Iron precipitation kinetics during microbial ferrous-ion oxidation

Abstract

The formation of ferric-ion precipitates, such as jarosite, has been extensively documented during the bioleaching process. These precipitates serve as pathways for unwanted iron to escape from the system in various processes. However, a significant accumulation over an extended period can hinder reaction kinetics and reduce the overall efficiency of bioleaching. Therefore, this study sought to investigate the kinetics of the ferric-ion precipitates that are formed through bacterial oxidation. Experiments were conducted using a mixed mesophilic culture in shake flasks, with temperatures set at 30, 35, and 40 °C in a shaking incubator maintained at a constant pH of 1.7. The experiments lasted 14 days, with an agitating speed of 120 rpm. Upon analysing the quantification results, the maximum quantity of ferric-ion solid precipitates that developed was 2.48 grams at a temperature of 40 °C. Additionally, the data indicated that ferric-ion precipitation began 24 hours into the process. The precipitates generated were characterized by dense, light ochreous yellow residues. The patterns created by the X-ray powder diffraction (XRD) of these crystals were identified as potassium jarosite (K-jarosite), with its chemical formula being [KFe3(SO4)2(OH)6]. The scanning electron microscopy (SEM) analysis of their shape showed clusters of spherical, oval, and/or rectangular, powdery particles, all without clear, sharp edges. The Fourier transform infrared (FTIR) spectra of these crystals revealed the vibrational frequencies of SO42−, H2O, OH, and Fe–O in the jarosite. Furthermore, the thermogravimetric analysis (TGA) tests indicated the loss of hydroxyl groups from K-jarosite and the complete decomposition of yavapaiite when heated. The formation of ferric precipitates occurred according to first-order kinetics. The estimated activation energy was 117.2 kJ/mol with a frequency factor (K) of 2.94 X 1020 mmol Fe3+.h-1, indicating that the process was endothermic, with an average [Fe3+] of threshold 1.22 g/L. The thermodynamic parameters obtained were entropy (ΔS) = 0.25 kJ/mol K, Gibbs free energy (ΔG) = 43.89, 42.64, and 41.39 kJ/mol at 30, 35, and 40 °C, respectively, and Enthalpy (ΔH) = 120 kJ/mol. These values suggest that the formation of ferric precipitates was non-spontaneous and required a considerable amount of energy to proceed towards spontaneity. This study revealed that the generation of iron precipitation during microbial ferrous-ion oxidation by mesophilic consortia follows first-order kinetics. This process is endothermic and non-spontaneous, necessitating energy to transition to a spontaneous state. The findings could provide valuable insights for biohydrometallurgical processes aimed at managing and controlling jarosite formation and accumulation, thereby minimizing metal losses.