Amorphous Fe Oxides Research Articles

Geochemical fractions of chromium, copper, and zinc and their vertical distribution in floodplain soil profiles along the Central Elbe River, Germany

The determination of geochemical fractions of heavy metals (HM) in soils is a key issue when studying their mobility. Therefore, we have determined the geochemical fractions and the vertical distribution of chromium (Cr), copper (Cu), and zinc (Zn) in seven floodplain soil profiles in relation to flooding conditions and relevant soil properties. These soil profiles represent two different soil groups (Mollic Fluvisols and Eutric Gleysols) which differ significantly in flooding duration and soil properties. The metals were fractionated sequentially to seven fractions as follows: F1: soluble+exchangeable, F2: easily mobilizable, F3: bound to Mn oxides, F4: bound to soil organic matter (SOM) (might include sulphides), F5: bound to low crystalline (amorphous) Fe oxides, F6: bound by crystalline Fe oxides, and F7: residual fraction.The total Cr, Cu, and Zn concentrations ranged from 54.4 to 134.1, 165.2 to 215.7, and 128.5 to 1097.8mgkg−1, respectively, and exceeded the precautionary values of the BBodSchV (1999). Total metal concentrations correlated significantly to soil organic carbon (SOC), total sulphur (St), and cation exchange capacity (CEC), as well as crystalline and amorphous Fe- and Mn-oxides. The residual fraction was dominant for Cr, the organic bound for Cu, and crystalline Fe oxides for Zn. The potential mobile fraction (PMF=∑F1–F6) ranged from 38.4 to 71.4, 63.9 to 85.1, and 51.5 to 83.3% of the total Cr, Cu, and Zn, respectively. However, the mobile fraction (MF=∑F1–F2) ranged from 0.96 to 1.84, 2.1 to 4.1, and 9.1 to 28.7% of the total concentrations of Cr, Cu, and Zn, respectively. The order of the PMF in the studied soil profiles was Cu>Zn>Cr, while the order of the MF was Zn>Cu>Cr. The PMF of the three metals was positively correlated with SOC and Mn oxides. The PMF was positively correlated with CEC (Cr, Zn), St (Cr), and Fe oxides (Zn). The PMF of Cu and Zn correlated negatively with clay. The MF of the three metals was correlated negatively with soil pH. The MF of Zn was correlated positively with clay, SOC, and St. The MF of Cr was correlated positively with clay, while it correlated negatively with SOC, St, CEC, and Fe–Mn oxides. The Fluvisols revealed higher total concentrations and potential motilities of Cr, Cu, and Zn compared to the Gleysols. However, the Gleysols had a higher metal mobility compared to Fluvisols due to their longer flooding duration. The solubility of Cr below the average water level in the studied profiles was higher than above the water level, while the solubility of Cu and Zn above the water level was higher than below the water level in both soil groups. The potential mobility of the studied metals (especially of Cu followed by Zn), and thus the transfer of these metals into the grassland and food chain, should be high, which might be harmful to the floodplain ecosystem.

Iron isotope geochemistry of biogenic magnetite-bearing sediments from the Bay of Vidy, Lake Geneva

Dissimilatory microbial iron oxide reduction (DIR) has been hypothesized to be an important respiratory pathway on early Earth, potentially generating significant quantities of Fe(II) that have been preserved in Proterozoic and Archean sedimentary rocks. In particular, DIR has been implicated in the formation of magnetite in Precambrian marine sediments. To date, however, only one modern sedimentary environment exists where in situ magnetite formation has been linked to DIR: the Bay of Vidy in Lake Geneva, Switzerland. Previous work at this locality has characterized a magnetic susceptibility anomaly that reflects the presence of fine-grained magnetite produced via microbial reduction of amorphous Fe(III) oxides that enter the Bay of Vidy from a nearby sewage treatment plant. In this study, we report on the Fe isotope composition of aqueous and solid-phase Fe in the Bay of Vidy sediments. Extensive Fe(III) reduction has occurred, resulting in the conversion of nearly all reactive (non-silicate) Fe(III) to a variety of Fe(II)-bearing phases, with mixed Fe valence magnetite being a minor but easily detectable component (0.5–8wt.%). Very little Fe isotope variation was observed in any solid phase Fe components, including magnetite, although significant fractionation was observed between aqueous and solid-phase Fe(II). Because Fe mass-balance was dominated by the solid phase, little net change in δ56Fe values for Fe(II)-bearing components was produced despite clear evidence for DIR. This study provides a basis for interpreting instances in the rock record where DIR was the driving force for Fe(II) production and magnetite formation, yet no significant deviations in δ56Fe values were preserved. A key implication of the results is that Fe isotope homogeneity is not sufficient to rule out a biological mechanism for magnetite formation, and this should be taken into account when examining the Precambrian rock record.

