<p>The increasing release of toxic pollutants into water bodies, especially heavy metal ions (HMs), poses severe environmental and health challenges. Among various remediation techniques, adsorption using functionalized carbon nanomaterials has emerged as an efficient and sustainable approach due to their high surface area, tunable porosity, and diverse surface functionalities. This review systematically evaluates recent advancements in the synthesis, surface functionalization, and adsorption behaviour of carbon-based nanomaterials such as graphene oxide (GO), carbon nanotubes (CNTs), biochar (BC), and activated carbon (AC) for the removal of HMs. Particular attention is given to the roles of oxygen-, nitrogen-, and sulfur-containing functional groups in enhancing adsorption via mechanisms such as electrostatic interactions, ion exchange, surface complexation, redox transformation, and precipitation. The novelty of this review lies in its focused and comparative analysis of how specific surface functional groups influence distinct adsorption mechanisms, capacities, and kinetic/isotherm behaviours across various carbonaceous nanomaterials. Additionally, it highlights recent advancements in multifunctional composite adsorbents such as GO–polymer hybrids, Metal organic frameworks (MOFs)–carbon composites, and metal oxide-functionalized BC that offer synergistic improvements in selectivity, reusability, and stability. By identifying current research gaps and synthesizing recent findings, this review provides a strategic framework for designing next-generation carbon nanomaterials for practical applications in wastewater treatment and environmental remediation.</p> Graphical abstract <p></p>