Anionic, hydrophilic carbon (Fe/C) and poly(acrylic acid)-supported (Fe/PAA) zerovalent iron nanoparticles were studied as a reactive material for the dehalogenation of chlorinated hydrocarbons in groundwater and soils. The transport of Fe/C nanoparticles was studied by elution through columns packed with model soils from different regions of the USDA soil textural triangle, and was compared to that of unsupported Fe nanoparticles. The Fe/C and Fe/PAA particles form colloidal suspensions that settle very slowly (hours to days) in water. Their anionic surface charge facilitates transport through soil- and sand-packed columns. Elution lengths from column breakthrough studies were compared with calculations based on the Tufenkji−Elimelech model (Environ Sci. Technol. 2004, 38, 529). It can be concluded from this comparison that nanoparticle diffusion is the dominant filtration mechanism, and that Fe/PAA and Fe/C particles have sticking coefficients on the order of 0.36 and ≤0.07, respectively, in sand and 0.05 and ≤0.01, respectively, in clay-rich Chagrin soil. In contrast, unsupported Fe nanoparticles rapidly agglomerate in water and are efficiently filtered by all of the soils tested, except for the clay-rich soil in which clay platelets may also act as an anionic support material. Trichloroethylene reduction by Pd-catalyzed Fe/C is rapid, and the reaction is unchanged by elution of a suspension of the material through a sand column.

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 degrees 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 degrees C in the initial stage of the reaction and that these particles are reduced to a mixture of alpha- and gamma-Fe nanoparticles above 600 degrees 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 approximately 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.

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 mirom 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-linearform 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.

Regulatory pressure has been increasing globally to address the issue of climate change. In particular, there are plans to reduce the use of hydrofluorocarbon (HFC) based working fluids across many applications, as HFCs are forecast to be significant contributors to global warming in the future. Therefore, there is a need to find low global warming potential (GWP) fluids suitable for organic rankine cycles (ORCs) in those systems where HFCs have historically been preferred. These are usually systems that require a non-flammable working fluid. A new ORC working fluid, cis-1,1,1,4,4,4-hexafluoro-2-butene, also called DR-2 (cis-CF3CH=CHCF3) has been developed which is nonflammable with very low GWP of 8.9 and an ozone depletion potential (ODP) of zero because it contains no chlorine or other halogen atoms other than fluorine. DR-2 also has a favorable toxicity profile based on testing to date. DR-2 is thermally stable in the presence of lubricant and metals, air and oxygen up to the maximum temperature tested of 250°C. DR-2 has a boiling point of 33.4°C and a relatively high critical temperature of 171.3°C, which result in relatively low vapor pressures and high cycle energy efficiencies. It can enable more environmentally sustainable ORC platforms to generate electrical power from widely available heat at higher temperatures and with higher energy efficiencies than incumbent working fluids.