A Flow Adsorption Microcalorimetry-Logistic Modeling Approach for Assessing Heterogeneity of Brønsted-Type Surfaces: Application to Pyrogenic Organic Materials
2018
Harvey, Omar R. | Leonce, Burke C. | Herbert, Bruce E.
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 (pKₐ, ₛᵤᵣf) for Brønsted-type surfaces. The ϕₛᵤᵣf, pKₐ, 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 pKₐ, 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 pKₐ, ₛᵤᵣf trajectories, the deprotonation range for all pyOM was pH = pKa,surf±2ϕsurf. Therefore, higher heterogeneity pyOMs deprotonate more readily at lower pH, over a wider range and (for similar pKₐ,ₛᵤᵣf and cation exchange capacity) are better cation/metal binding surfaces at pH<pKₐ,ₛᵤᵣf. The approach also facilitates the evaluation of surface and deprotonation characteristics for mixtures and more complex surfaces.
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