Comparative study of Cr(III) adsorption by carbon nanotubes and active carbons

The adsorption of trivalent chromium ions from aqueous solutions on the surface of carbon materials, namely, multiwall carbon nanotubes (NTs) and two samples of active carbon, is studied depending on pH and adsorbate concentration in the system. Isotherms of Cr(III) adsorption by the aforementioned materials are obtained. It is shown that chromium ions are predominantly bound by surface carboxyl groups. The adsorption of chromium ions reduces the electrokinetic potential of NTs and, at chromium concentrations C _Cr(III) > 10^–5 M, leads to the reversal of the surface charge. The adsorption value decreases in the series NT > Merck carbon > Norit carbon, in contrast to an increase in the adsorbate affinity to the adsorbent in this series, as determined from the slope of the initial section of the Langmuir isotherms. Small amounts of chromium ions sorbed at low concentrations in solution (C _Cr(III) ≤ 10^–5 M) are comparable with the concentration of hydrogen ions displaced from the surface, thus making it possible to suppose the existence of an ionexchange adsorption mechanism. As the concentration of Cr(III) increases, the equivalent displacement of H⁺ is violated, thereby indicating the development of other adsorption mechanisms (complexation).     

Separation and chromium speciation by single-wall carbon nanotubes microcolumn and inductively coupled plasma mass spectrometry

Microcolumn packed single-walled carbon nanotubes (SWCNTs) were used as solid phase extraction adsorbent for chromium speciation coupled to inductively coupled plasma mass spectrometry for detection. The effects of the experimental parameters, including pH of the solution, sample flow rate, volume and concentration of eluent, sample volume and interfering ions, on separation and determination of Cr(III) and Cr(VI) were investigated in detail. It was found that Cr(III) was selectively sorbed on the microcolumn packed with SWCNTs in the pH range from 2.0 to 4.0, while Cr(VI) remained in solution. The retained Cr(III) was subsequently eluted with 2.0 mL of 1.2 mol L^?1 nitric acid. Under the optimum conditions, the detection limits based on 3σ criterion were 0.01 ng mL^?1 and 0.024 ng mL^?1 for Cr(III) and Cr(VI), respectively. The relative standard deviations were less than 5.0% (n?=?9, c?=?1.0 ng mL^?1). The method was successfully applied to the speciation of chromium in real samples including natural and waste water. The recoveries of spiked samples were higher than 92.5%.     

Comparative Study of Chromium Biosorption by Mesorhizobium amorphae Strain CCNWGS0123 in Single and Binary Mixtures

The appearance of chromium in the aqueous effluent is a major concern for the modern industry. In this work, Mesorhizobium amorphae strain CCNWGS0123 was investigated as a biosorbent to remove chromium from aqueous solutions. The optimum pH for Cr(III) and Cr(VI) biosorption were 4 and 2, respectively. This isolate showed an experimental maximum Cr(III) adsorption capacity of 53.52 mg?L^?1, while the result was 47.67 mg?L^?1 for Cr(VI), with an initial 100 mg?L^?1 Cr ions and 1.0 g?L^?1 biomass. In terms of time equilibrium, Cr(III) ion was more readily adsorbed than Cr(VI) by this isolate. The biosorption data of both ions fit the Langmuir isotherm better than that of Freundlich model. Meanwhile, this organism exhibited a good capability to release Cr ions, with desorption efficiency of 70 % for Cr(III) and 76 % for Cr(VI). Fourier transform infrared spectroscopy analysis showed that –OH, –COO, –NH, amide I, and C=O were involved in Cr(III) and Cr(VI) binding. The biosorbent was further characterized by scanning electron microscopy and energy-dispersive X-ray spectrometry, which indicated an accumulation of chromium on the cellular level. In the binary mixtures, the removal ratio of total Cr and Cr(III) increased from pH?2 to 4. The highest removal ratio of the total Cr was observed in the 25/25 mg?L^?1 mixture at pH?4. In addition, the removal efficiency of Cr(VI) was closely influenced by Cr(III) in the mixture, decreasing to 23.57 mg?g^?1 in the 100/100 mg?L^?1 mixture system, due to the competition of Cr(III). The potential usage of the chromium-resistant rhizobium for the remediation of chromium-contaminated effluents has been demonstrated based on the above results.     

