Chromium speciation in solid matrices and regulation: a review

Magnetite Functionalized Nigella Sativa Seeds for the Uptake of Chromium(VI) and Lead(II) Ions from Synthetic Wastewater

The aim of the present study was to utilise pristine and magnetite-sucrose functionalized Nigella Sativa seeds as the adsorbents for the uptake of chromium(VI) and lead(II) ions from synthetic wastewater. Prestine Nigella Sativa seeds were labelled (PNS) and magnetite-sucrose functionalized Nigella Sativa seeds (FNS). The PNS and FNS composites were characterized by Fourier-transform infrared spectroscopy (FTIR) and X-ray powder diffraction (XRD). The FTIR analysis of both adsorbents revealed the presence of vibrations assigned to 1749 and 1739 cm-1 (-C=O) for ketonic group for both adsorbents. The amide (-NH) peak was observed at 1533 and 1527 cm-1 on FNS and PNS composites, respectively, whilst the carboxyl group (-COOH) were observed at 1408 cm-1 on both adsorbents. The XRD results of FNS and PNS composites showed a combination of spinel structure and$y$-Fe2O3 phase confirming the formation of iron oxide. The influence of operational conditions such as initial concentration, temperature, pH, and contact time was determined in batch adsorption system. The kinetic data of Cr(VI) and Pb(II) ions on both adsorbents was described by pseudo-first-order (PFO) model which suggested physisorption process. The sorption rate of Cr(VI) ions was quicker, it attained equilibrium in 20 min, and the rate of Pb(II) ions was slow in 90 min. Freundlich isotherm described the mechanism of Pb(II) ions adsorption on PNS and FNS composites. Langmuir best fitted the uptake of Cr(VI) ions on PNS and FNS. The results for both adsorbents showed that the removal uptake of Pb(II) ions increased when the initial concentration was increased; however, Cr(VI) uptake decreased when the initial concentration increased. The adsorption of Cr(VI) and Pb(II) ions on both adsorbents increased with temperature.

Speciation Analysis of Chromium in Chromium-Enriched Yeast by Ion Chromatography-Inductively Coupled Plasma Mass Spectrometry

Chromium-enriched yeast (CrY) is a popular Cr dietary supplement, but suitable speciation analysis of highly toxic Cr(VI) in CrY is not available. Ion chromatography-inductively coupled plasma mass spectrometry method was firstly developed and validated for the quantification of Cr(III) and Cr(VI). Ultrasound-assisted weakly alkaline EDTA solution combined with boiling was used to extract two Cr species in CrY. Two species were separated on two successive anion-exchange columns using a mobile phase of 0.6 mmol/L EDTA and 76 mmol/L NH₄NO₃ solution. The method was sensitive, accurate (92.4-100.9%), and precise (0.8-3.1%). Species of Cr(VI) were not found in CrY.

Modulation of the Bifunctional CrVI to CrIII Photoreduction and Adsorption Capacity in ZrIV and TiIV Benchmark Metal-Organic Frameworks

The presence of hexavalent chromium water pollution is a growing global concern. Among the currently applied technologies to remove CrVI, its adsorption and photocatalytic reduction to CrIII less mobile and toxic forms are the most appealing because of their simplicity, reusability, and low energy consumption. However, little attention has been paid to bifunctional catalysts, that is, materials that can reduce CrVI to CrIII and retain both hexavalent and trivalent chromium species at the same time. In this work, the dual CrVI adsorption–reduction capacity of two iconic photoactive water-stable zirconium and titanium-based metal–organic frameworks (MOFs) has been investigated: UiO-66-NH2 and MIL-125. The bifunctionality of photoactive MOFs depends on different parameters, such as the particle size in MIL-125 or organic linker functionalization/defective positions in UiO-66 type sorbents. For instance, the presence of organic linker defects in UiO-66 has shown to be detrimental for the chromium photoreduction but beneficial for the retention of the CrIII phototransformed species. Both compounds are able to retain from 90 to 98% of the initial chromium present at acidic solutions as well as immobilize the reduced CrIII species, demonstrating the suitability of the materials for CrVI environmental remediation. In addition, it has been demonstrated that adsorption can be carried out also in a continuous flux mode through a diluted photoactive MOF/sand chromatographic column. The obtained results open the perspective to assess the bifunctional sorption and photoreduction ability of a plethora of MOF materials that have been applied for chromium capture and photoreduction purposes. In parallel, this work opens the perspective to develop specific chemical encoding strategies within MOFs to transfer this bifunctionality to other related water remediation applications.

