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1.
A laboratory experiment with two sequenced columns was performed as a preliminary study for the installation of a permeable reactive barrier (PRB) at a site where a mixed ground water contamination exists. The first column contained granular zero valent iron (ZVI), the second column was filled with granular activated carbon (GAC). Trichloromethane (TCM, 930 μg/l) and chlorobenzene (MCB, 260 μg/l) were added to the ground water from the site as the main contaminants. Smaller amounts (<60 μg/l) of benzene, 1,2-dichloroethane, 1,1,2-trichloroethane (1,1,2-TCA), 1,1-dichloroethene (1,1-DCE), trichloroethene (TCE), tetrachloroethene (PCE), 1,2-dichloropropane (1,2-DCP), bromodichloromethane (BDCM), dibromochloromethane (DBCM), tribromomethane (TBM), vinyl chloride and chromate were also added to the water to simulate the complex contamination pattern at the site of interest. PCE, TCE, 1,1-DCE, DBCM, BDCM, TBM, MCB and chromate were remediated in contact with ZVI, while the remaining contaminants showed incomplete degradation. A fraction of 8–16.5% TCM was converted to dichloromethane (DCM). Remaining contaminant concentrations were efficiently sorbed by the GAC until breakthrough of DCM was observed after 1,230 exchanged pore volumes in the GAC. The results show that the complex mixture of contaminants can be remediated by a sequenced PRB consisting of ZVI and GAC and that DCM sorption capacity is the critical parameter for the dimensions of the GAC reactor.  相似文献   

2.
Reactive mixtures to be used in a permeable reactive barrier (PRB) for the treatment of low quality groundwater derived from a mine waste rock storage site were evaluated. Low pH drainage water from the site contained high concentrations of sulfate and dissolved metals, including Al, Co, Ni, and Zn. Column experiments were conducted to evaluate whether mixtures containing either peat moss (as an organic carbon source) or a mixture of peat moss and granular zero-valent iron (ZVI) filings, in addition to small amounts of lime and/or limestone, were suitable treatment materials for removing these metals from the water. The experimental results showed that the mixtures promote bacterially-mediated sulfate reduction and metal removal by precipitation of metal sulfides, metal carbonate/hydroxide precipitation, and adsorption under relatively high pH conditions (pH of 7–8). Both reactive mixtures removed influent dissolved metals to near or below the limit of detection in the effluent throughout the experiment; however, influent-level concentrations of the metals of interest gradually moved through the column containing peat alone, as the pH neutralizing ability in the mixture was consumed. In contrast, the column containing both peat and ZVI showed very little breakthrough of the influent metals, suggesting that the longevity of the mixture including ZVI will be much longer than the mixture containing peat alone. The results show that both reactive mixtures should be effective in a PRB installation as long as neutral pH conditions and microbial activity are maintained. The cost to performance ratio of the two reactive mixtures will be a key factor in determining which mixture is best suited for a particular site.  相似文献   

3.
Alachlor and metalaxyl contaminations of environmental and agricultural water and soil systems cause potential threats to human health. However, information on the pesticide adsorption quantity–intensity (Q/I) relationships in water and soil systems is limited. Therefore, adsorption behavior and the fate of alachlor and metalaxyl in the systems as influenced by the application of zerovalent iron (ZVI, Fe0) were investigated using the pesticide adsorption Q/I relationships. After treating ZVI in the systems, the concentration of alachlor rapidly decreased within a few days and then it disappeared at approximately 5–7 experiment days; whereas metalaxyl concentration was reduced by approximately 40–45% during the 28 day experimental period. In particular, ZVI adsorbed more metalaxyl in the aqueous system than in the soil-solution system. The alachlor concentration in the water and soil solution drastically decreased with increasing ZVI treatment, while metalaxyl concentration was relatively slow in its decrease. Adsorption mechanism of the pesticides in the soil-solution system was shown as multiple-site adsorption Q/I fitting. Buffering capacity (BC) of the pesticides increased with ZVI treatment in all sorption sites. The BC values in sandy soil were escalated with increasing ZVI application rates, so that the values were rather higher in sandy soil than in clayey soil. In addition, changes in alachlor concentration with applying ZVI were due to both dechlorination and adsorption; whereas metalaxyl concentration was dependent upon adsorption reaction. Thus, the pesticide adsorption Q/I relationships in different soil-solution systems were critically affected by the ZVI treatment.  相似文献   

