Microscale Zero-valent Iron Research Articles

Guar gum coupled microscale ZVI for in situ treatment of CAHs: Continuous-flow column study

A column study was performed under in situ conditions to evaluate to which extend the inactivation of the microscale zerovalent iron (mZVI) by guar gum occurs under continuous flow conditions. Five aquifer containing columns were set up under different conditions. Efficient removal of trichloroethene was observed for the column amended by mZVI. Stabilization of the mZVI with guar gum led to slightly reduced activity. More reduced reactivity was observed in the poisoned column containing guar gum stabilized mZVI. This confirms that soil microorganisms can degrade guar gum and that subsequent removal of the oligosaccharides by the groundwater flow (flushing effect) can reactivate the mZVI. After more than six months of continuous operation the columns were dismantled. DNA-based qPCR analysis revealed that mZVI does not significantly affect the bacterial community, while guar gum stabilized mZVI particles can even induce bacterial growth. Overall, this study suggests that the temporarily decreased mZVI reactivity due to guar gum, has a rather limited impact on the performance of in situ reactive zones. The presence of guar gum slightly reduced the reactivity of iron, but also slowed down the iron corrosion rate which prolongs the life time of reactive zone

Phosphate removal from aqueous solutions by nanoscale zero-valent iron

In this study, nanoscale zero-valent iron (NZVI) was synthesized by conventional liquid-phase chemical reduction methods without a support material and then characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The effect of NZVI particles on phosphate removal from aqueous solutions was examined. The results showed that the phosphate removal efficiency increased from 34.49% to 87.01% as the dosage of nanoscale iron particles increased from 100 to 600 mg L−1 with an initial phosphate concentration of 10 mg L−1, and the phosphate removal efficiency decreased from 72.89% to 51.39% as the initial phosphate concentration increased from 10 to 90 mg L−1, with 400 mg L−1 NZVI. Phosphate removal efficiencies of 99.41% and 95.09% were achieved at pH values of 2 and 4, respectively, with an initial phosphate concentration of 20 mg L−1 and 400 mg L−1 NZVI. The use of NZVI particles synthesized in a carboxymethyl cellulose (CMC)–water solution significantly enhanced phosphate removal from an aqueous solution compared with the use of NZVI synthesized in an ethanol–water solution. NZVI particles achieved 71.34% phosphate removal, which was remarkably higher than that of microscale zero-valent iron (MZVI) particles (16.35%) with 10 mg L−1 of phosphate and 400 mg L−1 iron. Based on the removal mechanism analysis performed in this study, we recommend that phosphate removal be accomplished by simultaneous adsorption and chemical precipitation. The XRD patterns of the NZVI before and after the reactions indicated the formation of crystalline vivianite (Fe3(PO4)2·8H2O) during the procedure.

Chemical and Biological Reduction of PCE by Pneumatic Fracturing and Injection of ZVI in Saprolite

AbstractThis article presents a case study of the source‐area treatment of tetrachloroethene (PCE) in a low‐permeability formation using zero‐valent iron (ZVI). Evidence of the stimulation of biological reduction processes within the treatment zone occurred. Pneumatic fracturing and injection of microscale ZVI slurry in the overburden and weathered bedrock zones was performed at a commercial brownfields redevelopment site in Maryland. A 20,000‐square‐foot source area impacted with PCE at concentrations greater than 15,000 µg/L was treated at depths ranging from 10 to 70 feet bgs. An average ZVI dosage of 0.0024 iron‐to‐soil mass ratio within the overburden zone led to a 75 percent decrease in PCE mass in less than one year. For the weathered bedrock zone, an average 0.0045 iron‐to‐soil mass ratio resulted in a 92 percent decrease in PCE mass during the same period. The reducing environment and hydrogen generated by the ZVI may have stimulated Dehalobacter populations, as evidenced by concentrations up to 104 cells per milliliter measured within the treatment area despite a groundwater pH as high as 9. The biological reductive dechlorination of the chlorinated ethenes explains the temporary increase in trichloroethene and cis‐1,2‐dichloroethene concentrations. © 2013 Wiley Periodicals, Inc.

