Aluminum mineral transformation drives reacidification and metal remobilization in a neutralized acidic pit lake

Lime neutralization is widely used to treat acid mine drainage (AMD), yet its long-term efficacy is often undermined by subsequent reacidification. A full-scale field study reveals the transformation of metastable aluminum (Al) minerals. This transformation acts as the key geochemical switch that governs this reversibility. During neutralization, Al precipitates as poorly crystalline hydroxysulfates, efficiently sequestering heavy metals via coprecipitation. However, when treatment stops, these metastable Al phases dissolve preferentially as reacidification begins. During reacidification, Al is released at 92.06 mg/(m²·d). This flux far exceeds that of all other measured metals, and Al accounts for more than 50% of the regenerated acidity. This dissolution converts Al from a primary metal sink into the dominant source of both metals and acidity. Our findings reveal that the stability of Al-bearing phases dictates the long-term success of AMD remediation, not short-term metal removal efficiency. We therefore argue for a paradigm shift toward strategies that ensure the formation of stable mineral phases to achieve sustainable AMD management.