Chromate Anions Research Articles

Tuning the Fe(II)/hydroxide Ratio during Synthesis of Magnetite Nanoparticles to Maximize Cr(VI) Uptake Capacity

The impact of hydroxyl excess as defined by the Fe(II)/hydroxide ratio during the synthesis of Fe3O4 nanoparticles by oxidative precipitation of FeSO4 was examined as a critical parameter determining the potential for Cr(VI) uptake from polluted water. Various samples were prepared by varying the OH− excess in the range of −0.10 up to +0.03 M and characterized according to their composition, morphology, and surface configuration. Their efficiency for Cr(VI) removal was evaluated by batch adsorption tests, carried out under similar conditions with drinking water purification in the concentration range below 10 mg/L. Results indicate that near the zero-excess point for hydroxyl balance, the uptake capacity for residual Cr(VI) concentration equal to 25 μg/L remains at very low levels (<0.5 mg/g). However, a small increase above +0.02 M features synthesized nanoparticles with an uptake capacity of 2.5 mg/g owed to the decrease in particles size (28 nm) and enhancement of the reducing potential (Fe2+/Fe3+ = 0.42). In addition, utilizing negative excess values below −0.05 M triggers a similar efficiency rise, although the morphology of the obtained aggregates is rather different. Such finding is attributed to a possible exchange mechanism between adsorbed sulfates and chromate anions that assist approach of Cr(VI) to the material’s surface. Overall, proper tuning of hydroxyl excess offers multiple options for the implementation of monodisperse magnetically responsive nanoparticles or larger aggregates with optimized purification efficiency in water technology.

Photoinduced adsorption of Cr(VI) ions in nano‐zinc oxide and nano‐zinc oxide/polypyrrole composite

AbstractZinc oxide‐polypyrrole composites (ZnO‐PPY) are novel materials that have attracted significant attention in recent years. These materials have the potential to remove hazardous pollutants from water effectively. Here, we explore facile synthesis, characterization, and adsorption capacity of hexavalent chromium anions, by nano‐ZnO and nano‐ZnO‐PPY composite. Cold precipitation of ZnO using sodium dodecyl sulfate as an additive prevents nanoparticle growth, which permits the composite preparation with PPY. The results demonstrated that the ZnO‐PPY structure was different than ZnO, more amorphous, less porous, and with a different morphology. New Fourier transform infrared spectroscopy vibrations observed in ZnO‐PPY hinted an interaction with ZnO, leading to a more acidic surface. This information could point to the composite formation driven by interactions between oxygen groups and pyrrole nitrogen. Both materials performed better under light, indicating a photoinduced adsorption phenomenon favored by oxygen vacancies. The new composite demonstrated a reduced adsorption capacity compared to ZnO at neutral and slightly acidic pHs due to electrostatic repulsion of the negatively charged surface and Cr(VI) anions. At alkaline pHs, the pyrrolic nitrogen's is able to interact with partially charged Cr‐complex and to carry out a ligand interchange even in a not favorable charge environment. The ZnO‐PPY composite, outperformed the zinc oxide capacity.

Use of Thermally Modified Jarosite for the Removal of Hexavalent Chromium by Adsorption

Jarosites are residues generated during the purification of zinc and are composed mainly of iron sulfates ((Na, K)Fe3(SO4)2(OH)6). Due to the large volume of jarosite generated during the process, these residues tend to be deposited in large land areas and are not used. In the present work, jarosite was used without heat treatment (JST) as an adsorbent of hexavalent chromium contained in a sample of wastewater from a chrome plating industry under the following conditions: C0 = 200 mg/L of Cr, T = 25 °C, and pH = 3. It was only possible to remove 34% of Cr (VI). Subsequently, a thermal treatment of a jarosite sample (JTT) was carried out at 600 °C. The heat-treated sample was later used as an adsorbent in the same conditions as those for JST. The maximum chromium removal was 53%, and the adsorption capacity was 10.99 mg/g. The experimental data were fitted to the Langmuir model and to the pseudo-second-order kinetic model. It was determined that the adsorption process involved electrostatic attractions between the surface of the positively charged adsorbent and the chromium anions contained in industrial wastewater.