Speciation Analysis of Chromium by Adsorptive Stripping Voltammetry in Tap and River Water Samples

An adsorptive stripping voltammetric method for speciation analysis of chromium in natural water samples has been developed. Ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) were used as complexing agents for Cr(III) present in the sample and formed as products of Cr(VI) reduction, respectively. Under optimum experimental conditions linear relations in the range from 1×10^?6 to 3×10^?5 mol L^?1 without accumulation and from 1×10^?9 to 1×10^?7 at 30 s accumulation time were obtained for Cr(III) and Cr(VI), respectively. For samples in which Cr(III) concentration is higher than 1×10^?6 mol L^?1 the Cr(III) and Cr(VI) were determined simultaneously in one voltammetric cell. For samples in which Cr(III) concentration is below 1×10^?6 mol L^?1 only Cr(VI) was selectively determined in the presence of Cr(III), which did not influence the Cr(VI) signal. The determination of Cr(III) and Cr(VI) was successful with the application of the proposed procedure in the presence of common foreign ions. The presented method was applied for the speciation of chromium in spiked tap and river water samples with satisfactory results.     

Cr(III)/Cr(VI) speciation in aerosol particles by extractive separation and thermal ionization isotope dilution mass spectrometry

 An isotope dilution mass spectrometric (IDMS) method, using the formation of positive thermal ions, was developed for Cr(III) and Cr(VI) speciation in aerosol particles. Cr(III) and Cr(VI) spike species, enriched in ⁵³Cr, were applied for the isotope dilution step. After leaching of filter collected aerosol samples by an alkaline solution at pH 13, species separation was carried out by extraction with a liquid anion exchanger in methyl isobutyl ketone. Cr(VI) in the organic phase was re-extracted into an ammoniacal solution and chromium was then isolated from both fractions of species by electrodeposition. Detection limits of 30 pg/m³ for Cr(III) and of 8 pg/m³ for Cr(VI) were achieved in atmospheric aerosols for volumes of air samples of about 120 m³. These low detection limits allowed the determination of chromium species in continental aerosol particles in dependence on different seasons. The Cr(III) /Cr(VI) ratio was always found to be about 0.3 whereas dust from soil erosion, which is probably the primary source of chromium in the atmosphere, showed higher ratios. This indicates that chromium is oxidized in the atmosphere. The accuracy of the method was demonstrated in two interlaboratory comparisons of Cr(VI) determinations in welding dust samples. The IDMS method also contributed to the certification of a corresponding standard reference material organized by the Standard Reference Bureau of the European Union. Chromium speciation, including the determination of elemental chromium Cr(0), was carried out in aerosols of different welding processes for stainless steel. These analyses showed distinct differences in the distribution of chromium species in the welding process and can be used as an exact calibration method for routine methods in this important field of monitoring corresponding working places. Received: 19 August 1996/Revised: 24 September 1996/Accepted: 28 September 1996     

Investigation of deceleration causes in gold electrowinning from aqueous cyanide solutions on porous carbon electrodes

Deceleration of gold electrowinning from model cyanide solutions on porous cathode of graphitized carbon felt was investigated. It was established that calcium ions present in the solution did not negatively affect the electrolysis rate. The main reason of the termination of gold electrowinning on the graphitized cathode was the corrosion of the stainless steel anode resulting in penetration into the cathode chamber of the electrolyzer of CrO₄²⁻ anions further reduced to Cr(III) and probably Cr(II) compounds. X-ray Photoelectron Spectroscopy investigation of the passivated carbon cathode showed that the film formed thereon consisted of Cr(III) compounds containing cyanide and hydroxy ligands. This film covers the active sites on the surface of the porous carbon cathode preventing the deposition thereon of ad-ions of gold(I). Inert aqua and hydroxo complexes of chromium(III) stronger impede the rate of gold(0) deposition on the porous carbon cathode than chromium(III) cyanide compounds. This fact originates presumably from the requirement of lower cathode potentials for the reduction of the former to labile chromium(II) complexes than those necessary for the thermodynamically stable anions [Cr(CN)₆]³⁻.     

