Removal of p-Nitrophenol in Aqueous Solution by Mixed Fe0/(Passivated Fe0) Fixed Bed Filters

An Experimental and Density Functional Theory Simulation Study of NO Reduction Mechanisms over Fe0 Supported on Graphene with and without CO

NO reduction over highly dispersed zerovalent iron (Fe0) supported on graphene (G), with and without the presence of CO in the reacting stream, was systematically studied using a fixed-bed reactor, and the reaction mechanism was examined with the aid of in situ Fourier transform infrared (FTIR) spectroscopy and density functional theory (DFT) calculations. The in situ FTIR results showed that NO adsorbed on the Fe0 site is reduced to form active surface oxygen species (O*), which is then reduced by carbon in graphene to form CO2. The presence of CO in the reacting stream helps to reduce the oxidized Fe(O) sites to regenerate Fe0 sites, making NO reduction easier. It was revealed that NO and CO2 are easily adsorbed on the active surface oxygen species (O*) to form nitrate and carbonate, inhibiting their reduction by CO and deactivating the catalyst. The DFT calculations results suggest that the role of Fe is to reduce the energy barrier of the NO adsorption and decomposition, which controls the formation of active surface oxygen species and N2. The combined FTIR and DFT results offer new insights into the possible mechanism of catalytic NO reduction over graphene loaded with Fe, with and without CO.

Influence of Synthesis Conditions on Physicochemical and Photocatalytic Properties of Ag Containing Nanomaterials

Silver (Ag) containing nanomaterials were successfully prepared by varying synthesis conditions to understand the influence of preparation conditions on the physicochemical and photocatalytic properties of these materials. Different analytical techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), Diffuse reflectance UV-vis spectra (DR UV-vis), X-ray photoelectron spectroscopy (XPS) measurements, and N2-physisorption were used to investigate the physicochemical properties of synthesized Ag containing nanomaterials. The samples (Ag-1 and Ag-2) prepared using AgNO3, NaHCO3, and polyvinylpyrrolidone (PVP) template exhibited pure Ag metal nanorods and nanoparticles; the morphology of Ag metal is influenced by the hydrothermal treatment. The Ag-3 sample prepared without PVP template and calcined at 250 °C showed the presence of a pure Ag2O phase. However, the same sample dried at 50 °C (Ag-4) showed the presence of a pure Ag2CO3 phase. Interestingly, subjecting the sample to hydrothermal treatment (Ag-5) has not resulted in any change in crystal structure, but particle size was increased. All the synthesized Ag containing nanomaterials were used as photocatalysts for p-nitrophenol (p-NP) degradation under visible light irradiation. The Ag-4 sample (pure Ag2CO3 with small crystallite size) exhibited high photocatalytic activity (86% efficiency at pH 10, p-NP concentration of 16 mg L−1, 120 min and catalyst mass of 100 mg) compared to the other synthesized Ag containing nanomaterials. The high photocatalytic activity of the Ag-4 sample is possibly due to the presence of a pure Ag2CO3 crystal structure with nanorod morphology with a low band gap energy of 1.96 eV and relative high surface area.

Environmental occurrence, toxicity concerns, and remediation of recalcitrant nitroaromatic compounds

Nitroaromatic compounds (NACs) are considered important groups of chemicals mainly produced by human and industrial activities. The large-scale application of these xenobiotics creates contamination of the water and soil environment. Despite applicability, NACs have been caused severe hazardous side effects in animals and human systems like different cancers, anemia, skin irritation, liver damage and mutagenic effects. The effective remediation of the NACs from the environment is a significant concern. Researchers have implemented physicochemical and biological methods for the remediation of NACs from the environment. Most of the applied methods are based on adsorption and degradation approaches. Among these methods, degradation is considered a versatile method for the subsequent removal of NACs due to its exceptional properties like simplicity, easy operation, cost-effectiveness, and availability. Most importantly, the degradation process does not generate hazardous side products and wastes compared to other methods. Hence, the importance of NACs, their remediation, and supreme attributes of the degradation method have encouraged us to review the recent progress and development for the removal of these perilous materials using degradation as a versatile method. Therefore, in this review, (i) NACs, physicochemical properties, and their hazardous side effects on humans and animals are discussed; (ii) Physicochemical methods, microbial, anaerobic bioremediation, mycoremediation, and aerobic degradation approaches for the degradation of NACs were thoroughly vetted; (iii) The possible mechanisms for degradation of NACs were investigated and discussed. (iv) The applied kinetic models for evaluation of the rate of degradation were also assessed and discussed. Finally, (vi) current challenges and future prospects of proposed methods for degradation and removal of NACs were also directed.

