This work involves the treatment of model groundwater containing arsenic with the use of chemically modified zeolite. A finite volume stirred tank reactor was used to study the effects of the variables; sorbent grain fraction and sorbent dose, on arsenic reaction kinetics. The results are modelled using the pseudo-second order and second-order reversible rate equations. Equilibrium arsenic uptake and residual sum of squares evaluated from the two models are compared. From them, the arsenic sorption onto the active sites is consistent with a pseudo-second order mechanism. The initial sorption rate which drives the uptake rate was found to be in the same order of magnitude for all the grain fractions but decreased with an increase in the sorbent dose. Furthermore, based on the pseudo-second order mechanism, Thiele moduli were determined and their values suggested that intraparticle diffusion may play a significant role in arsenic uptake.