Sensitive and selective spectrofluorimetric determination of chromium(VI) in water by fluorescence enhancement

A new fluorogenic method for the selective and sensitive determination of chromium(VI) in acidic water using rhodamine B hydrazide was developed. This method was based on the oxidation of non-fluorescent rhodamine B hydrazide by potassium dichromate in acidic aqueous conditions to give rhodamine B, which was highly fluorescent, as a product. With the optimum condition described, the fluorescence enhancement at 585 nm was linearly related to the concentration of chromium(VI) in the range of 5.0 × 10⁻⁸ to 2.0 × 10⁻⁶ mol L⁻¹ (2.60-104 ng mL⁻¹) with a correlation coefficient of R² = 0.9993 (n = 18) and a detection limit of 5.5 × 10⁻⁹ mol L⁻¹ (0.29 ng mL⁻¹). The R.S.D. was 2.2% (n = 5). The proposed method was also applied to the determination of chromium(VI) in drinking water, river water and synthetic samples.     

Column chromatographic speciation of chromium for Cr(VI) and several species of Cr(III)

Summary Chromium can be present in aqueous solution as Cr(VI) or in monomeric, dimeric, trimeric and higher polymeric forms of Cr(III). Many monomeric forms of Cr(III) are possible, with the water molecules of Cr(H₂O) ₆ ³⁺ substituted by anionic or neutral species. This proliferation of Cr(III) species makes the complete speciation of chromium a continuing challenge to the analyst. A simple and effective cation exchange procedure for the separation of various of these species uses a small glass column containing 1 mL of pre-treated cation exchange resin (Na⁺ form). Stepwise elution with solutions of perchloric acid, Ca²⁺ (pH=2) and La³⁺ (pH=2) separates Cr(VI) and seven Cr(III) species from CrX₃ to tetramer. Radiometric (Cr-51), spectrophotometric and other detection methods can be employed; the use of radiochromium gives the lowest detection limit.     

A rhodamine-based Hg2+-selective fluorescent probe in aqueous solution

A new rhodamine-based Hg²⁺-selective fluorescent probe (I) was designed and synthesized. Compound I displays excellent selective and sensitive response to Hg²⁺ over other transition metal ions in neutral aqueous solutions. I itself is a colorless, nonfluorescent compound. Upon addition of Hg²⁺ to its solution, the thiosemicarbazide moiety of I undergoes an irreversible desulfurization reaction to form the corresponding 1,3,4-oxadiazole (II), a colorful and fluorescent product, causing instantaneous development of visible color and strong fluorescence emission. Based on this mechanism, a fluorogenic probe for Hg²⁺ was developed. The fluorescence increases linearly with the Hg²⁺ concentration up to 0.8 μmol L⁻¹ with the detection limit of 9.4 nmol L⁻ (3σ).     

Simple and Highly Selective Catalytic Adsorptive Voltammetric Method for Cr(VI) Determination

A simple and highly selective and sensitive catalytic adsorptive stripping voltammetric procedure for determination of traces of Cr(VI) in the presence of a large excess of Cr(III) in environmental water samples is reported. To obtain a low detection limit the voltammetric procedure of chromium determination in the presence of DTPA and nitrate was exploited. For elimination of interference of Cr(III) ethylenediaminedisuccinic acid was used as a masking agent. At optimized conditions the calibration graph is linear from 2×10^?10 to 2×10^?8 mol L^?1 for accumulation time of 30 s. The validation of the procedure was performed by comparison of the results of analysis of river water samples with those obtained using other accepted voltammetric procedure.     

