Influences of redox transformation, metal complexation and aggregation of fulvic acid and humic acid on Cr(VI) and As(V) removal by zero-valent iron

This study investigated the removal kinetics and mechanisms of Cr(VI) and As(V) by Fe(0) in the presence of fulvic acid (FA) and humic acid (HA) by means of batch experiments. The focus was on the involvements of FA and HA in redox reactions, metal complexation, and iron corrosion product aggregation in the removal of Cr(VI) and As(V) removal by Fe(0). Synthetic groundwater was used as the background electrolyte to simulate typical groundwater. The results showed faster Cr(VI) removal in the presence of HA compared to FA. Fluorescence spectroscopy revealed that no redox reaction occurred in the FA and HA. The results of the speciation modeling indicate that the free Fe(II) concentration was higher in the presence of HA, resulting in a higher removal rate of Cr(VI). However, the removal of As(V) was inhibited in the HA solution. Speciation modeling showed that the concentration of dissolved metal-natural organic matter (metal-NOM) complexes significantly affected the aggregation of the iron corrosion products which in turn affected the removal of As(V). The aggregation was found to be induced by gel-bridging of metal-NOM with the iron corrosion products. The effects of metal-NOM on the aggregation of the iron corrosion products were further confirmed by TEM studies. Larger sizes of iron corrosion products were formed in the FA solution compared to HA solution. This study can shed light on understanding the relationships between the properties of NOM (especially the content of metal-binding sites) and the removal of Cr(VI) and As(V) by Fe(0).     

The effect of silica on the degradation of organohalides in granular iron columns

Dissolved silica species are naturally occurring, ubiquitous groundwater constituents with corrosion-inhibiting properties. Their influence on the performance and longevity of iron-based permeable reactive barriers for treatment of organohalides was investigated through long-term column studies using Connelly iron as the reactive medium. Addition of dissolved silica (0.5 mM) to the column feed solution led to a reduction in iron reactivity of 65% for trichloroethylene (TCE), 74% for 1,1,2-trichloroethane (1,1,2-TCA), and 93% for 1,1,1-trichloroethane (1,1,1-TCA), compared to columns operated under silica-free conditions. Even though silica adsorption was a gradual process, the inhibitory effect was evident within the first week, with subsequent decreases in reactivity over 288 days being relatively minor. Lower concentrations of dissolved silica species (0.2 mM) led to a lesser decrease (70%) in iron reactivity toward 1,1,1-TCA. The presence of dissolved silica species produced a shift in TCE product distribution toward the more highly chlorinated product cis-dichloroethylene (cis-DCE), although it did not appear to alter products originating from the trichloroethanes. The major corrosion products identified were magnetite (Fe3O4) or maghemite (gamma-Fe2O3) and carbonate green rust ([Fe4(2+)Fe(2)3+(OH)12][CO(3).2H2O]). Iron carbonate hydroxide (Fe(II)1.8Fe(III)0.2(OH)2.2CO3) was only found in the silica-free column, indicating that silica may hinder its formation. A comparison with columns operated under the same conditions, but using Master Builder iron as the reactive matrix, showed that Connelly iron is initially less reactive, but performs better than Master Builder iron over 288 days.     

One-dimensional model for biogeochemical interactions and permeability reduction in soils during leachate permeation

This paper uses the findings from a column study to develop a reactive model for exploring the interactions occurring in leachate-contaminated soils. The changes occurring in the concentrations of acetic acid, sulphate, suspended and attached biomass, Fe(II), Mn(II), calcium, carbonate ions, and pH in the column are assessed. The mathematical model considers geochemical equilibrium, kinetic biodegradation, precipitation-dissolution reactions, bacterial and substrate transport, and permeability reduction arising from bacterial growth and gas production. A two-step sequential operator splitting method is used to solve the coupled transport and biogeochemical reaction equations. The model gives satisfactory fits to experimental data and the simulations show that the transport of metals in soil is controlled by multiple competing biotic and abiotic reactions. These findings suggest that bioaccumulation and gas formation, compared to chemical precipitation, have a larger influence on hydraulic conductivity reduction.     

