Mechanistic insights and process optimization of perilla frutescens stem biochar for cadmium removal from zinc smelter wastewater

Cadmium (Cd), a hazardous heavy metal primarily emitted by smelting industries, presents significant environmental and health hazards due to its persistence and strong potential for bioaccumulation. Substantial amounts of Perilla frutescens stems (PFS), which constitute a major agricultural byproduct, are typically discarded or openly burned in fields, thereby generating smoke, particulate emissions, and greenhouse gases. This study explored a sustainable valorization pathway by converting PFS into biochar (PFSB) via pyrolysis at temperatures ranging from 300 to 750 °C, assessing its effectiveness as an adsorbent for Cd removal. Of the evaluated biochars, the sample produced at 450 °C (PFSB–450) demonstrated desirable physicochemical characteristics, such as a high C/N ratio, low H/C ratio, well-developed porosity, and a rich presence of oxygen-containing functional groups. Batch adsorption tests showed a peak Cd adsorption capacity of 75.74 mg/g, aligning with or exceeding those of other biochar-based adsorbents reported in the literature. Notably, this level of performance was attained using a straightforward pyrolysis technique without requiring chemical modification or intricate processing steps. The adsorption processes conformed to the Elovich and Langmuir models, signifying that both chemisorption and monolayer adsorption took place. Mechanistic studies identified electrostatic attraction, ion exchange, and Cd–O/Cd–π interactions as the principal adsorption mechanisms. Response surface methodology revealed that adsorbent dosage, initial pH, and reaction time played critical roles in determining Cd removal efficacy, while temperature was found to be insignificant. Under optimized operational parameters, PFSB–450 removed 80.84% of Cd from authentic zinc smelter wastewater samples. This work illustrates the dual advantages of mitigating air pollution through agricultural residue utilization and addressing heavy metal contamination in industrial effluents, promoting a scalable and cost-efficient solution consonant with circular economy practices.