Oxidation and sorption of arsenite by manganese dioxide as influenced by surface coatings of iron and aluminum oxides and calcium carbonate

Manganese dioxide (birnessite) was coated with two levels of Fe and Al oxides and CaCO₃, and the influence of these coatings on the surface features and the reactivity of MnO₂ with respect to the oxidation and sorption of As(III) (arsenite) was examined. For all untreated and coated MnO₂ samples, the depletion (oxidation plus sorption) of As(III) by the samples follows first-order kinetics. The rate constants are smaller for the samples with the high levels of coating of Fe and Al oxides and CaCO₃ on MnO₂ than they are for the untreated MnO₂ and the MnO₂ with the low levels of coating. The extent of masking of the electron-accepting sites on the MnO₂ for converting the toxic As(III) to the less toxic As(V) significantly varies with the kinds and levels of coatings. Coatings of Fe and A1 oxides and CaCO₃, on MnO₂ distinctively affect the sorption of As. Manganese oxide evidently catalyzes the sorption of As by Al oxide through oxidation of As(III) to As(V). The relative affinities of the oxides of Mn, Fe, and Al and CaCO₃, toward As(III) and As(V) account for the coating effects.     

Potential for the alleviation of arsenic toxicity in paddy rice using amorphous iron-(hydr)oxide amendments

A pot culture experiment was conducted to investigate the effects of amorphous iron-(hydr)oxide (Am-FeOH) amendments on arsenic (As) availability and its uptake by rice ( Oryza sativa L. cv. BR28) irrigated with As-contaminated water. A rhizobag system was established using 3.5 L plastic pots, each containing one central compartment for plant growth, a middle compartment and an outside compartment. Three levels of laboratory-synthesized Am-FeOH (0, 0.1 and 0.5% w/w) were used to amend samples of the As-free sandy loam paddy soil placed into each compartment of the rhizobag system. The soils were submerged with a solution containing 5 mg L⁻¹ As(V). Two-week-old rice seedlings were planted in the central compartments and cultured for 9 weeks under greenhouse conditions. The addition of 0.1% Am-FeOH to the soil irrigated with As-contaminated water improved plant growth, reduced the As concentration in the plants and enhanced Fe-plaque formation on the root surfaces. Analysis of soil solution samples collected during the experiment revealed higher pH levels and lower redox potentials in the soils amended with Am-FeOH at the onset of soil submergence, but later the soil solution collected from the 0.1% Am-FeOH treatment was slightly acidic and more oxidized than the solution from the 0% treatment. This indicated active functioning of the roots in the soil treated with 0.1% Am-FeOH. The concentrations of As(III) in the soil solution collected from the central compartment were significantly reduced by the Am-FeOH amendments, whereas in the soil treated with 0% Fe, As(III) accumulated in the rhizosphere, particularly during the late-cultivation period. The improvement in plant growth and reduction in As uptake by plants growing in the Am-FeOH treated soil could be attributed to the reduction of available As in the soil solution, mainly as a result of the binding of As to the Fe-plaque on the root surfaces.     

Potential for the alleviation of arsenic toxicity in paddy rice using amorphous iron-(hydr)oxide amendments

Abstract

A pot culture experiment was conducted to investigate the effects of amorphous iron-(hydr)oxide (Am-FeOH) amendments on arsenic (As) availability and its uptake by rice (Oryza sativa L. cv. BR28) irrigated with As-contaminated water. A rhizobag system was established using 3.5 L plastic pots, each containing one central compartment for plant growth, a middle compartment and an outside compartment. Three levels of laboratory-synthesized Am-FeOH (0, 0.1 and 0.5% w/w) were used to amend samples of the As-free sandy loam paddy soil placed into each compartment of the rhizobag system. The soils were submerged with a solution containing 5 mg L^?1 As(V). Two-week-old rice seedlings were planted in the central compartments and cultured for 9 weeks under greenhouse conditions. The addition of 0.1% Am-FeOH to the soil irrigated with As-contaminated water improved plant growth, reduced the As concentration in the plants and enhanced Fe-plaque formation on the root surfaces. Analysis of soil solution samples collected during the experiment revealed higher pH levels and lower redox potentials in the soils amended with Am-FeOH at the onset of soil submergence, but later the soil solution collected from the 0.1% Am-FeOH treatment was slightly acidic and more oxidized than the solution from the 0% treatment. This indicated active functioning of the roots in the soil treated with 0.1% Am-FeOH. The concentrations of As(III) in the soil solution collected from the central compartment were significantly reduced by the Am-FeOH amendments, whereas in the soil treated with 0% Fe, As(III) accumulated in the rhizosphere, particularly during the late-cultivation period. The improvement in plant growth and reduction in As uptake by plants growing in the Am-FeOH treated soil could be attributed to the reduction of available As in the soil solution, mainly as a result of the binding of As to the Fe-plaque on the root surfaces.     

