Two different approaches can be used to produce nanomaterials, e.g.
nZVI: top-down and bottom-upmethods. The former consists of the reduction
of the iron particle size through mechanical and/or chemical
processes, and includes milling, etching, and/or machining; while the
latter promotes the growth of the particles through chemical reactions,
positional and self-assembling, among others (Li et al., 2006).