Kinetics and mechanism of chromium electrodeposition from formate and oxalate solutions of Cr(III) compounds

Kinetics of multistep reaction of Cr(III) ions discharge to metal was studied on a stationary electrode and on a rotating disk electrode from the solutions containing formic acid or oxalic acid. The electroreduction of Cr(III) complex ions in aqueous solutions is shown to proceed via the formation of relatively stable intermediates—Cr(II) compounds which are partially removed into bulk solution. The effect of pH, organic ligand concentration and disk rotation velocity on the partial current density of chromium electroplating was demonstrated. The kinetic equations of the studied process were derived and compared with the experimental data. Kinetic parameters for the discharge of Cr(II) ions were calculated. The mechanism of chromium electrodeposition reaction was proposed. The electrodeposition of chromium from formate bath is suggested to proceed with the participation of hydroxocomplexes of bivalent chromium. The oxalate complexes of bivalent chromium directly discharge in the electrolytes containing oxalic acid. The partial polarization curves of chromium electrodeposition exhibit a current peak which may be caused by blocking the electrode surface with poorly soluble Cr(III) hydroxide.     

Adsorption of trivalent chromium from aqueous solutions onto activated carbon

The adsorption of chromium (III) onto activated carbon was investigated as a possible alternative method for its removal from aqueous solutions. The adsorption data were obtained in a batch adsorber and fitted the Langmuir adsorption isotherm well. The effect of pH on the adsorption isotherm was investigated at pH values of 2, 4, 5 and 6. It was found that at pH values below 2 the Cr(III) was not adsorbed and at pH values above 6.4 the Cr(III) was precipitated as Cr(OH)₃. Maximum adsorption occurred at pH 5. The pH plays a very important role in the adsorption of Cr(III) since Cr(III) can form different complexes in aqueous solutions. The adsorption capacity was increased by about 20% as the temperature was raised from 25 to 40°C. It was concluded that Cr(III) is adsorbed to an appreciable extent on activated carbon and that the adsorption is highly dependent upon pH.     

Effect of Cr(III) solution chemistry on electrodeposition of chromium

This work provides a new insight into the interaction of urea with formate during the chromium electrodeposition from a sulphate-based Cr(III) solution. The influence of solution chemistry on chromium electrodeposition in a Cr(III) bath containing sodium formate and urea as complexing agents was studied by FT-IR, XPS and AFM. The results show that good quality Cr coatings may be obtained only in those cases when the secondary ligand with the carbamidic group predominates over urea in the electrolyte. This suggests that electrodeposition of good quality chromium deposit is possible due to the formation of active chromium–carbamid complexes [Cr(carbamid)_n(H₂O)_6-n ]³⁺. These complexes delay the formation of the stable oligomeric species, and thus provide a prolonged working lifetime in the Cr(III) formate-urea electrolyte.     

Removal of chromium(III) from tannery effluents,using a system of packed columns of zeolite and activated carbon

The removal of chromium(III) in packed columns of zeolite and activated carbon has been studied. The process of Cr(III) exchange in 13X zeolite was optimized using mass transference parameters. In addition, the effects of pH, the presence of interfering ions and the anion associated with the chromium in the solution were studied. It was found that particle diameter controls the Cr(III) exchange in the zeolite, indicating that particle diffusion predominantly controls the process of Cr(III) exchange in 13X zeolite. A mixed system of zeolite and activated carbon columns increased the efficiency of chromium removal from diluted wastewater. This effect occurred due to the reduction of the organic matter (chemical oxygen demand), adsorption of chromium, and interfering ions on the activated carbon column. The activated carbon + zeolite column system emerges as an alternative method in Cr(III) removal from tannery effluents. Copyright © 2005 Society of Chemical Industry     

A systematic study of chromium solubility in the presence of organic matter: consequences for the treatment of chromium‐containing wastewater

