Community structure of microaerophilic iron-oxidizing bacteria in Japanese paddy field soils

ABSTRACT

Redox cycle of iron (Fe) is the central process in the biogeochemistry of paddy field soil. Although Fe(II)-oxidizing process is mediated by both abiotic and biotic reactions, microorganisms involved in the process have not been well studied in paddy field soil. The present study investigated the community structure of microaerophilic Fe(II)-oxidizing bacteria (FeOB) in the family Gallionellaceae in the plow layer of paddy fields located in the central (Anjo) and northeastern (Omagari) Japan since the members in the family are the typical FeOB in circumneutral freshwater environments and possibly have the significant role for Fe(II) oxidation in paddy field soils. A primer set targeting 16S rRNA gene of Gallionella-related FeOB was newly designed for the polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and quantitative PCR (qPCR) analyses. DGGE analysis showed significant differences in the band patterns between the field sites. Besides, minor differences were observed in the patterns between the soil depths (0–1 cm and below 1 cm) in the Anjo field, while the patterns were relatively stable in the Omagari field during the annual rice cultivation practices. In total 54 bands were sequenced and clustered into 20 operational taxonomic units (OTUs) on the basis of the 97% similarity. Eighteen out of twenty OTUs (50 of 54 bands) were affiliated within the FeOB cluster of Gallionellaceae, some of which were clustered with known microaerophilic FeOB, Ferrigenium kumadai, Ferriphaselus amnicola, ‘Sideroxydans lithotrophicus’ and ‘S. paludicola’. The number of the 16S rRNA gene copies was 10⁵–10⁷ and 10⁶–10⁸ copies g^?1 dried soil in the two paddy fields and negatively correlated to the contents of acetate-extractable Fe(II) in the soils during the rice cultivation period. These results suggested inhabitance of considerable number of diverse Gallionella-related FeOB and their potential involvement in the Fe(II)-oxidizing process of soil, especially during the rice cultivation period in the paddy field soils.     

Effects of the FeII/CuII interaction on copper aging enhancement and pentachlorophenol reductive transformation in paddy soil

The present study investigated copper aging and pentachlorophenol (PCP) reductive transformation under the effects of the Fe(II)/Cu(II) interaction in paddy soil in south China. Kinetic measurements demonstrated that the PCP reductive transformation rate (k) could be promoted in the presence of no more than 0.375 mM Cu(II) and inhibited in the presence of no less than 0.5 mM Cu(II). The highest k value in the presence of 0.25 mM Cu(II) corresponds to the lowest redox potential (E(p)) value of active Fe species. The increasing trend in E(p) of the active Fe species is consistent with the declining trend in the k value of PCP reduction and vice versa. Dissolved Cu(II) is gradually transformed into immobilized Cu species during PCP reduction, which indicates that Cu aging is enhanced by the Fe(II)/Cu(II) interaction. These findings improve our general understanding of the Fe(II)/Cu(II) interaction on soil iron redox chemistry for polychlorinated pesticide detoxification and heavy metal immobilization.     

Organic acids influence iron uptake in the human epithelial cell line Caco-2

It has previously been suggested that organic acids enhance iron absorption. We have studied the effect of nine organic acids on the absorption of Fe(II) and Fe(III) in the human epithelial cell line Caco-2. The effect obtained was dose-dependent, and the greatest increase (43-fold) was observed for tartaric acid (4 mmol/L) on Fe(III) (10 micromol/L). Tartaric, malic, succinic, and fumaric acids enhanced Fe(II) and Fe(III) uptake. Citric and oxalic acid, on the other hand, inhibited Fe(II) uptake but enhanced Fe(III) uptake. Propionic and acetic acid increased the Fe(II) uptake, but had no effect on Fe(III) uptake. Our results show a correlation between absorption pattern and chemical structure; e.g. hydroxyl groups, in addition to carboxyls, were connected with a positive influence. The results may be important for elucidating factors affecting iron bioavailability in the small intestine and for the development of foods with improved iron bioavailability.     

