Photocatalytic Reduction Of Hexavalent Chromium Research Articles

Enhanced photocatalytic reduction of hexavalent chromium by using piezo-photo active calcium bismuth oxide ferroelectric nanoflakes

The working mechanism of CBO nanoflakes for the reduction of Cr(vi): (a) in the presence of visible light only, and (b) the combined effect of visible light and ultrasonic vibrations.

Bio-Reduced Ag Nanoparticle Decorated on Zno for Enhancement of Photocatalytic Reduction of Hexavalent Chromium and Photocatalytic Degradation of Rhodamine B

Bio-Reduced Ag Nanoparticle Decorated on Zno for Enhancement of Photocatalytic Reduction of Hexavalent Chromium and Photocatalytic Degradation of Rhodamine B

Visible-light-driven Z-scheme protonated g-C3N4/wood flour biochar/BiVO4 photocatalyst with biochar as charge-transfer channel for enhanced RhB degradation and Cr(VI) reduction

For the simultaneous photocatalytic reduction of hexavalent chromium (Cr(VI)) and the degradation of rhodamine B (RhB), directional charge-transfer channels and efficient separation of photogenerated holes and electrons are important. Herein, a Z-scheme heterojunction photocatalyst, protonated g-C3N4/BiVO4 decorated with wood flour biochar (pCN/WFB/BiVO4), was prepared through a hydrothermal reaction and electrostatic self-assembly for Cr(VI) photoreduction and RhB photodegradation. The morphological features, crystalline structure, chemical composition, optical properties, specific surface area, and photoelectrochemical properties of the prepared samples were investigated. The pCN/WFB/BiVO4 photocatalyst exhibited superior removal performance when used to remove Cr(VI) and RhB separately or RhB-Cr(VI) system. The biochar bridge served as a charge-transfer channel between two semiconductors, and the electrons in protonated g-C3N4 (pCN) and BiVO4 achieved a charge balance. This led to the formation of a Z-scheme heterojunction, fast photogenerated charge separation, and a powerful redox ability. The pCN/WFB/BiVO4 photocatalyst provides new insight into the mechanisms responsible for boosting multicomponent photocatalytic reactions, while constituting a promising candidate for wastewater treatment.

Impacts of Mixing Mode on Photocatalytic Reduction of Hexavalent Chromium over Titanium Dioxide Nanomaterial under Various Environmental Conditions

This study explores the effects of initial Cr(VI) concentration, wavelength, hole-scavenger (absence and presence of salicylic acid), and oxygen conditions (aeration by air, nitrogen gas, and mechanical stir only) on photocatalytic reduction of hexavalent chromium over titanium dioxide photocatalyst and the chromic species distribution after photocatalysis. The experimental results show the existence of strong interactions between these factors. The factor of hole-scavenger was more important than the UV light wavelength condition for a reduction of 3 mg Cr(VI) L−1, whereas both factors became important when Cr(VI) concentration increased to 20 mgL−1. The higher the UV wavelength was, the less the amount of chromium retained on the TiO2 surface. The influence of oxygen-containing conditions in the solution on the reduction of 3 mgL−1 Cr(VI) was unobvious, whereas its influence became remarkable for the reduction of 20 mgL−1 Cr(VI) in the presence of SA. The interaction between oxygen-containing factor and other environmental factors, such as Cr(VI) concentration and scavenger presence (SA in this study), is a key factor about the degree of oxygen effect on Cr(VI) photo-reduction and the chromic species distribution. Simple stirring obtained better photocatalytic efficiency than aeration by air or nitrogen gas.

Photocatalytic Reduction of Hexavalent Chromium (Cr6+) Over BiOI Calcined at Different Temperature Under Visible Light Irradiation

AbstractSynthesis of photocatalysts with high charge separation and transfer efficiency are of huge significance in the photocatalytic waste water treatment technology. Herein, we report the phase transformation of bismuth oxyiodide through the facile calcination of BiOI microsphere at different temperatures. Different phases including BiOI, Bi4O5I2 and Bi5O7I were obtained as confirmed by different characterization techniques studied. The synthesized photocatalysts depicted high visible light absorption efficiency in a range of ∼ 655 to 420 nm. The photocatalytic activity of the synthesized photocatalyst were studied on the reduction of the highly carcinogenic and toxic hexavalent chromium as the representative heavy metal in waste water. Bi4O5I2 depicted high photocatalytic activity (k=0.0366 min−1) which was ascribed to high charge separation and transfer as evidenced in PL, photocurrent response and EIS. This study may furnish a new concept in designing a series of phases that are more advantageous in solar energy conversion.

