Application of response surface methodology to optimize the process variables for fluoride ion removal using maghemite nanoparticles

Adsorption of fluoride ion was done from its aqueous solution by using maghemite (γ-Fe₂O₃) nanoparticles. Effects of the major independent variables (temperature, adsorbent dose and pH) and their interactions during fluoride ion adsorption were determined by response surface methodology (RSM) based on three-level three-factorial Box–Behnken design (BBD). Optimized values of temperature, maghemite nanoparticle dose and pH for fluoride sorption were found as 313 K, 0.5 g/L, and 4, respectively. In order to investigate the mechanism of fluoride removal, various adsorption isotherms such as Langmuir, Freundlich, Temkin and Florry–Huggins were fitted. The experimental data revealed that the Langmuir isotherm gave a more satisfactory fit for fluoride removal. The adsorption process was rapid and obeyed pseudo-second-order kinetics. The values of thermodynamic parameters ΔG°, ΔH° and ΔS° indicated that adsorption was spontaneous and endothermic in nature.     

In situ study on the effects of Ni and Mo on the passive film formed on Fe–20Cr alloys by photoelectrochemical and Mott–Schottky techniques

Effects of alloying elements (Ni and Mo) on the structure of passive film formed on Fe–20Cr alloys in pH 8.5 buffer solution were explored by analyzing the in situ electronic properties measured using the photoelectrochemical technique and Mott–Schottky analysis. The passive film formed on Fe–20Cr–10Ni was found to be mainly composed of Cr-substituted γ-Fe₂O₃ from similarities in photocurrent response for the passive films formed on the alloy and Fe–20Cr. On the other hand, the photocurrent spectra for the passive films of Fe–20Cr–15Ni–(0, 4)Mo alloys exhibited the spectral components associated with NiO and Mo oxide (MoO₂ and/or MoO₃) in addition to that induced by Cr-substituted γ-Fe₂O₃. Mott–Schottky plots for the passive films formed on Fe–20Cr–(10, 15)Ni and Fe–20Cr–15Ni–4Mo confirmed that the passive films on Fe–20Cr–(10, 15)Ni–(0, 4)Mo alloys have a base structure of Cr-substituted γ-Fe₂O₃ with variation of densities of shallow and deep donors depending on the Ni and Mo contents in the alloys. We suggest that the passive film formed on Fe–20Cr–(10, 15)Ni and Fe–20Cr–15Ni–4Mo alloys are composed of (Cr, Ni, Mo)-substituted γ-Fe₂O₃ when the concentrations of Ni and Mo are below critical values. However, NiO and Mo oxide (MoO₂ and/or MoO₃) would be precipitated in the passive films when the concentrations of Ni and Mo exceed critical values.     

Simultaneous photocatalytic oxidation and adsorption for efficient As(III) removal by magnetic BiOI/γ-Fe2O3 core–shell nanoparticles

Addressing arsenite pollution in groundwater has drawn great attention. It is attractive to pre-oxidize highly mobile As(III) to relatively low-toxic As(V) with a subsequent adsorption separation process. Herein, BiOI anchoring on γ-Fe₂O₃ is performed to synthesize BiOI/γ-Fe₂O₃ core–shell nanoparticles for efficient removal of As(III) via a simultaneous photocatalytic oxidization–adsorption process. The physical and chemical structures of BiOI/γ-Fe₂O₃ are investigated by transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction measurements. The photoluminescence and electron spin resonance (ESR) characterization were employed to ascertain the possible reaction mechanism of visible-light-driven photocatalytic oxidation of As(III). Such BiOI/γ-Fe₂O₃ delivers a superior As(III) removal capability under visible light irradiation with an arsenic removal efficiency of 99.8% within 180 min, higher than those of BiOCl/γ-Fe₂O₃ (81.7%) and BiOBr/γ-Fe₂O₃ (98.9%). The optimal BiOI/γ-Fe₂O₃ (molar ratio of 2:1) is obtained by rationally adjusting the molar ratio of BiOI to γ-Fe₂O₃. The as-synthesized BiOI/γ-Fe₂O₃ performs well in a wide pH range of 2–8. Only coexisting PO₄^3? anions have a significant effect on the As(III) removal. The free radical trapping experiment and ESR results demonstrate that the ?O₂^? and h⁺ are the main active substances for the photocatalytic oxidation of As(III) on BiOI/γ-Fe₂O₃. This work not only gives a novel magnetic core–shell nanoparticle photocatalyst for efficient photocatalytic oxidation and adsorption of As(III) but also offers a new strategy to rationally design BiOX for its related practical applications.     

