@@@@Highlights •Hydrophilic fractions tend to have a higher density of Cu2+ complexation sites compared with hydrophobic acid fractions.•Cu-DOM fractions binding was depressed by Ca2+ and Mg2+, especially for hydrophilic base and hydrophobic acid fractions.•The spectral parameter ΔS225−275 and ΔS275−325 successfully quantified amount of Cu bounded onto HIA and HIB, respectively.•Mg2+ exhibited stronger competition with Cu for hydrophilic acid and base fractions binding sites compared with Ca2+.