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.

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.

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 (isoTGA), and 5-year in situ bidegradation data to develop energy-biodegradability relationships for a suite of plant- and manure-derived PyOM (n = 10). The net energy value (ΔE) for PyOM was between 4.0 and 175 kJ mol(-1); with manure-derived PyOM having the highest ΔE. Thermal-oxidation activation energy (Ea) requirements ranged from 51 to 125 kJ mol(-1), with wood-derived PyOM having the highest Ea requirements. We propose a return-on-investment (ROI) parameter (ΔE/Ea) 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.

Phosphate alteration of chloride behavior at the boehmite-water interface: New insights from ion-probe flow adsorption microcalorimetry.

Surface complexation of phosphate to aluminum oxyhydroxides can alter surface reactivity depending on the time-scale and mode of attachment. The effects of phosphate adsorption on reactivity of boehmite (γ-AlOOH) particles were investigated using ion-probe flow adsorption microcalorimetry (ipFAMC). Consistent with previous studies on adsorption energetics, probing the surface of pristine γ-AlOOH with chloride ions yielded endothermically unimodal temperature signals with a measured molar heat of exchange (ΔH(exc)) of -3.1 kJ/mol. However, when the surface of γ-AlOOH was probed with chloride following phosphate complexation, significant changes in surface reactivity resulted. Irrespective of phosphate loading, the typical endothermic response of the chloride-surface hydroxyl interaction was replaced with a multi-modal energy signature consisting of exothermic and endothermic features. These features indicate that in the presence of phosphate, the overall nature of the interaction of chloride with specific surface hydroxyls located on different exposed planes and their subsequent reactivity was transformed to a more complex environment accompanied by two or more short-lived secondary reactions. It was also shown that phosphate-promoted surface alteration of γ-AlOOH was highly selective to probing with chloride since no changes in reactivity were observed when nitrate was employed as the primary ion probe under identical experimental conditions.

Geochemical implications of gas leakage associated with geologic CO2 storage--a qualitative review.

Gas leakage from deep storage reservoirs is a major risk factor associated with geologic carbon sequestration (GCS). A systematic understanding of how such leakage would impact the geochemistry of potable aquifers and the vadose zone is crucial to the maintenance of environmental quality and the widespread acceptance of GCS. This paper reviews the current literature and discusses current knowledge gaps on how elevated CO(2) levels could influence geochemical processes (e.g., adsorption/desorption and dissolution/precipitation) in potable aquifers and the vadose zone. The review revealed that despite an increase in research and evidence for both beneficial and deleterious consequences of CO(2) migration into potable aquifers and the vadose zone, significant knowledge gaps still exist. Primary among these knowledge gaps is the role/influence of pertinent geochemical factors such as redox condition, CO(2) influx rate, gas stream composition, microbial activity, and mineralogy in CO(2)-induced reactions. Although these factors by no means represent an exhaustive list of knowledge gaps we believe that addressing them is pivotal in advancing current scientific knowledge on how leakage from GCS may impact the environment, improving predictions of CO(2)-induced geochemical changes in the subsurface, and facilitating science-based decision- and policy-making on risk associated with geologic carbon sequestration.

Reduction of Chromium(VI) mediated by zero-valent magnesium under neutral pH conditions.

In an effort to assess the potential use of ZVMg in contaminant treatments, we examined Cr(VI) reduction mediated by ZVMg particles under neutral pH conditions. The reduction of Cr(VI) was tested with batch experiments by varying [Cr(VI)](0) (4.9, 9.6, 49.9 or 96.9 µM) in the presence of 50 mg/L ZVMg particles ([Mg(0)](0) = 2.06 mM) at pH 7 buffered with 50 mM Na-MOPS. When [Cr(VI)](0) = 4.9 or 9.6 µM, Cr(VI) was completely reduced within 60 min. At higher [Cr(VI)](0) (49.9 or 96.9 µM), by contrast, the reduction became retarded at >120 min likely due to rapid ZVMg dissolution in water and surface precipitation of Cr(III) on ZVMg particles. Surface precipitation was observed only when [Cr(VI)](0) = 49.9 or 96.9 µM and increased with increasing [Cr(VI)](0). The effect of dissolved oxygen was negligible on the rate and extent of Cr(VI) reduction. Experimental results indicated that Cr(VI) was reduced not directly by ZVMg but by reactive intermediates produced from ZVMg-water reaction under the experimental conditions employed in this study. In addition, the observed rates of Cr(VI) reduction appeared to follow an order below unity (0.19) with respect to [Cr(VI)](0). These results imply that ZVMg-mediated Cr(VI) reduction likely occurred via an alternative mechanism to the direct surface-mediated reduction typically observed for other zero-valent metals. Rapid and complete Cr(VI) reduction was achieved when a mass ratio of [ZVMg](0):[Cr(VI)](0) ≥ 100 at neutral pH under both oxic and anoxic conditions. Our results highlights the potential for ZVMg to be used in Cr(VI) treatments especially under neutral pH conditions in the presence of dissolved oxygen.

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-CH(2)-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 R(50), for assessing biochar quality for carbon sequestration is proposed. The R(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 (n = 59), and soot-like black carbons. Comparison of R(50), with biochar physicochemical properties and biochar-C mineralization revealed the existence of a quantifiable relationship between R(50) and biochar recalcitrance. As presented here, the R(50) is immediately applicable to pre-land application screening of biochars into Class A (R(50) ≥ 0.70), Class B (0.50 ≤ R(50) < 0.70) or Class C (R(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 R(50), to an index-based degradation, and an economic model could provide a suitable framework in which to comprehensively assess soil carbon sequestration in biochars.

Metal interactions at the biochar-water interface: energetics and structure-sorption relationships elucidated by flow adsorption microcalorimetry.

Plant-derived biochars exhibit large physicochemical heterogeneity due to variations in biomass chemistry and combustion conditions. However, the influence of biochar heterogeneity on biochar-metal interaction mechanisms has not been systematically described. We used flow adsorption microcalorimetry to study structure-sorption relationships between twelve plant-derived biochars and two metals (K(+) and Cd(2+)) of different Lewis acidity. Irrespective of the biochar structure, sorption of K(+) (a hard Lewis acid) occurred predominantly on deprotonated functional groups via ion exchange with molar heats of adsorption (ΔH(ads)) of -4 kJ mol(-1) to -8 kJ mol(-1). By comparison, although ion exchange could not be completely ruled out, our data pointed to Cd(2+) (a soft Lewis acid) sorption occurring predominantly via two distinct cation-π bonding mechanisms, each with ΔH(ads) of +17 kJ mol(-1). The first, evident in low charge-low carbonized biochars, suggested Cd(2+)-π bonding to soft ligands such as -C ═ O; while the second, evident in low charge-highly carbonized biochars, pointed to Cd(2+)-π bonding with electron-rich domains on aromatic structures. Quantitative contributions of these mechanisms to Cd(2+) sorption can exceed 3 times that expected for ion exchange and therefore could have significant implications for the biogeochemical cycling of metals in fire-impacted or biochar-amended systems.