Reduction Of Cr Research Articles

Research on the fabrication of nanorod-structured phorphyrin/g-C3N4 as photocatalyst materials for the removal of Cr6+ ions in an aqueous solution

Due to their potential to expand the photon energy harvesting zones and improve charge separation, two semiconductors together have been shown to be a promising technique to increase the photocatalytic activity of photocatalysts. Through the use of monomeric porphyrin molecules and g-C3N4 nanomaterials, self-assembly of the hybird g-C3N4@porrphyrin nanorod materials under CTAB surfactant-assited conditions was demonstrated in this study. SEM, EDS, FTIR, and UV-vis were used to characterize the generated hybrid material. Under simulated sunlight irradiation, the hybrid mateial’s photocatalytic behaviour was examined for the photoreduction of Cr6+ ions. After 100 minutes of reaction time under the simulated solar spectrum, the results showed that the hybrid material demonstrated high photocatalytic performance against, with clearance percentage of almost 100%. The prepared photocatalyst was also highly stable with the reduction of Cr6+ removal efficiency of less than 10% after 4 cycles of photocatalytic testing.

Core-shell structured Ag-HAp@Bi2S3 as effective S-scheme heterojunction photocatalyst: Induced with internal polarization electric field effect and co-catalyst fashions

Considering the utilization of solar energy, the low efficiency of photocatalytic reactions is a crucial factor to take into account. One effective solution to this issue is the heterojunction method, deemed as an efficient approach for enhancing photocatalyst performance via optimal sunlight utilization. In this study, a HAp@Bi2S3 core-shell structure was synthesized using a simple microemulsion method, and subsequently, Ag nanoparticles were systematically anchored onto its surface. The Ag-HAp@Bi2S3 nanocomposite was utilized to concurrently achieve the photocatalytic degradation of Bis-phenol A (BPA) and the reduction of Cr6+ ions. The findings indicate that the heterojunction S-scheme and co-catalyst Ag have a significant role in charge separation and accelerating the photocatalytic reaction. Moreover, the presence of HAp as a polarizing substrate in this composite can create an internal polarized electric field, in addition to the electric field generated by the S-scheme heterojunction. This proposed nanocomposite exhibits potential in addressing the challenge of achieving rapid and simultaneous pollutant degradation.

Synthesis of five polyoxometalate-based compounds by using hydrothermal one pot method: Structures, supercapacitor, electrocatalytic and photocatalytic properties

Five polyoxometalate (POM) based compounds were synthesized by traditional hydrothermal method, namely {Co2(HTPT)2(H2O)2[Mo8O27]} (1), {Co2(TPT)2(H2O)6}(β-Mo8O26)·4H2O (2), {Co2(TPT)2(H2O)2(δ-Mo8O26)}·2H2O (3), {M(TPT)2(H2O)2}(Mo6O19) (M = Co 4, Ni 5) (TPT = 3-(4-terahydro-2H-pyran-4-yl)methyl)-4H-1,2,4-triazol-3-yl)pyridine). Compounds 1−3 were obtained in one pot, realizing efficient utilization of the hydrothermal method. The photocatalytic activities of compounds 1, 2, 4 and 5 were evaluated by the degradation of Methylene blue (MB) and Rhodamine B (RhB) and the reduction of Cr6+. The effects of the amount of catalyst and the pH of the system on the photocatalytic performance were also studied. Experimental studies have shown that compounds 1 and 4 also exhibit good bifunctional electrocatalytic activity for reducing NO2−, Cr6+ and H2O2, but also oxidizing AA (Ascorbic Acid). Compounds 1, 2, 4 and 5 can serve as amperometric sensors for NO2− and Cr6+. Compounds 1 and 4 own properties of capacitor materia.

