Ultrafine multi-metal (Zn, Cd, Pb) sulfide aggregates formation in periodically water-logged organic soil

This study investigates authigenic metal (Zn, Cd, and Pb) sulfides formed in the upper (4-20 cm) layer of severely degraded soil close to ZnPb smelter in CE Europe (southern Poland). The soil layer is circumneutral (pH 6.0-6.8), organic, occasionally water-logged, and contains on average 26,400 mg kg^-1 Zn, 18,800 mg kg^-1 Pb, 1300 mg kg^-1 Cd, and 2500 mg kg^-1 of sulfur. The distribution of the authigenic sulfide mineralization is uneven, showing close association with the remains of vascular plants (Equisetaceae, Carex, and herbs). A combination of focused ion beam (FIB) technology with scanning (SEM) and transmission electron microscopy (TEM) is used to reveal the structure and organization of the metal sulfides at micro- and nanoscale resolution. The sulfides form spheroidal and botryoidal porous aggregates composed of nanocrystalline (<5 nm) ZnCd sulfide solid solution and minor discrete PbS (galena) crystals up to 15 nm. The solid solution exists in a cubic (sphalerite) polytype over a whole Zn/Cd range. An intricate core-shell structure is found to be a characteristic feature of the aggregates in which high-Zn outer layers encapsulate Cd-rich sulfide core. PbS resides between the Cd-rich and Cd poor sulfide within nano sites of increased porosity. The study highlights the importance of nanoscale analyses for the prediction of metal behavior in soils. The sulfide self-organization into complex structures and Cd encapsulation inside high-Zn sulfide indicate the occurrence of a self-sustainable mechanism specific to polluted periodically water-logged soil that limits Cd mobility. However, as the reduced Cd mobility is obtained at the Zn expense, the soil gets Cd enriched relative to Zn over extended periods. Although the study proves PbS crystallization in the soil, the process seems environmentally irrelevant even at high Pb contents, being suppressed by other soil processes (e.g., Pb sorption on organic matter). Our findings are valuable in remediation strategies and the management of contaminated soils rich in organic matter that address the mobility of toxic metals and their transfer into living organisms.