## 4 Discussion  
The clear trend that the pH in soil solutions increased over time for all the 7 treatments suggests that proton consumption took place continuously during the period of the 15-day incubation experiment after mixing the soil with either the single or combined LMWOA solutions. This was accompanied by the enhancement of reducing conditions over time, as indicated by the consistent trend that the solution Eh decreased with increasing duration of the experiment. The temporal variation in both solution pH and Eh appeared to markedly complicate the mobilization-immobilization of various soil constituents, resulting in irregular variation in the EC value. Different organic acids or combinations of organic acids had differential effects on these chemical processes. However, as a whole, the EC value tended to decrease over time, and at the end of the experiment (the 15 th day), EC dropped to a very low value for all the treatments. This suggests that, at this point, hydrolytic polymerization of metal complexes dominated as a result of rising pH [27, 28]. For the single organic acid treatments (T1-T3), no removal of Fe, Mn, and Al from the solutions took place at this point because their concentrations remained high in the solutions. However, for the combined organic acid systems, removal of these metals from the solutions occurred, as evidenced by the facts that their \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Time** & **Element** & **As** & **Cd** & **Cr** & **Cu** & **Pb** & **Zn** & **Ni** \\ \hline
1 d & Al & 0.907 ** & 0.397 & 0.954 ** & 0.964 ** & 0.077 & 0.979 ** & 0.972 ** \\  & Fe & 0.941 ** & 0.351 & 0.929 ** & 0.923 ** & \(-\)0.026 & 0.944 ** & 0.943 ** \\  & Mn & 0.841 ** & 0.552 ** & 0.980 ** & 0.934 ** & 0.227 & 0.998 ** & 0.987 ** \\ \hline
3 d & Al & _0.986_ & ** & 0.744 ** & 0.961 ** & 0.889 ** & 0.232 & 0.965 ** & 0.943 ** \\  & Fe & 0.977 ** & 0.800 ** & 0.943 ** & 0.884 ** & 0.296 & 0.968 ** & 0.956 ** \\  & Mn & 0.978 ** & 0.850 ** & 0.983 ** & 0.944 ** & 0.392 & 0.993 ** & 0.974 ** \\ \hline
5 d & Al & 0.980 ** & 0.524 * & 0.967 ** & 0.732 ** & 0.248 & 0.962 ** & 0.923 ** \\  & Fe & 0.978 ** & 0.556 ** & 0.947 ** & 0.767 ** & 0.292 & 0.963 ** & 0.942 ** \\  & Mn & 0.989 ** & 0.619 ** & 0.970 ** & 0.865 ** & 0.418 & 0.991 ** & 0.971 ** \\ \hline
7 d & Al & 0.658 ** & \(-\)0.109 & 0.565 ** & 0.093 & \(-\)0.065 & 0.637 ** & 0.586 ** \\  & Fe & 0.925 ** & 0.249 & 0.890 ** & 0.643 ** & 0.366 & 0.934 ** & 0.923 ** \\  & Mn & 0.944** & 0.340 & 0.950 ** & 0.829 ** & 0.393 & 0.943 ** & 0.962 ** \\ \hline
15 d & Al & 0.790 ** & \(-\)0.270 & 0.864 ** & 0.322 & 0.077 & 0.852 ** & 0.808 ** \\  & Fe & 0.909 ** & \(-\)0.097 & 0.889** & 0.257 & 0.400 & 0.934 ** & 0.863 ** \\  & Mn & 0.838 ** & 0.216 & 0.894 ** & 0.116 & 0.414 & 0.636 ** & 0.970 ** \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Correlation coefficient between each selected trace metal and each of other elements investigated in this study (\(n=7\)). concentrations tended to decrease after either 5 or 7 days of the experiments. This suggests that the presence of multiple organic acid ligands enhanced the precipitation of solution-borne Fe, Mn, and Al. The lower initial pH for oxalic acid treatment can be attributed to the greater acid dissociation constant (Ka) or lower pKa, as compared to the other two individual acid treatments (Equations (1)-(7)) [29].