Shape Controlled Preparation,Characterization and Gas Sensitivities of Shuttle-like Cr-doped α-Fe_2O_3 Nanoparticles

α-Fe2O3 nanoparticles doped with various molar fractions of Cr^3＋ were synthesized by a forced hydrolysis route and were characterized by X-ray diffraction（XRD）,scanning electronic microscopy（SEM）,X-ray photoelectron spectroscopy（XPS） and inductive coupled plasma（ICP） techniques.The particles reserve shuttle-like shape in the presence of Cr^3＋.The crystallite sizes of Fe2O3 become smaller with the increased Cr^3＋ concentration in solution.The responses of Cr doped α-Fe2O3 sensors were studied towards reducing gases such as ethanol,methanol,acetone,gasoline and n-hexane.Gas sensors based on these materials have higher sensitivities and rapid response/recovery time to alcohol than to hydrocarbon.

Effect of oxide interactions on chromium speciation transformation during simulated municipal solid waste incineration

Chromium released during municipal solid waste incineration(MSWI)is toxic and carcinogenic.The removal of chromium from simulated MSWI flue gas by four sorbents(CaO,bamboo charcoal(BC),powdered activated carbon(PAC),and Al_(2)O_(3))and the effects of four oxides(SiO_(2),Al_(2)O_(3),Fe_(2)O_(3),and CaO)on chromium speciation transformationwere investigated.The results showed that the removal rates of total Cr by the four sorbents were Al_(2)O_(3)<CaO<PAC<BC,while the removal rates of Cr(Ⅵ)by the four sorbents were Al_(2)O_(3)<PAC<BC<CaO.CaO had a strong oxidizing effect on Cr(Ⅲ),while BC and PAC had a better-reducing effect on Cr(Ⅵ).SiO_(2)was better for the reduction of Na_(2)CrO_(4)and K_(2)CrO_(4)above 1000℃due to its strong acidity,and the addition of CaO significantly inhibited the reduction of Cr(Ⅵ).MgCrO_(4)decomposed above 700℃to form MgCr_(2)O_(4),and the reaction between MgCrO_(4)and oxides also existed in the form of a more stable trivalent spinel.Furthermore,when investigating the effect of oxides on the oxidation of Cr(Ⅲ)in CrC_(l3),it was discovered that CaO promoted the conversion of Cr(Ⅲ)to Cr(Ⅵ),while the presence of chlorine caused chromium to exist in the form of Cr(V),and increasing the content of CaO and extending the heating time facilitated the oxidation of Cr(Ⅲ).In addition,silicate,aluminate,and ferrite were generated after the addition of SiO_(2),Al_(2)O_(3),and Fe_(2)O_(3),which reduced the alkalinity of CaO and had an important role in inhibiting the oxidation of Cr(Ⅲ).The acidic oxides can not only promote the reduction of Cr(Ⅵ)but also have an inhibitory effect on the oxidation of Cr(Ⅲ)ascribed to alkali metals/alkaline earth metals,and the proportion of acidic oxides can be increased moderately to reduce the generation of harmful substances in the hazardous solid waste heat treatment.

Effect of Intergranular Precipitation on the Internal Oxidation Behavior of Cr–Mn–N Austenitic Stainless Steels

The effect of intergranular precipitation on the internal oxidation behavior of Cr–Mn–N austenitic steels at 1000 °C in dry air atmosphere was investigated using scanning electron microscope, transmission electron microscope, and X-ray diffraction analysis. The results show that intergranular M23C6 carbide morphologies play an important role on the internal oxidation behavior of Cr–Mn–N steels. During the period of the oxidation, both discontinuous chain-shaped and continuous film-shaped intergranular M23C6 carbides precipitated along the grain boundaries. Internal oxides of silica preferentially intruded into the matrix along grain boundaries with discontinuous M23C6 carbide particles, while silica was obviously restricted at the interfaces between the external scale and matrix on the occasion of continuous film-shaped M23C6 carbides. It is seemed that reasonable microstructure could improve the oxidation resistance of Cr–Mn–N steels.

