Abstract Since the introduction of pseudo-second-order (PSO) model for the description of adsorption kinetics in 1999, it has been widely applied in liquid-phase adsorption systems. An approaching equilibrium factor (Rw) was defined and deduced from the PSO model in this work. The adsorption characteristic curves were built up, in which three different zones of the PSO model were classified. Activated carbons with various particle sizes were prepared from fir wood with KOH activation for the adsorption of phenol, 4-chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), and methylene blue (MB) from water. Suitable ranges of the PSO model and the effect of adsorbent particle size on the Rw value were analyzed. It was found that the quantity k2qe was exactly the inverse of the half-life of adsorption process. It could be used as an indicator of kinetic performance because it was a key parameter affecting the fractional adsorption at any time. That is, k2qe, a newly defined rate index, could provide the related information between the static and dynamic behavior of adsorption processes in engineering practice.