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1.
Lacustrine sediments, submerged tailings, and their pore waters have been collected at several sites in Yellowknife Bay, Great Slave Lake, Canada, in order to investigate the biogeochemical controls on the remobilization of As from mining-impacted materials under different depositional conditions. Radiometric dating confirms that a mid-core enrichment of Pb, Zn, Cu and Sb corresponds to the opening of a large Au mine 60 a ago. This was evident even in a relatively remote site. Arsenic was enriched at mid-core, coincident with mining activity, but clearly exhibited post-depositional mobility, migrating upwards towards the sediment water interface (SWI) as well as down-core. Deep-water (15 m) Yellowknife Bay sediments that contain buried mine waste are suboxic, relatively organic-rich and abundant in microbes with As in pore waters and sediments reaching 585 μg/L and 1310 mg/kg, respectively. Late summer pore waters show equal proportions of As(III) and As(V) (16–415 μg/L) whereas late winter pore waters are dominated by As(III) (284–947 μg/L). This can be explained by As(III) desorption mechanisms associated with the conversion of FeS to FeS2 and the reduction of As(V) to As(III) through the oxidation of dissolved sulfide, both microbially-mediated processes. Processes affecting As cycling involve the attenuating efficiency of the oxic zone at the SWI, sediment redox heterogeneity and the reductive dissolution of Fe(hydr)oxides by labile organic matter, temporarily and spatially variable.  相似文献   

2.
This study investigates Sb speciation in sediments along the drainage of the Upper Peter adit at the Bralorne Au mine in southern British Columbia, Canada, and compares the behavior of Sb with that of As. The Upper Peter mineralization consists of native Au in quartz-carbonate veins with 1 wt.% sulfides dominated by pyrite and arsenopyrite although stibnite, the primary Sb-bearing sulfide mineral, can be locally significant. Dissolved Sb concentrations can reach up to 349 μg L−1 in the mine pool. Sediments were collected for detailed geochemical and mineralogical characterization at locations along the 350-m flow path, which includes a 100-m shallow channel within the adit, a sediment settling pond about 45 m beyond the adit portal and an open wetland another 120 m farther downstream. From the mine pool to the wetland outlet, dissolved Sb in the drainage drops from 199 μg L−1 to below the detection limit due to the combined effect of dilution and removal from solution. Speciation analyses using X-ray absorption near-edge structure (XANES) spectroscopy indicate that Sb(III)–S accounts for around 70% of total Sb in the sediments in the main pool at the far end of the adit. At a short distance (24 m) downstream of the main adit pool, however, Sb(III)–O and Sb(V)–O species represent ?50% of total Sb in the bulk sediments, indicating significant oxidation of the primary sulfides inside the adit. Although Sb appears largely oxidized in the bulk samples collected near the portal, Sb(III)–S species are nevertheless present in the <53-μm fraction, suggesting a higher oxidation rate for stibnite in the coarser grains, possibly due to galvanic interaction with pyrite. Secondary Sb species released from the sulfide oxidation are most likely sorbed/co-precipitated with Fe-, Mn-, and Al-oxyhydroxides along the flow channel in the adit and in the sediment settling pond, with the Fe phase being the dominant sink for Sb.  相似文献   

3.
Environmental contamination with As and Sb caused by past mining activities at Sb mines is a significant problem in Slovakia. This study is focused on the environmental effects of the 5 abandoned Sb mines on water, stream sediment and soil since the mines are situated in the close vicinity of residential areas. Samples of mine wastes, various types of waters, stream sediments, soils, and leachates of the mine wastes, stream sediments and selected soils were analyzed for As and Sb to evaluate their geochemical dispersion from the mines. Mine wastes collected at the mine sites contained up to 5166 mg/kg As and 9861 mg/kg Sb. Arsenic in mine wastes was associated mostly with Fe oxides, whereas Sb was present frequently in the form of individual Sb, Sb(Fe) and Fe(Sb) oxides. Waters of different types such as groundwater, surface waters and mine waters, all contained elevated concentrations of As and Sb, reaching up to 2150 μg/L As and 9300 μg/L Sb, and had circum-neutral pH values because of the buffering capacity of abundant Ca- and Mg-carbonates. The concentrations of Sb in several household wells are a cause for concern, exceeding the Sb drinking water limit of 5 μg/L by as much as 25 times. Some attenuation of the As and Sb concentrations in mine and impoundment waters was expected because of the deposition of metalloids onto hydrous ferric oxides built up below adit entrances and impoundment discharges. These HFOs contained >20 wt.% As and 1.5 wt.% Sb. Stream sediments and soils have also been contaminated by As and Sb with the peak concentrations generally found near open adits and mine wastes. In addition to the discharged waters from open adits, the significant source of As and Sb contamination are waste-rock dumps and tailings impoundments. Leachates from mine wastes contained as much as 8400 μg/L As and 4060 μg/L Sb, suggesting that the mine wastes would have a great potential to contaminate the downstream environment. Moreover, the results of water leaching tests showed that Sb was released from the solids more efficiently than As under oxidizing conditions. This might partly explain the predominance of Sb over As in most water samples.  相似文献   

