Chemistry Of Chromium Research Articles

High-temperature reaction chemistry of chromium and chlorine over Fe2O3 within sludge-incinerated fly ash

Fe2O3 is one of the main compositions of sludge-incinerated fly ash and shows reactivity for chromium removal from flue gas in the incinerator. Experiments and density functional theory (DFT) calculations were performed to investigate the high-temperature reaction chemistry of chromium and chlorine over Fe2O3 within fly ash. Fe2O3 exhibits outstanding chromium removal efficiency of 86.29 % in N2 atmosphere. The presence of HCl obviously inhibits chromium removal, the inhibition effect increases with increasing HCl concentration. Fe-Cr composite oxides are the main products of chromium-Fe2O3 high-temperature reaction. Chromium within the reaction products in N2 or HCl-containing atmosphere mainly exists in the form of Cr3+. O2 addition leads to the formation of a small amount of Cr6+. Fe2O3 exhibits excellent inhibition performance of chromium oxidation. DFT calculations suggest that CrCl3 and CrO2Cl2 are chemically adsorbed on Fe2O3 surface. Both O and Fe atoms of Fe2O3 surface serve as active sites for chromium adsorption. HCl is dissociatively adsorbed on the Fe2O3 surface, and reacted with chromium to form CrCl3. CrCl3 formation on the Fe2O3 surface is dominated by a four-step reaction pathway: Cr(ads) → CrCl(ads) → CrCl2(ads) → CrCl3(ads) → CrCl3. CrCl formation is the rate-determining step with the energy barrier of 39.51 kJ/mol.

Chromium/Photoredox Dual Catalyzed Synthesis of α-Benzylic Alcohols, Isochromanones, 1,2-Oxy Alcohols and 1,2-Thio Alcohols.

Herein, a Cr/Photoredox-catalyzed general synthetic strategy to access α-benzylic alcohols, isochromanones, oxy alcohols and thio alcohols is unveiled. Alkylation of aldehydes being a crucial C-C bond forming reaction, designing competent catalytic systems would render an attractive and decorated set of diverse alcohol motifs. Considering the challenges associated with classical organometallic chemistry, the strategy of dual catalysis is applied here to generate diverse alcohol motifs in a mild and efficient manner. The amalgamation of photocatalysis with chromium chemistry is chosen for this purpose to generate an environment with low basicity and thus, high chemoselectivity. With alkyl silanes as preferred coupling partners, this catalytic setup produces a broad substrate scope with an excellent functional group tolerance and displays a facile scale-up as well. Its application towards biologically relevant molecules and product diversification contributes to the synthetic utility of this method.

A comprehensive review on chromium chemistry along with detection, speciation, extraction and remediation of hexavalent chromium in contemporary science and technology

AbstractThe presence of hexavalent chromium in our environment is a potential mutagenic hazard for the whole animal and plant kingdom including humans. In the last five decades numerous physico‐chemical and bio‐chemical mechanisms have been proposed for the detection of chromium speciation, extraction and remediation of the same from contaminated environmental samples. This present review is dedicated on the discussions of detection of chromium speciation in various environmental samples viz., industrial wastes, workplace particulates, plants, aquatic animals and food samples. Different physical, chemical and biological analytical methods have been presented. Mechanisms of such detection, extraction/remediation techniques are also emphasised. The aim of this article is to comprehend the contemporary advancement in this field that may be applied from lab to industry levels.

