Adsorptive Removal of Cadmium Ions from Aqueous Media Using Coal Fly Ash Cross-Linked with Melamine–Formaldehyde Resin

Abstract

Cadmium (Cd²⁺) is a toxic heavy metal that poses serious health risks even at low concentrations. This review presents a comprehensive analysis of recent advances in adsorptive removal of Cd(II) using low-cost waste-derived materials, focusing on coal fly ash (CFA) cross-linked with melamine–formaldehyde (MF) resin. We summarize cadmium contamination issues and common treatment methods, then detail the synthesis, characterization, and performance of CFA–MF composite adsorbents. Key topics include sorbent preparation (e.g. MF resin polymerization around ash particles, carbonization of MF foam), material properties (surface area, functional groups), and adsorption behavior (isotherms, kinetics, thermodynamics). We compare Cd(II) uptake capacities and selectivity of modified CFA adsorbents, and discuss regeneration/reuse. Mechanistic aspects (chemisorption vs physisorption, pH and competition effects) are elucidated. Environmental and practical considerations (waste valorization, stability, scale-up) are critically assessed. The review cites over 40 recent studies and highlights that MF-modified coal ash offers a promising, sustainable solution for Cd(II) remediation in water.