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1.
This study investigated Cr(VI) reduction by dissolved Fe(II) in hyperalkaline pH conditions as found in fluid wastes associated with the U.S. nuclear weapons program. The results show that Cr(VI) reduction by Fe(II) at alkaline pH solutions proceeds very quickly. The amount of Cr(VI) removed from solution and the amount reduced increases with Fe(II):Cr(VI) ratio. However, the Cr(VI) reduction under alkaline pH condition is nonstoichiometric, probably due to Fe(II) precipitation and mixed iron(III)-chromium-(III) (oxy)hydroxides blocking Fe(II) surface sites, as well as removing Fe(II) from solution through O2 oxidation. After Cr(VI) was reduced to Cr(III), it precipitated out as mixed Fe(x)Cr1-xO3(solids) and various Fe(III) precipitates with an overall Cr:Fe ratio of 1:3; all Cr remaining in the solution phase was unreduced Cr(VI). EXAFS data showed that Cr-O and Cr-Cr distances in the precipitates equal to 1.98 and 3.01 A, respectively, consistent with the spinel-type structure as chromite.  相似文献   

2.
Kinetics of Cr(VI) reduction by carbonate green rust   总被引:1,自引:0,他引:1  
The kinetics of Cr(VI) reduction to Cr(III) by carbonate green rust were studied for a range of reactant concentrations and pH values. Carbonate green rust, [FeII4FeIII2(OH)12][4H2O x CO3], was synthesized by induced hydrolysis (i.e., coprecipitation) of an Fe(ll)/Fe(III) solution held at a constant pH of 8. An average specific surface area of 47 +/- 7 m2 g(-1) was measured for five separate batches of freeze-dried green rust precipitate. Heterogeneous reduction by Fe(II) associated with the carbonate green rust appears to be the dominant pathway controlling Cr(VI) loss from solution. The apparent stoichiometry of the reaction between ferrous iron associated with green rust ([Fe(II)GR]) and Cr(VI) was slightly higherthan the expected 3:1 ratio, possibly due to the presence of other oxidants, such as oxygen, protons, or interlayer carbonate ions. The rate of Cr(VI) reduction was proportional to the green rust surface area concentration, and psuedo-first-order rate coefficients (kobs) ranging from 1.2 x 10(-3) to 11.2 x 10(-3) s(-1) were determined. The effect of pH was small with a 5-fold decrease in rate with increasing pH (from 5.0 to 9.0). At low Cr(VI) concentrations (<200 microM), the rate of reaction was first order with respect to Cr(VI) concentration, whereas, at high Cr(VI) concentrations, rates appearto deviate from first-order kinetics and approach a constant value. Estimated amounts of surface Fe(II) and total Fe(II) suggest that the deviation from first-order kinetics observed at higher Cr(VI) concentrations and the 50-fold decrease in rate observed upon three sequential exposures to Cr(VI) is due to exhaustion of available Fe(II).  相似文献   

3.
Chromate is a widespread contaminantthat has deleterious impacts on human health, the mobility and toxicity of which are diminished by reduction to Cr(III). While biological and chemical reduction reactions of Cr(VI) are well resolved, reduction within natural sediments, particularly of arid environments, remains poorly described. Here, we examine chromate reduction within arid sediments from the Hanford, WA site, where Fe(III) (hydr)oxide and carbonate coatings limit mineral reactivity. Chromium(VI) reduction by Hanford sediments is negligible unless pretreated with acid; acidic pretreatment of packed mineral beds having a Cr(VI) feed solution results in Cr(III) associating with the minerals antigorite and lizardite in addition to magnetite and Fe(II)-bearing clay minerals. Highly alkaline conditions (pH > 14), representative of conditions near high-level nuclearwaste tanks, result in Fe(II) dissolution and concurrent Cr(VI) reduction. Additionally, Cr(III) and Cr(VI) are found associated with portlandite, suggesting a secondary mechanism for chromium retention at high pH. Thus, mineral reactivity is limited within this arid environment and appreciable reduction of Cr(VI) is restricted to highly alkaline conditions resulting near leaking radioactive waste disposal tanks.  相似文献   

