Tl(I) sequestration by pharmacosiderite supergroup arsenates: synthesis, crystal structures and relationships in Tl(I)–M(III)–As(V)–H2O (M = Al, Fe) system

Abstract

Due to their heteropolyhedral 3D open framework with cation exchange possibilities, pharmacosiderite supergroup arsenates play an essential role in the retention, mobility, and fate of various trace elements in the environment. However, the geochemical interaction with extremely toxic thallium (Tl) remains understudied. The formation of the compounds in the Tl(I)–M(III)–As(V)–H₂O (M(III) = Al, Fe) system results in the occurrence of poorly-crystalline thalliumpharmacosiderite, which was reported in the mining-impacted areas as well as in corresponding sediments and soils. Unfortunately, due to its low crystallinity, just a partial understanding of its key structural and compositional properties exists. Therefore, using hydrothermal synthesis (stainless steel autoclaves, autogenous pressure, T_max = 170 °C), we have synthesized good-quality synthetic analogue of thalliumpharmacosiderite (Tpsd), Tl_2.5Fe₄[(AsO₄)₃(OH)₄](OH)_1.5·3H₂O, and still-not discovered “thalliumpharmacoalumite” (Tpal), Tl_1.25Al₄[(AsO₄)₃(OH)₄](OH)_0.25·4H₂O single crystals. They were characterized using single-crystal X-ray diffraction (SC-XRD), powder X-ray diffraction (PXRD), Raman spectroscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM), providing more details on their chemical composition and crystal structure, thus bringing us one step further in better understanding of their structural and chemical properties and how they may relate to their formation in nature. Furthermore, Tl₃AsO₄ was resynthesized and its crystal structure and Raman spectrum were discussed, since it has a potential to be found in natural environments. Additionally, chemical characterization and Raman spectrum of a novel Tl-Fe-arsenate (Tl:Fe:As = 1:1:1) was mentioned. Consequently, the present research delivers useful insights on the role of pharmacosiderite supergroup arsenates in the environmental cycle of Tl.