Trivalent Chromium Research Articles

Trivalent Chromium Electrolytes with Organic and Inorganic Ligands: Use of Pulsed Currents

Hexavalent chromium salts are targeted by health authorities and in particular by the European regulation REACH because of their high toxicity for humans and the environment. Trivalent chromium salts are a promising alternative to hexavalent ones in the perspective of hard chromium coatings and have been extensively studied [1]. However, trivalent chromium ions bind with water molecules to form complexes resistant to electrochemical reduction, requiring ligands addition to evolve toward more favorable chemical equilibrium. This addition leads to a decrease of the coating properties, due to the incorporation of impurities such as carbon and oxygen. If carbon can be removed by the use of inorganic ligand, the presence of oxygen is more difficult to avoid [2], mostly due to hydroxide species incorporation induced by the pH increase at the interface. This phenomenon is the direct consequence of the important current part used to reduce protons during the plating process. It exists different ways to limit the pH elevation [3] such as the temperature, the electrolytes pH modification or the addition of specific chemical compounds, but an attractive way consists in current modulation i.e. using pulsed currents. The relaxation of the diffusion layer is known to reduce the competition between the diffusion of electrochemical species from the bulk and the proton discharge, which will limit the pH increase and restore the chromium concentration near the surface close to the initial one.Parameters of the pulse sequences tested in the present work were chosen from pH measurements [4] undertaken at the surface vicinity (less than 100 µm), and from the behavior of copper solutions working at high potentials where the competition between copper reduction and hydrogen takes place. Potentials were monitored at two-time scales: ms for the responses to current modulation (transient curves) and min for the average potential evolution all along the process. Finally, a comparison was made between inorganic and organic based trivalent chromium electrolytes, with a special care to cations presence, because of their influence on hydrogen discharge.[1] P. Benaben, Plating and Surface Finishing, p. 8, 2011.[2] A. Baral, R. Engelken, J. Electrochem. Soc., vol. 152, no 7, p. C504, 2005[3] J. Ji, W. C. Cooper, D. B. Dreisinger, E. Peters, J Appl Electrochem, vol. 25, no 7, p. 642-650, juill. 1995[4] M. Marcelet, Y-S. Chao, B. Vuillemin, M-P. Gigandet, C. Gleyzes, R. Oltra, J. Tardelli, J-Y. Hihn, ECS Meet. Abstr., vol. MA2019-02, no 21, p. 1046, sept. 2019

Hydrogen Limitation By Background Salts in the Case of Trivalent Chromium Electrolytes

The toxicity of some chemical compounds for humans and the environment encouraged health authorities to adopt new legislations. This is the case for hexavalent chromium salts, CMR known, which are targeted by the REACH European regulation. These salts are used in various surface treatments processes and their use is subjected to authorization. This situation has severely impacted the 'hard' chromium plating industry. Several projects all over the world deal with alternative solution and most of them are based on non toxic trivalent chromium salts, but more difficult to implement. The present work is undertaken within CRONOS 2024 project supported by a consortium of 20 companies and led by the IRT M2P (Institute for Technological Research Metallurgy Materials Processes in Metz). Trivalent chromium ions form very stable hexa-aqueous complexes in solution, difficult to reduce. It is therefore necessary to introduce complexing agents in order to facilitate their reduction [1]–[3]. These complexing agents, mostly organic, add a difficulty in the realization of trivalent chromium electrolytes since various phenomena must be considered. In addition, the chromium based electrolytes have a low faradic yield which induces permanent competition between its own reduction and the one of protons to dihydrogen. Excessive consumption of protons near the cathode leads to a pH rise which can cause precipitation and incorporation of chromium hydroxides.In order to improve the performance of trivalent chromium electrolytes, it is possible to adjust different parameters such as the nature and concentration of the complexing agents, the pH, the temperature, the addition of additives, etc. In this study, we focused mainly on the nature and concentration of the support salt. Previous works demonstrated that over a given concentration of Na2SO4 or K2SO4 support salts, a diffusion plateau appears, which can be attributed to the reduction of protons to dihydrogen [4]. Irrespective of the substrate nature, the composition and concentration of the support salts have an impact. It has been shown that a modulation of this plateau towards the most negative potential values would be able to help in decreasing the part of current attributed to the generation of dihydrogen. Moreover, as demonstrated by Phuong et al. [5], a comparison is possible with faradaic yields. Finally, the influence of organic and inorganic complexing agents has also been studied.[1] S. Imer et T. Varnali, Applied Organometallic Chemistry, 14(10) p. 660-669, (2000)[2] Z. Zeng, Y. Sun, et J. Zhang, Electrochemistry Communications, 11(2), p. 331-334 (2009)[3] Y. Song et D.-T. Chin, Electrochimica Acta 48(4) p. 349-356 (2002)[4] Y. Mukouyama, M. Kikuchi, et H. Okamoto Journal of Electroanalytical Chemistry 617(2) p. 179-184 (2008)[5] S. De, J. White, T. Brusuelas, C. Patton, A. Koh, et Q. Huang Electrochimica Acta 338 p. 135852 (2020)

