Oligo and polyuronic acids interactions with hypervalent chromium

Selective oxidation of galacturonic residues of oligo and polyuronic acids by Cr^VI affords CO₂/HCO₂H, oxidized uronic acid, and Cr^III as final redox products. Kinetic studies show that the redox reaction proceeds through a mechanism combining Cr^VI → Cr^IV → Cr^II and Cr^VI → Cr^IV → Cr^III pathways. The mechanism is supported by the observation of free radicals, CrO₂²⁺ and Cr^V as reaction intermediates. The EPR spectra show that five- and six coordinated oxo-Cr^V intermediates are formed. Penta-coordinated oxo-Cr^V species are present at any [H⁺], whereas hexa-coordinated ones are only observed at pH <1. At low pH Cr^V predominating species are coordinated by carboxylate groups and O^ring (g_iso = 1.9783/5). At pH 7.5, the predominating ones are those coordinated by alcoholate groups of the ligand (g_iso = 1.9800). Polygal can reduce Cr^VI and efficiently trap Cr^III. This behaviour represents an interesting model for the study of biomaterials, which possess a high proportion of polygal, in order to remove chromium from polluted waters.     

Kinetics and mechanism of the oxidation of chromium(III)–dipicolinic acid complex by N-bromosuccinimide

The kinetics of oxidation of the chromium(III)–dipicolinic acid complex [Cr^III(DPA)₂(H₂O)₂]^– by N-bromosuccinimide (NBS) in aqueous solution to Cr^VI have been studied spectrophotometrically over the 20–40 °C range. The reaction is first order with respect to both [NBS] and [Cr^III], and increases with pH over the 5.92–6.93 range. Thermodynamic activation parameters were calculated. It is proposed that electron transfer proceeds through an inner-sphere mechanism via coordination of [NBS] to chromium(III).     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. I. Fixation of chromium on wood

The kinetic behavior of CrO₃ in its reaction with wood has been elucidated. Various reactions take place between CrO₃ and the lignin and cellulose in wood. CrO₃ reacts with cellulose in a two-step reaction: the first step is an adsorption of Cr^VI onto the cellulose to form Cr^VI/cellulose activated complexes. The second step is a Cr^VI → Cr^III reduction taking place on the cellulose surface. The Cr^III formed is only physically adsorbed to the cellulose or very weakly bound as small amounts of Cr^III can be released into the reaction medium. The Cr^VI adsorbed by cellulose appears mainly to be reduced to Cr^III. The reaction of Cr^VI with lignin has been shown to be the composition of the three successive reaction of Cr₂O₇^2?, HCrO₄^?, and CrO₄^2? with the guaiacyl units of lignin. Insoluble and stable Cr^VI/lignin complexes in which chromium maintains its hexavalent oxidation state are formed. Rate constants and energies of activation for all the reactions have been determined. The fixation of CrO₃-derived compounds on wood has been explained as the combination of the various reactions investigated. The results indicate that 60% of Cr is fixed irreversibly to the lignin of wood as Cr^VI and 40% is weakly bound, probably just precipitated, on the cellulose surface as Cr^III of which small amounts can be released in a water medium. The complex Cr^VI and Cr^III species forming complexes with the guaiacyl units have been identified.     

Removal of chromate from aqueous solution by reduction with nanoscale Fe–Al layered double hydroxide

Nanoscale layered double hydroxides of Fe^II and Al^III (Fe–Al LDH) have been applied for removal of chromate (Cr^VI) from aqueous solution. Given the reaction stoichiometry, Cr^VI was completely reduced to Cr^III and coprecipitated with Fe^III and Al^III oxyhydroxides. The extent of Cr^VI removal decreased with increasing initial pH and decreasing molar ratio of Cr^VI/structural Fe^II in the LDH. The chromate reduction rate at different initial concentrations of Cr^VI was well described by the pseudo-second-order model with reaction rate constant ranging from 197.4 to 13.53 (mmol min)^?1. Initial pH and substitution of various amounts of Fe^III in the LDH structure had little effect on the reaction rate. Backtransformation of Cr^III to Cr^VI by birnessite Mn oxide (δ-MnO₂) after 40 days of reaction was less than 1% of the initial Cr (as Cr^III solid), indicating high stability of the final reaction products and high efficiency of nanoscale Fe–Al LDHs for removal of chromate from aqueous solution.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. III. Fixation of a Cr/As system on wood

