Performance and mechanisms of waste-based carbon adsorbents in heavy metal removal – an experimental and theoretical approach

Access to safe drinking water is increasingly challenged by rising demand, pollution, and climate change, with heavy metal pollutants having emerged as some of the major contributors to today's environmental degradation, intensifying the water crisis. The presence of hazardous metals like chromium, cadmium and lead is a widespread concern in aquatic ecosystems. This study investigates the removal of Pb²⁺, Cd²⁺, and Cr⁶⁺ ions under a set of predetermined conditions using waste-derived carbon adsorbents particularly granular activated carbon (GAC), GAC coated with chitosan (GAC-CS), <i>Prosopis juliflora</i> wood-derived biochar (PJBC), and banana stem char (BN char), through a combined experimental and theoretical approach. Characterization techniques (FESEM, FTIR, XRD, Raman, and BET) confirmed porous structures and functional groups responsible for adsorption. In addition, IR studies before and after were conducted to identify the possible mechanisms involved in the removal of these metals before and after adsorption. The removal efficiency was found to be in the order: Pb > Cd > Cr across all adsorbents. BN char consistently exhibited the highest adsorption capacity for all tested metals, with values of 16.50 ± 0.60 mg g^-1 for Cr⁶⁺, 186.16 ± 0.40 mg g^-1 for Cd²⁺, and 252.46 ± 0.60 mg g^-1 for Pb²⁺, followed in order by PJ biochar, GAC-CS, and GAC. To further understand the adsorption behaviour, Density Functional Theory (DFT) and QTAIM analyses were performed, which confirmed stronger binding of Pb²⁺ compared to Cd²⁺ and Cr⁶⁺ through charge transfer and orbital overlap. Although comparative studies on carbon-based adsorbents exist, simultaneous use of experimental evaluation and theoretical modelling has been rarely explored.