2.5. Kinetic modelling
The experimental kinetic data were fit to three mathematical models with estimation of the water and the solid diffusion coefficients. The three models consider the diffusivity in a unidirectional way, but they present different assumptions. Fick's model (Crank, 1975) considers the effective diffusivity (Deff) as a mean value, and it disregards its fluctuation during the osmotic process. Fito and Chiralt's model (Fito & Chiralt, 1997) also considers the Deff as a constant during the process. However, it takes into consideration, in addition to the diffusion process, the HDM mechanism, which is physically present in PVOD processes. The model of Barbosa Júnior et al. (Barbosa Júnior et al. 2013) considers the variation of Deff as a function of time.