Photochemical elimination of Cr(VI) from neutral–alkaline solutions

The elimination of Cr(VI) from aqueous solutions at neutral and low alkaline pH by photocatalytic reduction to Cr(III), using ZnO powder as catalyst, is investigated. The study has been performed by determination of the percentage of Cr(VI) eliminated in the presence of some species (i.e. Zn(II), Cd(II), Ni(II), Ca(II), CH₃COO^?, PO^3-₄, CO^2-₃, S^2- and NH₃) which usually are found in wastes together with Cr(VI). The results obtained show that the presence of some species prevents the alkalinization of the solution during irradiation, which increases the yield of Cr(VI) photoreduction. Other species, such as S^2- and NH 3 , are oxidized during illumination: thus they prevent electron–hole recombination and, consequently, increase the yield of the Cr(VI) elimination.     

Photocatalytical reduction of Cr(VI) over ZnO powder

The photocatalytical reduction of Cr(VI) in aqueous suspensions of ZnO under uv-illumination is studied. The amount of Cr(VI) photoreduced at different irradiation times, initial Cr(VI) concentrations, pH and temperatures have been determined. Different ZnO—doped samples have been used in order to improve the yield of the Cr(VI) photoreduction. Finally, some experiments under sunlight have been performed. The results obtained in the present work show that ZnO can be used as efficient catalyst for the photoreduction of Cr (VI) in aqueous solutions even under sunlight exposure.     

The adsorption and catalytic transformations of chromium on Mn substituted goethite

Goethite widely occurs in the environment and its role in photoreduction of toxic Cr(VI) to less toxic Cr(III) has been previously reported in the literature. However, natural goethite is rarely stoichiometric and usually contains certain substitutional cations, such as Mn(III) or Mn(IV), which, on the other hand, is an oxidant for Cr(III). In order to provide more insight into the fate of Cr(VI) in the environment, the influence of Mn-substitution in goethite on its Cr(VI) photoreduction capability needs to be clarified. The objectives of this study were to evaluate the adsorption and photo-transformations of Cr on Mn-substituted goethite with Mn/(Mn + Fe) molar ratios ranging from 0 to 0.1. The results showed that Cr adsorption and photo-catalytic transformation on goethite were strongly influenced by the Mn content of goethite. When the system was under illumination, Cr(VI) reduction was decreased with increasing Mn-substitution in goethite. The re-oxidation of photo-reduced product, i.e., Cr(III), at the Mn sites on the surface of goethite is responsible for the lower Cr(VI) photo-reduction capacity of Mn-substituted goethite. The role of goethite in the reduction of Cr(VI) may be overestimated unless its Mn-substituted counterparts are considered. The effectiveness of Mn sites in goethite to increase toxic Cr(VI) has important implications in determining the fate and threat of Cr(VI) contamination.     

Removal of Chromium(VI) and Chromium(III) from Aqueous Solution by Grainless Stalk of Corn

Abstract

Grainless stalk of corn (GLSC) was tested for removal of Cr(VI) and Cr(III) from aqueous solution at different pH, contact time, temperature, and chromium/adsorbent ratio. The results show that the optimum pH for removal of Cr(VI) is 0.84, while the optimum pH for removal of Cr(III) is 4.6. The adsorption processes of both Cr(VI) and Cr(III) onto GLSC were found to follow first-order kinetics. Values of k _ads of 0.037 and 0.018 min^?1 were obtained for Cr(VI) and Cr(III), respectively. The adsorption capacity of GLSC was calculated from the Langmuir isotherm as 7.1 mg g^?1 at pH 0.84 for Cr(VI), and as 7.3 mg g^?1 at pH 4.6 for Cr(III), at 20°C. At the optimum pH for Cr(VI) removal, Cr(VI) reduces to Cr(III). EPR spectroscopy shows the presence of Cr(V) + Cr(III)-bound-GLSC at short contact times and adsorbed Cr(III) as the final oxidation state of Cr(VI)-treated GLSC. The results indicate that, at pH ≈ 1, GLSC can completely remove Cr(VI) from aqueous solution through an adsorption-coupled reduction mechanism to yield adsorbed Cr(III) and the less toxic aqueous Cr(III), which can be further removed at pH 4.6.     

