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

For separation of organic colloids in fresh and saline waters, a cross -flow ultrafiltration system was built with a new type of membrane mad e 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 t he system for organic carbon studies was first tested by laboratory ex periments. In the conditions determined, we obtained acceptable values of dissolved organic carbon (DOC) blank (less than 4 mumol Cl-1) and minimal material adsorption on the membrane. Our special unit was appl ied to seawater samples collected from the bottom nepheloid layers (BN L) off the Rhone Delta (France). Material defined as colloidal matter in the size range of 0.01-0.7 mum was isolated by filtration on GF/F f ilters, followed by ultrafiltration. Organic carbon was measured in th e so-called dissolved (DOC < 0.7 mum), colloidal (0.01 < COC < 0.7 mum ), truly dissolved (tDOC < 0.01 mum) and particulate (POC > 0.7 mum) f ractions. COC displayed a wide range of concentrations, from 7 mumol C l-1 (COC/DOC = 7%) offshore, up to 88 mumol Cl-1 (COC/DOC = 49%) near the mouth, whereas truly dissolved carbon remains constant (tDOC = 89 +/- 10 mumol Cl-1). In fact, a strong negative correlation was found b etween 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 co ncentrations were positively correlated with POC concentrations, sugge sting a particulate origin of colloids in the Rhone BNL.