Oxidative degradation of non-ionic surfactant (TritonX-100) by chromium(VI)

Degradation of polyoxyethylene chain of non-ionic surfactant (TritonX-100) by chromium(VI) has been studied spectrophotometrically under different experimental conditions. The reaction rate bears a first-order dependence on the [Cr(VI)] under pseudo-first-order conditions, [TritonX-100]  [Cr(VI)] in presence of 1.16 mol dm⁻³ perchloric acid. The observed rate constant (k_obs) was 3.3 × 10⁻⁴ to 3.5 × 10⁻⁴ s⁻¹ and the half-life (t_1/2) was 33–35 min for chromium(VI). The effects of total [TritonX-100] and [H⁺] on the reaction rate were determined. Reducing nature of non-ionic TritonX-100 surfactant is found to be due to the presence of –OH group in the polyoxyethylene chain. It was observed that monomeric and non-ionic micelles of TritonX-100 were oxidized by chromium(VI). When [TritonX-100] was less than its critical micelle concentration (cmc) the k_obs values increased from 0.76 × 10⁻⁴ to 1.5 × 10⁻⁴ s⁻¹. As the [TritonX-100] was greater than the cmc, the k_obs values increases from 2.1 × 10⁻⁴ to 8.2 × 10⁻⁴ s⁻¹ in presence of constant [HClO₄] (1.16 mol dm⁻³) at 40 °C. A comparison was made of the oxidative degradation rates of TritonX-100 with different metal ion oxidants. The order of the effectiveness of different oxidants was as follows: permanganate > diperiodatoargentate(III) > chromium(VI) > cerium(IV).     

Chemically selective membrane optode for Cr(VI) determination in aqueous samples

A methodology for simultaneous preconcentration and determination of Cr(VI) from aqueous samples was developed using a membrane optode formed by physical inclusion of a Cr(VI) selective chromophore 1,5-diphenylcarbazide (DPC) into a plasticized cellulose triacetate matrix. The inclusion of an anion exchanger (Aliquat-336) was found to be effective for immobilization of both DPC and Cr(VI)-DPC complex in the optode matrix itself. The proportionality in intensity of the magenta color on the optodes loaded with varying amounts of Cr(VI) suggests its potential applications for screening of Cr(VI) in aqueous samples by visual colorimetry. On loading high amounts of Cr(VI) in the membrane optode, its color changes from magenta to yellow, which indicates the possibility of using it as a threshold detector for Cr(VI). The membrane optode was optimized in terms of obtaining maximum preconcentration efficiency for Cr(VI) and subsequent stable optical response proportional to the amount of Cr(VI) in the membrane optode sample. The membrane optodes were tested for Cr(VI) determination in tap water and seawater samples. Using this optode, Cr(VI) even at levels of 13.6 ppb could be quantitatively detected. The optodes developed in the present work were found to be stable, cost effective, easy to prepare and efficient for direct preconcentration and determination of Cr(VI) in a variety of aqueous samples using spectrophotometry. However, this membrane optode is for one time use only as the reaction of Cr(VI) with DPC is irreversible.     

Simultaneous determination of Cr(III) and Cr(VI) in water by reversed phase HPLC,after chelating with sodium diethyldithiocarbamate

Summary A method for simultaneous determination of Cr(III) and Cr(VI), using sodium diethyldithiocarbamate as chelating agent is given. At room temperature and pH 5.8 sodium diethyldithiocarbamate reacts with both Cr(III) and Cr(VI). Examination of this reaction by reversed phase high-performance liquid chromatography, makes it possible to correct for the interference between Cr(III) and Cr(VI) when determining the amount of Cr(III) present in the solution.     

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.     

Removal of Cr (VI) from aqueous solution using magnetic biochar synthesized by a single step method

Magnetic biochar, as an adsorbent, was synthesized by a single step method, where iron salt was directly mixed with pinewood sawdust by chemical co-precipitation and subsequently pyrolyzed at 700°C for Cr (VI) removal from aqueous solution. The effects of some important parameters including adsorbent dosage (0.4–2.8?g/L), pH (1–10) of the solution, contact time (0–1440 minutes), initial concentration (30–120?mg/L), and temperature (20–40°C) were investigated in batch experiments. Both pre- and post-adsorbents were characterized by SEM-EDX and XPS to investigate the adsorption mechanism. The maximum adsorption capacity of the tested magnetic biochar under the certain experimental conditions determined as optimal was 42.7?mg/g for Cr (VI). The adsorption data were proved to be suitable for the pseudo-second order model for kinetics and the Langmuir model for isotherms with correlation R²?=?0.9996 andR²?>?0.9980, respectively, after fitting with four kinetic models (pseudo-first order, pseudo-second order, W-M model, and Elovich) and three isotherm models (Langmuir, Freundlich, and Temkin). The characteristic analyses further verified that the efficient particle was a mixture of iron oxides in essence, and it had a strong effect on the spontaneous and endothermic adsorption process.     

