Hydrogarnet: a host phase for Cr(VI) in chromite ore processing residue (COPR) and other high pH wastes

For understanding both the environmental behavior and developing remediation treatments for chromium ore processing residue (COPR) it is important to identify all the potentially soluble sources of Cr(VI). Hydrogarnet has been identified as a major phase in COPR and it has been previously speculated that it has a capacity to host Cr(VI). Here we provide direct evidence of this capacity by demonstrating the incorporation of Cr(VI) into laboratory synthesized hydrogarnet. Electron microscopy and energy dispersive X-ray microanalysis show incorporation of approximately 17000-22000 mg Cr(VI) kg(-1) hydrogarnet. X-ray powder diffraction data show that peak intensities are altered by chromium substitution and that chromium substituted hydrogarnets have a smaller unit cell than the pure Ca-Al end member. This is consistent with substitution of hydroxyl tetrahedra by smaller chromate tetrahedra. Electron energy loss spectroscopy confirms the tetrahedral coordination and hexavalent oxidation state of chromium in the hydrogarnets. The maximum amount of hexavalent chromium that can be introduced synthetically corresponds to a replacement of about one out of every eight hydroxyl tetrahedral per unit cell by a CrO4(2-) tetrahedra and tallies closely with the amount of chromium measured in hydrogarnets from COPR. Chromium-bearing hydrogarnet is the most abundant crystalline phase in millions of tons of COPR contaminating land around Glasgow, Scotland, and was recently identified in COPR from sites in North America. Calculations based on its abundance and its Cr(VI) content indicate that hydrogarnet can host as much as 50% of the Cr(VI) found in some COPR samples.     

Uptake and reduction of Cr(VI) to Cr(III) by mesquite (Prosopis spp.): chromate-plant interaction in hydroponics and solid media studied using XAS

Chromium (Cr) is a well-established carcinogen that is a contaminant at half of the EPA Superfund sites in the United States. Two separate studies were performed to investigate the possibility that mesquite (Prosopis spp.), which is an indigenous desert plant species, can remove Cr from the environment via active transport systems to the aerial portions of the plant. The first study was performed by growing mesquite on solid media (agar) at Cr(VI) concentrations of 75 and 125 ppm. The accumulation found in the leaves under the present conditions indicated that mesquite could be classified as a hyperaccumulator of chromium. The second study was conducted to investigate the differences between the type of Cr ligand involved in Cr uptake with agar and hydroponic cultures. We used X-ray absorption spectroscopy (XAS) to determine the mechanisms involved in the uptake and binding of Cr(VI) in live mesquite tissue. The XAS results for this study showed that some of the supplied Cr(VI) was uptaken by the mesquite roots; however, the data analyses of the plant tissues demonstrated that it was fully reduced to Cr(III) in the leaf tissues. Experiments are currently being performed to evaluate the behavior of the Mesquite plant using lower Cr concentrations.     

Bifunctionalized mesoporous silicas for Cr(VI) reduction and concomitant Cr(III) immobilization

Surfactant-templated thiol-functionalized mesoporous silica adsorbents have been prepared by cocondensation of mercaptopropyltrimethoxysilane and tetraethoxysilane in the presence of cetyltrimethylammonium bromide, which were then partially oxidized to get bifunctionalized materials containing both thiol and sulfonic acid moieties (MCM-41-SH/SO3H). The resulting organic-inorganic hybrid was applied to the uptake of chromium species according to a reduction-sorption mechanism involving reduction of Cr(VI) by thiol groups and immobilization of Cr(III) onto sulfonic acid moieties. These processes were strongly affected by pH, and the optimal conditions for effective chromium sequestration resulted from a compromise between pH values low enough to ensure quantitative reduction of Cr(VI) and not too low to enable Cr(III) binding to sulfonate groups, which was best achieved at pH 2-3. The effect of the solid-to-solution ratio and the relative amounts of -SH and -SO3H groups was also discussed. Even if Cr(VI) reduction by thiol groups resulted in the formation of sulfonic acid moieties, their contentwas not high enough to ensure quantitative Cr(III) immobilization, which was only attained with materials containing already some sulfonic acid groups prior to contacting Cr(VI) solutions. Redox speciation of sulfur and chromium species was analyzed by X-ray photoelectron spectroscopy (XPS) and used to support the proposed mechanism.     

