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Nanoscale, zero-valent iron is a promising reagent for in situ reduction of a variety of subsurface contaminants, but its utility in full-scale remediation projects is limited by material costs. Iron nanoparticles (20-100 nm diameter) supported on carbon (C-Fe0) were synthesized by reacting iron salts, adsorbed or impregnated from aqueous solutions onto 80 m²/g carbon black, at 600-800 °C under Ar. Similar products were obtained by heating the reactants under air in a covered alumina crucible. X-ray powder diffraction patterns show that Fe3O4 particles are formed at 300-500 °C in the initial stage of the reaction and that these particles are reduced to a mixture of α and γ-Fe nanoparticles above 600 °C. When C-Fe0 was combined with carboxymethylcellulose in a 5:1 weight ratio in water, the resulting material had similar transport properties to previously optimized nanoiron/polyanion suspensions in water-saturated sand columns. At a 10:3 Fe/Cr mole ratio, C-Fe0 reduced a 10 ppm Cr(VI) solution to ∼1 ppm within three days. The surface area normalized first-order Cr removal rate was 1.2 h-1 m-2 under these conditions. These results demonstrate that reactive nanoiron with good transport properties in water-saturated porous media can be made in a scalable process from inexpensive starting materials by carbothermal reduction. [ABSTRACT FROM AUTHOR]

Sand-packed columns were used to study the transport of micro- and nanoiron particle suspensions modified with anionic polyelectrolytes. With microscale carbonyl iron powder (CIP), the profiles of initial and eluted particle diameters were compared with simulations based on classical filtration theory (CFT), using both the Tufenkji-Elimelech (TE) and Rajagopalan-Tien (RT) models. With particle size distributions that peaked in the submicron range, there was reasonable agreement between both models and the eluted distributions. With distributions that peaked in the 1.5 µm range, however, the eluted distributions were narrower and shifted to a smaller particle size than predicted by CFT. Apparent sticking coefficients depended on column length and flow rate, and the profile of retained iron in the columns did not follow the log-linear form expected from CFT. These observations could be rationalized in terms of the secondary energy minimum model recently proposed by Tufenkji and Elimelech (Langmuir 2005, 21,841). For microiron, sticking coefficients correlated well with particle zeta potentials and polyacrylate (PAA) concentration. With nanoscale iron particles, there was no apparent correlation between filtration length and total electrolyte concentration. However, mixtures of PAA with poly(4-styrenesulfonate) and bentonite clay significantly enhanced nanoiron transport, possibly by affecting the aggregation of the particles. [ABSTRACT FROM AUTHOR]

A five-year site investigation was conducted at a former chemical plant in Nanjing, China. The main contaminants were 1,2,4-trichlorobenzene (TCB) reaching concentrations up to 7300 μg/L, dichlorobenzene (DCB) isomers, monochlorobenzene (MCB), and benzene. Over time, these contaminants naturally attenuated to below regulatory levels under anaerobic conditions. To confirm the transformation processes and to explore the mechanisms, a corresponding laboratory microcosm study was completed demonstrating that 1,2,4-TCB was dechlorinated to 1,2-DCB, 1,3-DCB, and 1,4-DCB in approximately 2%/10%/88% molar proportions. The DCB isomers were dechlorinated via MCB to benzene, and, finally, benzene was degraded under prevailing sulfate-reducing conditions. Dechlorination could not be attributed to known dechlorinators Dehalobacteror Dehalococcoides, while anaerobic benzene degradation was mediated by microbes affiliated to a Deltaproteobacterium ORM2, previously associated with this activity. Unidentified organic compounds, possibly aromatic compounds related to past on-site production processes, were fueling the dechlorination reactions in situ. The microcosm study confirmed transformation processes inferred from field data and provided needed assurance for natural attenuation. Activity-based microcosm studies are often omitted from site characterization in favor of rapid and less expensive molecular surveys. However, the value of microcosm studies for confirming transformation processes, establishing electron balances, assessing cocontaminant inhibition, and validating appropriate monitoring tools is clear. At complex sites impacted by multiple compounds with poorly characterized transformation mechanisms, activity assays provide valuable data to incorporate into the conceptual site model to most effectively inform remediation alternatives [ABSTRACT FROM AUTHOR]