Electrically responsive structural transformations triggered by vapour and temperature in a series of pleochroic bis(oxalato)chromium(iii) complex salts

Complex salts of DABCO- and ABCO-based cations and bis(oxalato)chromium(iii) anions, prepared by a simple and environmentally friendly approach, exhibit stimuli-responsive structural transformations involving desolvation and resolvation processes.

Synthesis and validation of polystyrene-based polyethylenimine composite for Cr(VI) removal from aqueous solution: Performance and mechanism

With increasing groundwater and surface water contamination due to improper discharge of chromium-laden wastewaters, it is compelling to synthesize a material that can selectively remove Cr(VI) from the impaired water at neutral or near-neutral pH. In this study, various composites were synthesized through chemical grafting of polyethylenimine (PEI) polymer on the surface of INDION PA 800 polystyrene beads by varying the PEI loading and glutaraldehyde (GA) crosslinking for Cr(VI) removal. The adsorbent called PS-7 with 16% PEI and 2.5% GA crosslinking performed best, demonstrating Cr(VI) removal greater than 99% even at pH 9. The adsorbent showed significant trace Cr(VI) removal from the impaired solution containing competing anions at 500 times the concentration of Cr(VI) at neutral pH. Further, 1.0 g/L adsorbent showed 100% Cr(VI) removal from a solution containing 5 mg/L Cr(VI) at pH 7 and 9. PS-7 showed significant adsorption capacity of 355.26 and 143.6 mg/g for fixed-bed column runs at influent pH 3 and 5, respectively. An insightful investigation revealed that the use of hydrophobic matrix for PEI immobilization played an important role in Cr(VI) removal at neutral pH, as opposed to the other reported PEI-based adsorbents with hydrophilic matrix which were capable of removing chromate anions at acidic pH only. The phenomenon of redox reaction occurring in the adsorbent was validated by X-ray photoelectron spectroscopy, where Cr(VI) was reduced to Cr(III) with the help of electron-donating NH2 groups and the adsorbent got oxidized. Thus, PS-7 adsorbent holds immense potential for treating real-life Cr(VI)-contaminated waters.

Efficient removal of Cr (VI) by modified sodium alginate via synergistic adsorption and photocatalytic reduction

Conventional photocatalytic removal of hexavalent chromium (Cr (VI)) suffers from low metal uptake capacity, expensive photocatalysts, and slow desorption of trivalent chromium from the catalyst surface. In this study, calcium chloride and iron(III) nitrate were used to cross-link with polyethyleneimine grafted sodium alginate (SA) to acquire two materials, named as SPC and SPF, respectively, for the efficient removal of Cr (VI) from aqueous solutions via adsorption and photocatalytic reduction. FT-IR, DFT, and XPS results indicated that polyethyleneimine was successfully grafted onto the SA matrix and could electrostatically attractted hexavalent chromium anions in solution. Fe (III) cross-linking forms a highly porous structure at ambient temperature, which could overcome the polysaccharide collapse in the dehydration process. The UV–vis DRS, EIS, and photocurrent density characterizations of SA, SPC, and SPF suggested that SPF has smaller charge transfer resistance and is conducive to the separation of photogenerated charges, thus SPF exhibited superior performance in the photocatalytic reduction of Cr (VI). As a result, SPF is an efficient photocatalyst for the removal of Cr (VI) and provides new sights on polysaccharide-based photocatalysts applications.

Efficient Ion Separation from Environmental and Biological Samples Using a Novel Sorbent Based on Ni-Substituted ZIF-67: Optimization, Equilibrium, Kinetic, and Thermodynamic Study