微生物成矿技术在环境砷污染治理中的应用研究进展

近年来,微生物成矿技术成为环境污染治理领域研究热点之一.结合典型矿化菌与砷的成矿关联规律对微生物成矿作用固定砷的机制及环境污染治理中的应用进行归纳:(1)环境中的碳酸盐矿化菌、铁锰氧化菌及硫酸盐还原菌可通过诱导成矿的方式,直接促进含砷矿物的形成或生成其他矿物间接吸附砷,通过对砷的成矿产物和成矿因素分析,揭示微生物成矿机...     

影响土壤中铬, 锰的氧化还原反应的环境因素

Disposal of chromium(Cr) hexavalent form,Cr(VI),in soils as additions in organic fertilizers,liming materials or plant nutrient sources can be dangerous since Cr(VI) can be highly toxic to plants,animals,and humans.In order to explore soil conditions that lead to Cr(VI) generation,this study were performed using a Paleudult(Dystic Nitosol) from a region that has a high concentration of tannery operations in the Rio Grande do Sul State,southern Brazil.Three laboratory incubation experiments were carried out to examine the influences of soil moisture content and concentration of cobalt and organic matter additions on soil Cr(VI) formation and release and manganese(Mn) oxide reduction with a salt of chromium chloride(CrCl 3) and tannery sludge as inorganic and organic sources of Cr(III),respectively.The amount of Cr(III) oxidation depended on the concentration of easily reducible Mn oxides and the oxidation was more intense at the soil water contents in which Mn(III/IV) oxides were more stable.Soluble organic compounds in soil decreased Cr(VI) formation due to Cr(III) complexation.This mechanism also resulted in the decrease in the oxidation of Cr(III) due to the tannery sludge additions.Chromium(III) oxidation to Cr(VI) at the solid/solution interface involved the following mechanisms:the formation of a precursor complex on manganese(Mn) oxide surfaces,followed by electron transfer from Cr(III) to Mn(III or IV),the formation of a successor complex with Mn(II) and Cr(VI),and the breakdown of the successor complex and release of Mn(II) and Cr(VI) into the soil solution.     

Arsenate and Arsenite Sorption on Magnetite: Relations to Groundwater Arsenic Treatment Using Zerovalent Iron and Natural Attenuation

Magnetite (Fe₃O₄) is a zerovalent iron corrosion product; it is also formed in natural soil and sediment. Sorption of arsenate (As(V)) and arsenite (As(III)) on magnetite is an important process of arsenic removal from groundwater using zerovalent iron-based permeable reactive barrier (PRB) technology and natural attenuation. We tested eight magnetite samples (one from Phoenix Environmental Ltd, one from Cerac, Inc. and six from Connelly-GPM, Inc.) that contained from 79 to 100% magnetite. The magnetites were reacted in the absence of light with either As(V) or As(III) in 0.01 M NaCl at 23°C at equilibrium pH 2.5–11.5 for 24 h. As(V) sorption showed a continuous drop with increasing pH from 2.5 to 11.5; whereas, As(III) sorption exhibited maxima from pH 7 to 9. Equal amounts of As(V) and As(III) were sorbed at pH 5.6–6.8. Higher amounts of As(III) were sorbed by the magnetites than As(V) at pH values greater than 6.8. The solution speciation test did not show any chemical reduction of As(V) in any magnetite suspension, which is consistent with the X-ray Photoelectron Spectroscopy (XPS) study of a Connelly-GPM magnetite (CC-1048) suspension. Conversely, XPS results show that the As(III) is partially oxidized in the magnetite (CC-1048) suspension. This is also consistent with the batch test results that also show more oxidation occurring at alkaline pH. Complete oxidation of As(III) occurred in a synthetic birnessite (δ-MnO₂) suspension after 24 h of reaction. The minute impurities of Mn (possibly as an oxide form) in the magnetite samples may have been responsible for As(III) oxidation. In addition, the structural Fe(III) in magnetite and hydroxyl radicals in solution could also serve as oxidants for As(III) oxidation. The conversion of As(III) to As(V) in the magnetite suspensions would be beneficial in a remediation scheme for As removal, since As(V) is considered less toxic than As(III). Information from the present study can help predict the sorption behavior and fate of arsenic species in engineered PRB systems and natural environments.     

