24 Nanoscale zerovalent iron (NZVI) is one of the most extensively studied nanomaterials in 25 the fields of wastewater treatment and remediation of soil and groundwater. However, rapid 26 A c c e p t e d m a n u s c r i t p t 3 65 leading to initiation of the Fenton reaction (Fe(II) + H 2 O 2 → Fe(III) + HO * + OH -) and the 66 generation of strongly oxidizing hydroxyl radicals (HO * , 2.80 V). 67 Typical NZVI particles have a core-shell (Fe(0)-Fe (oxyhydr)oxide) structure with the 68

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... nanosized primary particles forming chain-like aggregates with sizes ranging up to a 69 few micrometers. 3 Moreover, the aggregates exhibit the magnetic properties of the primary 70 particles with the magnetism of the primary particles influencing the size and structure of 71 these assemblages. 32 The co-existence of the Fe(0) core and oxidized Fe surface layer 72 provides a unique reactive surface for the initial adsorption of contaminants and their 73 A c c e p t e d m a n u s c r i t p t 4 subsequent transformation on the particle surface via reductive or oxidative 74 pathways. 3,5,14,16,29,30 In accordance with contaminant adsorption, oxidation or reduction, Fe 75 may undergo oxidation (from Fe(0) to Fe(II) and Fe(II) to Fe(III)), precipitation (as Fe(II) 76 and/or Fe(III) solids) and, possibly, co-precipitation with ionic species. 8,33-35 Concomitantly, 77 it may also be necessary to consider reactions with other solution phase entities such as O 2, 78 (which, as noted above, may be reduced to H 2 O 2 , possibly due to adsorption to NZVI or its 79 (oxyhydr)oxide-coated surface), 28,29 sulfate (which may possibly be reduced to sulfide, 80 resulting in the formation of FeS solid phases on the NZVI surface) 35 and carbonate (which 81 may also promote the precipitation of ferrous carbonate minerals). 36 82 Many researchers have demonstrated that NZVI undergoes surface oxidation (or surface 83 passivation) when used for the removal of contaminants in variably oxic environments. 3,6,8,29 84 In addition, a field study has shown that 78-97% of Fe (0) injected was oxidized in a sediment 85 sample after 165 d at a remedial site contaminated with a dense non-aqueous phase liquid 86 (DNAPL). 37 Investigations have revealed that, depending on the environmental conditions 87 and types of contaminants present, NZVI surface layers (and eventually, the bulk of the solid) 88 transform to different Fe mineral phases, such as vivianite (Fe II 3 (PO 4 ) 2 ·8H 2 O), Fe(OH) 2 , 89 green rust (Fe(II)-Fe(III) layered double hydroxides with various interlayer anions (e.g. Cl -, 90 SO 4 2and CO 3 2-)), ferrihydrite, magnetite (Fe II 1 III 2 O 4 ), lepidocrocite (γ-Fe III OOH), goethite (α-91 Fe III OOH), mackinawite (Fe II S), and siderite (Fe II CO 3 ), as presented in Table 1. Moreover, 92 NZVI can be completely transformed to lepidocrocite in oxygenated water 38 and partially 93 transformed to Fe(OH) 2 in O 2 -free water in the absence of contaminants. 39 The passivation 94 kinetics and the nature of the products generated have a significant influence on the long-term 95 viability of the technology given that Fe(II)-containing minerals such as magnetite, 40-43 96 vivianite, 44-47 and green rust 48-50 and surface-bound Fe(II) 51,52 can further remove organic and 97 inorganic contaminants. In addition, Fe(III)-containing minerals (e.g., maghemite (γ-Fe 2 O 3 ), 53