In-situ synthesis of CN@La(OH)3 nanocomposite for improved the charge separation and enhanced the photocatalytic activity towards Cr(VI) reduction under visible light

In this work, we report for the first time a novel graphitic carbon nitride (CN) composited with different weight percentages (5–15%) of La(OH)3 (CN@La(OH)3) for photocatalytic reduction of hexavalent chromium (Cr(VI)) in aqueous solution. In situ fabrication of the CN@La(OH)3 photocatalysts were carried out via a hydrothermal method. The La(OH)3 nanoparticles were deposited onto the surface of CN nanosheets to form heterojunction, as confirmed by series of techniques. Compared to the pure CN and different weight percentages of CN@La(OH)3 nanocomposite, the CN@La(OH)3(10%) nanocomposite exhibited remarkable photocatalytic reduction performance for Cr(VI) under visible light illumination. Such excellent photocatalytic reduction activity ascribed to the more photocatalytic active sites, high visible light harvesting capacity and improved electron-hole separation and transfer efficiency which was confirmed by photocurrent, impedance and photoluminescence results. The photoreduction efficiency and the reduction rate constant was 98.7% and 0.0263 min−1 within 50 min. A possible reaction mechanism for the effective reduction of carcinogenic Cr(VI) is put forward tentatively. Moreover, the developed CN@La(OH)3(10%) nanocomposite also possessed high structural stability and recyclability after five photocatalytic cycles. This work may open up the way into the robust photocatalysis of Cr(VI) in wastewater treatment application.

Facile green synthesis of zero-valent iron nanoparticles using barberry leaf extract (GnZVI@BLE) for photocatalytic reduction of hexavalent chromium

In this study, zero-valent iron (GnZVI) was synthesized using barberry leaf extract (GnZVI@BLE). The physicochemical properties of the final products were characterized by FT-IR, SEM, TEM, and EDS techniques. The results of TEM analysis showed that the obtained iron zero-valent nanoparticles with a diameter between 20 and 40 nm and shell-core structures were successfully synthesized. The results of FT-IR confirmed the presence of various functional groups. The photocatalytic activity of synthesized nanoparticles was investigated by reduction of hexavalent chromium. Laboratory data showed that the presence of GnZVI@BLE as a nanocatalyst in the photocatalytic process could be reduction the hexavalent chromium (Cr (VI)). Photocatalytic data revealed that, when the dosage of nanoparticles was 0.675 g/L, the reduction efficiency of hexavalent chromium was 100%. The kinetics of the reaction follows a pseudo-second-order equation. The constant of reaction rate was 0.4 at pH 2 and 0.5 g/L concentration of GnZVI@BLE.

Simultaneous photocatalytic reduction of hexavalent chromium and oxidation of p-cresol over AgO decorated on fibrous silica zirconia

The co-existence of heavy metals and organic compounds including Cr(VI) and p-cresol (pC) in water environment becoming a challenge in the treatment processes. Herein, the synchronous photocatalytic reduction of Cr(VI) and oxidation of pC by silver oxide decorated on fibrous silica zirconia (AgO/FSZr) was reported. In this study, the catalysts were successfully developed using microemulsion and electrochemical techniques with various AgO loading (1, 5 and 10 wt%) and presented as 1, 5 and 10-AgO/FSZr. Catalytic activity was tested towards simultaneous photoredox of hexavalent chromium and p-cresol (Cr(VI)/pC) and was ranked as followed: 5-AgO/FSZr (96/78%) > 10-AgO/FSZr (87/61%) > 1-AgO/FSZr (47/24%) > FSZr (34/20%). The highest photocatalytic activity of 5-AgO/FSZr was established due to the strong interaction between FSZr and AgO and the lowest band gap energy, which resulted in less electron-hole recombination and further enhanced the photoredox activity. Cr(VI) ions act as a bridge between the positive charge of catalyst and cationic pC in pH 1 solution which can improve the photocatalytic reduction and oxidation of Cr(VI) and pC, respectively. The scavenger experiments further confirmed that the photogenerated electrons (e−) act as the main species for Cr(VI) to be reduced to Cr(III) while holes (h+) and hydroxyl radicals are domain for photooxidation of pC. The 5-AgO/FSZr was stable after 5 cycles of reaction, suggesting its potential for removal of Cr(VI) and pC simultaneously in the chemical industries.

