Kinetics of chromate reduction by pyrite and biotite under acidic conditions

The removal of chromate from aqueous solutions, using finely ground pyrite and biotite, was investigated by batch experiments. The kinetics and mechanism of chromate reduction are discussed here. Chromate reduction by pyrite was about 100 times faster than that by biotite, and was also faster at pH 3 than 4. When pyrite was used, more than 90% of the initial chromate was reduced within 4 h at pH 4, and within 40 min. at pH 3. However, with biotite more than 400 h was required for the reduction of 90% of the initial chromate. The results indicate that the rate of chromate reduction was strongly depending on the amount and dissolution rate of the Fe(II) in the minerals. The reduction of chromate at pH 4 resulted in the precipitation of (Cr, Fe)(OH)_3(s), which is believed to have limited the concentrations of dissolved Cr(III) and Fe(III) to less than the expected values. When biotite was used, the amounts of decreased Fe(II) and reduced Cr(VI) showed no stoichiometric relationship, which implies that not only was there chromate reduction by Fe(II) ions in the acidic solution, but also heterogeneous reduction of Fe(III) ions by structural Fe(II) in biotite. However, the results from a series of the experiments using pyrite showed that the concentrations of the decreased Fe(II) and the reduced Cr(VI) were close to the stoichiometric ratio of 3:1. This was because the oxidation of pyrite rapidly created Fe(II) ions, even in oxygenated solutions, and the chromate reduction by the Fe(II) ions was significantly faster than the Fe(II) ion oxygenation. When compared with the experimental sets controlled at an initial pH of 3, the pH of the biotite batch, which was not controlled, increased to 3.4. Because of the increase in the pH, Cr(VI) was not completely removed, and 25% (1.2–1.3 mg/L Cr(VI)) of the initial concentration remained for up to 1000 h. The pH increase is, in most cases, caused by the hydrolysis of clay minerals. However, in the pyrite batches, there was no difference in the variations of the chromate reduction in relation to the pH control. There was also no difference in the capacity and rate of Cr(VI) reduction in 0.01 M NaCl or Na₂SO₄ solutions. In the 0.01 M NaH₂PO₄ solution pyrite experiment, the Cr(VI) was not completely removed, despite the maintenance of the pH at 3. The dominant Fe species was about 10 mg/L Fe(III) and few Fe(II) ions existed in solution. The Fe phosphate (Fe₃(PO₄)₂ or FePO₄) coatings on the surface of pyrite prevented access of O₂ or Cr(VI). Therefore, the surface coatings are likely to have caused the deterioration of the Cr(VI) reduction capacity in the NaH₂PO₄ solution.     

The influence of citric acid,EDTA, and fulvic acid on U(VI) sorption onto kaolinite

Uranium(VI) sorption onto kaolinite was investigated as a function of pH (3–12), sorbate/sorbent ratio (1 × 10^?6–1 × 10^?4 M U(VI) with 2 g/L kaolinite), ionic strength (0.001–0.1 M NaNO₃), and pCO₂ (0–5%) in the presence or absence of 1 × 10^?2–1 × 10^?4 M citric acid, 1 × 10^?2–1 × 10^?4 M EDTA, and 10 or 20 mg/L fulvic acid. Control experiments without-solids, containing 1 × 10^?6–1 × 10^?4 M U(VI) in 0.01 M NaNO₃ were used to evaluate sorption to the container wall and precipitation of U phases as a function of pH. Control experiments demonstrate significant loss (up to 100%) of U from solution. Although some loss, particularly in 1 × 10^?5 and 1 × 10^?4 M U experiments, is expected due to precipitation of schoepite, adsorption on the container walls is significant, particularly in 1 × 10^?6 M U experiments. In the absence of ligands, U(VI) sorption on kaolinite increases from pH ～3 to 7 and decreases from pH ～7.5 to 12. Increasing ionic strength from 0.001 to 0.1 M produces only a slight decrease in U(VI) sorption at pH < 7, whereas 10% pCO₂ greatly diminishes U(VI) sorption between pH ～5.5 and 11. Addition of fulvic acid produces a small increase in U(VI) sorption at pH < 5; in contrast, between pH 5 and 10 fulvic acid, citric acid, and EDTA all decrease U(VI) sorption. This suggests that fulvic acid enhances U(VI) sorption slightly via formation of ternary ligand bridges at low pH, whereas EDTA and citric acid do not form ternary surface complexes with the U(VI), and that all three ligands, as well as carbonate, form aqueous uranyl complexes that keep U(VI) in solution at higher pH.     

