Simultaneous Photocatalytic Reduction Of Cr Research Articles

Porous ionic polymers as a photocatalyst: Self-assembly of donor-acceptor by regulating the valence state of photo-responsive groups

The world is on high alert against trace but extremely hazardous pollution of heavy metals and organic dyes in wastewater. Among various treatments, porous photocatalyst with dual functions of adsorptive enrichment and photodegradation are highly desirable yet challenging. Herein, we demonstrate a robust donor-acceptor (D-A) self-assembly by regulating the valence state of photo-responsive groups to construct porous ionic. The density function theory calculations reveal that the excitons induced on the neutral pyridine group (D) can be attracted by its neighboring cationic pyridinium one (A), thus the distinct charges separation and transfer enhancement can be realized in one PIP-based photocatalyst. Guided by this, a partially ionized pyridine-pyridinium (PB-PIPs) photocatalysts are fabricated, which show excellent adsorption capacity for the heavy metal Cr(VI) anion on pyridinium cations and rhodamine B (RhB) on the pyridine rings. Significantly, the target-adsorptions promote the accurate deliveries of excitons, that the simultaneous photocatalytic reduction of Cr(VI) and oxidation of RhB are achieved. The photodegradation performance of PB-PIPs under visible-light irradiation is superior to the state-of-the-art organic photocatalysts. Therefore, this convenient strategy of the valence state regulation provides a promising solution to the focus of synergistically visible-light harvesting, charges separating and transferring, and target-delivering in photocatalysis.

Efficient all-in-one removal of total chromium over nonconjugated polymer-inorganic ZnIn2S4 semiconductor hybrid

Chromium (Cr)-containing wastewater has caused a serious threat to the environment due to its high toxicity and mobility. The traditional Cr removal methods are generally based on an inconvenient two-step process with the first transformation of Cr(VI) to Cr(III) and the consecutive removal of Cr(III) by precipitation. Herein, we demonstrate the efficient all-in-one removal of total Cr through the simultaneous photocatalytic reduction of Cr(VI) to Cr(III) and in-situ fixation of Cr(III) over the nonconjugated polymer engineered ZnIn2S4 (P-ZIS) photocatalyst. By in-situ polyvinylpyrrolidone (PVP) modification of ZIS during the preparation process, the resulted P-ZIS can completely reduce Cr(VI) within 60 min under visible light irradiation. The kinetics of Cr(VI) reduction over P-ZIS is 2.8 times as that of pure ZIS, which is proved to be benefited from the enhanced light absorption, uplifted conduction band for strengthening reducibility, and accelerated charge carrier transfer. Moreover, as compared to ZIS, P-ZIS also exhibits significantly improved in-situ adsorption ability for Cr(III), thus resulting in efficient all-in-one elimination of total Cr within a single system. We show that this polymer engineered strategy could be a facile and versatile protocol for modulating the electronic structure and surface chemistry of the semiconductor photocatalysts towards complete, safe, and cost-efficient removal of Cr.

Synthesis of a stable iron(iii)–organic framework for a visible light induced simultaneous photocatalytic reduction of Cr(vi) and the degradation of organic dyes in water

A stable microporous Fe-MOF was synthesized as single-crystals and was applied as a bifunctional catalyst for the photocatalytic Cr(vi) reduction and organic dyes degradation.

Simultaneous photocatalytic Cr(VI) reduction and phenol degradation over copper sulphide-reduced graphene oxide nanocomposite under visible light irradiation: Performance and reaction mechanism

The contamination of water by synthetic organic molecules and trace metals is a growing challenge, in spite of the enormous research efforts being made in the field of water treatment. In this study, reduced graphene oxide-copper sulphide (rGO-CuS) nanocomposites of different rGO/CuS (2/1, 1/1, 1/2) molar ratios were fabricated via a facile one-step hydrothermal method. The nanocomposite materials, named hereafter as 2rGO-CuS, rGO-CuS and rGO-2CuS, were characterized using various analytical techniques, including X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS) and UV–visible spectrophotometry. The photocatalytic performance of the nanocomposites was assessed under visible light irradiation (λ > 420 nm) for the simultaneous photocatalytic reduction of Cr(VI) and phenol degradation. It was found that rGO-2CuS achieved a remarkable enhancement of the photocatalytic activity among the prepared nanocomposites for the degradation of phenol and reduction of Cr(VI). Therefore, the simultaneous photocatalytic phenol degradation and Cr(VI) reduction over rGO-2CuS sample was further investigated. The experimental results revealed that rGO-2CuS catalyst maintained good degradation efficacy of mixed pollutants after 6 runs and dissolved oxygen was found to be essential to promote Cr(VI) reduction and phenol degradation. A detailed photocatalytic activity under visible light irradiation mechanism was proposed based on quenching experiments and fluorescence measurements.

