Carbon and Nitrogen Isotope Fractionations During Biotic and Abiotic Transformations of 2,4-Dinitroanisole (DNAN).

2,4-Dinitroanisole (DNAN) is a main constituent in various new insensitive munition formulations. Although DNAN is susceptible to biotic and abiotic transformations, in many environmental instances, transformation mechanisms are difficult to resolve, distinguish, or apportion on the basis solely of analysis of concentrations. We used compound-specific isotope analysis (CSIA) to investigate the characteristic isotope fractionations of the biotic (by three microbial consortia and three pure cultures) and abiotic (by 9,10-anthrahydroquinone-2-sulfonic acid [AHQS]) transformations of DNAN. The correlations of isotope enrichment factors (ΛN/C) for biotic transformations had a range of values from 4.93 ± 0.53 to 12.19 ± 1.23, which is entirely distinct from ΛN/C values reported previously for alkaline hydrolysis, enzymatic hydrolysis, reduction by Fe2+-bearing minerals and iron-oxide-bound Fe2+, and UV-driven phototransformations. The ΛN/C value associated with the abiotic reduction by AHQS was 38.76 ± 2.23, within the range of previously reported values for DNAN reduction by Fe2+-bearing minerals and iron-oxide-bound Fe2+, albeit the mean ΛN/C was lower. These results enhance the database of isotope effects accompanying DNAN transformations under environmentally relevant conditions, allowing better evaluation of the extents of biotic and abiotic transformations of DNAN that occur in soils, groundwaters, surface waters, and the marine environment.

Pyrogenic Black Carbon Suppresses Microbial Methane Production by Serving as a Terminal Electron Acceptor.

Methane (CH4) is the second most important greenhouse gas, 27 times as potent as CO2 and responsible for >30% of the current anthropogenic warming. Globally, more than half of CH4 is produced microbially through methanogenesis. Pyrogenic black carbon possesses a considerable electron storage capacity (ESC) and can be an electron donor or acceptor for abiotic and microbial redox transformation. Using wood-derived biochar as a model black carbon, we demonstrated that air-oxidized black carbon served as an electron acceptor to support anaerobic oxidation of organic substrates, thereby suppressing CH4 production. Black carbon-respiring bacteria were immediately active and outcompeted methanogens. Significant CH4 did not form until the bioavailable electron-accepting capacity of the biochar was exhausted. An experiment with labeled acetate (13CH3COO-) yielded 1:1 13CH4 and 12CO2 without biochar and predominantly 13CO2 with biochar, indicating that biochar enabled anaerobic acetate oxidation at the expense of methanogenesis. Methanogens were enriched following acetate fermentation but only in the absence of biochar. The electron balance shows that approximately half (∼2.4 mmol/g) of biochar's ESC was utilized by the culture, corresponding to the portion of the ESC > +0.173 V (vs SHE). These results provide a mechanistic basis for quantifying the climate impact of black carbon and developing ESC-based applications to reduce CH4 emissions from biogenic sources.

Linear Free Energy Relationship for Predicting the Rate Constants of Munition Compound Reduction by the Fe(II)-Hematite and Fe(II)-Goethite Redox Couples.

Abiotic reduction by iron minerals is arguably the most important fate process for munition compounds (MCs) in subsurface environments. No model currently exists that can predict the abiotic reduction rates of structurally diverse MCs by iron (oxyhydr)oxides. We performed batch experiments to measure the rate constants for the reduction of three classes of MCs (poly-nitroaromatics, nitramines, and azoles) by hematite or goethite in the presence of aqueous Fe2+. The surface area-normalized reduction rate constant (kSA) depended on the aqueous-phase one-electron reduction potential (EH1) of the MC and the thermodynamic state (i.e., pe and pH) of the iron oxide-Feaq2+ system. A linear free energy relationship (LFER), similar to that reported previously for nitrobenzene, successfully captures all MC reduction rate constants that span 6 orders of magnitude: log(kSA)=(1.12±0.04)[0.53EH159mV-(pH+pe)]+(5.52±0.23). The finding that the rate constants of all the different classes of MCs can be described by a single LFER suggests that these structurally diverse nitro compounds are reduced by iron oxide-Feaq2+ couples through a common mechanism up to the rate-limiting step. Multiple mechanistic implications of the results are discussed. This study expands the applicability of the LFER model for predicting the reduction rates of legacy and emerging MCs and potentially other nitro compounds.

Thermodynamic Two-Site Surface Reaction Model for Predicting Munition Constituent Reduction Kinetics with Iron (Oxyhydr)oxides.

