Partitioning and speciation of chromium, copper, and arsenic in CCA-contaminated soils: influence of soil composition

This study focused on the influence of soil composition and physicochemical characteristics on the retention and partitioning of Cu, Cr and As in nine chromated copper arsenate (CCA) artificially contaminated soils. A statistical mixture design was used to set up the number of soils and their respective composition. Sequential extraction and modified solvent extraction were used to assess Cu and Cr partitioning and As speciation [As(III) or As(V)]. It was found that peat had a strong influence on CEC (232 meq/100 g), on buffer capacity and on Cu and Cr retention, whereas kaolinite's contribution to the CEC was minor (38 meq/100 g). Average metal retention in mineral soils was low (58% for Cu and 23% for Cr) but increased dramatically in highly organic soils (96% for Cu and 78% for Cr). However, both organic and mineral soils demonstrated a very high sorption of added As (71-81%). Levels of Cu and Cr in a soluble or exchangeable form (F1) in highly organic soils were very low, whereas the levels strongly bound to organic matter were much higher. Conversely, in mineral soils, 47% of Cu and 18% of Cr were found in F1. As a result, Cr and Cu in moderately and highly organic contaminated soils were present in less mobile and less bioavailable forms, whereas in mineral soils, the labile fraction was higher. The modified method used for selective determination of mineral As species in CCA-contaminated soils was found to be quantitative and reliable. Results revealed that arsenic was principally in the pentavalent state. Nevertheless, in organic soils, arsenite was found in significant proportions (average value of 29% in highly organic soils). This indicates that some reduction of arsenate to arsenite occurred since the original species in CCA is As(V).     

Arsenic speciation and mobilization in CCA-contaminated soils: influence of organic matter content

A laboratory incubation experiment was conducted to investigate the influence of organic matter content on arsenic speciation and mobilization in chromated copper arsenate (CCA)-contaminated soils. The study was performed with four synthetic CCA-contaminated soils, with a range of organic matter content (mixture of peat moss and poultry manure) varying between 0.5% and 15% (w/w), under unsaturated and aerobic conditions for 40 days. Changes in water-soluble arsenic speciation (As(V), As(III), MMAA, DMAA) were monitored over time in soil extracts by HPLC-ICP-MS and in the soil solid phase (As(III), As(V)) by a solvent extraction method. Irrespective of organic matter content, As(V) was the predominant soil bound and aqueous phase arsenic species. However, over 40 days, a high soil organic matter content (7.5% and 15%) was able to entail formation of soil bound As(III). Moreover, total water-soluble arsenic was positively correlated with dissolved organic carbon (r(2)=0.88). However, the organic matter content did not influence arsenic speciation in the soluble fraction; neither As(V) reduction nor arsenic biomethylation occurred within 40 days. An increase in dissolved organic carbon content promoted both As(V) and As(III) solubilization in soils. Also, over time, organic matter contents of 7.5% and 15% entailed the persistence of soluble As(V), likely due to the high content of dissolved organic compounds which prevented its sorption onto soil. Based on this data, the environmental risk of aerobic CCA-contaminated soils rich in organic matter may be due to an enhanced availability of soluble As(V) over time, rather than to the formation of the more toxic and more mobile As(III).     

The contribution of glomalin-related soil protein to Pb and Zn sequestration in polluted soil

The distribution of lead and zinc in glomalin-related soil protein (GRSP), a widespread glycoprotein presumably produced by arbuscular mycorrhizal fungi (AMF) in soil, and in some other soil fractions (soil organic matter - [SOM], carbonates, phosphates, etc.) was studied in soils from an area near a lead smelter that differed in SOM, carbonates and heavy metal (HM) content. Total GRSP represented 5.4-21.2% of the SOM and was positively correlated with the soil Pb and Zn concentrations (r=0.57 and 0.66, p=0.007 and p=0.001 for Pb and Zn, respectively). Pb and Zn were predominantly bound to carbonates and organic matter. The amount of lead bound to GRSP varied between 0.69 and 23.4 mg g(-1) DW GRSP which is 0.8-15.5% of the total soil Pb. The amount of GRSP-bound metal was positively correlated with the total concentration in the case of Pb (r=0.90, p=0.000) but the opposite was found for Zn (r=-0.41, p=0.048), indicating that GRSP predominantly binds Pb. The percentages of HM-GRSP in HM-SOM were variable and were not correlated with SOM content.     

