove toxic contaminants under UV light irradiation with good recycling performance up to 5 times.

Heavy Metal Removal from Aqueous Effluents by TiO2 and ZnO Nanomaterials

The presence of heavy metals in wastewater, such as Ni, Pb, Cd, V, Cr, and Cu, is a serious environmental problem. This kind of inorganic pollutant is not biodegradable for several years, and its harmful effect is cumulative. Recently, semiconductor nanomaterials based on metal oxides have gained interest due to their efficiency in the removal of heavy metals from contaminated water, by inducing photocatalytic ion reduction when they absorb light of the appropriate wavelength. The most commonly applied semiconductor oxides for these purposes are titanium oxide (TiO2), zinc oxide (ZnO), and binary nanomaterials composed of both types of oxides. The main purpose of this work is to critically analyse the existent literature concerning this topic focusing specially in the most important factors affecting the adsorption or photocatalytic capacities of this type of nanomaterials. In particular, photocatalytic activity is altered by various factors, such as proportion of polymorphs, synthesis method, surface area, concentration of defects and particle size, among others. After a survey of the actual literature, it was found that, although these metal oxides have low absorption capacity for visible light, it is possible to obtain an acceptable heavy metal reduction performance by sensitization with dyes, doping with metallic or nonmetallic atoms, introduction of defects, or the coupling of two or more semiconductors.

Synthesis of ZnO immobilized on recycled polyethylene terephtalate for sonocatalytic removal of Cr(VI) from synthetic, drinking waters and electroplating wastewater

In this study, Cr(VI) was removed sonocatalytically by the zinc oxide nanoparticle coated with polyethylene terephthalate (PET) fabricated through a facile co-precipitation method. The crystal structure, functional groups on the surface, morphology, surface composition and oxidation states of the nanomaterials were investigated by XRD, FTIR, SEM, EDX and XPS techniques. Environmental parameters including solution pH, catalyst dose, hexavalent chromium concentration, H₂O₂ content, purging gases, organic compounds and type and anions strength on the sonotocatalytic removal of Cr(VI) were also investigated. Additionally, the contribution of each process, reusability, Cr(VI) reduction from actual water and electroplating wastewater were evaluated. Under the optimal conditions, [Cr(VI)]₀=20 mg/L, nanocomposite loading=1.6 g/L and pH=5, 99.92% of Cr(VI) was removed within 60 min. By increaing, Cr(VI) concentration (5-50 mg/L), k_obs decreased to values between 0.1498 and 0.0063 min^-1 and the calculated electrical energy per order (E_Eo) increased from 148.68 to 3535.24 kWh.m^-3, respectively. The presence of purging gases, organic compounds and ionic strength negatively affected Cr(VI) reduction. Examination of radical scavengers showed that the most active radicals in Cr(VI) removal were O₂^•- and h⁺. The removal of the Cr(VI) using the US/ZnO-PET method (99.92%) was higher than that of the US/ZnO method (70.78%). The catalyst activity was well maintained up to eight consecutive cycles. In addition, the removal efficiency was approximately 72.23 and 68.55% for drinking water and real electroplating wastewater samples, respectively. The results of toxicity in the sonotocatalytic removal of Cr(VI) by Daphnia magna showed LC₅₀ and toxicity unit (TU) 48 h, which was equal to 81.46 and 1.227 vol percent.

Simultaneous photooxidation and photoreduction of phenol and Cr(VI) ions using titania modified with nanosilica

The photocatalytic process of phenol oxidation and Cr(VI) reduction in the presence of nano-silica modified titania was carried out. The activity of composites was tested using two different light sources. The photocatalysts with 10% of nanosilica showed the highest activity. The calcination temperature (200–800 oC) significantly determined the sensitivity of the obtained materials to the light source used. Photocatalysts alternately adsorbed and desorbed Cr(VI) ions from the reaction mixture during irradiation. In the one-component mixture, complete oxidation of phenol was observed using material calcined at 650 oC, after 3 h of UV-VIS irradiation. In the reaction mixture of Cr(VI) and phenol, the highest activity was demonstrated by photocatalyst calcined at 300 oC. The concentration of phenol decreased in proportion to the decrease of chromium ions. The obtained titania-silica composites showed oxidizing properties towards phenol and reductive properties toward Cr(VI) ions.

