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1.
《Applied Geochemistry》2004,19(7):1123-1135
About 200 samples from selected public supply, domestic, and observation wells completed in alluvial aquifers underlying the western Mojave Desert were analyzed for total dissolved Cr and Cr(VI). Because Cr(VI) is difficult to preserve, samples were analyzed by 3 methods. Chromium(VI) was determined in the field using both a direct colorimetric method and EPA method 218.6, and samples were speciated in the field for later analysis in the laboratory using a cation-exchange method developed for the study described in this paper. Comparison of the direct colorimetric method and EPA method 218.6 with the new cation-exchange method yielded r2 values of 0.9991 and 0.9992, respectively. Total dissolved Cr concentrations ranged from less than the 0.1 μg/l detection limit to 60 μg/l, and almost all the Cr present was Cr(VI). Near recharge areas along the mountain front pH values were near neutral, dissolved O2 concentrations were near saturation, and Cr(VI) concentrations were less than the 0.1 μg/l detection limit. Chromium(VI) concentrations and pH values increased downgradient as long as dissolved O2 was present. However, low Cr(VI) concentrations were associated with low dissolved O2 concentrations near ground-water discharge areas along dry lakes. Chromium(VI) concentrations as high as 60 μg/l occurred in ground water from the Sheep Creek fan alluvial deposits weathered from mafic rock derived from the San Gabriel Mountains, and Cr(VI) concentrations as high as about 36 μg/l were present in ground water from alluvial deposits weathered from less mafic granitic, metamorphic, and volcanic rocks. Chromium(III) was the predominant form of Cr only in areas where dissolved O2 concentrations were less than 1 mg/l and was detected at a median concentration of 0.1 μg/l, owing to its low solubility in water of near-neutral pH. Depending on local hydrogeologic conditions and the distribution of dissolved O2, Cr(VI) concentrations may vary considerably with depth. Samples collected under pumping conditions from different depths within wells show that Cr(VI) concentrations can range from less than the 0.1 μg/l detection limit to 36 μg/l in a single well and that dissolved O2 concentrations likely control the concentration and redox speciation of Cr in ground water.  相似文献   

2.
The development of a fast, effective, simple and low-cost procedure for chromium speciation is an analytical challenge. In this work, a new and simple method for speciation and determination of chromium species in different matrices was developed. Sepia pharaonis endoskeleton nano-powder was used as an adsorbent for the dispersive micro-solid-phase extraction. Finally, the desorbed chromium was determined using a graphite furnace atomic absorption spectrometer. The experimental results showed that Cr(III) could be quantitatively extracted by the adsorbent, while Cr(VI) adsorption was negligible. Concentrated H2SO4 and ethanol reduced Cr(VI)–Cr(III), and total chromium content was assessed as Cr(III). Then, the Cr(VI) concentration in the sample was calculated as the difference. The optimum conditions were obtained in terms of pH, adsorbent amount, contact time, and type, concentration and volume of eluent. Under the optimum conditions that involved the speciation of chromium ions from 25 mL of the water samples at pH 7.0 using 0.025 g of the adsorbent with contact time of 5 min, the method was validated in terms of linearity, precision and accuracy. The calibration curve was linear over the concentration range of 0.01–25.00 μg L?1 for Cr(III). The obtained limit of detection for the proposed method was 0.003 µg L?1. The maximum adsorption capacity of the adsorbent was found to be 995.57 mg g?1. The proposed method was validated by the speciation of Cr(III) and Cr(VI) in different real water and wastewater samples with satisfactory results.  相似文献   

