The adsorption of citric acid onto goethite, kaolinite, and illite was measured as
a function of pH (adsorption edges) and concentration (adsorption isotherms) at 25
degrees C. The greatest adsorption was onto goethite and the least onto illite. Adsorption
onto goethite was at a maximum below pH 5 and decreased as the pH was increased to
pH 9. For kaolinite, maximum adsorption occurred between pH 4.5 and pH 7, decreasing
below and above this pH region, while for illite maximum adsorption occurred between
about pH 5 and pH 7, decreasing at both lower and higher pH. ATR-FTIR spectra of citrate
adsorbed to goethite at pH 4.6, pH 7.0, and pH 8.8 were compared with those of citrate
solutions between pH 3.5 and pH 9.1. While the spectra of adsorbed citrate resembled
those of the fully deprotonated solution species, there were significant differences.
In particular the C[bond]O symmetric stretching band of the adsorbed species at pH
4.6 and 7.0 changed shape and was shifted to higher wave number. Further spectral
analysis suggested that citrate adsorbed as an inner-sphere complex at pH 4.6 and
pH 7.0 with coordination to the surface most probably via one or more carboxyl groups.
At pH 8.8 the intensity of the adsorbed bands was much smaller but their shape was
similar to those from the deprotonated citrate solution species, suggesting outer-sphere
adsorption. Insufficient citric acid adsorbed onto illite or kaolinite to provide
spectroscopic information about the mode of adsorption onto these minerals. Data from
adsorption experiments, and from potentiometric titrations of suspensions of the minerals
in the presence of citric acid, were fitted by extended constant-capacitance surface
complexation models. On the goethite surface a monodentate inner-sphere complex dominated
adsorption below pH 7.9, with a bidentate outer-sphere complex required at higher
pH values. On kaolinite, citric acid adsorption was modeled with a bidentate outer-sphere
complex at low pH and a monodentate outer-sphere complex at higher pH. There is evidence
of dissolution of kaolinite in the presence of citric acid. For illite two bidentate
outer-sphere complexes provided a good fit to all data.