Saiz P, Reizabal A, Wuttke S, Celaya-Azcoaga L, Valverde A, Fernández de Luis R. A State-of-the-Art of Metal-Organic Frameworks for Chromium Photoreduction vs. Photocatalytic Water Remediation

Hexavalent chromium (Cr(VI)) is a highly mobile cancerogenic and teratogenic heavy metal ion. Among the varied technologies applied today to address chromium water pollution, photocatalysis offers a rapid reduction of Cr(VI) to the less toxic Cr(III). In contrast to classic photocatalysts, Metal-Organic frameworks (MOFs) are porous semiconductors that can couple the Cr(VI) to Cr(III) photoreduction to the chromium species immobilization. In this minireview, we wish to discuss and analyze the state-of-the-art of MOFs for Cr(VI) detoxification and contextualizing it to the most recent advances and strategies of MOFs for photocatalysis purposes. The minireview has been structured in three sections: (i) a detailed discussion of the specific experimental techniques employed to characterize MOF photocatalysts, (ii) a description and identification of the key characteristics of MOFs for Cr(VI) photoreduction, and (iii) an outlook and perspective section in order to identify future trends.

Visible-light-driven CQDs/TiO2 photocatalytic simultaneous removal of Cr(VI) and organics: Cooperative reaction, kinetics and mechanism

CQDs/TiO₂ was synthesized with a coprecipitation method and characterized by XRD, SEM, TEM/HRTEM, BET, XPS, UV-Vis DRS and I-t curve technologies. UV-Vis DRS displayed that absorption spectrum of CQDs/TiO₂ enlarged to visible light zone, suggesting that CQDs/TiO₂ can be irradiated by visible light. I-t curve showed that photocurrent of CQDs/TiO₂ was higher than that of bare TiO₂, revealing that the doping of CQDs accelerated the transfer of photoelectrons and restrained the recombination of photoinduced carriers. Simultaneous removal of Cr(VI) and organics with CQDs/TiO₂ photocatalytic reaction was investigated and factors were optimized, and almost all Cr(VI) and organics were removed under the optimum conditions. Experimental results displayed that there was a distinct cooperation removal effect between Cr(VI) and organics in CQDs/TiO₂ photocatalytic reaction. XPS analysis proved that Cr(VI) was reduced to Cr(III) in situ on CQDs/TiO₂ surface. There were e^-, h⁺,·OH and ·O₂^- active species which were detected with DMPO in ESR test during CQDs/TiO₂ photocatalytic reaction, and scavenger experiment proved that e^- and h⁺ were the substantial reactants for Cr(VI) and organics, respectively. The pathway of photocatalytic simultaneous removal of Cr(VI) and organics underwent four steps: adsorption of Cr(VI) and organics on CQDs/TiO₂ surface; production of photo electrons and holes in visible light; reduction of Cr(VI) and oxidation of organics; desorption of Cr(III) and intermediates. Photocatalytic reaction kinetics of Cr(VI) and organics were both confirmed to pseudo first-order reaction. Life span and small scale real application tests both demonstrated that CQDs/TiO₂ had a potential application to wastewater containing Cr(VI) and organics.

Investigation on the role of graphene-based composites for in photocatalytic degradation of phenol-based compounds in wastewater: a review

The ineptitude of conventional water management systems to eradicate noxious compounds leads to the development of advanced treatment systems. The disclosure of graphene-based photocatalytic degradation for the eradication of phenolic compounds has become the "apple of the eye" for many researchers. This review article describes the advanced research progress during the period of 2008-2021 in graphene-based nanocomposites and discusses their different synthesis methods. We will also talk about the applications of nanocomposite in water splitting, dye degradation, solar fuel generations, and organic transformations. Multicomponent heterojunction structure, co-catalyst cohering, and noble metal coupling have been inspected to enhance the photocatalytic performance of graphene-based composite by increasing charge separation and stability. The photocatalytic system's remarkable stability has been described in terms of facile recyclability. The adsorption ability of phenolic compounds has been addressed in the form of Langmuir and Freundlich adsorption isotherm with various factors (pH, concentration, the intensity of light, the effect of catalyst, the effect of time, etc.). The purpose of this review is to survey mechanisms and processes that enlist graphene-based composite in terms of efficacy and dose of catalyst required to attain 99% degradation. Nanoparticles may cause toxicity and a pretext for their toxicity has been mentioned. Finally, it is anticipated that this article could allocate consequential knowledge to fabricating graphene-based composites that are in crucial demand of being discussed in future research.

