Hexavalent Chromium Species Research Articles

A novel approach to study ascorbic acid oxidation using hexavalent chromium species—theoretical and practical perspectives of the use of potentiometric titration technique

AbstractThis paper highlights the advantages of using the potentiometric titration technique as a valuable tool for studying the oxidation reaction of ascorbic acid by hexavalent chromium species in aqueous solutions. Particular attention was paid to the method of determining diagnostic points allowing the quantification of ascorbic acid in the sample under study. Additionally, the influence of experimental conditions, specifically the pH of the system and the concentration of the reactants, on the type of hexavalent chromium species involved in the interaction with ascorbic acid was analyzed. It has been shown that the method presented provides a simple, cost-effective, and rapid tool that can be widely used for the direct determination of ascorbic acid.

Superior comprehensive performance of modified activated carbon as a hexavalent chromium adsorbent

The presence of hexavalent chromium species (CrVI) in wastewater from manufacturing industries such as electroplating and leather production can pose serious health hazards. To address these concerns, this study developed a novel adsorbent based on activated carbon as the primary material to attract CrVI. Activated carbon has been modified with several other components to improve its comprehensive performance, including adsorption capacity, chemical stability, collectability, and reusability. Specifically, decoration with magnetite nanoparticles made it possible to collect the adsorbent magnetically and reuse it several times. On the one hand, the addition of chitosan not only increased the chemical stability of activated carbon, especially under acidic conditions, but also enhanced the Cr adsorption capacity at pH higher than 4, where adsorption of only activated carbon was significantly decreased, probably because the protonated amino groups attracted chromate anions. In addition, the co-existence of tannic acid did not increase the adsorption capacity significantly but appeared to promote the reductive adsorption of CrVI, where the reduction of CrVI means lowering the toxicity of Cr species. It was demonstrated that activated carbon modified with magnetite, chitosan, and tannic acid exhibited superior comprehensive performance that could be repeatedly used over a wide pH range as compared to the parent activated carbon.

Bi-functional photocatalytic heterostructures combining titania thin films with carbon quantum dots (C-QDs/TiO2) for effective elimination of water pollutants

In this study, carbon quantum dots (C-QDs), prepared via hydrothermal-microwave procedures, were successfully combined with nanostructured titania (TiO2). The photocatalytic oxidation/reduction activity of the C-QDs/TiO2 composite films was evaluated in the decomposition of organic-inorganic contaminants from aqueous solutions under UV illumination. Physicochemical characterizations were applied to investigate the crystal structure of the carbon quantum dots and the composites. It was found that the prepared C-QDs/TiO2 composites had great contribution to the photocatalytic reduction of hexavalent chromium (Cr+6) species and 4-Nitrophenol (PNP) as well as to the photocatalytic oxidation of methylene blue (MB) and Rhodamine B (RhB) dyes. The mechanism of the photocatalytic reaction was studied with trapping experiments, revealing that the electron (e−) radical species were powerfully supported for the photocatalytic reduction of Cr+6 and PNP and the holes (h+) are the main active species for the photocatalytic oxidation reactions.

Metal Content in Textile and (Nano)Textile Products.

Metals, metallic compounds, and, recently, metallic nanoparticles appear in textiles due to impurities from raw materials, contamination during the manufacturing process, and/or their deliberate addition. However, the presence of lead, cadmium, chromium (VI), arsenic, mercury, and dioctyltin in textile products is regulated in Europe (Regulation 1907/2006). Metal determination in fabrics was performed by inductively coupled plasma-mass spectrometry (ICP-MS) after microwave-assisted acid digestion. The ICP-MS procedure has been successfully validated; relative standard deviations were up to 3% and analytical recoveries were within the 90–107% range. The developed method was applied to several commercial textiles, and special attention has been focused on textiles with nanofinishing (fabrics prepared with metallic nanoparticles for providing certain functionalities). Arsenic content (in textile T4) and lead content (in subsamples T1-1, T1-2, and T3-3) were found to exceed the maximum limits established by the European Regulation 1907/2006. Although impregnation of yarns with mercury compounds is not allowed, mercury was quantified in fabrics T1-2, T5, and T6. Further speciation studies for determining hexavalent chromium species in sample T9 are necessary (hexavalent chromium is the only species of chromium regulated). Some textile products commercialised in Europe included in this study do not comply with European regulation 1907/2006.