Nb_2O_5 nanowires in-situ grown on carbon fiber: A high-efficiency material for the photocatalytic reduction of Cr(Ⅵ)

Niobium oxide nanowire-deposited carbon fiber（CF） samples were prepared using a hydrothermal method with amorphous Nb2O5·nH2O as precursor. The physical properties of the samples were characterized by means of numerous techniques, including X-ray diffraction（XRD）, energy-dispersive spectroscopy（EDS）, scanning electron microscopy（SEM）, transmission electron microscopy（TEM）, selected-area electron diffraction（SAED）, UV–visible spectroscopy（UV–vis）, N2 adsorption–desorption, Fourier transform infrared spectroscopy（FT-IR）, and X-ray photoelectron spectroscopy. The efficiency for the removal of Cr（VI） was determined.Parameters such as pH value and initial Cr（VI） concentration could influence the Cr（VI） removal efficiency or adsorption capacity of the Nb2O5/carbon fiber sample obtained after hydrothermal treatment at 160°C for 14 hr. The maximal Cr（VI） adsorption capacity of the Nb2O5 nanowire/CF sample was 115 mg/g. This Nb2O5/CF sample also showed excellent photocatalytic activity and stability for the reduction of Cr（Ⅵ） under UV-light irradiation： the Cr（VI） removal efficiency reached 99.9% after UV-light irradiation for 1 hr and there was no significant decrease in photocatalytic performance after the use of the sample for 10 repeated cycles. Such excellent Cr（VI） adsorption capacity and photocatalytic performance was related to its high surface area,abundant surface hydroxyl groups, and good UV-light absorption ability.

Oxidation of Cr(Ⅲ) on birnessite surfaces: The effect of goethite and kaolinite

Oxidation of Cr（Ⅲ） by manganese oxides may pose a potential threat to environments due to the formation of toxic Cr（Ⅵ） species. At present, it was still unclear whether the extent of Cr（Ⅲ） oxidation and fate of Cr（Ⅵ） would be changed when manganese oxides co-exist with other minerals, the case commonly occurring in soils. This study investigated the influence of goethite and kaolinite on Cr（Ⅲ） oxidation by birnessite under acidic p H condition（p H 3.5）and background electrolyte of 0.01 mol/L Na Cl. Goethite was found not to affect Cr（Ⅲ）oxidation, which was interpreted as the result of overwhelming adsorption of cationic Cr（Ⅲ） onto the negatively-charged birnessite（point of zero charge（PZC） 〈 3.0） rather than the positively-charged goethite（PZC = 8.8）. However, more Cr（Ⅵ） would be retained by the surface with the increase in addition of goethite because of its strong ability on adsorption of Cr（Ⅵ） at low p H. Moreover, either Cr（Ⅲ） oxidation or distribution of the generated Cr（Ⅵ）between the solid and solution phases was not affected by kaolinite（PZC 〈 3.0）, indicating its low affinity for Cr species. Reactions occurring in the present mixed systems were suggested, which could be partly representative of those in the soils and further indicates that the mobility and risk of Cr（Ⅵ） would be decreased if goethite was present.

Environmental Factors Affecting Chromium-Manganese Oxidation-Reduction Reactions in Soil

Disposal of chromium （Cr） hexavalent form, Cr（Ⅵ）, in soils as additions in organic fertilizers, liming materials or plant nutrient sources can be dangerous since Cr（Ⅵ） can be highly toxic to plants, animals, and humans. In order to explore soil conditions that lead to Cr（Ⅵ） generation, this study were performed using a Paleudult （Dystic Nitosol） from a region that has a high concentration of tannery operations in the Rio Crande do Sul State, southern Brazil. Three laboratory incubation experiments were carried out to examine the influences of soil moisture content and concentration of cobalt and organic matter additions on soil Cr（Ⅵ） formation and release and manganese （Mn） oxide reduction with a salt of chromium chloride （CrCl3） and tannery sludge as inorganic and organic sources of Cr（Ⅲ）, respectively. The amount of Cr（Ⅲ） oxidation depended on the concentration of easily reducible Mn oxides and the oxidation was more intense at the soil water contents in which Mn（Ⅲ/Ⅳ） oxides were more stable. Soluble organic compounds in soil decreased Cr（Ⅵ） formation due to Cr（Ⅲ） complexation. This mechanism also resulted in the decrease in the oxidation of Cr（Ⅲ） due to the tannery sludge additions. Chromium（Ⅲ） oxidation to Cr（Ⅵ） at the solid/solution interface involved the following mechanisms： the formation of a precursor complex on manganese （Mn） oxide surfaces, followed by electron transfer from Cr（Ⅲ） to Mn（Ⅲ or Ⅳ）, the formation of a successor complex with Mn（Ⅱ） and Cr（Ⅵ）, and the breakdown of the successor complex and release of Mn（Ⅱ） and Cr（Ⅵ） into the soil solution.

