Chromium Binding Research Articles

Speciation of Cr(III) and Cr(VI) in aqueous solution and their removal by oyster mushroom biomass

This study investigates the effectiveness of oyster mushroom (Pleurotus platypus) for the removal of Cr(III) and Cr(VI) from aqueous solutions by conducting a series of batch adsorption experiments. Results from the adsorption pH envelope experiments show that while the adsorption of Cr(III) increases with increasing pH, the adsorption of Cr(VI) decreases as the pH rises. These two distinct pH envelope patterns for Cr(III) and Cr(VI) suggest that the adsorption processes are primarily driven by the electrostatic force of attractions followed by the ligand exchange process. Findings from the adsorption isotherm experiments demonstrate that chromium binding to the adsorbent surface is better described by the Freundlich model than the Langmuir model, underscoring the presence of multiple surface binding sites. The study highlights the significance of identifying dominant chromium species before they are subjected to adsorptive removal from contaminated waters. J. Bangladesh Acad. Sci. 47(2); 267-275: December 2023

Hexavalent chromium reduction and bioremediation potential of Fusarium proliferatum S4 isolated from chromium-contaminated soil.

Microbial remediation, utilizing reduction of Cr(VI) to Cr(III), is considered a promising method for lowering toxic environmental chromium levels. In this study, a Cr(VI)-resistant fungal strain, Fusarium proliferatum S4 (F. proliferatum), was isolated from seriously chromium-polluted soil at Haibei Chemical Plant, China. This strain for treatment chromium-containing solution resulted in 100.00%, 93%, and 74% removal at initial concentrations of 10, 30, and 50mg L-1 Cr(VI), respectively, after 12days of treatment in a batch mode. Contributions of different cell fractions to Cr(VI) removal were explored. The Cr(VI) removal capacity of various cell components from strong to weak was as follows: cytoplasm, cell secretions, and cell debris. Observations obtained by scanning electron microscopy and transmission electron microscopy with energy dispersive X-ray spectroscopy revealed that not only the cell surfaces but also the intracellular contents were involved Cr through adsorption, reduction, or accumulation. Fourier transform infrared spectra indicated that a large number of functional groups (amino, carbonyl, carboxyl, and phosphate groups) participated in chromium binding on the cell surface. X-ray photoelectron spectroscopy confirmed the presence of Cr on the cell surface only as Cr(III). The results have important implications for an in-depth understanding of microbial chromate reduction by F. proliferatum. This study provides an insight into the microbial Cr(VI) bioreduction efficiency, and mechanisms in the chromium-contaminated environment.

Ultraviolet-visible and fluorescence spectra indicate the binding and transformation properties of hexavalent chromium in DOM solution

Dissolved organic matter (DOM) with complex functional groups affects the trace of binding and transformation of hexavalent chromium (Cr(VI)) in the environment. However, the mechanisms and quantitative indicators are still ambiguous. In this study, the perspectives of spectroscopy with synchronous and excitation-emission matrix (EEM) fluorescence, and ultraviolet-visible (UV–vis) were employed to probe the interaction of DOM and Cr(VI). The spectroscopic parameters with Dslope240–280 (the slope of the log-transformed absorbance in the range of 240–280 nm), DSlope325–375 (the slope of the log-transformed absorbance in the range of 325–375 nm), SR (the ratio of the slope of the log-transformed absorption coefficient with 275–295 nm to 350–400 nm), SFI (specific fluorescence intensities), AvgFI/AvgUVA (the ratio of average fluorescence to average UV), and SS (Stokes shift) of UV–vis and fluorescence were extracted to characterize the DOM properties. The UV–vis results showed carboxylic-like and total phenolic/carboxylic-like groups were contributed to the complexation of Cr, which resulted in forming macromolecular aggregates, however, the degree of participation of different functional groups was uneven and the binding sequence depended on the types of fractions especially the ratio of protein. Meanwhile, the EEM results proved interaction between DOM and Cr also affected the hydrophobicity and quantum yield of solutions. In addition, according to the change orders of substances and functional groups by Two-dimensional correlation spectroscopy (2DCOS) analysis, aromatic amino acids substances were much more sensitive binding onto Cr than fulvic-like fractions and humic-like fractions, and the mechanisms for Cr(VI) retention were developed. Statistical analysis results showed that SR and SFI were significantly correlated with the concentration of Cr(VI), and multiple linear regression models were developed to predict concentrations of Cr(VI). Moreover, it was found the goodness of fit of SFI was better than that of SR at low Cr concentrations, but opposite at high concentrations.

