Re‐use of Exhausted Ground Coffee Waste for Cr(VI) Sorption

Abstract

Exhausted ground coffee waste has been investigated as metal biosorbent for Cr(VI) from aqueous solution. Maximum metal sorption was found to occur at initial pH 3.0. Kinetic studies revealed that the initial uptake was quite rapid; nevertheless, it took five days to reach equilibrium. The value of the Langmuir maximum uptake was found to be 10.2 mg Cr(VI)/g waste. The sorbent is able to reduce hexavalent chromium to its trivalent form. A solution of 1 M NaOH was the most effective desorption agent and after 24 hours contact 42% of total chromium was desorbed in both hexavalent and trivalent oxidation states.     

Redox Dynamics of Mixed Metal (Mn,Cr, and Fe) Ultrafine Particles

The impact of particle composition on metal oxidation state, and on changes in oxidation state with simulated atmospheric aging, are investigated experimentally in flame-generated nanoparticles containing Mn, Cr, and Fe. The results demonstrate that the initial fraction of Cr(VI) within the particles decreases with increasing total metal concentration in the flame. In contrast, the initial Mn oxidation state was only partly controlled by metal loading, suggesting the importance of other factors. Two reaction pathways, one reductive and one oxidative, were found to be operating simultaneously during simulated atmospheric aging. The oxidative pathway depended upon the presence of simulated sunlight and O₃, whereas the reductive pathway occurred in the presence of simulated sunlight alone. The reductive pathway appears to be rapid but transient, allowing the oxidative pathway to dominate with longer aging times, i.e., greater than ～8 hours. The presence of Mn within the particles enhanced the importance of the oxidative pathway, leading to more net Cr oxidation during aging, implying that Mn can mediate oxidation by removal of electrons from other particulate metals.     

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.     

Speciation of As, Cr, Se and Hg under coal fired power station conditions

Coal combustion from power stations is an important anthropogenic contributor of toxic trace elements to the environment. Some trace elements may be emitted in range of valencies, often with varying toxicity and bioavailability. Hence, determination of trace element speciation in coals and their combustion products is important for conducting comprehensive risk assessments of the emissions from coal-fired power stations. This study focuses on speciation of selected trace elements, As, Cr, and Se, in coal combustion products and Hg in flue gas, which were sampled at one Australian power station. Different analytical methods such as secondary ion mass spectrometry (SIMS), ion chromatography-inductively coupled plasma mass spectrometry (IC-ICPMS) and X-ray absorption near edge structure spectrometry (XANES) were used to determine trace element speciation in coal and ash samples. Results showed that As, Cr and Se are all present in a range of valency states in coal. Concentrations of As and Se in the bottom ash as well as the more toxic hexavalent chromium were less than the detection limits. The more toxic As³⁺ form in fly ash was at 10% of the total arsenic, while selenium was mainly found in Se⁴⁺ form. Hexavalent chromium (Cr⁶⁺) in fly ash was 2.7% of the total fly ash chromium. Mercury speciation in flue gas was determined using the Ontario Hydro sampling train and analysis technique. Approximately 58% of the total mercury in flue gas was released in the elemental form (Hg⁰), which, among all mercury species, has the highest residence time in the environment due to lower solubility. This work summarises the performance of the selected analytical techniques for speciation of trace elements.     

Kinetics and thermodynamic studies of Cr(VI) adsorption using environmental friendly multifunctional zeolites synthesized from coal fly ash under mild conditions

Abstract

The Na-P1 zeolite was produced from coal fly ash and modified with different environmental friendly surfactants. The potential of these green modified zeolites was investigated as adsorbents for Cr(VI) ions in a batch system. XRD, SEM, XRF, and ICP-AES analyses were used for the characterization of raw materials and zeolite samples. The environmental friendly modified zeolites successfully immobilized different toxic elements in their framework inhibiting the transfer of these toxic elements to the surrounding liquid phase. The effects of various operational parameters on Cr(VI) removal were studied. The Hexamethylenediamine (HDTMA) and Ammonyx KP (KP) modified zeolites had larger chromium removal potential than the other samples at all temperatures. The effectiveness of Cr(VI) ions elimination became greater as the pH decreased and the adsorbent dose increased. The Freundlich, Langmuir, and Dubinin–Radushkevich isotherms were fitted to the equilibrium data. The Dubinin–Radushkevich and Langmuir models gave a better fitness to equilibrium data of HDTMA-Na-P1 and KP-Na-P1, respectively. The positive and high ΔH° values showed the endothermic nature of the total Cr(VI) sorption procedure and indicated that Cr(VI) adsorption onto HDTMA-Na-P1 and KP-Na-P1 is a chemisorption. The negative ΔS° values also showed that chromium ions were stable on the surface of adsorbents. The adsorption potential of the developed eco-friendly KP-Na-P1 was higher than those of other adsorbents reported in the literature.     