4.
The geochemical evolution of two acid mine effluents in Tharsis and La Zarza-Perrunal mines (Iberian Pyrite Belt, Huelva, Spain) has been investigated. In origin, these waters present a low pH (2.2 and 3.1) and high concentrations of dissolved sulphate and metals (Fe, Al, Mn, Cu, Zn, As, Cd, Co, Cr, Ni). However, the natural evolution of these acidic waters (which includes the bacterial oxidation of Fe(II) and the subsequent precipitation of Fe(III) minerals) represents an efficient mechanism of attenuation. This self-mitigating process is evidenced by the formation of schwertmannite, which retains most of the iron load and, by sorption, toxic trace elements like As. The later mixing with pristine waters rises the pH and favours the total precipitation of Fe(III) at pH 3.5 and, subsequently, Al compounds at pH 4.5, along with the sorption of trace metals (Mn, Zn, Cu, Cd, Co, Ni) until chemical equilibrium at circumneutral conditions is achieved.  相似文献   

5.
The oxidation of sulfide-rich rocks, mostly leftover debris from Cu mining in the early 20th century, is contributing to metal contamination of local coastal environments in Prince William Sound, Alaska. Analyses of sulfide, water, sediment, precipitate and biological samples from the Beatson, Ellamar, and Threeman mine sites show that acidic surface waters generated from sulfide weathering are pathways for redistribution of environmentally important elements into and beyond the intertidal zone at each site. Volcanogenic massive sulfide deposits composed of pyrrhotite and (or) pyrite + chalcopyrite + sphalerite with subordinate galena, arsenopyrite, and cobaltite represent potent sources of Cu, Zn, Pb, As, Co, Cd, and Hg. The resistance to oxidation among the major sulfides increases in the order pyrrhotite ? sphalerite < chalcopyrite ? pyrite; thus, pyrrhotite-rich rocks are typically more oxidized than those dominated by pyrite. The pervasive alteration of pyrrhotite begins with rim replacement by marcasite followed by replacement of the core by sulfur, Fe sulfate, and Fe–Al sulfate. The oxi dation of chalcopyrite and pyrite involves an encroachment by colloform Fe oxyhydroxides at grain margins and along crosscutting cracks that gradually consumes the entire grain. The complete oxidation of sulfide-rich samples results in a porous aggregate of goethite, lepidocrocite and amorphous Fe-oxyhydroxide enclosing hydrothermal and sedimentary silicates. An inverse correlation between pH and metal concentrations is evident in water data from all three sites. Among all waters sampled, pore waters from Ellamar beach gravels have the lowest pH (∼3) and highest concentrations of base metals (to ∼25,000 μg/L), which result from oxidation of abundant sulfide-rich debris in the sediment. High levels of dissolved Hg (to 4100 ng/L) in the pore waters probably result from oxidation of sphalerite-rich rocks. The low-pH and high concentrations of dissolved Fe, Al, and SO4 are conducive to precipitation of interstitial jarosite in the intertidal gravels. Although pore waters from the intertidal zone at the Threeman mine site have circumneutral pH values, small amounts of dissolved Fe2+ in the pore waters are oxidized during mixing with seawater, resulting in precipitation of Fe-oxyhydroxide flocs along the beach–seawater interface. At the Beatson site, surface waters funneled through the underground mine workings and discharged across the waste dumps have near-neutral pH (6.7–7.3) and a relatively small base-metal load; however, these streams probably play a role in the physical transport of metalliferous particulates into intertidal and offshore areas during storm events. Somewhat more acidic fluids, to pH 5.3, occur in stagnant seeps and small streams emerging from the Beatson waste dumps. Amorphous Fe precipitates in stagnant waters at Beatson have high Cu (5.2 wt%) and Zn (2.3 wt%) concentrations that probably reflect adsorption onto the extremely high surface area of colloidal particles. Conversely, crystalline precipitates composed of ferrihydrite and schwertmannite that formed in the active flow of small streams have lower metal contents, which are attributed to their smaller surface area and, therefore, fewer reactive sorption sites. Seeps containing precipitates with high metal contents may contribute contaminants to the marine environment during storm-induced periods of high runoff. Preliminary chemical data for mussels (Mytilus edulis) collected from Beatson, Ellamar, and Threeman indicate that bioaccumulation of base metals is occurring in the marine environment at all three sites.  相似文献   