Biosorption of chromium by dry algae Chlorella kessleri

Decontamination of environment according to traditional methods is not only economically inaccessible but also often highly environmentally harmful. It is necessary to apply methods that are environmentally friendly as possible. These methods include bioremediation, which uses organisms able to fight with high concentrations of pollutants to decontaminate the environment. In this study, we observed the biosorption of chromium from the aquatic environment using dried algae Chlorella kessleri. The results of the specific sorption at pH = 4.00 in the chromium model sample had a value of q = 5.9 mg / g, which represented a decrease in the chromium concentration by 74%. The specific sorption in the case of the native chromium sample q after 24 hours reached the highest value (q = 2.74 mg / g) at pH = 4.00. During the experiment, we observed a constant change in the content of photosynthetic pigments in the prepared solutions where it was shown that after 24 h exposure the yield of chlorophyll a was decrease by 95.91% compare to control. In the case of chlorophyll b, the same trend was demonstrated where 91.92% decrease of its content after 24 hours was observed. This type of dried alga has been shown to be a rapid biosorbent, in relatively short time intervals. The results of the study show that in the process of decontamination there was not only the binding of chromium to the cell surface but also its penetration through the cell wall.

Crosslinked poly(ionic liquid)s as selective receptors for Cr(VI) – Counter anion effect and application in treating drinking water and tannery effluents

A series of crosslinked poly(ionic liquid)s (C-PILs), with five different counter anions, were synthesised and investigated for their Cr(VI) binding properties. C-PIL with chloride as the counter anion (C-PIL-Cl) showed the highest sorption capacity (2.96 ± 0.03 mmol/g), while bis(trifluoromethanesulfonyl)imide led to minimum uptake. The chromium binding properties of C-PIL-Cl were investigated in detail, which revealed remarkable Cr(VI) selectivity against other anions. It could reduce the Cr(VI) concentration from 980 ppb to 28 ppb in a typical drinking water sample, which is below the WHO prescribed acceptable limit. The polymer could also remove about 90% of Cr(VI) from synthetic tannery effluent. SEM-EDX studies clearly revealed the chromium uptake and regeneration of the chromium sorbed polymer to be through anion exchange at the binding sites. Quantitative elution of the bound chromium from C-PIL-Cl was feasible with 1 M NaOH and 1 M NaCl solution. While the polymer regenerated with NaCl showed sorption properties similar to the virgin polymer, regeneration with NaOH led to reduced capacity. Microscopic investigations showed no effect on polymer morphology during desorption. The sorption kinetics followed pseudo second order and showed poor fit with pseudo first order model.

An experimental approach for the utilization of tannery sludge-derived Bacillus strain for biosorptive removal of Cr(VI)-contaminated wastewater.

Biosorption efficacy of Bacillus strain DPAML065, isolated from the tannery sludge, was appraised for the removal of toxic hexavalent chromium (VI) ions from synthetic wastewater. Effects of the process variable on biosorbent surface by variation in pH, metal Cr(VI) concentration and retention time were examined using batch experiments. The isolated Bacillus strain biosorbent was studied for its morphology and surface chemistry through FE-SEM, EDX and FTIR. It discloses that, the reduction mechanism of Cr(VI) during the process is mainly attributed to precipitation in addition to the functional groups (such as -COOH, -OH, C-O, P=O) present on the cellular matrix of Bacillus. Biochemical tests and 16s rRNA sequencing were also performed to identify the biosorbent at the genus level. A 95% Cr(VI) removal efficiency was procured by Bacillus strain DPAML065 biosorbent at pH6, incubation period 24h, 80mg/L initial feed concentration and operational temperature 35°C. Equilibrium behaviour of chromium binding follows the Langmuir isotherm model (R2 = 0.968) with an adsorption capacity of 106.38mg/g. Kinetic modelling disseminates that biosorption of Cr(VI) ions by Bacillus strain DPAML065 obeyed pseudo-second-order model (R2 = 0.984) rather than the pseudo-first-order model. Concisely, the results indicate that the Bacillus strain DPAML065 is a potential, economically feasible and eco-friendly biosorbent which can be effectively used for removal of chromium (VI) from wastewater.

