Surface Fe2 Research Articles

Fabrication of MOF-carbonized materials as ozone catalysts for water purification: Exploration of catalytic mechanisms

Due to their stability, the advent of MOF-derived catalysts ensures the applications of MOF-based materials, and they have been used in many catalytic systems. However, their application in ozone catalysis is limited, and the relevant mechanisms remain unclear. In this study, three MOF-carbonized materials (Me-MOF-C, Me: Fe, Cu, and Zn) were synthesized as ozone catalysts for water purification. When combined with ozone, all the Me-MOF-C promoted target removal compared to the Me-MOF/O3 systems, and Fe-MOF-C was optimal due to its superior ozone utilization efficiency. ROS quantification indicated that the •OH in the Fe-MOF-C/O3 system was approximately 48.0 times that of the single ozone system. Kinetic analysis demonstrated that over 90 % of the target was removed by •OH in the Fe-MOF-C/O3 system. Then, the catalytic mechanism of Fe-MOF-C was investigated. The surface hydroxyl and protonation effect of Fe-MOF-C presented limited activity. In comparison, H2O2 in the solution and Fe2+ on the Fe-MOF-C surface were more crucial. AcOH control experiments revealed a positive correlation between •OH and H2O2. H2O2 decomposition experiments further indicated that although the Fe-MOF-C/O3 system utilized H2O2 faster, only the coexistence of ozone and Fe-MOF-C could facilitate it more. Results of Fe2+ regulation suggested that it was related to the way of surface Fe2+ acted. The main contribution of Fe2+ to producing •OH was not through Fe2+ ozonation or reaction with H2O2, but it likely involved the efficient conversion of H2O2 into HO2ˉ, which subsequently decomposed ozone to produce •OH more efficiently. Finally, a possible catalytic pathway was proposed.

New insights into the catalyzed ozonation mechanism of a hydroxyl riched goethite: Contributions of H2O2 and surface Fe2+

Fe-based catalysts have been widely used to catalyze ozonation during water treatment. Among these catalysts, goethite (α-FeOOH) is of concern for the properties of low cost, easy availability, and excellent performance. In general, the surface hydroxyl groups of goethite are believed to be crucial catalytic sites. However, the contributions of Fe2+ on the goethite surface and H2O2 produced from pollutant degradation have rarely been mentioned. In this study, a goethite with rich surface hydroxyl was fabricated to remove aromatics by O3 catalysis. Probe and quenching experiments together proved that although the •OH was produced less than the O2•⁻ and 1O2 in the catalytic system, it contributed about 95 % of the target removal. EDTA complexation and heating experiments suggested that when the surface Fe2+ of the catalyst was destroyed, although the Fe-site hydroxyl or the vacancy hydroxyl of the catalyst was retained, its catalytic activity was reduced greatly. The outcomes of degradation kinetics of acetic acid further confirmed that, in the catalytic process, most of •OH originated from the reaction of Fe2+ reducing HO2• and Fe2+ ozonation, while the surface hydroxyl contributed less than 44.7 %. Moreover, as the main source of HO2•, the role of H2O2 produced from aromatics degradation cannot be ignored. The detection of H2O2 precursors in the degradation byproducts further confirmed this opinion. Finally, an efficient circular route for •OH production was proposed. This may be beneficial for the development of efficient Fe-based catalysts.

Influence of iron content in palladium catalysts supported on alumina and their reduction conditions on the hydrodechlorination of diclofenac in aqueous solutions