Phosphate Buffer Solubility and Oxidative Potential of Single Metals or Multielement Particles of Welding Fumes

To evaluate the chemical behavior and the health impact of welding fumes (WF), a complex and heterogeneous mixture of particulate metal oxides, two certified reference materials (CRMs) were tested: mild steel WF (MSWF-1) and stainless steel WF (SSWF-1). We determined their total chemical composition, their solubility, and their oxidative potential in a phosphate buffer (PB) solution under physiological conditions (pH 7.4 and 37 °C). The oxidative potential (OPDTT) of WF CRMs was evaluated using an acellular method by following the dithiothreitol (DTT) consumption rate (µmol DTT L−1 min−1). Pure metal salts present in the PB soluble fraction of the WF CRMs were tested individually at equivalent molarity to estimate their specific contribution to the total OPDTT. The metal composition of MSWF-1 consisted mainly of Fe, Zn, Mn, and Cu and the SSWF-1 composition consisted mainly of Fe, Mn, Cr, Ni, Cu, and Zn, in diminishing order. The metal PB solubility decreased from Cu (11%) to Fe (approximately 0.2%) for MSWF-1 and from Mn (9%) to Fe (<1%) for SSWF-1. The total OPDTT of SSWF-1 is 2.2 times the OPDTT of MSWF-1 due to the difference in oxidative capacity of soluble transition metals. Cu (II) and Mn (II) are the most sensitive towards DTT while Cr (VI), Fe (III), and Zn (II) are barely reactive, even at higher concentrations. The OPDTT measured for both WF CRMs extracts compare well with simulated extracts containing the main metals at their respective PB-soluble concentrations. The most soluble transition metals in the simulated extract, Mn (II) and Cu (II), were the main contributors to OPDTT in WF CRMs extracts. Mn (II), Cu (II), and Ni (II) might enhance the DTT oxidation by a redox catalytic reaction. However, summing the main individual soluble metal DTT response induces a large overestimation probably linked to modifications in the speciation of various metals when mixed. The complexation of metals with different ligands present in solution and the interaction between metals in the PB-soluble fraction are important phenomena that can influence OPDTT depletion and therefore the potential health effect of inhaled WF.

Chromium Pollution in European Water, Sources, Health Risk, and Remediation Strategies: An Overview

Chromium is a potentially toxic metal occurring in water and groundwater as a result of natural and anthropogenic sources. Microbial interaction with mafic and ultramafic rocks together with geogenic processes release Cr (VI) in natural environment by chromite oxidation. Moreover, Cr (VI) pollution is largely related to several Cr (VI) industrial applications in the field of energy production, manufacturing of metals and chemicals, and subsequent waste and wastewater management. Chromium discharge in European Union (EU) waters is subjected to nationwide recommendations, which vary depending on the type of industry and receiving water body. Once in water, chromium mainly occurs in two oxidation states Cr (III) and Cr (VI) and related ion forms depending on pH values, redox potential, and presence of natural reducing agents. Public concerns with chromium are primarily related to hexavalent compounds owing to their toxic effects on humans, animals, plants, and microorganisms. Risks for human health range from skin irritation to DNA damages and cancer development, depending on dose, exposure level, and duration. Remediation strategies commonly used for Cr (VI) removal include physico-chemical and biological methods. This work critically presents their advantages and disadvantages, suggesting a site-specific and accurate evaluation for choosing the best available recovering technology.

Assessing Chromium Contamination in Red Soil: Monitoring the Migration of Fractions and the Change of Related Microorganisms

The improper stacking of chromium (Cr) slag poses a great threat to the environment and human health. The toxicity of Cr in soil is not only related to its total amount, but also to its fractions. A simulated experiment was conducted in laboratory to assess the environmental risk of Cr fractions migration and distribution in red soil. The results showed the content of acid-soluble and reducible Cr significantly decreased (P < 0.05) in top layer but increased in middle and substratum layers over time. This indicated that acid-soluble and reducible Cr migrated downward with time and the relative mobility of acid-soluble Cr (0.038 mg/kg·d·m) was higher than that of reducible Cr (0.028 mg/kg·d·m). Furthermore, correlation analysis between microbial community and chromium fraction showed the relative abundance of Lysobacter, Flavihumibacter, Flavisolbacter, and Altererythrobacter was significantly (P < 0.05) correlated with acid-soluble and reducible fractions. Thus, these microorganisms might be evaluators to assess the migration of acid-soluble and reducible fractions in red soil. In summary, this study provided a new comprehension on remediation of Cr-contaminated soil by monitoring the migration of acid-soluble and reducible fractions and the changes of related microbial groups.

Direct Z-scheme FeVO4/BiOCl heterojunction as a highly efficient visible-light-driven photocatalyst for photocatalytic dye degradation and Cr(VI) reduction

In this work, potential applications of a direct Z-scheme FeVO₄/BiOCl heterojunction for photocatalytic degradation of organic dyes (methylene blue, MB and rhodamine B, RhB) and reduction of hexavalent chromium (Cr(VI)) ion under visible light irradiation were reported. Firstly, FeVO₄ and BiOCl were synthesized by using a microwave heating method. Then, the FeVO₄/BiOCl nanocomposites with different weight percentages of FeVO₄ (1, 3, 6.25, 12.5 and 25%wt) were fabricated by a method of modified wet impregnation. The photocatalytic degradation activities of the nanocomposites were investigated in parallel with pure BiOCl and FeVO₄. Among the as-prepared nanocomposites, the FeVO₄/BiOCl nanocomposite with 6.25%wt of FeVO₄ exhibited the highest photocatalytic dye degradation efficiency; 99.8% of RhB was degraded after being irradiated for 360 min, while 87.2% of MB was degraded. Similarly, this nanocomposite photocatalytically reduced 97.8% of Cr(VI) at a pH value of 3. The superior photocatalytic activity can be ascribed to the effective visible light absorption of the FeVO₄/BiOCl heterojunction and the suppression of the recombination process of photogenerated electron-hole pairs. Additionally, the improved charge migration and separation efficiencies between FeVO₄ and BiOCl through the direct Z-scheme charge transfer pathway are involved, as evidenced by the trapping experiments, and the UV-visible diffuse reflectance (UV-vis DRS), photoluminescence spectroscopy (PL) and electrochemical impedance spectroscopy analyses. Photocatalytic mechanisms of the direct Z-scheme FeVO₄/BiOCl heterojunction for the photodegradation of RhB and photoreduction of Cr(VI) have been proposed and discussed in greater detail.