Iron (oxyhydr)oxides comprise a significant portion of the redox-active fraction of soils and are key reductants for remediation of sites contaminated with munition constituents (MCs). Previous studies of MC reduction kinetics with iron oxides have focused on the concentration of sorbed Fe(II) as a key parameter. To build a reaction kinetic model, it is necessary to predict the concentration of sorbed Fe(II) as a function of system conditions and the redox state. A thermodynamic framework is formulated that includes a generalized double-layer model that utilizes surface acidity and surface complexation reactions to predict sorbed Fe(II) concentrations that are used for fitting MC reduction kinetics. Monodentate- and bidentate Fe(II)-binding sites are used with individual oxide sorption characteristics determined through data fitting. Results with four oxides (goethite, hematite, lepidocrocite, and ferrihydrite) and four nitro compounds (NB, CN-NB, Cl-NB, and NTO) from six separate studies have shown good agreement when comparing observed and predicted surface area-normalized rate constants. While both site types are required to reproduce the experimental redox titration, only the monodentate site concentration controls the MC reaction kinetics. This model represents a significant step toward predicting the timescales of MC degradation in the subsurface.

Electron Transfer Energy and Hydrogen Atom Transfer Energy-Based Linear Free Energy Relationships for Predicting the Rate Constants of Munition Constituent Reduction by Hydroquinones.

No single linear free energy relationship (LFER) exists that can predict reduction rate constants of all munition constituents (MCs). To address this knowledge gap, we measured the reduction rates of MCs and their surrogates including nitroaromatics [NACs; 2,4,6-trinitrotoluene (TNT), 2,4-dinitroanisole (DNAN), 2-amino-4,6-dinitrotoluene (2-A-DNT), 4-amino-2,6-dinitrotoluene (4-A-DNT), and 2,4-dinitrotoluene (DNT)], nitramines [hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and nitroguanidine (NQ)], and azoles [3-nitro-1,2,4-triazol-5-one (NTO) and 3,4-dinitropyrazole (DNP)] by three dithionite-reduced quinones (lawsone, AQDS, and AQS). All MCs/NACs were reduced by the hydroquinones except NQ. Hydroquinone and MC speciations were varied by controlling pH, permitting the application of a speciation model to determine second-order rate constants (k) from observed pseudo-first-order rate constants. The intrinsic reactivity of MCs (oxidants) decreased upon deprotonation, while the opposite was true for hydroquinones (reductants). The rate constants spanned ∼6 orders of magnitude in the order NTO ≈ TNT > DNP > DNT ≈ DNAN ≈ 2-A-DNT > DNP- > 4-A-DNT > NTO- > RDX. LFERs developed using density functional theory-calculated electron transfer and hydrogen atom transfer energies and reported one-electron reduction potentials successfully predicted k, suggesting that these structurally diverse MCs/NACs are all reduced by hydroquinones through the same mechanism and rate-limiting step. These results increase the applicability of LFER models for predicting the fate and half-lives of MCs and related nitro compounds in reducing environments.

Modeling the Reduction Kinetics of Munition Compounds by Humic Acids.

Dissolved organic matter (DOM) comprises a sizeable portion of the redox-active constituents in the environment and is an important reductant for the abiotic transformation of nitroaromatic compounds and munition constituents (NACs/MCs). Building a predictive kinetic model for these reactions would require the energies associated with both the reduction of the NACs/MCs and the oxidation of the DOM. The heterogeneous and unknown structure of DOM, however, has prohibited reliable determination of its oxidation energies. To overcome this limitation, humic acids (HAs) were used as model DOM, and their redox moieties were modeled as a collection of quinones of different redox potentials. The reduction and oxidation energies of the NACs/MCs and hydroquinones, respectively, via hydrogen atom transfer (HAT) reactions were then calculated quantum chemically. HAT energies have been used successfully in a linear free energy relationship (LFER) to predict second-order rate constants for NAC reduction by hydroquinones. Furthermore, a linear relationship between the HAT energies and the reduction potentials of quinones was established, which allows estimation of hydroquinone reactivity (i.e., rate constants) from HA redox titration data. A training set of three HAs and two NACs/MCs was used to generate a mean HA redox profile that successfully predicted reduction kinetics in multiple HA/MC systems.

Abiotic reduction of 3-nitro-1,2,4-triazol-5-one (NTO) and other munitions constituents by wood-derived biochar through its rechargeable electron storage capacity.

