Metal available sites on colloidal organic compounds in surface waters (Brazil)

Using sequential ultrafiltration (UF) of Amazonian surface waters, several fractions, defined by the molecular weight (MW) cut-off of the UF membranes, can be obtained: <5000 Dalton (Da) (or <5 K), 5000 Da 20 000 Da, 20 000 Da 100 000 Da, 100 000 Da 0.2 μm and 0.2 μm 0.45 μm fraction: the first one can be regarded as “true solution” while the others define colloidal classes. The metal binding capacity of these fractionated samples, using differential anodic pulse stripping voltammetry (DPASV), Cu2+ ion selective electrode (ISE) and H+ ISE procedure reveal that the 20 000 Da 100 000 Da fraction is the most efficient fraction for copper binding. The potentiometric results show pKCu spectra with one major peak near the 3 to 4 pKCu range in all fractions and several quite lower intensity peaks for higher pKCu values for the largest colloidal fractions only. The pKa distributions show pKa ranging from 3 to 4 and from 9 to 10, with intermediate smaller peaks. Concerning proton available sites, the results show that functional groups of organic colloids develop a weaker acidity for the low molecular weight organic compounds. The maximum proton available site density has been observed for the smallest molecules (<5000 Da); larger molecules show lower proton available site densities.