Evaluation of a cloud point extraction approach for the preconcentration and quantification of trace CuO nanoparticles in environmental waters

The cloud point extraction (CPE) of commercial copper(II) oxide nanoparticles (CuO NPs, mean diameter of 28 nm) in water samples was fully investigated. Factors such as Triton X-114 (TX-114) concentration, pH, incubation temperature and time, were optimized. The effects of CuO NP behavior like agglomeration, dissolution, and surface adsorption of natural organic matter, Cu²⁺, and coating chemicals, on its recovery were studied. The results indicated that all the CPE factors had significant effects on the extraction efficiency. An enrichment factor of ∼89 was obtained under optimum CPE conditions. The hydrodynamic diameter of CuO NPs increased to 4–5 μm upon agglomeration of NP-micelle assemblies, and decreased at pH >10.0 at which the extraction efficiency was also lowered. The solubility and therefore, the loss of NPs were greatly enhanced at pH <8.5 and in the first 60 min of incubation, whereas it declined at elevated incubation temperatures. Our results showed that the dissolved organic carbon (DOC) >5 mg C L⁻¹ and Cu²⁺ >2 times that of CuO NPs, lowered and enhanced the extraction efficiency, respectively. Pre-treatment of samples with 3% w v⁻¹ of hydrogen peroxide and 10 mM of ethylenediaminetetraacetic acid minimized the interferences posed by DOC and Cu²⁺, respectively. The decrease in CPE efficiency was also evident for ligands like poly(ethylene glycol). The TX-114-rich phase could be determined with either inductively coupled plasma mass spectrometry following microwave digestion, or graphite furnace atomic absorption spectrometry. The detection limits for CuO NPs were 0.02 and 0.06 μg L⁻¹ using these techniques, respectively. The optimum sample pre-treatment and CPE conditions were successfully applied to the river and wastewater samples. The relative recoveries of CuO NPs spiked at 5–100 μg L⁻¹ (as Cu) in these samples were in the range of between 59.2 and 108.2%. The approach demonstrates a robust analytical method for detecting trace levels of CuO NPs at their original states and assessing their exposure risks in real aquatic environments.