A Study of the Effect of Ligand-Metal Interactions on the Electrodeposition of Chromium from Trivalent Chromium Electrolytes

With electrolytic chromium coated steel (ECCS) technology, a thin layer of chromium and an outer layer of chromium oxide are deposited on packaging steel to provide corrosion protection and a suitable layer for adhesion of subsequent polymer coatings. In an ECCS plating line, the coating is deposited via a hexavalent chromium electrolyte. Hexavalent chromium has been found to be an environmental and health hazard and is therefore subject to restrictions under REACH legislation. A new coating process based on trivalent chromium chemistry has therefore been developed (TCCT).Coatings produced via trivalent chromium electrolytes have been found to provide comparable corrosion resistance to those achieved with the traditional Cr(VI)-based process [1]–[4]. However, there is limited understanding of the mechanism and kinetics of electrodeposition from trivalent chromium based electrolytes. This knowledge is key to optimizing process design of industrial plating lines. Formate ions are incorporated in the Cr3+ electrolyte as complexing agents to make Cr0 deposition possible. It has been shown that coating composition is dependent on the relative quantity of HCOO- ions present. The present study is comprised of two main research goals - to determine the complexes that form in solution as a function of the relative complexing agent (HCOO-) concentration and to determine the effect of this complex speciation on the deposition mechanism.Results of a multitechnique approach addressing the first of our research goals - complex speciation - will be presented. Density functional calculations have been used to establish the energetic feasibility of the reaction mechanisms suggested by results from UV Vis and FTIR spectroscopy and rotating cylinder cyclic voltammetry experiments. UV Vis and FTIR results show that a clear chemical transition occurs once the formate ion is introduced in the electrolyte – starting ratio 1:6 HCOO-:Cr3+. Beyond this point, no significant change in the spectrum apart from increasing absorbance up to a ratio of 12 HCOO-:Cr3+ is observed. These results suggest the formation of a single new complex in the solution upon introduction of the formate ion in the electrolyte solution which is not modified by further increase of the formate ion concentration.[1] M. Inman et al., ‘Green Process for Fuctional Trialent Crhromium Electroplating’, in 223rd ECS Meeting, 2012.[2] J. H. O. J. Wijenberg, M. Steegh, M. P. Aarnts, K. R. Lammers, and J. M. C. Mol, ‘Electrodeposition of mixed chromium metal-carbide-oxide coatings from a trivalent chromium-formate electrolyte without a buffering agent’, Electrochimica Acta, vol. 173, pp. 819–826, Aug. 2015.[3] J. E. Edy, H. N. McMurray, K. R. Lammers, and A. C. A. deVooys, ‘Kinetics of corrosion-driven cathodic disbondment on organic coated trivalent chromium metal-oxide-carbide coatings on steel’, Corrosion Science, vol. 157, pp. 51–61, Aug. 2019.[4] S. Hesamedini and A. Bund, ‘Trivalent chromium conversion coatings’, J Coat Technol Res, vol. 16, no. 3, pp. 623–641, May 2019.

De novo whole-genome assembly and resequencing resources for the roan (Hippotragus equinus), an iconic African antelope.

Roan antelope (Hippotragus equinus) is the second-largest member of the Hippotraginae (Bovidae), and is widely distributed across sub-Saharan mesic woodlands. Despite being listed as "Least Concern" across its African range, population numbers are decreasing with many regional Red List statuses varying between Endangered and Locally Extinct. Although the roan antelope has become an economically-important game species in Southern Africa, the vast majority of wild populations are found only in fragmented protected areas, which is of conservation concern. Genomic information is crucial in devising optimal management plans. To this end, we report here the first de novo assembly and annotation of the whole-genome sequence of a male roan antelope from a captive-breeding program. Additionally, we uncover single-nucleotide variants (SNVs) through re-sequencing of five wild individuals representing five of the six described subspecies. We used 10X Genomics Chromium chemistry to produce a draft genome of 2.56 Gb consisting of 16,880 scaffolds with N50 = 8.42 Mb and a BUSCO completeness of 91.2%. The draft roan genome includes 1.1 Gbp (42.2%) repetitive sequences. De novo annotation identified 20,518 protein-coding genes. Genome synteny to the domestic cow showed an average identity of 92.7%. Re-sequencing of five wild individuals to an average sequencing depth of 9.8x resulted in the identification of a filtered set of 3.4x106 bi-allelic SNVs. The proportion of alternative homozygous SNVs for the individuals representing different subspecies, as well as differentiation as measured by PCA, were consistent with expected divergence from the reference genome and among samples. The roan antelope genome is a valuable resource for evolutionary and population genomic questions, as well as management and conservation actions.