Using the method of wet impregnation of alumina with iron and palladium nitrates, 1Pd0.5Fe and 1Pd10Fe catalysts modified with iron oxides were prepared with a target content of 1 wt % Pd, 0.5 or 10 wt % iron. The catalysts were compared with each other and with the monometallic catalyst 1Pd in the hydrodechlorination (HDC) of diclofenac (DCF) in dilute aqueous solutions at 30°C in batch and flow reactors after high-temperature (320°C) and mild (30°C) reduction; the latter was carried out in a batch or flow reactor. Using X-ray photoelectron spectroscopy (XPS), it was shown that after reduction at 320°C the surface of catalysts contains mainly Pd0, Fe2+ and Fe3+. The surface Fe2+/Fe3+ ratio increases as the iron content decreases. The reduction of Pd2+ to Pd0 is possible already at 30°C, but it proceeds much worse on the surface of 1Pd0.5Fe compared to 1Pd10Fe. According to XPS data, temperature-programmed reduction and infrared spectroscopy of diffuse reflection of adsorbed CO, modification with iron oxides increases the palladium content on the surface compared to 1Pd, promotes the emergence of new Pd–O–Fe centers, and affects the ability of palladium to be reduced. These effects increase with increasing iron content. Iron-modified catalysts reduced at 320°C showed similar activity and stability in the conversion of DCP in flow-through and batch systems. Unlike 1Pd0.5Fe, the 1Pd10Fe catalyst is highly efficient and stable even after mild reduction at 30°C. Under flow conditions with comparable DCF conversion, it provides increased selectivity in the HDC reaction of diclofenac compared to 1Pd, which is also active in hydrogenation.

A crystallinity control strategy for controllable Heterogeneous-Homogeneous reaction and catalytic performance in wet-mechanochemical synthesized FeS2 Fenton system

In heterogeneous Fenton system, the Heterogeneous-Homogeneous reaction type induced by surface-bonded or dissolved catalytic center plays a vital role in determining Fenton performance. However, how to directionally control the reaction type with facile method still remains challenging. Herein, a novel crystallinity control strategy based on wet-mechanochemical synthesis combined with heat-treatment was applied to synthesize FeS2 nanoparticles with different crystallinity. The different crystalline FeS2 based Fenton system exhibited superior sulfamethoxazole removal performance at wide pH range (5.6–11) with •OH as dominating reactive oxygen species. The relative degradation contribution of homogeneous and heterogeneous Fenton reaction was semi-quantitatively evaluated, and the relationship between crystallinity indexes of FeS2 and degradation kinetic constants was established. It’s found that FeS2 crystallinity was directly correlated with its degradation efficiency, which first increased and then declined with crystallinity increasing. Crystallinity control strategy effectively determined dominating reaction type (homogeneous or heterogeneous) and further regulated catalytic performance in FeS2 Fenton system based on the interaction of specific surface area and Fe2+ dissolution ability. This work provides a novel crystallinity control strategy to build a correlative bridge among crystallinity, dominating reaction type and catalytic performance in FeS2 Fenton system.

Heterogeneous degradation of chloramphenicol antibiotic by peroxymonosulfate activated by FeS and FeS2: Mechanism and effects of catalysts dosage and pH

Antibiotics in the environmental waters pose long-term threat to ecological safety because of their persistent and potential toxic nature. FeS and FeS2 were proved to be efficient catalysts for the activation of peroxymonosulfate (PMS) and the generation of sulfate radical generation for refractory pollutant degradation. However, their performance for antibiotic degradation has not yet been comparatively and comprehensively investigated. Herein, the FeS/PMS and FeS2/PMS systems were developed to explore chloramphenicol (CAP) degradation under equal conditions. The results show that CAP was efficiently degraded in both two systems, with a removal efficiency exceeding 90 % within 120 min using 6 mM PMS and 0.6 g/L catalysts.Acceleration in degradation rate was observed in the FeS2/PMS system when the catalyst dosage ranged from 0.1 g/L to 1.0 g/L (constant PMS concentration) with a reaction rate constant (kobs) of 2.1-fold higher than that in the FeS/PMS system. However, the kobs were comparable with the PMS concentration range (constant catalyst dosage) in the two systems, suggesting that the catalysts were the rate-limiting step in CAP degradation process. The initial pH strongly affected CAP degradation in the FeS/PMS system but had little effect on that in the FeS2/PMS system. Surface Fe2+ played a dominant role in PMS activation, and S22−/S2− facilitated Fe2+ regeneration. Further, both FeS and FeS2 had stable activity for PMS activation during long-term use. This study proves the effectiveness of FeS and FeS2 for PMS activation for antibiotic degradation and differentiates their catalytic qualities, which will provide alternative heterogeneous catalysts for practical application.