Separation,preconcentration and speciation of chromium by solid phase extraction on immobilised ferron

A sensitive and simple method for determination of chromium species after separation and preconcentration by solid phase extraction (SPE) has been developed. For the determination of the total concentration of chromium in solution, Cr(VI) was efficiently reduced to Cr(III) by addition of hydroxylamine and Cr(III) was preconcentrated on a column of immobilised ferron on alumina. The adsorbed analyte was then eluted with 5?mL of hydrochloric acid and was determined by flame atomic absorption spectrometery. The speciation of chromium was affected by first passing the solution through an acidic alumina column which retained Cr(VI) and then Cr(III) was preconcentrated by immobilised ferron column and determined by FAAS. The concentration of Cr(VI) was determined from the difference of concentration of total chromium and Cr(III). The effect of pH, concentration of eluent, flow rate of sample and eluent solution, and foreign ions on the sorption of chromium (III) by immobilised ferron column was investigated. Under the optimised conditions the calibration curve was linear over the range of 2–400?µg?L^?1 for 1000?mL preconcentration volume. The detection limit was 0.32?µg?L^?1, the preconcentration factor was 400, and the relative standard deviation (%RSD) was 1.9% (at 10?µg?L^?1; n?=?7). The method was successfully applied to the determination of chromium species in water samples and total chromium in standard alloys.     

Speciation and determination of chromium ions by dispersive micro solid phase extraction using magnetic graphene oxide followed by flame atomic absorption spectrometry

A selective, simple and fast dispersive micro solid phase extraction method using magnetic graphene oxide (GO) as an efficient sorbent has been developed for the extraction, separation and speciation analysis of chromium ions. The method is based on different adsorption behaviour of Cr(VI) and Cr(III) species onto magnetic GO in aqueous solutions which allowed the selective separation and extraction of Cr(VI) in the pH range of 2.0–3.0. The retained Cr(VI) ions by the sorbent were eluted using 0.5 mL of 0.5 mol L^?1 nitric acid solution in methanol and determined by ?ame atomic absorption spectrometry. Total chromium content was determined after the oxidation of Cr(III) to Cr(VI) by potassium permanganate. All effective parameters on the performance of the extraction process were thoroughly investigated and optimised. Under the optimised conditions, the method exhibited a linear dynamic range of 0.5–50.0 µg L^?1 with a detection limit of 0.1 µg L^?1 and pre-concentration factor of 200. The relative standard deviations of 3.8% and 4.6% (n = 8) were obtained at 25.0 µg L^?1 level of Cr(VI) for intra- and inter-day analysis, respectively. The method was successfully applied to the speciation and determination of Cr(VI) and Cr(III) in environmental water samples.     

Electroanalytical Detection of Cr(VI) and Cr(III) Ions Using a Novel Microbial Sensor

A microbial sensor, namely carbon paste electrode (CPE) modified with Citrobacter freundii (Cf–CPE) has been developed for the detection of hexavalent (Cr(VI)) and trivalent (Cr(III)) chromium present in aqueous samples using voltammetry, an electroanalytical technique. The biosensor developed, demonstrated about a twofold higher performance as compared to the bare CPE for the chosen ions. Using cyclic voltammetry and by employing the fabricated Cf–CPE, the lowest limit of detection (LLOD) of 1x10⁻⁴ M and 5x10⁻⁴ M for Cr(VI) and Cr(III) ions respectively could be achieved. By adopting the Differential Pulse Cathodic Stripping Voltammetric technique, the LLOD could be further improved to 1x10⁻⁹ M and 1x10⁻⁷ M for Cr(VI) and Cr(III) ions respectively using the biomodified electrodes. The reactions occurring at the electrode surface‐chromium solution interface and the mechanisms of biosorption of chromium species onto the biosensor are discussed. The stability and utility of the developed biosensor for the analysis of Cr(VI) and Cr(III) ions in chromite mine water samples has been evaluated.     