4.
Permeable reactive barriers (PRBs) are used for groundwater remediation at contaminated sites worldwide. This technology has been efficient at appropriate sites for treating organic and inorganic contaminants using zero-valent iron (ZVI) as a reductant and as a reactive material. Continued development of the technology over the years suggests that a robust understanding of PRB performance and the mechanisms involved is still lacking. Conflicting information in the scientific literature downplays the critical role of ZVI corrosion in the remediation of various organic and inorganic pollutants. Additionally, there is a lack of information on how different mechanisms act in tandem to affect ZVI-groundwater systems through time. In this review paper, we describe the underlying mechanisms of PRB performance and remove isolated misconceptions. We discuss the primary mechanisms of ZVI transformation and aging in PRBs and the role of iron corrosion products. We review numerous sites to reinforce our understanding of the interactions between groundwater contaminants and ZVI and the authigenic minerals that form within PRBs. Our findings show that ZVI corrosion products and mineral precipitates play critical roles in the long-term performance of PRBs by influencing the reactivity of ZVI. Pore occlusion by mineral precipitates occurs at the influent side of PRBs and is enhanced by dissolved oxygen and groundwater rich in dissolved solids and high alkalinity, which negatively impacts hydraulic conductivity, allowing contaminants to potentially bypass the treatment zone. Further development of site characterization tools and models is needed to support effective PRB designs for groundwater remediation.  相似文献   

5.
为研究硝酸根对颗粒状铁降解三氯乙烯的影响,进行了柱实验和相应的反应铁腐蚀电位测定。在硝酸根离子存在条件下,铁的腐蚀电位相应升高,系统条件也因之发生变化,导致钝性的铁氧化物在铁表面生成。因而,三氯乙烯和硝酸根离子降解速率明显减小, 并且降解速率减小的程度与硝酸根离子的浓度成比例。当污染液流过反应柱时,硝酸根离子与铁反应, 被还原为氨根离子。该反应造成硝酸根离子的浓度梯度,使钝化区在柱中上移,从而影响了三氯乙烯的降解曲线。与三氯乙烯单独与铁反应相比,当含4 7 mg/L硝酸根的三氯乙烯溶液流经反应柱170 孔隙体积后,降解50% 三氯乙烯所需的时间(t50) 从小于4 h增加到大于10 h;而当三氯乙烯溶液中加入100 mg/L硝酸根离子,仅17 孔隙体积溶液流经反应柱后,三氯乙烯降解t50就已大于14 h。研究结果表明,由于硝酸根离子对铁的腐蚀电位和铁表面氧化膜的不利作用,在处理靶污染物为高浓度硝酸根离子和三氯乙烯共同污染的地下水时,铁渗透反应隔栅不是最佳选择。如果靶污染物中硝酸根离子浓度比较低,则在设计铁隔栅时应考虑到硝酸根离子造成的不利因素,相应增加铁墙的厚度,从而确保三氯乙烯的降解效果。  相似文献   

6.
An innovative setup of a permeable reactive barrier (PRB) was installed in Willisau, Switzerland to remediate chromate contaminated groundwater. Instead of a conventional continuous barrier, this PRB consists of cylinders installed in rows: a single row for lower expected CrVI-concentrations and an offset double row for higher expected CrVI-concentrations. The cylinders are filled with reactive grey cast-Fe shavings mixed with gravel to prevent extensive precipitation of secondary phases in the pore space. The treatment of the contaminants takes place both within the cylinders and in the dissolved FeII plume generated downstream of the barrier. Monitoring of the contamination situation over a period of 3 a provided evidence of the mobilization, transport and behavior of the contaminants in the aquifer. Groundwater and reactive material were sampled upstream, within and downstream of the barrier by a Multi-Port Sampling System (MPSS) that revealed the geochemical processes as a function of time and space. Comprehensive chemical analyses included sensitive parameters such as CrVI, FeII/FeIII, redox potential, dissolved O2 and pH. Several campaigns using multiple optical tracers revealed a rather complex hydrological regime at different scales, thereby complicating the barrier performance.  相似文献   