6.
Nearly half a century after mine closure, release of As from the Ylöjärvi Cu–W–As mine tailings in groundwater and surface water run-off was observed. Investigations by scanning electron microscopy (SEM), electron microprobe analysis (EMPA), synchrotron-based micro-X-ray diffraction (μ-XRD), micro-X-ray absorption near edge structure (μ-XANES) and micro-extended X-ray absorption fine structure (μ-EXAFS) spectroscopy, and a sequential extraction procedure were performed to assess As attenuation mechanisms in the vadose zone of this tailings deposit. Results of SEM, EMPA, and sequential extractions indicated that the precipitation of As bearing Fe(III) (oxy)hydroxides (up to 18.4 wt.% As2O5) and Fe(III) arsenates were important secondary controls on As mobility. The μ-XRD, μ-XANES and μ-EXAFS analyses suggested that these phases correspond to poorly crystalline and disordered As-bearing precipitates, including arsenical ferrihydrite, scorodite, kaňkite, and hydrous ferric arsenate (HFA). The pH within 200 cm of the tailings surface averaged 5.7, conditions which favor the precipitation of ferrihydrite. Poorly crystalline Fe(III) arsenates are potentially unstable over time, and their transformation to ferrihydrite, which contributes to As uptake, has potential to increase the As adsorption capacity of the tailings. Arsenic mobility in tailings pore water at the Ylöjärvi mine will depend on continued arsenopyrite oxidation, dissolution or transformation of secondary Fe(III) arsenates, and the As adsorption capacity of Fe(III) (oxy)hydroxides within this tailings deposit.  相似文献   

7.
In the present study, we investigated As behavior in a high-As hot spring (Sambe hot spring, Shimane, Japan) by coupling direct chemical speciation by synchrotron-based XAFS and HPLC–ICP-MS with microbial As-redox transformation gene analysis. The concentration of soluble As in the spring streamwater decreased immediately along the flow in correlation with Fe behavior, indicating that As in the streamwater was naturally attenuated in the streamwater. Iron XAFS analysis suggested deposition of Fe(III) oxyhydroxides along the flow. Thus, considering the strong affinity of As to Fe oxyhydroxides, the observed attenuation in As was possibly caused by sorption (or incorporation) of As on Fe(III) oxyhydroxides. Both dissolved As(III) and As(V) were present in the aqueous phase, and As(III) was rapidly oxidized to As(V) (<30 s) along the flow. The oxidation kinetics indicated the occurrence of biotic As(III) oxidation, because obtained As(III) oxidation rate (6.7–7.8 μM min−1) was much faster than the reported abiotic oxidation rates. Furthermore, the bacterial arsenite oxidase gene (aioA) was detected in DNA extracted from all samples (average of 2.0 × 105 copies dry g−1), which also supported potential attributes of biological As(III) oxidation in situ. In solid phase samples from sampling points analyzed by XAFS, most of the As existed as oxidized pentavalent form, As(V). This result indicated that this form was preferentially partitioned to the solid phase because of the much higher affinity of As(V) than of As(III) to Fe(III) oxyhydroxides. Considering the kinetic and microbiological findings, it is indicated that biotic process was predominantly responsible for As(III) oxidation at the present site, and this biotic As(III) oxidation to As(V) controlled the observed attenuation of As, because oxidized As(V) was removed from the aqueous phase by Fe(III) oxyhydroxides more efficiently.  相似文献   

8.
Sorption of contaminants such as arsenic (As) to natural Fe(III) (oxyhydr)oxides is very common and has been demonstrated to occur during abiotic and biotic Fe(II) oxidation. The molecular mechanism of adsorption- and co-precipitation of As has been studied extensively for synthetic Fe(III) (oxyhydr)oxide minerals but is less documented for biogenic ones. In the present study, we used Fe and As K-edge X-ray Absorption Near Edge Structure (XANES), extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, Mössbauer spectroscopy, XRD, and TEM in order to investigate the interactions of As(V) and As(III) with biogenic Fe(III) (oxyhydr)oxide minerals formed by the nitrate-reducing Fe(II)-oxidizing bacterium Acidovorax sp. strain BoFeN1. The present results show the As immobilization potential of strain BoFeN1 as well as the influence of As(III) and As(V) on biogenic Fe(III) (oxyhydr)oxide formation. In the absence of As, and at low As loading (As:Fe ≤ 0.008 mol/mol), goethite (Gt) formed exclusively. In contrast, at higher As/Fe ratios (As:Fe = 0.020-0.067), a ferrihydrite (Fh) phase also formed, and its relative amount systematically increased with increasing As:Fe ratio, this effect being stronger for As(V) than for As(III). Therefore, we conclude that the presence of As influences the type of biogenic Fe(III) (oxyhydr)oxide minerals formed during microbial Fe(II) oxidation. Arsenic-K-edge EXAFS analysis of biogenic As-Fe-mineral co-precipitates indicates that both As(V) and As(III) form inner-sphere surface complexes at the surface of the biogenic Fe(III) (oxyhydr)oxides. Differences observed between As-surface complexes in BoFeN1-produced Fe(III) (oxyhydr)oxide samples and in abiotic model compounds suggest that associated organic exopolymers in our biogenic samples may compete with As oxoanions for sorption on Fe(III) (oxyhydr)oxides surfaces. In addition HRTEM-EDXS analysis suggests that As(V) preferentially binds to poorly crystalline phases, such as ferrihydrite, while As(III) did not show any preferential association regarding Fh or Gt.  相似文献   

