Hierarchical quasi waxberry-like Ba5Si8O21 microspheres: Facile green rotating hydrothermal synthesis, formation mechanism and high adsorption performance for Congo red

Abstract

Towards the imperative water contamination by organic dyes, uniform hierarchical quasi waxberry-like Ba5Si8O21 microspheres (diameter: 0.65–1.30 µm) consisted of nanorods have been successfully synthesized via a facile eco-friendly and cost-effective rotating hydrothermal process (120 °C, 6.0 h) without any organic additives, templates or surfactants, using abundant BaCl2·2H2O and Na2SiO3·9H2O as the raw materials. Based on the impacts of the process parameters, a probable heat and mass transport enhanced rotating hydrothermal formation mechanism is proposed. The rotating homogeneous hydrothermal reactor provides vigorous macro- and micro-mixing of the solid and solution phase, thus favors the early stage dissolution-recrystallization, formation of the product nuclei at lower temperature, and final self-assembly for uniform waxberry-like microspheres with miniaturization of size. When employed as the adsorbents for the elimination of Congo red (CR) from simulated waste water, the adsorption data are well fitted with Langmuir isothermal model (qm: 1238.7 mg g−1). Meanwhile, the adsorption of CR is well described by the pseudo-second-order kinetic and intra-particle diffusion models, indicating the final diffusion at equilibrium stage within the mesopores of the real rate-limiting step in the whole adsorption process. The recycling test further confirms the Ba5Si8O21 microspheres as highly efficient adsorbents with satisfactory stability and reusability. As a noteworthy methodology, the heat and mass transport enhanced rotating hydrothermal synthesis presents new insights into the hierarchical porous silicate microspheres as a competitive candidate adsorbent for organic dyes removal, and also a potential new platform for heterogeneous catalysis.