Suitability of Urban Wastes for Crop Production in Zaria, Northern Nigeria: Bioavailability, Phytotoxicity, and Fractions of Micronutrients

The suitability of two composted solid urban wastes for crop production was evaluated in a pot experiment with sorghum (Sorghum bicolor) that focused on the geochemical fractions, bioavailability, and phytotoxicity of copper (Cu), manganese (Mn), and zinc (Zn). Total concentrations of Cu, Mn, and Zn in soil increased with increasing waste application, ranging from 1.6 to 48.2 mg kg−1 for Cu, 84 to 474 mg kg−1 for Mn, and 13.8 to 597 mg kg−1 for Zn. Waste application significantly increased pH and electrical conductivity (EC) of the soil. Copper, Mn, and Zn in the waste-amended soil were speciated into mobile (F1), easily mobilizable (F2), occluded in Mn oxides (F3), organically bound (F4), occluded in amorphous Fe oxides (F5), occluded in crystalline Fe oxides (F6), and residual (F7) fractions to assess the lability of the metals. On the average, the F4 was the most dominant Cu and Zn fraction, accounting for between 37 and 60% of total Cu and from 14 to 40% of total Zn concentrations, whereas F3 was the dominant Mn fraction closely followed by F4. The concentrations of Cu, Mn, and Zn in sorghum dry matter (DM) decreased with increasing waste application, probably induced by osmotic stress and ionic toxicity. Tissue Zn (Y-Zn) and Mn (Y-Mn) correlated significantly with the F1 and F2 fractions, but pH was an overriding factor in predicting Cu and Zn bioconcentration. Used as soil amendments, the application rate for these Zaria urban wastes should be limited to ≤10% (w/w basis), as Zn in the sorghum tissue reached the toxic limit just from one application of the waste to soil.

Chemical properties and mineralogy of soils with plinthite and petroplinthite in Iranduba (AM), Brazil

Large areas of Plinthosols with ferruginous materials such as plinthite and/or petroplinthite are fairly common in the Brazilian Amazon basin. This work was carried out to investigate the chemical behavior, mineralogical composition and weathering stage of four representative soil profiles with plinthite and petroplinthite, in Iranduba, AM (Central Amazon). Three well-drained soil profiles at high elevations were studied (P1, Plinthic Vetic Ferralsol; P2 and P3, Vetic Endopetric Plinthosol) and a contrasting poorly drained soil (P4 Haplic Plinthosol), located at low elevation. After profile descriptions, soil samples were collected from each horizon, air-dried, sieved (2 mm), and analyzed for particle-size distribution, pH, exchangeable cations (Al3+, Ca2+, Mg2+, K+, and Na+), as well as available P and total organic carbon (TOC) content. The minerals present in the clay and sand fractions, as well as in the ferruginous materials were identified by X-ray Diffraction (XRD). The weathering stage of these soils was assessed by means of Ki and Kr indexes, and the amounts of free and amorphous Fe and Al oxides by using dithionite citrate bicarbonate (DBC) and ammonium oxalate dissolution procedures, respectively. The results showed that all soils were extremely unfertile, with pH levels ranging between strong and moderate acidity, very low sum of bases and organic matter content, and of available P. The mineralogy of the soil profiles was very similar, mainly of the well-drained soils, with predominance of kaolinite and quartz in the clay and sand fractions, respectively. In the poorly-drained P4, 2:1 clay particles were also observed. These profiles can be considered highly developed according to the Ki index, however, the Ki value of P4 was higher, indicating that this soil was less developed than the others. In summary, these profiles with plinthite and petroplinthite can be characterized as highly developed and infertile soils and are, with exception of P4, well-drained.

