Transition metal complexes of polymeric amino ligands derived from triethyleneglycol dimethacrylate crosslinked polyacrylamides

Amino functions were incorporated into triethyleneglycol dimethacrylate (TEGDMA)-crosslinked (2–20 mol %) polyacrylamides by transamidation with ethylenediamine. The complexation of these polymeric ligands with the amino function in different structural environments was investigated toward Cr(III), Mn(II), Fe(III), Cu(II), Cd(II), and Pb(II) ions. The metal ion intake decreased with the extent of the crosslinking and followed the order Cu(II)>Cr(II)>Mn(II)>Fe(III)>Pb(II)>Cd(II). The pH dependence of complexation and distribution coefficient; time course and kinetics of complexation; swelling properties of the uncomplexed and complexed resins in water; recyclability; infrared (IR) spectra; electron paramagnetic resonance; ultraviolet visibility; magnetic susceptibility; and thermogravimetric and scanning electron microscopy studies were carried out. The kinetics of complexation was found to be first order. The swelling of the complexed resins is lower than that of the uncomplexed resins. Complexation resulted in the lowering of the IR absorption of the amino ligands. The IR data of the TEGDMA-crosslinked polyacrylamide amine and the corresponding Cr(III) complex reveals that the metal is coordinated with the ligand through the amino nitrogen. The SO⁼₄ is also coordinated. The number of coordinated nitrogen atoms varies as the hydrophilicity and flexibility of the polymer change with the crosslinking. Thus spectral and magnetic studies suggest a distorted octahedral geometry for Cu(II) complexes, octahedral geometry of the d³ system for Cr(III), and d⁵ high-spin octahedral geometry for Fe(III) and Mn(II) complexes. The thermal stabilities of the complexed resins increase with increase in the metal content. Surface morphology of the polymeric ligand changes on complexation. © 1996 John Wiley & Sons, Inc.     

VISCOMETRIC MEASUREMENT OF CHROMIUM(III)-POLYACRYLAMIDE GELS†

Gelled polymers are being used increasingly to modify the movement of injected fluids in secondary and enhaced oil recovery processes. A common gelation process involves the reduction of Cr(VI) to Cr(III) in the presence of polyacrylamide. The Cr(III) reacts or interacts with the polymer to form a gel network. Although correlations of gelation time with principal process variables have been obtained, viscometric data have not been reported during or after gelation. These data are needed for fluid flow calculations in surface equipment and estimation of flow behaviour in reservoir rocks.

A Weissenberg Rheogoniometer, with cone and plate geometry, was used to obtain viscometric data for the gelation of polyacrylamide and chromium (III). Solutions consisting of polyacrylamide polymer, sodium dichromate-dihydrate and sodium bisulfite were gelled under a steady shear field at constant temperature. The shear stress versus time profile for the galation process was interpreted to define a gelation time and to determine the apparent viscosity of the gelled fluid. The gelation time decreased as the applied shear rate increased up to about 14.25 sec^-1 and was affected by shear rate history. Viscometric properties of the gelled solutions were determined. Apparent viscosity of the gelled solutions decreased as the shear rate under which they were formed increased.

Post gelation studies indicated that gels exhibited a residual stress at zero shear rate and behaved as Bingham plastics under steady shear. Gels formed at low shear rates were more viscous than gels formed at high shear rates. However, the structure of these gels was susceptible to shear degradation.     

