Natural oxidation of black carbon in soils: Changes in molecular form and surface charge along a climosequence

The aim of this work was to investigate changes in molecular form and surface charge of black carbon (BC) due to long-term natural oxidation and to examine how climatic and soil factors affect BC oxidation. Black C was collected from 11 historical charcoal blast furnace sites with a geographic distribution from Quebec, Canada, to Georgia, USA, and compared to BC that was newly produced (new BC) using rebuilt historical kilns. The results showed that the historical BC samples were substantially oxidized after 130 years in soils as compared to new BC or BC incubated for one year. The major alterations by natural oxidation of BC included: (1) changes in elemental composition with increases in oxygen (O) from 7.2% in new BC to 24.8% in historical BC and decreases in C from 90.8% to 70.5%; (2) formation of oxygen-containing functional groups, particularly carboxylic and phenolic functional groups, and (3) disappearance of surface positive charge and evolution of surface negative charge after 12 months of incubation. Although time of exposure significantly increased natural oxidation of BC, a significant positive relationship between mean annual temperature (MAT) and BC oxidation (O/C ratio with r = 0.83; P < 0.01) explained that BC oxidation was increased by 87 mmole kg C⁻¹ per unit Celsius increase in MAT. This long-term oxidation was more pronounced on BC surfaces than for entire particles, and responded 7-fold stronger to increases in MAT. Our results also indicated that oxidation of BC was more important than adsorption of non-BC. Thus, natural oxidation of BC may play an important role in the effects of BC on soil biogeochemistry.     

Selenium levels in Lebanese environment

Very limited research has been conducted on selenium (Se) in Lebanese soils and forage crops but no work has been done on Se in water and locally produced vegetables and grains. This research was conducted in order to quantify Se levels and its availability in agricultural soils, vegetables and grains in Lebanon. Sixty-six (66) soil samples were collected from 33 selected sites in Lebanon: the Bekaa Valley, coastal and mountainous regions. Thirteen (13) different plant types (86 samples) were sampled from the same locations. Also, 13 spring water and 10 bottled water samples were collected. Soil samples were analyzed for their physical and chemical properties. Selenium was extracted from soils with: deionized–distilled water (Soluble-Se), KH₂PO₄-0.1 M (MKP-Se) and concentrated (HNO₃ + HCl) mixture (acid-Se). Plant Se was extracted by acid digestion on a hotplate. Selenium concentrations were measured by the inductively coupled plasma-mass spectrophotometer (ICP-MS). The values of Soluble-Se, MKP-Se and acid-Se ranged between 47 and 142, 147 and 400, and 1749 and 4713 μg/kg, respectively, with average values of 95, 306, and 3118 μg/kg and at a ratio of 1:3:30. Thus, Se extracted with deionized–distilled water is a good indicator for Se availability in the studied soils. The average Se concentration in plants was in the following order: radish > lettuce > cucumber > cabbage > parsley > alfalfa > onion (leaves) > broccoli > tomato > mint > chickpeas > wheat > onion (bulbs). The Se levels in water samples were in the safe range (less than 50 μg/L) and ranged between 2.14 and 17.6 μg/L. The levels of Se in the three soil extractants were positively correlated with each other and with organic matter content, salinity and phosphorus (P). Selenium levels in plant samples were positively correlated at a 0.01 significance level with clay and P content.     

Quantification of vanadium adsorption by German soils

Vanadium adsorption by soils representing different soil types from Germany has been studied. For 30 soils ‘Freundlich’ type sorption isotherms have been deduced from laboratory vanadium(V) adsorption experiments. The native adsorbed vanadium quantity of a soil (S₀) and the Freundlich parameters m and log k have been determined by non linear regression of the experimental data to the Freundlich model. Pronounced differences in vanadium adsorption of different soils exist and could be quantified. The vanadium adsorption data could be generalized by grouping the soils into four classes according to their vanadium adsorption properties. For each class (sandy soils, top soils, sub soils with pH < 5.5, and sub soils with pH > 5.5) mean Freundlich parameters m and log k have been calculated to be 0.59, 0.72, 0.52, 0.57 and 2.55, 2.89, 4.29, 3.41, respectively. These parameters can be used to estimate vanadium sorption properties of soils for which no vanadium sorption experiments are available. Aqua regia soluble vanadium contents of the studied soils (range 1.7–143 mg/kg; median 32 mg/kg) and leached vanadium concentrations from experiments without vanadium addition (range 0.08–37 µg/l; median 2.1 µg/l) are also given.     

