Arsenic contamination and secondary mineral evaluation in mine drainage using integrated acid–base accounting and toxicity characterisation leaching procedure: the case of Obuasi Mine, Ghana

Abstract

This study assessed arsenic (As) contamination and the characteristics of secondary minerals in mine drainage using integrated acid–base accounting (ABA) and toxicity characterisation leaching procedure (TCLP), aqueous and solid-phase media monitoring, as well as a textural and mineralogical analysis of tailings material at the Obuasi mine in Ghana. The neutralisation potential and maximum potential acidity from an ABA test; As and pH in TCLP leachate and mine drainage, particle size, trace and major elements in tailings, were analyzed using the relevant methodologies. The ABA results which indicated a high tendency for acid generation were interpreted using a four data point model developed for the purpose. The model allows an additional 40 kg CaCO3/ton amendment value to take care of any delayed acidification. Arsenic leaching simulations are described by the equation: [As] \(= {\text{A }}\left( {\frac{\text{NP}}{\text{MPA}}} \right)^{2} - \;{\text{B}}\left( {\frac{\text{NP}}{\text{MPA}}} \right)\; + \;{\text{C}}\). With the y-axis set to zero, pH simulation equations are defined by pH = K\(\left( {\frac{\text{NP}}{\text{MPA}}} \right)\), where the K and pH values ranging from 82 to 100 and 4.1 to 9.0 respectively, represent limits within which natural attenuation of As in drainage can be sustained. Expressing the pH equation above in the form, pH = (ln |R2| − lnK), the simulation equations defined the range 2.2–4.6 which conformed to pH range for which secondary minerals such as jarosite, schwertmannite and ferrihydrite, could occur as buffering agents in mine drainage. Also, molar value plot for Fe2O3 against Al2O3 indicate the precipitation of goethite and alunite into tailings, while increased fines and As enrichment in older tailings confirmed As attenuation. Such information is relevant for front-end planning for As impact remediation in mine drainage.