Formation and leachability of hexavalent chromium in the Al2O3-CaO-MgO-Cr2O3 system

The Al₂O₃-CaO-MgO-Cr₂O₃ system has immense potential for refractory castables applications. However, Cr(III) can potentially be oxidized to carcinogenic Cr(VI) during the usage, which can cause subsequent problems with disposal. Equilibrium experiments on the Al₂O₃-CaO-MgO-4wt%Cr₂O₃ system were performed at 1500?°C in air. The effect of MgO addition (0 and 20?wt%) on the formation and leachability of hexavalent chromium was investigated using XPS, XRD, SEM-EDS, the TRGS 613 standard Cr(VI) leaching test and multiple leaching tests. A Cr(VI)-containing phase Ca₄Al₆CrO₁₆ predominantly formed in the samples with 0 and 5?wt% MgO, while the Cr(III)-containing Mg(Al, Cr)₂O₄ spinel phase formed in the samples with 10 and 20?wt% MgO. Addition of MgO suppressed the formation of Cr(VI) while favored the formation of spinel phase. Concentrations of Cr(VI) in the leachates from TRGS 613 tests exceeded the European limit of 0.0002?wt% in all samples, although decreased significantly with 20?wt% MgO addition.     

Ozonation of a Complex Industrial Effluent: Oxidation of Organic Pollutants and Removal of Toxicity

This work deals with the ozonation of a chemical industry wastewater, which contains many complex organic pollutants and presents high chloride content and toxicity. Batch experiments were carried out until ozone absorption reached 0.1 to 3 gO₃/L. Ozonation promoted low to moderate levels of organic matter removal even when high ozone doses were applied. Organic matter removal tended to decrease when chloride content increased. Toxicity removal increased with the ozone dose up to 0.5 gO₃/L. High or complete toxicity removal was attained after ozonation of the wastewater samples. Ozonation was also very effective in removing the polycyclic aromatic compounds found in the wastewater.     

All-solid-state MoS2/WO3 photoelectrodes as high-efficiency catalysts for reductive/sorptive/recycling of hexavalent chromium

Photoelectrocatalysis (PEC) is an effective approach to eliminate carcinogen hexavalent chromium (Cr(VI)) in wastewater, in which high-performance catalysts are crucial. Herein, controlled growth of thin molybdenum disulphide (MoS₂) nanosheets on self-supported tungsten trioxide (WO₃) created an all-solid-state MoS₂/WO₃ heterojunction serving as electrode and catalyst simultaneously for removing Cr(VI). Countless small and thin MoS₂ nanoflakes build in a huge and porous interface for harvesting lights and adsorbing chromium species. And the highly conductive WO₃ substrate facilitates the transfer of those photoexcited-electron and therefore suppresses the recombination between electrons and holes. Furthermore, assisted by bias potential, electron streams from external circuit render an electron-rich interface at the MoS₂/WO₃ cathode, accelerating the Cr(VI) reduction by PEC. At −1.2 V, the PEC reduction efficiency of Cr(VI) reaches 100% within 30 min, surpassing the pristine WO₃ by 2.7 times. The generated Cr(III) ions can be immobilized on the porous MoS₂/WO₃ cathode through electrostatic attraction, enabling removal of total chromium. More importantly, the Cr(III) anchored to the catalyst can be effortlessly recovered by eluting with clean water, which also refreshes the MoS₂/WO₃ cathode. This study provides a new approach to fabricating photoelectrodes for sustainable PEC reduction and treatment of Cr(VI) containing wastewater.     

Combination of biological and chemical processes for the treatment of textile wastewater containing reactive dyes

The treatment of a segregated textile wastewater containing reactive dyes was investigated in two continuous‐flow process trains using ozonation and biological processes. The degree of decolorization and dissolved organic carbon (DOC) removal achieved by ozonation followed by aerobic treatment (two‐stage) was compared with that found when an anaerobic and aerobic pretreatment was added (four‐stage). Although the biological pretreatment reduced color by ～70%, similar amounts of ozone were required in both trains to achieve high degrees of overall removal of color and DOC. In both trains, ozonation increased biodegradability in the following aerobic reactor, however, in order to reach ～80% overall DOC removal, a specific ozone absorption (A*) of ～6 gO₃ gDOC_o^?1 was required and >50% of the DOC was mineralized in the ozone reactor. A comparison of cost estimates based on investment and operating costs for the process alternatives showed that a four‐stage train would reduce costs only if it enabled a decrease in A* to less than 2 gO₃ gDOC_o^?1. Difficulties in comparing treatment processes for segregated vs full‐stream wastewaters are discussed. Copyright © 2003 Society of Chemical Industry     