In Situ Investigation of Migration of Cations during Electrolyte Spreading on Chromium Coated Steels Electrodeposited from Trivalent-Chromium Electrolyte

Resistance to corrosion and inhibition of cathodic delamination of polymer topcoat makes chromium coated steels suitable for application in the packaging industry. These coatings were traditionally produced from hexavalent chromium electrolytes. These coatings are now being phased out due to the carcinogenic nature of hexavalent chromium used in the production process (by the REACH regulation in the European Union). Electrodeposition from trivalent chromium containing electrolyte is being considered as an alternative to replace these traditional hexavalent-chromium coated steels. The cathodic delamination behavior of polymer topcoat on these trivalent-chromium coated steels were studied in detail previously [1-3]. However, the factors that result in inhibition of cathodic delamination are still not well understood.The two important factors influencing the rate of cathodic delamination are the electron transport and cation migration on the surface. The aim of this study is to understand the influence of complex composition of these trivalent-chromium coatings on the electrical conductivity and ion migration on the surface of the coating at the microscopic scale. The electronic conductivity of the coatings was studied using current-sensing atomic force microscopy to understand electron transfer occurring at the cathodic delamination front. Cation migration during oxygen-reduction induced spreading of electrolyte on these trivalent-chromium coated steel was studied using in situ scanning Kelvin probe force microscopy. The coatings are found to highly resist electron transport. Heterogeneities observed in the microscopic scale using the current-sensing atomic force microscope are correlated to the behavior observed during electrolyte spreading. The kinetics of spreading observed at microscopic scales are similar to that observed using a scanning Kelvin probe at larger scales. The results demonstrate in situ scanning Kelvin probe force microscopy as an effective tool to study the migration of cations during electrolyte spreading and the local heterogeneities at the microscopic scale. Warren, David & N. McMurray, H & de Vooys, Arnoud. (2013). Localised SKP Studies of Cathodic Disbondment on Chromium/Chromium Oxide Coated Steel. ECS Transactions. 50. 67-78.Wint, N & de Vooys, Arnoud & Mcmurray, Hamilton. (2016). The corrosion of chromium based coatings for packaging steel. Electrochimica Acta. 203. 326-336.James E. Edy, Hamilton N. McMurray, Koen R. Lammers, Arnoud C.A. deVooys (2019). Kinetics of corrosion-driven cathodic disbondment on organic coated trivalent chromium metal-oxide-carbide coatings on steel. Corrosion Science. 157. 51-61.

Characterization of the Reduction Mechanism of Chromium(III) By in Situ Microgravimetry and Volumetric Determination of Hydrogen

