Synthesis and optimization of biochar compost nanocomposites for enhanced seed germination

Abstract

Conventional biochar–compost blends often exhibit uneven nutrient distribution, resulting in inconsistent agronomic performance. This study developed a post-composting mechanochemical strategy using high-energy planetary ball milling to convert bulk biochar-blended compost (BBC) into a nanosized organic fertilizer. The influence of solvent polarity, using water, ethanol, and hexane, was evaluated to determine its effect on structural modification and surface chemistry. Response Surface Methodology identified ethanol-assisted milling as the optimal condition for achieving structural homogeneity and uniform nanoparticle distribution, producing Nano-BBC with an average particle size of 230 nm. Characterization by FT-IR, XRD, and SEM confirmed significant nanoscale restructuring, including increased surface oxidation and enrichment of oxygen-containing functional groups. Elemental analysis further indicated that concentrations of potentially hazardous elements were below detectable limits, supporting its suitability for agricultural application. Germination assays using Raphanus sativus L showed that ethanol-assisted Nano-BBC achieved a germination index of 123%, compared to 96%, 98%, and 91% for water-milled, hexane-milled, and conventional BBC treatments, respectively. These results demonstrate that ethanol-assisted mechanochemical activation enhances nutrient availability and physicochemical stability while reducing phytotoxic effects. This solvent-assisted approach provides a scalable and sustainable pathway for upgrading organic waste into high-performance nano-fertilizers, contributing to improved nutrient-use efficiency and circular bioresource utilization in agriculture.