Characterization of Ultrafiltration-Fractionated Humic Acid Derived from Chinese Weathered Coal by Spectroscopic Techniques

Shuiqin Zhang,Yanting Li,Bingqiang Zhao,Liang Yuan

doi:10.3390/agronomy11102030

Abstract

Weathered coal is a widely used raw material of farm-oriented humic acid in China, while the high heterogeneity impedes its sufficient utilization. In this study, we fractionated the humic acid derived from Chinese weathered coal by ultrafiltration, and three fractions with the molecular range of ≥100 kDa, 10–100 kDa, ≤10 kDa were obtained. Subsequently, the chemical and spectral properties of the fractions were characterized by elemental analysis, potentiometric titration, Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR), and X-ray photoelectron (XPS) and other spectroscopy. The results showed that more than 60% of humic acid by weight was concentrated in the molecular range higher than 100 kDa, while only 3.25% was assigned to that with molecular weight lower than 10 kDa. The humic acid fraction with molecular weight higher than 100 kDa showed more carbon content, lower H/C atomic ratio, while higher E4/E6 ratio, more aromatic structure in FTIR, 13C NMR, and XPS spectra, indicating a higher degree of aromaticity and stronger hydrophobicity. Conversely, there were more carboxyl groups and aliphatic structures, while fewer condensed aromatic rings for the humic acid fraction with molecular weight less than 100 kDa. These differences provide a baseline for the better utilization of weathered coal.

Highlights

Weathered coal is a kind of low-rank coal, which is usually exposed to the Earth’s surface or buried at shallow depths
The fraction distribution showed that HAH fraction accounted for 61.88% of the whole humic acid (HA) derived from weathered coal, HAM for 20.95%, and HAL for 3.25%
These differences are likely because marine organic carbon and fulvic acid generally have much lower molecular weight and higher polarity than HAs derived from soil and weathered coal [36]

Summary

Introduction

Weathered coal is a kind of low-rank coal, which is usually exposed to the Earth’s surface or buried at shallow depths. The burial location determines the characteristics of this metamorphic coal. It has a low calorific value, while a high ash and moisture content, leading to a difficulty in the resource utilization of weathered coal in the current industry [1]. Abundant resource reserves of weathered coal are reported to occur in China, estimated at more than 10 billion tons [2]. Exploring an available way to use weathered coal is an urgent problem, and its solution will own a huge social and environmental value. The abundant reserve and high HA content contribute weathered coal to an important raw material of HA [5]

Methods

Results

Conclusion