Fine Soil Fraction Research Articles

Chemical and mineralogical changes in a Brazilian Rhodic Paleudult under different land use and managements

Changes in land use and management can affect the dynamic equilibrium of soil systems and induce chemical and mineralogical alterations. This study was based on two long-term experiments (10 and 27 years) to evaluate soil used for no-tillage maize cultivation, with and without poultry litter application (NTPL and NTM), and with grazed native pasture fertilized with cattle droppings (GrP), on the chemical and mineralogical characteristics of a Rhodic Paleudult in Southern Brazil, in comparison with the same soil under native grassland (NGr). In the four treatments, soil was sampled from the 0.0-2.5 and 2.5-5.0 cm layers. In the air-dried fine soil (ADFS) fraction (∅ < 2 mm), chemical characteristics of solid and liquid phases and the specific surface area (SSA) were evaluated. The clay fraction (∅ < 0.002 mm) in the 0.0-2.5 cm layer was analyzed by X-ray diffraction (XRD) after treatments for identification and characterization of 2:1 clay minerals. Animal waste application increased the total organic C concentration (COT) and specific surface area (SSA) in the 0.0-2.5 cm layer. In comparison to NGr, poultry litter application (NTPL) increased the concentrations of Ca and CECpH7, while cattle droppings (GrP) increased the P and K concentrations. In the soil solution, the concentration of dissolved organic C was positively related with COT levels. With regard to NGr, the soil use with crops (NTM and NTPL) had practically no effect on the chemical elements in solution. On the other hand, the concentrations of most chemical elements in solution were higher in GrP, especially of Fe, Al and Si. The Fe and Al concentrations in the soil iron oxides were lower, indicating reductive/complexive dissolution of crystalline forms. The X-ray diffraction (XRD) patterns of clay in the GrP environment showed a decrease in intensity and reflection area of the 2:1 clay minerals. This fact, along with the intensified Al and Si activity in soil solution indicate dissolution of clay minerals in soil under cattle-grazed pasture fertilized with animal droppings.

Hydromechanical model for internal erosion and its relationship with the stress transmitted by the finer soil fraction

We present a theoretical model to determine the hydromechanical boundary of an internally unstable soil subject to vertical seepage. The model is based in momentum balance equations, which consider that the system is divided into three components: water, finer soil fraction, and coarser soil fraction. The parameters of the model are as follows: the effective stress, the porosity of the soil, the friction angle between the coarse and fine fractions, and the proportion of the effective stress that is transmitted to the finer fraction (G*). Using laboratory data collected on a large permeameter, we demonstrate that the model is able to properly describe the observed behavior. Furthermore, we show that the value of G* is related to the value of D 15′/d 85′ proposed by Kezdi (Soil physics—selected topics. Elsevier, Amsterdam, 1979) and that it has the same trend as found experimentally by Skempton and Brogan (Geotechnique 44:449–460, 1994). The proposed model is a promising method to deduce an approximate value of critical hydraulic gradient that triggers internal erosion in a cohesionless soil of known particle size distribution curve.

A relatively simple scaling method for describing the unsaturated hydraulic functions of stony soils

AbstractFew if any methods exist to estimate the effects of stone content (stoniness) on the unsaturated soil hydraulic properties. A relatively simple scaling method is presented to estimate the hydraulic conductivity of unsaturated stony soils having different stone contents. A key assumption of the method is that van Genuchten's water retention parameters α and n of the fine soil fraction are the same as those of the stony soil. The method further assumes a linearly decreasing relationship between the saturated hydraulic conductivity and the stone content, based on previous numerical simulations. Using the proposed method, it is possible to calculate the hydraulic conductivity of unsaturated stony soils, knowing the saturated hydraulic conductivity of the fine soil fraction, the retention curve of the fine soil fraction, and the particular stoniness of the soil.

