Dithionite-citrate-bicarbonate Research Articles

Phosphorus Burial in Sediments Along the Salinity Gradient of the Patuxent River, a Subestuary of the Chesapeake Bay (USA)

We used a sequential extraction technique and 210Pb dating to determine the chemical form and amount of particulate phosphorus (PP) that is retained during burial in 1-m-long sediment cores collected along a salinity gradient from tidal freshwater to the mesohaline waters of the Patuxent River, a subestuary of the Chesapeake Bay. PP buried in the study sites with salinity values ≤3 was similar in concentration and form to PP entering the Patuxent from the watershed, suggesting efficient sequestration by the sediments at these low-salinity sites. PP extracted with citrate–dithionite–bicarbonate was the dominant form of PP at all salinities and all depths, and organic-P was the second most abundant fraction. We estimated that 81% of PP entering from the watershed is trapped in the sediments of the upper Patuxent subestuary and that the subtidal sediments retain three times as much PP as the marshes adjacent to the study sites.

Nanoparticles in natural systems I: The effective reactive surface area of the natural oxide fraction in field samples

Information on the particle size and reactive surface area of natural samples is essential for the application of surface complexation models (SCM) to predict bioavailability, toxicity, and transport of elements in the natural environment. In addition, this information will be of great help to enlighten views on the formation, stability, and structure of nanoparticle associations of natural organic matter (NOM) and natural oxide particles. Phosphate is proposed as a natively present probe ion to derive the effective reactive surface area of natural samples. In the suggested method, natural samples are equilibrated (⩾10 days) with 0.5 M NaHCO 3 (pH = 8.5) at various solid–solution ratios. This matrix fixes the pH and ionic strength, suppresses the influence of Ca 2+ and Mg 2+ ions by precipitation these in solid carbonates, and removes NOM due to the addition of activated carbon in excess, collectively leading to the dominance of the PO 4–CO 3 interaction in the system. The data have been interpreted with the charge distribution (CD) model, calibrated for goethite, and the analysis results in an effective reactive surface area (SA) and a reversibly bound phosphate loading Γ for a series of top soils. The oxidic SA varies between about 3–30 m 2/g sample for a large series of representative agricultural top soils. Scaling of our data to the total iron and aluminum oxide content (dithionite–citrate–bicarbonate extractable), results in the specific surface area between about 200–1200 m 2/g oxide for most soils, i.e. the oxide particles are nano-sized with an equivalent diameter in the order of ∼1–10 nm if considered as non-porous spheres. For the top soils, the effective surface area and the soil organic carbon fraction are strongly correlated. The oxide particles are embedded in a matrix of organic carbon (OC), equivalent to ∼1.4 ± 0.2 mg OC/m 2 oxide for many soils of the collection, forming a NOM–mineral nanoparticle association with an average NOM volume fraction of ∼80%. The average mass density of such a NOM–mineral association is ∼1700 ± 100 kg/m 3 (i.e. high-density NOM). The amount of reversibly bound phosphate is rather close to the amount of phosphate that is extractable with oxalate. The phosphate loading varies remarkably ( Γ ≈ 1–3 μmol/m 2 oxide) in the samples. As discussed in part II of this paper series ( Hiemstra et al., 2010), the phosphate loading ( Γ) of field samples is suppressed by surface complexation of NOM, where hydrophilic, fulvic, and humic acids act as a competitor for (an)ions via site competition and electrostatic interaction.

Quantification of the contribution of pedogenic magnetic minerals to magnetic susceptibility of loess and paleosols on Chinese Loess Plateau: Paleoclimatic implications

This study investigates the grain size–dependent properties of magnetic minerals in Chinese loess samples with an aim to better understand the contribution of pedogenesis to magnetic susceptibility enhancement of the paleosols. Samples of loess and paleosol were divided into two fractions, magnetic extracts and residues, by extraction in a weak magnetic field. Both the raw samples of loess and paleosol (i.e., before magnetic extraction) and the samples of residues after magnetic extraction were then separated into five grain size fractions by gravitational settling and centrifugation. Low‐field magnetic susceptibility (MS) and the frequency‐dependent MS of these samples were measured, and samples treated with citrate‐bicarbonate‐dithionite (CBD) to remove the iron oxides were also analyzed. For loess samples, coarse grains (≥2μm) contribute more than 50% of the magnetic signal. The ≤0.3 μm fraction, considered to be of pedogenic origin, contributes 22% (before magnetic extraction) or 36% (after magnetic extraction) of the magnetic signal in loess. A different pattern is found in the paleosol samples. For these samples, the ≤0.3 μm fraction contributes >60% (before magnetic extraction) or 71% (after extraction) to the bulk MS. In addition, 91% of the magnetic signal in paleosol S5 is CBD extractable. The CBD technique is able to extract pedogenic superfine‐grained magnetic particles and coarse dust magnetic particles. These results confirm that pedogenesis leads to enhanced MS and helps to quantify the multiple origins of the bulk magnetic material that defines loess and paleosol MS. This approach provides a reliable estimate of the contribution of pedogenic magnetic materials to MS. Our study indicates that multiple methods should be employed in analyzing paleoclimatic changes recorded by MS in the Chinese loess‐paleosol sequences.