Fabrication of a Plasmonic Heterojunction for Degradation of Oxytetracycline Hydrochloride and Removal of Cr(VI) from Water

A novel Ag/Ag2CO3/BiVO4 plasmonic photocatalyst was successfully prepared by depositing Ag nanoparticles on the surface of Ag2CO3/BiVO4 through the photoreduction reaction. Due to the existence of this novel heterojunction photocatalyst structure, not only can it prevent the photogenerated charge recombination, but the unique properties of Ag also have a great advantage in the absorption of light. The Ag/Ag2CO3/BiVO4 photocatalyst showed good catalytic performance in the degradation of oxytetracycline hydrochloride (OTH) and removal of Cr6+, and the degradation rate of OTH reached 98.0% after 150 min of illumination. The successful preparation of Ag/Ag2CO3/BiVO4 was confirmed by a series of characterization methods, and the importance of •OH and h+ radicals in the degradation of OTH was demonstrated. In addition, the photocatalytic mechanism of Ag/Ag2CO3/BiVO4 photocatalyst was systematically studied in terms of degradation of OTH and reduction of Cr6+. This study is of great importance for the development of novel plasmonic heterojunction photocatalysts and to meet future environmental requirements.

Reduction of Cr6+ formation in the stellite hardfacing operation by a novel thin film of zinc coating on a stellite filler rod

Reduction of Cr6+ formation in the stellite hardfacing operation by a novel thin film of zinc coating on a stellite filler rod

High-efficiency removal of hexavalent chromium from contaminated water using nitrogen-doped activated carbon: kinetics and isotherm study

Hexavalent chromium (Cr6+) is a carcinogenic pollutant found in different wastewater streams. The current study investigates the potential removal of Cr6+ from aqueous solutions by nitrogen-doped coconut granulated activated carbon (N-GAC). The raw GAC was chemically and thermally treated with urea (1:1 ratio) in an ethanol medium and calcined under nitrogen atmosphere at temperatures 400 °C and 600 °C. The optimized sample (N-GAC 400 °C) possessed a remarkable surface area (474.62 m2/g) while the chemical composition analysis revealed successful doping of nitrogen with no significant changes on the mesoporous morphology of the raw GAC. The adsorption studies revealed that Cr6+ adsorption capacity was higher for N-GAC 400 °C. The removal efficiency was significantly decreased with increasing the initial solution pH (optimum pH 2.2). The temperature has no significant effect on the adsorption capacity. The equilibrium experimental results were best fitted with Sips and Redlich-Peterson isotherm models with R2 ≥ 0.987 and a maximum adsorption capacity of 15.15 mg/g. The kinetic data agreed with the pseudo 2nd order model (R2 > 0.997). The isotherm and kinetic studies revealed that physisorption and chemisorption processes are involved in the removal process. The adsorption mechanism was mainly controlled by electrostatic interactions, ion exchange and reduction of Cr6+ to trivalent chromium. Accordingly, the N-GAC can be considered a promising material for the adsorptive removal of heavy metals from water.

Removal of CrO42-, a Nonradioactive Surrogate of 99TcO4-, Using LDH-Mo3S13 Nanosheets.

Removal of chromate (CrO42-) and pertechnetate (TcO4-) from the Hanford Low Activity Waste (LAW) is beneficial as it impacts the cost, life cycle, operational complexity of the Waste Treatment and Immobilization Plant (WTP), and integrity of vitrified glass for nuclear waste disposal. Here, we report the application of [MoIV3S13]2- intercalated layer double hydroxides (LDH-Mo3S13) for the removal of CrO42- as a surrogate for TcO4-, from ppm to ppb levels from water and a simulated LAW off-gas condensate of Hanford's WTP. LDH-Mo3S13 removes CrO42- from the LAW condensate stream, having a pH of 7.5, from ppm (∼9.086 × 104 ppb of Cr6+) to below 1 ppb levels with distribution constant (Kd) values of up to ∼107 mL/g. Analysis of postadsorbed solids indicates that CrO42- removal mainly proceeds by reduction of Cr6+ to Cr3+. This study sets the first example of a metal sulfide intercalated LDH for the removal of CrO42-, as relevant to TcO4-, from the simulated off-gas condensate streams of Hanford's LAW melter which contains highly concentrated competitive anions, namely F-, Cl-, CO32-, NO3-, BO33-, NO2-, SO42-, and B4O72-. LDH-Mo3S13's remarkable removal efficiency makes it a promising sorbent to remediate CrO42-/TcO4- from surface water and an off-gas condensate of nuclear waste.

