Effect of surfactants on the aggregation and stability of TiO2 nanomaterial in environmental aqueous matrices

The high likelihood of the coexistence of surfactants and engineered nano-materials (ENMs) in aqueous environments redoubles the need to study the effects of surfactants on the behavior and transport of nano-materials. In this study, we chose anionic sodium dodecyl sulfate (SDS) and nonionic nonylphenol ethoxylate (NPEO, Tergitol NP-9) to study the effect of surfactants on the behaviors of a typical TiO2 ENM in water environment. An adsorption isotherm and zeta potential study showed that both surfactants could be adsorbed onto the surface of nano-TiO2 but that only SDS can significantly decrease the zeta potential of TiO2. For a better simulation of the dynamic natural water environment, we conducted a 24-h batch experiment to study the aggregation behaviors of nano-TiO2 in the presence of SDS or NP-9 surfactants and recorded the sedimentation of the aggregates in solutions containing the surfactants over a 24h-period. SDS and NP-9 both reduced the growth of the TiO2 aggregates and retarded the aggregate sedimentation at surfactant concentrations ≥0.015% (w:v). The aggregation of TiO2 was far more remarkable in six environmental water matrices from different natural origins than in Milli-Q water, mainly because of the relatively high ionic strength of the former. The introduction of surfactants, however, still retarded the aggregation and sedimentation of TiO2 in the six environmental water matrices. SDS exerted a stronger reductive effect than NP-9. This study reveals that the presence of surfactants may increase the bioavailability and environmental risk of nanomaterial contaminants by retarding the aggregation and promoting the transport of nanomaterials in natural aqueous environments. •To better simulate the dynamic natural water environment, the aggregation of TiO2 nanoparticles was studied not by the widely applied static experimental method, but by a batch experiment conducted in a shaker.•Both ionic sodium dodecyl sulfate (SDS) and non-ionic nonylphenol ethoxylate (NPEO) surfactants can remarkably reduce the aggregation of nano-TiO2 and stabilize the aggregate when the surfactant concentration exceeds 0.015% (w:v).•The aggregation and sedimentation of nano-TiO2 are more remarkable in environmental aqueous matrices than in Milli-Q water, but surfactants are effective in reducing the aggregation and retarding the sedimentation of TiO2. [Display omitted]