Iron Oxyhydroxide Transformation in a Flooded Rice Paddy Field and the Effect of Adsorbed Phosphate.

The mobility and bioavailability of phosphate in paddy soils are closely coupled to redox-driven Fe-mineral dynamics. However, the role of phosphate during Fe-mineral dissolution and transformations in soils remains unclear. Here, we investigated the transformations of ferrihydrite and lepidocrocite and the effects of phosphate pre-adsorbed to ferrihydrite during a 16-week field incubation in a flooded sandy rice paddy soil in Thailand. For the deployment of the synthetic Fe-minerals in the soil, the minerals were contained in mesh bags either in pure form or after mixing with soil material. In the latter case, the Fe-minerals were labeled with 57Fe to allow the tracing of minerals in the soil matrix with 57Fe Mössbauer spectroscopy. Porewater geochemical conditions were monitored, and changes in the Fe-mineral composition were analyzed using 57Fe Mössbauer spectroscopy and/or X-ray diffraction analysis. Reductive dissolution of ferrihydrite and lepidocrocite played a minor role in the pure mineral mesh bags, while in the 57Fe-mineral-soil mixes more than half of the minerals was dissolved. The pure ferrihydrite was transformed largely to goethite (82-85%), while ferrihydrite mixed with soil only resulted in 32% of all remaining 57Fe present as goethite after 16 weeks. In contrast, lepidocrocite was only transformed to 12% goethite when not mixed with soil, but 31% of all remaining 57Fe was found in goethite when it was mixed with soil. Adsorbed phosphate strongly hindered ferrihydrite transformation to other minerals, regardless of whether it was mixed with soil. Our results clearly demonstrate the influence of the complex soil matrix on Fe-mineral transformations in soils under field conditions and how phosphate can impact Fe oxyhydroxide dynamics under Fe reducing soil conditions.

Iron and arsenic speciation and distribution in organic flocs from streambeds of an arsenic-enriched peatland.

Iron-rich organic flocs are frequently observed in surface waters of wetlands and show a high affinity for trace metal(loid)s. To date, spectroscopic speciation analyses of Fe and trace elements in these mineral-organic matter (OM) associations are missing. In this study, we investigated the speciation and distribution of Fe and As in flocs collected from low-flow streams (pH 5.3-6.3) of the naturally As-enriched peatland Gola di Lago (Switzerland) using (57)Fe Mössbauer spectroscopy and synchrotron X-ray techniques. The flocs were rich in acid carbohydrates and contained up to 22.1 wt % Fe, 34.9 wt % C, and 2620 mg/kg As. Mössbauer analyses revealed small quantities (<5%) of Fe(II) and Fe(III)-OM complexes and the predominance of ferrihydrite (∼ Fe(5)HO(8) · 4H2O, 51-59%) and lepidocrocite (γ-FeOOH, 34-46%). The latter was not observed by synchrotron X-ray diffraction, implying a coherent scattering domain size of <10 nm. Iron X-ray absorption spectroscopy (XAS) confirmed the Mössbauer results, and bulk As XAS indicated the prevalence of arsenate (71-84%) in the flocs. Shell-fit analyses showed that As was entirely sorbed to Fe(III)-(oxyhydr)oxides and that both arsenate and arsenite exclusively formed monodentate-binuclear ("bridging") complexes (R(As-Fe) = 3.31-3.34 Å). Microfocused X-ray fluorescence spectrometry documented a strong correlation between As and Fe in the flocs. These analyses also revealed intense As hotspots coinciding with abundant freshwater green algae (Closterium spp.). Microfocused As X-ray absorption near-edge structure spectra collected at algae-specific points identified up to 29% As(III), which, in combination with ∼ 5% As(III) detected at Fe-rich points, suggests As(V) bioreduction in the algae. Our findings imply that floc (bio)organics serve primarily as nucleation sites for the precipitation of nanocrystalline Fe(III)-(oxyhydr)oxides, rendering flocs effective sorbents for trace metal(loid)s. Thus, Fe-rich freshwater flocs likely play a pivotal role for the speciation and cycling of trace elements in wetlands.