The environmental fate of 3-nitro-1,2,4-triazol-5-one (NTO) and other insensitive munitions constituents (MCs) is of significant concern due to their high water solubility and mobility relative to legacy MCs. Plant-based biochars have been shown to possess a considerable electron storage capacity (ESC), which enables them to undergo reversible electron transfer reactions. We hypothesized biochar can act as a rechargeable electron donor to effect abiotic reduction of MCs repeatedly through its ESC. To test this hypothesis, MC reduction experiments were performed using wood-derived biochars that were oxidized with dissolved oxygen or reduced with dithionite. Removal of aqueous NTO, an anion at circumneutral pH, by oxidized biochar was minimal and occurred through reversible adsorption. In contrast, NTO removal by reduced biochar was much more pronounced and occurred predominantly through reduction, with concomitant formation of 3-amino-1,2,4-triazol-5-one (ATO). Mass balance and electron recovery with ferricyanide further showed that (1) the amount of NTO reduced to ATO was relatively constant (85-100 µmol per gram of biochar) at pH 6-10; (2) the fraction of biochar ESC reactive toward NTO was ca. 30% of that toward ferricyanide; (3) the NTO-reactive fraction of the ESC was regenerable over multiple redox cycles. We also evaluated biochar transformation of other MCs, including nitroguanidine (NQ), 2,4-dinitroanisole (DNAN), and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). While mass and electron balances could not be established due to sorption, DNAN and RDX reduction by reduced biochar was confirmed via detection of multiple reduction products. In contrast, NQ was not reduced under any of the conditions tested. This study is the first demonstration of organic contaminant degradation through biochar's rechargeable ESC. Our results indicate biochar is a regenerable electron storage medium and sorbent that can remove MCs from water through concurrent reduction and sorption, and is thus potentially useful for pollution control and remediation at military facilities.

Reductive Transformation of 3-Nitro-1,2,4-triazol-5-one (NTO) by Leonardite Humic Acid and Anthraquinone-2,6-disulfonate (AQDS).

3-Nitro-1,2,4-triazol-5-one (NTO) is a major and the most water-soluble constituent in the insensitive munition formulations IMX-101 and IMX-104. While NTO is known to undergo redox reactions in soils, its reaction with soil humic acid has not been evaluated. We studied NTO reduction by anthraquinone-2,6-disulfonate (AQDS) and Leonardite humic acid (LHA) reduced with dithionite. Both LHA and AQDS reduced NTO to 3-amino-1,2,4-triazol-5-one (ATO), stoichiometrically at alkaline pH and partially (50-60%) at pH ≤ 6.5. Due to NTO and hydroquinone speciation, the pseudo-first-order rate constants (kObs) varied by 3 orders of magnitude from pH 1.5 to 12.5 but remained constant from pH 4 to 10. This distinct pH dependency of kObs suggests that NTO reactivity decreases upon deprotonation and offsets the increasing AQDS reactivity with pH. The reduction of NTO by LHA deviated continuously from first-order behavior for >600 h. The extent of reduction increased with pH and LHA electron content, likely due to greater reactivity of and/or accessibility to hydroquinone groups. Only a fraction of the electrons stored in LHA was utilized for NTO reduction. Electron balance analysis and LHA redox potential profile suggest that the physical conformation of LHA kinetically limited NTO access to hydroquinone groups. This study demonstrates the importance of carbonaceous materials in controlling the environmental fate of NTO.

A Unified Linear Free Energy Relationship for Abiotic Reduction Rate of Nitroaromatics and Hydroquinones Using Quantum Chemically Estimated Energies.

Determining the fate of nitroaromatic compounds (NACs) in the environment requires the use of predictive models for compounds and conditions for which experimental data are insufficient. Previous studies have developed linear free energy relationships (LFERs) that relate the thermodynamic energy of NAC reduction to its corresponding rate constant. We present a comprehensive LFER that incorporates both the reduction and oxidation half-reactions through quantum chemically calculated energies. Environ Toxicol Chem 2020;39:2389-2395. © 2020 SETAC.

Reduction of 3-Nitro-1,2,4-Triazol-5-One (NTO) by the Hematite-Aqueous Fe(II) Redox Couple.

