The total area under the curve of PS' IR spectra, from 1800 to 900 cm -1 , were used to probe the extent of PS adsorption and desorption as a function of time. The Elovich and pseudo-first order (PFO) models were used for adsorption and desorption of PS on goethite, respectively, due to better fitting compared to other commonly used kinetic models. A simplified Elovich equation is written as 68 (Eq. 1)