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 mu m and 0.2 mu m << 0.45 mu m frac tion: the first one can be regarded as "true solution" while the others def ine colloidal classes. The metal binding capacity of these fractionated sam ples, using differential anodic pulse stripping voltammetry (DPASV), Cu2+ i on 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 pK(Cu) spectra with one major peak near the 3 to 4 pK(Cu) range in all fractions and several quite lower intensity pea ks for higher pK(Cu) values for the largest colloidal fractions only. The p K(a) distributions show pK(a) ranging from 3 to 4 and from 9 to 10, with in termediate smaller peaks. Concerning proton available sites, the results sh ow that functional groups of organic colloids develop a weaker acidity for the low molecular weight organic compounds. The maximum proton available si te density has been observed for the smallest molecules (< 5000 Da) larger molecules show lower proton available site densities. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.