Utilization of extracellular polymeric substances (EPS) immobilized in epoxy polymer as double ion exchanger biosorbent for removal of chromium from aqueous solution

Cation and industrial pollutant anions are removed from wastewater using organic cation and anion exchange resin. "Extracellular Polymeric Substance" (EPS) from bacterial extraction can accumulate cation and anion elements through biosorption by adsorption mechanism, ion exchange, formation of complex compounds and hydrogen bonds. EPS can be used as an biosorbent and ion exchange bioresin replacing organic resins, because EPS contains organic functional groups that are negatively charged (RCOOH, ROPO3H, ROPO3Na, ROSO3H, ROSO3Na, etc.) cation absorbers and positively charged (ROH, RCNH2HCOOH, etc.) anion absorber. EPS consists of 40-95% polysaccharide compounds, protein 1-60%, nucleic acids 1-10%, lipids 1-10% and the remaining amino acid polymers and other compounds. The tannery industry produces trivalent (Cr+3) chromium pollutants at levels of 15.2 ppm and hexavalent (CrO4-2 or Cr2O7-2) levels of 0.77 ppm which exceeds the standard quality for a total Cr of 0.6 ppm. Cr pollutants are very dangerous for human health. Research had been done on the use of immobilized EPS bioresin in epoxy polymers for chromium binding. EPS was extracted from bacterial activated sludge by centrifugation at 9000 rpm for 20 minutes at 4°C, the filtrate was EPS. The analysis showed EPS content were 16% fat, 12% carbohydrate, and 16% protein. The functional group analysis results with infrared ray spectroscopy (FTIR) showed EPS containing chemical bonds such as -CH, -OH, -NH, and -C=O which proved that EPS extraction contained RCOOH, ROH, and RCNH2HCOOH functional components which were exchanging components cations and anions. Epoxy polymers were prepared by mixing bisphenol A monomers and 1: 1 ratio epichlorohydrin. Immobilized EPS double ion exchange biorecin in epoxy polymers was prepared by mixing 200 mg EPS and 1800 mg epoxy. The binding of chromium ions in the resin was carried out by recirculating the chromium solution through a burette column filled with 2 rams of bioresin at pH 5, 6 and 7. The optimum results gave chromium ion absorption efficiency of 89.20% at pH 5. Column operations could be optimized by varied the amount of bioresin used.

Bidentate and Monodentate Binding of Chromium (VI) on the Acid Scavenger Exchange Resin, MP-Carbonate, as a Function of pH

Hexavalent chromium or Cr (VI) is a serious health risk that has been identified in drinking water and generally originates as industrial by-products and hazardous waste sites, although natural sources have been reported. Common methods to remove Cr (VI) employ adsorption, filtration, ion exchange technologies, and redox reactions. We report on a macro porous polystyrene anion exchange resin, a solid-supported equivalent of tetra-alkyl ammonium carbonate (Biotage® MP-Carbonate), to effectively remove Cr (VI) from synthetic wastewater with bidentate or monodentate binding modes of chromate, depending on the pH environment. Sorption capacity is pH dependent with 332, 161, and 163 mg CrO42− per gram of MP-Carbonate adsorbed at pH 2, 6.8, and 10, respectively. Experimental data indicate an exergonic and endothermic adsorption process. Static vs. dynamic reaction conditions are discussed. Density functional theory calculations parallel the Gibbs free energy results of the bidentate binding of the large chromate ion compared to carbonate with the ammonium ions. These studies identify the potential uses of MP-Carbonate in the remediation of Cr (VI) wastewater unrelated to the pH of the water source.