Temporal variations in the concentration and isotopic signature of ammonium- and nitrate–nitrogen in soils under a breeding colony of Black-tailed Gulls (Larus crassirostris) on Kabushima Island,northeastern Japan

Temporal variations in the concentration and N isotopic ratios of inorganic N (NH₄– and NO₃–N) as affected by the soil temperature regime together with the input of bird excreta were analyzed in a sedentary soil under a dense colony (1.6 nests/m²) of breeding Black-tailed Gulls (Laruscrassirostris: a ground-nesting seabird). Surface soil samples were taken monthly from mid-March to late July 2005 from Kabushima Island, Hachinohe, northeastern Japan. The spatial concentration of inorganic N in the soils varied considerably on all sampling dates. There may be a statistically significant trend, showing increased NH₄–N content from settlement up to early June when the input of fecal N attains its maximum, and then decreases towards the end of breeding activity (early August). Abundant NO₃–N was observed in all soils, particularly in the later stage of breeding (up to 3800 mg-N/kg dry soil), refuting earlier claims that nitrification is unimportant in the soils. δ¹⁵N values of NH₄ in the soils showed unusually high values up to +51‰, reflecting N isotope fractionation due to volatilization of NH₃ during the mineralization. Mean δ¹⁵N values of the monthly collected totals of NH₄ and NO₃ were not significantly different at the 5% level based on ANOVA and significant differences were observed only among the three means of NO₃–N collected in mid-March (settlement of colony: δ¹⁵N = −0.2 ± 3.5‰) and late July (later stages of breeding: δ¹⁵N = +22.1 ± 7.0‰, +23.3 ± 7.8‰) at the 1% and 5% levels by t-test, respectively. Such an observation of significantly increased δ¹⁵N values for NO₃–N in soils from the fledgling stage indicates the integration of denitrification coupled with nitrification under a limited supply of fecal N.     

Stoichiometry of a dissolution reaction of a trioctahedral vermiculite at pH 2.7

Transformation of vermiculite to hydroxy-interlayered vermiculites (HIV) significantly modifies the physicochemical properties of the original mineral. HIV is a common phase in acid soils, nevertheless its formation remains poorly understood. The main goal of this paper was to clarify the kinetics and process of interlayer aluminization of pure vermiculite using an experimental design. For this purpose, we monitored the dissolution of Na-saturated vermiculite in dilute HCL at pH 2.7, at 50 °C for 672 h in stirred flow-through reactors. Both reacted samples at different dissolution steps, and the leaching of elements, were investigated. The main result was a rapid change to hydroxy-interlayered vermiculite, with a decrease in CEC and a progressive displacement of d(0 0 1) reflection near 1.4 nm after K saturation, resulting from formation of hydroxy-interlayer material. Vermiculite was found to dissolve non-stoichiometrically for 500 h; after that, the release rate for Si, Mg and Al became stoichiometric with respect to vermiculite chemistry. By contrast, Fe sustained non-stoichiometric release throughout the whole experiment. At the steady state, i.e., after 500 h, a dissolution rate of 8.8 ± 0.1 × 10⁻¹¹ mol vermiculite m⁻² s⁻¹ was found with respect to Si. Both Al and Fe precipitated in the interlayer space, and their amounts calculated at the end of the experiment were 3.74 × 10⁻⁴ mol g⁻¹ of vermiculite for Al and 8.74 × 10⁻⁵ for Fe. The rate of interlayer aluminization increased for 60 h and then regularly decreased. Al-interlayering stopped after 288 h, but Fe still precipitated in the interlayer space.A comparison with the same mineral incubated for three years in acid soils revealed that the reaction was proton-promoted. The same pattern of CEC decrease and interlayer aluminization was observed, but the kinetics were slower due to soil environmental conditions.     

