Bioleaching is a promising and feasible option for metal extraction from sulfidic mine wastes. The main objective of this study was to investigate how the gas–liquid mass transfer phenomena affect the bioleaching of sulfidic mining wastes, with the ultimate goal of optimizing the gas supply. Experiments were performed in four continuous stirred-tank reactors (CSTRs) arranged in series with a total working volume of 114-L. Three different gas compositions were tested: (I) air, (II) air enriched with 0.5% v/v CO2, and (III) a mixture of N2 (69.5% v/v), O2 (30% v/v), and CO2 (0.5% v/v) in the first two reactors of the series. Results showed that during Condition I, the CO2 transfer rate was not sufficient, particularly in the primary reactor resulting in a low sulfide dissolution yield. Enriching air with 0.5% v/v of CO2 during Condition II increased both sulfide dissolution in the primary reactor and oxygen uptake rate (OUR) in the entire system, which caused oxygen transfer limitations. Sulfide dissolution was further enhanced during Condition III due to increased partial pressure of O2 in the first and second reactors, achieving a total cumulative sulfide dissolution yield of 81% and the dissolution of 86% of Co and 30% of Sb contained in the MW. This study emphasizes the importance of sufficient gas supply in the bioleaching process and demonstrates that gas–liquid mass transfer considerations are crucial for upscaling and cost optimization.