Efficient Heterogeneous Fenton-like Catalyst Research Articles

Facile construction of a novel S-scheme Fe2TiO5/BiVO4 photocatalyst for efficient visible light-driven fenton-like degradation of ciprofloxacin

The rational design and fabrication of stable and efficient heterogeneous Fenton-like catalyst is of great significance for wastewater treatment, but is still fraught with obstacles. In this paper, a novel S-scheme Fe2TiO5/BiVO4 nano-heterojunction with high visible-light photocatalytic performance was synthesized by an in-situ growth method. The optimized Fe2TiO5 (20 wt%)/BiVO4 composite exhibited remarkable photocatalytic activity and rapid degradation ability towards CIP (50 mg/L) with the addition of H2O2 (20 mM), achieving a removal efficiency of 94.2 % within 90 mins. This performance is significantly superior to that of pure BiVO4 (43.8 %) or pure Fe2TiO5 (48.2 %). The S-scheme heterojunction formed between Fe2TiO5 and BiVO4 facilitates efficient charge separation, and preserves the powerful photogenerated electrons in CB of Fe2TiO5 and holes in the VB of BiVO4, contributing to the fast degradation of CIP. The synergistic effects of the S-scheme heterojunction and H2O2 activation account for the excellent performance of Fe2TiO5/BiVO4 photocatalyst. The prepared photocatalyst demonstrates remarkable efficiency, non-selectivity, exceptional stability, and adaptability to various environmental conditions. This study proposes a novel approach for designing and synthesizing photo-assisted Fenton-like S-scheme photocatalysts for efficient removal of organic pollutants in wastewater.

Mechanochemical synthesis of sulfidated microscale zerovalent iron/graphene composite for enhanced degradation of tetracycline

This study introduces a novel ternary composite comprising sulfur (S), reduced graphene oxide (rGO), and microscale iron (mZVI) powder through a straightforward ball milling process. The composite was employed as an activator for H2O2 in the degradation of tetracycline (TC). Impressively, the S-mZVI/rGO composite exhibits significantly enhanced TC removal efficiency in the presence of H2O2 compared to bare mZVI and S-mZVI alone. In the S-mZVI/rGO/H2O2 system, 97.2 % of the TC can be removed within 60 min, with an apparent rate constant (kobs) of 0.054 min−1, which is 49.18 and 2.14 times than that in the mZVI/H2O2 (0.001 min−1) and S-mZVI/H2O2 system (0.025 min−1), respectively. Furthermore, the S-mZVI/rGO composite exhibits outstanding TC removal efficiency across a broad pH range (3 ∼ 9), consistently exceeding 90 % under optimal conditions. The much superior activity of S-mZVI/rGO/H2O2 process is attributed to both radical (OH) and non-radical pathways (1O2 and electron transfer), with the latter contributing more. The catalyst’s performance is also affected by the defective sites on the surface and the rapid cycle of Fe2+/Fe3+. Moreover, the degradation pathways of TC are proposed according to the intermediate products detected by UPLC-MS. Finally, the S-mZVI/rGO/H2O2 demonstrates high resistance to inorganic anions and natural organic matter in aquatic environments. Overall, S-mZVI/rGO is expected to be a cost-effective and highly efficient heterogeneous Fenton-like catalyst for the removal of organic contaminants from wastewater.

Utilizing sequence transformation of selective copper metal as an efficient heterogeneous Fenton-like catalyst for the degradation of aqueous methylene blue

Utilizing sequence transformation of selective copper metal as an efficient heterogeneous Fenton-like catalyst for the degradation of aqueous methylene blue

Cobalt Ferrite (CoFe2O4) Spinel as a New Efficient Magnetic Heterogeneous Fenton-like Catalyst for Wastewater Treatment

