Structural characteristics of humic acids derived from Chinese weathered coal under different oxidizing conditions.

Liping Zhou,Liang Yuan,Zhian Lin,Yanting Li,Bingqiang Zhao,Jingdong Mao

doi:10.1371/journal.pone.0217469

Abstract

Humic acids derived from Chinese weathered coal were oxidized with hydrogen peroxide (H2O2) under various conditions, and their chemical composition and structure were examined. The raw material humic acids (HA) and oxidized humic acids (OHAs) were characterized by elemental analysis and ultraviolet visible (UV-Vis), Fourier transform infrared (FTIR), and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Our results show that aromatic functional groups accounted for more than 70% of the HA and OHAs and there were significant differences in their structures and compositions. Compared to the HA, the average H and N contents of the OHAs decreased by 5.15% and 2.52%, respectively, and the average O content of those of the OHAs increased by 5.30%. The hydrophobicity index (HI) of HA is higher than those of the OHAs. Importantly, in the hypothesis test between the properties and preparation conditions of humic acid using SPSS, the partial η2 of the temperature, hydrogen peroxide concentration, liquid-solid ratio, and time were 0.809, 0.771, 0.748 and 0.729, respectively; thus, among the preparation conditions, temperature is the most important factor affecting the humic acids properties.

Highlights

Weathered coal is formed when near-surface or shallow-surface coal is exposed to physical and chemical weathering for a long period [1]
The C, H, N and O contents of humic acids (HA) is 47.00%, 4.89%, 1.04% and 33.56%, respectively, and those of the oxidized humic acids (OHAs) varied, the C content was 44.96–50.61%, the H content was 4.20–4.97%, the N content was 0.90–1.10%, and the O content was 33.56–36.52%
Compared to those of HA, the average C, H and N contents of the OHAs decreased by 0.34%, 5.15% and 2.52%, respectively, and the average O content of the OHAs increased by 5.30%

Summary

Introduction

Weathered coal is formed when near-surface or shallow-surface coal is exposed to physical and chemical weathering for a long period [1]. Due to the influence of long-term weathering, weathered coal has a high oxygen content and low calorific value. It is rich in humic substances and has a variety of functional groups, such as carboxyl, hydroxyl, phenolic hydroxy thiol groups, etc., resulting in a high capability to enhance bioactivity. Humic acids are economically important because of the abundant global reserves of weathered coal, i.e., approximately 100 billion tons in China alone [6]. The content of humic acids of weathered coal is greater than that of lignite and peat [7]. Research and applications of humic acids have mainly focused on the humic acids from peat and lignite [8,9,10,11,12], only relatively few reports were on the humic acids

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