7.
The Cr(VI) reduction behaviour of five different types of grey cast iron shavings meant for construction of a permeable reactive barrier (PRB) in Thun, Switzerland, was investigated by performing batch and column experiments. Closed system batch experiments clearly demonstrated that the largest shavings also containing the largest spherical C inclusions, were best qualified for the PRB due to their fast Cr(VI) reduction rate. However, the column experiment performed with this type of material revealed that a complete and long term Cr(VI) treatment by the planned PRB was questionable due to the site-specific high groundwater flow velocities and the almost O2 and CaCO3 saturated aquifer conditions. The experimentally observed Cr(VI) breakthrough is explained as a result of a decline of reactivity provoked by a strong passivation by observed Fe hydroxides (FeOOH-polymorphs) and carbonates (calcite and aragonite).The column experiment was simulated using the reactive transport modelling code CrunchFlow. Iron cycling and intra-aqueous reactions were incorporated into a previously described reaction network in order to model the strong decline in reactivity of grey cast iron. All key parameters like aqueous species concentrations and mineralogical evolution of the column were successfully modelled. The modelling results confirmed that the observed Cr(VI) breakthrough was caused by surface passivation of the Fe shavings. Complete oxidation of the initially present mass of shavings is not predicted to occur during the expected PRB lifetime of several decades. The model seems to be robust, and it is expected that an application of the calibrated model in 2D to field sites will allow a quantitative evaluation for the performance of planned PRB’s in such environments.  相似文献   

8.
This study focused on the ferric sulfate precipitates formed during the culture of Acidithiobacillus ferrooxidans (A. ferrooxidans) in a modified 9K medium by applying a potential control on the electrode. X-ray diffraction (XRD), environmental scanning electron microscope (ESEM), Raman spectroscopy (Raman) and Fourier Transform Infrared spectroscopy (FTIR) were carried out to characterize and identify the precipitates which were formed, respectively, in the electrochemical cultivation with a fixed cathode potential (bias-experiment) and in the conventional batch cultivation without cathode potential control (no-bias-experiment). The results indicated that K-jarosite presented in both experiments while NH4-jarosite and schwertmannite were only found in the no-bias-experiment. The formation of different precipitates could be attributed to the different growth statuses and rates of A. ferrooxidans and the different concentrations of Fe3+. In the bias-experiment, external electrons reproduced Fe2+ and promoted the growth of A. ferrooxidans, thus resulting in the low Fe3+ concentration and the rapid depletion of NH4 + as the nitrogen source, in which K-jarosite was preferentially formed. In the no-bias-experiment, the lower concentration of A. ferrooxidans was observed, which was due to the continuous consumption of Fe2+ by bacteria, thus resulting in the relatively higher Fe3+ and the NH4 + concentration in culture. The high concentration of Fe3+ favored the precipitation of the solid solution of K-NH4-H3O jarosite, and led to the formation of schwertmannite after K+ and NH4 + were depleted.  相似文献   

9.
An initial assessment of an old manufacturing site with groundwater impacted by trichloroethene (TCE) contamination in the metropolitan New York area showed that the TCE was being removed naturally by reductive dechlorination. However, complete dechlorination was not expected at the site because the process was progressing too slowly under transitional to aerobic conditions at a degradation constant of –0.0013 and a TCE half life of 533 days. A pilot test was conducted at the site in which a carbohydrate substrate (molasses) was injected into the groundwater to create an In-Situ Reactive Zone (IRZ). Post-pilot test groundwater sampling and analysis indicated that an IRZ was created successfully as the total organic carbon (TOC) content and conductivity increased significantly while oxidation-reduction (REDOX) potential and dissolved oxygen (DO) decreased. The created IRZ caused enhanced reductive dechlorination of TCE at the site, found to proceed with a degradation constant of –0.0158 and a TCE half life of 44 days.  相似文献   

