Equilibrium and kinetic studies of Se(VI) removal by Mg-Al layered double hydroxide doped with Fe2+

MgAl layered double hydroxide (Mg-Al LDH) doped with Fe2+ was found to be superior to undoped Mg-Al LDH in the removal of Se(VI) from aqueous solutions. For both systems, Se(VI) as SeO42- was removed through anion exchange with intercalated Cl-. In the Fe2+-doped Mg-Al LDH, however, some of the Se(VI) was reduced to Se(IV) upon oxidation of Fe2+ to Fe3+ in the LDH host layer to produce SeO32-, which was also adsorbed by the Fe2+-doped Mg-Al LDH through anion exchange. The reduction of Se(VI) to Se(IV) is advantageous for Se(VI) removal by Fe2+-doped Mg-Al LDH due to the larger charge density of SeO32-. The Fe2+-doped Mg-Al LDH effectively removed Se(VI) from an aqueous solution because of the anion exchange properties of Mg-Al LDH and activity of Fe2+ as a reducing agent. Se(VI) removal occurs through Langmuir-type adsorption, where the maximum adsorption and equilibrium adsorption constant were 1.4 mmol g(-1) and 1.6, respectively. Se(VI) removal is well expressed as a pseudo second-order reaction. The apparent rate constants at 10, 30, and 60 degrees C were 1.2 x 10(-3), 1.5 x 10(-3), and 2.2 x 10(-3) g mmol(-1) min(-1), respectively, and the apparent activation energy was 10.0 kJ mol(-1). The rate-determining step is chemical adsorption through anion exchange of SeO4 (2-) and SeO3 (2-) with intercalated Cl-.