Stability of potentially toxic elements in municipal sludge biochars modified by MgCl2 and phosphate

Municipal wastewater sludge can be pyrolyzed as biochars to better use nutrients and stabilize carbon compared with other typical technologies, such as landfill and incineration. However, sludge-derived biochars might contain large amounts of potentially toxic elements (PTEs), such as Zn, Cu, Cr, Ni, Pb, and As. The stability of PTEs in biochars might be improved by higher pyrolytic temperatures, which can be further improved by different modifications. Herein, PO₄-modification at 300 °C and Cl-modification at 700 °C were carried out, respectively, to enhance the stability of PTEs. Various leaching tests have been performed to assess the stability of PTEs in biochars, including the synthetic precipitation leaching procedure (SPLP), toxicity characteristic leaching procedure (TCLP), diethylenetriamine pentaacetate (DTPA) extraction, and in vitro simple bioaccessibility extraction test (SBET). The morphological structure, elemental mapping, and mineral formation of the pristine and modified biochars were studied by scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDS) and X-ray diffraction (XRD). Our results suggested that the leachability, mobility, plant-availability, and bioaccessibility of most PTEs were decreased by pyrolysis, yet the total contents of PTEs were elevated, especially at 700 °C. Generally, modification by phosphates and MgCl₂ enhanced the stability of PTEs in biochars. Nevertheless, it should be noted that higher bioaccessibility of PTEs was observed in biochars of P-modification than Cl-modification, which is associated with the dissolution of phosphate precipitates under acidic conditions (pH<2). Additionally, Cl-modification leads to higher plant-available Zn and Cu and bioaccessible Zn compared with the unmodified biochar produced at 700 °C.