Adsorptive removal of heavy metals from wastewater using emerging nanostructured materials: A state-of-the-art review

Heavy metals (HMs) are among the most hazardous water pollutants due to their toxic (when ionized in water) and non-biodegradable nature. Although there are several methods that can be used to remove HMs from contaminated water samples, adsorption is one of the most attractive techniques for water decontamination due to its several attractive attributes. However, the efficacy of the adsorptive removal of HMs from wastewater greatly depends on the characteristics of the utilized adsorbents. Recently, nanostructured adsorbents such as layered double/triple hydroxides (LDHs/LTHs), carbon nanotubes (CNTs), graphene-based materials, MXenes, metal organic frameworks (MOFs), and zeolitic imidazolate frameworks (ZIFs) have emerged as attractive tools for the removal of HMs from wastewater. Although there are a few review articles on each of these nanomaterials, comprehensive reviews documenting the applications of all these nanomaterials in one place are still greatly lacking. Additionally, adsorptive removal of HMs is a rapidly evolving field, requiring up-to-date comprehensive reviews in order to analyse, assess, and summarize the scattered information in recently published research articles, making it convenient for researchers and professionals to easily follow the progress in the field. Therefore, the main objective of this article is to comprehensively review recent studies on the adsorptive removal of HMs from contaminated water samples using nanostructured adsorbents (i.e., LDHs/LTHs, CNTs, graphene, MXenes, MOFs, ZIFs, and nanocomposites containing any of these nanomaterials). Besides their synthesis approaches, the adsorption performance of these nanomaterials towards HMs are extensively reviewed. Adsorption kinetics, isotherm, the effects of several adsorption parameters, adsorption thermodynamics, and reusability/recyclability of these nanostructured adsorbents are thoroughly and critically reviewed. Obstacles, challenges, limitations, and remaining knowledge gaps are identified, and future works to address them have been highlighted. Hence, the present article offers a state-of-the-art and comprehensive review of the various aspects of HMs removal from wastewater samples using nanostructured adsorbents.