ELEMENTAL COMPOSITION OF THE COLLOIDAL PHASE ISOLATED BY CROSS-FLOW FILTRATION FROM COASTAL SEAWATER SAMPLES

The colloidal fraction was isolated to a solid using cross-flow filtra tion from 165 1 of Massachusetts coastal seawater taken as part of the WHOI Intercomparison exercise in August 1994. The elemental compositi on of this sample was determined by ICP-MS. Comparisons to analyses ma de part way through the isolation process as part of the colloid inter comparison exercise, indicate that loss due to break-through and/or ad sorption occurred primarily when the 1-kD Millipore membranes were use d to concentrate the colloidal fraction. Retention of the colloidal ma terial was relatively high when the Desalinization membranes were used to concentrate the colloidal material. Overall loss for the complete isolation of the colloidal phase to a solid is estimated to be similar to 90%. Despite, this high degree of loss, it is still possible to me asure a large suite of elements using ICP-MS because of the removal of interfering sea salts. Elemental concentrations in the colloidal isol ate are generally similar to those found previously in isolates from c oastal waters off southern California except for elements such as U, I , Mo, and As which an greatly enriched, possibly due to the longer pro cessing time of the WHOI sample or higher organic content of the collo idal material. Elements presumed to exist as neutral or positively cha rged hydroxy species and those known to be complexed with organic liga nds have the highest relative concentrations when compared to seawater values in the colloidal phase. Reducible elements which are normally present largely as oxyanions, such as Mo, As, V, W, U and I, have high colloidal concentrations but small enrichments. These elements are en riched in the WHOI sample compared to SIO samples. When normalized to average shale, rare-earth elements (REE) in the colloidal material exh ibit a pattern similar to that of Buzzards Bay seawater with a flat li ght REE (LREE) pattern and an enrichment in heavy REE (HREE). When nor malized to this seawater, there is an enrichment in the HREE and a neg ative Gd anomaly characteristic of uptake on organic material.