## 4 Results and Discussion
### Diffusion Leaching Tests in FRC Specimens Made with RPF
#### 4.3.2 Long Term Behaviour: Dynamic Tank Leaching Test by UNE-EN 15863: 2015
Of all the represented elements: Cr, Zn, Se and Ba showed a \(pD_{e}\) value higher than 12.5 because they are components with low mobility. The determination of the leaching mechanisms occurring in the diffusion test is possible, based on the leaching of components previously measured and establishing whether the matrix of the test piece is dissolving during the conduct of the test. It is possible to determine whether leaching is diffusion controlled or whether other leaching mechanisms also contribute, for all individual components. Despite the usefulness of short-term tests, they do not provide information about the behaviour of contaminants in the medium or long-term. On the contrary, dynamic tests are usually employed for elucidating the dominant leaching mechanism from a stabilised/solidified (S/S) waste [46; 47]. As a matter of fact, the main release mechanisms are: (i) solubility; (ii) diffusion from the internal porosity of the matrix to the surface; and (iii) surface wash-off (where substances concentrated at the surface of the monoliths may be released at the first contact with water) [48]. Figure 5 illustrates the graphical representation of diffusion-controlled leaching (mg/m\({}^{2}\)) vs. time (days) for the identification of leaching mechanisms of the four heavy metals studied. The \(U_{availability}\) value obtained by the availability test is included to observe the maximum release level obtained for each heavy metal and each specimen is plotted in Figure 5, along with the obtained data from the diffusion test of cumulative leaching measurements (expressed in mg/m\({}^{2}\)) and its linear regression. Regarding the leaching behaviour of monolithic samples, there are three main basic transport and chemical mechanisms that usually control the leaching processes: surface wash off, where way the most soluble phases on the surface of the monolith are released quickly, diffusion transport of the solubilised species in the aqueous phase inside the monolith, and the surface dissolution of the monolith in contact with the aqueous phase that surrounds it (chemical mechanism control) [32; 47]. For determining the main leaching mechanism, the linear regression that fits the experimental points of cumulative fraction released versus time, or the logarithmic representation of cumulative flux versus time, can be used [49; 50; 51; 52]. According to [PERSON] et al. [47], if the slope of the straight line fitting the experimental data is greater than 0.65, then surface dissolution is the dominant leaching mechanism; if the _slope_ is close to 0.50 (0.35 < _slope_\(\leq\) 0.65) diffusion transport is the leaching mechanism and if the _slope_ is near 0 (_slope_\(\leq\) 0.35) a phenomenon of initial wash-off or depletion (if it happened in the middle or at the end of the test) is occurring. According to the data obtained, all element concentrations in the leachates are below the permissible limits prescribed by the SQD and used as a reference for shaped construction materials, such as concrete. The limits for heavy metals are shown in Table 5.  
According to the data in Table 5, Zn and Sb showed linear regression slopes that lead to the idea that the wash off mechanism occurred. A flat Zn curve appeared in the last steps of the diffusion test, indicating that the element has been depleted in dissolution. Ba presented a tendency towards a dissolution mechanism, except in the FRC-REF sample. In Figure 5, the cumulative leaching points follow the slope and the release levels increase along the steps. The element Cr presented lower values for the samples with RPF, corresponding to the wash off mechanism, whereas diffusion was detected for control specimens as has been observed in previous works: contaminants may be in the oxidised or reduced form (e.g., Chromium may be present as CrO\({}_{4}\)\({}^{-2}\) or Cr\({}^{+3}\)).