Abstract
In this study, humic acids (HAs) were extracted from Chinese lignite by adding KOH to a NaOH solution. The extraction yield of HAs was found to improve because of the synergistic effect imparted by the alkali mixture of sodium hydroxide (NaOH) and potassium hydroxide (KOH). The maximum yield was obtained at 150 min by adding the mixture of 0.750 M NaOH + 0.710 M KOH to Xianfeng lignite at 80 °C. The potassium (K), sodium (Na), nitrogen (N), oxygen (O), and iron (Fe) contents were determined by X-ray diffraction, scanning electron microscopy, and proximate and ultimate analysis. The oxygen-containing functional groups in HAs were identified by Fourier transform infrared spectroscopy. The addition of KOH resulted in higher oxygen/carbon and nitrogen/carbon ratios and oxygen-containing functional groups, as compared with that in NaOH alone. The extractants containing KOH could release HAs with a higher proportion of K, Fe, N contents, which is beneficial for HAs fertilizers prepared from the lignite. The release of the nutrients (K, Fe, N) which are essential for the crops is determined by the KOH dosages.
Highlights
Humic acids (HAs) are a key indicator of soil fertility because they could improve the soil characteristics by enhancing the water holding capacity, aeration, and aggregation
All the HAs fractions were stored in polyethylene bags until further characterizations by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray (SEM–EDX) spectroscopy, and X-ray diffraction (XRD)
The oxygen containing functional group of HAs and residual lignite were determined according to the information in the literature [16,17,18,19]
Summary
Humic acids (HAs) are a key indicator of soil fertility because they could improve the soil characteristics by enhancing the water holding capacity, aeration, and aggregation. Lignite was extracted using NaOH or KOH, and the raw coal was pretreated with oxidizing agents such as oxygen, potassium permanganate, hydrogen peroxide, nitric acid, and hydrochloric acid to produce humic fertilizers [1, 9]. Dilute NaOH or KOH was used to extract HAs from lignite, high sodium content in HAs was detrimental to soil and retarded plant growth [6]. Temperature, while less N-rich compounds and lower functional groups compounds were obtained using hot solvent and concentrated KOH solution. They concluded that higher temperatures were unsuitable for extraction of HAs from lignite. The molecular size distribution of the extracted compounds using NaOH + Na4P2O7 mixture was similar to those extracted using Na4P2O7 alone. Focused on investigating the influence of NaOH + KOH mixture on the extraction yield and characteristics of HAs
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