Simultaneous Health Risk Assessment of Potentially Toxic Elements in Soils and Sediments of the Guishui River Basin, Beijing

Simultaneous ecological and health risk assessments of potentially toxic elements in soils and sediments can provide substantial information on their environmental influence at the river-basin scale. Herein, soil and sediment samples were collected from the Guishui River basin to evaluate the pollution situation and the ecological and health risk of potentially toxic elements. Various indexes were utilized for quantitatively assessing their health risks. Pollution assessment by geo-accumulation index showed that Cd had “uncontaminated to moderately polluted” status in the soils and sediments. Potential ecological risk index showed that the Guishui River basin was at low risk in general, but Cd was classified as “moderate or considerable ecological risk” both in the soils and sediments. Health risk assessment calculated human exposure from soils and indicated that both non-carcinogenic and carcinogenic risks of the selected potentially toxic elements were lower than the acceptable levels. Health risks posed by potentially toxic elements bio-accumulated in fish, stemming from sediment resuspension, were also assessed. Non-carcinogenic hazard index indicated no adverse health effects on humans via exposure to sediments; however, in general, Cr contributed largely to health risks among the selected potentially toxic elements. Therefore, special attention needs to be paid to the Guishui River basin in the future.

Cysteamine-capped gold-copper nanoclusters for fluorometric determination and imaging of chromium(VI) and dopamine

Highly emissive cysteamine-capped gold-copper bimetallic nanoclusters (CA-AuCu NCs) with a quantum yield of 18% were synthesized via one-pot anti-galvanic reduction. The CA-AuCu NCs were characterized by HR-TEM, XPS, FTIR, MALDI-TOF mass spectrometry, DLS, and zeta potential analyses. The NCs are shown to be viable fluorescent probes for Cr(VI) ions and dopamine (DA) via quenching of the blue fluorescence, typically measured at excitation/emission wavelengths of 350/436 nm. During DA recognition, a dark brown color appears, which is distinguishable from that of Cr(VI) detection. The aggregation induced quenching due to electron transfer was demonstrated by photoluminescence, HR-TEM, FTIR, DLS, and zeta potential interrogations. In buffer of pH 7, response is linear in the 0.2 ~ 100 μM for Cr(VI) and from 0.4 ~ 250 μM for DA. The respective detection limits are 80 and 135 nM. The method was applied to the determination of both Cr(VI) and DA in (spiked) tap, lake and sea water, and in human urine samples. The low toxicity of CA-AuCu NCs was validated by the MTT assay, and their responses to Cr(VI) ions and DA was also proven by Raw 264.7 cell imaging. Graphical abstractCysteamine capped Au-Cu nanoclusters (CA-AuCu NCs) were synthesized via one-pot anti-galvanic reduction and utilized in sensing of Cr(VI) ions and dopamine (DA) with demonstrated real/urine and cell imaging applications.

Mitigation of chromium toxicity in Arabidopsis thaliana by sulfur supplementation

Chromium (Cr) contamination of soil and water has become a severe threat to human health. In this study, a series of experiments were conducted to examine the ameliorative effects of Cr toxicity, by exogenous 100 μM sodium sulfate. Our team has examined the plant growth, Cr content, chlorophyll, antioxidant index and soluble protein content, before and after the addition of sodium sulfate. The results showed that the addition of sulfur (S) can reduce the enrichment of Cr and the content of malonyldialdehyde (MDA) under Cr stress. After addition of S in the culture solution, the biomass and roots length of Arabidopsis thaliana increased under Cr stress. Furthermore, the content of chlorophyll, superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione (GSH), and soluble protein increased with the addition of sulfur. Transmission electron microscope observation point to that the chloroplasts can be damaged in leaf. All data demonstrate that S supplementation should help to alleviate the negative effects caused by both Cr(III) and Cr(VI) on Arabidopsis thaliana.