9.
This study examines some relevant limnological and hydrogeochemical characteristics of 22 mine pit lakes of the Iberian Pyrite Belt (IPB). The studied pit lakes include some of the largest and historically most important mines of the IPB (including Corta Atalaya and Cerro Colorado in Riotinto, Filón Norte, Filón Centro and Filón Sur in Tharsis, Aznalcóllar, or San Telmo) and many other of minor size. As a whole, these lakes constitute, at present, a large volume of highly acidic and metal-polluted water. Some of these pit lakes are very recent (e.g., Corta Atalaya, 2.5 a; Los Frailes, 6 a) and present a continuous hydrological and geochemical evolution, although many others were abandoned decades ago and show nearly constant water volume. Depth profiles obtained in several pit lakes (e.g., San Telmo, Confesionarios, Cueva de la Mora, Concepción) indicate that many of them have developed meromixis and show, at present, a permanent thermal and chemical stratification with a well defined chemocline separating an anoxic, Fe(II)-rich monimolimnion, and a well mixed, oxygenated and Fe(III)-rich mixolimnion. In the upper layer, the bacterial oxidation of Fe(II) competes with photoreductive processes which take place in the surface water, thus provoking diel cycles of Fe(II) concentration. The observed water chemistry reflects the oxidation and dissolution of pyrite and other sulphides and gangue aluminosilicates from the country rock. The pit lakes of the IPB cover a wide range of water compositions, from circumneutral and relatively low-metal (e.g., Los Frailes, pH 7.2, 0.07 mg/L Fe, 3.8 mg/L Mn, 30 mg/L Zn), to extremely acidic and metal(loid)-rich (e.g., Corta Atalaya, pH 1.2, 36.7 g/L Fe, 6.7 g/L Zn, 1.3 g/L Cu, 159 mg/L As). Most pit lakes, however, are comprised within the pH range of 2.2–3.6 and appear to be strongly buffered by the hydrolysis and precipitation of Fe(III) in the form of schwertmannite, which forms colloids that can sorb trace elements from the aqueous phase.  相似文献   

10.
《Applied Geochemistry》2004,19(11):1785-1800
Historical Au-ore exploitation at the Chéni mine in the Massif Central, France, generated 525,000 tonnes of finely ground mill tailings deposited in a heap that has spread with time into three settling basins. The tailings, which are rich in quartz (80%), mica and clay minerals (10% of illite, smectite, kaolinite and chlorite), feldspars (5%) but poor in carbonates (<1%), also contain sulphides (around 5%, mainly pyrite and arsenopyrite). Arsenic content of the tailings is around 6 g kg. This paper describes the geochemistry of drainage waters, with special attention paid to in situ values of the three major redox couples, namely Fe(II)/Fe(III), As(III)/As(V) and S(IV)/S(VI). The water samples range from acidic and oxidized (pH 2.9, Eh +700 mV) to moderate pH and weakly reducing (pH 7.6, Eh 15 mV). The waters are rich in SO4 and Ca and have variable As (0.05–95 mg L−1) and Fe concentrations (0.07–141 mg L−1). Reduced As(III) species predominate over As(V) species (As(III)/As(V) up to 21), whereas oxidized forms of Fe and S are favoured (Fe(II)/Fe(III) up to 0.5, and S(IV)/S(VI) up to 1).Thermodynamic calculations were performed with the PHREEQC and EQ3NR codes based on a revised As database to evaluate saturation indices (SI) of the waters in relation to the main minerals and define which redox couples control the redox state of the system. The important role of carbonates, though only present in small amounts, explains the acid buffering generated by the oxidation of sulphides for waters in the pH 7–7.5 range. Measured Eh appears to fall between the calculated Eh of the Fe(II)/Fe(III) couple and that of the As(III)/As(V) couple, illustrating redox disequilibrium.  相似文献   

