Exploring the interactions between antibiotics and soils/minerals is of great importance in resolving
their fate, transport, and elimination in the environment due to their frequent detection in wastewater,
river water, sewage sludge and soils. This study focused on determining the adsorption properties
and mechanisms of interaction between antibiotic ciprofloxacin and montmorillonite (SAz-1), a swelling
dioctahedral mineral with Ca2+ as the main interlayer cation. In acidic and neutral aqueous solutions,
a stoichiometric exchange between ciprofloxacin and interlayer cations yielded an adsorption capacity
as high as 330 mg/g, corresponding to 1.0 mmol/g. When solution pH was above its pKa2 (8.7), adsorption
of ciprofloxacin was greatly reduced due to the net repulsion between the negatively charged clay
surfaces and the ciprofloxacin anion. The uptake of ciprofloxacin expanded the basal spacing (d001) of
montmorillonite from 15.04 to 17.23Å near its adsorption capacity, confirming cation exchange within
the interlayers in addition to surface adsorption. Fourier transform infrared results further suggested
that the protonated amine group of ciprofloxacin in its cationic form was electrostatically attracted to
negatively charged sites of clay surfaces, and that the carboxylic acid group was hydrogen bonded to the
basal oxygen atoms of the silicate layers. The results indicate that montmorillonite is an effective sorbent
to remove ciprofloxacin from water.