For the first time, cobalt ferrite spinel (CoFe2O4) was used as a catalyst in the Fenton process for Remazol Red RR dye degradation in water. CoFe2O4 was synthesized via gel combustion using tris(hydroxymethyl)aminomethane as an alternative fuel in one step with a ratio of Ψ = 0.8. Its structural, surface optics, magnetic properties, and the optimal conditions of the Fenton reagents for dye degradation were evaluated. The saturation magnetization and remanence (Ms and Mr, respectively) for the as-prepared powder were 65.7 emu/g and 30.4 emu/g, respectively, and the coercivity (Hc) was 1243 Oe, indicating its ferromagnetic nature and suitability as a magnetic catalyst. Red Remazol RR dye degradation tests were performed using the Fenton process to evaluate the influence of the catalyst dosage and H2O2 concentration. The tests were performed in a batch reactor in the dark with constant agitation for 24 h. The best result was obtained using 1 g/L of catalyst with a dye degradation of 80.6%. The optimal concentration of H2O2 (1.0 M) resulted in 96.5% dye degradation. Nanoparticle recyclability testing indicated that the material could be satisfactorily reused as a catalyst for at least three cycles. The potential use of the CoFe2O4 synthesized in this study as a catalyst for dye degradation by the Fenton process was demonstrated.

CuS@Cu-CD composites as efficient heterogeneous Fenton-like catalysts for the photodegradation of tetracycline

CuS@Cu-CDs composites as Fenton-like photocatalysts for the degradation of tetracycline.

Novel immobilized polyoxometalate heterogeneous catalyst for the efficient and durable removal of tetracycline in a Fenton-like system

Polyoxometalates (e.g., phosphotungstic acid) provide a new viewpoint for the development of efficient heterogeneous Fenton-like catalysts due to their distinct chemical structure and catalytic properties. In this work, a polymer composite (PWA/PAN) was successfully prepared by doping phosphotungstic acid (PWA) as an active component in the polyaniline (PAN) matrix. The prepared composite was then applied to degrade tetracycline (TC) in a Fenton-like system. Characterization results confirmed that PWA was dispersed in the inorganic carrier with its unique and stable Keggin-type structure, and the inherent chain structure of PAN did not change markedly. Compared with common heterogeneous Fenton-like catalysts, PWA/PAN displayed better catalytic activity for H2O2 decomposition and TC removal. In the pH range of natural water (pH 6–11), PWA/PAN efficiently catalyzed H2O2 decomposition. Under optimal conditions (20 °C, pH0 7, catalyst dose of 0.5 g/L, and H2O2 concentration of 50 mM), the degradation efficiency of TC and the decomposition efficiency of H2O2 reached 100% and 0.1010 min−1 within 30 min, respectively. PWA/PAN exhibited excellent catalytic activity durability and structural stability. TC degradation efficiency by the PWA/PAN/H2O2 system did not distinctly decrease. In addition, the degradation mechanism of TC in the PWA/PAN/H2O2 system was revealed. Results disclosed that •OH was the main contributor to the degradation of TC, and it achieved most of mineralization of TC. This work is expected to tackle the common issues of easy deactivation, poor stability, and low H2O2 decomposition efficiency of Fenton-like catalysts at neutral pH and provide novel materials for the treatment of antibiotic pollution.

CuxNiyCo-LDH nanosheets on graphene oxide: An efficient and stable Fenton-like catalyst for dual-mechanism degradation of tetracycline

A highly efficient and stable heterogeneous Fenton-like catalyst was fabricated via growing CuxNiyCo-containing layered double hydroxide (LDH) nanosheets onto graphene oxide (GO) (CuxNiyCo-LDH/GO) by a facile co-precipitation method. The Cu2.5Ni0.5Co-LDH/GO with higher crystallinity and a larger specific surface area exhibited the highest catalytic activity in the degradation of tetracycline (TC). The TC degradation and mineralization efficiencies achieved 96.5% and 80.4%, respectively, in 40 min under the optimum conditions (60 mg/L of catalyst, 10 mM of H2O2 and pH = 10.2). The catalyst also exhibited excellent reusability with the degradation of TC remaining above 95% after 5 successive cycles. A dual-mechanism of TC degradation was proposed: attacked by ·OH primarily generated by catalyst surface and direct oxidation through the C-O-M bonds as the electron donor, leading to a high H2O2 utilization in the system. The degradation pathway of TC was also proposed based on the detected intermediates, and the overall toxicity of TC was effectively alleviated in Cu2.5Ni0.5Co-LDH/GO system according to toxicity prediction.