Layered Molybdenum (Meta)phosphate for Photoreduction of Hexavalent Chromium and Degradation of Methylene Blue under Sunlight Radiance

Noble metal, semiconductor, or metal-free nanomaterials have shown promising applicability as potential photocatalyst materials. A one-step process has been established for the synthesis of layered molybdenum (meta)phosphate [MoO₂(PO₃)₂] using a solvothermal method. The nanopowders were characterized by X-ray diffraction (XRD), UV-visible spectroscopy (UV-vis), scanning electron microscopy (SEM), infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL), surface area analysis (Brunauer-Emmett-Teller (BET)), electron spin resonance (ESR), and high-resolution transmission electron microscopy (HRTEM). Through this study, we demonstrate the use of MoO₂(PO₃)₂ as a photocatalyst for wastewater treatment. The photoreduction of toxic Cr⁶⁺ to Cr³⁺ by layered molybdenum (meta)phosphate is investigated using formic acid as a scavenger. This catalyst has also been used for photodegrading organic dyes like methylene blue. MoO₂(PO₃)₂ has been shown to complete photoreduction of toxic Cr⁶⁺ to Cr³⁺ in 6 min and achieved 78% degradation efficiency for methylene blue in 36 min. The reactive species trapping experiments revealed that the key active species like O₂ ^•-, ^•OH, and h⁺ can exist and play an important role in methylene blue photodegradation.

Process enhancing strategies for the reduction of Cr(VI) to Cr(III) via photocatalytic pathway

This discourse aimed at providing insight into the strategies that can be adopted to boost the process of photoreduction of Cr(VI) to Cr(III). Cr(VI) is amongst the highly detestable pollutants; thus, its removal or reduction to an innocuous and more tolerable Cr(III) has been the focus. The high promise of photocatalysis hinged on the sustainability, low cost, simplicity, and zero sludge generation. Consequently, the present dissertation provided a comprehensive review of the process enhancement procedures that have been reported for the photoreduction of Cr(VI) to Cr(III). Premised on the findings from experimental studies on Cr(VI) reductions, the factors that enhanced the process were identified, dilated, and interrogated. While the salient reaction conditions for the process optimization include the degree of ionization of reacting medium, available photogenerated electrons, reactor ambience, type of semiconductors, surface area of semiconductor, hole scavengers, quantum efficiency, and competing reactions, the relevant process variables are photocatalyst dosage, initial Cr(VI) concentration, interfering ion, and organic load. In addition, the practicability of photoreduction of Cr(VI) to Cr(III) was explored according to the potential for photocatalyst recovery, reactivation, and reuse reaction conditions and the process variables.

Incorporating nitrogen vacancies in exfoliated B-doped g-C3N4 towards improved photocatalytic ciprofloxacin degradation and hydrogen evolution

The e-BCN fabricated from the double calcination process exhibited significant photocatalytic activity towards photocatalytic ciprofloxacin degradation and hydrogen generation.