Synthesis of mesoporous maghemite with high surface area and its adsorptive properties

Mesoporous maghemite (γ-Fe₂O₃) with high surface area was prepared by the thermal decomposition of Fe–urea complex ([Fe(CON₂H₄)₆](NO₃)₃) with the aid of cetyltrimethyl ammonium bromide (CTAB), and its adsorption ability for the removal of fluoride was investigated. X-ray diffraction (XRD), nitrogen adsorption–desorption measurements, transmission electron micrograph (TEM) observations, and magnetic measurements show that the γ-Fe₂O₃ has a mesoporous structure and its crystallite size, specific surface area, and magnetic properties can be controlled by varying the content of CTAB in [Fe(CON₂H₄)₆](NO₃)₃. The maximum adsorption capacity of the mesoporous γ-Fe₂O₃ for fluoride is estimated to be 7.9 mg/g, which suggests that the mesoporous γ-Fe₂O₃ is an excellent adsorbent for fluoride.     

Removal of phenol from aqueous solution by adsorption onto hematite (α-Fe2O3): Mechanism exploration from both experimental and theoretical studies

Iron oxides in general and especially hematite, α-Fe₂O₃ have been proved promising materials for efficient removal of various organic pollutants. Herein, we report a successful preparation of hematite (α-Fe₂O₃) by a facile precipitation method and its potential application in the removal of phenol from wastewater. The prepared material was subjected to extensive characterization using a variety of techniques such as scanning electron microscope coupled with energy-dispersive X-ray spectroscopy (SEM/EDX), X-ray diffraction (XRD), and the Brunauer Emmett Teller (BET) method. The operating conditions were optimized to improve the adsorption process efficiently. The adsorption analysis showed an adsorption capacity of 16.17 mg g⁻¹ towards phenol at 30 °C. The reaction kinetics and potential rate-limiting steps were studied by Lagergren's pseudo-first-order and pseudo-second-order models, and it was found that the pseudo-second-order accurately described the adsorption kinetics. Freundlich and Langmuir adsorption isotherms models were applied, and the quality of the fittings clearly shows that the Langmuir model well describes the phenol adsorption on the hematite. The interaction mechanism between phenol and α-Fe₂O₃(0 0 1) surface was further addressed by Density Functional Theory (DFT) calculations and molecular dynamics (MD) simulations. Experimental and theoretical results indicate that there is strong evidence for the decisive effect of π–π interactions and H-bonds on the adsorption capacity.     

Banana peel: A green and economical sorbent for the selective removal of Cr(VI) from industrial wastewater

This study describes the use of banana peel, a commonly produced fruit waste, for the removal of Cr(VI) from industrial wastewater. The parameters pH, contact time, initial metal ion concentration, and temperature were investigated and the conditions resulting in rapid and efficient adsorption (95% within 10 min) were determined. The binding of metal ions was found to be pH dependent with the optimal sorption occurring at pH 2. The retained species were eluted with 5 mL of 2 M H₂SO₄. To elucidate the mechanism of the process, total amounts of chromium and Cr(VI) were analyzed using flame atomic absorption and ultraviolet–visible (UV–vis) spectroscopic techniques, respectively. The Langmuir and Dubinin–Radushkevich (D–R) isotherms were used to describe the partitioning behavior for the system at different temperatures. Kinetics and thermodynamics of Cr(VI) removal by banana peel were also studied. The influence of diverse ions on the sorption behavior revealed that only Fe(II) ions (of those tested) suppressed the sorption of Cr(VI) ions to some extent. The method was applied for the removal of Cr(VI) from industrial wastewater.     