Corrosion of structural constituents of 2017 aluminium alloy in acidic solutions containing inhibitors

AbstractThe objective of this study is to establish the corrosion behaviour of the most important structural constituents of the aluminium alloy 2017 in orthophosphoric acid solutions containing heteropolyoxomolybdate, tungstate and vanadate. These are potential candidates for replacing toxic hexavalent chromium species in stripping solution for anodic coatings. The corrosion rate of the alloy is estimated with linear polarisation method. It decreases from 0.58 mA cm−2 in uninhibited solution to 0.10 mA cm−2 in a solution containing heteropolyoxomolybdate species. Microscopic studies reveal that heteropolyoxomolybdate species inhibit corrosion of the matrix and intermetallic Al15(Fe,Mn)3(Si,Cu)2 but not Mg2Si. Intermetallic Al2Cu remains not corroded. Heteropolyoxotungstate species virtually do not inhibit the corrosion of the alloy. The solution containing vanadium species is not stable with time and the corrosion rate is not determined. Nevertheless, corrosion of the matrix is inhibited, but intermetallics Al15(Fe,Mn)3(Si,Cu)2 together with Mg2Si are dissolved. X‐ray photoelectron spectroscopy is used for examination of a corrosion product precipitated on the surface.

Eliminating the Formation of Hexavalent Chromium in Chrome Stripping Operations Using Pulse Reverse Processes

Chromium electrodeposits are commonly applied to aeronautical and automotive components as protective coatings from harsh and/or corrosive operational environments. Through normal use, these coatings may become damaged. During repair and refurbishment of these components, the parts are stripped of the existing chromium deposit before being recoated and reinstalled into various systems. Traditionally, the chrome stripping process has been facilitated by applying a potential bias to a workpiece in an alkaline electrolyte per MIL-STD-871B. While the alkaline pH may help to protect the underlying steel surface from corrosion, it preferentially favors the formation of the hexavalent chromium species, which is a carcinogen and an environmental toxin. In the conventional stripping process, hexavalent chromium ions build up in the stripping electrolyte over time, resulting in decreased stripping rates, high energy consumption and possible damage to the part. To minimize these effects, the bath must be purified or discarded and replaced. Prior to waste disposal all hexavalent chromium in the electrolytes must be reduced to trivalent form as the trivalent species is non-toxic, non-hazardous and more stable compared to the hexavalent species. Therefore, if the oxidation of chromium metal can be limited to the trivalent species during stripping operations, air quality regulations will be easier to meet and working conditions personnel would be greatly improved. Faraday is working to develop an electrolytic stripping process intended as a drop-in replacement for the conventional stripping process. The process seeks to eliminate the formation of hexavalent chromium while maintaining the material properties of the underlying substrate. Faraday will present work that demonstrates removal of chrome from high strength steel substrates using a weak acid electrolyte that results in the dissolution of the metal to its trivalent state. The effect of pulse and pulse/reverse electric fields on the efficacy of the stripping process will be presented in comparison to operation under constant voltage electric fields. The stripping process based on use of a weak acid electrolyte in conjunction with pulse/pulse reverse electric fields was demonstrated for flat and round specimens in a pilot scale cell using a simple oxalic acid electrolyte. In addition, studies were conducted to assess how age of the oxalic acid electrolyte impacts the stripping process using constant voltage electric fields. Faraday also developed a process for the recycling of spent alkaline stripping electrolytes loaded with hexavalent chromium ions. This process relies on the use of catalytic Au-Pd-Cu cathodes and pulse reverse process conditions. Results of the process in bench scale validation studies and lessons in the scaling of this technology are discussed. Acknowledgement: Funding for this work is gratefully acknowledged from Air Force SBIR Contract Number FA8222-16-C-0006.