Impact of carbon, oxygen and sulfur content of microscale zerovalent iron particles on its reactivity towards chlorinated aliphatic hydrocarbons

Zerovalent iron (ZVI) abiotically degrades several chlorinated aliphatic hydrocarbons (CAHs) via reductive dechlorination, which offers perspectives for in situ groundwater remediation applications. The difference in reactivity between ZVI particles is often linked with their specific surface area. However, other parameters may influence the reactivity as well. Earlier, we reported for a set of microscale zerovalent iron (mZVI) particles the disappearance kinetic of different CAHs which were collected under consistent experimental conditions. In the present study, these kinetic data were correlated with the carbon, oxygen and sulfur content of mZVI particles. It was confirmed that not only the specific surface area affects the disappearance kinetic of CAHs, but also the chemical composition of the mZVI particles. The chemical composition, in addition, influences CAHs removal mechanism inducing sorption onto mZVI particles instead of dechlorination. Generally, high disappearance kinetic of CAHs was observed for particles containing less oxygen. A high carbon content, on the other hand, induced nonreactive sorption of the contaminants on the mZVI particles. To obtain efficient remediation of CAHs by mZVI particles, this study suggested that the carbon and oxygen content should not exceed 0.5% and 1% respectively. Finally, the efficiency of the mZVI particles may be improved to some extent by enriching them with sulfur. However, the impact of sulfur content on the reactivity of mZVI particles is less pronounced than that of the carbon and oxygen content.

Adsorption and sequential degradation of polybrominated diphenyl ethers with zerovalent iron

The widely used flame retardants, polybrominated diphenyl ethers (PBDEs), have been regulated owing to their persistence and toxicity. However, the high and increasing accumulation amount of PBDEs in the environment raises a big concern for public safety. In this study, the removal processes of decabromodiphenyl ether (BDE-209) and monobromodiphenyl ether (BDE-3) with microscale zerovalent iron (MZVI) were investigated to get better understandings for the removal mechanism based upon adsorption and degradation. The removal kinetics of both compounds was analyzed and revealed two-step kinetics: a fast removal step at the beginning of the reaction and a follow-up slow removal step. By-products generated during the entire process followed a stepwise sequence. The content of brominated compounds on the surface of MZVI was measured. About 10–20% of BDE-209 and 15–30% of BDE-3 were adsorbed on MZVI. The adsorption of BDE-209 and BDE-3 on MZVI was confirmed through the Fourier transform infrared spectroscopy. Surface adsorption of PBDEs on MZVI dominates the removal mechanism in the beginning and further debromination with MZVI was found. Finally, about 70% of BDE-209 and 60% of BDE-3 was degraded by MZVI within about one month. Our findings provide evidences for understanding the removal mechanism of PBDEs with MZVI and its great longevity on the PBDE degradation, which can facilitate the remediation design.

Reactivity screening of microscale zerovalent irons and iron sulfides towards different CAHs under standardized experimental conditions

A standardized batch test procedure was developed and used to evaluate the reactivity of twelve newly designed microscale zerovalent iron (mZVI) particles and two biogenic iron sulfides towards a mixture of chlorinated aliphatic hydrocarbons (CAHs) and their breakdown products. For comparison, commercially available mZVIs, nanoscale zerovalent irons (nZVIs), iron sulfides (FeS) and granular zerovalent iron were also tested. Reactivity of the particles was based on observed (kobs) and mass normalized (kM) pseudo-first-order degradation rate constants, as well as specific surface area normalized reaction rate constants (kSA). Sorption characteristics of the particles were based on mass balance data. Among the new mZVIs, significant differences in reactivity were observed and the most reactive particles were identified. Based on kM data, nZVI degraded the examined contaminants one to two orders of magnitude faster than the mZVIs. kM values for biogenic iron sulfides were similar to the least reactive mZVIs. On the other hand, comparison of kSA data revealed that the reactivity of some newly designed mZVIs was similar to highly reactive nZVIs, and even up to one order of magnitude higher. kSA values for biogenic iron sulfides were one to two orders of magnitude lower than those reported for reactive mZVIs.

