Advances in biogenic calcium oxide: Green synthesis techniques, characterization, emerging applications, challenges, and future outlook

Nanoparticles are materials with at least one dimension in the 1–100 nm range, exhibiting unique physiochemical properties distinct from their bulk counterparts. This review critically examines recent advances in calcium oxide nanoparticles (CaO-NPs) synthesized from agricultural and biogenic wastes, highlighting their potential as sustainable, low-cost alternatives to conventionally mined CaO. The novel contribution of this work is a comparative evaluation of six green synthesis routes, including sol-gel, calcination-decomposition, microwave-assisted, precipitation, plant extract-mediated, and microorganism-assisted methods, with explicit focus on how synthesis parameters control particle size, morphology, crystallinity, surface basicity, and stability. A key finding is the strong correlation between calcination conditions and nanoparticle properties; higher temperatures and prolonged calcination improve crystallinity but often induce particle growth and agglomeration. Waste-derived CaO-NPs demonstrate multifunctional applications in catalysis, environmental remediation, agriculture, antimicrobial systems, and emerging membrane technologies. Despite these advances, significant research gaps persist, including limited process standardization, precursor variability, poor reproducibility, high energy consumption during calcination, and insufficient large-scale and membrane-based applications. Future research prioritize energy-efficient synthesis strategies, hybrid green processing methods, functionalized CaO nanocomposites, and scalable industrial implementation. These directions will enhance the sustainability, reproducibility, and practical applicability of biogenic CaO-NPs in circular-economy framework.