3-Nitro-1,2,4-triazol-5-one (NTO) is an insensitive munition compound (MC) that has replaced legacy MC. NTO can be highly mobile in soil and groundwater due to its high solubility and anionic nature, yet little is known about the processes that control its environmental fate. We studied NTO reduction by the hematite-Fe2+ redox couple to assess the importance of this process for the attenuation and remediation of NTO. Fe2+(aq) was either added (type I) or formed through hematite reduction by dithionite (type II). In the presence of both hematite and Fe2+(aq), NTO was quantitatively reduced to 3-amino-1,2,4-triazol-5-one following first-order kinetics. The surface area-normalized rate constant (kSA) showed a strong pH dependency between 5.5 and 7.0 and followed a linear free energy relationship (LFER) proposed in a previous study for nitrobenzene reduction by iron oxide-Fe2+ couples, i.e., log kSA = -(pe + pH) + constant. Sulfite, a major dithionite oxidation product, lowered kSA in type II system by ∼10-fold via at least two mechanisms: by complexing Fe2+ and thereby raising pe, and by making hematite more negatively charged and hence impeding NTO adsorption. This study demonstrates the importance of iron oxide-Fe2+ in controlling NTO transformation, presents an LFER for predicting NTO reduction rate, and illustrates how solutes can shift the LFER by interacting with either iron species.

Hydrogen Atom Transfer Reaction Free Energy as a Predictor of Abiotic Nitroaromatic Reduction Rate Constants: A Comprehensive Analysis.

A linear free energy model is presented that predicts the second-order rate constant for the abiotic reduction of nitroaromatic compounds (NACs). Previously presented models use the one-electron reduction potential EH1(ArNO2) of the NAC reaction ArNO2+e-→ArNO2•- . If EH1(ArNO2) is not available, it has been proposed that EH1(ArNO2) be computed directly or estimated from the gas-phase electron affinity (EA). The model proposed uses the Gibbs free energy of the hydrogen atom transfer (HAT) reaction ArNO2+H•→ArNOOH• as the parameter in the linear free energy model. Both models employ quantum chemical computations for the required thermodynamic energies. The available and proposed models are compared using experimentally determined second-order rate constants from 5 investigations from the literature in which a variety of NACs were exposed to a variety of reductants. A comprehensive analysis utilizing all the NACs and reductants demonstrate that the HAT energy model and the experimental one-electron reduction potential model have similar root mean square errors and residual error probability distributions. In contrast, the model using the computed EA has a more variable residual error distribution with a significant number of outliers. The results suggest that a linear free energy model utilizing computed HAT reaction free energy produces a more reliable prediction of the NAC abiotic reduction second-order rate constant than previously available methods. The advantages of the proposed HAT energy model and its mechanistic implications are discussed as well. Environ Toxicol Chem 2020;39:1678-1684. © 2020 SETAC.

Visualizing the distribution of black carbon's electron storage capacity using silver.

We have developed a method that combines chemical reduction, silver tagging, and electron microscopy (EM) for visualizing the electron storage capacity (ESC) of black carbon (BC). ESC is a BC's capacity to store and reversibly exchange electrons with abiotic and microbial agents, processes that are relevant to biochemistry, greenhouse gas production, contaminant fate, and remediation. In addition to the amount of electrons BC can store, the locations and spatial distribution of ESC on and inside biochar are critical for understanding the bioaccessibility of ESC and the kinetics of redox reactions involving BC. To locate the ESC in a BC particle, we fully reduced a BC, removed excess reductant, and applied silver ion (Ag+) as a tagging agent that diffused into BC to react with functional groups where electrons were stored (i.e., ESC) to form silver nanoparticles (nAg). The nAg deposited on and inside BC were then imaged using multiple EM techniques to visualize the locations and distribution of the ESC. The method is a new and potentially useful tool for investigating ESC production and for elucidating BC-mediated redox transformation.•Novel method to probe and assess the distribution of ESC on/within BC.•Visual confirmation of significant ESC both on the surface and in the interior of BC.•A new method to incorporate silver or other redox-sensitive elements into a carbon medium.

Visualizing electron storage capacity distribution in biochar through silver tagging.

Electron storage capacity (ESC) is the capacity of a black carbon to store and reversibly donate and accept electrons in redox processes. Electrochemical and chemical analyses have shown the ESC of black carbon (e.g., plant-based biochars) was on the order of a few mmol/g. However, it remains unknown where ESC is located. The spatial distribution of ESC is important because it controls the bioaccessibility of ESC and the rates of biochar redox reactions. Here we used silver to tag the ESC of a wood-derived biochar. Ag+ was allowed to diffuse into the pores of reduced biochar at a constant pH. Up to 2.49 mmol Ag+/g biochar (corresponding to 62% of its ESC) was reduced to Ago nanoparticles (nAg), which served as an ESC marker and was visualized by electron microscopy. Abundant and dense nAg were observed on the biochar surface. In addition, microtomed samples showed ubiquitous and well-dispersed nAg in the interior of biochar, which explains pore diffusion-limited redox reactions and the partial bioaccessibility of its ESC. In addition to probing ESC distribution in black carbon, this method represents a new, ESC-based approach to incorporate large quantities of Ag and other redox-active elements into carbon media for potential environmental applications.