Sludge Ozonation and its Application to a New Advanced Wastewater Treatment Process with Sludge Disintegration

The applicability of sludge ozonation on wastewater treatment processes was investigated to reduce the amount of excess sludge without losing phosphorus removal efficiency. Solubilization degree per ozone consumption for general sludge was in the range from 2.4 to 5.8 gSS/O₃ and from 4.1 to 7.7 gCOD/gO₃. Around 80 to 90% of solubilized organics was biodegradable at a solubilization degree of 0.3. Based on the experimental results, a lab-scale plant with sludge ozonation and phosphorus crystallization was constructed to investigate the treatment performance. Amount of excess sludge was reduced by 93% with almost complete removal of soluble BOD and phosphorus removal efficiency of more than 80%. The percentage of the effluent COD_Cr discharge increased from 10% to 14–17% after installing ozonation and crystallization because of the formation of non-biodegradable organic substances in ozonation process. Energy consumption of the innovative advanced process is comparable or can be even smaller than that of the conventional anaerobic/oxic (A/O) process in spite of the installation of ozonation and crystallization.     

Feasibility of Sludge Ozonation for Stabilization and Conditioning

A pilot-scale sludge treatment plant was built to investigate the feasibility of ozonation processes for waste activated sludge treatment. Ozonation of wastewater sludge resulted in mass reduction by mineralization as well as by supernatant and filtrate recycle. Another advantage of sludge ozonation is a significant improvement of settleability and dewaterability. Experimental results showed that mass reduction of 70% and volume reduction of 85% compared with the control sludge was achieved through the sludge ozonation at a dose of 0.5?gO₃/gDS. It is also interesting to note that the filterability deteriorates up to ozone dose of 0.2?gO₃/gDS and then improves considerably at a higher ozone dose. The filterability could be improved by chemical conditioning even at a low ozone dose. The economic feasibility by cost analysis reveals that ozonation processes can be more economical than other alternative processes for sludge treatment and disposal at small-sized wastewater treatment plants.     

Ozone-Based Processes Applied to Municipal Secondary Effluents

This study analyzes the performances of 2 methods of oxidation based on ozone, namely ozonation and ozone combined with hydrogen peroxide (O₃/H₂O₂), on two biotreated municipal wastewater effluents. The main parameters monitored to evaluate the effectiveness of the processes were Chemical Oxygen Demand (COD), Dissolved Organic Carbon (DOC) and Biochemical Oxygen Demand (BOD₅). Ozonation and O₃/H₂O₂ treatment removed 44% and 48%, respectively, of the COD, after 90 min, of the secondary effluent of Calafell wastewater treatment plant (Spain). On the secondary effluent from the Grasse wastewater treatment plant (France), these same treatments (O₃; O₃/H₂O₂) achieved, respectively, a degradation of 52% and 100% of the COD after 60 min. The transferred ozone dose (TOD) during Calafell and Grasse effluents' ozonation were 122 mg·L^?1 and 77 mg·L^?1 after 90 min, respectively. A low removal of DOC was monitored during both O₃ or O₃/H₂O₂ treatments applied to Calafell wastewater, respectively 12% and 14%. Better DOC reductions were obtained on the water of Grasse treated with O₃ or O₃/H₂O₂, respectively, 48% and 60%. In addition, ammonia nitrogen was oxidized to nitrate nitrogen thus giving rise to an over ozone consumption. And finally, both processes proceeded with an increase of pH values. These results highlight the strong dependency of O₃ or O₃/H₂O₂ treatment effectiveness in terms of dissolved organic matter (DOM) removal and ozone consumption on wastewater composition (organic and inorganic substances).     

The Use of para-Chlorobenzoic Acid (pCBA) as an Ozone/Hydroxyl Radical Probe Compound

Since OH^· radicals cannot be measured directly during an ozonation process, para-chlorobenzoic acid (pCBA) has been used recently as an OH^· radical probe compound during ozonation based on its very slow direct reaction with ozone and fast reaction with OH^· radicals. However, in experiments of this study it was shown that pCBA accelerated ozone decay. Furthermore, the formation of hydrogen peroxide was observed during this process. The formed H₂O₂ increases the decomposition of aqueous ozone and leads to enhanced formation of OH^· radicals. The chain reaction therefore changes to HO₂ ^? ion initiated decay of ozone instead of hydroxide ion, OH^?. Thus, an error in applying pCBA as a probe compound in low scavenger containing waters is likely to occur if the scavenging rate of pCBA makes up more than 5% of the total scavenging rate.     

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.     

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.     