Reductive dissolution of crystalline and amorphous Fe(III) oxides by microorganisms in submerged soil

Summary Reduction of Fe(III) of amorphous and crystalline Fe(III) oxides to Fe(II) in flooded soils was studied using ⁵⁹Fe(OH)₃ and ⁵⁹Fe₂O₃. The results indicated that Fe(III) in the amorphous oxide was readily amenable to microbial reduction in anaerobic soil condition whereas Fe(III) in the crystalline oxide was not. Following soil submergence, the native as well as the applied crystalline Fe(III) oxides were rapidly converted into the amorphous form. The transformation of the crystalline oxides to the amorphous form appears to be a prerequisite for the reduction of Fe(III) of the oxide. This transformation, probably through hydration, is also mediated by microorganisms.     

Cr(III) oxidation coupled with Mn(II) bacterial oxidation in the environment

Purpose  

Cr(III) oxidation to Cr(VI) significantly increases Cr mobility and toxicity and thus its environmental risks. Manganese (Mn) oxides may serve as the potential oxidants of Cr(III) in environment. Natural Mn oxides in the environment are believed to be derived from bacterial oxidation. The objective of this study was to examine the Cr(III) oxidation capacity of biogenic Mn oxide and the role of Mn-oxidizing bacteria in Cr(III) oxidation.     

Biogeochemical processes and arsenic enrichment around rice roots in paddy soil: results from micro‐focused X‐ray spectroscopy

The spatial distribution and speciation of iron (Fe), manganese (Mn) and arsenic (As) around rice roots grown in an As‐affected paddy field in Bangladesh were investigated on soil sampled after rice harvest. Synchrotron micro‐X‐ray fluorescence spectrometry on soil thin sections revealed that roots influence soil Fe, Mn and As distribution up to 1 mm away from the root–soil interface. Around thick roots (diameter around 500 µm), Mn was concentrated in discrete enrichments close to the root surface without associated As, whereas concentric Fe accumulations formed farther away and were closely correlated with As accumulations. Near thin roots (diameter < 100 µm), in contrast, a pronounced enrichment of Fe and As next to the root surface and a lack of Mn enrichments was observed. X‐ray absorption fine structure spectroscopy suggested that (i) accumulated Fe was mainly contained in a two‐line ferrihydrite‐like phase, (ii) associated As was mostly As(V) and (iii) Mn enrichments consisted of Mn(III/IV) oxyhydroxides. The distinct enrichment patterns can be related to the extent of O₂ release from primary and lateral rice roots and the thermodynamics and kinetics of Fe, Mn and As redox transformations. Our results suggest that in addition to Fe(III) plaque at the root surface, element accumulation and speciation in the surrounding rhizosphere soil must be taken into account when addressing the transfer of nutrients or contaminants into rice roots.     