Graphitic carbon nitride/NH2-MIL-101(Fe) composite for environmental remediation: Visible-light-assisted photocatalytic degradation of acetaminophen and reduction of hexavalent chromium

Herein, an efficient photocatalyst composed of graphitic carbon nitrate and iron-based metal-organic framework (g-C₃N₄/NH₂-MIL-101(Fe)) composite was fabricated by a solvothermal method for the degradation of acetaminophen (AAP) and reduction of Cr(VI) under sunlight illumination. The composite was confirmed by X-ray diffraction. UV-visible spectra showed that the bare g-C₃N₄, pure Fe-MOF, and composite harvest solar light effectively. The photocatalytic experiment indicated that the composite exhibited superior reduction efficiency of Cr(VI) (66%) compared to the bare g-C₃N₄ (35%) and pure Fe-MOF (51%) at pH 7. As the pH decreases from 9 to 2, the reduction efficiency increased. The highest Cr(VI) reduction (91%) was observed at pH 2. On the other hand, the catalyst degraded 94% of AAP at pH 7 compared to the bare g-C₃N₄ (42%) and pure Fe-MOF (60%) in the presence of hydrogen peroxide. A radical scavenger experiment endorsed that the generation of superoxide radicals was the main reason for the AAP degradation. The cyclic stability test indicated that there was no substantial decrease in the degradation efficiency of AAP after ten repeated cycles. The kinetic studies showed that the photodegradation of AAP and reduction Cr(VI) was well-fitted to the first-order kinetics. Gas chromatography-mass spectrometry analysis showed that hydroquinone, aliphatic carboxylic acids, monohydroxy, and dihydroxy paracetamol were the main products formed as a result of such degradation process. Therefore, the iron-based MOF and their composites can be used as effective photocatalysts for pollutants degradation.

Self-Assembled Silver Nanoparticles Decorated on Exfoliated Graphitic Carbon Nitride/Carbon Sphere Nanocomposites as a Novel Catalyst for Catalytic Reduction of Cr(VI) to Cr(III) from Wastewater and Reuse for Photocatalytic Applications

Silver nanoparticles decorated on an exfoliated graphitic carbon nitride/carbon sphere (AgNP/Eg-C₃N₄/CS) nanocomposites were synthesized by an adsorption method with a self-assembled process. These nanoparticles were characterized by different techniques like UV-visible (UV-vis) spectroscopy, photoluminescence (PL) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), thermal gravimetric analysis (TGA), Raman spectroscopy, scanning electron spectroscopy (SEM), transmission electron spectroscopy (TEM), electrochemical impedance spectroscopy (EIS), and ζ potential. AgNP/Eg-C₃N₄/CS nanocomposites showed a higher catalytic reduction activity for the conversion of Cr(VI) into Cr(III) with formic acid (FA) at 45 °C when compared to bulk graphitic carbon nitride (Bg-C₃N₄, Eg-C₃N₄, CS, and Eg-C₃N₄/CS). The kinetic rate constants were determined as a function of catalyst dosage, concentration of Cr(VI), pH, and temperature for the AgNP/Eg-C₃N₄/CS nanocomposite. This material showed higher reduction efficiency (98.5%, k = 0.0621 min^-1) with turnover frequency (0.0158 min^-1) for the reduction of Cr(VI) to Cr(III). It also showed great selectivity and high stability after six repeated cycles (98.5%). Further, the reusability of the Cr(III)-AgNP/Eg-C₃N₄/CS nanocomposite was also investigated for the photocatalytic degradation of methylene blue (MB) under visible light irradiation with various time intervals and it showed good degradation efficiency (α = 97.95%). From these results, the AgNP/Eg-C₃N₄/CS nanocomposite demonstrated higher catalytic activity, improved environmental friendliness, lower cost for the conversion of toxic Cr(VI) to Cr(III) in solutions, and also good reusability.

Highly surface activated carbon to remove Cr(VI) from aqueous solution with adsorbent recycling