Exploration of copper-cysteamine nanoparticles as an efficient heterogeneous Fenton-like catalyst for wastewater treatment

Copper-cysteamine (Cu-Cy) is a new nanoparticle that can be stimulated by UV light, microwaves, X-rays, and ultrasound to produce various types of reactive oxygen species (ROS) for cancer and infectious disease treatments. Most importantly, our recent studies revealed that Cu-Cy nanoparticles (NPs) can be used for wastewater treatment as an effective heterogeneous Fenton-like catalyst, thereby adding one more important practical application of this interesting material. Herein, for the first time, we describe the use of Cu-Cy NPs with the assistance of hydrogen peroxide (H2O2) as an oxidizing agent for the oxidative degradation of organic contaminants, particularly rhodamine B (RhB), methylene blue (MB), and 4-nitrophenol (4-NP), for wastewater treatment. Additionally, we compared two H2O2 oxidation methods for the treatment of RhB and 4-NP: oxidation using H2O2/Cu-Cy with or without microwave (MW) irradiation. Our findings show that the degradation rate enhanced when activated by MW irradiation. Furthermore, recycling experiments revealed that Cu-Cy has good stability and recyclability. Overall, it is believed that the Cu-Cy/H2O2 system studied in this work could be used in advanced oxidation processes toward the degradation of organic pollutants.

An efficient and low-cost magnetic heterogenous Fenton-like catalyst for degrading antibiotics in wastewater: Mechanism, pathway and stability

Metal-doped MgFe2O4 spinel ferrite synthesized from saprolite laterite nickel ore was verified as an efficient heterogeneous Fenton-like catalyst for degrading antibiotics including tetracycline (TC) and metronidazole (MNZ) in a “catalyst/oxalic acid (H2C2O4)/visible light (vis)” system. The degradation efficiencies reached over 95% and total organic carbon (TOC) removal efficiencies were nearly 50% of the two antibiotics within 210 min, under the optimal conditions, especially 90% catalytic activity of the fresh catalyst was maintained after five cycles, suggesting the ferrite possessed excellent degrading performance, cycling stability and applicability. Moreover, the degradation mechanism and pathway of TC were elucidated in detail. Results revealed that the [≡Fe(C2O4)3]3- complex ions formed by octahedral Fe3+ in spinel ferrite with oxalate ions on the surface of MgFe2O4, played the key role in production of ·OH radicals which decomposed antibiotic TC into small molecules even mineralized in three pathways. Cost-effective preparation, high catalytic performance and long cycle life may accelerate the practical application of the heterogeneous Fenton-like catalyst.

Design and synthesis of BiVO4@CuOx as a photo assisted Fenton-like catalyst for efficient degradation of tetracycline

The rational design and fabrication of stable and efficient heterogeneous Fenton-like catalyst is of great significance for wastewater treatment, but is also fraught with obstacles. In this paper, we synthesized BiVO4@CuOx composite catalyst through a simple hydrothermal method and in situ chemical deposition. The degradation of tetracycline (TC) is carried out as a main model pollutant through Fenton reaction assisted by light irradiation. The prepared catalyst exhibits superb efficiency and satisfactory stability for the degradation of TC owing to the synergistic effect of light irradiation and decomposition of H2O2. The factors influencing the degradation such as catalyst dosage, H2O2 concentration and initial pH value are also investigated. Moreover, a possible mechanism of degradation of TC in BiVO4@CuOx/H2O2 system is discussed based on the premise that •OH, •O2− and h+ are demonstrated to be the main active species via in-situ ESR measurements. This work puts forward a new suggestion for designing and synthesizing photo assisted Fenton-like catalyst to remove organic contaminants in sewage.

Facile one-step synthesis of 3D honeycomb-like porous chitosan bead inlaid with Mn[sbnd]Fe bimetallic oxide nanoparticles for enhanced degradation of dye pollutant