Experimental study and steady-state simulation of biogeochemical processes in laboratory columns with aquifer material

Packed bed laboratory column experiments were performed to simulate the biogeochemical processes resulting from microbially catalyzed oxidation of organic matter. These included aerobic respiration, denitrification, and Mn(IV), Fe(III) and SO(4) reduction processes. The effects of these reactions on the aqueous- and solid-phase geochemistry of the aquifer material were closely examined. The data were used to model the development of alkalinity and pH along the column. To study the independent development of Fe(III)- and SO(4)-reducing environments, two columns were used. One of the columns (column 1) contained small enough concentrations of SO(4) in the influent to render the reduction of this species unimportant to the geochemical processes in the column.The rate of microbially catalyzed reduction of Mn(IV) changed with time as evidenced by the variations in the initial rate of Mn(II) production at the head of the column. The concentration of Mn in both columns was controlled by the solubility of rhodochrosite (MnCO(3(S))).In the column where significant SO(4) reduction took place (column 2), the concentration of dissolved Fe(II) was controlled by the solubility of FeS. In column 1, where SO(4) reduction was not important, maximum dissolved Fe(II) concentrations were controlled by the solubility of siderite (FeCO(3(S))). Comparison of solid-phase and aqueous-phase data suggests that nearly 20% of the produced Fe(II) precipitates as siderite in column 1. The solid-phase analysis also indicates that during the course of experiment, approximately 20% of the total Fe(III) hydroxides and more than 70% of the amorphous Fe(III) hydroxides were reduced by dissimilatory iron reduction.The most important sink for dissolved S(-II) produced by the enzymatic reduction of SO(4) was its direct reaction with solid-phase Fe(III) hydroxides leading initially to the formation of FeS. Compared to this pathway, precipitation as FeS did not constitute an important sink for S(-II) in column 2. In this column, the total reacted S(-II) estimated from the concentration of dissolved sulfur species was in good agreement with the produced Cr(II)-reducible sulfur in the solid phase. Solid-phase analysis of the sulfur species indicated that up to half of the originally produced FeS may have possibly transformed to FeS(2).     

Nitrite reduction and formation of corrosion coatings in zerovalent iron systems

Batch tests were conducted to investigate nitrite reduction in a zerovalent iron (Fe0) system under various conditions. Nitrite at 1.4 mM initial concentration was slowly reduced to nitrogen gas in the first stage (days 1-6), which was mediated by an amorphous, Fe(II)-rich iron oxide coating. The second stage (days 7-14) featured a rapid reduction of nitrite to both ammonia and nitrogen gas and the formation of a more crystalline, magnetite form iron oxide coating. Water reduction by Fe0 occurred concurrently with nitrite reduction from the beginning and contributed significantly to the overall iron corrosion. Nitrite at 14 mM was found to passivate the surface of Fe0 grains with respect to nitrite reduction. Adding aqueous Fe2+ significantly accelerated reduction of nitrite by Fe0 to nitrogen gas with lepidocrocite as the main iron corrosion product. Substantially, though still substoichiometrically, 0.55 mol of Fe2+ were concomitantly consumed per 1.0 mol nitrite reduction, indicating that Fe0 was the main electron source. In the presence of Fe2+, nitrite reduction out-competed water reduction in terms of contributing to the overall iron corrosion. Results of this study help understand complicated interactions between water reduction and nitrite reduction, the roles of surface-bound Fe2+, and the evolution of the iron corrosion coating.     

