5:15 PM - 6:30 PM
[AGE27-P06] A laboratory test of heavy metal accumulation by using soil cooling and heating technique inducing dynamic soil water freezing and evaporation
Keywords:Soil cooling and heating cycle, heavy metal, soil freezing, liquid water and vapor transfer
Soil contamination by heavy metals has been recognized as a critical issue in recent years. A new technique to accumulate heavy metal in a thin soil layer and an adsorbent layer using an artificial soil cooling and heating cycle was proposed. Soil cooling causes soil freezing that dynamic liquid and solute transfer to the cooling plane occurs due to the low matric potential of frozen soil. Soil heating induces soil water evaporation and the solute is left in the heating plane. Given that, the soil a number of cooling and heating cycle results water soluble heavy metals accumulated near the cooling/heating plane and the adjacent adsorbent layer. In this study, we evaluated the possibility of the new technique, and influence of soil type, texture, initial water content, and addition of chelating solution on the efficiency of the technique with laboratory experiments.
30 cm height, 10 cm diameter PVC columns were filled with soils (Silica sand No. 7 or No. 9) contaminated by lead nitrate artificially. The lead concentration of the soils were 1000 mg kg-1. A heat exchanger was installed at both ends of the column to control the temperature, which represented a cooling and heating cycle. The cooling and heating temperatures were -10°C and 60°C. The duration of cooling and heating was set to 36 hours and 12 hours, respectively. Apatite was used as an adsorbent. The initial volumetric water content of the soil was either 0.15 m3 m-3 or 0.30 m3 m-3. Chelating agents (0.05 M and 0.15 M) were added as a substitute for water for some cases. TDR sensors were inserted to measure temperature and volumetric water content, and lead concentration was analyzed after the experiments.
The results of the laboratory tests showed that water and solutes tended to accumulate on the cooling and heating plane, indicating the feasibility of this technique. The amount of lead in soils decreased with the installation of the adsorbent layer, and the lead content in the adsorbent layer was 5.7 times higher than the initial value of the contaminated soil. The lead accumulation in the adsorbent increased when the moisture content was low and the particle size was fine. It is due to in part liquid water flux near the freezing front. The addition of chelating agent improved the mobility of lead and increased the accumulation of lead both on the cooling and heating plane and on the adsorbent. This indicates that the addition of chelating agent increases the accumulation efficiency of solute in this method. It was also found that low concentrations of chelating agents were sufficient to achieve the desired effect.
These results indicated that the effectiveness of the new technique in laboratory. Furthermore, the effect of changes in the soil environment on the accumulation efficiency of solutes was clarified. The findings obtained in this study are important information for the practical in-situ application of the new technique.
30 cm height, 10 cm diameter PVC columns were filled with soils (Silica sand No. 7 or No. 9) contaminated by lead nitrate artificially. The lead concentration of the soils were 1000 mg kg-1. A heat exchanger was installed at both ends of the column to control the temperature, which represented a cooling and heating cycle. The cooling and heating temperatures were -10°C and 60°C. The duration of cooling and heating was set to 36 hours and 12 hours, respectively. Apatite was used as an adsorbent. The initial volumetric water content of the soil was either 0.15 m3 m-3 or 0.30 m3 m-3. Chelating agents (0.05 M and 0.15 M) were added as a substitute for water for some cases. TDR sensors were inserted to measure temperature and volumetric water content, and lead concentration was analyzed after the experiments.
The results of the laboratory tests showed that water and solutes tended to accumulate on the cooling and heating plane, indicating the feasibility of this technique. The amount of lead in soils decreased with the installation of the adsorbent layer, and the lead content in the adsorbent layer was 5.7 times higher than the initial value of the contaminated soil. The lead accumulation in the adsorbent increased when the moisture content was low and the particle size was fine. It is due to in part liquid water flux near the freezing front. The addition of chelating agent improved the mobility of lead and increased the accumulation of lead both on the cooling and heating plane and on the adsorbent. This indicates that the addition of chelating agent increases the accumulation efficiency of solute in this method. It was also found that low concentrations of chelating agents were sufficient to achieve the desired effect.
These results indicated that the effectiveness of the new technique in laboratory. Furthermore, the effect of changes in the soil environment on the accumulation efficiency of solutes was clarified. The findings obtained in this study are important information for the practical in-situ application of the new technique.