Fig. 4

Three-dimensional excitation-emission matrix (3-D EEM) fluorescence spectra of whole humic acid samples and their fractions. Emission intensities were normalized on the maximum. Arrows indicate the positions of fluorescence peaks. H1–4 humic acid-specific fluorescence peaks, F1–3 fulvic acid-like fluorescence peaks [24], P protein-like fluorescence peaks [25, 26]