The addition of organic carbon and nitrate affects reactive transport of heavy metals in sandy aquifers

Organic carbon introduction in the soil to initiate remedial measures, nitrate infiltration due to agricultural practices or sulphate intrusion owing to industrial usage can influence the redox conditions and pH, thus affecting the mobility of heavy metals in soil and groundwater. This study reports the fate of Zn and Cd in sandy aquifers under a variety of plausible in-situ redox conditions that were induced by introduction of carbon and various electron acceptors in column experiments. Up to 100% Zn and Cd removal (from the liquid phase) was observed in all the four columns, however the mechanisms were different. Metal removal in column K1 (containing sulphate), was attributed to biological sulphate reduction and subsequent metal precipitation (as sulphides). In the presence of both nitrate and sulphate (K2), the former dominated the process, precipitating the heavy metals as hydroxides and/or carbonates. In the presence of sulphate, nitrate and supplemental iron (Fe(OH)(3)) (K3), metal removal was also due to precipitation as hydroxides and/or carbonates. In abiotic column, K4, (with supplemental iron (Fe(OH)(3)), but no nitrate), cation exchange with soil led to metal removal. The results obtained were modeled using the reactive transport model PHREEQC-2 to elucidate governing processes and to evaluate scenarios of organic carbon, sulphate and nitrate inputs.     

Humic acids enhance the microbially mediated release of sedimentary ferrous iron

Iron (Fe) is an essential element for many organisms, but high concentrations of iron can be toxic. The complex relation between iron, arsenic (As), bacteria, and organic matter in sediments and groundwater is still an issue of environmental concern. The present study addresses the effects of humic acids and microorganisms on the mobilization of iron in sediments from an arsenic-affected area, and the microbial diversity was analyzed. The results showed that the addition of 50, 100, and 500 mg/L humic acids enhanced ferrous iron (Fe(II)) release in a time-dependent and dose-dependent fashion under anaerobic conditions. A significant increase in the soluble Fe(II) concentrations occurred in the aqueous phases of the samples during the first 2 weeks, and aqueous Fe(II) reached its maximum concentrations after 8 weeks at the following Fe(II) concentrations: 28.95?±?1.16 mg/L (original non-sterilized sediments), 32.50?±?0.71 mg/L (50 mg/L humic acid-amended, non-sterilized sediments), 37.50?±?1.85 mg/L (100 mg/L humic acid-amended, non-sterilized sediments), and 39.00?±?0.43 mg/L (500 mg/L humic acid-amended, non-sterilized sediments). These results suggest that humic acids can further enhance the microbially mediated release of sedimentary iron under anaerobic conditions. By contrast, very insignificant amounts of iron release were observed from sterilized sediments (the abiotic controls), even with the supplementation of humic acids under anaerobic incubation. In addition, the As(III) release was increased from 50?±?10 μg/L (original non-sterilized sediments) to 110?±?45 μg/L (100 mg/L humic acid-amended, non-sterilized sediments) after 8 weeks of anaerobic incubation. Furthermore, a microbial community analysis indicated that the predominant class was changed from Alphaproteobacteria to Deltaproteobacteria, and clearly increased populations of Geobacter sp., Paludibacter sp., and Methylophaga sp. were found after adding humic acids along with the increased release of iron and arsenic. Our findings provide evidence that humic acids can enhance the microbially mediated release of sedimentary ferrous iron in an arsenic-affected area. It is thus suggested that the control of anthropogenic humic acid use and entry into the environment is important for preventing the subsequent iron contamination in groundwater.     

Utilisation of chemically stabilized arsenic-contaminated soil in a landfill cover

The aim of the study was to determine if an As-contaminated soil, stabilized using zerovalent iron (Fe⁰) and its combination with gypsum waste, coal fly ash, peat, or sewage sludge, could be used as a construction material at the top layer of the landfill cover. A reproduction of 2 m thick protection/vegetation layer of a landfill cover using a column setup was used to determine the ability of the amendments to reduce As solubility and stimulate soil functionality along the soil profile. Soil amendment with Fe⁰ was highly efficient in reducing As in soil porewater reaching 99 % reduction, but only at the soil surface. In the deeper soil layers (below 0.5 m), the Fe treatment had a reverse effect, As solubility increased dramatically exceeding that of the untreated soil or any other treatment by one to two orders of magnitude. A slight bioluminescence inhibition of Vibrio fischeri was detected in the Fe⁰ treatment. Soil amendment with iron and peat showed no toxicity to bacteria and was the most efficient in reducing dissolved As in soil porewater throughout the 2 m soil profile followed by iron and gypsum treatment, most likely resulting from a low soil density and a good air diffusion to the soil. The least suitable combination of soil amendments for As immobilization was a mixture of iron with coal fly ash. An increase in all measured enzyme activities was observed in all treatments, particularly those receiving organic matter. For As to be stable in soil, a combination of amendments that can keep the soil porous and ensure the air diffusion through the entire soil layer of the landfill cover is required.     