Use of a Hybrid Porous Carbon Material Derived from Expired Polysaccharides Snack/Iron Salt Exhibiting Magnetic Properties, for Hexavalent Chromium Removal

Nowadays, the scientific interest is focused more and more on the development of new strategies in recycling of waste products as well as on the development of clean technologies due to the increased environmental pollution. In this work we studied the valorization of an expired cheese-tomato flavor corn snack, which is polysaccharide food product, by producing advanced hybrid magnetic materials for environmental remediation purposes. The carbonization-chemical activation of this snack using potassium hydroxide leads to a microporous activated carbon with high surface area (SgBET ~800 m2/g). The magnetic hybrid material was synthesized via an in-situ technique using iron acetate complex as the precursor to produce iron based magnetic nanoparticles. The resulting material retains a fraction of the microporous structure with surface area SgBET ~500 m2/g. Such material consists, of homogenously dispersed magnetic isolated zero valent iron nanoparticles and of iron carbides (Fe3C), into the carbon matrix. The magnetic carbon exhibited high adsorption capacity in Cr(VI) removal applications following a pseudosecond order kinetic model. The maximum adsorption capacity was 88.382 mgCr(VI)/gAC at pH = 3. Finally, oxidation experiments, in combination with FT-IR, Mössbauer, and VSM measurements indicated that the possible Cr6+ removal mechanism involves oxidation of iron phases and reduction of Cr6+ to Cr3+.

CdTe and Ag nanoparticles co-modified TiO2 nanotube arrays for the enhanced wastewater treatment and hydrogen production

In this paper, the Z-scheme photocatalysts of TiO2 nanotube arrays (TiO2 NTs) modified by CdTe and Ag nanoparticles were prepared by a simple hydrothermal method. The photocatalytic performances of CdTe/TiO2 NTs and CdTe/Ag/TiO2 NTs toward the reduction of Cr6+ and the degradation of methylene blue (MB) were researched. The MB degradation efficiency reached 100.0% after 2 h visible light irradiation, and the removal of Cr6+ reached 95.9% after 3 h. The transient photocurrent and open-circuit voltage of CdTe/Ag/TiO2 NTs were higher than the other two samples. Furthermore, the effect of light intensity and pH on the photocatalytic performances was verified. The hydrogen production of the photocatalysts under visible light was also researched with the assistance of external voltage and sacrificial agents, which provides a new direction for the development and utilization of solar energy.

The performance of Cu2+ as dissolved cathodic electron-shuttle mediator for Cr6+ reduction in the microbial fuel cell

The study investigates the performance of Cu2+ as dissolved cathodic electron-shuttle mediator (dcESM) for simultaneous Cr6+ reduction and electricity generation in a microbial fuel cell (MFC) at pH 2 and 4 conditions. The dcESM behavior of Cu2+ on carbon cloth (CC) catalyzes the reduction of Cr6+ into Cr3+ at pH 2 by undergoing redox reactions. However, at pH 4, a simultaneous reduction of Cu2+ and Cr6+ was observed. Cyclic voltammetry studies were performed at pH 2 and 4 to probe the dcESM behavior of Cu2+ for Cr6+ reduction on CC electrode. Also, at pH 2, increasing the concentration of Cu2+ from 50 to 500 mg L− 1 favors the Cr6+ reduction by reducing the reaction time from 108 to 48 h and improving the current production from 3.9 to 6.2 mA m− 2, respectively. Nevertheless, at pH 4, the efficacy of Cr6+ reduction and electricity generation from MFC is decreased from 63 to 18% and 4.4 to 1.1 mA m− 2, respectively, by increasing the Cu2+ concentration from 50 to 500 mg L− 1. Furthermore, the performance of dcESM behavior of Cu2+ was explored on carbon felt (CF) and platinum (Pt) electrodes, and compare the results with CC. In MFC, at pH 2, with an initial concentration of 100 mg L− 1, the reduction of Cr6+ in 60 h is 9.6 mg L− 1 for CC, 0.2 mg L− 1 for CF, and 51.3 mg L− 1 for Pt cathodes. The reduction of Cr6+ (initial concentration of 100 mg L− 1) at pH 4 in 120 h is 44.7 mg L− 1 for CC, 32.1 mg L− 1 for CF, and 70.9 mg L− 1 for Pt cathodes. Maximum power densities of 1659, 1509, and 1284 mW m− 2 were achieved when CF, CC, and Pt, respectively were employed as cathodes in the MFC.

