Ultrafiltration of humic compounds through low molecular mass cut-off level membranes

Abstract

During ultrafiltration of humic compounds through Amicon YM2 flat membranes (cut-off level: 1 kDa), humic acid solutions presented retentions of 80–90% and fulvic acid of 60–70%. Fulvic acid retention on this membrane depends directly on pH and inversely on ionic strength. Calcein (2,7-Bis[bis(carboxymethyl)-aminomethyl]-fluorescein; 623 g/mol), an organic molecule tested for comparative purposes, exhibited a similar behavior. Experiments with humic acid were handicapped by precipitation or aggregation processes under low pH or high ionic strength conditions. Humic acid retention on PM10 membrane (cut-off level: 10 kDa) is the same as on YM2, despite the difference in cut-off levels. This anomalous result is attributed to the lipophilic character of the PM10 membrane. Both membranes are, therefore, highly efficient for the concentration of humic acid from typical freshwaters; a good recovery is also attained with fulvic acid on the YM2 membrane. Although YM2 is considered a hydrophilic, non-ionic membrane, salts containing polyvalent anions show high retentions under low ionic strength conditions, and the retention is proportional to the anion charge.