Kinetics-Driven Crystal Facet Evolution Mechanism of Atomically Ordered Intermetallic PtFe Nanocubes toward Electrochemical Catalysis.

Crystal structure engineering in nanoparticles has been regarded as a vital method in catalyst development and design. Herein, PtFe nanocubes, manufactured with ordered PtFe intermetallic structure and a desired facet of {202}, have been successfully prepared via the combination of selective deposition strategy and spatial barrier effect. In-situ X-ray photoelectron spectroscopy found that the growth of the high-index facet and formation of the nanocube for o-PtFe-202 materials arise from the surface Fe2+ modification stabilized effect and the selective deposition of Cl-, respectively. Moreover, density functional theory calculations and X-ray adsorption spectroscopies further proved that the improved oxygen reduction reaction activity and stability of o-PtFe-202 mainly originate from the synergistic effect of the desired high-index facet, ordered crystal structure, and resulting optimal d-band center of Pt. As expected, the o-PtFe-202 exhibits excellent mass activity (2.48 mA·ugPt-1) and specific activity (7.78 mA·cm-2), with only a 7.3% decrease in mass activity after 30 000 cycles.

Insights into sunlight-driven transformation of tetracycline by iron (hydr)oxides: The dominating role of self-generated hydrogen peroxide

Iron (hydr)oxides are abundant in surface environment, and actively participate in the transformation of organic pollutants due to their large specific surface areas and redox activity. This work investigated the transformation of tetracycline (TC) in the presence of three common iron (hydr)oxides, hematite (Hem), goethite (Goe), and ferrihydrite (Fh), under simulated sunlight irradiation. These iron (hydr)oxides exhibited photoactivity and facilitated the transformation of TC with the initial phototransformation rates decreasing in the order of: Hem > Fh > Goe. The linear correlation between TC removal efficiency and the yield of HO• suggests that HO• dominated TC transformation. The HO• was produced by UV-induced decomposition of self-generated H2O2 and surface Fe2+-triggered photo-Fenton reaction. The experimental results indicate that the generation of HO• was controlled by H2O2, while surface Fe2+ was in excess. Sunlight-driven H2O2 production in the presence of the highly crystalline Hem and Goe occurred through a one-step two-electron reduction pathway, while the process was contributed by both O2-induced Fe2+ oxidation and direct reduction of O2 by electrons on the conduction band in the presence of the poorly crystalline Fh. These findings demonstrate that sunlight may significantly accelerate the degradation of organic pollutants in the presence of iron (hydr)oxides.

Low-crystalline FeOx on carboxylic CNTs as high-performance Fenton-like catalysts: Influence of crystallinity and carbon matrix

A facile method to prepare the ultra-dispersed low-crystalline FeOx (2–5 nm) particles anchored on the carboxylic carbon nanotubes (CCNTs) was reported in this paper. The low-crystalline FeOx/CCNTs (LC-FeOx/CCNTs) system features excellent Fenton-like performance which is 24.3 times larger than that of conventional crystalline Fe2O3/CCNTs (C–Fe2O3/CCNTs). It also exhibits outstanding stability and versatility in the catalytic degradation of different types of (cationic and anionic) compounds. Through comprehensive mechanistic investigations, it's confirmed that crystallinity of iron oxide played a vital role in catalytic performance. Specifically, singlet oxygen (1O2) was detected as the main reactive intermediate by introducing surface oxygen vacancy (SOV) and Fe2+ as “Fenton-catalytic” dual reaction center to promote H2O2 decomposition, which was distinct from the common Fenton-like reaction path with HO• as the main reactive oxygen species. Furthermore, the carboxylic groups on the surface of carbon nanotubes (CNTs) complexed with metastable FeOx can accelerate the transformation of Fe(III)/Fe(II) so as to adjust the utilization of H2O2 via donor-acceptor coupling. This work addresses an imaginative leap forward the understanding of the role of crystallinity and matrix coupling in the design of Fenton-like catalyst.