Impact of soil weathering degree on silicon isotopic fractionation during adsorption onto iron oxides in basaltic ash soils, Cameroon

The sequestration of silicon in soil clay-sized iron oxides may affect the terrestrial cycle of Si. Iron oxides indeed specifically adsorb aqueous monosilicic acid (H₄SiO₄⁰), thereby influencing Si concentration in soil solution. Here we study the impact of H₄SiO₄⁰ adsorption on the fractionation of Si isotopes in basaltic ash soils differing in weathering degree (from two weathering sequences, Cameroon), hence in clay and Fe-oxide contents, and evaluate the potential isotopic impact on dissolved Si in surrounding Cameroon rivers. Adsorption was measured in batch experiment series designed as function of time (0-72 h) and initial concentration (ic) of Si in solution (0.61-1.18 mM) at 20 °C, constant pH (5.5) and ionic strength (1 mM). After various soil-solution contact times, the δ³⁰Si vs. NBS28 compositions were determined in selected solutions by MC-ICP-MS (Nu Plasma) in medium resolution, operating in dry plasma with Mg doping with an average precision of ±0.15‰ (±2σ_SEM). The quantitative adsorption of H₄SiO₄⁰ by soil Fe-oxides left a solution depleted in light Si isotopes, which confirms previous study on synthetic Fe-oxides. Measured against its initial composition (δ³⁰Si = +0.02 ± 0.07‰ (±2σ_SD)), the solutions were systematically enriched in ³⁰Si reaching maximum δ³⁰Si values ranging between +0.16‰ and +0.95‰ after 72 h contact time. The enrichment of the solution in heavy isotopes increased with increasing values of three parameters: soil weathering degree, iron oxide content, and proportion of short-range ordered Fe-oxide. The Si-isotopic signature of the solution was partly influenced by Si release, possibly through mineral dissolution and Si desorption from oxide surfaces, depending on soil type, highlighting the complex pattern of natural soils. Surrounding Cameroon rivers displayed a mean Si-isotopic signature of +1.19‰. Our data imply that in natural environments, H₄SiO₄⁰ adsorption by soil clay-sized Fe-oxides at least partly impacts the Si-isotopic signature of the soil solution exported to water streams.     

Effect of soil aging on arsenic fractionation and bioaccessibility in inorganic arsenical pesticide contaminated soils

Ingestion of As – contaminated soil by children is a growing concern in former agricultural lands converted to residential or recreational land use areas. The mobility and bioavailability of As is controlled by its reactions with soil particles. The degree and strength of As retention by soil constituents may vary greatly with time. The present authors hypothesize that aging results in reduced mobility of As thereby decreasing As release and its bioavailability. The present study is aimed at evaluating the effect of aging on soil As fractionation and bioaccessibility in a temperature and humidity-controlled greenhouse setting. The design allowed the evaluation of dynamic interactions between soils, pesticides, water, and plants. Therefore, 4 soil types (Immokalee, Millhopper, Pahokee Muck, and Orelia) were selected based on their potential differences in As reactivity. The soils were amended with the pesticide Na arsenate at two rates. Rice was used as the test crop. Soil samples collected after different time periods (0, 6 months, 1 a and 3 a) were extracted for soil-As forms via a sequential extraction technique. Bioaccessible As was extracted via an in vitro gastrointestinal method. At time 0, most of the extractable As in soil was in the soluble form, resulting in high bioaccessibility. As expected, soluble and exchangeable fractions decreased with time for up to 6 months, but remained constant thereafter. After 3 a of soil–pesticide equilibration, As bioaccessibility was still high in all the soils except for the Pahokee Muck. No significant difference in As bioaccessibility was observed between the soils. Arsenic was present predominantly as As(V) with 5–10% of the total dissolved As being present as As(III). Data obtained suggest that although aging had an impact on the geochemical forms, gastric pH was the sole important factor effecting As bioaccessibility.     

Analysis of raw soils and their re-suspended PM10 fractions: Characterisation of source profiles and enrichment factors

In this work, the inorganic chemical profiles of soil samples collected at different sites in the Salentum peninsula (Italy, Apulia region) are discussed. The samples were re-suspended in the laboratory, for PM10 sampling, using a ventilated wooden chamber and were then chemically analysed measuring the abundances of 17 elements. Different land use categories of soils (olive grove, arable land, vineyards, sand, and urban dust) were included in the 50 samples analysed: 45 collected in background areas and five collected in the urban area of Lecce. The objectives were to compare the chemical profiles of raw soil and re-suspended PM10 for different crustal sources and to estimate the potential improvements in the calculation of the enrichment factors of atmospheric PM10. The variability of elemental abundances in samples of the same category of soil collected in different zones was of the same order of magnitude as the differences observed between the various categories of soil. This allows the calculation of a weighted average composition of soil and urban dust and the corresponding weighted average composition of re-suspended PM10. In re-suspended PM10 from average background soil, all of the elements except Ca, Na, K and V have larger abundances with respect to raw soil. In urban dust, this is limited to Ca, V and Mg. The crustal enrichment factors (EFs) of atmospheric PM10 were evaluated by considering different reference elements and different reference tables. Results indicated that it is possible to apply a two-threshold (S₁ and S₂) scheme for the interpretation of EF, with thresholds derived from uncertainty in soil categories and from the choice of the reference element. A specific element is likely of crustal origin if EF < S₁ and likely of anthropogenic origin if EF > S₂. Between the two thresholds, the element can be considered of mixed origin. The thresholds vary according to the geological composition used in the evaluation of EF. If the average composition of local re-suspended soils is used, the thresholds are S₁ = 2 and S₂ = 4. If raw soil is used, the thresholds become S₁ = 5 and S₂ = 10. If the average upper-crust composition from literature data is used, the thresholds further increase to S₁ = 10 and S₂ = 20.     