3.
A soil geochemical survey was conducted in a 27,000-km2 study area of northern California that includes the Sierra Nevada Mountains, the Sacramento Valley, and the northern Coast Range. The results show that soil geochemistry in the Sacramento Valley is controlled primarily by the transport and weathering of parent material from the Coast Range to the west and the Sierra Nevada to the east. Chemically and mineralogically distinctive ultramafic (UM) rocks (e.g. serpentinite) outcrop extensively in the Coast Range and Sierra Nevada. These rocks and the soils derived from them have elevated concentrations of Cr and Ni. Surface soil samples derived from UM rocks of the Sierra Nevada and Coast Range contain 1700–10,000 mg/kg Cr and 1300–3900 mg/kg Ni. Valley soils west of the Sacramento River contain 80–1420 mg/kg Cr and 65–224 mg/kg Ni, reflecting significant contributions from UM sources in the Coast Range. Valley soils on the east side contain 30–370 mg/kg Cr and 16–110 mg/kg Ni. Lower Cr and Ni concentrations on the east side of the valley are the result of greater dilution by granitic sources of the Sierra Nevada.Chromium occurs naturally in the Cr(III) and Cr(VI) oxidation states. Trivalent Cr is a non-toxic micronutrient, but Cr(VI) is a highly soluble toxin and carcinogen. X-ray diffraction and scanning electron microscopy of soils with an UM parent show Cr primarily occurs within chromite and other mixed-composition spinels (Al, Mg, Fe, Cr). Chromite contains Cr(III) and is highly refractory with respect to weathering. Comparison of a 4-acid digestion (HNO3, HCl, HF, HClO4), which only partially dissolves chromite, and total digestion by lithium metaborate (LiBO3) fusion, indicates a lower proportion of chromite-bound Cr in valley soils relative to UM source soils. Groundwater on the west side of the Sacramento Valley has particularly high concentrations of dissolved Cr ranging up to 50 μg L−1 and averaging 16.4 μg L−1. This suggests redistribution of Cr during weathering and oxidation of Cr(III)-bearing minerals. It is concluded that regional-scale transport and weathering of ultramafic-derived constituents have resulted in enrichment of Cr and Ni in the Sacramento Valley and a partial change in the residence of Cr.  相似文献   

4.
The redox-sensitive stable isotope geochemistry of chromium bears the potential to monitor the attenuation of chromate pollution and to investigate changes in environmental conditions in the present and the past. The use of stable Cr isotope data as a geo-environmental tracer, however, necessitates an understanding of the reaction kinetics and Cr fractionation behaviour during redox transition and isotope exchange. Here, we report stable chromium isotope fractionation data for Cr(VI) reduction, Cr(III) oxidation and isotopic exchange between soluble Cr(III) and Cr(VI) in aqueous media. The reduction of Cr(VI) to Cr(III) with H2O2 under strongly acidic conditions shows a near-equilibrium isotope fractionation of Δ53/52Cr(Cr(III)-Cr(VI)) of −3.54 ± 0.35‰. At pH neutrality, however, the reduction experiments show a kinetic isotope fractionation Δ53/52Cr(Cr(III)-Cr(VI)) of −5‰ for the extent of reduction of up to 85% of the chromium. The oxidation of Cr(III) to Cr(VI) in alkaline media, using H2O2 as the oxidant, cannot be explained by a single, unidirectional reaction. Our experiments indicate that the involvement of the unstable intermediates Cr(IV) and Cr(V) and their disproportionation during redox reactions between Cr(III) and Cr(VI) influence the overall fractionation factor, depending on the prevailing pH conditions and the reaction rates. No detectable isotope exchange between soluble Cr(VI) and Cr(III) species at pH values of 5.5 and 7 was revealed over a timescale of days to weeks. This means that, at least within such a time frame, the isotopic composition of Cr(VI) in a natural system will not be influenced by equilibration with any Cr(III) and thus reveal the true extent of reduction, given that the Cr isotope composition of the source Cr(VI) and the fractionation factor for the prevailing conditions are known.  相似文献   

5.
Arctic regions experience metal pollution, despite their remote location, and the distribution and migration of those metals determine their potential impact on the local environment. Here, a High-Arctic catchment (Revelva, Svalbard) located remotely from human-induced pollution sources is studied with respect to the distribution and migration of chosen trace elements (Ag, Al, As, B, Ba, Bi, Ca, Cd, Co, Cr, Cu, Cs, Mo, Ni, Pb, Sb, Se, Sr, Tl, U, V and Zn) in surface waters. The metal concentrations fluctuated in 2010–2012 between 0.01 and 354 μg L?1, the highest mean-weighted concentration noted for Sr (42.5 μg L?1). The concentrations in the river water were likely influenced by both natural and human-activity-related processes. These factors can produce substances of the same chemical composition (e.g. carbon dioxide, sulphur dioxide and metals may be emitted both by a volcanic eruption and by industrial sources). Therefore, chemometric techniques were used in the current paper to distinguish the multiple sources of pollution in the Revelva catchment. The authors were seeking to determine whether there is indeed evidence for contamination, sufficient to cause environmental damage in polar region. As a result, it was shown that the long-range transport could play an important role in shaping the metal concentration profile of this Arctic tundra environment, capturing both the influence of volcanic eruptions within the region and the human activity in a range of distances from the study site.  相似文献   