Efficient visible-light-driven photocatalytic reduction of hexavalent chromium by three-dimensional heterostructure PANI@SnS2@carbon sphere

Tin sulfide (SnS2) is an excellent photocatalyst, which is widely used in reduction of heavy metal pollutants. However, its application is hindered by the rapid recombination of photogenerated carriers. In this study, a heterostructure composite composed of PANI, SnS2 and carbon sphere (PANI@SnS2@Cs) was prepared by a hydrothermal approach combined with in-situ oxidative polymerization to release the fast recombination of photogenerated carriers and improve the photoreduction performance. Compared with bare SnS2 and SnS2@Cs, PANI@SnS2@Cs not only provides more efficient photoreduction performance, but also possesses stronger adsorption capability. In neutral solution, 60 mg PANI@SnS2@Cs is capable of reducing 60 mL contaminated water with a concentration of 100 mg/L Cr(VI) within 30 min of solar light. Characterizations including transmission electron microscope and UV–Vis-NIR diffusive reflectance spectra were applied to provide evidences to confirm the heterostructure and the excellent optical performance of PANI@SnS2@Cs. Besides, the mechanism behind PANI@SnS2@Cs was investigated that the photogenerated e- in the lowest unoccupied molecular orbital (LUMO) of PANI can be transferred into the conduction band of SnS2 while h+ in valence band of SnS2 can be transferred into the highest occupied molecular orbital (HOMO) of PANI.

Synthesis of a novel, ternary AgI/CeO2@g-C3N4 nanocomposite with exceptional stability and reusability for visible light-assisted photocatalytic reduction of hexavalent chromium

This work demonstrates a highly efficient photocatalytic reduction of Cr(VI) to Cr(III) by using a AgI/CeO2@g-C3N4 nanocomposite, under visible light (Vis) irradiation. The as-prepared samples were characterized by FT-IR, XRD, FE-SEM, TEM, XPS, DRS, PL, PC, and BET. Optimization of the treatment conditions included systematic parameterization, such as initial solution pH, photocatalyst dosage, and Cr(VI) concentration were investigated in the reaction. Compared with pure g-C3N4, CeO2, AgI, and CeO2@g-C3N4 binary nanocomposite, the as-made AgI/CeO2@g-C3N4 ternary nanocomposite showed significantly higher photo-reduction efficiency. The ternary photocatalyst shows a 99.6% reduction efficiency for Cr(VI) (15 mg/L) within 75 min at pH = 3.0. The highest rate constant value (0.058 min−1) was calculated for ternary photocatalyst that is 2.1 times higher than CeO2@g-C3N4/Vis process. The improved activity could be related to the effective electron/hole (e-/h+) pairs separation by forming a hetero-junction structure. The XPS analysis of recovered photocatalyst shows the presence only of Cr(III), which indicated a complete photo-reduction of Cr(VI) over AgI/CeO2@g-C3N4 nanocomposite. Moreover, the as-made photocatalyst exhibited more than 91% reduction efficiency after four cycles under the optimum reaction conditions. Based on the radical trapping experiments, photo-generated electrons (e−), and superoxide radicals O2∙- were the dominant active species in photo-catalytic Cr(VI) removal.

