Removal of selenium from caustic solution by adsorption with Ca-Al layered double hydroxides

Removal of selenium from caustic solution by adsorption with Ca-Al Layered Double Hydroxides (Ca-Al LDHs) was proposed. Ca-Al LDHs with a hexagonal platelet structure were successfully synthetized by co-precipitation. 94% of Se(VI) was removed at 30 degrees 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 degrees C-70 degrees C), Ca-Al LDHs underwent an anion exchange with Se(VI) to form Ca4Al2O6SeO4(H2O)(10), with the d-spacing expanding from 7.95 angstrom to 10.15 angstrom. When increasing temperature above 80 degrees 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 degrees C. Desorption and regeneration results indicated that Ca-Al LDHs can be reused at least three times without significantly reducing its absorption capacity for Se(VI).