Binding and mobility of mercury in soils contaminated by emissions from chlor-alkali plants

Chlor-alkali plants are known to be an important source of Hg emissions to the atmosphere and related contamination of soils in their vicinity. In the present study, the results of Hg speciation and mobility of Hg in soils affected by Hg emissions from three chlor-alkali plants are compared. Solid phase mercury speciation analyses was carried out using a mercury-thermo-desorption technique with the aim of distinguishing elemental Hg [Hg(0)] from Hg(II)-binding forms. Mercury species in soil leachates were distinguished using an operationally defined method, which is based on the reactivity of soluble Hg compounds. Results show that the Hg(0) emitted from the plants could not be detected in any of the investigated soils. This indicates quantitative re-emission or oxidation of this Hg species in the atmosphere or soils. In most soils Hg was predominately bound to organic matter. Only in sandy soils deficient in organic matter was Hg, to a larger extent, sorbed onto mineral soil components. Leachable Hg in most soils occurred as non-reactive, soluble organic Hg complexes such as fulvic acid-bound Hg, and reach their highest values (90 microg kg(-1)) in soils rich in organic matter. Concentrations of reactive, soluble Hg compounds were highest in sandy soils where the content of organic matter was low. Leachability of Hg was found to be inhibited in soils with a high content of clayey soil components. The distribution of Hg in soil profiles suggests that migration of Hg to deeper soil layers (approx. 20 cm) is most effective if Hg is bound to soluble organic complexes, whereas reactive Hg or weak Hg complexes are effectively retained in the uppermost soil layer (5 cm) through sorption on mineral surfaces.     

Mercury characterization in a soil sample collected nearby the DOE Oak Ridge Reservation utilizing sequential extraction and thermal desorption method

A new attempt to characterize Hg speciation and to evaluate Hg mobility in soils was made by applying operationally defined speciation techniques coupled with fractionation of soil components to a soil sample collected just outside the Y-12 boundary of the Oak Ridge Reservation (ORR) site. The soil sample was fractionated based on redoximorphic features and particle size and a sequential extraction procedure and thermal desorption technique were then applied to the fractionated soil components. The redoximorphic concentration component was observed to have higher Hg concentrations than the redoximorphic depletion component in the soil, and fine particles contained higher concentrations of Hg compared with coarse particles. The preliminary results of using thermal desorption as well as the sequential extraction procedure suggested that Hg0 and other "easily" vaporized Hg species accounted for 10-30% of total Hg in the soil. Sequential extraction analysis showed that both soluble and bioavailable Hg fractions were relatively small proportions whereas the organic matter bound mercury fraction constituted the major form of Hg species in the sample. The results suggest that Hg retained in the redoximorphic concentrations was less volatile and labile than Hg in the redoximorphic depletions possibly due to the strong binding affinity of Fe/Mn oxides and organic matter to Hg.     