Processes for chromium electrodeposition based on hexavalent chromium compounds are more and more restricted by governmental laws1 due to the toxic and carcinogenic nature of Cr(VI) compounds. The most promising alternatives to replace hexavalent chromium solutions are trivalent chromium electrolytes considering process management, quality of the deposits and cost issues.2 However, many of these electrolytes suffer from low current efficiencies and reduced process stability compared to hexavalent chromium electrolytes. Organic additives are used to reduce the overvoltage of Cr(III) reduction and, hence, reach an acceptable current efficiency.3 In order to improve performance and reliability of the deposition process, a profound knowledge of the deposition mechanism and the effect of additives is crucial.In this work the stepwise reduction process of Cr(III) to metallic chromium is investigated by gravimetric and volumetric in-situ methods under variation of current density. Electrochemical quartz crystal microbalance (EQCM) was used to quantify the mass of deposited chromium metal.4 The amount of evolved hydrogen gas was determined with a gas burette in combination with a purpose-built setup for collecting the gas. Based on the results, the partial current densities of the two reduction steps of Cr(III) and the hydrogen evolution were calculated. A shift in the ratio of the partial current densities and an increased current efficiency of metal deposition was observed after the addition of saccharin, which is used as additive in chromium electrolytes for decorative applications.5 A mechanism on the mode of action of saccharin during chromium electrodeposition is proposed. European Chemicals Agency, Authorisation List, EC No. 215-607-8, 231-801-5, 231-889-5, 231-906-6, 232-140-5, 232-143-1, 234-190-3, 236-881-5. Z. X. Zeng, A. M. Liang, and J. Y. Zhang, Recent Pat. Mater. Sci., 2 (1), 50-57 (2009). D. Del Pianta, J. Frayret, C. Gleyzes, C. Cugnet, J. C. Dupin, and I. Le Hecho, Electrochim. Acta, 284, 234-241 (2018). M. Leimbach, C. Tschaar, U. Schmidt, and A. Bund, Electrochim. Acta, 270, 104-109 (2018). R. Katirci, Int. J. Surf. Sci. Eng., 10 (1), 73-85 (2016).

Preparation, microstructure, and thermophysical properties of Cr3+-modified cordierite high emissivity ceramic for metal thermal protection

The Cr3+-modified α-cordierite ceramics Mg2Al4-xCrxSi5O18 (x = 0, 0.2, 0.4, 0.6, 0.8, and 1, named MAS, MASCr0.2, MASCr0.4, MASCr0.6, MASCr0.8, and MASCr1, respectively) have been synthesized via the sol-gel process followed by cold isostatic pressing and sintering. The effects of trivalent chromium ion content in α-cordierite ceramic on microstructure and thermophysical properties were investigated. The MASCr0.2 ceramic exhibits the highest average emissivity value of 0.87 across the whole infrared range of 3–20 µm at 500 °C, especially at the vital infrared wavelength of 3–5 µm, the value can reach 0.92. The emissivity will decrease with the increase of doping amount owing to the growing precipitation of cristobalite and MgAl2O4 spinel phases. The lowest thermal conductivity 2.077 W/m K is discovered in MASCr0.2, a decrease of 23% compared to MAS. The MASCr0.4 ceramic with a close emissivity value compared to MASCr0.2 exhibits the TEC of 7.8 × 10−6 K−1 due to the multiphase structure.

Development of Ag decorated Au core-shell nanospheres for the detection of Cr(III) from environmental sample

Trivalent chromium ions are the abundant toxic metal, released to environment by various industrial effluent discharges. A sensitive and specific SPR technique for Cr(III) detection in trace levels from aqueous sample was developed in the present study. Here, Ag decorated Au nanoparticles (NPs) was functionalized with citrate by chemical-reduction technique. The Ag–Au NPs probe was analyzed via UV–visible spectrophotometer, TEM, EDAX, FTIR, DLS, and zetasizer. The Ag–Au NPs zeta potential was −36.66 mV and the particle size was 19 nm. The developed probe shown drastic variation in colour from dark pink to colourless with response time of 60 s after the interaction with Cr(III) ions. The absorbance at A526 showed good linearity with Cr(III) concentrations (10 nM - 100 µM). The LOD of Ag–Au NPs probe for Cr(III) detection was found to be 0.4 nM. The Ag–Au NPs sensor showed high specificity towards Cr(III) ions even in presence with competitive metal ions. In acidic pH and at temperature range of 25–50 °C the probe worked effectively. At last, the fabricated probe was applied for Cr(III) ion estimation in Cr(III) augmented environmental samples and the detection accuracy was determined by comparing with atomic absorption spectrophotometric analysis. Thus, the reported suggest that Ag–Au NPs could be applied for the selective detection of Cr(III) from environmental samples with excellent accuracy.

