Comparison of Pb stabilization in a contaminated calcareous soil by application of vermicompost and sheep manure and their biochars produced at two temperatures

Soil contamination with potentially toxic elements (PTEs) is increasing due to human activities including the widespread use of chemical fertilizers, sludge and industrial wastewater in agriculture. In situ stabilization of soil PTEs using organic amendments is a cost effective and efficacious method. A laboratory experiment was conducted to investigate the effect of two organic materials (sheep manure (SM) and vermicompost (VC)) and their biochars (SMB and VCB) produced at two pyrolysis temperatures (300 and 500 degrees C) at 2 wt. % on Pb immobilization in a contaminated calcareous soil. A sequential extraction procedure, mobility factor and release kinetics were used for evaluating efficiency of the amendments. Addition of the biochars resulted in a significant decrease in exchangeable (10.4-19.6%) and carbonate (3.8-10.5%) forms of soil Pb, while Pb bound to amorphous Fe oxides (4.3-52.9%) was increased. The application of VCB produced at 500 degrees C resulted in the greatest mobility factor reduction (11%), likely due to its characteristics such as high pH (10.5), high P (0.39%) and ash content (67.7%). Two first-order reactions model best fitted the Pb release data extracted by EDTA, indicating Pb release took place from two sections of the soil with different adsorption and solubility properties. The lowest amount of Pb extracted in Q(1) and Q(2) phases were observed in VCB (115.2 mg kg(-1)) and SMB (105.5 mg kg(-1)) produced at 500 degrees C treatments, respectively. Generally, the effect of biochars produced at higher temperature (500 degrees C) on the soil Pb stabilization was greater than those produced at lower temperature (300 degrees C), while the feedstocks (SM and VC) had no significant effect in stabilizing soil Pb.