The roles of iron and manganese oxides in controlling trace element retention in soils across a range of parent materials

Context: While human activities are the primary source of elevated soil metal concentration, natural geological processes can also contribute significantly. Aims: This study investigated the influence of parent rock lithology on the concentration of arsenic, chromium, copper, nickel, lead, and zinc in soils of the O&Higgins Region, central Chile. Methods: The soils developed on five distinct lithologies: andesites (intermediate volcanic rocks), felsic volcaniclastics, mixed (intermediate and felsic) volcanics, plutonic rocks, and metamorphic rocks. A total of 38 topsoil samples (0&;10&cm) and their corresponding 22 parent rock samples were collected from watersheds selected to represent background conditions. Selective dissolution using citrate&;bicarbonate&;dithionite and hydroxylamine hydrochloride was employed to quantify trace element association with iron oxides (FeOx) and manganese oxides (MnOx). Key results: Adsorption onto FeOx was the primary control on trace element retention across all soils. Notably, FeOx retained over 50% of total soil arsenic, reflecting the strong affinity between arsenate and FeOx surfaces, likely driven by binuclear inner-sphere complexation, whereby an arsenate ion replaces two hydroxyl ions on the FeOx surface. Conclusions: This study demonstrates that the influence of parent rock lithology on soil trace element content is not a direct relationship, but is instead mediated by the complex interplay between FeOx and MnOx. Implications: The novelty of this work lies in highlighting the differential influence of these oxide phases on the retention of As relative to divalent metals.