Iron Based Catalysts Used in Water Treatment Assisted by Ultrasound: A Mini Review

Environmental catalysts advance focused on lattice oxygen for the decomposition of harmful organic compounds

The recent industrial growth has made our lives more comfortable; however, it has led to an increase in the concentration of harmful compounds, such as carbon monoxide, volatile organic compounds (e.g., toluene), and phenolic compounds (e.g., phenol and cresol), in the environment. Catalytic oxidation using environmental catalysts is an important method for the removal of harmful compounds. To date, novel environmental catalysts have been developed from unique concepts based on solid-state ionics. In particular, the oxygen supply ability of a promoter can supply active oxygen from inside the lattice to the catalytically active site. Our catalysts exhibited high activity for the oxidation of harmful chemicals under moderate conditions in both the gaseous and liquid phases compared to conventional catalysts. This short review article describes our concepts of material design and our novel catalysts (ceria-zirconia (CeO2-ZrO2), apatite-type lanthanum silicate (La10Si6O27), and lanthanum oxyfluoride (LaOF) based catalysts).

Toxicity Effects of Engineered Iron Nanoparticles (Fe3O4) on the Growth, Cell Density, and Pigment Content of Chlorella vulgaris

This study investigated the effects of Fe₃O₄ nanoparticles released from synthesized thiourea catalyst on the biological aspects of Chlorella vulgaris. Fe₃O₄ concentrations (0, 10, 100, 250, 500, 750, and 1000 mg L^-1) were used for the exposure tests. Biological parameters of C. vulgaris, including cell density, cell viability, and pigment content, were assessed. Bioconcentration and bioaccumulation factors were evaluated for contaminated microalgae. Non-carcinogenic risks were then assessed using target hazard quotient (THQ) for potential risks in human consumptions. Findings showed that C. vulgaris cell number increased from 0 to 500 mg L^-1 of Fe₃O₄ concentration. Chlorophyll a represented a time-dependent response, and greatest values were detected in 250 and 500 mg L^-1 Fe₃O₄ at 4.2 and 4 mg/g, respectively. Chlorophyll b content showed a time-related manner in exposure to Fe₃O₄ with the highest values recorded at 250 mg L^-1 after 96 h. Moreover, bioaccumulation displayed a dose-dependent response at 15,000 µg/g dw in 1000 mg L^-1, whereas the lowest concentration was in the control group at 1700 µg/g dw. The bioconcentration factor showed a concentration-relevant decrease in all iron treatments and 10 mg L^-1 of Fe₃O₄ represented the greatest BCF at 327.3611. Non-carcinogenic risks illustrated negligible hazard (THQ < 1) and the largest EDI and THQ were calculated in 1000 mg L^-1 at 7.4332E-07 (mg kg^-1 day^-1) and 1.06189E-09, respectively. Together, iron is an essential trace element for biological purposes in aquatic systems, but in exceeding concentrations could impose toxicity effects to C. vulgaris populations.

Insights into the novel application of Fe-MOFs in ultrasound-assisted heterogeneous Fenton system: Efficiency, kinetics and mechanism

In this work, as a new strategy, ultrasound/H₂O₂/MOF system was firstly applied by environmental-benign Fe-MOFs (MIL-53, MIL-88B and MIL-101) for tetracycline hydrochloride removal. The syntheticFe-MOFs were characterized by XRD, FTIR, SEM, XPS, N₂ sorption-desorption isotherms and CO-FTIR. MIL-88B demonstrated the best catalytic performance because of its highest amount of Lewis acid sites. Influencing factors, contrast experiment, and corresponding dynamics were carried out to obtain the best experimental conditions and reaction system. Under optimal conditions ([Tetracyclinehydrochloride] = 10 mg/L, [MIL-88B] = 0.3 g/L, [H₂O₂] = 44 mM, [ultrasound power] = 60 W, and pH = 5.0), the-first-order kinetic rate constant k was calculated to be 0.226 min^-1, higher than the simple combination of the ultrasound system (0.004) and MIL-88B/H₂O₂ system (0.163), indicating the importance of synergistic effect between ultrasound and Fenton reaction. EPR test and quenching experiment proved that ·OH is mainly responsible for tetracycline hydrochloride removal. The major reaction path is the adsorption and decomposition of H₂O₂ by coordinative unsaturated iron sites on Fe-MOF, but it is not the only path. The direct decomposition of H₂O₂ and the cavitation effect caused by ultrasound also contribute to the generation of OH.

