Reactive transport modelling of Cr(VI) treatment by cast iron under fast flow conditions

The Cr(VI) reduction behaviour of five different types of grey cast iron shavings meant for construction of a permeable reactive barrier (PRB) in Thun, Switzerland, was investigated by performing batch and column experiments. Closed system batch experiments clearly demonstrated that the largest shavings also containing the largest spherical C inclusions, were best qualified for the PRB due to their fast Cr(VI) reduction rate. However, the column experiment performed with this type of material revealed that a complete and long term Cr(VI) treatment by the planned PRB was questionable due to the site-specific high groundwater flow velocities and the almost O₂ and CaCO₃ saturated aquifer conditions. The experimentally observed Cr(VI) breakthrough is explained as a result of a decline of reactivity provoked by a strong passivation by observed Fe hydroxides (FeOOH-polymorphs) and carbonates (calcite and aragonite).The column experiment was simulated using the reactive transport modelling code CrunchFlow. Iron cycling and intra-aqueous reactions were incorporated into a previously described reaction network in order to model the strong decline in reactivity of grey cast iron. All key parameters like aqueous species concentrations and mineralogical evolution of the column were successfully modelled. The modelling results confirmed that the observed Cr(VI) breakthrough was caused by surface passivation of the Fe shavings. Complete oxidation of the initially present mass of shavings is not predicted to occur during the expected PRB lifetime of several decades. The model seems to be robust, and it is expected that an application of the calibrated model in 2D to field sites will allow a quantitative evaluation for the performance of planned PRB’s in such environments.     

Enhanced microbial reduction of Cr(VI) and U(VI) by different natural organic matter fractions

Although direct microbial reduction of Cr(VI) and U(VI) is known, few studies have examined the kinetics and the underlying mechanisms of the reduction of these contaminants by different natural organic matter (NOM) fractions in the presence or absence of microorganisms. In this study, NOM was found to chemically reduce Cr(VI) at pH 3, but the reduction rates were negligible at pH ∼7. The abiotic reduction of U(VI) by NOM was not observed, possibly because of the presence of small amounts of nitrate in the reactant solution. However, all NOM fractions, particularly the soil humic acid (HA), enhanced the bioreduction of Cr(VI) or U(VI) in the presence of Shewanella putrefaciens CN32. The reduction rates varied greatly among NOM fractions with different chemical and structural properties: the polyphenolic-rich NOM-PP fraction appeared to be the most reactive in abiotically reducing Cr(VI) at a low pH, but soil HA was more effective in mediating the microbial reduction of Cr(VI) and U(VI) under anaerobic, circumneutral pH conditions. These observations are attributed to an increased solubility and conformational changes of the soil HA with pH and, more importantly, its relatively high contents of polycondensed and conjugated aromatic organic moieties. An important implication of this study is that, depending on chemical and structural properties, different NOM components may play different roles in enhancing the bioreduction of Cr(VI) and U(VI) by microorganisms. Polycondensed aromatic humic materials may be particularly useful in mediating the bioreduction and rapid immobilization of these contaminant metals in soil.     

Isotopic fractionation and reaction kinetics between Cr(III) and Cr(VI) in aqueous media

