Ciprofloxacin sorption by dissolved organic carbon from reference and bio-waste materials

Amphoteric pharmaceuticals are released to the environment in treated wastewater, medicated aquaculture feed, and through land disposal of livestock wastes and municipal biosolids. These wastes are rich in dissolved organic carbon (DOC), which may bind and potentially enhance pharmaceutical transport in the environment. To assess this potential, the antimicrobial ciprofloxacin (CIP) was chosen as a model amphoteric pharmaceutical to quantify DOC–water sorption coefficients (Kdoc, L kg−1 DOC) for four reference humic materials with varying chemical properties, digested and undigested municipal biosolids, treated municipal wastewater, and beef lagoon wastewater. For reference humic materials, the predominance of sorption by cation exchange was exemplified by decreasing sorption with increasing pH and ionic strength, as well as highly predictable relationships between the sorbed CIP concentration normalized to the DOC charge versus the free aqueous concentration of the CIP cation. In contrast to humic materials, however, sorption to DOC from digested biosolids was low and showed no pH dependence, suggesting that additional weaker mechanisms contributed to binding interactions. CIP had no quantifiable affinity for DOC from undigested biosolids, treated wastewater, or beef lagoon wastewater. With Kdoc values of ∼104 L kg−1 estimated for CIP concentrations reported in biosolids (∼20–60 μg kg−1 wwt), the DOC measured for digested biosolids in this study (40 mg L−1) could increase CIP mobility by up to ∼15%, thus DOC-facilitated transport should be considered in environmental fate assessments for amphoteric pharmaceuticals. However, the potential for waste-derived DOC to enhance CIP transport would have been greatly overestimated using information derived only from reference humic materials.