The treatment objective for wastewater containing Cr(VI) and/or Cr(III) is the reduction of Cr(VI) to the less toxic Cr(III) and the sequestering of Cr(III) from the water streams. The presence of organic matter in the wastewater and in a bioreactor environment may alter the solubility behavior of Cr(III), resulting in poor sequestering and Cr(III) removal from the soluble phase. Systematic experimental solubility results for Cr(III) were obtained in the presence of organic molecules often encountered in industrial effluents containing chromium as well as in biological treatment systems. Chromium solubility curves were determined experimentally from synthetic solutions of Cr(III) in contact with organic acids (acetic, oxalic, citric and ascorbic), amino acids (alanine and aspartic), proteins (albumin and casein) and synthetic nutrient media (yeast extract, nutrient broth and peptones). Experimental data sets were generated for each one of the above organics at three different initial organics concentrations in order to quantify the role of the relative abundance of organic matter in the solubility behavior of Cr(III). A progressive increase of trivalent chromium solubility was systematically observed when the relative abundance of organic ligands to Cr(III) exceeds 10, indicating the existence of a threshold above which the solubility of Cr(III) increases significantly. The hexacoordinate and octahedral character of Cr(III) complexes may explain the need for ligand excess also pointing to a treatment strategy suggesting the relative abundance of organic moieties should be kept below the above threshold. Copyright © 2007 Society of Chemical Industry     

Properties and preparation of amorphous chromium carbide electroplates

The electrochemical and corrosion behavior of chromium electroplates formed in sulfuric acid solutions of Cr(III) in the presence of oxalates was studied by measuring steady-state polarization curves in 0.5 M H₂SO₄ solution. These electroplates demonstrate no active dissolution region and their open-circuit potentials are located in the passivity region, i.e. shifted substantially in the positive direction as compared with those of metallurgical chromium and electroplates from standard chrome-plating baths. The results of X-ray photoelectron spectroscopy (XPS) studies evidence that the passive metal surface layer several nanometers thick consists of chromium oxides with incorporated carbides formed during electroplating. It is supposed that the peculiarities of the corrosion and electrochemical behavior of the deposits under study can be attributed to the presence in them of chromium carbides, which operate as cathodic agents. At the same time, the formation of these carbide compounds during the cathodic deposition of chromium electroplates from sulfuric acid Cr(III) solutions in the presence of sodium oxalate is a result of electrocatalytic activity of metal chromium. The latter assumption is confirmed by XPS analysis of surface layers formed during the exposure of chromium to sulfuric acid solutions containing organic substances.     

ABS塑料三价黑铬电镀工艺与故障处理

介绍了ABS塑料电镀三价黑铬的工艺流程,主要包括除油,亲水,粗化,中和,预浸,催化,加速,化学镀镍,活化,镀铜,微蚀,镀光亮镍,镀三价黑铬,钝化和烘干.给出了三价黑铬镀液的配方,阐述了各组分的作用及工艺奈件对镀层的影响,介绍了常见故障的处理方法.     

Chrome dyeing using anionic bivalent chromium complexes as mordants

Chrome dyeings are normally produced by first applying an acid dye containing ligand groups and then afterchroming with hexavalent chromium derivatives such as dichromate salts. The latter can damage wool fibres and also are toxic components in dyehouse effluents, so afterchroming with mixtures of Cr(III) salts and organic acids has been studied. It has been shown that the anionic complexes that form from trivalent chromium and the organic acids are fibre substantive at pH 2–4 and have the correct stability profile for exchanging ligands to bind the chromium with ligand groups in the dye. Dyeings produced generally show colour yields and fastness properties similar to the normal Cr(VI) dyeing process; the exception are these chromophores which undergo oxidative change in the dichromate process.     

