Bioaccessibility and availability of trace elements coprecipitated with aluminum-substituted iron oxides

Iron oxides nanoparticles are one of the main reservoirs for trace elements in soils and sediments. Trace elements can coprecipitate with iron oxides through the formation of surface complexes or incorporation to the solid phase structure. This study aimed to evaluate the sorption stability and bioaccessibility of trace elements coprecipitated with Fe-oxides, and to assess the influence of Al-substitution and Hg presence on their environmental mobility. Iron oxides were precipitated from a solution containing arsenic (As), lead (Pb), antimony (Sb), cadmium (Cd), lanthanum (La), ytterbium (Yb), and mercury (Hg) in the presence and absence of aluminum (Al). The sorption stability of the multiple trace elements coprecipitated was evaluated through BCR and SBET extraction procedures. Low extraction percentages (< 10%) of As, Cd, and Sb were observed, indicating that more than 90% of their total contents were likely retained through strong surface adsorption and incorporation into the iron oxide structure. In contrast, the higher extraction percentages of La, Pb, and Yb (up to 86%, 41%, and 37%, respectively) were attributed to weaker surface adsorption and limited incorporation into the iron oxide structure. It was assumed that there is a limit for trace elements incorporation to iron oxides and the remaining not incorporated ions would be only weakly adsorbed and easier extractable. Indeed, at high trace element concentrations, Cd and Pb sorption stability decreased, increasing their extraction percentages. Arsenic and Cd coprecipitated with Al-goethite were more stable than with lepidocrocite or Al-free-goethite. Conversely, La and Yb coprecipitated with Al-goethites were less stable. The human risk assessment by oral intake revealed that the high bioaccessible content of As in iron oxides represents risk for children health (HQ > 1) in every sample. The results were considered useful for improving the comprehension of interactions between iron oxides and trace elements in natural systems, including aspects such as sorption mechanisms, stability, bioavailability, and toxicity.