Enrichment of metals in soils subjected to different land uses in a typical Mediterranean environment (Murcia City, southeast Spain)

Industrialization, urbanization, and agricultural practices are 3 of the most important sources of metal accumulations in soils. Concentrations of Cr, Mn, Ni, Cu, Pb, Zn and Cd were determined in surface soils collected under different land uses, including urban (UR), industrial (IN-1 and IN-2), agricultural (AG), abandoned unused (AB), and natural (NA) sites to examine the influence of anthropogenic activities on metals in soils formed in a typical Mediterranean environment. The highest concentrations of Cr, Cd, and Pb observed in the NW industrial area (IN-2) were 63.7, 3.34 and 2330 mg metal kg⁻¹ soil, for each metal, respectively. The SW industrial area (IN-1) contained the highest Zn content at 135 mg kg⁻¹. However, soils with the highest concentrations of Ni and Cu were located in AG sites at 30.9 and 64.9 mg kg⁻¹ soil, respectively. Sampling locations with the highest concentrations of Mn were identified in AB sites. Using the concentrations of metals at the NA sites as the baseline levels, soils collected from all other land uses in the study area exhibited significantly higher total contents of Zn, Mn, Cr and Ni. Metal enrichment was attributed to fertilizer and pesticide applications, industrial activities, and metal deposition from a high volume of vehicular traffic (for Pb and Cd). High concentrations of Mn in some samples were attributed to parent materials. The study demonstrated that anthropogenic activities associated with various land uses contribute to metal accumulation in soils and indicated a need to closely monitor land management practices to reduce human and ecological risks from environmental pollution.     

Global distributions of Fe,Al, Cu,and Zn contained in Earth's derma layers

To improve the usefulness and accuracy of modeling Earth's anthrobiogeochemical metal cycles, global maps at approximately 1° × 1° are produced of the concentrations and masses of Fe, Al, Cu, and Zn contained in continental sediments and soils. The maps generated utilize inverse distance weighting (IDW) and cokriging to generate new estimates for geospatially weighted mean global concentrations for these metallic micronutrients. Sediment metal concentration maps are generated from IDW of sediment samples; global soil maps are produced via cokriging upon an underlying parent rock dataset composed of both surface bedrock and sediment samples. Derived are independent estimates for the global mean concentrations in continental sediments (Fe = 3.1 wt.%, Al = 6.1 wt.%, Cu = 45 μg/g, Zn = 86 μg/g) and soils (Fe = 2.5 wt.%, Al = 3.9 wt.%, Cu = 17 μg/g, Zn = 50 μg/g). While continental sediment concentrations for Cu are within the range of previous estimates, Zn concentrations are relatively higher, ~ 20 μg/g above previous estimates. Fe and Al are slightly depleted (~ 1 wt.%) in continental sediments relative to previous estimates, likely ascribable to sampling bias and error inherent in the comparative methodologies. Besides an estimated global mean, metal concentrations in soils are also broken down by FAO soil group. Metal masses in sediments and soils remain within 30% of previous, non-spatial estimates. These maps also illustrate the discernable spatial variability across the Earth's surface. Despite data gaps, maps of metal mass show regional patterns such as the high quantities of Al in the soils and biomass of the Amazonia and Congo regions. Concentrations of metals are relatively high in the anthrosols of China. Finally, this analysis highlights those areas for which generating and providing publically available geochemical data should be prioritized. For instance, gypsisols, lixisols, and nitisols have little to no analytical data available on metal contents. A sensitivity analysis suggests that the most poorly constrained soil metal concentrations occur in the thick, old tropical soils of central Africa and the anthrosols of eastern China.     