Photocatalytic removal of cyanide and Cr(IV) from wastewater in the presence of each other by using TiO 2 /UV

Cyanide (CN) and chromium (VI) are hazardous, toxic pollutants whose presence in water and wastewater can seriously affect human and its environment, hence the necessity of controlling these contaminants. In this work, by using TiO 2 /UV, the simultaneous photocatalytic reduction of Cr(VI) and oxidation of CN in aqueous suspensions were investigated. Variations of this study were pH, pollutant concentration, and reaction time. CN and total chromium were determined by Titrimetric method, and Colourimetric method, respectively. Maximum removal of chromium and CN occurs at pH 2.3 and 10, respectively. In the binary system, the electron-hole recombination is blocked and as a result, total efficiency increases. An increase in the initial concentrations of CN and Cr(VI) led to a rise in the reduction efficiency of Cr(VI) and CN up to an optimum amount, and then it went into decline as the concentration of the pollutants increased any further. The maximum quantum efficiency of Cr(VI) photoreduction with CN being present was calculated to be 23% at room temperature. Furthermore, the kinetic experimental was investigated and it became evident that the pseudo-first-order kinetic model is capable of defining Cr(VI) and CN both. Finally, based on the obtained results, a TiO 2 system is proposed to remove both pollutants.

All-solid-state Z-scheme Co9S8/graphitic carbon nitride photocatalysts for simultaneous reduction of Cr(VI) and oxidation of 2,4-dichlorophenoxyacetic acid under simulated solar irradiation

An all-solid-state Z-scheme heterojunction photocatalyst, Co9S8/graphitic carbon nitride (Co9S8/g-C3N4), was synthesized via a hydrothermal method. The photocatalyst was used in the simultaneous photocatalytic reduction of Cr(VI) and oxidation of 2,4-dichlorophenoxyacetic acid (2,4-D). Co-Nx bonds in Co9S8/g-C3N4 promoted the formation of a well-combined interface between Co9S8 and g-C3N4. The CoNx bonds served as an electron-transfer medium in Co9S8/g-C3N4, leading to the formation of a Z-scheme photocatalytic system. Photocatalytic reactions were carried out under simulated solar irradiation and involved simultaneous Cr(VI) reduction and 2,4-D oxidation. Co9S8/g-C3N4 showed better photocatalytic activity than pristine g-C3N4 and Co9S8 owing to its effective separation of photo-generated charge-carriers and high redox potential. Compared with the single pollutant system, photocatalytic efficiencies over Co9S8/g-C3N4 were enhanced in the Cr(VI)/2,4-D composite pollutant system. This was because the simultaneous reduction and oxidation reactions promoted the separation of charge carriers. This study presents a simple, economic, and efficient method to synthesize an all-solid-state Z-scheme Co9S8/g-C3N4 photocatalytic system. The system has good potential in the solar light photocatalytic removal of complex pollutants.

Simultaneous Photocatalytic Reduction of Cr(Ⅵ) and Oxidation of SSA by Carbon Nitride