After routine alkali treatment of industrial wastewaters, a subsequent purification step is required before discharging the resulting effluent due to the remaining trace amount of chromium(VI) at milligram per liter levels. In the present study, a simple one-step synthesis was carried out to fabricate a new adsorbent of NixCoyZIF-67 for adsorption and removal of wastewaters containing chromium(VI) efficiently. To find optimum adsorption conditions of chromium(VI), the sample pH effect, initial concentration of chromium(VI), temperature, contact time, and adsorbent amount were examined. At 298 K, an adsorption capacity of 30.3 mg/g was calculated. Analyzing the mechanism of removal indicated that the chromium(VI) uptake by NixCoyZIF-67 fits with the Langmuir and Dubinin–Radushkevich isotherms well and follows a pseudo-second-order kinetic approach. Also, a positive ΔG and negative ΔH and ΔS indicated adsorption that is endothermic and occurs spontaneously. Electrostatic attractions of chromium(VI) anions by the positively charged surface of the adsorbent and ion exchange with the hydroxyl groups of the adsorbent surface can be ascribed as the mechanism of adsorption. Under the optimized conditions, the detection limit was 0.003 mg/L, and the quantification limit was 0.010 mg/L. The linear range of the calibration plot was between 0.010 and 2.0 mg/L (R2 > 0.9900). The intraday and interday RSDs% at two concentration levels of 0.05 and 0.20 mg/L were less than 3.4%. Eventually, the applicability of the presented method was evaluated for measuring trace amounts of chromium(VI) in blood samples. Acceptable relative recoveries of 82–93% were achieved. Overall, it can be concluded that NixCoyZIF-67 can be introduced as a promising sorbent for the determination of chromium(VI) into complex biological fluids and its decontamination from industrial wastewaters.

Secondary Role of Aliphatic and Heterocyclic Amines in the Formation of Low-Temperature Amine-Bearing U, Np, and Pu(VI) Chromates.

Uranyl compounds with tetrahedral oxoanions demonstrate a significant structural and topological diversity. Complexes of transuranium elements with such anions are not equally well-represented in the literature. To answer the question about the structural similarity in a series of An6+ complexes with XO42- anions, we synthesized and studied 10 new U, Np, and Pu chromates with outer-sphere organic cations. The structural analysis and comparison with the literature data shows that the Np and Pu complexes are generally based on the same structural blocks as the uranyl compounds. Moreover, the chromate anion does not show any unique structural role as compared to the sulfate and selenate ions. As a result, the neptunium and plutonium chromates contain 1D and 2D structural units similar to those found in the uranyl sulfates and selenates. The templating role of the outer-sphere cations in the actinyl complexes with tetrahedral oxoanions is also not evident, and there is no clear correlation between the nature of the outer-sphere cations and the topology of the structural units.

Nanocrystalline erdite from iron-rich sludge: Green synthesis, characterization and utilization as an efficient adsorbent of hexavalent chromium

A low-temperature hydrothermal process was developed to synthesize erdite adsorbent from a solid waste sludge contained 10.2% Fe, 6.2% Al and 1.4% Si, alongside 59.5% water content. At 90℃, adding Na2S and NaOH could convert it into erdite nanorods with a diameter of 80 nm and a length of 100 nm. In the sludge, only Fe oxyhydroxide was involved in the formation of erdite, and the other Al/Si-bearing compounds were dissolved in an alkaline medium. The dissolved Al/Si-bearing compounds were further removed, forming faujasite so that the used medium was purified and then entirely recycled into the next conversion stage. No secondary waste was generated in the pilot-scale conversion, and the adsorption efficiency of the prepared products to wastewater with a high initial Cr(VI) concentration of 1000 mg/L was more than 99.5%. The adsorption data complied with the pseudo-second-order kinetics. During the wastewater treatment, hexavalent chromium anion diffused to erdite surface and replaced OH/SH groups of adjacent structural Fe to form a stable complex ligand. In addition, the redox reaction between hexavalent chromium and the -SH group occurred to generate a trivalent chromium complex on the Fe/S-bearing flocs surface.

Transcriptome analysis reveals antioxidant defense mechanisms in the red swamp crayfish Procambarus clarkia after exposure to chromium

Chromium (Cr) as a chromate anion has a strong redox capacity that seriously threatens the ecological environment and human health. Cr can contaminate water and impart toxicity to aquatic species. Procambarus clarkii is an important food source that once represented a large proportion of the aquaculture industry due to its rapid reproduction and high economic value. However, there have been reports on the death of P. clarkii due to heavy metal pollution. The underlying mechanism regarding heavy metal toxicity was studied in this paper. The transcriptome data of hemocytes extracted from P. clarkii injected with Cr were analyzed by high-throughput sequencing and compared to the control group. In total, 48,128,748 clean reads were obtained in the treatment group and 56,480,556 clean reads were obtained in the control group. The reads were assembled using Trinity and the identified unigenes were then annotated. Then, 421 differentially-expressed genes (DEGs) were found, 170 of which were upregulated and 251 downregulated. Many of these genes were found to be related to glutathione metabolism and transportation. The glutathione metabolic pathway of P. clarkii was thus activated by Cr exposure to detoxify and maintain body function. Validation of DEGs with quantitative real-time PCR confirms the changes in gene expression. Thus, this study provides data supporting a glutathione-focused response of P. clarkii to exposure to heavy metals.