Ultrasound-assisted coagulation for Microcystis aeruginosa removal using Fe3O4-loaded carbon nanotubes

Harmful cyanobacterial blooms are increasing environmental issues and require novel removal technology since the required doses of algaecides may cause further environmental pollution or treatment facility damage. Herein, we firstly introduce the combination of ultrasound and Fe₃O₄/CNTs as an alternative strategy to enhance coagulation for the removal of Microcystis aeruginosa cells in water. It remarkably enhanced cyanobacterial cell removal and microcystins control, compared with sonication alone (40 kHz ultrasonic bath, 4.2 mJ mL⁻¹). 94.4% cyanobacterial cells were removed using 20 second sonication with 20 mg L⁻¹ Fe₃O₄/CNTs, Al₂(SO₄)₃ coagulation (20 μM). Both sonication time and catalyst dose significantly influenced the cyanobacterial removal. Ultrasound with Fe₃O₄/CNTs only induced a slight increase of cell permeability, which may contribute to the effective control of DOC and microcystins' release in water. The enhanced settlement of the cyanobacterial cells may result from the moderate oxidation on the cell surface. This study suggested a novel ultrasound-Fe₃O₄/CNT process to promote cyanobacteria removal with efficient DOC and microcystin release control, which is a green and safe technology for drinking water treatment.

The combination of sonication and Fe₃O₄/CNTs were applied on Microcystis aeruginosa removal for the first time.

Sonocatalytic degradation of butylparaben in aqueous phase over Pd/C nanoparticles

In the present work, the sonocatalytic degradation of butylparaben was investigated using Pd immobilized on carbon black as the sonocatalyst. The presence of 25 mg/L 10Pd/C significantly increased the removal rate of butylparaben and the observed kinetic constant increased from 0.0126 to 0.071 min^-1, while the synergy index between sonolysis and adsorption was 70.7%. The BP degradation followed pseudo-first-order kinetics with the apparent kinetic constant decreased from 0.071 to 0.030 min^-1 when the initial concentration of butylparaben increased from 0.5 to 2 mg/L. The process was being favored slightly under alkaline conditions. The presence of organic matter (20 mg/L humic acid) reduced the apparent kinetic constant more than two times. The addition of chlorides up to 250 mg/L did not significantly reduce the rate of reaction, while the presence of 250 mg/L bicarbonates reduced the observed kinetic constant from 0.071 to 0.0472 min^-1. The prepared catalyst retains the efficiency after five subsequent experiments since the apparent kinetic constant was only slightly decreased from 0.071 to 0.059 min^-1.

Synergistic degradation of PNP via coupling H2O2 with persulfate catalyzed by nano zero valent iron

Simultaneous activation of H₂O₂ and persulfate by nanoscaled Fe⁰ shows synergistic effect for degradation of p-nitrophenol with generating both hydroxyl and sulfate radicals in a wide initial pH range.

Catalytic Liquid-Phase Oxidation of Phenolic Compounds Using Ceria-Zirconia Based Catalysts

Catalytic liquid-phase oxidation using a catalyst and oxygen gas (Catalytic wet air oxidation, CWAO) is one of the most promising technology to remove hazardous organic compounds in wastewater. Up to now, various heterogeneous catalysts have been reported for phenolic compounds decomposition. The CeO₂-ZrO₂ based catalysts have been recently studied, because CeO₂-ZrO₂ works as a promoter which supplies active oxygen species from inside the lattice to the active sites. Since it is difficult to dissolve oxygen gas into water, the use of the promoter is effective for realizing the high catalytic activity at moderate conditions. Also, CeO₂-ZrO₂ shows high resistance for the metal leaching during the catalytic reaction in the liquid-phase. This article reviews the studies of the catalytic liquid-phase oxidation of phenolic compounds using CeO₂-ZrO₂ based catalysts.