Sorption of Ciprofloxacin to Goethite
In this study the sorption of ciprofloxacin (CIP) to the iron oxide goethite was studied under varying conditions like time, pH, concentration, and competitive anions in order to understand the sorption behavior of CIP to goethite and to get information about sorption mechanisms. Experiments were performed by adding CIP at concentration of 250-10000 μg kg-1 to a solution (0.023 M NaCl) with 50 g L-1 goethite at a fixed pH. After 24 hour equilibration time the samples were centrifuged, filtered and analyzed by HPLC with fluorescent detection.
Kinetic studies showed that equilibrium was reached immediately and that the sorption remained constant between 0.5 and 72 hour of contact time.
The solution pH exerted a marked influence on the CIP sorption on goethite. The Kd achieved the maximum at pH 5 (450-500 kg kg-1 for CIP concentration of 1000μg kg-1 and
750-850 kg kg-1 for CIP concentration of 2500 μg kg-1) and decreased rapidly at both lower
and higher pH range. The Kd at pH 4 and 6 was only about 200 kg kg-1 at both CIP concentrations, and below pH 3.5 and above 7 no sorption was observed.
The sorption is linear at pH 7.5 and follows a Langmuir isotherm at pH 4.5, what can be explained by saturation effects at lower pH where the sorption is about two times higher than at pH 7.5.
A strong competition between H2 PO4- and CIP for the sorption on goethite was found. The
Kd of CIP decreased from ca. 600 kg kg-1 to 45 kg kg-1 at 2.2 mM kg-1 added H2 PO4-. On
the other hand the sorption of CIP was unaffected by SO42- and NO3- addition.
Ligand exchange was found to be the most suitable sorption mechanism to explain this sorption behavior. The steep decrease of sorption at pH higher than 5 cannot be explained and suggests that there is maybe another specific interaction, i.e. interaction different than that attributable to ligand exchange, between CIP and goethite surfaces.