Heterogeneous photocatalytic reduction of hexavalent chromium by modified Ag, Cu co-doped tungsten oxide nanoparticles

A highly efficient photocatalyst of Ag, Cu co-doped tungsten oxide nanoparticles (WO3) with 5.0 mol% Ag and 0.5 mol% Cu contents was synthesised by using a simple co-precipitation method at a low temperature. The nanoparticles had a large specific surface area, a rectangular shape and a controlled size. The Ag metal ions acted as loading on the surface, and the Cu metal ions disappeared as decorated into the WO3 structure to enhance electron trapping and reduce the bandgap energies. This enhanced the photocatalytic reduction, a Cr6+ solution. The intense and narrow peak of XRD patterns corresponded to strong crystallinity of monoclinic WO3 with rectangular particles of 30–40 nm. In addition, the high value of the specific surface area of co-doped WO3 (34.0 m2/g) was key to enabling Cr6+ ions to adsorb to the large active sites of the catalyst. A TEM micrograph of the modified WO3 confirmed that the Ag hemisphere particles deposited on the surface of the WO3 and Cu2+ also might be incorporated into the WO3 microstructure. Results from the X-ray photoelectron spectroscope (XPS) showed a drastic shift of binding energy for all oxidation states. This was consistent with EDX and the results of spontaneous element mapping. DRS-UV-vis spectrum of Ag, Cu-WO3 annealed the red-shift to the wavelength of ca. 475 nm, and it revealed bandgap energy of 2.61 eV that helped to increase the rate of electron generation. The photoreduction activities of Cr6+ over the optimum catalyst showed extremely fast decay: 82.38% in only 15 min under low-energy visible light radiation. Eventually, the optimum sample decayed the percentage of C6+ around 8 times as much as pure WO3.

Fabrication of In2O3-SnS2 Core–Shell Heterojunction for Efficient Photocatalytic Removal of Hexavalent Chromium

The development of heterojunction composites with core–shell structure could effectively facilitate the separation of carriers. In this work, a novel In2O3-SnS2 (IOS) core–shell heterojunction photocatalyst was successfully synthesized for the efficient photocatalytic reduction of hexavalent chromium (Cr(VI)), namely, the In2O3 nanorods were obtained through successive hydrothermal and carbonization process using urea and glucose as template. Then, SnS2 nanosheets were successfully in situ coated onto In2O3 nanorods. The spectroscopic characterization and photo-electrochemistry test indicated that the IOS core–shell heterojunction could effectively accelerate the separation and transport of carriers and suppress the recombination of carriers. The photocatalytic performance of IOS was evaluated by photocatalytic reduction Cr(VI). The results showed that IOS-4 samples almost completely removed Cr(VI) (20 mg/L), within 90 min under visible light, which was superior to pure In2O3 and SnS2. Furthermore, IOS samples also possess excellent stability, the removal efficiency was still maintained at 90% after five cycles. This work provided a reliable method for designing core–shell heterojunctions for the photocatalytic removal of Cr(VI) under visible light.

Boosting charge separation and nitrogen vacancies in graphitic carbon nitride by implanted strontium vanadate for highly efficient photocatalytic reduction of hexavalent chromium.

Strontium vanadate nanoparticles embedded graphitic carbon nitride (g-C3N4/Sr2V2O7) was facilely prepared in situ via a hydrothermal method. It was shown that the Sr2V2O7 nanoparticles implanted into g-carbon nitride had a small size and high distribution. Importantly, compared with some other photocatalysts, the as-prepared g-C3N4/Sr2V2O7 nanohybrid showed excellent photocatalytic activity for reduction of Cr(vi), and as high as 99% efficiency for Cr(vi) reduction (100 mg L−1) was reached within 8 min. Moreover, its activity was hardly changed after five cycles, demonstrating that the developed g-C3N4/Sr2V2O7 nanohybrid was highly stable and promising an efficacious disposal of Cr(vi) in water. It was confirmed that the improved charge separation owing to more nitrogen vacancies in the hybrid was the main reason for the improved performance of the g-C3N4-Sr2V2O7 nanohybrid.

Furfural mediated synthesis of silver nanoparticles for photocatalytic reduction of hexavalent chromium