Fate of Cr(III) during Ozonation of Secondary Municipal Wastewater Effluent

ABSTRACT

In the present study, the fate of trivalent chromium (Cr(III)) during ozonation of ultrapurified water and wastewater effluent was investigated. In experiments conducted in phosphate buffered ultrapurified water, O₃ alone in excess was inefficient to oxidize Cr(III) (only about 10–15% of total Cr(III) content), while in presence of the secondary oxidant, OH radical, almost all Cr(III) was oxidized to hexavalent chromium (Cr(VI)). In a wastewater effluent, spiked with Cr(III), only about 10–20% of Cr(III) was oxidized with specific ozone doses in the range 0.15–1.5 gO₃/gDOC, although O₃ and OH radical were both available for reaction. Cr(VI) formation was monitored in parallel with the abatement of some common micropollutants, reacting with differing apparent second-order rate constants with ozone, decreasing in the order carbamazepine>> benzotriazole> atrazine> p-chlorobenzoic acid (pCBA). Carbamazepine and benzotriazole were abated to >80% for specific O₃ doses of 0.3 gO₃/gDOC and 0.8 gO₃/gDOC, respectively. The more ozone-resistant compounds (atrazine and pCBA) required a specific ozone dose of about 1.25 gO₃/gDOC for the same relative abatement. At this specific ozone dose (i.e., 1.25 gO₃/gDOC), only about 20% of Cr(III) was oxidized to Cr(VI), whereas only 10% of Cr(III) was oxidized to Cr(VI) at a more realistic specific ozone dose for enhanced wastewater treatment for micropollutant abatement (0.5 gO₃/gDOC). Therefore, for typical Cr(III) levels in municipal wastewaters, effluent ozonation only leads to toxicologically insignificant Cr(VI) concentrations.     

A study of the uptake and gelation reactions of Cr(III) oligomers with polyacrylamide

The reaction rates of Cr(III) oligomers with partially hydrolyzed (PAAm) polyacrylamide were measured. The oligomers were prepared in relatively pure form using liquid chromatography, allowed to react with polymer, quenched, and then separated using equilibrium dialysis. In the pH range of 4–5, Cr(III) concentrations in the range of 8–30 ppm and polymer concentrations in the range of 4000–20,000 ppm, reaction rates increased with increasing oligomer size, concentration, and pH. Gelation rates determined using rheological measurements were found to follow closely the Cr(III) reaction rates. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1381–1391, 1997     

VISCOMETRIC MEASUREMENT OF CHROMIUM(III)-POLYACRYLAMIDE GELS†

Gelled polymers are being used increasingly to modify the movement of injected fluids in secondary and enhaced oil recovery processes. A common gelation process involves the reduction of Cr(VI) to Cr(III) in the presence of polyacrylamide. The Cr(III) reacts or interacts with the polymer to form a gel network. Although correlations of gelation time with principal process variables have been obtained, viscometric data have not been reported during or after gelation. These data are needed for fluid flow calculations in surface equipment and estimation of flow behaviour in reservoir rocks.

A Weissenberg Rheogoniometer, with cone and plate geometry, was used to obtain viscometric data for the gelation of polyacrylamide and chromium (III). Solutions consisting of polyacrylamide polymer, sodium dichromate-dihydrate and sodium bisulfite were gelled under a steady shear field at constant temperature. The shear stress versus time profile for the galation process was interpreted to define a gelation time and to determine the apparent viscosity of the gelled fluid. The gelation time decreased as the applied shear rate increased up to about 14.25 sec^?1 and was affected by shear rate history. Viscometric properties of the gelled solutions were determined. Apparent viscosity of the gelled solutions decreased as the shear rate under which they were formed increased.

Post gelation studies indicated that gels exhibited a residual stress at zero shear rate and behaved as Bingham plastics under steady shear. Gels formed at low shear rates were more viscous than gels formed at high shear rates. However, the structure of these gels was susceptible to shear degradation.     

Removal of Arsenic(III), Chromium(III) and Iron(III) Traces from Hydrofluoric Acid Solutions by Specialty Anion Exchangers

The removal of As(III), Fe(III), and Cr(III) at trace levels from HF solutions by means of specialty ion exchange resins has been investigated. These impurities are usually found in technical‐grade HF, and they need to be removed to prepare metal‐free HF for the semiconductor industry. It was assumed that Fe(III) and As(III) species in dilute HF were present in anionic form, while Cr(III) was probably in neutral form, CrF₃. First, a selection of specialty ion exchangers was performed. Then, fixed‐bed experiments were carried out to check the ability of selected resins to reach the impurity levels required in SEMI C29 for 5 wt.% HF (5 ppb of As, and 10 ppb of Cr and Fe). The effect of the flow rate and the HF concentration on the metal removal was studied with Purolite D‐3777 and Fuji PEI‐CS‐07 resins respectively. Fuji PEI‐CS‐07 showed the best performance for Fe(III) removal, even at high HF concentration (25 wt.%). A strong decrease in the Cr(III) and As(III) removal capacity with increasing concentration of HF was observed.     

