Evaluation of Hydrotalcites Produced in Seawater for Remediation of Metalliferous Acid Mine Drainage and Submarine Placement

Abstract

Uncontrolled release of acid mine drainage (AMD) causes widespread detrimental impacts on the receiving environment. Thus, effective treatment to neutralise AMD effluent pH and capture a suite of metals is required. In-situ hydrotalcite (HTC) precipitation is an emerging technology for AMD remediation. HTC has an inherent capacity to accommodate a range of cations and anions during in situ formation, offering a method of broad-spectrum contaminant removal. This study explored the feasibility of using seawater as an Mg source and synthetic AMD in HTC formation. The HTC was formed from a stoichiometric combination of synthetic AMD and seawater. While three initial stoichiometric M2+:M3+ ratios of 2:1, 3:1, and 4:1 were investigated, only HTC with an M2+:M3+ ratio of 2:1 was generated, as confirmed by both mineralogical and geochemical analyses. Importantly, the HTC was demonstrated to effectively remove a suite of metals present in AMD such as Cu, Zn, Al, and Mn with removal rates of between 99.97 to 99.99%. The HTC precipitate contained ≈6.6% Cu and 4.1% Zn, and thus shows the potential, if required, for future metal recovery. Since submarine placement is often used in metal mining and processing operations proximal to the coast, the stability of the HTC precipitate in seawater was also investigated. Importantly, only 0.2% of the Cu and 1.1% of the Zn within the HTC were subsequently leaching in decreasing increments into seawater over 30 days with decreasing increments after the initial seven days. This indicates robust element retention and confirms the potential of HTC for AMD remediation with direct submarine placement.