3.2.1. Adsorption kinetics
Fig. 2 shows the adsorption kinetics of oxidized graphite to lubricating oil by batch techniques. The amount of adsorbed oxidized graphite is significantly with increasing contact time within 12 h, and then the slightly adsorption is observed after 24 h. As shown in the inset of Fig. 2, the adsorption capacity (Qt, mg/g) of lubricating oil is plotted as a function of the reaction time. Obviously, the adsorption capacities increase with adsorption time until saturation. The adsorption kinetics processes of lubricating oil on oxidized graphite are described by the pseudo-first- order and pseudo-second- order kinetic models:
equation(1)
ln(qe−qt)=lnqe−kf×t
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equation(2)
t/qt=1/(ks×qe2)+t/qe
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where qt (mg/g) and qe (mg/g) represent the sorption amount of lubricating oil at time t and equilibrium time, respectively; kf (h− 1) and ks (g (mg · h)− 1) are the kinetic rate constants of the pseudo-first-order and pseudo-second-order equations, respectively. The fitted results are showed in Fig. 2 and Table 1. As shown in Fig. 2, the fitted results matches well with the experimental data by using pseudo-second-order kinetic model with high correlation efficient (R2 = 0.999) compared to the pseudo-first-order kinetic order model (R2 = 0.892) (Table 1). These fitted results indicate that the sorption of lubricating oil on oxidized graphite is predominated by both physisorption and mass transport reactions [45].