Métodos de concentração de minerais 2:1 secundários na fração argila visando sua identificação por difratometria de raios x

Além dos baixos teores normalmente encontrados na fração argila dos solos sob clima tropical e subtropical, o tamanho reduzido e a baixa cristalinidade dos minerais 2:1 secundários dificultam sua identificação por difratometria de raios X (DRX). Este estudo objetivou avaliar métodos químicos e físico de concentração de minerais 2:1 secundários na fração argila para facilitar a identificação por DRX, incluindo a natureza dos minerais quanto ao local de formação de cargas permanentes (lâmina tetraedral ou octaedral). Coletaram-se amostras de dois Cambissolos originados de argilito da Formação Guabirotuba na Bacia Sedimentar de Curitiba (PR): horizontes A, Bi, C1 (1,2 a 1,5 m), C2 (2,2 a 2,5 m), C3 (3,2 a 3,5 m) e C4 (4,2 a 4,5 m). Após remoção da matéria orgânica e dispersão da terra fina seca ao ar, a fração argila foi submetida a tratamentos sequenciais com ditionito-citratobicarbonato (DCB) (amostra desferrificada - remoção de óxidos de Fe pedogenéticos) e com soluções de NaOH a quente, em diferentes concentrações (0,5; 1,0; 1,5; 2,5; 3,5; 4,0; 4,5 e 5,0 mol L-1), para extração de gibbsita e caulinita, em diferentes graus. A fração argila desferrificada também foi submetida à separação física (centrifugação) em argila grossa (0,2 a 2 m) e fina (< 0,2 m). Foram realizados tratamentos auxiliares para identificar as espécies minerais 2:1 na fração argila: saturação com Mg e solvatação com etilenoglicol; saturação com K e secagem ao ar e aquecimento a 550 ºC; e saturação com Li (teste de Greene-Kelly). Os resultados mostraram que o método clássico de extração da caulinita, com solução de NaOH 5,0 mol L-1 a quente, não deve ser aplicado para concentração de minerais 2:1 secundários, pois também removeu grande parte desses minerais. O tratamento com DCB e com solução de NaOH 3,5 mol L-1 possibilitou, com maior eficiência, a concentração e identificação de minerais 2:1 secundários por DRX nas amostras dos horizontes A, Bi e C1. Nas amostras tomadas em maiores profundidades (horizontes C2, C3 e C4), devido aos maiores teores desses minerais e ao menor tamanho dos cristais (argila fina), a solução menos concentrada de NaOH (1,5 mol L-1) foi mais eficiente para esse propósito. No horizonte A, os minerais 2:1 concentraram-se na fração argila grossa, compatível com o maior grau de intemperismo desse horizonte. Identificou-se esmectita com hidroxi-Al entrecamadas nos horizontes mais superficiais (A e Bi) e esmectita nas amostras do horizonte C. A saturação com Li permitiu a identificação das esmectitas dioctaedrais montmorilonita e beidelita/nontronita. As adaptações ao métodopadrão (NaOH 5 mol L-1) favoreceram a concentração de minerais 2:1 secundários na fração argila dos solos; a concentração da solução de NaOH deve ser maior para horizontes com menor teor do mineral.

Influence of the aging of Fe oxides on the decline of magnetic susceptibility of the Tertiary red clay in the Chinese Loess Plateau

