Cosmetic nanomaterials in the environment: nano-zinc oxide and zinc-influence on soil microorganisms

This study aimed to investigate the impact of water-containing sunblock products with different residual quantities of bulk ZnO or ZnO nanoparticles (NPs) on soil microorganisms using a bioassay toxicity experiment. The two forms of ZnO were studied at different concentrations ranging from 0 to 10 mg L −1 , and leachates obtained from the water disposal during a handwashing simulation experiment were also evaluated, along with raw sunblocks containing both bulk ZnO and ZnO NPs (at 50% and 100%). The key characteristics of each type of ZnO material were analyzed using multiple analytical techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and ultraviolet–visible spectroscopy (UV–Vis). The outcomes revealed that the stability of ZnO nanoparticles was considerably high, with the highest dissolution rate estimated after 36 h as 0.19% of the sunblock's overall ZnO NPs concentration. The different forms of pure ZnO used in the bacterial bioassay demonstrated that the Zn concentration of 10 mg L −1 exhibited the largest inhibition zone area compared to the other treatments. The disc diffusion bioassay findings confirmed that ZnO NPs are active components with greater toxicity than bulk ZnO. These results demonstrated that the antimicrobial effect was exclusively due to the nano-specific influence at higher concentrations. However, additional research is needed to understand better the environmental effects of different types of ZnO particles disposed of by sunblock users. Examining how such substances react in actual environmental conditions is crucial, as they contain various diverse ingredients that may cause varying reactions compared to bulk ZnO particles.