Fig. 1

a XPS of the C 1s region of the DOM shown in black, 8.3 initial C:Mn molar ratio samples in blue, 1.4 initial C:Mn molar ratio sample in yellow, and untreated HMO shown in red. The region is broken into three distinct species: the most oxidized is the C that is double bonded to O labeled as C=O at 288.2 eV, the middle species is labeled as C–O and C–N at 286.1 eV, and the least oxidized carbon is the C–C or C–H carbon at 284.6 eV. b The C atomic percent of each sample based on every element detected with XPS. c The most reduced C species, the C–O and C–N associated C, and the most oxidized C (C=O), each expressed by relative atomic percent of the total C signal as a function of the initial C:Mn molar ratio