Reduction Efficiency Of Cr Research Articles

Enhanced Removal of Hexavalent Chromium in the Presence of H2O2 in Frozen Aqueous Solutions.

The reductive transformation of Cr(VI) to Cr(III) by H2O2 in ice was compared with that in water. The reduction of Cr(VI) was significant at -20 °C (ice), whereas the reduction efficiency was very low at 25 °C (water). This enhanced reduction of Cr(VI) in ice was observed over a wide range of H2O2 concentration (20-1000 μM), pH (3-11), and freezing temperature (-10 to -30 °C). The observed molar ratio of consumed [H2O2] to reduced [Cr(VI)] in ice was in close agreement with the theoretical (stoichiometric) molar ratio (1.5) for H2O2-mediated Cr(VI) reduction through proton-coupled electron transfer (PCET). The synergistic increase in Cr(VI) reduction in water by increasing the H2O2 and proton concentrations confirms that the freeze concentration of both H2O2 and protons in the liquid brine is primarily responsible for the enhanced Cr(VI) reduction in ice. In comparison, the one-electron reduction of Cr(VI) to Cr(V) and subsequent reoxidation of Cr(V) to Cr(VI) is the major reaction mechanism in aqueous solution. The reduction efficiency of Cr(VI) by H2O2 in the frozen aqueous electroplating wastewater was similar to that in the frozen aqueous deionized water, which verifies the enhanced reduction of Cr(VI) by freezing in real Cr(VI)-contaminated aquatic systems.

K4Nb6O17·4.5H2O: A novel dual functional material with quick photoreduction of Cr(VI) and high adsorptive capacity of Cr(III)

A series of orthorhombic phase K4Nb6O17·4.5H2O was synthesized via a hydrothermal approach. When presented in an acidic pH range, K4Nb6O17·4.5H2O showed a strong ability in quick reduction from Cr(VI) to Cr(III). The resulted Cr(III) ions were removed by an effective adsorption through simply adjusting the solution pH from strong acidity to near neutrality, owing to the sample's unique nano-sheet structure with a wide layer spacing. The Cr(III) ions adsorbed onto samples were released again for reusing by eluting with 1molL−1 HCl solution, and K4Nb6O17·4.5H2O regenerated by immersing in a KOH solution. The reduction efficiency of Cr(VI) was still up to 98% after irradiation for 60min, and the removal efficiency of Cr(III) ions was as high as 83% even after five cycles. Therefore, K4Nb6O17·4.5H2O is clearly demonstrated to be an excellent dual functional material with quick photoreduction of Cr(VI) and high adsorptive capacity of Cr(III). The relevant materials reported herein might be found various environment-related applications.

Biological reduction and removal of Cr(VI) with microbial consortia collected from local water treatment plant

Application of microbes in water treatment is a well-established technique. A few microbes are tolerant to toxic heavy metals. Cr(VI) tolerant microbes have practical importance of detoxifying Cr(VI) to Cr(III). The water treatment plant situated at Indian Institute of Technology Guwahati (IITG), India, employs both physical and biological processes to treat domestic wastewater. The microbial consortia collected from this plant is employed to investigate Cr(VI) reduction to Cr(III) and removal behaviour from model effluent. The results reveal that microbial consortia efficiently grow at wide pH range from 4 to 10. Microbial growth is inhibited even with addition of 5 mg L –1 Cr(VI) to the nutrient media over the entire pH range. Reduction efficiency of Cr(VI) to Cr(III) increases with decreasing solution pH, while total chromium removal is higher at elevated pH. Maximum chromium removal is achieved at initial pH of 6. Higher pH may result in Cr(III) precipitation. Therefore, chromium removal is possibly the combined effect of precipitation as well as bioaccumulation.