AWWA Water Science Author Spotlight

Having recently published an article in AWWA Water Science, Anthony Kennedy answered questions from the publication's editor-in-chief, Kenneth L. Mercer, about the research. Stannous Chloride Reduction–Filtration for Hexavalent and Total Chromium Removal From Groundwater Anthony Kennedy, Rachel Croft, Leah Flint, and Miguel Arias-Paic At the time of the research, I worked at the United States Bureau of Reclamation (USBR). I currently work at Corona Environmental Consulting, developing a research focus related to advanced water treatment processes and process optimization. Anthony enjoys backcountry skiing in Grand Teton National Park, March 2021. I've had two great mentors so far in my career: Professor Summers at CU Boulder and Dr. Miguel Arias-Paic at USBR. Professor Summers’ best advice was to focus on what is important and to not get caught up in minutia that can degrade your confidence. Dr. Arias-Paic said that for most problems we face in water treatment, a technological solution already exists; we just need to be more creative in how we apply those technologies. The primary motivations for the research were the cost and logistical challenges associated with brine generated from ion exchange treatment. Although ion exchange brine volume from regenerations can be significantly minimized, it still contains high levels of total dissolved solids and hexavalent chromium, in addition to other metals such as arsenic, vanadium, and uranium, and must ultimately face hazardous waste disposal. With stannous chloride, rapid-sand-filter backwash waste can also contain high levels of the less toxic trivalent chromium, but without the high total dissolved solids, making it more suitable for discharge to wastewater collection systems or sludge-drying ponds. In my personal life, I'm very conscious of my consumption and environmental impact. In my research, I try to simplify technologies and processes such that they use a more sustainable material or consume fewer materials, chemicals, etc., compared with conventional technologies, with the goal of meeting the same treatment target. Three challenges are (1) actual implementation of water treatment research; (2) widespread adoption and implementation of direct potable reuse to mitigate the effects of climate change on the drinking water industry; and (3) funding for drinking water, wastewater, and water reuse infrastructure. In July 2021, Anthony visited Parco Naturale Puez Odle in South Tyrol, Italy. I grew up near Pittsburgh, which has seen a comeback due in large part to a hardworking, motivated, but also modest and humble, populace. I take the same approach with my research—just sound, thorough research that can be used to improve the design, operation, and potentially the sustainability of water treatment facilities. To learn more about Anthony's research, visit the article, available online at https://doi.org/10.1002/aws2.1174.

Improvement of hydrothermal stability and chrome uptake of leather: Amino-terminated hyper-branched polymer as high exhausted tanning auxiliary

Carboxyl-terminated hyper-branched polyester is extensively used as high exhaustion chrome tanning auxiliary owing to its strong coordination ability with trivalent chrome. Owing to lower electronegativity of N than O, the coordination ability with trivalent chrome for hyper-branched polyamide is stronger than polyester. Herein, in this paper, an amino-terminated hyper-branched polyamide (HBPN) is synthesized and used as high exhaustion chrome in tanning of pickling pigskin (HBPN–Cr). The effects of utilization of HBPN in denaturation temperature, shrinkage temperature, chrome uptake, and microstructure structure of wet-blue leather are studied. Moreover, the anti-oxidation stability of wet-blue leather is preliminary studied by heat treatment. The results show that incorporation of 1% HBPN in pickling process can significantly improve chrome uptake to 83.32%. The hydrothermal stability of wet-blue leather is improved due to the formation of higher crosslinking degree of collagen fibers. In addition, for the wet-blue leather without unsaturated fatliquoring agent, through heat treatment, hexavalent chrome does not appear after 24 ​h heat treatment. On the contrary, the chrome complex changes from hydroxyl copolymerization to more stable oxygen copolymerization or chrome oxide for C–Cr and HBPN-Cr tanned leather, respectively. These results will build a data support for the further study of hyper-branched polymer chrome tanning auxiliaries.