4.
We demonstrated that mesoporous titania beads of uniform size (about 450 μm) and high surface area could be synthesized via an alginate biopolymer template method. These mesoporous titania beads could efficiently remove Cr(VI), Cd(II), Cr(III), Cu(II), and Co(II) ions from simulated wastewater with a facile subsequent solid-liquid separation because of their large sizes. We chose Cr(VI) removal as the case study and found that each gram of these titania beads could remove 6.7 mg of Cr(VI) from simulated wastewater containing 8.0 mg·L(-1) of Cr(VI) at pH = 2.0. The Cr(VI) removal process was found to obey the Langmuir adsorption model and its kinetics followed pseudo-second-order rate equation. The Cr(VI) removal mechanism of titania beads might be attributed to the electrostatic adsorption of Cr(VI) ions in the form of negatively charged HCrO(4)(-) by positively charged TiO(2) beads, accompanying partial reduction of Cr(VI) to Cr(III) by the reductive surface hydroxyl groups on the titania beads. The used titania beads could be recovered with 0.1 mol·L(-1) of NaOH solution. This study provides a promising micro/nanostructured adsorbent with easy solid-liquid separation property for heavy metal ions removal.  相似文献   

5.
In this study, new coprecipitation methods were developed using a combination of N-benzoyl-N-phenylhydroxylamine (BPHA) as a chelating reagent and copper(II) and iron(III) as coprecipitate carriers. The determinations of Fe(III), Cr(III) and Cr(VI) with Cu(II)–BPHA coprecipitant and of Cu(II), Pb(II), Cr(III) and Cr(VI) with Fe(III)–BPHA coprecipitant were made by flame atomic absorption spectrometry. The surface morphology of coprecipitants was examined by scanning electron microscopy. The coprecipitation conditions for each method such as the effect of pH, the amounts of carrier element and BPHA, sample volume and matrix effects were examined. The optimum pH for each method was found to be 6. The detection limits of the methods changed in the range of 0.23–3.2 μg L−1 with an enrichment factor of 100. The RSD % was ?4.2. The accuracy of the methods was confirmed by analysing the certified reference material. The developed methods were applied for the determination of analytes in food samples.  相似文献   

6.
This study shows that calcium silicate/aluminate-based materials can be carbonated to produce sorbents for metal removal. The material chosen for investigation, cement clinker, was accelerated carbonated, and its structural properties were investigated using X-ray diffraction (XRD), scanning electron microscopy, thermal gravimetric and differential thermal analysis, nuclear magnetic resonance spectroscopy, and nitrogen gas adsorption techniques. The principal carbonation reactions involved the transformation of dicalcium silicate, tricalcium silicate, and tricalcium aluminate into a Ca/Al-modified amorphous silica and calcium carbonate. It was found that carbonated cement had high acid buffering capacity, and maintained its structural integrity within a wide pH range. The uptake of Pb(II), Cd(II), Zn(II), Ni(II), Cr(II), Sr(II), Mo(VI), Cs(II), Co(II), and Cu(II) from concentrated (1000 mg L(-1)) single-metal solutions varied from 35 to 170 mg g(-1) of the carbonate cement. The removal of metals was hardly effected by the initial solution pH due to the buffering capability of the carbonated material. The kinetics of Pb, Cd, Cr, Sr, Cs, and Co removal followed a pseudo-second-order kinetic model, whereas the equilibrium batch data for Cu fitted the pseudo-first-order rate equation. PHREEQC simulation supported by XRD analysis suggested the formation of metal carbonates and silicates, calcium molybdate, and chromium (hydro)oxide. Cesium was likely to be adsorbed by Ca/Al-modified amorphous silica.  相似文献   

