Use of waste iron metal for removal of Cr(VI) from water

Cr(VI) removal from water was evaluated using waste iron particles in batch experimental mode. The reaction rates were inversely proportional to the initial Cr(VI) concentrations, and the reaction rates of Cr(VI) removal with the waste iron metal were faster than those with Peerless iron, a commercial zero-valent iron. The loss in iron reactivity due to the oxidation, from Fe(0) to Fe(II), ultimately to Fe(III), could be recovered by adding iron-reducing consortium (IRC) to the oxidized iron. Bacterial reduction of Cr(VI) also helped to decrease the aqueous concentration of Cr(VI), but the reduction of oxidized iron by IRC and the consequent reduction of Cr(VI) to Cr(III) by the reduced iron was more significant. Thus, reusing waste iron metal for Cr(VI) removal can reduce the cost of reactive media. Furthermore, the addition of IRC to the waste iron metal can accelerate the removal rate of Cr(VI), and can recover the reactivity of irons which were oxidized by Cr(VI).     

Investigation of the removal mechanism of Cr(VI) in groundwater using activated carbon and cast iron combined system

Zero-valent iron (Fe⁰) has been widely used for Cr(VI) removal; however, the removal mechanisms of Cr(VI) from aqueous solution under complex hydrogeochemical conditions were poorly understood. In this research, the mixed materials containing cast iron and activated carbon were packed in columns for the treatment of aqueous Cr(VI)-Cr(III) in groundwater with high concentration of Ca²⁺, Mg²⁺, HCO₃ ⁻, NO₃ ⁻, and SO₄ ²⁻. We investigate the influences of those ions on Cr(VI) removal, especially emphasizing on the reaction mechanisms and associated precipitations which may lead to porosity loss by using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) techniques. The results show that the precipitations accumulated on the material surface were (Fe/Cr) (oxy)hydroxide, mixed Fe(III)-Cr(III) (oxy)hydroxides, Fe₂O₃, CaCO₃, and MgCO₃. During these reactions, the Cr(VI) was reduced to Cr(III) coupled with the oxidated Fe⁰ to Fe(II) through the galvanic corrosion formed by the Fe⁰-C and/or the direct electron transfer between Fe⁰ and Cr(VI). In addition, Cr(VI) could be reduced by aqueous Fe(II), which dominated the whole removal efficiency. The primary aqueous Cr(III) was completely removed together with Cr(III) reduced from Cr(VI) even when Cr(VI) was detected in the effluent, which meant that the aqueous Cr(III) could occupy the adsorption sites. In general, the combined system was useful for the Cr(VI)-Cr(III) treatment based on galvanic corrosion, and the hardness ions had a negative effect on Cr(VI) removal by forming the carbonates which might promote the passivation of materials and decrease the removal capacity of the system.

     

零价铁对水中六价铬还原性能及沉淀污泥中铬的固定化

采用零价铁还原水中六价铬,考察了溶液初始pH、铁粉投量、反应温度及水体中常见阴阳离子对还原效率的影响,得出其工艺参数为:初始pH=3.0,反应时间t=30 min,反应温度为室温,Fe0与Cr(VI)质量比为1 000: 1。将还原后产生的三价铬以沉淀形式从水中彻底分离出来,并对其进行了碱激发矿渣固定化处理,考察了固定化样品中Cr在不同环境条件下的浸出水平,结果表明,固定化处理能有效控制沉淀污泥中Cr的浸出。     

Reduction of hexavalent chromium by ascorbic acid in aqueous solutions

Hexavalent chromium is a priority pollutant in the USA and many other countries. Reduction of Cr(VI) to Cr(III) is environmentally favorable as the latter species is not toxic to most living organisms and also has a low mobility and bioavailability. Reduction of Cr(VI) by ascorbic acid (vitamin C) as a reductant was studied using potassium dichromate solution as the model pollutant. Effects of concentration of vitamin C, pH, temperature, irradiation and reaction time on the reduction of Cr(VI) were examined. Cr(VI) might be reduced by vitamin C not only in acidic conditions but also in weakly alkaline solutions. The reduction of Cr(VI) by vitamin C might occur not only under irradiation but also in the dark. Vitamin C is an important biological reductant in humans and animals, and not toxic. It is water-soluble and can easily permeate through various types of soils. The results indicate that vitamin C could be used in effective remediation of Cr(VI)-contaminated soils and groundwater in a wide range of pH, with or without sunlight.     

