Remediation of Pb, Zn, Cu, Cd, and as Contaminated Agricultural Soils Using Biochar–Magnetic Nanocomposite: A Systematic Review

Agricultural soil contamination by heavy metals particularly lead (Pb), zinc (Zn), copper (Cu), cadmium (Cd), and arsenic (As) represents a critical environmental and public health challenge in Nigeria and across sub-Saharan Africa. Mining operations, agro-industrial inputs, and indiscriminate waste disposal have progressively elevated the concentrations of these toxic elements in farmlands, threatening food security, ecosystem integrity, and human wellbeing. Despite a growing body of literature on conventional remediation approaches, there remains a pressing need for cost-effective, sustainable, and multipurpose alternatives suitable for resource-constrained settings. This review synthesises current knowledge on the sources, pathways, and health implications of Pb, Zn, Cu, Cd, and As contamination in Nigerian agricultural soils, and critically evaluates the potential of biochar–magnetic nanocomposites (BMNCs) as a transformative remediation technology. Biochar, particularly rice husk-derived biochar, offers high surface area, abundant functional groups (hydroxyl, carboxyl, silanol), and significant capacity for metal immobilisation and soil carbon sequestration. Magnetic iron oxide nanoparticles (Fe₃O₄ and γ-Fe₂O₃) complement these properties through their superparamagnetic characteristics, high surface-area-to-volume ratio, and ease of separation post-application. When combined into nanocomposites, these materials demonstrate synergistic mechanisms—surface complexation, ion exchange, precipitation, and electrostatic attraction—that significantly outperform individual components in multi-metal remediation scenarios. The review identifies critical research gaps in field-scale validation, long-term stability assessments, and risk evaluations for BMNC applications in Nigerian agro ecosystems. Recommendations for future research and policy are presented.