Counter-Current Metal Leaching and Precipitation for Soil Remediation

Chemical acid leaching is an effective technique for extracting toxic metals from the finest fractions of polluted soils. Nevertheless, the use of large quantities of reagents and process water results in prohibitive operating costs. The purpose of this study was to evaluate the technical and economic advantages of recirculating water in a counter-current leaching process (CCLP). Five 1-h sulfuric acid extraction steps (at pH = 1.5) followed by three 5-min water-washing steps were applied to the fine particle fraction (<0.125 mm) of an industrial soil polluted by Cd (13.2 mg·kg−1), Cu (3 100 mg·kg−1), Mn (685 mg kg−1), Pb (550 mg·kg−1), and Zn (2 840 mg·kg−1). The leaching experiments were carried out at ambient temperature using a 10% soil suspension and in 1-L working volume stirred tank reactors. This paper presents results of conventional and counter-current leaching process (CCLP) tests and shows that the CCLP yields removal results for Cu (85%), Zn (86%), Mn (75%), and Cd (90%) that are similar to those obtained using the conventional leaching process. Moreover, the CCLP uses half of the quantity of acid and one-eighth of the amount of water that the conventional process uses. Metal precipitation with NaOH and Ca(OH)2 was applied to treat the acidic leachates, and good metal removal yields were achieved with both reagents. However, the large consumption of chemicals implies high operating costs. In addition, the precipitation causes considerable sludge production, particularly when using Ca(OH)2. Overall, the CCLP coupled to metal precipitation using NaOH and water recycling appears to be the most attractive option for the removal of toxic metals from this industrial soil.

Grassland fire effect on soil organic carbon reservoirs in a semiarid environment

Abstract. The aim of this work was to investigate the effect of an experimental fire used for grassland management on soil organic carbon (SOC) stocks. The study was carried out on Hyparrhenia hirta (L.) Stapf (Hh) grassland and Ampelodesmos mauritanicus (Desf.) T. Durand &amp;amp; Schinz (Am) grasslands located in the north of Sicily. Soil samples were collected at 0–5 cm before and after the experimental fire, and SOC was measured. During the grassland fire, soil surface temperature was monitored. Biomass of both grasses was analysed in order to determine dry weight and its chemical composition. The results showed that SOC varied significantly with vegetation type, while it is not affected in the short term by grassland fire. Am grassland stored more SOC compared with Hh grassland thanks to lower content in the biomass of the labile carbon pool. No significant difference was observed in SOC before and after fire, which could be caused by several factors: first, in both grassland types the measured soil temperature during fire was low due to thin litter layers; second, in a semiarid environment, a higher mineralization rate results in a lower soil carbon labile pool; and third, the SOC stored in the finest soil fractions, physically protected, is not affected by fire.

Clay minerals in the stream floodplain soils in undisturbed landscapes of the Southern Taiga (with the soil of the state central forest nature and biosphere reserve as an example)

An X-ray diffraction technique was used to study the composition of clay material from floodplain soddy- and mucky gley soils formed in the valleys of small streams in the reserve. The content of nonexpandable minerals was higher and the labile mineral content was lower in the fine fractions of the floodplain soils than in similar fractions of upland soils found in adjacent positions. Two types of disordered mixed-layered not found in upland soils-namely, illite-chlorites and chlorite-vermiculites with a variable proportion of individual layeres-were detected in the clay material. Both types of mixed-layer minerals are supposed to originate from stony fragments containing phyllosilicates of supergenic or post-magmatic origin.

Sorption of organic carbon compounds to the fine fraction of surface and subsurface soils

Dissolved organic carbon (DOC) transported from the soil surface is stabilized in deeper soil profiles by physico-chemical sorption processes. However, it is unclear how different forms of organic carbon (OC) compounds common in soil organic matter interact with soil minerals in the surface (A) and subsurface (B) horizons. We added four compounds (glucose, starch, cinnamic acid and stearic acid) to the silt- and clay-sized fraction (fine fraction) of A and B horizons of eight soils from varying climates (3 temperate, 3 tropical, 1 arctic and 1 sub-arctic). Equilibrium batch experiments were conducted using 0 to 100mgCL−1 of 14C-labeled compounds for 8h. Sorption parameters (maximum sorption capacity, Qmax and binding coefficient, k) calculated by fitting sorption data to the Langmuir equation showed that Qmax of A and B horizons was very similar for all compounds. Both Qmax and k values were related to sorbate properties, with Qmax being lowest for glucose (20–500mgkg−1), highest for stearic acid (20,000–200,000mgkg−1), and intermediate for both cinnamic acid (200–4000mgkg−1) and starch (400–6000mgkg−1). Simple linear regression analysis revealed that physico-chemical properties of the sorbents influenced the Qmax of cinnamic acid and stearic acid, but not glucose and starch. The sorbent properties did not show predictive ability for binding coefficient k. By using the fine fraction as sorbent, we found that the mineral fractions of A horizons are equally reactive as the B horizons irrespective of soil organic carbon content.