10.
Microbial sulfidogenesis plays a potentially important role in Fe and As biogeochemistry within wetland soils, sediments and aquifers. This study investigates the specific effects of microbial sulfidogenesis on Fe mineralogy and associated As mobility in mildly acidic (pH 6) and mildly basic (pH 8) advective-flow environments. A series of experiments were conducted using advective-flow columns, with an initial solid-phase comprising As(III)-bearing ferrihydrite-coated quartz sand. Columns for each pH treatment were inoculated with the sulfate-reducing bacteria Desulfovibrio vulgaris, and were compared to additional abiotic control columns. Over a period of 28 days, microbial sulfidogenesis (as coupled to the incomplete oxidation of lactate) caused major changes in Fe mineralogy, including replacement of ferrihydrite by mackinawite and magnetite at the in-flow end of the inoculated columns. At pH 8, the Fe2+ produced by electron transfer between sulfide and ferrihydrite was mainly retained near its zone of formation. In contrast, at pH 6, much of the produced Fe2+ was transported with advecting groundwater, facilitating the downstream Fe2+-catalyzed transformation of ferrihydrite to goethite. At both pH 6 and pH 8, the sulfide-driven reductive dissolution of ferrihydrite and its replacement by mackinawite at the in-flow end of the inoculated columns resulted in substantial mobilization of As into the pore-water. At pH 8, this caused the downstream As concentrations within the inoculated columns to be greater than the corresponding abiotic column. However, the opposite occurred under pH 6 conditions, with the Fe2+-catalyzed transformation of ferrihydrite to goethite in the inoculated columns causing a decrease in downstream As concentrations compared to the abiotic column. Although thermodynamically favorable at intermediate times and depth intervals within the inoculated columns, solid As sulfide phases were undetectable by As XANES spectroscopy. Our findings show that microbial sulfidogenesis can trigger significant As mobilization in subsurface environments with advective groundwater flow. The results also demonstrate that formation of mackinawite by sulfidization of ferric (hydr)oxides is not effective for the immobilization of As, whereas the Fe2+-catalyzed transformation of ferrihydrite to goethite under mildly acidic conditions may mitigate As mobility.  相似文献   

11.
The electrical charge transport was examined in an Fe-rich amphibole, arfvedsonite, using frequency dependent AC resistivity (impedance spectroscopy) and thermopower Θ measurements in the temperature range 30–800°C. Two different semiconducting charge transfer mechanisms were observed which are due to volume conduction for measurements parallel and perpendicular to the [001] direction; they arise probably from a conduction mechanism related to lattice defects, both with activation energies EA ≈ 0.4 eV. The extrapolated DC conductivity, for a fixed temperature, along [001] is about 5–6 times higher than that perpendicular to [001]. From the temperature independent positive Θ values it follows that hole conduction occurs, and a hopping-type charge transport is acting, probably effected by electron hopping Fe2+ → Fe3+. Possible charge transfer paths are discussed. 57Fe Mössbauer spectra enabled to determine the relative concentrations of Fe2+ and Fe3+. From a comparison of these data and from Θ results it is concluded that the main fraction of Fe2+ and Fe3+ take part in long range charge transport.  相似文献   

12.
《Geochimica et cosmochimica acta》1999,63(19-20):3171-3182
The oxidation rate of pyrite at pH 7, 25°C and at constant partial pressure of oxygen (0.21 and 0.177 atm) was measured in the presence of the Fe(III)-chelators NTA, oxalate, leucine, EDTA, citrate, IDA and the Fe(III)-reductant ascorbic acid. With the exception of leucine and EDTA, non-reducing Fe(III)-chelators increased the oxidation rate relative to the reference state of formation of the Fe(OH)2+ complex at pH 7. The rate increase was proportional to the logarithm of the conditional stability constant of the ligands for the complexation of Fe3+. No effect on the oxidation rate was observed in the presence of EDTA, which shifted the redox potential of the redox couple Fe2+/Fe3+ to a value below that in the absence of any ligand at pH 7. Ascorbic acid decreased the pyrite oxidation rate by a factor of 5 at ascorbic acid concentrations between 10−4 and 10−2 mol L−1. Comparison of the rate constants for the oxidation of ascorbic acid by surface bound Fe(III) in the absence and presence of pyrite shows that the pyrite surface accelerates this reaction by a factor of 10. The oxidation of both pyrite and ascorbic acid is of fractional order with respect to ascorbic acid (HAsc): rpy=0.55 c(HAsc)−0.35 rHAsc=3.6 c(HAsc)0.59. Both the results from experiments with Fe(III)-chelating ligands and the Fe(III)-reductant, suggest a very efficient interference in the electron cycling between Fe(II) and Fe(III) at the pyrite surface. The interference seems to be mainly related to the reductive side of the iron cycling. It is therefore concluded that the electron transfer between ferric iron and pyritic sulfur limits the pyrite oxidation rate at pH 7.  相似文献   