The redox-sensitive stable isotope geochemistry of chromium bears the potential to monitor the attenuation of chromate pollution and to investigate changes in environmental conditions in the present and the past. The use of stable Cr isotope data as a geo-environmental tracer, however, necessitates an understanding of the reaction kinetics and Cr fractionation behaviour during redox transition and isotope exchange. Here, we report stable chromium isotope fractionation data for Cr(VI) reduction, Cr(III) oxidation and isotopic exchange between soluble Cr(III) and Cr(VI) in aqueous media. The reduction of Cr(VI) to Cr(III) with H₂O₂ under strongly acidic conditions shows a near-equilibrium isotope fractionation of Δ^53/52Cr_{(Cr(III)-Cr(VI))} of −3.54 ± 0.35‰. At pH neutrality, however, the reduction experiments show a kinetic isotope fractionation Δ^53/52Cr_{(Cr(III)-Cr(VI))} of −5‰ for the extent of reduction of up to 85% of the chromium. The oxidation of Cr(III) to Cr(VI) in alkaline media, using H₂O₂ as the oxidant, cannot be explained by a single, unidirectional reaction. Our experiments indicate that the involvement of the unstable intermediates Cr(IV) and Cr(V) and their disproportionation during redox reactions between Cr(III) and Cr(VI) influence the overall fractionation factor, depending on the prevailing pH conditions and the reaction rates. No detectable isotope exchange between soluble Cr(VI) and Cr(III) species at pH values of 5.5 and 7 was revealed over a timescale of days to weeks. This means that, at least within such a time frame, the isotopic composition of Cr(VI) in a natural system will not be influenced by equilibration with any Cr(III) and thus reveal the true extent of reduction, given that the Cr isotope composition of the source Cr(VI) and the fractionation factor for the prevailing conditions are known.     

Biosorption of Cr(III) and Cr(VI) species from aqueous solution by Cabomba caroliniana: kinetic and equilibrium study

This study reports the potential ability of non-living biomass of Cabomba caroliniana for biosorption of Cr(III) and Cr(VI) from aqueous solutions. Effects of contact time, biosorbent dosage, pH of the medium, initial concentration of metal ion and protonation of the biosorbent on heavy metal–biosorbent interactions were studied through batch sorption experiments. Cr(III) was sorbed more rapidly than Cr(VI) and the pH of the medium significantly affected the extent of biosorption of the two metal species differently. Surface titrations showed that the surface of the biosorbent is positively charged at low pH while it is negatively charged at pH higher than 4.0. Protonation of the biosorbent increased its capacity for removal of Cr(III), while decreasing that of Cr(VI). FT-IR spectra of the biosorbent confirmed the involvement of –OH groups on the biosorbent surface in the chromium removal process. Kinetic and equilibrium data showed that the sorption process of each chromium species followed pseudo second-order kinetic model and both Langmuir and Freundlich isothermal models. A possible mechanism for the biosorption of chromium species by non-living C. caroliniana is suggested.     

天然黄铁矿除Cr（Ⅵ）中Cr2S3物相的发现

利用天然黄铁矿处理含Cr(Ⅵ)废水实验,在反应中与反应后黄铁矿表面和胶体沉淀物中明显出现Cr2p的XPS能谱峰,能拟合出574.7～575.1eV、576.6～576.9eV和578.2～578.7eV3个峰位,分别代表Cr2S3、Cr2O3和CrO3物相。在实验过程中新发现Cr2S3沉淀物相,表明在常温常压下的水溶液介质中也能产生铬的硫化物物相。充分利用这些含铬物相,能使天然黄铁矿包括天然磁黄铁矿在还原Cr(Ⅵ)的同时,无需加碱就可将Cr(Ⅲ)沉淀转移到胶体沉淀物中,处理后的上清液中全Cr含量低于1.5mg/L的国家允许排放标准。天然黄铁矿自身溶解出的重金属含量很低,不影响处理后的出水水质。开发利用Cr2S3等含铬沉淀物相,便于推广应用天然黄铁矿和磁黄铁矿还原Cr(Ⅵ)同时沉淀Cr(Ⅲ)的一步法除铬新工艺。     

自然状态与加热改性的黄铁矿处理含铬(VI)废水的实验研究

最近几年,人们对一些天然矿物在处理重金属污染物过程中所表现出的高效性、经济性和安全性倍加青睐,并逐渐发展成环境矿物材料研究的方向［１,２］。国外对某些矿物吸附机理也有较系统的评述,有关含铬矿物与有机物的作用也有报道［１,３］。１　实验部分（１）试样来源：黄铁矿取自硫铁矿矿山,经鉴定与粉碎,淘洗,烘干,筛分,磁选后获得。（２）含铬（ＶＩ）废水：用分析纯重铬酸钾（Ｋ２Ｃｒ２Ｏ７）和蒸馏水配制。（３）实验仪器：反应体系的酸度用ＰＨＳ３Ｃ型酸度计测定,反应平衡时的吸光度用ＵＶ１６０Ａ分光光度计测定,…     