Quantitative C NMR of whole and fractionated Iowa Mollisols for assessment of organic matter composition

Both the concentrations and the stocks of soil organic carbon vary across the landscape. Do the amounts of recalcitrant components of soil organic matter (SOM) vary with landscape position? To address this question, we studied four Mollisols in central Iowa, two developed in till and two developed in loess. Two of the soils were well drained and two were poorly drained. We collected surface-horizon samples and studied organic matter in the particulate organic matter (POM) fraction, the clay fractions, and the whole, unfractionated samples. We treated the soil samples with 5 M HF at ambient temperature or at 60 °C for 30 min to concentrate the SOM. To assess the composition of the SOM, we used solid-state nuclear magnetic resonance (NMR) spectroscopy, in particular, quantitative ¹³C DP/MAS (direct-polarization/magic-angle spinning), with and without recoupled dipolar dephasing. Spin counting by correlation of the integral NMR intensity with the C concentration by elemental analysis showed that NMR was ?85% quantitative for the majority of the samples studied. For untreated whole-soil samples with <2.5 wt.% C, which is considerably less than in most previous quantitative NMR analyses of SOM, useful spectra that reflected ?65% of all C were obtained. The NMR analyses allowed us to conclude (1) that the HF treatment (with or without heat) had low impact on the organic C composition in the samples, except for protonating carboxylate anions to carboxylic acids, (2) that most organic C was observable by NMR even in untreated soil materials, (3) that esters were likely to compose only a minor fraction of SOM in these Mollisols, and (4) that the aromatic components of SOM were enriched to ∼53% in the poorly drained soils, compared with ∼48% in the well drained soils; in plant tissue and particulate organic matter (POM) the aromaticities were ∼18% and ∼32%, respectively. Nonpolar, nonprotonated aromatic C, interpreted as a proxy for charcoal C, dominated the aromatic C in all soil samples, composing 69-78% of aromatic C and 27-36% of total organic C in the whole-soil and clay-fraction samples.     

Adsorption and surface complexation modeling of palladium,rhodium and platinum in surficial semi-arid soils and sediment

Sorption isotherms, time-dependent adsorption and surface complexation modeling studies were used to investigate the post-depositional mobility of three of the platinum group-elements (Pd, Rh, and Pt) in semi-arid soil and sediment samples with varying surface properties. The acidity constants (Log K_a1 and Log K_a2), optimized from batch titration data, ranged from 4.69 to 5.34 for Log K_a1 and from −6.51 to −7.61 for Log K_a2, suggesting the occurrence of both protonation and deprotonation reactions on the solid surfaces. Partition coefficients and removal rates of the metals had a general trend of Pd > Pt > Rh. The sediment sample, with the highest clay content and exchangeable cation concentrations, also had the highest affinity for the metals. The times required for sediment to adsorb 63% of the metals were 2.63 h, 4.08 h and 10.64 h for Pd, Pt and Rh, respectively. The FITEQL program successfully optimized the conditional binding constants of the metals on the solids from batch adsorption data. The constants decreased in the order of Pd > Rh > Pt, which was consistent with the observed high affinity of the solids for Pd. The modeling results also showed that aqueous Pd was the least sensitive to pH followed by Rh and Pt. However, metal adsorption below the points of zero net proton charges (ca. pH 6.7) is attributable to the involvement of permanent negatively charged binding sites in the adsorption process. Notably, partition coefficients, removal rates and conditional binding constants all showed a high affinity of Pd for the solids. A similarity between the model outputs and the batch adsorption data indicates the suitability of the model for describing the mobility and retention of the three metals in semi-arid soils and sediments.     