The electrodeposition of chromium from chromium(III) solutions — a study using microelectrodes

Potential sweep and pulse experiments at a carbon microdisc electrode (radius 4 m) have been used to investigate chromium plating from aqueous solutions of chromic ions containing sulphate or thiocyanate ions and buffered at pH 3.3. While chromium deposition occurs in many media, it is shown that the plating reaction is always in competition with other cathodic reactions including hydrogen evolution. This leads to loss in current effciency and, in some circumstances, to precipitation of chromic hydroxide on the cathode and, hence, contamination of the metal. In addition, it is demonstrated that the chromium(III) exists in solution as mixtures of complexes, not all of which are electroactive within the accessible potential range. Hence the investigation confirms that the quality of electroplates will vary strongly with the choice of medium. A commercial chromium(III) plating bath, Envirochrome, has also been studied and it is concluded that its properties are superior to simple sulphate or thiocyanate solutions.     

Remediation of chromium(VI) at polypyrrole-coated titanium

The application of conducting polypyrrole (PPy) and polyaniline (PAni) coated substrates in remediation of chromium, Cr(VI), is an area of considerable interest. Here, we discuss the implementation of PPy-coated titanium as a new material for the reduction of Cr(VI) to the less toxic trivalent state, Cr(III). An alkaline-peroxide based etching process was used to ensure the adhesion of the PPy coatings to the underlying titanium. The PPy films showed excellent resistance to acidic Cr(VI) solutions and remained adherent after continuous exposure to the solutions. In order to optimise the remediation process a number of experimental parameters were investigated, including the thickness of the PPy coating, the reduction potential used in pre-treatment of the PPy and the degree of solution agitation. The durability of the materials on exposure to the Cr(VI) test solutions made them suitable for repeated remediation experiments. Following several test-runs, the cleanup efficiency of the material was found to decrease slightly, however, increasing the exposure/experiment time resulted in significantly improved cleanup ability.     

Nonisothermal decomposition kinetics of chromium–polyacrylate complexes prepared from aged Cr(III) solutions

Thermogravimetric studies of poly(acrylic acid) (PAA) and its chromium–polyacrylate complexes prepared from Cr(III) solutions aged for different times were undertaken. Six kinetic methods were used to investigate the thermal decomposition behavior of these materials. The applied methods were the Coats–Redfern, Horowitz–Metzger, MacCallum–Tanner, van Krevelen, Madhusudanan–Krishnan–Ninan, and Criado methods. The activation energy values obtained with the Coats–Redfern, Horowitz–Metzger, and MacCallum–Tanner methods were in good agreement with one another, and those obtained with the van Krevelen and Madhusudanan–Krishnan–Ninan methods were found to be 10–20 kJ/mol larger. The apparent activation energies increased for the complexes prepared from Cr(III) solutions aged for longer times. An analysis of the experimental results suggested that the actual decomposition mechanisms of PAA and the metal–polymer complexes were a D_n deceleration type. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Metal chelates with some cellulose derivatives. Part I. Preparation and characterization of chromium (III)–carboxymethyl cellulose complexes

A systematic spectrophotometric study on the complexation of chromium (III) ion with sodium carboxymethyl cellulose (CMC) was carried out. The effects of the degree of substitution (DS) of the polymer, the concentrations of Cr(III) and CMC solutions, the pH and temperature on the complex formation were studied in the aqueous state. CMC complexes with Cr(III) were characterized by elemental analysis, magnetic moment and spectral (Vis and IR) data. The results showed that CMC (L) chelated to the metal ion according to the formula CrL₂ · 2H₂O. The ligand field parameters, namely D_q, B and β were calculated; the β-values indicate strong covalency in the ligand σ bond. The IR spectra revealed that the chelating sites of CMC are not only the carboxymethyl groups, via the ether- and the carboxyl-oxygen atoms, but also the secondary hydroxyl groups.     

