Effects of competing counterions with different charge-to-size ratios on potential-determining ion (pdi; H+, OH-) adsorption at mineral/water interfaces were resolved in mixtures of aqueous solutions of NaCI and NaC10(4) solutions. These effects were monitored on two synthetic goethite (alpha-FeOOH) particle prep-arations with distinct charge uptake capacities arising from differences in surface roughness. Charge development at these mineral surfaces was chiefly explored by high precision potentiometric titrations at 25 C. These measurements confirmed that the greater charge-to-size ratio chloride ion not only promoted greater surface charge, but also had pronounced effects in perchlorate-dominated solutions. Cryogenic X-ray photoelectron spectroscopic measurements confirmed that perchlorate retains signifi-cant loadings at the goethite surface, even in the presence of chloride. Molecular dynamics simulations of the (1 1 0) plane of goethite exposed to these mixed solutions showed that chloride compressed the interfacial region containing electrolyte ions. Perchlorate, on the other hand, is not only present over a thicker region of the interface but also promotes an additional outer-sphere sodium species. These findings were used to develop a thermodynamic adsorption model predicting charge development at these mineral surfaces. The model involves a new formulation accounting for coexisting ion-specific regions each with their distinct compact plane capacitance values. The model can predict charge development in any mixtures of NaCl and NaC104 contacted with goethite particles of contrasting charge uptake capacities without any additional parameters. This model can also be applied to a broader range of material surfaces. (C) 2013 Elsevier Inc. All rights reserved.