Uptake of heavy metals by vegetable plants grown on contaminated soils, their bioavailability and speciation

This research aimed to investigate the bioavailability of 9 metals (Cr, Mn, Fe, Ni, Cu, Zn, Mo, Cd, and Pb) to vegetable crops (spinach, lettuce, carrot and radish) cultivated in compost soils at different levels of metal contamination. The uptake and accumulation of these metals by the plants were examined. The elemental speciation using SEC-UV-ICP-MS and Nanospray Mass Spectrometry had been performed to characterize the metal containing species induced in the plants exposed to metal stress. In order to evaluate potential health risks arising from ingestion of the metal contaminated plants, the oral bioaccessibility i.e. the use of an in vitro physiologically based extraction test (PBET) simulating the transition of the metal pollutants in the plants into the human gastrointestinal system was undertaken. It was found that, with the exception of Cr, metal concentrations (Mn, Fe, Ni, Cu, Zn, Mo and Cd) in lettuce, spinach, carrot and radish depended on the concentrations of the total metal in the soils in which the plants were grown. For Pb, the amounts accumulated in the leafy vegetables also depended on their levels of contamination in the soils while the root vegetables had rather low uptake and the uptake levels did not increase when higher levels of contamination were applied. Mn, Fe and Zn were relatively easily mobilised from soils to plants; they tended to accumulate in all plants studied at high concentrations. The elements which were more enriched in leaves included Mn and Zn (in all plant types), and Fe and Cd (only in the root vegetables). In contrast, Fe, Ni, Cu, Mo and Pb were accumulated more in roots of the leafy vegetables. Among all plants studied, it was observed that carrot had low uptake for all elements (Cr, Ni, Cu, Mo, Cd and Pb), except for Mn, Fe, Zn which were found in all plants. The metal mobilised from soil to plant as indicated by the metal contents accumulated in the plants decreased in the order Mn >> Zn > Fe > Cu > Mo > Ni > Cd > Pb Cr. The metal bioavailability to plants was assessed by measuring transfer factor (TF) values of the metals based on total metal contents in the soils. It was found that the order of TF values was Mn > Zn >> Cd > Ni > Cu > Mo Pb > Cr Fe. The mean TF values of each element irrespective of plant types were 1.93, 1.77, 0.485, 0.194, 0.111, 0.052, 0.045, 0.037 and 0.036 for Mn, Zn, Cd, Ni, Cu, Mo, Pb, Cr, and Fe, respectively. Hence, Mn and Zn were most bioavailable to plants i.e. they can be transferred from soils to plants more easily than Ni, Cu, Mo, Pb, Cr and Fe. Whereas, the bioavailability of Cd was relatively moderate. In addition, the results enabled the development of statistical regression models that are suited to predict metal uptake by plants. It indicated that the relationship between the TF values and the extractable soil metals followed the power regression curve. However, there were some cases in which it did not follow the power regression curve but a linear model, these are; Mn (for carrot leaves and radish roots), Mo (for spinach roots and carrot roots), and Cd (for lettuce leaves, spinach roots and leaves and carrot roots). In the multi-elemental speciation study, it was found that a common association of the metals (Cd, Cu, Mo, Pb, and Zn) to the high molecular weight (MW) fractions (8160 Da) was observed in all plant extracts. The lower MW fractions of approximately 1000 — 3000 Da of Cd, Cu, Mo, Ni, Pb, and Zn containing compounds were found to be present in all plant extracts. Iron was not detected in the roots of carrot and radish, but present as both high MW (8200 Da) and low MW (2500 Da) compounds in the leaves of spinach and lettuce. To characterize the individual metal containing species present in the plant samples, the Nanospray Mass Spectrometry was employed. Unfortunately, no evidence from this analysis can confirm that these compounds are related to the phytochelatin family. The PBET (Physiologically-Based Extraction Test) resul.