October 21st, 2013 --
Particle size: The particle size of ZVI is the primary and the most important aspects in choosing a type of ZVI. The reductive dechlorination of chlorinated compounds is a surface area mediated reaction, meaning that in theory, the greater the surface area, the more reactive a ZVI particle is. For example, granular iron is less reactive than micro-iron powder which is less reactive than nano-iron particles. While reactivity is extremely important, having a particle that is too reactive can be a negative side effect because ZVI reacts with oxygen in air and water and can be expended before the particles interact with contamination. As reductive dechlorination is a surface area mediated reaction, the reaction only takes place when the ZVI is in contact with the contaminant of concern. A balance between reactive particles (ie, micron, sub-micron and nano-iron) and fairly less unreactive particles is necessary for in-situ remediation.
Particle motility: When particles are larger than the pore space in the subsurface, the ZVI will be intercepted and unable to move. These larger particle sizes are useful for permeable reactive barriers. When particles are much smaller than the pore space in the subsurface, the ZVI particles are affected by Brownian motion which means that particles interact with the soil and are more likely to get stuck in the soil. In order to avoid the interaction of soil and the ZVI particle, as well as to stop agglomeration of the particles, a dispersing agent such as a polymer, is necessary to limit particle-particle interactions as well as particle-soil interaction. Particles that are large enough to avoid Brownian motion, but small enough to not be intercepted in the pore space are optimal for in-situ reductive dechlorination. Low micron ZVI, 1-5 micron particle size, are optimal to achieve this result.
Synthesis: The synthetic method used to produce the ZVI particles is also crucial for cost, reactivity, and the ability of the particle to transport through the subsurface. The chemical and physical synthesis of ZVI particles will impart differing reactivity, particle shape, and impurities in the ZVI particles which can have both positive and negative effects on remediation properties. Nano-Iron synthesis is usually expensive and difficult to scale up due to the inherent reactivity of the particles as well as the cost and quality of the reagents necessary to create the small particle size. The effective in-situ size of these particle is significantly larger than the specified size due to brownian motion and the resulting agglomeration. Precipitated Iron, precipitation, while increasing the surface area of the ZVI particle, can create porosity in the particle that create micro-environments that trap water and blind the surface to contaminant reduction that can reduce the effective reactivity of the ZVI particles. Further, precipitated iron, in these micro-environments generate elevated pH which frequently leads to a deposit of carbonate on the ZVI surface. Milled Iron, generated mechanically from larger iron particles generally is available to sizes between 10 microns upwards as a rule the surfaces have an oxide surface which is rough and uneven. The reactivity of milled irons vary greatly depending on the stock and the milling mechanism (CAS of Cast Iron Powder - 7439-89-6). Carbonyl-Iron (CAS 13463-40-6), generated chemically from a solution of ferrous salts presents itself under an electron microscope much like an onion, spherical and smooth. A more reactive material, generally available in sizes ganging from sub-micron to under 10 microns.
Cost Normalization: To evaluate the right iron product for your project the normalized cost should be considered ($/square meter of surface area). The most inexpensive product by the pound is frequently not the most inexpensive normalized product or the correct material for optimal results (see table)