Elemental composition of the colloidal phase isolated by cross-flow filtration from coastal seawater samples

Abstract

The colloidal fraction was isolated to a solid using cross-flow filtration from 165 1 of Massachusetts coastal seawater taken as part of the WHOI Intercomparison exercise in August 1994. The elemental composition of this sample was determined by ICP-MS. Comparisons to analyses made part way through the isolation process as part of the colloid intercomparison exercise, indicate that loss due to break-through and/or adsorption occurred primarily when the 1-kD Millipore membranes were used to concentrate the colloidal fraction. Retention of the colloidal material 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 ~ 90%. Despite, this high degree of loss, it is still possible to measure a large suite of elements using ICP-MS because of the removal of interfering sea salts. Elemental concentrations in the colloidal isolate are generally similar to those found previously in isolates from coastal waters off southern California except for elements such as U, I, Mo, and As which are greatly enriched, possibly due to the longer processing time of the WHOI sample or higher organic content of the colloidal material. Elements presumed to exist as neutral or positively charged hydroxy species and those known to be complexed with organic ligands 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 enriched in the WHOI sample compared to SIO samples. When normalized to average shale, rare-earth elements (REE) in the colloidal material exhibit a pattern similar to that of Buzzards Bay seawater with a flat light REE (LREE) pattern and an enrichment in heavy REE (HREE). When normalized to this seawater, there is an enrichment in the HREE and a negative Gd anomaly characteristic of uptake on organic material.