<p>The utilization of low-cost seaweed biomass waste for biochar (BC) production has been demonstrated to be an effective solution in remediating heavy metals (HMs)-contaminated soil, thereby enhancing soil health and mitigating the environmental risks associated with HMs contamination. This study successfully synthesized and characterized a novel <i>Enteromorpha</i>-derived BC nanoparticles (MEBC-900) from <i>Enteromorpha</i> by wet-mechanochemical sand milling and applied it to remediate Cd²⁺- and Pb²⁺-contaminated soil. Sand milling significantly increased the specific surface area of MEBC-900 (466.5 m²/g) by reducing it into nanosized particles and enriching it with abundant O- and N-functional groups. These modifications improved its adsorption for HMs, achieving 68.9&#xa0;mg/g for Cd²⁺ and 170.8&#xa0;mg/g for Pb²⁺ through the surface complexation, electrostatic interaction and precipitation. The adsorption kinetics and isotherm analysis conformed to the pseudo-second-order and Langmuir models, confirming that monolayer chemisorption is the primary mechanism. Pot experiments demonstrated that MEBC-900 improved soil conditions, including increasing soil pH, organic matter, and nutrient level while promoting microbial community. Furthermore, MEBC-900 effectively enhanced the immobilization of Cd²⁺ and Pb²⁺ in soil, reducing their accumulation in maize by 52% and 43%, respectively. It also facilitated oxidative stress in maize by reducing malondialdehyde levels by 40% and enhancing plant enzymatic activity under Cd²⁺ and Pb²⁺ stress conditions. This study presents a promising and low-cost sand-milled adsorbent (MEBC-900) for the remediation of HM-contaminated soil.</p>