INVESTIGATING THE EFFECT OF AGING AND TIME ON THE FATE AND TRANSPORT OF LEAD IN ARTIFICIALLY CONTAMINATED TROPICAL SOILS

Incubation and Batch desorption experiments were conducted on Ultisol and a Mollisol to assess the effect of aging temperature and contact time on Pb desorption in A and B horizon soils. Ultisol and Mollisol from Humboldt and San Luis Obispo County, California were incubated at room temperature, 50 and 70 ⁰C for 3 hours, four weeks and two months in the laboratory.

After each aging period, samples were extracted with 10^-4 M H₂SO₄ solution, equilibrated for 24 hours, centrifuged and filtered through a 0.45 µm filter. A separate subsample of the B-Horizon supernatant was passed through a 0.22 µm filter to assess the role of aging temperature and time on colloidal facilitated transport.

Increase in aging temperature and fixation time was observed to enhance Pb solubility and bioavailability in the A horizon soils. The Mobility of Pb in the B horizon was found to be significantly retarded. XRD analysis showed no phase change in the bulk soil upon heating; while Kd computations showed partitioning of Pb in the solid phase of the Mollisol was higher than that of the Ultisol. These results allow for assessing the risk associated with mobility of Pb in tropical soils and shows that shallow groundwater in areas overlain by Ultisol is probably more liable to Pb contamination.