Because there was no flux measurement as done in the experiments of in our case Fick's 1 st law cannot be applied to derive 𝐷 . Our fitted 𝐷-value thus corresponds to an apparent diffusion coefficient, according to 2 nd Fick's law. To be noted, elemental concentrations measured via X-ray fluorescence correspond to the total local concentration, given by the sum of (i) all mobile species present in the pore water plus (ii) all immobile (sorbed, precipitated) species. Accordingly, the fitted 𝐷-value cannot be a priori equated to 𝐷 as defined in Eq. ( ), which refers only to diffusion in the aqueous phase (pore water).