The kinetic behavior of a Cr/As system in its reaction with wood is described, as are its reactions with lignin and carbohydrates of wood for Cr^VI alone. Chromium^III arsenate complexes with the guaiacyl units of lignin, as well as the usual chromic acid/guaiacyl unit complexes already characterized, was formed. The interference of As concentration on the various zones of fixation of chromium are identified. Complexes between guaiacol and chromium arsenates indicate that in this Cr/As system Cr^III can also be linked to the guaiacyl units of lignin when in the form of CrAsO₄·6H₂O contrary to what obtained with systems not containing As. An initial increase in the rate of reduction Cr^VI → Cr^III by the carbohydrates fraction in wood is ascribed to the catalytic effect of the arsenic. The percentages of chromium fixed onto lignin and cellulose of wood compare well with experimental values.     

Kinetics and mechanism of the oxidation of nitrilotris(methylenephosphonato)chromium(III) by periodate

The kinetics of oxidation of nitrilotris(methylenephosphonato)chromium(III), Cr^IIINTMP, by periodate to yield Cr^VI have been studied spectrophotometrically over the 5.80–6.85 pH range at 22–33 °C. The reaction rate, which is first-order with respect to [Cr^IIINTMP] and [IO^– ₄] and inversely dependent on [H⁺], obeys the rate law:-d[Cr^IIINTMP/dt=kKK_h[IO^- ₄] [Cr^III]_T/K_h+ [H⁺] +KK_h[IO^- ₄] The values of the intramolecular electron transfer, k, and the formation constant of the intermediate complex, K, were determined at various temperatures. The hydrolysis constant for Cr^IIINTMP, K _h , was determined spectrophotometrically and is in agreement with the value estimated from the kinetic data. The activation parameters were calculated from the temperature dependence of the specific rate constants. A mechanism is proposed in which the hydroxo complex, [CrHNTMP(OH)]^3–, is the reactive species. The results support a mechanism where intramolecular electron transfer is the rate-determining step.     

Trace metal-ion catalysis: kinetics and mechanism of oxidation of l-ascorbic acid by chromium(VI) in phosphate buffers

Summary The kinetics of oxidation of l-ascorbic acid (H₂A) by Cr^VI with and without added Cu^II conforms to the stoichiometry represented by the equation: 2Cr^VI + 3H₂A 2Cr^III + 3A + 6H⁺ where A is dehydroascorbic acid. The mode of the electron transfer from H₂A to Cr^VI is suggested to involve oxidative decomposition of an intermediate complex. Catalysis by Cu^II is indicated via complexation of the catalyst and substrate. The inhibitory effects of Cl^–, NO _f3 ^p– and SO _f4 ^p2– ions indirectly support complexation between Cr^VI and H₂A.     

Sol–Gel Processed Functional Nanosized TiO2 and SiO2-Based Films for Photocatalysts and Other Applications

Titania and silica-based porous coatings have been produced via sol–gel route in the presence of pore-creating agents PEG and viscous solvent -terpineol, or template agents CTAB, as well as triblock copolimer Pluronic P123. Porous titania films were characterised by HR TEM, UV-Vis, XRD, ellipsometry, ARS methods. The dispersion of the refractive index, the porosity (32–39%) and the thickness of the samples were estimated by integrating sphere transmission and reflection spectra with 3D angular resolved light scattering. The catalytic activity of mesoporous TiO₂ and TiO₂/ZrO₂ as well as SiO₂-benzophenone films in the process of Cr^VI to Cr^III and Ag⁺ to Ag⁰ photoreduction have been studied.     

