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.