Kinetics of photocatalytic degradation of trichloroethylene in aqueous colloidal solutions of TiO2 and WO3 nanoparticles

Performance and Mechanisms of Sulfidated Nanoscale Zero-Valent Iron Materials for Toxic TCE Removal from the Groundwater

Trichloroethylene (TCE) is one of the most widely distributed pollutants in groundwater and poses serious risks to the environment and human health. In this study, sulfidated nanoscale zero-valent iron (S-nZVI) materials with different Fe/S molar ratios were synthesized by one-step methods. These materials degraded TCE in groundwater and followed a pathway that did not involve the production of toxic byproducts such as dichloroethenes (DCEs) and vinyl chloride (VC). The effects of sulfur content on TCE dechlorination by S-nZVI were thoroughly investigated in terms of TCE-removal efficiency, H₂ evolution, and reaction rate. X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) characterizations confirmed Fe(0) levels in S-nZVI were larger than for zero-valent iron (nZVI). An Fe/S molar ratio of 10 provided the highest TCE-removal efficiencies. Compared with nZVI, the 24-h TCE removal efficiencies of S-nZVI (Fe/S = 10) increased from 30.2% to 92.6%, and the Fe(0) consumed during a side-reaction of H₂ evolution dropped from 77.0% to 12.8%. This indicated the incorporation of sulfur effectively inhibited H₂ evolution and allowed more Fe(0) to react with TCE. Moreover, the pseudo-first-order kinetic rate constants of S-nZVI materials increased by up to 485% compared to nZVI. In addition, a TCE degradation was proposed based on the variation of detected degradation products. Noting that acetylene, ethylene, and ethane were detected rather than DCEs and VC confirmed that TCE degradation followed β-elimination with acetylene as the intermediate. These results demonstrated that sulfide modification significantly enhanced nZVI performance for TCE degradation, minimized toxic-byproduct formation, and mitigated health risks. This work provides some insight into the remediation of chlorinated-organic-compound-contaminated groundwater and protection from secondary pollution during remediation by adjusting the degradation pathway.

Superior trichloroethylene removal from water by sulfide-modified nanoscale zero-valent iron/graphene aerogel composite

Sulfide-modified nanoscale zero-valent iron (S-nZVI) is a promising material for removal of organic pollutants from water, but S-nZVI nanoparticles (NPs) easily agglomerate and have poor contact with organic contaminants. Herein, we propose a new S-nZVI/graphene aerogel (S-nZVI/GA) composite which exhibits superior removal capability for trichloroethylene (TCE) from water. Three-dimensional porous graphene aerogel (GA) can improve the efficiency of electron transport, enhance the adsorption of organic pollutants and restrain the agglomeration of the core-shell S-nZVI NPs. The TCE removal rates of FeS, nZVI, GA and S-nZVI were 27.8%, 42%, 63% and 75% in 2 hr, respectively. Furthermore, TCE was completely removed within 50 min by S-nZVI/GA. The TCE removal rate increased with increasing pH and temperature, and TCE removal followed the pseudo-first-order kinetic model. The results demonstrate the great potential of S-nZVI/GA composite as a low-cost, easily separated and superior monolithic adsorbent for removal of organic pollutants.

Photocatalytic degradation of local dyeing wastewater by iodine-phosphorus co-doped tungsten trioxide nanocomposites under natural sunlight irradiation

In the present work, one-step green synthesis of WO₃ based on the interaction of ammonium paratungstate and Spondias mombin leaves extract is reported. Different concentrations of iodine and phosphorus in the range of (2%, 5% and 10%) were firstly incorporated into the prepared WO₃ nanoparticles to obtain Iodine doped and Phosphorus doped WO₃ nanoparticles respectively. Subsequently, iodine and phosphorus co-doped WO₃ nanocomposites was prepared using a wet impregnation method followed by calcination at high temperature. The nanomaterials were characterized by HRSEM, HRTEM, BET, UV-Visible, EDS, XRD and XPS. The photo-oxidation of dyeing wastewater by the synthesized WO₃ nanomaterials were tested and assessed using Total organic carbon (TOC) and Chemical oxygen demand (COD) as indicator parameters. XRD and HRSEM analysis demonstrated the formation of only monoclinic phase of WO₃ irrespective of the dopants. The UV-Visible diffuse reflectance spectroscopy showed the band gap energy of 2.61 eV for undoped WO₃ and 2.02 eV for I-P co-doped WO₃ nanocomposites. The surface area of I-P co-doped WO₃ (416.18 m²/g) was higher than the undoped WO₃ (352.49 m²/g). The XPS demonstrated interstitial and substitution of oxygen (O^2-) vacancies in WO₃ by I^- and P³⁺ and formed I-P-WO(_3-x). The I-P co-doped WO₃ exhibited higher catalytic activities (93.4% TOC, 95.1% COD) than the undoped (54.9% TOC, 79.2% COD) due to the synergistic effects between the two dopants. The experimental data better fitted to pseudo-second order than first order and pseudo-first order model. This study demonstrated the enhanced photocatalytic performance of I-P co-doped WO₃ nanocomposites under sunlight.