Removal of Cr(VI) from aqueous solution using magnetite/non-oxidative graphene composites: Synergetic effect of Cr(VI) on dyes removal

Non-oxidative graphene (nOG) synthesized from natural graphite powder was modified with magnetite (Fe₃O₄) for removal of Cr(VI) and dyes in aqueous solution. The adsorption behavior of Cr(VI) on Fe₃O₄/nOG (M-nOG) was systematically investigated, and the simultaneous adsorption of Cr(VI) and dyes such as methylene blue (MB) and rhodamine B (RhB) was evaluated. Adsorption kinetic and isotherm of Cr(VI) were fitted well with pseudo-second-order model and Sips model, respectively. For the binary system, Cr(VI) removal was not affected with increasing the dye concentration, whereas the adsorption capacity of both MB and RhB was enhanced with increasing the concentrations of Cr(VI).     

Boron-Doped Diamond Electrode Performance in Cr(VI) Reduction Using Synthetic and Plating Wastewater

The goal of this work was to evaluate the effectiveness of iron and boron-doped-diamond (BDD) as cathodic electrodes on the reduction of Cr(VI) in synthetic and wastewater samples. The pH and electrolyte composition were varied, and the effect on the Cr(VI) reduction rate was measured. The optimized conditions from the synthetic water Cr(VI) reduction experimental data, were tasted on electroplating wastewater. The results indicated that both a pH of 2 and the use of NaCl as an electrolyte significantly increase the Cr(VI) reduction rate for all synthetic systems, especially the iron-BDD system. The Cr(VI) reduction rate in Fe-BDD systems was also affected by nitrate and sulfate ions. In the case of electroplating wastewater, Cr(VI) reduction by BDD cathodes was faster than with iron cathodes, achieving a complete reduction of 180 mg Cr(VI)/L in 25 min, with 40% less sludge produced. The elemental composition of sludge was analyzed using SEM/EDS and X-ray spectroscopy to confirm that iron and chromium precipitated out of the solution. The sludge had a chemical composition of (31.9%) Fe₂O₃, (29.6%) FeOOH, (21%) FeO, and (17.4%) FeSO₄. Therefore, BDD as an electrode material effectively reduces Cr(VI) in electroplating wastewater, and can be effectively scaled up to industrial applications.     

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.     

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.     

SnS2/TiO2 nanocomposites with enhanced visible light-driven photoreduction of aqueous Cr(VI)

SnS₂/TiO₂ nanocomposites have been synthesized via microwave assisted hydrothermal treatment of tetrabutyl titanate in the presence of SnS₂ nanoplates in the solvent of ethanol at 160 °C for 1 h. The physical and chemical properties of SnS₂/TiO₂ were studied by XRD, FESEM, EDS, TEM, XPS and UV–vis diffuse reflectance spectra (DRS). The photocatalytic activity of SnS₂/TiO₂ nanocomposites were evaluated by photoreduction of aqueous Cr(VI) under visible light (λ>420 nm) irradiation. The experimental results showed that the SnS₂/TiO₂ nanocomposites exhibited excellent reduction efficiency of Cr(VI) (~87%) than that of pure TiO₂ and SnS₂. The SnS₂/TiO₂ nanocomposites were expected to be a promising candidate as effective photocatalysts in the treatment of Cr(VI) wastewater.     

Thermal evolution of Al2O3–CaO–Cr2O3 castables in different atmospheres

Al₂O₃–CaO–Cr₂O₃ castables are required for various furnaces linings due to their excellent corrosion resistance. However, toxic and water-soluble Cr(VI) could be generated in these linings during service. In this study Al₂O₃–CaO–Cr₂O₃ castables were prepared and heated at 300–1500 °C in air and coke bed to simulate actual service conditions. The formations of various phases were investigated by XRD and SEM-EDS. The Cr(VI) compounds CaCrO₄ and Ca₄Al₆CrO₁₆ formed in air at 300–900 °C and 900–1300 °C respectively, while C₁₂A₇ and CA₂ were generated rather than forming Cr(VI) compounds in coke bed at 700–1300 °C. However, at 1500 °C, nearly all the chromium existed in the form of (Al_1-xCr_x)₂O₃ solid solution in both atmosphere. As a result, the specimens treated in air contained 185.0–1697.8 mg/kg of Cr(VI) at 500–1300 °C but only 17.2 mg/kg of Cr(VI) at 1500 °C, whereas specimens treated in coke bed exhibited extremely low Cr(VI) concentration in the whole temperature range studied. Moreover, in coke bed, the mutual diffusion between Cr₂O₃ and Al₂O₃ was suppressed and a trace of Cr₂O₃ would even be reduced to form chromium-containing carbides on its surface, which would hindered the sintering process and hence lower the density as well as strength of the castables.     