Fractionation of caesium (Cs) in coniferous forest soil in central Sweden

Sequential extraction procedure (SEP) was applied for fractionation of Chernobyl fallout ¹³⁷Cs bound onto soils of a coniferous forest ecosystem located in central Sweden. Results of sequentially extracted ¹³⁷Cs fractions demonstrated that 8% (mean value) of the total deposited ¹³⁷Cs was water soluble (F1) and 13% was NH₄OAc extractable (F2). Oxidation of F2 residuals by H₂O₂ led to a release of 15% of soil-bound ¹³⁷Cs (F3). Acid digestion of F3 residuals showed a possibility of releasing an extra amount of soil-bound ¹³⁷Cs, 22% of the total soil ¹³⁷Cs inventory (F4). These two fractions (F3 and F4) include strongly bound ¹³⁷Cs that seems to require longer biodegradation processes by soil microflora and microfauna before becoming available for uptake by plants and fungi. More than 37% of the total soil ¹³⁷Cs inventory was bound onto soil residuals in a non-extractable form that includes slowly degradable organic matter and other soil residual compartments. The distribution coefficient (K_d) was rather low and shows an inverse relation with the increase of percentage of soil organic matter, which indicates a week binding of ¹³⁷Cs onto forest soil. In contrast, chemical fractionation of soil bound ¹³⁷Cs showed a substantial fraction of ¹³⁷Cs was strongly bound onto soil as organically bound ¹³⁷Cs. Apparently, the binding processes of radiocaesium onto forest soil seems to be time dependent.     

The importance of organic matter distribution and extract soil:solution ratio on the desorption of heavy metals from soils

The lability (mobility and bioavailability) of metals varies significantly with soil properties for similar total soil metal concentrations. We studied desorption of Cu, Ni and Zn, from 15 diverse, unamended soils. These studies included evaluation of the effects of soil:solution extraction ratio and the roles of soil properties on metal desorption. Dcsorption was examined for each metal by computing distribution coefficients (Kd) for each metal in each soil where Kd = [M]soil/[M]solution, Results from soil:solution ratio studies demonstrated that Kd values for the metals tended to increase with increasing soil:solution ratio. This result also held true for distribution of soil organic matter (SOM). Because the soil:solution ratio has a significant effect on measured metal distributions, we selected a high soil:solution ratio to more closely approach natural soil conditions. Copper showed strong affinity to operationally defined dissolved organic matter (DOM). In this study, DOM was operationally defined based on the total organic carbon (TOC) content in 0.45-microm or 0.22-microm filtrates of the extracts. The Kd of Cu correlated linearly (r2 = 0.91) with the Kd of organic matter (Kd-om) where the Kd-om is equal to SOM as measured by Walkley-Black wet combustion and converted to total carbon (TC) by a factor of 0.59. These values representing solid phase TC were then divided by soluble organic carbon as measured by TOC analysis (DOM). The conversion factor of 0.59 was employed in order to construct Kd-om values based on solid phase carbon and solution phase carbon. SOM plays a significant role in the fate of Cu in soil systems. Soil-solution distribution of Ni and Zn, as well as the activity of free Cu2+, were closely related to SOM, but not to DOM. Kd values for Ni, Zn and free Cu2+ in a particular soil were divided by the SOM content in the same soil. This normalization of the Kd values for Ni, Zn, and free Cu2+ to the SOM content resulted in significant improvements in the linear relationships between non-normalized Kd values and soil pH. The semi-empirical normalized regression equations can be used to predict the solubility of Ni and Zn and the activity of free Cu2+ as a function of pH.     

The influence of soluble organic matter from municipal solid waste compost on trace metal leaching in calcareous soils

The use of municipal solid waste (MSW) compost as fertilizer may cause increased leaching due to its high content of trace metals and thus pose a threat to groundwater quality. The effect of MSW compost application on trace metal leaching in calcareous soils has been studied in soil column experiments under laboratory conditions using three soils from the study area in the Gaza Strip and Israel. Higher levels of organic matter in solution (TOMS), nitrate, and the trace metals Cu, Ni and Zn were found in the leachates of a sandy soil and, to a lesser extent, a loamy soil, to which MSW compost had been applied at a rate of 65 Mg ha(-1) (dry weight basis). Nevertheless, the majority of water-soluble trace metal species from compost accumulated in the topsoil rather than washing out, with the exception of aqueous Ni species. Ni concentrations exceeded the maximum allowable limits for drinking water (in Germany: 50 microg l(-1)) at peak times in the leachates from sandy soil, while all other trace metals remained far below the corresponding limits. The highest absolute concentrations of trace metals were found for the leaching of Cu from compost-amended sandy soil (100 microg l(-1)). For Cd, Pb and Hg no evidence of downward movement was found in any assay. Gel filtration studies of the collected soil leachates showed that all trace metals encountered in the leachates existed mostly as organic complexes. In sandy soil most of the water-soluble organic matter added with the compost had leached from the rootzone after a year's equivalent of rainfall, while TOMS mobility was greatly reduced in the loamy soil. The makeup of the TOMS in the sandy soil and its metal-binding capacity was strongly influenced by compost-derived dissolved organic matter (DOM) as observed by FTIR spectrometry. Hence the vertical displacement of trace metals (Cu, Ni, Zn) in these calcareous soils seemed to result primarily from the presence of mobile metal-organic complexes in the soil solution after compost addition. Further studies are required to validate these findings in the field, especially to assess the risk of Cu and Ni leaching in sandy soil.     

