Green synthesis of magnetic biochar from iron-containing sludge and spent coffee grounds for the removal of lead ions from groundwater

Heavy metal contamination in aquatic environments poses serious risks to human health and ecosystems. Lead, a toxic and potentially carcinogenic metal commonly found in industrial wastewater, can bioaccumulate and damage multiple human organ systems even at low concentrations. This study investigated the use of magnetic biochar derived from iron-containing sludge and spent coffee residues (SCR) via one-step pyrolysis at 700 °C, for the removal of Pb (II) from water. The synthesized biochar was characterized using FE-SEM-EDS, XRD, BET, VSM, and FTIR. Experimental parameters were systematically optimized. Under optimal conditions, the biochar achieved 88.5% Pb(II) ion removal with an adsorption capacity of 22 mg/g from a 20 mg/L Pb(II) solution, and 86% removal with a capacity of 181.5 μg/g from real contaminated groundwater containing 170.3 μg/L Pb(II), within 40 min in batch mode. The adsorption process was analyzed using Langmuir and Freundlich isotherm models. The good fit with both Langmuir and Freundlich models indicates mixed surface characteristics, where monolayer adsorption on homogeneous sites coexists with heterogeneous multilayer adsorption. Kinetic analysis followed the pseudo-second-order model, indicating chemisorption as the dominant mechanism. The synthesized biochar can be regenerated and reused using 0.1 M HNO3 or NaOH, and its adsorption mechanism was proposed. This study demonstrates that the biochar is a cost-effective, sustainable, and efficient adsorbent for toxic heavy metal removal, with magnetic separability supporting its potential for large-scale wastewater treatment.