Arsenic removal from water employing heterogeneous photocatalysis with TiO2 immobilized in PET bottles

Arsenic oxidation (As(III) to As(V)) and As(V) removal from water were assessed by using TiO₂ immobilized in PET (polyethylene terephthalate) bottles in the presence of natural sunlight and iron salts. The effect of many parameters was sequentially studied: TiO₂ concentration of the coating solution, Fe(II) concentration, pH, solar irradiation time; dissolved organic carbon concentration. The final conditions (TiO₂ concentration of the coating solution: 10%; Fe(II): 7.0 mg l⁻¹; solar exposure time: 120 min) were applied to natural water samples spiked with 500 μg l⁻¹ As(III) in order to verify the influence of natural water matrix. After treatment, As(III) and total As concentrations were lower than the limit of quantitation (2 μg l⁻¹) of the voltammetric method used, showing a removal over 99%, and giving evidence that As(III) was effectively oxidized to As(V). The results obtained demonstrated that TiO₂ can be easily immobilized on a PET surface in order to perform As(III) oxidation in water and that this TiO₂ immobilization, combined with coprecipitation of arsenic on Fe(III) hydroxides(oxides) could be an efficient way for inorganic arsenic removal from groundwaters.     

Mercury in contaminated sediments and pore waters enriched in sulphate (Tagus Estuary,Portugal)

Three sediment cores, collected nearby the effluent of a chlor-alkali industry, were sliced in 0.5-cm layers and centrifuged for pore water extraction. Mercury, Fe and Mn were determined in the solids as total concentration, hydroxylamine extractable fraction and HCl extractable fraction. Sulphur was determined in the HCl extraction. Total and reactive mercury, chlorinity, S(2-), SO(4)(2-), total Fe, and total Mn were measured in pore waters. The solids contained 3.0-60 nmol g(-1) of total Hg and pore waters 70-5800 pM of total Hg and 1.8-76 pM of reactive mercury. Pore waters presented 2.3-94 times more sulphate than the overlying estuarine waters due to the input from the industry. In layers where hydroxylamine extractable Fe exhibited a broad maximum (precipitation of Fe-oxides) sulphate was reduced to S(2-). The competition between the high content of SO(4)(2-) and Fe(III) as electron acceptors, in chemical reactions occurring in the upper sediments, may explain the co-existence of S(2-) and Fe-oxides in the same layers. Mercury was detected in the hydroxylamine extracts (20-29 nmolg(-1)) in the layers where Fe-oxides were formed, and reactive dissolved Hg showed minimum concentrations. The excess of sulphate in pore waters favoured the abundant Fe-oxides in the upper solid sediments, which appear to work as a barrier limiting the escape of mercury to the water column.     