Arsenic species formed from arsenopyrite weathering along a contamination gradient in Circumneutral river floodplain soils.

Arsenic is a toxic trace element, which commonly occurs as contaminant in riverine floodplains and associated wetlands affected by mining and ore processing. In this study, we investigated the solid-phase speciation of As in river floodplain soils characterized by circumneutral pH (5.7-7.1) and As concentrations of up to 40.3 g/kg caused by former mining of arsenopyrite-rich ores. Soil samples collected in the floodplain of Ogosta River (Bulgaria) were size-fractionated and subsequently analyzed using a combination of X-ray fluorescence (XRF) spectrometry, powder X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and selective chemical extraction of poorly crystalline mineral phases. Arsenic and Fe were found to be spatially correlated and both elements were strongly enriched in the fine soil particle size fractions (<2 µm and 2-50 µm). Between 14 and 82% of the total As was citrate-ascorbate extractable. Molar As/Fe ratios were as high as 0.34 in the bulk soil extracts and increased up to 0.48 in extracts of the fine particle size fractions. Arsenic K-edge XAS spectra showed the predominance of As(V) and were well fitted with a reference spectrum of As(V) adsorbed to ferrihydrite. Whereas no As(III) was detected, considerable amounts of As(-I) were present and identified as arsenopyrite originating from the mining waste. Iron K-edge XAS revealed that in addition to As(V) adsorbed to ferrihydrite, X-ray amorphous As(V)-rich hydrous ferric oxides ("As-HFO") with a reduced number of corner-sharing FeO6 octahedra relative to ferrihydrite were the dominating secondary As species in the soils. The extremely high concentrations of As in the fine particle size fractions (up to 214 g/kg) and its association with poorly crystalline Fe(III) oxyhydroxides and As-HFO phases suggest a high As mobilization potential under both oxic and anoxic conditions, as well as a high bioaccessibility of As upon ingestion, dermal contact, or inhalation by humans or animals.

Speciation of Zn in blast furnace sludge from former sedimentation ponds using synchrotron X-ray diffraction, fluorescence, and absorption spectroscopy.

Blast furnace sludge (BFS), an industrial waste generated in pig iron production, typically contains high contents of iron and various trace metals of environmental concern, including Zn, Pb, and Cd. The chemical speciation of these metals in BFS is largely unknown. Here, we used a combination of synchrotron X-ray diffraction, micro-X-ray fluorescence, and X-ray absorption spectroscopy at the Zn K-edge for solid-phase Zn speciation in 12 BFS samples collected on a former BFS sedimentation pond site. Additionally, one fresh BFS was analyzed for comparison. We identified five major types of Zn species in the BFS, which occurred in variable amounts: (1) Zn in the octahedral sheets of phyllosilicates, (2) Zn sulfide minerals (ZnS, sphalerite, or wurtzite), (3) Zn in a KZn-ferrocyanide phase (K(2)Zn(3)[Fe(CN)(6)](2)·9H(2)O), (4) hydrozincite (Zn(5)(OH)(6)(CO(3))(2)), and (5) tetrahedrally coordinated adsorbed Zn. The minerals franklinite (ZnFe(2)O(4)) and smithsonite (ZnCO(3)) were not detected, and zincite (ZnO) was detected only in traces. The contents of ZnS were positively correlated with the total S contents of the BFS. Similarly, the abundance of the KZn-ferrocyanide phase was closely correlated with the total CN contents, with the stoichiometry suggesting this as the main cyanide phase. This study provides the first quantitative Zn speciation in BFS deposits, which is of great relevance for environmental risk assessment, the development of new methods for recovering Zn and Fe from BFS, and potential applications of BFS as sorbent materials in wastewater treatment.

