On-site characterization of humic-rich hydrocolloids and their metal loading by means of mobile size-fractionation and exchange techniques

Humic-rich hydrocolloids and their metal loading in selected Gen-nan bog-wa ters have been characterized by a novel on-site approach. By use of an on-l ine multistage ultrafiltration (MST-UF) unit equipped with conventional pol yethersulfone (PES)-based flat membranes (nominal cut-off 0.45, 0.22, and 0 .1 mum, or 100, 50, 10, 5, 3 kDa) the hydrocolloids could be fractionated o n-site in both sub-particulate and macromolecular size ranges. Characteriza tion (dissolved organic carbon (DOC), metals) of the colloid fractions obta ined this way was performed off-site by use of conventional instrumental me thods (carbon analyzer, AAS, ICP-OES, and TXRF (total reflection X-ray fluo rescence)). Major DOC fractions of the hydrocolloids studied were found to be in the size range <5 kDa. The assessed metals (Al, Cu, Fe, Mn, Pb, and Z n) were, however, predominantly enriched in the macromolecular and sub-part iculate range, depending on the metal and the sample, respectively. In addi tion, metal species bound to these hydrocolloids were kinetically character ized on-site by use of competitive ligand (EDTA (ethylenediaminetetraacetat e)) and metal (Cu(II)) exchange; the EDTA complexes formed and the metal io ns exchanged were separated by means of a small time-controlled tangential- flow UF unit (cut-off I kDa). Bound metal fractions, in particular Al and F e, reacted only slowly (500 to 1000 min) with EDTA; the conditional availab ility was 60-99%, depending on the hydrocolloid. In contrast, the Cu(II) ex change of colloid-bound metal species approached equilibrium within 5-10 mi n, with characteristic exchange constants, K-ex, of the order of 0.01 to 90 for the metals (Fe < Al < Zn < Mn approximate to Ca approximate to Mg). Th e results were clearly dependent on the water investigated. Thus detailed i nformation on the conditional kinetic and thermodynamic stability of colloi d-bound metal species could be obtained from competitive EDTA and Cu(II) ex change under on-site conditions.