Comparative study of dissolved copper speciation using DGT and CLE-AdSV in a contaminated urban river estuary and marina

Abstract

A comprehensive study was conducted in a river estuary and marina in Auckland, New Zealand, to examine the in-situ speciation of copper (Cu) complexes across freshwater, estuarine, and marine environments using the diffusive gradient in thin films (DGT) technique. The study employed both open-pore (OP) and restrictive-pore (RP) diffusive gels of varying thicknesses to assess the effectiveness of DGT hydrogels and the role of the apparent diffusive boundary layer (ADBL) in determining the lability of Cu complexes in natural waters. The DGT findings were compared with results from competing ligand exchange–adsorptive stripping voltammetry (CLE-AdSV) and a speciation prediction model (WHAM/Model-VII). The study highlighted the critical importance of accounting for the ADBL, as its exclusion led to an average underestimation of Cu-complex lability by approximately 50 %. Given the minimal difference in outcomes between DGT-OP and DGT-RP, the use of standard DGT-OP is recommended. Further analysis revealed a strong correlation between DGT-labile Cu (CuDGT-lab), the weaker Cu-L₂ complexes detected by CLE-AdSV, and the inorganic Cu species predicted by WHAM/Model-VII, the latter two based on discrete samples. A strong relationship was also noted between Cu_DGT-lab and the inorganic Cu measured by CLE-AdSV. The study provided compelling evidence that the in-situ ligand pool consists of a mixture of terrestrial humic substances and marine-derived organic ligands. These marine ligands form highly stable Cu-L₁ complexes along the salinity gradient, which were mostly DGT-inert. The DGT results indicated that the lability of in-situ Cu complexes decreased with increasing salinity, with over 60 % of Cu in freshwater being DGT-labile. Overall, the study demonstrates that the DGT technique is a robust and straightforward method for environmental monitoring and risk assessment across diverse physico-chemical water conditions.