Attapulgite/oyster shell composite reduces cadmium and lead bioavailability in acidic agricultural soils through synergistic adsorption and pH regulation

Heavy metal contamination in acidified agricultural soils poses significant environmental and health risks. This study presents the development of a cost-effective dual-functional adsorbent, the attapulgite-oyster shell (ATP-OS) composite, engineered for the simultaneous removal of co-contaminated Cd(II) and Pb(II) ions. The ATP-OS composite was synthesized through thermal treatment at 700 °C, facilitating a synergistic interaction between attapulgite (ATP) and oyster shell (OS) powders. Characterization revealed enhanced dispersibility, increased active site density, and an expanded specific surface area, contributing to superior adsorption capacities of 197 mg g^-1 for Cd(II) and 1459 mg g^-1 for Pb(II), outperforming conventional adsorbents. Kinetic studies indicated that Pb(II) adsorption occurred at a significantly faster rate than Cd(II), with the adsorption processes aligning well with the pseudo-second-order model, suggesting chemisorption as the primary mechanism. Langmuir isotherm analysis confirmed monolayer adsorption behavior for both metal ions, corroborating the formation of stable PbCO₃ and CdCO₃ precipitates on the adsorbent surface. The composite demonstrated high adsorption efficiency across a broad pH range, despite significant influences from increased environmental pH and competitive adsorption by co-existing metal ions, underscoring its robustness in complex environmental conditions. In practical applications, ATP-OS demonstrated significant pH-neutralizing abilities, increasing soil pH values by 2.30-3.07 units and effectively reducing the concentrations and bioavailability of Cd(II) and Pb(II) in acidic agricultural soils. This study highlights the ATP-OS composite's potential as an effective solution for mitigating heavy metal pollution in agricultural settings, contributing to sustainable soil management practices.