Boron boosted Fe3O4 activated peracetic acid for removing sulfamethazine: Role of boron and mechanism

BackgroundNowadays, antibiotic pollution is increasingly serious and posing a huge threat to ecology and human health. Therefore, it is vital to find effective approaches to remove antibiotics. MethodsIn this study, boron (B) was adopted to accelerate the surface and solution Fe2+ recycle in Fe3O4/peracetic acid (PAA) system, which showed excellent performance for antibiotics removal. Significant findingsAccording the XPS analysis and experiments detection, both recycle of surface and solution Fe2+ could be enhanced in the presence of boron in Fe3O4/PAA system. 74.0% sulfamethazine (SMT) could be removed within 60 min in Fe3O4/PAA system while 93.4% could be removed within only 20 min in B/Fe3O4/PAA system and the value of k in B/Fe3O4/PAA system (0.136 min−1) was also much higher than that in Fe3O4/PAA system (0.028 min−1). The effect of boron, Fe3O4, PAA dosage and initial pH on removing SMT in Fe3O4/PAA system were also conducted. Quenching experiments and EPR experiments demonstrated that ·OH, organic radicals (R-O·) and Fe(IV) exhibited 63.1%, 17.3%, and 16.2% role in removing SMT, respectively. Possible SMT degradation pathway and the toxicity of transformation products were analyzed. B/Fe3O4/PAA exhibited superior resistance to inorganic ions and nature organic matters, good recycle ability, excellent adaptability to various pollutants, and efficient performance for antibiotics removal in natural fresh water.

Effect of morphology especially leaf-like morphology on surface Fe2+ content of α-Fe2O3 in photo-assisted fenton-like degradation of organic contaminants

Surface Fe2+ content has become a limiting step of α-Fe2O3 Photo-assisted Fenton catalytic activity, and a high-efficiency process is urgently required to enhance it. Herein, α-Fe2O3 catalysts with three different morphologies (cube, sphere, and leaf-like) were successfully synthesized under hydrothermal conditions and presented quite different Photo-assisted Fenton-like activities for Rhodamine B (RhB) degradation. Unexpectedly, catalysts show certain activity, especially leaf-like α-Fe2O3 (L-Fe2O3) showed the highest RhB catalytic degradation activity. The mechanistic study has found that the morphology will change the surface Fe2+ content of α-Fe2O3, which will affect the Fe2+/Fe3+ cycle and the role of surface Fe2+ in a Photo-assisted Fenton-like system proved by the results of XPS measurements and 2,2-bipyridine (BPY) quenching experiment. In addition, the L-Fe2O3 catalyst represented broad applicability and stability, which is beneficial for the actual industrial application.

Experimental Characterization and Pore-Scale Modeling of Iron Precipitation in Shale Reservoirs by Interacting with Hydraulic Fracturing Fluid

