Activation of Nitro-Humic Substances from Lignite using Solid-Phase Nitro-Humification Process Assisted by Nitrogen Enrichment and Ozone Oxidation

 In this work, solid-phase nitro-humification process was developed to activation of lignitic nitro-humic substances (NHSs) by using the two-step sequential treatment assisted by nitrogen enrichment (using KOH and urea as a humic acid activator and a nitrogen enrichment agent, respectively) and ozone oxidation in fixed-bed reactor. The changes in main elements, spectral index of humification (E4/E6), surface functionalization, and textural properties were determined by CHNOS analysis, ultraviolet-visible (UV-VIS), Fourier-transform infrared (FT-IR) spectroscopy, and specific surface area (SSA) analysis based on BET, BJH and t-plot models, respectively. The results revealed the increasing 2.25 and 2.94-folds on the yield of alkali and water-soluble NHSs, respectively, compared to the conventional alkaline extraction method. A 14.5% reduction in the carbon content owing to ozone oxidation and 8.15% nitrogen enrichment resulting from urea pretreatment led to an ideal humification ratio (C/N ratio=5.6) and a higher degree of oxidation (O/C ratio=0.94). Also, E4/E6 ratio up to 6.8 and salt index of 46.2% for NHSs were at an acceptable level. The decomposition of double bonds of aromatic carbons and the effective transformation of nitrogen into amide and organic nitrogen forms due to ozone oxidation were well proven by elemental analysis of CHNOS and FT-IR spectroscopy. As a result of ozone oxidation, the main pores in lignite were mainly turned into mesopores. Collectively, the results demonstrated that ozone oxidation, in addition to enhancing the yield and quality of NHSs, provide an end product with a competitive value and a lower price than other commercial nitrogenous humic fertilizers.