A continuous approach for the determination of Cr(VI) in sediment and soil based on the coupling of microwave-assisted water extraction, preconcentration, derivatization and photometric detection

Improving the practicality of recombinant Escherichia coli biosensor in detecting trace Cr(VI) by modifying the cryogenic storage conditions of biosensors and applying simple pretreatment

Hexavalent chromium (Cr(VI)) is a global environmental pollutant. To reduce the risk caused by Cr(VI), a simple, accurate, reproducible, and inexpensive method for quantifying Cr(VI) in water and soil should be developed. In this study, three types of recombinant Escherichia coli biosensors (namely T7-lux-E. coli, T3-lux-E. coli, and SP6-lux-E. coli biosensor) containing promoters (T7, T3, and SP6), chromate-sensing regulator chrB, and the reporter gene luxAB were constructed. This study investigated the effects of cryogenic freezing temperature and time on trace Cr(VI) measurement by using recombinant E. coli biosensors. The results indicated that the activity of thawed frozen SP6-lux-E. coli cells stored at -20 °C for 270 days did not differ from that of freshly prepared cells. Turbidity and conductivity in water samples and organic matter in soil interfered with Cr(VI) measurement using the biosensor. The SP6-lux-E. coli biosensor exhibited a wide measurement range and a low deviation of <5% for measuring Cr(VI) in various Cr(VI)-contaminated water and soil samples and required only a simple pretreatment or extraction process even after 270-day storage at -20 °C. To the best of our knowledge, this is the first study to report the use of recombinant biosensors for accurately measuring Cr(VI) in both water and soil.

Selective Separation of Chromium Species from Soils by Single-Step Extraction Methods: a Critical Appraisal

A critical appraisal of single-step extraction procedures of chromium species from soil was done in terms of their selectivity towards Cr(III) and Cr(VI) species. Samples of natural mineral and organic soil and samples of soil enriched with different chromium compounds of various solubility (in liquid or solid form) were used to simulate contamination of soil by liquid and solid wastes. The efficiency of extraction of Cr(III) and Cr(VI) species with various reagents, e.g. acetic acid, chelating agents (EDTA, DTPA) or inorganic salts (phosphates and carbonates), was evaluated on the basis of recovery results obtained for enriched samples. None of used reagents allow for quantitative extraction of added Cr(III) form. Procedures based on extraction of soil with Na₂CO₃ at room and elevated temperature (90-95 °C) were suitable for extraction of Cr(VI) species from mineral soil, whereas for organic soil, the procedure based on extraction with Na₂CO₃ at room temperature was recommended. The developed extraction procedures were validated using certified reference material (CRM 041 soil) and applied for analysis of contaminated soil samples. The studies showed that the physical state of waste, initial form and oxidation state of chromium and soil properties influenced the final chromium species and their mobility in soil, which have an impact on contamination of environment. The analysis of contaminated soil samples from a tannery area showed that the share of Cr(VI) was very low (only 0.8-4.5%) despite the high total content of chromium, which confirmed that chromium was present in immobile forms.

On-line coupling of dynamic microwave-assisted extraction with high-speed counter-current chromatography for continuous isolation of nevadensin from Lyeicnotus pauciflorus Maxim

An on-line method based upon dynamic microwave-assisted extraction (DMAE) coupled with high-speed counter-current chromatography (HSCCC) was developed for continuous isolation of nevadensin from Lyeicnotus pauciflorus Maxim. The DMAE parameters were optimized by means of the Box-Behnken design. The maximum extraction yield was achieved using 30:1 ml/g of liquid-solid ratio, 10 ml/min of solvent flow rate and 200 W of microwave power. The crude extracts were then separated by HSCCC with a two-phase solvent system composed of n-hexane-ethyl acetate-methanol-water (7:3:5:5, v/v/v/v). 13.0mg of nevadensin was isolated from 15.0 g original sample by HSCCC with five times sample injection in 12h, and the isolation yield of nevadensin was 0.87 mg/g. The average purity of nevadensin was higher than 98.0%. The chemical structure of collected fraction was identified by HPLC, ESI-MS and (1)H NMR. The results indicated that this on-line method was effective and fast for high-throughput isolation of nevadensin from L. pauciflorus Maxim.