Arsenic Remediation Field Study Using a Sulfate Reduction and Zero‐Valent Iron PRB

AbstractToxic and carcinogenic effects of arsenic in drinking water continue to impact people throughout the world and arsenic remains common in groundwater at cleanup sites and in areas with natural sources. Advances in groundwater remediation are needed to attain the low concentrations that are protective of human health and the environment. In this article, we present the successful use of a permeable reactive barrier (PRB) utilizing sulfate reduction coupled with zero‐valent iron (ZVI) to remediate the leading edge of a dissolved arsenic plume in a wetland area near Tacoma, Washington. A commercially available product (EHC‐M®, Adventus Americas Inc., Freeport, Illinois) that contains ZVI, organic carbon substrate, and sulfate was injected into a reducing, low‐seepage‐velocity aquifer elevated in dissolved arsenic and iron from a nearby, slag‐containing landfill. Removal effectiveness was strongly correlated with sulfate concentration, and was coincident with temporary redox potential (Eh) reductions, consistent with arsenic removal by iron sulfide precipitation. The PRB demonstrates that induced sulfate reduction and ZVI are capable of attaining a regulatory limit of 5 µg/L total arsenic, capturing of 97% of the arsenic entering the PRB, and sustaining decreased arsenic concentrations for approximately 2 years, suggesting that the technology is appropriate for consideration at other sites with similar hydrogeochemical conditions. The results indicate the importance of delivery and longevity of minimum sulfate concentrations and of maintaining sufficient dissolved organic carbon and/or microscale ZVI to precipitate FeS, a precursor phase to arsenic‐bearing pyrite that may provide a stable, long‐term sink for arsenic.

1,4-Dioxane degradation by oxidation and sonication in the presence of different-sized ZVI in open-air system

We investigated the effect of particle size during oxidation and the sono-Fenton process in an open-air system (without air supply) containing microscale zero-valent iron (MZVI) and nanoscale zero-valent iron (nZVI) on the degradation of 1,4-dioxane (1,4-D) at neutral pH. Time-dependent concentrations of 1,4-D, H2O2, and ionic Fe were measured as a function of MZVI and nZVI loading during oxidation and sono-Fenton (without addition of H2O2). The optimal loading of MZVI and nZVI for 1,4-D degradation were determined to be 0.5 g/L during both oxidation and sono-Fenton, and 0.2 and 0.1 g/L during both oxidation and sono-Fenton, respectively. Overall, the optimal loading of MZVI in both the oxidation and sono-Fenton system showed better 1,4-D degradation efficiency compared with the optimal loading of nZVI for 6 h: approximately 60.00 and 76.65% removal efficiencies for MZVI oxidation and sono-Fenton, respectively, while approximately 47.13 and 60.98% removal efficiencies for nZVI oxidation and sono-Fenton, respectively. Consequently, the smaller size of ZVI did not enhance 1,4-D degradation efficiency during the oxidation and sono-Fenton at neutral pH.

Reactivity recovery of guar gum coupled mZVI by means of enzymatic breakdown and rinsing

Microscale zerovalent iron (mZVI) reduces chlorinated aliphatic hydrocarbons (CAHs) to harmless compounds, but the sedimentation of the mZVI particles in the injection fluid limits the injectability of the particles during field applications. In this study, mZVI particles in suspension were stabilized by green polymer guar gum, which had a positive impact on mZVI stability, but decreased the reactivity of the particles towards CAHs by 1 to 8 times. Guar gum (GG) was found to adsorb onto the mZVI surface, inhibiting contact between the chlorinated compounds and the reactive iron surface. Indications were found for intermolecular hydrogen bonding between mZVI and the guar gum. Subsequent addition of commercially available enzymes resulted in the cleavage of the polysaccharide guar gum into lower molecular fragments, but not in improved reactivity. The reactivity recovery of guar gum coupled mZVI was recovered after intensive rinsing of the iron particles, removing the guar gum fragments from the particles. Overall, this study shows that CAHs can be treated efficiently by guar gum stabilized mZVI after reactivation by means of enzymatic breakdown and rinsing.

Synergistic effect of microscale zerovalent iron particles combined with anaerobic sludges on the degradation of decabromodiphenyl ether

Polybrominated diphenyl ethers (PBDEs) are widely used flame retardants. Owing to their toxicity and increasing accumulation in the environment, the fate of PBDEs in nature is of serious concern. The combined effects of microscale zerovalent iron (MZVI) and anaerobic sludge in decabromodiphenyl ether (BDE-209) degradation were investigated. The co-incubation resulted in 63% and 29% enhancement of removal ability when compared to the single component conditions. By-products generated during the entire process followed a stepwise sequence with non-uniform accumulation rates. Microbes hindered the accessibility of MZVI to BDE-209 and reduced the removal ability in the initial stage (<12h). According to the analysis of the microbial community change, co-incubation with MZVI leads to the enrichment of heterotrophic microbial populations bearing nitrate- or iron-reducing activities. The interaction between MZVI and microbes contributed to the synergistic effect. Our findings provide evidence for this synergistic effect and offer an alternative for developing better remediation strategies.