Industrial effluents contain various toxic substances and the removal or conversion of these substances to non-toxic products is highly crucial. The work emphasized on the green synthesis of silver nanoparticles with furfural, a promising derivative of lignocellulosic biomass (FAgNPs) for exploring its catalytic efficiency in the photocatalytic reduction of hexavalent chromium [Cr(VI)]. A peak at 410 nm was obtained with a molar ratio of 1:1 (silver nitrate to furfural) at pH 9. The effect of initial concentration of Cr(VI), pH, catalyst dosage, time on reduction of Cr(VI) has been investigated. The photocatalytic efficiency of 1 g/L FAgNPs was studied and 9% to 88% Cr(VI) reduction was observed with decreasing Cr(VI) concentration at pH 2. Under UV illumination, electron–hole pairs generated in FAgNPs resulted in the migration of photo generated electrons for the reduction process which is highly pH dependent. The photocatalytic reduction process can be explained by Langmuir–Hinshelwood model and fits into pseudo-first order kinetics. HaCaT cells exposed to Cr(VI) after being treated with FAgNPs for various time period exhibited an increase in cell viability with time which further substantiated its efficiency. Therefore, the study was an attempt to prepare a nanomaterial by a simple, facile method for the photocatalytic reduction of hexavalent chromium with a focus on environmental remediation.

A novel Au/g-C3N4 nanosheets/CeO2 hollow nanospheres plasmonic heterojunction photocatalysts for the photocatalytic reduction of hexavalent chromium and oxidation of oxytetracycline hydrochloride

Au/g-C3N4/CeO2 plasmonic heterojunction has been successfully fabricated, which was constructed by forming the three-dimensional hollow CeO2 mesoporous nanospheres on the two-dimensional Au/g-C3N4 nanosheets. The photocatalysts not only exhibited excellent catalytic reduction activity against Cr6+, but also oxidized oxytetracycline hydrochloride (OTH), and OTH catalytic degradation route was suggested by the degradation products determined by GC–MS. The improvement of photocatalytic performance comes from the high absorption capability of visible light and effective interface charge separation. Finally, it speculated that the Au/g-C3N4/CeO2 heterostructures followed Z-scheme charge transfer mechanism in this study, which is verified by the analysis of experiment and theoretical calculation results.

Noble-metal-free ultrathin MXene coupled with In2S3 nanoflakes for ultrafast photocatalytic reduction of hexavalent chromium

The conversion of highly toxic heavy metal hexavalent chromium (Cr(VI)) in the natural environment into low-toxic trivalent chromium (Cr(III)) is one of the research topics of great concern. Herein, a binary noble-metal-free photocatalyst consisting of ultrathin MXene (Ti3C2) and indium sulfide (In2S3) is prepared for ultrafast photocatalytic reduction of Cr(VI) to Cr(III) under visible light. A simple electrostatic assembly and reflow method uniformly coats the In2S3 nanoflakes on the surface of hollow Ti3C2 nanospheres, which effectively boost the separation of charges and thus enhance the photocatalytic performance. Thus, the prepared heterojunction can reduce Cr(VI) to Cr(III) completely within 6 min with an ultrafast rate constant of k = 0.739 min−1. More importantly, the appropriate band structure ensures the stable existence of Cr(III). Therefore, it provides a new insight for economic MXene-based binary photocatalysts for efficient environmental remediation.

Fabrication of copper phthalocyanine/reduced graphene oxide nanocomposites for efficient photocatalytic reduction of hexavalent chromium

Copper(II) phthalocyanine (CuPc) and non-peripheral octamethyl-substituted copper(II) phthalocyanine (N-CuMe2Pc) were combined with reduced graphene oxide (rGO) via a precipitation method to form CuPc/rGO and N-CuMe2Pc/rGO nanocomposites, respectively. CuPc nanorods are distributed on rGO, and N-CuMe2Pc exists as nanorods and nanoparticles on rGO. The Cr(VI) removal ratio of N-CuMe2Pc/rGO exposed in simulated sunlight is 99.0% with a fast photocatalytic reaction rate of 0.0320 min−1, which is approximately 1.5 times faster than that of CuPc/rGO (0.0215 min−1) and far surpasses that of pristine phthalocyanine and rGO. As an electron acceptor, rGO can suppress the recombination of photo-induced electron-hole pairs and also can provide a large surface area for Cr(VI) removal, both of which are beneficial to the reducing capacity of the nanocomposites. The higher removal efficiency of N-CuMe2Pc/rGO compared with that of CuPc/rGO is attributed to the higher specific surface area, higher light harvesting, higher conductivity and more negative lowest unoccupied molecular orbital level of N-CuMe2Pc/rGO. The N-CuMe2Pc/rGO nanocomposite shows excellent photochemical recyclability which is essential for application in wastewater treatment.

