Green synthesis of metal nanocatalyst (MNC)is an updated green nanotechnology. There are several methods for the green synthesis of metal nanocatalyst, one of these methods used plants and microorganism extract and defined as biosynthesis of nanoparticles.  Biological entities or their extracts are used for bio-reduction of metallic particles. The nanocatalyst produced by green synthesis differ from those using physical and chemical methods. Green synthesis of MNC is easy, efficient, nontoxic,  eco-friendly, cost-effective, eliminates the use of toxic chemicals,  and consume less energy. Several biological agents were employed for green synthesis of MNC such as plants, fungi, bacteria, and algae . Among the different agents available of MNC synthesis, the plant extract is a rather simple and easy process to produce nanoparticles at large scale relative to bacteria or fungi mediated synthesis. Additionally, various plant parts, including leaves, stems, fruits,  and roots  have been used for the  synthesis of metal nanoparticles. Nowadays, honey bees , and human cells have been shown to be innovative resources for producing safe and nontoxic nanoparticles. This review discusses, through the recently published researches, the different methods for green synthesis of MNC, their particle sizes produced by different bio-extracts, and their environmental applications in different fields as well as their application in water, wastewater treatment. 

Mohamed Nageeb Rashed  is professor of analytical  and environmental chemistry, former vice-dean for environmental affairs , Faculty of Science, Aswan  University, Egypt.  His research interest has been analytical and environmental chemistry with special emphasis in  advanced oxidation treatment; photocatalysis, nanocatalyst , nanocomposite  and adsorption techniques for water and wastewater treatment. Prof. Rashed supervised several M.Sc. and  Ph.D. thesis in the field of analytical and environmental chemistry. He selected as external examiner for several Ph.D. thesis in analytical chemistry from India, Kasakhastan, Malisia and Bostuana. Prof. Rashed published about 95 scientific papers in peer reviewed international journals. He participated as invited and plenary speaker in 30 international conferences worldwide. He acts as editor-in-chief and an editorial board member in several international journals. Prof.Rashed enrolled among World's top 2% scientists by Stanford University, USA 2020-2022. He awarded the Egyptian State Award for Environmental Researches in the years 2001, and Aswan University Merit Award for Basic Science year 2020.