Bioreduction of Cr(VI) by Bacillus sp. QH-1 isolated from soil under chromium-containing slag heap in high altitude area

A chromium-reducing strain QH-1, identified as Bacillus sp., was isolated from soil under chromium-containing slag heap in Qinghai high altitude area, China. The strain was found to resist 200 mg/L Cr(VI), and Cr(VI) negatively affects the metabolic activity of the cells, as well as the cell morphology of Bacillus sp. QH-1. The reduction efficiency of Cr(VI) at concentrations of Cr(VI) 25 mg/L, 50 mg/L, 100 mg/L and 200 mg/L were 99.48 %, 65.99 %, 23.22 % and 6.99 %, respectively, decreasing with increasing initial Cr(VI) concentration. This indicates that the toxicity of Cr(VI) increased with concentration. Energy dispersive X-ray analysis revealed that there was insoluble Cr(III) generated during Cr(VI) reduction. In order to apply strain QH-1 to remove Cr(VI) from groundwater, factors of concentration of electron donors (glucose) and temperature were investigated in a synthetic medium. The results demonstrated that glucose could promote the reduction of Cr(VI) by this strain, and the general trend of Cr(VI) reduction increased with temperature within the range of 4 to 37 °C. Cr(VI) was reduced effectively at 25 °C and 37 °C, and all of Cr(VI) was reduced after 96 h at 37 °C, while the reduction was slow at 4 °C and 15 °C, and it almost ceased after about 120 h. These results could be potentially useful for the bioremediation of Cr(VI) in groundwater.

Reduction of Cr(VI) in aqueous solution with DC diaphragm glow discharge

This paper investigated the reduction of Cr(VI) in aqueous solution with direct current diaphragm glow discharge (DGD). The glow discharge sustained around the hole on a quartz tube which divided the electrolyte cell into two parts. The reduction efficiencies of Cr(VI) under different applied voltages, initial conductivities, hole diameters, hole numbers, initial pH values and initial concentrations were systematically studied. The results showed that the reduction efficiency of Cr(VI) increased with the increase of applied voltage, initial conductivity, hole diameter and hole number. The different initial pH values showed less effects on the reduction of Cr(VI). The reduction efficiency decreased with the increasing initial concentration. In addition, the simultaneous reduction of Cr(VI) and decolorization of acid orange (AO) with DGD were also fulfilled. Furthermore, the energy efficiency for Cr(VI) reduction with DGD was calculated and compared with those in photocatalysis and other glow discharge reactor.

Hexavalent chromium reduction by Escherichia coli in the presence of ferric iron

The potential of Cr(VI) reduction by Escherichia coli in the presence of soluble Fe(III) was investigated to explore the chemo-biologically mediated reduction process under anaerobic condition. The reduction efficiency of Cr(VI) reached 95% within 24 h. The influences of experimental parameters, including initial pH, temperature, Fe(III) dosage, carbon source, and chelating agent, were also investigated. The highest efficiency of reduction was observed when pH was 5.8 and temperature was 32°C. Amendments of culture medium with Fe(III) and citric-3Na enhanced Cr(VI) reduction, while the addition of EDTA-2Na inhibited the process. Analysis showed that soluble Fe(III) enhanced the reduction process by shuttling electrons from bio-reduced Fe(II) to Cr(VI) in a coupled biotic-abiotic cycle and hence, Cr(VI) was reduced to Cr(III) followed by deposition to sludge.

Photoreduction of Cr(VI) from acidic aqueous solution using TiO2-impregnated glutaraldehyde-crosslinked alginate beads and the effects of Fe(III) ions

To overcome the limitation of powder TiO2, the TiO2-impregnated glutaraldehyde-crosslinked alginate beads (TIGCAB) were prepared. The TIGCAB were characterized by DC, DBM, SEM, BET, XRD, Raman and TG. These synthesized beads were applied to reduce toxic Cr(VI) to Cr(III) in acidic aqueous solution under UV irradiation. In the system without TIGCAB or UV light, no chromium reduction was observed. The reduction rate of Cr(VI) was about 8% when the system was exposed to UV irradiation with blank beads. Photoreduction rate of Cr(VI) decreased with increasing pH. The reduction efficiency of Cr(VI) decreased with the increase of initial Cr(VI) concentration in the range of 5–20mgL−1. Fe(III) ions improved the photoreduction of Cr(VI) because they can promote the trapping of electrons and holes. Moreover, TIGCAB can maintain full photoreduction activity for at least three cycles. The results show that TIGCAB are promising materials for the photoreduction of Cr(VI) from acidic aqueous solution in environmental pollution cleanup.