Metal mobility by de-icing salt from an infiltration trench for highway runoff

Soil from an infiltration trench for highway runoff was leached in columns alternately with NaCl and de-ionised water to simulate the runoff of de-icing salt into the trench followed by snowmelt or rainwater. Simultaneously, two columns with the same soil were leached with de-ionised water throughout the experiment. In addition, the groundwater below the infiltration trench was sampled on some occasions. The column leachate and groundwater were split into two sub samples, one was filtered though a 0.45 μm filter; both were analysed for Pb, Cd, Zn, Fe and total organic carbon (TOC). The column experiment showed clearly that an extensive mobilisation of Pb occurred in low electrolyte water leaching following NaCl leaching. The high Pb concentration coincided with peaks in Fe and TOC concentrations and implied colloid-assisted transport. Conversely, Cd and Zn concentrations were raised in the NaCl leachate and a high correlation with Cl showed that Cl complexes are important for the mobilisation, although a pH effect and ionic exchange cannot be excluded. Only 0.15% and 0.06% of the total amount of Pb was leached from the columns leached with alternating NaCl and deionised water confirming the usual hypotheses about the high immobility of Pb in soils. However, on one occasion when the ionic strength and pH was the lowest measured the concentration of Pb in groundwater sampled from 2.5 m depth was 27 μg L⁻¹ in the dissolved phase (<0.45 μm) and 77 μg L⁻¹ in the particle phase (>0.45 μm). These Pb concentrations are almost 3 and 8 times above the Swedish limit for drinking water quality. Accordingly, in spite of the immobility of Pb the accumulation in roadside soils is so large that groundwater quality is threatened. In conclusion, the study suggests that roadside soils impacted by NaCl from de-icing operations contribute Pb to groundwater by colloid-assisted transport.     

Global calibration of a novel,branched GDGT-based soil pH proxy

Recently, 6-methyl branched glycerol dialkyl glycerol tetraethers (brGDGTs) were separated from 5-methyl brGDGTs, which are used in brGDGT-based proxies. Here we analyzed brGDGTs in 27 soil samples along the 400 mm isoline of mean annual precipitation in China by using tandem 2D liquid chromatography. The fractional abundance of 6-methyl brGDGTs showed a positive correlation with soil pH, while that of 5-methyl brGDGTs decreased with increasing soil pH. The abundance ratio of 6-/5-methyl brGDGTs, namely the isomerization of branched tetraethers (IBT), was calculated. The correlation of IBT with pH (pH = 6.33 − 1.28 × IBT; R² 0.89; root mean squared error, RMSE, 0.24) was much stronger than that of the traditionally used cyclization index of branched tetraethers (CBT) with pH (R² 0.52; RMSE 0.49) and comparable with that of CBT′ with pH (R² 0.88; RMSE 0.25). Compiling all available data from 319 soil samples resulted in a global calibration: pH = 6.53 − 1.55 × IBT (R² 0.72; RMSE 0.65), which has a better correlation than the CBT_5ME-pH proxy (R² 0.63; RMSE 0.78), but a weaker correlation than the CBT′-pH proxy (R² 0.85; RMSE 0.52). Our result suggests that the IBT is a promising indicator for soil pH, particularly in cases when some compounds in the CBT′ index cannot be determined.     

