Quantification of Mn oxide transformation using dissolved Mn as a metric may overlook reduced Mn on the solid phase or Mn that has re-oxidize on the mineral surface. Therefore, Xray photoelectron spectroscopy (XPS) data was obtained from the Mn oxide chips before and after microfluidic experiments to determine the contribution of Mn(II), Mn(III) and Mn(IV) by curve fitting of the high-resolution XPS Mn 3p spectra as described by Ilton et al. See Text S4, SI for details on XPS instrumentation and measurement.