Kinetics and time-dependent Langmuir modeling of 4-nitrophenol adsorption onto Mansonia sawdust

Often time's adsorption of large molecules onto untreated lignocellulosic materials is viewed as a two stage process and has frequently been characterized only by kinetic models while the rate limiting step of adsorption is determined only at various stages of the adsorption process. In this study the kinetics and the contribution of diffusion processes to 4-nitrophenol adsorption onto untreated sawdust was examined and the overall rate limiting step evaluated.The adsorption profile showed an initial rapid uptake of 4-nitrophenol which decreased and became almost constant after 5 min of contact. Analysis of the adsorption profile with the intraparticle diffusion equation and fractional 4-nitrophenol uptake with time showed that the profile can be divided into three different stages. The rate determining step of 4-nitrophenol adsorption was then evaluated based on the activation energies of each processes along with their activation parameters (ΔG*, ΔH* and ΔS*). The results revealed that external mass transfer was the overall rate limiting step with activation parameters Ea = 21.11, ΔH* = 23.75 and ΔS* = 144.97. Time dependent Langmuir modeling was carried out to optimize process parameters.

► Determination of diffusion process involved in 4-nitrophenol adsorption.
► Determination of activation energies of kinetic and diffusion process.
► Evaluation of activation parameters of kinetics and diffusion processes.
► Determination of overall rate determining step.
► Time dependent Langmuir modeling to optimize process parameters.