Chromium speciation in solid matrices and regulation: a review

In recent years, the extensive use of chromium in industrial processes has led to the promotion of several directives and recommendations by the European Union, that try to limit and regulate the presence of Cr(VI) in the environment and to protect industrial workers using chromium and end-users of manufactured products. As a consequence, new standard methods and analytical procedures have been published at the EU level for Cr(VI) determination in soil, sludge, sediment, and similar waste materials, workplace atmospheres, cement, packaging materials, industrially produced samples, and corrosion-protection layers on some components of vehicles and electrical and electronic equipment. The objective of this article is to summarize the different directives and recommendations and to critically review the currently existing standard methods and the methods published in the literature for chromium speciation in the above mentioned solid matrices, putting the emphasis on the different extraction procedures which have been developed for each matrix. Particular attention has been paid to Cr(III) and Cr(VI) inter-conversions that can occur during extraction and efforts to minimize these unwanted reactions. Although the use of NaOH-Na(2)CO(3) solutions with hot plate extraction seems to be the more widespread procedure, species transformation can still occur and several studies suggest that speciated isotope-dilution mass spectrometry (SIDMS) could be a suitable tool for correction of these interconversions. Besides, recent studies have proved the role of Cr(III) in chromium toxicology. As a consequence, the authors suggest an update of standard methods in the near future.

Removal of chromium Cr(VI) by low-cost chemically activated carbon materials from water

Low-cost, chemically activated carbon materials, Pellet-600 and PVA-300, were prepared at relatively low temperatures and show more effective removal efficiency of Cr(VI) from water than commercially available activated carbons tested. The Pellet-600 is a pelletized carbon material with high mesoporous volumes and surface area, and the PVA-300 is composed of a high surface area carbon coating on a fiberglass mat substrate. A much faster adsorption kinetics and a much higher adsorption capacity for Cr(VI) are achieved by the Pellet-600. At very low concentrations of Cr(VI), the PVA-300 displays a strong adsorption ability for Cr(VI). XPS data show an increase in the atomic ratio of Cr/C and oxidation of carbon materials after adsorption of Cr(VI). These results suggest that a high content of mesopores with a high surface area and surface functional groups greatly improve the Cr(VI) removal efficiency from water.

Microwave-assisted extraction coupled online with derivatization, restricted access material cleanup, and high-performance liquid chromatography for determination of formaldehyde in aquatic products

A rapid technique based on microwave-assisted extraction (MAE) coupled online with derivatization, restricted access material cleanup, and high-performance liquid chromatography (HPLC) was developed for the determination of formaldehyde in aquatic products. Formaldehyde was first extracted with water under the action of microwaves and then directly introduced into a derivatization reservoir containing 2,4-dinitrophenylhydrazine (DNPH). The formaldehyde-DNPH derivative (100 μL) was loaded into a restricted access material (RAM) precolumn for online cleanup. Subsequently, the analyte was transferred from the precolumn to an analytical column and determined by UV absorption spectrum at 352 nm. The limit of detection (LOD) was 0.27 mg kg(-1). The intraday and interday precisions expressed as RSDs were 3.5% and 5.0%, respectively. This method was applied to determine the presence of formaldehyde in various aquatic products. The results were in agreement with those obtained by the state standard method (steam-distillation and offline HPLC analysis) used in China and higher than those obtained by the online ultrasound-assisted extraction (UAE) method. The recoveries obtained by analyzing 11 spiked aquatic products were in the range of 70.0%-105.0%. The online technique was demonstrated to be rapid with little consumption of samples and reagents.

The determination of organochlorine pesticides based on dynamic microwave-assisted extraction coupled with on-line solid-phase extraction of high-performance liquid chromatography

A rapid technique based on dynamic microwave-assisted extraction coupled with on-line solid-phase extraction of high-performance liquid chromatography (DMAE-SPE-HPLC) has been developed. A TM(010) microwave resonance cavity built in the laboratory was applied to concentrate the microwave energy. The sample placed in the zone of microwave irradiation was extracted with 95% acetonitrile (ACN) aqueous solution which was driven by a peristaltic pump at a flow rate of 1.0 mL min(-1). The extraction can be completed in a recirculating system in 10 min. When a number of extraction cycles were completed, the extract (1 mL) was diluted on-line with water. Then the extract was loaded into an SPE column where the analytes were retained while the unretained matrix components were washed away. Subsequently, the analytes were automatically transferred from the SPE column to the analytical column and determined by UV detector at 238 nm. The technique was used for determination of organochlorine pesticides (OCPs) in grains, including wheat, rice, corn and bean. The limits of detection of OCPs are in the range of 19-37 ng g(-1). The recoveries obtained by analyzing the four spiked grain samples are in the range of 86-105%, whereas the relative standard deviation (R.S.D.) values are <8.7% ranging from 1.2 to 8.7%. Our method was demonstrated to be fast, accurate, and precise. In addition, only small quantities of solvent and sample were required.