Lead and zinc in the structure of organic and mineral soil components

In addition to the more reactive forms, metals can occur in the structure of minerals, and the sum of all these forms defines their total contents in different soil fractions. The isomorphic substitution of heavy metals for example alters the dimensions of the unit cell and mineral size. This study proposed a method of chemical fractionation of heavy metals, using more powerful extraction methods, to remove the organic and different mineral phases completely. Soil samples were taken from eight soil profiles (0-10, 10-20 and 20-40 cm) in a Pb mining and metallurgy area in Adrianópolis, Paraná, Brazil. The Pb and Zn concentrations were determined in the following fractions (complete phase removal in each sequential extraction): exchangeable; carbonates; organic matter; amorphous and crystalline Fe oxides; Al oxide, amorphous aluminosilicates and kaolinite; and residual fractions. The complete removal of organic matter and mineral phases in sequential extractions resulted in low participation of residual forms of Pb and Zn in the total concentrations of these metals in the soils: there was lower association of metals with primary and 2:1 minerals and refractory oxides. The powerful methods used here allow an identification of the complete metal-mineral associations, such as the occurrence of Pb and Zn in the structure of the minerals. The higher incidence of Zn than Pb in the structure of Fe oxides, due to isomorphic substitution, was attributed to a smaller difference between the ionic radius of Zn2+ and Fe3+.

Efficient removal of trace arsenite through oxidation and adsorption by magnetic nanoparticles modified with Fe–Mn binary oxide

Magnetic nanoparticles (MNPs) modified simultaneously with amorphous Fe and Mn oxides (Mag-Fe–Mn) were synthesized to remove arsenite [As(III)] from water. Mag-Fe–Mn particles were fabricated through heterogeneous nucleation technique by employing the maghemite as the magnetic core and Fe–Mn binary oxide (FMBO) as the coating materials. Powder X-ray diffraction, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy were utilized to characterize the hybrid material. With a saturation magnetization of 23.2 emu/g, Mag-Fe–Mn particles with size of 20–50 nm could be easily separated from solutions with a simple magnetic process in short time (within 5 min). At pH 7.0, 200 μg/L of As(III) could be easily decreased to below 10 μg/L by Mag-Fe–Mn particles (0.1 g/L) within 20 min. As(III) could be effectively removed by Mag-Fe–Mn particles at initial pH range from 4 to 8 and the residual As was completely oxidized to less toxic arsenate [As(V)]. The co-occurring redox reactions between Mn oxide and As(III) was confirmed by XPS analysis. Chloride, sulfate, bicarbonate, and nitrate at common concentration range had negligible influence on As(III) removal, whereas, silicate and phosphate reduced the As(III) removal by competing with arsenic species for adsorption sites. As(III) removal was not obviously affected by natural organic matter (up to 8 mg/L as TOC). Mag-Fe–Mn could be regenerated with ternary solution of NaOH, NaCl, and NaClO. Throughout five consecutive cycles, the adsorption and desorption efficiencies maintained above 98% and 87%, respectively. Mag-Fe–Mn had a larger adsorption capacity for As(III) (47.76 mg/g) and could remove trace As(III) more thoroughly than MNPs modified solely with either Fe or Mn oxide due to the synergistic effect of the coating Fe and Mn oxides. This research extended the potential applicability of FMBO to a great extent and provided a convenient approach to efficiently remove trace As(III) from water.

Metal forms in sediments from Arctic coastal environments in Kandalaksha Bay, White Sea, under separation processes

This study focuses on sediments from small restricted exchange environments along the Karelian shore of Kandalaksha Bay (White Sea, Russian Arctic), which are known as separating basins and are characterised by contrasting oxidising conditions within the water column and the occurrence of anoxia. In the basins that were studied, no significant contamination by trace heavy metals (Pb, Cu, Zn and Cr, in particular) was detected. The comparative study of the two most bioavailable metal forms, namely, labile (acid soluble) and organically bound (alkali soluble) forms, indicated that acetic acid and sodium pyrophosphate released 3–11% and 2–12%, respectively, of the total metal content from sediments. The most bioavailable parts of metals are weakly bound to organic matter and, to a greater extent, associated with easily soluble amorphous Fe-oxides. Among the studied elements, most of the bioavailable Zn and Cu was most likely bound to organic substances, whereas bioavailable Cr and Mn were controlled to a greater extent by the formation of Fe-oxyhydroxide. The elements studied could be arranged in the following decreasing order of average potential bioavailability: Cu > Zn > Mn > Fe > Cr > Pb. In the separating basins, the relative proportion of labile bioavailable metals is enhanced in relation to the neighbouring open coastal sea.