Carbon nitride is a novel nonmetal semiconductor photocatalyst, which has developed into an ideal environmental treatment material in recent years. Graphite carbon nitride(g-C3N4) was prepared through pyrolysis melamine, and the structure, morphology and optical properties of samples were characterized by X-ray diffraction(XRD), transmission electron microscopy(TEM) and UV-Vis diffuse reflectance spectra(UV-Vis DRS). The potential application of g-C3N4 in the simultaneous photocatalysis reduction of Cr(Ⅵ) and oxidation of sulfosalicylic acid(SSA) was further explored. And the effects of different conditions such as catalyst dosage, pH and initial concentration ratio of Cr(Ⅵ) with SSA on the simultaneous photocatalysis were also investigated. The results showed that when the catalyst dosage was 0.5 g·L-1, pH=2, the initial concentration ratio of Cr (VI) and SSA was 1:4(10 mg·L-1:40 mg·L-1), optimal simultaneous photocatalysis efficiency was achieved, which was more than 3 times higher than that of the separated photoreduction or photooxidation reactions. Within 3 hours, the reduction ratio of Cr(Ⅵ) and oxidation ratio of SSA could reach 98.9% and 93.4%, respectively. The mechanism of simultaneous photocatalysis was discussed. Cr(Ⅵ) was reduced by electrons and SSA was oxidized by the combined function of hole, O2·- and·OH under visible light.

Insight into highly efficient simultaneous photocatalytic removal of Cr(VI) and 2,4-diclorophenol under visible light irradiation by phosphorus doped porous ultrathin g-C3N4 nanosheets from aqueous media: Performance and reaction mechanism

Carbon nitride (g-C3N4) has attracted great attention for its wide applications in hydrogen evolution and photocatalytic degradation. In this study, phosphorus doped porous ultrathin carbon nitride nanosheets (PCN-S) were prepared successfully via the element doping and thermal exfoliation method. The prepared PCN-S was characterized by XRD, SEM, TEM, N2-adsorption-desorption measurement, FT-IR, XPS, UV–vis diffuse reflectance spectra, photoluminescence (PL), photocurrent response (I-t) and EIS. The results show that PCN-S owns regular crystal structure of g-C3N4, large specific surface areas and nanosheet structure with lots of in-plane pores on its surface, excellent chemical stability, and broad light response to the whole visible light region, which was attributed to the doping of phosphorus element. Under visible light irradiation, the photocatalytic reduction of Cr(VI) over different samples indicated that the P doping and porous nanosheet structure play an important role for the enhanced performance of PCN-S. The reason was that P element doping can broaden the visible light response region, and large specific surface areas from the porous nanosheet structure can provide quantities of active sites for the photocatalytic reaction. Then the detailed study on the PCN-S for simultaneous photocatalytic reduction of Cr(VI) and oxidation of 2,4-diclorophenol (2,4-DCP) was conducted. The experiments results show that low pH value and enough dissolved oxygen were found to promote Cr(VI) reduction and 2,4-DCP oxidation. The detailed photocatalytic mechanism was proposed. The strategies used in this study could provide new insight into the design of g-C3N4 based materials with high photocatalytic activity, and present potential for the treatment of Cr(VI)/2,4-DCP or other mixed pollutants in wastewater.

Characterization and catalytic performance of B-doped, B–N co-doped and B–N–F tri-doped TiO2 towards simultaneous Cr(VI) reduction and benzoic acid oxidation

B-doped, B–N co-doped and B–N–F tri-doped TiO2 catalysts were prepared via a sol–gel method using H3BO3, NH4Cl and NH4F as B, N and N–F dopant precursors, respectively. N-doped, N–F co-doped and a pure TiO2 (undoped) samples were also prepared by the same method as blank samples, for comparison. The catalysts were evaluated for the simultaneous photocatalytic reduction of Cr(VI) and oxidation of benzoic acid (BA). XRD analysis shows the formation of pure TiO2 anatase phase in all cases, with crystallite size varying from 10.1nm to 14.6nm for all doped samples. UV–vis DRS spectra show narrowing of the band gap (Eg) for all doped samples, compared to the undoped TiO2. Concomitant presence of N and F ions i.e. in N–F co-doped and B–N–F tri-doped TiO2 catalysts, causes the most pronounced decrease of Eg while B–N co-doped and B–N–F tri-doped TiO2 have lower Eg values than mono B-doped but higher than N-doped and N–F co-doped TiO2. B and N doping also causes an increase of specific surface area and a decrease in mean pore diameter. Electron Paramagnetic Resonance (EPR) spectroscopy was employed for a detailed investigation of the structure and photoexcited species formed in these catalysts. The EPR data show that N-doping forms Nb photoexcited species which react as holes, Ti4+–O− oxygen vacancies (OVs) and Ti3+ lattice and surface defects. B-doping enhances the formation of Ti4+–O− oxygen vacancies and the quantities of O2H and OH radicals. In B-doped and B–N co-doped samples, boron causes the most prominent induction of Ti3+ lattice centers. Boron in interstitial sites, with oxygen atoms [BO3] behaves as a three-electron donor leading to the formation of B3+ and reduction of Ti4+ to Ti3+. Under UV–vis irradiation, Cr(VI) reduction followed the trend: TB1>TBN1>TBNF1>TNF1>TN1>undoped TiO2; while benzoic acid (BA) oxidation trend was: TB1>TBNF1>TBN1>TNF1>TN1>undoped TiO2. Cr(VI) reduction and BA oxidation rates are in agreement with EPR-detected Ti4+–O− oxygen vacancies and O2H, OH photoexcited radicals, respectively.