Influences of water vapor on Cr(VI) reduction by gaseous hydrogen sulfide

In Situ Gaseous Reduction (ISGR) using hydrogen sulfide (H2S) is a technology developed for soil remediation by reductive immobilization of contaminants such as hexavalent chromium (Cr(VI)). Deploying the technology requires us to obtain a much-improved understanding of the interactions among the contaminants, H2S, and various soil components. In this study, Cr(VI) reduction by gaseous H2S was examined under various relative humidities (0-96.7%), concentrations of Cr(VI) (127-475 microg/g of solid), and H2S (0-800 ppm(v)) and by using Cr(VI) compounds with different solubilities. Glass beads with various average diameters (GA = 0.600 mm; GB = 0.212-0.300 mm; and GC = 0.106 mm) and silica (SA = 0.075-0.150 mm) were used as matrices to support K2CrO4, CaCrO4, PbCrO4, or BaCrO4, and reduction of these compounds by gaseous H2S was monitored by Cr(VI) analysis following extractions with distilled water or hot alkali solution. The results showed that Cr(VI) reduction relied on both the relative humidity of the gaseous stream and the size of particles onto which Cr(VI) was deposited. The relative humidity required for fast Cr(VI) reduction was 85% for GA, 61% for GB, 6% for SA, and 0% for GC. It was believed that a water film formed on the particle surfaces under appropriate humidity conditions, resulting in Cr(VI) compound dissolution and subsequent reduction. For nonsoluble Cr(VI) compounds including PbCrO4 and BaCrO4, no reduction by H2S was observed, even at high relative humidity (96.7%), due to lack of dissolution. This study indicated that ISGR treatment in soils requires appropriate moisture content in the subsurface or maintaining a suitable humidity in the treatment gas stream to maximize chromium immobilization.     

Reduction and removal of aqueous Cr(VI) by glow discharge plasma at the gas-solution interface

Aqueous chromium(VI) reduction and removal induced by glow discharge taking place at the gas-solution interface in an argon atmosphere was studied. The effect of initial pH and hydroxyl radical scavenger (ethanol) on the reduction efficiency was examined. High reduction efficiency was obtained when initial pH ≤ 2.0 or ≥ 8.0. In particular, addition of ethanol into the solution substantially increased the reduction efficiency and facilitated chromium removal from the solution in the form of sediment after discharge. The optimum pH values for Cr(VI) removal were within 6.0-7.0. Fourier transform-infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analysis confirmed that the main constituent of the sediment is chromium hydroxide.     

Spectroscopic investigation of Cr(III)- and Cr(VI)-treated nanoscale zerovalent iron

The reaction of hexavalent chromium (Cr(VI)) with zerovalent iron (Fe0) during soil and groundwater remediation is an important environmental process. This study used several techniques including X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy to investigate nanometer scale Fe0 particles (nano Fe0) treated with Cr(III) and Cr(VI). X-ray diffraction and XPS analyses of oxidized nano Fe0 showed the crystalline Fe(III) phase is composed of lepidocrocite (gamma-FeOOH). Results of XPS Cr 2p data and Cr K-edge X-ray absorption near edge spectroscopy (XANES) provided evidence that Cr(VI) was entirely reduced to Cr(III) by nano Fe0 with no residual Cr(VI) after reaction. In addition, XPS and XANES results of Cr(III) precipitated as Cr(OH)3 in the presence of corroding nano Fe0 were nearly identical to the Cr(VI)-nano Fe0 reaction product. Detailed analysis of XPS O 1s line spectra revealed that both Cr(III)- and Cr(VI)-treated nano Fe0 yielded a predominantly hydroxylated Cr(OH)3 and/ or a mixed phase CrxFe(1 - x)(OH)3 product. The structure of the Cr(III)- and Cr(VI)-treated nano Fe0 determined using extended X-ray absorption fine structure spectroscopy (EXAFS) revealed octahedral Cr(III) with Cr-O interatomic distances between 1.97 and 1.98 A for both Cr(III) and Cr(VI) treatments and a pronounced Cr-Cr second interatomic shell at 3.01 A. Our results suggest that the reaction product of Cr(VI)-treated nano Fe0 is either a poorly ordered Cr(OH)3 precipitate or possibly a mixed phase CrxFe(1 - x)(OH)3 product, both of which are highly insoluble under environmental conditions.     