Carcinogenic Chromium(VI) Compounds Formed by Intracellular Oxidation of Chromium(III) Dietary Supplements by Adipocytes

Chromium(III) nutritional supplements are widely consumed for their purported antidiabetic activities. X‐ray fluorescence microscopy (XFM) and X‐ray absorption near‐edge structure (XANES) studies have now shown that non‐toxic doses of [Cr₃O(OCOEt)₆(OH₂)₃]⁺ ( A ), a prospective antidiabetic drug that undergoes similar H₂O₂ induced oxidation reactions in the blood as other Cr supplements, was also oxidized to carcinogenic Cr^VI and Cr^V in living cells. Single adipocytes treated with A had approximately 1 μm large Cr hotspots containing Cr^III, Cr^V, and Cr^VI (primarily Cr^VI thiolates) species. These results strongly support the hypothesis that the antidiabetic activity of Cr^III and the carcinogenicity of Cr^VI compounds arise from similar mechanisms involving highly reactive Cr^VI and Cr^V intermediates, and highlight concerns over the safety of Cr^III nutritional supplements.     

Kinetics of the oxidation of diaqua(nitrilotriacetato)-chromium(III) by periodate in aqueous solutions

Summary The kinetics of oxidation of [Cr^III(nta)(H₂O)₂] (nta is nitrilotriacetate) by periodate obey the rate law d[Cr^VI]/dt=(k₂[IO ₄ ^– ]+k₃[IO ₄ ^– ]²)[Cr^III(nta)(H₂O)₂] under fixed conditions. The activation parameters are reported and we propose that electron-transfer proceeds via an inner-sphere mechanism.     

Kinetics and Mechanism of Oxidation of Chromium(III)‐guanosine 5‐Monophosphate Complex by Periodate

The kinetics of oxidation of the chromium(III)‐guanosine 5‐monophosphate complex, [Cr^III(L)(H₂O)₄]³⁺(L = guanosine 5‐monophosphate) by periodate in aqueous solution to Cr^VI have been studied spectrophotometrically over the 25–45 °C range. The reaction is first order with respect to both [IO₄^?] and [Cr^III], and increases with pH over the 2.38–3.68 range. Thermodynamic activation parameters have been calculated. It is proposed that electron transfer proceeds through an inner‐sphere mechanism via coordination of IO₄^? to chromium(III).     

Performance of a photocatalytic hybrid system coupled with a stainless steel membrane for removal of humic acid

The objective of this study was to evaluate the performance of a photocatalysis/H₂O₂/metal membrane hybrid system in the degradation of humic acid. A metal membrane of nominal pore size 0.5 μm was used in the experiment for separation of TiO₂ particles. Hydrogen peroxide was tested as an oxidant. The efficiency of removal of COD_Cr and color increased rapidly for initial hydrogen peroxide concentrations up to 50 mg L⁻¹. The efficiency of removal of COD_Cr and color by 50 mg L⁻¹ initial hydrogen peroxide concentration was approximately 95 and 98%, respectively. However, addition of hydrogen peroxide over 50 mg L⁻¹ inhibited the efficiency of the system. Addition of hydrogen peroxide to a UV/TiO₂ system enhanced efficiency of removal of COD_Cr and color compared with no addition of hydrogen peroxide. This may be ascribed to capture electrons ejected from TiO₂ and to the production of OH radicals. Application of the metal membrane in the UV/TiO₂/H₂O₂ system enhanced the efficiency of removal of COD_Cr and color because of adsorption by the metal membrane surface and the production of OH radicals. By application of a metal membrane with a nominal pore size of 0.5 μm, TiO₂ particles were effectively separated from the treated water by metal membrane rejection. The photocatalytic metal membrane had much less resistance than the humic acid, TiO₂, and humic acid/TiO₂ because of the degradation of humic acid by the photocatalytic reaction.     