Assessment of transport of two polyelectrolyte-stabilized zero-valent iron nanoparticles in porous media

This study investigated the breakthrough patterns of carboxymethyl cellulose- and polyacrylic acid-stabilized zero-valent iron (Fe(0)) nanoparticles (NZVI) from packed sand columns under a range of pore water velocities of 0.02, 0.2 and 1 cm min(-1) and NZVI influent concentrations of 0.1, 0.5 and 3 g L(-1). The NZVI effluent relative concentrations of both types of particles decreased with slower flow velocities and increasing particle concentrations. PAA-NZVI exhibited slower elution from the columns than CMC-NZVI under identical experimental conditions, and this is attributed to more rapid aggregation kinetics of PAA-NZVI. The elution patterns of PAA-NZVI showed a stronger trend of gradually increasing effluent concentrations with flushing of additional pore volumes, especially at low flushing velocities and higher influent particle concentrations and this phenomenon too can be attributed to increasing aggregate sizes with time which caused decreases in the values of the single collector efficiency and thus the deposition rate constant. A 7 nm increase in CMC-NZVI aggregate size over 60 min was observed using nanoparticle tracking analysis. The reduction in colloidal stability due to aggregation of CMC- and PAA-NZVI was verified using sedimentation tests, and it was found that PAA-NZVI were less stable than CMC-NZVI. There were also notable inherent differences in the two NZVI particles. The CMC-NZVI were monodisperse with a mean diameter of 5.7 ± 0.9 nm, whereas PAA-NZVI had a bimodal particle size distribution with a small sub-population of particles with mean size of 30 ± 21 nm and a more abundant population of 4.6 ± 0.8 nm diameter particles. Furthermore, PAA-NZVI had a lower surface potential. These characteristics are also responsible for the different elution patterns CMC- and PAA-NZVI.     

As(III) removal from groundwaters using fixed-bed upflow bioreactors

The application of biological oxidation of iron and manganese, as a potential treatment method for the removal of arsenic from contaminated groundwaters, was examined in this paper. This method was based on the growth of certain species of indigenous bacteria, which are capable of oxidizing the soluble iron and manganese ions; the oxidized forms can be subsequently removed from the aqueous stream by over 97%, through their transformation to insoluble oxides and separation by a suitable filter medium. Arsenic was removed by around 80%, under certain conditions, which were found to be sufficient for Fe(II) removal (dissolved oxygen 2.7 mg/l, redox 280-290 mV, pH 7.2, U 8.25 m/h). The specific treatment technique presents several advantages towards conventional physicochemical treatment methods, such as enhanced coagulation or direct adsorption since: (a) it does not require the addition of other chemicals for oxidizing and removing As(III), (b) it does not require close monitoring of a breakthrough point, as in conventional column adsorption processes and (c) it could find application for the removal of, at least, three groundwater contaminants (Fe, Mn, As).     

Heavy metals incidence in the application of inorganic fertilizers and pesticides to rice farming soils

The concentrations of Cd, Co, Cu, Ni, Pb, Zn, Fe and Mn in different inorganic fertilizers (urea, calcium superphosphate, iron sulphate and copper sulphate) and in pesticides (two herbicides and one fungicide) are evaluated together with the contribution of these metals in soils from their use. The study was made in rice farming areas to the north of Albufera Natural Park (Valencia, Spain). The results obtained show that superphosphate is the fertilizer that contains the highest concentrations of Cd, Co, Cu and Zn as impurities. Copper sulphate and iron sulphate have the most significant concentrations of Pb, and are the only fertilizers in which Ni was detected. The three pesticides analysed show similar Cd contents and the highest levels of Fe, Mn, Zn, Pb and Ni are found in the herbicides. The most significant additions of heavy metals as impurities that soil receives from agricultural practices, are Mn, Zn, Co and Pb. Three contamination indexes have been applied to provide a basis for comparison of potential heavy metal toxicity. These results denote the potential toxicity of heavy metals in the studied soils.     