Chemically Modified CaO/Fe3O4 Nanocomposite by Sodium Dodecyl Sulfate for Cr(III) Removal from Water

A CaO/Fe₃O₄ nanocomposite was modified by sodium dodecyl sulfate (SDS) and used for Cr(III) removal from aqueous solution. The physical and surface characteristics of the adsorbent were studied by different analysis techniques. The effects of key parameters such as pH, contact time, temperature, initial concentration of Cr(III) ions, and adsorbent dose were investigated at a fixed mixing rate. Parameters were optimized to attain the best possible removal efficiency of Cr(III) ions. The maximum adsorption capacities obtained from the Langmuir model were determined. The results of equilibrium and kinetic studies indicate that the adsorption process follows the Langmuir isotherm model and the pseudo‐second‐order kinetic model. The thermodynamic study demonstrated that the adsorption process was suitable, spontaneous, and exothermic.     

Correlation between the electrochemical properties of colloidal oxides and supported oxides on metallic substrates

Mixed oxide electrodes of Ti(IV) and Cr(III) were prepared by calcining Cr(OH)₃ layers deposited on metallic titanium supports. This treatment produced a mixed oxide film of TiO₂–Cr₂O₃ covered with a layer of pure Cr₂O₃. The electrochemical response (cyclic voltammetry) shows the presence of two or three oxidation peaks depending on the electrode preparation conditions. One peak may be interpreted as the oxidation of Cr(III) to Cr(VI) species and the appearance of other peaks is due to the presence of chromium atoms in oxidation states higher than (III). The results of chemical analyses, electrophoretic mobilities and acid-base potentiometric titrations on calcined Cr(OH)₃ powders shows that the calcination step in air produced the decomposition of the Cr(III) hydroxide to the Cr(VI) oxide. Soluble Cr(VI) compounds were found in equilibrium with the suspended powder oxide which markedly affected the shape of the titrations curves. From the amount of Cr(VI) present in solution it was possible to correct the experimental ₀-pH curves. These corrected data indicated that Cr(VI) soluble species adsorbed at the Cr₂O₃/electrolyte interface.     

Adsorption behaviors of Cr(III) on carboxylated collagen fiber

Carboxylated collagen fiber (CCF) was prepared by modifying collagen fiber with glyoxylic acid. The structure of CCF was characterized and analyzed by scanning electron microscopy and Fourier transform infrared spectroscopy. CCF was an effective adsorbent for the removal of Cr(III) from aqueous solutions, and the adsorption capacity of Cr(III) increased by 74.13% after modification. The adsorption isotherm, as well as the kinetics of the adsorption of Cr(III) on CCF, was studied. The results showed that the adsorption isotherm of Cr(III) could be well described by the Freundlich equation, and the adsorption capacity increased slightly with increasing temperature, but the influence was not so obvious. The adsorption kinetic investigations indicated that the adsorption rate of Cr(III) on CCF could be well described by the pseudo‐second‐order rate model, and that the adsorption capacity calculated using the pseudo‐second‐order rate model was close to that from actual measurements. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40285.     

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.     

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.     

Photocatalytic reduction of chromium and oxidation of organics by polyoxometalates

The photocatalytic reduction of Cr(VI) to the less toxic Cr(III) is presented in the presence of the polyoxometalates (POM) PW₁₂O₄₀³⁻ or SiW₁₂O₄₀⁴⁻ as photocatalyst and an organic substrate (salicylic acid or propan-2-ol) as electron donor. Cr(VI), as dichromate, is reduced to Cr(III), according to the 6:1 stoichiometry of PW₁₂O₄₀⁴⁻ versus Cr₂O₇²⁻ indicated from experiments in the dark. Increase of POM or salicylic acid (SA) concentration accelerates, till a saturation value, both the reduction of metal and the oxidation of the organic, suggesting that these two conjugate reactions act synergistically. The photocatalytic action of POM is not so important in the case of highly concentrated solutions of organics that exhibit direct photochemical reduction of Cr(VI), i.e. propan-2-ol (i-prOH), while it becomes important at low concentrations of i-prOH, especially for organics that do not react directly photochemically with Cr(VI), such as SA. Increase of Cr(VI) concentration enhances consumption of SA and Cr(VI) till an optimum value, due to inner filter effect. The method is suitable for a range of chromium concentration from 5–100 ppm achieving complete reduction of Cr(VI) to Cr(III) up to non-detected traces (>98%). The presence of oxygen does not influence the efficiency of SA and Cr(VI) consumption. In contrast to the semiconductor-based heterogeneous photocatalysis, the POM-based homogeneous process seems superior in the frame that: (i) it remains catalytic throughout illumination by providing more active sites and (ii) among the two POM used, the one that is more efficient in the degradation of the organic, that is PW₁₂O₄₀³⁻ compared to SiW₁₂O₄₀⁴⁻, is also more efficient in reducing Cr(VI), due to a kinetic effect, and a compromise is not needed.     