Developing a sustainable, efficient and recyclable heterogeneous Fenton-like catalyst is important to wastewater treatment. Herein, well-dispersed MnO2 and Fe3O4 nanoparticles inlaid in chitosan beads (MnO2-Fe3O4/CH) was firstly fabricated and employed in the degradation of methylene blue (MB). The bead was prepared via a facile one-step method by dropwise addition of chitosan-metal salt solution into alkaline solution. Comparing with monometallic chitosan beads (MnO2/CH, Fe3O4/CH) and naked MnO2-Fe3O4, MnO2-Fe3O4/CH displayed significantly higher activity for MB degradation with the assistance of hydrogen peroxide (H2O2), finally removing 96.8% MB under the optimal conditions (50 mg L−1 MB, 4.0 g L−1 catalyst, 30 g L−1 H2O2, pH = 7, 60 min). Based on a series of characterizations, the large surface area (60.1 m2 g−1), well-developed porosity (0.3 cm3 g−1), and intensified electron transport of MnO2-Fe3O4/CH consequently enhanced the catalytic performance via a synergistic effect. Because the specific porous structure of MnO2-Fe3O4/CH facilitated the adsorption/diffusion of reactants and exposure of active sites. Meanwhile, the electron transfer from Mn3+ to Fe3+ accelerated the Fe3+/Fe2+ cycle, which favored the production of dominant reactive species hydroxyl radical for MB degradation. Besides, the magnetic beads could be easily collected from the solution and reused for five times with a negligible leaching.

FeS₂ Loaded Porous SiO₂ Ball as a Tweezers Recoverable Heterogeneous Fenton Catalyst with Enhanced Recyclability.

We report a novel FeS₂ loaded large SiO₂ sphere (diameter of 3 cm) via double immobilization with (3-mercaptopropyl)trimethoxysilane (KH590) as a highly efficient, stable, and tweezers-recoverable heterogeneous Fenton-like catalyst (FeS₂/KH590/SiO₂@KH590). Notably, as-prepared composite catalyst exhibits a significant recycling improvement (10 runs) over free FeS₂ powder (3 runs). The outstanding recyclability and stability of FeS₂/KH590/SiO₂@KH590 can be attributed to the passivation of FeS₂ nanoparticles on SiO₂ surface by KH590 and subsequent prevention of Fe2+ leaching. Overall, our work provides a new avenue towards fabricating composite Fenton-like catalyst with high stability, enhanced recyclability and the advantage of easy separation.

Enhanced heterogeneous Fenton-like degradation of refractory organic contaminants over Cu doped (Mg,Ni)(Fe,Al)2O4 synthesized from laterite nickel ore

The heterogeneous Fenton-like catalyst (Mg,Cu,Ni)(Fe,Al)2O4 was synthesized via a coprecipitation method using laterite nickel ore leaching solution as raw material. The effects of CuCl2·2H2O addition and calcination temperature on the microstructures and degradation properties of the obtained products were investigated. Results showed that higher calcination temperature could promote the migration of Cu2+ ions from CuO to the spinel ferrite lattice and occupied octahedral sites. The degradation efficiencies (η) of various types of low-concentration dyes and tetracycline were higher than 95%, which was mainly due to the accelerated generation of OH radicals by the synergistic effect of Fe3+ and Cu2+ ions in octahedral sites of the formed (Mg,Cu,Ni)(Fe,Al)2O4. Moreover, after five consecutive degradation cycles, the η of RhB was still close to 100%, TOC removal efficiency was maintained around 40% and the concentrations of metallic ions in degraded solutions were all lower than the national effluent discharge standard (GB8978-1996), confirming the as-obtained (Mg,Cu,Ni)(Fe,Al)2O4 was an eco-friendly heterogeneous Fenton-like catalyst with excellent stability and reusability. This study may provide an effective reference for large scale preparing efficient heterogeneous Fenton-like catalysts from natural minerals in treating the wastewater contaminated by refractory organics.

Copper-promoted heterogeneous Fenton-like oxidation of Rhodamine B over Fe3O4 magnetic nanocatalysts at mild conditions.

Rhodamine B (RhB) is used in various industries and its effluent must be effectively treated because of its harmful and carcinogenic nature. In this work, ionothermally synthesized Cu-doped Fe3O4 magnetic nanoparticles (Cu-Fe3O4 MNPs) were found to be a highly efficient heterogeneous Fenton-like catalyst for complete decolorization of the RhB solution with H2O2 at pH ~ 7 and 25 °C. The effects of the catalyst loading, initial concentrations of RhB and H2O2, co-existing natural organic matter and inorganic salts, reaction temperature, and radical scavengers on the catalytic performance of Cu-Fe3O4 were investigated. Monte-Carlo simulations revealed that copper dopants facilitated the activation of H2O2 via adopting a terminal end-on adsorption mode and increased collision frequency by bringing the RhB molecules closer to H2O2 and the magnetite surface. These theoretical calculations provide new insight into the promotional effect of copper dopants in magnetite at molecular level.