As(III) removal from aqueous solutions using non-stoichiometric coprecipitation with iron(III) sulphate and filtration or flotation

A study of As(III) removal from aqueous solutions was carried out, using iron (III) sulphate as a coprecipitation agent. When the initial As(III) concentration was 3775 mg l(-1), maximum As(III) removal was achieved at pH 9 and with a molar ratio of Fe(III)/As(III) equal to 11. Under these conditions the formed precipitates showed highest stability, relative to the fixation of As(III). Flotation, which can be applied as a subsequent solid/liquid separation method, was found to be particularly effective for solutions containing low As(III) concentrations (相似文献   

Microbial reduction of nitrate in the presence of nanoscale zero-valent iron

Microbial reduction of nitrate in the presence of nanoscale zero-valent iron (NZVI) was evaluated to assess the feasibility of employing NZVI in the biological nitrate treatment. Nitrate was completely reduced within 3 d in a nanoscale Fe(0)-cell reactor, while only 50% of the nitrate was abiotically reduced over 7 d at 25 °C. The removal rate of nitrate in the integrated NZVI-cell system was unaffected by the presence of high amounts of sulfate. Efficient removal of nitrate by Fe(II)-supported anaerobic culture in 14 d indicated that Fe(II), which is produced during anaerobic iron corrosion in the Fe(0)-cell system, might act as an electron donor for nitrate. Unlike abiotic reduction, microbial reduction of nitrate was not significantly affected by low temperature conditions. This study demonstrated the potential applicability of employing NZVI iron as a source of electrons for biological nitrate reduction. Use of NZVI for microbial nitrate reduction can obviate the disadvantages associated with traditional biological denitrification, that relies on the use of organic substrates or explosive hydrogen gas, and maintain the advantages offered by nano-particle technology such as higher surface reactivity and functionality in suspensions.     

An in situ study of the effect of nitrate on the reduction of trichloroethylene by granular iron

The effect of nitrate on the reduction of TCE by commercial granular iron was investigated in column experiments designed to allow for the in situ monitoring of the iron surface film with Raman spectroscopy. Three column experiments were conducted; one with an influent solution of 100 mg/l nitrate+1.5 mg/l TCE, and two control columns, one saturated directly with 100 mg/l nitrate solution, the other pre-treated with Millipore water prior to the introduction of a 100 mg/l nitrate solution. In the presence of nitrate, TCE adsorbed onto the iron, but there was little TCE reduction to end-products ethene and ethane. The iron used (Connelly, GPM, Chicago) is a product typical of those used in permeable granular iron walls. The material is covered by an air-formed high-temperature oxidation film, consisting of an inner layer of Fe(3)O(4), and an outer, passive layer of Fe(2)O(3). In the control column pre-treated with Millipore water, the passive Fe(2)O(3) layer was removed upon contact with the water in a manner consistent with an autoreduction reaction. In the TCE+nitrate column and the direct nitrate saturation column, nitrate interfered with the removal of the passive layer and maintained conditions such that high valency protective corrosion species, including Fe(2)O(3) and FeOOH, were stable at the iron surface. The lack of TCE reduction is explained by the presence of these species, as they inhibit both mechanisms proposed for TCE reduction by iron, including catalytic hydrogenation, and direct electron transfer.     

Effective remediation of grossly polluted acidic, and metal-rich, spoil heap drainage using a novel, low-cost, permeable reactive barrier in Northumberland, UK

A permeable reactive barrier (PRB) for remediation of coal spoil heap drainage in Northumberland, UK, is described. The drainage has typical chemical characteristics of pH<4, [acidity]>1400 mg/L as CaCO3, [Fe]>300 mg/L, [Mn]>165 mg/L, [Al]>100mg/L and [SO4]>6500 mg/L. During 2 years of operation the PRB has typically removed 50% of the iron and 40% of the sulphate from this subsurface spoil drainage. Bacterial sulphate reduction appears to be a key process of this remediation. Treatment of the effluent from the PRB results in further attenuation; overall reductions in iron and sulphate concentrations are 95% and 67% respectively, and acidity concentration is reduced by an order of magnitude. The mechanisms of attenuation of these, and other, contaminants in the drainage are discussed. Future research and operational objectives for this novel, low-cost, treatment system are also outlined.     