Electrochemical regeneration of chrome etching solution

A metal surface is chromatized with a chromic acid solution to obtain a good adherence of polymer coatings. In this process Cr(VI) is reduced to Cr(III). The oxidation strength of the solution decreases during use. The chrome solution needs to be regenerated and purified. A new anode material, namely boron-doped diamond, was used to investigate the oxidation of Cr(III) to Cr(VI). It was found that the current efficiency for Cr(III) oxidation decreases with increasing total current density. The current density of Cr(III) oxidation increases linearly with increasing Cr(III) concentration and is practically independent of the Cr(VI) concentration. It was concluded that the diffusion of Cr(III) is the rate-determining step for the Cr(III) oxidation at Cr(III) concentrations form 40 to 160 mol m^–3. The surface of the boron-doped diamond shows no signs of chemical corrosion or mechanical destruction. A filter-press type cell divided into two compartments by a cation exchange membrane was proposed. A cost calculation was carried out for the oxidation of 1.28 mmol s^–1 Cr(III) in a 40 mol m^–3 chrome(III) solution. Factors affecting the feasibility of this process include the costs of chemical waste disposal, the costs of chromic acid, government legislation and to a great extent the costs of the new anode material.     

Crosslinking reaction of polyacrylamide with chromium(III)

Summary The crosslinking of initially non-hydrolyzed polyacrylamide with chromium in its +3 valence state was investigated in this work. IR spectra showed that the gelation occurred when the polyacrylamide was hydrolyzed, which implies that the gelation should take place at elevated temperature under neutral condition. The gelation process was monitored by means of viscosity measurement. The addition of acetic acid to the system might lead to the delay of gelation, owing to the synergistic effect of chelating with Cr(III) and acidity.     

Identification of Cr oxidation state active in ethylene polymerization on Cr/SiO2 at room temperature

High temperature treatment under vacuum or in a pure N₂ atmosphere can be used instead of a reduction step to activate Cr/SiO₂ catalysts used for ethylene polymerization. After this procedure, the Cr species active over a range of temperature has been identified as Cr(III) using CO adsorption (IR triplet bands at 2228, 2214, and 2202 cm^–1) and an ethylene polymerization activation energy of 12.1 kcal/mol. The Cr(III) species has also been shown to be present after activation using CO reduction, the more commonly used procedure. Cr(II) sites are always present when activation involves reduction with CO and become increasingly active as the temperature rises above room temperature.     

Removal of hexavalent chromium in wastewater by polyacrylamide modified iron oxide nanoparticle

Iron oxide nanoparticle has been successfully modified by polyacrylamide and the polyacrylamide modified magnetic nanoparticles (PMMNs) were applied to remove Cr(VI) in wastewater. The vibrating sample magnetometer (VSM) spectra indicated the large saturation magnetization and superparamagnetic property of the PMMNs. This made the polyacrylamide modified iron oxide easily separate with liquid phase. Scanning electron microscope (SEM) results showed that both the synthesized iron oxide and the PMMNs were nanoscale. Batch adsorption studies had been carried out to determine the effect of pH, contact time, Cr(VI) initial concentration, and coexisting salts on the adsorption of Cr(VI). Maximum removal (98.30%) was observed from an initial concentration of 100 mg L^?1 Cr(VI) at pH 3.0, 30°C. This process followed pseudo‐second‐order kinetics model and the equilibrium time was 40 min. The experimental data fitted the Langmuir isotherm better than Freundlich. Maximum adsorption amount of Cr(VI) by PMMN was 35.186 mg g^?1. The effect of coexisting salts on Cr(VI) removal was not apparent even the concentration of salt was 10 times as big as the low concentration, 0.01M. It had been proposed that the mechanism of Cr(VI) uptake onto PMMN was adsorption‐coupled reduction. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40945.     

