Direct evidence for the occurrence of indigenous cadmium-based nanoparticles in paddy soils

Speciation of heavy metal-based nanoparticles (NPs) in paddy soils greatly determines their fate and potential risk towards food safety. However, quantitative understanding of such distinctive species remains challenging, because they are commonly presented at trace levels (e.g., sub parts-per-million) and extremely difficult to be fractionated in soil matrices. Herein, we propose a state-of-art non-destructive strategy for effective extraction and quantification of cadmium (Cd)-NPs - the most widespread heavy metal in paddy soils - by employing single particle inductively coupled plasma mass spectrometry (spICP-MS) and tetrasodium pyrophosphate (TSPP) as the extractant. Acceptable extraction efficiencies (64.7-80.4 %) were obtained for spiked cadmium sulfide nanoparticles (CdS-NPs). We demonstrate the presence of indigenous Cd-NPs in all six Cd-contaminated paddy soils tested, with a number concentration ranging from 2.20 × 10⁸ to 3.18 × 10⁹ particles/g, representing 17.0-50.4 % of the total Cd content. Furthermore, semi-spherical and irregular CdS-NPs were directly observed as an important form of the Cd-NPs in paddy soils, as characterized by transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (TEM-EDX). This research marks a significant step towards directly observing indigenous Cd-NPs at trace levels in paddy soil, offering a useful tool for quantitative understanding of the biogeochemical cycling of heavy metal-based NPs in complex matrices.