Arsenic solubility and distribution in poultry waste and long-term amended soil

The purpose of this study was to quantify the solubility and distribution of As among solid-phase components in poultry wastes and soils receiving long-term poultry waste applications. Arsenic in the water-soluble, NaOCl-extractable (organically bound), NH(2)OH x HCl-extractable (oxide bound) and residual fractions were quantified in an Upper Coastal Plain soil (Neshoba County, MS) that received annual waste applications. After 25 years, As in the amended soil had a mean of 8.4 mg kg(-1) compared to 2.68 mg kg(-1) for a non-amended soil. Arsenic in the amended soil was mainly in the residual fraction (72% of total), which is generally considered the least bioavailable fraction. Arsenic in poultry waste samples was primarily water-soluble (5.3-25.1 mg kg(-1)), representing 36-75% of the total As. To assess the extent of spatial heterogeneity, total As in a 0.5-ha area within the long-term waste-amended field was quantified. Soil surface samples were taken on 10-m grid points and results for total As appeared negatively skewed and approximated a bimodal distribution. Total As in the amended soil was strongly correlated with Fe oxides, clay and hydroxy interlayered vermiculite concentrations, and negatively correlated with Mehlich III-P, mica and quartz contents.     

Evaluation of metal concentrations in edible vegetables grown in compost amended soil

Agricultural uses of compost usually have a positive effect on the yield of vegetable crops for human consumption. However, compost that contains heavy metals can transfer these components to soils and plants. To evaluate the contamination levels of metals in soil, compost, and edible vegetables, the Mn, Zn, Pb, Cd, Cu, and Ni total contents were measured. Metal availability in soils, as well as other variables – the pH, CEC (cation exchange capacity), total nitrogen, organic carbon, particle size distribution and mineralogy of the clay fraction – were examined in the soil samples. The analysed compost samples were produced from urban solid waste, cattle manure, and edible vegetable and tree pruning residues. The values of pH, CEC, total nitrogen, organic matter, exchangeable hydrogen and carboxylic groups were measured in the compost samples. Of the six metals examined in the soils, in general, Mn and Zn attained the highest concentrations, followed by Cu. Relatively high Mn, Zn, Cu, Cd and Pb concentrations were found in the soils. Metal concentrations extracted with DTPA were below the critical levels in soils, i.e. the levels above which toxicity is likely. In general, Zn, Pb, Cd, Cu and Ni concentrations in compost were lower than those reported by other workers, while Mn levels were within the range for this metal in compost. The results showed that there was an effect of the vegetable type (p < 0.01) for all the parameters examined. High Pb concentrations were found in lettuce and chive as compared with the tolerance limit for this metal in fresh vegetables in Brazil. Cadmium concentrations were also enhanced in the fresh vegetables compared with the typical concentrations of metals in plants. Zinc, Cu, Cd, and Ni concentrations were lower than the tolerance limits established for foods by the Brazilian legislation.     