Tessier连续提取法用于土壤铬分析的Cr(Ⅵ)-Cr(Ⅲ)转化及适用性

为揭示Tessier连续提取法应用于土壤总铬(TCr)和六价铬[Cr(Ⅵ)]结合态分析的适用性,通过液相和土壤相实验,分别研究了提取液自身还原性组分及土壤残留还原剂在各个提取步骤中导致的Cr(Ⅵ)-Cr(Ⅲ)转化问题。结果表明,提取液中还原性组分会在铁锰氧化物结合态和有机结合态的检测中导致Cr(Ⅵ)被还原为Cr(Ⅲ),可还原的Cr(Ⅵ)最大量分别为50.88和0.54 mg;经还原修复后的铬污染土壤中残留还原剂会在可交换态和碳酸盐结合态的检测中导致Cr(Ⅵ)的还原。提取过程中Cr(Ⅵ)-Cr(Ⅲ)的转化限制了Tessier连续提取法在铬污染土壤中的应用。当用于了解铬污染土壤TCr、Cr(Ⅵ)结合态的分布时,Cr(Ⅵ)铁锰氧化物结合态和有机结合态检测结果均显著低于真实值,TCr结合态的检测结果重现性较差。当用于评价修复后土壤中Cr(Ⅵ)结合态变化时,可交换态、碳酸盐结合态、铁锰氧化物结合态和有机结合态的检测结果均可能显著低于真实值。研究可为利用Tessier连续提取法准确评估铬污染土壤修复效果提供参考。     

Effect of amorphous silica and silica sand on removal of chromium(VI) by zero-valent iron

The effect of two surfaces (amorphous silica and silica sand) on the reduction of chromium(VI) by zero-valent iron (Fe(0)) was investigated using batch reactors. The amendment of both surfaces significantly increased the rate and extent of Cr(VI) removal. The rate enhancement by amended surfaces is presumed to result from scavenging of Fe(0)-Cr(VI) reaction products by the provided surfaces, which minimized surface deactivation of Fe(0). The rate enhancing effect was greater for silica compared to sand, and the difference is attributed to silica's higher surface area, greater affinity for reaction products and pH buffering effect. For a given mass of Fe(0), the reactivity and longevity of Fe(0) to treat Cr(VI) increased with increasing dose of silica. Elemental analyses of the reacted iron and silica revealed that chromium removed from the solution was associated with both surfaces, with its mass distribution being approximately 1:1 per mass of iron and silica. The overall result suggests reductive precipitation was a predominant Cr(VI) removal pathway, which involves initial reduction of Cr(VI) to Cr(III), followed by formation of Cr(III)/Fe(III) hydroxides precipitates.     

Transportation and distribution of chromium in the anaerobic sludge treating the chromium-containing wastewater

The chromium distribution and transportation in the anaerobic activated sludge was investigated using a sequential extraction method. The results showed that Cr(VI) in aqueous solution was reduced by the metabolic product of SRB and form indissoluble Cr(OH)₃ in a solid phase. More than 99% of Cr(III) in the sludge was in a stable residual form (RES) of bio-reduction. The Cr(VI) was mainly present in RES and organic form (OR). With increasing Cr(VI) concentration, Cr(VI) was transferred from RES into OR and even exchangeable form (EXCH). Meanwhile, sulphate and co-existing metal ions affected the occurrence form of Cr(VI).     