It has been reported that ∼60% of total U.S. hydrocarbon production comes from shale reservoirs. Understanding of reactive transport is of fundamental importance to the application in subsurface systems of natural shales that have rich compositions of carbonate, clay, and sulfide, which have high reactivity with water. In this study, we focus on the interaction between pyrite (sulfide) and hydraulic fracturing fluid in shale to investigate the potential impact of iron precipitation on fluid transport. We first conducted the experiments with pyrite samples to calibrate the reaction rate constants for pyrite oxidation (at the pyrite surface) and Fe2+ oxidation (in solution). The obtained reaction rate constants were utilized to establish the pore-scale numerical model to track these oxidation reactions. In other words, the reaction rate constants of pyrite surface oxidation and Fe2+ oxidation were calibrated by matching the results of numerical simulations with the experimental measurements of ion concentrations. By doing so, we could also obtain confidence in our developed numerical simulator. In numerical simulation case 1, where the reactions of pyrite oxidation and Fe2+ oxidation mainly occurred, the transport patterns in the systems were investigated based on the digital rock image model. In numerical simulation case 2, the level-set method was coupled with the reactive transport model to simulate iron(III) hydroxide precipitation on the pyrite surface. The precipitation patterns in the digital rock image model were investigated under different Damköhler numbers (DaII). Under the larger DaII, the precipitated iron(III) hydroxides had a longer dendritic shape, and the precipitation pattern was highly random. The quantified pore-scale parameters obtained from this study are expected to improve continuum-scale models to accurately predict the potential impact of the interaction between pyrite in shale and hydraulic fracturing fluid.

Fe-doping induced surface Fe2+/Fe3+ cycle and activated redox-inert TiO2 for enhanced Hg(II) electrochemical sensing: An efficient strategy to strengthen the redox activity

Recently, Fe-based metal oxide with a variable-valence ability (i.e., the Fe2+/Fe3+ cycle) can participate in the redox of target heavy metal ions (HMIs) and enhance the electrochemical signal, which have attracted significant attention. However, it has not yet been proved whether iron-based metal oxides with variable-valence ability can activate the variable valence behavior of inert metal oxides (i.e., TiO2) and enable them to participate in the redox of target HMIs. Herein, we develop an efficient Fe-doped strategy to activate TiO2 nanoparticles for the electrochemical detection of Hg(II). TiO2 nanoparticles with the 5% Fe-doped content (FT5) possess the best detection sensitivity of 400.63 μA μM−1 cm−2 for Hg(II), which is dramatically higher than that of pure TiO2. The synergistic effects of enhanced adsorption by OVs and promoted redox activity by surface Fe2+/Fe3+ and Ti3+/Ti4+ cycle help FT5 to obtain an excellent electrochemical detection performance of Hg(II). In detail, Fe doping tune the concentration of oxygen vacancies (OVs) in TiO2 nanoparticles, which contributes to improving the adsorption ability of Hg(II). The exposed OVs on the surface of Fe-doped TiO2 nanoparticles form numerous hydroxyl groups (-OH) in water, and the hydroxyl groups can bond with Hg(II), tremendously accelerating the capture of Hg(II). Upon successfully obtaining OVs, the Ti3+ species are created in TiO2, achieving the activation of TiO2. Moreover, it is found that large amount surface Fe2+/Fe3+ and Ti3+/Ti4+ cycle on FT5 can accelerated the redox of Hg(II) and then favor to electrochemical detection performance. This study emphasizes that doping transition metal elements with variable valence states can control OVs concentration and successfully activate inert metal oxides.

Two-step electrodeposition synthesis of iron cobalt selenide and nickel cobalt phosphate heterostructure for hybrid supercapacitors

Exploring novel heterostructure with multiscale nanoarchitectures and modulated electronic structure is crucial to improve the electrochemical properties of electrode materials for supercapacitors (SCs). In this study, a two-step electrodeposition approach which involves suitable efficient procedures, is leading to in-situ preparation of iron cobalt selenide (Fe0.4Co0.6Se2) @ nickel cobalt phosphate (NiCo(HPO4)2·3H2O, denoted as NiCo-P) hybrid nanostructure on carbon cloth (CC) substrate. Particularly, depositing two-dimensional (2D) NiCo-P nanosheets on the surface of Fe0.4Co0.6Se2 nanobelts results in formation of well-organized Fe0.4Co0.6Se2@NiCo-P nanocomposite with large surface area, hierarchical porous nanoarchitecture as well as numerous electroactive sites, leading to enhanced electroactivity and accelerated mass/electron transfer. Benefiting from its unique nanoarchitecture and synergistic effect of two components, the obtained free-standing Fe0.4Co0.6Se2@NiCo-P electrode demonstrates gravimetric capacity (Cm)/volumetric capacity (Cd) of 202.3 mAh/g/319.6 mAh cm−3 at 1 A g−1 and good cyclic stability (83.9% capacity retention over 5000 cycles), which are superior to those of pure Fe0.4Co0.6Se2 and NiCo-P electrodes. Impressively, it was established that an aqueous hybrid supercapacitor (HSC) based on Fe0.4Co0.6Se2@NiCo-P and rape pollen derived hierarchical porous carbon (RPHPC) achieves gravimetric energy density (Em)/volumetric energy density (Ed) of 64.4 Wh kg−1/10.7 mWh cm−3 and a long cycle life with 90.3% capacity retention over 10,000 cycles. This report offers a perspective to design selenide/phosphate heterostructure on conducting substrate for electrochemical energy storage applications.