High spatial resolution quantitative imaging by cross-calibration using Laser Ablation Inductively Coupled Plasma Mass Spectrometry and Synchrotron micro-X-ray Fluorescence technique.

High spatial resolution, quantitative chemical imaging is of importance to various scientific communities, however high spatial resolution and robust quantification are not trivial to attain at the same time. In order to achieve microscopic chemical imaging with enhanced quantification capabilities, the current study links the independent and complementary advantages of two micro-analytical techniques - Synchrotron Radiation-based micro X-ray Fluorescence (SR-microXRF) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). A cross-calibration approach is established between these two techniques and validated by one experimental demonstration. In the presented test case, the diffusion pattern of trace level Cs migrating into a heterogeneous geological medium is imaged quantitatively with high spatial resolution. The one-dimensional line scans and the two-dimensional chemical images reveal two distinct types of geochemical domains: calcium carbonate rich domains and clay rich domains. During the diffusion, Cs shows a much higher interfacial reactivity within the clay rich domain, and turns out to be nearly non-reactive in the calcium carbonate domains. Such information obtained on the micrometer scale improves our chemical knowledge concerning reactive solute transport mechanism in heterogeneous media. Related to the chosen demonstration study, the outcome of the quantitative, microscopic chemical imaging contributes to a refined safety assessment of potential host rock materials for deep-geological nuclear waste storage repositories.

Isolation and purification of Cu-free methanobactin from Methylosinus trichosporium OB3b.

BACKGROUND: The isolation of highly pure copper-free methanobactin is a prerequisite for the investigation of the biogeochemical functions of this chalkophore molecule produced by methane oxidizing bacteria. Here, we report a purification method for methanobactin from Methylosinus trichosporium OB3b cultures based on reversed-phase HPLC fractionation used in combination with a previously reported resin extraction. HPLC eluent fractions of the resin extracted product were collected and characterized with UV-vis, FT-IR, and C-1s NEXAFS spectroscopy, as well as with elemental analysis and ESI-MS. RESULTS: The results showed that numerous compounds other than methanobactin were present in the isolate obtained with resin extraction. Molar C/N ratios, mass spectrometry measurements, and UV-vis spectra indicated that methanobactin was only present in one of the HPLC fractions. On a mass basis, methanobactin carbon contributed only 32% to the total organic carbon isolated with resin extraction. Our spectroscopic results implied that besides methanobactin, the organic compounds in the resin extract comprised breakdown products of methanobactin as well as polysaccharide-like substances. CONCLUSION: Our results demonstrate that a purification step is indispensable in addition to resin extraction in order to obtain pure methanobactin. The proposed HPLC purification procedure is suitable for semi-preparative work and provides copper-free methanobactin.

Time-dependent changes of zinc speciation in four soils contaminated with zincite or sphalerite.

The long-term speciation of Zn in contaminated soils is strongly influenced by soil pH, clay, and organic matter content as well as Zn loading. In addition, the type of Zn-bearing contaminant entering the soil may influence the subsequent formation of pedogenic Zn species, but systematic studies on such effects are currently lacking. We therefore conducted a soil incubation study in which four soils, ranging from strongly acidic to calcareous, were spiked with 2000 mg/kg Zn using either ZnO (zincite) or ZnS (sphalerite) as the contamination source. The soils were incubated under aerated conditions in moist state for up to four years. The extractability and speciation of Zn were assessed after one, two, and four years using extractions with 0.01 M CaCl(2) and Zn K-edge X-ray absorption fine structure (XAFS) spectroscopy, respectively. After four years, more than 90% of the added ZnO were dissolved in all soils, with the fastest dissolution occurring in the acidic soils. Contamination with ZnO favored the formation of Zn-bearing layered double hydroxides (LDH), even in acidic soils, and to a lesser degree Zn-phyllosilicates and adsorbed Zn species. This was explained by locally elevated pH and high Zn concentrations around dissolving ZnO particles. Except for the calcareous soil, ZnS dissolved more slowly than ZnO, reaching only 26 to 75% of the added ZnS after four years. ZnS dissolved more slowly in the two acidic soils than in the near-neutral and the calcareous soil. Also, the resulting Zn speciation was markedly different between these two pairs of soils: Whereas Zn bound to hydroxy-interlayered clay minerals (HIM) and octahedrally coordinated Zn sorption complexes prevailed in the two acidic soils, Zn speciation in the neutral and the calcareous soil was dominated by Zn-LDH and tetrahedrally coordinated inner-sphere Zn complexes. Our results show that the type of Zn-bearing contaminant phase can have a significant influence on the formation of pedogenic Zn species in soils. Important factors include the rate of Zn release from the contaminant phases and effects of the contaminant phase on bulk soil properties and on local chemical conditions around weathering contaminant particles.

