Parameters influencing ciprofloxacin, ofloxacin, amoxicillin and sulfamethoxazole retention by natural and converted calcium phosphates.

The retention of four antibiotics, ciprofloxacin, ofloxacin, amoxicillin and sulfamethoxazole by a natural phosphate rock (francolite) was studied and compared with a converted hydroxyapatite powder. The maximum sorption capacities were found to correlate with the molecular weight of the molecules. The mechanisms of sorption depended mostly on the charge of the antibiotic whereas the kinetics of the process was sensitive to their hydrophobic/hydrophilic character. The two materials showed slightly distinct affinities for the various antibiotics but exhibited similar maximum sorption capacities despite different specific surface areas. This was mainly attributed to the more pronounced hydrophobic character of the francolite phase constituting the natural phosphate. These data enlighten that the retention properties of these mineral phases depend on a complex interplay between the inter-molecular and molecule-solid interactions. These findings are relevant to understand better the contribution of calcium phosphates in the fate and retention of antibiotics in soils.