<p>Cadmium (Cd) contamination in water and soil poses severe ecological and health risks due to its high mobility and bioaccumulation potential, yet existing adsorbents such as pristine biochar and geopolymers suffer from limited adsorption capacity, poor chemical reactivity, or dense microstructures that hinder mass transfer. To address these drawbacks, we rationally designed a hierarchical porous biochar/geopolymer composite (BC/PGP) via a one‑step calcination strategy, where hemp straw was incorporated during geopolymer synthesis to simultaneously engineer a mesoporous architecture and enrich surface functionality. The resulting BC/PGP exhibits a significantly enhanced specific surface area (35.56 m²/g) and well‑developed mesoporosity, outperforming both pristine biochar and geopolymer. Systematic kinetic and isotherm studies reveal a chemisorption‑dominated process with a maximum Cd(II) uptake of 29.85&#xa0;mg/g, which is substantially higher than those of most geopolymer‑based adsorbents reported to date. Mechanistic investigations, including XPS analysis, confirm that Cd(II) removal proceeds via synergistic electrostatic attraction, ion exchange, surface complexation, and precipitation, a multi‑pathway mechanism that leverages both the biochar’s porous scaffold and the geopolymer’s chemical affinity. Moreover, sand‑soil column tests demonstrate that BC/PGP effectively immobilizes Cd(II) and restricts its downward migration, underscoring its practical applicability for in‑situ remediation. This work offers a low‑cost, eco‑friendly, and structurally engineered composite that not only overcomes the intrinsic limitations of individual components but also provides a promising alternative to conventional adsorbents for treating cadmium‑polluted water and soil.</p> Graphical Abstract <p></p>