Establishing and elucidating reduction as the removal mechanism of Cr(VI) by reclaimed limestone residual RLR (Modified Steel Slag)

The viability of utilizing Reclaimed Limestone Residual RLR (Modified Steel Slag) to remove hexavalent chromium Cr(VI) from the aqueous phase was investigated. A physical characterization of RLR showed that it is composed of various minerals some of which can reduce and others adsorb Cr(VI). Preliminary results showed that RLR significantly reduced the concentration of Cr(VI) from the aqueous phase. Adsorption competition tests with orthophosphate (HP04(2-)) and sulfate (SO4(2-)) showed that Cr(VI) was still effectively reduced from solution regardless of the competing anions present. Kinetic tests based on the relationship d[Cr(VI)]/dt = kCr[RLR]alpha[Cr(VI)]beta showed that under initially neutral to basic conditions kCr = 3.45+/-(0.25) x 10(-4) mg0.4 L(-0.4) h(-1), alpha = 0.9, and beta = -0.3, while under initially acidic conditions kCr = 5.65+/-(1.055) x 10(-1) mg(-0.4) L0.4 h(-1), alpha = 2.2, and beta = -0.8. Stirred batch tests with RLR in deionized water showed significant drops in the redox potential (Eh), and in the presence of oxygen Eh values dropped to between 50 and 100 mV while in the absence of oxygen Eh values as low as -200 mV were observed. These results lead to the conclusion that redox mechanisms were responsible for the reduction of hexavalent chromium by RLR.     

Fate of Cu, Cr, and As during combustion of impregnated wood with and without peat additive

The E.U. Directive on incineration of waste regulates the harmful emissions of particles and twelve toxic elements, including copper, chromium, and arsenic. More information is critically needed on the speciation and behavior of these trace elements during combustion, including the effects of different process variables, as well as of different fuels and fuel mixtures. Using a 15 kW pellets-fueled grate burner, experiments were performed to determine the fate of copper, chromium, and arsenic during combustion of chromate copper arsenate (CCA) preservative wood. The effects of co-combustion of CCA-wood with peat were also studied since peat fuels previously have proved to generally reduce ash related problems. The fate and speciation of copper, chromium, and arsenic were determined from analysis of the flue gas particles and the bottom ash using SEM-EDS, XRD, XPS, and ICP-AES. In addition, chemical equilibrium model calculations were performed to interpret the experimental findings. The results revealed that about 5% copper, 15% chromium, and 60% arsenic were volatilized during combustion of pure CCA-wood, which is lower than predicted volatilization from the individual arsenic, chromium, and copper oxides. This is explained by the formation of more stable refractory complex oxide phases for which the stability trends and patterns are presented. When co-combusted with peat, an additional stabilization of these phases was obtained and thus a small but noteworthy decrease in volatilization of all three elements was observed. The major identified phases for all fuels were CuCrO2(s), (Fe, Mg, Cu)(Cr, Fe, Al)04(s), Cr2O3(s), and Ca3(AsO4)2(s). Arsenic was also identified in the fine particles as KH2AsO4(s) and As2O3(s). A strong indication of hexavalent chromium in the form of K2CrO4 or as a solid solution between K3Na(CrO4)2 and K3Na(SO4)2 was found in the fine particles. Good qualitative agreement was observed between experimental data and chemical equilibrium model calculations.     

