Effect of amendments on chemical immobilization of heavy metals in sugar mill contaminated soils

A bulk soil sample collected from the vicinity of PSM (Premier Sugar Mill) Mardan was amended with diammonium phosphate (DAP), triple super phosphate (TSP), Farm Yard Manure (FYM) and poultry manure (PM) in 1.5 kg soil in a 2 L plastic pot. Both DAP and TSP were added at 230 mg per kg (460 kg per ha) soil whereas the organic amendments (FYM and PM) were added at the rate of 10% by weight of soil. The air dried samples in pots were brought to field moisture content (0.33 bar water content) by the addition of either HIE (Hayatabad Industrial Estate) or PSM in two separate sets of experiments. The experimental pots were arranged in randomized complete design with three replicates under laboratory conditions during March to May (Temperature varying between 25 to 30 degree C). Treated and control pots were incubated for 90 days al 0.33 bar ca 25% moisture and the moisture deficit during the incubation time was adjusted by adding PSM and HIE effluents in their respective set of experimental pots. Soil samples were collected after 15, 30, 45 and 90 d to determine the effect of amendments on AB-DTPA extractable metals. The results showed that AB-DTPA extractable Cd, Or, Cu, Ni and Cd increased significantly with lime and the maximum values were noted after 90 days incubation whereas the Fe, Mn and Zn content in soil increased with time but the increase was not significant. It was further noted that the increase over time in metal was not pronounced when supplied with amendments indicating their ability to chemically stabilize it compared to unamended soils. Higher values of all the heavy metals were noted in unamended soil. By comparing the different amendments, it was observed that FYM was effective in reducing the extractability/phytoavailability of all the metals under study except Pb whereby DAP was most effective as a stabilizing agent in the soil. It was concluded that in calcareous soil, FYM and DAP can be used to reduce the risk of phytotoxicity of heavy metals in contaminated soil or using wastewater for irrigation.