单斜与六方磁黄铁矿处理含Cr(VI)废水过程中pH值变化规律

与六方磁黄铁矿相比,单斜磁黄铁矿Fe缺位较普遍。在初始pH值分别为3.40～9.66和3.47～9.66较宽范围内,利用单斜磁黄铁矿和六方磁黄铁矿处理含Cr(VI) 废水,当反应达到充分平衡时,废水的pH值分别变化在3.61～4.47和5.39～6.57范围内。六方磁黄铁矿除Cr(VI)效果明显不及单斜磁黄铁矿,但被氧化的六方磁黄铁矿除Cr(VI)效率有所 提高。电化学分析表明,在酸性介质中处理的反应过程为H+的消耗过程,而在碱性介质中 则为OH 的消耗过程。根据pH值的这一变化规律,可自行调节处理过程中水质的酸性变化 ,能节省传统工艺中需要加碱以中和处理后酸性水的环节,具有一定的实际应用价值。     

镍钛改性膨润土对铬的吸附性能研究

以钠基膨润土为原料,制备镍钛交联改性膨润土、镍钛有机复合改性膨润土,并应用于含铬模拟废水的处理。探讨了改性膨润土的用量、pH值、吸附时间等最佳使用条件,比较了原土、交联改性土、有机复合改性土对铬的吸附效果。结果表明改性土的吸附效果明显优于原土,在最佳实验条件下,交联改性土、有机复合改性土对Cr(Ⅵ)的去除率分别达到了87%和96%。两种改性土对铬吸附行为均符合Langmuir吸附等温方程,饱和吸附量和Lang-muir常数分别为3.1827mg/g、8.5543mg/g和3.5007、1.2738。     

Treatment of Cr(VI) contaminated water with Pannonibacter phragmitetus BB

Pannonibacter phragmitetus BB was utilized to treat hexavalent chromium [Cr(VI)] contaminated water. Cr(VI) concentration of the contaminated water (pH 10.85) was 534 mg/L. With the inoculum size ranging from 1 to 20 %, P. phragmitetus BB completely reduced Cr(VI) within 27 h when the initial medium concentration exceeded 20 g/L. The lag time of bio-reduction by Cr(VI)-induced cells was 24 h, which was longer than the non-Cr(VI)-induced cells. Under the agitation condition, an obvious bio-reduction lag phase existed and Cr(VI) was completely reduced within 24 h. However, the lag phase was not observed under the static condition, Cr(VI) was reduced continuously after inoculation and Cr(VI) was completely reduced after 27 h incubation. The main chromium components after Cr(VI) reduction were Cr(OH)₃, Cr₂O₃ and CrCl₃. The results of this study are fundamentally significant to the application of P. phragmitetus BB in the treatment of Cr(VI) contaminated water.     

Removal of Cr(VI) and phenol coupled with the reduction of sulfate by sulfate-reducing bacteria sludge

Biological treatment of industrial wastewater containing heavy metal and organic pollutant has attracted extensive attention. In this study, Cr(VI) reduction coupled with phenol degradation was investigated by the sulfate-reducing bacteria (SRB) sludge with addition of zero-valent iron (ZVI). The results showed that the SRB wet sludge (SWS) had a good bioactivity in the reduction of Cr(VI) only when the initial concentration of Cr(VI) was below 60 mg L^?1. The addition of ZVI significantly enhanced the bioactivity and reusability of SWS, and the reduction percentage of Cr(VI) achieved 98% after SWS was successively used for seven cycles. SWS coupled with ZVI showed a high activity in phenol degradation, with more than 94% phenol being degraded in each cycle. However, in the simultaneous removal of Cr(VI) and phenol, phenol degradation was inhibited due to the toxicity of Cr(VI) to phenol degrading microbes in SWS. On the other hand, reduction of sulfate and Cr(VI) was not affected by the presence of phenol, with more than 95% of sulfate and Cr(VI) being removed at the end of the 5th cycle. This study enriches our understanding on the applications of the SRB sludge in the removal of organic and inorganic contaminants in wastewater.     

