Characterization of humic substances derived from swine manure-based compost and correlation of their characteristics with reactivities with heavy metals

Various composts contain a significant amount of humic substances including humic acid (HA) and fulvic acids (FAs). The FA fraction in soils is considered to be sensitive to agronomic and environmental factors. In this study, three fractions of humic substances, HA (MW > 1000 Da), FA (MW > 1000 Da), and FA (MW < 1000 Da) were extracted from swine manure-based compost and characterized, and then, their reactivities were correlated with heavy metals. Compositions of the three fractions of humic substances were characterized by elemental and total acidity analyses and electron spin resonance (ESR), Fourier transform infrared (FTIR), and 13C nuclear magnetic resonance with cross-polarization and magic-angle spinning spectroscopic techniques. Elemental analyses indicated that HA has higher contents of C, H, N, and S than those of FAs. However, FA (MW > 1000) and especially FA (MW < 1000) have higher contents of O than that of HA (MW > 1000). The g values of the ESR spectra of the three fractions showed that the organic free radical characteristics and the widths of the spectra and free radical concentrations of the three fractions are significantly different. The FTIR spectra indicated that HA (MW > 1000) is abundant in C=C bonds while FA (MW > 1000), especially FA (MW < 1000), are abundant in C=O bonds. In addition, 13C NMR spectra indicate that carboxyl contents of FA (MW > 1000), especially FA (MW < 1000), are higher than that of HA (MW > 1000). The sequence of the reactivity in terms of acidic functional groups was FA (MW < 1000) > FA (MW > 1000) > HA (MW > 1000). Elemental and functional group compositions of the three fractions significantly correlated with reported reactivities with heavy metals. The application of swine manure-based compost containing HA and FAs fractions to soil and associated environments may thus significantly affect the concerned reactions with organic and inorganic compounds including pollutants.