Production of gaseous mercury in tropical hydromorphic soils in the presence of ferrous iron: a laboratory study

In this paper, we demonstrate that reduction of Hg(II) to Hg⁰ under anaerobic conditions, followed by volatilization of Hg⁰ to the atmosphere, might be responsible for the removal of Hg from tropical hydromorphic soils. We conducted a series of kinetic batch experiments in which we added Hg(II) to anoxic suspensions of a soil clay fraction and haematite nanoparticles. The clay fraction came from three depths in a hydromorphic soil by the Leblond River in French Guiana, South America: close to the upper and lower boundaries (30–40 cm and 90–100 cm) and within the central part (60–70 cm) of the frequently water‐saturated clay horizon. We prepared a second set of soil clay fraction suspensions with Fe(III) citrate, whose reduction acted as a source of dissolved Fe(II) to investigate the influence of Fe(II) on the production of Hg⁰. Reduction of Hg(II) to Hg⁰ occurred with all samples amended with Fe(III) citrate. Laboratory experiments with haematite suspensions demonstrated that adsorption of Fe(II) to the haematite surface created very reactive sites for the reduction of Hg(II), while in the absence of haematite particles, no production of Hg⁰ occurred. The greatest production of Hg⁰ was found for the depth intervals 30–40 cm and 90–100 cm, where the total mercury concentration exhibits a local minimum. The observed pseudo‐first order rate constants for the 90–100 cm depth sample were close to rate constants reported for abiotic reduction of Hg(II) by Fe(II) adsorbed on mineral surfaces. Significant production of Hg⁰ was found for the 90–100 cm depth interval sample, both with and without Fe(III) citrate. A biotic pathway as well as abiotic reduction by Fe(II) might be involved in the reduction of Hg(II) to Hg⁰.     

Abiotic reduction of pendimethalin and trifluralin in controlled and natural systems containing Fe(II) and dissolved organic matter

The environmental fate of dinitroaniline herbicides is poorly understood, despite their classification as Persistent Bioaccumulative Toxins by the U.S. Environmental Protection Agency. This study investigated the abiotic reduction of pendimethalin and trifluralin in controlled laboratory systems in the presence of Fe(II) and fulvic acids isolated from various surface waters and in sediment pore waters containing naturally abundant levels of dissolved Fe(II) and dissolved organic matter (DOM). It was found that Fe(II) was necessary for pendimethalin and trifluralin reduction to occur in controlled systems and that higher concentrations of DOM slowed Fe(II)-mediated reactions. Pendimethalin and trifluralin reduction in natural pore waters was roughly an order of magnitude slower compared to controlled Fe(II)-DOM solutions, indicating that the reactive Fe(II) species responsible for reduction are concentration-limited in natural pore waters relative to controlled systems. The results show that caution must be exercised when extrapolating results from controlled system reactions to natural systems and that abiotic reduction of both trifluralin and pendimethalin is observed within 3-7 days in anaerobic sedimentary pore waters containing high concentrations of both dissolved Fe(II) and DOM.     

Atrazine photodegradation in aqueous solution induced by interaction of humic acids and iron: photoformation of iron(II) and hydrogen peroxide

The photochemical formation of Fe(II) and hydrogen peroxide (H 2O 2) coupled with humic acids (HA) was studied to understand the significance of iron cycling in the photodegradation of atrazine under simulated sunlight. The presence of HA significantly enhanced the formation of Fe(II) and H 2O 2, and their subsequent product, hydroxyl radical ( (*)OH), was the main oxidant responsible for the atrazine photodegradation. During 60 h of irradiation, the fraction of iron presented as Fe(II) (Fe(II)/Fe(t)) decreased from 20-32% in the presence of the Fe(III)-HA complex to 10-22% after adding atrazine. The rate of atrazine photodegradation in solutions containing Fe(III) increased with increasing HA concentration, suggesting that the complexation of Fe(III) with HA accelerated the Fe(III)/Fe(II) cycling. Using fluorescence spectrometry, the quenching constant and the percentage of fluorophores participating in the complexation of HA with Fe(III) were estimated by the modified Stern-Volmer equation. Fourier transform infrared spectroscopy (FTIR) offered the direct evidence that Fe(III)-carboxylate complex could be formed by ligand exchange of HA with Fe(III). Based on all the information, a possible reaction mechanism was proposed.     