Draft Genome Assembly of the Freshwater Apex Predator Wels Catfish (Silurus glanis) Using Linked-Read Sequencing.

The wels catfish (Silurus glanis) is one of the largest freshwater fish species in the world. This top predator plays a key role in ecosystem stability, and represents an iconic trophy-fish for recreational fishermen. S. glanis is also a highly valued species for its high-quality boneless flesh, and has been cultivated for over 100 years in Eastern and Central Europe. The interest in rearing S. glanis continues to grow; the aquaculture production of this species has almost doubled during the last decade. However, despite its high ecological, cultural and economic importance, the available genomic resources for S. glanis are very limited. To fulfill this gap we report a de novo assembly and annotation of the whole genome sequence of a female S. glanis. The linked-read based technology with 10X Genomics Chromium chemistry and Supernova assembler produced a highly continuous draft genome of S. glanis: ∼0.8Gb assembly (scaffold N50 = 3.2 Mb; longest individual scaffold = 13.9 Mb; BUSCO completeness = 84.2%), which included 313.3 Mb of putative repeated sequences. In total, 21,316 protein-coding genes were predicted, of which 96% were annotated functionally from either sequence homology or protein signature searches. The highly continuous genome assembly will be an invaluable resource for aquaculture genomics, genetics, conservation, and breeding research of S. glanis.

Impact of volatile organic compounds on chromium containing atmospheric particulate: insights from molecular dynamics simulations

The effect of volatile organic compounds (VOCs) on chromium-containing atmospheric particles remains obscured because of difficulties in experimental measurements. Moreover, several ambiguities exist in the literature related to accurate measurements of atmospheric chromium concentration to evaluate its toxicity. We investigated the interaction energies and diffusivity for several VOCs in chromium (III)-containing atmospheric particles using classical molecular dynamics simulations. We analyzed xylene, toluene, ascorbic acid, carbon tetrachloride, styrene, methyl ethyl ketone, naphthalene, and anthracene in Cr(III) solutions, with and without air, to compare their effects on solution chemistry. The interaction energy between Cr(III) and water changed from 48 to 180% for different VOCs, with the highest change with anthracene and the lowest change with naphthalene. The results revealed no direct interactions between Cr(III) particles and the analyzed volatile organic compounds, except ascorbic acid. Interactions of Cr(III) and ascorbic acid differ significantly between the solution phase and the particulate phase. The diffusion of Cr(III) and all the VOCs also were observed in a similar order of magnitude (~ 10−5 cm2/s). The results can further assist in exploring the variation in chromium chemistry and reaction rates in the atmospheric particles in the presence of VOCs.

A new modeling of the dissolution of chromia in Na2O-SiO2 liquids

A model of dissolution of chromia in binary Na2O-SiO2 liquids is presented in this work. Data available in the literature on the acid/base properties and the chemistry of chromium in such liquids were used in order to refine the model. Physical phenomena such as migration of ions and the resulting electric field, the mechanical equilibrium of the liquid and the interface recession caused by the dissolution of chromia are also implemented in the model. Numerical simulation has been compared to experimental measurements of Na2O, SiO2 and Cr2O3 concentration profiles after 1 h of immersion of chromia in a (Na2O)0.25(SiO2)0.75 liquid at 1150 °C. The phenomena having a significant influence on the dissolution rate of chromia are highlighted in the last part of this paper.