生物铁修复Cr（VI）污染土壤的性能及机理

以活性污泥与生物铁作为修复剂,对比时间、pH、污泥投加量、Cr（VI）初始浓度对活性污泥、生物铁去除土壤中Cr（VI）的影响;考察生物铁对土壤中Cr（VI）的还原率;采用SEM、XRD、XPS对生物铁修复前后的Cr（VI）污染土壤进行表征分析。实验结果表明：在土壤中,生物铁对Cr（VI）的修复效果优于活性污泥,当pH为6.0,Cr（VI）初始浓度为300 mg·kg-1,生物铁和活性污泥的投加量均为19.57 mg·g-1,修复时间为45 d时,土壤中Cr（VI）经活性污泥与生物铁修复后浓度分别为31.42、19.69 mg·kg-1,去除率分别为89.52%、94.12%。生物铁修复土壤Cr（VI）以还原作用为主,吸附为辅。Cr（VI）污染土壤被生物铁修复后出现了FeOOH、Fe3O4以及Fe3O4与FeCr2O4的混合物,部分Cr（VI）被还原为Cr（III）,铁被氧化为Fe2+、Fe3+。     

Influence of complex reagents on removal of chromium(VI) by zero-valent iron

The removal of Cr(VI) by zero-valent iron (Fe(0)) and the effect of three complex reagents, ethylenediaminetetraacetic acid (EDTA), NaF and 1,10-phenanthroline, on this reaction were investigated using batch reactors at pH values of 4, 5 and 6. The results indicate that the removal of Cr(VI) by Fe(0) is slow at pH 5.0 and that three complex reagents play different roles in the reaction. EDTA and NaF significantly enhance the reaction rate. The zero-order rate constants at pH 5.0 were 5.44 microM min(-1) in the presence of 4mM EDTA and 0.99 micrM min(-1) in the presence of 8 mM NaF, respectively, whereas that of control was only 0.33 micrM min(-1), even at pH=4.0. This enhancement is attributed to the formation of complex compounds between EDTA/NaF and reaction products, such as Cr(III) and Fe(III), which eliminate the precipitates of Cr(III), Fe(III) hydroxides and Cr(x)Fe(1-)(x)(OH)(3) and thus reduce surface passivation of Fe(0). In contrast, 1,10-phenanthroline, a complex reagent for Fe(II), dramatically decreases Cr(VI) reduction by Fe(0). At pH=4.0, the zero-order rate constant in the presence of 1mM of 1,10-phenanthroline was 0.02 micrM min(-1), decreasing by 99.7% and 93.9%, respectively, compared with the results in the presence and absence of EDTA. The results suggest that a pathway of the reduction of Cr(VI) to Cr(III) by Fe(0) may involve dissolution of Fe(0) to produce Fe(II), followed by reduction of Cr(VI) by Fe(II), rather than the direct reaction between Cr(VI) and Fe(0), in which Fe(0) transfers electrons to Cr(VI).     

Chromium species behaviour in the activated sludge process

The purpose of this research was to compare trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) removal by activated sludge and to investigate whether Cr(VI) reduction and/or Cr(III) oxidation occurs in a wastewater treatment system. Chromium removal by sludge harvested from sequencing batch reactors, determined by a series of batch experiments, generally followed a Freundlich isotherm model. Almost 90% of Cr(III) was adsorbed on the suspended solids while the rest was precipitated at pH 7.0. On the contrary, removal of Cr(VI) was minor and did not exceed 15% in all experiments under the same conditions. Increase of sludge age reduces Cr(III) removal, possibly because of Cr(III) sorption on slime polymers. Moreover, the decrease of suspended solids concentration and the acclimatization of biomass to Cr(VI) reduced the removal efficiency of Cr(III). Batch experiments showed that Cr(III) cannot be oxidized to Cr(VI) by activated sludge. On the contrary, Cr(VI) reduction is possible and is affected mainly by the initial concentration of organic substrate, which acts as electron donor for Cr(VI) reduction. Initial organic substrate concentration equal to or higher than 1000 mgl(-1) chemical oxygen demand permitted the nearly complete reduction of 5 mgl(-1) Cr(VI) in a 24-h batch experiment. Moreover, higher Cr(VI) reduction rates were obtained with higher Cr(VI) initial concentrations, expressed in mg Cr(VI) g(-1) VSS, while decrease of suspended solids concentration enhanced the specific Cr(VI) reduction rate.     