Catalytic Adsorptive Stripping Voltammetric Procedure for Determination of Total Chromium in Environmental Materials

A sensitive catalytic adsorptive stripping voltammetric procedure for determination of traces of total chromium in environmental samples is reported. The method is based on the preconcentration of a Cr(III)? H₂DTPA complex by adsorption at the HMDE from an acetate buffer solution at the potential ?1.0 V vs. Ag/AgCl. Total chromium was determined as Cr(III) after reduction of Cr(VI) to Cr(III) by NaHSO₃. In order to stabilize the signal of Cr(III) the measurements were performed at 5 °C. The calibration graph for chromium for an accumulation time of 60 s was linear in the range from 5×10^?10 to 5×10^?8 mol L^?1. The relative standard deviation for a chromium concentration of 1×10^?8 mol L^?1 was 3.9% (n=5). The detection limit for accumulation time of 60 s was about 8×10^?11 mol L^?1. The validation of the procedure was performed by the analysis of the certified reference materials.     

Speciation of Cr(VI)/Cr(III) by electrothermal atomisation AAS after electrodeposition on a L'vov platform

Summary The pyrolysed graphite L'vov platform of a tube furnace is considered as an electrode for the electrodeposition and speciation of chromium by electrothermal atomisation atomic absorption spectrometry (ETA-AAS). Firstly, a preliminary study of the Cr(VI)/Cr(III) voltammetric behavior at pH 4.70 on a glassy-carbon electrode is carried out. Secondly, the L'vov platform is used as a cathodic macro-electrode for the selective preconcentration of Cr(VI)/Cr(III) on a mercury film. Speciation of Cr(VI)/Cr(III) is carried out on the basis of the electrolysis potential (E_e): at pH 4.70 and E_e=–0.30 V, only Cr(VI) is reduced to Cr(III) and accumulated as Cr(OH)₃ by adsorption on a mercury film; at E_e=–1.80 V both Cr(VI) and Cr(III) are accumulated forming an amalgam with added mercury(II) ions. Once the film has been formed, the platform is transferred to a graphite tube to atomise the element. The reliability of the method was tested for the speciation of chromium in natural waters and it proves to be highly sensitive thanks to the electroanalytical step. In all samples, the Cr(VI) concentration was less than the detection limit (0.15 ng ml^–1), and the concentration of Cr(III) agrees with those of total chromium. The analytical recovery of Cr(VI) added to water samples [3.50 ng ml^–1 of Cr(VI)] was 105±6.2%.     

Study of radioactive labelled chromium (III) ion adsorption on stearic acid monomolecular layer

A study of chromium(III) ion adsorption on a stearic acid monomolecular layer (MML), spread on the surface of a⁵¹CrCl₃ aqueous solution, is presented. From the analysis of the experimental results it can be concluded that MML saturation is achieved at CrCl₃ concentrations above 10^?6M. When the maximum adsorption pH value of Cr(III) ions on MML is 6.7, MML is completely ionized and chromium ion is bonded as Cr(OH) ₂ ⁺ . The kinetics of Cr(III) ion adsorption on MML could be expressed as A^S=A _∞ ^S (1?e^{?k t}).     

Preconcentration and Determination of Chromium Species Using Octadecyl Silica Membrane Disks and Flame Atomic Absorption Spectrometry

A novel and selective method for the fast determination of trace amounts of chromium species in water samples has been developed. The procedure is based on the selective formation of chromium diethyldithiocarbamate complexes at different pH in the presence of Mn(II) as an enhancement agent of chromium signals followed by elution with organic eluents and determination by atomic flame absorption spectrometry. The maximum capacity of the employed disks was found to be (396±3) µg and (376±2) µg for Cr(III) and Cr(VI), respectively. The detection limit of the proposed method is 49 and 43 ng·L^?1 for Cr(III) and Cr(VI), respectively. The proposed method was successfully applied for determination of chromium species Cr(III) and Cr(VI) in different water samples.     

