An ultraclean cross-flow ultrafiltration technique for the study of trace metal phase speciation in seawater

A series of laboratory and field studies were conducted to test suitability and optimal sampling conditions of an Amicon ultrafiltration system for the collection of colloidal material for trace metal phase speciation studies in marine environments. A cleaning procedure is required between each sample processed to eliminate carry-over artifacts and achieve a low system blank. Mass-balance recovery of 88–109% was achieved for ultrafiltration of trace metals (e.g., Cd, Cu, Ni, Pb, Mn, Fe, Zn, Hg) from estuarine samples. Results of sample storage experiments indicate that ultrafiltration should ideally be performed within 4 h of sample collection to prevent phase speciation shifts. The concentration of a number of trace metals in the permeate fraction, when plotted as a function of concentration factor, was found to fit the steady-state macromolecular permeation model of Kilduff and Weber (1992). A retentate concentration factor between 5 and 10 was optimal for most metals based on modeling the permeate metal concentration. For Galveston Bay, the colloidal fraction (1 kDa-0.45 μm) accounted for 65–85% of the Cu, 35–40% of the Ni, and 30–95% of the Pb in the filtered (< 0.45 μm) fraction. Colloidal Cu, Cuc (1 kDa-0.2 μm) was 64%, Nic was 6%, Pbc was 9% of the filtered (< 0.2 μm) fraction in deep Pacific water samples from the colloid intercomparison exercise.