Understanding soluble arsenate removal kinetics by zerovalent iron media

Zerovalent iron filings have been proposed as a filter medium for removing arsenic compounds from potable water supplies. This research investigated the kinetics of arsenate removal from aqueous solutions by zerovalent iron media. Batch experiments were performed to determine the effect of the iron corrosion rate on the rate of As(V) removal. Tafel analyses were used to determine the effect of the As(V) concentration on the rate of iron corrosion in anaerobic solutions. As(V) removal in column reactors packed with iron filings was measured over a 1-year period of continuous operation. Comparison of As(V) removal by freely corroding and cathodically protected iron showed that rates of arsenate removal were dependent on the continuous generation of iron oxide adsorption sites. In addition to adsorption site availability, rates of arsenate removal were also limited by mass transfer associated with As(V) diffusion through iron corrosion products. Steady-state removal rates in the column reactor were up to 10 times faster between the inlet-end and the first sampling port than between the first sampling port and the effluent-end of the column. Faster removal near the influent-end of the column was due to a faster rate of iron oxidation in that region. The presence of 100 microg/L As(V) decreased the iron corrosion rate by up to a factor of 5 compared to a blank electrolyte solution. However, increasing the As(V) concentration from 100 to 20,000 microg/L resulted in no further decrease in the iron corrosion rate. The kinetics of arsenate removal ranged between zeroth- and first-order with respect to the aqueous As(V) concentration. The apparent reaction order was dependent on the availability of adsorption sites and on the aqueous As(V) concentration. X-ray absorption spectroscopy analyses showed the presence of iron metal, magnetite (Fe3O4), an Fe(III) oxide phase, and possibly an Fe(II,III) hydroxide phase in the reacted iron filings. These mixed valent oxide phases are not passivating and permit sustained iron corrosion and continuous generation of new sites for As(V) adsorption.     

Chromium-removal processes during groundwater remediation by a zerovalent iron permeable reactive barrier

Solid-phase associations of chromium were examined in core materials collected from a full-scale, zerovalent iron permeable reactive barrier (PRB) at the U.S. Coast Guard Support Center located near Elizabeth City, NC. The PRB was installed in 1996 to treat groundwater contaminated with hexavalent chromium. After eight years of operation, the PRB remains effective at reducing concentrations of Cr from average values >1500 microg L(-1) in groundwater hydraulically upgradient of the PRB to values <1 microg L(-1) in groundwater within and hydraulically downgradient of the PRB. Chromium removal from groundwater occurs at the leading edge of the PRB and also within the aquifer immediately upgradient of the PRB. These regions also witness the greatest amount of secondary mineral formation due to steep geochemical gradients that result from the corrosion of zerovalent iron. X-ray absorption near-edge structure (XANES) spectroscopy indicated that chromium is predominantly in the trivalent oxidation state, confirming that reductive processes are responsible for Cr sequestration. XANES spectra and microscopy results suggest that Cr is, in part, associated with iron sulfide grains formed as a consequence of microbially mediated sulfate reduction in and around the PRB. Results of this study provide evidence that secondary iron-bearing mineral products may enhance the capacity of zerovalent iron systems to remediate Cr in groundwater, either through redox reactions at the mineral-water interface or by the release of Fe(II) to solution via mineral dissolution and/or metal corrosion.     

Field column study using zerovalent iron for mercury removal from contaminated groundwater

Passive in situ remediation technologies, for example, permeable reactive barriers, PRBs, are an attractive and less expensive alternative compared to conventional pump and treat systems for groundwater remediation. Field column experiments were conducted to evaluate the removal of dissolved mercury from groundwater using zerovalent iron as the reactive media. Two column tests were conducted over a 6-week period, which simulated 2 and 10 years of groundwater flow through a potential full-scale treatment system. The influent groundwater pH was 7.8-9.5. The groundwater was reduced with an Eh, corrected to the standard hydrogen electrode, ranging from 0 to 120 mV over the trial period. Prior to treatment the total mercury concentration of the groundwater was approximately 40 microg L(-1). Effluent from the 10-year simulation contained approximately 0.5 microg/L of mercury during the first 3 weeks and increased to as much as 4 microg L(-1) by the end of the testing period. Effluent from the 2-year simulation was generally < 0.1 microg L(-1). Profile sampling of the 2-year simulation suggests that most of the mercury removal occurred in the initial 50% of the 20 cm column. Mineralogical studies, conducted using SEM/EDS and X-ray absorption spectroscopy (XAS), confirm the accumulation of mercury onto a zerovalent iron surface in this 20-cm zone. These analyses indicate that mercury accumulated as a mercury sulfide with a stoichiometery similar to those of cinnabar and metacinnabar (HgS).     

