Challenges to predicting the fate of emerging classes of organic micropollutants in subsurface environments

Widespread detection of emerging micropollutants (e.g. antibiotics, hormones, personal care products) in soil and aquatic environments has raised serious concerns that necessitate improved understanding of processes controlling their environmental fate. However, existing models and reaction mechanisms are inadequate for predicting the fate of micropollutants that possess complex structures and functional groups not usually present in more commonly studied contaminants. Here, we describe recent efforts to characterize processes and mechanisms contributing to the fate of two widely detected classes of antibiotic micropollutants (sulfonamides, fluoroquinolones). First, we describe a novel microbially mediated-abiotic transformation mechanism for the antibiotic sulfamethoxazole (SMX) in subsurface environments. Rapid dissipation of SMX is observed in ironreducing soil microcosms, and mechanistic studies demonstrate that SMX transformation occurs via abiotic reactions of sorbed Fe(II) with an isoxazole group in the SMX structure, a moiety not previously reported to be amenable to reductive transformation in soil environments. The complex poly-ionogenic structures of some micropollutants have complicated efforts to predict sorption and speciation in subsurface environments. For example, the unique characteristics of zwitterionic fluoroquinolone (FQ) structures (presence of both positively and negatively charged groups) lead to complex electrostatic interactions with charged surfaces that cannot be accounted for by isotherm or pointcharge surface complexation models. Here, we describe the application of a charge-distribution (CD) surface complexation model for zwitterionic species sorbing to variably charged oxide minerals. Model formulation includes functional group-specific inner- and outer-sphere bonding interactions with mineral surfaces, in accordance ATR-FTIR spectroscopic observations. CD model predictions agree closely with measurements of FQ sorption to three oxide soil minerals (TiO2, E-FeOOH, and ,-AlOOH) collected over a wide range of pH, ionic strength and FQ concentrations.