Biotite dissolution and Cr(VI) reduction at elevated pH and ionic strength

The effects of elevated pH, ionic strength, and temperature on sediments in the vadose zone are of primary importance in modeling contaminant transport and understanding the environmental impact of tank leakage at nuclear waste storage facilities like those of the Hanford site. This study was designed to investigate biotite dissolution under simulated high level waste (HLW) conditions and its impact on Cr(VI) reduction and immobilization. Biotite dissolution increased with NaOH concentrations in the range of 0.1 to 2 mol L^-1. There was a corresponding release of K, Fe, Si, and Al to solution, with Si and Al showing a complex pattern due to the formation of secondary zeolite minerals. Dissolved Fe concentrations were an order of magnitude lower than the other elements, possibly due to the formation of green rust and Fe(OH)₂. The reduction of Cr(VI) to Cr(III) also increased with increased NaOH concentration. A homogeneous reduction of chromate by Fe(II)_aq released through biotite dissolution was probably the primary pathway responsible for this reaction. Greater ionic strengths increased biotite dissolution and consequently increased Fe(II)_aq release and Cr(VI) removal. The results indicated that HLW would cause phyllosilicate dissolution and the formation of secondary precipitates that would have a major impact on radionuclide and contaminant transport in the vadose zone at the Hanford site.     

焦作地区浅层地下水铬(Ⅵ)污染机理及迁移预测

通过对焦作地区浅层地下水中铬(Ⅵ)污染物分布特征进行调查,分析了研究区浅层地下水中铬(Ⅵ)的污染机理,并运用Visual MODFLOW建立地下水流模型及溶质运移模型,模拟预测了浅层地下水中铬(Ⅵ)的迁移规律。结果表明：研究区浅层地下水铬(Ⅵ)污染严重,污染源是位于老君庙西南方向的焦作某电厂堆灰场,主要原因是露天堆放的粉煤灰中的铬(Ⅵ)污染物在长期淋滤作用下下渗污染含水层。气候条件、包气带岩性、地下水化学环境以及人为因素等也间接使浅层地下水铬(Ⅵ)浓度升高;模拟结果显示在未来的五年时间内,受地形和地下水动力场的影响,浅层地下水中铬(Ⅵ)的迁移方向与地下径流方向一致,沿大沙河水流方向上扩散速度更快,污染区域面积逐渐增大。     

Optimization of a PRB structure with modified chitosan restoring Cr(VI)-contaminated groundwater

Hexavalent chromium is a soluble, mobile, and highly toxic metal ion in groundwater. Adsorption by permeable reactive barriers (PRBs) with special sorbent is a common method to remove hexavalent chromium. A series of experiments have been performed to remove hexavalent chromium in groundwater under PRB with modified chitosan. Therefore, in this paper, the authors first estimated adsorption characteristics of the modified chitosan in a column test, and then calibrated the PRB adsorption parameters in a sandbox test, and finally designed an optimal width, length, and depth of the PRB with the same reactive media in a three-dimensional aquifer. The results showed that the modified chitosan might be a potential adsorption medium. The design schemes can meet the water quality standard of 0.1 mg/L Cr(VI). Heterogeneity of dispersion is a crucial factor when designing the PRB. Therefore, the design of the PRB structures can be appropriate and serve as reference for groundwater remediation.     