The Speciation and Morphology of Chromium Oxide Nanoparticles in a Diffusion Flame

The impact of combustion conditions on the formation of chromium oxide aerosol has been studied in a hydrogen diffusion flame seeded with Cr(CO)₆ vapor. Specifically, the effects of the precursor concentration, the flame temperature and the flame residence time on properties of the postflame chromium oxide aerosol were investigated. Each of these flame parameters was found to affect the particle size distribution and/or particle morphology. These effects were partially associated with the fact that the chromium speciation in the final flame products was sensitive to the precursor concentration and the flame temperature. The flame residence time was found to affect the crystallinity of the Cr(III) particles in the postflame chromium oxide aerosol. Conversion of Cr(III) particles to Cr(VI) in the high temperature zone of the flame was proposed to explain changes in the particle size distribution, composition, and morphology. The proposed mechanism was able to explain the experimental findings satisfactorily.     

高温燃烧过程中Cr的氧化机理及控制方法

Cr作为一种有害元素广泛地存在于化石燃料、生活垃圾和工业固体废物中。其中,化石燃料、相当大部分的生活垃圾以及一部分的工业固体废物都通过燃烧过程来回收能量或者进行减容和无害化处理。因此,通过燃烧过程人类向自然界中排放了大量的Cr。最重要的是,热处置过程中的高温氧化环境容易使低迁移性和毒性的Cr（Ⅲ）氧化为高迁移性和毒性的Cr（Ⅵ）。所以,热处置过程中Cr的反应、行为以及产物中Cr的存在形态得到了越来越多的研究。本文综述了燃料中Cr的存在形态、燃烧过程中Cr的氧化机理、燃烧过程中Cr氧化的控制方法以及燃烧产物中Cr的固化方法四方面内容,为减少燃烧过程中的Cr对环境的危害提供理论基础。     

Coals as sorbents for the removal and reduction of hexavalent chromium from aqueous waste streams

The aim of this study is to demonstrate the potential of coals as a low-cost reactive barrier material for environmental protection applications, with the ability to prevent leaching of toxic Cr(VI) and other transition metals. Depending upon the type of ion and the surface functionalities, the uptake can involve ion sorption, ion exchange, chelation and redox mechanisms with the surface functionalities being considered as partners in electron transfer processes. The capacity for Cr(VI) uptake of low rank coals and oxidized bituminous coals has been found to lie within the range 0.2-0.6 mM g⁻¹. Air oxidation of bituminous coals can increase their Cr(VI) removal capacities. The effect of air oxidation of coals on uptake capacity was more pronounced for Cr(VI) than Cr(III), but less than for Hg(II) and the other ions (Ca²⁺, Ba²⁺, Zn²⁺, Cd²) investigated. As previously found for Hg(II), redox mechanisms play an important role in Cr(VI) uptake, with sorption of the resultant Cr(III) being aided by the functionalities arising from oxidation of the coal surface. In acidic media, much of the resultant Cr(III) is exchanged back into solution by hydrogen ions, but some of the sorbed chromium is irreversibly bound to the coal. The reduction of Cr(VI) alone is often considered a satisfactory solution in view of Cr(III) being essentially non-toxic.     

Comparison of ethylene polymerization catalyzed over Cr(VI)/silica and Cr(II)/silica catalysts

Summary Ethylene polymerization and the morphology of produced high density polyethylene (HDPE) catalyzed over. Cr(VI)/silica and Cr(II)/silica were studied. Cr/silica catalyst with Cr loading of 1 wt % has been prepared by impregnating an aqueous solution of CrO₃ onto silica having specific surface area of 301 m²/g and pore volume of 1.64 ml/g. The rate profiles and the morphologies of polyethylene polymerized over Cr(VI) and Cr(II)/silica were different. The process of active site formation influences the rate profiles. The shape of polyethylene particles polymerized with Cr(II)/silica resembled the original shape of catalyst particles. However, Cr(VI)/silica catalyst particles were fractured inhomogeneously during the polymerization. The variation of molecular weight and molecular weight distribution at various polymerization times indicated that the formation of active sites of Cr(VI)/silica was accomplished gradually during polymerization.     