Optical determination of Cr(VI) using regenerable, functionalized sol-gel monoliths

Transparent, pyridine-functionalized sol-gel monoliths have been formed and their use in Cr(VI) sensing applications is demonstrated. The monoliths were immersed in acidic Cr(VI)-containing solutions, and the Cr(VI) uptake was monitored using UV-vis and atomic absorption spectroscopies. At concentrations at the ppm level, the monoliths exhibit a yellow color change characteristic of Cr(VI) uptake, and this can be measured by monitoring the absorption change at about 350 nm using UV-vis spectroscopy. Concentrations at the ppb level are below the limit of detection using this wavelength of 350 nm for measurement. However, by adding a diphenylcarbazide solution to monoliths that have been previously immersed in ppb-level Cr(VI) solutions, a distinct color change takes place within the gels that can be measured at about 540 nm using UV-vis spectroscopy. Concentrations as low as 10 ppb Cr(VI) can be measured using this method. The monoliths can then be regenerated for subsequent sensing cycles by thorough washing with 6.0 M HCl. The factors affecting monolith uptake of Cr(VI) have been explored. In addition, the gels have been characterized using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) measurements.     

Adsorption Studies of Cr(III) & Cr(VI) on Lead Sulphide,Copper Sulphide & Zinc Sulphide-Determination Cr(III) in the Presence of Cr(VI)

Abstract

The adsorption studies of Cr(VI) in presence of Cr(III) on the sulphide of Lead, Zinc and Copper has been studied. It has been found that in case of lead sulphide 100% adsorption of Cr(VI) took place at pH 4.0 and of Cr(III) at pH 7.0. While in case of zinc sulphide the 100% adsorption of Cr(VI) took place at pH 4.5 and of Cr(III) at pH 6.5. In case of copper sulphide 100% adsorption of Cr(VI) took place at pH 5.0 and of Cr(III) at pH 7.0. This difference in adsorption at different pH values forms the basis for the determination of these ions. The method is accurate.     

Highly efficient removal of Cr(VI) from wastewater via adsorption with novel magnetic Fe3O4@C@MgAl-layered double-hydroxide

Novel magnetic Fe₃O₄@C@MgAl-layered double-hydroxide (LDH) nanoparticles have been successfully prepared by the chemical self-assembly methods. The properties of surface functional groups, crystal structure, magnetism and surface morphology of magnetic nanoparticles were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermal gravity-differential thermal gravity (TG-DTG), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The adsorption studies of the novel adsorbent in removing heavy metals Cr (VI) from waste water showed that the maximum absorption amount of Cr(VI) was 152.0 mg/g at 40℃ and pH 6.0. The excellent adsorption capacity of the Fe₃O₄@C@MgAl-LDH nano-absorbents plus their easy separation, environmentally friendly composition and reusability makes them more suitable adsorbents for the removal of metal ions from waste water.     

A novel flow injection chemiluminescence determination of Cr(VI) with Dichlorotris(1,10-phenanthroline)ruthenium(II)

A selective novel reverse flow injection system with chemiluminescence detection (rFI-CL) for the determination of Cr(VI) in presence of Cr(III) with Dichlorotris (1,10-phenanthroline)ruthenium(II), (Ru(phen)₃Cl₂), is described in this work. This new method is based on the oxidation capacity of Cr(VI) in H₂SO₄ media. First, the Ruthenium(II) complex is oxidized to Ruthenium(III) complex by Cr(VI) and afterwards it is reduced to the excited state of the Ruthenium(II) complex by a sodium oxalate solution, emitting light inside the detector. The intensity of chemiluminescence (CL) is proportional to the concentration of Cr(VI) and, under optimum conditions, it can be determined over the range of 3-300 μg L⁻¹ with a detection limit of 0.9 μg L⁻¹. The RSD was 8.4% and 1.5% at 5 and 50 μg L⁻¹, respectively. For the rFI-CL method various analytical parameters were optimized: flow rate (1 mL min⁻¹), H₂SO₄ carrier concentration (20% w/V), Ru(phen)₃Cl₂ concentration (5 mM) and sodium oxalate concentration (0.1 M). The effect of Cr(III), Fe(III), Al(III), Cd(II), Zn(II), Hg(II), Pb(II), Ca(II) and Mg(II), was studied. The method is highly sensitive and selective, allowing a fast, on-line determination of Cr(VI) in the presence of Cr(III). Finally, the method was tested in four different water samples (tap, reservoir, well and mineral), with good recovery percentage.     