Chromium speciation using paper-based analytical devices by direct determination and with electromembrane microextraction

In this study, we developed and compared three different methods for chromium speciation in water samples using microfluidic paper-based analytical devices (μPADs). In all methods, detection was based on the complexation reaction of Cr(VI) with diphenylcarbazide on the μPADs. Cr(III) ions were oxidized to Cr(VI) by Ce(IV) prior to colorimetric detection on the μPADs. In the first method, oxidization of Cr(III) to Cr(VI) in the solution containing both trivalent and hexavalent chromium was performed using a batch procedure to obtain total chromium. A dual electromembrane extraction (DEME) technique for simultaneous preconcentration and extraction of chromium species and a single electromembrane extraction (SEME) for preconcentration and extraction of Cr(VI)/total chromium [quantified as Cr(VI) content after oxidation of Cr(III) ions to Cr(VI)] were used in the second and third methods, respectively. The electromembrane extraction was based on the electrokinetic migration of cationic Cr(III) and anionic Cr(VI) toward the cathode and anode, respectively, into the two different hollow fibres. Octanol-1 and bis(2-ethylhexyl) phosphate (DEHP) in octanol-1 (0.7% v/v) were the most suitable supported liquid membranes for extraction of Cr(VI) and Cr(III), respectively. Among these methods, SEME showed the lowest limits of detection for both analytes. Under optimized conditions, linear calibrations were obtained for Cr(III) from 3 to 30 μg L^-1 and for Cr(VI) from 3 to 70 μg L^-1. The detection limits were 1.0 μg L^-1 and 0.7 μg L^-1 for Cr(III) and Cr(VI), respectively. Our developed method was applied to analyse water samples spiked with different concentrations of Cr(III) and Cr(VI) at the parts-per-billion (ppb) level. The statistical evaluation showed that the proposed method agreed well with the validation method, i.e., inductively coupled plasma atomic emission spectroscopy (ICP-AES).

Magnetic nanostructures for preconcentration, speciation and determination of chromium ions: A review

Magnetic nanoparticles based solid-phase extraction is a new analytical technique based on the use of magnetic sorbents for the preconcentration and quantification of different inorganic and organic species. The present review concentrates on recent developments that have been built in magnetic nanostructures-based solid phase extraction, speciation and quantification of chromium ions. Besides, a description of the preparation, characterization as well as applications of various types of magnetic nanostructures, either with an inorganic or organic coating of the magnetic core, is presented. In addition, the most important analytical characteristics such as preconcentration factor, linear range, and limits of detection were carefully reported and compared. On the other hand, the removal of the chromium ions by magnetic solid phase extraction was not discussed in the review.

Microbial reduction fate of chromium (Cr) in aqueous solution by mixed bacterial consortium

Groundwater contaminated by Cr(VI) requires effective remediation to prevent adverse environmental impacts. The biodegradation of Cr(VI) has been documented for several decades, but little remains known about the removal fate of chromium, including the main species of reductase (sites) and functional genes involved in Cr(VI) reduction in mixed bacterial consortium. Cr(VI) reduction in this study was verified to be an enzyme-mediated process. Meanwhile, Cr(VI) reduction of different cell components demonstrated that the extracellular enzyme was the main active substance, and the distribution of Cr after experiment was quantified using mass balance calculation. Furthermore, the optimal pH for reduction was 8.0, with the reduction rate decreasing with increasing initial Cr(VI) concentrations. The co-existing oxyanions had little effect on Cr(VI) reduction, while the presence of other heavy metals had a relatively significant influence. The evolutionary behavior of microbial community structure and functional genes affected by Cr(VI) were also analyzed, which provided new insights on the underlying mechanisms involved in bioreduction in this study. These results generated new understanding of the reduction mechanisms on the Cr-relevant bacterial species and genes, which would be helpful in designing strategies for the bioremediation of Cr(VI) contaminated water.

Groundwater diffuse pollution in functional urban areas: The need to define anthropogenic diffuse pollution background levels

Groundwater status in highly urbanized areas is particularly affected by anthropogenic influence due to diffuse pollution deriving from many sources. This makes very often challenging to determine whether the observed groundwater conditions are the result of localized pollutant sources (PS-Point Sources). In the EU legislative framework it is accepted that, when Natural Background Levels (NBLs) of undesirable elements are proven to be higher than specific Groundwater Quality Standards (GQSs), NBLs should be assumed as GQSs. No procedure is instead defined when anthropogenic Diffuse Pollution Background levels (DBPLs) are higher than GQSs and make unfeasible any remediation strategy. Among the many contaminants affecting groundwater, the chlorinated solvents, tetrachloroethylene (PCE), trichloroethylene (TCE) and trichloromethane (TCM) among the organics and hexavalent chromium, among the inorganics, having been widely used in several industries all over Europe, are very often the most prevalent contaminants in soil and groundwater. Aim of this paper is to discuss a multivariate statistical approach to address the issue of identification of anthropogenic Diffuse Pollution Background Levels. With such aim, an area of about 1600 km², including the Functional Urban Area of Milan, was considered and 10 independent geochemical datasets, provided by local and regional agencies, and covering the period 2003-2014 were merged into a single database after homogenization and multiple quality checks. A total of 618,258 chemical analyses from 3477 sampling wells were considered, being all samples collected and analyzed through internally consistent protocols. The analysis enabled to identify five main clusters, having specific hydrogeological characteristics, different temporal profiles and pollutant background concentration levels, which were also found to respond differently to meteo-climatic changes. This study offers a robust knowledge basis for drafting a diffuse pollution management plan of the area.