Role of Anionic Chromium Species in Leather Tanning

Chrome tanned leathers are definitely unique in comparison with leather made from any other known tanning agents, especially in terms of thermal stability, cost and its reactive mechanism with collagen fibers. In our current studies, self basifying chrome tanning materials masked with different percentages of organic acid were prepared and applied after the de-liming stage of leather processing. This eliminated the need for pickling and basification steps. Tanned leathers resisted shrinkage up to 103 and 105±2°C while conventional chrome tanned leathers resisted up to 108±2°C. Also, interaction of anionic chrome species in tanning was studied. It was observed that the percentage of anionic species in the experimental chrome tanning material was higher than conventional chrome tanning material and the shrinkage temperature achieved by application of experimental tanning material proves that anionic species do involve in tanning. Tanned leathers were crusted and analysed for strength and organoleptic properties.

Boric-acid-functionalized luminescent sensor for detection of chromate ions in aqueous solution

The development of highly fluorescent materials for detection of environmental contaminants like chromate anions (Cr2O72−) is of great importance. Here, boric acid-coated Eu(III) metal–organic framework (Eu-MOF-B) was successfully synthesized using one pot hydrothermal conditions and further extensively characterized by several spectroscopic and microscopic techniques. The fluorescence spectroscopy investigations indicated the dual-fluorescence emission along with high stability in water. In case of presence of Cr2O72− ion in aqueous samples, the red fluorescence of Eu-MOF-B is selectively quenched but blue fluorescence is enhanced. Hence, the complex could be a potential fluorescent sensor for sensitive detection of Cr2O72− with a low limit of detection of 0.17 ppm.

Self-Ordering of Porous Anodic Alumina Fabricated by Anodizing in Chromic Acid at High Temperature

Anodizing Al in chromic acid is a very useful surface finishing process for corrosion protection and nanoscale porous structure fabrication, whereas the self-ordering of porous anodic alumina (PAA) in chromic acid has never been found to date. Herein, we provide a self-ordered PAA film possessing numerous sub100-nm-scale characteristic bumps through anodizing in chromic acid at high temperature. Anodizing of high purity Al plates in a 0.3 M chromic acid solution at conventional low temperatures, such as 293 K, leads to the formation of a disordered PAA film, whereas anodizing at a high temperature of 348 K causes the self-ordering behavior of the pore structure. The PAA film grown in the initial stage possessed the highest regularity, and it decreased with anodizing time due to pore branching during anodizing. A highly ordered PAA film measuring approximately 340 nm in interpore distance can be fabricated by short-term, two-step anodizing in chromic acid at 348 K and 120 V. The ordered PAA film possesses a characteristic nanostructure consisting of hexagonally arranged 100-nm-scale pores and sub100-nm-scale disordered bumps on their pore walls without any electrolyte chromate anion.