Unraveling the partial failure of a permeable reactive barrier using a multi-tracer experiment and Cr isotope measurements

At a Cr(VI) contaminated site in Thun, Switzerland, a permeable reactive barrier (PRB) was installed in 2008. Downstream Cr(VI) concentrations did not indicate any sign of its successful operation more than 2 years after PRB installation. The cause for this potential PRB failure was investigated by performing Cr isotope measurements and a multi-tracer experiment. The combination of reactive (Cr isotopes) and non-reactive tracers allowed characterizing the groundwater flow regime in the vicinity of the PRB in detail. In particular, it could be confirmed that most of the Cr(VI) load is currently bypassing the barrier, whereas only a minor Cr(VI) load is flowing through the PRB. Fitting of observed breakthrough curves using a conventional advection dispersion model resulted in average linear flow velocities of 13–15 m/day for the bypassing Cr(VI) load and 4–5 m/day for the Cr(VI) flowing through the barrier. Using a Rayleigh fractionation model a Cr(VI) reduction efficiency of 77–98% was estimated for the Cr(VI) load that is flowing through the barrier. In contrast, a value of 0–23% was estimated for the current overall PRB reduction efficiency. It is concluded that the PRB bypass and the low overall Cr(VI) reduction efficiency are caused by a limited PRB permeability inherited from skin effects that occurred during PRB emplacement.     

Interaction of U(VI) with Äspö diorite: A batch and in situ ATR FT-IR sorption study

Pristine diorite drill cores, obtained from the Äspö Hard Rock Laboratory (HRL, Sweden), were used to study the retention properties of fresh, anoxic crystalline rock material towards the redox-sensitive uranium. Batch sorption experiments and spectroscopic methods were applied for this study. The impact of various parameters, such as solid-to-liquid ratio (2–200 g/L), grain size (0.063–0.2 mm, 0.5–1 mm, 1–2 mm), temperature (room temperature and 10 °C), contact time (5–108 days), initial U(VI) concentration (3 × 10⁻⁹ to 6 × 10⁻⁵ M), and background electrolyte (synthetic Äspö groundwater and 0.1 M NaClO₄) on the U(VI) sorption onto anoxic diorite was studied under anoxic conditions (N₂). Comparatively, U(VI) sorption onto oxidized diorite material was studied under ambient atmosphere (pCO₂ = 10^−3.5 atm). Conventional distribution coefficients, K_d, and surface area normalized distribution coefficients, K_a, were determined. The K_d value for the U(VI) sorption onto anoxic diorite in synthetic Äspö groundwater under anoxic conditions by investigating the sorption isotherm amounts to 3.8 ± 0.6 L/kg which corresponds to K_a = 0.0030 ± 0.0005 cm (grain size 1–2 mm). This indicates a weak U sorption onto diorite which can be attributed to the occurrence of the neutral complex Ca₂UO₂(CO₃)₃(aq) in solution. This complex was verified as predominating U species in synthetic Äspö groundwater by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Compared to U sorption at room temperature under anoxic conditions, U sorption is further reduced at decreased temperature (10 °C) and under ambient atmosphere. The U species in aqueous solution as well as sorbed on diorite were studied by in situ time-resolved attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy. A predominant sorbing species containing a UO₂(CO₃)₃⁴⁻ moiety was identified. The extent of U sorption onto diorite was found to depend more on the low sorption affinity of the Ca₂UO₂(CO₃)₃(aq) complex than on reduction processes of uranium.     

Soil-geochemical factors controlling the distribution and oral bioaccessibility of nickel,vanadium and chromium in soil

Geogenic nickel (Ni), vanadium (V) and chromium (Cr) are present at elevated levels in soils in Northern Ireland. Whilst Ni, V and Cr total soil concentrations share common geological origins, their respective levels of oral bioaccessibility are influenced by different soil-geochemical factors. Oral bioaccessibility extractions were carried out on 145 soil samples overlying 9 different bedrock types to measure the bioaccessible portions of Ni, V and Cr. Principal component analysis identified two components (PC1 and PC2) accounting for 69% of variance across 13 variables from the Northern Ireland Tellus Survey geochemical data. PC1 was associated with underlying basalt bedrock, higher bioaccessible Cr concentrations and lower Ni bioaccessibility. PC2 was associated with regional variance in soil chemistry and hosted factors accounting for higher Ni and V bioaccessibility. Eight percent of total V was solubilised by gastric extraction on average across the study area. High median proportions of bioaccessible Ni were observed in soils overlying sedimentary rock types. Whilst Cr bioaccessible fractions were low (max = 5.4%), the highest measured bioaccessible Cr concentration reached 10.0 mg kg⁻¹, explained by factors linked to PC1 including high total Cr concentrations in soils overlying basalt bedrock.     