Trace metal behaviour in estuarine and riverine floodplain soils and sediments: a review

This paper reviews the factors affecting trace metal behaviour in estuarine and riverine floodplain soils and sediments. Spatial occurrence of processes affecting metal mobility and availability in floodplains are largely determined by the topography. At the oxic-anoxic interface and in the anoxic layers of floodplain soils, especially redox-sensitive processes occur, which mainly result in the inclusion of metals in precipitates or the dissolution of metal-containing precipitates. Kinetics of these processes are of great importance for these soils as the location of the oxic-anoxic interface is subject to change due to fluctuating water table levels. Other important processes and factors affecting metal mobility in floodplain soils are adsorption/desorption processes, salinity, the presence of organic matter, sulphur and carbonates, pH and plant growth. Many authors report highly significant correlations between cation exchange capacity, clay or organic matter contents and metal contents in floodplain soils. Iron and manganese (hydr)oxides were found to be the main carriers for Cd, Zn and Ni under oxic conditions, whereas the organic fraction was most important for Cu. The mobility and availability of metals in a floodplain soil can be significantly reduced by the formation of metal sulphide precipitates under anoxic conditions. Ascending salinity in the flood water promotes metal desorption from the floodplain soil in the absence of sulphides, hence increases total metal concentrations in the water column. The net effect of the presence of organic matter can either be a decrease or an increase in metal mobility, whereas the presence of carbonates in calcareous floodplain soils or sediments constitutes an effective buffer against a pH decrease. Moreover, carbonates may also directly precipitate metals. Plants can affect the metal mobility in floodplain soils by oxidising their rhizosphere, taking up metals, excreting exudates and stimulating the activity of microbial symbionts in the rhizosphere.     

Physico-chemical and biological parameters determine metal bioavailability in soils

The Netherlands Stimulation program on System-oriented Ecotoxicological Research focused on three study areas, including two floodplains and a peaty grassland. All three areas were polluted with metals, with total soil concentrations often exceeding Dutch Intervention Values. The floodplain areas showed a homogeneous distribution of metal pollution, while pollution in the peaty area was more heterogeneous. This study aimed at establishing possible general trends in metal bioavailability by combining results obtained at the three different study sites. Available metal concentrations, measured as pore water or 0.01 M CaCl₂ extractable concentrations in soil, were lowest in the floodplain soils, probably due to the high pH (> 7.0) and high organic matter (8-30%) and clay contents (13-42%). In the peaty soil, having a lower soil pH (4.5-6.5) but higher organic matter contents (38-60%), in some but not all samples Cu concentrations in pore water and Cu and Pb concentrations in 0.01 CaCl₂ extracts were higher than in non-polluted reference areas. Plants in the floodplain areas had only low metal concentrations in their leaves, but soil invertebrates and small mammals did contain elevated concentrations in their body. Cd showed high levels in earthworms, snails and small mammals, while also Cu levels were sometimes increased in earthworms, millipedes and small mammals from the floodplain areas. Earthworms from the peaty area contained increased levels of Cu and Pb. These results suggest that metal bioavailability cannot be predicted from available concentrations in pore water or 0.01 M CaCl₂ soil extracts, but requires measurement of biota and more insight into the biodynamics of metal uptake.     

Quantification of copper doses to settlement plates in the field using diffusive gradients in thin films

A study of carbon (C) storage in the 0-0.75-m profile of soils subtending various types of grasslands on the Qinghai-Tibetan Plateau showed that the organic carbon content of the 1,627,000 km2 of such lands in the region reaches 33.52 Pg of C. Organic carbon is mainly stored in the meadow and steppe soils of the Plateau, which combined, represents 23.2 Pg of C stored in organic form. This represents 23.44% of China's total organic soil-stored carbon and 2.5% of the global pool of soil carbon as of 1996. Carbon emissions from the grassland soils were estimated based on the two major modes of emission: (i) natural soil respiration and (ii) shifts in net C flux to/from soil due to land-use changes and their potential influence on organic matter decomposition. Annual soil respiration-driven CO2 emissions from the grassland soils of the plateau reached 1.17 Pg C year(-1), accounting for 26.4% of China's total soil respiration and 1.73% of global soil respiration. Because the grassland area accounts for 1.02% of the global terrestrial land and 16.9% of China's total terrestrial land, this CO2 emission rate is significantly higher than the country's mean annual rate (approx. 4.2 Pg C year(-1)) and even higher than the global mean rate (approx. 68 Pg C year(-1)). In the last 30 years, approximately 3.02 Pg C have been emitted from the grassland soils of the plateau due to land-use changes and grassland degradation. The total CO2 emissions rate from the grassland soils of the plateau reached 1.27 Pg C year(-1). Protecting grasslands on the Qinghai-Tibetan Plateau is of great importance in limiting global climate change.     

