Source indicator of commercial humic products: UV-Vis and fluorescence proxies

Over the last decades, commercial humic products (HPs) from various resources found multiple applications in agriculture and environmental technologies. The key factor defining chemical properties and biological activity of HPs is their organic matter origin. Thus, there is a need to find diagnostic criteria for differentiating organic sources of HPs. The objective was to determine indicators using absorption and fluorescence spectra in combination with derivative spectroscopy that might serve as tools to compare HPs from various environments, both as bulk materials and their humic acid (HA) fractions. HA-like fractions were isolated from commercially available humates, industrially manufactured from a number of raw source materials, such as: fossils (brown coal and leonardite), peat, lake-bottom sediment, and organic waste material (lignosulphonate). They were analyzed using chemical, fluorescence, and UV-Vis measurements. Elemental composition and ash content were determined. The blue shift of fluorescence band was defined with excitations at 310 and 270 nm or 355 and 310 nm. The following indices have been calculated from absorption spectra: specific absorbances normalized by the HA content, absorbance ratios E (2):E (3), E (4):E (6), E (270/400) E (280/472), E (280/664), Delta log K, and the spectral slope ratio Sr. In addition, we implement new indices: the ratio of amplitudes in the first-order (FDR280/240) and in the second-order derivative spectra (SDR267/280). Absorption spectra of HA-like fractions isolated from HPs varying in organic matter origin demonstrated similar wavelength-dependent character. However, some HA-like fractions demonstrate weak maxima in the derivative spectra with enhanced spectral resolution at 230 and 280 nm. This effect is most probably due to the presence of low molecular weight phenolic compounds, products of lignin decomposition. HA-like fractions isolated from peat- and lignosulphonate-derived HPs show fluorescence maximum at shorter wavelengths than HA-like fractions from coal- and sapropel-derived HPs. Under excitation at 310 nm, certain peat- and lignosulphonate- originated samples manifest significant blue shift of emission band. In contrast, HA-like fractions from coal- and sapropel-derived HPs exhibit excitation-independent fluorescence band position or its small shift to longer wavelengths. Information extracted from absorption and fluorescence spectra can be useful to discriminate organic matter source for HPs from coalified materials (fossils), peat, and lignosulphobate. Indices with the highest descriptive ability, showing strong loadings in principal component analysis, are as follows: specific absorbance SUVA(254), absorbance ratios E (270/400), E (280/472,) derivative indices FDR280/240 and SDR267/280, wavelength of fluorescence maximum, and the presence of blue shift.