Sediment characterization and its implications for arsenic mobilization in deep aquifers of eastern Croatia

Sediments from two boreholes drilled in the Drava and Sava depressions were analyzed to determine the origin of the elevated arsenic (As) concentrations in groundwaters in eastern Croatia. The composition of the sediments was investigated by total chemical analysis and sequential extraction. In addition, groundwater samples were taken from eight producing wells, and hydrochemical and trace element and As speciation analyses were performed. The concentrations of As in the sediment were high (up to 471 and 65.1mg/kg, in the <0.065mm fraction in the boreholes in the Drava and Sava depression respectively) supporting the hypothesis proposed in a previous hydrochemical study that the Middle and Upper Pleistocene sediments are the main source of As in eastern Croatian groundwaters. Mineralogical and chemical analyses showed that sediments from the Drava and the Sava boreholes differed, supporting different origins of the sediments in the boreholes concerned: the provenance of PVc-3 sediments being the Alps, and of G1 sediments, the Bosnian mountains. In both boreholes, the large amount of As was bound to amorphous Fe and Mn (hydr)oxides (on average 32% and 60% in boreholes PVc-3 and G1 respectively). Sediment and groundwater analyses indicate that reductive dissolution of iron (hydr)oxides and concomitant release of sorbed or co-precipitated As into the groundwater is ready to occur. Since the groundwaters in eastern Croatia are generally anoxic and reducing conditions, in which S2− and Fe2+ are stable, prevail the concentration of As released by reductive dissolution of iron (hydr)oxides may be controlled by the secondary formed sulfides which could act as a sink rather than as a source of As in the groundwater. Depending on local geomorphological and hydrogeochemical conditions, in the zones with a longer groundwater retention time, pH dependant As desorption from iron (hydr)oxides can occur and other trace elements such as Mo may become mobile.

Modeling Selenite Adsorption Envelopes on Oxides, Clay Minerals, and Soils using the Triple Layer Model

Selenite adsorption behavior was investigated on amorphous Al and Fe oxides, clay minerals: kaolinite, montmorillonite, and illite, and 45 surface and subsurface soil samples from the southwestern and midwestern regions of the United States as a function of solution pH. Selenite adsorption decreased with increasing solution pH. The triple layer model, a chemical surface complexation model, was able to describe selenite adsorption as a function of solution pH by simultaneously optimizing both inner‐sphere and outer‐sphere selenite surface complexation constants. The fit of the triple layer model to selenite adsorption by soils was much improved over that obtained previously by optimizing solely an inner‐sphere selenite surface complexation constant and the protonation constant in the constant capacitance model. In this previous application, the deprotonation constant had been neglected; thereby, preventing the reactive surface hydroxyl group from deprotonating; a chemically unrealistic situation. The selenite surface speciation predicted using the triple layer model was in agreement with that obtained for other strongly adsorbing anions such as molybdate. Direct spectroscopic investigations of selenite surface configuration are needed to corroborate the species predicted by the modeling approach.

What do results of common sequential fractionation and single-step extractions tell us about P binding with Fe and Al compounds in non-calcareous sediments?

Correct identification of P forms together with their main Fe and Al binding partners in non-calcareous sediments is of crucial importance for evaluation of P cycling in water bodies. In this paper, we assess extraction methods frequently used for this purpose, i.e., a sequential five-step fractionation (water, bicarbonate buffered dithionite solution (BD), NaOH, HCl, nitric-perchloric acid), ascorbate extraction (pH ∼7.5), and oxalate extraction (pH ∼3), directly on a range of laboratory prepared Fe and Al minerals enriched with adsorbed P. Extraction selectivity and efficiency for particular P, Fe and Al forms were also verified by specific combinations of these extraction methods applied on freshwater sediment samples. In the sequential fractionation, BD was highly effective in dissolving both amorphous and crystalline Fe (hydr)oxides and the associated P, while neither FeS nor Al (hydr)oxides were dissolved. The following NaOH extraction effectively dissolved both amorphous and crystalline Al (hydr)oxides. The high solubilizing power of BD and NaOH to dissolve crystalline Fe and Al oxides that have only a small P-sorption ability prevents the use of resulting Fe/P and Al/P ratios as simple predictors of total P sorption capacity of sediments and soils. Ascorbate non-selectively extracted small proportions of FeS and amorphous Fe and Al (hydr)oxides, but significant amounts of adsorbed P, which hinders its use for the characterization of P forms in non-calcareous sediments. Similar nonselective characteristics were found for oxalate extractions. As oxalate extracts most of the adsorbed phosphate, it is not possible to use it unambiguously to determine specific Fe/P and Al/P ratios of active complexes. However, this method is convenient (and more selective than NaOH step in the sequential fractionation) for the determination of amorphous Al (hydr)oxides.