11.
Mining residues from the Sb mine of Bournac in the upper Orb River valley (Southern France), constitute an important source of As and Sb pollution. Arsenic concentrations are as high as 78 μg/L and Sb reaches 32 μg/L in the small creek draining the tailings impoundment. Although both metalloids occur mainly in oxidized form in the creek water, their behaviour differs significantly. Iron oxides are the main carrier phases for both elements in the suspended particulate matter. In oxic conditions the two elements are mainly present in water in oxidised form As(V) and Sb(V) and both field studies and laboratory experiments indicate a higher affinity of As(V) than Sb(V) for the solid phase. In the pool, which receives the water from Bournac Creek, the reductive dissolution of Fe-oxides is linked to the oxidation of small pyrite grains transported from the tailings dump. In oxic conditions Sb is released to solution more efficiently than As. Conversely, in anoxic conditions, mobilisation of As is greater than that of Sb. This is attributed to the reduction of As, which favours its mobility. Whatever the conditions, the activity of bacteria naturally present in the sediments enhances the remobilization of Sb in oxidizing conditions and that of As in reducing conditions.  相似文献   

12.
《Applied Geochemistry》2004,19(7):995-1009
Published literature on preservation procedures for stabilizing aqueous inorganic As(III/V) redox species contains discrepancies. This study critically evaluates published reports on As redox preservation and explains discrepancies in the literature. Synthetic laboratory preservation experiments and time stability experiments were conducted for natural water samples from several field sites. Any field collection procedure that filters out microorganisms, adds a reagent that prevents dissolved Fe and Mn oxidation and precipitation, and isolates the sample from solar radiation will preserve the As(III/V) ratio. Reagents that prevent Fe and Mn oxidation and precipitation include HCl, H2SO4, and EDTA, although extremely high concentrations of EDTA are necessary for some water samples high in Fe. Photo-catalyzed Fe(III) reduction causes As(III) oxidation; however, storing the sample in the dark prevents photochemical reactions. Furthermore, the presence of Fe(II) or SO4 inhibits the oxidation of As(III) by Fe(III) because of complexation reactions and competing reactions with free radicals. Consequently, fast abiotic As(III) oxidation reactions observed in the laboratory are not observed in natural water samples for one or more of the following reasons: (1) the As redox species have already stabilized, (2) most natural waters contain very low dissolved Fe(III) concentrations, (3) the As(III) oxidation caused by Fe(III) photoreduction is inhibited by Fe(II) or SO4.  相似文献   

13.
《Applied Geochemistry》2002,17(4):431-443
A steady state geochemical model has been developed to assist in understanding surface-catalysed oxidation of aqueous Fe(II) by O2(aq), which occurs rapidly at circumneutral pH. The model has been applied to assess the possible abiotic removal of Fe(II)(aq) from alkaline ferruginous mine water discharges using engineered reactors with high specific-surface area filter media. The model includes solution and surface speciation equilibrium, oxidation kinetics of dissolved and adsorbed Fe(II) species and mass transfer of O2(g). Limited field data for such treatment of a mine water discharge were available for model development and assessment of possible parameter values. Model results indicate that an adsorption capacity between 10−6 and 10−5 mol l−1 is sufficient for complete removal, by oxidation, of the Fe(II)(aq) load at the discharge. This capacity corresponds approximately to that afforded by surface precipitation of Fe(III) oxide onto plastic trickling filter media typically used for biological treatment of wastewater. Extrapolated literature values for microbial oxidation of Fe(II)(aq) by neutrophilic microbial populations to the simulated reactor conditions suggested that the microbially-mediated rate may be several orders-of-magnitude slower than the surface-catalysed oxidation. Application of the model across a range of mine water discharge qualities shows that high Fe(II)(aq) loadings can be removed if the discharge is sufficiently alkaline. Additional reactor simulations indicate that reactor efficiency decreases dramatically with pH in the near acid region, coinciding with the adsorption edge for Fe2+ on Fe oxyhydroxide. Alkaline discharges thus buffer pH within the range where Fe(II)(aq) adsorbs onto the accreting Fe hydroxide mineral surface, and undergoes rapid catalytic oxidation. The results suggest that the proposed treatment technology may be appropriate for highly ferruginous alkaline discharges, typically associated with abandoned deep coal mines.  相似文献   

