Conversion of Weathered Coal into High Value-Added Humic Acid by Magnetically Recoverable Fe3O4/LaNiO3 Nanocatalysts under Solid-Phase Grinding Conditions

The Fe3O4/LaNiO3 composite, synthesised with the sol-gel method, is considered to be an excellent nanocatalyst for the production of high value-added humic acids from oxidised weathered coal under solid phase milling process conditions. Under optimum process conditions (1% catalyst, 10% activator, 60 min grinding), 48.4% of the weathered coal can be oxidised to produce humic acid. The prepared Fe3O4/LaNiO3 catalyst was characterized by HRTEM, XRD, and XPS, etc. The heterojunction structure that can promote the electron transfer between the components of the composite material was formed with the recombination of Fe3O4 and LaNiO3. The activation of surface oxygen species and adsorbed oxygen could be enhanced with the help of electron transfer between components. Compared to the blank sample or the LaNiO3 catalyst alone, the molecular weight of the humic acid produced using the Fe3O4/LaNiO3 composite catalyst was significantly lower (maximum heavy mean molecular weight decreased from 59.7 kDa to 5.5 kDa) and the number of reactive groups in humic acid increased (to seven times that of the blank sample). Oxygen-free vacuum experiments indicated that O2 has an indispensable effect on its excellent catalytic performance in the Fe3O4/LaNiO3 system. In addition, Fe3O4/LaNiO3 could be used at least six times by simple magnetic separation. The development and preparation of perovskite composite catalysts provide a promising approach to the environmentally friendly development and application of weathered coal, as well as an effective method to resolve the associated environmental pollution.