A novel waste activated sludge multistage utilization strategy for preparing carbon-based Fenton-like catalysts: Catalytic performance assessment and micro-interfacial mechanisms

Waste activated sludge (WAS) contains many anionic functional groups which can interact with heavy metal ions through electrostatic action and complexation reactions. The transition metals adsorbed in WAS can catalyze sludge pyrolysis in anaerobic conditions and improve structural properties of organic matter. In this work, a multistage WAS utilization process for preparing the carbon-based Fenton-like catalysis materials is proposed. More specifically, WAS is firstly used as an adsorbent for heavy metals (Cu and Ni) removal, and then complexes are converted into heterogeneous Fenton-like carbon-based catalysts through oxygen-free pyrolysis. The mechanisms of interactions between extracellular polymeric substances (EPS) and metals are investigated, and the physicochemical properties of sludge-based carbons (SBC) are comprehensively characterized using varies techniques. It is found that WAS is an excellent adsorbent for Cu and Ni removal, which is mainly due to the coordination and electrostatic interactions between EPS and heavy metals. Cu and Ni adsorbed in WAS significantly improved the porous structure of SBC. Both adsorption and catalytic oxidization of Cu/Ni-SBC contribute the removal of E2 in real wastewater. The E2 removal mechanism is explored by electron-spin resonance spectroscopy (ESR) analysis, and it is found that both of .O2− and .OH radicals are responsible for E2 degradation in Cu(II)-SBC-H2O2, while .O2− radicals contributes to E2 degradation in Ni(II)-SBC-H2O2 system, so the former performed better than the latter in total removal of E2. Besides, Cu(II) and Cu(I) are both formed in Cu(II)-SBC during the oxidation process, while only Ni(II) is found in the Ni(II)-SBC-H2O2 process, confirming that different catalytic oxidation reactions are occurred in the Cu(II)-SBC-H2O2 and Ni(II)-SBC-H2O2 processes. This study facilitates a great strategy to the sludge multi-stage circulating utilization and a better understanding about the role of the Cu/Ni existed in SBC during the estrogens removal process.