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).     

Removal of Chromium and Organic Pollutants from Industrial Chrome Tanning Effluents by Electrocoagulation

The treatment of industrial chrome tanning effluents by electrocoagulation (EC) in a laboratory‐scale reactor was investigated. Mild‐steel (MS) electrodes have been found to outperform aluminum (Al) electrodes in reducing the Cr(III) concentration to <2 mg L^–1. The conversion of Fe(II) to Fe(III) is slow in the lower pH range (<6), and OH^– ions generated during EC are amply available for Cr(III) removal by precipitation in the case of the MS electrode. Formation of Al(OH)₃(s) in competition with Cr(OH)₃(s) while consuming the OH^– ion is a cause for lower Cr(III) removal with Al. EC with the MS electrode and chemical coagulation (CC) with addition of alkali proved to be equally efficient for removing Cr(III).     

Chemical and electrochemical considerations on the removal process of hexavalent chromium from aqueous media

Two methods were used to remove Cr(VI) from industrial wastewater. Although both are based in the same general reaction: 3Fe(II)_(aq) + Cr(VI)_(aq) ; 3Fe(III)_(aq) + Cr(III)_(aq) the way in which the required amount of Fe(II) is added to the wastewater is different for each method. In the chemical method, Fe(II)_(aq) is supplied by dissolving FeSO₄ · 7(H₂O)_(s) into the wastewater, while in the electrochemical process Fe(II)_(aq) ions are formed directly in solution by anodic dissolution of an steel electrode. After this reduction process, the resulting Cr(III)_(aq) and Fe(III)_(aq) ions are precipitated as insoluble hydroxide species, in both cases, changing the pH (i.e., adding Ca(OH)_2(s)). Based on the chemical and thermodynamic characteristics of the systems Cr(VI)–Cr(III)–H₂O–e^– and Fe(III)–Fe(II)–H₂O–e^– both processes were optimized. However we show that the electrochemical option, apart from providing a better form of control, generates significantly less sludge as compared with the chemical process. Furthermore, it is also shown that sludge ageing promotes the formation of soluble polynuclear species of Cr(III). Therefore, it is recommended to separate the chromium and iron-bearing phases once they are formed. We propose the optimum hydraulic conditions for the continuous reduction of Cr(VI) present in the aqueous media treated in a plug-flow reactor.     

CaAl2Cr2O7: Formation,synthesis, and characterization of a new Cr(III) compound under air atmosphere in the Al2O3–CaO–Cr2O3 system

Despite huge potential, Al₂O₃–CaO–Cr₂O₃ system has been one of the least investigated one due to the generation of carcinogenic and toxic Cr(VI) compounds. Herein, we investigated the system under air atmosphere varying Cr₂O₃ while keeping Al₂O₃:CaO ratio constant in order to identify the Cr(VI) dominant region, eventually to avoid it. The Ca₄Al₆Cr^VIO₁₆ phase predominantly formed in the air atmosphere with Cr₂O₃ content up to ∼12 mol%. However, an unprecedented Cr(III) phase appeared under air at higher Cr₂O₃ content (26.43 mol%). We then synthesized the new polycrystalline ternary Cr(III) compound (CaAl₂Cr₂O₇) at 1500 °C under air atmosphere for the first time. A trigonal symmetry of hexagonal crystal family with space group P3 (143), lattice parameters a = b = 7.7909 Å and c = 7.6506 Å were determined from the X-ray powder diffraction pattern study. Electron microscope studies revealed uniform hexagonal microcrystals with similar lattice parameters. Most significantly, the binding energies of 586.1 and 576.2 eV for Cr2p_1/2 and Cr2p_3/2 respectively implied the +3 oxidation state of Cr in this compound.