Removal of selenium from caustic solution by adsorption with CaAl layered double hydroxides

Removal of selenium from caustic solution by adsorption with CaAl Layered Double Hydroxides (CaAl LDHs) was proposed. CaAl LDHs with a hexagonal platelet structure were successfully synthetized by co-precipitation. 94% of Se(VI) was removed at 30 °C for 30 min with NaOH concentration of 1 mol·L−1 and adsorbent dosage of 40 g·L−1. Temperature played an important role in Se(VI) removal. At low temperature (30 °C–70 °C), CaAl LDHs underwent an anion exchange with Se(VI) to form Ca4Al2O6SeO4(H2O)10, with the d-spacing expanding from 7.95 Å to 10.15 Å. When increasing temperature above 80 °C, Ca4Al2O6SeO4(H2O)10 began to decomposed to Ca3Al2O6(H2O)6 and Ca(OH)2, resulting in SeO42− into the liquor. Kinetic data were well fitted to pseudo-second-order model, indicating Se(VI) removal process was controlled by the chemisorption. The adsorption isotherms were in good agreement with Langmuir isotherm model, suggesting Se(VI) removal was a monolayer adsorption. The maximum adsorption capacity was calculated to be 138.76 mg·g−1 at 30 °C. Desorption and regeneration results indicated that CaAl LDHs can be reused at least three times without significantly reducing its absorption capacity for Se(VI).