Оptimization of the chrome plating bath operation during the restoration of vehicle parts

Introduction. In the process of chrome plating of car parts, there is a change in the composition of the chrome plating bath, which affects the stability of the ongoing processes – the performance and quality of the chrome coating. Therefore, it is necessary to constantly monitor the chrome plating bath for the presence of foreign elements that can be formed during the operation of the bath during the reaction of the solution with the anode or cathode.Materials and methods. During the research, the necessary equipment was used, which made it possible to determine with sufficient accuracy the content of trivalent chromium and its effect on the electrolysis process – the performance and quality of chrome coatings. A well-known technique was used to determine the quality and performance.Results. When conducting studies of the obtained cold self-regulating chromium plating electrolyte, it was found that trivalent chromium has a significant effect on the stability of the chromium plating bath. The content of which can vary within 2...25 g/l during operation. As a result, it was found that the optimal amount of trivalent chromium is its content in the chromium plating bath from 2 to 15 g/l. With a lower or higher content, the electrolysis performance and the quality of the chrome coating decrease. It was also found that in order to maintain the optimal amount of trivalent chromium and increase the time for stable operation of the chromium bath, it is necessary to observe the ratio of the area of the anode and cathode (the coated surface of the part), the value of which is within 3...4.Discussion and conclusion. As a result of the conducted research, it will make it possible, under certain conditions, to carry out the process of chromium deposition from a cold self-regulating electrolyte, which will allow for stable chromium deposition at high productivity and the necessary quality of the coatings obtained. The main condition is to control and maintain the amount of trivalent chromium in the chromium plating electrolyte in the bath.

Design of BiOBr0.25I0.75 for synergy photoreduction Cr(VI) and capture Cr(III) over wide pH range

Conversion of hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)) is an effective way to reduce its environmental risk, especially via photoreduction process. However, over a wide range of pH values, it is still a great challenge to achieve a high removal rate, and the disposal of produced Cr(III) should be concerned. In this work, we implemented a high removal rate at 98% for Cr(VI) and total chromium (Cr(T)) over a wide pH range (4–10) through the synergistic effect of adsorption, photoreduction and immobilization on the surface of BiOBr0.25I0.75. The substitution of bromine by iodine reduced the adsorption energy of Cr(VI) on BiOBr0.25I0.75, promoting the adsorption of Cr(VI). Meanwhile, the introduced iodine upshifted the conduction band (CB), enhancing the reduction ability for Cr(VI) to Cr(III). The negative surface of BiOBr0.25I0.75 can capture Cr(III), achieving a high removal rate for Cr(T). The pH-independent feature for Cr(VI) and Cr(III) removal make BiOBr0.25I0.75 a potential material for chromium-containing wastewater treatment. This work provides an effective strategy for removing chromium over a wide pH range.

Study and applicability of photoluminescence properties of Cr4+: LiInGeO4

This paper reports a series of Cr4+: LiInGeO4, based on a high-temperature solid-phase synthesis method and the effect of LiInGeO4-doped Cr4+ ions on the spectral properties and luminescence colors in the near-infrared region. Under 808 nm excitation, the emission spectra showed a broad band at 1000–1500 nm, with a maximum peak occurring at 1218 nm due to the 3T2→3A2 transition of Cr4+ ions. The lifetime decreases with the increasing amount of Cr4+ ions, from 8.30035 μs to 6.656 μs. The chromium's chemical composition and valence state in LiInGeO4 were analyzed by x-ray photoelectron spectroscopy. The results showed that the chromium in LiInGeO4 was tetravalent, and no trivalent chromium was found. The results show that LiInGeO4 crystals can have promising applicability in anti-counterfeiting due to the possibility of both up-conversion and down-conversion glowing.

Trivalent chromium removal from tannery wastewater with low cost bare magnetic Fe3O4 nanoparticles

In this work, a new method is proposed to remove Cr (III) in tanning wastewater with low cost bare magnetic Fe3O4 nanoparticles. It has achieved ultra-high chromium capture capacity under high selectivity. The maximum capture capacity of magnetic Fe3O4 nanoparticles for Cr (III) in chromium containing tanning wastewater is 456.20 mg/g at optimal pH 8. In a pilot scale apparatus running continuously at 18 L/h for 8 h, the chromium removal efficiency could be stabilized above 98.5% with chromium concentration reduced from 1065 mg/L to about 15 mg/L at the primary treatment. After secondary treatment, the outlet chromium concentration drops to 0.2 mg/L (lower than 0.5 mg/L, the maximum allowable concentration of Cr total in tanning wastewater determined by China), and the chromium removal efficiency is steadily above 97.8%. The results revealed that the method could be used economically as an efficient technique for removal of Cr (III) in chromium containing leather tanning wastewater and has great potential in heavy metal wastewater treatment.

