Recovery of Single-Atom-Doped Layered Dioxides via Facile Hydrothermal Carbonization of Excess Sludge for Catalytic Pollutant Degradation

The conversion of metal-rich excess sludge into functional materials under mild conditions is an environmentally friendly and value-added sludge disposal approach. In this study, we propose a facile method for in situ hydrothermal recovery of a carbon-supported single-atom-doped catalyst (SA-LDO@C) from excess activated sludge by regulating the pH and temperature under mild conditions. The as-prepared SA-LDO@C catalyst showed high catalytic performance for peroxymonosulfate (PMS) activation to degrade tetracycline (TC). Multiple characterizations demonstrated the formation of LDO and metal monatomic structures in SA-LDO@C, and adjusting the parameters of the hydrothermal reaction of sludge, particularly the initial pH, can regulate the structure and composition of SA-LDO@C. Catalytic degradation of TC by PMS activation by SA-LDO@C revealed that the metal monatomic structure jointly promoted PMS activation, whereas the hydrothermal carbon structure may serve as a dispersion carrier or an electron transporter. In addition to its high catalytic activity, SA-LDO@C has excellent catalytic stability during cycling experiments. This is the first finding of monatomic structures in SA-LDO@C and may open a new avenue for high-value disposal of intractable excess sludge.