Fe cycling in the Shale Hills Critical Zone Observatory, Pennsylvania: An analysis of biogeochemical weathering and Fe isotope fractionation

During weathering, Fe in primary minerals is solubilized by ligands and/or reduced by bacteria and released into soil porewaters. Such Fe is then removed or reprecipitated in soils. To understand these processes, we analyzed Fe chemistry and isotopic composition in regolith of the Shale Hills watershed, a Critical Zone Observatory in central Pennsylvania overlying iron-rich shale of the Rose Hill Formation. Elemental concentrations were measured in soil from a well-drained catena on a planar hillslope on the south side of the catchment. Based upon X-ray diffraction and bulk elemental data, loss of Fe commences as clay begins to weather ∼15cm below the depth of auger-refusal. More Fe(III) was present than Fe(II) in all soil samples from the ridge top to the valley floor. Both total and ferrous iron are depleted from the land surface of catena soils relative to the bedrock. Loss of ferrous Fe is attributed mostly to abiotic or biotic oxidation. Loss of Fe is most likely due to transport of micron-sized particles that are not sampled by porous-cup lysimeters, but which are sampled in stream and ground waters. The isotopic compositions (δ56Fe, relative to IRMM-014) of bulk Fe and 0.5N HCl-extracted Fe (operationally designed to remove amorphous Fe (oxyhydr)oxides) range between −0.3‰ and +0.3‰, with Δ56Febulk-extractable values between ∼0.2‰ and 0.4‰. Throughout the soils along the catena, δ56Fe signatures of both bulk Fe and HCl-extracted Fe become isotopically lighter as the extent of weathering proceeds. The isotopic trends are attributed to one of two proposed mechanisms. One mechanism involves Fe fractionation during mobilization of Fe from the parent material due to either Fe reduction or ligand-promoted dissolution. The other mechanism involves fractionation during immobilization of Fe (oxyhydr)oxides. If the latter mechanism is true, then shale – which comprises one quarter of continental rocks – could be an important source of isotopically heavy Fe for rivers.

Chemical Fractionation and Contamination Intensity of Trace Elements in Stream Sediments at the Sarcheshmeh Porphyry Copper Mine, SE Iran

This paper presents chemical fractionation and contamination intensities of trace elements in stream sediments at the Sarcheshmeh mine, southeastern Iran, which is one of the world’s largest Oligo-Miocene porphyry copper deposits. Evaluation of environmental pollution indices and maximum probable background concentrations revealed that As, Cu, Cd, Mo, Pb, Sb, Se, S, and Zn are highly concentrated in the contaminated sediments, while Cr, Co, Ni, Fe, and Mn show lower enrichment values. Discharges of industrial effluents (especially those contaminated by tailings), reject waste from the semi-autogenous mill, and rock waste drainages are the main anthropogenic contaminant sources. High values of As, Cu, Fe, Mo, Pb, and Zn were associated with the oxidizable, primary sulfide, and residual sediment fractions. Relatively high percentages of Co (>92 %), Cr (>58 %), Cu (>79 %), Fe (>40 %), Mn (>97 %), Ni (>87 %), and Zn (>83 %) in the sediments associated with the rock waste drainages were readily released during the extraction of water-soluble, exchangeable, and carbonate fractions. Sediments that received reject waste drainages were also polluted by As (>351.7 mg kg−1), Cu (>1.58 %), Mo (>91.8 mg kg−1), Pb (>291.8 mg kg−1), and Zn (>762.4 mg kg−1). A large percentage of these contaminants were found to be adsorbed and co-precipitated with amorphous Fe-oxides and carbonate phases. The chemical fractionation pattern of the potentially hazardous trace elements corresponded well with the mineralogical composition of the contaminated sediments.

