Extractability and phytotoxicity of heavy metals present in petrochemical industry sludge

Bioavailability of heavy metals present in industrial sludges and their subsequent phytotoxicity are crucial parameters to assess the associated health hazards and suitability for land application. The present study is an effort to determine the extractability of heavy metals present in different phases of the sludges, coming out of two different operations involved in petrochemical industry viz. spent caustic treatment (SCT) and waste water treatment (WWT) following the BCR sequential extraction procedure. The maximum amount of Cd and Cu was found associated with oxidizable phase, whereas Cr and Ni were best recovered in residual fractions of both the sludges. Maximum Pb was recovered in oxidizable and residual phase in the WWT and SCT sludges, respectively. The stabilization treatment undergone by sludges strongly influenced the heavy metal distribution and the phases to which they were associated. The total metal concentration in both the sludges did not exceed the limit set out by the European Legislation and was found as Cd = 0.449, Pb = 3.340, Ni = 6.530, Cr = 21.087, & Cu = 27.129 μg g⁻¹ and Cd = 0.549, Pb = 5.664, Ni = 7.161, Cr = 27.096, & Cu = 35.479 μg g⁻¹ in the SCT and WWT sludges, respectively. Phytotoxicity of the sludges was assessed against the germination index and the relative root and shoot growth. Sludge leachates did not adversely affect the seed germination and the early seedling growth of Mung (Phaseolus mungo) and Gram (Cicer arietinum), indicating that these metals were concentrated in the non-bioavailable fractions of sludges.