Photoredox processes in the Cr(VI)–Cr(III)–oxalate system and their environmental relevance

Irradiation of the [Cr(C₂O₄)₃]³⁻ complex or the chromate(VI)–oxalate mixture, or the ternary system composed of Cr(III), Cr(VI) and oxalate, leads to chromium photoreductions in consequence of the ligand to metal charge transfer (LMCT) excitations induced by artificial solar radiation. In the case of the Cr(III) complex, the photoreduction involves the innersphere electron transfer leading to the formation of the Cr(II) species and the C₂O₄⁻ radicals. On reacting with molecular oxygen, Cr(II) is oxidised to Cr(III) catalysing thereby the oxalate substitution reaction. Moreover, under specific conditions, Cr(II) can be also oxidised to Cr(VI). Chromate(VI) is not photoreducible, but in the presence of oxalate, or other sacrificial electron donor, the outersphere photoinduced electron transfer (PET) produces Cr(V) species and the C₂O₄⁻ radicals. This initiates a series of thermal reactions leading to the formation of Cr(III) and oxidized oxalate (CO₂). In the system composed of [Cr(C₂O₄)₃]³⁻ and chromate(VI), the acidic medium and anoxic conditions favour the Cr(VI) photoreduction, whereas alkaline oxygenated solutions assist the Cr(VI) photoproduction. When an approximately neutral solution equilibrated with the ambient air is irradiated intermittently, Cr(VI) is consumed and/or produced, accordingly to the time sequence of exposure and dark periods. The oscillations of Cr(VI) concentrations are accompanied by continuous oxidation of oxalate, playing the role of the sacrificial electron donor. The effects of solution pH, molecular oxygen, concentrations of reagents and cations on the reaction rates were investigated. The results of this paper revealed that the Cr(III)/Cr(VI) system under environmental conditions behaves as the photocatalytic one catalysing the oxidation of oxalate or other organic matter by molecular oxygen, contributing thereby to the abatement of pollution.     

Effect of pH on the removal of Cr(III) and Cr(VI) from aqueous solution by modified polyethyleneimine

A novel flocculant with the capacities of reduction and chelation was prepared in this paper. The flocculant, called polyethyleneimine–sodium xanthogenate (PEX), was synthesized by modifying polyethyleneimine with carbon disulfide and sodium hydroxide. The effect of pH on the removal of Cr(III) and Cr(VI) from aqueous solution with PEX was investigated by using flocculation experiments. The results showed that in the single-ion system (only including Cr(III) or Cr(VI) in the solution), the final Cr(III) decreased with the increase in pH from 2.0 to 10.0, while the final Cr(VI) increased at first and then decreased with the increase in this pH range studied. The removal of Cr(III) was not desirable at pH lower than 7.0, whereas the final Cr(VI) concentration reached the minimum value of 0.145 mg/L at pH 2.0. In the mixture system of Cr(III) and Cr(VI), the variation tendency for the removal of Cr(III) or Cr(VI) was very similar to that obtained in the single-ion system. The oxidation–reduction potential, zeta potential, and final pH in the supernatant were also measured to analyze the above results. Furthermore, FTIR spectra revealed that dithiocarboxylic acid groups on the macromolecular chains of PEX played a major role in the Cr(VI) reduction and Cr(III) chelation.     

Bacterial cellulose/PANi mat for Cr(VI) removal at acidic pH

Remediation of hexavalent chromium - Cr(VI) at acidic pH using polyaniline coated bacterial cellulose porous mat (BC/PANi) is presented and the possible mechanism is discussed. The efficacy of BC/PANi mats in remediation of Cr(VI) was studied by varying pH (pH 1, 2, 3, and 5) and initial Cr(VI) concentrations (250–1000 ppm) of the solution. The BC/PANi (50 mg) mat was able to completely reduce 2000 ppm Cr(VI) into Cr(III) in a 20 ml solution at pH ~ 1 in 24 h. An increasing chromium removal efficiency was observed with decreasing solution pH; reaching a maximum removal capacity of ~920 mg/g at pH 1. The proposed mechanism of negatively charged Cr(VI) ions removal by BC/PANi mat is adsorption and simultaneous reduction into Cr(III), followed by desorption of Cr(III) from the mat. The role of temperature and co-existing anions like sulphate, nitrate and chloride found in industrial sludge were also investigated for removal efficiency of Cr(VI) at acidic pH ~ 1. The adsorption kinetics of Cr(VI) on polyaniline surface followed a pseudo-second-order model with reduction of Cr(VI) into Cr(III) as rate-limiting step. The reduced Cr(III) from the media was further recovered by neutralizing the pH of the solution.     

