Retention of Critical Metals (Ga, La, Pt) Bound to Organic Matter by Ultrafiltration Membranes Evaluated Using HPSEC-ICP-MS

Aqueous speciation can be used to explain and even predict the effects of metal contaminants on aquatic organisms. Yet, speciation is difficult to estimate in the presence of natural organic matter (NOM): a mixture of heterogeneous polymeric ligands. Partial ultrafiltration is a fast and affordable emerging technique which allows the experimental determination of metal complexation by NOM. Its applicability is based on the premise that metal-NOM complexes do not pass the low molecular weight cutoff membrane. Their retention was evaluated for samples containing different NOMs (10 mg C L-1) and a mixture of Ga, La, and Pt (360 nM) using high performance size exclusion chromatography coupled to an inductively coupled plasma mass spectrometer. The results show a satisfying separation performance, with better discrimination by the 2 kDa membrane compared to the 3 kDa membrane. The proportions of metal-NOM complexes passing into the ultrafiltrates were 0.0 ± 0.0, 0.7 ± 1.4, and 14.1 ± 11.9% for Ga, La, and Pt, respectively. Furthermore, experimentally determined molecular weight cutoffs for NOM were below the nominal values: 0.4 and 0.5 kDa instead of 2 and 3 kDa, respectively. Overall, the results support the applicability of partial ultrafiltration and show its great potential for speciation measurements in natural waters.