Polyethylenimine-modified fungal biomass as a high-capacity biosorbent for Cr(VI) anions: sorption capacity and uptake mechanisms

Heavy metal pollution in the aqueous environment is a problem of global concern. Biosorption has been considered as a promising technology for the removal of low levels of toxic metals from industrial effluents and natural waters. A modified fungal biomass of Penicillium chrysogenum with positive surface charges was prepared by grafting polyethylenimine (PEI) onto the biomass surface in a two-step reaction. The presence of PEI on the biomass surface was verified by FTIR and X-ray photoelectron spectroscopy (XPS) analyses. Due to the high density of amine groups in the long chains of PEI molecules on the surface, the modified biomass was found to possess positive zeta potential at pH below 10.4 as well as high sorption capacity for anionic Cr(VI). Using the Langmuir adsorption isotherm, the maximum sorption capacity for Cr(VI) at a pH range of 4.3-5.5 was 5.37 mmol/g of biomass dry weight, the highest sorption capacity for Cr(VI) compared to other sorbents reported in the literature. Scanning electronic microscopy (SEM) provided evidence of chromium aggregates formed on the biomass surface. XPS results verified the presence of Cr(III) on the biomass surface in the pH range 2.5-10.5, suggesting that some Cr(VI) anions were reduced to Cr(III) during the sorption. The sorption kinetics indicated that redox reaction occurred on the biomass surface, and whether the converted Cr(III) ions were released to solution or adsorbed on the biomass depended on the solution pH. Sorption mechanisms including electrostatic interaction, chelation, and precipitation were found to be involved in the complex sorption of chromium on the PEI-modified biomass.     

Electroreduction of Cr(VI) to Cr(III) on reticulated vitreous carbon electrodes in a parallel-plate reactor with recirculation

The reduction of Cr(VI) to Cr(III) is achieved in a flow-by, parallel-plate reactor equipped with reticulated vitreous carbon (RVC) electrodes;this reduction can be accomplished by the application of relatively small potentials. Treatment of synthetic samples and field samples (from an electrodeposition plant) results in final Cr(VI) concentrations of 0.1 mg/L (i.e., the detection limit of the UV-vis characterization technique used here) in 25 and 43 min, respectively. Such concentrations comply with typical environmental legislation for wastewaters that regulate industrial effluents (at presenttime = 0.5 mg/L for discharges). The results show the influence of the applied potential, pH, electrode porosity, volumetric flow, and solution concentration on the Cr(VI) reduction percentage and on the required electrolysis time. Values for the mass transfer coefficient and current efficiencies are also obtained. Although current efficiencies are not high, the fast kinetics observed make this proposed treatment an appealing alternative. The lower current efficiency obtained in the case of a field sample is attributed to electrochemical activation of impurities. The required times for the reduction of Cr(VI) are significantly lower than those reported elsewhere.     

皮革中六价铬成因初探

综述了皮革中 Cr( )的成因。以试验为基础 ,重点研究了在皮革加工过程中的加脂剂、p H值、加热及光照等条件对皮革中 Cr( )形成的影响     

Enhanced reduction of Cr(VI) by direct electric current in a contaminated clay

The probable relation between diffuse double-layer processes and redox reactions that enhance degradation or conversion of contaminants under an applied electric field were examined in a clay medium. Kaolinite clay, precontaminated with hexavalent chromium, was the test soil medium. Analyte, containing ferrous iron, was transported through the kaolinite clay using direct electric current. The Cr(VI) reduction to Cr(III) was followed by measuring the soil redox potential and pH at discrete locations in the clay bed. The post-test distribution of Cr showed significantly more Cr(III) than Cr(VI) at low to slightly acidic pH distribution (2 < pH < 6) in clay. The stoichiometric analyses of measured chromium and iron species concentrations versus the measured redox potentials were compared to Nernst equation predictions of an equivalent aqueous system. An average of +0.37 V shift was measured from the linear Nernstian prediction of cell potential. The applied electric field appeared to provide additional "cathodic current" to drive forth the redox reactions. The redox potential shift was explained by possible overpotential development at the clay-water interfaces due to double-layer polarization under the applied field.     