Preparation, characterization of N–I co-doped TiO2 and catalytic performance toward simultaneous Cr(VI) reduction and benzoic acid oxidation

Four N–I co-doped TiO2 catalysts having Ti:N/I molar ratios of 1:1, 1:3, 1:5 and 1:10 were prepared via a sol–gel method using NH4I as N–I dopant precursor. A pure TiO2 (undoped) sample was also prepared by the same method for comparison. The catalysts were evaluated for the simultaneous photocatalytic reduction of Cr(VI) and oxidation of benzoic acid (BA). TiO2 anatase phase was formed for all N–I co-doped catalysts as shown by XRD. UV–vis diffuse reflectance spectra showed that N–I co-doping resulted in increased absorption at visible wavelengths and a decrease of the band gap energy (Eg). The smallest Eg value of 2.34eV was observed for the 1:5 Ti:N/I molar ratio. The structure and photodynamics of the TiO2 catalysts was investigated in detail by Electron Paramagnetic Resonance (EPR) spectroscopy. The EPR data showed: [i] formation of non-photoactive NO centers and photoinduced Nb paramagnetic species as a result of N doping, [ii] photoinduced Ti3+ surface ions, and [iii] formation of surfacial oxygen O2− radical ions and trapped holes TiO4+–O−. The Nb species act upon the narrowing of the band gap and the production of photogenerated electrons, i.e. Ti3+ surface ions. These Ti3+ surface ions have a key role, capturing gas O2 and supporting both reduction and oxidation process. Cr(VI) reduction by the N–I co-doped catalysts followed the trend: TNI5>TNI10>TNI1>TNI3>TiO2[undoped] which correlates with the concentration of Nb species formed and the narrowing of Eg values. Oxidation of benzoic acid (BA) followed a more complex trend as follows: TNI1>TNI3>TNI10>TNI5 which is the reverse of the trend for trapped holes TiO4+–O−. Finally progressive microwave EPR saturation experiments show that Nb species are located deeper in the TiO2 lattice in particles with narrower Eg values.

Simultaneous Photocatalytic Reduction of Cr(VI) and Oxidation of Benzoic Acid in Aqueous N-F-Codoped TiO2Suspensions: Optimization and Modeling Using the Response Surface Methodology

The simultaneous photocatalytic reduction of Cr(VI) and oxidation of benzoic acid (BA) in aqueous suspensions using N-F-codoped TiO2and simulated solar irradiation were investigated in the present study. Chemometric optimization tools such as response surface methodology (RSM) and experimental design were used to model and optimize selected operational parameters of the simultaneous photocatalytic reduction of Cr(VI) and oxidation of BA. RSM was developed by considering a central composite design with three input variables, that are, N-F-codoped TiO2mass, ratio of Cr/BA, and pH. The removal of Cr(VI) and BA in binary systems, containing both Cr(VI) and BA, showed a synergistic photocatalytic decontamination as BA significantly facilitated Cr(VI) reduction, whereas Cr(VI) accelerated also BA degradation. Due to the anionic-type adsorption onto TiO2and its acid-catalyzed photocatalytic reduction, the removal of Cr(VI) decreased with increasing pH, while the degradation of BA followed also the same trend. Under the optimum conditions (N-F-TiO2) = 600 mg L−1, ratio of Cr(VI)/BA = 5, pH = 4, the removal for both Cr and BA followed a pseudo first-order kinetic model. It was found that the selected variables have significant effect both on Cr(VI) removal and BA degradation efficiency. The results revealed the feasibility and the effectiveness of using N-F-codoped TiO2as photocatalyst for simultaneous decontamination of Cr(VI) and organic pollutants such as BA due to the appropriate oxidation and reduction ability of the photogenerated h+VB-e−CBpairs.

