Mechanistic insights into heavy metal adsorption on pyrolytic char from waste tires using experimental and DFT approaches

The contamination of irrigation water with heavy metals poses critical threats to soil health, crop productivity, and food safety, as these toxic elements can accumulate in agricultural systems and enter the food chain. Addressing this challenge requires the deployment of efficient and sustainable treatment strategies to ensure water quality complies with irrigation safety standards. This study explores the adsorption-based removal of lithium (Li), manganese (Mn), boron (B(OH)3), lead (Pb), nickel (Ni), cadmium (Cd), cobalt (Co), and zinc (Zn) from synthetic wastewater using gravel, activated carbon (AC), and pyrolytic char (PC) derived from waste tires. These metals were selected due to their toxicity, environmental persistence, and potential bioaccumulation in crops. Batch adsorption experiments conducted over 24, 48, and 72 h revealed that PC achieved the highest removal efficiencies, including complete or near-complete elimination of Pb, Ni, Cd, and Co, and substantial reductions in Mn (94%), B(OH)3 (91%), Zn (94%), and Li (84%) after 72 h. AC also performed effectively, particularly for Pb (95%), Ni (85%), Cd (78%), and Co (80%). In contrast, gravel exhibited limited adsorption capacity, attributed to its quartz-dominated, inert mineral composition. The structural and spectroscopic analyses confirmed PC’s advantageous features, i.e., porosity, surface functionality, and partial graphitization, while density functional theory (DFT) calculations highlighted its favorable electronic properties and strong ion-surface interactions. These findings highlight PC’s superior potential as a cost-effective and sustainable adsorbent for remediating heavy metal-contaminated wastewater, thereby promoting safe irrigation practices and long-term agricultural sustainability. Moreover, the integration of adsorption performance testing with theoretical modeling offered novel mechanistic insights into ion-surface interactions, strengthening the understanding of multi-metal removal in irrigation-relevant wastewater. Pyrolytic char (PC) derived from waste tires removed > 99% of Pb2+, Ni2+, Cd2+, and Co2+ from wastewater. PC outperformed activated carbon (AC) and gravel in metal ion adsorption. DFT showed PC has high reactivity and strong ion interactions. π–cation and electrostatic forces drive adsorption on PC. Waste tire-derived PC offers a low-cost water treatment option.