Encapsulation on rhodochrosite stabilizes toxic CdS nanoparticles in aqueous oxidation systems

Manganese (Mn) redox cycling and phase variation reactions play a crucial role in natural water settings. Rhodochrosite (MnCO₃), a mineral commonly found in oxygen-deprived environments, develops a surface oxide film upon exposure to oxygen. This Mn oxide film significantly influences the fate of nanoparticles within its proximity. Employing atomic force microscopy (AFM), this study examined the growth of the Mn oxide film on MnCO₃ and the encapsulation of cadmium sulfide nanoparticles (CdS-NPs). Results revealed the gradual development of a nanometer-thick oxide film on MnCO₃ over time in aerobic conditions, with the rate of film formation correlated to the solution's ionic strength. The oxide film on MnCO₃ encapsulated pre-adsorbed CdS-NPs, either through embedding or covering. Intriguingly, CdS-NPs were found to enhance the growth of the Mn oxide film, contributing to the fixation of CdS-NPs. Furthermore, an ultrasonic desorption protocol verified the stability of CdS-NPs encapsulated by the Mn oxide film on MnCO₃. This study elucidates a novel mechanism for immobilizing CdS-NPs in aqueous oxidizing conditions, providing valuable insights into the behavior and distribution of toxic nanoparticles in environmental contexts. ENVIRONMENTAL IMPLICATION: This study classifies cadmium sulfide nanoparticles (CdS-NPs) as "hazardous material" due to the inherent toxicity of cadmium, posing risks to both ecological and human health. The research addresses environmental concerns by exploring the interaction between CdS-NPs and manganese (Mn) redox cycling. The formation of a Mn oxide film, encapsulating CdS-NPs, suggests a mechanism for limiting the dispersion of these hazardous nanoparticles in oxidizing water. This provides valuable insights for managing the environmental impact of CdS-NPs, offering a proactive strategy to mitigate their adverse effects in natural systems.