Reconsideration to the deactivation of TiO 2 catalyst during simultaneous photocatalytic reduction of Cr(VI) and oxidation of salicylic acid

Photocatalytic reduction/oxidation and deactivation of TiO 2 photocatalyst was investigated in the systems composed of Cr(VI) and salicylic acid. The selection of analysis method of Cr(IV) was very important to the monitoring of the photocatalytic process. It was found that as previously reported, serious deactivation of TiO 2 catalyst in the simultaneous photo-reduction of Cr(VI) and oxidation of salicylic acid was incorrectly observed if the Cr(VI) level was analyzed by directly monitoring the absorbance at characteristic 348 nm band of Cr(VI), because it seriously suffers from the interferences of the intermediates generated from the degradation of salicylic acid. By using an appropriate method to determine the Cr(VI) concentration, it was observed that all the added Cr(VI) could be reduced, not showing marked deactivation of the photocatalyst. A long time photocatalytic reduction of Cr(VI) under UV illumination induced the deposition of Cr(III) species on the surface of TiO 2 particles, which could cause a mild deactivation of the photocatalyst. However, the accompanied oxidation of salicylic acid was demonstrated to depress the deactivation effect of the deposited Cr(III) species on the photocatalytic activity of the TiO 2 photocatalyst.

Simultaneous photocatalytic reduction of Cr(VI) and oxidation of bisphenol A induced by Fe(III)–OH complexes in water

Simultaneously photocatalytic reduction of Cr(VI) and oxidation of bisphenol A (BPA) in aqueous solution in the presence of Fe(III)–OH complexes were investigated under a 250 W metal halide lamp ( λ ≥ 365 nm). Synergy effect of the simultaneous photocatalytic oxidation and reduction of both pollutants was achieved. The effects of initial pH value, initial concentration of BPA, Cr(VI) and Fe(III) were preliminarily investigated. The results showed that both photocatalytic reduction of Cr(VI) and degradation of BPA could occur simultaneously in the Fe(III)/Cr(VI)/BPA ternary system, and the rates of photocatalytic reduction of Cr(VI) and the oxidation of BPA were more rapid at a low pH range of 2.0–3.0. The increase of the initial concentration of Fe(III) was favorable to both photocatalytic reduction of Cr(VI) and oxidation of BPA. The reduction efficiency of Cr(VI) decreased with increasing initial concentrations of Cr(VI) and BPA, but the degradation efficiency of BPA was not changed obviously at different Cr(VI) concentrations.

Simultaneous photocatalytic reduction of Cr(VI) and oxidation of phenol over monoclinic BiVO 4 under visible light irradiation

BiVO 4 powder with monoclinic structure was prepared and used as a visible-light catalyst simultaneously for the photooxidation of phenol and the photoreduction of Cr(VI). The photocatalytic efficiency was found to be rather low for either single phenol solution or single Cr(VI) solution. However, the photocatalytic reduction of Cr(VI) and photocatalytic oxidation of phenol proceed more rapidly for the coexistence system of phenol and Cr(VI) than for the single process, showing synergetic effect between the oxidation and reduction reactions. For the simultaneous photocatalytic reduction–oxidation process, the first-order kinetic constant of phenol degradation was 0.0314 min −1, being about six times higher than that for the photocatalytic process of single phenol. This result reveals the feasibility of using Cr(VI) as the electron scavenger of mBiVO 4-mediated photocatalytic process of phenol degradation, and gives us an enlightenment to employ other semiconductor with a better visible light response but with a more positive band edge to efficiently degrade organic pollutants. This is the first report for simultaneous photocatalytic reduction of Cr(VI) and removal of phenol under visible light irradiation using photocatalyst mBiVO 4.