Aging Process Reduces Arsenic Availability in Precipitates Due to Mineralogical Transformations in Fe and Al (Hydr)oxides

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ABSTRACT Crystallinity changes of iron (hydr)oxides can occur due to many factors such as pH alterations, temperature, and aging. These alterations can influence solubility and therefore, the mobilization of potentially toxic elements (PTE) that are associated to iron (Fe) compounds. In this study, precipitates of pure and aluminum (Al) – iron (hydr)oxides with isomorphic substitution that were coprecipitated with arsenic (As) were evaluated regarding As availability and bioaccessibility after 7 years of aging. Both Fe (II) and Fe (III) were used, and both speciation of As were assessed: As (III) and As (V). Chemical composition of these compounds was assessed by using aqua regia and HNO3 digestion. Single chemical extractions with Mehlich-1, CaCl2 (0.01 M), citric acid (2%) and HNO3 0.43 M were performed to evaluate availability and bioaccessibility of As. The leaching behavior of the synthetic compounds was repeated in the aged compounds, compared with the fresh precipitates. Crystallinity of iron (hydr)oxides was tested by dithionite-citrate-bicarbonate and oxalate extractions. The association between As and well crystallized iron oxides is greater when aluminum is in the system, while in the absence of Al, As associated more with oxides of lower crystallinity. Leaching of As was greater in fresh precipitated materials when the primary source was Fe (III), while the aging process did not affect the Fe (II) precipitates. This study reinforces the importance of the long-term evaluation of iron compounds when incorporating trace elements, since the mobilization of them can promote health injuries.