## 4 Conclusions  
The performance of the stabilization treatment for As-contaminated soil using 5% of either AMDS, steel slag, CMDS, and cement with long-term (over 50 years) exposure was studied using an up-flow percolation column test. The results showed that the stabilization of As in the contaminated soil significantly reduced the release of As from the soil. The concentrations of As and other substances (Na, Mg, K, Ca, and Fe) were initially higher in the percolation test and gradually decreased to become almost negligible over time.  
Stabilizers improved the As binding capacity in the soil for longer than 50 years of exposure. The most effective stabilizers for average As reduction (or hindering efficiency) with respect to the control were: steel slag (55.0%) < AMDS (74.2%) < cement (78.1%) < CMDS (81.5%). In addition, the release of As from the stabilized soil towards the end of the percolation test was reduced to below 10 \(\upmu\)g/L. Moreover, leaching of As in the TCLP test followed a similar trend to that in the percolation test. However, the TCLP leaching concentration was higher than that in the percolation column test. This study shows that a stabilization treatment can achieve satisfactory durability and is a reliable technique for the long-term remediation of heavy metal-contaminated soil. Although these results are highly promising, further studies are required to evaluate the optimal parameters (soil pH, organic matter, stabilizer content, stabilizer size, mixing process, and aging period) before this technique can be widely applied to contaminated soils.  
The following are available online at [[https://www.mdpi.com/article/10](https://www.mdpi.com/article/10)]([https://www.mdpi.com/article/10](https://www.mdpi.com/article/10)) 3390/app11177859/s1, Text S1: Measurement of soil characteristics. Text S2: Reagent and materials. Text S3: Chemical analysis of samples. Text S4: Alkali metals and iron concentration in the effluents. Table S1: Major heavy metals present in the contaminated soil. Table S2: The exposure time (in years) at different liquid-to-solid (L/S) ratios calculated by using Equation (1) for each soil column. Figure S1: Performing leaching tests for arsenic release from soil during (A) column percolation test and (B) toxicity characteristic leaching procedure (TCLP). Figure S2: The eluates' redox potential (Eh)  
Figure 8: The TCLP vs percolation column test for the release of arsenic from contaminated and stabilized soils. Note that the comparisons are based on the same L/S ratio (i.e., cumulative mass release at 20 L/S for the percolation test and the mass release at 1:20 solid-to-liquid ratio for the TCLP test).  
and pH values (obtained from Figures 2 and 4) plotted on a Pourbaix diagram for arsenic species. Figure S3: Eluate concentration curves for the release of various metals (Na, Mg, K, Ca, and Fe) from different soil columns as a function of L/S ratio in the percolation test (mean concentrations were obtained from three replicates). Figure S4: Percentage composition (%) of different stabilizing agents used in this study. The component portions were analyzed using X-ray fluorescence spectrometry (XRF-1800, SHIMADZU, Kyoto, Japan).  
Conceptualization, G.P. and S.-W.J.; methodology, G.P. and S.-W.J.; validation, G.P., J.C. and S.-W.J.; resources, J.C.; writing--original draft preparation, G.P.; writing--review and editing, J.C. and S.-[PERSON]; visualization, G.P.; supervision, S.-W.J.; project administration, S.-W.J.; funding acquisition, J.C. All authors have read and agreed to the published version of the manuscript.  
This work was supported by Korea Environment Industry & Technology Institute (KEITI) through Subsurface Environmental Management (SEM) Project, funded by Korea Ministry of Environment (MOE) (Grant number:2019002470001).  
Not applicable.