ADSORPTION OF PHARMACEUTICALS AND PERSONAL CARE PRODUCTS (PPCPs) TO NANO-SCALE OXIDES

Pharmaceuticals and personal care products (PPCPs) have been detected in sediment-freshwater systems, where nanometer-size (10^-9 m) oxides are likely important in the transport of these compounds. To investigate the interactions between PPCPs and oxide nanoparticles (NPs), batch adsorption experiments were performed using six NPs (TiO₂, Fe₂O₃, MgO, SiO₂, Al₂O₃, and CaO) and seven PPCPs: salicylic acid, propanolol, carbamazepine, bisphenol-a, diclofenac, caffeine, and ibuprofen. Composite PPCP solutions (1-10ppm) were batched with 50mg of each NP. Equilibrium aqueous concentrations were determined by LC/MS after mixing, and sorption was quantified by comparison to control samples. Preliminary data were fit with linear isotherms.

Results from this study illustrate how sorption can be controlled by the surface charge of the oxide NP and the overall charge on the attendant PPCPs. Over the pH range of the experiments (~6-8), PPCPs are either cationic, neutral, or anionic, and the net surface charge of oxide NPs depends on their PZC (point of zero charge; the pH value below which the NP has a net positive surface charge). For example, sorption of anionic ibuprofen and diclofenac to negatively-charged SiO₂ (PZC=2) was negligible (K_d<0.1 L/kg), while sorption coefficients for these compounds to positively-charged MgO (PZC=12) were 43±8 L/kg and 394±66 L/kg, respectively.

Results from the study also demonstrate that compound charge and NP PZC are not the only parameters controlling sorption. For example, the cationic compound propanolol was the only compound with significant sorption to SiO₂ (K_d=10±6 L/kg), but its sorption to positively-charged MgO was even greater (67±33 L/kg). Diclofenac was highly sorbed to Fe₂O₃ (PZC=~7; K_d=361±80 L/kg) and MgO (PZC=12; K_d=394±66 L/kg) compared to TiO₂ (PZC=7.2; K_d=4.4±0.7 L/kg), CaO (PZC=~8; K_d=9.5±1.0), and Al₂O₃ (PZC= 9.1; K_d=9.6±0.9). Bisphenol-A exhibited much greater sorption to MgO (K_d=119±9 L/kg) than the other five NP (1.6 to 11 L/kg). Sorption coefficients for carbamazepine (<0.1 to 6.0 L/kg) and caffeine (0.6 to 2.9 L/kg) were relatively consistent for all six NP. Future work is planned to investigate the possible effects of nonlinear or competitive sorption, PPCP complexation, and NP aggregation.