Speciation and Separation of Trace Quantities of Hexavalent and Trivalent Chromium Species in Aqueous Extract of Wild Leafy Vegetables Using Multistep Pre-concentration Method

The presence of Cr(VI) in edible parts of vegetable plants might be representing a probable health risk to humans and animals. On the other hand, the presence of Cr(III) is beneficial for human health. A new method has been employed for the separation and pre-concentration of Cr(III)/Cr(VI) present in the aqueous extract (soup) of wild vegetables prior to determining with graphite furnace atomic absorption spectrometry. For the pre-concentration of Cr(III) and Cr(VI) in two sets of aqueous extract of each vegetable, complexing reagents, 8-hydroxyquinoline (oxine) and ammonium pyrrolidinedithiocarbamate (APDC), was added respectively. Then the tunable solvent solution (TSS) was added based on 1,8-diazabicyclo[5.4.0]undec-7-ene and decanol (1:1) in an aqueous system. The role of different variables for a proposed method such as the amount of ligands (oxine and APDC), pH of the solution, volume of TSS, purging time, and pressure of CO2 was investigated. The accuracy for the quantification of Cr(III) and Cr(VI) was determined by the standard addition method at optimized values of all factors, and > 98% of both species were obtained. The LOD and LOQ of Cr(III)/Cr(VI) were found to be 0.048/0.072 and 0.16/0.24 μg L−1, respectively. The data indicate that Cr(III) is found > 65% and Cr(VI) as < 35% in aqueous extract of all selected leafy vegetables, which is a good sign for public awareness to maintain human health by using these wild vegetables. These results will also be helpful to the researchers compiling the nutritional data sheet.

Industrial steel waste as an iron source to promote heterogeneous and homogeneous oxidation/reduction reactions

This work examines the mill scale (MS) of hot rolling, a waste product formed in industrial steel processing. MS was evaluated as an iron source in promoting the oxidation of Reactive Red 120 azo dye in location-sourced and factory textile wastewater through a Fenton reaction (FR), as well as reducing hexavalent chromium species to its trivalent form in the presence of an organic ligand (citric acid). The MS was characterised by various techniques and the results showed that iron is the major compound in its composition and occurs mainly in the form of oxides (wüstite, hematite and magnetite). In addition, it has ferromagnetic properties, which facilitate its separation. In the oxidation tests it was observed that acidic pH conditions promoted a positive influence on the reaction efficiency and that the presence of H2O2 can limit the leaching of iron into the solution. The best experimental conditions for colour removal from a RR 120 dye solution containing 35 mg L−1 were 15 g L−1 of MS, 0.5 mM H2O2, pH 3, 25 °C, 200 ± 2 rpm and 120 min. Furthermore, the homogeneous Fenton reaction showed a significant contribution to decolourisation, but the heterogeneous phase cannot be neglected since it is predominant at the beginning of the reactions. In the continuous process, the influence of the H2O2 concentration and the feed rate was evaluated, obtaining an optimal dose of 0.5 mM H2O2 for the RR dye 120 and 0.7 mM H2O2 for the real textile effluent, with a flow rate of 0.25 mL min−1 and pH 3 for both. Subsequently, the residue was evaluated as the source of iron in the process of reducing Cr(VI) to Cr(III). An increase in the efficiency of the reaction in the presence of citric acid was observed, associated with the removal of the surface oxide layer through the formation of soluble Fe(III)Cr(III)-ligand complexes and the consequent increase of the iron redox cycle, reducing it to sequencing the reduction of chromium. 100% reduction was obtained in 30 min of reaction using 20 g L−1 of MS, at pH 3 and Cr(VI):citric acid ratio of 1:4. In continuous operation mode, 100% Cr(VI) reduction was obtained during 50 h (1.0 mL min−1 flow rate), dropping to 20% with increasing flow rate to 4.0 mL min−1 after 200 h of assay.