Zinc fractionation in contaminated soils by sequential and single extractions: influence of soil properties and zinc content.

We studied the fractionation of zinc (Zn) in 49 contaminated soils as influenced by Zn content and soil properties using a seven-step sequential extraction procedure (F1: NH4NO3; F2: NH4-acetate, pH 6; F3: NH3OHCl, pH 6; F4: NH4-EDTA, pH 4.6; F5: NH4-oxalate, pH 3; F6: NH4-oxalate/ascorbic acid, pH 3; F7: residual). The soils had developed from different geologic materials and covered a wide range in soil pH (4.0-7.3), organic C content (9.3-102 g kg(-1)), and clay content (38-451 g kg(-1)). Input of aqueous Zn with runoff water from electricity towers during 26 to 74 yr resulted in total soil Zn contents of 3.8 to 460 mmol kg(-1). In acidic soils (n = 24; pH <6.0), Zn was mainly found in the mobile fraction (F1) and the last two fractions (F6 and F7). In neutral soils (n = 25; pH > or =6.0), most Zn was extracted in the mobilizable fraction (F2) and the intermediate fractions (F4 and F5). The extractability of Zn increased with increasing Zn contamination of the soils. The sum of mobile (F1) and mobilizable (F2) Zn was independent of soil pH, the ratio of Zn in F1 over F1+F2 plotted against soil pH, exhibited the typical shape of a pH sorption edge and markedly increased from pH 6 to pH 5, reflecting the increasing lability of mobilizable Zn with decreasing soil pH. In conclusion, the extractability of Zn from soils contaminated with aqueous Zn after decades of aging under field conditions systematically varied with soil pH and Zn content. The same trends are expected to apply to aqueous Zn released from decomposing Zn-bearing contaminants, such as sewage sludge or smelter slag. The systematic trends in Zn fractionation with varying soil pH and Zn content indicate the paramount effect of these two factors on molecular scale Zn speciation. Further research is required to characterize the link between the fractionation and speciation of Zn and to determine how Zn loading and soil physicochemical properties affect Zn speciation in soils.

Plant availability of zinc and copper in soil after contamination with brass foundry filter dust: effect of four years of aging.

We investigated the effect of 4 yr of aging of a noncalcareous soil contaminated with filter dust from a brass foundry (80% w/w ZnO, 15% w/w Cu0.6Zn0.4) on the chemical extractability of Zn and Cu and their uptake by barley (Hordeum vulgare L.), pea (Pisum sativum L.), and sunflower (Helianthus annus L.). Pot experiments were conducted with the freshly contaminated soil (2250 mg kg-1 Zn; 503 mg kg-1 Cu), with the contaminated soil aged for 4 yr in the field (1811 mg kg-1 Zn; 385 mg kg-1 Cu), and with the uncontaminated control soil (136 mg kg-1 Zn; 32 mg kg-1 Cu). In comparison with the uncontaminated soil, the growth of barley and pea was clearly reduced in both contaminated soils, while toxicity symptoms did not systematically vary from the freshly contaminated to the 4 yr aged soil. The sunflower did not grow in the contaminated soils. The slow oxidative dissolution of the brass platelets led to an increase in the solubility and the plant uptake of Cu from the freshly contaminated to the 4 yr aged soil. In an earlier study, we found that the fine-grained ZnO dissolved in the field soil within 9 mo and that about half of the released Zn was incorporated into a layered double hydroxide phase and about half was adsorbed to the soil matrix. These changes in Zn speciation did not lead to a reduction of the Zn contents in the shoots and roots of barley and pea grown in the aged soil as compared with the freshly contaminated soil.