To enhance the inferior removal capability of aqueous Cr(VI) by commercial activated carbon under neutral conditions. The emerging ball milling technology was employed and the removal efficiency of Cr(VI) by ball-milled highly activated carbon (HAC) increased from 68.3% to 99.0% under pH 6 and from 42.7% to 77.8% under pH 7 compared to pristine activated carbon (AC), respectively. Raman spectra and Boehm's titration results signified that the enhanced Cr(VI) removal performance of HAC under neutral conditions was associated with the enriched surface acid functional groups, in which the content of COOH groups increased from 0.31 mmol/g to 0.97 mmol/g. Two Cr(VI) removal mechanisms were proposed established on the acid and alkalic solution washed chromium-loaded HAC, involving the reduction of Cr(VI) to Cr(III) subsequently accompany with the formation of chromium hydroxides on the surface and inside the pores of HAC, and the bonding of CrO₄^2- on the surface COOH groups, as confirmed by SEM-EDX element mapping and specific surface area and porosity measurements. The Pseudo-second order model and Freundlich model fitted the adsorption kinetic and isotherm of AC and HAC well severally, suggesting that the specific interaction of Cr(VI) with the HAC surface and the Cr(VI) removal was multi-layer adsorption. Thermodynamic study exhibited the spontaneity of Cr(VI) removal on ball-milled HAC was increased. Reusability and regeneration studies of HAC denoted the potential application on Cr(VI) uptake under neutral conditions.

A novel visible light-driven TiO2 photocatalytic reduction for hexavalent chromium wastewater and mechanism

Titanium dioxide (TiO₂) photocatalyst was prepared with a sol-gel method and its characterizations were analyzed TiO₂ photocatalytic reduction of Cr⁶⁺ was investigated in visible light irradiation and reduction mechanisms were calculated. Prepared TiO₂ is anatase with a bandgap of about 2.95 eV. Experimental results display that almost 100% of Cr⁶⁺ is removed by visible light-driven TiO₂ photocatalytic reduction after 120 min when Cr₂O₇^2- initial concentration is 1.0 mg·L^-1, TiO₂ dosage is 1.0 g·L^-1, and pH value is 3. In acidic aqueous solution, HCrO₄^- is the dominant existing form of Cr⁶⁺ and is adsorbed by TiO₂, forming a complex catalyst HCrO₄^-/TiO₂ with an increase in wavelength to the visible light zone, demonstrated by UV-Vis diffuse reflection spectroscopy. Based on X-ray photoelectron spectroscopy data, it can be deduced that Cr⁶⁺ is adsorbed on the surface of TiO₂ and then reduced to Cr³⁺ in situ by photoelectrons. Self-assembly of HCrO₄^-/TiO₂ complex catalyst and self-reduction of Cr⁶⁺ in situ are the key steps to start the visible light-driven TiO₂ photocatalytic reduction. Furthermore, TiO₂ photocatalytic reduction of Cr⁶⁺ fits well with pseudo-first-order kinetics and has the potential application to treat chemical industrial wastewater.

Nano-remediation of toxic heavy metal contamination: Hexavalent chromium [Cr(VI)]

The inexorable industrialization and modern agricultural practices to meet the needs of the increasing population have polluted the environment with toxic heavy metals such as Cr(VI), Cu²⁺, Cd²⁺, Pb^2+, and Zn²⁺. Among the hazardous heavy metal(loid)s contamination in agricultural soil, water, and air, hexavalent chromium [Cr(VI)] is the most virulent carcinogen. The metallurgic industries, tanneries, paint manufacturing, petroleum refineries are among various such human activities that discharge Cr(VI) into the environment. Various methods have been employed to reduce the concentration of Cr(VI) contamination with nano and bioremediation being the recent advancement to achieve recovery at low cost and higher efficiency. Bioremediation is the process of using biological sources such as plant extracts, microorganisms, and algae to reduce the heavy metals while the nano-remediation uses nanoparticles to adsorb heavy metals. In this review, we discuss the various activities that liberate Cr(VI). We then discuss the various conventional, nano-remediation, and bioremediation methods to keep Cr(VI) concentration in check and further discuss their efficiencies. We also discuss the mechanism of nano-remediation techniques for better insight into the process.

