Mobility, bioavailability and ecological risk assessment of cadmium and chromium in soils contaminated by paper mill wastes

Contamination and associated risks posed by cadmium (Cd) and chromium (Cr) were characterized in the soils being continuously exposed by paper mill waste discharge in Assam, India. Soil samples from 20 (S-1 to S-20) sites surrounding a paper mill were analyzed by sequential extraction technique for Cd and Cr to assess the potential mobility and bioavailability of these heavy metals in the soils. Cadmium was mostly found in its carbonate fraction (37.14%) whereas; Cr was mostly available in Fe-Mn oxide bound fraction (34.72%). Relatively low amounts of Cd and Cr were associated with the water soluble fractions. The pollution load index (P i) values of Cd exceeded the maximum permissible limit i.e. 1 in all the sites but, for Cr it exceeded in all sites but S-7, S-13 and S-16. The mobility factor values ranged from 47.4% to 87.1% for Cd and 12.8–67.2% for Cr. High mobility factor values represent relatively higher biological availability in the soil samples and thus could pose environmental threats in the area surrounding the paper mill. The high values of other contamination factors, viz. individual and global contamination factors, potential ecological risk factor, ecological risk index and geo-accumulation index indicated that the studied area was widely polluted by Cd and Cr as a result of the effluents discharged from the paper mill.