Influence of septic system wastewater treatment on titanium dioxide nanoparticle subsurface transport mechanisms

Engineered nanomaterials (ENMs) are commonly incorporated into food and consumer applications to enhance a specific product aspect (i.e., optical properties). Life cycle analyses revealed ENMs can be released from products during usage and reach wastewater treatment plants (WWTPs), with titanium dioxide (TiO₂) accounting for a large fraction. As such, food grade (FG) TiO₂, a more common form of TiO₂ in wastewater, was used in this study. Nanomaterials in WWTPs have been well characterized, although the problematic septic system has been neglected. Elution and bioaccumulation of TiO₂ ENMs from WTTPs in downriver sediments and microorganisms has been observed; however, little is known about mechanisms governing the elution of FG TiO₂ from the septic drainage system. This study characterized the transport behavior and mechanisms of FG TiO₂ particles in porous media conditions after septic waste treatment. FG and industrial grade (IG) TiO₂ (more commonly studied) were introduced to septic tank effluent and low-ionic strength electrolyte solutions prior to column transport experiments. Results indicate that FG TiO₂ aggregate size (200-400 nm) remained consistent across solutions. Additionally, elution of FG and IG TiO₂ was greatest in septic effluent at the higher nanoparticle concentration (100 ppm). FG TiO₂ was well retained at the low (2 ppm) concentration in septic effluent, suggesting that particles that escape the septic system may still be retained in drainage field before reaching the groundwater system, although eluted particles are highly stabilized. Findings provide valuable insight into the significance of the solution environment at mediating differences observed between uniquely engineered nanomaterials. Graphical abstract.