In-situ measurement of labile Cr(III) and Cr(VI) in water using diffusive gradients in thin-films (DGT)

The toxicity and bioavailability of Cr depends on its speciation in the aquatic environment. Here, we developed a new method for simultaneously obtaining in-situ data on labile Cr(III) and Cr(VI) using diffusive gradients in thin films (DGT) and high-performance liquid chromatography hyphenated to inductively coupled plasma mass spectrometry (HPLC-ICP-MS). A Zr-oxide DGT was applied to accumulate both labile Cr(III) and Cr(VI). The elution of Cr species was carried out with 50 mM EDTA-2Na at pH 9.5 for 1.5 h. Agilent Bio WAX anion-exchange chromatography was used to separate Cr species in the slightly alkaline mobile phase containing 40 mM NH₄NO₃ at pH 7.4. An ICP-MS was used to quantitatively measure Cr within 4.5 min. Method detection limits were 0.05 μg/L for Cr(III) and 0.02 μg/L for Cr(VI). Labile Cr(III) and Cr(VI) was accurately quantified in synthetic solutions for pH in the range of 5-8 and ionic strength ranging from 10 to 100 mmol L^-1. The method allows quantification of labile Cr(III) and Cr(VI) in natural water and was consistent with results of when a separate measurement method based on DGT was used. This study was an attempt at simultaneous in-situ quantification of labile Cr(III) and Cr(VI), and will facilitate in-situ labile Cr speciation analysis in the environment.

Bioavailability and risk estimation of heavy metal(loid)s in chromated copper arsenate treated timber after remediation for utilisation as garden materials

There is increasing concern about the use of chromated copper arsenate (CCA) treated timber due to the possible leaching of toxic metals or metalloids. CCA-treated timber waste are currently stockpiled across Australia with limited information about their risks to the environment or human health. In this study, the treatment and utilisation of CCA-treated timber waste as garden mulch, garden retaining walls, and soil additive were investigated. Iron materials were used as immobilising agents. The bioavailability of Cr, Cu and As to Spinacia oleracea from CCA-treated timber, before and after treatment, was determined in the context of human health risk assessment. The results showed that the iron-based treatments resulted in significant decreases in the concentrations of Cu and As in spinach grown in CCA-treated timber in soil. Analyses of CCA derived Cu and As in spinach showed that they accumulated in the roots rather than in the leaves. The risks of toxicity to humans varied for different utilisation scenarios and the immobilisation amendments were shown to reduce carcinogenic and non-carcinogenic risks. The information obtained in this study can inform development of utilisation options for CCA-treated timber wastes.

Determination of hexavalent Cr in river sediments by speciated isotope dilution inductively coupled plasma mass spectrometry

During a sampling campaign in September 2015, elevated chromium (Cr) concentrations were observed in sediments of industrially exposed sites of the Sava River (stainless steel production and impact of metallurgic industry). To verify if sediments also contained hazardous hexavalent Cr (Cr(VI)), there was a need to develop a sensitive and reliable analytical procedure for its determination. In the determination of Cr(VI) in environmental samples, it is necessary to evaluate the suitability of the applied analytical methodology for each individual sample matrix. In these studies, the use of isotopically enriched Cr tracers importantly contribute to the validity of the obtained results. In the present work, an analytical procedure was optimised for the extraction of total Cr(VI) from sediments and the content of Cr(VI) determined by high performance liquid chromatography coupled to inductively coupled plasma mass spectrometry (HPLC-ICP-MS). To leach the total amount of Cr(VI) from sediments, an ultrasound-assisted extraction procedure was applied at 80 °C, using 2% NaOH + 3% Na₂CO₃ as the extraction solution. The addition of 0.4 mol L^-1 MgCl₂ prevented oxidation of Cr(III) during the extraction step. To control for species interconversion and for an accurate calculation of Cr(VI) concentration by speciated isotope dilution (ID)-ICP-MS, the alkaline extract was doubly spiked with enriched ⁵⁰Cr(VI) and ⁵³Cr(III). The accuracy of the determination of Cr(VI) was verified by analysing the certified reference material CRM 041 (Cr(VI) in soil), and by spike recovery test. Low limits of detection (LOD) and quantification (LOQ) (1.25 and 4.0 μg Cr(VI) kg^-1, respectively) and good repeatability of measurement (relative standard deviation better than ±4.8%) were obtained. The analytical data revealed that Cr(VI) concentrations in sediments of the Sava River did not represent any known environmental hazard.

Support Tool for Identifying In Situ Remediation Technology for Sites Contaminated by Hexavalent Chromium

Sites contaminated by hexavalent chromium raise concerns relating to the toxicity of the pollutant, as well as for the increased solubility of its compounds, which helps it to seep into aquifers. Chemical and biological in situ treatment technologies, with good potential in terms of environmental sustainability, have recently been designed and implemented on a wide scale. A useful support tool is shown in the manuscript in the preliminary phase of assessing possible technologies applicable according to the site-specific characteristics of sites. The actual efficacy of the technologies identified should nevertheless be verified in laboratory trials and pilot tests.