In situ remediation of arsenic in simulated groundwater using zerovalent iron: laboratory column tests on combined effects of phosphate and silicate

We performed three column tests to study the behavior of permeable reactive barrier (PRB) materials to remove arsenic under dynamic flow conditions in the absence as well as in the presence of added phosphate and silicate. The column consisted of a 10.3 cm depth of 50:50 (w:w, Peerless iron:sand) in the middle and a 10.3 cm depth of a sediment from Elizabeth City, NC, in both upper and lower portions of the 31-cm-long glass column (2.5 cm in diameter) with three side sampling ports. The flow velocity (upflow mode) was maintained at 4.3 m d(-1) during the 3-4-month experiments. As expected, dissolved As concentrations in different positions of the column generally followed the order: column influent > bottom port effluent > middle port effluent > top port effluent > column effluent. The steady-state As removal in the middle Peerless iron and sand mixture zone might be attributed to the continuous supply of corroded iron in the form of iron oxides and hydroxides that served as the sorbents for both As(V) and As(III). Consistent with previous batch study findings, dissolved phosphate (0.5 or 1 mg of P L(-1)) and silicate (10 or 20 mg of Si L(-1)) showed strong inhibition for As(V) and As(III) (1 mg of As(V) L(-1) + 1 mg of As(III) L(-1) in 7 mM NaCl + 0.86 mM CaSO4) removal by Peerless iron in the column tests. The presence of combined phosphate and silicate resulted in earlier breakthrough (C = 0.5C0) and earlier complete breakthrough of dissolved arsenic relative to absence of added phosphate and silicate in the bottom port effluent. Competition between As(V)/As(III) and phosphate/silicate forthe sorption sites on the corrosion products of Peerless iron seems to be the cause of the observations. This effect is especially important in the case of silicate for designing a PRB of zerovalent iron for field use because dissolved silicate is ubiquitous in terrestrial waters.     

Arsenate and arsenite adsorption and desorption behavior on coprecipitated aluminum:iron hydroxides

Although arsenic adsorption/desorption behavior on aluminum and iron (oxyhydr)oxides has been extensively studied, little is known about arsenic adsorption/desorption behavior by bimetal Al:Fe hydroxides. In this study, influence of the Al:Fe molar ratio, pH, and counterion (Ca2+ versus Na+) on arsenic adsorption/desorption by preformed coprecipitated Al:Fe hydroxides was investigated. Adsorbents were formed by initial hydrolysis of mixed Al3+/ Fe3+ salts to form coprecipitated Al:Fe hydroxide products. At Al:Fe molar ratios < or = 1:4, Al3+ was largely incorporated into the iron hydroxide structure to form a poorly crystalline bimetal hydroxide; however, at higher Al:Fe molar ratios, crystalline aluminum hydroxides (bayerite and gibbsite) were formed. Although approximately equal As(V) adsorption maxima were observed for 0:1 and 1:4 Al:Fe hydroxides, the As(III) adsorption maximum was greater with the 0:1 Al: Fe hydroxide. As(V) and As(III) adsorption decreased with further increases in Al:Fe molar ratio. As(V) exhibited strong affinity to 0:1 and 1:4 Al:Fe hydroxides at pH 3-6. Adsorption decreased at pH > 6.5; however, the presence of Ca2+ compared to Na+ as the counterion enhanced As( retention by both hydroxides. There was more As(V) and especially As(III) desorption by phosphate with an increase in Al:Fe molar ratio.     