Ligand-assisted degradation of carbon tetrachloride by microscale zero-valent iron

Degradation of carbon tetrachloride (CT) by microscale zero-valent iron (ZVI) was investigated in batch systems with or without organic ligands (ethylenediaminetetraacetic acid (EDTA), citric acid, tartaric acid, malic acid and oxalic acid) at pHs from 3.5 to 7.5. The results demonstrated that at 25°C, the dechlorination of CT by microscale ZVI is slow in the absence of organic ligands, with a pseudo-first-order rate constant of 0.0217h−1 at pH 3.5 and being further dropped to 0.0052h−1 at pH 7.5. However, addition of organic ligands significantly enhanced the rates and the extents of CT removal, as indicated by the rate constant increases of 39, 31, 32, 28 and 18 times in the presence of EDTA, citric acid, tartaric acid, malic acid and oxalic acid, respectively, at pH 3.5 and 25°C. The effect of EDTA was most significant; the dechlorination of CT at an initial concentration of 20mgl−1 increased from 16.3% (no ligands) to 89.1% (with EDTA) at the end of 8h reaction. The enhanced CT degradation in the presence of organic ligands was primarily attributed to the elimination of a surface passivation layer of Fe(III) (hydr)oxides on the microscale ZVI through chelating of organic ligands with Fe(III), which maintained the exposure of active sites on ZVI surface to CT.

Assessing the impact of nano- and micro-scale zerovalent iron particles on soil microbial activities: Particle reactivity interferes with assay conditions and interpretation of genuine microbial effects

The effects of nano-scale and micro-scale zerovalent iron (nZVI and mZVI) particles on general (dehydrogenase and hydrolase) and specific (ammonia oxidation potential, AOP) activities mediated by the microbial community in an uncontaminated soil were examined. nZVI (diameter 12.5nm; 10mgg−1 soil) apparently inhibited AOP and nZVI and mZVI apparently stimulated dehydrogenase activity but had minimal influence on hydrolase activity. Sterile experiments revealed that the apparent inhibition of AOP could not be interpreted as such due to the confounding action of the particles, whereas, the nZVI-enhanced dehydrogenase activity could represent the genuine response of a stimulated microbial population or an artifact of ZVI reactivity. Overall, there was no evidence for negative effects of nZVI or mZVI on the processes studied. When examining the impact of redox active particles such as ZVI on microbial oxidation–reduction reactions, potential confounding effects of the test particles on assay conditions should be considered.

Effects of pH and dissolved oxygen on Cr(VI) removal in Fe(0)/H2O systems

The effects of pH and dissolved oxygen (DO) on aqueous Cr(VI) removal by micro-scale zero-valent iron (Fe(0)/H2O system) were investigated. Batch experiments were conducted at pH 4.0, 5.0 and 6.0 under oxic and anoxic conditions. Column experiments were performed at pH 5.0 and 7.5 under oxic condition. Spectroscopic analyses were applied to explain the mechanism of Cr(VI) removal using X-ray absorption near-edge structure (XANES), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Results showed that the kinetics of Cr(VI) removal were fastest at pH 5 under both oxic and anoxic conditions. As a rule, Cr(VI) removal were faster under oxic conditions than under anoxic conditions. Column experiments showed that Cr(VI) removal was about 1.7-fold higher at pH 5 than at pH 7.5. XANES (X-ray absorption near edge structures) results showed that Fe(0) reduced Cr(VI) to Cr(III) under both oxic and anoxic conditions. X-ray diffraction patterns of the Cr(VI)–Fe(0) reaction products suggested partial formation of chromite (FeCr2O4) at pH 5 and 6 under oxic conditions. However, nano-sized clusters of Cr(III)/Fe(III) hydroxide/oxyhydroxide were formed on the surface of Fe(0) under anoxic conditions. These results indicate that the presence of oxygen in solution plays an important role in control of the kinetic of Cr(VI) removal and in development of various Cr(VI) reduction products.