7.
Pyrite, an iron sulfide, occurs in many soils and sediments, making it an important natural reductant of toxic metal pollutants. This study investigated the processes leading to aqueous Cr(VI) reduction by pyrite in a closed thermostated (25 +/- 0.1 degrees C) system and under an argon atmosphere. Synthetic pyrite suspensions were reacted with a range of Cr(VI) solutions from 0 to 7 x 10(-4) M and at pH 2-8. Metal species concentrations were continuously monitored during a period lasting approximately 20 h. Preliminary experiments carried out in acidic media without Cr(VI) have shown that some pyrite dissolution occurred. Then, metal species concentration changes with time during pyrite oxidation by Cr(VI) solutions exhibited two distinct trends depending on the complete or incomplete Cr(VI) removal. As long as chromate existed in solution, the Cr-(Ill) to Fe(lIl) ratio was found to be an effective parameter to investigate the pyrite reaction stoichiometry with Cr(VI). Experimental values close to 2 suggest that sulfur compounds with oxidation states between 0 and 2 should be formed during pyrite oxidation by Cr(VI). If Cr(VI) was completely reduced from solution, then the pyrite oxidation by Fe(lll) ions took place to generate ferrous ions.  相似文献   

8.
Binding of phytic acid to Zn(II) free glycinin was not observed in 0.5 M KCl at pH 6.2. The addition of varying quantities of phytic acid to a Zn(II)- glycinin system at pH 6.2 ([KCl]=0.5 M ) initially resulted in binding of phytate and increased binding of Zn(II) to glycinin probably as a phytate-Zn(II)-glycinin complex. Further addition of phytate resulted in precipitation of zinc and protein. Bovine serum albumin also showed increased affinity for Zn(II) owing to the presence of phytic acid. Phytic acid-Zn(II) precipitates have a capacity of removing glycinin from solution, presumably by surface adsorption. The presence of soluble Zn(II) inhibits this adsorption. Bovine serum albumin is removed from solution by phytic acid-Zn(II) precipitates only when soluble zinc is present.  相似文献   

9.
Cr(VI) photocatalytic reduction experiments over TiO2 particles under near UV irradiation in the presence of excess oxalate were performed at acid pH (2 and 3) and under air and N2 bubbling. Initial photonic efficiencies for Cr(VI) reduction are nearly the same under aerobic and anaerobic conditions, but show a significant increase at the lowest pH. At pH 2, the addition of oxalate facilitates Cr(VI) reduction, hindering the electron-shuttle mechanism taking place in pure water. The oxalate synergistic effect at pH 2 is lower than that previously found for EDTA and negligible at pH 3. Chromium(V) oxalate concentration profiles were obtained by EPR spectroscopy in the presence of excess oxalate at pH 1.5. Coordinated Cr(V) complexes [Cr(V)(O)(Ox)2]-, [Cr(V)(OH2)(Ox)2]-, and [Cr(V)(O)(OH)2(Ox)]- were identified, on the basis of the comparison of their corresponding g values with recent literature data. The kinetic analysis of the temporal evolution of the paramagnetic Cr(V) species indicates also an effective photocatalytic degradation of chromium(V) oxalate complexes. This new evidence reinforces previous findings regarding sequential one-electron-transfer processes in Cr(VI) photocatalytic reduction, suggesting that this route may represent a general behavior for the Cr(VI) reduction over UV-irradiated TiO2 particles.  相似文献   

10.
Information on chromium (Cr) oxidation states is essential for the assessment of environmental and health risks associated with the overall life-cycle of chromated copper arsenate (CCA) treated wood products because of differences in toxicity between trivalent [Cr(III)] and hexavalent [Cr(VI)] chromium compounds. Hypothetical Cr(VI) fixation products were investigated in CCA type C treated sawdust of aspen and red pine during or following preservative fixation by extraction with Cr(VI)-specific extractants. Cr(VI) was found only in alkaline extracts of treated wood. A major source of Cr(VI) was method-induced oxidation of fixed Cr(III) during alkaline extraction, as confirmed by demonstrated oxidation of Cr(III) from CrCl3 treated wood. Oxidation of nontoxic and immobile Cr(III) to toxic and mobile Cr(VI) was facilitated by the presence of wood at pH > 8.5. Thermodynamic equilibrium between Cr(III) and Cr(VI) is affected by pH, temperature, rates of dissolution of CrIII) compounds, and oxygen availability. Results of this study recommend against alkaline extraction protocols for determination of Cr(VI) in treated wood. This Cr oxidation mechanism can act as a previously unrecognized route for generation of hazardous Cr(VI) if CCA treated wood is exposed to alkaline conditions during its production, use, or waste management.  相似文献   