UV and visible light photodegradation effect on Fe–CNT/TiO2 composite catalysts

Using multi-walled carbon nanotube (CNT) as an one-dimensional support, we have succeeded in uniformly anchoring of TiO2 and Fe nanoparticles at its surface. The as-prepared Fe–CNT/TiO2 composite photocatalysts have been investigated by degrading methylene blue (MB) under UV and differently intensified visible light irradiation. The ability of CNT to store and shuttle electrons, and Fe nanoparticles demonstrate its capability to serve as a yield and transfer electrons on demand to separate h + /e − pairs. Moreover, the MB photodegradation increase with an increase of visible light intensity can be ascribed to the enhancement MB cationic radical. In addition, chemical oxygen demand (COD) of piggery waste and reduction efficiency of Cr (IV) was done at regular intervals, which gave a good idea about mineralization of wastewater.

Photoreduction of Cr(VI) in Fe(III)-lactate System

The photochemical reduction of Cr(VI) in aqueous solutions containing Fe(III)-lactate complex was preliminarily investigated. The effects of initial pH, initial concentrations of Fe (III), lactate and Cr(VI) were studied in detail and the photoreduction mechanism of Cr(VI) was discussed preliminarily. The results indicate that Cr(VI) can be effectively photochemically reduced by Fe(III)-lactate complex, and the optimum pH for photoreduction of Cr(VI) is 3.0. The reduction efficiency of Cr(VI) decreases with increase of the initial concentrations of Cr(VI) over the range of 12.0~48.0 μM. The initial rate of Cr (VI) photoreduction increases with increase of the initial concentrations of Fe (III) or lactate. The photoreduction reaction is accordance with the law of apparent first-order dynamics reaction.

Preliminary Study on Photocatalytic Degradation of Methyl Orange and Cr(VI) Compound System by Titania-Bearing Blast Furnace Slag

Using calcined sulfate-modified titanium-bearing blast furnace slag (STBBFS) as photocatalyst, decolorization efficiency of methyl orange (MO) in the presence of Cr(VI) were studied. The effect of solution pH on the decolorization efficiency of MO and reduction efficiency of Cr(VI) have been investigated. The results indicate that the acidic solutions are favorable for the photocatalytic oxidation of MO in the presence of Cr(VI), and the optimum pH for oxidation of MO is 1.5. The increasing of photocatalytic activities in the compound system can be attributed to five main reasons: (1) the redox reaction between Cr(VI) and MO; (2) adsorption of Cr(VI) species and dye molecule onto STBBFS surface; (3) the visible light irradiation; (4) Cr(VI) species reduced to Cr(III) by Mn2+ in STBBFS photocatalyst; (5) Moreover, the addition of Cr(VI) species able to act as electron scavengers to catalyst surface promotes the effective separation of electron-hole, and hence promote the increase of decolorization efficiency of MO and reduction efficiency of Cr(VI) in Cr(VI)-MO system under visible irradiation. UV-vis spectral analysis indicated that MO was completely mineralized in the presence of Cr(VI) after 240 min. FTIR spectral analysis showed that all these characteristic peaks of Cr(VI) and MO disappear after photoreaction, indicating the degradation of Cr(VI) and MO.