14.
The Mokrsko Stream in the central Czech Republic is an oxic and slightly alkaline stream that drains a natural As geochemical anomaly. Although long-term monitoring has characterized the general seasonal trends in trace element concentrations (i.e., As, Mo, Cu, Zn) in this stream, little is known about solubility controls and sorption processes that influence diel cycles in trace element concentrations. Trace elements (including As species, Cu, Mn, Mo, Pb and Sb) and other parameters were monitored over two 24-h periods in unfiltered and filtered (0.1 μm) samples collected in August 2010 and June 2011. Copper and Pb were predominantly (>92% of the mass) associated with the particulate fraction (>0.1 μm). Arsenic, Mo and Sb were predominantly (>88% of the mass) in the “dissolved” (<0.1 μm) form. Particulate-associated elements displayed up to a factor of 13 differences between minimum and maximum concentrations, most likely due to increased streamflow related to rainfall events. Dissolved concentrations of the trace metal cations (Cu, Fe, Mn and Pb) were consistently low and displayed no diel trends. Dissolved As(V), Mo and Sb varied on a diel cycle, with increased concentrations (up to 36%) during the late afternoon and decreased concentrations during the nighttime. Diel trends in trace anionic elements are explained by temperature-dependent sorption, as the diel changes in pH during base flow were very small (0.07 std. units). Very low concentrations of As(III), which have been shown to vary in a diel cycle, were attributed to enhanced hydraulic exchange with As(III)-rich hyporheic water during rainfall events.  相似文献   

15.
Arsenic and Antimony in Groundwater Flow Systems: A Comparative Study   总被引：3，自引：0，他引：3  
Arsenic (As) and antimony (Sb) concentrations and speciation were determined along flow paths in three groundwater flow systems, the Carrizo Sand aquifer in southeastern Texas, the Upper Floridan aquifer in south-central Florida, and the Aquia aquifer of coastal Maryland, and subsequently compared and contrasted. Previously reported hydrogeochemical parameters for all three aquifer were used to demonstrate how changes in oxidation–reduction conditions and solution chemistry along the flow paths in each of the aquifers affected the concentrations of As and Sb. Total Sb concentrations (SbT) of groundwaters from the Carrizo Sand aquifer range from 16 to 198 pmol kg−1; in the Upper Floridan aquifer, SbT concentrations range from 8.1 to 1,462 pmol kg−1; and for the Aquia aquifer, SbT concentrations range between 23 and 512 pmol kg−1. In each aquifer, As and Sb (except for the Carrizo Sand aquifer) concentrations are highest in the regions where Fe(III) reduction predominates and lower where SO4 reduction buffers redox conditions. Groundwater data and sequential analysis of the aquifer sediments indicate that reductive dissolution of Fe(III) oxides/oxyhydroxides and subsequent release of sorbed As and Sb are the principal mechanism by which these metalloids are mobilized. Increases in pH along the flow path in the Carrizo Sand and Aquia aquifer also likely promote desorption of As and Sb from mineral surfaces, whereas pyrite oxidation mobilizes As and Sb within oxic groundwaters from the recharge zone of the Upper Floridan aquifer. Both metalloids are subsequently removed from solution by readsorption and/or coprecipitation onto Fe(III) oxides/oxyhydroxides and mixed Fe(II)/Fe(III) oxides, clay minerals, and pyrite. Speciation modeling using measured and computed Eh values predicts that Sb(III) predominate in Carrizo Sand and Upper Floridan aquifer groundwaters, occurring as the Sb(OH)30 species in solution. In oxic groundwaters from the recharge zones of these aquifers, the speciation model suggests that Sb(V) occurs as the negatively charged Sb(OH)6− species, whereas in sufidic groundwaters from both aquifers, the thioantimonite species, HSb2S4 − and Sb2S4 2−, are predicted to be important dissolved forms of Sb. The measured As and Sb speciation in the Aquia aquifer indicates that As(III) and Sb(III) predominate. Comparison of the speciation model results based on measured Eh values, and those computed with the Fe(II)/Fe(III), S(-II)/SO4, As(III)/As(V), and Sb(III)/Sb(V) couples, to the analytically determined As and Sb speciation suggests that the Fe(II)/Fe(III), S(-II)/SO4 couples exert more control on the in situ redox condition of these groundwaters than either metalloid redox couple.  相似文献   

16.
This study examines the removal of dissolved metals during the oxidation and neutralization of five acid mine drainage (AMD) waters from La Zarza, Lomero, Esperanza, Corta Atalaya and Poderosa mines (Iberian Pyrite Belt, Huelva, Spain). These waters were selected to cover the spectrum of pH (2.2–3.5) and chemical composition (e.g., 319–2,103 mg/L Fe; 2.85–33.3 g/L SO4=) of the IPB mine waters. The experiments were conducted in the laboratory to simulate the geochemical evolution previously recognized in the field. This evolution includes two stages: (1) oxidation of dissolved Fe(II) followed by hydrolysis and precipitation of Fe(III), and (2) progressive pH increase during mixing with fresh waters. Fe(III) precipitates at pH < 3.5 (stages 1 and 2) in the form of schwertmannite, whereas Al precipitates during stage 2 at pH 5.0 in the form of several hydroxysulphates of variable composition (hydrobasaluminite, basaluminite, aluminite). During these stages, trace elements are totally or partially sorbed and/or coprecipitated at different rates depending basically on pH, as well as on the activity of the SO4= anion (which determines the speciation of metals). The general trend for the metals which are chiefly present as aqueous free cations (Pb2+, Zn2+, Cu2+, Cd2+, Mn2+, Co2+, Ni2+) is a progressive sorption at increasing pH. On the other hand, As and V (mainly present as anionic species) are completely scavenged during the oxidation stage at pH < 3.5. In waters with high activities (> 10−1) of the SO 4= ion, some elements like Al, Zn, Cd, Pb and U can also form anionic bisulphate complexes and be significantly sorbed at pH < 5. The removal rates at pH 7.0 range from around 100% for As, V, Cu and U, and 60–80% for Pb, to less than 20% for Zn, Co, Ni and Mn. These processes of metal removal represent a significant mechanism of natural attenuation in the IPB.  相似文献   