天然磁铁矿和商用Fe3O4修复Hg(II)污染地下水的模拟研究

目前汞污染地下水修复面临很大的技术和成本挑战,亟需发展修复效果好、经济效益性高的汞污染地下水修复技术和修复材料。通过批量实验和光谱分析探究了天然磁铁矿和商用Fe_3O_4对Hg(Ⅱ)的去除效率和去除机制,并分析了两种材料对模拟地下水中Hg(Ⅱ)的吸附和脱附行为。结果表明,天然磁铁矿和商用Fe_3O_4对Hg(Ⅱ)的去除受pH、Hg(Ⅱ)初始浓度、Cl-等因素的影响;二者对Hg(Ⅱ)的去除均符合准二级动力学模型和Freundlich模型;天然磁铁矿对Hg(Ⅱ)的去除机制主要是羟基络合与物理吸附,而商用Fe_3O_4对Hg(Ⅱ)的去除主要是化学还原与物理吸附。二者对模拟地下水中Hg(Ⅱ)的去除率分别达90%和95%,具有修复Hg(Ⅱ)污染地下水的应用潜力。     

Iron speciation in soils and soil aggregates by synchrotron-based X-ray microspectroscopy (XANES,μ-XANES)

Iron speciation in soils is still poorly understood. We have investigated inorganic and organic standard substances, diluted mixtures of common Fe minerals in soils (pyrite, ferrihydrite, goethite), soils in a forested watershed which constitute a toposequence with a hydrological gradient (Dystric Cambisol, Dystric Planosol, Rheic Histosol), and microsites of a dissected soil aggregate by X‐ray Absorption Near Edge Spectroscopy (XANES) at the iron K‐edge (7112 eV) to identify different Fe(II) and Fe(III) components. We calculated the pre‐edge peak centroid energy of all spectra and quantified the contribution of different organic and inorganic Fe‐bearing compounds by Linear Combination Fitting (LCF) conducted on the entire spectrum (E = 7085–7240 eV) and on the pre‐edge peak. Fe‐XANES conducted on organic and inorganic standards and on synthetic mixtures of pyrite, ferrihydrite and goethite showed that by calculating the pre‐edge peak centroid energy, the Fe(II)/Fe(III) ratio of different Fe‐bearing minerals (Fe sulphides, Fe oxyhydroxides) in mineral mixtures and soils can be quantified with reasonable accuracy. A more accurate quantification of the Fe(II)/Fe(III) ratio was possible with LCF conducted on the entire XANES spectrum. For the soil toposequence, an increased groundwater influence from the Cambisol to the Histosol was reflected in a larger contribution of Fe(II) compounds (Fe(II) silicate, Fe monosulphide, pyrite) and a smaller contribution of Fe(III) oxyhydroxides (ferrihydrite, goethite) to total iron both in the topsoil and the subsoil. In the organic topsoils, organically bonded Fe (33–45% of total Fe) was 100% Fe(III). For different microsites in the dissected aggregate, spatial resolution ofμ‐XANES revealed different proportions of Fe(II) and Fe(III) compounds. Fe K‐edge XANES andμ‐XANES allows an approximate quantification of Fe(II) and Fe(III) and different Fe compounds in soils and (sub)micron regions of soil sections, such as mottles, concretions, and rhizosphere regions, thus opening new perspectives in soil research.     

Implications of iron solubilization on soil phosphorus release in seasonally flooded forests of the lower Orinoco River, Venezuela

The microbial reduction of Fe oxides is thought to contribute with the release of P in sedimentary environments. However, secondary reactions of the bioproduced Fe(II) with P in solution, can lead to a decrease in the soluble P concentration. In this study, we examined how the reduction of Fe(III) affects the soluble P concentration, when the soils of a seasonally flooded forest gradient are subjected to anaerobic conditions. Soil samples were collected during the dry season from two zones subjected to different flooding intensity: MAX and MIN zones that were inundated 8 and 2 months per year, respectively. When anaerobic conditions were applied to soils from both zones, a clear stimulatory effect on the Fe(III) reduction was observed. However, bioproduced Fe(II) underwent secondary chemical reactions, masking the extend of Fe(III) reduction of these soils. Iron was reduced mainly during the first 15 days of the anaerobic incubation and it was stimulated by a pulse of labile carbon. Iron dissolution did not lead to an increase of the soluble P content. However, in both zones P was high and positively correlated with Fe(II), implying that soil P mobilization was linked to Fe dissolution. In the MIN zone, soluble P concentration decreased, probably as a consequence of the secondary reactions of solubilized P with other non-redox sensitive soils elements. Fe solubilization also had an effect on the activity of acid phosphatase and consequently in the solubilization of P from the organic pool. In conclusion, the P cycle in these soils is strongly coupled to C and Fe cycles.     