Magnetic properties, free and active Fe oxides, grain size distribution and mineral assemblage of a Quaternary loess–Tertiary red clay (TRC) section in Lingtai County in the Chinese Loess Plateau were studied. The results suggest that the TRC of the Lingtai section shares similar aeolian characteristics with the overlying Quaternary loess-paleosol sequence (QLPS), but the former is generally more intensively weathered than the latter, as indicated by its higher citrate–bicarbonate–dithionite (CBD) extractable Fe (Fe d), finer grain size and lower content of easily weathered primary minerals. However, magnetic susceptibility ( χ lf) and magnetic remanence ( χ arm, SIRM and SOFT etc.) of the main part of the TRC are significantly lower, implying the decline of ferrimagnetic minerals (FM). In contrast, hematite and goethite in the TRC is significantly enhanced, as indicated by its higher HIRM. The acid ammonium oxalate (AAO) extractable Fe (Fe o) of the whole section sharply decreases with increasing age. Fe o and Fe o/Fe d values of the section are significantly correlated with χ lf. Especially in the TRC, the low values of Fe o and Fe o/Fe d correspond to weak χ lf. This suggests a significant influence of the age-related transformation of Fe oxides on the decline of FM in the TRC. Lower FM and higher hematite and/or goethite in the TRC further suggest the transformation of FM into hematite with the aging of Fe oxides during post-depositional processes, though more evidence is needed to understand the processes involved. χ lf of the TRC in the Chinese Loess Plateau mostly cannot be regarded as a promising paleoclimatic proxy because its weak magnetism does not correlate to its strong pedogenesis.

Characterization and Quantification of Iron Oxides Occurring in Low Concentration in Soils

Iron (Fe) oxides control plant nutrient and contaminant redistribution in soils, but their phase identification and quantification is complicated by their low concentration and poor crystallinity and by the high detection limits of the current techniques. Cyclic voltammetry using a carbon paste electroactive electrode was tested to characterize soil iron oxides. Twenty‐eight clays (<2 µm) from eight soils varying in parent material and pedological development were separated, and Fe phases were determined by x‐ray diffraction (XRD) before and after preconcentration treatment with 5M sodium hydroxide (NaOH). Morphology of Fe phases was determined by electron microscopy, and total extractable Fe phases in soil were determined by citrate bicarbonate dithionite. The preconcentration treatment was effective in phase identification. Goethite and hematite were found, depending upon origin of the soil clays. The samples with less than 2% hematite needed preconcentration. Three percent goethite in clays was on the edge of the XRD limit. The electrochemical signals of re‐oxidation of Fe2+ released from reductive dissolution of free Fe oxides was quantitatively related to their content in the sample. Voltammetry effectively quantified goethite and hematite or both in a mixture with detection limit as low as 0.025 mg 100−1 mg in clay. The technique can be used for quick estimate of low content of Fe oxides in clays and can be adapted for free Fe estimation in soils.

A method to concentrate boehmite in bauxite by dissolution of gibbsite and iron oxides

Some bauxites contain boehmite which is not dissolved by lower temperature Bayer extraction and which may cause precipitation of additional boehmite and an associated reduced recovery of alumina. For the mineralogical and morphological characterisation of boehmite in bauxite, it is necessary to concentrate boehmite by dissolving gibbsite and iron oxides while preserving boehmite intact. There is no standard method to concentrate boehmite from bauxite without affecting the properties of boehmite. Therefore we developed a laboratory method to concentrate the boehmite in diverse bauxites using “one standard procedure”. Five molar NaOH containing dissolved Al(III) was used to dissolve gibbsite and kaolin in bauxite by shaking at 80 °C for 48 h. A mass balance calculation and mineralogical analysis by XRD showed that gibbsite and kaolin were completely dissolved and boehmite and some other minerals (anatase, quartz) were unaffected. Boehmite was further concentrated by removing iron oxides from the caustic digest residues by the reduction–complexation dithionite–citrate–bicarbonate method. Sand size quartz was removed by sedimentation. After these pre-treatments boehmite was sufficiently concentrated for detailed analysis by XRD and electron microscopy. Results for bauxites from Guinea, Jamaica, Queensland (Australia) and Western Australia are presented to illustrate the effectiveness of this procedure.

MINERALOGIA E TEORES DE CROMO, NÍQUEL, COBRE, ZINCO e CHUMBO NOS SOLOS NO ENTORNO DO ATERRO SANITÁRIO DA CAXIMBA EM CURITIBA-PR

Sete perfis de solos do entorno do Aterro Sanitário da Caximba em Curitiba-PR, foram coletados para caracterização e quantificação dos minerais da fração argila e determinação dos teores totais e trocáveis de Cr, Ni, Cu, Zn e Pb. A fração argila foi estudada por difratometria de raios-X, análise térmica e extrações químicas com oxalato de amônio, ditionito-citrato-bicarbonato de sódio. Os teores totais e trocáveis dos metais pesados foram determinados nas amostras de solo por espectroscopia de absorção atômica, após digestão ácida (H2SO4, HF, HNO3 e HClO4 concentrados) e extração com solução de BaCl2 1 mol dm-3, respectivamente. O predomínio de caulinita, com teores variando de 548,5 a 702,1 g kg-1, e a ocorrência de vermiculita com hidroxi entre camadas na fração argila determinaram os relativamente altos valores de CTC dos solos. Os teores de óxidos de Fe foram baixos, evidenciando a natureza do material de origem (granito/ganisse e sedimentos argilo-siltosos) e as condições de redução e remoção de Fe para os solos sob influência de hidromorfismo. De maneira geral, verificou-se incremento nos teores de metais pesados nos solos do entorno do aterro sanitário. Contudo, o nível de contaminação não foi muito expressivo, relacionado às baixas concentrações de Cr, Ni, Cu, Zn e Pb no chorume; facilidade de lixiviação nos solos hidromórficos e; altos teores de cátions divalentes e trivalentes (Ca2+, Mg2+ e Al3+) no complexo sortivo dos solos.