6.
《Applied Geochemistry》2000,15(9):1383-1397
Water pollution arising from base metal sulphide mines is problematic in many countries, yet the hydrogeology of the subsurface contaminant sources is rarely well-characterized. Drainage water pumped from an active F–Pb mine in northern England has unusual chemistry (alkaline with up to 40 mg.l−1 Zn) which profoundly impacts the ecology of the receiving watercourse. Detailed in-mine surveys of the quantity and quality of all ground water inflows to the mine were made. These revealed major, temporally persistent heterogeneities in ground water quality, with three broad types of water identified as being associated with distinct hydrostratigraphic units. Type I waters (associated with the Firestone Sill aquifer) are cool (<10°C), Ca–HCO3–SO4 waters, moderately mineralized (specific electrical conductance (SEC)≤410 μS.cm−1) with <4 mg.l−1 Zn. Type II waters (associated with the Great Limestone aquifer) are warmer (≈15°C), of Ca–SO4 facies, highly mineralized (SEC≤1500 μS.cm−1) with ≤40 mg.l−1 Zn. Type III waters (in the deepest workings) are tepid (>18°C), of Ca–HCO3–SO4 facies, intermediately mineralized (SEC≤900 μS.cm−1) with ≤13 mg.l−1 Zn, and with significant Fe (≤12 mg.l−1) and Pb (≤8 mg/l). Monotonic increases in temperature and Cl− concentration with depth contrast with peaks in total mineralization, SO4 and Zn at medium depth (in Type II waters). Sulphate, Pb and Zn are apparently sourced via oxidation of galena and sphalerite, which would release each metal in stoichiometric equality with SO4. However, molal SO4 concentrations typically exceed those of Pb and Zn by 2–3 orders of magnitude, which mineral equilibria suggest is due to precipitation of carbonate “sinks” for these metals. Contaminant loading budgets demonstrate that, although Type II waters amount to only 25% of the total ground water inflow to the mine, they account for almost 60% of the total Zn loading. This observation has important management implications for both the operational and post-abandonment phases of the mine life cycle.  相似文献   

7.
Thirty-three bottom sediments were collected from the Potomac and Anacostia rivers, Tidal Basin, and Washington Ship Channel in June 1991 to define the extent of trace metal contamination and to elucidate source areas of sediment contaminants. In addition, twenty-three sediment samples were collected directly in front of and within major storm and combined sewers that discharge directly to these areas. Trace metals (e.g., Cu, Cr, Cd, Hg, Pb, and Zn) exhibited a wide range in values throughout the study area. Sediment concentrations of Pb ranged from 32.0 μg Pb g?1 to 3,630 μg Pb g?1, Cd from 0.24 μg Cd g?1 to 4.1 μg Cd g?1, and Hg from 0.13 μg Hg g?1 to 9.2 μg Hg g?1, with generally higher concentrations in either outfall or sewer sediments compared to river bottom-sediments. In the Anacostia River, concentration differences among sewer, outfall, and river sediments, along with downriver spatial trends in trace metals suggest that numerous storm and combined swers are major sources of trace metals. Similar results were observed in both the Tidal Basin and Washington Ship Channel. Cadminum and Pb concentrations are higher in specific sewers and outfalls, whereas the distribution of other metals suggests a more diffuse source to the rivers and basins of the area. Cadmium and Pb also exhibited the greatest enrichment throughout the study area, with peak values located in the Anacostia River, near the Washington Navy Yard. Enrichment factors decrease in the order: Cd>Pb>Zn>Hg>Cu>Cr. Between 70% and 96% of sediment-bound Pb and Cd was released from a N2-purged IN HCl leach. On average, ≤40% of total sedimentary Cu was liberated, possibly due to the partial attack of organic components of the sediment. Sediments of the tidal freshwater portion of the Potomac estuary reflect a moderate to highly components area with substantial enrichments of sedimentary Pb, Cd, and Zn. The sediment phase that contains these metals indicates the potential mobility of the sediment-bound metals if they are reworked during either storm events or dredging.  相似文献   

