Temperature Dependences of IR Spectra of Humic Substances of Brown Coal

Dmitry Volkov,Mikhail Proskurnin,Olga Rogova

doi:10.3390/agronomy11091822

Dmitry Volkov, Mikhail Proskurnin + Show 1 more

Open Access

https://doi.org/10.3390/agronomy11091822

Copy DOI

Abstract

The capabilities of temperature-monitored IR spectroscopy for studying the organic matter and mineral composition of humic substances (HS) were tested. Temperature dependences of the mid-IR spectra of humic substances heated in the air in the range 25–215 °C (298–488 K, with a step of 2.5 °C)—for three commercially available samples isolated from brown coal (leonardite)—were performed. The characteristic bands were identified, and their changes in band maxima positions and intensities were compared. From the viewpoint of interpretation of HS components, the spectra were divided into regions of quartz lattice region (800–260 cm−1), quartz overtone region (1270–800 cm−1), humic substance organic matter region (1780–1270 cm−1), quartz combination region (2800–1780 cm−1), CH-speciation region (3100–2800 cm−1), and hydrogen-speciation region (4000–3100 cm−1) thus selected to contain the dominating type of bands. For the first time, a reversible change in the frequencies of the band maxima in IR spectra upon heating was observed, which can be interpreted as forming structures with a particular order in the studied humic substances in the dry state. For a single sample, both the band-shift scale and the functional dependence of the various bands on temperature differ significantly. The approach differentiates crystalline quartz bands, amorphous silica, and HSOM/surface groups experiencing a different temperature behavior of the band maxima and their intensities. Band-maximum temperature dependence can be considered more stable to changes in experimental conditions than band maxima at a single temperature, thus providing a more detailed HS structure analysis without HS decomposition or destruction.

Highlights

Humic substances (HS) are a predominant species of organic matter in natural and human-made environments such as sewage and landfills [1,2,3,4]
Diamond crystal with a lowered pressure screw with a flat end Before recording the spectra with heating, an empty ATR crystal spectrum was recorded at 25 ◦ C as a background
The IR spectra of the studied HS samples are shown in Figure 1; more details are given in the Supplementary Materials (Figures S1–S11, see below)

Summary

Introduction

Humic substances (HS) are a predominant species of organic matter in natural (soil and waters) and human-made environments such as sewage and landfills [1,2,3,4]. HS of various origins—soil, sediments, peat, lignites, brown coal—are relevant resources [1,3] involved in soil [9,12], chemical, and biochemical processes of brown coal treatment [13,14] and biomass treatment [15] These processes require a large volume of information of HS systems, which requires the development of corresponding analysis and characterization methods. IR spectroscopy is a developed method to study HS and is used to characterize and assess functional (mainly carboxylic and hydroxylic) groups and whole HS organic matter (HSOM) [16,17,18,19,20,21].

Objectives

Methods

Results

Discussion

Conclusion