## 3 Results and Discussion  
### Percolation Column Test Results  
#### 3.1.1 Change in pH, EC and Eh of Effluent  
The column effluents were collected and the pH, EC, and Eh were measured immediately. The eluate pH varied from fairly neutral to an alkaline pH, ranging between 6.6 and 12.8, depending on the soil stabilizer (Figure 2). The eluent pH applied in the column was 5.8. The first collected eluates (at 0.2 L/S ratio representing a 0.2 year exposure) of AMDS-, CMDS-, and steel slag-stabilized soil and the control soil columns resulted in similar slightly alkaline pH values of 8 (pH range; 8.1-8.3). However, the pH of the cement-stabilized soil was significantly higher (approximately 12) than that of the other stabilized soils. The higher eluate pH from the cement-stabilized soil is due to the alkaline nature of cement. Cement-based materials generally have higher pH values (of more than 12) because they contain oxide minerals and alkali metals [36; 37]. The eluate concentration of calcium (Ca) in the cement-stabilized soil was 2-8 times higher than in the other eluates. A similar increase in soil pH occurs after the soil is stabilized with cement [38; 39]. The pH of the eluates gradually decreased (albeit only slightly) by one unit when the cumulative L/S ratio reached 50 (representing more than 50 years of exposure). The eluate pH of the control, steel slag-stabilized, and cement-stabilized soil column decreased by approximately one unit or more (8.2 to 7.3 in the control, 8.3 to 7.3 in the steel slag, 12.8 to 11.4 in the cement) while those of AMDS- and CMDS-stabilized soils did not decrease (8.1 to 7.7 in AMDS, and 8.3 to 8.1 in CMDS) over the entire experimental period (Figure 2). These results indicate that, in the initial stage of percolation, some alkaline components present in contaminated or stabilized soil are partially removed with the eluent. The EC of the column effluent eluted from the AMDS-, steel slag-, and CMDS-stabilized soils sharply decreased during the initial elution phase (from a cumulative L/S ratio of 0.2 to 2) (Figure 3). However, it sluggishly decreased to an almost constant value after 2 L/S (i.e., after an exposure of 2 years or more). However, the EC of the cement-stabilized soil in the initial phase was approximately 6.5. A gradual decrease in the eluate EC was observed in the cement-stabilized soil until the end of the percolation test. The results imply that stabilized soils should be carefully monitored and groundwater use should be prohibited for the initial two years (or five years for cement-stabilized soil) if the ions are not treated before its use, because high levels of ionic species were leached from stabilized soil in this initial period. Figure 2: Eluate curves for the pH as a function of the L/S ratio in the column percolation test using different stabilizers within the contaminated soils. The redox potential (Eh) measures the tendency of the environment to oxidize or reduce the substrate. The Eh was also assessed during the percolation test period. The Eh values for the various column effluents at 0.2 L/S were in the order of: cement (251.1 mV) < control (373.4 mV) < CMDS (411.0 mV) \(\leq\) steel slag (412.2 mV) \(\leq\) AMDS (419.2 mV) (Figure 4). Our results imply the contaminated or stabilized soils are in the categories of either oxidized (Eh; >400 mV) or moderately reduced (Eh; +100 to +400 mV) environments, regardless of their positive Eh values [40]. AMDS-, steel slag-, and CMDS-stabilized soil had a relatively higher oxidative strength than the control or the cement-stabilized soils. The pH and Eh are the most important factors in controlling As species, because they affect the chemical characteristics of different arsenic forms (As\({}^{3+}\) and As\({}^{5+}\)) [41].