Heavy metals uptake by vegetables growing in sewage irrigated soil: relationship with heavy metal fractionation in soil

Concentrations of the heavy metals Cd, Ni, Cr, Zn, Mn, Co and Cu in soils and vegetable samples (i.e. green chili and gourd) taken from six vegetable fields in the vicinity of Lahore, Pakistan were measured. These soils have been irrigated for a long time with untreated sewage effluents. A control site was selected that has history of fresh canal water irrigation. The sequential extraction procedure developed by Tessier was adopted to demarcate five metal fractions: exchangeable, acid soluble, reducible, oxidizable and residual. The extractants and digests were analysed by atomic absorption spectrometry. The fractionation procedure showed that all the metals were dominant (>50%) in the residual phase in control as well as in waste waster irrigated soils. The concentrations of all the metals in edible parts of the vegetables collected from wastewater irrigated soils, were above critical levels. The total metal concentrations in wastewater irrigated soils followed the order Mn>Co>Zn>Cr>Ni>Cu>Cd, while in control soils the order was Mn>Zn>Ni>Co>Cr>Cu>Cd. In order to understand the uptake of metals from soil to vegetables, correlation analyses were performed between metal concentrations in different fractions of soil and their concentrations in vegetables. Correlation analysis was performed at 95% and 99% confidence level. The meaningful significant negative correlation was observed between Cd-F5-Cd-chili, Cr-Total-Cr-gourd, Mn-F5-Mn-Chili,Mn- Gourd, Co-F5,Co- Total-Co-chili, Cu-Total-Cu-Chili which indicate the non availability of total content and residual fraction to studied vegetables. The positive correlation was observed between Cd-F1-Cd-Chili, Cd-F1-Cd-gourd, Ni-F1,Ni- F3-Ni-Gourd, Cr-F1,Cr- F3,Cr- F4-Cr-Chili, Cr-F2-Cr-Gourd, Zn-F1,Zn- F2-Zn-Gourd, Mn-F3-Mn-Chili, Mn-F1-Mn-Gourd, Co-F2-Co-Chili, Cu-F1-Cu-Chili. The positive correlation, especially for the first (exchangeable) second (acid soluble) and for the third (reducible) extraction steps, was obtained which indicate the bioavailability of these metal fractions to plant.