Hydrothermal carbonization of sewage sludge and in-situ preparation of hydrochar/MgAl-layered double hydroxides composites for adsorption of Pb(II)

Hydrothermal carbonization is a novel technology for dewatering of sewage sludge. Layered double hydroxides (LDHs) are a widely used functional material, which can be prepared by hydrothermal method. In this study, we proposed a new process combining hydrothermal carbonization of sludge and in-situ preparation of LDHs. The addition of Mg and Al salts (as LDHs precursor) to the slurry before hydrothermal treatment can act as a sludge conditioner to effectively accelerate the sludge sedimentation. The obtained products hydrochar/LDHs (HL) composites with various mass ratios were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that the well-crystallized LDHs slice is thinner after combining with hydrochar. The HL products can be used as a highly effective adsorbent for removal of Pb2+ from water. The adsorption experiment results showed that the adsorption behavior of Pb2+ onto HL composites can be well expressed by a pseudo-second-order kinetic model and Langmuir isotherm with the maximum adsorption capacity of 62.441 mg/g. Ionic strength had no significant effect on the adsorption capacity. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectra indicated that the adsorption mechanisms of Pb2+ by HL composites mainly included electrostatic interaction and surface coprecipitation. Therefore, this is a cost-effective method to obtain a high efficiency adsorbent. •Hydrothermal treatment of sludge and in-situ preparing layered double hydroxides.•Layered double hydroxides precursor solution can act as sludge conditioner.•The composites has a higher adsorption capacity of Pb(II) than other adsorbents.•The microstructure of the composites and their adsorption mechanism are deciphered.•This is a cost-effective method to obtain a high efficiency adsorbent.