Evaluation of a single extraction test to estimate the human oral bioaccessibility of potentially toxic elements in soils: Towards more robust risk assessment

Intake of soil by children and adults is a major exposure pathway to contaminants including potentially toxic elements (PTEs). However, only the fraction of PTEs released in stomach and intestine are considered as bioaccessible and results from routine analyses of the total PTE content in soils, therefore, are not necessarily related to the degree of bioaccessibility. Experimental methods to determine bioaccessibility usually are time-consuming and relatively complicated in terms of analytical procedures which limits application in first tier assessments. In this study we evaluated the potential suitability of a recently developed single extract method (ISO-17586:2016) using dilute (0.43M) nitric acid (HNO₃) to mimic the bioaccessible fraction of PTEs in soils. Results from 204 soils from Portugal, Brazil and the Netherlands including all major soil types and a wide range of PTEs' concentrations showed that the extraction efficiency using 0.43M HNO₃ of Ba, Cd, Cu, Ni, Pb and Zn in soils is related to that of in vitro methods including the Simple Bioaccessibility Extraction Test (SBET) and Unified BARGE Method (UBM). Also, differences in the degree of bioaccessibility resulting from differences in parent material, geology and climate conditions did not affect the response of the 0.43M HNO₃ extraction which is a prerequisite to be able to compare results from different soils. The use of 0.43M HNO₃ as a first screening of bioaccessibility therefore offers a robust and representative way to be included in first tier standard soil tests to estimate the oral bioaccessibility.

CAPSULE

The single dilute (0.43M) nitric acid extraction can be used in first tier soil risk assessment to assess both geochemical reactivity and oral bioaccessibility of PTEs.

Chromium speciation in foodstuffs: A review

Numerous critical reviews have evaluated exposure to toxic and carcinogenic hexavalent chromium (Cr(VI)) from a number of pathways; including workplace air, cement and packaging materials. The contribution of foodstuffs to dietary Cr(VI) has been increasingly under investigation, however no summary of this work has been carried out. The objective of this article is to review the last twenty years of chromium speciation research in foodstuffs. Alkaline extraction, used for chromium speciation in other solids, is the most widely-reported procedure. Previous measurement of Cr(VI) in foodstuffs is questionable due to the reducing power of organic matter and antioxidants, leading to the development of speciated isotope dilution mass spectrometry (SIDMS) techniques to monitor interconversions. Evaluation of the genotoxicity of trivalent chromium (Cr(III)), which acts through a different pathway to that of Cr(VI), requires reconsideration towards measurement of Cr(III), which is present at higher concentrations in foodstuffs following reduction of the more-bioavailable Cr(VI).

Determination of Cr(III) solids formed by reduction of Cr(VI) in a contaminated fractured bedrock aquifer: Evidence for natural attenuation of Cr(VI)

Hexavalent chromium Cr(VI) is toxic and can be highly mobile in many aquifer systems. Redox reactions with naturally occurring minerals and organic compounds can reduce Cr(VI) to Cr(III), forming labile Cr(III) oxyhydroxide precipitates, which is a natural attenuation process. In fractured bedrock aquifers, reduction of Cr(VI) in the rock matrix can enhance attenuation beyond that from matrix diffusion only, and potentially reduce back diffusion if concentrations in fractures decline following source reduction via natural processes or engineered remediation. In this study, we develop an extraction method for labile Cr(III) precipitates from Cr(VI) reduction using 5% hydrogen peroxide (H₂O₂). Combining Cr(III) extractions with an established sodium hydroxide (NaOH) method for determination of Cr(VI) concentrations in rock porewater, a measure of the labile Cr(III) and Cr(VI) fractions in geologic samples is achieved. The methods were applied to cores from a contaminated groundwater system in fractured porous bedrock in order to assess the effectiveness of natural attenuation and whether Cr(VI) mass that diffused into the bedrock matrix was undergoing reduction. Detailed vertical distributions display two depth intervals with corresponding elevated concentrations of Cr(VI) in the porewater and extractable total Cr. The correspondence of Cr(VI) and labile Cr(III) provides evidence for reduction of Cr(VI) contamination in the bedrock matrix. Mineralogical analysis suggests that Fe(II)-bearing minerals, chlorite and biotite are the most likely candidates for natural reductants. This study provides evidence for the natural attenuation of anthropogenic Cr(VI) contamination in the porewater of a fractured bedrock aquifer, and it outlines a quantitative method for evaluating the effectiveness of natural attenuation in groundwater systems.

Determination of chromium (VI) in primary and secondary fertilizer and their respective precursors

Hexavalent chromium species (Cr(VI)) are often carcinogenic, of high acute toxicity, highly mobile, and thus pose a severe risk to health and environment. Fertilizers usually contain significant amounts of chromium. Therefore, a reliable analysis of chromium and the fraction of Cr(VI) are crucial for safe use of fertilizers. This problem is expected to increase in the future, since more and more recycled fertilizers emerge due to increasing fertilizer demand and respective supply risks. However, existing analytical methods have been developed for conventional fertilizers and have to be tested whether they are suitable for the new materials. Thus, we performed a wet-chemical extraction for Cr(VI) on several matrices as well as respective quality control experiments including spiking with Cr(III) and Cr(VI) compounds. We found the Cr(VI) amounts to be below 2 mg/kg except for a thermally post-treated sewage sludge ash (SSA) that showed 12.3 mg/kg. The presence of organic matter e.g. in sludge or precipitated struvite caused a reduction of spiked Cr(VI) and thus no satisfying recovery for quality control. Cr(VI) reduction was also observed for SSA, presumably due to the presence of Fe(II) compounds. Even though the tested procedure can be hampered in some recycled fertilizer matrices, it might be adapted to be applicable also for these complex samples.