Improvement of the NH2OH-HCl-HOAc method for extracting manganese and iron oxides in Japanese Andisols and other soil types in Japan

Abstract

We evaluated the validity of Tessier’s method as applied to the extraction of manganese (Mn) and iron (Fe) oxides in Japanese Andisols and other soil types in Japan. Using the original Tessier’s extractant mixture, 0.04 mol L^?1 hydroxylamine hydrochloride in 25% acetic acid (0.04 mol L^–1 NH₂OH-HCl in 25% HOAc), we found that substantial amounts of short-range-ordered Fe oxides were not extracted from allophanic Andisol samples and that considerable amounts of total Fe oxides were not extracted from all soil types. Relatively high extraction pH and large amounts of short-range-ordered Fe oxides in the Andisol samples might be responsible for incomplete extraction. Stoichiometric calculation indicated that the concentration of NH₂OH-HCl might be insufficient for complete extraction of Fe oxides. The extracted amounts of Mn and Fe increased with increasing concentration of NH₂OH-HCl in the extractant, and most of the Mn and Fe oxides in the soil samples, including samples with as much as 5.6% Fe, were extracted with 0.6 mol L^–1 NH₂OH–HCl in 25% HOAc. As judged from the simultaneous dissolution of aluminum (Al) and silicon (Si) minerals, extraction selectivity of Fe oxides with 0.6 mol L^–1 NH₂OH-HCl in 25% HOAc was comparable to that of the original Tessier’s method and better than that of a modified Community Bureau of Reference (BCR) sequential extraction procedure or a method using an extractant consisting of a mixture of oxalate and ascorbate, especially for Andisol samples.     

对矿区和冶炼区土壤重金属污染进行化学和矿物学特征分析

The binding of metallic contaminants （Pb, Cd, and Zn） and As on soil constituents was studied on four highly contaxninated alluvial soil profiles from the mining/smelting district of Pribram （Czech Republic） using a combination of mineralogical and chemical methods. Sequential extraction analysis （SEA） was supplemented by mineralogical investigation of both bulk samples and heavy mineral fractions using X-ray diffraction analysis （XRD） and scanning electron microscopy with an energy dispersive X-ray spectrometer （SEM/EDS）. The mineralogy of Fe and Mn oxides was studied by voltammetry of microparticles （VMP） and diffuse reflectance spectrometry （DRS）. Zinc and Pb were predominantly bound in the reducible fraction attributed to Fe oxides and Mn oxides （mainly birnessite, Na4Mn14O27.9H2O）, which were detected in soils by XRD and SEM/EDS. In contrast, Cd was the most mobile contaminant and was predominantly present in the exchangeable fraction. Arsenic was bound to the residual and reducible fractions （corresponding to Fe oxides or to unidentified Fe-Pb arsenates）. SEM/EDS observations indicate the predominant affinity of Pb for Mn oxides, and to a lesser extent, for Fe oxides. Thus, a more suitable SEA procedure should be used for these mining-affected soils to distinguish between the contaminant fraction bound to Mn oxides and Fe oxides.     

Minerals controlling arsenic distribution in floodplain soils

As a consequence of intensive mining of the western Erzgebirge since medieval times, floodplain soils of the Mulde river contain large concentrations of arsenic (As) (>50 mg kg⁻¹). Arsenic in soil is often bound to poorly crystalline Fe and Mn (hydr)oxides, which may dissolve under reducing conditions. Part of the As may also exist in primary minerals, predominately sulphides, or in secondary minerals formed upon weathering. In order to better understand the impact of seasonal flooding, we surveyed As‐bearing mineral phases, especially of iron (Fe) (hydr)oxides. Because Fe (hydr)oxides are clay‐sized, soil samples were fractionated into six particle‐size fractions. The fractions were digested with aqua regia for determination of total element concentrations, extracted with hydroxylammonium chloride (NH₃OHCl; selective for Mn (hydr)oxides and NH₄ oxalate), and analysed by X‐ray diffraction and scanning electron microscopy. The largely similar distribution of As and lead (Pb) suggested the potential co‐existence of the two elements in primary or secondary mineral phases. However, neither As–Pb minerals nor any other As mineral were detected. Association with Mn oxides was negligible. The predominant As‐bearing phases were poorly crystalline Fe (hydr)oxides, which also incorporated large amounts of Pb and were affected by redox dynamics.     

