Lead stabilization in a polluted calcareous soil using cost-effective biochar and zeolite amendments after spinach cultivation

Biochar has been shown to be an effective organic soil amendment for the in-situ immobilization of lead (Pb). Little research has been done on the effects of biochar and zeolite interactions on Pb immobilization in contaminated calcareous soils. Therefore, the aim of this study was to investigate the effects of different levels of zeolite (2% and 4%, weight:weight) and biochars (3%, weight:weight) prepared from green compost (GB), poultry manure (PB), and municipal waste (MB) on the content of Pb in spinach shoots and Pb stabilization in a greenhouse experiment conducted using a Pb-contaminated calcareous soil. Application of GB and PB significantly decreased the Pb mobility factor (MF) from 2.8% to 4.6% and the cumulative Pb release (diethylenetriamine pentaacetate (DTPA) extraction) from 16.8% to 20.1% in the calcareous soil, which were further enhanced by the combined addition of zeolite, with Pb MF reduction from 4.4% to 8.4% and cumulative Pb release reduction from 24.8% to 28.6%. The enhancement of soil Pb immobilization was attributed to the properties of the amendments, such as high pH, ash content, and phosphorus content (10.5-11.0, 390-435 g kg^-1, and 2.5-4.3 g kg^-1, respectively). Results of the soil Pb sequential extraction and Pb release kinetics were highly and significantly correlated with spinach shoot Pb content. Soil chemical data showed that the application of MB with 4% zeolite was the most effective treatment for immobilizing Pb (8.4% reduction in Pb MF and 28.6% reduction in cumulative DTPA-extractable Pb), which is in agreement with the results of the spinach shoot Pb (89.0% reduction in Pb uptake).