EQUILIBRIA PROCESSES IN AQUEOUS SOLUTIONS OF K2CrO4 – HNO3 – KNO3 – H2O AND K2Cr2O7 – NaOH – KNO3 – H2O SYSTEMS

The equilibria processes in aqueous solutions of CrO42– – H+ – H2O and Cr2O72– – OH– – H2O systems were studied by pH‑potentiometric titration, mathematical modeling and UV–Vis. spectroscopy. It was established that in the CrO42– – H+ – H2O systems with acidity ZН = ν(H+)/ν(Cr(VI)) = 0–2.5 the processes of dichromate and hydrochromate anion formation and hydrolytic conversion of dichromate to hydrochromate take place, for which the logarithms of the equilibrium concentration constants were calculated by the quasi-Newton method (CLINP 2.1 software; 95 % confidence probability). The calculated values of the logarithms of the concentration equilibrium constants lgKC reliably agree with the literature data. The calculated lgKC were used to build of chromium(VI) anions distribution diagrams depending on ZH, and ZOH in solutions. For the first time, by the Pitzer method the thermodynamic equilibria constants of hydrochromate HCrO4– (lgK10 = 6,94), the dichromate anion Cr2O72– (lgK20 = 15,49) formation processes from the CrO4 2– and H+ ions, and the logarithm of equilibrium constant of the interconversion of the dichromate anion to the hydrochromate anion (lgK30 = –1,61) were calculated. Mathematical modeling and UV–Vis. spectroscopy show that the composition of anions in Cr2O72– – OH– – H2O solutions with alkality ZOH = ν(OH–)/ν(Cr(VI)) = 0–2.5 is identical to CrO42– – H+ – H2O systems. It is established that the experimental dependencies pH = f(Z) for the Cr2O72– – OH– – H2O system can be reliably reproduced by hydrolysis reactions of dichromate anion to hydrochromate anion and by subsequent neutralization to chromate anion with equilibrium constants calculated for processes in CrO42– – H+ – H2O solutions with same ionic strengths. The quantitative composition of chromium(VI) solutions was confirmed qualitatively by UV–Vis. spectroscopy.

Novel Zn(II) coordination polymer based on a semi-rigid tricarboxylate acid ligand: synthsis, structure, and fluorescence recognition of acetylacetone and chromium(VI) anions

A novel Zn(II)-coordination polymer [Zn(HL)(bpe)0.5]·H2O] (CP 1) (H3L ​= ​4-(2′,3′-dicarboxylphenoxy) benzoic acid, bpe ​= ​1,2-bis(4-pyridyl) ethylene) has been successfully prepared. In CP 1, a dinuclear Zn2(C2OO)4 secondary building unit (SBU) is found, which is further connected by HL2− ligand to induce a two-dimensional (2-D) layer cage structure with (4•52) (4•53•72) topology. CP 1 exhibits excellent fluorescence property. Interestingly, it can also act as a highly efficient fluorescence based sensor for simultaneously detecting acetylacetone (Hacac) and oxoanion Cr(VI) ions with high sensitivity and selectivity in aqueous solution through fluorescence quenching. The detection limits of CP 1 toward Hacac and Cr2O72−, CrO42− are as low as 1.92 ​ppm, 2.23 ​μM and 2.57 ​μM, respectively. Furthermore, the possible sensing mechanism in both proposed sensing systems has also been discussed in more detail.

An Ion Chromatography Method for Simultaneous Quantification of Chromate, Arsenate, Selenate, Perchlorate, and Other Inorganic Anions in Environmental Media.

Chromium (Cr) (VI) is a toxic, mutagenic, and carcinogenic water pollutant. The standard ion chromatography (IC) method for quantification of Cr (VI) in water samples is Environmental Protection Agency Method 218.7, which requires postcolumn derivatization with 1,5-diphenylcarbazide and UV-Vis spectroscopy detection. Method 218.7 is Cr (VI) specific; thus, it does not allow detection of co-occurring natural and anthropogenic anions in environmental media. In this study, we developed an isocratic IC method with suppressed conductivity detection, a Metrohm Metrosep A Supp 7 column, and sodium carbonate/acetonitrile as mobile phase for simultaneous quantification of Cr (VI), , As (V) as arsenate, Se (VI) as selenate, and the common anions F–, Cl–, , , and . The determination coefficient for every analyte was >0.99 and the method showed good accuracy in quantification. Cr (VI), As (V), Se (VI), and limit of detection and limit of quantification were 0.1–0.6 μg/L and 0.5–2.1 μg/L, respectively. Recovery of Cr (VI) in various aqueous samples (tap water, surface water, groundwater, and wastewater) was between 97.2% and 102.8%. Overall, most analytes showed acceptable recovery (80–120%) in the environmental samples tested. The IC method was applied to track Cr (VI) and other anion concentrations in laboratory batch microcosms experiments with soil, surface water, and anaerobic medium. The IC method developed in this study should prove useful to environmental practitioners, academic and research organizations, and industries for monitoring low concentrations of multiple anions in environmental media, helping to decrease the sample requirement, time, and cost of analysis.

