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Zero Valent Iron - Normalizing and Valueing Surface Area Claims
The application of Zero Valent Iron (ZVI) is a function of true available surface area and reactivity. Varying claims of surface area abound as do reactivity claims.
Synthesis: The synthetic method used to produce the ZVI particles is 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 particles 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, these micro-environments generate elevated pH which frequently leads to a deposit of carbonate on the ZVI surface. Sponge (Precipitated Iron) Geochemical changes such as pH increases and oxygen elimination, occur in water passing across ZVI, however, in the case of sponge iron the effects are magnified due to the microenvironments both on the surface and the claimed surface areas within the particle. These changes can lead to precipitation of secondary minerals onto the reactive surface which can influence the reactivity and permeability of the ZVI system quickly. These redox conditions permits precipitation of secondary minerals from ions typically present in ground water as well as some ground water contaminants. The typically secondary minerals formed on the surface and internally insponge iron are magnetite (Fe3O4), hematite(α-Fe2O3)
Milled Iron, generated mechanically from larger iron particles, generally is available in sizes of 10 microns and upward. As a rule, the surfaces of the milled iron particle have an oxide surface which is rough and uneven. The reactivity of milled irons varies 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 ranging from sub-micron to under 10 microns.
Although we see a variety of reactivity claims, each iron product will behave differently based on the type of iron and more importantly, the geochemistry. In carbonate rich waters or waters with sulfates the efficacy of the porous sponge irons is greatly reduced. In nearly all applications a spherical particle is preferred. That said the reactivity of ZVI is not easily enumerated as seen in the reactivity caculation for PCE. Specifically, the competition for reactive sites for a particular compound is significant in the determination of reactivity.
rPCE = (-k3 -k1) X K p X [PCE]/(1 Kt x [TCE] + Kd x transDCE] + K dc x [Dichloroacetylene]
Iron type, geochemistry, competition for reaction sites, particle size, distribution and delivery process all play a role in the design and implimentation of a ZVI based process.