Investigation of metal speciation in crude oil contaminated marine environments using a novel DGT technique

Crude oil spills and discharges from refineries, mining, and industrial activities can introduce trace metals into marine environments. Determining trace metal concentrations and speciation in seawater is challenging due to the complex matrix and high salinity. This study developed a novel approach using modified diffusive gradients in thin-films (DGT) to measure labile concentrations and speciation of Co, Ni, Cu, Cd, Pb, and Zn in crude oil-contaminated seawater. Five DGT devices (open pore, restricted pore, 1000 and 3500 MWCO dialysis membranes, and Nafion 112) were tested. Results showed that crude oil impacts the lability and mobility of metals, with Nafion 112 excluding negatively charged complexes. Diffusion coefficients were measured in diffusive hydrogels, dialysis membranes, and Nafion using a diffusion cell. Metal speciation was investigated in synthetic seawater with 1 % and 4 % crude oil/water ratios (OWR). For Co, Ni, Zn, and Pb, similar labile concentrations across DGT devices indicated low molecular weight complexes dominated. Labile Cu decreased significantly with increased oil content, while other metals showed varying degrees of lability. The findings suggest that crude oil influences metal speciation through complexation with organic ligands, affecting their bioavailability in marine environments. Compared to measurements by equilibrium microdialysis, similar concentrations for Pb and Zn suggested their weak complexes with oil-soluble organic chemicals. However, microdialysis measured higher concentrations of Co, Cu, Ni, and Cd, indicating partially non-labile, low molecular weight complexes existed. Nafion 112 diffusion coefficients were 16-38 times lower than those in diffusive gel. Only free metal ions and potentially some positively charged inorganic/small organic complexes are likely to be measured by Nafion-DGT. This work has demonstrated that crude oil level and metal characteristics greatly influence metal speciation, and that small, labile organic complexes play a crucial role in controlling the mobility and availability of metals in oil-contaminated seawater.