Rare earth elements association with predominant elements in solids precipitated during alkalinization of acid mine water

Rare Earth Elements (REE) scavenging during remediation of acid mine water using Passive Treatment Systems (PTS) resulted in PTS to be considered as a potential source of REE. This has sparked interest in the possible retrieval of REE from the solids precipitated along PTS. The development of an efficient retrieval method requires first to assess the REE scavenging mechanisms and to identify any possible association/correlation of REE with predominant elements (i.e. Fe, Al, Mn, Ca, S) present in the precipitated solids. These first steps were here tackled using Laser Ablation (LA) Inductively Coupled Plasm-Mass Spectrometry (ICP-MS) in combination to Scanning Electron Microscope/Energy Dispersive X-ray spectroscopy (SEM/EDX). PTS was simulated by batch alkalinization experiments, which were performed to sequentially increase the pH of an acid mine water sample through CaCO3 addition. The solids precipitated up to pH≈4 and between pH 4–6 were collected separately for characterization. Results revealed a medium positive correlation (R2 > 0.5) between Ca and S in the solids collected up to pH≈4 indicating gypsum precipitation which was also confirmed by SEM/EDX. In addition, a strong positive correlation (R2 > 0.7) between Ca, S and light REEs (LREEs) was observed pointing also to an association of LREEs with gypsum. In the solid collected between pH 4–6, LREEs and Gd showed strong positive correlations with Fe and medium positive correlations with Al. The observed correlations indicate that during the alkalinization of acid mine water by CaCO3, gypsum acts as a host mineral for LREEs at low pH (up to pH≈4) while at higher pH (between pH 4–6), Fe and Al oxyhydroxides appear to be the preferential host minerals for LREEs, Gd and Er. This implies that for the possible recovery of REE from PTS, gypsum can be targeted mostly in the first layers of the PTS and especially for LREEs whereas Fe- and Al- oxyhydroxides can be targeted for all REE further on in the PTS, i.e. where higher pH are obtained.