Wear and corrosion of Co-Cr coatings electrodeposited from a trivalent chromium solution: Effect of heat treatment temperature

Electrodeposition from a trivalent chromium bath was employed to obtain the cobalt‑chromium coatings with a thickness of ~25 μm. The effect of heat treatment temperature on characteristics, mechanical properties , and electrochemical behavior of the coatings, were examined. Heat treatment was performed in a wide range of temperatures (200–800 °C). Heat-treating below 400 °C increased the crystallinity of alloy coatings and had a negligible effect on nodular and cracked morphology. Raising the heat-treating temperature changed the X-ray diffraction patterns by thickening the surface oxide films , which consisted of different oxides (e.g., cobalt chromite spinel). The surface nodules were faded out above 700 °C. The heat-treating below 300 °C decreased the hardness and wear resistance of Co-Cr electrodeposits. The 400 °C heat-treated sample had the highest hardness of ~1000 HV, and the best tribological behavior. The weight loss and coefficient of friction of this sample were 24 and ~2 times smaller than the as-deposited coating. The hardness and wear resistance dropped again with further raising the heat-treating temperature. While detachment of coating during sliding occurred at low-temperature heat-treated samples, the abrasion was the main mechanism of material loss at those heat-treated at higher temperatures. According to the corrosion results, the optimum heat-treating temperature that the highest corrosion resistance could be achieved was 300–500 °C. The corrosion current density of 400 °C heat-treated coating was ~3.7 times smaller than the as-deposited Co-Cr film. • The relatively thick Co-Cr coatings were produced from a trivalent chromium bath. • Effect of heat treatment on Co-Cr films' characteristics and properties was studied. • Different types of oxides and morphologies were observed at various temperatures. • The 400 °C heat-treated sample showed the highest hardness and wear resistance. • Optimum heat treatment temperatures for the best corrosion resistance was 300–500 °C.

Oxidation behavior of trivalent chromium ions on boron-doped diamond electrodes

Boron-doped diamond (BDD) films (boron content: 17,000 ppm) were grown by hot-filament chemical vapor deposition. To investigate whether BDD films are suitable anode materials in the chromium electroplating process, the oxidation behavior from Cr3+ to Cr6+ was investigated on a BDD electrode. The current efficiency of the BDD electrode for Cr3+ oxidation was similar to that of lead alloy anode, the conventional anode in chromium electroplating processes. In strong acid solution (pH = −0.39), Cr3+ ions were readily oxidized on the BDD and lead alloy electrodes but were not oxidized on platinum and iridium oxide electrodes. Furthermore, during electrolysis for 50 h using the BDD anode in a hexavalent chromium electroplating bath, the Cr3+ ion concentration was maintained below 1.5 g/L. According to these results, the BDD anode is a feasible alternative to lead alloy in hexavalent chromium electroplating.

Microstructure and flow accelerated corrosion resistance of Cr coatings electrodeposited in a trivalent chromium bath

In this study, Cr coatings were electrodeposited on carbon steel substrates by a trivalent Cr salt. The microstructure of the Cr coatings and their flow accelerated corrosion (FAC) resistance in simulated secondary circuit environments (10 MPa and 150 °C) of nuclear power plants were investigated. The results showed that the as-deposited coating was amorphous and crack-free. Annealing at 200 °C resulted in the decrease of Cr (OH) 3 and the increase of Cr and Cr 2 O 3 on the coating surface , which improved the corrosion resistance of the coating. The subsequent FAC test showed that the Cr coatings significantly improved the FAC resistance of the carbon steel substrate. The coating annealed at 200 °C exhibited better FAC resistance. The Cr-rich oxide film formed on the coatings was mainly Cr 2 O 3 , with a small amount of Cr and CrO 2 . Due to the galvanic effects caused by pinhole defects, abnormally grown annular deposits formed on the coating surface. • Uniform and dense Cr coatings were electrodeposited from a trivalent Cr bath. • Annealing at 200 °C reduced Cr(OH) 3 on the coating surface and increased Cr and Cr 2 O 3 . • The Cr-rich oxide film formed in the corrosion was Cr 2 O 3 and a small amount of CrO 2 . • The interface between the coating and the substrate formed Cr-rich corrosion areas.