17.
Waters from abandoned Sb-Au mining areas have higher Sb (up to 2138 μg L−1), As (up to 1252 μg L−1) and lower Al, Zn, Li, Ni and Co concentrations than those of waters from the As-Au mining area of Banjas, which only contain up to 64 μg L−1 As. In general, Sb occurs mainly as SbO3− and As H2AsO4−. In general, waters from old Sb-Au mining areas are contaminated in Sb, As, Al, Fe, Cd, Mn, Ni and NO2−, whereas those from the abandoned As-Au mining area are contaminated in Al, Fe, Mn, Ni, Cd and rarely in NO2−. Waters from the latter area, immediately downstream of mine dumps are also contaminated in As. In stream sediments from Sb-Au and As-Au mining areas, Sb (up to 5488 mg kg−1) and As (up to 235 mg kg−1) show a similar behaviour and are mainly associated with the residual fraction. In most stream sediments, the As and Sb are not associated with the oxidizable fraction, while Fe is associated with organic matter, indicating that sulphides (mainly arsenopyrite and pyrite) and sulphosalts containing those metalloids and metal are weathered. Arsenic and Sb are mainly associated with clay minerals (chlorite and mica; vermiculite in stream sediments from old Sb-Au mining areas) and probably also with insoluble Sb phases of stream sediments. In the most contaminated stream sediments, metalloids are also associated with Fe phases (hematite and goethite, and also lepidocrocite in stream sediments from Banjas). Moreover, the most contaminated stream sediments correspond to the most contaminated waters, reflecting the limited capacity of stream sediments to retain metals and metalloids.  相似文献   

18.
Oxidation of As(III) by natural manganese (hydr)oxides is an important geochemical reaction mediating the transformation of highly concentrated As(III) in the acidic environment such as acid mine drainage (AMD) and industrial As-contaminated wastewater, however, little is known regarding the presence of dissolved Fe(II) on the oxidation process. In this study, oxidation of As(III) in the absence and presence of Fe(II) by MnO2 under acidic conditions was investigated. Kinetic results showed that the presence of Fe(II) significantly inhibited the removal of As(III) (including oxidation and sorption) by MnO2 in As(III)-Fe(II) simultaneous oxidation system even at the molar ratio of Fe(II):As(III) = 1/64:1, and the inhibitory effects increased with the increasing ratios of Fe(II):As(III). Such an inhibition could be attributed to the formation of Fe(III) compounds covering the surface of MnO2 and thus preventing the oxidizing sites available to As(III). On the other hand, the produced Fe(III) compounds adsorbed more As(III) and the oxidized As(V) on the MnO2 surface with an increasing ratio of Fe(II):As(III) as demonstrated in kinetic and XPS results. TEM and EDX results confirmed the formation of Fe compounds around MnO2 particles or separated in solution in Fe(II) individual oxidation system, Fe(II) pre-treated and simultaneous oxidation processes, and schwertmannite was detected in Fe(II) individual and Fe pre-treated oxidation processes, while a new kind of mineral, probably amorphous FeOHAs or FeAsO4 particles were detected in Fe(II)-As(III) simultaneous oxidation process. This suggests that the mechanisms are different in Fe pre-treated and simultaneous oxidation processes. In the Fe pre-treated and MnO2-mediated oxidation pathway, As(III) diffused through a schwertmannite coating formed around MnO2 particles to be oxidized. The newly formed As(V) was adsorbed onto the schwertmannite coating until its sorption capacity was exceeded. Arsenic(V) then diffused out of the coating and was released into the bulk solution. The diffusion into the schwertmannite coating and the oxidation of As(III) and sorption of both As(V) and As(III) onto the coating contributed to the removal of total As from the solution phase. In the simultaneous oxidation pathway, the competitive oxidation of Fe(II) and As(III) on MnO2 occurred first, followed by the formation of FeOHAs or FeAsO4 around MnO2 particles, and these poorly crystalline particles of FeOHAs and FeAsO4 remained suspended in the bulk solution to adsorb As(III) and As(V). The present study reveals that the formation of Fe(III) compounds on mineral surfaces play an important role in the sorption and oxidation of As(III) by MnO2 under acidic conditions in natural environments, and the mechanisms involved in the oxidation of As(III) depend upon how Fe(II) is introduced into the As(III)-MnO2 system.  相似文献   

