Mechanochemical approach to get layered double hydroxides: Mechanism explore on crystallite growth

•Layered double hydroxides were prepared by a mechanochemical approach.•The mechanism of solid state reactions has been explored.•The reaction process during grinding has been inferred.•Crystallite growth is the rate-controlling process.•LDHs from mechanochemical approach are good adsorbents for F− anions. In this paper, the mechanochemical approach, which includes solid state reactions and hydrothermal treatment, has been proposed to synthesize magnesium–aluminum–layered double hydroxides (Mg–Al–LDHs). Specially, the reaction process of solid state reactions has been explored, and it presents that crystallite growth is the rate-controlling process. The hydrothermal treatment is performed after solid state reactions, on one hand, the crystallinity and monodispersity of final LDHs particles can be improved, on the other hand, such treatment can tailor the particle size efficiently. Furthermore, the relationship between particle size and hydrothermal conditions (time and temperature) has been systematically investigated, which indicates that the particle size and monodispersity can be effectively controlled. The crystallite growth along a–b plane and c-axis has been emphatically discussed, and the results show that under relatively low temperatures such as 100°C, the gradual growth along c-axis has been found in the range of 48h, and high temperatures will hider its growth on the contrary. Crystal growth along a–b plane could be accelerated by higher hydrothermal temperature and longer treatment time. Our studies also show that during the hydrothermal treatment, such events as aggregation, disaggregation and particle growth, occur in series or in parallel with time. At last, the Mg–Al–CO3–LDHs samples (synthesized at 100°C for 24, 36 and 48h) which were acid activated by HCOOH were used to adsorb fluoride ions present in aqueous solution.