Resolving humic and fulvic acids in binary systems influenced by adsorptive fractionation to Fe-(hydr)oxide with focus on UV–Vis analysis

•The UV–Vis spectroscopic method developed can identify the fingerprints of humic and fulvic acids in their mixtures.•Preferential adsorption of acidic polar moieties decreased the specific UV–Vis absorbance of humic and fulvic acids.•The shape of UV–Vis spectra of humic and fulvic acids was little affected by adsorptive fractionation.•The UV–Vis method performs well in quantifying fractionated samples with the absolute error of 5 mgC L−1.