Highly efficient detoxification of Cr(VI) by brown coal and kerogen: Process and structure studies

•Brown coal and kerogen showed high capacity for reducing Cr(VI) to Cr(III).•Cr species on the surface of adsorbents were main Cr(III) which was not desorbed.•The structure of brown coal and kerogen was altered due to oxidization of Cr(VI) and load of Cr(III).

In this study, different from previous reports focused on removal efficiency of Cr(VI), the structural variations of brown coal and kerogen concerned with Cr(VI) removal were studied. The process conditions of Cr(VI) removal were optimized, and the reduction of Cr(VI) to Cr(III) was evaluated. In addition, the variations of functional groups in brown coal and kerogen before and after Cr(VI) detoxification were characterized by FTIR, XPS, SS-NMR, TEM-EDS, and SEM-EDS. The results indicated that brown coal and kerogen had highly adsorption efficiency for Cr(VI) (maximum 4.67 mmol/g), and 80.1% of adsorbed Cr(VI) was reduced to Cr(III) while 90.4% of them was held on the adsorbents. It also revealed that the content of –CH3, –CH2, CO and phenolic hydroxyl groups in adsorbents decreased, but the content of CO and O–CO groups increased after Cr(VI) detoxification. Overall, structure identified of brown coal and kerogen in this work could provide important basic data for further application in wastewater treatment.