pH control on the colloidal association of trace elements with iron and organic matter along the hydrological continuum of a tropical river floodplain

Colloidal transport of trace metals is a major biogeochemical feature of organic-rich ("humic") watersheds in the Equatorial Belt (10° N-10° S), a region critical for global land-to-ocean carbon (C) export. Yet, floodplains and short hydrological continuums in these systems remain poorly characterized due to their remoteness and logistical challenges. Here, we examined colloidal (1 kDa-0.22 μm) and small particulate (0.22-3.1 μm) fractions of major and trace elements across contrasting aquatic environments of the Ogooué River floodplain (Gabon, western Africa) during the wet season. Our goals were to quantify element partitioning along the hydrological continuum, identify organic versus mineral colloidal carriers, and evaluate pH control on colloidal stability. Three solute groups emerged based on their association with Fe and dissolved organic carbon (DOC): (i) weakly colloidal oxyanions and alkalis (< 30 %), (ii) moderately colloidal divalent metals, P, Se, and Cr (30-80 %), and (iii) strongly colloidal trivalent and tetravalent hydrolysates (≥ 80 %). Low-mobility elements such as Fe, Al, Pb, and U were predominantly associated with Fe oxyhydroxide colloids, whereas divalent trace metals were mainly stabilized in solution through complexation with organic ligands. pH was the dominant factor controlling element speciation-higher pH enhanced DOC-metal complexation and Fe/Al-oxyhydroxide association, but reduced colloidal fractions of alkalis and oxyanions through desorption and groundwater inputs. Despite spatial heterogeneity, the colloidal behavior observed across the Ogooué floodplain closely resembled that of other tropical and boreal humic rivers, revealing a broadly consistent pattern of colloid-mediated metal transport. Anticipated hydrological changes under future climate scenarios are likely to alter this colloidal equilibrium, with implications for trace-metal mobility and C export to the Atlantic Ocean.