Highly crystalline anatase TiO2 nanocuboids as an efficient photocatalyst for hydrogen generation

Highly crystalline anatase titanium dioxide (TiO2) nanocuboids were synthesized via a hydrothermal method using ethylenediamine tetraacetic acid as a capping agent. The structural study revealed the nanocrystalline nature of anatase TiO2 nanocuboids. Morphological study indicates the formation of cuboid shaped particles with thickness of ∼5 nm and size in the range of 10-40 nm. The UV-visible absorbance spectra of TiO2 nanocuboids showed a broad absorption with a tail in the visible-light region which is attributed to the incorporation of nitrogen atoms into the interstitial positions of the TiO2 lattice as well as the formation of carbonaceous and carbonate species on the surface of TiO2 nanocuboids. The specific surface areas of prepared TiO2 nanocuboids were found to be in the range of 85.7-122.9 m2 g-1. The formation mechanism of the TiO2 nanocuboids has also been investigated. Furthermore, the photocatalytic activities of the as-prepared TiO2 nanocuboids were evaluated for H2 generation via water splitting under UV-vis light irradiation and compared with the commercial anatase TiO2. TiO2 nanocuboids obtained at 200 °C after 48 h exhibited higher photocatalytic activity (3866.44 μmol h-1 g-1) than that of commercial anatase TiO2 (831.30 μmol h-1 g-1). The enhanced photoactivity of TiO2 nanocuboids may be due to the high specific surface area, good crystallinity, extended light absorption in the visible region and efficient charge separation.

Magnetite Functionalized Nigella Sativa Seeds for the Uptake of Chromium(VI) and Lead(II) Ions from Synthetic Wastewater

The aim of the present study was to utilise pristine and magnetite-sucrose functionalized Nigella Sativa seeds as the adsorbents for the uptake of chromium(VI) and lead(II) ions from synthetic wastewater. Prestine Nigella Sativa seeds were labelled (PNS) and magnetite-sucrose functionalized Nigella Sativa seeds (FNS). The PNS and FNS composites were characterized by Fourier-transform infrared spectroscopy (FTIR) and X-ray powder diffraction (XRD). The FTIR analysis of both adsorbents revealed the presence of vibrations assigned to 1749 and 1739 cm-1 (-C=O) for ketonic group for both adsorbents. The amide (-NH) peak was observed at 1533 and 1527 cm-1 on FNS and PNS composites, respectively, whilst the carboxyl group (-COOH) were observed at 1408 cm-1 on both adsorbents. The XRD results of FNS and PNS composites showed a combination of spinel structure and$y$-Fe2O3 phase confirming the formation of iron oxide. The influence of operational conditions such as initial concentration, temperature, pH, and contact time was determined in batch adsorption system. The kinetic data of Cr(VI) and Pb(II) ions on both adsorbents was described by pseudo-first-order (PFO) model which suggested physisorption process. The sorption rate of Cr(VI) ions was quicker, it attained equilibrium in 20 min, and the rate of Pb(II) ions was slow in 90 min. Freundlich isotherm described the mechanism of Pb(II) ions adsorption on PNS and FNS composites. Langmuir best fitted the uptake of Cr(VI) ions on PNS and FNS. The results for both adsorbents showed that the removal uptake of Pb(II) ions increased when the initial concentration was increased; however, Cr(VI) uptake decreased when the initial concentration increased. The adsorption of Cr(VI) and Pb(II) ions on both adsorbents increased with temperature.

Morphology-Controlled Synthesis of α-Fe2O3 Nanocrystals Impregnated on g-C3N4-SO3H with Ultrafast Charge Separation for Photoreduction of Cr (VI) Under Visible Light

Surface functionalization and shape modifications are the key strategies being utilized to overcome the limitations of semiconductors in advanced oxidation processes (AOP). Herein, the uniform α-Fe₂O₃ nanocrystals (α-Fe₂O₃-NCs) were effectively synthesized via a simple solvothermal route. Meanwhile, the sulfonic acid functionalization (SAF) and the impregnation of α-Fe₂O₃-NCs on g-C₃N₄ (α-Fe₂O₃-NCs@CN-SAF) were achieved through complete solvent evaporation technique. The surface functionalization of the sulfonic acid group on g-C₃N₄ accelerates the faster migration of electrons to the surface owing to robust electronegativity. The incorporation of α-Fe₂O₃-NCs with CN-SAF significantly enhances the optoelectronic properties, ultrafast spatial charge separation, and rapid charge transportation. The α-Fe₂O₃-HPs@CN-SAF and α-Fe₂O₃-NPs@CN-SAF nanocomposites attained 97.41% and 93.64% of Cr (VI) photoreduction in 10 min, respectively. The photocatalytic efficiency of α-Fe₂O₃-NCs@CN-SAF nanocomposite is 2.4 and 2.1 times higher than that of pure g-C₃N₄ and α-Fe₂O₃, respectively. Besides, the XPS, PEC and recycling experiments confirm the excellent photo-induced charge separation via Z-scheme heterostructure and cyclic stability of α-Fe₂O₃-NCs@CN-SAF nanocomposites.