Significance of Conformation Changes During the Binding and Release of Chromium(III) from Human Serum Transferrin

Abstract Objectives Transferrin, Tf, the protein that transports iron from the blood to the tissues via endocytosis, is believed to also transport chromium(III), Cr(III). Recently, the presence of multiple conformations was suggested by spectroscopic studies. The objective of this work is confirm whether various conformers of Cr(III)2-Tf exist and their potential significance for Cr(III) transport. Methods Cr(III) was added to apoTf in a buffered solution at pH 7.4 containing 25 mM bicarbonate at 37 °C. After time intervals, ultraviolet spectra were collected, or aliquots were removed and frozen for analysis by electron paramagnetic resonance (EPR) spectroscopy, which can distinguish free Cr(III) and Cr(III) bound to the two metal binding sites of Tf. To model the acidification of the endosome that triggers release of metal ions from Tf, the Cr(III)2-Tf conformer solutions were acidified by the addition of hydrochloric acid to pH 4.5 or 5.5. At time intervals after acidification, samples were again analyzed by ultraviolet and EPR spectroscopies. Results A combination of electronic and EPR studies reveal that the addition of Cr(III) to apoTf at near neutral pH in the presence of 25 mM bicarbonate results in the rapid binding of two Cr(III) accompanied and then followed by a series of conformation changes in Cr(III)2-Tf. These multiple conformations give rise to different spectroscopic properties and upon acidification different rates of Cr(III) release. Conclusions The conformer of Cr(III)2-Tf used in most previous studies and giving rise to EPR features at g ∼ 5.1, 5.4, and 5.6 forms too slowly to be physiologically relevant; however, two previously unknown conformers of Cr(III)2-Tf, giving rise to an EPR feature at g ∼2 and at g ∼ 5.4, respectively, were identified. The latter of these conformers has a lifespan similar to the turnover time of transferrin and releases Cr(III) rapidly, suggesting it is probably the most physiologically significant conformer of Cr(III)2-Tf. Funding Sources The University of Alabama College of Arts and Sciences Research Award.

Biosorption performance of date palm empty fruit bunch wastes for toxic hexavalent chromium removal

Biosorption ability of date palm empty fruit bunch (DPEFB) was examined for the removal of toxic hexavalent chromium (Cr6+) ions from synthetic wastewater. The pretreated DPEFB biosorbent was studied for its morphology and surface chemistry through Scanning electron microscopy, Energy dispersive elemental analysis and Fourier transform infrared spectroscopy. Effect of biosorption parameters such as pH, biosorbent dosage, contact time, temperature, initial feed concentration and agitation speed on the Cr6+ ions removal efficiency by DPEFB was critically evaluated. The isoelectric point for the DPEFB sorbent was observed at pH 2, above which it was dehydronated to capture the positively charged Cr6+ ions. Batch biosorption studies showed that an optimal chromium removal efficiency of 58.02% was recorded by the DPEFB biosorbent for pH 2, dosage 0.3 g, 100 rpm agitation speed, 120 min contact time, 50 mg/L initial feed concentration and 30 °C operational temperature. Thermodynamic analysis showed that the binding of Cr6+ ions on DPEFB surface was exothermic, stable and favorable at room temperature. Equilibrium behavior of chromium binding on DPEFB was more aligned to Temkin isotherm (R2 = 0.9852) highlighting the indirect interactions between Cr6+ ions and the biosorbent. Kinetic modeling revealed that the biosorption of Cr6+ ions by DPEFB obeyed pseudo-second order model than the pseudo-first order and intra-particle diffusion models. Reusability studies of the DPEFB sorbent showed that NaNO3 was an effective regenerant and the biosorbent can be efficiently reused up to three successive biosorption-desorption cycles for chromium removal. In summary, the results clearly showed that the DPEFB biowaste seems to be an efficient, economic and eco-friendly biosorbent for sustainable removal of toxic hexavalent chromium ions from domestic and industrial wastewater streams.