Removal of Chromium from Synthetic and Real Effluents Using Uncalcined MgAl-NO3 Layered Double Hydroxide: Column Study and Modeling

In this study, the sorption ability of MgAl-NO₃ layered double hydroxide prepared by coprecipitation method to remove Cr(VI) from aqueous solution was investigated in fixed-bed column. The sorbent was characterized by XRD, SEM, FTIR, BET surface area, and pH_zpc. The effects of bed height (2–5 cm), flow rate (5–20 mL/min), inlet Cr(VI) concentration (30–100 mg/L), and solution pH (3–8) on the breakthrough characteristics of the column were determined, and the optimum conditions were observed at 3.5 cm, 10 mL/min, 50 mg/L, and a pH of 6. Moreover, the performance of the sorbent to remove Cr(VI) from wastewater was also examined at optimum conditions. The results show high removal and good selectivity for Cr(VI) in wastewater containing large amounts of coexisting anions.     

Ozone treatment of textile effluents and dyes: effect of applied ozone dose,pH and dye concentration

The ozonation of wastewater supplied from a treatment plant (Samples A and B) and dye‐bath effluent (Sample C) from a dyeing and finishing mill and acid dye solutions in a semi‐batch reactor has been examined to explore the impact of ozone dose, pH, and initial dye concentration. Results revealed that the apparent rate constants were raised with increases in applied ozone dose and pH, and decreases in initial dye concentration. While the color removal efficiencies of both wastewater Samples A and C for 15 min ozonation at high ozone dosage were 95 and 97%, respectively, these were 81 and 87%, respectively at low ozone dosage. The chemical oxygen demand (COD) and dissolved organic carbon (DOC) removal efficiencies at several ozone dose applications for a 15 min ozonation time were in the ranges of 15–46% and 10–20%, respectively for Sample A and 15–33% and 9–19% respectively for Sample C. Ozone consumption per unit color, COD and DOC removal at any time was found to be almost the same while the applied ozone dose was different. Ozonation could improve the BOD₅ (biological oxygen demand) COD ratio of Sample A by 1.6 times with 300 mg dm^?3 ozone consumption. Ozonation of acid dyes was a pseudo‐first order reaction with respect to dye. Increases in dye concentration increased specific ozone consumption. Specific ozone consumption for Acid Red 183 (AR‐183) dye solution with a concentration of 50 mg dm^?3 rose from 0.32 to 0.72 mg‐O₃ per mg dye decomposed as the dye concentration was increased to 500 mg dm^?3. © 2002 Society of Chemical Industry     

Inhibited Cr(VI) formation in Cr2O3-containing refractory castable using reactive MgO as hydraulic binder

Reactive MgO was used in the first time as alternative hydraulic binder of calcium aluminate cement (CAC) to prepare Cr₂O₃-bearing refractory castable. The formation of Cr(VI), and physical and mechanical properties of MgO-bonded refractory castables after heat-treating were investigated. Microstructural characterization and phase composition analyses on the heat-treated MgO-bonded refractory castable matrices resulted in a comprehensive understanding of the mechanism for the inhibition of Cr(VI), and of the strength development during firing. The results indicate that compared with CAC, Cr(VI) levels were 6.7–28.1 times lower using reactive MgO after firing at 700–1300 °C. The in situ Mg(Cr,Al)₂O₄ spinel formed from the preferential interactions among MgO and Cr₂O₃ and Al₂O₃ would be the main reason leading to the inhibited Cr(VI) formation and strength development during firing.     

Effects of chromium (VI) on the activities of ureolytic mixed culture

BACKGROUND: Ureolytic microbial carbonate precipitation (MCP) is a novel process for removing excess calcium from industrial effluents. This process is based on the hydrolization of urea to provide a suitable medium for the precipitation of Ca as CaCO₃. RESULT: A toxicity identification evaluation was conducted on synthetic wastewater simulating wastewater from paper recycling to determine the inhibition or toxicity of chromium(VI) ions on the activities of a ureolytic mixed culture (UMC) with respect to the removal rate of COD and removal of Ca²⁺ in batch reactors. The 50% inhibiting concentration (IC₅₀) and the 25% inhibiting concentration (IC₂₅) values of Cr(VI) on UMC were determined as 40 and 18 mg L^?1, respectively, for an exposure time of 1 day. The inhibitory effects of Cr(VI) were lower on UMC after longer exposure times of 3, 4 and 5 days. COD removal rate, ammonium production and Ca removal of samples with 512 mg Cr(VI) L^?1 were significantly inhibited. CONCLUSION The present study has shown that the effects of different metallic species, organic toxicants and other environmental factors should be taken into account when removing problematic Ca and other possible elements from the environment using UMC. Copyright © 2008 Society of Chemical Industry