The immobilization of heavy metals by a tropical andepts treated with lime and organic composts

Heavy metal solubility in soils has been reported to be altered by an addition of lime and/or organic materials. This research was intended to evaluate the effects of some lime or organic materials on the extractability of heavy metals in an andic soil (Andepts), which is well-known to have an extremely high adsorption capacity with respect to organic compounds and phosphates. Soil sample, collected from South Lampung, Indonesia, lime materials comprising CaCO3, Ca(OH)2, and CaMg(CO3)2, and composts of plant leaves (“alang-alang”, corn, cassava, and soybean) were used. The soil sample (air-dry, 2-mm sieved) was treated independently with lime (at a level to increase its pH about 2 units) or with organic composts (at 10 Mg ha-1). The mixture was spiked with a standard solution containing 10 mg kg-1 Cu, Cd, Mn, and Zn and incubated for 8 weeks at the soils field moisture capacity. The heavy metals were extracted with DTPA and measured with flame AAS. The results showed that the extractable heavy metals in limed soils (except Cd) were in general greater than those in unlimed soils, probably due to increased soluble organic materials which might have complexed with heavy metals with the increase in soil pH. The organic compost generally did not affect the extractability of heavy metals except Mn that increased with compost addition in samples not spiked with heavy metals, particularly in samples treated with soybean compost. The insignificance difference in the extractability of heavy metals between the spiked and unspiked soil samples suggested that the Andepts was a good adsorber for heavy metals, except Cd. The majority of Cd added was recovered both in lime- and in compost-treated soils. Based on the percent recovery values, the preference of the soil towards the heavy metals followed the sequence of Mn > Zn > Cu > Cd.