Short-term effects of biosolid and municipal solid waste applications on heavy metals distribution in a degraded soil under a semi-arid environment

Digested biosolid (SS) and municipal solid waste (MSW) were surface-applied to a degraded carbonated soil, under semi-arid environment, at rates of 0 and 80 Mg/ha, to determine the changes in organic matter and in the distribution of heavy metals in the topsoil, 1 year after its application. Waste application slightly increased the organic matter content and improved the composition of humic fractions in the treated soils, mainly in the MSW amended plots. A sequential extraction method (Tessier et al., 1979) was used to determine the distribution of Cd, Cr, Cu, Ni, Pb and Zn in both the waste and the amended.soils. Waste application had little effect on the total concentration of Ni and Cr in the treated soils as a consequence of the low availability of these metals in the wastes. A considerable increase of Cd, Cu, Pb and Zn was observed as a consequence of the high content and/or high availability of these metals in the wastes. The more labile fraction (exchangeable fraction) of all metals studied increased slightly (< 1.5 mg/kg) when SS and MSW were added. However, a remarkable increase in the Fe/Mn oxide fraction of Cd, Cu and Pb and in the organic fraction of Zn were noted in treated plots, this increase being higher in the MSW treated soils.     

Quantification and speciation of mercury in soils from the Tripuí Ecological Station, Minas Gerais, Brazil

Contents of total mercury, organic carbon, total sulfur, iron, aluminum and grain size and clay mineralogy were used along with Pearson's correlation and Hg thermal desorption technique to investigate the presence, distribution and binding behavior of Hg in soils from three depths from the Tripuí Ecological Station, located near Ouro Preto, Minas Gerais State, Brazil. The soils studied had predominantly medium and fine sand texture (0.59-0.062 mm), acid character and Hg contents ranging from 0.09 to 1.23 microg/g. The granulometric distribution revealed that Hg is associated with coarse sand (2-0.59 mm) and silt and clay (<0.062 mm) and presents similar Hg concentrations in both fractions. Mercury distribution in soil profiles showed that Hg was homogeneously distributed throughout the depths at most sites. Hg thermal desorption curves show that mercury occurs not only as Hg2+ predominantly bound to organic components in most of the samples, but also in the form of cinnabar in some. Pearson's correlation confirmed that mercury is associated with organic matter and sulfur and possibly with sulfur-bearing organic matter in most samples.     

Lead and Cu in contaminated urban soils: extraction with chemical reagents and bioluminescent bacteria and yeast

Twenty urban soil samples, with a wide range of Pb (14-5323 mg/kg) and Cu (8-12987 mg/kg), were used to compare the operational speciation of a five-step sequential leach with the bioavailability determined with bioluminescent Pb (RN4220(pTOO24)) and Cu (MC1061(pSLcueR/pDNPcopAluc)) specific bacterial biosensors and a Cu specific yeast sensor. The bioavailable Pb concentrations were all similar or lower than the first sequential leach step (1M NaOAc). In contrast, in some samples the bioavailable concentrations of Cu clearly exceeded even the second sequential leach step (0.1 M Na4P2O7). With the yeast sensor 12/20 samples were below detection, however, the yeast sensor was capable of detecting all high Cu concentrations. The biosensors used in this study are not capable of detecting the natural soil concentrations of Pb and Cu in the studied area.     