Surface and microanalytical studies of whetlerite charcoals: 1. Effects of aging

ASC Whetlerite charcoals impregnated with copper and chromium have been studied by several surface and microanalytical techniques to determine the effects of aging on the composition and microstructure. X-ray photoelectron spectroscopic (XPS) studies of absorbers not deliberately aged, showed that most of copper on the inside of each particle was present as Cu(I). Transmission microscopy showed that some of this copper is present as crystallites with diameters up to 0.1 μm. Natural or artificial aging in a humid atmosphere resulted in the number and size of such crystallites and a migration of some of the copper impregnants to the outer surface of the particle, where it is bonded largely as Cu(OH)₂. Chromium is present in the adsorber in both Cr(III) and Cr(VI) oxidation states and it undergoes little change during aging.     

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.     

Removal of hexavalent chromium ion from aqueous solution using nanoscale zero-valent iron particles immobilized on porous silica support prepared by polymer template method

Porous silica supported nanoscale zero-valent iron was prepared by a polymer template method in order to effectively remove a hexavalent chromium ion (Cr(VI)) in an aqueous solution. It did not show a deterioration of Cr(VI) removal efficiency, which could be caused by the surface oxidation and agglomeration of nanoscale zero-valent iron (NZVI) particles. Porous silica by the polymer template method showed quite unique structure, which we named as quasi-inverse opal silica (QIOS), and it showed high surface area (375.4m²/g) and fine pore size (76.5 nm). NZVI immobilized on the surface of QIOS (NZVI@QIOS) was added to an aqueous Cr(VI) solution at 0.025 g/L, and it showed over 96% Cr(VI) removal efficiency. Such a high removal efficiency of Cr(VI) was maintained over two weeks after preparation (92% after 16 days). Morphology of porous silica supported nanoscale zero-valent iron was analyzed by TEM and FE-SEM. Identification of the reaction compounds produced by the reaction of Cr(VI) and zero-valent iron (Fe(0)) was made by the application of XPS.     

Effect of the catalyst supports on the removal of perchloroethylene (PCE) over chromium oxide catalysts

The oxidation of perchloroethylene (PCE) was investigated over chromium oxide catalysts supported on TiO₂, Al₂O₃, SiO₂, SiO₂–Al₂O₃ and activated carbon. The phase of chromium oxide on the catalyst surface is critical for the oxidation of PCE. The catalytic activity of PCE removal enhances as the formation of Cr(VI) species on the catalyst surface increases. The surface area and the type of the catalyst supports were also essential for high performance in the PCE oxidation. In addition, the structure of Cr(VI) on the catalyst surface also plays an important role for the decomposition of PCE. The polymerized Cr(VI) mainly formed by the interaction of metals with the support is the active reaction site for the present reaction system. CrO_x/TiO₂ reveals the strongest PCE removal activity among the catalysts examined in the present study. This revised version was published online in July 2006 with corrections to the Cover Date.     

THERMODYNAMIC EQUILIBRIA OF THE EXTRACTION OF CHROMIUM(VI) WITH TRI-N-OCTYLPHOSPHINE OXIDE FROM AQUEOUS SOLUTIONS

ABSTRACT

The distribution of chromium( VI ) between aqueous solutions and the kerosene solutions of tri-n-octyl-phosphine oxide (TOPO) have been studied at 25 °C and pH 2-4, in which range the predominant forms of Cr(VI) are HCrO₄ and Cr₂O₇ ^2?. The extraction equilibrium is influenced by the total chromium(VI) concentration, aqueous pH value, and total extractant concentration. The thermodynamic equilibria among all species in the aqueous phase were used for the analysis of distribution data, and the activity coefficients of ionic species were estimated according to Debye-Hiickel or Bromley equation. By numerical method, the complexes of Cr(VI ) with TOPO formed in the organic phase were found to be H₂CrO₄ (TOPO) and H₂Cr₂O₇ (TOPO) ₃. The corresponding extraction constants were 2019 and 3.69× 10⁸, respectively.     

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.     