Reversibilidade do fósforo não-lábil em solos submetidos à redução microbiana e química: I - alterações químicas e mineralógicas

Alterações no potencial redox (Eh) de solos, como ocorrem em solos hidromórficos, causam alterações significativas nas características químicas e mineralógicas dos solos e têm forte influência sobre o comportamento do P, alterando a relação entre suas formas lábil e não-lábil. O objetivo deste trabalho foi verificar o efeito de condições de baixo Eh na dissolução/reversibilidade de formas não-lábeis de P e solubilização/comportamento dos oxihidróxidos de Fe e Al. Para isso, amostras de 11 solos foram incubadas com 300 mg kg-1 de P por 30 dias, com posterior exaustão do P-lábil por extrações sucessivas com resina de troca aniônica (RTA). Posteriormente, elas foram submetidas a um tratamento denominado redução microbiana com 0,1 mol L-1 de sacarose, por 60 dias, com trocas desta solução a cada 15 dias. Os resultados obtidos foram comparados com outros quando a mesma dose de P foi aplicada após essa redução microbiana, seguida por extrações sucessivas do P-lábil com RTA. Mediram-se, ainda, em ambas as condições, na solução de sacarose em equilíbrio com os solos, os valores de Eh e pH, teores de Fe, Al e P, ao final de cada período de 15 dias, por ocasião da troca da solução. As amostras originais - não tratadas - dos solos foram, também, submetidas à complexação/redução com oxalato de amônio (Ox) ou com citrato-ditionito-bicarbonato de sódio (CDB). Foram feitas quatro extrações sucessivas com Ox e duas como CDB, determinando-se, após cada extração, em ambos os métodos, os valores de Eh, pH e os teores de Fe, Al e P. Posteriormente, as amostras residuais dos solos submetidos aos tratamentos com Ox e CDB receberam 300 mg kg-1 de P. A diminuição dos valores de Eh pela adição de sacarose ou pelas extrações com Ox e CDB não favoreceu a reversibilidade de formas não-lábeis de P. A adição de P aos solos aparentemente promoveu maior estabilidade para a goethita, restringindo sua interferência no valor de Eh do solo, como, também, estabilizou a gibbsita, que passou a não interferir no pH do solo. Os resultados permitem aventar a hipótese de que o P aplicado proporcionaria efeito restritivo ao comportamento de oxihidróxidos.

Reversibilidade de fósforo não-lábil em solos submetidos à redução microbiana e química: II - extrações sucessivas do fósforo pela resina de troca aniônica

A formação de P não-lábil a partir de formas lábeis, adsorvidas aos oxihidróxidos de Fe e Al, é razão para o baixo efeito residual da fertilização fosfatada em solos tropicais. A reversibilidade do P não-lábil para formas lábeis nesses solos pode ser favorecida pela redução do Fe3+ e, ou, pela diminuição da atividade dos oxihidróxidos de Fe e Al. Neste trabalho, objetivou-se avaliar a adsorção e dessorção de P, sua transformação em não-lábil e reversibilidade dessa forma em amostras de solos submetidas à redução microbiana ou química. Para isso, amostras de 11 solos foram homogeneizadas com 300 mg kg-1 de P na forma de NaH2PO4 em solução, incubadas por 30 dias e, então, submetidas a dez extrações sucessivas de P com resina de troca aniônica (RTA) (tratamento sem redução). Em seguida, outras amostras dos solos foram submetidas à ambiente redutor em solução de sacarose 0,1 mol L-1 com posterior aplicação da doses de P (300 mg kg-1) e as dez extrações sucessivas de P (tratamento redução microbiana). Depois, outras amostras foram reduzidas/complexadas com oxalato de amônio (Ox) ou com citrato-ditionito-bicarbonato (CDB), e os solos/resíduos receberam a mesma dose de P dos experimentos anteriores. Foram então incubados por 30 dias e submetidos às extrações sucessivas com RTA (tratamento redução química). A capacidade máxima de adsorção de P (CMAP) dos solos mostrou-se mais dependente da goethita (com 70,8 % de contribuição para seu valor) do que da gibbsita (com contribuição de 29,2 %). A correlação negativa entre os teores de P obtidos na segunda extração com a RTA, nos solos em condição natural, sem redução, e os teores de gibbsita sugere que este, e não a goethita, é o oxihidróxido responsável pela maior restrição à dessorção do P. Os valores de CMAP, estimados por meio do P remanescente (P-rem), mostraram, nas amostras submetidas ao tratamento com redução microbiana, pequena alteração para o grupo dos solos menos oxídicos, com menor CMAP. Todavia, nos solos mais oxídicos, com maior CMAP, o efeito prévio da sacarose foi o aumenta do P-rem (diminuição da CMAP) 10 vezes em relação aos solos para o tratamento sem redução. Entretanto, a redução gerada pela sacarose não alterou a dessorção do P anteriormente adsorvido. A expectativa de que ocorreria significativa reversibilidade de P não-lábil com a redução microbiana ou química dos solos não se concretizou, demonstrando a grande estabilidade dessas formas.

