Nickel Immobilization in a Contaminated Calcareous Soil with Application of Organic Amendments and Their Derived Biochars

Nickel (Ni) contamination of soils is an ever-increasing problem due to use of poor quality irrigation water and other industrial activities. The aim of this study was to evaluate the application of two common organic soil amendments (vermicompost and sheep manure) and their derived biochars produced at two temperatures (300 degrees C and 500 degrees C) on Ni immobilization in a Ni-contaminated calcareous soil. Immobilization of Ni was evaluated using a sequential extraction procedure and desorption kinetic models. The results of this study showed that the application of biochars produced from vermicompost and sheep manure at 5% (w/w) significantly enhanced the stabilization of Ni in the calcareous soil, while the application of unpyrolyzed organic amendments did not. The application of the biochars significantly enhanced Ni occurring in the residual mineral fractions (3-6% increase) and decreased the DPTA-extractable Ni released over 24 h (1-11% decrease) compared to the control. However, the application unpyrolyzed organic amendments significantly enhanced DPTA-extractable Ni released (16-21% increase) compared to the control, likely due to significant enhancement of Ni in the soil organic matter fraction (33-75% increase). The biochars produced at 500 degrees C were significantly more effective in enhancing Ni stabilization than those produced at 300 degrees C, likely due to their higher ash and calcium carbonate content and lower organic matter content, which promotes Ni sorption and precipitation.