Kinetics and mechanism of the chromic oxidation of 3-O-methyl-d-glucopyranose

The oxidation of 3-O-methyl-d-glucopyranose (Glc3Me) by Cr^VI in acid medium yields Cr^III, formic acid and 2-O-methyl-d-arabinose as final products when a 50-times or higher excess of Glc3Me over Cr^VI is used. The redox reaction takes place through the combination of Cr^VI → Cr^IV → Cr^II and Cr^VI → Cr^IV → Cr^III pathways. Intermediacy of free radicals and Cr^II in the reaction was demonstrated by the observation of induced polymerization of acrylamide and detection of CrO₂²⁺ formed by reaction of Cr^II with O₂. Intermediate oxo-Cr^V–Glc3Me species were detected by EPR spectroscopy. In 0.3–0.5 mol/L HClO₄, intermediate Cr^V rapidly decompose to the reaction products, while, at pH 5.5–7.5, where the redox processes are very slow, five-coordinate Cr^V bis-chelates of the pyranose and furanose forms of Glc3Me remain more than 15 h in solution. The C1–C2 bond cleavage of Glc3Me upon reaction with Cr^VI distinguishes this derivative from glucose, which is oxidized to gluconic acid.     

Chromium(IV) Stabilisation in New Ceramic Matrices by Coprecipitation Method: Application as Ceramic Pigments

In this paper the stabilization of Cr⁴⁺ in new ceramic matrices as willemite Zn₂SiO₄ and galium‐gadolinium garnet GGG (Gd₃Ga₅O₁₂) is studied and compared with the chromium‐sphene ceramic pigment using the traditional solid reaction and an unconventional coprecipitation method.The chromate amount etched from the powders using Cr^III and Cr^VI precursors on ceramic and coprecipitation routes are analysed in order to evaluate the environmental impact on the waste water. Cr‐willemite and codoped Ca, Cr‐GGG produces solid solutions of Cr⁴⁺ ion occupying tetrahedral and dodecahedral sites, respectively, while Cr⁴⁺ into sphene lattice occupies octahedral sites. Cr‐willemite are stable when used as ceramic pigment in habitual tile glazes but Cr‐GGG unstabilses and produces green colour associated to Cr³⁺ solved in the glaze. Coprecipitate powders are more reactive than ceramic powders but in Cr‐GGG stabilizes the metastable Ca₃Cr₂O₈ oxide. Chromate amount in washing hot water are not dependent of the chromium precursor but depends of the allowed reactivity level and the reaction media.     

An IR study of NO adsorption on a CrOx/ZrO2 catalyst

The adsorption of NO at room temperature (RT) on a CrO_x/ZrO₂ catalyst has been investigated by IR spectroscopy and compared with data previously obtained by ESR spectroscopy and chemical titrations. Prior to NO adsorption, the sample was heated in O₂ at 923 K (average oxidation number of chromium, ñ = 5.5) and reduced in CO from 373 to 623 K (ñ varying from 5.5 to 2.5). The adsorption of NO on samples heated in O₂ yields mainly dinitrosyls of Cr^III, arising from the reduction of Cr^V. The adsorption of NO on samples reduced at 373 K yields mainly dinitrosyls of Cr^III, in addition to N₂O, nitrites and nitrates. The admission of NO on fully reduced samples (ñ = 2.5) yields dinitrosyls of Cr^III and mono- and dinitrosyls of Cr^II, in addition to N₂O, nitrites and nitrates. Upon evacuation of the latter sample at 423 K, the adsorbed NO oxidized most of the Cr^II to Cr^VI (ñ = 4.2) and, in fact, the nitrosyls of Cr^III are the only species detected by IR on NO readmission after evacuation treatment, in addition to N₂O_ads, nitrites and nitrates. Blank experiments on pure ZrO₂ show that the dismutation of NO leading to N₂O and nitrites (or nitrates) takes place on sites of the ZrO₂ support. When a fully reduced sample is further reacted with water at 853 K, mainly dinitrosyls and mononitrosyls of Cr^III are detected upon exposure to NO, since reaction with water selectively oxidized Cr^II to chromia-like species.     

