nanomaterials Review Green Synthesis of Iron Nanoparticles and Their Environmental Applications and Implications Sadia Saif 1,2,*, Arifa Tahir 1 and Yongsheng Chen 2,* 1 Department of Environmental Science, Lahore College for Women University, Lahore 54000, Pakistan; [email protected] 2 School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA * Correspondence: [email protected] (S.S.); [email protected] (Y.C.); Tel.: +92-332-4543310 (S.S.); +1-404-894-3089 (Y.C.) Academic Editor: Thomas Nann Received: 12 August 2016; Accepted: 7 November 2016; Published: 12 November 2016 Abstract: Recent advances in nanoscience and nanotechnology have also led to the development of novel nanomaterials, which ultimately increase potential health and environmental hazards. Interest in developing environmentally benign procedures for the synthesis of metallic nanoparticles has been increased. The purpose is to minimize the negative impacts of synthetic procedures, their accompanying chemicals and derivative compounds. The exploitation of different biomaterials for the synthesis of nanoparticles is considered a valuable approach in green nanotechnology. Biological resources such as bacteria, algae fungi and plants have been used for the production of low-cost, energy-efficient, and nontoxic environmental friendly metallic nanoparticles. This review provides an overview of various reports of green synthesised zero valent metallic iron (ZVMI) and iron oxide (Fe2O3/Fe3O4) nanoparticles (NPs) and highlights their substantial applications in environmental pollution control. This review also summarizes the ecotoxicological impacts of green synthesised iron nanoparticles opposed to non-green synthesised iron nanoparticles. Keywords: iron nanomaterials; sustainable green nanotechnology; environmental pollution; environmental toxicology 1. Introduction Nanotechnology is the ability to measure, see, manipulate and manufacture things on an atomic or molecular scale, usually between one and 100 nanometres. These tiny products also have a large surface area to volume ratio, which is their most important feature responsible for the widespread use of nanomaterials in mechanics, optics, electronics, biotechnology, microbiology, environmental remediation, medicine, numerous engineering fields and material science [1]. Different protocols have been designed for the production of metallic nanoparticles. Currently, two main approaches are used to synthesize nanoparticles, referred to as the top-down and bottom-up approaches. Briefly, in the top-down approach, nanoparticles are produced by size reduction of bulk material by lithographic techniques and by mechanical techniques such as machining and grinding, etc., while, in bottom-up approach, small building blocks are assembled into a larger structure, e.g., chemical synthesis [2]. However, the most acceptable and effective approach for nanoparticle preparation is the bottom-up approach, where a nanoparticle is “grown” from simpler molecules known as reaction precursors. In this way, it is likely possible to control the size and shape of the nanoparticle depending on the subsequent application through variation in precursor concentrations and reaction conditions (temperature, pH, etc.) [3]. Physical and chemical methods are being used extensively for production of metal and metal oxide nanoparticles. However, this production requires the use of very reactive and toxic reducing agents Nanomaterials 2016, 6, 209; doi:10.3390/nano6110209 www.mdpi.com/journal/nanomaterials Nanomaterials 2016, 6, 209 2 of 26 Nanomaterials 2016, 6, 209 2 of 26 Physical and chemical methods are being used extensively for production of metal and metal suchoxide as nanoparticles. sodium borohydride However, and this hydrazine production hydrate, requires which the causeuse of undesired very reactive detrimental and toxic impacts reducing on theagents environment, such as sodium plant andborohydride animal life and it supports.hydrazine Researchershydrate, which continue cause efforts undesired to develop detrimental facile, effectiveimpacts on and the reliable environ greenment, chemistry plant and processes animal life for theit supports. production Researchers of nanomaterials. continue Various efforts to organisms develop actfacile, as clean,effective eco-friendly and reliable and green sustainable chemistry precursors processes to for produce the production the stable ofand nanomaterials. well functionalised Various nanoparticles.organisms act Theseas clean, may eco include-friendly bacteria, and sustainable actinomycetes, precursors fungi, yeast, to produc viruses,e the etc. stable [4,5]. 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The green nanotechnology-based production processes operate underMany green recent conditions studies without have indicated the intervention the potential of toxic of ironchemicals. nanoparticles (NPs) for environmental remediation.Many recent Nanoscale studies materials have indicated such as nanoadsorbents, the potential of nanocatalysts, iron nanoparticles nanofiltration, (NPs) for and environmental nanobiocides suchremediation. as metal andNanoscale metal oxidematerials nanoparticles such as are nanoadsorbents, currently being employednanocatalysts, for remediationnanofiltration, of water and andnanobiocides wastewater such pollutants. as metal Among and metal these oxide metallic nanoparticles nanoparticles, are iron currently nanoparticles being employed (FeNPs) have for promisingremediation advantages of water that and can wastewater combat environmental pollutants. pollution.Among Thethese interest metallic in nanoscale nanoparticles, zero-valent iron ironnanoparticles (nZVI) in environmental(FeNPs) have remediationpromising advantages is increasing that due can to thecombat reactivity environmental of nanoscale pollution. iron having The a largeinterest surface in nanoscale area to volumezero-valent ratio iron [6,7 ].(nZVI The) production in environmental of iron nanomaterials,remediation is increasing such as metallic due to iron the andreactivity oxide of of nanoscale iron via a iron more having convenient a large greener surface route, area to is volume a great stepratio forward[6,7]. 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Green Green Routes Routes for for the the Synthesis Synthesis of of Metallic Metallic Iron Iron Nanoparticles Nanoparticles 2.1. Synthesis by Biocompatible Green ReagentsReagents Biopolymers:: ResearchResearch has has been been performed performed to utilize to utilize non-toxic non synthetic-toxic synthetic biocompatible biocompatible materials formaterials the synthesis, for the synthesis, as well as as for well stabilisation as for stabil ofisation magnetic of magnetic nanoparticles nanoparticles polymer compositespolymer composites (Table1). In(Table this scenario,1). In this He scenario, et al. [8] He used et wateral. [8] solubleused water starch soluble for stabilisation starch for of stabil bimetallicisation Fe/Pd of bimetallic nanoparticles. Fe/Pd Starchnanoparticles. is a hydrophilic Starch is polymer, a hydrophilic which polymer, consists of which ~20% consists amylose; of in ~20% this study,amylose; it was in this found study that, starchit was found that starch plays a significant role in dispersion and stabilisation of iron nanoparticles. In Nanomaterials 2016, 6, 209 3 of 26 plays