Sonochemical assisted preparation of ZnS–ZnO/MCM-41 based on blast furnace slag and electric arc furnace dust for Cr (VI) photoreduction

Iron/steel making industry is a weed that produces large quantities of slag and dust. The objective of the present study was to develop a procedure for obtaining and characterizing photocatalysts derived from this waste for chromium remediation. The MCM-41 was synthesized via sodium silicate (Na₂SiO₃) derived from Blast Furnace Slag (BFS), and ZnO and ZnS were synthesized based on zinc extracted from Electric Arc Furnace Dust (EAFD). Subsequently, ZnO/ZnS were sono-chemically loaded on the MCM-41 and were tested for the Cr (IV) photoreduction. The resultant ZnO, ZnS, MCM-41, and composites were characterized by X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDX), N2adsorption–desorption isotherms, Fourier-transform infrared (FT-IR) spectrometry, Dynamic Light scattering, and Transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). A regular hexagonal structure of typical mesoporous MCM-41 had been proven by small-angle XRD, HRTEM, and N2 adsorption–desorption. The photoreduction activity of ZnS–ZnO/MCM-41 nanocomposite has obvious efficiency compared to ZnO and ZnO/MCM-41, achieving a 94% photoreduction of Cr (VI) in 180 min under UV irradiation. The slight activity loss after 4 cycles (84.7%) reveals the good photoreduction properties of catalysts. Based on these results, ZnS–ZnO/MCM-41 composite material seems to be high efficiency, green, stable, environment, and economical alternative to be used as a photocatalyst for the reduction of Cr (VI).     

Removal of Cr(VI) from Aqueous Solution: Electrocoagulation vs Chemical Coagulation

Abstract

Hydrolyzed products of Al(III) have affinity below pH_zpc for oppositely charged mono and bi‐nuclear species of hexavalent chromium. This study investigates the comparative performance of electrocoagulation (EC) and chemical coagulation (CC) for the removal of Cr(VI) from aqueous solution. The highest removal of Cr(VI) achieved with EC was about 42% with 4.36 mA/cm² current density. Cathodic adsorption of chromium boosted up Cr(VI) removal during EC. Simultaneous electro‐ and chemical‐dissolution lead to high current efficiency of about 178%. Both the pH and the coagulant dosage have a significant impact on Cr(VI) removal in the pH ranges from 4.9 to 7.0. CC with alum and aluminum sulfate (AS) removed about 11% and 12% of Cr(VI). Co‐adsorption of divalent SO₄ ^2? ions with Cr(VI) is responsible for the lower removal observed with chemical coagulants. About 0.061 and 0.099 mole of SO₄ ^2? was adsorbed per mole Al in the precipitate in the pH range 4.9 to 7.0 with AS and alum. A higher coagulant dosage increases the removal of Cr(VI) but adversely affects the removal efficiency (Cr(VI) removed per unit of Al dosing). Cell current density (CD) has shown little effect on Cr(VI) removal and the pH elevation at the same charge density. Higher initial Cr(VI) concentration improves the removal efficiency as the species of Cr(VI) is acidic in solution and decreases the pH elevation rate.     

Reduction of Cr(VI) to Cr(III) and removal of total chromium from wastewater using scrap iron in the form of zerovalent iron(ZVI): Batch and column studies

This work experimentally investigates Cr(VI) reduction to Cr(III) using waste scrap iron in the form of zerovalent iron (ZVI) collected from the mechanical workshop of the Institute, both in batch and continuous operation. The reduction of Cr(VI) to Cr(III) was found to be complete (～100%) depending on the experimental conditions. Lower pH values favour Cr(VI) reduction. Two concurrent reactions take place, that is reduction of Cr(VI) by Fe⁰ (ZVI) and by Fe²⁺ generated due to H⁺ corrosion of iron. Maximum around 22%, 11% and 2% Cr(III) remained dissolved in solution while the experiments were carried out at initial pH of 2, 4.67 and 7. Higher ZVI loading increases Cr(VI) reduction rate, however, consumption of iron is noted to be higher. The results indicate that the bed is exhausted rapidly at higher pH, initial Cr(VI) concentration and flow rate. This is attributable to predominance passivation of ZVI surface forming Cr(III)–Fe(III)‐oxide layer. SEM analysis of ZVI before and after the experiments confirms formation of passive oxide on iron surface is responsible for deterioration of Cr(VI) reduction efficiency due to its blanketing effect.     