Spectroscopic investigation of magnetite surface for the reduction of hexavalent chromium

The reduction of Cr(VI) to Cr(III) by magnetite in the presence of added Fe(II) was characterized through batch kinetic experiments and the effect of Fe(II) addition and pH were investigated in this study. The addition of Fe(II) into magnetite suspension improved the reductive capacity of magnetite. Eighty percent of Cr(VI) was reduced by magnetite (6.5 g l(-1)) with Fe(II) (80 mg l(-1)) within 1 h, while 60% of Cr(VI) was removed by magnetite only. However, the extent of improved reductive capacity of magnetite with Fe(II) was less than that predicted by the summation of each reduction capacity of magnetite and Fe(II). The reduction of Cr(VI) in the magnetite suspension with Fe(II) increased with the increase of molar ratio of Fe(II) to Cr(VI) (0.6, 1, 1.5, 2.3) in the range of 0-2.3 and with the decrease of pH in the range of pH 8.0-5.5. The speciation of chromium, iron, and oxygen on the surface of magnetite was investigated by X-ray photoelectron spectroscopy. Cr 2p3/2, Fe 2p3/2, and O 1s peaks were mainly observed at 576.7 and 577.8 eV, at 711.2 eV, and at 530.2 and 531.4 eV, respectively. The results indicates that Cr(III) and Fe(III) were the dominant species on the surface of magnetite after reaction and that the dominant species covered the magnetite surface and formed metal (oxy)hydroxide.     

Column study of Cr(VI) removal by cationic hydrogel for in-situ remediation of contaminated groundwater and soil

Column experiments were conducted for examining the effectiveness of the cationic hydrogel on Cr(VI) removal from groundwater and soil. For in-situ groundwater remediation, the effects of background anions, humic acid (HA) and pH were studied. Cr(VI) has a higher preference for being adsorbed onto the cationic hydrogel than sulphate, bicarbonate ions and HA. However, the adsorbed HA reduced the Cr(VI) removal capacity of the cationic hydrogel, especially after regeneration of the adsorbents, probably due to the blockage of adsorption sites. The Cr(VI) removal was slightly influenced by the groundwater pH that could be attributed to Cr(VI) speciation. The 6-cycle regeneration and reusability study shows that the effectiveness of the cationic hydrogel remained almost unchanged. On average, 93% of the adsorbed Cr(VI) was recovered in each cycle and concentrated Cr(VI) solution was obtained after regeneration. For in-situ soil remediation, the flushing water pH had an insignificant effect on the release of Cr(VI) from the soils. Multiple-pulse flushing increased the removal of Cr(VI) from the soils. In contrast, more flushing water and longer operation may be required to achieve the same removal level by continuous flushing.     

PBTCA稳定零价纳米铁的制备及其去除水中Cr(Ⅵ)

以2-膦酸丁烷-1,2,4三羧酸(PBTCA)为稳定剂,通过FeCl3.6H2O与NaBH4反应,利用液相还原法制备稳定纳米级零价铁颗粒(P-NZVI),并用透射电子显微镜(TEM)、扫描电子显微镜(SEM)及X射线衍射(XRD)进行表征,颗粒平均粒径为73 nm。考察了Cr(Ⅵ)溶液初始浓度、pH、NZVI投加量、温度等条件对Cr(Ⅵ)去除效果的影响,并与同等条件下不加稳定剂制备的纳米铁(N-NZVI)进行对比。结果表明:Cr(Ⅵ)的去除率随温度和纳米铁投加量增加而升高,随pH和Cr(Ⅵ)溶液初始浓度升高而降低。在相同实验条件下,P-NZVI对Cr(Ⅵ)的去除效果明显优于N-NZVI,表明改性后纳米铁在地表水原位修复领域具有较好的应用前景。     

Kinetics of the reduction of hexavalent chromium with the brown seaweed Ecklonia biomass

The dead biomass of the brown seaweed, Ecklonia sp., is capable of reducing toxic Cr(VI) into less toxic or nontoxic Cr(III). However, little is known about the mechanism of Cr(VI) reduction by the biomass. The objective of this work was to develop a kinetic model for Cr(VI) biosorption, for supporting our mechanism. The reduction rate of Cr(VI) increased with increasing total chromate concentration, [Cr(VI)], and equivalent concentration of organic compounds, [OCs], and decreasing solution pH. It was found that the reduction rate of Cr(VI) was proportional to [Cr(VI)] and [OCs], suggesting the simple kinetic equation -d[Cr(VI)]/dt=k[Cr(VI)][OCs]. When considering the consumption of organic compounds due to the oxidation by Cr(VI), an average rate coefficient of 9.33 (+/-0.65)microM(-1)h(-1) was determined, at pH 2. Although the function of the pH could not be expressed in a mechanistic manner, an empirical model able to describe the pH dependence was obtained. It is expected that the developed rate equation could likely be used for design and performance predictions of biosorption processes for treating chromate wastewaters.     