XANES spectroscopy studies of Cr(VI) reduction by thiols in organosulfur compounds and humic substances

The reduction of Cr(VI) by the thiol-containing compounds cysteine and glutathione and by reduced sulfur in humic substances was monitored with sulfur and chromium X-ray absorption near-edge structure (XANES) spectroscopy in chromium-contaminated soils. Reaction of humic acids with Cr(VI) resulted in a reduction of the peak area of thiols and an increase in the peak area of disulfides in the sulfur XANES spectra. Analysis of the sulfur XANES spectra in various systems indicates that the reduction of Cr(VI) by humic substances involves a thiol/disulfide redox couple analogous to that of the Cr(VI) reduction by the simple thiol-containing compounds cysteine and glutathione. A fraction of the hexavalent chromium present in industrially-contaminated soils was not reducible by thiols. Reduction of Cr(VI) to Cr(III) in soils by thiols has little effect on the pH of the system in contrast to the pH decrease resulting from reduction by Fe(II).     

柳絮纤维生物质炭的制备及其对染料废液中Cr(Ⅵ)的吸附性能

针对目前酸性媒染染料废水中的六价铬Cr(VI)对水体环境污染严重的问题,以柳絮纤维为原料,通过限氧裂解法制备了KOH活化生物质炭(CBK)、NaOH活化生物质炭(CBN),采用吸附批实验法研究模拟染料废液pH值、吸附剂投放量、温度效应等对柳絮纤维生物质炭吸附处理Cr(VI)的影响,利用动力学和热力学相关模型对吸附过程进行拟合,探究柳絮纤维生物质炭对Cr(VI)的吸附机制。结果表明:CBK较CBN比表面积显著增大,表面吸附位点增多;在模拟废液pH值为2时,CBK、CBN对Cr(VI)的理论最大吸附量分别为82.68、47.16 mg/g,且吸附过程符合Freundlich热力学模型和准二级动力学模型,吸附过程主要为多分子层吸附,同时还伴随着化学吸附,该吸附反应是自发进行且为吸热反应,温度升高可显著提高柳絮纤维生物质炭对Cr(VI)的吸附量。     

黄背木耳对基质中铬的富集研究

通过添加不同浓度硝酸铬,探讨四川地区黄背木耳主栽品种(上海1号和黄耳10号)基质与子实体中铬的相关性,明确子实体对基质中铬的富集能力,为制订基质中铬限量标准提供技术支撑。实验采用在基质中人为添加不同浓度硝酸铬的方式,设计添加0、1、5、10、20 mg/kg共计5个不同浓度,按正常栽培方法进行种植,分别取5个浓度的1~3潮毛木耳子实体进行验证,验证方法采用微波消解-电感耦合等离子体-质谱法(ICP-MS),并对验证数据进行SPSS方差及回归分析。结果表明,基质灭菌前后铬含量无明显差异,1~3潮子实体对基质中铬的富集能力差异规律不明显,子实体中铬含量随基质中铬含量增加而增加,但增长趋势随着基质中铬含量的增加渐缓。两个品种子实体对基质中铬的富集方程分别为,黄耳10号:y=0.7ln(x)+0.2221,R²=0.9129;上海1号:y=1.3324ln(x)-0.9914,R²=0.9574,该回归方程可作为限量指标设定的参考。     

ICP-MS与GF-AAS测定饮用水中镍、铬、镉、铅含量的对比分析

电感耦合等离子质谱法(inductively coupled plasma massspectrometry,ICP-MS)与石墨炉原子吸收光谱法(graphite furnace atomic absorptionspectrometry,GF-AAS)是灵敏度较高的两种检验水中重金属含量的方法。本试验采用两种方法对天然饮用水,自来水、矿泉水、白开水、纯净水中水样品中铅、镉、铬、镍的检出限、精密度和加标回收率等数据进行对比分析,结果发现在检测杂质较多的水样时相对标准偏差较高,准确度也会相应的下降,石墨炉原子吸收光谱法影响较为严重。电感耦合等离子质谱法样品制备简单、检测过程干扰因素较少、检测过程方便,一次进样就可以得到多种元素的检测结果,检测速度远快于石墨炉原子吸收光谱法。电感耦合等离子质谱法优于石墨炉原子吸收光谱法,且各元素的检测结果稳定,精密度和加标回收率波动小,满足现代社会大批量检测的需求,检测准确性高,更适合检测重金属元素。     