Depletable peroxidase-like activity of Fe3O4 nanozymes accompanied with separate migration of electrons and iron ions

As pioneering Fe3O4 nanozymes, their explicit peroxidase (POD)-like catalytic mechanism remains elusive. Although many studies have proposed surface Fe2+-induced Fenton-like reactions accounting for their POD-like activity, few have focused on the internal atomic changes and their contribution to the catalytic reaction. Here we report that Fe2+ within Fe3O4 can transfer electrons to the surface via the Fe2+-O-Fe3+ chain, regenerating the surface Fe2+ and enabling a sustained POD-like catalytic reaction. This process usually occurs with the outward migration of excess oxidized Fe3+ from the lattice, which is a rate-limiting step. After prolonged catalysis, Fe3O4 nanozymes suffer the phase transformation to γ-Fe2O3 with depletable POD-like activity. This self-depleting characteristic of nanozymes with internal atoms involved in electron transfer and ion migration is well validated on lithium iron phosphate nanoparticles. We reveal a neglected issue concerning the necessity of considering both surface and internal atoms when designing, modulating, and applying nanozymes.

In-situ reduction-passivation synthesis of magnetic octahedron accumulated by Fe@Fe3O4-C core@complex-shell for the activation of persulfate

The iron-based heterogeneous catalysts, especially the zero valent iron (ZVI) is an eco-friendly and cost-effective activator for persulfate (PS) excitation. However, the oxidation of surface ZVI reduces its service lifetime. Herein, a series of magnetic octahedrons accumulated by Fe@Fe3O4-C core@complex-shell were constructed by in-situ reduction-passivation of the MIL-101(Fe). The catalysts exhibited good activity, high stability and magnetic recyclability. The optimized Fe@Fe3O4-C-60 with largest proportion of active iron achieved a 100 % conversion of Rhodamine B (RhB) within 4 min and high mineralization efficiency of 667.34 % in 30 min. Moreover, Fe@Fe3O4-C-60 maintained its activity even after 3 times of cycling test and could be easily magnetic separation. The rate constant was 1.23 min−1 for Fe@Fe3O4-C-60+PS in first 3 min, which was 1.6, 5 and 323 times of Fe0+PS, FeSO4+PS and Fe3O4+PS. Fe0 and Fe2+ were identified as active sites for SO4•- generating, while the carbon shell was responsible for the high stability. The synergistic effect between Fe0 core and Fe3O4-C complex shell realized the sustained regeneration of active surface Fe2+ and inhibition leaching of Fe.