The reactivity of cis-platin. Spectroscopic properties of products isolated from the [cis-Pt(NH3)2Cl2–quercetin] and [cis-Pt(NH3)2Cl2–CrVI–quercetin] systems

New coordination compounds: [Pt^IIQ] and [Pt^IV–Cr^IIIQ₂], where Q = quercetin, were isolated from the [Pt(NH₃)₂Cl₂–quercetin] and [Pt(NH₃)₂Cl₂–Cr^VI–quercetin] systems, respectively. Structures are proposed on the basis of i.r., n.m.r. and deconvoluted electronic spectra.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. IV. Fixation of CCA to wood

The kinetic behavior of copper–chromium–arsenic (CCA) wood preservatives in their reaction with wood, lignin, cellulose, and their respective simple model compounds, guaiacol and D(+)-glucose, has been elucidated. Copper chromates^VI/lignin guaiacyl units and chromium^III arsenates/lignin guaiacyl units, which are stable and insoluble complexes formed during the reaction, have been characterized. No copper arsenates were formed during the reactions (which were carried out using a CCA of type C). The sequence, nature, and combined effect of the reactions of CCA with wood, lignin, and cellulose are similar to that already described for the “CrO₃ alone,”¹ Cu/Cr², and Cr/As³ systems. Rate constants, energies of activation, and Arrhenius equations have been obtained and reported. The distribution of Cr^VI, Cr^III, As^v, and Cu²⁺ in CCA-C-treated wood has been calculated. The results obtained have allowed meaningful comparisons of several commercial CCA formulations.     

Kinetic studies on the oxidation of [N-phenylethylethylenediaminetriacetate]chromium(III) by periodate

Summary Kinetic studies of the oxidation of [Cr^IIIZ(H₂O)](Z=N-phenylethylethylenediaminetriacetate) by periodate ion, to produce chromium(VI), were carried out in aqueous solutions. The reaction is first order with respect to both total chromium(III) and total periodate, and the rate is inversely dependent upon H⁺ in the 5.43–7.02 pH range. The reaction may follow a two-step inner-sphere electron transfer mechanism. The activated parameters are reported. Steric effects of the phenyl ring account for the smaller electron-transfer rate constants for [Cr^IIIZ(H₂O)] compared to [Cr^III(TOH)(H₂O)], (TOH=N-(2-hydroxyethyl)ethylenediaminetriacetate).     

Colloidal Surface Active Maghemite Nanoparticles for Biologically Safe CrVI Remediation: from Core‐Shell Nanostructures to Pilot Plant Development

The present study is aimed at the exploration of achievable improvements for Cr^VI ex situ and in situ water remediation by using novel naked colloidal maghemite (γ‐Fe₂O₃) nanoparticles (surface active maghemite nanoparticles, SAMNs). The reliability of SAMNs for Cr^VI binding and removal was demonstrated, and SAMN@Cr^VI complex was characterized, as well as the covalent nature of the absorption was unequivocally proved. SAMNs were structurally and magnetically well conserved after Cr^VI binding. Thus, in consideration of their affinity for Cr^VI, SAMNs were exploited in a biological model system, mimicking a real in situ application. The assay evidenced a progressive reduction of revertant colonies of Salmonella typhimurium TA100 strain, as maghemite nanoparticles concentration increased, till the complete suppression of Cr^VI mutagen effect. Finally, an automatic modular pilot system for continuous magnetic removal and recovery of Cr^VI from water is proposed. SAMNs, thanks to their colloidal, binding, and catalytic properties, represent a promising tool as a reliable nanomaterial for water remediation by Cr^VI.