In Situ Low-Field Nuclear Magnetic Resonance Reveals Dynamic Adsorption of Paramagnetic Heavy Metals on Microplastics

The co-occurrence of microplastics and heavy metals poses significant environmental risks. However, conventional analytical methods are ex situ, destructive, and unable to capture real-time adsorption dynamics. We present a low-field nuclear magnetic resonance (LF-NMR) technique that enables the in situ, nondestructive, time-resolved quantification of paramagnetic metal adsorption onto microplastics. We applied this method to monitor the adsorption of Cu(II) and Cr(III) onto poly(vinyl chloride) and polystyrene microplastics under diverse water chemistry conditions. Validation using inductively coupled plasma mass spectrometry confirmed the excellent accuracy of the proposed method, which exhibited high linearity (R2 > 0.99), an average recovery of 100.7%, and detection limits of approximately 0.1 mg L–1. The method showed robust sensitivity and stability across varying salinity (0–250 mM) and pH (3.5–6.5) with minimal interference. LF-NMR revealed a two-stage adsorption mechanism characterized by rapid surface binding followed by slower intraparticle diffusion. Critically, the nondestructive nature preserved the microplastic–paramagnetic metal complexes for subsequent characterization, overcoming the limitations of conventional destructive techniques. This real-time approach bridges quantitative detection with mechanistic understanding, providing a powerful tool for elucidating microplastic–metal interactions and showing strong potential for nanoplastic and aging studies.