# Removal of Methylene Blue from Aqueous Solution by Bone Char
## 3 Results and Discussion  
Figure 1a shows that the XRD pattern of bone char fits well with the hydroxyapatite phase. The SEM image in Figure 1b shows the accidented surface of bone char, indicating a potentially large specific surface area. The photograph and surface areas of the bone char samples are shown in Figure 2a and Table 1, respectively. From Table 1, it can be seen that bone char with the smallest size has the biggest total and external surface area but has no pores on its surface. Further pore diameter detection shows that the pore size of bone char is between 1.7 and 75 nm, including major mesopores (2\(\sim\)50 nm) and a small number of pores <2 nm (micropores) or >50 nm (macropores) in diameter [15]. The chemical structure of the methylene blue molecule is shown in Figure 2b. The length of a methylene blue molecule is 13.82 A [35] or 14.47 A [36], and the width is approximately 9.5 A. The length depends on the chloride ion's locations, namely whether it connects to the intermediate sulfur located in the center of the molecule or one of the two nitrogen atoms located at sides. Considering the length of the methylene blue molecule and the pore sizes of bone char, methylene blue molecules can enter into all of the pores of bone char. As shown in Table 1, there is no association between removal rates and bone char particle size when the particle sizes are below 1.0 mm, while bone char samples with particle sizes between 0.25\(\sim\)0.50 and 0.50\(\sim\)0.80 mm have almost the same removal rate for methylene blue. The external specific surface areas of the two samples are not proportional to the removal rates. This may result from the intraparticle diffusion caused by the micropores on bone char with particle sizes of 0.50\(\sim\)0.80 mm. The results indicate that the adsorption process is restricted by not only the external surface but also the intraparticle diffusion. While bone char with a larger particle size of 0.80-1.00 mm has the biggest micropore specific surface area, the removal rate is slightly reduced. This is because the adsorption performance of bone char mainly depends on external diffusion. Similar results were reported in other literatures [28,37]. The errors of adsorption data listed in Table 1 were due to the existence of dynamic adsorption equilibrium. Figure 2c shows that a rather fast adsorption of methylene blue occurs on the bone char surface during the first few minutes (0\(\sim\)10 min), followed by two slow stages (10\(\sim\)40 min and 40\(\sim\)80 min). The second slow adsorption stage is ascribed to the decreasing diffusion path within the narrow \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Bone Char Size (mm)**} & \multicolumn{3}{c}{**Specific Surface Area (m\({}^{2}\) g\({}^{-1}\))**} & \multirow{2}{*}{**Removal Rate (\%)**} \\ \cline{2-2} \cline{4-5}  & **Total** & & **External** & **Micropore** \\ \hline
0.25\(\sim\)0.50 & 119.21 & 119.21 & - & 45.72 \(\pm\) 0.95 \\
0.50\(\sim\)0.80 & 116.03 & 115.16 & 0.87 & 45.86 \(\pm\) 0.57 \\
0.80\(\sim\)1.00 & 113.13 & 107.73 & 5.40 & 42.35 \(\pm\) 0.66 \\ \hline \end{tabular}
\end{table} 
Table 1: Effect of specific surface area on the removal rate of methylene blue by different sizes of bone char (\(T\): 303 K, \(t\): 24 h). Figure 1: (a) XRD pattern of bone char and (b) SEM image [15] of bone char. mesopores with possible pore blockage, further reducing diffusion [23]. In view of the length and width of the methylene blue molecule, large numbers of methylene blue dimers in water, and the micropore size of bone char, one methylene blue molecule or its dimer is enough to clog these microporous orifices. Therefore, the adsorption rate of the third stage is quite low. At last, the adsorption reaches its equilibrium at about 80 min. In this situation, the uptake rate on the surface region is much faster than the intraparticle diffusion rate in the pores [38; 39].