Lead (Pb) retention in sandy sediments impacted by acid mine drainage derived from abandoned smelter residues

This study evaluates the Pb retention capacity of AMD-affected and unaffected sediments to assess the role of sediment transformation by AMD exposure in Pb retention. Samples were collected from impacted sites in San Antonio Oeste, Argentina, where Pb-rich smelter residues have been abandoned since 1984. Leaching tests and sorption experiments, including kinetics, isotherms and pH-dependent tests, were performed to verify and quantify Pb retention in six sediments (four classified as AMD-affected and two as unaffected). According to the leaching test, only the AMD-affected samples with pH less than 6 showed Pb release between 39 and 45 mg kg^-1 Pb. Sediments have shown favorable conditions for Pb retention under field pH conditions. Fortunately, the sorption experiments indicated that the AMD-affected sediments had higher sorption capacities than the unaffected ones. This is likely due to the presence of Fe (oxy)hydroxide coatings in non-reactive minerals contributing to a greater availability of sorption sites. According to the Langmuir sorption model, the sediments most affected by AMD showed a maximum sorption capacity up to four times higher than that of unaffected ones. pH played a crucial role in the retention mechanisms: under neutral to alkaline conditions, Pb retention was dominated by precipitation in immobile forms. However, under acidic conditions (pH < 6), Pb retention occurred primarily by chemisorption, a less stable mechanism that may contribute to the gradual mobilization of Pb into the underlying sediments and, potentially, the phreatic aquifer. Predicting Pb behavior and planning remediation requires understanding of mineralogical and geochemical influences in AMD-affected sites.