Uniform features of organic carbon and trace metal colloidal carriers in humic surface waters: A case study of Cameroon

Organic matter (OM) and Fe/Al oxy(hydr)oxides are two of the most important drivers for trace element (TE) transport in surface waters, occurring both as colloids and particles. Distinguishing between these two trace metal carriers remains a challenge and requires significant instrumental investment. However, empirical methods such as ultrafiltration and dialysis can be suitable for assessing the colloidal status of trace metals in pristine, organic- and Fe-rich waters, particularly when transport to high-resolution analytical facilities is cost-prohibitive. In this study, we conducted an on-site assessment of colloids and particles in humid tropical region (Cameroon, Central Africa), selecting several representative humic and Fe-rich surface waters, including a large river, its tributaries, small streams, and stagnant soil water, all sampled during the long dry season (February). Within this hydrological continuum, we analyzed the size distribution of trace elements and solute chemical compositions using filtrates and ultrafiltrates with progressively smaller pore sizes (3.1 μm, 0.7 μm, 0.45 μm, 0.025 μm), along with dialysis through 25 kDa and 1 kDa (∼1.4 nm) membranes. The objective was to differentiate the organic and organo-ferric colloids/particles responsible for TE transport in these waters. The two main constituents of the studied colloids and particles, Fe and organic carbon (OC), showed independent behavior across the filtration and ultrafiltration sequence, representing two distinct carriers of trace metals. Based on TE concentration patterns relative to Fe or OC during filtration, ultrafiltration, and dialysis, we identified two groups of elements significantly affected (>20-30%) by size separation: (1) Elements preferentially associated with Fe (P, Al, Ti, V, Pb, Ba, Ga, light REE, Nb, Zr, Th, U) and (2) elements more closely linked to OC than Fe (Mn, Ni, Co, Sr, Cd, Y, heavy REE). Some elements, such as Cr and the middle REEs, were associated with both large Fe oxyhydroxide colloids/particles and low molecular weight organic complexes. Contrary to expectations, there was no significant or systematic variation in the major colloidal constituents (OC, Fe, Al) or TE partitioning coefficients between colloids, particles and low-molecular weight forms across the different surface waters (rivers, streams, and forest pool). The long water residence time and strong connectivity of lentic and lotic waters to subsoil and groundwater during the dry season likely contribute to the homogeneity in colloidal and particulate TE transport. We conclude that the partitioning of TEs between organic and organo-ferric colloidal pools is a common feature across humic and iron-rich surface waters. At the same time, TE concentration levels and the pattern of TE-Fe or OC concentration change across the filtration sequence, depend on the surrounding lithology and soil types.