Green Synthesis of Gold, Silver, and Iron Nanoparticles for the Degradation of Organic Pollutants in Wastewater

Review Green Synthesis of Gold, Silver, and Iron Nanoparticles for the Degradation of Organic Pollutants in Wastewater Brajesh Kumar Post Graduate Department of Chemistry, TATA College, Kolhan University, Chaibasa 833202, India; [email protected] or [email protected]; Tel.: +91-8757618562 Abstract: The green synthesis of nanoscale materials is of special interest to researchers all over the world. We describe a simple, robust, inexpensive, and environmentally friendly approach to the synthesis of gold, silver, and iron nanoparticles using a variety of biomolecules/phytochemicals as potential reducers and stabilizers. The green approach to the controlled synthesis of nanoparticles with different morphologies is based on the use of plant extracts. Green synthesized nanoparticles can be used as catalysts, photocatalysts, adsorbents, or alternative agents for the elimination of various organic dyes. The kinetic enhancement of nanoparticles for the degradation/removal of dyes could provide significant and valuable insights for the application of biochemically functionalized nanoparticles in engineering. In this review, current plant-mediated strategies for preparing nanoparticles of gold, silver, and iron are briefly described, and morphologically dependent nanoparticles for the degradation of organic pollutants in wastewater are highlighted. Overall, the approach presented in the article supports environmental protection and is a promising alternative to other synthesis techniques. Keywords: green synthesis; nanoparticles; UV-Vis spectroscopy; photocatalyst; wastewater treatment Citation: Kumar, B. Green Synthesis of Gold, Silver, and Iron Nanoparticles for the Degradation of Organic 1. Introduction Pollutants in Wastewater. J. Compos. Sci. 2021, 5, 219. https://doi.org/ Currently, nanotechnology is one of the most promising research fields regarding 10.3390/jcs5080219 the development of nanomaterials and especially nanoparticles (NPs) ranging from 1 to 100 nm in size. It has been the center of attention due to the synthesis of nanoparticles Academic Editor: Pankaj Attri of various materials to be used in different sectors thanks to their unique physical and chemical properties when compared to the corresponding bulk materials. These distinctive Received: 6 April 2021 properties, related to the high surface to volume ratio of nanoparticles or quantum effects, Accepted: 30 July 2021 have attracted notable interest from chemists, biologists, physicists, and engineers [1]. Published: 16 August 2021 In the past decade, the synthesis of engineered nanoparticles including metals, metal oxides, non-metals, lipids, and polymers, as well as various nanocomposites, has attracted Publisher’s Note: MDPI stays neutral increasing interest for various applications, in emerging technologies and in consumer with regard to jurisdictional claims in products such as optical, electrical, and catalytic devices, sensors, electronic devices, and published maps and institutional affil- other products used for personal care, biomedicine, antibacterial purposes, agriculture, iations. renewable energy, water and soil treatment, etc. [2,3]. Inorganic nanoparticles, especially those that are purely made from gold, silver, copper, etc., have unique optical and photo- thermal properties due to their well-known localized surface plasmon resonance (LSPR) and their broad absorption spectrum in the visible region of electromagnetic radiation. Copyright: © 2021 by the author. Gold and silver nanoparticles have enhanced optoelectronic properties, stability, and Licensee MDPI, Basel, Switzerland. biocompatibility. Therefore, they have been used in products within different industrial This article is an open access article fields, such as medicine, food, biomaterials, sensing, dye-sensitized solar cells, catalysis, distributed under the terms and and photocatalysis [4]. conditions of the Creative Commons In the last few decades, a great deal of research on nanoparticle synthesis has empha- Attribution (CC BY) license (https:// sized their use in biological procedures and applications in the engineering and biomedical creativecommons.org/licenses/by/ fields. Bottom–up approaches are preferred over top–down approaches for the fabrication 4.0/). J. Compos. Sci. 2021, 5, 219. https://doi.org/10.3390/jcs5080219 https://www.mdpi.com/journal/jcs J. Compos. Sci. 2021, 5, x FOR PEER REVIEW 2 of 27 In the last few decades, a great deal of research on nanoparticle synthesis has empha- J. Compos. Sci. 2021, 5, 219 2 of 26 sized their use in biological procedures and applications in the engineering and biomedical fields. Bottom–up approaches are preferred over top–down approaches for the fabrication of nanoparticles, and it can be carried out