Removal of arsenic from groundwater by zerovalent iron and the role of sulfide

Several recent investigations have shown encouraging potential for the removal of arsenic (As) from groundwater by granular zerovalent iron (Fe0). In contrast to previous studies conducted, we have investigated the applicability of this method and the nature of As bonding under conditions with dissolved sulfide. Three column tests were performed over the period of 1 year using solutions with either As(V) or As(II) (2-200 mg/L) in the input solution. Arsenic outflow concentrations decreased from initially 30-100 microg/L to concentrations of below 1 microg/L with time. XANES (X-ray absorptions near edge structure) and EXAFS (expanded X-ray absorption fine structure) spectra indicated that As in the solid phase is not only directly coordinated with oxygen, as is the case in adsorbed or coprecipitated arsenite and arsenate. Samples with high sulfur content showed additional bonding, for which Fourier transformations of EXAFS data exhibited a peak between 2.2 and 2.4 A. This bonding most likely originated from the direct coordination of sulfur or iron with As, which was incorporated in iron sulfides orfrom adsorbed thioarsenites. The formation of this sulfide bonding supports the removal of As by Fe0 because sulfide production by microbial sulfate reduction is ubiquitous in permeable reactive barriers composed of Fe0.     

Mechanisms of uranium interactions with hydroxyapatite: implications for groundwater remediation

The speciation of U(VI) sorbed to synthetic hydroxyapatite was investigated using a combination of U LIII-edge XAS, synchrotron XRD, batch uptake measurements, and SEM-EDS. The mechanisms of U(VI) removal by apatite were determined in order to evaluate the feasibility of apatite-based in-situ permeable reactive barriers (PRBs). In batch U(VI) uptake experiments with synthetic hydroxyapatite (HA), near complete removal of dissolved uranium (>99.5%) to <0.05 microM was observed over a range of total U(VI) concentrations up to equimolar of the total P in the suspension. XRD and XAS analyses of U(VI)-reacted HA at sorbed concentrations < or = 4,700 ppm U(VI) suggested that uranium(VI) phosphate, hydroxide, and carbonate solids were not present at these concentrations. Fits to EXAFS spectra indicate the presence of Ca neighbors at 3.81 A. U-Ca separation, suggesting that U(VI) adsorbs to the HA surfaces as an inner-sphere complex. Uranium(VI) phosphate solid phases were not detected in HA with 4700 ppm sorbed U(VI) by backscatter SEM or EDS, in agreement with the surface complexation process. In contrast, U(VI) speciation in samples that exceeded 7000 ppm sorbed U(VI) included a crystalline uranium(VI) phosphate solid phase, identified as chernikovite by XRD. At these higher concentrations, a secondary, uranium(VI) phosphate solid was detected by SEM-EDS, consistent with chernikovite precipitation. Autunite formation occurred at total U:P molar ratios > or = 0.2. Our findings provide a basis for evaluating U(VI) sorption mechanisms by commercially available natural apatites for use in development of PRBs for groundwater U(VI) remediation.     

Significance of iron(II,III) hydroxycarbonate green rust in arsenic remediation using zerovalent iron in laboratory column tests