Nano-sized Fe_2O_3/Fe_3O_4 facilitate anaerobic transformation of hexavalent chromium in soil–water systems

The purpose of this study is to investigate the effects of nano-sized or submicro Fe_2O_3/Fe_3O_4 on the bioreduction of hexavalent chromium（Cr（VI）） and to evaluate the effects of nano-sized Fe_2O_3/Fe_3O_4 on the microbial communities from the anaerobic flooding soil.The results indicated that the net decreases upon Cr（VI） concentration from biotic soil samples amended with nano-sized Fe_2O_3（317.1 ± 2.1 mg/L） and Fe_3O_4（324.0 ± 22.2 mg/L） within21 days,which were approximately 2-fold of Cr（VI） concentration released from blank control assays（117.1 ± 5.6 mg/L）.Furthermore,the results of denaturing gradient gel electrophoresis（DGGE） and high-throughput sequencing indicated a greater variety of microbes within the microbial community in amendments with nano-sized Fe_2O_3/Fe_3O_4 than the control assays.Especially,Proteobacteria occupied a predominant status on the phylum level within the indigenous microbial communities from chromium-contaminated soils.Besides,some partial decrease of soluble Cr（VI） in abiotic nano-sized Fe_2O_3/Fe_3O_4 amendments was responsible for the adsorption of nano-sized Fe_2O_3/Fe_3O_4 to soluble Cr（VI）.Hence,the presence of nano-sized Fe_2O_3/Fe_3O_4 could largely facilitate the mobilization and biotransformation of Cr（VI） from flooding soils by adsorption and bio-mediated processes.

Stabilization of chromium(Ⅵ) by hydroxysulfate green rust in chromium(Ⅵ)-contaminated soils

Chromium(Cr)-contaminated soils pose a great environmental risk, with high solubility and persistent leaching of Cr(Ⅵ). In this study, hydroxysulfate green rust(GR_(SO4)), with the general formula Fe(Ⅱ)_(4) Fe(Ⅲ)_(2)(OH)_(12) SO_(4)·8 H_(2) O, was evaluated for its efficiency in Cr(Ⅵ) stabilization via Cr(Ⅵ) reduction to Cr(Ⅲ) in four representative Cr(Ⅵ)-spiked soils. The initial concentrations of phosphate buffer-extractable Cr(Ⅵ)(Cr(Ⅵ)b) in soils 1, 2, 3, and 4 were 382.4, 575.9, 551.3, and 483.7 mg kg^(-1), respectively. Reduction of Cr(Ⅵ) to Cr(Ⅲ) by structural Fe(Ⅱ)(Fe(Ⅱ)s) in GR_(SO4) in all studied soils was fast,wherein the application of GR_(SO4) markedly decreased the amount of Cr(Ⅵ)bat the Cr(Ⅵ)b/Fe(Ⅱ)s stoichiometric mole ratio of 0.33. The kinetics of Cr(Ⅵ)reduction by GR_(SO4) could not be determined as this reaction coincided with the release of Cr(Ⅵ) from soil during the experiment. The concentration of Cr(Ⅵ)bdecreased, as the Cr(Ⅵ)b/Fe(Ⅱ)s ratio decreased from 0.46 to 0.20, generally to below 10 mg kg^(-1). Back-transformation of the generated Cr(Ⅲ)was examined in the presence of manganese oxide birnessite at the birnessite/initial Cr(Ⅲ) mole ratio of 4.5. The results of batch tests showed that only 5.2% of the initial Cr(Ⅲ) was converted to Cr(Ⅵ) after two months, while under field capacity moisture conditions, less than 0.05% of the initial Cr(Ⅲ) was oxidized to Cr(Ⅵ) after six months. The results illustrated that remediation of Cr(Ⅵ)-contaminated soils would be fast, successful, and irreversible with an appropriate quantity of fresh GR_(SO_(4)).