Hardwood Tree Growth after Eight Years on Brown and Gray Mine Soils in West Virginia

Surface coal mining in Appalachia disturbs hundreds of hectares of land every year with the removal of valuable and ecologically diverse eastern deciduous forests. After the passage of the Surface Mining Control and Reclamation Act in 1977, coal mine operators began planting a variety of grasses and legumes as a fast and economical way to reestablish a permanent vegetative cover to meet erosion and site stabilization requirements. However, soil compaction and competitive forage species have arrested the recolonization of native hardwood tree species on these reclaimed sites. Three 2.8-ha demonstration plots were established at Catenary Coal's Samples Mine in Kanawha County, West Virginia, of weathered brown sandstone and unweathered gray sandstone. Half of each plot was compacted. Each plot was hydroseeded with a low-competition herbaceous cover and planted with 11 hardwood tree species. After eight growing seasons, average tree volume index was nearly 10 times greater for trees grown in the brown sandstone treatments, 3853 cm, compared with 407 cm in gray sandstone. Trees growing on compacted treatments had a lower mean volume index, 2281 cm, than trees growing on uncompacted treatments, 3899 cm. Average pH of brown sandstone was 5.2 to 5.7, while gray sandstone was 7.9. The gray sandstone had much lower fine soil fraction (<2-mm) content (40%) than brown sandstone (70%), which influenced nutrient- and water-holding capacity. Brown sandstone showed significantly greater tree growth and survival and at this stage is a more suitable topsoil substitute than gray sandstone on this site.

Evaluation of Bouwer-Rice Large-Particle Correction Procedure for Soil Water Characteristic Curves

Abstract The Bouwer-Rice correction method to account for large particles excluded during laboratory testing to measure the soil water characteristic curve (SWCC) was evaluated on samples of well-graded alluvium. A large-scale hanging column apparatus was used so that tests could be conducted on specimens containing all particle sizes. The analyses show that SWCCs measured on the fraction of alluvium finer than the No. 4 U.S. sieve (4.8 mm) can be corrected reliably to represent the SWCC of bulk soil or fractions of bulk soil corresponding to different large-particle thresholds. The method can also be used reliably to correct SWCCs measured on soils prepared with different large-particle thresholds (e.g., finer than 25 mm, 12.5 mm, or 4.8 mm). Dry density of the finer fraction being tested must be carefully controlled to match the dry density of the finer fraction in the soils containing large particles. An equation is described for computing the dry density of the finer fraction in a bulk soil. A simplified version of the Bouwer-Rice method is also proposed and evaluated. In this method, a SWCC test is conducted on the finer soil fraction with dry density matching that anticipated in the field, the van Genuchten equation is fit to the measured SWCC to define the shape parameters α and n, and then the SWCC for the field application is computed using the fitted α and n from the test on the finer fraction, saturated volumetric water content of the bulk soil in the field, and a residual water content of zero. Analyses show that this modified Bouwer-Rice method is simpler and results in an accurate representation of the SWCC of soil containing large particles.

Nitrogen input quality changes the biochemical composition of soil organic matter stabilized in the fine fraction: a long-term study