Adsorption of Cr(VI) and As(III) on coaly activated carbon in single and binary systems

In single component system, the adsorption of Cr(VI) and As(III) increase with contact time. Solution pH is found influencing the adsorption. Cr(VI) removal is found to be maximum (98%) at pH = 2. While, As(III) removal is found to be maximum at pH = 6 (77.2%). The adsorption capacity of Cr(VI) is greater than that of As(III) in single component system. Several adsorption isotherms were used to fit the equilibrium data. The adsorption kinetic data of Cr(VI) and As(III) were analyzed and is found fitting well in a pseudo-second-order equation both in single and binary systems. In binary system, the adsorption of As(III) is generally higher than that in single system. The pseudo-second-order rate constant k₂ is 0.5037 g/mg min in binary system larger than 0.0782 g/mg min in single system. However, the presence of As(III) in solution does not significantly influence the capacity of Cr(VI) adsorption on coaly activated carbon (CAC). The complexation between Cr(VI) and As(III) influence the adsorption, resulting in increased adsorption of As(III). The complexation structure of As(III), Cr(VI) and CAC was proposed as A-Cr(VI)-As(III) (A represents the adsorption site on the CAC).     

Enhanced adsorptive removal of Cr(VI) in aqueous solution by polyethyleneimine modified palygorskite

Polyethyleneimine (PEI) modified palygorskite (Pal) was used for the adsorption of Cr(VI) in aqueous solution. The absorbent was characterized by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). Characterized results confirmed that the Pal has been successfully modified by PEI. The modification of PEI increased the Cr(VI) adsorption performance of the Pal by the adsorption combined reduction mechanism, and amino groups of the adsorbent play the main role in the enhanced Cr(VI) adsorption. The maximum adsorption capacity was 51.10 mg·g⁻¹ at pH 4.0 and 25 °C. The adsorption kinetics of Cr(VI) on the adsorbent conforms to the Langmuir isotherm model. The maximum adsorption occurs at pH 3, and then the adsorption capacity of PEI-Pal was decreased with the increase of pH values. The adsorption kinetics of Cr(VI) on PEI-Pal was modeled with pseudo-second-order model. The addition of Cl⁻, SO₄²⁻ and PO₄³⁻ reduced the Cr(VI) adsorption by competition with Cr(VI) for the active sites of PEI-Pal. The Cr(VI) saturated PEI-Pal can be regenerated in alkaline solution, and the adsorption capacity can still be maintained at 30.44 mg·g⁻¹ after 4 cycles. The results demonstrate that PEI-Pal can be used as a potential adsorbent of Cr(VI) in aqueous solutions.     

Adsorption Characteristics of Chromium(VI) Ions onto Fe(III)-Coordinated Amino-Functionalized Poly(glycidylmethacrylate)-Grafted TiO2-Densified Cellulose

An investigation was conducted with a newly developed adsorbent, iron(III)- coordinated amino-functionalized poly(glycidylmethacrylate)-grafted TiO₂-densified cellulose (Fe(III)-AM-PGDC) on the removal of chromium(VI) from aqueous solution. Batch experiments were performed under various conditions of time, pH, concentration, dose, ionic strength, and temperature. Adsorption of Cr(VI) on Fe(III)-AM-PGDC was dominated by ion exchange or outer-sphere complexation. The maximum adsorption capacity was found to be 109.76 mg g^?1. Thermodynamic study showed that adsorption of Cr(VI) onto Fe(III)-AM-PGDC is more favored. The complete removal of Cr(VI) from electroplating wastewater was achieved by the adsorbent. The adsorbent did not lose its adsorption capacity even after the fourth regeneration.     

Spectroscopic characterization of the reduction and removal of chromium (VI) by tropical peat and humin

The reduction of Cr(VI) to Cr(III) in aqueous solution, and the subsequent removal of Cr(III) using in natura peat or its humin fraction, was characterized by electron paramagnetic resonance (EPR), ultraviolet–visible (UV–Vis) spectroscopy and X-ray photoelectron (XPS) spectroscopy. EPR spectroscopy g-factors ranging from 2.0029 to 2.0030 indicated the presence of organic free radicals (OFR) associated with carbon atoms. The spin densities of the samples increased with pH in the range pH 2.0–pH 6.0. An XPS spectroscopy peak at 579.0 eV was attributed to Cr(VI), and peaks at 576.8 eV and 677.3 eV to two different chemical forms of Cr(III). Removal of chromium increased with the reduction of Cr(VI) to Cr(III), and removal rates after 50 h were 13% (peat) and 15% (humin) at pH 6.0, and 70% (peat) and 80% (humin) at pH 2.0. The greater efficiency of the humin fraction in the reduction/removal process is discussed in terms of the chemical structures of these materials.     