Ameliorative effect of co-application of Bradyrhizobium japonicum EI09 and Se to mitigate chromium stress in Capsicum annum L.

The present study was conducted to explore the potential of Bradyrhizobium japonicum EI09 (EI09) and selenium (Se) alone or in combination to mitigate hexavalent chromium (Cr6+) stress in Capsicum annum L. Chromium stressed plants exhibited significant reduction in biomass, chlorophyll content and gas exchange characteristics. The inoculated seedlings subjected to Cr6+stress showed improvement in growth, proline content, gas exchange attributes and total soluble proteins. Likewise, inoculated C. annum seedlings exhibited augmented activity of ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD) under Cr6+ stress. The Cr6+ stress mitigation in inoculated seedlings was ascribed to reduction in malondialdehyde (MDA) content, hydrogen peroxide (H2O2) besides increase activity of flavonoids, proline, phenolic content along with modulation of antioxidative enzymes. The growth-enhancing attributes of bacteria such as indole acetic acid (IAA) content and 1-aminocyclopropane-1-carboxylate deaminase (ACCD) activity enhanced growth in Cr6+-stressed plants. Moreover, co-treatment of EI09 and 5 µM Se effectively mitigated Cr (VI) stress in C. annum plants. Current studies provide a novel insight into potential of B. japonicum EI09 and Se in reduction of Cr6+ toxicity in C. annum plants.

Carbon dots derived from flax straw for highly sensitive and selective detections of cobalt, chromium, and ascorbic acid

Fluorescent detections of cobalt, chromium, and ascorbic acid by carbon dots are of importance for human health and environment. Green synthesis of fluorescent carbon dots from biomass is essential for their sustainable applications. Herein, carbon dots were successfully prepared by a simple hydrothermal method using flax straw as carbon source. The obtained carbon dots possess a much higher quantum yield (20.7%) and excitation-dependent photoluminescence behavior. The “on-off” fluorescence principle, quenching of the fluorescence intensity of the carbon dots in the presence of Co2+ or Cr6+ based on static quenching effect and inner filter effect, is extended to “on-off-on” principle for detection of ascorbic acid based on the reduction of Cr6+ to Cr3+ by ascorbic acid. The linear ranges for detections of Co2+, Cr6+, and ascorbic acid are 0–500, 0.5–80, and 0–200 μM, and the limits of the corresponding detections are 0.38, 0.19, and 0.35 μM, respectively. Compared with most reported fluorescence detections, our linear ranges are significantly wider and our detection limits are much lower for the detections of Co2+, Cr6+, and ascorbic acid. This sensing platform is highly sensitive and selective to monitor Co2+ and Cr6+ in real water and ascorbic acid in vitamin C tablets with remarkable practicality.