In situ construction of low permeable barrier in soil to prevent pollutant migration by applying weak electric field

In order to prevent vertical migration of pollutant in soil matrix, this study firstly proposed to construct an in situ low permeable barrier (LPB) through synchronously transporting calcium and carbonate. After LPB construction, the soil permeability was declined tenfold. Exchangeable calcium (37.3%) and calcium bonding to carbonate (41.7%) respectively alleviated flocculation of microaggregates and cementation of marcoaggregates. Accordingly, smaller particles (<1 mm) aggregated into bigger ones (>2 mm) after electrokinetic remediation. The other soil characters like pH, moisture, and bacterial communities were well preserved after remediation. In addition, the pollutant prevention was divided into two phases as unsaturated phase and saturated phase. In unsaturated phase, phenol, F^-, Cd²⁺, and Ni²⁺ in filtrate were all lower than 0.1 mg, and Cr₂O₄^2--Cr discharged from LPB was 1/5.1 than that from initial soil. In saturated phase, LPB prevented 4.3-12.1 fold pollutant than initial soil. Taken together, proposed method could effectively prevent vertical migration of pollutants, indicating significant values for saving soil remediation cost or avoiding contamination of underground water.

Coupled Hydro-Biogeochemical Processes Controlling Cr Reductive Immobilization in Columbia River Hyporheic Zone

An experiment and modeling study was conducted to investigate coupled hydro-biogeochemical processes controlling reductive immobilization of groundwater Cr in the hyporheic zone (HZ) at the U.S. Department of Energy's Hanford Site, where dynamic surface water-groundwater exchange occurs on a daily basis. Experiments were performed to calibrate kinetic models, and the calibrated models were incorporated into a multicomponent reactive transport model to simulate Cr redox transformation and immobilization under field hydrological conditions. The results revealed that the rates of Cr(VI) reduction, Cr(III) accumulation, and Cr(VI) release to the river are mostly affected by dynamic sediment redox conditions represented by Fe(II) reactivity, which is controlled by its cyclic interaction with O₂ carried by river water, microbial activities, and the supply and bioavailability of organic carbon (OC) that is present in the HZ and/or carried by transport. In addition, the HZ geophysical properties including hydraulic conductivity and the thickness of the top alluvial layer have a significant influence on Cr reactive transport and immobilization by controlling residence times for reactions and the supply rates of O₂, Cr, and OC into the HZ. The results provide important insights into the dynamic redox environments in the HZ that can reductively immobilize contaminants.

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.

Elemental sulfur amendment decreases bio-available Cr-VI in soils impacted by leather tanneries

This study investigated the potential use of elemental S (S(0)) to convert Cr-VI to Cr-III which should decrease the bio-availability hence, toxicity of Cr-VI in soils. The bio-available fraction of Cr in soil was measured by phosphate buffer extraction (PBE) and the results showed that the fraction is about 10% of the total Cr-VI and varied from 12.8 to 42.5 mg kg(-1). The addition of 4.0 mg g(-1) S(0) decreased PBE Cr-VI to <0.4 mg kg(-1) limit established for Cr-VI toxicity in soils. Synchrotron-based X-ray absorption near-edge structure (XANES), X-ray fluorescence (XRF) and micro-XRD revealed that Cr-III was the dominant species (99% of total Cr) and Cr was retained by hematite and goethite in soil. Fe-containing minerals may have provided sufficient protection to render the dominant Cr-III species biochemically inert to redox processes in soils. It is concluded that S(0)amendment is a promising approach to remediate Cr-VI contaminated soils.

A novel strategy for Cr(III) and Cr(VI) analysis in dietary supplements by speciated isotope dilution mass spectrometry

In recent years, Cr speciation in dietary supplements has become decisive in the evaluation of their health risks. Despite being an beneficial micronutrient, Cr(III) can be toxic at living organisms at high concentrations, while Cr(VI) is known to be highly toxic and carcinogenic. The main objective of this work was to optimize an analytical methodology for the extraction and accurate quantification of Cr(III) and Cr(VI) in dietary supplements. The extraction of Cr species was carried out with 50mM EDTA solution on a hotplate under optimized conditions. Special attention was paid to bidirectional species transformations. No noticeable oxidation of Cr(III) into Cr(VI) was observed and the reduction to Cr(III) only occurred at very high Cr(VI) concentrations. Cr(III) as Cr(EDTA)(-) complex was chromatographically separated from Cr(VI), retained as CrO4(2-), on an anion exchange column coupled to inductively coupled plasma mass spectrometry (LC-ICP-MS). The limit of quantification (0.08µgg(-1)) was below the limit established for Cr enriched yeasts by the European Union. Eleven dietary supplements were analyzed and Cr(III) and Cr(VI) quantification was carried out by external calibration monitoring (52)Cr isotope and by speciated isotope dilution mass spectrometry (SIDMS) adding (50)Cr(III) and (53)Cr(VI) spikes. Total Cr was also quantified by ICP-MS and mass balance between total Cr and the sum of Cr(III) and Cr(VI) was achieved. In eight of the eleven tested supplements Cr(III) calculated amounts were higher than those indicated by the manufacturer, but only one of them exceeded the 250µgday(-1) recommended by World Health Organization (WHO). In contrast, it is worth noting that Cr(VI) amounts beyond the recommendations of the European Union for Cr enriched yeasts were found in five supplements. These results revealed that more accurate and rigorous quality assurance protocols should be applied to the testing of the final products, including the analysis of both Cr(III) and Cr(VI).