Interaction of lead and chromium with chitin and chitosan

The interaction of the natural marine polymer chitin and its deacetylated derivative chitosan with lead and chromium has been investigated. The uptake of lead and chromium was determined from changes in concentration as measured by atomic absorption spectroscopy. A significant uptake of Pb(II) on both chitosan and chitin was observed. However, the uptake of Pb(II) on chitin was approximately 21% of that on chitosan. The number of Natoms in chitin and chitosan and per number of Pb(II) ions sorbed was 115 and 29, respectively. The number density of flakes observed in the scanning electron microscope and characterized by an intense Pb signal in energy dispersive analysis of x rays (EDAX) was greater on the surface of chitosan [containing 1.7 × 10^?4 mole Pb(II)/g chitosan] than chitin [containing 3.5 × 10^?5 mole Pb(II)/g chitin] after equilibration with Pb(II) solution. The bonding state of lead on chitosan as determined by electron spectroscopy for chemical analysis (ESCA) is similar to the bonding of lead in PbO based on the Pb 4f_7/2 binding energy. A significant shift in the O 1s binding energy from 532.2 to 531.4 eV was observed for chitosen after equilibration with Pb(II) solution. The caculated values of the N/Pb ratio from ESCA spectra were 0.5 and 11, for chitosan and chitin, respectively. A significant uptake of Cr(III) on chitosan was observed and a significant increase in the pH of solutions of Cr(III) on equilibration with chitosan occurred. A high number density of nodules characterized by an intense Cr signal in EDAX was observed in chitosan [containing 2.5 mole Cr(III)/g chitosan] after equilibration with Cr(III) solution. The calculated values of the N/Cr ratio from ESCA spectra was 18 for chitosan.     

Brown ceramic pigments based on chromium(III)-doped titanite obtained by spray pyrolysis

Cr-doped titanite (CaTiSiO₅) pigments were synthesized through spray pyrolysis of aerosols generated from aqueous solutions containing colloidal silica, calcium chloride, titanium(IV) oxychloride and chromium(III) nitrate. This process yielded amorphous powders with spherical morphology and broad size distribution (<10 μm) after thermal decomposition at 600 °C. The titanite phase was obtained by further calcination at 800 °C without any addition of flux agents. The brown color of the pigments can be attributed mainly to the existence of Cr(IV) ions occupying both, octahedral positions of Ti(IV) and tetrahedral position of Si(IV), together with a small amount of Cr(III) present as Cr₂O₃. The optimum pigment obtained by this method corresponded to a Cr/titanite mole ratio of 0.04.     

Chemical composition and structural transformations of amorphous chromium coatings electrodeposited from Cr(III) electrolytes

Amorphous chromium coatings were electrodeposited from Cr(III)-based solutions containing organic (HCOONa) or phosphorus-containing (NaH₂PO₂) additives. Their structure was studied by a combination of X-ray diffraction (XRD), valence-to-core X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) at the Cr K-edge. Metalloid atoms (C or P) incorporated in electroplates structure are chemically bonded to chromium (i.e. are located in the first coordination shell). Upon annealing at elevated temperatures in vacuum, these amorphous coatings crystallize into a mixture of phases containing metallic chromium and chromium carbides or chromium phosphides. Quantitative analysis of valence-to-core XES data demonstrates that the average local structure of chromium in the amorphous coatings does not change significantly during crystallization.     

High-temperature electroplating of chromium from molten FLINAK

Electroplating of chromium has been investigated in molten FLINAK (46.5 LiF-11.5 NaF-42.0 KF; mol %) over a temperature range of 600 to 1000°C. When Cr(III) is the electroactive species employed, a considerable amount of Cr(II) must be generated before chromium metal is deposited. Various parameters were tested to establish the optimum conditions for obtaining a smooth and thick chromium plating. Bath temperature was found to be a most important parameter; at temperatures above 870°C a smooth plate can be obtained, while at lower temperatures the deposits consist of extensive dendrites embedded in the salt which is the first reduction product of Cr(III).     