11.
12.
A novel l-cysteine functionalized silica gel adsorbent (SG-Cys) was successfully synthesized and characterized by Fourier transform infrared (FTIR) spectra and elemental analysis. An efficient method for simultaneous determination of heavy metals [V (V), Cr (VI), Cu (II), As (V), Cd (II), and Pb (II)] was developed by inductively coupled plasma-mass spectrometry (ICP-MS) coupled with pre-concentration with the prepared SG-Cys adsorbent. The experimental parameters, including the solution pH, sample flow rate, eluent volume, eluent type, and concentration, have been systematically optimized to obtain higher enrichment factors of target ions. Under the optimum conditions, the enrichment factors of V (V), Cr (VI), Cu (II), As (V), Cd (II), and Pb (II) reached 123, 100, 86, 106, 97, and 94, respectively, and the detection limits were as low as 1.8–3.7 ng L?1. The proposed method has been applied to the determination of trace heavy metals in water, rice, wheat, corn, and tea samples, which exhibited a satisfactory recovery in the range of 90.6–106.0 %.  相似文献   

13.
Nanoscale zerovalent iron (NZVI) was aged over 30 days in suspension (2 g/L) with different anions (chloride, perchlorate, sulfate, carbonate, nitrate), anion concentrations (5, 25, 100 mN), and pH (7, 8). During aging, suspension samples were reacted periodically with 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA) and Cr(VI) to determine the time scales and primary mode of NZVI reactivity loss. Rate constants for 1,1,1,2-TeCA reduction in Cl(-), SO(4)(2-), and ClO(4)(-) suspensions decreased by 95% over 1 month but were generally equivalent to one another, invariant of concentration and independent of pH. In contrast, longevity toward 1,1,1,2-TeCA depended upon NO(3)(-) and HCO(3)(-) concentration, with complete reactivity loss over 1 and 14 days, respectively, in 25 mN suspensions. X-ray diffraction suggests that reactivity loss toward 1,1,1,2-TeCA in most systems results from Fe(0) conversion into magnetite, whereas iron carbonate hydroxide formation limits reactivity in HCO(3)(-) suspensions. Markedly different trends in Cr(VI) removal capacity (mg Cr/g NZVI) were observed during aging, typically exhibiting greater longevity and a pronounced pH-dependence. Notably, a strong linear correlation exists between Cr(VI) removal capacities and rates of Fe(II) production measured in the absence of Cr(VI). While Fe(0) availability dictates longevity toward 1,1,1,2-TeCA, this correlation suggests surface-associated Fe(II) species are primarily responsible for Cr(VI) reduction.  相似文献   

14.
Aluminum-rich, hyperalkaline (pH > 13.5) and saline high-level nuclear waste (HLW) fluids at elevated temperatures (>50 degrees C), that possibly contained as much as 0.41 mol L(-1) Cr(VI), accidentally leaked to the sediments at the Hanford Site, WA. These extreme conditions promote base-induced dissolution of soil minerals which may affect Cr(VI)aq mobility. Our objective was to investigate Cr(VI)aq transport in sediments leached with HLW simulants at 50 degrees C, under CO2 and O2 free conditions. Results demonstrated that Cr(VI)aq fate was closely related to dissolution, and Cr(VI)aq mass loss was negligible in the first pore volumes but increased significantly thereafter. Similar to dissolution, Cr(VI)aq attenuation increased with increasing fluid residence time and NaOH concentration but decreased with Al concentrations in the leaching solutions. Aqueous Cr(VI) removal rate half-lives varied from 1.2 to 230 h with the fastest at the highest base concentration, lowest Al concentration, greatest reaction time, and lowest Cr(VI) concentration in the leaching solution. The rate of Cr(VI) removal (normalized to 1 kg of solution) varied from 0.83 x 10(-9) (+/-0.44 x 10(-9)) to 9.16 x 10(-9) (+/-1.10 x 10(-9)) mol s(-1). The predominant mechanism responsible for removing Cr(VI) from the aqueous phase appears to be homogeneous Cr(VI) reduction to Cr(III) by Fe(II) released during mineral dissolution. Cr(VI)aq removal was time-limited probably because it was controlled by the rate of Fe(II) release into the soil solution upon mineral dissolution, which was also a time-limited process, and other processes that may act to lower Fe(II)aq activity.  相似文献   