Fabrication of highly efficient magnesium silicate and its adsorption behavior towards Cr(VI)

Heavy metal ions pollution has caused worldwide attention due to its toxicity to people's health. Existing as chromate anions, Cr(VI) is difficult to remove from polluted water. Owing to their low cost, stability, environmentally benign nature, and excellent properties, silicate materials are widely used in catalysis and adsorption. In this work, magnesium silicates with sepiolite and talc phase were synthesized through facile hydrothermal route using diatomite as substrate and silicon source. The Cr removal capacity of wire-like Mg4Si6O15(OH)2-diatomite exhibited a Cr adsorption capacity of 1292 mg/g, which was much higher than the other reported inorganic adsorbents. The adsorption followed Freundlich and Temkin models well, indicating that Cr species adsorbed onto adsorbent was a favorable, exothermic multilayer chemisorption process. The first-principles density functional theory (DFT) calculation results revealed the interaction between adsorptive sites of adsorbents and Cr species, also demonstrated ion-exchange behavior between interlayer Mg ion and Cr(III). We believe these low-cost synthesized adsorbents have great potentials for practical applications in the treatment of heavy metal ions polluted wastewater.

Corrosion inhibition of aluminum in sodium hydroxide solutions using some inorganic anions

The rates of corrosion of aluminum and hydrogen gas production in sodium hydroxide solutions without and with the additions of chromate, phosphate, and tungstate anions were examined. Potentiometric, weight loss, thermometric, gasometric, galvanostatic polarization, and electrochemical impedance spectroscopy techniques were used. The data show that these anions favored oxide film formation and mitigate hydrogen production. The inhibition efficiency, η, was calculated and found to depend on the anion-type and concentration, as well as, temperature. The inhibition mechanism is assumed to take place through an adsorption process obeying Temkin's isotherm. The thermodynamic parameters for the adsorption process K ads and ΔG°ads are calculated and discussed. The surface examination was investigated using SEM and EDX spectroscopy.

Cetylpyridinium bromide (CPB)-treated sugarcane bagasse for the removal of chromate in aqueous solution

Modification of sugarcane bagasse (SB) by cetylpyridinium bromide (CPB) creating surfactant-modified SB (SBC) for the removal of chromate has been studied. The CPB adsorbed increased in line with the increased CPB concentrations, and infrared spectroscopy showed preserved SB structure after modification. SB showed little adsorption capacity in the adsorption study; however, the adsorption of chromate is proportional to the amount of CPB adsorbed on SB, possibly due to the attached CPB molecules on the SB. CPB molecules create a bilayer formation on SB, generating anion exchange sites for chromate to be adsorbed on SBC. The adsorption behavior of chromate on SBC followed the Langmuir equation model better than the Freundlich model. A better adsorption capacity was noted at the lower pH value of the chromate solution. In conclusion, surfactant-modified SB could be a potential alternative adsorbent for removing toxic chromate anion in water.

Enhanced adsorption of Cr(VI) under neutral conditions using a novel adsorbent with preorganized diquaternary ammonium structure

In this study, polystyrene microspheres with diquaternary ammonium cation structure N+-C-C-N+ were designed and synthesized to realize the efficient removal of Cr(VI) in neutral wastewater. The key adsorption sites, N+-C-C-N+, can match the charge and the configuration of chromate anion, which greatly improves the ability of associating chromate anions under neutral conditions. Density functional theory (DFT) was used to compute the adsorption energies between Cr(VI) and N+-C-C-N+, which proved the structure's effectiveness. The adsorbent can work under a wide pH range and greatly improved the efficiency of removing Cr(VI) from wastewater under neutral and alkaline conditions. The adsorbent was characterized with Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Batch adsorption experiments show that the removal efficiency of Cr(VI) remains above 95% in pH values range from 2.0 to 12.0. The maximum capacity of the adsorption achieved within 40 min and conformed to the pseudo-second-order kinetic. According to the Langmuir model, the maximum capacity reached to 89.77 mg g−1 at pH 7.0. Regeneration tests show that adsorbent has good reusability and can be used in the repeated adsorption. FTIR and XPS analysis demonstrated the electrostatic interactions mechanism between the N+-C-C-N+ and Cr(VI) proposed for the adsorption process.