Polyethylenimine Functionalized Multi‐walled Carbon Nanotubes for Electrochemical Detection of Chromium(VI)

Multi‐walled carbon nanotubes (MWCNTs) functionalized with polyethylenimine (PEI) were synthesized and characterized by dispersibility, field‐emission scanning electron microscope (FE‐SEM), FT‐IR and thermogravimetric Analyzer (TGA). The glassy carbon electrodes modified by MWCNT‐PEI composite were used for sensitive and selective detection of chromium (VI). A linear response was obtained over a wide range of Cr(VI) concentrations (0.002–20 µmol L^?1) with the detection limit of 0.0006 µmol L^?1 (S/N=3). The proposed electrodes were used successfully for Cr(VI) detection in three real water samples.     

Adsorption of U (VI) ions from aqueous solution using silicon dioxide nanopowder

The adsorption kinetics for removal of uranium (V1) from aqueous solution using silicon dioxide nanopowder (nano-SiO₂) was investigated in batch and continuous techniques. Pseudo-first order and pseudo-second order were used to analyze the kinetics of batch experiments. In continuous technique the important parameters (initial concentration, flow rate and bed height) on the breakthrough curves were studied and the adsorption kinetics was analyzed using Thomas and Yoon and Nelson kinetic models. The comparison between the kinetic models was evaluated by the correlation coefficients (r²). The results indicated that the batch experiments fitted well with pseudo second-order kinetic model. The comparison of the experimental breakthrough curve to the breakthrough profile obtained from Thomas and Yoon and Nelson methods showed a satisfactory fit for silicon dioxide nanopowder.     

Highly efficient adsorption of Cr(VI) from aqueous solution by Fe3+ impregnated biochar

Biochar (BC) has been widely used as a low-cost adsorbent for the removal of contaminants from water and soil. However, the adsorption ability of BC towards heavy metal oxyanions (e.g., Cr(VI)) is relatively low due to the negatively charged surface of BC. In this study, pristine BC was impregnated with Fe³⁺ to improve its Cr(VI) adsorption capability. Fe³⁺-impregnated BC (Fe³⁺-BC) was successfully synthesized by a simple impregnation method and used for the removal of Cr(VI) from aqueous solution. Various factors affecting the adsorption, such as impregnation ratio, pH, adsorbent dosage, contact time, temperature, and the presence of humic acid, were investigated in detail. Results showed that Fe³⁺-BC had strong adsorption ability to Cr(VI) with a maximum adsorption capacity of 197.80 mg/g, which were not only significantly higher than that of the pristine BC, but also were superior to many previously reported adsorbents. It was favorable for Cr(VI) adsorption under the condition of acidic and high temperature. The adsorption data obeyed Sips and Langmuir isotherms and the kinetic data were well described by the pseudo-first-order kinetic model. The results herein revealed that the Fe³⁺-impregnated BC had a good potential as a highly efficient material for adsorption of Cr(VI) from aqueous solution.     

