Mg-Al 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°C were 1.2 × 10-3, 1.5 × 10-3, and 2.2 × 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 SeO42- and SeO32- with intercalated Cl-.
ASJC Scopus subject areas
- Chemical Engineering(all)