Synthesis, characterization, computational study, DNA binding and molecular docking studies of chromium (III) drug-based complexes

Chromium(III) drug-based metal complexes [Cr(CTZ)(NO3)3]∗9H2O(1), [Cr(MCNZ)(NO3)3] (2), and [Cr(PNZ)(NO3)2] NO3∗5H2O (3) were synthesized and characterized by FTIR, UV–Vis, mass spectroscopy and Thermogravimetric Analysis (TGA). The Cr(III):Ligand stoichiometric ratio is 1:1; with a mono-dentate imidazole ring bonding via a nitrogen atom active site. The geometric structure and spectroscopic properties of the complexes were investigated by using Density Functional Theory (DFT) and Time Dependent Density Functional Theory (TD-DFT). The experimental and theoretical investigations indicated that the complexes have distorted octahedral symmetries. The simulated UV–Vis and IR spectra are in satisfactory agreement with the experimental ones. The main absorption bands are related to Ligand-to-Ligand Charge Transfer (LLCT) and/or Ligand-to-Metal Charge Transfer (LMCT) transitions. DNA binding using UV–Vis spectroscopy was applied to investigate the binding mode of these complexes. The complexes bind non-covalently as groove binders. Molecular docking score between the three complexes with Human Insulin Receptor (PDB code = 4OGA) gave high docking scores in the order: complex (2) > complex (3) > complex (1).

Chromium removal from aqueous solutions using new silica gel conjugates of desferrioxamine or diethylenetriaminepentaacetic acid.

Desferrioxamine (DFO) and diethylenetriaminepentaacetic acid (DTPA) conjugated with silica gel (IDFOSG and IDTPASG, respectively) were evaluated as adsorbents for chromium in aqueous solutions. Different parameters affecting adsorption such as pH, sorbent dosage, contact time, sample volume and potential of interfering ions have been optimized. The optimum pH for chromium binding was 4 for 100mg of adsorbents at 5min of table shaking with 5mL sample volume of chromium solutions. Langmuir adsorption model described the removal of chromium ions. The adsorption capacity for chromium was 90% for IDFOSG and 83% for IDTPASG in single solutions, and at least 75% in multielemental solutions. Considering the removal efficacy, regeneration and stability, DFO-grafted silica gel was generally superior to its DTPA counterpart and may be applied to the removal of traces of chromium species from natural waters.

Significance of conformation changes during the binding and release of chromium(III) from human serum transferrin

Trivalent chromium has been proposed to be transported in vivo from the bloodstream to the tissues via endocytosis by transferrin (Tf), the major iron transport protein in the blood. While both Cr(III) binding and release from Tf have been proposed to be too slow to be physiologically relevant, recent kinetic studies under physiological conditions demonstrate that Cr(III) binding and release are sufficiently fast to occur during the time of the endocytosis cycle (circa 15 min). Consequently, the release of Cr(III) from human and bovine serum Tf has been examined under conditions mimicking an endosome during endocytosis. These studies have also found that Cr(III)2-Tf can exist in multiple conformations giving rise to different spectroscopic properties and different rates of Cr(III) release. Time-dependent spectroscopic studies of the binding and release of Cr(III) from human serum Tf have been used to identify three different conformations of Cr(III)2-Tf. The conformation of Cr(III)2-Tf used in most previous studies forms too slowly to be physiologically relevant and slowly releases Cr(III) in endosomal pH range. The conformation formed between 5 min to 60 min after the addition of Cr(III) to apoTf at pH 7.4 in 25 mM bicarbonate resembles the conformation of Cr(III)2-Tf in its complex with Tf receptor (TfR) and loses Cr(III) rapidly at endosomal pH, although not as fast as the Tf-TfR complex. The significance of these conformations and the potential role of Tf in detoxification of Cr(III) are described.

