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 (<i>py</i>OM) are greatly influenced by surface and deprotonation characteristics. We present an energetics-based, logistic modeling approach for quantifying surface homogeneity (ϕ_surf) and surface acidity (<i>p</i>K_{<i>a</i>, surf}) for Brønsted-type surfaces. The ϕ_<i>surf</i>, <i>p</i>K_{<i>a</i>, surf} and associated deprotonation behavior of <i>py</i>OM were quantified across feedstock (honey mesquite, HM; pine, PI; cord grass, CG) and heat-treatment-temperatures (HTT; 200–650 °C). At HTT200, lower ϕ_surf [HM (0.86) > PI (0.61) > CG (0.42)] and higher <i>p</i>K_{<i>a</i>, 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 <i>py</i>OM. Surface acidity of CG increased at HTT550/650 °C with no effect on ϕ_surf; while the surface heterogeneity of both wood <i>py</i>OMs increased, the acidity of HM increased and that of PI decreased. Despite different HTT-induced ϕ_surf and <i>p</i>K_{<i>a</i>, surf} trajectories, the deprotonation range for all <i>py</i>OM was pH = pKa,surf±2ϕsurf. Therefore, higher heterogeneity <i>py</i>OMs deprotonate more readily at lower pH, over a wider range and (for similar <i>p</i>K_{<i>a</i>,surf} and cation exchange capacity) are better cation/metal binding surfaces at pH<<i>p</i>K_{<i>a</i>,surf}. The approach also facilitates the evaluation of surface and deprotonation characteristics for mixtures and more complex surfaces

Generalized Two-Dimensional Perturbation Correlation Infrared Spectroscopy Reveals Mechanisms for the Development of Surface Charge and Recalcitrance in Plant-Derived Biochars

Fundamental knowledge of how biochars develop surface-charge and resistance to environmental degradation is crucial to their production for customized applications or understanding their functions in the environment. Two-dimensional perturbation-based correlation infrared spectroscopy (2D-PCIS) was used to study the biochar formation process in three taxonomically different plant biomass, under oxygen-limited conditions along a heat-treatment-temperature gradient (HTT; 200–650 °C). Results from 2D-PCIS pointed to the systematic, HTT-induced defragmenting of lignocellulose H-bonding network and demethylenation/demethylation, oxidation, or dehydroxylation/dehydrogenation of lignocellulose fragments as the primary reactions controlling biochar properties along the HTT gradient. The cleavage of OH^...O-type H-bonds, oxidation of free primary hydroxyls to carboxyls (carboxylation; HTT ≤ 500 °C), and their subsequent dehydrogenation/dehydroxylation (HTT > 500 °C) controlled surface charge on the biochars; while the dehydrogenation of methylene groups, which yielded increasingly condensed structures (R–CH₂–R →RCH–R →RCR), controlled biochar recalcitrance. Variations in biochar properties across plant biomass type were attributable to taxa-specific transformations. For example, apparent inefficiencies in the cleavage of wood-specific H-bonds, and their subsequent oxidation to carboxyls, lead to lower surface charge in wood biochars (compared to grass biochars). Both nontaxa and taxa-specific transformations highlighted by 2D-PCIS could have significant implications for biochar functioning in fire-impacted or biochar-amended systems

An Index-Based Approach to Assessing Recalcitrance and Soil Carbon Sequestration Potential of Engineered Black Carbons (Biochars)

The ability of engineered black carbons (or biochars) to resist abiotic and, or biotic degradation (herein referred to as recalcitrance) is crucial to their successful deployment as a soil carbon sequestration strategy. A new recalcitrance index, the <i>R</i>₅₀, for assessing biochar quality for carbon sequestration is proposed. The <i>R</i>₅₀ is based on the relative thermal stability of a given biochar to that of graphite and was developed and evaluated with a variety of biochars (<i>n</i> = 59), and soot-like black carbons. Comparison of <i>R</i>₅₀, with biochar physicochemical properties and biochar-C mineralization revealed the existence of a quantifiable relationship between <i>R</i>₅₀ and biochar recalcitrance. As presented here, the <i>R</i>₅₀ is immediately applicable to pre-land application screening of biochars into Class A (<i>R</i>₅₀ ≥ 0.70), Class B (0.50 ≤ <i>R</i>₅₀ < 0.70) or Class C (<i>R</i>₅₀ < 0.50) recalcitrance/carbon sequestration classes. Class A and Class C biochars would have carbon sequestration potential comparable to soot/graphite and uncharred plant biomass, respectively, whereas Class B biochars would have intermediate carbon sequestration potential. We believe that the coupling of the <i>R</i>_<i>50</i>, to an index-based degradation, and an economic model could provide a suitable framework in which to comprehensively assess soil carbon sequestration in biochars

Modeling the Role in pH on Contaminant Sequestration by Zerovalent Metals: Chromate Reduction by Zerovalent Magnesium

The role of pH in sequestration of Cr(VI) by zerovalent magnesium (ZVMg) was characterized by global fitting of a kinetic model to time-series data from unbuffered batch experiments with varying initial pH values. At initial pH values ranging from 2.0 to 6.8, ZVMg (0.5 g/L) completely reduced Cr(VI) (18.1 μM) within 24 h, during which time pH rapidly increased to a plateau value of ∼10. Time-series correlation analysis of the pH and aqueous Cr(VI), Cr(III), and Mg(II) concentration data suggested that these conditions are controlled by combinations of reactions (involving Mg0 oxidative dissolution and Cr(VI) sequestration) that evolve over the time course of each experiment. Since this is also likely to occur during any engineering applications of ZVMg for remediation, we developed a kinetic model for dynamic pH changes coupled with ZVMg corrosion processes. Using this model, the synchronous changes in Cr(VI) and Mg(II) concentrations were fully predicted based on the Langmuir–Hinshelwood kinetics and transition-state theory, respectively. The reactivity of ZVMg was different in two pH regimes that were pH-dependent at pH < 4 and pH-independent at the higher pH. This contrasting pH effect could be ascribed to the shift of the primary oxidant of ZVMg from H+ to H2O at the lower and higher pH regimes, respectively

Discrimination in Degradability of Soil Pyrogenic Organic Matter Follows a Return-On-Energy-Investment Principle

A fundamental understanding of biodegradability is central to elucidating the role(s) of pyrogenic organic matter (PyOM) in biogeochemical cycles. Since microbial community and ecosystem dynamics are driven by net energy flows, then a quantitative assessment of energy value versus energy requirement for oxidation of PyOM should yield important insights into their biodegradability. We used bomb calorimetry, stepwise isothermal thermogravimetric analysis (<i>iso</i>TGA), and 5-year in situ bidegradation data to develop energy-biodegradability relationships for a suite of plant- and manure-derived PyOM (<i>n</i> = 10). The net energy value (Δ<i><i>E</i></i>) for PyOM was between 4.0 and 175 kJ mol^–1; with manure-derived PyOM having the highest Δ<i><i>E</i></i>. Thermal-oxidation activation energy (<i>E</i>_a) requirements ranged from 51 to 125 kJ mol^–1, with wood-derived PyOM having the highest <i>E</i>_a requirements. We propose a return-on-investment (ROI) parameter (Δ<i><i>E</i>/E</i>_a) for differentiating short-to-medium term biodegradability of PyOM and deciphering if biodegradation will most likely proceed via cometabolism (ROI < 1) or direct metabolism (ROI ≥ 1). The ROI-biodegradability relationship was sigmoidal with higher biodegradability associated with PyOM of higher ROI; indicating that microbes exhibit a higher preference for “high investment value” PyOM