13.
Chlorophenols are persistent toxins in the natural environment. In this investigation, 2,4-dichlorophenol (2,4-DCP) and pentachlorophenol (PCP) in aqueous media were degraded using an electrokinetic process (EK) and an electro-Fenton process (EF) using stainless steel and graphite as the anode and cathode, respectively. Chlorophenols were degraded via direct electrolysis at the surface of the electrode in the EK process. However, in the EF process, the degradation mechanism includes direct electrolysis and oxidation by hydroxyl radicals. The optimal conditions were a current density of 0.75 mA/cm2 and an air flow of 0.7 l/min at pH 4. Under the optimal conditions, the 2,4-DCP and PCP removal rates in the EF process were 80.18 and 64.03 %, respectively. The mineralization efficiencies of 2,4-DCP and PCP were 78.23 and 75.77 %, respectively. The results of dechlorination reveal that almost all of the chlorines were released, but some were retained in the intermediates. The dechlorination efficiency revealed that the EF and EK4 processes two chlorines from 2,4-DCP. They released four or five chlorines and four chlorines from PCP, respectively. The kinetic results provide evidence of pseudo-first degradation. The rate constant (k cp) declined as pHi was increased from 4 to 10. The k cp values reveal that the pH is an important factor that affects the degradation efficiency in the electrochemical process.  相似文献   

14.
Aqueous oxidation of sulfide minerals to sulfate is an integral part of the global sulfur and oxygen cycles. The current model for pyrite oxidation emphasizes the role of Fe2+-Fe3+ electron shuttling and repeated nucleophilic attack by water molecules on sulfur. Previous δ18O-labeled experiments show that a variable fraction (0-60%) of the oxygen in product sulfate is derived from dissolved O2, the other potential oxidant. This indicates that nucleophilic attack cannot continue all the way to sulfate and that a sulfoxyanion of intermediate oxidation state is released into solution. The observed variability in O2% may be due to the presence of competing oxidation pathways, variable experimental conditions (e.g. abiotic, biotic, or changing pH value), or uncertainties related to the multiple experiments needed to effectively use the δ18O label to differentiate sulfate-oxygen sources. To examine the role of O2 and Fe3+ in determining the final incorporation of O2 oxygen in sulfate produced during pyrite oxidation, we designed a set of aerated, abiotic, pH-buffered (pH = 2, 7, 9, 10, and 11), and triple-oxygen-isotope labeled solutions with and without Fe3+ addition. While abiotic and pH-buffered conditions help to eliminate variables, triple oxygen isotope labeling and Fe3+ addition help to determine the oxygen sources in sulfate and examine the role of Fe2+-Fe3+ electron shuttling during sulfide oxidation, respectively.Our results show that sulfate concentration increased linearly with time and the maximum concentration was achieved at pH 11. At pH 2, 7, and 9, sulfate production was slow but increased by 4× with the addition of Fe3+. Significant amounts of sulfite and thiosulfate were detected in pH ? 9 reactors, while concentrations were low or undetectable at pH 2 and 7. The triple oxygen isotope data show that at pH ? 9, product sulfate contained 21-24% air O2 signal, similar to pH 2 with Fe3+ addition. Sulfate from the pH 2 reactor without Fe3+ addition and the pH 7 reactors all showed 28-29% O2 signal. While the O2% in final sulfate apparently clusters around 25%, the measurable deviations (>experimental error) from the 25% in many reaction conditions suggest that (1) O2 does get incorporated into intermediate sulfoxyanions (thiosulfate and sulfite) and a fraction survives sulfite-water exchange (e.g. the pH 2 with no Fe3+ addition and both pH 7 reactors); and (2) direct O2 oxidation dominates while Fe3+ shuttling is still competitive in the sulfite-sulfate step (e.g. the pH 9, 10, and 11 and the pH 2 reactor with Fe3+ addition). Overall, the final sulfate-oxygen source ratio is determined by (1) rate competitions between direct O2 incorporation and Fe3+ shuttling during both the formation of sulfite from pyrite and from sulfite to final sulfate, and (2) rate competitions between sulfite and water oxygen exchange and the oxidation of sulfite to sulfate. Our results indicate that thiosulfate or sulfite is the intermediate species released into solution at all investigated pH and point to a set of dynamic and competing fractionation factors and rates, which control the oxygen isotope composition of sulfate derived from pyrite oxidation.  相似文献   