水稻土中铁还原与无机磷有效性的关系

采集我国吉林省吉林市(1号)、四川省邛崃市(2号)、江西省安福县(3号)及广东省雷州半岛(4号)等地区的四种典型水稻土,通过模拟厌氧培养试验,研究在厌氧还原条件下不同水稻土中Fe(II)产生量、有效性磷浓度及A l-P、Fe-P和O-P等无机磷形态变化情况。结果表明,淹水后四种水稻土中Fe(II)含量均有不同程度的增加,土壤有效磷浓度也呈现相同的变化趋势。淹水60天后在1、2、3、4号土壤中,Fe(II)的净增加量分别为5.5mg g-1,4.3mg g-1,2.1mg g-1和3.7mg g-1;有效磷的增加量分别为50 mg kg-1,18.6 mg kg-1,23 mg kg-1和12.4 mg kg-1。厌氧培养30天内土壤Fe(II)产生量与有效磷浓度的变化呈极显著的相关关系。在1、2、3、4号土壤中,Fe(II)与有效磷的相关系数依次为0.9679、0.9744、0.8949和0.7501。     

Enhancement of callose production by a combination of aluminum and iron in suspension-cultured tobacco (Nicotiana tabacum) cells

In nutrient medium, aluminum (A1) enhances ferrous ion [Fe(II)] -mediated per oxidation of lipids, which results in the loss of the plasma membrane integrity and the accumulation of A1 in tobacco cells. Under these conditions, the mechanism of callose production and possible involvement of callose in the accumulation of Al were investigated. Callose production was enhanced by both Al and Fe(II), but not by A1 or Fe(II) alone, and the enhancement was inhibited by a lipophilic antioxidant, suggesting that the enhancement of callose production is caused by the A1-enhanced, Fe(II)-mediated peroxidation of lipids. The enhancement of callose production depended on the presence of external Ca²⁺ in the treatment medium. The activity of β-l,3-glucan synthase in the microsomes was increased several times by the addition of Ca²⁺ in the assay medium, although the activity in the microsomes was reduced by the treatment of cells with Al and Fe(II) together. Therefore, it is likely that callose production is enhanced by exogenous Ca²⁺ via the AI-enhanced, Fe(II)-mediated peroxidation of lipids. During the exposure of the cells to Al and Fe(II), callose production started and increased simultaneously with Al accumulation. However, the digestion of callose in the cell wall materials prepared from the A1-treated cells by laminarinase did not release A1, suggesting that callose is not involved in the binding or trapping of A1.     

Interaction of Oxidation Products from Caffeic Acid with Fe(III) and Fe(II)

Abstract

Phenolic substances in the soil–plant system can be oxidized by metal ions, inorganic components, molecular oxygen as well as by phenoloxidases, giving rise to the formation of products of low or high molecular weight. Interactions of these products with iron, in both reduced and oxidized form, can affect the iron mobility in soil and rhizosphere, and thus its availability to plants. Here we report the results of a study on the complexing and reducing activity of the oxidation products from caffeic acid (CAF), obtained via electrochemical means, towards Fe(III) and Fe(II) in aqueous solution in the 3.0–6.0 pH range. The HPLC analysis of the filtered solutions after the CAF oxidation showed the formation of two main groups of products: (i) CAF oligomers formed through radicalic reactions which do not involve the double bond of the CAF lateral chain and (ii) products where this bond is involved. These oxidation products (COP) were found to interact with both Fe(III) and Fe(II) with formation of soluble and insoluble Fe(III)‐, and Fe(II)‐COP complexes. The COP were found to be able to reduce Fe(III) to Fe(II) mainly at pH < 4.0. A low redox activity was observed at pH ≥ 4.5 due to Fe(III) hydrolysis reactions as well as to the decrease in the redox potential of the Fe(III)/Fe(II) couple. Formation of hydroxy Fe(III)‐COP polymers occurs at pH > 3.5.     