Preconcentration and spectrophotometric determination of low concentrations of malachite green and leuco-malachite green in water samples by high performance solid phase extraction using maghemite nanoparticles

A novel and sensitive extraction procedure using maghemite nanoparticles (γ-Fe₂O₃) modified with sodium dodecyl sulfate (SDS), as an efficient solid phase, was developed for removal, preconcentration and spectrophotometric determination of trace amounts of malachite green (MG) and leuco-malachite green (LMG). Combination of nanoparticle adsorption and easily magnetic separation was used to extraction and desorption of MG and LMG. The adsorption capacity was evaluated using both the Langmuir and Freundlich adsorption isotherm models. Maghemite nanoparticles were prepared by co-precipitation method and their surfaces were modified by SDS. The size and properties of the produced maghemite nanoparticles was determined by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and BET analysis. MG and LMG became adsorbed at pH 3.0. LMG was oxidized to MG by adsorption on maghemite nanoparticles. The adsorbed MG was then desorbed and determined spectrophotometrically. The calibration graph was linear in the range 0.50-250.00 ng mL⁻¹ of MG and LMG with a correlation coefficient of 0.9991. The detection limit of the method for determination of MG was 0.28 ng mL⁻¹ and the relative standard deviation (R.S.D.) for 10.00 and 50.00 ng mL⁻¹ of malachite green was 1.60% (n = 3) and 0.86% (n = 5), respectively. A preconcentration factor of 50 was achieved in this method. The Langmuir adsorption capacity (q_max) was found to be 227.3 mg g⁻¹ of the adsorbent. The method was applied to the determination of MG in fish farming water samples.     

Preparation of carbon coated Fe3O4 nanoparticles for magnetic separation of uranium

Uranium(VI) was removed from aqueous solutions using carbon coated Fe₃O₄ nanoparticles (Fe₃O₄@C). Batch experiments were conducted to study the effects of initial pH, shaking time and temperature on uranium sorption efficiency. It was found that the maximum adsorption capacity of the Fe₃O₄@C toward uranium(VI) was ∼120.20 mg g⁻¹ when the initial uranium(VI) concentration was 100 mg L⁻¹, displaying a high efficiency for the removal of uranium(VI) ions. Kinetics of the uranium(VI) removal is found to follow pseudo-second-order rate equation. In addition, the uranium(VI)-loaded Fe₃O₄@C nanoparticles can be recovered easily from aqueous solution by magnetic separation and regenerated by acid treatment. Present study suggested that magnetic Fe₃O₄@C composite particles can be used as an effective and recyclable adsorbent for the removal of uranium(VI) from aqueous solutions.     

Preparation,Characterization, and Electrochemical Performance of the Hematite/Oxidized Multi-Walled Carbon Nanotubes Nanocomposite

In this research work, a hematite (α-Fe₂O₃) nanoparticle was prepared and then mixed with oxidized multi-walled carbon nanotubes (O-MWCNT) to form a stable suspension of an α-Fe₂O₃/O-MWCNTs nanocomposite. Different characterization techniques were used to explore the chemical and physical properties of the α-Fe₂O₃/O-MWCNTs nanocomposite, including XRD, FT-IR, UV-Vis, and SEM. The results revealed the successful formation of the α-Fe₂O₃ nanoparticles, and the oxidation of the MWCNT, as well as the formation of stable α-Fe₂O₃/O-MWCNTs nanocomposite. The electrochemical behaviour of the α-Fe₂O₃/O-MWCNTs nanocomposite was investigated using cyclic voltammetry (CV) and linear sweep voltammetry (LSV), and the results revealed that modification of α-Fe₂O₃ nanoparticles with O-MWCNTs greatly enhanced electrochemical performance and capacitive behaviour, as well as cycling stability.     