Heavy metal contamination of the soils used for stocking raw materials in the former ILVA iron-steel industrial plant of Bagnoli (southern Italy)

The total contents and the chemical and mineralogical forms of the metals Fe, Al, Cu, Co, Cr, Pb, Zn, Ni and Mn in the horizons of a soil profile, representative of an area devoted to stocking raw materials in the dismantled iron-steel industrial plant of ILVA of Bagnoli (Naples), were studied by physical and chemical methods. The geological setting of the study area is the result of volcanic activity in the Phlegrean Fields, a group of polygenic volcanoes to the west of Naples, which give rise to the parent soil material. Soil morphology appeared to be strongly disturbed by the occurrence and stratification of materials used in the industrial process. Fine sediments illuviation down the profile resulted in the occurrence of silt and clay coatings. The total contents of Cu, Co, Cr, Pb, Zn and Ni, in the whole soil samples, especially in the surface layers, were above the regulatory levels (Cu 120, Co 20, Cr 150, Pb 100, Zn 150, Ni 120 mg kg(-1)) stated by the Italian Ministry of Environment for soils in public, private and residential areas, and below the levels (Cu 600, Co 250, Cr 800, Pb 1000, Zn 1500, Ni 500 mg kg(-1)) outlined for soils and subsoils of industrial and commercial areas (Gazzetta Ufficiale della Repubblica Italiana, 1999). Speciation of heavy metals and the determination of the different chemical pools in the fraction < 2 mm identified the large presence of elements trapped in the mineralogical structure of oxides and silicates and occluded in easily reducible manganese or iron oxides. A constant amount of Cu was associated with organic compounds. A significant amount of Zn (> 20%) was extracted in diluted acetic acid solution, indicating that the element was present in a more readily and potentially available form. In the clay fraction (< 2 microm) heavy metals were associated with both amorphous and crystalline iron forms. The presence of iron-rich clay coatings was evident in the illuvial pores of deeper horizons. Enrichment in Cu, Co, Cr and Zn of the coatings was observed. Possible translocation of metals down through the soil profile mainly bound to fine particles of relatively inert forms of iron is hypothesised. The dispersion in water of the clay fraction resulted in an average percentage dispersion of approximately 20% with a peak of 41.7% at 68-72 cm depth. Magnetite, goethite, hematite, calcite and quartz mixed with K-feldspars, clynopyroxenes and mica occurred in the coarse sand fractions (2-0.2 mm) of the soil samples from all the surface horizons. Talcum and goethite together with clay minerals at 1.4 nm, kaolinite and illite were found in the clays (< 2 microm).     

Binding of mercury in soils and attic dust in the Idrija mercury mine area (Slovenia)

Total Hg concentrations and Hg speciation were determined in soils and attic dust in a 160 km2 area around Idrija mercury mine. Attic dust as well as a sample of soil was collected at 100 locations. Mercury phases were separated into cinnabar and non-cinnabar compounds via a thermo-desorption technique. The amount of the non-cinnabar fraction is important since it is potentially bioavailable and results are needed for further risk assessment studies. The concentrations of Hg in attic dust are many times higher than in surrounding soils and the attic dust/soil ratio changes with distance. The highest concentration ratios were identified at the greatest distance from the source of pollution and the lowest close to the source of pollution. This confirms the impact of air emissions on the wider area around Idrija. Furthermore the spatial mercury distribution in the attic dust shows that the influence of atmospheric emissions caused by the Idrija smelter resulted in impacts on the environment on a regional scale. The portions of non-cinnabar compounds increase with distance from the mercury source in both sampling media. Non-cinnabar fractions were found to be enriched in distant areas where fine grained material was deposited. There were two different transport mechanisms of dust particles and gaseous Hg(0) during the mercury production period. Obviously coarse grained particles, with mostly cinnabar-bound Hg settled in the immediate vicinity of the smokestack of the smelter, whereas the fine grained fraction could be dispersed further ahead. This is represented by the percentage of cinnabar-bound Hg in attic dust and soil decreasing with distance from the smelter. Gaseous Hg(0) is probably bound to fine and ultrafine aerosols with longer residence time against deposition. The consequence is that fine grained material with Hg2+ and Hg0 prevails in remote localities and is bound in soils and dust with matrix and organic matter as non-cinnabar mercury compounds. The distributions of mercury species in attic dust and soils along the Idrijca River show that in the region from Idrija to Spodnja Idrija the portions of cinnabar and non-cinnabar are about equal, while in the upper and in the lower Idrijca valley non-cinnabar bound mercury prevails. The applicability of attic dust for tracing the mercury halo in the Idrija area was successfully shown.     