Fabrication of an AMC/MMT Fluorescence Composite for its Detection of Cr(VI) in Water.

Hexavalent chromium species, Cr(VI), which can activate teratogenic processes, disturb DNA synthesis and induce mutagenic changes resulting in malignant tumors. The detection and quantification of Cr(VI) is very necessary. One of the rapid and simple methods for contaminant analysis is fluorescence detection using organic dye molecules. Its application is limited owing to concentration quenching due to aggregation of fluorescent molecules. In this study, we successfully intercalated 7-amino-4-methylcoumarin (AMC) into the interlayer space of montmorillonite (MMT), significantly inhibited fluorescence quenching. Due to enhanced fluorescence property, the composite was fabricated into a film with chitosan to detect Cr(VI) in water. Cr(VI) can be detected in aqueous solution by instruments excellent, ranging from 0.005 to 100 mM with a detection limit of 5 μM.

Investigation of Surface Interactions between Volatile Chromium Species and Ceramics

Volatile chromium species are formed when chromium containing materials are exposed to high temperature (>500°C) oxidizing environments. These volatile species are often accompanied by detrimental effects, such as cathode poisoning in solid oxide fuel cells and condensation of toxic hexavalent chromium species. The reactive vaporization of chromium species and their detrimental effects are well understood, but less is known about how these species interact with and condense onto various materials. This study investigates how volatile chromium species interact with ceramics over a range of temperatures and exposure times. Preliminary work performed on aluminosilicate fibers suggests that chromium speciation depends on factors such as temperature, chromium loading of a surface, and what material the volatile chromium is interacting with. Additionally, the color of condensed chromium species on these fibers has been linked with the oxidation state of chromium, allowing inferences to be made to known chromium compounds such as chromate, chromium trioxide, and chromia. To extend this work, volatile chromium was generated by flowing humidified air over chromia powder at various temperatures up to 900°C for various time intervals up to 100 hours. Vapor collection materials, including pure and mixed silica and alumina ceramic fibers and coupons, were placed downstream at temperatures ranging from 50-900C. Post exposure characterization of ceramic surfaces was performed using x-ray photoelectron spectroscopy, x-ray diffraction, inductively coupled plasma mass spectroscopy, and colorimetry to obtain information about chromium speciation in the form of oxidation states and phase formation. Results will be presented and discussed in context of advancing fundamental understanding of condensation mechanisms for volatile chromium species.

Mathematical Model of the Catalytic Effect of Chromium(VI) on Hypochlorite Disproportionation in Chlorate Electrolysis

The effect of chromium(VI) on the kinetics of disproportionation of hypochlorous acid and hypochlorite was established in the solution for the electrolytic production of chlorate. The hexavalent chromium species Cr2O72−, HCrO4− and CrO42−present in the solution catalyze the disproportionation reaction. In both the absence and presence of chromium(VI), disproportionation is a third-order reaction with respect to HClO and ClO−, and a first-order reaction with respect to the hexavalent chromium species. In the presence of chromium(VI) ions, four parallel reactions probably take place in the solution i.e. uncatalyzed disproportionation and three parallel reactions catalyzed by Cr2O72−, HCrO4− and CrO42−ions. Most likely, the hexavalent chromium species do not change the sequence of elementary reactions in the disproportionation mechanism but only speed up the rate-determining step through interaction with its reactants. In the chlorate production process, as chromium(VI) concentration increases, the optimum pH which ensures the maximum rate of disproportionation is shifted to an acid environment. This is due to an increase in the concentration of the catalytically most active species HCrO4− with increasing acidity of the solution. A mathematical model of the kinetics of the chromium(VI)-catalyzed disproportionation of hypochlorite and hypochlorous acid into chlorate was set up. Good agreement was obtained between theoretical and experimental data.

