A review on the adsorptive removal of heavy metals from wastewater using eco-friendly adsorbents

Water contaminated with heavy metals poses serious health risks to humans and ecosystems, necessitating innovative and sustainable treatment methods. Among various techniques, adsorption is widely acknowledged for its efficiency, low cost, easy implementation, and environmentally friendly process that effectively removes a variety of pollutants even at low levels without producing toxic byproducts. The adsorption process benefits from the use of inexpensive materials with porous structures and high cation exchange capacities, such as natural clays, modified clays (also known as organoclays), and bio-adsorbents. These widely available, non-toxic, and biodegradable materials efficiently capture metal ions, including Cu²⁺, Pb²⁺, Cd²⁺, and Zn²⁺, primarily due to their large specific surface areas, ion exchange capabilities, and functional groups that facilitate the complexation of metal ions. This comprehensive review highlights key developments in the application of these eco-friendly adsorbents for heavy metal remediation over the last two decades (2005–2025). The research under consideration demonstrates that for heavy metal adsorption, the Langmuir isotherm model fits data better than the Freundlich model. Furthermore, the adsorption process is best described by the pseudo-second-order kinetic model, with removal efficiencies of up to 84% for natural clays, 88% for modified clays, and 84% for bio-adsorbents. These results underline the strong potential of clays and bio-adsorbents for sustainable and effective treatment of heavy metal-contaminated waters.