Effective Removal of Nitrotoluene Compounds from Aqueous Solution Using Magnetic-Activated Carbon Nanocomposites (<Emphasis Type="Italic">m</Emphasis>-Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>@ACCs)

In this study, regular-shaped magnetic-activated carbon nanocomposite (m-Fe₃O₄@ACCs) was synthesized and characterized with X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and the vibrating sample magnetometer (VSM) and was used as adsorbents for the removal of nitrotoluene compounds (NTCs) from water and industrial wastewater. The effective parameters on adsorption process, such as solution pH, shaking speed, contact time, and adsorbent dosage were optimized and the optimum amounts were 7 300 rpm, 10 min, and 1.2 g L^–1, respectively. The contact time and adsorbent dosage are dependent parameters and hence were studied simultaneously. The results showed no significant loss in the adsorption capacity, and the adsorption efficiency of m-Fe₃O₄@ACCs could still be 90% in the 9th cycle. The equilibrium adsorption isotherm followed the Langmuir isotherm model describes the monolayer adsorption of NTCs on m-Fe₃O₄@ACCs, and the maximum adsorption capacities (q_m) for 2-nitrotolouene, 2,6-dinitrotoluene, 2,4-dinitrotoluene, and 3,4-dinitrotoluene were found to be 144.93, 142.86, 166.67, and 153.85 mg g^?l, respectively. The proposed process was successfully applied for the removal of NTCs from tap water and nitration process wastewater.     

The Thermal Conversion of Lepidocrocite (γ-FeOOH) Revisited

The thermal conversion of lepidocrocite (γ-FeOOH) into maghemite (γ-Fe₂O₃)and hematite (α-Fe₂O₃) has been studied by dynamic (DSC) and static heating experiments. Dynamic heating defines two main regions: conversion of lepidocrocite to maghemite (endothermal signal peaking at 255°C) and conversion of maghemite to hematite (exothermal signal peaking at 450°C). In addition, an exotherm following the lepidocrocite to maghemite endotherm is observed. The maghemite phase appears as porous aggregates of nanocrystals characterized by an extensive spin-canting. We suggest that the additional exotherm is associated with structural changes and a decreasing extent of spin-canting in the maghemite phase. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fe3O4-CuO-activated carbon composite as an efficient adsorbent for bromophenol blue dye removal from aqueous solutions

Dye wastewater from industries is posing tremendous health hazards. The lethal dyes can be eliminated using nanomaterials and scientific approach like adsorption which is facile, cheap, safe as well as ecofriendly. Fe₃O₄-CuO-AC composite was prepared by a hydrothermal method and used for the removal of dyes in wastewater. The composite material was characterized by various techniques such as XRD, SEM, EDS, TEM and FT-IR. The Fe₃O₄-CuO-AC composite was used to treat five types of dyes in water. Fe₃O₄-CuO-AC composite showed the highest adsorption capability for bromophenol blue (BPB) dye. The effects of initial concentration, pH, the amount of adsorbent and temperature were also studied. The optimum conditions were found to be 20 ppm dye concentration, pH 9, an adsorbent dose of 0.06 gL^─1 at 65 °C. A removal efficiency of 97% was obtained for BPB dye during 120 min of adsorption. Kinetic studies indicated that a pseudo-second order is the most suitable model for the adsorption process. The Fe₃O₄-CuO-AC composite showed better adsorption capacity as compare to Fe₃O₄-AC except for the Methyl green dye. The maximum adsorption capacity was found to be 88.60 mg/g for BPB. Additionally, the thermodynamic parameters (ΔS°, ΔH° and ΔG°) showed that the process was spontaneous and exothermic. All the above results revealed that the Fe₃O₄-CuO-AC compositecan be an effective adsorbent for removing dyes from wastewater.     

Multitracer study on adsorption of metal ions on α-Fe2O3

pH dependence of the adsorption of Na, Sc, Ga, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, and Pd on -Fe₂O₃ from a 0.1 mol dm^–3 NaCl solution was studied by using a multitracer technique. Desorption of the metal ions from the -Fe₂O₃ with the adsorbed metal ions at pH 11 was also studied by lowering the pH of the suspensions. The desorption curve of each element was in good agreement with the adsorption curve except for Ru and Rh under conditions studied. Adsorption kinetics showed that the adsorption of most metal ions increases with shaking time before an adsorption equilibrium is attained. An increase in the adsorption was also observed with an elevation in temperature for the elements, suggesting that the adsorption is involved in chemisorption.     