Application of iron/aluminum bimetallic nanoparticle system for chromium-contaminated groundwater remediation

When the nanoscale zero valent iron (nZVI) is used for the reduction of hexavalent chromium (Cr6+) to trivalent chromium (Cr3+) in groundwater, the reduction efficiency is decreased due to the passivation of reactive sites by precipitation. The bimetallic nanoparticle (BNP) can be created with the addition of the second metal to achieve a higher activity and reduce the occurrence of the ferrous/ferric hydroxide precipitation. In this study, the iron-coated aluminum (Fe/Al) BNP and aluminum-coated iron (Al/Fe) BNP systems were designed for remediating Cr6+-contaminated groundwater. The chemical liquid-phase deposition and co-reduction method was applied to produce BNPs. Cr6+ removal rate by Fe/Al BNPs was directly proportional to the saturation concentration and reactive sites, which caused a higher Cr6+ removal rate. The pseudo-first-order kinetic model could be used to describe the Cr6+ adsorption mechanism by Fe/Al BNPs. Results show that Fe/Al BNPs and Al/Fe BNPs could reduce Cr6+ to Cr3+, and the removal efficiencies for Cr6+ were 1.47 g/g BNP and 0.07 g/g BNP, respectively. Detection of Cr3+ in the aqueous phase was observed during the Cr6+ removal process. Results from X-ray diffraction (XRD) analysis confirmed that Cr(OH)3 was present on the surface of BNPs. Main mechanisms caused Cr6+ removal included reduction, precipitation, and adsorption. The reduction of Cr6+ produced OH−, which created alkaline environment and facilitated the formation of chromium hydroxide precipitates [Cr(OH)3]. Thus, the migration of Cr3+ was prevented and the environmental risk was reduced. BNP had a higher activity and stability, and it was applicable for Cr6+-contaminated site remediation.

Microbial Chromate Reductases: Novel and Potent Mediators in Chromium Bioremediation-A Review

Heavy metal pollution from the growing industrialization are a significant cause of environmental concern.Chromium (Cr) is commonly used in the production of stainless steel, textile dyeing, electroplating, as nuclear coolants and largely in chrome tanning of hides and skins. About 90% of leather is produced by chrome tanning and the leather industry contributes to an overload of Cr toxicity in tannery effluents. Accumulation of Cr6+ is carcinogenic, genotoxic and teratogenic to organisms. Biological methods are ‘green’ approaches for chromium bioremediation and microorganisms are the desired candidates for pollution abatement. Microbial chromate reduction is mediated by chromate reductases (ChrRs) which may be expressed constitutively or inducibly. ChrRs have been produced by a number of bacteria, fungi and yeasts and may be extracellular or localized in the membrane or cytosol. ChrRs are dependent on electron donors such as reduced Nicotinamide adenine dinucleotide (NADH) or reduced Nicotinamide adenine dinucleotide phosphate (NADPH) or reduced Glutathione (GSH) as cofactors. In chromate reduction by ChrRs, Cr6+ undergo one electron transfer to produce an unstable Cr5+ radical that is converted to stable and less toxic Cr3+. Putative ChrR genomic sequences have been studied with 99% sequence similarity in Gram negative bacteria. ChrRs are valuable resources in different environments for chromate reduction. This review is to discuss the expression and characteristics of ChrRs and their mechanisms in reduction of Cr6+ toxicity in order to provide a comprehensive understanding of this novel class of enzymes for promisingapplications in Cr bioremediation.

Study of electrode modification and microbial concentration for microbial fuel cell effectivity from molasses waste and reduction of heavy metal Cr (VI) by continue dual chamber reactor

The rapid growth of textile industry has resulted in increasing amounts of heavy metal chromium waste. Metal waste production reaches 80% of the total amount of water used in the batik process. Microbial Fuel Cell (MFC) is a promising alternative because of its ability to reduce metal waste Cr6+ while processing organic waste and producing bioelectric. In addition, molasses still contain 50-60% sugar which has the potential to be degraded. Shewanella oneidensis MR-1 was used in this research due to its high productivity. MFC used dual chamber continue to optimize the process. The objective of this research is to analyze the effect of carbon modification and bacterial concentration on MFC electricity production, reduction of Cr6+ and reduction of BOD. Carbon electrode was modified with rice husks and strong acid-base to improve MFC performance. The higher concentration of bacteria and modification of electrodes could produce greater electricity and can reduce more hexavalent chromium metal Cr6+ also able to reduce the most optimum BOD. Variable of 1011 cells/mL bacteria with electrode modification produced the best performance by producing electricity that was relatively more stable and the most optimum with total power density of 690.9mW/m2, reduced heavy metals Cr6+ to Cr3+ up to 92.78% and BOD removal of 33.11%.