The effects of iron plaque and phosphorus on yield and arsenic accumulation in rice

The effects of arsenate, Fe2+, and phosphate on amount and composition of Fe-oxide plaque at the rice-root surface and on the yield and arsenic accumulation in rice (cv. BRRI dhan33) were studied in a replicated pot-culture experiment. Arsenic in the form of Na2HAsO4 was applied at concentrations of 0, 15 and 30 mg kg−1 in combination with P and/or Fe at 0 and 50 mg kg−1, from KH2PO4 and FeSO4, respectively. Root, grain and straw yields and their As, Fe and P concentrations were determined. The Fe-oxide plaque was extracted from the plant roots using dithionite-citrate-bicarbonate (DCB) and NH4-oxalate extractions. The addition of Fe2+ reduced the toxic effect of As in flooded-rice culture and resulted in reduced grain-As accumulation and increased grain yields. The effect of applied phosphate was the opposite, in that it resulted in higher As concentrations in both grain and straw and lower grain yields. The effects of both Fe and P can be explained based on their impacts on adsorption of As onto soil and rice-plaque Fe-oxides and the subsequent As solubility and availability for uptake by rice. These reactions have important implications to rice-crop management and the natural variability in soils and irrigation-water characteristics that might impact As uptake by rice.

The speciation of iron in desert dust collected in Gran Canaria (Canary Islands): Combined chemical, magnetic and optical analysis

Atmospheric dust collected on filters at a coastal site in Gran Canaria has been analysed by a combination of chemical, magnetic and optical methods with the aim of determining the iron speciation. The fraction of total iron as particulate (oxyhydr)oxides, determined by the citrate–bicarbonate–dithionite method, was 0.39 ± 0.11 (mean ± s.d.); the fraction of (oxyhydr)oxide iron in ferrimagnetic form, through analysis of the saturation magnetisation, was 0.053 ± 0.038 (mean ± s.d.); and the fraction of haematite iron with respect to the iron in haematite + goethite form, by diffuse reflectance spectroscopy measurements, was 0.47 ± 0.12 (mean ± s.d.). Consistent with these findings, low temperature in-phase and out-of-phase AC susceptibility measurements reveal also the presence of paramagnetic iron, most likely in silicates with ionic substitution, and indicate that, while magnetite or haematite particles may be present in the dust, their particle size should be very small, as the typical magnetic transitions characteristic of large crystals of these oxides are practically impossible to detect. The comparison of the Fe/Al elemental ratios with typical crustal values indicates that the great majority of captured dust iron has a non-anthropogenic origin. Although no significant correlations have been found between the analysed dust properties and the dust provenance, the obtained magnetic data corresponding to the dust collected at this site may be useful, as a middle step, in future magnetic monitoring studies of the iron biogeochemical cycle.

Removal of Hydrogen Sulfur from Coal-Derived Gas by Iron Oxides in Various Oxisols

Five oxisols sampled from Taiwan were applied to remove sulfur species from coal-derived gas. Sorption of H2S was conducted by loading oxisols into a fixed–bed reactor under a temperature of 500°C. All of the oxisols had some sorption ability under these experimental conditions. The sulfur sorption capacities of the five oxisols decreased in the following trend: Tungwei > Shouka > Tamshui > Loupi > Pingchen. The difference of sulfur capacity for the tested oxisols was attributed to the dithionite–citrate–bicarbonate (DCB) extractable iron. For all oxisols, the sulfur sorption capacity derived from breakthrough time experiments is in line with those measured by elemental analysis. Sulfur recovery of the five oxisols ranges from 93.4 to 103.1%. Sulfur sorption capacity of oxisols attributed to DCB extractable iron oxides is close to that obtained by calculation (breakthrough time) and elemental analysis. It is suggested that the major mechanism of sulfur uptake by oxisol is attributed to the reaction between DCB extractable iron oxides and H2S. Higher CO concentration has a positive effect on the sorption of H2S, whereas H2 appears to have a negative effect. The water-shift reaction can explain this result.

