Study on protonation of fulvic acid using differential absorption spectroscopy coupled with Gaussian fitting method

The protonation properties of soil fulvic acid in aquatic systems were investigated using multi-peaks Gaussian fitting method based on UV-Vis differential absorption spectra. At the pH range of 3.0~11.0, the characteristic peaks were located at 235nm, 320nm, 280nm and 360nm in differential absorption spectra of SFA. Six Gaussian peaks were fitted from the differential absorption spectra of SFA (R²>0.983): A0 (211.19nm), A1 [(238.62±1.13) nm], A2 [(274.78±1.50) nm], A3 [(308.31±3.74) nm], A4 [(353.72±3.67) nm], A5 [(419.44±1.64) nm] and A6 [(389.82±2.57) nm]. The changes of the locations and widths of the Gaussian peaks A1~A6 ranged 3.18~10.50nm (σ=1.13~3.74nm) and 3.36~19.08nm (σ=1.33~9.54nm), respectively. The location changes of Gaussian peaks A1, A2, A5 and A6 (3.18~5.50nm), which were induced by the changes of pH, were less than that of Gaussian peaks A3 and A4 (10.50~10.13nm). Moreover, the width changes of Gaussian peaks A1and A2 (3.36~8.49nm), which were induced by the changes of pH, were less than that of Gaussian peaks A3~A6 (10.86~19.08nm). Gaussian peaks A1~A3were related to phenolic, carboxyl and phenolic chromophores, respectively. Gaussian peaks A4~A6 were associated with the changes in molecular conformation and affected by molecular charge transfer and internal reactions between chromophores. The differential absorbance of characteristic peaks, ∆A(235), ∆A(280), ∆A(320) and ∆A(360), in differential absorption spectra had significantly positive relationship with pH (R²=0.855~0.995). The ln∆A(360) in the study could be used to reflect the changes in Gaussian peak A4 with the changes of pH (R²=0.938). The results showed that differential absorption spectroscopy could be applied to investigate the protonation properties of trace fulvic acid in aquatic systems, which provided basis for exploring of the interaction mechanisms between dissolved organic matters and environmental contaminants.