Simultaneous photocatalytic reduction of Cr(VI) and oxidation of bisphenol A induced by Fe(III)–OH complexes in water

Simultaneously photocatalytic reduction of Cr(VI) and oxidation of bisphenol A (BPA) in aqueous solution in the presence of Fe(III)–OH complexes were investigated under a 250 W metal halide lamp ( λ ≥ 365 nm). Synergy effect of the simultaneous photocatalytic oxidation and reduction of both pollutants was achieved. The effects of initial pH value, initial concentration of BPA, Cr(VI) and Fe(III) were preliminarily investigated. The results showed that both photocatalytic reduction of Cr(VI) and degradation of BPA could occur simultaneously in the Fe(III)/Cr(VI)/BPA ternary system, and the rates of photocatalytic reduction of Cr(VI) and the oxidation of BPA were more rapid at a low pH range of 2.0–3.0. The increase of the initial concentration of Fe(III) was favorable to both photocatalytic reduction of Cr(VI) and oxidation of BPA. The reduction efficiency of Cr(VI) decreased with increasing initial concentrations of Cr(VI) and BPA, but the degradation efficiency of BPA was not changed obviously at different Cr(VI) concentrations.

Chromium adsorption and Cr(VI) reduction to trivalent chromium in aqueous solutions by soya cake

Chromium as Cr(VI) is a industrially produced pollutant. Hexavalent chromium can be reduced to the trivalent state using various reductive agents or it can be removed from solution by surface-active adsorbents. In this study, both of these methods were evaluated using soya cake. A high efficiency for reduction of Cr(VI) to trivalent chromium was observed at pH<1. Increasing the temperature, also increased the yield. Experimentally, the optimum time and soya cake mass were 5 h and 0.7 g, respectively. In the second treatment method, a high efficiency for adsorption of chromium was also observed at pH<1. The favorable temperature for adsorption was found to be 20 °C. Experimentally, the best time was 1h and with increasing soya cake mass up to 30 g, the adsorption efficiency was increased. Dissolution of LiCl in the experimental solutions, increased the efficiency of adsorption, however, this effect was not observed in the case of KCl. Langmuir isotherm constants, Q and b, for ground soybeans, were found to be 2.8×10 −4 mg/mg and 0.623, respectively. Freundlich isotherm constants, K f and n, were found to be 1.4×10 −4 and 4.99, respectively.

Photochemical Reduction of Cr(VI) in Aqueous Solutions Containing Fe(III)-hydroxy Complexes

Hexavalent Cr is acutely toxic and carcinogenic. The photochemical reduction of Cr(VI) in aqueous solutions containing Fe(III)-hydroxy Complexes was preliminarily investigated. The effects of pH, the initial concentrations of Fe(III) and Cr(VI) on the photochemical reduction of Cr(VI) with a high-pressure mercury lamp at 25°C was also investigated. The photochemical reduction efficiency of Cr(VI) decrease with increasing the initial Cr(VI) concentration. Optimum pH for photochemical reduction of Cr(VI) by Fe(III)-hydroxy complexes is 2.5-3.0. The initial Fe(III) concentrations also have effects on the reduction efficiency. Compared with the results of the experiments carried out by Hug, our research has shown Fe(III)-hydroxy complexes can reduced Cr(VI) without adding any catalyst, implying little secondary pollution and a potential of utilizing solar energy although the rate is not so high.

Cr(VI) Reduction in Aqueous Solutions by Using Synthetic Iron Sulphide

This study was carried out to reduce Cr(VI) in aqueous solutions by using synthetic iron sulphide. For that purpose, the effects of acid content, contact time, initial Cr(VI) concentration, temperature of the solution, particle size and dosage of iron sulphide on the Cr(VI) reduction were investigated. Reduction extent of Cr(VI) is, to a great extent, dependent on the amount of acid. The Cr(VI) in the solution containing two times the H2SO4 with respect to stoichiometric amount of Cr(VI) was completely reduced within 45 min by using 2.5 g litre−1 iron sulphide at 25 °C. The reduction efficiency of Cr(VI) was increased with temperature of solution. The variation in particle size of iron sulphide did not affect the reduction yield of Cr(VI). The total reduction capacity of synthetic iron sulphide was found to be 237.6 mg Cr(VI) g−1. The results related to shrinking particle model showed that the reduction reaction was chemically controlled and activation energy for the process was calculated as 38.4 kJ mol−1.