Recovery of Cr(VI) and removal of cationic metals from chromium slag using a modified bipolar membrane system

A bipolar membrane electrodialysis (BMED) system equipped with a ‘back-to-back’ chromium slag compartment was fabricated to simultaneously recover hexavalent chromium (Cr(VI)) as Na2CrO4 and remove trivalent chromium (Cr(III)), Fe3+, Mg2+ and Ca2+ from chromium slag. Desorption experiments showed that the solution pH had a significant effect on the heavy-metal desorption efficiency from chromium slag. The BMED system can effectively recover more than 70.6% of Cr(VI) and simultaneously remove more than 99.9% of Fe3+, 87.8% of Mg2+, 26.2% of Ca2+ and 23.1% of Cr(III). When the number of ‘back-to-back’ chromium slag compartment modules was increased from one to two and three, the current efficiency increased from 11.2% to 17.2% and to 39.1%, respectively, while the amount of recovered heavy metals increased by factors of two and three, respectively. An analysis of the binding states showed that different chromium states were interchanging, which is beneficial to the Cr(VI) recovery and the Cr(III) removal. The experimental results indicated that the BMED system is as an effective technique for the simultaneous recovery of Cr(VI) and removal of Cr(III) from chromium slag and other types of solid waste contaminated with heavy metals.

Feasibility of Remediation Lead, Nickel, Zinc, Copper, and Cadmium-Contaminated Groundwater by Calcium Sulfide

Metal contamination in groundwater often occurs in various industrial processes. Studies have confirmed that polysulfide could reduce hexavalent chromium to trivalent chromium, achieving the effect of in situ stabilization. For other metal contamination in groundwater, whether polysulfide also had a stabilizing ability to achieve in situ remediation. This research focused on metals in addition to chromium that often contaminated groundwater, including lead, nickel, zinc, copper, and cadmium, to explore the feasibility of using calcium polysulfide (CaSx) as an in situ stabilization technology for these metals’ contamination of groundwater. Results showed that CaSx had a great removal efficiency for metals lead, nickel, zinc, copper, and cadmium. However, for nickel, zinc, copper, and cadmium, when CaSx was added excessively, complexes would be formed, causing the result of re-dissolution, in turn reducing the removal efficiency. As it is difficult to accurately control the dosage of agents for in situ groundwater remediation, the concentration of re-dissolved nickel, zinc, copper, and cadmium may not be able to meet the groundwater control standards. CaSx had high lead removal efficiency and for a concentration of 100 mg/L, the dose of calcium polysulfide was more than the amount of 1/1200 (volume ratio of CaSx to groundwater). In addition, the removal rate was almost 100% and it would not cause re-dissolution due to excessive CaSx dosing. CaSx can be used as an in situ stabilization technique for lead-contaminated groundwater.

Characterization of cobalt containing and cobalt-free trivalent chromium passivations on γ-ZnNi coated steel substrates

The corrosion performance was studied for standard trivalent chromium passivations and cobalt-free trivalent chromium passivations on steel substrates. The steel substrates were either polished or unpolished and coated with γ-phase ZnNi prior to passivation. For unpolished substrates, cobalt-free specimens had 50 ± 8 cracks/mm, a thickness of 69 ± 4 nm, and notable porosity compared to cobalt-containing passivations which had no cracks, a thickness of 40 ± 5 nm, and no porosity. For polished substrates, cobalt-free passivations had 50 ± 3 cracks/mm, a thickness of 38 ± 2 nm, and notable porosity compared to cobalt-containing passivations with 16 ± 2 cracks/mm, thickness of 48 ± 2 nm, and no porosity. After 1000 hours of salt-spray exposure, cobalt-free passivations had visible white rust on 4% of the surface area while cobalt-containing passivations had 22% of the surface area covered by white rust. The thickness of both passivations increased on both polished and unpolished specimens after salt spray exposure. Electrochemical testing showed that the cobalt-free passivation had the least change before and after salt spray exposure. The difference in corrosion performance was attributed to the greater porosity in the cobalt-free passivations influencing the amount of corrosion inhibiting species present.