Removal of Gaseous Hexavalent Chromium Species in a SOFC System By a Chromium Getter

Chromium containing alloys release hexavalent chromium species, which degrade SOFC stacks, and are hazardous for human health. Cathode air preheaters are a known source for hexavalent chromium, as they have a large surface area, consisting of austenitic steel, containing 22% chromium. The preheaters in this study use 0.5 – 1.0 m² steel surface, with maximum wall temperatures of 700 – 900 °C. These preheaters release 0.05 – 0.1 g hexavalent chromium in 1000 h. Improved alloy selection reduces this by a factor 15 – 20. Higher reduction is required, depending on the operating temperatures in the SOFC stack, and for automotive applications such as SOFC APU’s. Hexavalent chromium at end of life is a hazardeous waste, and might complicate the recycling strategy for used vehicles. The design and performance of a Chromium Getter is presented: this component captures hexavalent chromium, as released by a cathode air preheater or another steel component. The Getter is a catalyst: it reduces hexavalent chromium to trivalent chromium, which is stored on a Ti/TiO2 surface in the component. Lab tests were executed during 1000 h, at 750°C, 4 g/s mass flow, 3% moisture. The source of the hexavalent chromium is a plate heat exchanger, with contains 0.6 m² heat exchanging walls, consisting of austenitic alloy. These walls were kept at 780°C overall. This was done for accelerated testing. Chromium was analyzed at 7 collection runs, each taking 20h. During the collection runs, the outflowing hot gas was cooled in a Liebig cooler. The Liebig coolers were rinsed with acid, and the total chromium amount in the acid was measured using ICP-OES. The hexavalent fraction was determined by photometry after derivatization. The concentration of hexavalent chromium species at the outlet of the Chromium Getter is below the analytical detection limit during the 1000h test. This corresponds to a conversion by a factor 650 or higher, as compared to measurements on heat exchangers without chromium getter. Post mortem analysis shows that the chromium stored on the active surface is trivalent chromium, as desired. Therefore, the component can be recycled as regular metal at the end of life. The storage capacity of the component is calculated for a 1.2 kWe SOFC system, using a Chromium Getter with 0.17 l volume. Figure 1

Kinetics and Thermodynamic Study on Novel Modified–Mesoporous Silica MCM-41/Polymer Matrix Nanocomposites: Effective Adsorbents for Trace CrVI Removal

It is no longer news that the world is in the midst of a freshwater crisis, with the future health of both humans and the environment being threatened by heavy metal pollution. Accordingly, attempts at removing the toxic heavy metals from water are attracting considerable attention around the world. As a result, for the first time, amine-modified MCM-41/polymer matrix nanocomposites (m-MCM-41/PMNCs) have been applied to remove hexavalent chromium species (CrVI) from aqueous solution. Poly(methyl methacrylate), poly(vinyl alcohol), nylon-6, and polystyrene, which are environmentally benign and economically accessible polymer matrices, were used for fabricating four different types of m-MCM-41/PMNCs. Owing to the abundance of chemically active functional groups on the surface of these m-MCM-41/PMNCs, excellent adsorption performance was observed. Maximum metal removal was observed at low pH (2–3) and with the efficiency of CrVI removal being in the range of 61.78–85.71%. The kinetic modeling and equilibrium...

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.

