Mechanism study of Cr(III) immobilization in the process of Cr(VI) removal by Huolinhe lignite

• The C = O and O-C = O groups are abundant in HLoxi.
• The reduced Cr(III) ions are bonded to oxygenic functional groups in HLoxi and electrons are delocalized over -O-Cr-O- domain.
• The Cr bound by HLoxi can be completely recovered using alkaline hydrogen peroxide.

Lignite showed high capacity for reducing Cr(VI) to Cr(III) and hence removed Cr(VI) from aqueous solution, but the reduced Cr(III) cannot be released from adsorbent afterwards especially under acidic condition. In order to know why Cr(III) ions are immobilized within lignite adsorbent and how to recovery Cr species, the interaction of Cr(III) ions with lignite oxidized by Cr(VI) was studied by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The data obtained from these studies indicate that Cr(III) ions are bonded to oxygenic functional groups in lignite oxidized by Cr(VI) and electrons are delocalized over -O-Cr-O- domain. It is also revealed that the O atoms of C = O and C-O groups in the adsorbent matrix are bonded to Cr(III) ions along with the decrease of electron density, while the O atoms of O-C = O group are along with the increase of electron density. Based on these important structural information, we developed a method to treat Cr-loaded lignite further and to recover the Cr species.