Sedimentary evidence of soil organic matter input to the Curuai Amazonian floodplain

A multi-proxy study has been performed on a sediment core from the Curuai floodplain, Central Amazonia. The combination of elemental, isotopic and molecular analysis of a 110 cm core (a record of ca. the last 100 yr) allowed reconstruction of the hydrological conditions of organic matter (OM) deposition. Two units could be delineated. The first (UI) was composed of three sub-units: UIa (0–15 cm), composed of highly degraded organic particles originating from the surrounding soil and indicative of restricted transport; UIb (15–48 cm), during which the region was permanently flooded and the material stored came from soil runoff, mainly from alluvial forest; and UIc (48–88 cm) composed of material from Amazon River suspended sediment, itself originating from OM degradation in forest soil. In UII (88–111 cm), the OM originated mainly from the forest soil and other plant remains in the floodplain. The data reveal that, during the four distinct depositional periods, the sedimentary OM alternated between land derived soil and alluvial vegetation due to changes in hydrodynamics.     

Metal and arsenic distribution in soil particle sizes relevant to soil ingestion by children

Ingestion of soil is a common behaviour in young children as a means of exploring their surroundings. Much attention has been given to remediation of point-source polluted sites with regard to potential health risks for children. However, because of diffuse pollution and long-range atmospheric deposition, soil contaminant levels are generally increased in urban areas compared to their rural counterparts, even in areas located away from any point sources of pollution. Intake of urban soil can thereby result in significant amounts of the child’s daily metal intake. In the present study, soil samples were collected from 25 playgrounds around urban Uppsala, Sweden and analysed for contents of Al, As, Fe, Cr, Cu, Cd, Hg, Mn, Ni, Pb, W and Zn. Prior to aqua regia digestion, the samples were wet-sieved in order to separate soil particle fractions representing deliberate (<4 mm) and involuntary (<50 μm) soil ingestion by children, as well as a third size fraction of 50–100 μm representing soil that is easily transported by suspension. While the metal and As contents in the 50–100 μm fraction were similar to those of the <4 mm fraction, the <50 μm fraction had metal and As contents on average one and a half times higher than those of the <4 mm fraction. The metal and As contents correlated negatively with the sand content in both particle size fractions <4 mm and 50–100 μm, suggesting a general decrease in metal and As content with increasing sand content. However, a positive correlation was found between sand content and the metal and As contents of the finest fraction (<50 μm), suggesting that when the sand content is high, the bulk of the sorbed elements are on the finest particles. The difference between metal and As contents in the different size fractions was greater in the soil sample with the highest sand content than in the sample with the lowest sand content. This implies that texture is a significant factor in metal and As distribution in soils with moderate metal and As contents, when the number of binding sites associated with small particles is low. Tolerable daily intake (TDI) values for Pb and As were exceeded at all sites, and at two sites for Cd, for children with pica behaviour. A high ingestion rate of mainly small particles could also result in the TDI value for Pb being exceeded at 10 sites and that for As at one site. This study also found that soil analysis by the procedure recommended by Swedish authorities accurately represents the metal intake from deliberate soil ingestion, whereas involuntary soil ingestion of mainly small particles could result in metal intakes which are up to twice as high.     

Soil pH governs production rate of calcium carbonate secreted by the earthworm Lumbricus terrestris

Lumbricus terrestris earthworms exposed to 11 soils of contrasting properties produced, on average, 0.8 ± 0.1 mg_CaCO3 earthworm^?1 day^?1 in the form of granules up to 2 mm in diameter. Production rate increased with soil pH (r² = 0.68, p < 0.01). Earthworms could be a significant source of calcite in soils.     

