A Comprehensive Review on Green and Eco-Friendly Nano-Adsorbents for the Removal of Heavy Metal Ions: Synthesis, Adsorption Mechanisms, and Applications

Purpose of Review: Environmental pollution stemming from industrial, agricultural, and domestic activities is constantly increasing. The removal of these hazardous pollutants from the environment is inevitable, so finding and developing convenient, cost-effective, and biocompatible treatment methods is imperative. As emerging adsorbents, green nanoparticles (NPs) have received significant attention in recent years due to their biocompatibility and promising capability for removing pollutants such as heavy metals from aqueous solutions. This study aims to provide a comprehensive and coherent review of the heavy metals removal via an adsorption technique using green nanoparticles, focusing on their synthesis and adsorption mechanisms. Recent Findings: Currently, plants and microorganisms are used to synthesize green nano-adsorbents. The antioxidant compounds in the extracts of different parts of the plants could be used as reducing agents for the synthesis of zero-valent metal nanoparticles. Moreover, they can be applied to the surface of nanoparticles, which enhances the stability of synthesized nanoparticles. In addition to plant-derived compounds, microorganisms can play a significant role in synthesizing green nanoparticles. Summary: For decades, human health and the environment have been threatened by exposure to heavy metals caused by the activities of mines, industries, and factories. Therefore, there is a need to provide solutions to remove these pollutants from the environment. One of the effective solutions is the adsorption method. The efficiency of this method is strongly influenced by the selection of suitable adsorbents. In recent years, special attention has been paid to nano-adsorbents. Utilizing green nano-adsorbents, as opposed to conventional materials, is a critical strategy for reducing environmental pollutants, particularly for heavy metal adsorption from contaminated water and wastewater. This paper also discusses the sources and occurrence of heavy metals, as well as a number of environmental issues of methods that employ green and eco-friendly nano-adsorbents for heavy metals removal, including (i) the type of heavy metals and their use pattern, (ii) influencing factors, (iii) heavy metal analysis methods and their potential toxicity, and (iv) different conventional and cutting-edge nanotechnologies for water and wastewater treatment. The literature review, which covered the years 2002–2023, provided a critical illustration of current concerns about heavy metal contamination and removal efforts, with a focus on green nano-adsorbents and the use of these environmentally friendly materials.