Design and Fabrication of Novel Sheet-like Iron Vanadate Photocatalyst for the Photoreduction of Chromium

In past days, the occurrence of toxic heavy metal ions into the water and soil environment causes major health risk to the living organisms. In this work, we mainly focused on the photoreduction of hexavalent chromium (Cr6+) using novel sheet-like Fe2V4O13 photocatalyst under visible light irradiation. The sheet-like Fe2V4O13 was tailored through hydrothermal process using ferric chloride and sodium metavanadate precursors without addition of any templates. The surface morphology, elemental analysis and various physical properties are characterized by numerous spectroscopic techniques. Interestingly, the sheet-like Fe2V4O13 demonstrated proficient photocatalytic performances towards the reduction of Cr6+ into Cr3+. The obtained UV-visible spectroscopy results portrayed that sheet-like Fe2V4O13 could reduce above of Cr6+ solution within 40 min. Moreover, the sheet-like Fe2V4O13 holds very good stability even after five consecutive cycles. This study could open new insights for the design novel nanostructured binary metal oxides for environmental applications.

Chromium Ion Redox Processes in Aqueous Solution during Treatment with Atmospheric Pressure Direct-Current Discharge in Oxygen

The kinetic features of the oxidation–reduction processes of chromium ions during the treatment of potassium dichromate aqueous solutions with concentrations of 0.092–0.39 mmol/L in atmospheric-pressure dc discharge in oxygen at currents of 20–60 mA have been investigated. The solution served as the cathode of the discharge. It has been found that the discharge induces reversible Cr6+ ↔ Cr3+ redox reactions of chromium ions. The limiting degree of reduction of Cr6+ ions slightly depends on the discharge current and decreases with an increase in the initial concentration of the solution. The apparent rate constants of oxidation and reduction have been found, and the energy efficiency of the process has been evaluated. The obtained data are compared with previously obtained results for electric discharge in air and argon. It has been shown that the discharge in argon is the most effective for carrying out the reduction process.

Design of novel solar-light driven sponge-like Fe2V4O13 photocatalyst: A unique platform for the photoreduction of carcinogenic hexavalent chromium

In past days, the occurrence of toxic heavy metal ions into the water and soil environment causes a major health risk to the living organisms. In this work, we mainly focused on the photoreduction of hexavalent chromium (Cr6+) using novel sponge-like Fe2V4O13 photocatalyst under visible light irradiation. The sponge-like Fe2V4O13 was tailored through hydrothermal process using ferric chloride and sodium metavanadate precursors without the addition of any templates. The surface morphology, elemental analysis and various physical properties are characterized by numerous spectroscopic techniques. Interestingly, the sponge-like Fe2V4O13 demonstrated proficient photocatalytic performances towards the reduction of Cr6+ into Cr3+. The obtained UV–visible spectroscopy results portrayed that sponge-like Fe2V4O13 could reduce above of Cr6+ solution within 40 min. The effect of operational reaction parameters such as catalyst dosage, initial Cr6+ concentration and pH of the solution was optimized. Moreover, the sponge-like Fe2V4O13 holds very good stability even after five consecutive cycles. This study could open new insights for the design novel nanostructured binary metal oxides for environmental applications.