Effect of steel-making slag as a soil amendment on arsenic uptake by radish (Raphanus sativa L.) in an upland soil

The steel-making slag (SMS), a by-product of steel manufacturing process with an alkaline pH (11–12) and high amount of iron (Fe) and calcium (Ca) oxides, was used to reduce arsenic (As) phytoextractability. The by-product was selected as an alternative to commercial Fe oxides, which can decrease plant uptake, but they are expensive if used as amendments of contaminated arable soils. SMS was applied at rates 0, 2, 4, and 8 Mg ha⁻¹ to an As (1 N HCl-extractable As 25 mg kg⁻¹) contaminated soil prepared by mixing non-contaminated soil and mine tailings and cropped to radish (Raphanus sativa L.) seeding. Calcium hydroxide (Ca(OH)₂), a common liming material in Korea, was applied at the same rates for comparison. Steel-making slag more effectively suppressed radish As uptake and increased yield than Ca(OH)₂ due to stronger As immobilization because it significantly increased extractable Fe concentration and decreased extractable As. The SMS-treated soil showed an apparent increase in As chemisorbed by Fe and Al oxides and hydroxides of surface soil, As associated at the Fe and Al oxides and hydroxides of internal surfaces of soil aggregates, and Ca-associated As. The steel-making slag can be a good soil amendment to suppress As phytoextractability and improve nutrient balance in As-contaminated soil.     

Nature of redox concentrations in a sequence of agriculturally developed acid sulfate soils in Thailand

Potential acid sulfate soils (PASS) are drained for agriculture, resulting in the formation of active acid sulfate soils (AASS), which gradually evolve into post-active acid sulfate soils (PAASS). Various redox concentrations (precipitates, costings, and mottles) occur in these soils as a result of pedogenic processes including biological activity and effects of land management. Although several studies have determined the mineralogy and geochemistry of ASS, the mineralogy and geochemistry of redox concentrations occurring in a sequence of ASS through PASS to PAASS have not been investigated. This study examined the mineralogy and geochemistry of redox concentrations and matrices within 5 PASS, 8 AASS, and 5 PAASS in Thailand. The labile minerals were predominantly controlled by oxidation status and management inputs. The unoxidized layers of PASS, AASS, and PAASS contained pyrite and mackinawite. The oxidation of Fe sulfides caused acidification and accumulation of yellow redox concentrations of jarosite and Fe (hydr)oxides at shallow depths. As the soils became well developed, they were recognized as PAASS, and the jarosite and goethite transformed to hematite. As ASS were drained, Co, Mn, Ni, and Zn moved downward and were associated with Fe sulfides and Mn oxides in the unoxided layer. Concentrations of As, Cu, Cr, Fe, and V did not change with depth because these elements became associated with jarosite and Fe (hydr)oxides in yellow and red redox concentrations, as well as the root zone, in the partly oxidized layer of AASS and PAASS. Arsenic was associated with pyrite under reducing conditions.     

An improved selective extraction method for Mn oxides and occluded metals with emphasis on applicability to Andisols

Abstract

It has been showed that Chao’s method [extraction with 0.1 mol L^?1 hydroxylamine hydrochloride (NH₂OH-HCl) at pH 2.0 for 30 min], which is commonly used to extract manganese (Mn) oxides and occluded heavy metals from soil samples, is not suitable for Andisols because of low solubility, and thus low extractability, of Mn oxides in such soils. Therefore, a new method is evaluated here, for extracting Mn oxides and occluded heavy metals from Andisols, Entisols and Inceptisols. The method has three steps: (1) reduction of Mn oxides with 0.01 mol L^?1 NH₂OH-HCl (pH 5.0) for 16 h, (2) recovery of re-adsorbed metals by short-time extraction with 0.5 mol L^?1 ammonium chloride in 0.02 mol L^?1 hydrochloric acid, and (3) washing with ultrapure water. This method achieves a higher rate of extraction of Mn oxides than does Chao’s method, especially from Andisol samples. Standard addition experiments showed that both the new method and Chao’s method can successfully extract released cadmium (Cd), cobalt (Co), nickel (Ni) and zinc (Zn) from Mn oxides with little re-adsorption. The selectivity of Mn oxide extraction by the new method, indicated by the rate of extraction of iron (Fe) oxides and the aluminum (Al)/Mn and silicon (Si)/Mn extraction ratios, is comparable to that of Chao’s method. Thus, the new method should be useful for extracting Mn oxides and occluded Cd, Co, Ni, and Zn from soil samples. Moreover, because the new method achieved nearly complete extraction of NH₂OH-HCl reactive Mn oxides even from Andisol samples, the method is more applicable to Andisol samples than Chao’s method.     