Factors influencing degradation of trichloroethylene by sulfide-modified nanoscale zero-valent iron in aqueous solution

Sulfide-modified nanoscale zero-valent iron (S/NZVI) has been considered as an efficient material to degrade trichloroethylene (TCE) in groundwater. However, some critical factors influencing the dechlorination of TCE by S/NZVI have not been investigated clearly. In this study, the effects of Fe/S molar ratio, initial pH, dissolved oxygen and particle aging on TCE dechlorination by S/NZVI (using dithionite as sulfidation reagent) were studied. Besides, the feasibility of reactivation of the aged-NZVI by sulfidation treatment was looked into. The results show that the Fe/S molar ratio and initial pH significantly influenced the TCE dechlorination, and a higher TCE dechlorination was observed at Fe/S molar ratio of ∼60 under alkaline condition. Spectroscopic analyses demonstrate that the enhanced TCE dechlorination was associated with the presence of FeS on the surface of S/NZVI. Dissolved oxygen had little effect on TCE dechlorination by S/NZVI, revealing that the FeS layer could be able to alleviate the surface passivation of NZVI caused by oxidation. Aging of S/NZVI up to 10-20 d only slightly decreased the dechlorination efficiency of TCE. Although an obvious drop in dechorination efficiency was observed for the S/NZVI aged for 30 d, it still exhibited a higher reactivity than the bare NZVI. This indicates that sulfidation of NZVI did prolong its lifetime. Additionally, sulfidation treatment was used to reactivate the aged NZVI, and the results show that the reactivated NZVI even had higher reactivity than the fresh NZVI, suggesting that sulfidation treatment would be a promising method to reactivate the aged NZVI.

Photocatalytic behaviour of WO3/TiO2-N for diclofenac degradation using simulated solar radiation as an activation source

In this study, the photocatalytic removal of an emerging contaminant, diclofenac (DCF) sodium, was performed using the nitrogen-doped WO₃/TiO₂-coupled oxide catalyst (WO₃/TiO₂-N). The catalyst synthesis was accomplished by a sol-gel method using tetrabutyl orthotitanate (C₁₆H₃₆O₄Ti), ammonium p-tungstate [(NH₄)₁₀H₂W₁₂O₄₂·4H₂O] and ammonium nitrate (NH₄NO₃) as the nitrogen source. For comparison, TiO₂ and WO₃/TiO₂ were also prepared under similar conditions. Analysis by X-ray diffraction (XRD), N₂ adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance UV-Vis spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) were conducted to characterize the synthesized materials. The photocatalytic efficiency of the semiconductors was determined in a batch reactor irradiated with simulated solar light. Residual and mineralized DCF were quantified by high-performance liquid chromatography, total organic carbon analysis and ion exchange chromatography. The results indicated that the tungsten atoms were dispersed on the surface of TiO₂ as WO₃. The partial substitution of oxygen by nitrogen atoms into the lattice of TiO₂ was an important factor to improve the photocatalytic efficiency of WO₃/TiO₂. Therefore, the best photocatalytic activity was obtained with the WO₃/TiO₂-N_0.18 catalyst, reaching 100% DCF transformation at 250 kJ m^-2 and complete mineralization at 400 kJ m^-2 of solar-accumulated energy.

Degradation of trichloroethylene by hydrodechlorination using formic acid as hydrogen source over supported Pd catalysts

An advanced method for the degradation of trichloroethylene (TCE) over Pd/MCM-41 catalysts through a hydrogen-transfer was investigated. Formic acid (FA) was used instead of gaseous H2 as the hydrogen resource. As a model H-carrier compound, FA has proven to yield less by-products and second-hand pollution during the reaction. Several factors have been studied, including: the property of catalyst supports, Pd loading and size, temperature, initial concentrations of FA and TCE (potential impact on the reaction rates of TCE degradation), and FA decomposition. The intrinsic kinetics for TCE degradation were measured, while the apparent activation energies and the reaction orders with respect to TCE and FA were calculated through power law models. On the basis of kinetics, we assumed a plausible reaction pathway for TCE degradation in which the catalytic degradation of TCE is most likely the rate-determining step for this reaction.

Optical and photocatalytic properties of novel heterogeneous PtSe2-graphene/TiO2 nanocomposites synthesized via ultrasonic assisted techniques

Novel material PtSe2-graphene/TiO2 nanocomposites were successfully synthesized through a facile ultrasonic assisted method. The prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) with an energy dispersive X-ray (EDX),transmission electron microscopy (TEM), Raman spectroscopic analysis, UV-vis absorbance spectra and UV-vis diffuse reflectance spectra (DRS) analysis were obtained. The catalytic behavior was investigated through the decomposition of rhodamine B (Rh.B) as a standard dye. Enhanced photocatalytic activities were observed by increasing the weight% of graphene in the PtSe2-graphene/TiO2 nanocomposites. We observed that the coupling of TiO2 with PtSe2-graphene alter the optical properties by observing a precise band gap in the visible range.