Characterization of the Fe(III)—fulvic acid reaction by Mössbauer spectroscopy

Fulvic acids have been isolated from a sandy loam (Countesswells series) and a clay soil (Tipperty series) and the products of their reaction with different amounts of iron over a range of pH from 0.5 to 11 analyzed by Mössbauer spectroscopy. Three distinct types of spectral component were detected at 77 K, a sextet from magnetically dilute Fe(III) and doublets from Fe(II) and Fe(III), the last arising from both organic complexes and poorly crystalline oxide species. In iron-fulvic acid mixtures the proportion of iron as Fe(II) increased as the pH was lowered from 5 to 1 by the addition of hydrochloric or nitric acid at all Fe to fulvic acid ratios (1:5 to 1:500). When the pH was lowered below 1 the amounts of Fe(II) decreased with the lower Fe to fulvic acid ratios, but increased with the higher ratios. The amounts of the Fe(III) component contributing to a doublet signal decreased with decreasing Fe:fulvic acid ratios. At low iron concentrations the iron appears to be strongly bound to the fulvic acid, but when the iron content is of the order of 1–2% uncomplexed Fe(III) species can be present. At pH > 2 these are hydrolysed ions which form poorly-crystalline oxides at higher pH. This was confirmed by analysis of spectra at 4.2 K. At pH < 2 free ions are present in solution. In solutions with high fulvic acid contents (greater than 100-fold excess) the reactions with iron are completely reversible, but in solutions with a lower proportion of fulvic acid to iron, where free ions are present, there is a lack of reversibility.     

土壤铁氧化物-亚铁的相互作用及其环境影响研究进展

铁氧化物和溶液相亚铁常在厌氧土壤环境中共存。铁氧化物能够加快亚铁的氧化速率，且控制亚铁氧化成矿产物的类型，同时，亚铁与铁氧化物组成的系统是一种良好的还原剂，能够有效还原重金属及降解有机污染物。另一方面，亚铁能够催化铁氧化物晶相转变，导致铁氧化物结构和表面性质发生改变，进而影响相关重金属、有机质的环境行为。本文综述了铁氧化物催化亚铁氧化成矿、铁氧化物-亚铁系统还原污染物以及亚铁催化铁氧化物相变的反应机制及影响因素，最后，对未来在自然土壤中研究铁氧化物-亚铁界面反应及其环境影响进行了展望。     

与Fe(III) 共存的Fe(II) 分光光度法测定

Fe(Ⅱ)的含量及变化与土壤和沉积物的氧化还原性质关系密切。当与Fe(Ⅲ)共存时,Fe(Ⅱ)的测定往往受到干扰。本文研究了常用显色剂2,2’-联吡啶和菲洛嗪(Ferrozine)测定土壤Fe(Ⅱ)时存在的问题及解决办法。结果表明,Fe(Ⅲ)可与显色剂作用形成络合物,该络合物对Fe(Ⅱ)测定所用波段的光线具有吸收作用,从而使Fe(Ⅱ)浓度被过高估计。Fe(Ⅲ)对Fe(Ⅱ)测定的干扰程度与其浓度及所选显色剂有关。当以2,2’-联吡啶为显色剂时,单位浓度Fe(Ⅲ)(1.0 mg/L)将导致Fe(II)的测定值比实际值高0.012 mg/L;而当菲洛嗪为显色剂时,单位浓度Fe(Ⅲ)引起的Fe(Ⅱ)高估值在0.010～0.032 mg/L之间。F-能够抑制Fe(Ⅲ)-显色剂络合物的形成。当F-的加入量超过Fe(Ⅲ)的4倍时,F-能有效地消除Fe(Ⅲ)的干扰。实际样品的测定结果表明,改进的Fe(Ⅱ)分光光度法能够满足土壤及沉积物中Fe(II)的准确测定。     

Effect of Metal Ions on the Entomopathogenic Nematode Heterorhabditis Bacteriophora Poinar (Nematoda: Heterohabditidae) Under Laboratory Conditions

The effect of sixteen metal ions: Al, Cd, Co(II), Cr(III), Cr(VI), Cu(II), Fe(III), Li, Mg, Mn(II), Mo(VI), Ni(II), Pb(II), Se(IV), V(V), and Zn on the mortality and infectivity ofHeterorhabditis bacteriophora were observed over a 96 hr period. All ions except Pb(II) even at naturally unrealistic concentrations did not cause the mortality of the nematodes. A weak vitalizing effect could eventually be observed with Mn(II), Mg, Fe(III) and Ni(II) (Table 1). However, such treatment generally lowered infectivity of the nematodes with respect to wax moth caterpillars.Galleria mellonella. This effect was particularly significant with Ni(II) and Pb(II).     

Kinetic Speciation of Ni(II) in Model Solutions and Freshwaters: Competition of Al(III) and Fe(III)

The competing ligand exchange method was used to investigate the competitive binding of Ni(II) by Al(III) and Fe(III) in model aqueous solutions and freshwaters. Graphite furnace atomic absorption spectrometry and adsorptive cathodic stripping voltammetry were used to monitor the rate of uptake of the Ni by Chelex 100 chelating resin and dimethylglyoxime as the competing ligands, respectively. The results have revealed that Ni(II)–humate complexes were more labile in presence of the mixture of Al(III) and Fe(III), compared to the lability of the Ni(II)–humate complexes when only one of the two, Al(III) or Fe(III), was present. The environmental significance of this work is that in model solutions simulating freshwater containing humic substances and the target trace metal Ni(II) and cations, Al(III) and Fe(III), the competitive binding of Ni(II), Al(III) and Fe(III) by humic substances makes Ni(II)–humate complexes labile, releasing free Ni²⁺–aqua complex, which reported to be toxic.     