15.
Oxidation and dehydrogenation processes for heat-treated anthophyllites were investigated using Mössbauer and infrared spectroscopy. At temperatures from 350°C to about 650°C, Fe2+ at the M1 and M3 sites oxidizes, yielding Fe3+ + one electron. A proton from the (OH)– is liberated and combines with this electron to form a hydrogen atom; and some Fe2+ ions at the M2 and M4 sites exchange with Mg at the M1 and M3 sites and then are oxidized in a similar way; at higher temperature, OH remaining in the (MgMgMg/Fe3+)-(OH)-configuration are dehydrogenated by decomposition of the amphibole to orthopyroxene and quartz. During oxidation and dehydrogenation of anthophyllite, there is disordering of Mg and Fe at the M1, M2, M3 and M4 sites in all samples studied. When all Fe2+ is oxidized, the site occupancies of at the M4 and M1, M2, M3 sites become identical, indicating that Mg and Fe3+ are completely disordered at these sites.  相似文献   

16.
A series of laboratory batch experiments was conducted to evaluate the potential for treatment of acid mine drainage (AMD) using organic C (OC) mixtures amended by zero-valent Fe (Fe0). Modest increases in SO4 reduction rates (SRRs) of up to 15% were achieved by augmenting OC materials with 5 and 10 dry wt% Fe0. However, OC was essential for supporting SO4 reducing bacteria (SRB) and therefore SO4 reduction. This observation suggests a general absence of autotrophic SRB which can utilize H2 as an electron donor. Sulfate reduction rates (SRRs), calculated using a mass-based approach, ranged from −12.9 to −14.9 nmol L−1 d−1  g−1 OC. Elevated populations of SRB, iron reducing bacteria (IRB), and acid producing (fermentative) bacteria (APB) were present in all mixtures containing OC. Effective removal of Fe (91.6–97.6%), Zn (>99.9%), Cd (>99.9%), Ni (>99.9%), Co (>99.9%), and Pb (>95%) was observed in all reactive mixtures containing OC. Abiotic metal removal was achieved with Fe0 only, however Fe, Co and Mn removal was less effective in the absence of OC. Secondary disordered mackinawite [Fe1+xS] was observed in field-emission scanning electron microscopy (FE-SEM) backscatter electron micrographs of mixtures that generated SO4 reduction. Energy dispersive X-ray (EDX) spectroscopy revealed that Fe–S precipitates were Fe-rich for mixtures containing OC and Fe0, and S-rich in the absence of Fe0 amendment. Sulfur K-edges determined by synchrotron-radiation based bulk X-ray absorption near-edge structure (XANES) spectroscopy indicate solid-phase S was in a reduced form in all mixtures containing OC. Pre-edge peaks on XANES spectra suggest tetragonal S coordination, which is consistent with the presence of an Fe–S phase such as mackinawite. The addition of Fe0 enhanced AMD remediation over the duration of these experiments, however long-term evaluation is required to identify optimal Fe0 and OC mixtures.  相似文献   

17.
Experiments were conducted under static batch and dynamic flow conditions to evaluate the sorption of FeII onto three goethites (G1, G2 and G3) having different crystal habits, morphologies and surface properties. Results reveal that G1 exhibited the highest FeII sorption extent and lowest kinetic rate constant, which may result from higher surface site density, surface roughness and edge surface faces. Surface complexation modeling parameters derived from batch experiments were combined with hydrodynamic parameters to simulate breakthrough curves in goethite-coated sand packed columns. The total sorbed amount of FeII at complete breakthrough was in agreement with that expected from the batch experiments, except for G1. Sorption breakthrough predictions that make use of surface complexation parameters accurately predicted FeII mobility in G2 and G3 columns, but poorly in G1 column. Experiments at various flow rates in G1 columns represented different amounts of FeII sorbed at complete breakthrough, thereby underscoring the impact of kinetic sorption. Moreover, Fe dissolution/re-precipitation or FeII-induced transformation of goethite was suspected at the lowest flow rate in the G1 column. The influence of goethite phase specific reactivity on FeII sorption under batch versus advective–dispersive flow is herein demonstrated. These findings have strong implications to assess transport of FeII and environmental contaminants both in natural and engineered systems.  相似文献   

