## 3 Results and Discussion
### Adsorption Method  
The above-mentioned results of the two-step neutralization treatment revealed that most of Fe (ferric iron) and Cu were separated and removed as the precipitates from the real AMD water derived from the Robule Lake at pH 4 and 7, respectively. However, approx. 30 mg L\({}^{-1}\) of Mn till remained in the treated AMD water. Generally, the Mn\({}^{2+}\) ions removal from water can be induced by the neutralization treatment at pH 9 or above, which is often more than the pH value for the effluent standard. In this case, it is necessary for discharge to the river or surrounding areas to lower pH to below the effluent standard. Thus, the adsorption method using chitosan, a kind of polysaccharide, was investigated as a new approach for Mn\({}^{2+}\) ion removal. To confirm the Mn\({}^{2+}\) ion removal capacity of chitosan, the adsorption removal tests were firstly carried out using the simulated Mn solutions (0.5 mM). Mn\({}^{2+}\) ion removal from the aqueous solution at a higher pH (more than 6.5) was identified, while Mn removal capacity by chitosan was not confirmed below pH 5. Figure 5 shows change in the Mn removal rate when various input amounts of chitosan (0.5, 1, 2, 4 g) were added to 50 mL of the Mn solutions with pH 6.7 and 7. In case of 0.5 g of chitosan input, approx. 30 mass % of Mn removal rate was indicated. One gram of chitosan input increased the removal rate to 50 mass %. Finally, Mn removal rate reached 90 mass % when 4 g of chitosan was applied to the adsorbent for 0.5 mM of Mn solution. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & & \multicolumn{6}{c}{**Element**} \\ \cline{2-9}
**Sample Location (Test Duration)** & **Fe** & **Mn** & **Cu** & **Zn** & **Cd** & **Co** & **Ni** & **Ca** \\ \cline{2-9}  & & & \multicolumn{6}{c}{**Concentration, mg L\({}^{-1}\)**} \\ \hline Robule Lake (0 h) & 461.8 & 83.6 & 45.3 & 16.9 & 0.055 & 1.05 & 0.52 & 453 \\ \hline \multicolumn{9}{c}{**First neutralization step, pH 4**} \\ \hline Thickener A (1 h) & 16.0 & 66.7 & 35.9 & 15.0 & 0.050 & 0.92 & 0.44 & 797 \\ Thickener A (2 h) & 11.5 & 65.3 & 36.6 & 14.7 & 0.050 & 0.94 & 0.46 & 861 \\ Thickener A (3 h) & 12.5 & 60.5 & 38.1 & 15.0 & 0.051 & 0.97 & 0.47 & 772 \\ Thickener A (4 h) & 7.5 & 64.4 & 37.0 & 15.2 & 0.052 & 0.97 & 0.46 & 783 \\ Thickener A (5 h) & 0.7 & 79.7 & 45.4 & 15.6 & 0.054 & 0.99 & 0.49 & 516 \\ Thickener A (6 h) & 8.3 & 74.2 & 40.3 & 15.0 & 0.050 & 0.91 & 0.43 & 860 \\ \hline \multicolumn{9}{c}{**Second neutralization step, pH 7**} \\ \hline Thickener B (3 h) & 0.078 & 25.9 & 0.38 & 0.43 & 0.011 & 0.11 & 0.047 & 1430 \\ Thickener B (4 h) & 0.016 & 25.9 & 0.20 & 0.38 & 0.011 & 0.12 & 0.053 & 1107 \\ Thickener B (5 h) & 0.017 & 27.9 & 0.20 & 0.36 & 0.011 & 0.13 & 0.064 & 1298 \\ Thickener B (6 h) & 0.044 & 29.4 & 0.31 & 0.45 & 0.012 & 0.13 & 0.079 & 1312 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Concentration of elements in AMD neutralized in the two-step neutralization treatment on the semi industrial plant. Figure 5 shows change in Mn\({}^{2+}\) ion concentration and the amount of Mn removal as a function of input amount of chitosan. Due to the adsorption removal using chitosan, the Mn concentration in solutions reduced from 25 mg L\({}^{-1}\) (0.5 mM) to approx. 3 mg L\({}^{-1}\) with an increase in chitosan input. Chitosan input increased the solution pH to around 8. Chitosan, a cationic polymer obtained from chitin, has a lot of amino groups (-NH\({}_{2}\)). Increase in the solution pH may be caused by the consumption of H+ due to transferring -NH\({}_{2}\) to -NH\({}^{3+}\). Consequently, these results revealed that chitosan had a good performance for Mn removal from the simulated solution. Then, the application of chitosan to Mn removal from the real AMD water was attempted (Figure 6). In this experiment, the real AMD water sample, which was derived from the Robule Lake, was used. From this AMD sample, almost all of the Fe and Cu were already removed due to the two-step neutralization treatment at pH 4 and 7, respectively.