8.
The risk of groundwater contamination by chromate at a former chromite ore processing industrial site in Rivera (Switzerland) was assessed by determining subsoil Cr(VI) concentrations and tracking naturally occurring Cr(VI) reduction with Cr isotopes. Using a hot alkaline extraction procedure, a total Cr(VI) contamination of several 1000 kg was estimated. Jarosite, KFe3((SO4)x(CrO4)1−x)2(OH)6, and chromatite (CaCrO4) were identified as Cr(VI) bearing mineral phases using XRD, both limiting groundwater Cr(VI) concentrations. To track assumed Cr(VI) reduction at field scale δ53Cr values of contaminated subsoil samples in addition to groundwater δ53Cr data are used for the first time. The measurements showed a fractionation of groundwater δ53Cr values towards positive values and subsoil δ53Cr towards negative values confirming reduction of soluble Cr(VI) to insoluble Cr(III). Using a Rayleigh fractionation model, a current Cr(VI) reduction efficiency of approximately 31% along a 120 m long flow path was estimated at an average linear groundwater velocity of 3.3 m/d. Groundwater and subsoil δ53Cr values were compared with a site specific Rayleigh fractionation model proposing that subsoil δ53Cr values can possibly be used to track previous higher Cr(VI) reduction efficiency during the period of industrial activity. The findings strongly favor monitored natural attenuation to be part of the required site remediation measures.  相似文献   

9.
《Applied Geochemistry》2004,19(8):1255-1293
In order to investigate the mechanism of As release to anoxic ground water in alluvial aquifers, the authors sampled ground waters from 3 piezometer nests, 79 shallow (<45 m) wells, and 6 deep (>80 m) wells, in an area 750 m by 450 m, just north of Barasat, near Kolkata (Calcutta), in southern West Bengal. High concentrations of As (200–1180 μg L−1) are accompanied by high concentrations of Fe (3–13.7 mg L−1) and PO4 (1–6.5 mg L−1). Ground water that is rich in Mn (1–5.3 mg L−1) contains <50 μg L−1 of As. The composition of shallow ground water varies at the 100-m scale laterally and the metre-scale vertically, with vertical gradients in As concentration reaching 200 μg L−1 m−1. The As is supplied by reductive dissolution of FeOOH and release of the sorbed As to solution. The process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit. In well waters, thermo-tolerant coliforms, a proxy for faecal contamination, are not present in high numbers (<10 cfu/100 ml in 85% of wells) showing that faecally-derived organic matter does not enter the aquifer, does not drive reduction of FeOOH, and so does not release As to ground water.Arsenic concentrations are high (≫50 μg L−1) where reduction of FeOOH is complete and its entire load of sorbed As is released to solution, at which point the aquifer sediments become grey in colour as FeOOH vanishes. Where reduction is incomplete, the sediments are brown in colour and resorption of As to residual FeOOH keeps As concentrations below 10 μg L−1 in the presence of dissolved Fe. Sorbed As released by reduction of Mn oxides does not increase As in ground water because the As resorbs to FeOOH. High concentrations of As are common in alluvial aquifers of the Bengal Basin arise because Himalayan erosion supplies immature sediments, with low surface-loadings of FeOOH on mineral grains, to a depositional environment that is rich in organic mater so that complete reduction of FeOOH is common.  相似文献   

10.
The heavy metal content of particulate matter was investigated in the city of Guangzhou in southern China. Samples of urban foliage near 36 pedestrian bridges were analyzed to determine their Zn, Pb, Cu, Cr, V, Ni, and Co contents after digestion in a mixture of strong acids composed of HNO3, HCl, HF, and HClO4. The results revealed a severe heavy metal pollution compared with the background levels in Chinese soils, except for Co and V. The mean concentrations of Zn (1,024 mg kg?1), Pb (233 mg kg?1), Cu (203 mg kg?1), Cr (118 mg kg?1), V (41.9 mg kg?1), Ni (41.4 mg kg?1), and Co (11.3 mg kg?1) in urban dust were higher than the reference levels, and were highest in samples located near high-traffic areas. Multivariate statistical methods (correlation analysis, principal-components analysis, and clustering analysis) were used to identify the possible sources of the metals. Three main pollutant sources are assigned: Zn, Cu and Ni levels were strongly correlated and were possibly related to combustion processes and vehicles; Pb, Cr and Co were mainly derived from traffic sources, combined with soil sources; and V mainly originated from natural sources.  相似文献   

