Evaluation of the Mobility and Bioaccessibility of Potentially Toxic Metal Contents in Urban Soil of Lahore, Pakistan

Abstract

Present study is based on the measurement of potentially toxic metal contents (Cd, Cr, Pb, Cu, Co, Zn, Mn and Fe) in the urban soil of Lahore, Pakistan, employing five extraction methodologies aimed at the evaluation of the mobility and bioaccessibility of the metals in soil. Concentrations of the metals in the soil samples were determined by flame atomic absorption spectrophotometer and the metal data were subjected to the statistical analysis. Among the metals, Fe, Mn, Zn and Pb showed relatively higher contributions in various soil extracts. On the average basis, aqua regia and glycine extracts revealed comparatively higher concentrations for most of the metals; however, Cd, Pb, Co and Zn exhibited higher bioaccessibility and mobility/bioavailability in the soil while Cr, Fe and Mn were least mobile and bioavailable. Significant and strong positive correlations were found between Zn-Cu, Zn-Fe, Zn-Cr, Mn-Cr, Pb-Cr, Pb-Cu, Pb-Co, Co-Cd, Fe-Cr, Fe-Co and Fe-Mn in various soil extracts. Quartile distribution manifested that most of the metals demonstrated relatively broad spread in their concentrations. Physiochemical parameters and soil texture were also estimated and most of the soil samples were found to be sandy loam in texture. The pollution index associated with the metal levels was assessed by computing contamination factors (C f ), geoaccumulation index (I geo ), enrichment factor (EF) and modified degree of contamination (mC deg ). In comparison with the earth crust levels, mean concentrations of Cd, Pb and Zn in the soil were found to be higher in the present study. The pollution indices (C f , I geo , EF) unfolded severe to significant contamination, accumulation and anthropogenic enrichment of Cd, Pb and Zn (along with Cu and Co, in some cases) in the urban soil of Lahore, while mC deg associated with the metal levels in the soil divulged moderate contamination on the average. Multivariate analyses which include principal component analysis (PCA) and cluster analysis (CA) were performed for source identification/apportionment of the measured metal levels in the soil. PCA and CA of the pseudo-total metal levels pointed towards mostly natural contributions for Fe-Mn-Co, whereas Zn-Cu-Cr-Pb-Cd were predominantly anthropogenic in origin in the soil. Potential health risks associated with the metal levels were also assessed for ingestion and dermal adsorption along with the hazard quotient/index. No significant human health risk/hazard was associated with the prevailing metal levels in the urban soil of Lahore.