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Table S1. Fitting parameters obtained with Eq. (S2) for the curves of charge developed by the humic substance molecule QHS (meq/gHS) against pH (see Fig.  S2).

Table S2. Recovered fitting parameters of UV spectra of DHS using Eq. (S3).

Table S3. Spectral redshift emission maxima of the TRANES (Δλt), isoemissive points (λie) and decay time of interconversion (τic) for DHS at different excitation wavelength.

Figure S1. Curves of charge developed, QHS (meq/gHS) against pH (solid circles) of N2-bubbled solutions of 8 mg of purified Aldrich humic acid (AHA) and 11-months vermicomposted humic acid-like (VC11M) at 25°C. The solid red line represents the data fitting with Eq. (S1). The fitting residuals are represented below.

Figure S2. Fluorescence decay curves of air-saturated 10 mg·L−1 DHS solutions obtained with excitation at 340 nm (Nanoled®) and emission at 450 nm. Emission bandwith 12 nm.

Figure S3. Excitation and emission fluorescence spectra of air-saturated 10 mg·L−1 DHS solutions. Excitation and emission slits width 5 nm.

Figure S4. Time-resolved area normalized emission spectra (TRANES) of DHS obtained between 0 (black line) and 15 ns (dark gray line) after excitation with Nanoleds® at 340 and 460 nm, respectively.

Figure S5. Relationship between the specific molar absorptivity (280) of the dissolved humic substances (DHS) with: the emission maxima shift between the final and initial TRANES, Δλ(t), at both excitation wavelengths (e.g. 340 and 460 nm); and with the TRANES isoemissive wavelength, λie, observed by excitation at 460 nm. The MWHS x-scale (gray line) calculated with Eq. (5) is also shows for comparison. DHS abbreviations as in Fig.  1.

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