Biogeochemical weathering of serpentinites: An examination of incipient dissolution affecting serpentine soil formation

Serpentinite rocks, high in Mg and trace elements including Ni, Cr, Cd, Co, Cu, and Mn and low in nutrients such as Ca, K, and P, form serpentine soils with similar chemical properties resulting in chemically extreme environments for the biota that grow upon them. The impact of parent material on soil characteristics is most important in young soils, and therefore the incipient weathering of serpentinite rock likely has a strong effect on the development of serpentine soils and ecosystems. Additionally, porosity generation is a crucial process in converting rock into a soil that can support vegetation. Here, the important factors affecting the incipient weathering of serpentinite rock are examined at two sites in the Klamath Mountains, California. Serpentinite-derived soils and serpentinite rock cores were collected in depth profiles from each sampling location. Mineral dissolution in weathered serpentinite samples, determined by scanning electron microscopy, energy dispersive spectrometry, electron microprobe analyses, and synchrotron microXRD, is consistent with the order, from most weathered to least weathered: Fe-rich pyroxene > antigorite > Mg-rich lizardite > Al-rich lizardite. These results suggest that the initial porosity formation within serpentinite rock, impacting the formation of serpentine soil on which vegetation can exist, is strongly affected both by the presence of non-serpentine primary minerals as well as the composition of the serpentine minerals. In particular, the presence of ferrous Fe appears to contribute to greater dissolution, whereas the presence of Al within the parent rock appears to contribute to greater stability. Iron-oxidizing bacteria present at the soil–rock interface have been shown in previous studies to contribute to the transition from rock to soil, and soils and rock cores in this study were therefore tested for iron-oxidizing bacteria. The detection of biological iron oxidation in this study indicates that the early alteration of these Fe-rich minerals may be mediated by iron-oxidizing bacteria. These findings help provide insight into the incipient processes affecting serpentinite rock weathering, important to the development of extreme serpentine soils and the biota that grow on them.     

Phytoliths as quantitative indicators for the reconstruction of past environmental conditions in China II: palaeoenvironmental reconstruction in the Loess Plateau

Quantitative reconstruction of the climatic history of the Chinese Loess Plateau is important for understanding present and past environment and climate changes in the Northern Hemisphere. Here, we reconstructed mean annual temperature (MAT) and mean annual precipitation (MAP) trends during the last 136 ka based on the analysis of phytoliths from the Weinan loess section (34°24′N, 109°30′E) near the southern part of the Loess Plateau in northern China. The reconstructions have been carried out using a Chinese phytolith–climate calibration model based on weighted averaging partial least-squares regression. A series of cold and dry events, as indicated by the reconstructed MAT and MAP, are documented in the loess during the last glacial periods, which can be temporally correlated with the North Atlantic Heinrich events. Our MAT and MAP estimations show that the coldest and/or driest period occurred at the upper part of L2 unit (Late MIS 6), where MAT dropped to ca 4.4 °C and MAP to ca 100 mm. Two other prominent cold-dry periods occurred at lower Ll-5 (ca 77–62 ka) and L1-1 (ca 23–10.5 ka) where the MAT and MAP decreased to about 6.1–6.5 °C and 150–370 mm, respectively, ca 6.6–6.2 °C and 400–200 mm lower than today. However, the highest MAT (average 14.6 °C, max. 18.1 °C) and MAP (average 757 mm, max. 1000 mm) occurred at Sl interval (MIS 5). During the interstadial of L1-4–L1-2 (MIS 3) and during the Holocene warm-wet period, the MAT was about 1–2 °C and MAP 100–150 mm higher than today in the Weinan region. The well-dated MAT and MAP reconstructions from the Chinese Loess Plateau presented in this paper are the first quantitatively reconstructed proxy record of climatic changes at the glacial–interglacial timescale that is based on phytolith data. This study also reveals a causal link between climatic instability in the Atlantic Ocean and climate variability in the Chinese Loess Plateau.     

The Diavik waste rock project: Water flow through mine waste rock in a permafrost terrain

A field experiment is being carried out at the Diavik diamond mine in northern Canada to investigate the influence of unsaturated flow behavior on the quality of drainage from mine waste rock piles in a region of continuous permafrost. This paper is part of a series describing processes affecting the weathering of waste rock and transport of reaction products at this site; here the focus is on unsaturated water flow and its role in mass loading. Two 15 m-high instrumented test piles have been built on 60 m by 50 m collection systems, each consisting of lysimeters and a large impermeable high-density polyethylene (HDPE) liner. Collection lysimeters are installed nearby to investigate infiltration in the upper 2 m of the waste rock. Porosity, water retention curves, and hydraulic conductivity functions are estimated from field measurements and for samples ranging in size from 200 cm³ to 16 m³. Net infiltration in 2007 is estimated to have been 37% of the rainfall for mean annual rainfall conditions. Early-season infiltration freezes and is remobilized as the waste rock thaws. Wetting fronts migrate at rates of 0.2–0.4 m d⁻¹ in response to common rainfall events and up to 5 m d⁻¹ in response to intense rainfall. Pore water and non-reactive solutes travel at rates of <10⁻² to 3 × 10⁻² m d⁻¹ in response to common rainfall events and up to 0.7 m d⁻¹ in response to intense rainfall. Time-varying SO₄ mass loading from the base of the test piles is dictated primarily by the flow behavior, rather than by changes in solute concentrations.     

