Novel multi-isotope tracer approach to test ZnO nanoparticle and soluble Zn bioavailability in joint soil exposures

Here we use two enriched stable isotopes, 68Znen and 64Znen (>99%), to prepare 68ZnO nanoparticles (NPs) and soluble 64ZnCl2. The standard LUFA 2.2 test soil was dosed with 68ZnO NPs and soluble 64ZnCl2 to 5 mg kg–1 each, plus between 0 and 95 mg kg–1 of soluble ZnCl2 with a natural isotope composition. After 0, 1, 3, 6, and 12 months of soil incubation, earthworms (Eisenia andrei) were introduced for 72 h exposures. Analyses of soils, pore waters, and earthworm tissues using multiple collector inductively coupled plasma mass spectrometry allowed the simultaneous measurement of the diagnostic 68Zn/66Zn, 64Zn/66Zn, and 68Zn/64Zn ratios, from which the three different isotopic forms of Zn were quantified. Eisenia andrei was able to regulate Zn body concentrations with no difference observed between the different total dosing concentrations. The accumulation of labeled Zn by the earthworms showed a direct relationship with the proportion of labeled to total Zn in the pore water, which increased with longer soil incubation times and decreasing soil pH. The 68Znen/64Znen ratios determined for earthworms (1.09 ± 0.04), soils (1.09 ± 0.02), and pore waters (1.08 ± 0.02) indicate indistinguishable environmental distribution and uptake of the Zn forms, most likely due to rapid dissolution of the ZnO NPs.