Release of trivalent chromium from serum transferrin is sufficiently rapid to be physiologically relevant

Transferrin, the major iron transport protein in the blood, apparently also transports trivalent chromium in vivo via endocytosis. Recent in vitro studies have, however, suggested that the release of chromic ions from serum transferrin is too slow to be biologically relevant. Consequently, the release of chromium(III) from human serum transferrin has been examined under conditions mimicking an endosome during endocytosis. At pH 4.5 and 5.5, the release of Cr(III) from transferrin occurs rapidly from the weak binding site. While appreciably slower, the release of Cr(III) from the tighter site in the presence of biological chelating agents is potentially sufficiently fast to be physiologically relevant. When Cr(III)-loaded transferrin is added to soluble transferrin receptor, the interaction with the receptor results in Cr(III) in both the weak and tight binding sites giving rise to an EPR signal similar to that of the weak binding site; concurrently, the loss of Cr(III) from both binding sites becomes rapid at acidic pH, more rapid than from either site in the absence of the receptor. Loss of Cr(III) from the transferrin-transferrin receptor complex, thus, is easily sufficiently rapid for transferrin to serve as the physiological transporter of Cr(III) from the bloodstream to the tissues. However, detailed studies of conformational changes of transferrin associated with the binding and release of chromium along with investigations of how and at what rate Cr(III) is transported from the endosome will be required before this question of whether transferrin transport Cr(III) in vivo can be definitively resolved.

Nano-Bio sequential removal of hexavalent chromium using polymer-nZVI composite film and sulfate reducing bacteria under anaerobic condition

Abstract A sequential method for removing Cr(VI) was studied using polymer-nano zero valent iron (nZVI)-based composite film and live biomass of sulfate reducing bacteria (SRB) under anaerobic conditions. The polymer-nZVI composite film was made using layer-by-layer coating of chitosan, polyethylene glycol (PEG) blend, poly (sodium 4-styrene sulphonate) solution (PSS), and nZVI on a glass slide. The conditions for the removal of Cr(VI) through polymer-nZVI nanocomposite film were optimized using Response Surface Methodology (RSM). SRB was grown in Baar’s medium was adapted to 10 mg L−1 of Cr(VI) and the live biomass obtained thereof was used for removal of Cr(VI). Cr(VI) removal capacity of 394 mg g−1 was observed at optimal conditions by nano-bio sequential removal process using polymer-nZVI-based composite film and SRB. The XRD, SEM-EDX, and FT-IR spectral data demonstrated the reduction of Cr(VI) and binding of chromium (Cr) species on the polymer-nZVI-based composite film and SRB biomass. The applicability of the sequential Cr(VI) removal process using polymer-nZVI-based composite film and SRB was successfully verified in Cr(VI)-spiked environmental water samples.

The effects of the glycation of transferrin on chromium binding and the transport and distribution of chromium in vivo

Chromium (III) has been shown to act as a pharmacological agent improving insulin sensitivity in rodent models of obesity, insulin resistance, and diabetes. To act in beneficial fashion, chromium must reach insulin-sensitive tissues. Chromium is transported from the bloodstream to the tissues by the iron-transport protein transferrin. When blood concentrations of glucose are high (as in a diabetic subject), transferrin can be glycated, modifying its ability to bind and transport iron. However, the effects of glycation of transferrin on its ability to bind and transport Cr have not been examined previously. Storage of transferrin at 37°C in the presence and absence of glucose has significant effects on the binding of Cr. Transferrin stored in the absence of glucose only binds one equivalent of Cr tightly, compared to the normal binding of two equivalents of Cr by transferrin. Glycated transferrin (stored in the presence of glucose) binds two equivalents of Cr but the changes in its extinction coefficient at 245nm that accompany binding suggest that the Cr-bound transferrin possesses a conformation that deviates appreciably from untreated transferrin. These changes have dramatic effects, greatly reducing the ability of transferrin to transport Cr in vivo in rats. The results suggest that glycation of transferrin in subjects with high blood glucose concentrations should reduce the ability of Cr from pharmacological agents to enter tissues.