Effect of Ligand-Complex Interactions on the Electrodeposition of Mixed Chromium Metal-Carbide-Oxides from Trivalent Chromium Electrolytes

Electrolytic chromium coated steel (ECCS) is packaging steel coated with a thin layer of chromium with an outer layer of chromium oxide. This coating system provides corrosion protection in addition to good adhesion properties due the outer oxide layer. The coating for ECCS is deposited via a hexavalent chromium electrolyte which has been found to be an environmental and health hazard and is therefore subject to restrictions under REACH legislation. A new coating process based on trivalent chromium chemistry has therefore been developed.According to prior studies, coatings comparable in terms of corrosion resistance to those achieved with the traditional Cr(VI)-based process have been produced with this process [1]–[4]. However, at present there is limited understanding of the mechanism and kinetics of electrodeposition from trivalent chromium based electrolytes. This knowledge is key to optimize the process design for industrial plating lines. In the present study, we investigate the effect of the chemistry of the Cr(III) complex – as determined by the relative concentration of the formate complexing agent and temperature – on the electrodeposition of mixed chromium metal-carbide-oxide coatings on low carbon steel in a rotating cylinder electrode (RCE) setup.In this study, low carbon steel samples are electroplated in an aqueous Cr(III) based electrolyte bath with a deposition time of 0.8 s with applied current densities ranging from 20 to 90 A dm-2. The pH of the electrolyte is maintained at 2.6. A rotation speed of 776 RPM is used. SEM and XPS analysis is also performed to determine the effect of the above parameters on the microstructure and composition of the deposits. The amount of Cr deposited is determined using XRF measurements. Relative chromium metal, oxide and carbide contents are determined via XPS analysis. UV-Vis spectra of each of the electrolytes are also measured.Optical surface analysis shows that of the three regimes distinguishable as a function of current density [2], homogeneous, bright coatings are deposited in regime II (Figure 1). The current range over which acceptable coatings are deposited is significantly wider with higher relative ligand concentrations. Lower applied currents are required with lower bath temperatures. Increasing the chromium ion concentration (at a fixed relative ligand concentration) results in increased Cr metal deposition and an associated decrease in the carbide content of the coating. At higher bath temperatures, carbide formation is favoured. Results also indicate a threshold ligand-to-complex ratio at ~2.0 [HCOO-]/[Cr3+] above which a bright, homogeneous coating is deposited. This threshold corresponds with UV/Vis results which indicate the formation of a new complex species in the electrolyte at that ratio (Figure 2). Figure 1

Dinitrogen Activation and Functionalization with Chromium

The study of metal‐dinitrogen complexes has lent valuable insight into the nature of dinitrogen (N2) reduction to ammonia (NH3), known as “nitrogen fixation.” Even so, understanding this difficult transformation continues to be an elusive goal for chemists. The N2chemistry of chromium (Cr) is exceptional for its diversity, rarity, and richness of depth. Hans Karsch opined in 1977 that “Almost all transition metals are known to form complexes with molecular nitrogen, but in the case of some metals ‐ notably chromium ‐ the examples are few and far between.” Although that sentiment hasn't changed much in 42 years, recent advances in the field of Cr‐N2chemistry have begun to illuminate that elusive relationship. Herein we cover the theory and experimental basis for chromium's coordination, activation, reduction, and catalysis of N2to NH3. An emphasis will be placed on work towards understanding catalytic systems for chromium mediated dinitrogen reduction.

Are free radicals actually responsible for enhanced oxidation of contaminants by Cr(VI) in the presence of bisulfite?

Recently, the technology for the remediation of Cr(VI) pollutant via bisulfite has been found to be effective for fast elimination of co-contaminants especially in acidic solution, where free radicals (i.e., sulfate and/or hydroxyl radicals) are proposed to act as dominant oxidants. Here, it was demonstrated that high-valent Cr intermediate played a primary role in the Cr(VI)/bisulfite system through applying methyl phenyl sulfoxide (PMSO) as a probe. PMSO was effectively transformed in the Cr(VI)/bisulfite system with appreciable generation of methyl phenyl sulfone (PMSO2) product, while PMSO was oxidized by free radicals to hydroxylated and/or polymeric products rather than PMSO2. The involvement of high-valent Cr species was further supported by the formation of 18O-labeled PMSO2 in 18O labeling experiments, where the incorporation of 18O from solvent water H218O into PMSO2 was likely resulted from competitive oxygen exchange of Cr-oxo species with water. The relative contribution of high valent Cr species versus free radicals was evaluated based on the yield of PMSO2, which was dependent on the solution chemistry such as [Cr(VI)]:[bisulfite] ratio and dissolved oxygen. This work advances the understanding of chromium chemistry involved in the Cr(VI)/bisulfite system. These findings have important implications on the application of this “waste control by waste” technology for environmental decontamination.