Variations of δSi and Ge/Si with weathering and biogenic input in tropical basaltic ash soils under monoculture

In soils, silicon released by mineral weathering can be retrieved from soil solution through clay formation, Si adsorption onto secondary oxides and plant uptake, thereby impacting the Si-isotopic signature and Ge/Si ratio of dissolved Si (DSi) exported to rivers. Here we use these proxies to study the contribution of biogenic Si (BSi) in a soil-plant system involving basaltic ash soils differing in weathering degree under intensive banana cropping. δ³⁰Si and Ge/Si ratios were determined in bulk soils (<2 mm), sand (50-2000 μm), silt (2-50 μm), amorphous Si (ASi, 2-50 μm) and clay (<2 μm) fractions: δ³⁰Si by MC-ICP-MS Nu Plasma in medium resolution, operating in dry plasma with Mg doping (δ³⁰Si vs. NBS28 ± 0.12‰ ± 2σ_SD), Ge/Si computed after determination of Ge and Si concentrations by HR-ICP-MS and ICP-AES, respectively. Components of the ASi fraction were quantified by microscopic counting (phytoliths, diatoms, ashes). Compared to fresh ash (δ³⁰Si = −0.38‰; Ge/Si = 2.21 μmol mol⁻¹), soil clay fractions (<2 μm) were enriched in light Si isotopes and Ge: with increasing weathering degree, δ³⁰Si decreased from −1.19 to −2.37‰ and Ge/Si increased from 4.10 to 5.25 μmol mol⁻¹. Sand and silt fractions displayed δ³⁰Si values close to fresh ash (−0.33‰) or higher due to saharian dust quartz deposition, whose contribution was evaluated by isotopic mass balance calculation. Si-isotopic signatures of bulk soils (<2 mm) were strongly governed by the relative proportions of primary and secondary minerals: the bulk soil Si-isotopic budget could be closed indicating that all the phases involved were identified. Microscopic counting highlighted a surface accumulation of banana phytoliths and a stable phytolith pool from previous forested vegetation. δ³⁰Si and Ge/Si values of clay fractions in poorly developed volcanic soils, isotopically heavier and Ge-depleted in surface horizons, support the occurrence of a DSi source from banana phytolith dissolution, available for Si sequestration in clay-sized secondary minerals (clay minerals formation and Si adsorption onto Fe-oxide). In the soil-plant system, δ³⁰Si and Ge/Si are thus highly relevant to trace weathering and input of DSi from phytoliths in secondary minerals, although not quantifying the net input of BSi to DSi.     

Environmental impact of ancient small-scale mercury ore processing at Pšenk on soil (Idrija area, Slovenia)

The Idrija mine was the second largest Hg mine in the world surpassed only by the Almaden mine in Spain. It has been estimated that almost 145,000 tons of Hg was produced during operation (1490-1995) of the mine. In the first decade of Hg mining in Idrija the ore was roasted in piles; after that it was roasted for 150 years, until 1652, in earthen vessels at various sites in the woods around Idrija. Pšenk is one out of 21 localities of ancient roasting sites established on the hills surrounding Idrija and one of the largest localities of roasting vessel fragments. The unique way of roasting very rich Hg ore at this site has resulted in soil contamination and considerable amounts of waste material that potentially leach Hg into the surrounding environment. The main aim of this study was to determine the distribution and the forms of Hg in contaminated soils in order to evaluate potential environmental risk. Detailed soil sampling was performed on 37,800 m² area to establish the extent of Hg pollution and to investigate Hg transformations and transport characteristics through the 400 a-long period. A total of 156 soil (0-15 cm and 15-30 cm) and SOM (soil organic matter) samples were collected from 73 sampling points. Three soil profiles were sampled to determine vertical distribution of Hg. The main Hg phases were determined by the Hg-thermo-desorption technique. The measured Hg contents in soil samples in the study area vary from 5.5 to almost 9000 mg/kg with a median of 200 mg/kg. In SOM, Hg contents range from 1.4 to 4200 mg/kg with a median of 20 mg/kg. Extremely high Hg contents were found in soil profiles where the metal reaches 37,020 mg/kg. In general, Hg concentrations in all three profiles show a gradual decrease with depth with the minimum values between 140 mg/kg and 1080 mg/kg. The Hg-thermo-desorption curves indicate the presence of Hg in the form of cinnabar and that of Hg bound to organic or mineral soil matter. The distribution of Hg species in soil and SOM samples show almost equal distribution of cinnabar and non-cinnabar Hg compounds. The non-cinnabar fraction shows a little increase with depth, but cinnabar represents a high portion of total Hg (about 40%). Large amounts of potentially mobile and transformable non-cinnabar Hg compounds exist at the roasting site, which are potentially bioavailable.     