Phosphorus sorption-desorption characteristics of ditch sediments from different land uses in a small headwater catchment in the central Sichuan Basin of China

Investigation of phosphorus (P) sorption-desorption characteristics of drainage ditch sediments is important for better understanding on sediment P transport behaviors in ditches. Surface ditch sediment samples were collected from headwater subcatchment of forestland, sloping cropland, paddy field, and residential area in a representative catchment in the central Sichuan Basin. These sediment samples were used for determination of P sorption-desorption characteristics by a batch equilibrium technique. Results showed that the maximum P sorption capacities (Q(m)) in the sediments ranged from 159.7 to 263.7 mg/kg, while higher Q(m) were observed in the ditch sediments from the paddy fields. The Q(m) was significantly and positively correlated with oxalate-extractable Fe and Al oxides (r=0.97 and 0.98, p 50 g/kg) would not increase P sorption capacity in the ditch sediments. The ditch sediments featured a linear desorption curve, suggesting that P release risk would be enhanced with the increase of the P adsorption. The P desorption rate was positively correlated with Olsen P (r = 0.94, p < 0.01), but negatively related to the fine particle-size fractions (r = -0.92, p < 0.01), the sum of the amorphous Fe and Al oxides (r = -0.67, p < 0.05) and the P sorption capacity (r = -0.59, p < 0.05). The ditch sediments from residential area had a higher P release risk than that from the other ditches of forestland, sloping cropland and paddy field. The P sorption index (PSI) derived from single-point measurement was significantly correlated with the P sorption capacity (r = 0.99, p < 0.01), and could be used for estimating Q(m) as 1.64 times PSI plus 24.0 (Q(m) = 1.64 PSI + 24.0) for similar sediments with highly calcareous soils and sediments. Ditch cleaning and sediment removal for the ditch in residential area were recommended in this area to reduce the P release risk.

Suitability of urban wastes for crop production in Zaria, northern Nigeria: bioavailability and geochemical fractions of potentially toxic elements

A savannah soil amended with two urban wastes and planted with sorghum (Sorghum bicolor) in a pot experiment was evaluated for chemical forms and phytoavailability of Cr, Ni and Pb. The metals were partitioned into seven operationally defined chemical fractions: mobile (F1), easily mobilisable (F2), occluded in Mn oxides (F3), organically bound (F4), occluded in amorphous Fe oxides (FeOxam, F5), occluded in crystalline Fe oxides (FeOxc, F6), and residual (F7). Although the application of the urban wastes increased the total concentrations of the metals in the soil, there were no corresponding increases in Cr and Ni uptake by sorghum except for Pb uptake. Waste application increased mobile Ni (F1) but decreased the residual form (F7). The organically bound Pb (F4) was increased at the expense of residual Pb (F7). With respect to Cr, waste application increased Cr form occluded in non-crystalline Fe oxides (F5) in the soil. Bioassay of the test crop indicated that the uptake of Pb by sorghum (Y-Pb) correlated significantly with Pb occluded in non-crystalline and crystalline Fe oxides (F5 and F6). Since a single application of urban wastes to this savannah soil significantly increased Pb uptake by sorghum, the consequences of long-term applications under urban and peri-urban agriculture (UPA) should be of great concern; particularly with respect to children’s exposure to Pb through consumption of food and vegetables.

Stable iron isotope fractionation between aqueous Fe(II) and model Archean ocean Fe–Si coprecipitates and implications for iron isotope variations in the ancient rock record

Iron isotope fractionation between aqueous Fe(II) (Fe(II)aq) and two amorphous Fe(III) oxide–Si coprecipitates was investigated in an aqueous medium that simulated Archean marine conditions, including saturated amorphous silica, low sulfate, and zero dissolved oxygen. The equilibrium isotope fractionation (in 56Fe/54Fe) between Fe(II)aq and Fe(III)-Si coprecipitates at circum-neutral pH, as inferred by the three-isotope method, was −3.51±0.20 (2σ)‰ and −3.99±0.17 (2σ)‰ for coprecipitates that had Fe:Si molar ratios of 1:2 and 1:3, respectively. These results, when combined with earlier work, indicate that the equilibrium isotope fractionation factor between Fe(II)aq and Fe(III)–Si coprecipitates changes as a function of Fe:Si ratio of the solid. Isotopic fractionation was least negative when Fe:Si=1:1 and most negative when Fe:Si=1:3. This change corresponds with changes in the local structure of iron, as revealed by prior spectroscopic studies. The kinetics of isotopic exchange was controlled by movement of Fe(II) and Si, where sorption of Fe(II) from aqueous to solid phase facilitated atom exchange, but sorption of Si hindered isotopic exchange through blockage of reactive surface sites. Although Fe(II)–Fe(III) isotopic exchange rates were a function of solid and solution compositions in the current study, in all cases they were much higher than that determined in previous studies of aqueous Fe(III) and ferrihydrite interaction, highlighting the importance of electron exchange in promoting Fe atom exchange. When compared to analogous microbial reduction experiments of overlapping Fe(II) to Fe(III) ratios, isotopic exchange rates were faster in the biological experiments, likely due to promotion of atom exchange by the solid-phase Fe(II) produced in the biological experiments. These results provide constraints for interpreting the relatively large range of Fe isotope compositions in Precambrian marine sedimentary rocks, and highlight important differences between modern and ancient marine environments due to the absence or presence of dissolved silica. Evidence can be found in the Fe isotope compositions of the ancient rock record for both abiologic and biologic processes, distinction of which becomes apparent when sedimentological and diagenetic processes are fully explored, as well as Fe mass balance.