On-line coupling of dynamic microwave-assisted extraction with high-performance liquid chromatography for determination of andrographolide and dehydroandrographolide in Andrographis paniculata Nees

A novel technique based on dynamic microwave-assisted extraction (DMAE) coupled on-line with high-performance liquid chromatography (HPLC) through a flow injection interface has been developed for determination of andrographolide and dehydroandrographolide in Andrographis paniculata Nees. A TM(010) microwave resonance cavity built in the laboratory was applied to concentrating the microwave energy. An extraction vessel was placed in microwave irradiation zone. The extraction was performed in a recirculating system. When a number of extraction cycles were completed, the fractional extract (20muL) was driven to the analytical column by 65% aqueous methanol and was measured by diode array detector (DAD) at 225nm. The optimized extraction conditions are follows: extraction solvent 60% aqueous methanol; microwave forward power 80W; extraction time 6min; extraction solvent flow-rate 1.0mLmin(-1). The detection and quantification limits obtained are 0.5 and 1.7microgmL(-1) for andrographolide and 0.6 and 1.9microgmL(-1) for dehydroandrographolide, respectively. The within-day and between-day precision (RSD) are 2.1% and 3.7% for andrographolide and 1.7% and 4.1% for dehydroandrographolide, respectively. Mean recoveries for andrographolide and dehydroandrographolide are 97.7% and 98.7%, respectively. Compared with ultrasonic extraction used in the Chinese pharmacopoeia, the proposed method was demonstrated to obtain higher extraction yield in a shorter time. In addition, only small quantities of solvent (5mL) and sample (10mg) were required.

Dynamic microwave-assisted extraction coupled with on-line spectrophotometric determination of safflower yellow in Flos Carthami

A rapid dynamic microwave-assisted extraction and on-line detection by spectrophotometry is proposed for the determination of safflower yellow in Flos Carthami. A high pressure and a peristaltic pump were used to deliver the solvent. A TM010 microwave resonance cavity was applied to concentrate the microwave energy and the forward power about 60 W was enough for the extraction. Other extraction conditions also were examined and optimized. In this work, the extraction process can be monitored by measuring the absorption of safflower yellow in the extract, which would be convenient for rapid optimization of the extraction process. The detection and quantification limits are 8 and 27 microg mL(-1), respectively. The within-day and between-day precision (R.S.D.) are 1.6-3.2% and 2.8-4.2%, respectively. Compared with off-line detection, the proposed method may provide more rapid measurement and is more convenient for obtaining continuous measurements.

Speciation of chromium by in-capillary derivatization and electrophoretically mediated microanalysis

An electrophoretic method for chromium speciation analysis--as Cr(III) and Cr(VI)--based on in-capillary derivatization with 1,5-diphenylcarbazide (DPC) is here proposed. As Cr(III) does not react with DPC, it was oxidized also in-capillary to Cr(VI) by Ce(IV). For this purpose, a capillary electrophoresis (CE) mode called electrophoretically mediated microanalysis (EMMA) based on sequential injection of sample and reagents--namely, DPC, sample and Ce(IV)--was employed. The conditions of both reactions--Cr(III) oxidation and Cr(VI)-DPC derivatization--were optimized in order to quantify separately the Cr(VI)-DPC complex from the original Cr(VI) in the sample and that from oxidation of Cr(III) to Cr(VI). The electrophoretic conditions were independently optimized for variables influencing the resolution and those affecting sensitivity. The method thus developed was applied to the determination of Cr(III) and Cr(VI) in glass material, for which different sample preparation methods--namely, EPA method 3060A, ultrasound-assisted leaching and microwave-assisted digestion--were tested. Microwave-assisted digestion was found to be the best sample preparation alternative in terms of efficiency of the step--99.6 and 98.3% for Cr(VI) and Cr(III), respectively--and procedure time--20 min. The complete method was validated with the certified reference material BAM-S004.

On-line dynamic extraction and automated determination of readily bioavailable hexavalent chromium in solid substrates using micro-sequential injection bead-injection lab-on-valve hyphenated with electrothermal atomic absorption spectrometry

