**5. Soil adsorption barrier for retarding transport of heavy metals**

#### **5.1 Distribution coefficients**

The soil adsorption performance of heavy metals, which directly affect migration, has been of great interest among engineers. Many reports about adsorption parameters for various soil types have been published. In particular, the Japan Atomic Energy Agency (JAEA) has established a database summarizing the results of soil adsorption tests using radioactive isotopes [39]. This section overviews soil adsorption parameters (distribution coefficients) of radioactive isotopes, such as Hg, Cd, Pb, Se, and Cs, as references. Cr and As do not have radioactive isotopes and JAEA does not support their distribution coefficients. The distribution coefficients are collected with previous references targeted to soils in major countries (**Figures 8**–**14**) [40–55].

The distribution coefficients of any heavy metals take a wide range of values. Here, the geometric mean value in each soil type against heavy metals is presented as a representative value in **Figures 8**–**14**. The distribution coefficient indicates a soil

#### **Figure 9.**

*Distribution coefficients of lead arranged from JAEA database.*

**Figure 10.**

*Distribution coefficients of selenium arranged from JAEA database.*

**Figure 11.** *Distribution coefficients of mercury arranged from JAEA database.*

*Engineering Measures for Isolation and Sequestration of Heavy Metals in Waste as Safe… DOI: http://dx.doi.org/10.5772/intechopen.102872*

#### **Figure 12.**

*Distribution coefficients of cesium arranged from JAEA database.*

**Figure 13.** *Distribution coefficients of trivalent or quinquevalent arsenic.*

**Figure 14.** *Distribution coefficients of trivalent or hexavalent chromium.*

type's ability to adsorb a heavy metal. The larger the distribution coefficient, the greater the retardation of the chemical substance transport, resulting in superior barrier performance against the transport. The distribution coefficients against Cd, Pb Hg, and Cs are relatively high, whereas those against Se, As, and Cr are lower. This is because the chemical forms of Se, As, and Cr in water are anions that are hard to adsorb to the soil surface with a negative charge and whose transport is hard to retard. Therefore, the environmental impacts of the transport of Se, As, and Cr with such small distribution coefficients should be carefully evaluated.

Se, As, and Cr in water can have anionic forms with different ionic valences, depending on environmental conditions, such as pH and oxidation–reduction potential. Numerous review studies, including this chapter, describe in broad strokes the distribution coefficients of heavy metals, but they hardly investigate the differences in distribution coefficients of heavy metals with different ionic valences. However, some scientific papers investigate the effects of ionic valence on distribution coefficients. For example, hexavalent chromium has smaller distribution coefficients than trivalent chromium [54], but both have values in the range of 100 mL/g or less at maximum [55]. Kumpiene et al. [56] review the stabilization mechanisms of As, Cr, Cu, Pb, and Zn. Especially, they discuss the stabilization of As, which is dependent on seven factors iron compounds, aluminum oxides, manganese oxides, organic matter, alkaline materials, clay minerals, and sulfides. The fact that As can be adsorbed on Fe has been considered a reason why As and Pb included in slags do not leach into water [57–59].

Soil pollution is a global environmental problem. As and Cr are relatively common as causative pollutants, so, their findings are collected and shared among not only researchers but also practitioners. In contrast, Se is a relatively minor substance in soil pollution and waste management, and thus far there are few studies on Se. Further studies on Se, As, and Cr are needed to accurately manage human risk because their distribution coefficients are small and environmental pollution by them is easily spread.

#### **5.2 Soil adsorption barrier for gaseous Hg**

Soil adsorption is also effective for retarding gaseous substances and preventing their diffusion. In general, there are three methods for evaluating the distribution coefficients of gaseous substances—(i) dynamic adsorption column, (ii) gravimetry, and (iii) constant volume [60]. The constant volume method has commonly been used to obtain adsorption isotherms. However, gases often adsorb on the container surface or leak from the plug, so obtained isotherm data need to be compensated using the losses of the gases in a blank test. In the previous studies that evaluate the adsorption abilities of adsorbents against some volatile organic compounds (VOCs) using Tedlar bags [61], the compensated isotherms at the equilibrium state can be exactly calculated because the losses of the VOCs in the Tedlar bags are mostly due to adsorption on the surface.

Gaseous Hg, however, would not only adsorb on the container surface but also leak from the plug, so, the equilibrium state cannot be reached. This characteristic of gaseous Hg makes the evaluation of its distribution coefficients difficult. Therefore, a testing method to evaluate adsorption abilities under a nonequilibrium state caused by the leakage should be established.

Ishimori et al. [62] suggested the constant volume method for evaluating the adsorption characteristics of soils and adsorbents against gaseous Hg under a nonequilibrium state. They formulated the phenomenon of nonequilibrium soil adsorption with leakage using the Langmuir sorption model and the diffusive leakage *Engineering Measures for Isolation and Sequestration of Heavy Metals in Waste as Safe… DOI: http://dx.doi.org/10.5772/intechopen.102872*

**Figure 15.**

*Kinetics of gaseous Hg during adsorption tests for decomposed granite soil (a) and calcium bentonite (b). Plots: Experimental results, solid lines: Fitting results to a governing equation, dashed lines: Estimation results with neither Hg adsorption on container surface nor leakage from sealing plug.*

model, resulting in an estimation of distribution coefficients of gaseous mercury by fitting experimental data to the governing equation (see **Figure 15**). Finally, the adsorption isotherms for sand, granite soil, calcium bentonite, and mordenite are estimated as shown in **Figure 16**. Then their distribution coefficients values for gaseous mercury are obtained as 56.3, 2070, 7140, and 3490 mL/g, respectively. It is noted that their values are obtained from the initial slopes of their adsorption isotherms when the equilibrium concentrations are zero.

Hg-coning wastes will be disposed of in landfill sites in the near future due to the signing of the Minamata Convention on Mercury. Soil adsorption barriers are an effective containment method to retard the transport and minimize the emission of mercury. The distribution coefficient is the most important parameter for providing the required containment barrier performance in landfills. The soil adsorption characteristics against both the aqueous and gaseous forms of mercury are insufficiently investigated thus far. It is well known that distribution coefficients depend not only on the type of soil and the adsorbents but also on environmental conditions, such as pH, oxidation–reduction potential, temperature, and coexisting aqueous or gaseous substances.