Effects of citrate on the dissolution and transformation of biotite,analyzed by chemical and atomic force microscopy

The aim of this study was to investigate the dissolution and transformation characteristics of phyllosilicate under low molecular weight organic acids in the farmland environment (pH 4.0–8.0). Changes of dissolution and morphology of biotite were evaluated using chemical extraction experiments and in situ/ex situ atomic force microscopy (AFM) with fluids of citric acid (CA) solution at pH 4.0, 6.0, and 8.0. Results of extracting experiments show that CA solutions contributed to the release rate of potassium (K), silicon (Si), and aluminum (Al) from biotite relative to a control aqueous solution. In situ AFM observations indicate that the dissolution of biotite from the biotite (0 0 1) surface occurred on the terrace, segment, and fringe of pits, while new etch pits did not readily form on biotite (0 0 1) surfaces in aqueous solutions. However, dissolution rates of terraces can be greatly accelerated with the help of citrate. In pH 4.0 CA solution, 70 min dissolution reactions of biotite (0 0 1) surfaces result in more etch pits than in pH 6.0 and 8.0 solutions. In addition, the transformation of biotite occurred simultaneously with the dissolution process. Secondary coating was observed on the biotite (0 0 1) surface after 140 h of immersion in a weak acid environment. Thus, the protons have a dominant role in the dissolution process of biotite with organic (carboxyl) acting as a catalyst under acidic condition. Based on the theory of interactions on a water–mineral interface in a weak acid environment, dissolution of biotite starts from defect/kink sites on the surface, one layer by one layer, and develops along the [h k 0] direction. A secondary coating that forms on the biotite (0 0 1) surface may restrain the formation and growth of etch pits, whereas this process may have a positive role on the stability of soil structure during long-term soil management.     

Fluctuations in late Neoproterozoic atmospheric oxidation — Cr isotope chemostratigraphy and iron speciation of the late Ediacaran lower Arroyo del Soldado Group (Uruguay)

Oxygenation of the Earth's atmosphere occurred in two major steps, near the beginning and near the end of the Proterozoic Eon (2500 to 542 Ma ago), but the details of this history are unclear. Chromium isotopes in iron-rich chemical sediments offer a potential to highlight fine-scale fluctuations in the oxygenation of the oceans and atmosphere and to add a further dimension in the use of redox-sensitive tracers to solve the question regarding fluctuations of atmospheric oxygen levels and their consequences for Earth's climate. We observe strong positive fractionations in Cr isotopes (δ⁵³Cr up to + 5.0‰) in iron-rich cherts and banded iron formation horizons within the Arroyo del Soldado Group (Ediacaran; Uruguay) that can be explained by rapid, effective oxidation of Fe(II)-rich surface waters. These fluctuations are correlated with variations in ratios of highly reactive iron (Fe_HR) to total iron (Fe_tot) which indicate a predominance of anoxic water columns (Fe_HR/Fe_tot > 0.38) during the onset of oxidation pulses. We favor the scenario by which isotopically heavy Cr(VI) entered the basin after pulses of oxidative weathering on land and in which Fe(II) accumulated in the water column. Neodymium isotopes reveal that these oxygenation pulses were followed by increased influxes to the basin of continental crust-derived detrital components of Paleoproterozoic (Nd T_DM model ages = 2.1–2.2 Ga) provenance typical of the Rio de la Plata Craton. The association of positive δ⁵³Cr–ferruginous (Fe_HR/Fe_tot > 0.38) stratigraphic intervals with low-diversity acritarch assemblages dominated by Bavlinella faveolata strongly support models postulating a stratified, eutrophic Neoproterozoic ocean. Thus, even within a few million years of the Precambrian–Cambrian boundary, paleoceanographic conditions resembled more those of Paleoproterozoic oceans than Phanerozoic and present oceans. This highlights the sheer magnitude of ecological changes at the Precambrian–Cambrian transition, changes which ultimately led to the demise of the Precambrian world and the birth of the metazoan-dominated Phanerozoic.     