We examined the corrosion products of zerovalent iron used in three column tests for removing arsenic from water under dynamic flow conditions. Each column test lasted 3-4 months using columns consisting of a 10.3-cm depth of 50:50 (w:w, Peerless iron:sand) in the middle and a 10.3cm depth of a sediment from Elizabeth City, NC, in both upper and lower portions of the 31-cm-long glass column (2.5 cm in diameter). The feeding solutions were 1 mg of As(V) L(-1) + 1 mg of As(III) L(-1) in 7 mM NaCl + 0.86 mM CaSO4 with or without added phosphate (0.5 or 1 mg of P L(-1)) and silicate (10 or 20 mg of Si L(-1)) at pH 6.5. Iron(II,III) hydroxycarbonate green rust (or simply, carbonate green rust) and magnetite were the major iron corrosion products identified with X-ray diffraction for the separated fractions (5 and 1 min sedimentation and residual). The presence of carbonate green rust was confirmed by scanning electron microscopy (hexagonal morphology) and FTIR-photoacoustic spectroscopy (interlayer carbonate stretching mode at 1352-1365 cm(-1)). X-ray photoelectron spectroscopy investigation revealed the presence of predominantly As(V) at the surface of corroded iron particles despite the fact that the feeding solution in contact with Peerless iron contained more As(III) than As(V) as a result of a preferential uptake of As(V) over As(III) by the Elizabeth City sediment. Extraction of separated corrosion products with 1.0 M HCI showed that from 86 to 96% of the total extractable As (6.9-14.6 g kg(-1)) was in the form of As(V) in agreement with the XPS results. Combined microscopic and macroscopic wet chemistry results suggest that sorbed As(III) was partially oxidized by the carbonate green rust at the early stage of iron corrosion. The column experiments suggest that either carbonate green rust is kinetically favored or is thermodynamically more stable than sulfate green rust in the studied Peerless iron corrosion systems.     

In situ remediation of groundwater contaminated by heavy- and transition-metal ions by selective ion-exchange methods

Laboratory studies were conducted to investigate the feasibility of using ion-exchange resins in permeable reactive barriers (PRBs) for the remediation of groundwater contaminated by heavy and transition metals. Ion-exchange resins represent an essentially neglected class of materials which may, in addition to iron, activated carbon, and zeolites, prove effective for use in PRBs. Four resins were considered: two commercially available resins, Duolite GT-73 (Rohm and Haas) and Amberlite IRC-748 (Rohm and Haas), and two solvent-impregnated resins (SIRs). The SIRs were prepared from Amberlite IRA-96 (Rohm and Haas) and two different thiophosphoric extractants. All four resins are able to reduce cadmium, lead, and copper concentrations from 1000 microg/L (typical for contaminated groundwaters) to below 5 microg/L. Significantly, all of the resins are effective for the capture of cadmium, copper, and lead, even in the presence of CaCl2 and clay. Because of their high hydraulic conductivity, the use of these resins in clusters of wells, as an alternative to continuous walls, is considered in the design of effective PRBs. Numerical solution of the groundwater flow equations shows that, depending on the well configuration, most (or all) of the contaminated groundwater can pass through the resins. These results demonstrate the possibility of using selective ion-exchange resins as an effective, active material in PRBs for in situ groundwater remediation.     

Combined removal of chlorinated ethenes and heavy metals by zerovalent iron in batch and continuous flow column systems

The combined removal of chlorinated ethenes and heavy metals from a simulated groundwater matrix by zerovalent iron (ZVI) was investigated. In batch, Ni (5-100 mg L(-1)) enhanced trichloroethene (TCE, 10 mg L(-1)) reduction by ZVI (100 g L(-1)) due to catalytic hydrodechlorination by bimetallic Fe0/Ni0. Cr(VI) or Zn (5-100 mg L(-1)) lowered TCE degradation rates by a factor of 2 to 13. Cr(VI) (100 mg L(-1)) in combination with Zn or Ni (50-100 mg L(-1)) inhibited TCE degradation. Addition of 20% H2(g) in the headspace, or of Zn (50-100 mg L(-1)), enhanced TCE removal in the presence of Ni and Cr(VI). Sorption of Zn to ZVI alleviated the Cr(VI) induced inhibition of bimetallic Fe0/Ni0 apparently due to release of protons necessary for TCE hydrodechlorination. In continuous ZVI columns treating tetrachloroethene (PCE, 1-2 mg L(-1)) and TCE (10 mg L(-1)), and a mixture of the metals Cr(VI), Zn(II), and Ni(II) (5 mg (L-1)), the PCE removal efficiency decreased from 100% to 90% in columns operated without heavy metals. The PCE degradation efficiency remained above 99% in columns receiving heavy metals as long as Ni was present. The findings of this study indicate the feasibility and limitations of the combined treatment of mixtures of organic and inorganic pollutants by ZVI.     