The chemical composition of soil organic matter (SOM) is a key determinant of its biological stability. Our objective in this study was to evaluate the effects of various sources of supplemental N on the chemical composition of SOM in the fine (<5 μm) mineral fraction. Treatments were fallow, maize/soybean in rotation, and continuous maize receiving no fertilizer (maize0N), synthetic fertilizer N (maize + N), or composted manure (maize + manure). The chemical structures in SOM associated with the fine fraction were determined using XANES spectroscopy at the C and N K-edges, which was assessed using multidimensional scaling. Analysis of amino sugar biomarkers were used to evaluate the fungal:bacterial contributions to the SOM. The addition of N to soils (i.e., maize + N, maize + manure, and maize/soybean treatments) resulted in the enrichment of proteinaceous compounds. Soils which did not receive supplemental N (i.e., fallow and maize0N treatments) were enriched in plant-derived compounds (e.g., aromatics, phenolics, carboxylic acids and aliphatic compounds), suggesting that decomposition of plant residues was constrained by N-limitation. Microbial populations assessed by amino sugar biomarker ratios showed that the highest contributions to SOM by bacteria occurred in the maize + manure treatment (high N input), and by fungi in the fallow treatment (low N input). The SOM in the maize + N and maize/soybean treatments was enriched in N-bonded aromatics; we attribute this enrichment to the abiotic reaction of inorganic N with organic C structures. The SOM in the maize + manure treatment was enriched in pyridinic-N, likely as a result of intense microbial processing and high SOM turnover. The presences of signals for ketone and pyrrole compounds in XANES spectra suggest their use as biomarkers for microbially transformed and stabilized SOM. The SOM in the maize + manure treatment was enriched in ketones which are likely microbial by-products of fatty acid catabolism. Pyrrole compounds, which may accumulate over the long term as by-products of protein transformations by an N-limited microbial community, were dominant in the fallow soil. A combination of molecular spectroscopy and biomarker analysis showed that the source of supplemental N to soil influences the stable C- and N-containing compounds of SOM in a long-term field study. Indeed, any increase in N availability allowed the microbial community to transform plant material into microbial by-products which occur as stable SOM compounds in the fine soil fraction.

Surface dispersion of uranium in the Al-Hayt area, NE Arabian Shield

Al-Hayt area, although located on the Cenozoic basaltic plateau of Harrat Khaybar, is threatened by uranium contamination emanating mainly from the rhyolite hills of Asfar Thwelil just outside the eastern boundary of the lava field. This outcrop is an exhumed volcanic dome belonging to the Shammar Group of Late Proterozoic age. The rock units in this dome are porphyritic and flow rhyolites with some ignimbrites. The flow rhyolite is especially enriched in uranium and much of this is concentrated in thin vertical veins of uraninite and uranophane. Radiation measurements were carried out in the field using a portable gamma ray differential spectrometer that was capable of counting radiation at 5 spectral intervals representing uranium, thorium, potassium and total radiation (TC1 & TC2). Four traverses containing 67 points were made across the hills of Asfar Thwelil and nearby plains, and some measurements were also carried out in various parts of the Al-Hayt Town itself. Radiation measurements were also carried out in several parts of the eastern and northern shield for the sake of comparison. Soil samples for chemical analysis were also collected along the traverses and from the town and its main quarry. The samples were sieved to 3 sizes (coarse, medium and fine) and analyzed for 34 elements using the Neutron Activation technique. Results of the chemical analyses reveal the concentration of uranium and thorium in the fine soil fraction; although this may indicate adsorption onto clay minerals, a similar trend by immobile elements such as Zr, Hf, Ta, REE and Co rules out this possibility and points to heavy mineral concentration in the fine fraction as the main cause. Radiation was measured in 5 exploration trenches, and the upper layers (alluvium) were found to be the least radioactive, while layers containing salt and calcrete were highly enriched in radioactive elements. Correlation coefficients from soil samples are strongly in favor of the concentration of U & Th with other incompatible elements in heavy minerals such as zircon. Factor analysis of the same samples provides a number of underlying causes that control their chemistry; most important of all is the heavy mineral content followed by the influence of basaltic detritus form the Harrah.

Metal extractability patterns to evaluate (potentially) mobile fractions in periurban calcareous agricultural soils in the Mediterranean area—analytical and mineralogical approaches