Effect of resident microbiota on the solubilization of gold in soil from the Tomakin Park Gold Mine, New South Wales, Australia

The processes influencing the solubilization and observed mobility of Au in soil were studied using a combination of geochemical and microbiological techniques. In this study, we demonstrate for the first time that biotic processes mediated by the resident microbiota are likely to control the mobilization of Au in auriferous soils and other regolith materials. Microcosms with auriferous soils from the Tomakin Park Gold Mine in temperate south eastern New South Wales, Australia, were incubated under biologically active versus inactive (sterilized) conditions. The soils were incubated oxic and anoxic, unamended and Au pellet- or cycloheximide amended for 70 days in a 1:4 (w:v) aqueous slurry at 25 °C in the dark. In biologically active unamended Ah- and B-horizon microcosms up to 80 wt.% of total Au was detected in solution after 45 days of incubation. In biologically active Au pellet amended microcosms Au was liberated from the soil and also from added Au pellets. Scanning electron microscopy and nucleic acid staining combined with confocal stereo laser microscopy revealed the presence of bacterial biofilms on Au pellets incubated in the biologically active microcosms. The biologically inactive microcosms displayed no or significantly reduced Au solubilization. After 40-50 days of incubation Au was generally re-adsorbed to the solid soil fractions. The results of sequential extractions conducted with dried slurry samples collected from the biologically active Ah-horizon microcosms after 0, 10, 20, 30, 40, and 68 days of incubation indicated a continuous microscale solubilization and re-adsorption of Au. In samples taken after 40 days of incubation more than 80 wt.% of the Au was extracted from the operationally defined organic fraction, which appears to act as a final re-adsorption site for Au in the soil. In samples taken after 10 days of incubation from microcosms amended with 100 μg g⁻¹ (d.w. soil) of Au as AuCl₄⁻ 95 wt.% of the Au was associated with the organic fraction. To establish a mechanistic link between Au dissolution and re-adsorption with the activity of the heterotrophic bacterial community, analysis of the community structure based on carbon utilization patterns using was conducted. The bacterial community structure changed from a carbohydrate- and polymer-utilizing to a carboxylic- and amino acid utilizing community concurrently with the change from Au solubilization to re-adsorption. The bacterial community in the early stages of incubation (0-30 days) apparently produced an excess of amino acids, which are known to form stable amino acid Au complexes. The bacterial community in the later stages of incubation (after 40-50 days) metabolized these Au complexing ligands and Au, which apparently became unstable in the solution, was re-adsorbed to the solid soil fractions.     

Origin and mobility of hexavalent chromium in North-Eastern Attica,Greece

An integrated framework that is comprised of field surveys of groundwater, surface water and soils, laboratory process experiments and hydrologic and geochemical modeling is used to identify the origin (anthropogenic versus geogenic sources), fate and transport of hexavalent Cr in Tertiary and Quaternary deposits of Oropos plain in Greece. Groundwater and soils were analyzed in May 2008 and exhibited considerable Cr concentrations. Mineralogical analysis and micro-XRF analysis of the heavy soil fractions (metallic components) showed Cr bearing phases like chromites, Cr-silicate phases with positive correlation between Si, Al, Fe and Cr soil concentrations. Column experiments showed the Cr(VI) desorption ability of soils, e.g. concentration of 20 μg L⁻¹ was detected after the application of 50 mm of rain. The groundwater model simulated the variability of Cr concentrations emanating from both anthropogenic and geogenic sources, successfully using rate constants obtained from the laboratory experiments, e.g. 4.24 nM h⁻¹ for serpentine soil and 0.77 nM h⁻¹ for soil in alluvial deposits. The mineralogical and geochemical results support a geogenic origin for Cr in soils and groundwater of Oropos plain while modeling results suggest that contaminants transported by Asopos River have affected only the upper layers of the subsurface in the vicinity of the river. The framework can be used to establish background concentrations or clean up levels of Cr-contaminated soils and groundwater.     

The role of rare rainstorms in the formation of calcic soil horizons on alluvial surfaces in extreme deserts

Soils in similar geomorphic settings in hyperarid deserts (< 50 mm yr⁻¹) should have similar characteristics because a negative moisture balance controls their development. However, Reg soils in the hyperarid southern Negev and Namib deserts are distinctly different. Soils developed on stable alluvial surfaces with only direct input of rainfall and dust depend heavily on rainfall characteristics. Annual rainfall amount can be similar (15-30 mm), but storm duration can drastically alter Reg soil properties in deserts. The cooler fall/winter and dry hot summers of the southern Negev Desert with a predominance brief (≤ 1 day) rainstorms result in gypsic-saline soils without any calcic soil horizon. Although the Namib Desert receives only 50-60% of the southern Negev annual rainfall, its rainstorm duration is commonly 2-4 days. This improves leaching of the top soil under even lower annual rainfall amount and results in weeks-long grass cover. The long-term cumulative effect of these rare rain-grass relationships produces a calcic-gypsic-saline soil. The development of these different kinds of desert soils highlights the importance of daily to seasonal rainfall characteristics in influencing soil-moisture regime in deserts, and has important implications for the use of key desert soil properties as proxies in paleoclimatology.     