Solid waste removes toxic liquid waste: adsorption of chromium(VI) by iron complexed protein waste

The leather processing industry generates huge amounts of wastes, both in solid and liquid form. Fleshing from animal hides/skins is one such waste that is high in protein content. In this study, raw fleshing has been complexed with iron and is used for removal of chromium(VI). The effect of pH and the initial concentration of chromium(VI) on the removal of Cr(IV) by iron treated fleshing is presented. Iron treatment is shown to greatly improve adsorption of the fleshing for hexavalent chromium. The ultimate adsorption capacity of iron treated fleshing is 51 mg of chromium(VI) per gram of fleshing. That of untreated fleshing is 9 mg/g such that iron treatment increases the adsorption capacity of fleshing by 10-fold. The measured adsorption kinetics is well described by a pseudo-second-order kinetic model. The uptake of chromium(VI) by fleshing is best described by the Langmuir adsorption isotherm model. X-ray photoelectron spectroscopic (XPS) studies show that the iron is incorporated into the protein matrix. Shifts in XPS spectra suggest that dichromate binding occurs with iron at active adsorption sites and that iron treated fleshing removes chromium(VI) without reducing it to chromium(III).     

Nutritional Assessment of Processing Effects on Major and Trace Element Content in Sea Buckthorn Juice (Hippophaë rhamnoides L. ssp. rhamnoides)

ABSTRACT:  Processing effects on the mineral content were investigated during juice production from sea buckthorn ( Hippophaë rhamnoides L. ssp . rhamnoides, Elaeagnaceae) using berries from 2 different growing areas. The major and trace elements of sea buckthorn berries and juices were determined by atomic absorption spectroscopy (AAS)—(calcium, iron, magnesium, potassium, sodium) and inductively coupled plasma–mass spectrometry (ICP-MS)—(arsenic, boron, chromium, copper, manganese, molybdenum, nickel, selenium, zinc). Potassium is the most abundant major element in sea buckthorn berries and juices. The production process increased the potassium content in the juice by about 20%. Moreover, the processing of juice increased the value of manganese up to 32% compared to the content in berries. During industrial juice production, the technological steps caused a loss of about 53% to 77% of the chromium concentration, 50% of the copper content, 64% to 75% of the molybdenum amount, and up to 45% of the iron concentration in the final juice product. Consumption of sea buckthorn juice represents a beneficial source of chromium, copper, manganese, molybdenum, iron, and potassium for the achievement of the respective dietary requirements.     

Photochemical coupling reactions between Fe(III)/Fe(II), Cr(VI)/Cr(III), and polycarboxylates: inhibitory effect of Cr species

The roles of chromium species on photochemical cycling of iron and mineralization of polycarboxylates are examined in the presence of Cr(VI) or Cr(III) at pH 2.2-4.0. Under UV irradiation, Cr(III) altered the redox equilibrium of iron species, leading to the shift of the photosteady state toward Fe(II). After a longer time of illumination, total organic carbon (TOC) approached a steady state in the presence of Cr(III) or Cr(VI), whereas oxalate was thoroughly mineralized in the absence of Cr species. The TOC of steady state was closely related to the kind of polycarboxylates, Cr species dosages, pH and O2 atmosphere, but hardly affected by more addition of Fe(III). ESI-MS data indicates that several Cr-oxalate complexes formed in the photochemical reactions, which are responsible for protecting oxalate against further oxidation. A mechanism is proposed for the inhibitory effect of Cr species on oxidation of oxalate and Fe(II). The present study may provide a new insight into the dual environmental effects induced by Cr contaminants especially at heavily chromium-contaminated and dissolved organic matter (DOM)-rich sites.