Impact of Pyrolysis Temperature and Application Amount of Sewage Sludge Biochar on the Speciation and Bioavailability of Cd and Pb in Paddy Soil

In order to investigate the effect of various pyrolysis temperatures and application amount on the remediation of Cd–Pb contaminated soil, dried sewage sludge was pyrolyzed at 300 °C (SSB300) and 500 °C (SSB500), and soil incubation experiments were conducted for 60 days with two biochars (SSB300 and SSB500) and three different application amounts (1%, 3%, and 5%). The chemical speciation and bioavailability of Cd and Pb in paddy soil were determined by BCR and DTPA. Results showed that sewage sludge-based biochar (SSB) slightly increased the soil pH, due to the neutral SSB. The specific surface area of SSB500 was increased by 40.06 m2/g compared to SSB300. FTIR showed a decrease in the hydroxyl vibrational peak of SSB500 compared to SSB300; in addition, the intensity of the aliphatic-CH2 peak decreased with increasing pyrolysis temperature, indicating a decrease in non-polar aliphatic functional groups on the surface of SSB. The incubation experiments showed that the addition of 5% SSB500 had the best performance for DTPA-extractable Cd (48 mg.kg−1 to 12.81 mg.kg−1) and Pb (90.23 mg.kg−1 to 15.91 mg.kg−1). And 5% SSB500 amendment more remarkably transformed Cd and Pb from the acid-soluble state to the residual state than other treatments, demonstrating that the high pyrolysis temperature and application amount had great influence for transformation of Cd and Pb. In addition, the microbial community in the soil was significantly changed by 5% SSB500 application. At the phylum level, Chloroflexi is the dominant species, due to its strong tolerance in Cd-contaminated soil. At the genus level, the relative abundance of Thiobacillus and Defluviicoccus increased, which would enhance inorganic ion transport and metabolism functions to promote passivation and stabilization of heavy metals throughout the remediation process.Graphical abstract