Stabilization of Chromium in Synthetic Slags with FeSO4 and FeS2

In this work, the effects of the slag basicity (mass ratio CaO to SiO2) and the addition of FeSO4 or FeS2 into the slag on the stability of the mineralogical species in the slag containing chromium compounds were studied. The chemical stability was evaluated by leaching the slags with an acid solution. The main Cr-compounds in slags with low basicity (CaO/SiO2 = 1) and FeSO4 were FeCr2O4 and Ca3Cr2Si3O12, while Cr3S4 and Ca3Cr2Si3O12 were formed in slags with FeS2. The slags with high basicity (CaO/SiO2 = 2) and FeSO4 contained CaCr2O4, FeCr2O4 and Ca5(SiO4)2SO4. The slags with FeS2 and high basicity produced FeS·Cr2S3, FeCr2O4 and Ca3Cr2Si3O12. The results showed that the lowest chromium concentration levels in the leaching liquors corresponded to slags with CaO/SiO2 = 1 and high FeS2 contents, owing to the stable binding of chromium in the compounds FeCr2O4, Cr3S4 and Ca3Cr2Si3O12.

Effective bioleaching of chromium in tannery sludge with an enriched sulfur-oxidizing bacterial community

In this study, a sulfur-oxidizing community was enriched from activated sludge generated in tannery wastewater treatment plants. Bioleaching of tannery sludge containing 0.9–1.2% chromium was investigated to evaluate the effectiveness of the enriched community, the effect of chromium binding forms on bioleaching efficiency, and the dominant microbes contributing to chromium bioleaching. Sludge samples inoculated with the enriched community presented 79.9–96.8% of chromium leaching efficiencies, much higher than those without the enriched community. High bioleaching efficiencies of over 95% were achieved for chromium in reducible fraction, while 60.9–97.9% were observed for chromium in oxidizable and residual fractions. Acidithiobacillus thiooxidans, the predominant bacteria in the enriched community, played an important role in bioleaching, whereas some indigenous heterotrophic species in sludge might have had a supporting role. The results indicated that A. thiooxidans-dominant enriched microbial community had high chromium bioleaching efficiency, and chromium binding forms affected the bioleaching performance.

Binding of Chromium(III) to Transferrin Could Be Involved in Detoxification of Dietary Chromium(III) Rather than Transport of an Essential Trace Element.

Cr(III) binding to transferrin (Tf; the main Fe(III) transport protein) has been postulated to mediate cellular uptake of Cr(III) to facilitate a purported essential role for this element. Experiments using HepG2 (human hepatoma) cells, which were chosen because of high levels of the transferrin receptor, showed that Cr(III) binding to vacant Fe(III) -binding sites of human Tf effectively blocks cellular Cr(III) uptake. Through bio-layer interferometry studies of the Tf cycle, it was found that both exclusion and efflux of Cr2 (III) Tf from cells was caused by 1) relatively low Cr2 Tf affinity to cell-surface Tf receptors compared to Fe2 Tf, and 2) disruption of metal release under endosomal conditions and post-endosomal Tf dissociation from the receptor. These data support mounting evidence that Cr(III) is not essential and that Tf binding is likely to be a natural protective mechanism against the toxicity and potential genotoxicity of dietary Cr through blocking Cr(III) cellular accumulation.

Binding of Chromium(III) to Transferrin Could Be Involved in Detoxification of Dietary Chromium(III) Rather than Transport of an Essential Trace Element

AbstractCrIII binding to transferrin (Tf; the main FeIII transport protein) has been postulated to mediate cellular uptake of CrIII to facilitate a purported essential role for this element. Experiments using HepG2 (human hepatoma) cells, which were chosen because of high levels of the transferrin receptor, showed that CrIII binding to vacant FeIII‐binding sites of human Tf effectively blocks cellular CrIII uptake. Through bio‐layer interferometry studies of the Tf cycle, it was found that both exclusion and efflux of Cr2IIITf from cells was caused by 1) relatively low Cr2Tf affinity to cell‐surface Tf receptors compared to Fe2Tf, and 2) disruption of metal release under endosomal conditions and post‐endosomal Tf dissociation from the receptor. These data support mounting evidence that CrIII is not essential and that Tf binding is likely to be a natural protective mechanism against the toxicity and potential genotoxicity of dietary Cr through blocking CrIII cellular accumulation.