Influence of a polymer sunscreen additive on the transport and retention of titanium dioxide nanoparticles in water-saturated porous media†
Titanium dioxide nanoparticles (nTiO2) are utilized in an array of consumer products including paints, sunscreens, cosmetics, and food. These products typically contain stabilizing agents that may alter nTiO2 fate when released into the environment. The objective of this study was to investigate the effects of TEGO carbomer, a polymeric stabilizing agent used in sunscreen, on the transport and deposition behavior of nTiO2 in porous media. Aqueous nTiO2 suspensions at pH 5.0 or 7.5 ± 0.2 were introduced into water-saturated columns packed with Federal Fine Ottawa sand. In the absence of carbomer, nTiO2 was not detected in effluent samples at pH 5, which was below the estimated point of zero charge (PZC) of nTiO2 (pH 6.3), while greater than 80% elution of nTiO2 was observed at pH 7.5. The addition of 3 mg L−1 carbomer decreased the PZC from 6.3 to less than 5, and resulted in greater than 94% elution of nTiO2 at pH 5 and 7.5. A nanoparticle transport model that incorporates a first-order, maximum retention capacity term was able to capture column breakthrough and retention data. Model results indicate that the presence of carbomer reduced the average retention capacity of the solid phase from 3.40 to 1.10 μg TiO2 g−1 sand, irrespective of solution chemistry changes. These findings demonstrate the substantial impact that polymeric stabilizing agents can have on the fate of nTiO2 in porous media, potentially enhancing nTiO2 mobility in the environment and reducing the efficiency of filtration systems for nTiO2.