11.
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.  相似文献   

12.
The adsorptive removal of Cr(VI) was studied using activated carbon derived from Leucaena leucocephala (ACLL). The physico-chemical properties of ACLL were determined using proximate analysis and N2 BET surface area analysis. The N2 BET surface area of ACLL was determined to be 1131 m2 g?1. The point of zero charge (pHpzc) of 5.42 indicated that ACLL surface was positively charged for pH below the pHPZC, attracting anions. The effect of experimental operating parameters such as time of contact, ACLL dose, pH, initial concentration and temperature was investigated. The optimum values of parameters such as concentration of 100 mg L?1, 300 mg of ACLL dose, time of contact of 60 min, pH of 4 indicated the maximum Cr(VI) uptake of 13.85 mg g?1. The pseudo-second-order kinetic model best fitted with the Cr(VI) adsorption data. Adsorptive removal of Cr(VI) onto ACLL satisfactorily fitted in the order of Redlich–Peterson > Freundlich > Langmuir > Temkin adsorption isotherm model. The thermodynamic parameters showed the adsorption of Cr(VI) onto ACLL was an endothermic and spontaneously occurred process.  相似文献   

13.
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14.
《Applied Geochemistry》2003,18(9):1297-1312
The concentrations of As in surface- and up to 90 °C ground waters in a tholeiite flood basalt area in N-Iceland lie in the range <0.03–10 μg/kg. With few exceptions surface waters contain <0.5 μg/kg As whereas ground waters generally contain >0.5 μg/kg As. The As content of ground waters increases on the whole with rising temperature. Arsenic is highly mobile in the basalt-water environment of the study area. An insignificant fraction of the As dissolved from the rock is taken up into secondary minerals. Arsenic is less mobile than B but considerably more mobile than Na which has the highest mobility among the major aqueous components. A significant fraction of the As in the basalt occurs in an easily soluble form. The As hosted in the primary minerals is expected to be concentrated in the titano-magnetite. This mineral is stable in contact with both surface- and ground waters and does not, therefore, supply As to the water, explaining the difference in mobility between As and B. Aqueous As concentrations are a reflection of water/rock ratios, i.e. how much rock a given quantity of water has dissolved. This ratio increases with increasing temperature and increasing residence time of the water in contact with the rock. The distribution of As species has been calculated on the assumption of equilibrium at the redox potential retrieved from measurement of aqueous Fe(II) and Fe(III) concentrations. These calculations indicate that pentavalent As is stable in surface waters and in ground waters with an in situ pH of <10 and would occur mostly as H2AsO4− and HAsO4−2. In higher pH ground waters the concentrations of the arsenite species H2AsO3− is significant at equilibrium, up to 65% of the total dissolved As.  相似文献   

15.
Samples collected in December 1990 and July 1991 show that dissolved Cd, Cu, Ni, and Zn distributions in the Gulf of the Farallones are dominated by mixing of two end-members: (1) metal-enriched San Francisco Bay water and (2) offshore California Current water. The range of dissolved metal concentrations observed is 0.2–0.9 nmol kg?1 for Cd, 1–20 nmol kg?1 for Cu, 4–16 nmol kg?1 for Ni, and 0.2–20 nmol kg?1 for Zn. Effective concentrations in fresh water discharged into San Francisco Bay during 1990–1991 (estimated by extrapolation to zero salinity) are 740–860 μmol kg?1 for silicate, 21–44 μmol kg?1 for phosphate, 10–15 nmol kg?1 for Cd, 210–450 nmol kg?1 for Cu, 210–270 nmol kg?1 for Ni, and 190–390 nmol kg?1 for Zn. Comparison with effective trace metal and nutrient concentrations for freshwater discharge reported by Flegal et al. (1991) shows that input of these constituents to the northern reaches of San Francisco Bay accounts for only a fraction of the input to Gulf of the Farallones from the estuary system as a whole. The nutrient and trace metal composition of shelf water outside a 30-km radius from the mouth of the estuary closely resembles that of California Current water further offshore. In contrast to coastal waters elsewhere, there is little evidence of Cd, Cu, Ni, and Zn input by sediment diagenesis in continental shelf waters of California.  相似文献   