Reductive Dechlorination of Chloroacetanilide Herbicide (Alachlor) Using Zero-Valent Iron Nanoparticles

Abstract Nanoscale zero-valent iron (NZVI) was used for the first time to reductively degrade the chloroacetanilide herbicide, alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl) acetanilide], in laboratory studies. NZVI was synthesized by the borohydride reduction method and characterized using transmission electron microscopy and Brunauer–Emmett–Teller surface area analysis. Resulting particles had an average diameter of 35 nm and a Brunauer–Emmett–Teller surface area of 25 m2 g−1. Alachlor degradation in the presence of NZVI was rapid and complete. Parallel comparison trials with commercial microscale zero-valent iron (surface area = 1–2 m2/g−1) were also conducted. Surface area-normalized rate constants (kSA) for NZVI and microscale zero-valent iron were found to be 38.5 × 10−5 and 3.8–7.7 × 10−5 L h−1/m−2, respectively. A reaction product was identified as dechlorinated alachlor [N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide]. Based on results obtained in the research, the authors stipulate that N...

Transport in Porous Media of Highly Concentrated Iron Micro- and Nanoparticles in the Presence of Xanthan Gum

The ability of xanthan gum to act as a delivery vehicle for the transport in porous media of highly concentrated nano- and microscale zerovalent iron (NZVI and MZVI, respectively) slurries was investigated. Sand-packed column experiments were performed injecting iron suspensions at a concentration of 20 g/L, amended with xanthan gum (3 g/L), at different ionic strength values (6 x 10(-3) mM or 12.5 mM) in 0.46 m long columns. Breakthrough curves of iron, obtained by in-line continuous measurement of magnetic susceptibility, under each experimental condition showed that normalized elution concentration at the end of the injection (i.e., after 7 or 26 pore volumes) is higher for MZVI (>0.94) than for NZVI (>0.88). Additional susceptibility measurements along the column and pressure drop also confirmed that MZVI is more easily eluted than NZVI. Moreover, water flushing after the iron injection phase lead to recoveries of over 95% for MZVI, and over 92% for NZVI of the total injected iron mass. The tests proved that xanthan gum is an excellent stabilizing agent and delivery vehicle of ZVI particles and has a high potential for use in real scale remediation interventions.

Remediation of Soil Contaminated with Pyrene Using Ground Nanoscale Zero-Valent Iron

The sites contaminated with recalcitrant polycyclic aromatic hydrocarbons (PAHs) are serious environmental problems ubiquitously. Some PAHs have proven to be carcinogenic and hazardous. Therefore, the innovative PAH in situ remediation technologies have to be developed instantaneously. Recently, the nanoscale zero-va-lent iron (ZVI) particles have been successfully applied for dechlorination of organic pollutants in water, yet little research has investigated for the soil remediation so far. The objective in this work was to take advantage of nanoscale ZVI particles to remove PAHs in soil. The experimental factors such as reaction time, particle diameter and iron dosage and surface area were considered and optimized. From the results, both microscale and nanoscale ZVI were capable to remove the target compound. The higher removal efficiencies of nanoscale ZVI particles were obtained because the specific surface areas were about several dozens larger than that of commercially microscale ZVI particles. The optimal parameters were observed as 0.2 g iron/2 mL water in 60 min and 150 rpm by nanoscale ZVI. Additionally, the results proved that nanoscale ZVI particles are a promising technology for soil remediation and are encouraged in the near future environmental applications. Additionally, the empirical equation developed for pyrene removal efficiency provided the good explanation of reaction behavior. Ultimately, the calculated values by this equation were in a good agreement with the experimental data.

Comparison of biotic and abiotic treatment approaches for co-mingled perchlorate, nitrate, and nitramine explosives in groundwater

Biological and abiotic approaches for treating co-mingled perchlorate, nitrate, and nitramine explosives in groundwater were compared in microcosm and column studies. In microcosms, microscale zero-valent iron (mZVI), nanoscale zero-valent iron (nZVI), and nickel catalyzed the reduction of RDX and HMX from initial concentrations of 9 and 1 mg/L, respectively, to below detection (0.02 mg/L), within 2 h. The mZVI and nZVI also degraded nitrate (3 mg/L) to below 0.4 mg/L, but none of the metal catalysts were observed to appreciably reduce perchlorate (∼ 5 mg/L) in microcosms. Perchlorate losses were observed after approximately 2 months in columns of aquifer solids treated with mZVI, but this decline appears to be the result of biodegradation rather than abiotic reduction. An emulsified vegetable oil substrate was observed to effectively promote the biological reduction of nitrate, RDX and perchlorate in microcosms, and all four target contaminants in the flow-through columns. Nitrate and perchlorate were biodegraded most rapidly, followed by RDX and then HMX, although the rates of biological reduction for the nitramine explosives were appreciably slower than observed for mZVI or nickel. A model was developed to compare contaminant degradation mechanisms and rates between the biotic and abiotic treatments.