The metal — Metal interactions in biological systems Part III. Daphnia magna

The toxicity of single metal ions: Al, Co(II), Cr(III), Cu(II), Fe(III), Mg, Mn(II), Mo(VI), Ni(II), Se(VI), V(V) and Zn and the following pairs of them: Al-Co, Al-Mg, Al-Mo, Al-Se, Al-Zn, Cr-Co, Cr-Mg, Cr-Mo, Cr-Se, Cr-Zn, Cu-Co, Cu-Mg, Cu-Mo, Cu-Se, Cu-Zn, Fe-Co, Fe-Mg, Fe-Mo, Fe-Se, Fe-Zn, Mn-Co, Mn-Mg, Mn-Mo, Mn-Se, Mn-Zn, Ni-Co, Ni-Mg, Ni-Mo, Ni-Se, Ni-Zn, V-Co, V-Mg, V-Mo, V-Se, V-Zn, Zn-Co, Zn-Mg, Zn-Mo, and Zn-Se on Daphnia magna was examined. The most prominent antagonism in the toxicity was observed in the following ion pairs: Al-Mo(VI), Cr(III)-Co(II), Cr(III)-Mg, Cr(III)-Mo(VI), Cr(III)-Se(VI), Cr(III)-Zn, Fe(III)-Se(VI), Mn(II)-Mg, Mn(II-Se(VI), Zn-Mg and Zn-Mo(VI). The strong synergism was found for the following ion systems: Cr(III)-Se(VI), Cr(III)-Zn, Fe(III)-Se(VI), Mn(II)-Zn, Mn(II)-Se(VI), Ni(II)-Co(II), Ni(II)-Mo(VI), Ni(II)-Se(VI), Ni(II)-Zn, V(V)-Co(II), V(V)-Mo(VI), V(V)- Se(VI), and V(V)-Zn. Synergism and antagonism in toxicity were dependent on water hardness as well as on the ion concentration. Adaptive procesess of the animals to the toxic environment could also be observed. Thus, the toxicity of the single ions and their pairs was not linear with respect to time.     

Iron compounds and oil biodegradation in overmoistened contaminated soils: A review of publications

In Russia, the areas of oil pollution gradually shift toward the north into the zone of increased moistening with widespread hydromorphic soils. In this zone, the role of the rapid aerobic degradation of hydrocarbons decreases, while that of slow anaerobic degradation increases. The biological reduction of Fe(III) only proceeds at the expense of the energy of oxidation of organic substances, including oil hydrocarbons, in the oil-polluted soils. This favors the development of technogenic gleying. In contrast to the uncontaminated background soils, in which gleying is correctly considered a degradation process, the same process in the oil-contaminated mineral soils plays a positive role, because it is accompanied by the oxidation of organic pollutants, which otherwise penetrate into rivers and lakes with water flows. The role of Fe(III) reduction may be significant: at one of the oil-spill sites, one-third of the organic pollutants degraded within twelve years after an accident in the anaerobic zone due to Fe(III) reduction. Both iron hydroxides and clay minerals enriched in Fe(III) participate in the reduction processes. In the anaerobic zone, the destruction of organic pollutants begins several years after the relevant natural microorganisms become active. The reduction of Fe(III) reaches its maximum faster than the process of methanogenesis. Upon the soil’s cooling in the winter, the reduction of Fe(III) is replaced by the spontaneous formation of iron oxides (oxidogenesis). Thus, alternating reduction ↔ oxidation reactions proceed in the soils with a contrasting temperature regime. Iron oxides formed in the winter are reduced to Fe(II) in the summer and, thus, resume the associated oxidation of organic pollutants upon the stagnant moisture regime. Therefore, upon monitoring of hydromorphic oilcontaminated soils, special attention should be paid to the forms of iron compounds.     

