Different co-transport forms of ciprofloxacin with montmorillonite colloids in porous media as affected by pH and cations

Continuous release of antibiotics into the environment poses a potential threat to human and animal health. The existing forms of antibiotics in soil and groundwater determine their fate, transport, and ecological risks. However, antibiotic transport forms (dissolved and colloidal) as affected by mineral colloids have not been well investigated to date.

Ciprofloxacin (CIP), one of the most frequently detected fluoroquinolones (FQs), and montmorillonite, ubiquitous in soil and groundwater, were chosen as the representative antibiotic and colloid. The co-transport forms of CIP with montmorillonite colloids under different pH and cations were systematically investigated in saturated quartz sand porous media. Additionally, solute (TSM) and colloid (TKRSM) transport models were used to simulate CIP and colloid transport and further reveal their co-transport mechanisms.

More than 50% of recovered CIP was transported in colloidal form under different pH conditions due to the high adsorption capacity of montmorillonite colloids for CIP. However, in terms of the total amount, massive deposited colloids inhibited CIP transport under acidic conditions, resulting in 17.21% less CIP recovered. Conversely, under neutral and alkaline conditions, montmorillonite colloids slightly promoted the transport of CIP mainly owing to enhanced desorption. Compared with pH, cations (Na⁺/Ca²⁺) have more effects on CIP co-transport with colloids. At a low NaCl concentration, mobile colloids enhanced CIP transport. However, CIP was mainly transported in dissolved form and inhibited by immobile colloids with the recovery rates decreasing by 39.58%, 32.66%, and 21.78%, respectively, at 0.01 M Na⁺, 0.001 M Ca²⁺, and 0.01 M Ca²⁺. With the increasing ionic strength and valence, the inhibitory effect weakened. Notably, CIP was remobilized with the release of colloids under NaCl solution, whereas not under CaCl₂ solution, which could be explained by DLVO theory and straining effect.

Colloid mobility and their sorption capacity jointly determined their impact on antibiotic transport, and the form, not just the total amount, should be considered carefully when evaluating the potential environmental risk of antibiotics.