Direct contact membrane distillation of refining waste stream from precious metal recovery: Chemistry of silica and chromium (III) in membrane scaling

Precious metals, such as platinum group metals (PGMs) with distinct catalytic activity, are widely used as active components in various industrial catalysts. It is, therefore, highly desirable to recover these valuable components from the end-of-life products. We explored treatment of refining wastewater from precious metals recovery using direct contact membrane distillation (DCMD). The role of various initial pH of refining wastewater on DCMD performance was assessed. Results suggested that hydrochloride acid (HCl) and high-quality water can be reclaimed from the real refining wastewater by adjusting initial pH. Furthermore, DCMD water flux decline was mainly caused by silica and chromium (III) scaling, which was dependent on initial pH of refining wastewater. Silica scaling was responsible for the decrease of DCMD performance when the initial pH of refining wastewater increased from original 0.03 to 5 and 7. Silica oligomers in the concentrated feed with various initial pH were identified using mass spectra. Dichlorotetraaquochromiun was identified by X-ray photoelectron spectroscopy and ultraviolet and visible absorbance spectra as the main species contributing to the green colour and scaling on the PTFE membrane surface. Our results suggest that DCMD can be used as a promising and feasible solution for resource recovery from acidic refining waste stream.

Highly Continuous Genome Assembly of Eurasian Perch (Perca fluviatilis) Using Linked-Read Sequencing

The Eurasian perch (Perca fluviatilis) is the most common fish of the Percidae family and is widely distributed across Eurasia. Perch is a popular target for professional and recreational fisheries, and a promising freshwater aquaculture species in Europe. However, despite its high ecological, economical and societal importance, the available genomic resources for P. fluviatilis are rather limited. In this work, we report de novo assembly and annotation of the whole genome sequence of perch. The linked-read based technology with 10X Genomics Chromium chemistry and Supernova assembler produced a draft perch genome ∼1.0 Gbp assembly (scaffold N50 = 6.3 Mb; the longest individual scaffold of 29.3 Mb; BUSCO completeness of 88.0%), which included 281.6 Mb of putative repeated sequences. The perch genome assembly presented here, generated from small amount of starting material (0.75 ng) and a single linked-read library, is highly continuous and considerably more complete than the currently available draft of P. fluviatilis genome. A total of 23,397 protein-coding genes were predicted, 23,171 (99%) of which were annotated functionally from either sequence homology or protein signature searches. Linked-read technology enables fast, accurate and cost-effective de novo assembly of large non-model eukaryote genomes. The highly continuous assembly of the Eurasian perch genome presented in this study will be an invaluable resource for a range of genetic, ecological, physiological, ecotoxicological, functional and comparative genomic studies in perch and other fish species of the Percidae family.

Paramagnetic Chromium(II) Complexes and Chromium(IV) Nitrides with Bulky Alkylcyclopentadienyl Ligands