Isolation of Phyllosilicate–Iron Redox Cycling Microorganisms from an Illite–Smectite Rich Hydromorphic Soil

The biogeochemistry of phyllosilicate–Fe redox cycling was studied in a Phalaris arundinacea (reed canary grass) dominated redoximorphic soil from Shovelers Sink, a small glacial depression near Madison, WI. The clay size fraction of Shovelers Sink soil accounts for 16% of the dry weight of the soil, yet contributes 74% of total Fe. The dominant mineral in the clay size fraction is mixed layer illite–smectite, and in contrast to many other soils and sediments, Fe(III) oxides are present in low abundance. We examined the Fe biogeochemistry of Shovelers Sink soils, estimated the abundance of Fe redox cycling microorganisms, and isolated in pure culture representative phyllosilicate–Fe oxidizing and reducing organisms. The abundance of phyllosilicate–Fe reducing and oxidizing organisms was low compared to culturable aerobic heterotrophs. Both direct isolation and dilution-to-extinction approaches using structural Fe(II) in Bancroft biotite as a Fe(II) source, and O2 as the electron acceptor, resulted in recovery of common rhizosphere organisms including Bradyrhizobium spp. and strains of Cupriavidus necator and Ralstonia solanacearum. In addition to oxidizing biotite and soluble Fe(II) with O2, each of these isolates was able to oxidize Fe(II) in reduced NAu-2 smectite with as the electron acceptor. Oxidized NAu-2 smectite or amorphous Fe(III) oxide served as electron acceptors for enrichment and isolation of Fe(III)-reducing microorganisms, resulting in recovery of a strain related to Geobacter toluenoxydans. The ability of the recovered microorganisms to cycle phyllosilicate–Fe was verified in an experiment with native Shovelers Sink clay. This study confirms that Fe in the native Shovelers Sink clay is readily available for microbial redox transformation and can be cycled by the Fe(III)-reducing and Fe(II)-oxidizing microorganisms recovered from the soil.

Phyto-availability of chromium to fluted pumpkin (Telfairia occidentalis Hook F.) in an ultisol

The present study was conducted to determine the influence of Cr on some agronomic characters of Telfairia occidentalis nutrient content and uptake and some chemical properties of soil. In the greenhouse trial, chromium nitrate [Cr(N03)2] was applied at rates of 0, 50, 100 and 200 mg per 5 kg sieved and air-dried soil obtained from a depth of 0-15cm. The rates of 0, 20, 40 and 80 kgha-1 equivalent to pot rates were used in the field trial. Results showed that the soil used was texturally sandy loam and an ultisol as revealed by its low base saturation. In the greenhouse the soil pH, N, K, Mg, Ca, Na, Fe, Mn, Zn, free Fe and Al oxides, organic carbon, effective cation exchange capacity, exchangeable acidity, amorphous Fe and Al oxides content of the soil decreased inconsistently at various levels of Cr treatments except available P, which appreciated inconsistently. With the exception of soil pH, organic carbon, available P and amorphous Fe oxide, which increased at various levels of Cr concentrations, all other soil chemical properties determined, declined inconsistently in the field trial. The amorphousAl oxide however remained stable in the field trial. The Cr content of the soil increased with the levels of Cr treatments when compared with the control in the trials. The N, P, K, Mg, Ca, Na, Fe, Mn and Zn content of shoot and root as well as their uptake also decreased consistently with increasing Cr treatments. In addition, the Cr content as well as uptake by the shoot and root also increased consistently with increased rates of the Cr applied in the trials with the minimum levels of the Cr content and uptake recorded at the control treatments. As the Cr concentration increased, the crude protein content of both shoot and root consistently decreased with highest crude protein content recorded in the shoot compared to the root. A decrease in the dry matter yield with increased Cr treatments in shoot and root was recorded in the trials. Results also showed that the Cr influenced the height, collar girth, leaf area and number of leaves with control treatments higher than other treatments at final harvest.