Novel Speciation Method Based on Diffusive Gradients in Thin-Films for in Situ Measurement of Cr(VI) in Aquatic Systems

Hexavalent chromium (Cr(VI)) is much more toxic and mobile than the trivalent species (Cr(III)) and consequently, in situ monitoring of Cr(VI) can improve the understanding of Cr biogeochemistry and toxicity in ecosystems. The passive diffusive gradients in thin-films (DGT) technique is a powerful tool for determining metal(loid) speciation, but a binding phase that absorbs only one specific species of Cr is needed. N-Methyl-d-glucamine (NMDG) functional resin was incorporated into the DGT binding phase for selective measurement of Cr(VI). This NMDG-DGT sampler exhibited a theoretically linear accumulation of Cr(VI), with negligible accumulation (<5%) of Cr(III), even after 72 h deployment. The good prediction of Cr(VI) concentration in synthetic freshwater with NMDG-DGT, even in the presence of 10-time more Cr(III), further indicated the sampler's reliability in selective detection of Cr(VI). Moreover, its high capacity for Cr(VI), which exceeded 230 μg cm(-2), facilitates measurement of Cr(VI) in both uncontaminated natural waters and in slightly and heavily contaminated (ppm level) waters. Field deployment of the NMDG-DGT sampler in such waters allowed accurate measurement of time-averaged Cr(VI) concentration, indicating its robustness for in situ measurements of Cr speciation and its potential for further application in the risk assessment of Cr.

Quantification of Cr(VI) in soil samples from a contaminated area in northern Italy by isotope dilution mass spectrometry

The aims of the work were to detect and quantify hexavalent chromium in 14 soil samples from an area in Lombardia (northern Italy) contaminated by two polluted water plumes. Cr(VI) was extracted from the solid samples by applying focused microwaves in an alkaline medium after Cr(III) complexation with EDTA. Cr(VI) was reduced to Cr(III) when previously reported extraction conditions for the analysis of certified reference materials were used, and Cr(VI) could not be reliably quantified in the soil samples. The influence of organic matter and iron contents in the samples on the reduction of Cr(VI) was subsequently studied using a new set of soil samples with different iron and organic matter concentrations. Isotope dilution mass spectrometry (IDMS) measured two different enriched stable isotopes of Cr (54 and 53) to evaluate the reduction extent of hexavalent chromium during the analytical procedure. The extraction conditions were optimized to obtain the lowest amount of Cr(VI) reduction and quantify Cr(VI) in the polluted soil samples from Lombardia.

Recycling of ladle slag in cement composites: Environmental impacts

In the present work compact and ground cement composites in which 30% of cement by mass was replaced by ladle slag were investigated for their chemical and physico-mechanical properties. To evaluate long-term environmental impacts, leachability test based on diffusion, which combined both, diffusion and dissolution of contaminants, was performed in water and saline water. Total element concentrations and Cr(VI) were determined in leachates over a time period of 180days. At the end of the experiment, the mineralogical composition and the physico-mechanical stability of cement composites was also assessed. The results revealed that Cr(III) and Cr(VI) were immobilized by the hydration products formed in the cement composites with the addition of ladle slag. Cr(VI) content originating from the cement was also appreciably reduced by Fe(II) from minerals present in the added ladle slag, which thus had significant positive environmental effects. Among metals, only Mo and Ba were leached in elevated concentrations, but solely in ground cement composites with the addition of ladle slag. Lower V concentrations were observed in leachates of ground than compact composite. It was demonstrated that the presence of ladle slag in cement composites can even contribute to improved mortar resistance. The investigated ladle slag can be successfully implemented in cement composites as supplementary cementitious material.

Risk assessment of exposure to particulate output of a demolition site

Whilst vehicular and industrial contributions to the airborne particulate budget are well explored, the input due to building demolition is relatively unknown. Air quality is of importance to human health, and it is well known that composition of airborne particles can have a significant influence on both chronic and acute health effects. Road dust (RD) was collected before and after the demolition of a large building to elucidate changes in elemental profile. Rainfall and PM10 mass concentration data aided interpretation of the elemental data. Quantification of Al, As, Ba, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Rh, S, Si, Sn, Ti, V and Zn was carried out. It was found that only Al, K, Mg, Si and S increased in concentration across all size fractions after the building demolition. Risk assessment was then carried out on elements with applicable reference dose values to assess the potential health risks due to the demolition. Significant risk to children was observed for chromium and aluminium exposure. PM10, monitored 40 metres from the demolition site, indicated no abnormal concentrations during the demolition; however, rainfall data were shown to affect the concentration of PM10. The elemental data observed in this study could possibly indicate the role of increased sulphur concentrations (in this case as a result of the demolition) on the buffer capacity of RD, hence leaching metals into rainwater.