Activation of Phillips Cr/silica catalysts: IV. Mobility of Cr(VI)

The saturation coverage behavior of Cr(VI) on silica has been used to study the mobility of hexavalent chromium during commercial activation of the Phillips catalyst in a fluidized bed. Cr(VI) was found to migrate not only within each silica particle, but even between particles. The mechanism of transfer is thought to be particle-to-particle contact during the fluid bed calcining process, and Cr migration was observed at temperatures as low as 300–400 °C in these experiments. Transfer can take place even in the absence of oxygen, which suggests that redox cycling is probably not the main method of mobility. However, once the silica is calcined at high temperatures the migration is stopped at low temperatures, indicating that the thermal history of the surface is important in the transfer process. Migration of Cr on and between alumina particles behaved in much the same way, and Cr transfer was also observed between silica and alumina in both directions. Cr(III) on deactivated commercial Cr/silica catalysts could be reclaimed to Cr(VI) surprisingly easily by co-activation with virgin support. The spreading out of Cr(VI) onto a fresh unoccupied surface probably provides a powerful force to reoxidize the otherwise seemingly inert alpha-Cr₂O₃ crystallites. The mobility has important consequences for commercial manufacture and use of the catalyst.     

Reduction of Cr(VI) by palladized biomass of Desulfovibrio vulgaris NCIMB 8303

Palladized biomass of Desulfovibrio vulgaris (Bio‐Pd(0)) reduced Cr(VI) to Cr(III) at an initial rate four‐fold higher than chemically‐prepared Pd(0) metal. Optimal Cr(VI) reduction by suspended Bio‐Pd(0) occurred at pH 3, whereas pH did not affect the rate of Cr(VI) reduction by Bio‐Pd(0) immobilized in agar beads. The rate of Cr(VI) reduction was concentration‐dependent below 300 µmol dm^?3, and application of enzyme kinetics, considering Bio‐Pd(0) as an ‘artificial enzyme’, gave an apparent K_m (K_mapp) of approx. 650 µmol dm^?3 and V_max of 1667 nmol h^?1 mg Pd(0) for suspended Bio‐Pd(0). The potential of Bio‐Pd(0) as a method for the treatment of Cr(VI)‐wastes is discussed. Copyright © 2005 Society of Chemical Industry     

Adsorption of chromium by activated carbon from aqueous solution

Adsorption isotherms of Cr(III) and Cr(VI) ions on two samples of activated carbon fibres and two samples of granulated activated carbons from aqueous solutions in the concentration range 20–1000 mg/l have been studied. The adsorption isotherms have been determined after modifying the activated carbon surfaces by oxidation with nitric acid, ammonium persulphate, hydrogen peroxide and oxygen gas at 350°C and after degassing at different temperatures. The adsorption of Cr(III) ions increases on oxidation and decreases on degassing. On the other hand, the adsorption of Cr(VI) ions decreases on oxidation and increases on degassing. The increase of Cr(III) and the decrease of Cr(VI) on oxidation and the decrease of Cr(III) and the increase of Cr(VI) on degassing have been attributed to the fact that the oxidation of the carbon surface enhances the amount of acidic carbon–oxygen surface groups while degassing eliminates these surface groups. Thus while the presence of acidic surface groups enhances the adsorption of Cr(III) cations, it suppresses the adsorption of Cr(VI) anions.     

Chrome dyeing using anionic bivalent chromium complexes as mordants

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

Mechanisms of Cr(VI) removal from water by various types of activated carbons

The removal mechanisms of Cr(VI) from water using different types of activated carbons, produced from coconut shell, wood and dust coal, were investigated in this project. Different types of activated carbons have different surface characteristics. The coconut shell and dust coal activated carbons have protonated hydroxyl groups on the surface (H‐type carbons), while the surface of the wood‐based activated carbon has ionised hydroxyl groups (L‐type carbons). The adsorption kinetics of chromium onto the activated carbons at pH values ranging from 2 to 6 were investigated. It was found that the optimum pH to remove total chromium was 2 for wood‐based activated carbon, while for coconut shell and dust coal activated carbons, the optimum pH was around 3–4. The difference in the optimum pH for different activated carbons to remove Cr(VI) from water can be explained by the different surface characteristics and capacity of the activated carbons to reduce Cr(VI) to Cr(III). © 1999 Society of Chemical Industry     

Adsorption of trivalent chromium from aqueous solutions onto activated carbon

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