Kinetics of soluble chromium removal from contaminated water by zerovalent iron media: corrosion inhibition and passive oxide effects

Permeable reactive barriers containing zerovalent iron are being increasingly employed for in situ remediation of groundwater contaminated with redox active metals and chlorinated organic compounds. This research investigated the effect of chromate concentration on its removal from solution by zerovalent iron. Removal rates of aqueous Cr(VI) by iron wires were measured in batch experiments for initial chromium concentrations ranging from 100 to 10 000 microg/L. Chromate removal was also measured in columns packed with zerovalent iron filings over this same concentration range. Electrochemical measurements were made to determine the free corrosion potential and corrosion rate of the iron reactants. In both the batch and column reactors, absolute rates of chromium removal declined with increasing chromate concentration. Corrosion current measurements indicated that the rate of iron corrosion decreased with increasing Cr(VI) concentrations between 0 and 5000 microg/L. At a Cr(VI) concentration of 10 000 microg/L, Tafel polarization diagrams showed that chromium removal was affected by its diffusion rate through a passivating oxide film and by the ability of iron to release Fe2+ at anodic sites. In contrast, water reduction was not mass transfer limited, but chromium did decrease the exchange current for the hydrogen evolution reaction. Even at the most passivating concentration of 10 000 microng/L, effluent Cr(VI) concentrations in the column reactors reached a steady state, indicating that passivation had also reached a steady state. Although chromate contributes to iron surface passivation, the removal rates are still sufficiently fast for in situ iron barriers to be effective for Cr(VI) removal at most environmentally relevant concentrations.     

Demonstration of combined zero-valent iron and electrical resistance heating for in situ trichloroethene remediation

The effectiveness of in situ treatment using zero-valent iron (ZVI) for nonaqueous phase or significant sediment-associated contaminant mass can be limited by relatively low rates of mass transfer to bring contaminants in contact with the reactive media. For a field test in a trichloroethene (TCE) source area, combining moderate-temperature subsurface electrical resistance heating with in situ ZVI treatment was shown to accelerate TCE treatment by a factor of about 4 based on organic daughter products and a factor about 8 based on chloride concentrations. A mass-discharge-based analysis was used to evaluate reaction, dissolution, and volatilization processes at ambient groundwater temperature (~10 °C) and as temperature was increased up to about 50 °C. Increased reaction and contaminant dissolution were observed with increased temperature, but vapor- or aqueous-phase migration of TCE out of the treatment zone was minimal during the test because reactions maintained low aqueous-phase TCE concentrations.     

Oxygen and superoxide-mediated redox kinetics of iron complexed by humic substances in coastal seawater

Complexes with terrestrially derived humic substances represent one of the most reactive pools of dissolved Fe in natural waters. In this work, redox kinetics of Fe-humic substance complexes (FeL) in simulated coastal seawater were investigated using chemiluminescence techniques with particular attention given to interactions with dioxygen (O2) and superoxide (O2?-). Although rate constants of FeIIL oxidation by O2 (5.6-52 M-1 · s-1) were 4-5 orders of magnitude less than those for O2?- (6.9-23 × 105 M-1 · s-1),O2 is likely to outcompete O2?- for FeIIL oxidation in coastal seawaters where steady-state O2?- concentrations are generally subnanomolar. Rate constants for FeIIIL reduction by O2?- of 1.8-5.6 × 104 M-1 · s-1 were also determined. From the balance of FeIIL oxidation rates and O2?- -mediated FeIIIL reduction rates, steady-state FeIIL concentrations were estimated to be in the subpicomolar to picomolar range, which is generally lower than measured in situ Fe(II) concentrations under relevant conditions. This suggests that (i) processes other than O2?- -mediated reduction (such as photochemical ligand-to-metal charge transfer) may be responsible for Fe(II) formation, (ii) the in situ ligands differ significantly from the humic substances used in this work, and/ or (iii) the influence of other environmental factors such as pH and temperature on Fe redox kinetics may have to be considered.     

