Using Industrial Sewage Sludge-Derived Biochar to Immobilize Selected Heavy Metals in a Contaminated Calcareous Soil

Many studies have indicated the effect of biochar on the fractionation of heavy metals in acidic soils, while little information is available on their effects on contaminated calcareous soils. Applying biochar products from sewage sludge pyrolysis as soil amendment was investigated in this study with special attention paid to fractionation of Pb and Cd in calcareous soil around a lead–zinc mine when pyrolysis temperature and biochar application rates were changed. The biochar feedstock was industrial sewage sludge, collected from the Baharan industrial park, using an anaerobic–anoxic–aerobic treatment process at two different temperatures (300 and 600 °C), and then adopted as amendment for the soil. The results revealed that with increasing pyrolysis temperature, the biochar’s levels of N, H and O decreased, while its amount of C increased significantly. The highest rate of the biochar application (8%) had lower pH compared to the control soil for both biochars. The soil EC with 8% biochar 300 °C was less than biochar 600 °C, perhaps due to the higher amount of ash in the biochar produced at 600 °C. The results showed that biochar 600 was more porous than biochar 300. The exposed porous structure made the pores more accessible for the adsorbate particles. Therefore, the biochar produced at 600 °C, more effectively, reduced the exchangeable form of Pb and Cd. The organic forms of Pb and Cd increased with increasing levels of biochar application, and this trend was higher in the treated soil with biochar produced at 600 °C due to high organic carbon content. The results showed that most of Pb and Cd existed in the residual form after application of biochar, especially at 600 °C, resulting in a significant reduction in their bioavaliability.