Analysis of stability behavior of carbon black nanoparticles in ecotoxicological media: Hydrophobic and steric effects

The stability of carbon black nanoparticles (CB-NPs) was investigated in five different ecotoxicity test media for fish, daphnia, and algae (i.e., ISO Test water, Elendt M4 medium, OECD TG 201 medium, AAP medium, and Bold's Basal Medium) in the presence and absence of Suwannee River Humic Acid (SRHA) as a function of time. Hydrodynamic size, particle sedimentation rate, and visual images of suspensions were analyzed for 96 h, and the SRHA concentration was varied from 0 to 10 mg/L. Zeta potential and water contact angle of CB-NP, and SRHA sorption to CB-NPs were also examined to complement stability analyses. CB-NPs always exhibited negative zeta potential regardless of media type and SRHA concentration, and became more negative in the presence of SRHA due to SRHA sorption. Moreover, CB-NPs were found to be hydrophobic in the absence of SRHA, whereas they became hydrophilic when SRHA was adsorbed. Stability test results showed that regardless of test media, the hydrodynamic size increased fast and the sedimentation rate was high in the absence of SRHA, indicating poor stability of the CB-NPs. Conversely, the presence of SRHA substantially increased the stability of CB-NPs over 96 h, regardless of the SRHA concentration and test media type. Stability trends in the presence and absence of SRHA were not consistent with predictions from classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. However, extended and modified DLVO theories, that also accounted for hydrophobic-attractive forces due to the inherent nature of CB-NPs and steric repulsive forces associated with the brush-like conformation of SRHA adsorbed to CB-NPs, better described CB-NPs' stability in the absence and presence of SRHA, respectively.