Colloid-borne americium migration in Gorleben groundwater: significance of iron secondary phase transformation

The mobility of actinides in natural water may be enhanced by colloid-mediated transport. In this context the reversibility of actinide colloid interaction is a key factor. Iron is an element that can generate colloids under conditions found in natural waters. In this paper, the impact of hematite and the low-crystalline precursor 2-line ferrihydrite on colloid-mediated transport of americium(III) is investigated. Am(III)-containing iron colloids are generated from two different approaches, namely contact between the two in aqueous solution or coprecipitation of Am(III) during iron colloid generation. Dissolved organic carbon (DOC), especially humic substances, has a strong influence on the stability of inorganic colloids. In addition, humic substances interfere in the distribution and kinetics of exchange between groundwater and sediments. Four groundwaters from the Gorleben aquifer system are used with DOC concentrations varying between 0.9 and 81.6 mgC/L together with Pleistocene Aeolian quartz sand from this site. Batch and column experiments are conducted under near-natural conditions (Ar + 1% CO2). To study the influence of kinetics, contact times up to one month are studied. The dynamic light-scattering investigations show that the colloidal stability of the 2-line ferrihydrite increases with increasing DOC concentration. The low-crystalline iron colloids have a marginal influence on the Am(III) transport due to reversibility of americium sorption. Contrary to this, the crystalline hematite generated from coprecipitation of Am(III) leads to an increase of unretarded colloid-mediated Am(III) transport up to a factor of almost five. Chemical characterization of these hematite colloids shows that Am(III) is structurally entrapped in the hematite. The distribution of Am(III) and 2-line ferrihydrite between groundwater and sand sediment remained in disequilibrium even after one month. This shows that the kinetics of Am(III) distribution between the different phases (bulk solution/colloidal form/ sediment) is a key issue.     

Dehalogenation of polybrominated diphenyl ethers and polychlorinated biphenyl by bimetallic, impregnated, and nanoscale zerovalent iron

Nanoscale zerovalent iron particles (nZVI), bimetallic nanoparticles (nZVI/Pd), and nZVI/Pd impregnated activated carbon (nZVI/Pd-AC) composite particles were synthesized and investigated for their effectiveness to remove polybrominated diphenyl ethers (PBDEs) and/or polychlorinated biphenyls (PCBs). Palladization of nZVI promoted the dehalogenation kinetics for mono- to tri-BDEs and 2,3,4-trichlorobiphenyl (PCB 21). Compared to nZVI, the iron-normalized rate constants for nZVI/Pd were about 2-, 3-, and 4-orders of magnitude greater for tri-, di-, and mono-BDEs, respectively, with diphenyl ether as a main reaction product. The reaction kinetics and pathways suggest an H-atom transfer mechanism. The reaction pathways with nZVI/Pd favor preferential removal of para-halogens on PBDEs and PCBs. X-ray fluorescence mapping of nZVI/Pd-AC showed that Pd mainly deposits on the outer part of particles, while Fe was present throughout the activated carbon particles. While BDE 21 was sorbed onto activated carbon composites quickly, debromination was slower compared to reaction with freely dispersed nZVI/Pd. Our XPS and chemical data suggest about 7% of the total iron within the activated carbon was zerovalent, which shows the difficulty with in-situ synthesis of a significant fraction of zerovalent iron in the microporous material. Related factors that likely hinder the reaction with nZVI/Pd-AC are the heterogeneous distribution of nZVI and Pd on activated carbon and/or immobilization of hydrophobic organic contaminants at the adsorption sites thereby inhibiting contact with nZVI.     

Nitrate reduction by zerovalent iron: effects of formate, oxalate, citrate, chloride, sulfate, borate, and phosphate