P-aminophenol catalysed production on supported nano-magnetite particles in fixed-bed reactor: Kinetic modelling and scale-up

The aim of this work was to investigate on the possibility to use nano-magnetite particles supported on waste biomass as heterogeneous catalyst for the production of p-aminophenol starting from a well-known pollutant, p-nitrophenol, in fixed-bed reactors. The kinetic and the thermodynamic of the process was firstly studied in batch system, subsequently a first scale-up was performed using a glass column packed with the supported catalyst. The experimental data obtained with the column were interpreted in light of a suitable dynamic model. The Langmuir-Hinshelwood mechanism well described the process, obtaining from the data fitting a surface rate kinetic constant k = 2.68 × 10^-6 mol/m^2·h, an adsorption equilibrium constants for PNP and BH₄^- species equal to 20.07 l/mol and 1.83 l/mol, at 25 °C. The Eyring equation was used to fit the apparent kintic constant variation with the temperature, to estimate thermodynamic parameters, obtaining a ΔH = - 1145.68 kJ/mol and ΔS = -315.02 kJ/K·mol. The process was then simulated in PROII environment, investigating the influence of initial PNP flowrate, NaBH₄/PNP and reactor length/diameter ratios on PNP conversion, on required duty to maintain isothermal conditions and on pressure drops in the reactor.

Enhancement of ball-miling on pyrite/zero-valent iron for arsenic removal in water: A mechanistic study

In this study, the effect of ball milling on pyrite (FeS₂) promoting arsenic (As) removal by zero-valent iron (Fe⁰) was investigated. The influences of different mass ratios of ball-milled FeS₂/Fe⁰, the dosage of ball-milled FeS₂/Fe⁰ used and initial pH value were evaluated by batch experiments. The results showed that the ball-milled FeS₂/Fe⁰ system had a higher total As removal efficiency than the mixed FeS₂-Fe⁰ system, ball-milled FeS₂ and ball-milled Fe⁰ systems in equal mass. Higher As removal efficiency in ball-milled FeS₂/Fe⁰ system was primarily related to the accelerated corrosion of Fe⁰, which was supported by the determination of total Fe²⁺ release and electrochemical experiments. SEM-EDS and XPS characterizations revealed that there were iron sulfides (Fe(II)-S and Fe(III)-S) produced on the surface of Fe⁰ in ball-milled FeS₂/Fe⁰, which could facilitate the electron transfer of Fe⁰ and enhanced the corrosion of it. BET test also indicated that ball-milled FeS₂/Fe⁰ possessed a higher specific surface area than ball-milled Fe⁰. In addition, the results also showed the optimum mass ratio of FeS₂ and Fe⁰ in ball-milled FeS₂/Fe⁰ to remove As ([As(III)] = 2 mg/L) was 1:1, and the optimum dosage was 0.5 g/L, thereby indicating the optimal As:Fe⁰ molar ratio was about 1:168. And the removal rate of As by ball-milled FeS₂/Fe⁰ was faster in acidic condition than that in alkaline condition. These findings suggest that Fe⁰-based arsenic removal efficiency can be enhanced by ball-milling with FeS₂, making it more feasible for remediation of arsenic-polluted water.

Effect of pyrite on enhancement of zero-valent iron corrosion for arsenic removal in water: A mechanistic study

In this study, the enhanced effect of pyrite (FeS₂) on zero-valent iron (Fe⁰) corrosion for arsenic (As) removal was investigated in a combined-Fe⁰/FeS₂ system. The effects of different Fe⁰/FeS₂ composition, dosage and initial pH were evaluated by batch experiments. Results showed that the best combination ratio of Fe⁰:FeS₂ (w/w) was 1:1 and the optimal dosage of mixture was 2.0 g/L. The combination of Fe⁰ and FeS₂ in a system significantly enhanced the reactivity of Fe⁰ for effective As removal within a broad pH range (3.0-9.0). The effective As removal in the combined-Fe⁰/FeS₂ system was primarily ascribed to being enhanced corrosion of Fe⁰ by addition of FeS₂. SEM and XRD characterizations strongly verified this point. Specifically, the mechanism study (the releases of Fe²⁺ and total Fe ion, variations of pH values as well as XPS characterization) suggested that FeS₂ in the combined-Fe⁰/FeS₂ system could alleviate the passivation of Fe⁰ (pH_ini 3.0-5.0) and accelerate the dissolution of pristine oxide film that coated on Fe⁰ surface (pH_ini 6.8-9.0). Besides, FeS₂ in combined-Fe⁰/FeS₂ system could also accelerate the reactions between Fe⁰ to O₂ at pH_ini 3.0-9.0. These phenomena were well explained by a galvanic couple between Fe⁰ and FeS₂, where FeS₂ was a cathode and Fe⁰ was an anode. Consequently, electrons released from Fe⁰ that mediated by FeS₂ to oxide film, passivation layer and O₂ were accelerated in combined-Fe⁰/FeS₂ system and thereby enhanced the corrosion of Fe⁰ for efficient As removal. Our findings suggest that utilizing FeS₂ to enhance the corrosion of Fe⁰ would be a promising technology for remediation of As-contaminated water.

