Agricultural waste-derived nanoparticles for heavy metal removal from wastewater: a comprehensive review.

Heavy metal contamination of aquatic environments remains a major environmental challenge due to the persistence, toxicity, and bioaccumulative nature of metals such as Pb(II), Cd(II), Cr(VI), Hg(II), and As(V). Agricultural waste-derived nanoparticles have recently emerged as promising sustainable adsorbents because of their low cost, high surface area, tunable surface chemistry, and strong metal-binding capability. This review critically evaluates recent advances in the synthesis, characterization, adsorption performance, and removal mechanisms of nano-engineered adsorbents produced from agricultural residues including rice husk, sugarcane bagasse, coconut shell, corn stover, and banana peel. Comparative analysis demonstrates that nano-functionalization significantly enhances adsorption capacity, kinetics, selectivity, and regeneration efficiency compared with conventional adsorbents such as activated carbon and mineral sorbents. Among the most promising systems, magnetic biochar nanocomposites and chemically activated nano-biochars exhibit particularly high removal efficiencies for Pb(II), Cr(VI), Cd(II), and As(V), owing to synergistic mechanisms involving electrostatic attraction, ion exchange, surface complexation, pore filling, and redox interactions. The review further demonstrates that synthesis route strongly influences adsorbent performance, with pyrolysis, hydrothermal carbonization, chemical activation, co-precipitation, and microwave-assisted synthesis producing distinct surface properties and adsorption behaviors. Beyond batch adsorption studies, this review highlights the growing importance of continuous-flow fixed-bed systems and breakthrough modeling for evaluating practical scale-up potential. Dynamic adsorption models including Thomas, Adams-Bohart, and Yoon-Nelson are discussed as essential tools for predicting column performance and reactor design. Existing life-cycle assessment and techno-economic studies additionally indicate that agricultural waste-derived nano-adsorbents can provide lower carbon footprints and reduced treatment costs compared with many commercial materials, particularly when low-energy synthesis routes and recyclable magnetic systems are employed. Critical challenges remain regarding synthesis standardization, nanoparticle aggregation, regeneration stability, environmental safety, and long-term fate of released nanomaterials. Future research should prioritize pilot-scale continuous-flow applications, multi-contaminant wastewater systems, standardized adsorption protocols, integrated life-cycle and techno-economic assessment, and development of multifunctional recyclable nanocomposites. Overall, agricultural waste-derived nanoparticles demonstrate strong potential as scalable, environmentally sustainable, and economically viable materials for next-generation heavy metal remediation technologies.