Possible mechanisms of CIP-like compounds adsorbed on
minerals have been previously proposed. Formation of surface
complexes between zwitterionic/anionic CIP and aluminum/iron
hydrous oxides was attributed to adsorption [21]. This interaction
may also occur between CIP and the positively charged layer edges
of montmorillonite. Because the fraction of edge to surface area is
commonly rather low in clay minerals, such a mechanism would
make aminor contribution to adsorption by montmorillonite, especially
in acidic aqueous solution. A second mechanism involving
cation bridging, a bonding between the carboxylate group of CIP
and the interlayer cations of clay minerals was pointed out in a
study of enrofloxacin adsorption on Mg-saturated montmorillonite
in an aqueous solution with pH 6 [15]. By comparing the adsorption
properties of CIP with a protonated amine group and phenylpiperazine
(i.e., a structurally related probe compound possessing an
amine group only) on Na-saturated kaolinite and montmorillonite,
a cation-exchange mechanism was suggested [18]. Although the
detection of CIP compounds in soils and their potential impacts
on the ecosystems and human and animal health are obvious,
the dominant interaction in the adsorption processes of the CIPmontmorillonite
system remains uncertain.