Phosphorus-doped g-C3N4/SnS nanocomposite for efficient photocatalytic reduction of aqueous Cr(VI) under visible light

In this study, a novel phosphorus-doped g-C3N4/SnS (P-CN/SnS) nanocomposite for photocatalytic reduction of hexavalent chromium (Cr(VI)) in aqueous solution was successfully synthesized via a hydrothermal method. The SnS nanoparticles were loaded onto the surface of P-CN nanosheets to form tight interfaces, as confirmed by morphology observations. It was found that the P-CN/SnS nanocomposite displayed higher photocatalytic ability under visible light compared to non-composite catalysts. The unique structure of the P-CN/SnS nanocomposite provides excellent photocatalytic activity for Cr(VI), which is likely attributable to a large surface area, a wide spectral response, and the inhibition of electron-hole recombination. The best photocatalytic activity was exhibited by P-CN/SnS (30 wt% of SnS on P-CN), which completely removed Cr(VI) (100 mg/L) from solution within 60 min. Moreover, the high structural stability of the nanocomposite ensured four cycles with little activity loss. The outstanding performance of this new material provides new insights into the photocatalysis of Cr(VI).

Fabrication of effective visible-light-driven ternary Z-scheme ZnO-Ag-BiVO4 heterostructured photocatalyst for hexavalent chromium reduction

A simple hydrothermal technique was used for the synthesis of a ternary ZnO-Ag-BiVO4 nanocomposite. The morphology, structure and optical performance of ZnO-Ag-BiVO4 were analyzed by XPS, TEM, XRD, FT-IR, FE-SEM, PL, and DRS. The sharp peaks of ZnO-Ag-BiVO4 indicated the good crystalline nature of the photocatalyst. FE-SEM images depicted the hexagonal structure of ZnO and the plate-like structure of BiVO4. The heterostructured catalyst Ag-ZnO-BiVO4 is capable of reducing 20 ppm of Cr(VI) from potassium dichromate in 100 mL aqueous solution to 97% within 70 min under visible light. The first-order rate constant value of 0.0371 min−1 and a 97% reduction for the ZnO-Ag-BiVO4 catalyst show its maximum photoreduction efficiency. The optimum pH for effective Cr(VI) reduction on ZnO-Ag-BiVO4 is 5 and it is steady and recyclable. The photo generated electron acts mainly in the photocatalytic reduction of hexavalent chromium. The photocurrent analysis shows that the ZnO-Ag-BiVO4 composite material can enhance the photoexcited charge production, separation and migration, which improved the photocatalytic reduction of Cr(VI). Proposed Z-scheme mechanism of ternary ZnO-Ag-BiVO4 clearly explains the electron-hole separation and higher efficiency of this nanocomposite in the photoreduction.

Fabrication of CdS/P2MoxW18-x nanospheres with type II heterostructure for photocatalytic reduction of hexavalent chromium

A serial of CdS/POM (POM denotes polyoxometalate: P2MoxW18-x, x = 3, 9, 15) nanospheres were fabricated through electrostatic self-assembly method. For the first time, the semiconductor properties of the three polyoxometalates were investigated to direct the rational design of the type-II heterojunction photocatalysts. The morphology and semiconducting properties of the as-prepared materials were characterized by using X-ray powder diffraction, scanning electron microscopy, UV–vis diffuse reflectance spectra, Mott-Schottky plots, surface photovoltage spectroscopy and time-resolved photocurrent curves. The band gap of P2Mo3W15, P2Mo9W9 and P2Mo15W3 is taken respectively to be 2.34 eV, 2.26 eV and 2.15 eV. The average diameters of CdS and CdS/P2MoxW18-x (X = 3, 9, 15) nanospheres are all around 80–100 nm, and CdS/P2MoxW18-x (X = 3, 9, 15) nanospheres have better photoelectric performance than pure CdS. The formation of type II heterostructure between CdS and P2MoxW18-x could promote the separation and transfer of photoinduced electron-hole pairs, so the photocatalytic activities of CdS/POM nanospheres for Cr(VI) reduction were improved significantly. Among them, the CdS/P2Mo15W3 photocatalyst exhibited the highest photocatalytic yield of 64% for the Cr(VI) removal. Our study provides new insight into developing novel CdS/P2MoxW18-x nanocomposite photocatalysts for environmental applications.