A Flow Adsorption Microcalorimetry-Logistic Modeling Approach for Assessing Heterogeneity of Brønsted-Type Surfaces: Application to Pyrogenic Organic Materials.

Biogeochemical functioning of oxides and pyrogenic organic matter ( pyOM) are greatly influenced by surface and deprotonation characteristics. We present an energetics-based, logistic modeling approach for quantifying surface homogeneity (ϕsurf) and surface acidity ( pK a, surf) for Brønsted-type surfaces. The ϕ surf, pK a, surf and associated deprotonation behavior of pyOM were quantified across feedstock (honey mesquite, HM; pine, PI; cord grass, CG) and heat-treatment-temperatures (HTT; 200-650 °C). At HTT200, lower ϕsurf [HM (0.86) > PI (0.61) > CG (0.42)] and higher pK a, surf [CG (4.4) > PI (4.2) > HM (4.1)] for CG indicated higher heterogeneity and lower acidity for Brønsted-type surface moieties on grass versus wood pyOM. Surface acidity of CG increased at HTT550/650 °C with no effect on ϕsurf; while the surface heterogeneity of both wood pyOMs increased, the acidity of HM increased and that of PI decreased. Despite different HTT-induced ϕsurf and pK a, surf trajectories, the deprotonation range for all pyOM was pH = [Formula: see text]. Therefore, higher heterogeneity pyOMs deprotonate more readily at lower pH, over a wider range and (for similar pK a,surf and cation exchange capacity) are better cation/metal binding surfaces at pH< pK a,surf. The approach also facilitates the evaluation of surface and deprotonation characteristics for mixtures and more complex surfaces.