Acidity and metallic elements release from AMD-affected river sediments: Effect of AMD standstill and dilution

In acid mine drainage (AMD) polluted rivers, considerable fraction of potential toxic elements are temporarily sequestered by sediments. There are two main potential environmental hazards associated with the sediments, acidity liberation and re-mobilization of metallic elements, during environmental conditions change. The effects of AMD standstill and water dilution on metallic elements migration were assessed in an AMD standstill test and a dialysis experiment. Maintaining AMD standstill, often occurring in AMD damming process, could induce the occurrence of iron secondary minerals precipitation along with attenuation of dissolved elements and a decrease in water pH value. Both field sediments and lab precipitates were confirmed as being dominant with schwertmannite which was the most important source and sink for acidity and metallic elements. The mechanism of cation heavy metals scavenging implied by FTIR results mostly depended on the exchanging of H⁺ from surface hydroxyl groups (-OH) in schwertmannite-rich sediments. For arsenic oxyanion, its adsorption included surface complexation with iron hydroxyl groups at the mineral surface, as well as anion exchange of SO₄^2- present in the structure. The quantities of acidity release differed significantly from 20 to 3714 mol H⁺/t depending on the iron hydroxyl minerals type and their contents in the corresponding sediments in 35 d dialysis, with the release rate well fitted by the second order model. Slight degree of phase transformation in schwertmannite dominant sediment had resulted in a high risk of metallic element release during the 35 d dilution duration. The significant risk of metallic elements release was ranked in the order of Cd > Mn > Zn > Pb, and with more than 89% of Cd released from FS6 and 82% from LPS1. Relatively, Cu and As in sediments were much more stable. Overall, damming was an effective and low cost pretreatment strategy for AMD pollution control. Knowledge of the characteristics of iron secondary minerals in river sediments is essential premise for both comprehensive assessment of site contamination status and effective remediation strategy decision.