16.
Tropical peat soils present higher ash content than those generated at temperate climate areas. Therefore, this study evaluated the characteristics of a Brazilian organic soil (OS), commercialized as peat, as well as its capacity in removing Cr(VI) from contaminated waters. The OS is composed of 35.5 wt% of organic matter and 56 wt% of inorganic fraction (ash), which is formed by minerals and phytoliths rich in silica (29.2 wt%) and alumina (23.6 wt%). The Cr(VI) removal tests were carried out in batch and column systems using OS and solutions of Cr(VI) prepared with distilled water and groundwater. Batch tests revealed that the organic substances in the OS caused the reduction of Cr(VI) to Cr(III), with an efficiency depending on solution pH. At pH 5.0 the Cr(VI) removal was 0.45 mg g?1 in 24 h; whereas at pH 2.0, this removal increased to 1.10 mg g?1. Since this redox reaction is very slow, the removal of Cr(VI) at pH 5.0 increased to around 2 mg g?1 after 5 days. The removal of Cr(VI) was more effective in the column tests than in the batch test due to the greater solid/solution ratio, and their half-lives were 4.4 and 26.2 h, respectively. Chemical analysis indicated that Cr(VI) was reduced by the humic substances of OS, followed by the precipitation and/or adsorption of Cr(III) into the organic and inorganic components, as anatase. The presence of Cr(III) increased the stability of anatase structure, avoiding its transformation into rutile, even after being heated at 800 °C/2 h.  相似文献   

17.
Chromium (Cr) is a heavy metal that exists in soils in two stable oxidation states, +III and +VI. The trivalent species is an essential nutrient, whereas the hexavalent species is highly toxic. This study investigated the environmental impact of CrIII potentially released into soil from wastes and various materials by determining the risk of oxidation of initially soluble inorganic CrIII into hazardous CrVI. The principal aim was to describe the pH-dependent mechanisms that regulate 1) the formation of CrVI from the easily soluble CrIII and 2) the potential bioavailability of CrIII and that of CrVI species produced in the oxidation of CrIII in agricultural soil (fine sand, organic carbon 3.2%). The amount of CrVI formed in oxic soil conditions was regulated by two counteracting reactions: 1) oxidation of CrIII into CrVI by manganese oxide (MnIVO2) and 2) the subsequent reduction of CrVI by organic matter back to CrIII. The effect of pH on this net-oxidation of CrIII and on the chemical availability of both CrIII and CrVI species was investigated in soil samples incubated with or without excessive amounts of synthetic MnO2, over the chemically adjusted pH range of 3.9–6.3 (+22 °C, 47 d). In soil subsamples without added MnO2, the net-oxidation of CrIII into CrVI (1 mM CrCl3 in soil suspensions, 1:10 w/V) was negligible. As for the MnO2-treated soils, at maximum only 4.7% of added CrIII was oxidized – regardless of the high oxidation potential of these subsamples. The lowest production of CrVI was observed under acidic soil conditions at pH ∼4. At low pH, the net-oxidation diminished as result of enhanced reduction of CrVI back to CrIII. At higher pHs, the oxidation was limited by enhanced precipitation (or adsorption) of CrIII, which lowered the overall amount of CrIII susceptible for oxidation. Moreover, the oxidation reactions by MnO2 were inhibited by formation of Cr(OH)3 coverage on its surface. The pH-dependent chemical bioavailability of added CrIII differed from that of the CrVI formed. At elevated pHs the chemical availability of CrIII decreased, whereas that of CrVI produced increased. However, the risk of CrVI formation through oxidation of the easily soluble inorganic CrIII was considered to be low in agricultural soils high in organic matter and low in innate MnO2.  相似文献   