Chemical reduction of nitrate by nanosized iron: kinetics and pathways

This study was conducted to investigate chemical reduction of nitrate by nanoscale zero-valent iron (ZVI) in aqueous solution and related kinetics and pathways. In the last decade, employment of micro-scale ZVI has gained its popularity in nitrate reduction. To further study chemical reduction of nitrate, nanosized iron was synthesized and tested in this work. It has a size in the range of 50-80 nm and a BET surface area of 37.83 m(2)g(-1). Chemical reduction of nitrate by nanosized iron under various pHs was carried out in batch experiments. Experimental results suggest that nitrate reduction by nanosized ZVI primarily is an acid-driven surface-mediated process. A stronger acidic condition is more favorable for nitrate reduction. Results of the kinetics study have indicated that a higher initial concentration of nitrate would yield a greater reaction rate constant. Additional test results also showed that the reduction rate of nitrate increased as the dose of nanosized ZVI increased. In all tests, reaction rate equations developed do not obey the first- or pseudo-first-order reaction kinetics with respect to the nitrate concentration. Based on the research findings obtained, two possible reaction pathways for nitrate reduction by nanoscale iron particles have been proposed in this work.

A new method to produce nanoscale iron for nitrate removal

This article proposes a novel technology combining electrochemical and ultrasonic methods to produce nanoscale zero valent iron (NZVI). With platinum placed in the cathode and the presence of the dispersion agent, 0.2g/l cetylpyridinium chloride (CPC), a cation surfactant, in the solution, the nanoscale iron particle was successfully produced with diameter of 1–20 nm and specific surface area of 25.4m2/g. The produced NZVI was tested in batch experiments for nitrate removal. The results showed that the nitrate reduction was affected by pH. Al low pH, nitrate was shown faster decline and more reduction in term of g NO 3 − –N/g NZVI. The reaction was first order and kinetic coefficients for the four pHs were directly related to pH with R 2 >0.95. Comparing with microscale zero-valent iron (45μm, 0.183m2/g), microscale zero-valent iron converted nitrate to ammonia completely, but NZVI converted nitrate to ammonia partially from 36.2 to 45.3% dependent on pH. For mass balance of iron species, since the dissolved iron in the solution was very low (<1mg/l), Electron Spectroscopy for Chemical Analysis (ESCA) was used for identification of oxidation state of the surface species on the NZVI and Fe2O3 was recognized. Thus the reaction mechanisms can be determined.

Congener-Specific Dechlorination of Dissolved PCBs by Microscale and Nanoscale Zerovalent Iron in a Water/Methanol Solution

Polychlorinated biphenyl (PCB)-contaminated sediments remain a significantthreatto humans and aquatic ecosystems. Dredging and disposal is costly, so viable in situ technologies to dechlorinate PCBs are needed. This study demonstrates that nanoscale zerovalent iron (ZVI) dechlorinates PCBs to lower-chlorinated products under ambient conditions, provides insight into structure-activity relationships between PCB isomers, and compares the reactivity of nanoscale ZVI to that of palladized microscale ZVI. Six PCB congeners were studied (22', 34', 234, 22'35', 22'45', and 33'44') to compare the initial rate of dechlorination of each and to monitor the order in which chlorines are removed. Using 200 g/L of nanoscale ZVI in a 30% MeOH/water mixture, observed surface-area-normalized pseudo-first-order PCB dechlorination rate constants ranged from 1 x 10(-6) to 5.5 x 10(-4) L yr(-1) m(-2) depending on the PCB congener tested. Using 200 g/L of palladized (0.05 wt %) microscale ZVI, surface-area-normalized pseudo-first-order PCB dechlorination rate constants were significantly faster and ranged from 3.8 x 10(-2) to 1.7 x 10(-1) L yr(-1) m(-2), but these rates were not sustainable. For nanoscale ZVI, nonorthosubstituted congeners had faster initial dechlorination rates than orthosubstituted congeners in the same homologue group. Chlorines in the para and meta position were predominantly removed over chlorines in the ortho position, which suggests that more-toxic coplanar PCB congeners are not likely to form from less-toxic noncoplanar, orthosubstituted congeners. Complete dechlorination was not observed over the course of the experiments. PCB dechlorination is rapid enough that nanoscale ZVI may offer novel in situ remedial alternatives for PCB-contaminated sediments.