3. For the basic cases (pH = 9 and pH = 11) the two-reaction model was needed to fit the apparent steady state that was approached. Here the competing species W was taken to be NaOH. The physical interpretation of these results is that ultimately all adsorption sites S will be utilised, some by X, and the remainder by W. The ratio of the amounts of X and W adsorbed depends on the ratio 𝑘 ! /𝑘 " as well as the starting stoichiometric ratios 𝑟 % and 𝑟 & . Further details can be found in the Supplementary Material (Fig. ).