Chromium(III) Ion Selective Electrode Based on Oxalic Acid Bis(Cyclohexylidene Hydrazide)

A poly(vinyl chloride) membrane sensor based on oxalic acid bis (cyclohexylidene hydrazide) as membrane carrier was prepared and investigated as a Cr(III)‐selective electrode. The electrode reveals a Nernstian behavior (slope 19.8±0.4 mV decade^?1) over a wide Cr(III) ion concentration range 1.0×10^?7–1.0×10^?2 mol dm^?3 with a very low limit of detection (i.e., down to 6.3×10^?8 mol dm^?3). The potentiometric response of the sensor is independent of the pH of the test solution in the pH range 1.7–6.5. The electrode possesses advantage of very fast response, relatively long lifetime and especially good selectivity to wide variety of other cations. The sensor was used as an indicator electrode, in the potentiometric titration of chromium ion and in the determination of Cr(III) in waste water and alloy samples.     

Determination of chromium(III) and total chromium in natural waters using a surface ion-imprinted silica gel as selective adsorbent

A sensitive and selective method has been developed to determine Cr(III) and total Cr in natural water samples by ICP-AES with a Cr(III)-imprinted aminopropyl-functionalised silica gel adsorbent. The Cr(III)-imprinted and non-imprinted adsorbent were prepared by an easy one-step reaction with a surface imprinting technique. Their maximum static adsorption capacities for Cr(III) were 11.12 mg g^?1 and 3.81 mg g^?1, respectively. The relative selectivity factors (α _r) for Cr(III)/Co(II), Cr(III)/Au(III), Cr(III)/Ni(II), Cr(III)/Cu(II), Cr(III)/Zn(II), and Cr(III)/Cr(VI), were 377, 21.4, 15.4, 27.7, 26.4, and 31.9, respectively. Under the optimal conditions, Cr(III) can be absorbed quantitatively, but Cr(VI) was not retained. Total chromium was obtained after reducing Cr(VI) to Cr(III) with hydroxyammonium chloride. The detection limit (3σ) for Cr(III) was 0.11 ng mL^?1. The relative standard deviation was 1.2%. The proposed method has been validated by analysing two certified reference materials and successfully applied to the determination and speciation of chromium in natural water samples with satisfactory results.     

Preconcentration of total chromium on Dowex 50W-X8 resin loaded with 2-amino-benzenethiol

Application of Dowex 50W-X8 loaded with 2-amino-benzenethiol for preconcentration of total chromium (Cr(VI) and Cr(III)) in water samples and subsequent determination by inductively coupled plasma-atomic emission spectrometry was studied. The reagent 2-amino-benzenethiol loaded onto the resin effectively reduced Cr(VI) to Cr(III) and total chromium (both Cr(VI) and Cr(III)) formed chelate complex with the reagent in the Cr(III) valence state. Experimental parameters such as preconcentration time, solution flow rates, pH, and concentration of the eluent were optimized. The method has been applied for the determination of total chromium in seawater samples in the range of 0.1–200?µg?L^?1. A detection limit of 0.3?µg?L^?1 was achieved, and the relative standard deviation was about 5%.     

Speciation Analysis of Cr(III) and Cr(VI) after Solid Phase Extraction Using Modified Magnetite Nanoparticles

A new solid phase extraction (SPE) method has been developed for the speciation of Cr(III) and Cr(VI). This method is based on the adsorption of Cr(VI) on modified alumina‐coated magnetite nanoparticles (ACMNPs). Total chromium in different samples was determined as Cr(VI) after oxidation of Cr(III) to Cr(VI) using H₂O₂. The chromium concentration has been determined by flame atomic absorption spectrometric (FAAS) technique and amount of Cr(III) was calculated by substracting the concentration of Cr(VI) from total chromium concentration. The effect of parameters such as pH, amount of adsorbent, contact time, sample volume, eluent type, H₂O₂ concentration and cetyltrimethylammonium bromide (CTAB) concentration as modifier on the quantitative recovery of Cr(VI) were investigated. Under the optimal experimental conditions, the preconcentration factor, detection limit, linear range and relative standard deviation (RSD) of Cr(VI) were 140 (for 350 mL of sample solution), 0.083 ng mL^?1, 0.1‐10.0 ng mL^?1 and 4.6% (for 5.0 ng mL^?1, n = 7), respectively. This method avoided the time‐consuming column‐passing process of loading large volume samples in traditional SPE through the rapid isolation of CTAB@ACMNPs with an adscititious magnet. The proposed method was successfully applied to the determination and speciation of chromium in different water and wastewater samples and suitable recoveries were obtained.     