Reduction of toxic Cr(VI)-humic acid in an ionic liquid

Remediation of soil contaminated by toxic hexavalent chromium species associated with humic acid (Cr(VI)-HA) and absorbed Cr(VI) in pores frequently experiences technical difficulties. In the present work, a feasibility study for extraction of the Cr(VI) species from a molecular sieve MCM-41 (Mobil Composition of Matter No. 41) that was used to simulate the pore system of soil, with a green solvent (ionic liquid), 1-butyl-3-methylimidazolium chloride ([C4mim][Cl]), was carried out. After a 30-min extraction, approximately 70% of the Cr(VI) species can be extracted. By component fitted X-ray absorption near edge structure (XANES) spectroscopy, about 48% of the Cr(VI)-HA are reduced to form less toxic species (Cr(III)-HA) during extraction with [C4mim][Cl]. Note that the Cr-O in the [C4mim][Cl] phase has a slightly greater bond distance (BD) (0.162nm) possibly due to the fact of that the fraction of Cr(III) is increased in the extraction process. The non-extractable chromium remaining in MCM-41 has a much greater fraction of Cr(III) (78%) and its BD is further increased to 0.195nm. The coordination numbers of chromium for the 1st shell Cr-O in the Cr(VI) and Cr(III) species are in the range of 2.4–2.9, suggesting that chromium is chelated with HA and adsorbed in MCM-41. The 1H NMR data also suggest that the enhanced reduction of the Cr(VI) species may be related to interactions between chromium species and electron-rich imidazole ring of the [C4mim]+. This work also exemplifies that the fate of toxic chromium species in the complicated remediation of contaminated soils can be revealed in a molecule-scale study by synchrotron X-ray absorption spectroscopy.

Remediation of hexavalent chromium contamination in chromite ore processing residue by sodium dithionite and sodium phosphate addition and its mechanism

Large amounts of chromite ore processing residue (COPR) wastes have been deposited in many countries worldwide, generating significant contamination issues from the highly mobile and toxic hexavalent chromium species (Cr(VI)). In this study, sodium dithionite (Na2S2O4) was used to reduce Cr(VI) to Cr(III) in COPR containing high available Fe, and then sodium phosphate (Na3PO4) was utilized to further immobilize Cr(III), via a two-step procedure (TSP). Remediation and immobilization processes and mechanisms were systematically investigated using batch experiments, sequential extraction studies, X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). Results showed that Na2S2O4 effectively reduced Cr(VI) to Cr(III), catalyzed by Fe(III). The subsequent addition of Na3PO4 further immobilized Cr(III) by the formation of crystalline CrPO4·6H2O. However, addition of Na3PO4 simultaneously with Na2S2O4 (via a one-step procedure, OSP) impeded Cr(VI) reduction due to the competitive reaction of Na3PO4 and Na2S2O4 with Fe(III). Thus, the remediation efficiency of the TSP was much higher than the corresponding OSP. Using an optimal dosage in the two-step procedure (Na2S2O4 at a dosage of 12× the stoichiometric requirement for 15 days, and then Na3PO4 in a molar ratio (i.e. Na3PO4: initial Cr(VI)) of 4:1 for another 15 days), the total dissolved Cr in the leachate determined via Toxicity Characteristic Leaching Procedure (TCLP Cr) testing of our samples was reduced to 3.8 mg/L (from an initial TCLP Cr of 112.2 mg/L, i.e. at >96% efficiency).

Surfactant-promoted enhancement in bioremediation of hexavalent chromium to trivalent chromium by naturally occurring wall algae

The discharge of hexavalent chromium [Cr(VI)] has resulted in significant pollution because of the presence of Cr(VI) oxyanions. They are highly water-soluble, cell-permeable and transportable in water sources. To eliminate the toxic effects, various chemical and biological methods have been developed. Bioremediation can be used as an effective tool over conventional chemical processes for the removal of toxic hexavalent chromium species. Blue-green algae-mediated bioremediation is gaining more interest due to its availability and efficiency. In this method, [Cr(VI)] is reduced to relatively less toxic [Cr(III)] as a consequence of oxidation of organic components present in the water extract of wall algae. The percent removal of Cr(VI) is increased by the addition of the anionic surfactant sodium dodecyl sulfate (SDS) and the neutral surfactant Triton-X-100 (TX-100). It was found that the percent removal efficiency of unanalyzed, TX-100 and SDS catalyzed (3 × 10−2 M) systems are approximately 47, 54 and 80 %, respectively, in 8 days. The efficiency of these systems increases with the increase in the equivalent amount of added surfactant (3 × 10−1 M). The reaction proceeds to almost completion (96 %) with the increase in the added surfactant (3 × 10−1 M). The optimum pH of the reaction was found to be 2 and the temperature 40 °C. The amount of Cr(III) formed was measured by the fluorescence technique with a known dye selective to Cr(III), and the presence of soluble Cr(VI) in the mixture was estimated by a diphenylcarbazide kit. This method was further used to determine the removal efficiency of the SDS-catalyzed system in collected [Cr(VI)]-contaminated water sources, i.e. a pond beside the Sukinda valley in Cuttack, India, and pond water from the local area, sea water from Cuttack and Digha, W.B., India, as well as from the tanning industry. All the observations were recorded using UV–Vis, fluorescence and FTIR spectrophotometry.