Metal-doped (Cu,Zn)Fe2O4 from integral utilization of toxic Zn-containing electric arc furnace dust: An environment-friendly heterogeneous Fenton-like catalyst

Pure metal-doped (Cu,Zn)Fe2O4 was synthesized from Zn-containing electric arc furnace dust (EAFD) by solid-state reaction using copper salt as additive. The effects of pretreated EAFD-to-Cu2(OH)2CO3·6H2O mass ratio, calcination time, and calcination temperature on the structure and catalytic ability were systematically studied. Under the optimum conditions, the decolorization efficiency and total organic carbon (TOC) removal efficiency of the as-prepared ferrite for treating a Rhodamine B solution were approximately 90.0% and 45.0%, respectively, and the decolorization efficiency remained 83.0% after five recycles, suggesting that the as-prepared (Cu,Zn)Fe2O4 was an efficient heterogeneous Fenton-like catalyst with high stability. The high catalytic activity mainly depended on the synergistic effect of iron and copper ions occupying octahedral positions. More importantly, the toxicity characteristic leaching procedure (TCLP) analysis illustrated that the toxic Zn-containing EAFD was transformed into harmless (Cu,Zn)Fe2O4 and that the concentrations of toxic ions in the degraded solution were all lower than the national emission standard (GB/31574—2015), further confirming that the as obtained sample is an environment-friendly heterogeneous Fenton-like catalyst.

Oxygen vacancy enhancing Fenton-like catalytic oxidation of norfloxacin over prussian blue modified CeO2: Performance and mechanism

To develop an efficient heterogeneous Fenton-like catalyst is of great importance for degrading organic pollutants. CeO2 was selected as the catalyst carrier. Prussian blue (PB) was chose as the iron resource for its sensitivity to H2O2 and low toxicity. PB modified CeO2 composite was successfully fabricated and used for Fenton-like oxidation of norfloxacin (NOR) in this study. The characteristics of the catalysts demonstrated that the doping of PB distorted the lattice locally and increased the surface area of CeO2 obviously. The XPS analysis also indicated that chemically supported catalysts PB/CeO2 with more Ce3+ was beneficial to Fenton-like catalytic reaction. The degradation tests showed that the PB/CeO2 significantly enhanced the removal of NOR which indicated a synergistic effect between PB and CeO2. The reason should be mainly attributed to the synergetic catalysis of H2O2 by Fe3+/Fe2+ and Ce3+/Ce4+ redox couples. At the same time, PB/CeO2 composite showed well reusability and wide pH value range of 2–9 with fairly low concentration of iron ions. The reaction mechanisms were identified to be OH oxidation and improvement of oxygen vacancies (OVs).

Fe3O4/graphene aerogels: A stable and efficient persulfate activator for the rapid degradation of malachite green

Encapsulation metal oxides into carbon frameworks is a good strategy to synthesis high activity and stable catalyst. Here, Fe3O4 nanoparticles (∼20 nm) were firmly encapsulated in the graphene aerogels by a simple and environmentally friendly method (Fe3O4/GAs), for activating persulfate (PS) to degrade malachite green (MG) under simulated sunlight. A strong electron conduction was generated between the Fe3O4 nanoparticles and graphene sheets to improve the cycle of Fe(II)/Fe(III), and the MG degradation over a wide pH rage (3–9) was enhanced greatly. The MG molecule was decomposed into 12 intermediates and two possible pathways was proposed. More importantly, toxicity test and Toxicity Estimation Software (T.E.S.T.) proved that the toxicity of MG can be effectively controlled by Fe3O4/GAs + PS + light system. In addition to the high catalytic activity, Fe3O4/GAs exhibited a good stability and reusability due to the strong interaction between Fe3O4 and graphene layers. The degradation efficiency remained above 87% after six cycles, and the leaching amount of iron in each cycle was less than 0.125 wt%. SO4•- was the dominate radical for MG degradation and the heterogeneous Fenton-like reaction was mainly performed on the surface of catalyst. This work lay a foundation for applying Fe3O4/GAs as a highly efficient, stable and reusable heterogeneous Fenton-like catalyst for future applications.