The use of compost in afforestation of Mediterranean areas: Effects on soil properties and young tree seedlings

In Mediterranean frequently burnt areas, fire and erosion result in the decrease of soil fertility, so afforestation is a major concern. We carried out an in situ experiment of compost amendment to improve survival and growth of planted tree seedlings. One-year-tree seedlings of native species (Quercus ilex, Pinus halepensis and Pinus pinea) were planted on a frequently burnt calcareous site. Three rates of fresh co-composted sewage sludge and greenwastes (control without compost, 20 and 40 kg m(-2) of compost) were incorporated into the soil at each seedling stem. Changes of soil properties and tree development were studied during 3 years (2001-2003) and 2 years (2002-2003) respectively. The compost improved survival of Quercus ilex and Pinus pinea seedlings in severe drought conditions, but had no effect on Pinus halepensis. For all species seedling length and radial growth and NPK nutrition were increased for both rates of amendment. Amendment improved soil fertility, but available P concentration increased 13 fold in the neighbouring soil of seedlings amended at the maximal rate compared to control. However, amendment did not significantly increase concentrations of Cd, Cr, Ni and Pb in soils or tree seedlings. It increased Cu and Zn total and available concentrations in soils, while foliar Cu and Zn concentrations in the seedlings remained similar in all plots. Compost can efficiently help afforestation of dry soils with low organic matter content. However, sewage sludge concentrations in P, and to a lesser extent in Cu and Zn, limit rates of application that can be applied without environmental hazard.     

Arsenic biogeochemistry and human health risk assessment in organo-arsenical pesticide-applied acidic and alkaline soils: an incubation study

Organo-arsenical compounds are considered non-carcinogenic, and hence, are still allowed by the regulatory agencies for use in agriculture as pesticides. Due to rapid encroachment of suburban areas into former agricultural lands, the potential for human exposure to soil-arsenic has increased tremendously in recent years. However, insufficient data is available on the stability of organo-arsenicals in soils; as to whether they remain in an organic form, or are converted over time to potentially carcinogenic inorganic forms. A static incubation study was conducted to estimate soil speciation and in-vitro bioavailability (i.e., bioaccessibility) of arsenic as a function of soil properties. Two chemically variant soil types were chosen, based on their potential differences with respect to arsenic reactivity: an acid sand with minimal arsenic retention capacity and an alkaline clay loam with relatively high concentrations of Fe/Al and Ca/Mg. The soils were amended with dimethylarsenic acid (DMA) at three rates, 45, 225 and 450 mg/kg, and incubated for 1 year. A sequential extraction scheme was employed to identify the geochemical forms of arsenic in soils, which were correlated with the in-vitro bioavailable fractions of arsenic. Human health risk calculated in terms of excess cancer risk (ECR) showed that risk assessment based on bioaccessible arsenic concentrations instead of the traditional total soil arsenic is a more realistic approach. Results showed that soil properties (such as pH, Fe/Al content and soil texture) of the two soils dictated the geochemical speciation, and hence, bioaccessibility of arsenic from DMA, indicating that the use of organic arsenicals as pesticides in mineral soils may not be a safe practice from a human health risk perspective.