Highly humified nitrogen-functionalized lignite activated by urea pretreatment and ozone plasma oxidation

Ehsan Sarlaki, Mohammad Hossein Kianmehr*, Marzieh Ghorbani, Ali Mashaallah Kermani, Keyvan Asefpour Vakilian, Irini Angelidaki, Yajing Wang, Vijai Kumar Gupta, Junting Pan, Meisam Tabatabaei, Mortaza Aghbashlo

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)


A cost-effective and energy-efficient nitro-humification process (urea pretreatment and ozone plasma oxidation) was developed for nitrogen functionalization and activation of the humic substances (HSs) of lignite. Different nitro-humification metrics, i.e., solubility, physicochemical properties, oxidation degree, nitrogen functionalization, spectral ratios, and surface functionalization, along with energy efficiency and economic profitability, were determined for the processed lignite. The best solubility metrics for the lignite were obtained under 3 wt% ozone, 10 wt% KOH, and 20 wt% urea. The selected conditions could increase alkali- and water-soluble HSs by 2.25 and 2.94 times, respectively, compared with the conventional alkaline extraction method. The process could result in a balanced distribution between nitrogen binding forms. The functionalized lignite showed a 14.5% reduction in the carbon content due to ozone oxidation and an 8.15% nitrogen enrichment due to urea pretreatment, leading to an ideal C/N ratio (5.6%) and a higher O/C ratio (0.94). The humification index (E4/E6 ratio of 6.8) and salt index (46.21%) were acceptable. The process could increase the cation exchange capacity of the lignite by 3.83 times while enhancing its carboxylic and phenolic groups by 215.87% and 85.71%, respectively. The crystallinity index of the processed lignite was increased by 2 times, and the availability of its macro-micronutrients and rare earth elements was increased by 6.35 times and 43.38%, respectively. The nitro-humic products showed lower heavy metals with higher micropores-mesopores in a globular-like and heterogeneous porous structure. The proposed method could increase the process energy efficiency by 59.91% while discounting the production costs by 43%.

Original languageEnglish
Article number140978
JournalChemical Engineering Journal
Early online date22 Dec 2022
Publication statusPrint publication - 15 Jan 2023

Bibliographical note

Publisher Copyright: © 2022 Elsevier B.V.


  • Alkali and water solubility
  • Humic substances
  • Lignite activation
  • Nitro-humification
  • Ozone oxidation
  • Urea pretreatment


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