Valorization of shell waste as a nature-based soil amendment for stabilization of heavy metals in agricultural soils: Toward safe agricultural recultivation.

This study evaluated abalone and pen shell wastes as nature-based soil amendments for Pb- and Zn-contaminated agricultural soils. Natural and calcined shells were incorporated into contaminated soils and cured for four weeks, followed by metal leaching tests (0.1 M HCl), sequential extraction, and mechanistic analysis using SEM-EDX. Soil functional changes were assessed using a Soil Health Index (SHI), and a pot experiment was conducted to evaluate crop safety and agronomic feasibility under a stabilization-capping system using lettuce. Calcined shell amendments, rich in reactive CaO, reduced leachable Pb and Zn by more than 97% at 2 wt%. Sequential extraction showed that the mobile Pb fraction (F1+F2) decreased from 16.0% in untreated soil to approximately 4.6% after stabilization, accompanied by a shift toward more stable fractions. SEM-EDX analysis indicated the formation of low-solubility pozzolanic reaction products (e.g., C-S-H phases) in calcined-shell treatments, contributing to effective metal immobilization. SHI analysis revealed a trade-off, with stabilized soils exhibiting a 14-36% decline relative to pre-stabilization conditions due to elevated pH and salinity. However, under the stabilization-capping configuration, upward metal transport was effectively inhibited, resulting in non-detectable Pb levels in lettuce tissues and Zn concentrations within safe limits. Furthermore, this system successfully restored crop productivity, increasing lettuce fresh biomass by 60-94% compared to the untreated soil. Overall, this study demonstrates that recycling shell waste enables effective heavy metal stabilization, and that coupling stabilization with capping provides a realistic and practical framework for the safe agricultural use of amended soils.