15.
A flow-through pilot-scale system was tested for removal of Cr(VI) from contaminated groundwater in Glendale, California. The process consisted of the reduction of Cr(VI) to Cr(lll) using ferrous sulfate followed by coagulation and filtration. Results indicated that the technology could reduce influent Cr(VI) concentrations of 100 microg L(-1) to below detectable levels and also remove total Cr (Cr(VI) plus Cr(lll)) to very low concentrations (< 5 microg L(-1)) under optimized conditions. Complete reduction of Cr(VI) to Cr(lll) was accomplished with Fe(ll) doses of 10-50 times the Cr(Vl) concentration even in the presence of significant dissolved oxygen levels. The overall Cr removal efficiency was largely determined by the filterability of Cr(lll) and Fe(lll) precipitates, of which a relatively high filtration pH (7.5-7.6) and high filter loading rate (6 gpm ft(-2)) had negative impacts. The pilot system was able to operate for an extended time period (23-46 h depending on the Fe:Cr mass ratio) before turbidity breakthrough or high head loss. Backwash water was effectively settled with low doses (0.2-1.0 mg L(-1)) of high molecular weight polymer. Backwash solids were found to be nonhazardous bythe toxicity characteristic leaching procedure but hazardous by the California waste extraction test.  相似文献   

16.
Aqueous U(VI) reduction by hydrogen sulfide was investigated by batch experiments and speciation modeling; product analysis by transmission electron microscopy (TEM) was also performed. The molar ratio of U(VI) reduced to sulfide consumed, and the TEM result suggested that the reaction stoichiometry could be best represented by UO2(2+) + HS- = UO2+ S* + H+. At pH 6.89 and total carbonate concentration ([CO32-]T) of 4.0 mM, the reaction took place according to the following kinetics: -d[U(VI)]/dt = 0.0103[U(VI)][S2-]T0.54 where [U(VI)] is the concentration of hexavalent uranium, and [S2-]T is the total concentration of sulfide. The kinetics of U(VI) reduction was found to be largely controlled by [CO32-]T (examined from 0.0 to 30.0 mM) and pH (examined from 6.37 to 9.06). The reduction was almost completely inhibited with the following [CO32-]T and pH combinations: [(> or = 15.0 mM, pH 6.89); (> or = 4.0 mM, pH 8.01); and (> or = 2.0 mM, pH 9.06)]. By comparing the experimental results with the calculated speciation of U(VI), it was found that there was a strong correlation between the measured initial reaction rates and the calculated total concentrations of uranium-hydroxyl species; we, therefore, concluded that uranium-hydroxyl species were the ones being reduced by sulfide, not the dominant U-carbonate species present in many carbonate-containing systems.  相似文献   