Easily available chromium(II) acetate was used for an entry into chromium chemistry: sodium tetraisopropylcyclopentadienide (Na4Cp), sodium 1,2,4‐tri(tert‐butyl)cyclopentadienide (NaCp′′′), or lithium pentaisopropylcyclopentadienide (Li5Cp) convert the acetate to the acetate‐bridged dimers [4CpCr(µ‐OOCCH3)]2 (1a), [Cp′′′Cr(µ‐OOCCH3)]2 (1b), or [5CpCr(µ‐OOCCH3)]2 (1c) in yields well above 70 %. These acetates could be converted into the halide‐bridged dimers [4CpCr(µ‐Cl)]2 (2a), [4CpCr(µ‐Br)]2 (3a), [Cp′′′Cr(µ‐Br)]2 (3b), or [5CpCr(µ‐Br)]2 (3c), [4CpCr(µ‐I)]2 (4a), or [Cp′′′Cr(µ‐I)]2 (4b) with the corresponding trimethylsilyl halides in nearly quantitative yield. With bis(trimethylsilyl)sulfate the acetate 1a gave crude bis(tetraisopropylcyclopentadienylchromium)sulfate (5a). From the bromide 3b and sodium bis(trimethylsilyl)amide the mononuclear chromium(II) silylamide [Cp′′′CrN(SiMe3)2(THF)] (7b) was obtained as a tetrahydrofuran adduct. Upon reaction with sodium azide, the iodide 4a afforded the tetranuclear chromium(IV) nitride cubane [(4CpCr)3(µ3‐N)4CrOH] (8a) after extraction with heptane containing traces of water. The analogous chromium nitride derivative [(Cp′′′Cr)3(µ3‐N)4CrBr] (9b) was obtained from the bromide 3b and sodium azide.

Cytotoxic, antiglycation and carbonic anhydrase inhibition studies of chromium(III)-aroylhydrazine complexes

In order to further reveal the chemistry and biochemistry of chromium(III) complexes, the present work illuminates the formation of chromium(III) complexes with aroylhydrazine ligands with their physical, chemical and spectral studies. Another significant contribution of this study is the evaluation of the cytotoxic activity, antiglycation property and carbonic anhydrase inhibition study of synthesized chromium(III)-aroylhydrazine complexes. Synthesis and structural investigation of aroylhydrazine ligands (1-7) and their chromium(III) complexes (1a-7a) were carried out by using elemental analysis (C, H, N), physical (conductivity measurements) and spectral (EI-Mass, ESI-Mass, FTIR and UV-Visible) methods. These physical, analytical and spectral data supports that all chromium(III)-aroylhydrazine complexes exhibit an octahedral geometry in which ligand exhibits as a bidentate coordination and two water molecules coordinated at equatorial positions with general formula [Cr(L)2(H2O)2]Cl3. Cytotoxic investigations shows that synthesized chromium(III)-aroylhydrazine complexes were not found to be toxic against normal cells so these compounds were further studied for other biological activities. Moreover, aroylhydrazine ligands and their chromium(III) complexes were examined for their antiglycation activity in which ligands were found inactive whereas chromium(III)-aroylhydrazine complexes showed significant inhibition of the process of protein glycation. Similarly, in carbonic anhydrase inhibition studies all aroylhydrazine ligands were observed inactive while some of chromium(III)-aroylhydrazine complexes showed potential in carbonic anhydrase inhibition.

Raman investigation of an Fe(II)-containing solution influencing chromium chemistry in a thin film on aluminium

A modified trivalent chromium conversion coating formed in an Fe(II)-containing solution has been investigated by Raman spectroscopy. Fe(II)-containing salts revealed a promising additive to solve the presence of chromate constituent in a trivalent chromium conversion coating. However, the chloride solution can induce the chromate generation in such modified coating due to the presence of the dissolved oxygen in the solution.

(Invited) Low Wear Index Chromium Coatings Electroplated from a Functional Reach-Compliant Trivalent Chromium Chemistry

