**2. Concept of sequestration/isolation in environmentally sound resource circulation**

The main objectives of the sequestration/isolation of hazardous substances are to reduce and avoid the long-term exposure of humans and the environment to hazardous substances. Environmentally sound resource circulation entails challenges for the actors involved, such as requirements that manufacturers implement cleaner and resource-efficient production. It also means customers should purchase long-life products and recycle their waste. Recycling activity should include only the safe circulation of resources, and hazardous substances must be sequestrated from human activity.

The scheme of sequestration of hazardous substances should be consistent with the strategy for a final sink [1]. A final sink is a process in a manner that satisfies the acceptance level of a substance flow as low as the natural environment and the acceptance level of exposure of substances for human health. Hazardous organic chemicals are not expected to be mineralized, and it is hard to secure their long-term immobilization in an isolation site (or a landfill to be upgraded for sequestration). Therefore, the final sink for these substances must occur through physical or chemical destruction. This indicates that the potential of an isolation site to be a final sink is limited to toxic heavy metals or inorganic substances.

Isolation/sequestration should be designed under the multi-barrier approach. The major elements of this approach are (i) containment of toxic substances by stabilization and insolubilization with chemical or physical measures, (ii) avoidance of the release of toxic substances from containment, including exposure to moisture, (iii) retardation of the migration of hazardous substances within a site and the environment, and (iv) early warning of the potential release of a toxic substance by monitoring of containment structures, gases/leachates, and the environment.

#### **3. Containment of toxic heavy metals for safe sequestration**

Most countries have legal regulations for the disposal of waste containing hazardous heavy metals. Waste that meets certain criteria can be disposed of in landfills equipped with emission control measures (e.g., leachate treatments and gas collection) [2, 3]. Otherwise, those waste products must be delivered to facilities with containment functions. Pretreatment to detoxify and immobilize waste containing heavy metals is an essential measure to reduce potential emissions reasonably.

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

Air pollution control (APC) residues, which are generated through thermal treatments of waste, such as incineration, gasification, and pyrolysis, are commonly classified as hazardous materials owing to the high leaching potential of toxic metals. Before APC residues are disposed of, pretreatments are required in many countries to prevent the release of toxic metals into the environment [4]. The available pretreatments can be categorized into three groups—(i) physical or chemical separation, (ii) solidification/stabilization (S/S), and (iii) thermal treatment [5]. Chemical stabilization using organic chelating agents, such as piperazine-based or dithiocarbamatebased agents, is often preferred because such treatments are simple, do not require pretreatments, such as pH control, and remain stable across a wide pH range [6, 7]. On the other hand, these treatments are significantly more expensive than other forms of chemical stabilization [8]. In addition, organic components derived from chelating agents induce long-term leachate problems treatments at landfill sites [9].

#### **3.1 Cement solidification**

Cement solidification is a widely used containment technique around the world. The purpose is to avoid the leakage of toxic substances into the environment. The target chemical substances are diverse, such as heavy metals, F, B, and even radioactive substances. Hiraoka and Takeda [10] investigated the effects of cement solidification on the leaching amounts of Hg and Cd in waste sludges in relation to compressive strength. They suggested that the solidification of landfill waste containing heavy metals is safe when the cement amount is over 150 kg/m<sup>3</sup> and the compressive strength is over 0.98 MPa.

Cement solidification was also applied to radioactive cesium-contaminated APC residue generated after the Fukushima Daiichi nuclear disaster in 2011. Radioactive cesium is hardly precipitated in the alkaline condition in a cement mixture and cannot be chemically stabilized, although the solubilities of heavy metals are reduced in alkaline conditions (**Figure 1**). To reduce the leachability of radioactive Cs, cement solidification should be prepared on a large scale with a value of 1 m<sup>3</sup> so that the specific surface area contacting water is limited. Another improvement is the use of blast furnace cement rather than ordinary Portland cement. Since cement includes Cr as a material component, solidified pieces can potentially leach hexavalent chromium.

**Figure 1.** *Relationship between solubility and pH for heavy metal [11].*

Blast furnace cement can provide a reductive condition, resulting in the transformation from hexavalent to trivalent chromium leaching. In addition, blast furnace cement is effective for reducing volume expansion and maintaining the long-term containment performance of cement solidifications. APC residue complicates the solidification mechanism of cement because of coexisting reactive chemicals, such as calcium, aluminum, and sulfur. Their chemical components contribute to form an expansive mineral known as ettringite. Volume expansion due to ettringite will generate cracks on the surfaces of solidified pieces, thus increasing the specific surface area. The reduction of both specific surface area and volume expansion is an essential design criterion for controlling the cement solidification of hazardous wastes.

#### **3.2 Solidification by magnesium oxide**

Magnesium oxide (MgO) is also an effective binder for solidifying wastes containing heavy metals. There are two methods of producing MgO. One is to bake the natural magnesium carbonate included in dolomite and then crush it. The other is to precipitate Mg ions in seawater as hydroxides and then dehydrate them at high temperatures. This means that magnesium oxide is a safe insolubilizer free from toxic chemicals, which is remarkably different from cement-containing Cr.

When MgO is dissolved in water, magnesium hydroxide is precipitated so that the pH reaches around 10.5 at an equilibrium state. At this pH level, some heavy metals can exhibit the lowest solubility, as shown in **Figure 1**. This has been considered a reason why MgO has a greater ability than cement to immobilize heavy metals. On the other hand, MgO cannot give such a large compressive strength to solidified pieces compared with cement, and MgO is about 8–10 times more expensive than cement. Therefore, it is necessary to optimize the amount and field of usage of MgO. **Figure 2** shows the results of batch leaching tests using an APC residue solidified with blast furnace cement or magnesium oxide. The ratio of ash (A) to binder (B) in weight is parametrically changed. Solidification remarkably has reduced the leachability of Cd, Zn, and F compared with raw APC residue. However, the leachability of Pb cannot be reduced by cement solidification even by increasing the amount of cement. In contrast, magnesium oxide can reduce the leaching amount under specified conditions.