Speciation,separation and enrichment of Cr(III) and Cr(VI) in environmental samples by ion-pair solvent extraction using a β-diketone ligand

A simple speciation, separation and enrichment method has been developed for the determination of Cr(III) and Cr(VI) ions in different samples by ion-pair solvent extraction with a β-diketone ligand, 2-(4-methoxybenzoyl)-N′-benzylidene-3-(4-methoxyphenyl)-3-oxo-N-phenyl-propono hydrazide (MBMP). Cr(III) was separated from Cr(VI) as Cr(III)-(MBMP)-perchlorate ternary ion-pair complex. The influences of various analytical parameters including pH, amount of reagent, shaking time, sample volume and ionic strength on the recovery of Cr(III) and/or Cr(VI) were investigated. Total chromium was obtained after reducing Cr(VI) to Cr(III) with NH₂OH?·?HCl. Recoveries were found to be higher than 95% and the relative standard deviation (RSD) was less than 2%. The method detection limit based on 3σ criterion for Cr(III) was found to be 0.32?µg?L^?1. The formed ternary ion-pair complex, Cr(III)?:?MBMP?:?2ClO₄, has a molar ratio of 1?:?1?:?2. The developed method has been applied successfully to the speciation of chromium in various natural water, soil, sediment and hair samples with satisfactory results.     

Chemically Modified Carbon Paste Electrode for Determination of Sulfate Ion by Potentiometric Method

Four Schiff base complexes of different metal ions, M=Cr(III), Mn(III), Fe(III), and Co(III), were studied to characterize their ability as sulfate ion carriers in carbon paste electrode (CPE). The modified CPE electrode with Schiff base complex of Cr(III), N,N′‐ethylenebis(5‐hydroxysalicylideneiminato) chromium(III) Chloride, showed good response characteristics to SO₄^2? ion. The proposed electrode exhibits a Nernstian slope of 28.9±0.4 mV per decade for SO₄^2? ion over a wide concentration range from 1.5×10^?6?4.8×10^?2 M, with a detection limit of 9.0×10^?7 M. The CPE electrode manifested advantages of relatively fast response time, suitable reproducibility and life time and, most important, good potentiometric selectivity relative to a wide variety of other common inorganic anions. The potentiometric response of the electrode is independent of the pH of the test solution in the pH range 4.0–9.0. The proposed electrode was used as an indicator electrode in potentiometric titration of sulfate with Ba²⁺ ion, the determination of zinc in zinc sulfate tablet and also determination of sulfate content of a mineral water sample.     

Mass spectrometric investigations of the kinetic stability of chromium and copper complexes with humic substances by isotope-labelling experiments

Isotope-labelling exchange experiments were carried out to investigate the kinetic stability of Cr(III) complexes with humic substances (HS). To compare the results with those of an ion, not expected to form kinetically stable HS complexes with respect to its electron configuration, Cu(II) was investigated under the same conditions. HS solutions of different origin were therefore spiked with ⁵³Cr(III) or ⁶⁵Cu(II) after saturation of HS with chromium and copper of natural isotopic composition. In fractions of metal/HS complexes with different molecular weight, obtained by ultrafiltration and HPLC/ICP-MS using size exclusion chromatography (SEC), respectively, the isotope ratios of chromium and copper were determined by ICP and thermal ionisation mass spectrometry. Distinct differences in the isotopic composition of chromium were found in the permeate of the ultrafiltration compared with the corresponding unseparated solution, which indicates kinetically stable Cr(III)/HS complexes. On the other hand, the copper isotopic composition was identical in the permeate and the unseparated solution, which shows that a total exchange of Cu²⁺ ions took place between free and HS complexed copper ions. The SEC/ ICP-MS experiments also resulted in a different isotopic distribution of chromium in the chromatographically separated complexes whereas the copper complexes, separated by SEC, showed identical isotopic composition. The kinetic stability of Cr(III)/HS complexes could be explained by the d³ electron configuration of Cr³⁺ ions, a fact which is well known from classical Cr(III) complexes, and influences substantially the mobility of this heavy metal in the environment.