and mineral dissolution, leading to Acid Mine Drainage (AMD) [3,17]. AMD significantly increases heavy metal solubility, posing risks to human health and ecosystems, while abandoned facilities present further structural risks, including dumps instability [4,17]. In alignment with circular economy principles, the mining sector has shifted toward reinterpreting tailings as secondary sources of critical metals [2,15,16]. This paradigm shift is driven by the depletion of high-grade primary ores and the increasing demand for strategic materials required for the global energy transition [5,12]. Cobalt (Co) has emerged as a high-priority element due to its essential role in rechargeable battery technologies and electromobility [6,7,16]. Research indicates that copper and nickel tailings often contain cobaltiferous pyrite with grades comparable to low-grade primary deposits [6,7]. Recovering cobalt from these sources mitigates environmental liabilities and reduces dependence on primary mining, which is often hindered by geopolitical and social constraints [7,15]. However, conventional hydrometallurgical recovery remains limited by high operational costs and secondary effluent generation [5,15]. Microbial bioleaching offers a sustainable alternative for metal recovery from low-grade residues [5,12]. This process utilizes acidophilic chemolithotrophic microorganisms to catalyze the oxidation of reduced iron and sulfur, generating reactive lixiviant agents that release target metals [3,11]. Bioleaching systems are increasingly recognized for their low carbon footprint, reduced energy consumption, and compatibility with in situ or column-based processing [12,15,16]. Despite these advantages, tailings bioleaching is often constrained by slow initial kinetics, low indigenous microbial populations, and suboptimal physicochemical conditions [9,10,11,14]. Consequently, recent research has focused on optimized microbial inoculation strategies using preadapted consortia to reduce lag phases and enhance sulfide oxidation rates [9–11,13]. Successful inoculation depends on timing, microbial composition, and the hydrodynamic conditions within the system [11,14,18] among main factors. Furthermore, integrated approaches such as acid curing are employed to neutralize acid consumption and facilitate microbial establishment [8,15]. Within this context, the present study evaluates various inoculation strategies for the bioleaching of cobalt-bearing sulfidic tailings. The primary objective is to determine how specific inoculation approaches influence metal solubilization efficiency, particularly regarding cobalt recovery. Through column bioleaching experiments and systematic monitoring, this work aims to develop optimized biotechnological strategies for tailings valorization, supporting sustainable mining practices and environmental stewardship [12,15,16,18]. 2. Materials and methods 2.1.Microbial consortium acquisition and selection To identify microbial consortia suitable for bioleaching, microbiological scouting was conducted at the San José de Pucobre flotation plant (Copiapó, Chile). Samples collected from various stages of the flotation process served as initial inoculum for batch cultures. These cultures were incubated at 30, 45, and 55 °C in KMD media (0,99 g/l (NH4)2SO4, 0,15 g/l NaH2PO4*2H2O, 0,1 MgSO4*7H2O, 0,021 g/l CaCl2*2H2O, 0,05 g/l KHPO4*2H2O, supplemented with 4 g/l FeSO4*7H2O or/and 0,1% S°) to evaluate microbial growth alongside iron and sulfur-oxidizing
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