Chemical Speciation of Zinc and Lead in Soils and its Effects on Bioaccessibility

Soils with metal concentrations above regulatory guidelines (e.g., Canadian Soil Quality Guidelines) may pose health risks to humans (especially children), plants, animals and soil microorganisms. To study the toxicity and assess the risks of metal contaminated soils to the aforementioned receptors, researchers add metal salts to uncontaminated soils. This commonly used method of metal addition to soils sometimes fails to mimic field contaminated soils with regards to metal concentration and speciation. In addition, it creates artifacts that may skew metal toxicity and risk assessment results. Consequently, two alternative solid metal addition methods (annealed metal and metal oxide) were compared to the widely used metal-salt method. The objective was to identify the method that adequately mimics environmentally relevant Zn, Pb and Ni concentrations and Zn speciation. After spiking three soils with three different metal mixtures and incubating for 30 days, total metal concentration in the spiked soils were measured with X-ray fluorescence (XRF); Zn speciation was characterized using X-ray powder diffraction (XRD) and X-ray absorption near edge structure (XANES) spectroscopy. Speciation of Zn depended on spiking method and soil properties, especially pH. Each spiking method produced between 1-3 Zn species that occur in smelter-affected soils. Comparatively, the metal-salt spiking method failed to replicate the environmentally relevant Zn and Ni concentrations. Also, it was found to have several drawbacks that may present serious challenges to toxicity and risk assessment research. The oxide spiking method produced metal concentrations that were higher than the expected values. The acidic pH of the test soils ensured the dissolution of ZnO, which led to the formation of secondary Zn species. Franklinite was the synthetic mineral produced from the annealing process and it was found to be very stable even at pH 3.4. None of the metal addition methods evaluated in this study adequately mimicked the selected field conditions. The hypothesis that metal bioaccessibility is controlled by metal speciation and not metal concentration was tested in the second part of this study. A combination of bioaccessibility extractions, XANES and LCF were used to identify the species of Zn and Pb in the spiked and smelter affected soils that contribute Zn and Pb ions towards bioaccessibility. After conducting in vitro digestion tests on the soil samples, XANES and LCF analyses were carried out on the residual pellets to identify the species of Zn and Pb that remained after digestion of the soils in pH 1.4 and 6.3 gastric and duodenal fluids, respectively. In the residual pellets from the simulated stomach, ZnO, sphalerite (ZnS), outer-sphere Zn, PbO, and hydrocerussite [Pb3(CO3)2(OH)2] were below detection limits, indicating that the total metal fraction of these species contributed to the bioaccessible metal concentrations in the stomach. The high relative abundance of some of these metal species (e.g., ZnO) translated to high bioaccessible metal concentrations. Conversely, franklinite (ZnFe2O4) and Zn incorporated into a hydroxy interlayer mineral (Zn-HIM) were detected in the residual pellets from both the simulated stomach and duodenum. This suggests that franklinite and Zn-HIM are sparingly soluble, and released relatively low concentrations of Zn ions into the digestive fluids. The residence time of Zn-Al layered double hydroxide (Zn Al LDH) in soils was found to affect the release of Zn ions into the simulated digestive fluids. One field sample (stabilized soil) and one spiked sample (oxide spiked WTRS soil) contained Zn-Al LDH; however, after digestion, Zn-Al LDH was detected in residual pellets of the field sample, but not the spiked sample. The resistance of Zn-Al LDH in the field sample to dissolution is probably due to longer residence time, which allowed silication in the hydroxide layers