Manganese-coated granular oyster shells: a novel approach for heavy metal removal from urban stormwater runoff

This study introduces manganese oxide (MnO_x)-coated granular oyster shells (MnO_x@GOS) as an advanced adsorbent for heavy metal removal from urban stormwater runoff. The MnO_x coating increased the surface area of ​​granular oyster shell (GOS) by 132% (from 1.0534 to 2.4420 m²/g), enhancing the adsorption capacity. Selective adsorption followed the order of Pb(II) > Cu(II) > Zn(II) > Ni(II), influenced by ionic radius compatibility, hydration energy, and MnO_x redox interactions. Kinetic studies showed that the adsorption process was fit to the pseudo-second-order (R² = 0.9590–0.9931), confirming chemisorption as the dominant mechanism. The highest kinetic rate constant (K₂ = 0.0766 g/mg·min) was observed for Ni(II), whereas Pb(II) exhibited the strongest affinity. The Freundlich model provided a better fit (R² = 0.9900–0.9969) than the Langmuir model (R² = 0.9433–0.9826), confirming that adsorption primarily occurred through a multilayer process on a heterogeneous surface. Fixed-bed column studies demonstrated over 90% removal efficiency for Pb(II) and Cu(II) over eight days, confirming MnO_x@GOS as an effective adsorbent. With abundant oyster shell waste and a simple synthesis process, MnO_x@GOS has great potential for cost-effective, large-scale use in low impact development (LID) systems such as bioretention cells and permeable pavements. However, ensuring long-term stability, efficient regeneration, and consistent performance under a wide range of environmental conditions remains a key challenge for real-world deployment.