Removal process and mechanism of hexavalent chromium by adsorption-coupled reduction with marine-derived Aspergillus niger mycelial pellets

In order to remove hexavalent chromium(Cr(Ⅵ)) from solutions efficiently, the mycelial pellets with a marine-derived fungus Aspergillus niger as a biosorbent were prepared. The effects of removal process parameters such as solution pH, initial Cr(Ⅵ) concentration and biomass concentration on Cr(Ⅵ) removal process were investigated. The results showed that Cr(Ⅵ) removal rate up to 100% could be achieved under optimized conditions, which indicated the excellent Cr(Ⅵ) removal performance of the Asp...     

高温燃烧过程中Cr的氧化机理及控制方法

Cr作为一种有害元素广泛地存在于化石燃料、生活垃圾和工业固体废物中。其中,化石燃料、相当大部分的生活垃圾以及一部分的工业固体废物都通过燃烧过程来回收能量或者进行减容和无害化处理。因此,通过燃烧过程人类向自然界中排放了大量的Cr。最重要的是,热处置过程中的高温氧化环境容易使低迁移性和毒性的Cr（Ⅲ）氧化为高迁移性和毒性的Cr（Ⅵ）。所以,热处置过程中Cr的反应、行为以及产物中Cr的存在形态得到了越来越多的研究。本文综述了燃料中Cr的存在形态、燃烧过程中Cr的氧化机理、燃烧过程中Cr氧化的控制方法以及燃烧产物中Cr的固化方法四方面内容,为减少燃烧过程中的Cr对环境的危害提供理论基础。     

Electrolytic removal of hexavalent chromium from aqueous solutions

The removal of hexavalent chromium from a synthetic solution and from an industrial sample by electrolysis using stainless steel plate and titanium mesh cathodes is studied. It is found that the process proceeds in two steps: (1) electroreduction of hexavalent chromium to trivalent chromium in highly acidic medium and (2) electrochemical precipitation of trivalent chromium in an alkaline medium. A comparative study of electroreduction, electrochemical reduction and chemical reduction of hexavalent chromium is carried out and the electroreduction process is shown to be a clean technique. It is concluded that, in the absence of addition of any chemical as reducing agent, the Cr(III) solution formed on the reduction of Cr(VI) can be recycled in other industrial processes. Copyright © 2003 Society of Chemical Industry     

Preparation and characterization of water‐soluble polymers and their utilization in chromium sorption

In this article, we study the sorption of chromium from aqueous solutions using water‐soluble polymers (WSPs): poly[2‐(acryloyloxy) ethyl] trimethylammonium chloride, P(ClAETA); poly[2‐(methacryloyloxy) ethyl] trimethylammonium methyl sulfate, P(SAETA); and poly(sodium 4‐styrenesulfonate), P(NaSS). These WSPs were obtained by radical polymerization and purified by fractionation through ultrafiltration membranes with different molar mass cut‐offs (30 and 100 kDa). The characterization was carried out by thermogravimetric analysis (TGA), FTIR, and ¹H‐NMR spectroscopies and scanning electron microscopy/energy dispersive X‐ray spectroscopy. The chromium retention properties of the polymers were determined in terms of pH, optimal polymer concentration, and the effect of interfering ions. The results show yields above 80% for all of the synthesized WSPs. Characterization by spectroscopy confirmed the chemical structure of the polymers. TGA shows thermal decomposition temperatures of 264 and 324 °C for P(ClAETA) and P(SAETA), respectively. In the case of P(NaSS), the first thermal decomposition begins at approximately 450 °C. Maximum retention of Cr(VI) (100%) by the polymers P(ClAETA) and P(SAETA) was achieved at pH 9, and the maximum retention of Cr(III) (100%) was achieved by P(NaSS) at pH 3. The optimal polymer:Cr molar ratio obtained was 20:1. The retention of chromium(VI) was decreased by the presence of interfering ions, and the hydrodynamic flux was almost constant during ultrafiltration. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45355.