This study presents the results of ultrafiltration experiments (0.20 μm-300,000 Da-5000 Da-1000 Da) performed on natural rich-organic waters (30-40 mg l-1 of dissolved organic carbon) sampled in wetland area of Cameroon (Nsimi-Zoetele site). A very strong decrease in all cation concentrations (major and trace elements) except Si was observed after filtration. Speciation calculations using data available in the literature for metal-humic substance complexation observations suggest that ultrafiltration performed on this water source at a pH of 4.74 induces a very strong retention of cations only weakly bound to humic substances in the aqueous solution [Viers, J., Dupre, B., Polve, M., Schott, J., Dandurand, J.L., Braun, J.J., 1997. Chemical weathering in the drainage basin of a tropical watershed (Nsimi-Zoetele site, Cameroon): comparison between organic-poor and organic-rich waters. Chem. Geol., 140, 181-206]. To minimize this artifact, ultrafiltration must be performed at low pH (=3) or with the addition of high concentrations of a complexing metal (e.g., Lanthanum). The goal of these ultrafiltration experiments is to anticipate the affinity of several major and trace elements to form organo-metallic complexes with humic substances. Experiments using Sr and Ba isotopes at fixed ratios were entered in order to determine the exchangeable fraction of base cation. Natural Sr isotopic ratio (87Sr/86Sr) in different filtrates and retentates as well as isotopic ratios of Ba and Sr (86Sr/84Sr and 138Ba/135Ba) in spiked and filtered samples appear to remain constant. These results suggest that there is only one source of Sr in these natural waters and that it is present in an exchangeable form (i.e., free ion and/or complexed with organic matter). By analogy, we suppose that elements such as Ca or Mg are not present in organic or mineral colloids but in an exchangeable position. This is in agreement with the hypothesis of the filtration artifact. In the filtration experiment, performed at pH 3, more than 95% of Al, Ga, Fe, U, Th, Y and REEs, κ 50% of Cr and V, 25% of Cu, 10% of Co, and ≤ 5% of Ca, Mg, Na, K, Mn, Ba, Rb and Sr are complexed with organic material. According to the data gathered in this study and the results of speciation calculations, the order and the overall constants (log K) for the formation of metal-humate complexes are the following: Al, Ga, Fe, Th, U, Y, REEs (more than 7) >> Cr (5.5) >> Co (3) > Rb, Ba, Sr, Mn, Mg (~ 2). These data are obtained for both low ionic strength and low metal concentrations. Using the filtration experiment performed with an addition of La, we observe that REEs appear to be complexed with humic substances via two types of site. The first site has a strong affinity for the REEs but is not abundant. So the complexation by this site will be important when the REEs concentration is low. The second type of site is much more abundant but has a much smaller affinity for the complexation of REEs. This site will dominate when the REEs concentration is high. The first site remains unassigned but the second should be related to the carboxylic functional groups.
