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

Abstract

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 (ϕ) and surface acidity ( pK ) for Brønsted-type surfaces. The ϕ, pK 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 ϕ [HM (0.86) > PI (0.61) > CG (0.42)] and higher pK [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 ϕ; while the surface heterogeneity of both wood pyOMs increased, the acidity of HM increased and that of PI decreased. Despite different HTT-induced ϕ and pK 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 and cation exchange capacity) are better cation/metal binding surfaces at pH< pK . The approach also facilitates the evaluation of surface and deprotonation characteristics for mixtures and more complex surfaces.

Biochar Phosphorus Release Is Limited by High pH and Excess Calcium

“…T he distinct physical, chemical, and surface properties of biochar as a matrix have to be considered in understanding and assessing its nutrient value (Farrell et al, 2013; Shepherd et al, 2017; Harvey et al, 2018). The typical high porosity (e.g., 62.5–85.1%; Gray et al, 2014) and high pH of biochar (e.g., pH 7–11; Qambrani et al, 2017) means that extraction methods optimized for soil are not necessarily appropriate for biochar.…”

Structure–Energy–Photochemical Activity Relationships in Fluorophoric Water-Extracted Organic Matter from (Un)charred Plant Materials

“…To the best of our knowledge, the apparent heterocyclic ring-opening mechanism reflected in HM400-derived (vs HM25-derived) w DOM (or its impact on reactivity) has never been reported in previous w DOM studies. However, there is precedence from charcoal studies that point to very different behaviors for leguminous and/or nitrogen-rich feedstocks (like HM) compared to their nonleguminous and/or nitrogen-deficient counterparts. , For instance, Knicker et al found that while charcoals from nonleguminous and/or nitrogen-deficient feedstocks tended toward increased levels of aromatic condensation, higher alkyl C contents are typical of charcoals from leguminous and/or nitrogen-rich feedstocks. Knicker et al further noted that this higher alkyl C content in charcoals from leguminous and/or nitrogen-rich feedstocks would render them more susceptible to biological or chemical oxidation and subsequently dissolution than would be commonly assumed.…”

“…Knicker et al 35 further noted that this higher alkyl C content in charcoals from leguminous and/ or nitrogen-rich feedstocks would render them more susceptible to biological or chemical oxidation and subsequently dissolution than would be commonly assumed. Harvey et al 34 found that while the diversity in R groups attached to the carboxyl decreased in grass charcoals at HTTs between 350 and 650 °C, that in HM charcoals increased over the same HTT range. The difference in the trajectory of R group diversity with charring was attributed to HTT-promoted condensation of the soft tissue-dominated grass feedstock compared to HTT-promoted disintegration of the more thermally resistant, nitrogen-rich HM feedstock.…”

Sorption Dynamics and Energetics of Cinnamic Acid and Its Derivatives at the Ferrihydrite–Water Interface Determined by Flow-Adsorption Microcalorimetry

“…Scanning electron microscope images showed that ferrihydrite grains had an average size of 186 ± 86 μm (Figure S2). The surface charge of ferrihydrite was assessed using the ion-probe flow adsorption microcalorimetry method of Harvey et al 25 Probing experiments were conducted at pH 4, 5, 6, 7, 9, and 10 using K + and Na + as the cationic probes, whereas the anionic probes were Cl − and NO 3 − (Figure S3). Flow Adsorption Microcalorimetry Experiments.…”

“…The similarity in the energetics between Cl/NO 3 (+3.15 J g −1 ) and NO 3 /Cl (−3.10 J g −1 ) is congruent with flow adsorption microcalorimetry studies of simple ion exchange reactions, which are completely reversible and occur via outer-sphere electrostatic interactions of anions (or cations) with metal oxides 23,27 and other charged surfaces. 21,25 The endothermic response of Cl/NO 3 relative to the exothermic response of NO 3 /Cl is reflective of the smaller ionic radius of Cl. 21,28 The unimodal shape and comparable reaction duration (30−35 min) of the Cl/NO 3 and NO 3 /Cl thermograms are also consistent with the simple reaction dynamics of outer-sphere ion exchange.…”