Arsenic(III) and iron(II) co-oxidation by oxygen and hydrogen peroxide: Divergent reactions in the presence of organic ligands

Iron-catalyzed oxidation of As(III) to As(V) can be highly effective for toxic arsenic removal via Fenton reaction and Fe(II) oxygenation. However, the contribution of ubiquitous organic ligands is poorly understood, despite its significant role in redox chemistry of arsenic in natural and engineered systems. In this work, selected naturally occurring organic ligands and synthetic ligands in co-oxidation of Fe(II) and As(III) were examined as a function of pH, Fe(II), H₂O₂, and radical scavengers (methanol and 2-propanol) concentration. As(III) was not measurably oxidised in the presence of excess ethylenediaminetetraacetic acid (EDTA) (i.e. Fe(II):EDTA < 1:1), contrasting with the rapid oxidation of Fe(II) by O₂ and H₂O₂ at neutral pH under the same conditions. However, partial oxidation of As(III) was observed at a 2:1 ratio of Fe(II):EDTA. Rapid Fe(II) oxidation in the presence of organic ligands did not necessarily result in the coupled As(III) oxidation. Organic ligands act as both iron speciation regulators and radicals scavengers. Further quenching experiments suggested both hydroxyl radicals and high-valent Fe species contributed to As(III) oxidation. The present findings are significant for the better understanding of aquatic redox chemistry of iron and arsenic in the environment and for optimization of iron-catalyzed arsenic remediation technology.     

Redox reactions in the Fe-As-O2 system

We have examined two redox reactions involving arsenic and iron at near-neutral pH: the reduction of As(V) by Fe(II) under anoxic conditions, and the co-oxidation of As(III) during Fe(II) oxygenation. We also considered the impact of goethite, pH buffers, and radical scavengers on these reactions. In a series of anoxic experiments, Fe(II) was found to reduce As(V) in the presence of goethite, but not in homogeneous solution. The reaction rate increased with increasing pH and Fe(II) concentration, but in all cases was relatively slow. In aerobic experiments, the kinetics of Fe(II) oxygenation at neutral pH, and the corresponding oxidation of As(III) were found to depend heavily on pH buffer type and concentration. The classic formulation of Fe(II) oxidation by oxygen, involving four single-electron transfers, was reviewed and found to be inadequate for explaining observed oxidation of Fe(II) and As(III). Widely cited rate constants for Fe(II) oxygenation originate from experiments conducted in carbonate buffer, and do not match observations made in phosphate, MES, or HEPES systems. In phosphate buffer, Fe(II) oxidation is rapid and dependent on phosphate concentration. In MES and HEPES buffers, Fe(II) oxidation is much slower due to the lack of labile ferrous iron species. Oxygenation of Fe(II) appears to proceed through different mechanisms in phosphate and MES or HEPES systems. In both cases, reactive intermediary species are produced which can oxidize As(III). These oxidants are not the hydroxyl radical, but may be Fe(IV) species.     

A comparison of the low frequency electrical signatures of iron oxide versus calcite precipitation in granular zero valent iron columns

Geophysical methods have been proposed as technologies for non-invasively monitoring geochemical alteration in permeable reactive barriers (PRBs). We conducted column experiments to investigate the effect of mineralogy on the electrical signatures resulting from iron corrosion and mineral precipitation in Fe0 columns using (a) Na2SO4, and (b) NaHCO3 plus CaCl2 mixture, solutions. At the influent interface where the reactions were most severe, a contrasting time-lapse electrical response was observed between the two columns. Solid phase analysis confirmed the formation of corrosion halos and increased mineralogical complexity in the corroded sections of the columns compared to the minimal/non-corroded sections. We attribute the contrasting time-lapse signatures to the differences in the electrical properties of the mineral phases formed within the two columns. While newly precipitated/transformed polarizable and semi-conductive iron oxides (mostly magnetite and green rust) increase the polarization and conductivity of the sulfate column, the decrease of both parameters in the bicarbonate column is attributed to the precipitation of non-polarizable and non-conductive calcite. Our results show that precipitate mineralogy is an important factor influencing the electrical properties of the corroded iron cores and must be considered if electrical geophysical methods are to be developed to monitor PRB barrier corrosion processes in situ.