VARIABILIDADE DAS CARACTERÍSTICAS QUÍMICAS E MINERALÓGICAS DE SOLOS DA REGIÃO METROPOLITANA DE CURITIBA (PR)

O propósito deste trabalho foi estudar as variações nos teores totais de elementos e atributos mineralógicos do solo em curtas distâncias horizontais de amostras (repetições) dos horizontes A e B em cinco áreas na Região Metropolitana de Curitiba. A fração silte+argila foi estudada por difratometria de raios-X, análise termodiferencial, análise termogravimétrica e análises químicas. Os teores totais de Fe, Al, Mg, Zn, Mn, Cu, Ni, Pb e Cr foram determinados após a digestão ácida das amostras (HF, HNO3, H2SO4 e HClO4). Os teores de Fe e Al foram determinados após extração das amostras com oxalato de amônio (OA), ditionito-citrato-bicarbonato (DCB) e solução de NaOH 0,5 mol L-1. Mesmo sendo mais estáveis que os teores trocáveis de nutrientes, verificou-se variações nos atributos mineralógicos entre as repetições coletadas em pontos próximos em uma mesma área, principalmente, para solos originados de rochas sedimentares. Os valores dos coeficientes de variação (CV) foram inversamente proporcionais à estabilidade da característica analisada: teores totais de elementos e teores de minerais de alta cristalinidade apresentaram menores CV; comportamento oposto foi verificado para os teores de óxidos de Fe e Al de baixa cristalinidade. As características cristalográficas da caulinita e gibbsita foram homogêneas.

A mosaic of nonallophanic Andosols, Umbrisols and Cambisols on rhyodacite in the southern Brazilian highlands

Soils in the highlands of north-eastern Rio Grande do Sul (Brazil) show dark-coloured topsoils which are light and very rich in organic matter. They are distributed in a mosaic of grassland, forest with Araucaria angustifolia and several anthropogenic vegetation forms. The climate is humid and temperate without marked dry periods. We characterized and classified 27 soils to investigate the relationships between present and past vegetation pattern and soil properties. All soils are strongly acidic with high clay contents, low bulk densities and most of them show high phosphate retentions. X-ray diffraction indicates the presence of quartz, kaolinite, secondary chlorite and gibbsite. Selective extractions with pyrophosphate (p), dithionite–citrate–bicarbonate (d) and acid oxalate (o) give in most horizons the following relationships: Fe d > Fe p > Fe o and Al p > Al d > Al o. Despite the possible Al release from gibbsite, we attribute the high Al p concentrations mainly to high amounts of Al–humus complexes. The absence of allophane and imogolite was confirmed by the very low Si o and Si d concentrations. 21 soils have andic properties according to World Reference Base for Soil Resources (WRB) [IUSS Working Group WRB, 2006. World Reference Base for Soil Resources 2006. World Soil Resources Reports, 103. FAO, Rome.] at least for some depth range, 15 of them fulfil the minimum thickness and depth requirements of Andosols. They belong to the aluandic type and are the first detected Andosols in South America outside the areas of recent volcanism in the Andes. The other soils are Umbrisols (11) and Cambisols (1). In grassland soils, andic properties start at or near the soil surface, in forest soils they are only present in subsurface layers, especially in buried A horizons. The latter differ from their actual topsoils by some characteristics which they have in common with the grassland soils like deep black colours and low melanic indexes. It is known from pollen analyses that the Araucaria forests are invading the grasslands since about 1000 years. In this environment a grass vegetation seems to favour formation and permanence of Andosols whereas a change into the forest starts a process of losing andic properties. Most soils without andic properties or with andic properties of insufficient thickness only fail the required minimum concentrations of Al o and Fe o. On the other hand, they all show similar concentrations of Al d and Fe d. Therefore we conclude that the loss of andic properties with forest proliferation is caused by crystallization of Al and Fe oxides which seems to be easier under forest.