In vivo staining of reduced iron by 2,2′ bipyridine in rice exposed to iron toxicity

A protocol for a novel method to visualize Fe(II) in rice tissues is proposed. The method is based on the selective formation of a purple‐red color complex of 2,2′ bipyridine and Fe(II). Rice genotypes were exposed to 18 mM Fe(II) in nutrient solution for 2 d. Root systems of intact plants were subsequently placed in 2,2′ bipyridine solutions. The formation of the [Fe(bipy)<$>_3^{2+}<$>] color complex was visualized using bifocal microscopy. The method may improve the selection of genotypes during breeding for Fe‐toxicity resistance of rice.     

EFFECTIVENESS OF DIFFERENT FOLIAR IRON APPLICATIONS TO CONTROL IRON CHLOROSIS IN ORANGE TREES GROWN ON A CALCAREOUS SOIL

The effectiveness on controlling Fe chlorosis in orange trees grown on calcareous soils was tested. The treatments were Fe(II) sulfate (500 mg Fe L^?1), sulfuric acid (0.5 mM H₂SO₄), Fe(III)-chelate (Hampiron 654 GS, 120 mg Fe L^?1) and distilled water as a control. A non-ionic wetting agent was used in all treatments. The use of frequent foliar sprays alleviated Fe chlorosis in orange trees. Sprays of Fe(II) sulfate increased the concentrations of chlorophyll, Fe and zinc in leaves and improved fruit size and quality compared to fruits of control trees. Sprays of Fe(III)-chelate also increased leaf chlorophyll and Fe concentrations and improved fruit quality, but did not increase fruit size. Sprays of sulfuric acid alone slightly increased leaf chlorophyll and Fe concentrations, without improving fruit size and quality. These results suggest that foliar sprays with Fe could help to avoid yield and quality losses caused by Fe chlorosis in citrus orchards. Furthermore, these treatments could be done with relatively cheap materials such as solutions containing Fe(II) sulfate.     

Catalysis of Dissolved and Adsorbed Iron in Soil Suspension for Chromium（Ⅵ） Reduction by Sulfide

The kinetics of Cr（Ⅵ） reduction by sulfide in soil suspensions with various pHs, soil compositions, and Fe（Ⅱ） concentrations was examined using batch anaeroblc experimental systems at constant temperature. The results showed that the reaction rate of Cr（Ⅵ） reduction was in the order of red soil 〈 yellow-brown soil 〈 chernozem and was proportional to the concentration of HCl-extractable iron in the soils. Dissolved and adsorbed iron in soil suspensions played an important role in accelerating Cr（Ⅵ） reduction. The reaction involved in the Cr（Ⅵ） reduction by Fe（Ⅱ） to produce Fe（ⅡI）, which was reduced to Fe（Ⅱ） again by sulfide, could represent the catalytic pathway until about 70% of the initially present Cr（Ⅵ） was reduced. The catalysis occurred because the one-step reduction of Cr（Ⅵ） by sulfide was slower than the two-step process consisting of rapid Cr（Ⅵ） reduction by Fe（Ⅱ） followed by Fe（Ⅲ） reduction by sulfide. In essence, Fe（Ⅱ）/Fe（Ⅲ） species shuttle electrons from sulfide to Cr（Ⅵ）, facilitating the reaction. The effect of iron, however, could be completely blocked by adding a strong Fe（Ⅱ）-complexing ligand, 1,10-phenanthroline, to the soil suspensions. In all the experiments, initial sulfide concentration was much higher than initial Cr（Ⅵ） concentration. The plots of In e[Cr（Ⅵ）] versus reaction time were linear up to approximately 70% of Cr（Ⅵ） reduction, suggesting a first-order reaction kinetics with respect to Cr（Ⅵ）. Elemental sulfur, the product of sulfide oxidation, was found to accelerate Cr（Ⅵ） reduction at a later stage of the reaction, resulting in deviation from linearity for the In c[Cr（Ⅵ）] versus time plots.