Cr(VI) uptake mechanism of Bacillus cereus

In this study, we investigated the Cr(VI) uptake mechanism in an indigenous Cr(VI)-tolerant bacterial strain -Bacillus cereus through batch and microscopic experiments. We found that both the cells and the supernatant collected from B. cereus cultivation could reduce Cr(VI). The valence state analysis revealed the complete transformation from Cr(VI) into Cr(III) by living B. cereus. Further X-ray absorption fine structure and Fourier transform infrared analyses showed that the reduced Cr(III) was coordinated with carboxyl and amido functional groups from either the cells or supernatant. Scanning electron microscopy and atomic force microscopy observation showed that noticeable Cr(III) precipitates were accumulated on bacterial surfaces. However, Cr(III) could also be detected in bacterial inner portions by using transmission electron microscopy thin section analysis coupled with energy dispersive X-ray spectroscopy. Through quantitative analysis of chromium distribution, we determined the binding ratio of Cr(III) in supernatant, cell debris and cytoplasm as 22%, 54% and 24%, respectively. Finally, we further discussed the role of bacterium-origin soluble organic molecules to the remediation of Cr(VI) pollutants.     

Biogeochemical influence on transport of chromium in manganese sediments: experimental and modeling approaches

Hexavalent chromium (Cr(VI)) was reduced to immobile and nontoxic Cr(III) by a dissimilatory metal reducing bacteria, Shewanella alga Simidu (BrY-MT) ATCC 55627. A series of kinetic batch and dynamic column experiments were conducted to provide an understanding of Cr(VI) reduction by the facultative anaerobe BrY-MT. Reduction of Cr(VI) was rapid (within 1 h) in columns packed with quartz sand and bacteria, whereas Cr(VI) reduction by BrY-MT was delayed (57 h) in the presence of beta-MnO2-coated sand. A mathematical model was developed and evaluated against data obtained from column experiments. The model takes into account (1) advective-dispersive transport of Cr(III), Cr(VI), lactate, and protein (mobile and immobile bacteria); (2) first-order kinetic adsorption of Cr(III) and lactate; (3) conversion of solid phase beta-MnO2 to solid phase MnOOH due to oxidation of Cr(III); (4) dual-Monod kinetics, where Cr(VI) is the electron acceptor and lactate is the electron donor. The breakthrough data for Cr(III), Cr(VI), lactate, and protein (mobile and immobile bacteria) were fitted simultaneously. The breakthrough data are well described by the mathematical model that considers the above processes. This result demonstrates the ability of the coupled hydrobiogeochemical model to simulate chromium transport in complex reactive systems.     

Reduction of Cr(VI) by malic acid in aqueous Fe-rich soil suspensions

Detoxification of Cr(VI) through reduction by organic reductants has been regarded as an effective way for remediation of Cr(VI)-polluted soils. However, such remediation strategy would be limited in practical applications due to the low Cr(VI) reduction rate. In this study, the catalytic effect of two Fe-rich soils (Ultisol and Oxisol) on Cr(VI) reduction by malic acid was evaluated. As the results shown, the two soils could obviously accelerate the reduction of Cr(VI) by malic acid at low pH conditions, while such catalytic effect was gradually suppressed as the increase in pH. After reaction for 48 h at pH 3.2, Oxalic acid was found in the supernatant of Ultisol, suggesting the oxidization of hydroxyl in malic acid to carboxyl and breakage of the bond between C₂ and C₃. It was also found that the catalytic reactivity of Ultisol was more significant than that of Oxisol, which could be partly attributed to the fact that the amount of Fe(II) released from the reductive dissolution of Ultisol by malic acid was larger than that of Oxisol. With addition of Al(III), the catalytic effect from Ultisol was inhibited across the pH range examined. On the contrary, the presence of Cu(II) would increase the catalytic effect of Ultisol, which was more pronounced with the increase in pH. This study proposed a potential way for elimination of the environmental risks posed by the Cr(VI) contamination by use of the natural soil surfaces to catalyze Cr(VI) reduction by the organic reductant such as malic acid, a kind of organic reductant originating from soil organic decomposition process or plant excretion.     