Influence of iron plaque on uptake and accumulation of Cd by rice ( Oryza sativa L.) seedlings grown in soil

Iron plaque is ubiquitously formed on the root surfaces of rice. However, little is known about the role of iron plaque in Cd movement from soil to the plant aboveground parts. A pot experiment was conducted to investigate the influence of iron plaque in Cd uptake and accumulation by rice seedlings in soil. Rice seedlings were pre-cultivated in solution culture for 16 days. Two seedlings were transplanted in a nylon bag containing no substrate but surrounded by soil amended with Fe and Cd combined at rates of 0, 1, or 2 g Fe kg − 1 and 0, 2.0, or 10 mg Cd kg − 1 soil. Fe was added to induce different amounts of iron plaque, and Cd to simulate Cd-polluted soils. Plants were grown for a further 43 days and then harvested. The length of the longest leaf and SPAD values of the newly mature leaves were measured during plant growth. Fe and Cd concentrations were determined in dithionite–citrate–bicarbonate (DCB) soil extracts and in plant roots and shoots. Shoot and root dry weights were significantly affected by Fe supply level but not by added Cd. Root dry weight declined with increasing Fe supply but shoot dry weight decreased at 2 g Fe kg − 1 and increased at 1 g Fe kg − 1 (except at 2 mg Cd kg − 1 ). The length of the longest leaf and SPAD values of the newly mature leaves were significantly affected by plant growth stage and added Fe and Cd. Fe tended to diminish the negative effect of Cd on these two parameters. Cd concentrations in DCB extracts increased with increasing Cd and Fe supply. In contrast, external Fe supply markedly reduced shoot and root Cd concentrations and there was generally no significant difference between the two Fe supply levels. Shoot and root Cd concentrations increased with increasing Cd addition. Root Cd concentrations were negatively correlated with root Fe concentrations. The proportion of Cd in DCB extracts was significantly lower than in roots or shoots. The results indicate that enhanced Fe uptake by plants can diminish the negative effects of Cd to some extent and that iron plaque on root surfaces is of little significance in affecting uptake and accumulation of Cd by rice plants.

Heterogeneous reduction of Tc(VII) by Fe(II) at the solid–water interface

Experiments were performed herein to investigate the rates and products of heterogeneous reduction of Tc(VII) by Fe(II) adsorbed to hematite and goethite, and by Fe(II) associated with a dithionite–citrate–bicarbonate (DCB) reduced natural phyllosilicate mixture [structural, ion-exchangeable, and edge-complexed Fe(II)] containing vermiculite, illite, and muscovite. The heterogeneous reduction of Tc(VII) by Fe(II) adsorbed to the Fe(III) oxides increased with increasing pH and was coincident with a second event of Fe 2 + (aq) adsorption. The reaction was almost instantaneous above pH 7. In contrast, the reduction rates of Tc(VII) by DCB-reduced phyllosilicates were not sensitive to pH or to added Fe 2 + (aq) that adsorbed to the clay. The reduction kinetics were orders of magnitude slower than observed for the Fe(III) oxides, and appeared to be controlled by structural Fe(II). The following affinity series for heterogeneous Tc(VII) reduction by Fe(II) was suggested by the experimental results: aqueous Fe(II) ∼ adsorbed Fe(II) in phyllosilicates [ion-exchangeable and some edge-complexed Fe(II)] ≪ structural Fe(II) in phyllosilicates ≪ Fe(II) adsorbed on Fe(III) oxides. Tc-EXAFS spectroscopy revealed that the reduction products were virtually identical on hematite and goethite that were comprised primarily of sorbed octahedral TcO 2 monomers and dimers with significant Fe(III) in the second coordination shell. The nature of heterogeneous Fe(III) resulting from the redox reaction was ambiguous as probed by Tc-EXAFS spectroscopy, although Mössbauer spectroscopy applied to an experiment with 56Fe-goethite with adsorbed 57Fe(II) implied that redox product Fe(III) was goethite-like. The Tc(IV) reduction product formed on the DCB-reduced phyllosilicates was different from the Fe(III) oxides, and was more similar to Tc(IV) oxyhydroxide in its second coordination shell. The heterogeneous reduction of Tc(VII) to less soluble forms by Fe(III) oxide-adsorbed Fe(II) and structural Fe(II) in phyllosilicates may be an important geochemical process that will proceed at very different rates and that will yield different surface species depending on subsurface pH and mineralogy.

