Ultrafiltration of seawater with a zirconium and aluminum oxide tubular membrane: application to the study of colloidal organic carbon distribution in an estuarine bottom nepheloid layer.

International audienceFor separation of organic colloids in fresh and saline waters, a cross-flow ultrafiltration system was built with a new type of membrane made of zirconium and aluminum oxides. This membrane had a tubular design (16 cm long, 4 cm wide) and a porosity of 10 nm. The suitability of the system for organic carbon studies was first tested by laboratory experiments. In the conditions determined, we obtained acceptable values of dissolved organic carbon (DOC) blank (less than 4 μmol Cl−1) and minimal material adsorption on the membrane. Our special unit was applied to seawater samples collected from the bottom nepheloid layers (BNL) off the Rhône Delta (France). Material defined as colloidal matter in the size range of 0.01–0.7 μm was isolated by filtration on GF/F filters, followed by ultrafiltration. Organic carbon was measured in the so-called dissolved (DOC 0.7 μm) fractions. COC displayed a wide range of concentrations, from 7 μmol Cl−1 (COC/DOC = 7%) offshore, up to 88 μmol Cl−1 (COC/DOC = 49%) near the mouth, whereas truly dissolved carbon remains constant (tDOC = 89 ± 10 μmol Cl−1). In fact, a strong negative correlation was found between COC concentrations near the sediment and the distance from the mouth, indicating that in the area investigated, the major part of the colloidal material does not reach the open ocean. Furthermore, COC concentrations were positively correlated with POC concentrations, suggesting a particulate origin of colloids in the Rhône BNL