Chalcopyrite passivation phenomenon drastically slows the copper extraction rate in hydrometallurgical processes; currently, scientists have reached an apparent consensus about the causes of this phenomenon. A great deal of information on this subject can be found in the literature, but there are also abundant differences in the experimental conditions applied and contradictions in the conclusions reached, making it difficult to obtain a global and structured view of the real advances in the research. This paper comprises an ordered classification of the available information regarding the unresolved and the often-ignored issues addresses by electrochemistry and surface analysis techniques.

Currently, the high demand for copper is in direct contrast with the decrease in the mineral grade and, more significantly, the concerns regarding the environmental impact that arise as a result of processing such low-grade materials. Consequently, new mineral processing concepts are needed. This work explores the chemical dissolution of chalcopyrite concentrate at ambient pressure and moderate temperatures in a deep eutectic solvent. Copper and iron are dissolved without changing their oxidation state, without solvent pH change, and stabilized as a chloride complex with no evidence of passivation. Chemical equilibria of the metallic chloride complexes limit the dissolution, and the step that is rate-controlling of the kinetics is the interdiffusion of species in the solvent. The chemical mechanism may involve initial chloride adsorption at positive sites of the solid surface, pointing out the importance of surfaces states on chalcopyrite particles. A model based on a shrinking particle coupled with pseudo-second-order increase in the liquid concentration of copper describes the dissolution kinetics and demonstrates the importance of the liquid to solid ratio. Iron and copper can be recovered separately from the solvent, which highlights that this concept is an interesting alternative to both redox-hydrometallurgy and pyrometallurgy to obtain copper by the processing of chalcopyrite concentrate.

Nanobubble technology has emerged as a transformative approach in bioprocessing, significantly enhancing mass-transfer efficiency for effective microbial activity. Characterized by their nanometric size and high internal pressure, nanobubbles possess distinct properties such as prolonged stability and minimal rise velocities, allowing them to remain suspended in liquid media for extended periods. These features are particularly beneficial in bioprocesses involving aerobic strains, where they help overcome common obstacles, such as increased culture viscosity and diffusion limitations, that traditionally impede efficient mass transfer. For instance, in an experimental setup, nanobubble aeration achieved 10% higher soluble chemical oxygen demand (sCOD) removal compared to traditional aeration methods. Additionally, nanobubble-aerated systems demonstrated a 55.03% increase in caproic acid concentration when supplemented with air nanobubble water, reaching up to 15.10 g/L. These results underscore the potential of nanobubble technology for optimizing bioprocess efficiency and sustainability. This review delineates the important role of the mass-transfer coefficient (k_L) in evaluating these interactions and underscores the significance of nanobubbles in improving bioprocess efficiency. The integration of nanobubble technology in bioprocessing not only improves gas exchange and substrate utilization but also bolsters microbial growth and metabolic performance. The potential of nanobubble technology to improve the mass-transfer efficiency in biotechnological applications is supported by emerging research. However, to fully leverage these benefits, it is essential to conduct further empirical studies to specifically assess their impacts on bioprocess efficacy and scalability. Such research will provide the necessary data to validate the practical applications of nanobubbles and identify any limitations that need to be addressed in industrial settings.

Bilge water generated on sea ships is a mixture of water and hydrocarbons that must be further separated and treated. In this work, an innovative electrochemical reactor, with parallel plate electrodes, is operated to achieve separation and flotation of oily traces, electroreduction of contained metals and electro-oxidation of dissolved organic matter. All the environmental parameters measured in the study (i.e. colour, turbidity, COD and Pb–Zn concentration) were reduced by the process (average reduction for the 2-h experiences 80, 70, 50 and 40%, respectively). Increasing the imposed current favours all the processes occurring in the cell; however, there exists an optimum in terms of energy consumption for target organic load removal.

The volumetric mass transfer coefficients (k_La) of oxygen during sorption and desorption were analyzed using nanobubbles (NBs) of air and pure oxygen under various experimental conditions. The results showed that oxygen NBs achieved an increase in dissolved oxygen (DO) levels during absorption, reaching peaks of 30–34 mg∙L⁻¹ and stabilizing at 31.3 ± 0.2 mg∙L⁻¹, with a volumetric mass transfer coefficient of 0.105 ± 0.002 min⁻¹. In comparison, air NBs showed a lower efficiency, with peak DOs of 8∙10 mg∙L⁻¹ and k_La of 0.048 ± 0.001 min⁻¹. In desorption studies, oxygen NBs had higher DO retention, reducing from 30.0 mg∙L⁻¹ to 15.0 mg∙L⁻¹ in 300 min, with a k_La of 0.042 ± 0.003 min⁻¹, while air NBs decreased more rapidly, with a k_La of 0.028 ± 0.002 min⁻¹. When oxygen was used, k_La outperformed air in both absorption and desorption, with a higher k_La during absorption, a lower k_La during desorption, and higher stability. In addition, the results show that the residence time has an important impact on the performance of NBs, showing that the direct influence of the flow dynamics and surface/to/volume ratio influences the value of k_La. The results highlight the superior performance of oxygen NBs versus air NBs in terms of mass transfer efficiency and stability and highlight the effect of residence time and NB composition in applications requiring efficient oxygen transfer, given the promising prospects for the development of advanced aeration technologies in industrial and environmental contexts.