Removal of humic matter interference in the determination of Cr(VI) in soil extracts by the diphenylcarbazide method

Alkaline digestion of soil samples, which is recommended for minimizing Cr(III)–Cr(VI) interconversions during the extraction of Cr(VI), may also solubilize humic matter (HM). The latter is responsible for both positive and negative interference in the analysis of Cr(VI) in the extract by the diphenylcarbazide (DPC) method. Humic compounds indeed absorb light at 540 nm as the Cr-DPC product and are also able to rapidly reduce Cr(VI) under the pH conditions of the standard DPC method. To prevent any risk of interference and make the DPC method applicable to soil extracts, a new protocol is proposed. This consists of three successive steps: (1) extraction of Cr(VI) by the U.S.EPA method 3060A in the presence of Mg²⁺; (2) batch removal of solubilised HM by the XAD-7 sorbent at pH 3.0 ± 0.1 with 3–5 min contact time; (3) analysis of Cr(VI) with the DPC method at pH 3.0 ± 0.1. The application of this new protocol to the soil product SQC-012 Lot 4 certified by R.T. Corporation (RTC, USA) gave significantly lower Cr(VI) concentrations and smaller variability compared to certified values (46.5 ± 2.3 instead of 153 ± 32.6 mg/kg). The new protocol was validated by Cr(VI) and Cr(III) spikes either to the soil/extractant suspension or to the exctract. Cr(VI) results in the wide range of acceptance limits (104–202 mg/kg) reported by RTC were only obtained when the U.S.EPA method 3060A without the addition of Mg²⁺ and DPC analysis at pH 1.0 were applied. The latter procedure appears questionable since it leads to largely variable results which reflect the complex role played by humic matter in the determination of Cr(VI) and the instability of Cr(III) during the extraction.     

Immersed single-drop microextraction interfaced with sequential injection analysis for determination of Cr(VI) in natural waters by electrothermal-atomic absorption spectrometry

Single-drop microextraction (SDME) and sequential injection analysis have been hyphenated for ultratrace metal determination by Electrothermal-Atomic Absorption Spectrometry (ETAAS). The novel method was targeted on extraction of the Cr(VI)-APDC chelate and encompasses the potential of SDME as a miniaturized and virtually solvent-free preconcentration technique, the ability of sequential injection analysis to handle samples and the versatility of furnace autosamplers for introducing microliter samples in ETAAS.     

A highly selective colorimetric and fluorescent chemosensor for Cr2+ in aqueous solutions

A new rhodamine-based chemosensor was synthetized through a modified copper-catalyzed [3+2]-cycloaddition of an azidocoumarin with an alkynyl-rhodamine. Its sensing properties toward various metal cations in aqueous solutions were investigated by colorimetric changes, UV–vis and fluorescence spectroscopies. The sensor exhibited a high selectivity for Cr²⁺ over Cr³⁺ and other divalent cations such as Cu²⁺, Mg²⁺, Zn²⁺, Ca²⁺, Cd²⁺, Co²⁺, Hg²⁺ and Ni²⁺. The linear range of detection by fluorescence spectroscopy is 0.07–3.5 mM, with a detection limit of ca. 64 μM. The binding mode of Cr²⁺ with the sensor was rationalized through experimental evidences.     

Polyacrylonitrile/Aminated Polymeric Nanosphere Nanofibers as Efficient Adsorbents for Cr(VI) Removal

In this work, polyacrylonitrile/aminated polymeric nanosphere (PAN/APN) nanofibers were prepared by electrospinning of monodispersed aminated polymeric nanospheres (APNs) for removal of Cr(VI) from aqueous solution. Characterization results showed that obtained PAN/APNs possessed nitrogen functionalization. Furthermore, the adsorption application results indicated that PAN/APN nanofibers exhibited a high adsorption capacity of 556 mg/g at 298 K for Cr(VI) removal. The kinetic data showed that the adsorption process fits the pseudo-second order. A thermodynamic study revealed that the adsorption of Cr(VI) was spontaneous and endothermic. The coexisting ions Na⁺, Ca²⁺, K⁺, Cl⁻, NO₃⁻ and PO₄³⁻ had little influence on Cr(VI) adsorption, while SO₄²⁻ in solution dramatically decreased the removal performance. In the investigation of the removal mechanism, relative results indicated that the adsorption behavior possibly involved electrostatic adsorption, redox reaction and chelation. PAN/APN nanofibers can detoxify Cr(VI) to Cr(III) and subsequently chelate Cr(III) on its surface. The unique structure and nitrogen functionalization of PAN/APN nanofibers make them novel and prospective candidates in heavy metal removal.