A set of periurban calcareous agricultural Mediterranean soils was spiked with a mixture of Cd, Cu, Pb and Zn at two levels within the limit values proposed by current European legislation, incubated for up to 12months, and subjected to various one-step extraction procedures to estimate mobile (neutral salts) and potentially mobile metal fractions (complexing and acidic extraction methods). The results obtained were used to study metal extractability patterns according to the soil characteristics. The analytical data were coupled with mineralogical investigations and speciation modelling using the Visual Minteq model. The formation of soluble metal-complexes in the complexing extracts (predicted by the Visual Minteq calculations) led to the highest extraction efficiency with complexing extractants. Metal extractability patterns were related to both content and composition of carbonate, organic matter, Fe oxide and clay fractions. Potentially mobile metal fractions were mainly affected by the finest soil fractions (recalcitrant organic matter, active lime and clay minerals). In the case of Pb, scarce correlations between extractable Pb and soil constituents were obtained which was attributed to high Pb retention due to the formation of 4PbCO3·3PbO (corroborated by X-ray diffraction). In summary, the high metal proportion extracted with complexing agents highlighted the high but finite capacity to store potentially mobilizable metals and the possible vulnerability of these soils against environmental impact from metal accumulation.

Initial soil organic carbon concentration influences the short-term retention of crop-residue carbon in the fine fraction of a heavy clay soil

Among factors controlling decomposition and retention of residue C in soil, effect of initial soil organic C (SOC) concentration remains unclear. We evaluated, under controlled conditions, short-term retention of corn residue C and total soil CO2 production in C-rich topsoil and C-poor subsoil samples of heavy clay. Topsoil (0–20 cm deep, 31.3 g SOC kg−1 soil) and subsoil (30–70 cm deep, 4.5 g SOC kg−1 soil) were mixed separately with 13C–15N-labeled corn (Zea mays L.) residue at rates of 0 to 40 g residue C kg−1 soil and incubated for 51 days. We measured soil CO2–C production and the retention of residue C in the whole soil and the fine particle-size fraction (<50 μm). Cumulative C mineralization was always greater in topsoil than subsoil. Whole-soil residue C retention was similar in topsoil and subsoil at rates up to 20 g residue C kg−1. There was more residue C retained in the fine fraction of topsoil than subsoil at low residue input levels (2.5 and 5 g residue C kg−1), but the trend was reversed with high residue inputs (20 and 40 g residue C kg−1). Initial SOC concentration affected residue C retention in the fine fraction but not in the whole soil. At low residue input levels, greater microbial activity in topsoil resulted in greater residue fragmentation and more residue C retained in the fine fraction, compared to the subsoil. At high residue input levels, less residue C accumulated in the fine fraction of topsoil than subsoil likely due to greater C saturation in the topsoil. We conclude that SOC-poor soils receiving high C inputs have greater potential to accumulate C in stable forms than SOC-rich soils.

Long-term assessment of natural attenuation: statistical approach on soils with aged PAH contamination

Natural attenuation processes valorization for PAH-contaminated soil remediation has gained increasing interest from site owners. A misunderstanding of this method and a small amount of data available does not encourage its development. However, monitored natural attenuation (MNA) offers a valuable, cheaper and environmentally friendly alternative to more classical options such as physico-chemical treatments (e.g., chemical oxidation, thermal desorption). The present work proposes the results obtained during a long-term natural attenuation assessment of historically contaminated industrial soils under real climatic conditions. This study was performed after a 10year natural attenuation period on 60 off-ground lysimeters filled with contaminated soils from different former industrial sites (coking industry, manufactured gas plants) whose initial concentration of PAH varied between 380 and 2,077mgkg(-1). The analysed parameters included leached water characterization, soil PAH concentrations, evaluation of vegetation cover quality and quantity. Results showed a good efficiency of the PAH dissipation and limited transfer of contaminants to the environment. It also highlighted the importance of the fine soil fractions in controlling PAH reactivity. PAH dissipation through water leaching was limited and did not present a significant risk for the environment. This PAH water concentration appeared however as a good indicator of overall dissipation rate, thereby illustrating the importance of pollutant availability in predicting its degradation potential.

