Coupling electrokinetic technique with hydrothermal carbonization for phosphorus-enriched hydrochar production and heavy metal separation from sewage sludge

Hydrothermal carbonization is a promising treatment technology for wet organic solid wastes, due to its energyefficient production of functionalized carbonaceous materials. However, the usage of sewage sludge (SS) derived hydrochar as phosphorus (P) fertilizer for agricultural soil is inhibited by the presence of heavy metals (HMs). Appropriate HM removal technology is required prior to its land application. In this study, an innovative FeCl3-assisted electrokinetic technique coupling with hydrothermal carbonization (FEK-HTC) is proposed to produce SS hydrochars with reduced HM content and increased P-retention. FEK-HTC treatment showed synergistic effects in terms of P retention and HM content reduction (56-64 % for Ni, Cu, and Zn). Furthermore, the toxicity of the remaining HMs was dramatically reduced, e.g., by 39.35 % in the case of Zn. Additionally, FEK-HTC transformed P species in hydrochars into highly bio-available form. Electrokinetic treatment resulted in decreased pH values of SS feedstock and promoted the solubilization and erosion of Ca-associated P, resulting in higher non-apatite inorganic phosphorus contents (12.47 to 23.19 mg.g(-1)) compared to hydrochars from untreated sludge. The P migration patterns during the FEK-HTC treatment were assessed based on the combined result of chemical extraction, P-31 NMR, X-ray absorption near edge structure (XANES), and diffusive gradients in thin film (DGT). DGT analysis showed that the addition of treated hydrochars greatly increased the available P content in the soil. The results provide new insights into applying FEK-HTC technique for SS management.