Since the beginning of the Iron Age, the discovery of iron has proved a boon to the human race through the role it played in the evolution of various civilizations [1]. The twentieth century has witnessed the enormous applications of steel, an alloy of iron, as the hardest material used for the development of large infrastructures. Similarly, iron-based salts like FeSO₄ and FeCl₃ are being used in primary treatment like coagulation and flocculation of wastewater [2]. Iron is characterized by unique redox properties which could be exploited under ambient atmospheric conditions [3]. At the same time, the emergence of nanotechnology explored the various prospects of nanoscaled iron particles and rendered the nanoscale zerovalent iron (nZVI) the most effective properties such as high reactivity, better mobility than microscale zerovalent iron (mZVI) particles, intrinsic magnetic interactions, good adsorption capacities, low toxicity, and cost to act as a versatile engineered nanomaterial for environmental remediation [4]. The high reactivity of nZVI is attributed to inherent strong reducing tendencies of Fe(0) and is proficiently exploited for its reaction with a number of inorganic and organic substrates such as heavy metal ions, dyes, drugs, halogenated hydrocarbons, and reduction of organic compounds [5]. In recent years, nZVI has been progressively utilized in groundwater remediation and hazardous waste treatment. However, the applications of bare nZVI in catalytic processes are retarded by the surface passivation of nZVI on contact with air/moisture and/or by aggregation of nZVI [6]. The use of immobilizers as support and to act as stabilizing agents increases the catalytic efficiencies of nZVI [7, 8]. Thus, this chapter focuses on the mechanism and applications of nZVI in development of efficient nanocomposite materials as heterogeneous catalysts to perform the Fenton process for the environmental remediation of wastewaters containing complex organic materials.

The reactivity and applicability of nZVI as an efficient environment remediating agent depends on its size, capping material, surface oxide layer, support material, and so on. Therefore, the process employed for manufacturing of nZVI plays a significant role in deciding the efficiency of the nanocatalyst formed [1]. Literature supports the higher reactivity of bare nZVI (10–10³ times) than granular ZVI, owing to its high surface energy and magnetic properties [9]. A thin layer of oxide is formed on its surface when in contact with air and moisture. A mixed Fe(0)/Fe(II)/Fe(III) phase appeared on the surface when nZVI came in contact with water with major phase as lepidocrocite, that is, FeOOH [10, 11]. Core–shell particles, with protective oxide layer of appropriate thickness not prohibiting the transfer of electrons from the iron core, are more stable than the bare pyrophoric iron nanoparticles (FeNPs) and thus are found advantageous in practical applicability [1]. Prevention of agglomeration by using support or dispersing agents, capping of nZVI, and improving colloidal properties by using organic polymers may also result in enhanced efficiencies of nZVI [12]. Two general approaches are used for the synthesis of nZVI: top-down approach, that is, reducing the size of bulk iron to nanoscale; or bottom-up approach, that is, building nanoiron from atoms formed from ions or molecules [13].