Uranium Deposition in a Weathered Fractured Saprolite/Shale

Chemical analysis and scanning electron microscopy (SEM) microanalysis were carried out on cores of contaminated geological material collected around four closed waste disposal ponds to examine the extent of nitric acid extractable U (U(NA)) association with P, S, and extractable Fe, Al, and Mn oxides within deeply weathered fractured shale. The solid phase in many regimes on the site has been exposed to highly buffered acidic (< 3.5) groundwater and has been aggressively weathered. Higher correlations occur between U(NA) and total P and S (r2 = 0.76, 0.69, respectively), citrate bicarbonate dithionite extractable Fe (Fed) and Al (Ald) (r2 = 0.87, 0.80, respectively), and acid oxalate extractable or amorphous/poorly crystalline Fe (Feo) (r2 = 0.63) in core material from a field plot known as Area 1 compared to core material from another field plot known as Area 3. In Area 3 core material, linear regression analysis of U(NA) and total P and S, and Fed, Ald and Feo gave r2 values of 0.67, 0.4, 0.06, 0.24, and 0.45, respectively. These results showed similar relationships with SEM-wavelength dispersive spectroscopy (WDS) mapping of this material. It is noteworthy that Area 1 geological material has not been as aggressively weathered as Area 3 material due to its physical location from the waste source. In all of the cores, most of the Fe and Al oxides were crystalline, while most of the Mn oxides were amorphous. The greater adsorption and/or fixation of anion complexes of P-U (uranium phosphate) and S onto Fe and Al oxides from Area 1 cores compared to Area 3 core material is probably due to a higher amount of crystalline Fe and Al oxides compared to amorphous Fe and Al oxides and higher Al substitution in Fe oxides in Area 1. This unique study illustrates the relationships between U(NA), total P and S, and Al, Fe and Mn oxides in fractured shale under field conditions which can be used in planning remediation of this site and other similar sites.

Influence of extractable soil manganese on oxidation capacity of different soils in Korea

We examined the relationship between soil oxidation capacity and extractable soil manganese, iron oxides, and other soil properties. The Korean soils examined in this study exhibited low to medium Cr oxidation capacities, oxidizing 0.00–0.47 mmol/kg, except for TG-4 soils, which had the highest capacity for oxidizing added Cr(III) [>1.01 mmol/kg of oxidized Cr(VI)]. TG and US soils, with high Mn contents, had relatively high oxidation capacities. The Mn amounts extracted by dithionite-citrate-bicarbonate (DCB) (Mnd), NH2OH·HCl (Mnh), and hydroquinone (Mnr) were generally very similar, except for the YS1 soils, and were well correlated. Only small proportions of either total Mn or DCB-extractable Mn were extracted by NH2OH·HCl and hydroquinone in the YS1 soils, suggesting inclusion of NH2OH·HCl and hydroquinone-resistant Mn oxides, because these extractants are weaker reductants than DCB. No Cr oxidation test results were closely related to total Mn concentrations, but Mnd, Mnh, and Mnr showed a relatively high correlation with the Cr tests (r = 0.655–0.851; P < 0.01). The concentrations of Mnd and Mnh were better correlated with the Cr oxidation tests than was the Mnr concentration, suggesting that the oxidation capacity of our soil samples can be better explained by Mnd and Mnh than by Mnr. The first component in principal components analysis indicated that extractable soil Mn was a main factor controlling net Cr oxidation in the soils. Total soil Mn, Fe oxides, and the clay fraction are crucial for predicting the mobility of pollutants and heavy metals in soils. The second principal component indicated that the presence of Fe oxides in soils had a significant relationship with the clay fraction and total Mn oxide, and was also related to heavy-metal concentrations (Zn, Cd, and Cu, but not Pb).

Selectivity of reagents used to extract iron from soil

The selectivity of the Tamm and Mehra-Jackson reagents to iron (hydr)oxides was verified chemically by comparing the efficiencies of the two methods of iron extraction: parallel (commonly adopted) and sequential, i.e., by calculating the difference ΔFe = [Fedit − (Feox(paral) + Fedit(seq))]. The expected equality of the extracted iron (i.e., ΔFe ∼ 0) is rarely observed. A positive balance reaching ΔFe = (0.5−0.8)% predominates upon iron extraction from forest soils of the Cis-Ural region. This is probably due to the stable Fe(II)-oxalate forming in the course of the Tamm extraction, which is incompletely dissolved by the dithionite-citrate-bicarbonate (DCB). On the contrary, a negative balance with ΔFe reaching −0.4% predominates in the steppe soils of the Stavropol region. This may be caused by Fe(II) minerals (pyrite and siderite) acting as catalyzers in Tamm’s extract, while being weakly soluble in the DCB. To follow the additivity principle for the Tamm and Mehra-Jackson extracts, we suggest a modification of the Schwertmann criterion KSch = (Feox: Fedit(paral)), which sometimes exceeds 1, to K Sch m = Feox: (Feox + Fedit(seq)), which is always less than 1. The values of the modified Schwertmann criterion better agree with the color of the Cis-Ural forest soils expressed in the CIE-L*a*b* optical system as compared with the initial Schwertmann criterion. In the steppe soils, the use of the modified criterion makes it possible to replace the illogical values of KSch = 1.11−1.37 by the quite acceptable K Sch m = 0.68−0.83.