Effect of 3-D distribution of ZVI nanoparticles confined in polymeric anion exchanger on EDTA-chelated Cu(II) removal

Millispherical nanocomposites are promising for water decontamination combining the high reactivity of the confined nanoparticles and the excellent hydrodynamic properties of the supporting host. However, the effect of three-dimensional (3-D) distribution of the nanoparticles inside the host on the performance of the nanocomposite was highly dependent on the specific decontamination process. In this study, four D201-ZVI nanocomposites from peripheral to uniform 3-D distributions of nZVI were prepared to evaluate the effect of 3-D distribution of the confined nanoparticles inside the host beads on the removal of EDTA-chelated Cu(II). The performance of Cu(II) removal increased with the 3-D distribution tailoring towards the peripheral region, which was also validated under various solution chemistry conditions in terms of initial pH, DO, and coexisting sulfate. The mechanism underlying the 3-D distribution effect may be ascribed to three perspectives. First, the dissolution of Fe was also higher from the peripherally distributed nZVI nanocomposites compared with the uniform ones. In addition, SEM-EDS analysis revealed the immobilization of Cu occurred at limited depth from the outermost surface of the composite beads, leading to the low spatial utilization of the inner core region. Furthermore, XRD and XPS analyses demonstrated the higher chemical utilization of nZVI for the outer-distributed nanocomposites owing to the shortened pathway for mass transfer. This study shed new light on the design and development of tunable nanocomposites of improved reactivity for water decontamination processes.

Mesoporous TiO2 with WO3 functioning as dopant and light-sensitizer: A highly efficient photocatalyst for degradation of organic compound

The suitable doping or modification on TiO₂ holds promise for improving charge separation and extending light absorption range. Here, WO₃ modifying reduced band gap mesoporous TiO₂ (WO₃/RM-TiO₂) due to WO₃ doping was successfully fabricated by immersing mesoporous TiO₂ nanoparticles in the peroxotungstic acid sol with controllable reaction time (0-1 h). The W⁶⁺ ions were first incorporated into the TiO₂ lattice to form WOTi bonds, resulting in the formation of WO₃ doping TiO₂. Then, WO₃ nanoparticles gradually formed and attached on the TiO₂ surface, constructing a novel heterojunction catalyst with WO₃ serving as both dopant and light-sensitizer for TiO₂. Photocatalytic activity of the resulting WO₃/RM-TiO₂ depends on the immersing duration in the peroxotungstic acid. The BET analysis shows that 0.5 h-WO₃/RM-TiO₂ has the largest pore volume of 0.491 cm³ g^-1 and the highest surface area of 82.3 m² g^-1, whereas these values decline with prolonged immersing duration. As expected, the optimal efficiency in removing p-nitrophenol (PNP) is achieved over 0.5 h-WO₃/RM-TiO₂ under visible light irradiation, which is 2.33 times that of the unmodified M-TiO₂. This should be attributed to the suitable WO₃ doping and WO₃ modification.

Premagnetization enhancing the reactivity of Fe0/(passivated Fe0) system for high concentration p-nitrophenol removal in aqueous solution

In order to strengthen the treatment efficiency of Fe⁰ based system for high concentration wastewater treatment, Fe⁰ particles were passivated by concentrated nitric acid, and a premagnetization Fe⁰/(passivated Fe⁰) system was setup for high concentration p-nitrophenol (PNP) removal in this study. The significant parameters of this system were optimized. Under the optimal conditions, the premagnetization Fe⁰/(passivated Fe⁰) system could obtain high k_obs value for PNP removal (0.100 min^-1) and COD removal (15.0% after 60 min) for high concentration PNP (500 mg/L) treatment. In addition, five control experiments were set up to confirm the advantage of the premagnetization Fe⁰/(passivated Fe⁰) system. The results suggest that passivated Fe⁰ particles could be stimulated better than Fe⁰ particles by premagnetization process, and the premagnetization Fe⁰/(passivated Fe⁰) systems is much superior to the other five control systems. Furthermore, the pathway for PNP destruction treated by 6 different systems was also proposed according to intermediates determination by High Performance Liquid Chromatography (HPLC) and UV-vis spectrum, and the carbon mass balance was demonstrated according to the COD and HPLC analyses. Finally, the characteristics of (premagnetization) Fe⁰ and passivated Fe⁰ was detected by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and vibrating sample magnetometer (VSM), and the mechanism of premagnetization effectively enhancing the reactivity of Fe⁰/(passivated Fe⁰) system (better than that of Fe⁰ system) was proposed. Consequently, the premagnetization for reactivity improvement of Fe⁰/(passivated Fe⁰) system is a promising technology to enhance the efficiency of this system for high concentration wastewater treatment.