Characterization and evaluation of a tropical peat for the removal of Cr(VI) from solution

Tropical peat soils present higher ash content than those generated at temperate climate areas. Therefore, this study evaluated the characteristics of a Brazilian organic soil (OS), commercialized as peat, as well as its capacity in removing Cr(VI) from contaminated waters. The OS is composed of 35.5 wt% of organic matter and 56 wt% of inorganic fraction (ash), which is formed by minerals and phytoliths rich in silica (29.2 wt%) and alumina (23.6 wt%). The Cr(VI) removal tests were carried out in batch and column systems using OS and solutions of Cr(VI) prepared with distilled water and groundwater. Batch tests revealed that the organic substances in the OS caused the reduction of Cr(VI) to Cr(III), with an efficiency depending on solution pH. At pH 5.0 the Cr(VI) removal was 0.45 mg g^?1 in 24 h; whereas at pH 2.0, this removal increased to 1.10 mg g^?1. Since this redox reaction is very slow, the removal of Cr(VI) at pH 5.0 increased to around 2 mg g^?1 after 5 days. The removal of Cr(VI) was more effective in the column tests than in the batch test due to the greater solid/solution ratio, and their half-lives were 4.4 and 26.2 h, respectively. Chemical analysis indicated that Cr(VI) was reduced by the humic substances of OS, followed by the precipitation and/or adsorption of Cr(III) into the organic and inorganic components, as anatase. The presence of Cr(III) increased the stability of anatase structure, avoiding its transformation into rutile, even after being heated at 800 °C/2 h.     

Assessing the effectiveness of nanoscale zero-valent iron particles produced by green tea for Cr(VI)-contaminated groundwater remediation

Nanoscale zero-valent iron particles(NZVI) produced by using green tea(GT) extract as a reductant can remove Cr(Ⅵ) from water effectively,which can be utilized in groundwater remediation.In order to define the reaction mechanism and removal effect in the aquifer,in this study,GT-NZVI particles were prepared and measured by some characterization methods to define their surface performance,and then batch and one-dimensional experiments were carried out to reveal the reaction properties of GT-NZVI ...     

Interaction between Cr(VI) and a Fe-rich soil in the presence of oxalic and tartaric acids

This study examined the interaction between Cr(VI) and a Fe-rich soil in the presence of low-molecular-weight organic acids as a function of pH. Oxalic and tartaric acids were chosen since they existed in soils commonly. Batch experiments showed that adsorption of Cr(VI) by the soil within the pH range examined was inhibited in the presence of oxalic acid, which was more pronounced when the initial ratio of [oxalic acid]/[Cr(VI)] was raised from 1:1 to 2:1. With the addition of tartaric acid, concentration of Cr(VI) in equilibrium solutions was far less than that of single adsorbate system across the pH wide (2.5–5.5), which was noticeable especially at low pH. The results were attributed to Cr(VI) adsorption and, particularly, the soil surface catalyzed reduction of Cr(VI) to Cr(III) by tartaric acid. The data reported in this paper suggested that the mobility, the bioavailability, and the toxicity of Cr(VI) in soil environments might be greatly affected by pH, the presence and nature of low-weight-molecular organic acids (oxalic and tartaric acids).     

Natural Cr(VI) contamination of groundwater in the Cecina coastal area and its inner sectors (Tuscany,Italy)

A detailed hydrogeochemical study of groundwater in the Cecina coastal plain (Livorno province, Italy) and its inner sectors was undertaken in 2008, as chemical analyses carried out on groundwater since 2006 have revealed Cr(VI) concentrations of up to 49 μg/L (well above the permissible limit of 5 μg/L). Ophiolite outcrops are present throughout the study area, and their fragments likely represent a significant portion of the existing multilayered aquifer skeleton. Waters delivered by the serpentinite outcrops have a typically Mg–HCO₃ composition, whereas those of the coastal plain are prevailingly of the Ca/Mg–HCO₃ type with significant Mg contents. Significant NO₃ contamination characterises the studied coastal plain, and an interesting negative correlation exists between Cr(VI) and both NO₃ and SO₄ deriving from the widespread use of (NH₄)₂SO₄ as a farm fertilizer. Chromium speciation calculations carried out using the EQ3NR code reveal that the prevailing Cr(VI) species in solution is CrO₄ ^2?; however, CaCrO₄° and MgCrO₄° neutral complexes represent significant percentages (up to 42 %). These findings suggest that the mobility and consequently the bioavailability of Cr(VI) can be significantly enhanced by these neutral complexes, which are not considered to be affected by adsorption/desorption processes. The Cr(VI) source, investigated by means of the Mg/SiO₂ molar ratio, seems to be represented mainly by Mg-bearing minerals of the chlorite group. Petrographic observations confirm the occurrence of this mineral group. The interaction between rainwater and the local serpentinite rock was simulated at different $P_{{{\text{CO}}_{ 2} }}$ and $P_{{{\text{O}}_{ 2} }}$ conditions by reaction path modelling using the EQ3/6 software package. $P_{{{\text{O}}_{ 2} }}$ was varied in accordance with the assumption that redox conditions are determined in part by NO₃. Results are in good agreement with experimental data on spring waters and subordinately with data on some coastal plain groundwater, which plot in a rather wide $P_{{{\text{CO}}_{ 2} }}$ and $P_{{{\text{O}}_{ 2} }}$ field. Although the dissolved Cr content is mostly of natural origin, fertilization may affect its fate.     

