Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Review MXenes (two-dimensional metal carbides) as emerging nanomaterials for water purification: Progress, challenges and prospects Ihsanullah Ihsanullah Center for Environment and Water, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia HIGHLIGHTS • A broad overview of MXenes and MXene-based nanomaterials in water purification is presented. • The potential of MXenes for the up- take of selected pollutants is critically evaluated. • Removal mechanisms and regenera- tion capability of MXenes are ap- praised. • Current challenges and future pro- spects of MXenes in water treatment are highlighted. GRAPHICAL ABSTRACT ARTICLE INFO Keywords: MXenes 2D materials Metal carbides Water treatment Adsorption Nanomaterials ABSTRACT Two-dimensional (2D) transition metal carbides (MXenes) have emerged as a fabulous nanomaterial and an ideal candidate for a host of applications. The tremendous characteristics of MXenes i.e., high surface area, activate metallic hydroxide sites, biocompatibility, ease of functionalization, high metallic conductivity and hydro- philicity make them the best aspirant for applications in environmental remediation, energy storage, electronics, sensors, and catalysis. MXenes exhibited substantial sorption selectivity and efficient reduction capability for numerous pollutants present in water. This paper critically reviews the recent advancement in applications of MXenes in water treatment with an emphasis on adsorption–reduction properties of MXenes and MXene-based composites. The removal of selected pollutants i.e., heavy metals, dyes, and radionuclides by MXenes and MXene-based composites are portrayed in detail by elaborating the removal mechanism and regeneration ability. The key challenges and issues associated with the synthesis and applications of MXenes and MXene-based composites in water purification are highlighted. Lastly, research directions are provided to guarantee the synthesis and applications of MXenes in a more effective way. This review may provide an insight into the applications of MXenes for water purification in future. 1. Introduction Since the discovery of graphene in 2004, various two-dimensional (2D) materials have gained considerable attention from the research community, owing to their tremendous properties [1–3]. In 2011, a new class of 2D transition metal carbides, nitrides, and carbonitrides, named MXenes (pronounced “maxines”) were discovered by re- searchers at Drexel University [4–7]. Ever since their discovery, MXenes have grabbed the interest of scientists and academicians due to their various fascinating mechanical, magnetic, electronic and chemical properties [3]. MXene nanomaterials have a general formula of M n+1 X n T x (n =1–3), where M, as an early transition metal (such as W, Mo, Cr, Ta, V, Nb, Hf, Zr, Ti, Y, or Sc), X is carbon and/or nitrogen, and T denotes the surface termination groups such as fluorine (eF), oxygen (]O), chlorine (eCl) and hydroxyl (eOH), and x represents the number of surface functionalities [8–13]. The thickness of MXenes is usually in https://doi.org/10.1016/j.cej.2020.124340 Received 7 December 2019; Received in revised form 1 February 2020; Accepted 3 February 2020 E-mail addresses: ihsankhan@kfupm.edu.sa, engr.ihsan.dir@gmail.com. Chemical Engineering Journal 388 (2020) 124340 Available online 04 February 2020 1385-8947/ © 2020 Elsevier B.V. All rights reserved. T