A novel and miniaturized micro-sequential injection bead-injection lab-on-valve (microSI-BI-LOV) fractionation system was developed for on-line microcolumn soil extraction under simulated environmental scenarios and accurate monitoring of the content of easily mobilisable hexavalent chromium in soil environments at the sub-low parts-per-million level. The flow system integrates dynamic leaching of hexavalent chromium using deionized water as recommended by the German Standard DIN 38414-S4 method; on-line pH adjustment of the extract by a 0.01 mol L(-1) Tris-HNO(3) buffer solution; isolation of the chromate leached from the matrix constituents onto a Q Sepharose strong anion-exchanger freshly packed into the microconduits of the microSI-assembly; air-segmented elution of the sorbed species by a 40 microL plug of 0.5 mol L(-1) NH(4)NO(3) (pH 8) eluent; and detection by electrothermal atomic absorption spectrometry (ETAAS). The effect of simulated acidic rain on the accessibility of chromate forms for plant uptake was also investigated. The proposed approach offers several advantages over conventional speciation/fractionation protocols in the batch mode, including immediate separation with concomitant preconcentration of the released chromate, minimization of Cr(vi) to Cr(iii) interconversion risks, enhanced accuracy, and non-existence of re-adsorption/re-distribution problems along with a detailed pattern of the kinetics of the leaching process. The reliability of the proposed method was evaluated via spiking of a moderately polluted agricultural soil material (San Joaquin Soil-Baseline Trace Element Concentrations) with water-soluble Cr(vi) salts at different concentration levels. The potential of the microSI-BI-LOV set-up with renewable surfaces for flame-AAS determination of high levels of readily bioavailable chromate in contaminated soils is also addressed.

A novel dynamic approach for automatic microsampling and continuous monitoring of metal ion release from soils exploiting a dedicated flow-through microdialyser

In this paper, a novel concept is presented for automatic microsampling and continuous monitoring of metal ions in soils with minimum disturbance of the sampling site. It involves a hollow-fiber microdialyser that is implanted in the soil body as a miniaturized sensing device. The idea behind microdialysis in this application is to mimic the function of a passive sampler to predict the actual, rather than potential, mobility and bioavailability of metal traces. Although almost quantitative dialysis recoveries were obtained for lead (> or = 98%) from aqueous model solutions with sufficiently long capillaries (l > or = 30 mm, 200 microm i.d.) at perfusion rates of 2.0 microL min(-1), the resistance of an inert soil matrix was found to reduce metal uptake by 30%. Preliminary investigation of the potential of the microdialysis analyser for risk assessment of soil pollution, and for metal partitioning studies, were performed by implanting the dedicated probe in a laboratory-made soil column and hyphenating it with electrothermal atomic absorption spectrometry (ETAAS), so that minute, well-defined volumes of clean microdialysates were injected on-line into the graphite furnace. A noteworthy feature of the implanted microdialysis-based device is the capability to follow the kinetics of metal release under simulated natural scenarios or anthropogenic actions. An ancillary flow set-up was arranged in such a way that a continuous flow of leaching solution--mild extractant (10(-2) mol L(-1) CaCl2), acidic solution (10(-3) mol L(-1) HNO3), or chelating agent (10(-4) or 10(-2) mol L(-1) EDTA)--was maintained through the soil body, while the concentration trends of inorganic (un-bound) metal species at the soil-liquid interface could be monitored at near real-time. Hence, relevant qualitative and quantitative information about the various mobile fractions is obtained, and metal-soil phase associations can also be elucidated. Finally, stimulus-response schemes adapted from neurochemical applications and pharmacokinetic studies are to be extended to soil research as an alternative means of local monitoring of extraction processes after induction of a chemical change in the outer boundary of the permselective dialysis membrane.

Automated Sequential Injection-Microcolumn Approach with On-Line Flame Atomic Absorption Spectrometric Detection for Implementing Metal Fractionation Schemes of Homogeneous and Nonhomogeneous Solid Samples of Environmental Interest

An automated sequential injection (SI) system incorporating a dual-conical microcolumn is proposed as a versatile approach for the accommodation of both single and sequential extraction schemes for metal fractionation of solid samples of environmental concern. Coupled to flame atomic absorption spectrometric detection and used for the determination of Cu as a model analyte, the potentials of this novel hyphenated approach are demonstrated by the ability of handling up to a 300 mg sample of a nonhomogeneous sewage amended soil (viz., CRM 483). The three steps of the endorsed Standards, Measurements, and Testing sequential extraction method have been also performed in a dynamic fashion and critically compared with the conventional batchwise protocols. The ecotoxicological relevance of the data provided by both methods with different operationally defined conditions is thoroughly discussed. As compared to traditional batch systems, the developed SI assembly offers minimal risks of sample contamination, the absence of metal re-distribution/readsorption, and dramatic saving of operational times (from 16 h to 40-80 min per partitioning step). It readily facilitates the accurate manipulation of the extracting reagents into the flow network and the minute, well-defined injection of the desired leachate volume into the detector. Moreover, highly time-resolved information on the ongoing extraction is given, which is particularly relevant for monitoring fast leaching kinetics, such as those involving strong chelating agents. On-line and off-line (for Cu, Pb, and Zn) single extraction schemes are also proven to constitute attractive alternatives for fast screening of metal pollution in solid samples and for predicting the current, rather than the potential, element bioavailability by the assessment of the readily mobilizable metal forms.