Comparative analysis of biological versus chemical synthesis of palladium nanoparticles for catalysis of chromium (VI) reduction

The discharge of hexavalent chromium [Cr(VI)] from several anthropogenic activities leads to environmental pollution. In this study, we explore a simple yet cost effective method for the synthesis of palladium (Pd) nanoparticles for the treatment of Cr(VI). The presence of elemental Pd [Pd(0)] was confirmed by scanning electron microscope (SEM), electron dispersive spectroscopy and X-ray diffraction (XRD). We show here that the biologically synthesized nanoparticles (Bio-PdNPs) exhibit improved catalytic reduction of Cr(VI) due to their size being smaller and also being highly dispersed as compared to chemically synthesized nanoparticles (Chem-PdNPs). The Langmuir–Hinshelwood mechanism was successfully used to model the kinetics. Using this model, the Bio-PdNPs were shown to perform better than Chem-PdNPs due to the rate constant (kbio = 6.37 mmol s−1 m−2) and Cr(VI) adsorption constant (KCr(VI),bio = 3.11 × 10−2 L mmol−1) of Bio-PdNPs being higher than the rate constant (kchem = 3.83 mmol s−1 m−2) and Cr(VI) adsorption constant (KCr(VI),chem = 1.14 × 10−2 L mmol−1) of Chem-PdNPs. In addition, product inhibition by trivalent chromium [Cr(III)] was high in Chem-PdNPs as indicated by the high adsorption constant of Cr(III) in Chem-PdNPs of KCr(III),chem = 52.9 L mmol−1 as compared to the one for Bio-PdNPs of KCr(III),bio = 2.76 L mmol−1.

Investigation of Ferrochromium Wastes for their Hazardous Hexavalent Chromium Content

In this study, ferrochromium slag (FS) and electric-arc furnace dust (EAFD) samples were examined for their hazardous hexavalent chromium content mainly by X-ray photoelectron spectroscopy (XPS) analysis. According to the experimental results, magnesium-silicate constitutes the main framework of both FS and EAFD samples. Highly crystalline FS contains 3.11%Cr2O3 as spinel phases while this constituent is much higher, 15.44%, in EAFD. The general XPS spectra show that both samples mainly consist of O, Mg and Si elements. In addition, FS also includes aluminum while carbon, calcium and potassium are the other major elements for EAFD. The peaks appeared in the range of 577-589 eV confirm chromium content of the samples. Besides, the peaks appeared at 1022 and 1045 eV point out zinc content of EAFD. Based on the XPS atomic ratio, Cr contents of FS and EAFD were obtained as 0.505 and 0.987%, respectively while EAFD further contains 2.594% Zn. The detailed XPS analysis indicates that chromium exists as two oxidation states, trivalent and hexavalent forms at 2p orbitals. Specifically, for 2p3/2 orbitals, trivalent and hexavalent chromium were assigned at 577.1 and 579.2 eV, respectively whereas 586.9 and 588.9 eV for 2p1/2 orbitals. Furthermore, the corresponding intensities of these peaks are much higher for EAFD. By comparing the peak intensities, the relative proportions of Cr (VI) were calculated as 49.44 and 49.53% for FS and EAFD, respectively. The leaching tests show that solubilities of all the metals examined except Cr are far below the toxicity limits for hazardous waste and even below the ranges of non-hazardous waste. Specifically, chromium contents of the filtrates were determined as 0.862 and 268.050 mg/l for FS and EAFD, respectively. Considering the toxicity limits of chromium, EAFD can be surely labeled as a highly hazardous waste whereas, on the other hand, FS is within the limits of non-hazardous waste. In conclusion, it can be said that the hazardous hexavalent chromium content must be reduced to immobile trivalent forms for the dust sample prior to disposed in landfills.