Enhanced Li+ adsorption by magnetically recyclable iron-doped lithium manganese oxide ion-sieve: Synthesis, characterization, adsorption kinetics and isotherm

The Li+ adsorption from aqueous solution by lithium-ion sieve has become one of the most promising methods due to the high efficiency and selectivity towards lithium ion (Li+). However, the industrial application of manganese oxide ion-sieve is limited due to its difficult separation and decrease of adsorption capacity resulting from manganese dissolution loss. In this paper, the magnetically recyclable Fe-doped manganese oxide lithium ion-sieves with spinel-structure were proposed and prepared from LiMn2-xFexO4 synthesized by solid state reaction method. The effects of calcination temperature, calcination time and Fe doping amounts on the phase compositions, dissolution loss and adsorption performance of lithium ion-sieve precursors were systematically studied, and the influences of solution pH value, initial Li+ concentration and adsorption temperature on the adsorption performance were investigated. The adsorption mechanism was further discovered through adsorption kinetics and thermodynamics. The results show that the adsorption capacity of lithium ion-sieves could reach to 34.8 mg·g–1 when the calcination temperature, time and Fe doping content were controlled at 450 °C, 6 h, and 0.05, respectively. The Mn dissolution loss was reduced to 0.51%, much lower than the undoped lithium ion-sieve (2.48%), which is attributed to the inhibition of disproportionation reaction with the increasing proportion of Mn4+ in the skeleton. The adsorption process conformed to the pseudo-second-order kinetics equation and Langmuir isothermal adsorption model. Furthermore, the recycling performance of Fe-doped lithium ion-sieve showed that the adsorption capacity could remain 22.5 mg·g–1 (about 70%) after five cycles, which is greater than that of undoped lithium ion-sieve (about 50%), and the recovery of lithium ion-sieve can be realized by magnetic separation in an applying magnetic field.