Mercury speciation in soils of the industrialised Thur River catchment (Alsace,France)

Methylmercury (MeHg) and total Hg (THg) concentrations in soil profiles were monitored in the Thur River basin (Alsace, France), where a chlor-alkali plant has been located in the city of Vieux-Thann since the 1930s. Three soil types were studied according to their characteristics and location in the catchment: industrial soil, grassland soil and alluvial soil. Contamination of MeHg and THg in soil was important in the vicinity of the plant, especially in industrial and alluvial soil. Concentrations of MeHg reached 27 ng g⁻¹ and 29,000 ng g⁻¹ for THg, exceeding the predictable no effect concentration. Significant ecotoxicological risk exists in this area and remedial actions on several soil types are suggested. In each type of soil, MeHg concentrations were highest in topsoil, which decreased with depth. Concentrations of MeHg were negatively correlated with soil organic matter and total S, particularly when MeHg concentrations exceeded 8 ng g⁻¹. Under these conditions, MeHg concentrations in soil seemed to be influenced by THg, soil organic matter and total S concentrations. It was found that high MeHg/THg ratios (near 2%) in soil were mainly related to the combined soil environmental conditions such as low THg concentrations, low organic C/N ratios (<11) and relatively low pH (5–5.5). Nevertheless, even when the MeHg/THg ratio was low (∼0.04%), MeHg and THg concentrations were elevated, up to 13 ng g⁻¹ and to 29,000 ng g⁻¹, respectively. Thus, both THg and MeHg concentrations should be taken into account to assess potential environmental risks of Hg.     

Bioavailability and mobility of trace metals in phosphogypsum from Aqaba and Eshidiya,Jordan

As, Cd, Cr, Cu, U, and Zn in Aqaba and Eshidiya phosphogypsum showed relative uniformity in particle size presented by coarse (>0.212 mm); medium (0.212–0.053 mm); and fine (<0.053 mm) in the stacks by age of deposition. Jordan phosphogypsum contains low concentrations of As, Cd, Cr, Cu, U, and Zn compared to those reported worldwide. The bioavailability and mobility of trace metals were estimated using aqua regia leaching experiments. Leaching results have shown that around 3% of the As, 1% of the Cd, 5% of the Cr, 9% of the Cu, 4% of the U, and 3% of the Zn are transferred to the surrounding aquatic environment and/or soils. Mobility of trace metals in phosphogypsum was classified into three degrees: elements with high mobility were Cu and U; those with moderate mobility were As and Zn; and those with low mobility were Cd and Cr. It can be concluded that As, Cd, Cr, Cu, U, and Zn were not only uniformly distributed in the stack, but they are not leached from the phosphogypsum stacks in any significant amount, and then they are not easily transferred to the surrounding aquatic environment and/or soils.     

Mercury in European agricultural and grazing land soils

Agricultural (Ap, A_p-horizon, 0–20 cm) and grazing land soil samples (Gr, 0–10 cm) were collected from a large part of Europe (33 countries, 5.6 million km²) at an average density of 1 sample site/2500 km². The resulting more than 2 × 2000 soil samples were air dried, sieved to <2 mm and analysed for their Hg concentrations following an aqua regia extraction. Median concentrations for Hg are 0.030 mg/kg (range: <0.003–1.56 mg/kg) for the Ap samples and 0.035 mg/kg (range: <0.003–3.12 mg/kg) for the Gr samples. Only 5 Ap and 10 Gr samples returned Hg concentrations above 1 mg/kg. In the geochemical maps the continental-scale distribution of the element is clearly dominated by geology. Climate exerts an important influence. Mercury accumulates in those areas of northern Europe where a wet and cold climate favours the build-up of soil organic material. Typical anthropogenic sources like coal-fired power plants, waste incinerators, chlor-alkali plants, metal smelters and urban agglomerations are hardly visible at continental scales but can have a major impact at the local-scale.     

