Comparative evaluation of adsorption kinetics and isotherms of a natural product removal by Amberlite polymeric adsorbents

Adsorption mechanism of a natural compound, alizarin (1,2-dihydroxyanthraquinone), onto three types
of microporous polymeric adsorbents (XAD-4, XAD-7, XAD-16) has been proposed using Langmuir, Freundlich
and Redlich–Petersen isotherms. Adsorption capacity and optimum adsorption isotherms were
predicted by linear least squares and non-linear regression method. Adsorption kinetics was proposed
by pseudo-first and second second order models. The adsorption capacity of XAD-16 was the highest at
0.0424 mg/mg, with initial alizarin concentration of 200 mg/L. Pseudo-second-order kinetics was more
appropriate in explaining the adsorption mechanism than pseudo-first-order. Over the studied concentration
ranges, only XAD-4 adsorption can be reasonably described by the three isotherms. XAD-16 data
is only best-fitted to Langmuir and Redlich–Petersen isotherms. Non-linear method proved a better way
to predict the equilibrium isotherm parameters. The combination of parameters, such as specific surface
area, pore diameter, polarity of the network of the resins, the solubility and polarity of the adsorbate, are
the significant parameters for optimum adsorption process.