18.
Many studies have shown that the concentration of aqueous Fe2+ increases in surface waters during exposure to sunlight and attribute this phenomenon either to photoreductive dissolution of ferric minerals/colloids or to ligand-to-metal charge transfer within organic complexes of Fe3+. In a multi-summer study of iron redox cycling in a relatively high pH stream (Middle Crow Creek, MCC) that drains a mostly-granitic watershed at an altitude of 2400 m, aqueous Fe3+ (not Fe2+) concentrations were correlated with both sunlight and temperature. A steady state model fails to explain the [Fe2+] and [Fe3+] data from this stream. However, Fe2+ concentrations can be explained using a simple kinetic model in which rate constants for oxidation and reduction were obtained by fitting data from in situ oxidation experiments, including first-order thermal (nonphotochemical) reduction of Fe3+. Rate constants obtained from experiments in the dark result in too much Fe2+ to match the data from illuminated experiments, requiring a net photooxidation process to explain [Fe3+] measured in MCC. The organic content of MCC results in high concentrations of Fe–DOM complexes that not only act as a reservoir contributing to daily changes in [Fetot] as measured by our methods, but whose photochemistry may contribute highly oxidizing reactive oxygen species to the stream. In situ studies suggest that photochemical reduction of organically bound Fe3+ occurs, followed by thermal release of Fe2+ to the water column and subsequent rapid re-oxidation.  相似文献   

19.
Discharge of Fe(II)-rich groundwaters into surface-waters results in the accumulation of Fe(III)-minerals in salinized sand-bed waterways of the Hunter Valley, Australia. The objective of this study was to characterise the mineralogy, micromorphology and pore-water geochemistry of these Fe(III) accumulations. Pore-waters had a circumneutral pH (6.2–7.2), were sub-oxic to oxic (Eh 59–453 mV), and had dissolved Fe(II) concentrations up to 81.6 mg L−1. X-ray diffraction (XRD) on natural and acid-ammonium-oxalate (AAO) extracted samples indicated a dominance of 2-line ferrihydrite in most samples, with lesser amounts of goethite, lepidocrocite, quartz, and alumino-silicate clays. The majority of Fe in the samples was bound in the AAO extractable fraction (FeOx) relative to the Na-dithionite extractable fraction (FeDi), with generally high FeOx:FeDi ratios (0.52–0.92). The presence of nano-crystalline 2-line ferrihydrite (Fe5HO3·4H2O) with lesser amounts of goethite (α-FeOOH) was confirmed by scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX), and transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED). In addition, it was found that lepidocrocite (γ-FeOOH), which occurred as nanoparticles as little as ∼5 lattice spacings thick perpendicular to the (0 2 0) lattice plane, was also present in the studied Fe(III) deposits. Overall, the results highlight the complex variability in the crystallinity and particle-size of Fe(III)-minerals which form via oxidation of Fe(II)-rich groundwaters in sand-bed streams. This variability may be attributed to: (1) divergent precipitation conditions influencing the Fe(II) oxidation rate and the associated supply and hydrolysis of the Fe(III) ion, (2) the effect of interfering compounds, and (3) the influence of bacteria, especially Leptothrix ochracea.  相似文献   

20.
《Applied Geochemistry》1999,14(4):531-541
An empirical kinetic rate law appropriate for many ground waters (neutral pH, aerobic) has been determined for the aqueous oxidation of trichloroethene (TCE), one of the most volumetrically important chlorinated hydrocarbon pollutants. Mass balances were monitored by measuring both the rate of disappearance of TCE and the rate of appearance of CO2 and Cl−. Dilute buffer solutions were used to fix pH and stoichiometrically sufficient amounts of dissolved O2 were used to make the reactions pseudo zero-order in O2. Using a standard chemical kinetic approach, two orders-of-magnitude in initial TCE concentration were spanned and the resulting double-log plot (log concentration vs. log initial rate) was used to determine the rate constant (k=5.77±1.06×10−7 s−1) and “true” (i.e., with respect to concentration, not time) reaction order (nc=0.85±0.03) for the rate law. By determining rate constants over the temperature interval 343–373 K, the Arrhenius activation energy (Ea) for the reaction was determined to be 108.0±4.5 kJ/mol. The rate law and derived kinetic parameters may be used in reactive transport simulators in order to account for aqueous oxidation of TCE as a function of temperature.  相似文献   

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