19.
The extent of historical U mining impacts is well documented for the North Cave Hills region of Harding County, South Dakota, USA. While previous studies reported watershed sediment and surface water As and U concentrations up to 90× established background concentrations, it was unclear whether or how localized changes in sediment redox behavior may influence contaminant remobilization. Five pore-water equilibration samplers (peepers) were spatially and temporally deployed within the study area to evaluate seasonal solid–liquid As and U distributions as a function of sediment depth. Pore-water and solid phase As and U concentrations, Fe speciation, Eh and pH were measured to ascertain specific geochemical conditions responsible for As and U remobilization and transport behavior. At a mine overburden sedimentation pond adjacent to the mine sites, high total aqueous As and U concentrations (4920 and 674 μg/L, respectively) were found within surface water during summer sampling; however pond dredging prior to autumn sampling resulted in significantly lower aqueous As and U concentrations (579 and 108 μg/L, respectively); however, both As and U still exceeded regional background concentrations (20 and 18 μg/L, respectively). At a wetlands-dominated deposition zone approximately 2 km downstream of the sedimentation pond, pore-water geochemical conditions varied seasonally. Summer conditions promoted reducing conditions in pore water, resulting in active release of As(III) to the water column. Autumn conditions promoted oxidizing conditions, decreasing pore-water As (Aspw) 5× and increasing Upw 10×. Peak U pore-water concentrations (781 μg/L) were 3.5× greater than determined for the surface water (226 μg/L), and approximately 40× background concentrations. At the Bowman–Haley reservoir backwaters 45 km downstream from the mine sites, As and U pore-water concentrations increased significantly between the summer and autumn deployments, attributed to increased Fe reduction processes. Geochemical modeling suggests solid-phase Fe reduction promotes the liberation of pore-water As and U via suppressing the formation of thioarsenite. Intermittent hydrological processes facilitate As and U transport and deposition throughout the watershed, while biogeochemical-influenced redox changes cycle As and U between pore and surface water within localized environments.  相似文献   

20.
Permanganate (MnO4−) has widely been used as an effective oxidant for drinking water treatment systems, as well as for in situ treatment of groundwater impacted by various organic contaminants. The reaction stoichiometry of As(III) oxidation by permanganate has been assumed to be 1.5, based on the formation of solid product, which is putatively considered to be MnO2(s). This study determined the stoichiometric ratio (SR) of the oxidation reaction with varying doses of As(III) (3-300 μM) and MnO4− (0.5 or 300 μM) under circumneutral pH conditions (pH 4.5-7.5). We also characterized the solid product that was recovered ∼1 min after the oxidation of 2.16 mM As(III) by 0.97 mM MnO4− at pH 6.9 and examined the feasibility of secondary heterogeneous As(III) oxidation by the solid product. When permanganate was in excess of As(III), the SR of As(III) to Mn(VII) was 2.07 ± 0.07, regardless of the solution pH; however, it increased to 2.49 ± 0.09 when As(III) was in excess. The solid product was analogous to vernadite, a poorly crystalline manganese oxide based on XRD analysis. The average valence of structural Mn in the solid product corresponded to +III according to the splitting interval of the Mn3s peaks (5.5 eV), determined using X-ray photoelectron spectroscopy (XPS). The relative proportions of the structural Mn(IV):Mn(III):Mn(II) were quantified as 19:62:19 by fitting the Mn2p3/2 spectrum of the solid with the five multiplet binding energy spectra for each Mn valence. Additionally, the O1s spectrum of the solid was comparable to that of Mn-oxide but not of Mn-hydroxide. These results suggest that the solid product resembled a poorly crystalline hydrous Mn-oxide such as (MnII0.19MnIII0.62MnIV0.19)2O3·nH2O, in which Mn(II) and Mn(IV) were presumably produced from the disproportionation of aqueous phase Mn(III). Thermodynamic calculations also show that the formation of Mn(III) oxide is more favorable than that of Mn(IV) oxide from As(III) oxidation by permanganate under circumneutral pH conditions. Arsenic(III), when it remained in the solution after all of the permanganate was consumed, was effectively oxidized by the solid product. This secondary heterogeneous As(III) oxidation consisted of three steps: sorption to and oxidation on the solid surface and desorption of As(V) into solution, with the first step being the rate-limiting process as observed in As(III) oxidation by various Mn (oxyhydr)oxides reported elsewhere. We also discussed a potential reaction pathway of the permanganate oxidation of As(III).  相似文献   

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