Saffari, Moazallahi, & Mashayekhi investigated the effects of pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More temperature and chlorinated additives on the fate of heavy metals in sewage sludge (SS) biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More. The goal was to see how these factors affected the amount of heavy metals left in the biochar, what forms they took, and if they would likely leach out.  

The researchers heated SS with either PVC or FeCl3, which act as sources of chlorine, to 400°C or 600°C. Adding these chlorinated materials helped reduce the heavy metals arsenic (As), chromium (Cr), and zinc (Zn) in the biochar. PVC was better at reducing As and Zn, especially at higher temperatures, while FeCl₃ worked best for Cr.  PVC changed As and Zn into forms that are less likely to be taken up by plants or animals, reducing their risk to the environment. FeCl₃ did this for Cr, making it more stable. Tests showed that both additives helped reduce how much of the heavy metals could leach out, with PVC doing the best job for As and Zn, and FeCl₃ for Cr.  

To see the overall environmental risk, the team calculated an ecological risk index (RI). PVC treatments had lower RI values, meaning less risk, while FeCl₃ sometimes increased the risk, especially at higher temperatures.  The study showed that co-pyrolysis with chlorinated additives is a good way to reduce the risk from heavy metals in SS biochar. PVC at 600°C was best for reducing As and Zn, while FeCl₃was better at stabilizing Cr.

The authors emphasize choosing the right additive depending on which heavy metal needs to be controlled to make the process as environmentally friendly as possible.  

SOURCE: Saffari, M., Moazallahi, M., & Mashayekhi, R. (2025). The fate and mobility of chromium, arsenic and zinc in municipal sewage sludge during the copyrolysis process with organic and inorganic chlorides. Scientific Reports, 15(1), 2986. https://doi.org/10.1038/s41598-025-87169-3