Abstract
Humic substances are the major components of soil organic matter. Among the three humic substance components (humic acid, fulvic acid, and humin), humin is the most insoluble in aqueous solution at any pH value and, in turn, the least understood. Humin has poor solubility mainly because it is tightly bonded to inorganic soil colloids. By breaking the linkage between humin and inorganic soil colloids using inorganic or organic solvents, bulk humin can be partially soluble in alkali, enabling a better understanding of the structure and properties of humin. However, the structural relationship between bulk humin and its alkaline-soluble (AS) and alkaline-insoluble (AIS) fractions is still unknown. In this study, we isolated bulk humin from two soils of Northeast China by exhaustive extraction (25 to 28 times) with 0.1 mol L-1 NaOH + 0.1 mol L-1 Na4P2O7, followed by the traditional treatment with 10 % HF-HCl. The isolated bulk humin was then fractionated into AS-humin and AIS-humin by exhaustive extraction (12 to 15 times) with 0.1 mol L-1 NaOH. Elemental analysis and solid-state 13C cross-polarization magic angle spinning nuclear magnetic resonance (13C CPMAS NMR) spectroscopy were used to characterize and compare the chemical structures of bulk humin and its corresponding fractions. The results showed that, regardless of soil types, bulk humin was the most aliphatic and most hydrophobic, AS-humin was the least aliphatic, and AIS-humin was the least alkylated among the three humic components. The results showed that bulk humin and its corresponding AS-humin and AIS-humin fractions are structurally differed from one another, implying that the functions of these humic components in the soil environment differed.
Highlights
Humic substances are the most ubiquitous and widespread natural nonliving organic compounds in the environment (Senesi, 2010)
The results showed that, regardless of soil types, bulk humin was the most aliphatic and most hydrophobic, AS-humin was the least aliphatic, and AIS-humin was the least alkylated among the three humic components
The C content followed the order AS-humin > AIS-humin > bulk humin, and for H and N, the order was bulk humin > AIS-humin >AS-humin. This led to the H/C ratio decreasing in the order bulk humin > AIS-humin > AS-humin, whereas the reverse was observed for the C/N ratio
Summary
Humic substances are the most ubiquitous and widespread natural nonliving organic compounds in the environment (Senesi, 2010). By breaking the linkage between humin and inorganic soil colloids using inorganic (e.g., HF, H2SO4, and Na2S2O3) and organic (e.g., MIBK, urea, and DMSO) solvents, it is possible to recover and separate bulk humin into soluble and insoluble fractions (Pallo, 1993; Stevenson, 1994; Rice, 2001; Song et al, 2005; Spaccini et al, 2006; Song et al, 2011). To the best of our knowledge, the structural relationship between bulk humin and its alkaline-soluble and alkaline-insoluble fractions is still unknown
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