Impact of monsoonal climatic change on Holocene overbank flooding along Sushui River,middle reach of the Yellow River,China

In the semiarid loess regions, slackwater deposition of overbank flooding over the piedmont alluvial plains was episodic and alternated with dust accumulation and soil formation throughout the Holocene. The records of past hydrological events are therefore preserved within the architecture of loess and soils and are protected from subsequent erosion and destruction. Several Holocene loess–soil sequences with the deposits of overbank flooding over the semiarid piedmont alluvial plains in the southeast part of the middle reaches of the Yellow River drainage basin were investigated by field observation, OSL and C¹⁴ dating, measurement of magnetic susceptibility, particle-size distribution and chemical elements. This enables the reconstruction of a complete catalog of Holocene overbank flooding events at a watershed scale and an investigation of hydrological response to monsoonal climatic change as well. During the Holocene, there are six episodes of overbank flooding recorded over the alluvial plain. The first occurred at 11,500–11,000 a BP, i.e. the onset of the Holocene. The second took place at 9500–8500 a BP, immediately before the mid-Holocene Climatic Optimum. After an extended geomorphic stability and soil formation, the third overbank flooding episode came at about 3620–3520 a BP, i.e. the late stage of the mid-Holocene Climatic Optimum, and the floodwater inundated and devastated a Bronze-age town of the Xia Culture built on the alluvial plain, and therefore the town was abandoned for a period of ca 100 years. During the late Holocene, the alluvial plain experienced three episodes of overbank flooding at 2420–2170, 1860–1700 and 680–100 a BP, respectively. The occurrence of these overbank flooding episodes corresponds to the anomalous change in monsoonal climate in the middle reaches of the Yellow River drainage basin when rapid climate change or climatic decline occurs. During at least the last four episodes, both extreme floods and droughts occurred and climate departed from its normal condition, which was defined as a balanced change between the northwestern continental monsoon and southeastern maritime monsoon over time. Great floods occurred as a result of extreme rainstorms in summers caused by rare intensive meridianal airflows involving northwestward moving tropical cyclone systems from the Pacific. These results could be applied to improve our understanding of high-resolution climatic change, and of hydrological response to climatic change in the semiarid zones.     

Dissolution kinetics and biodurability of tremolite particles in mimicked lung fluids: Effect of citrate and oxalate

The effect of citrate and oxalate on tremolite dissolution rate was measured at 37 °C in non-stirred flow-through reactors, using modified Gamble’s solutions at pH 4 (macrophages), 7.4 (interstitial fluids) and 5.5 (intermediate check point) containing 0, 0.15, 1.5 and 15 mmol L⁻¹ of citrate or oxalate. The dissolution rates calculated from Si concentration in the output solutions without organic ligands depend on pH, decreasing when the pH increases from −13.00 (pH 4) to −13.35 (pH 7.4) mol g⁻¹ s⁻¹ and following a proton-promoted mechanism. The presence of both ligands enhances dissolution rates at every pH, increasing this effect when the ligand concentration increases. Citrate produces a stronger effect as a catalyst than oxalate, mainly at more acidic pHs and enhances dissolution rates until 20 times for solutions with 15 mmol L⁻¹ citrate. However, at pH 7.4 the effect is lighter and oxalate solutions (15 mmol L⁻¹) only enhances dissolution rates eight times respect to free organic ligand solutions. Dissolution is promoted by the attack to protons and organic ligands to the tremolite surface. Magnesium speciation in oxalate and citrate solutions shows that Mg citrate complexes are more effective than oxalate ones during the alteration of tremolite in magrophages, but this tendency is the opposite for interstitial fluids, being oxalate magnesium complexes stronger. The biodurability estimations show that the destruction of the fibers is faster in acidic conditions (macrophages) than in the neutral solutions (interstitial fluid). At pH 4, both ligands oxalate and citrate reduce the residence time of the fibers with respect to that calculated in absence of ligands. Nevertheless, at pH 7.4 the presence of ligands does not reduce significantly the lifetime of the fibers.