Capture and recovery of heavy metals in trap minerals: incorporation mechanisms of Tl, Pb and Zn in pyrochlore and hollandite structures

Two pyrochlores with the compositions (falling in the field of microlite and elsmoreite), and a hollandite (cryptomelane) were synthesized in the form of microcrystalline powders and used through imbibition experiments solutions containing Tl(I), Pb(II) and Zn(II) with concentrations ranging from 0.01 to 1M. The chemical composition of both treated and untreated samples was given by PIXE analysis; the eventual presence of water species occurring in the tunnel environment was detected by means of FTIR measurements. All the three sorbent materials were able to incorporate conspicuous amounts of Tl, in the range of 1-1.2 apfu for the most concentrated solutions. Microlite and cryptomelane incorporated Pb and Zn at lower levels, around 0.2-0.3 apfu, while elsmoreite showed very limited ability to absorb the latter metals. XRD analysis showed that the octahedral frameworks remain quite stable after the ion exchanges, while the tunnel environment may undergo drastic changes in electron density distribution due to the uptake of metals. Water species were found to play an important role in pyrochlores, since they can stabilize the structure by balancing the charge of other ions and participate in the ion exchange reactions. Despite the strong disordering phenomena occurring in the tunnels, EXAFS analysis allowed to achieve a quite accurate overview of the local structure of the selected heavy metals. The results showed that heavy metal ions do not occupy conventional crystallographic sites, but they are rather displaced along the tunnel axis or towards the tunnel walls, with disorder occurring around a constellation of satellite positions. Finally, a series of preliminary imbibition experiments using 10-20 ppm solutions of Tl(I), Pb(II) and Zn(II) was performed. The analysis of the residual solutions revealed that in most cases the three sorbents can remove up to 99% of metal in less than one minute.