Abstract
Humic acid (HA) is extremely important for understanding the geochemical cycle of pollutants in different environments. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has performed molecular-level analysis of two standard HAs from the Suwannee River (SRHA) and leonardite (LEHA) and HA from Jiufeng forest in Beijing (JFHA), which is impossible for other conventional instruments. Regardless of the source of HA, compounds containing more heteroatoms (such as nitrogen and sulfur) have a higher degree of unsaturation and aromaticity. JFHA, SRHA, and LEHA from soil, river, and leonardite, respectively, are arranged in order from the lowest to highest degree of humification, according to molecular unsaturation and aromaticity of HAs. Soil HA is more labile and contains many large molecular weight compounds with low unsaturation. Regardless of unsaturation, molecules of River HA have a homogeneous molecular mass distribution and contain many plant-derived lignin- and tannin-like compounds, which are more stable than lipid and more labile than condensed aromatics. Leonardite HA with a high degree of humification contains a large number of compounds with high aromaticity and more heteroatoms and has low lability. Our results reveal the diversity of humic acid at molecular level because of different degree of humification and the lability. These conclusions are significant for understanding the role of humic acid from different sources in pollutant transformation and the geochemical cycle at the molecular level.
Highlights
Humic substances (HS) are the most widespread natural organic matter (NOM) in soil, water, and sediment
leonardite-derived HA sample (LEHA) has the lowest H/C and the largest molecular mass and this may be because the condensed aromatic structure with low H/C is abundant in LEHA and has a large molecular weight. is is consistent with the previous conclusion that refractory humic acid fractions are marked by increases in the condensed aromatic components [40]
In the v-K diagram, the aromaticity of the compounds shows such a rule: the aromaticity of the compounds in lower layer is higher than that in upper layer [30]. e compound distribution of SRHA shows a rectangle in the v-K diagram, which indicates that the molecular distribution is relatively homogeneous (Figure 1)
Summary
Humic substances (HS) are the most widespread natural organic matter (NOM) in soil, water, and sediment. HS are derived mainly from plant, organism, and animal tissues [1], while the biomaterials lost their initial structures in chemical and biological degradation. HA generally represents the major fraction of HS [6] and is mainly derived from typical environments rich in biochemical reactions such as soil, rivers, and leonardite. River, and leonardite are three typical sources of HAs. Soil HAs are mainly derived from the (bio)chemical degradation of plant and animal residues and synthetic activity from microorganisms and account for 20% of the total organic matter [6, 9, 10]. Leonardite-derived HA accounts for 10–80% of leonardite organic matter, depending on its maturity level [18] and is further along the diagenetic path of humification. ey are much older and contain more condensed aromatics than soil HAs [6]. ere are a number
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