Chromium(VI) Biosorption and Bioaccumulation by Live and Acid-Modified Biomass of a Novel Morganella morganii Isolate

Conventional methods of chromium removal are often insufficient for the remediation of chromium-contaminated natural environments, necessitating the development of alternative strategies. In this paper, we report the isolation of a novel Morganella morganii strain capable of reducing hexavalent chromium to its less-toxic and less-soluble trivalent form. Cr(VI) reduction by this strain was evaluated in both acidic environments and conditions reflecting natural freshwater sources. The isolate achieved equilibrium within 3 h and displayed a specific uptake rate of 24.30 ± 1.67 mg Cr(VI)/g biomass following HCl treatment. Without acid treatment, a reduction of over 90% was recorded within 72 h for an initial Cr(VI) concentration 20 mg/L, corresponding to a Cr(VI) removal capacity of 19.36 ± 1.89 mg/g. Absorption data of acid-treated STB5 biomass most closely followed the Toth and Langmuir models. FTIR results indicate that hydroxyl groups and extracellular or cell membrane polysaccharides may be potential adsorption sites for hexavalent chromium. Our results suggest that the isolate may be used in situ for treatment of polluted freshwater environments.     

Determination of hexavalent chromium in South African cements and cement-related materials with electrothermal atomic absorption spectrometry

The selective extraction of Cr(VI) in a cement matrix, based on treatment with 0.1 M Na₂CO₃ solution and subsequent determination of chromium by electrothermal atomic absorption spectrometry has been proposed. The proposed method has been applied to South African cement clinkers, natural gypsum, limestone and certified reference materials. The limit of detection of chromium determination in cement samples was found to be 0.14 μg g⁻¹. Between 30% and 80% of the total chromium in South African cement clinkers are Cr(VI) compounds and 8-26% of the total amount of Cr(VI) species is water soluble. The analytical performance of the proposed method has been verified by the analysis of BCS-CRM, a sulphate-resisting Portland cement, and the results obtained were in good agreement with the certified values.     

The electrochemical study of a chromium plating bath. I. Reactions leading to solution-free species

The electrode reactions occurring at C, Cr, Cu and Ni cathodes in the standard electroplating solution for chromium, 200 g dm^?3 CrO₃ and 2 g dm^?3 H₂SO₄, have been reinvestigated. This paper emphasises the study of those reactions which lead to solution-free products; these occur at potentials positive to that necessary for film or metal deposition. For each cathode, theI-E curve shows a wave or peak for the reduction of chromium (VI) to chromium(III) and it is demonstrated that this process is mass transport controlled, apparently by the transport of sulphate ions to the electrode surface. At Cr, Ni and Cu, the potential of the chromium(VI)/chromium(III) couple is determined by the removal of an insulating oxide film which forms spontaneously when the metal is immersed in the bath. In certain conditions it is also possible to define a well formed wave for proton reduction.     

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.     

Biological Cr(VI) reduction in a trickling filter under continuous operation with recirculation

BACKGROUND: Hexavalent chromium (Cr(VI)) is toxic to humans, animals and plants. Conventional treatment technologies reduce Cr(VI) to the less toxic and mobile Cr(III), but these methods are usually expensive and generate secondary waste. Microbial Cr(VI) reduction has recently gained attention as a detoxification process, since it enables Cr(VI) reduction through relatively cheap and simple methods. The aim of this work was to investigate the mechanism and the performance of biological Cr(VI) reduction using mixed cultures originated from industrial sludge under continuous operation with recirculation in a pilot‐scale trickling filter. RESULTS: Biological Cr(VI) reduction was studied using a pilot‐scale trickling filter filled with plastic media under continuous operation with recirculation and the use of indigenous bacterial population. The effect of the organic carbon (electron donor) concentration was examined for constant Cr(VI) influent concentration at about 5.5 mg L^?1 and volumetric flow rates ranging from 60 to 900 mL min^?1. The highest reduction rate achieved was 1117 g Cr(VI) m^?2 d^?1 for a volumetric flow rate of 900 mL min^?1. The system's reduction capacity was significantly affected by chromate loadings, resulting in frequent backwashing of the filter. The determination of the reduction mechanism was also studied using batch cultures of free suspended cells and culture supernatant. CONCLUSION: The high reduction rates combined with the low operating cost indicate that the above technology can be a viable solution for the treatment of industrial chromate effluents. Copyright © 2008 Society of Chemical Industry