Sorption of chromium(III) and chromium(VI) on lead sulfide

The sorption of chromium(III) and chromium(VI) on lead sulfide has been investigated in dependence on pH, time of sorption and the concentrations of sorbate and sorbent. The mechanisms of the sorption of Cr³⁺ and CrO ₄ ^2– traces on lead sulfide are discussed; a difference between CrO ₄ ^2– sorption on PbS and -Fe₂O₃ has been found. Sulfates and molybdates affect the removal of chromates from aqueous solutions. Lead sulfide carrier prepared in this work was also used for the preconcentration of chromium(III) and chromium(VI) from tap water.     

Nanostructured spinel-type M(M = Mg,Co, Zn)Cr2O4 oxides: novel adsorbents for aqueous Congo red removal

Nanostructured spinel-type M(M = Mg, Co, Zn)Cr₂O₄ oxides with novel adsorbents for aqueous Congo red removal were synthesized by a polyacrylamide gel method and studied for their phase structure, microstructure, adsorption performance, and multiferroic behavior. The phase structure and purity analysis revealed that the nanostructured spinel-type M(M = Mg, Co, Zn)Cr₂O₄ oxides presented a spinel-type cubic structure, and the formation of a secondary phase such as Cr₂O₃, MgO, ZnO, or Co₃O₄ was not observed. The microstructure characterization confirmed that the spinel-type MCr₂O₄ oxides grew from fine spherical particles to large rhomboid particles. Adsorption experiments of spinel-type MCr₂O₄ oxides for adsorption of Congo red dye were fitted well with the pseudo-second-order kinetics. The adsorption capacity of the ZnCr₂O₄ oxide (44.038 mg/g, pH 7, temperature 28 °C, initial dye concentration 30 mg/L) was found to be higher than that of MgCr₂O₄ oxide (43.592 mg/g, pH 7, temperature 28 °C) and CoCr₂O₄ oxide (28.718 mg/g, pH 7, temperature 28 °C). The effects of initial adsorbent concentration, initial dye concentration, pH, and temperature between the ZnCr₂O₄ oxide and Congo red dye at which optimal removal occurs, were performed. The thermodynamic studies confirmed that a high temperature favors the adsorption of Congo red dye onto ZnCr₂O₄ oxide studied. The nanostructured spinel-type M(M = Mg, Co, Zn)Cr₂O₄ oxides that exhibited high adsorption performance for adsorption of Congo red dye can be ascribed to the synergistic effect of electrostatic interaction, pore filling, and ion exchange. The present work suggested that the nanostructured spinel-type M(M = Mg, Co, Zn)Cr₂O₄ oxides have excellent adsorption performance and multiferroic behavior, which shows potential applications for removal of the Congo red dye from wastewater, magnetic memory recording media, magnetic sensor, energy collection and conversion device, and read/write memory.     

Humic Acid Removal by Combining the Magnetic Property of Maghemite with the Adsorption Property of Nanosized Hydroxyapatite

Maghemite incorporated hydroxyapatite (HAP) nanocomposite was prepared by in situ precipitation of the calcium phosphate phase in an iron oxide colloidal suspension. The resultant nanocomposite was characterized by x-ray diffraction, Fourier transform infrared spectrometer, transmission electron microscope, N₂ adsorption analysis, and vibrating sample magnetometry. The potential of HAP/γ-Fe₂O₃ nanocomposite for HA adsorption from aqueous solution was evaluated by batch experiments and adsorption kinetic tests. HA adsorption amount on the adsorbent decreased with increasing solution pH and the presence of KNO₃ and alkali-earth metal ions resulted in enhanced HA adsorption. HA adsorption onto HAP/γ-Fe₂O₃ nanocomposite could be well described by Freundlich and Sips models, while HA adsorption process on the adsorbent obeyed pseudo-second-order kinetics and the adsorption rates decreased with increasing initial HA concentration. This study showed that the HAP/γ-Fe₂O₃ nanocomposite could be used as an efficient and magnetically separable adsorbent for the removal of HA from aqueous solution. Subsequent studies demonstrated that the HA-loaded HAP/γ-Fe₂O₃ nanocomposite could be further applied for the highly efficient adsorption of methylene blue (MB) and separated from the medium by a simple magnetic process.     

