Solid-phase humification of Lignite for activation of nitro-humified substances via ozone oxidation: humification efficiency and nitrogen transformation

This study is the first attempt to solid-phase humification of Lignite for activation of nitro-humified substances (NHSs) towards improve the cleaner application of lignites in agriculture and the environment. Ozone oxidation and additives of urea and KOH were used as accelerator of humification process, nitrogen enrichment and humic acid activating agent, respectively. In order to elucidate the solid-phase humification performance of lignite, indicators such as total humic acids (THA) and the water-soluble humic acids (WHA), nitrogen/oxygen content, C/N ratio, degree of oxidation (O/C) obtained from CHNOS elemental analysis, and carboxylic and phenolic functional groups content were evaluated. Toward the confirmation of changes of the oxygen-nitrogen containing functional groups as well as effective nitrogen transformation in the humification process, FT-IR spectroscopy and analysis of distribution percentage of incorporated bonded nitrogen forms were studied. Experiments and data collection of this research were completely conducted in 2021-year in the college of Abouraihan-University of Tehran and in the laboratories located in department of biosystems engineering. In continuation, response surface methodology based on the central composite design (CCD-RSM) was used to determine the optimal conditions, and mathematical models were developed to accurately predict the changes of THAs and the WHAs. Under the optimal conditions of 8 g/h of ozonation rate, 15 min of oxidation time, 14.85 %wt. of KOH and 30 %wt. of urea, the yield of THA and WHA were 69.02% and 53.32%, respectively which were significantly higher than the control samples (30% and 7%, respectively). The uniform distribution of binded nitrogen forms in the NHSs showed that 47% of the total nitrogen content in the NHSs was in the form of ammonium nitrogen, 16% in the form of amide nitrogen and 37% in the form of highly bonded organic nitrogen with very strong bonds. By assist of ozone humification, the content of phenolic and carboxylic acid groups increased by 85.71% and 215.87%, respectively. The optimized process resulted in superior quality indicators, including less carbon, more nitrogen and consequently lower C/N ratio, higher O/C ratio, lower aromatic carbon, and more aliphatic and carboxylic carbon for developed NHSs.