Effects of ageing on rare earth elements retention and the structure of secondary mineral phases formed during acid mine water alkalinization

Acid Mine Drainage (AMD) remediation with CaCO₃ in passive treatment systems (PTS) results in the precipitation of gypsum and various Fe and Al minerals, which immobilize toxic metals and rare earth elements (REE). The poorly crystalline and metastable formed phases can transform into more stable forms over time, potentially affecting REE scavenging. This study investigated how ageing affects solid-phase transformations and REE scavenging. Batch alkalinization experiments with CaCO₃, mimicking PTS, were set up by sequentially increasing the pH of two synthetic AMD waters: medium Fe (Syn_MFe) and low Fe (Syn_LFe). Suspensions were aged for one year at pH∼4 (Syn_MFe only) and at pH∼6 (both solutions), with results compared to previously published data obtained after two weeks. Changes due to ageing were assessed by supernatant analysis (ICP-MS) and solid-phase characterization (XRD, LA-ICP-MS, and SEM/EDX). Ageing for one year at pH∼4 did not affect REE removal in Syn_MFe. In contrast, increased REE removal was observed by ageing at pH∼6 in both AMD solutions, driven by a gradual pH increase of 0.6 to 1.2 units due to the slow dissolution of excess CaCO₃ remaining after two weeks. Gypsum remained the only crystalline phase, while Fe- and Al-phases, potentially jarosite(H), schwertmannite, basaluminite, ferrihydrite and gibbsite, stayed amorphous. At pH∼4, MREE associations shifted from gypsum to Fe-S-O phases without affecting overall REE removal. Increased REE removal at pH∼6 was attributed to pH-dependent sorption, likely on basaluminite and gibbsite. The results of the present study show that the effectiveness of REE scavenging in AMD PTS is likely to improve over time due to further limestone dissolution increasing the pH, which would reduce the environmental REE risks.