17.
Chromite ore processing residue (COPR), derived from the so-called high lime processing of chromite ore, contains high levels of Cr(III) and Cr(VI) and has a pH between 11 and 12. Ferrous sulfate, which is used for remediation of Cr(VI) contamination in wastewater and soils via reduction to Cr(III) and subsequent precipitation of iron(III)/chromium(III) hydroxide, has also been proposed for remediation of Cr(VI) in COPR. Instead, however, addition of FeSO4 to the infiltrating solution in column experiments with COPR greatly increased leaching of Cr(VI). Leached Cr(VI) increased from 3.8 to 12.3 mmol kg(-1) COPR in 25 pore volumes with 20 mM FeSO4, reaching solution concentrations as high as 1.6 mM. Fe(II) was ineffective in reducing Cr(VI) to Cr(III) because it precipitated when it entered the column due to the high pH of COPR, while Cr(VI) in solution was transported away with the infiltrating solution. The large increase in leaching of Cr(VI) upon infiltration of sulfate, either as FeSO4 or Na2SO4, was caused by anion exchange of sulfate for chromate in the layered double hydroxide mineral hydrocalumite, a process for which scanning electron microscopy with energy-dispersive X-ray microanalysis provided direct evidence.  相似文献   

18.
This paper describes the investigation of chitosan and poly(allylamine) (PAH) for the creation of a multi-film, color-based dipstick for the detection of metal ions in solution. Thin, colored films of chitosan and PAH cross-linked with hexamethylene 1,6-di(aminocarboxysulfonate) (HDACS) are created where color is due to film thickness and optical interference effects. The films are investigated for their ability to selectively detect aqueous metal ions via changes in thickness and/or color. Chitosan-HDACS films were selective for Cr(VI) over all other metal ions tested including Cr(acac)3 and Cr(NO3)3 x 9H2O, and PAH-HDACS films were selective for Cu(II) and Cu(I) salts over all other metal ions tested. The irreversible, selective changes due to metal ion solutions were not caused by varying the pH. Potomac River water was also tested using the two films, with results indicating the presence of Cu(II) in the aqueous sample.  相似文献   

19.
Surface complexation of copper(II) on soil particles: EPR and XAFS studies   总被引:1,自引:0,他引:1  
The interactions of transition metals with natural systems play an important role in the mobility and the bioavailability of these metals in soils. In this study, the adsorption of copper(II) onto natural soil particles was studied as a function of pH and metal concentration. The retention capacity of soil particles was determined at pH 6.2 to be equal to 6.7 mg of copper/g of solid. The Langmuir and Freundlich isotherm equations were then used to describe the partitioning behavior of the system at different pH values. A combination of EPR, extended X-ray absorption fine structure (EXAFS), and X-ray absorption near-edge structure (XANES) spectroscopies was used to probe the Cu atomic environment at the soil particles/aqueous interface. The spectroscopic study revealed that copper(II) ions are held in inner-sphere surface complexes. It also revealed that Cu was in an octahedral coordination with first-shell oxygen atoms. A weak tetragonal distortion was pointed out due to the Jahn-Teller effect, with a mean Cu-Oequatorial bond distance of 1.96 A and a Cu-Oaxial bond distance of 2.06 A. A detailed analysis of the spectroscopic data suggested that Cu(II) was bonded to organic matter coated onto the mineral fraction of soil particles.  相似文献   

20.
酿酒废弃葡萄皮渣对Cr(VI)的吸附能力研究   总被引:1,自引:1,他引:0       下载免费PDF全文
邹磊 《现代食品科技》2012,28(8):930-932
利用酿酒后废弃葡萄皮渣(WGP)作为吸附剂,对溶液中六价铬离子Cr(Ⅵ)进行吸附试验,研究了吸附时间、粒径、用量、溶液pH、Cr离子浓度对吸附率的影响。结果如下:WGP对Cr(Ⅵ)的吸附4 h就基本达到平衡,吸附率为87.97%;粒径小于60目的WGP表现出更强的吸附能力;吸附率与WGP用量正相关,用量1 g以上吸附率趋于稳定在88%左右;WGP对Cr(VI)的吸附率随着pH值增加而下降,直至pH为3时,吸附率趋于平衡;溶液中Cr浓度越高,葡萄皮渣对Cr(Ⅵ)的吸附率越高。研究表明:酿酒后葡萄皮渣具有较强吸附铬Cr(Ⅵ)的能力,可作为铬吸附剂,用于废水治理。  相似文献   

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