Highly efficient,economic, and recyclable glutathione decorated magnetically separable nanocomposite for uranium(VI) adsorption from aqueous solution

A green and environment-friendly magnetically separable nanocomposite, glutathione@magnetite was fabricated sonochemically through the functionalization of Fe₃O₄ by glutathione which was well characterized using Fourier-transform infrared spectroscopy, ultravoilet-visible spectroscopy, scanning electron microscope, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, thermogravimetric analysis, vibrating sample magnetometer, Brunauer-Emmett-Teller, and high-resolution transmission electron microscope. The parameters affecting adsorption including pH, temperature, contact time, initial adsorbate concentration, and adsorbent amount were optimized by batch experiments. The magnetic glutathione@magnetite was applied for the removal of uranium(VI) in water with maximum adsorption capacity found to be 333.33 mg/g in 120 min at a neutral pH at 25 °C showing high efficiency for U(VI) ions. Furthermore, adsorption results obtained from UV-vis spectroscopy were validated by inductively coupled plasma optical emission spectroscopy. The thermodynamic parameters, viz Gibbs free energy (ΔGº), standard enthalpy change (ΔHº), and standard entropy change (ΔSº) of the process were calculated using the Langmuir constants. The pseudo-second-order kinetics model is seen to be applicable for describing the uptake process using a kinetics test. Moreover, desorption studies reveals that glutathione@magnetite can be used repeatedly, and removal efficiency shows only a small decrease after six cycles. Thus, glutathione@magnetite acts as a potential adsorbent for the removal of U(VI) from the water with great adsorption performance.     

Adsorption of microamounts of ruthenium on hydrous iron oxides

The adsorption of Ru on amorphous Fe(OH)₃, -Fe₂O₃ and Fe₃O₄ have been measured as a function of the pH and the time of aging. The adsoprtion of Ru increases markedly in the 3–5.5 pH range. At higher pH values, -Fe₂O₃ shows different behaviour with respect to Ru adsorption. The influence of EDTA, citrate and oxalate on the adsorption of Ru on Fe₃O₄ has also been investigated. Possible mechanisms of the adsorption of Ru on hydrous iron oxides are discussed in the light of the results obtained in the course of this study and of those of other researchers.     

Microwave-assisted one-step hydrothermal synthesis of pure iron oxide nanoparticles: magnetite, maghemite and hematite

A simple, rapid, one-step synthesis way of pure iron oxide nanoparticles: magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃) and hematite (α-Fe₂O₃) was investigated. Nanoparticles were prepared by microwave synthesis, from ethanol/water solutions of chloride salts of iron (FeCl₂ and FeCl₃) in the presence of sodium hydroxide NaOH. X-ray powder diffraction (XRD), Transmission Electron Microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize these nanoparticles.     

Promoting effect of Mg in supported Mo/HBeta–Al2O3 catalyst for cross-metathesis of ethene and butene-2 to propene

Promoting effects of Mg in heterogeneous Mo/HBeta–Al₂O₃ catalyst have been carefully studied for cross-metathesis of ethene and butene-2 to propene. The catalyst shows good stability with Mg content in the range of 1–2 wt%. Such effect may be attributed to the elimination of weak acid sites through introduction of Mg which suppresses the side olefin oligomerization reaction, as evidenced from NH₃-TPD and ¹H MAS NMR results. Addition of more Mg content to 3 wt% may change the state and reducibility of Mo species, as indicated from the UV–vis, UV-Raman and H₂-TPR measurements. The increasing difficulty for the reduction of Mo(VI) species is closely related with the poor performance of 3 wt% Mg–4Mo/HBeta–30% Al₂O₃ catalyst in the metathesis reaction.