Recent studies have shown that zerovalent iron (Fe0) may potentially be used as a chemical medium in permeable reactive barriers (PRBs) for groundwater nitrate remediation; however, the effects of commonly found organic and inorganic ligands in soil and sediments on nitrate reduction by Fe0 have not been well understood. A 25.0 mL nitrate solution of 20.0 mg of N L(-1) (1.43 mM nitrate) was reacted with 1.00 g of Peerless Fe0 at 200 rpm on a rotational shaker at 23 degrees C for up to 120 h in the presence of each of the organic acids (3.0 mM formic, 1.5 mM oxalic, and 1.0 mM citric acids) and inorganic acids (3.0 mM HCl, 1.5 mM H2SO4, 3.0 mM H3BO3, and 1.5 mM H3PO4). These acids provided an initial dissociable H+ concentration of 3.0 mM available for nitrate reduction reactions under conditions of final pH < 9.3. Nitrate reduction rates (pseudo-first-order) increased in the order: H3PO4 < citric acid < H3BO3 < oxalic acid < H2SO4 < formic acid < HCl, ranging from 0.00278 to 0.0913 h(-1), corresponding to surface area normalized rates ranging from 0.126 to 4.15 h(-1) m(-2) mL. Correlation analysis showed a negative linear relationship between the nitrate reduction rates for the ligands and the conditional stability constants for the soluble complexes of the ligands with Fe2+ (R2 = 0.701) or Fe3+ (R2 = 0.918) ions. This sequence of reactivity corresponds also to surface adsorption and complexation of the three organic ligands to iron oxides, which increase in the order formate < oxalate < citrate. The results are also consistent with the sequence of strength of surface complexation of the inorganic ligands to iron oxides, which increases in the order: chloride < sulfate < borate < phosphate. The blockage of reactive sites on the surface of Fe0 and its corrosion products by specific adsorption of the inner-sphere complex forming ligands (oxalate, citrate, sulfate, borate, and phosphate) may be responsible for the decreased nitrate reduction by Fe0 relative to the chloride system.     

Role of humic acid and ouinone model compounds in bromate reduction by zerovalent iron

Experiments were conducted to examine the role of humic acid and quinone model compounds in bromate reduction by Fe(0). The reactivity of Fe(0) toward bromate declined by a factor of 1.3-2.0 in the presence of humic acid. Evidence was obtained that the quick complexation of humic acid with iron species and its adsorption passivated the iron surface and decreased the rate of bromate reduction by Fe(0). On the other hand, in the long run, the reduced functional groups present in humic acid were observed to regenerate Fe(II) and reduce bromate abiotically. Compared with the case of humic acid only, the simultaneous presence of Fe(II) and humic acid significantly increased the bromate removal rate. Fe(III)/Fe(II) acted as a catalyst in the oxidation of humic acid by bromate. Anthraquinone-2,6-disulfonate (AQDS) and lawsone did not cause any significant effect on the bromate reduction rate by Fe(0). However, the redox reactivity of lawsone in the presence of Fe(III) was evident, while AQDS did not show any under the tested conditions. The difference was attributable to the presence/ absence of reducing functional groups in the model compounds. The electron spin resonance further demonstrated that the redox functional groups in humic acid are most likely quinone-phenol moieties. Although the bromate reduction rate by regenerated Fe(II) is a few times slower than that by Fe(0), the reactive Fe(II) can be, alternatively, reductively formed to maintain iron surface activation and bromate reduction to prolong the lifetime of the zerovalent iron.     

"Forced mass balance" technique for estimating in situ transformation rates of sorbing solutes in groundwater

A method for estimating in situ transformation rates of sorbing solutes in groundwater is presented. The method utilizes a novel data processing technique called "forced mass balance" (FMB) to remove the effects of transport processes from reactant and product concentrations measured during single-well, "push-pull" tests. The effectiveness of the FMB technique was evaluated by quantifying errors in derived rates obtained by applying FMB to simulated push-pull test data generated by a numerical model. Results from simulated tests indicated that errors in derived rates increase as the test duration, groundwater velocity, and ratio of reactant to product retardation factors increase. In addition, errors in derived rates increase as the reaction rate constant and aquifer dispersivity decrease. As a demonstration, the FMB technique was used to derive an in situ reductive dechlorination rate for trichlorofluoroethene (TCFE) using data from a field push-pull test. Error analyses indicated that the in situ TCFE transformation rate was underestimated by a factor of 1.1-2. Thus, the FMB technique makes it possible to estimate in situ transformation rates of sorbing solutes and when FMB is coupled with computer modeling to estimate errors in derived in situ rates.