#### **3.3 Stabilization/solidification of mercury-containing waste**

According to the framework of the Minamata Convention, the national scheme for the appropriate disposal of Hg-containing waste is required. Due to its environmental effects, Hg-containing waste must be stabilized prior to disposal in HgS form and/or solidified with a polymer or cement to reduce leaching and volatilization. **Figures 3** and **4** show the long-term leaching and volatilization behaviors of processed mercury. Hg-containing waste specimens are first stabilized with sulfide as metacinnabar [13, 14], which has extremely low solubility in water. The specimens are then solidified with one of four binders—sulfur polymer (SP), low-alkaline cement A (CA), lowalkaline cement B (CB), or low-alkaline cement B containing a water-reducing agent (CB+). Here, low-alkaline cement A has hauynite as the main component, and lowalkaline cement B has high-volume fly ash and silica fume. Stabilized Hg-consisting waste solidified with a sulfur polymer exhibits the lowest leaching and volatilization. Low-alkaline-cement-based binders effectively confine Hg but have lower performance than sulfur polymer. pH may significantly affect Hg leachability [15]. Leaching from a piece solidified by low-alkaline-cement binders increases under acidic

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

**Figure 2.**

conditions, whereas that solidified with sulfur polymer increases under alkaline conditions. On the other hand, Hg volatilization increases with temperature except for waste solidified with sulfur polymer. Sulfur polymer is effective for decreasing the volatilization rate due to elevated temperature.

#### **3.4 Stabilization by diatomite addition**

Among available pretreatments, cement-based S/S is commonly used worldwide [16, 17]. In this process, calcium–silicate–hydrate (C–S–H) gel is formed by the reaction between amorphous silica (SiO2nH2O) and calcium hydroxide (Ca(OH)2) (pozzolanic reaction) in the cement [18]. Toxic metals can be immobilized by the C–S–H gel via sorption, incorporation, and encapsulation owing to the high microporosity and high surface area [19, 20]. APC residues usually contain high amounts of Ca as a sorbent and reactant for the removal of acidic components in exhaust gas [21]. APC residues often show high pH due to the presence of alkaline Ca compounds [22, 23], and the solubility of amorphous silica increases at alkaline pH [24]. Owing to the high Ca content and alkaline pH provided by APC residues, the addition of amorphous silica to APC residues may induce C–S–H gel formation via pozzolanic reactions for

**Figure 3.** *Results of long-term leaching tests: Effects of pH on Hg leaching.*

**Figure 4.** *Results of long-term volatilization tests: Effects of temperature on the accumulated amount of volatilized Hg.*

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

metal immobilization. Thus, both the treatment cost and the use of chemical agents might be reduced by using inexpensive silicon materials instead of cement. We considered diatomite as a natural pozzolanic additive [25] for lead immobilization in APC residues owing to its high amorphous silica content [26, 27], relative abundance [28], and low cost compared to Portland cement [29].

Assessment of the impact of diatomite addition on Pb immobilization in APC residues (**Figure 5**) indicates that Pb leaching from weathered APC residues decreased as time and temperature increased. This is attributed to the increase in the hydration reaction of cementitious materials as the temperature increases [30, 31]. At each weathering temperature, Pb leaching from stabilized APC residues decreased as diatomite doses increased. The leaching amount of Pb from 14-day stabilized APC residues with 0%, 5%, or 10% diatomite addition was reduced by 18–67%, 67–90%, or 80– 99%, respectively. Consequently, the leaching amount of Pb dropped below 0.3 mg/L (Japanese criterion for landfill disposal) after 14 days of curing with the addition of 10% diatomite at 70°C.

**Figure 6** shows the X-ray diffraction (XRD) patterns of raw APC residues and 14 day cured APC residues following the addition of 10% diatomite at 70°C. The peak

**Figure 5.**

*Leaching concentrations of Pb from cured APC residues. APC residue under the temperature of (a) 25°C, (b) 50°C, (c) 75°C.*

**Figure 6.**

*XRD patterns of raw and cured APC residues by 10% of diatomite at 70°C for 14 days.*

intensities of CaClOH and Ca(OH)2 significantly decreased by weathering with diatomite, indicating that they reacted to diatomite and were consumed by the **Figure 6** XRD patterns of raw and cured APC residues by 10% of diatomite at 70°C for 14 days pozzolanic reaction. New peaks of C–S–H gel in the stabilized APC residues were not confirmed, as the residues were below the detectable level of XRD analysis or crystallization was incomplete [32]. Even though C–S–H gel formation was not detected in the XRD analysis, the amount was sufficient to immobilize 99% Pb in the APC residues.

Diatomite, consisting mainly of amorphous silica, was used as a pozzolanic additive for Pb immobilization in APC residues instead of cement. The results showed that the leaching amount of Pb from the stabilized APC residues was reduced by C–S–H gel formation via the pozzolanic reaction among Ca(OH)2, CaClOH, and diatomite. Consequently, the leaching amount of Pb dropped below 0.3 mg/L. This study showed the feasibility of using diatomite to immobilize Pb in APC residues. Although 10% diatomite was added to the APC residues, the volume increase is supposed to be lower than that in cement-based S/S [33]. From the viewpoint of landfill management, this treatment would reduce the use of chelating agents while suppressing the increase in volume. If wastes containing amorphous silica can be used to immobilize metals in APC residues, this method has the potential to be a low cost and environmentally friendly solution.