Heavy metal release from contaminated soils: comparison of column leaching and batch extraction results.

Heavy metals in soils may adversely affect environmental quality. In this study, we investigated the release of Zn, Cd, Pb, and Cu from four contaminated soils by column leaching and single and sequential batch extractions. Homogeneously packed soil columns were leached with 67 mL/g 10(-2) M CaCl2 to investigate the exchangeable metal pool and subsequently with 1400 mL/g 10(-2) M CaCl2 adjusted to pH 3 to study the potential of metal release in response to soil acidification. In two noncalcareous soils (pH 5.7 and 5.1), exchange by Ca resulted in pronounced release peaks for Zn and Cd that were coupled to the exchange of Mg by Ca, and 40 to 70% of total Zn and Cd contents were rapidly mobilized. These amounts compared well with exchangeable pools determined in single and sequential batch extractions. In two soils with near-neutral pH, the effluent concentrations of Zn and Cd were several orders of magnitude lower and no pronounced elution peaks were observed. This behavior was also observed for Cu and Pb in all four soils. When the soils were leached at pH 3, the column effluent patterns reflected the coupling of CaCO3 dissolution (if present) and other proton buffering reactions, proton-induced metal release, and metal-specific readsorption within the soil column. Varying the flow rate by a factor of five had only minor effects on the release patterns. Overall, Ca exchange and subsequent acidification to pH 3 removed between 65 and 90% of total Zn, Cd, Pb, and Cu from the four contaminated soils.

Slow formation and dissolution of Zn precipitates in soil: a combined column-transport and XAFS study.

Recent spectroscopic studies have demonstrated the formation of layered double hydroxides (LDH) and phyllosilicates upon sorption of Zn2+, Ni2+, and Co2+ to clay minerals and aluminum oxides at neutral to alkaline pH and at relatively high initial metal concentrations (>1 mM). The intention of the present study was to investigate whether such phases also form in soil under slightly acidic conditions and at lower metal concentrations. Columns packed with a loamy soil were percolated with aqueous solutions containing 0.1 or 0.2 mM Zn, Ni, Co, and Cd in a 10 mM CaCl2 background at pH 6.5. Metal breakthrough curves indicated a rapid initial sorption step, resulting in retarded breakthrough fronts, followed by further slow metal retention during the entire loading period of 42 days (7000 pore volumes). Total metal sorption and the contribution of slow sorption processes decreased in the order Zn > Ni > Co > Cd. Leaching the reacted soil with 10 mM CaCl2 at pH 6.5 remobilized 8% of the total retained Zn, 15% of Ni, 21% of Co, and 77% of Cd. Subsequent leaching with acidified influent (pH 3.0) remobilized most of the remaining metals. X-ray absorption fine-structure (XAFS) spectroscopy revealed that slow Zn sorption was due to the formation of a Zn-Al LDH precipitate. Although Ni, Co, and Cd concentrations were too low for XAFS analysis, their leaching patterns suggest that part of Ni and Co were also incorporated in solid phases, while most sorbed Cd was still present as exchangeable sorption complex after 42 days. A small but significant percentage of the sorbed metals (2-5%) remained in the soil, even after leaching with more than 3000 pore volumes at pH 3.0, which may suggest micropore diffusion or incorporation into more stable mineral phases.