18.
A simple and accurate method for the determination of Cr, Ni, Cu and Zn at μg g?1 levels in milligram‐sized bulk silicate materials is reported using isotope dilution high‐resolution inductively coupled plasma‐mass spectrometry (HR‐ICP‐MS) with a flow injection system. Silicate samples with Cr, Ni, Cu and Zn spikes were digested with HF‐HBr and Br2, and subsequently decomposed at 518 K in a Teflon bomb. In this procedure, all sulfides and chromite, major hosts of these elements, were completely decomposed, thus allowing for isotope equilibration between the sample and spike. Magnesium and Al fluorides formed after the digestion of the sample were removed by centrifugation, and the supernatant was directly aspirated into a HR‐ICP‐MS at a mass resolution of 7500, where interfering oxide ions, ArO+, CaO+, TiO+, CrO+ and VO+, were separated from Cr+, Ni+, Cu+ and Zn+. No matrix effects were observed down to a dilution factor of 50. Detection limits for these elements in silicate samples were < 0.04 μg g?1. The effectiveness of the technique was demonstrated by the analysis of 13 to 40 mg test portions of USGS and GSJ silicate reference materials with a major element composition ranging from andesite to peridotite, in addition to 8‐23 mg of the Smithsonian reference Allende. Both the reproducibility and the deviation from the reference value for most reference materials of various rock types were < 9%, and thus confirm that the method gives accurate analytical results for small sample sizes over a wide range of Cr, Ni, Cu and Zn contents. This method is, therefore, suitable for analysing small and/or precious bulk samples, such as meteorites, mantle peridotites and mineral separates, and for the characterisation of silicate and sulfide minerals for use as calibration samples in secondary ion mass spectrometry or laser ablation ICP‐MS.  相似文献   

19.
Sediment core segments from Sylvan Lake, Lake Champlain and Lake Canadarago were dated radiometrically with 210Pb and 137Cs. Their respective sedimentation rates were determined to be 0.11, 0.14 and 0.52 g cm?2 yr?1. For the two lakes of lower sedimentation the variations of selected elemental abundances as function of depth were analyzed. Two groupings were found: Al, K, Ti, Rb and Zr were correlated among themselves but reflected different variations in the input of terrigenous erosion material to the lakes. The Cu, Zn and Pb correlated among themselves showed similar depth dependence with increasing concentrations toward the top which can be attributed to cultural pollution. Recent ‘excess’ fluxes to the sediments above the natural contribution by clastic material were derived for the location of the cores, which for Cu, Zn and Pb amounted to 3.8, 24 and 16 μg cm?2 yr?1 respectively for Sylvan Lake and 4.9, 20 and 16 μg cm?2 yr?1 for Lake Champlain. The corresponding 210Pb flux was 3.3 and 2.3 dpm cm?2 yr?1, respectively for the two lakes.Approximate residence times in the water column were obtained for trace metals at the Lake Champlain location. Short residence times estimated for Pb (< 0.15 yr) and Cu (< 0.4 yr) indicate fast removal, whereas those for Zn (1.0 ± 0.3 yr) and Cr (2.0 ± 0.5 yr) appeared to be dominated by the water residence time.  相似文献   

20.
We examined the relationship between soil oxidation capacity and extractable soil manganese, iron oxides, and other soil properties. The Korean soils examined in this study exhibited low to medium Cr oxidation capacities, oxidizing 0.00–0.47 mmol/kg, except for TG-4 soils, which had the highest capacity for oxidizing added Cr(III) [>1.01 mmol/kg of oxidized Cr(VI)]. TG and US soils, with high Mn contents, had relatively high oxidation capacities. The Mn amounts extracted by dithionite-citrate-bicarbonate (DCB) (Mnd), NH2OH·HCl (Mnh), and hydroquinone (Mnr) were generally very similar, except for the YS1 soils, and were well correlated. Only small proportions of either total Mn or DCB-extractable Mn were extracted by NH2OH·HCl and hydroquinone in the YS1 soils, suggesting inclusion of NH2OH·HCl and hydroquinone-resistant Mn oxides, because these extractants are weaker reductants than DCB. No Cr oxidation test results were closely related to total Mn concentrations, but Mnd, Mnh, and Mnr showed a relatively high correlation with the Cr tests (r = 0.655–0.851; P < 0.01). The concentrations of Mnd and Mnh were better correlated with the Cr oxidation tests than was the Mnr concentration, suggesting that the oxidation capacity of our soil samples can be better explained by Mnd and Mnh than by Mnr. The first component in principal components analysis indicated that extractable soil Mn was a main factor controlling net Cr oxidation in the soils. Total soil Mn, Fe oxides, and the clay fraction are crucial for predicting the mobility of pollutants and heavy metals in soils. The second principal component indicated that the presence of Fe oxides in soils had a significant relationship with the clay fraction and total Mn oxide, and was also related to heavy-metal concentrations (Zn, Cd, and Cu, but not Pb).  相似文献   

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