Faraday Technology Inc. is a research, development and engineering firm developing electrochemical innovations based has pulse/pulse reverse electrolytic principles [1]. One of Faraday’s current activities is directed towards the development of an innovative REACH-compliant plating process for deposition of functional chrome coatings, based on replacement of the conventional hexavalent chromium plating chemistry with a non-toxic trivalent chromium chemistry. Hexavalent chromium plating has been used for many decades to produce hard, durable coatings with excellent wear and corrosion resistance properties. However, hexavalent chromium has come under increasing scrutiny due to the toxic nature of the bath, effects on the environment, and workers’ health. Faraday has demonstrated that the chrome coatings prepared using the FARADAYIC(R) Process in a trivalent chromium chemistry have equivalent functional properties to chrome coatings produced with a hexavalent chromium bath. The data demonstrates equivalent or superior: 1) plating rate, 2) Knoop hardness, 3) current efficiency, 4) hydrogen embrittlement behavior, 5) adhesion, 6) corrosion resistance, 7) porosity, 8) thickness, 9) Taber Abrasion, Ball on Flat Reciprocating and Dithering wear resistance, with no hexavalent chromium formation in the bath. This paper will discuss the 2013 EPA Green Chemistry Challenge Award winning work on the development of a REACH-compliant plating process for wear resistant, functional coatings from a single, simple-to-control trivalent-based electrolyte. Specifically, this paper will discuss electrochemical deposition approaches to improve (decrease) the wear index of the chrome coating, to meet the performance from conventional chrome coating processes. Acknowledgements: This study is supported by the US Army (W911NF-11-2-0014) and private sources. [1] E. J. Taylor “Adventures in Pulse/Pulse Reverse Electrolytic Processes: Explorations and Applications in Surface Finishing” J. Applied Surface Finsihing, 3(4), 178-189 (2008).

Chromate Formed in a Trivalent Chromium Conversion Coating on Aluminum

This present study employs aluminum substrates to investigate the formation of Cr(VI) species during a trivalent chromium conversion coating process. The study had a particular focus on understanding the influences of copper in the substrate and O2, ZrF62− and F− in the bath on the formation of Cr(VI) species, which were detected by Raman spectroscopy. Comparison of electropolished aluminum and sputtering-deposited aluminum substrates revealed greatly increased rates of coating growth associated with an enrichment of copper impurity in the electropolished substrate that was revealed by transmission electron microscopy. With respect to chromium chemistry in a developed coating, the presence of dissolved oxygen and long conversion treatment times promoted the formation of Cr(VI) species that are generated by oxidation of Cr(III) species. The Cr(III) species are oxidized by H2O2, which was produced by oxygen reduction reaction. The generation of H2O2 was demonstrated by analysis of the treatment bath using UV spectrophotometry.

Chromium Oxidation-Reduction Chemistry at Soil Horizon Interfaces Defined by Iron and Manganese Oxides

Soil interfaces, including those defined by horizons, are a largely unstudied area in soils research, especially in terms of oxidation–reduction properties and processes. Despite this lack of research, understanding these interfaces is essential to the complete understanding of chromium chemistry in soils. Chromium is most commonly found in its non-toxic trivalent [Cr(III)] and its toxic hexavalent [Cr(VI)] forms under normal environmental conditions. In soils, both Cr(III) and Cr(VI) can undergo oxidation–reduction processes as a result of microbes, organic C, or Fe(III)/Mn(III,IV) (hydr)oxides, and can exist as soluble ions in the soil solution, sorbed ions on the soil surface, soluble organic complexes, or precipitated compounds. Quantitatively determining each of these fractions of Cr can refine our understanding of how soil chemical properties and their differences across horizon interfaces may influence element speciation. This study addresses the question of how oxidation/reduction of Cr changes in mineralogically different soil horizons. The A and B horizons from seven soils encompassing a wide range of soil properties, were used in a series of leaching experiments which were set up as follows. Using a mechanical vacuum extractor, Cr(III) and Cr(VI) solutions were leached through A horizons, B horizons, and A horizons stacked on B horizons; the A horizon leachate was also leached through the B horizons in an attempt to dissect the mechanisms of interfacial oxidation–reduction processes. The Cr speciation results of the study indicate that there are significant interfacial properties affecting reduction of Cr(VI) in soils, but no interfacial effects on Cr(III) oxidation.

Mechanism-based design of labile precursors for chromium(I) chemistry.

Dinitrogen complexes of the type TpR,RCr–N2–CrTpR,R are not the most labile precursors for Cr(I) chemistry, as they are sterically protected from obligatory associative ligand substitution. A mononuclear alkyne complex – TptBu,MeCr(η2-C2(SiMe3)2) – proved to be much more reactive.