Groundwater water remediation by static diffusion usinG nano-zero valent metals ( fe 0 , cu 0 , al 0 )

Zero valent metals (ZVM) and ZVM combinations, including magnetic ZVM [1–4] and reduced ZVM [5–7], are highly efficient water treatment agents [8–12]. They will remove ions, chemicals, compounds, and biota from water. The principal ZVM used in pilot and commercial water treatment are Fe and Fe + Cu [13–17]. Prices for ZVM powders are a function of commodity prices, particle size, particle shape (e.g., dendritic, spherical, platy, irregular, etc), manufacturing method, source, and quantity ordered. Current (June 2014) prices (k$ FOB t) for powders are (i) 13,000–400,000 nm (Fe = 0.6–1.6; Cu = 1–7; Al = 5–20), (ii) <3,000 to >8,000 nm (Fe = 5–15; Cu = 9–18; Al = 9–850), (iii) 10–1000 nm (Fe = 30–1100; Cu = 9–1000; Al = 9–900). n-ZVM powders are either used to treat water in a reactor [10], or are injected into an aquifer [10, 17], or are placed in a permeable reactive barrier (PRB) within the aquifer [10, 18]. Use of n-ZVM in a fixed (packed) bed reactor (where all the feed water flows through the n-ZVM) is impractical, as n-Fe rapidly corrodes and expands to form hydrated, low-density hydroxides (Fe(OH) 2 , Fe(OH) 3 ) and peroxides (FeOOH). This results [10] in a decrease in porosity, decrease in permeability, increase in the proportion of dead end pores, and a decrease in pore throat size. Associated gas bubble formation (O 2 , H 2 [10]) results in permeability reduction [18, 19] due to gas occlusion switching water flow from viscous flow to Knudsen diffusion [20–22]. The net effect is a major reduction in permeability (and water flow rate) over a short time period (Fig. 1.1a) in the reactor, or aquifer [10]. Fluid flow (Q f , m m s) = k p D f [20–22]. A list of abbreviations is provided in Appendix 1.A. The dominant fluid flow mechanism switches over a period of 2–6 weeks from viscous flow to Knudsen diffusion due to the generation and presence of trapped nano/micron-sized gas bubbles [10, 20–22]. Changing the reactor type to a diabatic diffusion reactor (where a body of water overlies a static body of ZVM, and all water enters and leaves the reactor through the water body), mimics the situation that occurs in an aquifer, during remediation, following n-ZVM injection. A policy of groundwater abstraction, treatment, and reinjection allows the remediation to be undertaken in a short time period in a controlled environment without creating long-term damage to the aquifer [10]. In an unconfined, diabatic, diffusion environment (e.g., shallow contaminated aquifer or soil), the principal controls on Eh, pH, EC (electrical conductivity), and remediation are [10]: (i) flowing water space velocity (SV = Q fr /W zvm ); (ii) the stored water to ZVM ratio, [SWZ = S w /W zvm ]; (iii) the water composition; (iv) atmospheric/groundwater temperature fluctuations; (v) atmosphere composition variation (principally humidity); (vi) atmospheric pressure fluctuation; (vii) vertical infiltration recharge (associated with storm events) into the aquifer/soil; (viii) water losses from the aquifer/soil, due to evaporation, leaching, and the interaction of the n-ZVM (and n-ZVM products) with minerals and biota; and (ix) porosity occlusion resulting from the movement of displaced air as the water levels rise and fall (during and following infiltration recharge) [20–23].