Tracking Changes in Organic-Copper Speciation during Wastewater Treatment Using LC-ICPMS-ESIMS

Abstract

Wastewater is a significant source of copper to freshwater environments, which can severely harm aquatic life. The bioavailability and toxicity of copper in water are influenced by its complexation with dissolved organic matter (DOM). Speciation models, like the biotic ligand model (BLM) that guides Cu regulations, assume DOM is dominated by humic substances. Research suggests that anthropogenic compounds in wastewater discharge may be important copper binding ligands, although their identities remain largely unknown. To address this knowledge gap, we identified and quantified organic copper species isolated from 24 h composite wastewater samples by solid phase extraction (SPE) using liquid chromatography (LC) with inductively coupled plasma mass spectrometry (ICPMS) and electrospray ionization mass spectrometry (ESIMS). Analyses of samples across different stages of treatment revealed the net removal of Cu (73%) and DOC (66%). LC-ICPMS showed that certain complexes were selectively removed, while others evaded removal or were generated during treatment. Relatively hydrophobic complexes decreased in abundance from the initial to the secondary treatment stage. In contrast, more hydrophilic organic Cu complexes, likely formed during treatment, showed a significant increase from the secondary to the tertiary stage. The molecular mass and formula of seven discrete chromatographically resolved complexes were identified by LC-Orbitrap MS. Six were detected only in wastewater, and one was detected in all the wastewater and river samples. Identification of these compounds provides additional evidence for the formation of new copper-binding ligands during treatment and confirms the presence of nitrogen- and sulfur-containing compounds with copper-chelating properties in the wastewater. These findings demonstrate the utility of LCMS approaches for identifying and quantifying distinct organic-copper species in wastewater, as well as tracking their changes and removal during the treatment process.