Beryllium-10 concentrations of tektites from the Ivory Coast and from Central Europe: Evidence for near-surface residence of precursor materials

By using accelerator mass spectrometry, we measured ¹⁰Be (T_1/2 = 1.5 Ma) concentrations in nine Ivory Coast (IVC) tektites, in six soil samples collected near the Bosumtwi impact crater, the likely source region, and in a depth profile taken through a 23 g moldavite. In the core of the moldavite sample we also measured an upper limit on the ³⁶Cl (T_1/2 = 0.3 My) concentration. The average ¹⁰Be concentration in IVC tektites of (22 ± 11) × 10⁶ atom/g exceeds reasonable limits for a meteoritic component or cosmic-ray production in situ after tektite formation. The ¹⁰Be must be meteoric, which implies that IVC tektites formed from soils or sediments. Corrected to the time of formation (ToF) 1.07 Ma ago and for a small in situ component, the average ¹⁰Be concentration of (35 ± 7) × 10⁶ atom/g (1 − σ mean) is considerably lower than those of contemporary Bosumtwi soils, ∼250 × 10⁶ atom/g, or of Australasian tektites at their ToF, 0.8 Ma B.P. near Lake Bosumtwi today the soil column is only ∼1 m thick. If the landscape was similar 1.07 Ma ago, then the total thickness of the tektite formation zone probably did not exceed 10 m. With increasing depth below the surface of the moldavite, the ¹⁰Be concentrations decrease rapidly owing to the presence of a surface component, probably of recent origin. The main interior mass of the sample contains ∼0.8 × 10⁶ atom ¹⁰Be/g and fewer than 0.1 × 10⁶ atom ³⁶Cl/g, little of which can be meteoritic. Although not definitive, consideration of several possible cosmic-ray exposure histories suggests that about half the interior ¹⁰Be has a meteoric origin, which if corrected to the time of formation yields a concentration compatible with those measured in typical contemporary soils. The observations are consistent with the formation of three of the four main tektite groups from surface soils or sediments.     

Formation of Zn-rich phyllosilicate, Zn-layered double hydroxide and hydrozincite in contaminated calcareous soils

Recent studies demonstrated that Zn-phyllosilicate- and Zn-layered double hydroxide-type (Zn-LDH) precipitates may form in contaminated soils. However, the influence of soil properties and Zn content on the quantity and type of precipitate forming has not been studied in detail so far. In this work, we determined the speciation of Zn in six carbonate-rich surface soils (pH 6.2-7.5) contaminated by aqueous Zn in the runoff from galvanized power line towers (1322-30,090 mg/kg Zn). Based on 12 bulk and 23 micro-focused extended X-ray absorption fine structure (EXAFS) spectra, the number, type and proportion of Zn species were derived using principal component analysis, target testing, and linear combination fitting. Nearly pure Zn-rich phyllosilicate and Zn-LDH were identified at different locations within a single soil horizon, suggesting that the local availabilities of Al and Si controlled the type of precipitate forming. Hydrozincite was identified on the surfaces of limestone particles that were not in direct contact with the soil clay matrix. With increasing Zn loading of the soils, the percentage of precipitated Zn increased from ∼20% to ∼80%, while the precipitate type shifted from Zn-phyllosilicate and/or Zn-LDH at the lowest studied soil Zn contents over predominantly Zn-LDH at intermediate loadings to hydrozincite in extremely contaminated soils. These trends were in agreement with the solubility of Zn in equilibrium with these phases. Sequential extractions showed that large fractions of soil Zn (∼30-80%) as well as of synthetic Zn-kerolite, Zn-LDH, and hydrozincite spiked into uncontaminated soil were readily extracted by 1 M NH₄NO₃ followed by 1 M NH₄-acetate at pH 6.0. Even though the formation of Zn-precipitates allows for the retention of Zn in excess to the adsorption capacity of calcareous soils, the long-term immobilization potential of these precipitates is limited.