**4. Weathering and attenuation of waste in landfills: impact of mineral compositions on metal leachability**

Waste containing toxic metals, which are commonly immobilized before disposal, will be stabilized by a chemical or microbial weathering process in landfills. For example, precipitation of insoluble metal sulfide in the presence of sulfides produced

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

by sulfate-reducing bacteria is one of the major mechanisms by which toxic metals are immobilized in landfills [34]. In the initial stage of the weathering process, calcite (CaCO3) generated by carbonation via atmospheric CO2 immobilizes toxic metals by absorption on the surface and/or incorporation into the crystal structure [35, 36]. Thus, these processes have non-negligible impacts on metal mobilities at waste landfills. In this context, the impacts of mineral compositions of waste in a landfill on the leaching behaviors of heavy metals were analyzed. The obtained waste was composed of the following**—**cover soils (0**–**0.15 m), waste layers (0.15–47.6 m), embankment (47.6–50.9 m), and gravel layers (50.9 m–56 m). Mineral compositions identified by XRD analysis (**Table 1**) showed that the waste in this landfill mainly consisted of calcite (CaCO3), gypsum (CaSO42H2O), and quartz (SiO2). The contents of heavy metals (Cu, Mn, Ni, and Zn) and their leachability (**Figure 7**) revealed that the waste at 31.3 m depth had high contents and high leaching concentrations of Cu, Mn, Ni, and Zn. The core sample contained weddellite (CaC2O4 2H2O) and whewellite (CaC2O4H2O). Waste incineration residues are known to contain small amounts of organic acids, such as oxalic acid (C2H2O4) [37]. Calcium oxalate (weddellite and/or whewellite) can be formed in waste incineration residues by the coprecipitation of Ca2 <sup>+</sup> ion and oxalic acid [36]. It is also considered that calcium oxalate in sludge produced by the neutralization of waste acid using calcium hydroxide has been landfilled. Another possible source of calcium oxalate is waste from the ceramics industry. The waste also contains iron oxide (see **Table 1**) that, like calcium oxalate, is used for ceramic glazes. Thus, oxalic acid and calcium oxalate could be in the waste. If oxalic acid has a high capability of extracting heavy metals, it might enhance the leachability of heavy metals in waste landfills. Compared to mineral acids, the contents of oxalic acid (organic acid) do not seem high, owing to its precipitation as a metal–oxalate complex [22]. On the other hand, the mobility of heavy metals seems to be significant in this landfill according to the solubility of the metal–oxalate complex (**Table 2**) [38]. Moreover, the stability constant of this complex is higher than that of calcium oxalate (**Table 2**), suggesting that the mobility of heavy metals might be enhanced by the


**Table 1.** *Mineral compositions of a waste obtained from the*

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


**Table 2.**

*Solubility and stability constant of metal oxalate in water.*

formation of a metal–oxalate complex via a substitution reaction between Ca2+ ion in calcium oxalate and divalent metals.

Our assessment revealed the high leachability of Cu, Mn, Ni, and Zn from waste containing calcium oxalate. Calcium oxalate could be in waste from possible sources, such as incineration residues, sludge produced by waste acid treatment, and ceramics. This suggests that the existence of specific minerals, such as calcium oxalate (weddellite and/or whewellite), might enhance the leachability or mobility of heavy metals in landfills to some extent.