TiO2-based catalysts for photocatalytic reduction of aqueous oxyanions: State-of-the-art and future prospects

Nowadays, an increasing discharge of oxyanions to the natural environment has been attracting worldwide attention. TiO₂-based photocatalysis is regarded as one of the most promising technologies for the conversion of toxic oxyanions (such as chromate, nitrate, nitrite, bromate, perchlorate and selenate) to harmless and/or less toxic substances in contaminated waters. Various types of TiO₂-based catalysts have been developed, and each of them exhibits its own advantages in catalytic reduction of oxyanions. However, the application of these nanostructured TiO₂ in real water bodies remains a challenge, with limitations associated with sunlight harvesting abilities, production costs, reuse stability and exposure risks. Herein, we aim to present a critical review on reported TiO₂-based photocatalytic reduction of aqueous oxyanions, provide a comprehensive understanding of the possible reaction pathways of formed active species, and evaluate the reduction performance of different types of TiO₂-based catalysts. In addition, the impact of operating parameters (such as solution pH, temperature, dissolved oxygen and coexisting substances) on catalytic reduction performance is discussed. Furthermore, the perspectives of TiO₂-based photocatalytic reduction of oxyanions are also proposed.

Review of advanced sensor devices employing nanoarchitectonics concepts

Many recent advances in sensor technology have been possible due to nanotechnological advancements together with contributions from other research fields. Such interdisciplinary collaborations fit well with the emerging concept of nanoarchitectonics, which is a novel conceptual methodology to engineer functional materials and systems from nanoscale units through the fusion of nanotechnology with other research fields, including organic chemistry, supramolecular chemistry, materials science and biology. In this review article, we discuss recent advancements in sensor devices and sensor materials that take advantage of advanced nanoarchitectonics concepts for improved performance. In the first part, recent progress on sensor systems are roughly classified according to the sensor targets, such as chemical substances, physical conditions, and biological phenomena. In the following sections, advancements in various nanoarchitectonic motifs, including nanoporous structures, ultrathin films, and interfacial effects for improved sensor function are discussed to realize the importance of nanoarchitectonic structures. Many of these examples show that advancements in sensor technology are no longer limited by progress in microfabrication and nanofabrication of device structures - opening a new avenue for highly engineered, high performing sensor systems through the application of nanoarchitectonics concepts.

One-Pot Synthesis of Pd-Based Ternary Pd@CdS@TiO2 Nanoclusters via a Solvothermal Route and Their Catalytic Reduction Efficiency toward Toxic Hexavalent Chromium

In this work, we report the synthesis of Pd-based ternary Pd@CdS@TiO₂ nanocomposites using molecular precursors. This method is facile, less time-consuming, and cost-effective. This catalyst is prepared within 2 h by a solvothermal route using molecular precursors. Information about the phase, morphologies, elemental mapping, and composition of the nanocomposites was obtained using various characterization techniques. The catalytic activity of the as-prepared Pd-based ternary Pd@CdS@TiO₂ nanocomposites exhibits effective reduction efficiency for the conversion of toxic Cr(VI) to Cr(III) using formic acid as a reducing agent within 5-7 min. To the best of our knowledge, this is the first report on Pd-based ternary Pd@CdS@TiO₂ nanocomposites prepared by a solvothermal route and used as catalysts toward the reduction of hexavalent chromium at room temperature.

Controllable synthesis of graphitic carbon nitride nanomaterials for solar energy conversion and environmental remediation: the road travelled and the way forward

This review discusses advances in the synthesis and design of g-C₃N₄-based nanomaterials and their various photocatalytic and photoredox applications.