Synergistic effect and mechanism of catalytic degradation toward antibiotic contaminants by amorphous goethite nanoparticles decorated graphitic carbon nitride

Cost-effective ultrafine amorphous goethite nanoparticles with size of around 5 nm decorated graphitic carbon nitride (g-C3N4/FeOOH) heterojunction was fabricated by a simple chemical reaction which exhibited efficient photo-Fenton-like activity for antibiotic tetracycline (TC) degradation in the presence of H2O2 and visible light irradiation. Within the reaction time of 60 min, approximately 88% of TC molecules can be degraded and this value was obviously higher than those in the corresponding Fenton-like and photocatalytic processes. Compared with those common iron-based oxide counterparts, the photo-Fenton-like performance of g-C3N4/FeOOH was also significantly higher. In addition, the efficient degradation of TC in actual water bodies was observed for g-C3N4/FeOOH through using natural sunlight to drive the photo-Fenton-like reaction. Meanwhile, g-C3N4/FeOOH can maintain stable performance in the repeated experiment and the effects of different conditions (TC concentration, H2O2 concentration, catalyst dosage, initial pH, reaction temperature, and anion coexistence) were revealed. The improved separation and transfer of photogenerated charge carriers and accelerated conversion from surface Fe3+ to Fe2+ brought about enhanced catalytic activity of g-C3N4/FeOOH. The holes, O2− and OH radicals were major oxide species and possible TC degradation mechanism was revealed. This study might give some important inspiration for development of efficient heterogeneous Fenton-like catalysts in the treatment of antibiotic wastewaters.

Copper-Cysteamine Nanoparticles as a Heterogeneous Fenton-Like Catalyst for Highly Selective Cancer Treatment

Herein, for the first time, we report copper-cysteamine (Cu-Cy) nanoparticles having Cu1+ instead of Cu2+ as an efficient heterogeneous Fenton-like catalyst for highly selective cancer treatment. Initial measurements of Cu-Cy's hydroxyl radical generation ability show that it behaves as a Fenton-like reagent in the presence of H2O2 (100 μM) at pH 7.4, and that its Fenton-like activity is dramatically enhanced under acidic conditions (pH 6.5 and 5.5). Notably, Cu-Cy exhibits high stability and minimal copper release during the Fenton-like reaction, demonstrating its potency as a heterogeneous Fenton-like catalyst with a low cytotoxic effect. Through extensive in vitro studies, Cu-Cy NPs are found to generate a significantly higher level of ROS, thereby causing significantly more destruction to cancerous cells than to normal cells without the need for exogenous additives, such as H2O2. To the best of our knowledge, the average IC-50 value of Cu-Cy to cancer cells (11 μg/mL) is the lowest among reported heterogeneous Fenton-like nanocatalyst so far. Additionally, compared to cancer cells, Cu-Cy NPs display substantially higher IC-50 value toward normal cells (50 μg/mL), suggesting high selectivity. Overall, Cu-Cy NPs can participate in heterogeneous Fenton-like activity with elevated H2O2 under acidic conditions to produce significantly higher levels of hydroxyl radicals in cancer cells when compared to normal cells, resulting in selective cytotoxicity to cancer cells.

Graphitic carbon nitride (g-C3N4) as an efficient metal-free Fenton-like catalyst for degrading organic pollutants: the overlooked non-photocatalytic activity.

Graphitic carbon nitride (g-C3N4) has attracted a large amount of research, mainly being used as a photocatalyst, but its Fenton-like catalytic performance has been overlooked. In this paper, the dark Fenton-like catalytic performance of g-C3N4 was evaluated by degrading rhodamine B over a wide pH range. The results showed that the g-C3N4, which was synthesized by conventional urea pyrolysis without any modification, was an efficient metal-free heterogeneous Fenton-like catalyst. The highest activity occurred under a weakly alkaline condition of about pH 10. The experiment of catalyst recycling indicated that g-C3N4 had long-term stability. The reactive oxidizing species of HO·, generated by the g-C3N4 activating H2O2, was identified by EPR and further supported by a scavenging experiment of HO· using isopropanol as the scavenger. The HNO3 oxidation of g-C3N4 resulted in catalytic deactivation, implying the catalytic activity originated from the surface reduced groups of g-C3N4. The structure of synthesized g-C3N4 before and after the HNO3 oxidation was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, and a possible catalytic mechanism was proposed.