Study of polyacrylic acid adsorption on the interface of hydroxyapatite – electrolyte solutions

The adsorption of polyacrylic acid (PAA) on hydroxyapatite (HAP) from HAP saturated solutions was
studied in this work as a function of PAA concentration, temperature, pH and ionic strength. In all
experiments, the adsorption was studied under conditions of negatively charged adsorbent and
adsorbate, respectively. These conditions ensured that adsorption was not electrostatically favored.
The solution composition was found to influence significantly the maximum of the adsorbed amount.
In fact, the adsorption of PAA, was found to increase with ionic strength, due to the decrease of
double layer thickness of HAP particles. The effect of temperature and ionic strength on the kinetics
of adsorption are also reported. Adsorption isotherms obtained at 30oC were found to be Langmuirtype,
while at higher temperatures they were of the Brunauer-Emmett-Teller (BET) type. The
absorbed amount of PAA was found to decrease with temperature, while it increased with ionic
strength. For pH values lower than 6.5, the adsorption increased while for pH between 6.5 and 9 the
adsorbed amount remained almost constant.