Molecular mass distributions of dissolved organic carbon and associated metals in waters from Rio Negro and Rio Solimões

Abstract

Waters from two tributaries of the Amazon River, the Rio Negro and Rio Solimões, were collected near Manaus (Brazil), and their dissolved organic carbon (DOC) were concentrated and size fractionated by ultrafiltration. Hollow fiber ultrafiltration columns were used to separate humic compounds (and associated metal ions) from larger volumes of water in the field. The metal composition of each fraction was determined primarily by ICP-AES, after preconcentration. Flat membrane ultrafiltration was used to obtain sharper molecular mass cut-off levels in the laboratory. About 30-40% of the DOC in the Rio Negro water was within the mass range of 1-10 kDa. This fraction was composed mainly of humic compounds, which are the carriers for most of the metal ions studied (e.g. Al, Fe, Ti, Cu, Mn, Zn, Ca, Mg). Humic and fulvic acids, isolated from these waters, were characterized by IR-spectrometry and potentiometric titrations. Water from the Rio Solimões showed lower retention of DOC (≊ 20%) and associated metal ions, probably due to the more pronounced sorption of the high-molecular mass humic compounds on suspended particles. Despite the high load of clay particles in this water, no evidence of such particles in colloidal size ranges was observed. © 1994.