Impact of miscanthus cultivation on trace metal availability in contaminated agricultural soils: Complementary insights from kinetic extraction and physical fractionation

In order to assess the impact of in situ energy crop cultivation on Cu, Pb, Zn and Cd availabilities in an area contaminated by atmospheric fallout, a parcel divided in two parts was sampled: one under perennial miscanthus since three years and the other staying under annual crops used as reference. Metal availability parameters determined using EDTA extractions at equilibrium and kinetic extractions were related to metal localization in size fractions after physical fractionation. Extraction at equilibrium as well as kinetically defined labile pools highlighted significantly lower Cu and Pb availabilities in the soil under miscanthus compared to the annual crop reference while physical fractionations highlighted an increased localization of these metals in the fine size fractions. For Zn and Cd, changes in metal availability were highlighted only through the kinetic parameters of extraction rate constants while localization in size fractions changed for Cd only. Indeed for these metals extraction at equilibrium and kinetically defined labile and slowly labile pools failed to show some changes. Organic carbon percentage was significantly higher in the 20–50μm and 200–2000μm fractions for the miscanthus sample underlying the importance of no-tillage and organic carbon inputs. Pearson correlations among metal availability parameters and metal and organic carbon localizations were used to identify the more important fractions involved in the metal availability. Results revealed that available metal pools were related to metal localization in the finer soil fractions (<50μm) and to the organic carbon localization in the 20–50μm and 200–2000μm fractions.

Organic amendments as strategy to increase organic matter in particle-size fractions of a semi-arid soil

The distribution of organic matter (OM) in soil particle-size fractions is a key process in the dynamic and persistence of soil organic carbon. In order to study the distribution of organic amendments among the different soil particle-size fractions in a semi-arid, degraded soil (a sandy loam Haplic Regosol), a 9-month-incubation experiment was carried out after the application of composted organic amendments (90Mgha−1) differing in nature (farmyard manure, cFYM; vine pruning wastes, cPW, and sewage sludge, cSS). A physical fractionation based on the separation of soil according to particle size was carried out and chemical and microbiological parameters were measured both in the whole soil and in the particle-size fractions. After amendment, all the particle-size fractions in the amended soils showed higher organic carbon (Corg) and N concentration than the control soil. At the end of the experiment, the amount of Corg in the silt-sized (2–63μm) and clay-sized (0.1–2μm) fractions did not show significant differences among amended soils, indicating that the accumulation and protection of the OM in the fine fractions of this semi-arid soil are not affected by the nature of the composted amendment. Furthermore, the fine fractions (silt and clay) of the amended soils showed intermediate C/N ratios, suggesting that the organic compounds of these fractions are derived from both microorganisms and organic amendments. At the end of the experiment, the soils amended with cPW showed the highest Corg content in the whole soil and in the coarse sand fraction (200–2000μm). In contrast, soils amended with cSS showed the highest Corg loss among treated soils and showed in general the highest microbial and enzymatic activities in the coarse sand fraction. These results highlight the importance of the recalcitrance of the amendment (high C/N ratio) to avoid fast degradation of non-protected OM in the coarse sand fraction. In conclusion, the type of the composted organic amendment did not influence the OM accumulation in fine fractions of the studied semi-arid soil, but was determinant for the degradation of OM in the coarse sand fractions.

Toxic Metal Removal from Polluted Soil by Acid Extraction

Sulfuric acid leaching is a promising technique to extract toxic metals from polluted soils. The objective of this study was to define the optimum sulfuric acid leaching conditions for decontamination of the fine particle fraction (<125 μm) of an industrial soil polluted by Cd (16.8 mg kg−1), Cu (3,350 mg kg−1), Pb (631 mg kg−1) and Zn (3,010 mg kg−1). Batch leaching tests in Erlenmeyer shake flasks showed that a soil pulp pH between 1.5 and 2.0 using a solid concentration (SC) ranging from 5 to 20 % is adequate to efficiently solubilize toxic metals. Leaching tests performed at different temperatures (20, 40, 60 and 80 °C) also revealed that it is not beneficial to heat the soil suspension during the leaching treatment. The application in a 1-L stirred tank reactor of five consecutive 1-h leaching steps at 10 % SC and ambient temperature, followed by three water washings steps resulted in the following metal extraction yields: 30 % As, 90 % Cd, 43 % Co, 7 % Cr, 88 % Cu, 75 % Mn, 26 % Ni, 18 % Pb and 86 % Zn. The decontaminated soil conformed to Quebec norms for commercial and industrial use of soil.

