Unlocking the fertilizer potential of low-grade coal: the role of synthesis conditions in tailoring the properties of ammoniated nitrohumic acids

The conversion of low-grade coals (LGCs) into value-added fertilizer products presents a promising avenue for proper utilization of natural resources. The object of this study is the synthesis of ammoniated nitrohumic acids (ANHAs) from an Iranian LGC to enhance soil nitrogen storage. The demineralized LGCs were subjected to a two-stage pretreatment comprising H₂O₂ oxidation followed by HNO₃ treatment, with and without an initial thermal step. The treated low-grade coals (TLGCs) were subjected to NH₄OH and the extracted ANHAs were incorporated into a sandy loam soil for an incubation experiment to monitor nitrate levels over time. Despite an 11% weight loss, the optimized H₂O₂/HNO₃ pretreatment more than doubled the carboxyl group content of the NHA compared to the HNO₃ pretreatment alone, as confirmed by FTIR spectroscopy. Response Surface Methodology (RSM) analysis for optimizing the H₂O₂-assisted oxidation of LGC identified contact duration as the most significant parameter for carboxyl group content, followed by temperature and H₂O₂ concentration, which exhibited comparable influence. The solid-to-solution ratio demonstrated no significant effect. Soil incubation experiments revealed that the synthesized ANHA with a nitrogen content of 6.53% served as an effective slow-release nitrogen source. Over 40 days, the nitrogen release percentage was inversely proportional to the application rate, decreasing from 59.8% at 0.1% to 17.5% at 1%. Crucially, even at the lowest application rate of 0.1%, the released nitrogen (38.2 mg NO₃-N kg^{- 1}) fell well within the optimal range of 20-40 mg NO₃-N kg^{- 1}. These results demonstrate that the ANHAs produced from LGCs under optimized chemical pretreatments are a highly promising fertilizer for boosting soil nitrogen content.