Influence of Shewanella oneidensis MR-1 Bacterial Metabolism Process On Waste Treatment Of Cr And Mn Metals In Reactor Microbial Fuel Cell (MFC)

Chromium (Cr) and manganese (Mn) are heavy metals which have high oxidation potential and cause mutagenesis and carcinogenic to living organisms. Microbial Fuel Cell (MFC) appears as one of the applications in the processing of Cr and Mn metals. MFC is bioreactor which converts chemical energy of organic compounds into electrical energy through the catalytic reaction of microorganisms in anaerobic conditions. The aim of this research are to find out the role of Shewanella oneidensis MR-1 bacteria on manufacture electrical energy by utilizing organic wastes, and to find out the concentration of Shewanella oneidensis MR-1 on the most influential anode side for the reduction of metallic Cr, Mn, and its mixed metal on the cathode side by using a MFC. The research method used molasses with bacteria concentration of 10%; 12.5%; 15% (v/v) on the anode side, while on the cathode side was used Cr(VI), Mn(VII) solutions, and mixtures of Cr and Mn solutions with the ratio of 1:0; 0:1; 1:1 (w/w). 15% bacteria concentration on mixture of metal solution resulted the highest electricity production, which was 46.33 (mW/m2), and the best reduction of Cr6+ to Cr3+ 82.15%, while for the reduction of Mn7+ to Mn2 was 61.17%.

A systematic look at chromium isotopes in modern shells – implications for paleo-environmental reconstructions

Abstract. The chromium isotope system (53Cr ∕ 52Cr, expressed as δ53Cr relative to NIST SRM 979) in marine biogenic and non-biogenic carbonates is currently being evaluated as a proxy for the redox state of the ocean. Previous work has concentrated on using corals and foraminifera for this purpose, but investigations focusing on the behavior of Cr in bivalves as potential archives are lacking. Due to their often good preservation, fossil marine biogenic carbonates have the potential to serve as useful archives for the reconstruction of past ocean redox fluctuations and eventually link those to climatic changes throughout Earth's history. Here, we present an evaluation of the Cr isotope system in shells of some modern bivalves. Shell species from Lucidinadae, Cardiidae, Glycimerididae and Pectenidae, collected systematically from one Mediterranean location (Playa Poniente, Benidorm, Spain) over a 3-year period reveal δ53Cr values ranging from 0.15 ‰ to 0.65 ‰, values that are systematically below the local seawater δ53Cr value of 0.83±0.05 ‰. This attests to a significant reduction of dissolved seawater chromium in the process leading to calcification and thus for control of Cr isotope fractionation during biological routes. A similar, constant offset in δ53Cr values relative to surface seawater is observed in shells from Mytilius edulis from an arctic location (Godhavn, Disko Bay, Greenland). Chromium concentrations in the studied shells are significantly controlled by organic matter and typically range from 0.020 to 0.100 ppm, with some higher concentrations of up to 0.163 ppm recorded in Pectenidae. We also observe subtle, species-dependent differences in average Cr isotope signatures in the samples from Playa Poniente, particularly of Lucidinadae and Cardiidae, with considerably depressed and elevated δ53Cr values, respectively, relative to the other species investigated. Intra-species heterogeneities, both in Cr concentrations and δ53Cr values, are favorably seen to result from vital effects during shell calcification rather than from heterogeneous seawater composition. This is because we observe that the surface seawater composition in the particular Playa Poniente location remained constant during the month of July of the 3 years we collected bivalve samples. Intra-shell heterogeneities – associated with growth zones reflecting one to several years of growth, both in δ53Cr and Cr concentrations – are observed in a sample of Placuna placenta and Mimachlamys townsendi. We suspect that these variations are, at least partially, related to seasonal changes in δ53Cr of surface seawaters. Recognizing the importance of organic substances in the bivalve shells, we propose a model whereby reduction of Cr(VI) originally contained in the seawater as chromate ion and transported to the calcifying space, to Cr(III), is effectively adsorbed onto organic macromolecules which eventually get included in the growing shell carbonates. This study, with its definition of statistically sound offsets in δ53Cr values of certain bivalve species from ambient seawater, forms a base for future investigations aimed at using fossil shells as archives for the reconstruction of paleo-seawater redox fluctuations.