Metal fractionation of cadmium, lead and arsenic of geogenic origin in topsoils from the Marrancos gold mineralisation, northern Portugal

The Marrancos gold mineralisation has a chemical assemblage of Fe-As-Se-Bi-Au-Ag-Te-(Cu-Pb-Zn-Sn-W). The -200 mesh of 144 topsoil samples was analysed by ICP-MS to determine total contents of 53 elements that include potentially harmful elements like Cd, As and Pb. The soil geochemistry shows that some trace elements occur in considerably high concentrations. On the basis of data for total metal concentrations, 10 topsoil samples were selected to carry out a metal fractionation study using a selective extraction method. A set of four leaches of increasing strength was used sequentially in the soil samples. Across the study area, there is some evidence of past mining and exploration activity, indicating that these soils may be locally disturbed. The shallow mineralised quartz veins were exploited for gold by the Romans. Several galleries were constructed during the 2nd World War, probably for the exploration of quartz-cassiterite-wolframite veins. However, the main mineralised body in depth was never explored. The results of metal fractionation show different partitions for the three elements. Total Cd concentrations in these soils are low, with a median value of 0.1 mg/kg. In average, 12% of total Cd is adsorbed by clays and/or co-precipitated with carbonates, and 19% is bounded to Fe-Mn oxyhydroxides. However, the low concentrations indicate that the metal does not represent an immediate risk to human health. For Pb, metal fractionation shows that, on average, 22% of Pb is adsorbed by amorphous Fe and Mn oxides, but the samples from the northern part of the area have the major fractions of Pb in soluble forms. The low probability of exposure in this part of the study area decreases the risk posed by this heavy metal. Total As concentrations in the Marrancos soils are extremely high. A large area has As concentrations above 1,000 mg/kg. For As, metal fractions in the sulphide phase vary between 84 and 98% in the studied samples. But one sample has 20% of total As in easily reducible forms, corresponding to a partial concentration of 1,800 mg/kg that has a high probability of being bioavailable. The most labile As forms occur at the southern part of the area, where the probability of exposure is higher and the risk of human health increases in the same order. From the three studied potentially harmful elements, As is certainly the element of concern.

Kinetics of plutonium and americium sorption to natural clay

Kinetics of Pu(IV) and Am(III) sorption from natural groundwater to three types of clays were studied at trace concentrations of the elements. Higher Kd values were determined for sorption of Pu than of Am, and no clear dependence of the Kd values and the kinetic coefficients on the composition of the clays can be deduced. Kinetic data evaluated by models for six different control processes indicated a sorption mechanism controlled by Pu or Am diffusion in the inert layer on the surface of the clays. Apart from the kinetics of the elements sorption, time-dependent changes in their bonding nature were also studied using a sequential extraction. It was found that Pu(IV) was predominantly associated with amorphous and crystalline Fe oxides as well as natural organic matter sites on the clays, whereas in the case of Am(III) the exchangeable and carbonate sites played the principal role.

Arsenic Speciation and Phytoavailability in Contaminated Soils Using a Sequential Extraction Procedure and XANES Spectroscopy

In this study, a sequential extraction procedure (SEP) and X-ray absorption near edge structure (XANES) spectroscopy were used to determine the solid-phase speciation and phytoavailability of arsenic (As) of historically contaminated soils from As containing pesticides and herbicides and soils spiked with As in the laboratory. Brassica juncea was grown in the contaminated soils to measure plant available As in a glasshouse experiment. Arsenic associated with amorphous Fe oxides was found to be the dominant phase using both SEP and XANES spectroscopy. Arsenic predominantly existed in arsenate (As(V)) form in the soils; in a few samples As was also present in arsenite (As(III)) form or in scorodite mineral. Arsenic concentration in shoots showed significant (p < 0.001-0.05) correlations with the exchangeable As (r = 0.85), and amorphous Fe oxides associated As evaluated by the SEP (r = 0.67), and As associated with amorphous Fe oxides as determined by XANES spectroscopy (r = 0.51). The results show that As in both fractions was readily available for plant uptake and may pose a potential risk to the environment. The combination of SEP and XANES spectroscopy allowed us the quantitative speciation of As in the contaminated soils and the identification of valence and mineral forms of As. Such detailed knowledge on As speciation and availability is vital for management and rehabilitation of As-contaminated soils.