Natural arsenic attenuation via metal arsenate precipitation in soils contaminated with metallurgical wastes: II. Cumulative evidence and identification of minor processes

Abstract Accurate identification of individual As species in contaminated environments is critical because the toxicology, mobility and adsorptive properties of this element may vary substantially with its chemical forms and oxidation states. The goal of this work was to relate the geochemical behavior of As in soils contaminated by a lead smelter in Mexico, with its chemical speciation, and to achieve direct identification of low-solubility poorly-crystalline metal arsenates. Arsenic was identified as the most mobile trace element in the wastes from the smelting plant. Arsenic solubility in soils was significantly lower than its solubility in wastes, showing natural attenuation of this element. Its solubility in soil was quantitatively described in selected samples through thermodynamic equilibrium modeling. The results indicated that As solubility is controlled by solid Pb and Cu arsenate formation. The behaviors of the sequential chemical extractions were consistent with the presence of the predicted arsenates. Microscopic evidence of the formation of solid metal arsenates were obtained in fine soil fractions of selected samples with high As contents, by using the following complementary techniques: X-ray diffraction, scanning electron microscopy and transmission electron microscopy, both coupled with energy dispersive X-ray spectroscopy, and the latter with a high angle annular dark field detector. All results supported the formation of low-solubility Pb arsenates as controlling As mobility in the samples studied, in which As(V) adsorption to Fe (hydr)oxides was not the dominant process of natural attenuation.

Comparison of nuclear magnetic resonance methods for the analysis of organic matter composition from soil density and particle fractions

Environmental contextThe association of specific organic matter (OM) compounds with clay mineral surfaces is believed to protect these compounds from degradation and thus result in long-term protection in soil. The molecular-level composition of soil OM associated with soil fractions was measured and compared using solid-state 13C nuclear magnetic resonance (NMR) and solution-state 1H NMR methods. Combining these methods allowed more detailed characterisation of OM associated with different soil fractions and will improve the understanding of OM dynamics in soil. AbstractOrganic matter (OM) associated with fine soil fractions is hypothesised to be protected from complete biodegradation by soil microbes. It is therefore important to understand the structure and stage of decomposition of OM associated with various soil fractions. Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy has been used extensively to investigate the OM composition of soils and soil fractions. Solution-state 1H NMR spectroscopy has not been used as much but is an emerging tool for analysing soil OM because 1H NMR spectra are often better resolved and provide information that complements the structural information obtained from solid-state 13C NMR experiments. This study compares one-dimensional solution-state 1H NMR and solid-state 13C NMR methods for assessing the degradation and composition of OM in three different soils, and their light and clay-size fractions. The alkyl/O-alkyl degradation parameter was consistent across all NMR methods and showed that OM associated with clay-size fractions were at more advanced stages of degradation as compared to that in light density soil fractions. Solution-state 1H and diffusion edited (DE) 1H NMR results showed the presence of high concentrations of microbial-derived peptidoglycan and peptide side-chains in clay-sized fractions. Lignin was also identified in clay-sized fractions using solid-state 13C and solution-state 1H NMR techniques. The combination of solid-state 13C and solution-state 1H NMR methods provides a more detailed analysis of OM composition and thereby facilitates a better understanding of the fate and preservation of OM in soil.

In situ electro-osmotic cleanup of tar contaminated soil—Removal of polycyclic aromatic hydrocarbons

An in situ electro-osmosis experiment was set up in a tar contaminated clay soil in Olst, the Netherlands, at the site of a former asphalt factory. The main goal of this experiment was to remove polycyclic aromatic hydrocarbons (PAHs) from the contaminated clay layer by applying an electric gradient of 12Vm−1 across the soil over an electrode distance of 1m. With the movement of water by electro-osmosis and the addition of a non-ionic surfactant (Tween 80), the non-polar PAHs were dragged along by convection and removed from the fine soil fraction. Soil samples were taken at the start and after 159 days at the end of the experiment. Water at the electrode wells was sampled regularly during the course of the experiment. The results reflect the heterogeneity of the soil characteristics and show the PAH concentrations within the experimental set up. After first having been released into the anolyte solution due to extraction by Tween 80 and subsequent diffusion, PAH concentrations increased significantly in the electrode reservoirs at the cathode side after 90 days of experiment. Although more detailed statistical analysis is necessary to quantify the efficiency of the remediation, it can be concluded that the use of electro-osmosis together with a non-ionic surfactant is a feasible technique to mobilize non-polar organic contaminants in clayey soils.