Pencil graphite synergistic improvement of zero-valent iron composite for the removal of diclofenac sodium in aqueous solutions: Kinetics and comparative study

Pencil graphite (PG) was used as a carrier material to increase the stability, dispersity, and removal capacity of zero-valent iron (Fe0) in an aqueous solution. Freshly synthesized composite adsorbent from Fe0 and pencil graphite (Fe0-PG) was established. The morphology of Fe0 and Fe0-PG was characterized using FTIR, SEM, TEM, XRD, and BET techniques. The adsorption mechanism and potential removal of Diclofenac sodium (DCF) onto the surface of Fe0 and Fe0-PG composite in an aqueous solution were investigated in terms of pH, contact time, initial DCF concentration, adsorption dose, and temperature. The adsorption data fit better the pseudo-second-order kinetic and Langmuir isotherm model via both adsorbents (R2 > 0.992). The experimental results indicated that the maximum adsorption capacity qmax of DCF via Fe0-PG, obtained from Langmuir isotherm was 48.1 mg.L−1, and it is an extraordinary value in comparison with that of Fe0 (23.1 mg.L−1). The maximum removal efficiency was achieved at pH = 6 for both adsorbents (Fe0: 51% and Fe0-PG: 92%). The calculated value of the activation energy Ea for adsorption of DCF via Fe0-PG composite is 112 kJ.mol−1, which signifies the chemisorption nature of the adsorption mechanism. The data suggest that the process for both sorbates is endothermic and spontaneous in nature at high temperatures. Higher environmental temperature is more favorable for uptake of DCF onto the Fe0-PG surface. PG was found to be a promising synergistic material, and Fe0-PG shows enhancement for DCF removal in aqueous solutions compared with the bare Fe0 NPs.

A potential industrial waste–waste synchronous treatment scheme of utilizing copper slag flotation tailings to remediate Cr(VI)-containing wastewater

Untreated Cr(VI)-containing wastewater and long-term stockpiled copper slag flotation tailings (CSTs) cause massive irreversible soil and water pollution. The current study explored an environmentally friendly method for aqueous Cr(VI) remediation using CST, which was improved by reduction calcination with anthracite as the reductant. X-ray diffraction (XRD), Fourier infrared (FTIR), and scanning electron microscopy (SEM) coupled with energy-dispersive spectrometry (EDS) indicated that reduction calcination could promote the transformation of fayalite to zero-valent iron (Fe0), which is beneficial to enhance the reduction ability of CST for Cr(VI). The CST calcined with 20% anthracite at 1300 °C for 40 min had the best Cr(VI) remediation performance. Compared with raw CST without Cr(VI) remediation ability, anthracite-calcined CST (ACST) can achieve 100% Cr(VI) remediation efficiency. Moreover, ACST exhibited excellent removal capacity for As(III), Pb(II), and Cd(II). Multi-characterizations supported that the Fe0 in ACST provides electrons to promote the reduction of Cr(VI) to Cr(III), and then Cr(III) was deposited on the surface of Fe0 in the form of hydroxide precipitation. The remediation mechanism could be attributed to the joint contribution of reduction, precipitation, and coprecipitation. The present study provides new insights into the remediation of Cr(VI)-containing wastewater and the reuse of CST.

Green Synthesis of Magnetite-Based Catalysts for Solar-Assisted Catalytic Wet Peroxide Oxidation

A novel synthesis method under green philosophy for the preparation of some magnetite-based catalysts (MBCs) is presented. The synthesis was carried out in aqueous media (i.e., absence of organic solvents) at room temperature with recovery of excess reactants. Terephthalic acid (H2BDC) was used to drive the synthesis route towards magnetite. Accordingly, bare magnetite (Fe3O4) and some hybrid magnetite-carbon composites were prepared (Fe3O4-G, Fe3O4-GO, and Fe3O4-AC). Graphene (G), graphene oxide (GO), and activated carbon (AC) were used as starting carbon materials. The recovered H2BDC and the as-synthetized MBCs were fully characterized by XRD, FTIR, Raman spectroscopy, XPS, SQUID magnetometry, TGA-DTA-MS, elemental analysis, and N2-adsorption-desorption isotherms. The recovered H2BDC was of purity high enough to be reused in the synthesis of MBCs. All the catalysts obtained presented the typical crystalline phase of magnetite nanoparticles, moderate surface area (63–337 m2 g−1), and magnetic properties that allowed their easy separation from aqueous media by an external magnet (magnetization saturation = 25–80 emu g−1). The MBCs were tested in catalytic wet peroxide oxidation (CWPO) of an aqueous solution of metoprolol tartrate (MTP) under simulated solar radiation. The Fe3O4-AC materials showed the best catalytic performance among the prepared MBCs, with MTP and total organic carbon (TOC) removals higher than 90% and 20%, respectively, after 3 h of treatment. This catalyst was fairly successfully reused in nine consecutive runs, though minor loss of activity was observed, likely due to the accumulation of organic compounds on the porous structure of the activated carbon and/or partial oxidation of surface Fe2+ sites.