Extractability of manganese and iron oxides in typical Japanese soils by 0.5 mol L−1 hydroxylamine hydrochloride (pH 1.5)

We investigated the extractability of manganese (Mn) and iron (Fe) oxides from typical Japanese soils (Entisols, Inceptisols, and Andisols) by 0.5?mol?L^?1 hydroxylamine hydrochloride (NH₂OH-HCl) extraction (pH 1.5; 16?h shaking at 25°C; soil:solution ratio 1:40), referred as to HH_mBCR, which is Step 2 (used for the reducible fraction) of the modified BCR (Community Bureau of Reference) sequential extraction procedure. The HH_mBCR procedure extracted almost all Mn oxides from the non-Andisol samples, but failed to extract a part of the Mn oxides from some Andisol samples. The procedure extracted most short-range ordered Fe oxides from non-Andisol samples, but it extracted only 7.5% and 13% of the short-range ordered Fe oxides from allophanic and non-allophanic Andisol samples, respectively. This remarkably low extractability of Fe oxides suggests that the HH_mBCR method is not suitable for extracting oxide-occluded heavy metals from Andisols. Since the extraction rate of short-range ordered Fe oxides from various soils with the extractant was negatively correlated with the amounts of oxalate- and pyrophosphate-extractable Al even when the variability of the extraction pH was reduced by increasing the soil:solution ratio from 1:40 to 1:500, the extractability of Fe oxides would be negatively affected by the presence of active Al, including allophane/imogolite, amorphous Al, and Al-humus complexes. Because these Al constituents are abundant in Andisols, they would be at least partially responsible for the lower extractability of Fe oxides by HH_mBCR from Andisols.     

Chromium and arsenic in contaminated soils (Review of publications)

In the last decades, the chromium clarke in the world’s soils has been revised and reduced; at present, it is equal to 70 mg/kg. No maximal permissible concentration is accepted for the total chromium content in the soils of Russia; it appears reasonable to use the Western European and North American standards in Russia and to take the average value of the maximal permissible concentration equal to 200 mg Cr/kg. Chromium toxicity depends on its oxidizing status. The hazardous effect decreases with the reduction of Cr(VI) to Cr(III). There are various chemical reducers of Cr(VI), including sulfides, dissolved organic substance, aqueous Fe(II) and minerals enriched in Fe(II), and Fe(0). As-containing ore tailings represent a powerful source of technogenic arsenic. Significant environment contamination with natural As is registered in a number of Asian countries. The maximal permissible concentration of total arsenic is equal to 2 mg/kg in Russian soils; it is probably underestimated, because it is lower than the As clarke in soil (5 mg/kg). The approximately permissible concentration (APC) values for As look more reasonable. Arsenic toxicity depends on its oxidation degree: As(III) is 2–3 times more toxic than As(V).     

Arsenic from Groundwater to Paddy Fields in Bangladesh: Solid–Liquid Partition, Sorption and Mobility

The arsenic contamination of Bangladesh groundwater involves heavy arsenic inputs to irrigated rice fields. Beside adsorption on soil colloids, iron–arsenic co-precipitation phenomena can affect arsenic retention in soils. In paddy fields of Satkhira District, Bangladesh, the study of the arsenic and iron forms in the irrigation waters and in soils at different times and distances from the irrigation well evidenced that a higher Fe/As ratio in the well water was related to a faster oxidation of Fe(II) and As(III) in water and to a close Fe–As association in soils, together with a greater accumulation of arsenic and poorly ordered iron oxides. The concentration of arsenic and of labile iron forms decreased with the distance from the well and with the depth, as well as the reversibility of arsenic binding. The fate of the arsenic added to the soils by irrigation hence resulted strongly influenced by iron–arsenic co-precipitation, depending on the Fe/As ratio in water. Irrigation systems favouring the sedimentation of the Fe–As flocks could help in protecting the rice from the adverse effects of dissolved arsenic.