Extraction and Chemical Characterization of Humic Acid from Nitric Acid Treated Lignite and Bituminous Coal Samples

Abstract

Currently, conversion of coal into alternative fuel and non-fuel valuable products is in demand and growing interest. In the present study, humic acid was extracted from two different ranks of coal, i.e., low rank and high rank (lignite and bituminous), through chemical pretreatment by nitric acid. Samples of lignite and bituminous coal were subjected to nitric acid oxidation followed by extraction using KOH and NaOH gravimetric technique. The chemical pretreatment of both types of coal led to enhanced yield of humic acid from 21.15% to 57.8% for lignite low-rank coal and 11.6% to 49.6% bituminous high rank coal. The derived humic acid from native coal and nitric acid treated coal was analyzed using elemental analysis, E4/E6 ratio of absorbance at 465 nm and 665 nm using UV-Visible spectrophotometry and Fourier transformed infrared spectroscopy FTIR. The chemical characteristics of coal treated with nitric acid have shown increased molecular weight and improved aromaticity with more oxygen and nitrogen and lower C, H, and sulphur content. The E4/E6 ratio of nitric acid-treated low and high ranks of coal were high. The FTIR spectroscopic data of nitric acid-treated lignite coal indicates an intensive peak of carboxyl group at 2981.84 cm, while bituminous coal was shown in cooperation of N-H group at 2923.04 cm. SEM was performed to detect the morphological changes that happen after producing humic acid from HNO3 treatment and native coal. The humic acid produced from HNO3 treated coal had shown clear morphological changes and some deformations on the surface. SEM-EDS detected the major elements, such as nitrogen, in treated humic acid that were absent in raw coal humic acid. Hence, the produced humic acid through HNO3 oxidation showed a more significant number of humic materials with improved efficiency as compared to native coal. This obtained humic acid can be made bioactive for agriculture purposes, i.e., for soil enrichment and improvement in growth conditions of plants and development of green energy solution.

Understanding the Role of Humic Acids on Crop Performance and Soil Health

“…Most commercially-produced HA are from lignite and coal due to their more extensive deposits, especially in oil producing regions. Humic acids from these sources tend to have more carbon but less oxygen and nitrogen content as well as low carboxylic and phenolic functional groups, which mostly contribute to HA physical, chemical and biological activities (Fatima et al, 2021). Recent development in HA research has focused on efficient extraction methods that produce high HA yield with many crop growthpromoting functional groups (phenolic and carboxylic groups).…”

“…Pre-treatment of lignite and coal with acid breaks down the complex compounds, thus reducing them to weaker acidic functional groups (Barhoumi et al, 2019). Nitric acid (HNO 3 ) has been used to increase the yield of HA produced from lignite and bituminous coal following alkaline extraction (Zara et al, 2017;Boral et al, 2021;Fatima et al, 2021). In addition to increased HA yield, they also found an increase in nitrogen and oxygen contents and functional groups such as carboxylic, phenolic and amine groups.…”

Extraction and Characterization of Humic Acid Based on Coal Mining Waste

“…Further interpreting the FTIR spectra, the absorption bands within 2974–2940 cm −1 and 2862–2665 cm −1 show the presence of aliphatic and methoxy groups with valence fluctuations, υ C‐H 44. The same information is given in 45, where humic acids were extracted from brown coals of Balochistan. Low absorption bands in the range of 2538–2206 cm −1 and 2000–1800 cm −1 are explained by the presence of azo compounds.…”

Low-Rank Coal as a Source of Humic Substances for Soil Amendment and Fertility Management

“…Good adsorptive properties of LRC have aroused intense interest for its potential as a versatile environmental adsorbent. The utilization of the coal-based HS in soil remediation [45,[62][63][64] and water treatment systems (municipal wastewater and acid mine drainage) [65][66][67] are recently well-documented. Detoxification studies by research groups led by Qi and by Skłodowski [58,68] have employed LRC as an attractive low-cost adsorbent for the removal of different pollutants from the aquatic and terrestrial environments.…”