through physical, chemical, and bio-as- sistedof nanoparticles, methods. Surface and it-modified can be carried metal nanoparticles out through physical,can be synthesized chemical, by and the bio-assisted reduction ofmethods. metal ions Surface-modified followed by the capping metal nanoparticles or surface functionalization can be synthesized of nanoparticles; by the reduction agents of suchmetal as ionsgallic followed acid, citric by theacid, capping polyphenols, or surface starch, functionalization polysaccharides, of proteins, nanoparticles; terpenoids, agents saponins,such as gallic surfactants, acid, citric and acid, polymers polyphenols, have been starch, used polysaccharides, to enhance the stability proteins, of terpenoids, nanopar- ticlessaponins, [5]. The surfactants, stability of and the polymers surfaces haveof organically been used capped to enhance metal the nanoparticles stability of nanoparti-could be influecles [nced5]. The differently stability ofby theenvironmental surfaces of organicallyconditions cappedsuch as metaltemperature, nanoparticles pH, humidity, could be theinfluenced presence differently of interfering by environmental ions, and light conditions when compared such as to temperature, the typical pH,inert humidity, conditions the forpresence nanoparticles of interfering [1]. ions, and light when compared to the typical inert conditions for nanoparticlesTo date, various [1]. popular physical and chemical methods have been involved in na- noparticleTo date, synthesis, various including popular solution physical-phase and chemicalsynthesis, methods chemical havereduction, been involvedthe sol–gel in technique,nanoparticle evaporation synthesis,– includingcondensation, solution-phase hydrothermal synthesis, synthesis, chemical the wet reduction, chemical the method, sol–gel sonochemicaltechnique, evaporation–condensation, synthesis, the microwave hydrothermalmethod, the laser synthesis, ablation the method, wet chemical and ball method, mill- sonochemical synthesis, the microwave method, the laser ablation method, and ball ing [6], in which a large amount of solvent is required in order to obtain pure and well- milling [6], in which a large amount of solvent is required in order to obtain pure and defined nanoparticles. However, these methods are costly and also create various prob- well-defined nanoparticles. However, these methods are costly and also create various prob- lems for the ecosystem and the environment. To overcome these issues, safe, inexpensive, lems for the ecosystem and the environment. To overcome these issues, safe, inexpensive, environmentally friendly, green methods that can be easily scaled up for the large-scale environmentally friendly, green methods that can be easily scaled up for the large-scale production of nanoparticles are recommended using various plant materials/biomole- production of nanoparticles are recommended using various plant materials/biomolecules cules due to the generation of environmentally benign products and byproducts after the due to the generation of environmentally benign products and byproducts after the com- completion of nanoparticle synthesis [7]. Furthermore, these methods do not require pletion of nanoparticle synthesis [7]. Furthermore, these methods do not require harsh harsh conditions such as high temperature, pressure, and energy or the use of toxic chem- conditions such as high temperature, pressure, and energy or the use of toxic chemicals icals (Figure 1). Therefore, various natural materials adopted for the biosynthesis of nano- (Figure1). Therefore, various natural materials adopted for the biosynthesis of nano- and micro-scaled inorganic materials having different compositions, sizes, shapes, and and micro-scaled inorganic materials having different compositions, sizes, shapes, and controlledcontrolled dispersity dispersity are are needed; needed; this this remains remains an an unexplored unexplored area area of of nanotechnology nanotechnology re- re- searchsearch but but is is recommended recommended due toto thethe potentialpotential benefits benefits of of simplicity simplicity and and sustainability sustainability [8]. [8].Extracts Extracts of plantsof plants or theiror their parts, parts, such such as roots, as roots, leaves, leaves, fruits, fruits, flowers, flowers, bark, bark, and and seeds, seeds, have havebeen been extensively extensively screened screened for their for abilitytheir ability to promote to promote the biosynthesis the biosynthesis of different of different kinds of kindsmetallic of metallic NPs, oxide NPs, NPs, oxide and NPs, chalcogenide and chalcogenide NPs [9]. NPs [9]. FigureFigure 1. 1. SchematicSchematic representation representation for the biosynthesisbiosynthesis of of different differen morphologicalt morphological

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