Separating monovalent anions of similar size and charge, such as acetate (Ac⁻) and chloride (Cl⁻), remains highly challenging in membrane processes. Using molecular dynamics simulations, we explored functionalized graphene surfaces with tunable fractions of protonated amine (-NH₃⁺) and deprotonated carboxyl (-COO⁻) groups (ratios 0:10 to 10:0) in a NaAc/NaCl mixed solution. We identified a charge-regulated selectivity reversal process: -COO⁻-rich, negatively charged surfaces showed only a weak Cl⁻ preference (α_Cl⁻_/Ac⁻ = 1.06–2.71), whereas -NH₃⁺-dominated, positively charged surfaces achieved strong Ac⁻ selectivity, up to 14.86, for an 8:2 -NH₃⁺:-COO⁻ ratio. Decomposing permeability into solubility and diffusivity showed that the Ac⁻ advantage on -NH₃⁺-rich surfaces was more than 80 % diffusion-dominated, whereas the small Cl⁻ preference on -COO⁻ surfaces reflected slight, counterbalancing changes in solubility and diffusivity. Interfacial hydration analysis linked the selectivity to the dehydration difference between Ac⁻ and Cl⁻. The maximum Ac⁻/Cl⁻ selectivity coincided with the largest ΔN_Ac⁻_-Cl⁻ between the bulk and the interface. Together, our work reveals interfacial dehydration-controlled diffusion as the main mechanism for separating Ac⁻ from Cl⁻ on charge-regulated graphene surfaces and offers a quantitative design rule recommending setting the -NH₃⁺:-COO⁻ ratio near 8:2 to optimize membrane functionalization for challenging monovalent anion separations.