Effects of indigenous bacteria on Cr(VI) reduction in Cr-contaminated sediment with industrial wastes

Black, clay-like sediments have been obtained from the area of the pigment manufacturing factories in Dongducheon city, Korea. These sediments were contaminated by heavy metals, especially chromium (700 mg/kg). Indigenous bacteria in the sediments were isolated to investigate their ability to reduce Cr(VI) to Cr(III). The enriched bacterial consortium reduced over 99% of dissolved Cr(VI) in 96 h from the onset of the experiments under anaerobic condition, while there was no change in Cr(VI) concentration until 300 h in abiotic controls. Total amount of dissolved Cr decreased simultaneously when Cr(VI) was reduced, which was likely due to precipitation of Cr(OH)₃ after microbial reduction of Cr(VI) to Cr(III). Under aerobic condition, only 30% of dissolved Cr(VI) was reduced by indigenous bacteria until 900 h. The reduction of Cr(VI) did not accompany bacterial growth since the amount of protein did not show a significant change with time both in the presence and absence of O₂. These indigenous bacteria may play a role in the treatment of Cr(VI)-contaminated sediments.     

Aqueous Cr(VI) reduction by pyrite: Speciation and characterisation of the solid phases by X-ray photoelectron, Raman and X-ray absorption spectroscopies

Optical microscopy, confocal Raman micro-spectrometry, X-ray photoelectron micro-spectroscopy (XPS) and synchrotron based micro-X-ray fluorescence (XRF), micro-X-ray absorption near edge spectroscopy (XANES) and micro-extended X-ray absorption fine structure (EXAFS) were used to investigate the reduction of aqueous Cr(VI) by pyrite. Special emphasis was placed on the characterisation of the solid phase formed during the reaction process. Cr(III) and Fe(III) species were identified by XPS analyses in addition to non-oxidised pyrite. Optical microscopy images and the corresponding Raman spectra reveal a strong heterogeneity of the samples with three different types of zones. (i) Reflective areas with E_g and A_g Raman wavenumbers relative to non-oxidised pyrite are the most frequently observed. (ii) Orange areas that display a drift of the E_g and A_g pyrite vibration modes of −3 and −6 cm⁻¹, respectively. Such areas are only observed in the presence of Cr(VI) but are not specifically due to this oxidant. (iii) Bluish areas with vibration modes relative to a corundum-like structure that can be assigned to a solid solution Fe_2−xCr_xO₃, x varying between 0.2 and 1.5. The heterogeneity in the spatial distribution of chromium observed by optical microscopy and associated Raman microspectroscopy is confirmed by μ-XRF. In agreement with both solution and XPS analyses, these spectroscopies clearly confirm that chromium is in the trivalent state. XANES spectra in the iron K-edge pre-edge region obtained in rich chromium areas reveal the presence of ferric ion thus revealing a systematic association between Cr(III) and Fe(III). In agreement with Raman analyses, Cr K-edge EXAFS can be interpreted as corresponding to Cr atoms involved in a substituted-type hematite structure Fe_2−xCr_xO₃.