The impact of humic acid on chromium phytoextraction by aquatic macrophyte Lemna minor

Studies assessing chromium phytoextration from natural waters rarely consider potential implications of chromium speciation in the presence of ubiquitous humic substances. Therefore, the present study investigated the influence of environmentally relevant concentration of humic acid (TOC = 10 mg L−1) on chromium speciation (Cr = 0.15 mg L−1) and consequently on phytoextraction by aquatic macrophyte duckweed Lemna minor. In absence of humic acid, only hexavalent chromium was present in water samples and easily taken up by L. minor. Chromium uptake resulted in a significant reduction of growth rate by 22% and decrease of chlorophyll a and chlorophyll b contents by 48% and 43%, respectively. On the other hand, presence of humic acid significantly reduced chromium bioavailability (57% Cr uptake decrease) and consequently it did not cause any measurable effect to duckweed. Such effect was related to abiotic reduction of hexavalent chromium species to trivalent. Hence, findings of our study suggest that presence of humic acid and chromium speciation cannot be neglected during phytoextraction studies.

Determination of hexavalent chromium in aqueous and alkaline soil extracts

A fast, simple, and reliable method for determination of hexavalent chromium species in soil aqueous and alkaline extracts has been developed. The procedure is based on the ion-pair extraction of Cr(VI) with methyltrioctylammonium chloride (Aliquat 336) into xylene. The extract is analyzed by flame (FAAS) or electrothermal atomic absorption spectrometry (ETAAS). The method allows the detection of up to 2 mg kg$^{-1}$ Cr(VI) by FAAS and up to 25 $\mu $g kg$^{-1}$ Cr(VI) by ETAAS. The relative standard deviation varied in the range 2%-12% depending on the Cr(VI) levels in the samples. The analysis of certified reference material CRM041-30G Chromium VI--Soil and spike experiments showed recoveries from 91% to 106%. The procedure was applied to industrially polluted soils.

Role of chemical hardness in the adsorption of hexavalent chromium species onto metal oxide nanoparticles

We employed chemical hardness to explain the adsorption behaviour of Cr(VI) anions on metal oxide sorbents. In the absence of electrostatic attraction at pH values around the point of zero charge, silica and titania nanoparticles adsorbed a very small amount of Cr(VI) anions. This lack of adsorption is because the magnitude of the chemical hardness is too high to pair with the less hard Cr(VI) anions. In contrast, maghemite nanoparticles with relatively weak values of hardness adsorbed 0.021mol of Cr(VI) per one mole of OH groups on the particle surface, 2.7 times that of titania. The magnitude of the chemical hardness decreases in the order of SiO2≫TiO2>γ-Fe2O3, which is determined by the hardness of constituting metal cations. This result indicated that the adsorption onto an oxide surface is governed by metal cations in that their chemical hardness determines the basicity of surface OH groups, which regulates adsorption as well as protonation/deprotonation. The hardness match between a sorbent and a sorbate may be interpreted as their chemical affinity. We demonstrated that the chemical affinity is the principal driving force for adsorption of Cr(VI) anions onto the oxides.