Thermally enhanced adsorption and persulfate oxidation-driven regeneration on FeCl3-activated biochar for removal of microcystin-LR in water

In this study, a cyclic process of adsorption and persulfate (PS) oxidation-driven regeneration using FeCl3-activated biochar (FA-BC) was suggested as a novel remediation process to remove microcystin-LR (MC-LR) from water. For enhancing overall treatment efficiency and cost effectiveness, the impacts of temperature on adsorption and PS oxidation-driven regeneration were investigated. The increase of temperature resulted in the increase of MC-LR adsorption rate on FA-BC due to the enhanced MC-LR diffusivity in water. Moreover, the MC-LR oxidation and PS reaction rates during the PS regeneration on FA-BC were remarkably improved by factors of 3.4 and 3.5 with increasing temperature from 20 °C to 50 °C. Both diffusion and desorption of MC-LR from FA-BC were thought to be the key factors for controlling the MC-LR oxidation rate during the PS regeneration of MC-LR. In addition, the decrease of pH (from 10 to 4) and increase of PS concentration (from 100 to 400 mg/L) enhanced the regeneration efficiency for MC-LR-spent FA-BC. The four cycles of adsorption-PS regeneration (200 mg/L PS, pH 6, and 50 °C) resulted in 92.81% regeneration efficiency in DI water and 82.89% in lake water. However, the four cycles of adsorption-PS regeneration led to the reduction of surface area (from 835 to 413 m2/g), oxidation of carbon surface and slight reduction of Fe0 on FA-BC. In overall, the cyclic adsorption-PS regeneration at higher temperature could provide practical reuse of FA-BC for cost-effective treatment of aqueous MC-LR.

Persistently high Cr6+ removal rate of centi-sized iron turning owing to tribocatalysis

Fe0 has been found a good material for removing heavy metal from waste water due to high efficiency, low cost, facile operation. But its application has been impeded by its rapidly decreasing reduction rate with increasing treatment time owing to easily oxidization during reduction. This study found that a centi-sized commercial iron turning with a thin layer of amorphous iron oxides (AIO) possesses persistently high Cr6+ removal rate of 98% in the testing time (36 h). The tribocatalysis arising from the friction between the solution and above-said AIO could account for above-mentioned results. During the first tribocatalysis stage, the electrons of Fe0 in iron turning were stimulated to higher energy and more easily went through the interface Fe0/AIO, resulting in increasing Cr6+ reduction efficiency, and the reaction from the Fe0 inside AIO to Fe2+ and Fe3+. During the next tribocatalysis stage, Since there is weak bonding between Fe0 and FeO due to lattice mismatch, the surface AIO dropped from iron turning in case of friction. Thus, much more Fe0 was exposed to Cr6+ solution and much higher Cr6+ removal rate has been obtained. More importantly, the high Cr6+ removal rate can be persistently kept. It has also been found that iron with larger friction area favors in tribocatalysis than one with less friction area. All in all, this work has found that an effective way-tribocatalysis-to lift the possibility that Fe0 go through potential barrier to react with heavy metal ions, and to timely remove the covered materials on the surface of Fe0, and therefore resulting in persistently high Cr6+ removal rate. This work has also given a clear picture for tribocatalysis, facilitating people to understand the mechanism of tribocatalysis.