Influences of redox transformation, metal complexation and aggregation of fulvic acid and humic acid on Cr(VI) and As(V) removal by zero-valent iron

This study investigated the removal kinetics and mechanisms of Cr(VI) and As(V) by Fe(0) in the presence of fulvic acid (FA) and humic acid (HA) by means of batch experiments. The focus was on the involvements of FA and HA in redox reactions, metal complexation, and iron corrosion product aggregation in the removal of Cr(VI) and As(V) removal by Fe(0). Synthetic groundwater was used as the background electrolyte to simulate typical groundwater. The results showed faster Cr(VI) removal in the presence of HA compared to FA. Fluorescence spectroscopy revealed that no redox reaction occurred in the FA and HA. The results of the speciation modeling indicate that the free Fe(II) concentration was higher in the presence of HA, resulting in a higher removal rate of Cr(VI). However, the removal of As(V) was inhibited in the HA solution. Speciation modeling showed that the concentration of dissolved metal-natural organic matter (metal-NOM) complexes significantly affected the aggregation of the iron corrosion products which in turn affected the removal of As(V). The aggregation was found to be induced by gel-bridging of metal-NOM with the iron corrosion products. The effects of metal-NOM on the aggregation of the iron corrosion products were further confirmed by TEM studies. Larger sizes of iron corrosion products were formed in the FA solution compared to HA solution. This study can shed light on understanding the relationships between the properties of NOM (especially the content of metal-binding sites) and the removal of Cr(VI) and As(V) by Fe(0).     

Simultaneous photocatalytic reduction of Cr(VI) and oxidation of phenol over monoclinic BiVO4 under visible light irradiation

BiVO4 powder with monoclinic structure was prepared and used as a visible-light catalyst simultaneously for the photooxidation of phenol and the photoreduction of Cr(VI). The photocatalytic efficiency was found to be rather low for either single phenol solution or single Cr(VI) solution. However, the photocatalytic reduction of Cr(VI) and photocatalytic oxidation of phenol proceed more rapidly for the coexistence system of phenol and Cr(VI) than for the single process, showing synergetic effect between the oxidation and reduction reactions. For the simultaneous photocatalytic reduction-oxidation process, the first-order kinetic constant of phenol degradation was 0.0314 min-1, being about six times higher than that for the photocatalytic process of single phenol. This result reveals the feasibility of using Cr(VI) as the electron scavenger of mBiVO4-mediated photocatalytic process of phenol degradation, and gives us an enlightenment to employ other semiconductor with a better visible light response but with a more positive band edge to efficiently degrade organic pollutants. This is the first report for simultaneous photocatalytic reduction of Cr(VI) and removal of phenol under visible light irradiation using photocatalyst mBiVO4.     

Effect of N-hydroxyethyl-ethylenediamine-triacetic acid (HEDTA) on Cr(VI) reduction by Fe(II)

The complexation of Fe(II) with organic ligand results in the decrease of redox potential, and enhances the reduction ability of Fe(II). An important example is the use of Fe(II)-organic complexes to accelerate Cr(VI) reduction. Dissolved O(2) and light can potentially affect Cr(VI) reduction; however, these two factors have not been adequately evaluated. A batch technique was used to investigate the Cr(VI) reduction as influenced by the light and dissolved O(2) using N-hydroxyethyl-ethylenediamine-triacetic acid (HEDTA) and Fe(II) solutions. The oxidation of Fe(II) by dissolved O(2) was rapid in the presence of HEDTA at low pH; nonetheless, the oxidation proceeded slowly when HEDTA was absent. Although Cr(VI) could be reduced by free Fe(II) at low pH, the reaction was considerably slower than that of systems involving HEDTA. The enhancement of Cr(VI) reduction by Fe(II) in the presence of high concentrations of HEDTA was achieved as a result of two processes. First, HEDTA acted as a ligand for expediting electron transfer between Fe(II) and Cr(VI). Secondly, HEDTA served as a reductant for Cr(VI) under illumination.