Bio-electrochemical potential and mineralogy of metal rich acid mining lake sediment: the “Robule” lake case study

Abstract

Mining for base metals and some critical raw materials is associated with acid, sulphate, and metal-rich waters (AMD). At the points where these waters are accumulated or discharged, the secondary minerals are precipitated, mostly iron-oxyhydroxides, forming characteristic orange sediments toxic to the environment. The sediment or sludge is prone to leaching of metals and sulphate, thus requiring proper disposal to maintain the waste stability. In this study, we assessed the AMD lake “Robule” sediment electrochemical properties and associated mineralogy in sediment microbial fuel cell (SMFC). The SEM–EDS and XRPD analysis revealed schwertmannite, jarosite, goethite and gypsum as the most abundant mineral forms with efflorescent sulphates salts precipitated on top of the air-open cathode. In the acid lake mine water dominant aquatic species are SO42−, Mg2+, AlSO4+, and Fe3+ iron mostly as various sulphate complexes. Although oxidized forms prevail in the lake water and sediment, the possibility of generating electrical current is, to our knowledge, for the first time registered for this type of mining waste. A closed-circuit voltage of 439 ± 60 mV (at 220 Ω external resistance), an average current density of 210 ± 28.6 mA/m2 and a maximal power density of 29 mW/m2 was measured. The absence of sulphide minerals suggests that the biological oxidation of organic molecules at the anode and the iron cycle drives electricity production. The stability of the SMFC environment and the absence of hydrogen sulphide build-up while generating electrical current opens new directions for sustainable management of AMD and the associated sediment.