Resolving natural organic matter and nanoplastics in binary or ternary systems via UV-Vis analysis

Nanoplastics (NPs) and natural organic matter (NOM) are ubiquitous and usually present simultaneously in the environment. Both NPs and NOM can be adsorbed to minerals such as iron-(hydr)oxides, with such interactions being important for controlling their fate in the environment. However, the quantification of NPs and NOM in mixtures remains challenging even under controlled conditions in laboratory studies. In this research, a UV-Vis method was established to quantify concentrations of NOM, such as humic acid (HA) and fulvic acid (FA), and polystyrene NPs (PSNPs) in mixtures. In addition, both original NOM samples and those recovered following adsorptive fractionation using an iron oxide (goethite, α-FeOOH) were mixed separately with PSNPs and their concentrations were further calculated via the developed UV-Vis method. The UV-Vis method performed well (recovery of 100 ± 16 %) with original NOM and PSNPs system at detection limits of 20.8 and 7.4 mgC L^-1, respectively. Particularly, for original FA and PSNPs systems with carboxylic groups (PSNPs-COOH, 200 nm), a similar recovery rate could be obtained at detection limits of only 2.5 and 1.9 mgC L^-1, respectively. For fractionated NOM and PSNPs systems, detection limits (31.2 mgC L^-1 and 27.5 mgC L^-1, respectively) are increased to reach the same accuracy. Furthermore, the UV-Vis method can be used to estimate the proportion of HA that is adsorbed to PSNPs. The relative errors are < 13.7 % when the mass ratios of PSNPs and HA was between 1.6:1 and 8:1 and HA concentration was higher than 4.6 mgC L^-1. This method developed can be applied to future laboratory research to investigate the interaction between NOM, NPs, and minerals.