# Nascent Rice Husk as an Adsorbent for Removing Cationic Dyes from Textile Wastewater
## 3 Results and Discussion
### Kinetic Studies  
The experimental data for MB and CV adsorption onto RH as a function of reaction-time were represented in Figure 3, with PFO and PSO kinetic model fits. The rates of adsorption for both dyes (MB and CV) were fast during the first 15 min; 16.56 mg/g (MB) and 18.20 mg/g (CV) were adsorbed, respectively. Equilibrium was attained within 3 h for MB but the adsorbed amounts for CV increased continuously during 24 h. The fast increase in the adsorbed dye amount was ascribed to enough vacant adsorption sites at the beginning, leading to easy access of dyes to adsorption sites [57; 58]. The slow adsorption-rate at equilibrium was owed to overloading of favorable active sites [8]. Figure 1: Impact of thermal treatment on methylene blue (MB) adsorption capacity onto rice husk. Figure 2: Field emission-scanning electron microscope photos of rice husk thermally treated. (**a**) Room temperature, (**b**) 75 \({}^{\circ}\)C, (**c**) 100 \({}^{\circ}\)C, (**d**) 300 \({}^{\circ}\)C, (**e**) 500 \({}^{\circ}\)C. (Magnification: \(\times\)200; Scale: 200 \(\upmu\)m). (**f**) Digital image of rice husk used in this study. To investigate the mechanism involved in the adsorption process, the PFO and PSO models demonstrated a relatively high correlation coefficient (R\({}^{2}\)) for MB and CV as depicted in Table 1. The PFO model best represents MB adsorption, divulging that the sorption rate involves either intra-particle or film-diffusion [59; 60]. The PSO kinetic model best fits CV adsorption, indicating a chemisorption process involving ionic exchange and valence forces via the splitting or swapping of electrons between the RH and CV [61]. The kinetic data was also explored using an IPD model. In order to explore the relevance of the IPD model, the amount adsorbed, \(q_{t}\), vs. the square root of time, t\({}^{1/2}\), adsorption of MB and CV onto RH at different conditions are mapped according to the theory proposed by [PERSON] and [PERSON] [52]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**adsorbate**} & \multicolumn{3}{c}{**Pseudo-First-Order Kinetics**} & \multicolumn{3}{c}{**Pseudo-Second-Order Kinetics**} \\ \cline{2-7}  & \(q_{e}\) **(mg/g)** & \(k_{1}\) **(1/h)** & \(R^{2}\) & \(q_{e}\) **(mg/g)** & \(k_{2}\) **(g/mg/h)** & \(R^{2}\) \\ \hline MB & 17.1038 & 1.6835 & 0.9848 & 18.1906 & 0.1537 & 0.9641 \\ CV & 18.9546 & 2.8671 & 0.9185 & 20.2015 & 0.2275 & 0.9649 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Pseudo-first-order and pseudo-second-order model parameters for methylene blue (MB) and crystal violet (CV) adsorption onto rice husk. \(q_{e}\): the amount of adsorbed dyes at equilibrium (mg/g); \(k_{1}\): PFO rate constant (1/h); \(k_{2}\): PSO rate constant (g/mg/h). Figure 3: The kinetic profile represented by the pseudo-first order and pseudo-second-order for (**a**) methylene blue (MB) adsorption and (**b**) crystal violet (CV) adsorption  
Figure 4a presents that the rate limiting process is not only influenced by intra-particle diffusion during the adsorption of MB onto RH. The adsorption process of MB involved three stages of diffusion mechanisms. The first being the diffusion of MB through the solution to the external surface of the RH, forming a boundary layer. The passage of the MB adsorption plot through the origin indicated that intra-particle diffusion controlled the adsorption-rate [62]; this was consistent with the PFO observed for the MB adsorption process. Secondly, the slow adsorption of MB through internal pores, which limited the adsorption-rate; and thirdly, the retarded rate of intra-particle diffusion caused by low MB concentration, which helps attain final equilibrium. [63, 64]. MB adsorption at the final stage involved the lowest slope of 0.2062 and the highest intercept of 16.401, indicating the largest boundary layer diffusion effect [65]. Conversely, the straight-line CV adsorption plot was observed not to pass through the origin, indicating that the adsorption-rate was not only controlled by intra-particle diffusion [66].