New method for efficient removal of Cr(VI) by recoverable magnetic nitrogen-doped carbon aerogel microspheres: Kinetics and mechanism

The efficient removal of hexavalent chromium from solution is a challenging task, in this paper, the nZVI and nitrogen co-doped magnetic carbon aerogel microsphere composites (nZVI@NCAs) were prepared via the microemulsion polymerization and subsequent carbonization process, and were expected to satisfactory performance in the removal of Cr(VI). From batch experiments, the Cr(VI) removal process by nZVI@NCA900 fitted better with the pseudo-second-order model. The optimal nZVI@NCA900 was superior to that of most magnetic carbon-substrate composites, displaying high Cr(VI) adsorption capacity of 266.79 mg·g −1 at 318 K. Additionally, it was found that Cr(VI) was reduced to the low-toxicity Cr(III) from the results analyzed by XPS, and nZVI@NCA900 exhibited Cr(VI) reduction performance of 33.92% Cr(VI) reduced to Cr(III). The reaction between the nZVI@NCAs and Cr(VI) was pH-dependence, endothermic and spontaneous. Removal mechanisms might involve multiple processes. The nZVI particles on the surface acted as reductants to reduce Cr(VI) to Cr(III) and form Cr(III)/Fe(III) hydroxides to precipitate on nZVI@NCAs. Simulation results by DFT prove that N is an active site and show positive effect in the adsorption system. N atoms were used as active sites to stabilize the interaction with Cr(VI) via adsorption attributed to electrostatic and act as electron donors for reduction of Cr(VI). The nZVI particles encapsulated by carbon aerogels could provide magnetic stability after acid reaction, which was beneficial to the separation of adsorbents after reaction and reuse for many cycles. These results provide a new view for developing magnetic carbon materials to remove Cr(VI) from contaminated water.