Experimental Validation of Hydrogen Atom Transfer Gibbs Free Energy as a Predictor of Nitroaromatic Reduction Rate Constants.

Nitroaromatic compounds (NACs) are a class of prevalent contaminants. Abiotic reduction is an important fate process that initiates NAC degradation in the environment. Many linear free energy relationship (LFER) models have been developed to predict NAC reduction rates. Almost all LFERs to date utilize experimental aqueous-phase one-electron reduction potential ( EH1) of NAC as a predictor, and thus, their utility is limited by the availability of EH1 data. A promising new approach that utilizes computed hydrogen atom transfer (HAT) Gibbs free energy instead of EH1 as a predictor was recently proposed. In this study, we evaluated the feasibility of HAT energy for predicting NAC reduction rate constants. Using dithionite-reduced quinones, we measured the second-order rate constants for the reduction of seven NACs by three hydroquinones of different protonation states. We computed the gas-phase energies for HAT and electron affinity (EA) of NACs and established HAT- and EA-based LFERs for six hydroquinone species. The results suggest that HAT energy is a reliable predictor of NAC reduction rate constants and is superior to EA. This is the first independent, experimental validation of HAT-based LFER, a new approach that enables rate prediction for a broad range of structurally diverse NACs based solely on molecular structures.

Zerovalent iron-sand filtration can reduce the concentration of multiple antimicrobials in conventionally treated reclaimed water.

Irrigation with reclaimed water is increasing in areas that lack access to, and infrastructure for, high-level treatment and distribution. Antimicrobial residues are known to persist in conventionally treated reclaimed water, necessitating the investigation of reuse site-based mitigation options to further reduce these contaminants. We examined the effectiveness of a 50:50 volume/volume, particle matched, micro-scale zerovalent iron (ZVI)-sand filter in reducing concentrations of mixtures of antimicrobials present in pH-unadjusted conventionally treated reclaimed water. Twelve antimicrobials (azithromycin, ciprofloxacin, erythromycin, linezolid, oxacillin, oxolinic acid, penicillin G, pipemidic acid, sulfamethoxazole, triclocarban, tetracycline and vancomycin) were quantified using high performance-liquid chromatography-tandem mass spectrometry in reclaimed water, and ZVI-sand filtered reclaimed water, in a two-month long greenhouse-based experiment. Data were analyzed using a non-parametric rank-based approach. ZVI-sand filtration significantly reduced concentrations of azithromycin, ciprofloxacin, oxolinic acid, penicillin G, sulfamethoxazole, linezolid, pipemidic acid and vancomycin. Azithromycin, the antimicrobial with the highest median concentration (320 ng/L), was reduced to below the limit of detection after ZVI-sand filtration. Inorganic element (antimony, beryllium, cadmium, chromium, iron, lead, selenium and thallium) and water quality (free and total chlorine, nitrates, nitrites, pH and total dissolved solids) analyses showed that ZVI-sand filtered reclaimed water quality (nitrate, salinity, and inorganic elements) met the recommended guidelines for agricultural irrigation with reclaimed water. Based on our initial results, ZVI-sand filtration may be a promising basis for a point-of-use filtration system for reclaimed water irrigation on small-scale farms.

A pilot-scale, bi-layer bioretention system with biochar and zero-valent iron for enhanced nitrate removal from stormwater.

Nitrogen (N) removal in conventional bioretention systems is highly variable owing to the low nitrate (NO3-) elimination efficiency. We hypothesized that amending bioretention cells with biochar and zero-valent iron (ZVI) could improve the NO3- removal performance. A well-instrumented, bi-layer pilot-scale bioretention cell was developed to test the hypothesis by investigating its hydrologic performance and NO3- removal efficacy as affected by biochar and ZVI amendments. The cell containing 18% (v/v) wood biochar in the vadose zone and 10% (v/v) ZVI in the saturation zone was monitored for 18 months of field infiltration tests using synthetic stormwater amended with bromide (tracer) and NO3-. Compared to the Control cell without amendments, the Biochar/ZVI cell increased water retention by 11-27% and mean residence time by 0.7-3.8 h. The vadose zone of the Biochar/ZVI cell removed 30.6-95.7% (0.6-12.7 g) of NO3-N from the influent, as compared with -6.1-89.6% (-0.1-2.9 g) by that of the Control cell. While the performance varied with synthetic storm events and seasons, in all cases the Biochar/ZVI cell resulted in greater NO3- removal than the Control cell. This improvement was presumably due to biochar's ability to improve water retention, facilitate anoxic conditions, increase residence time, and provide electrons for microbial denitrification. The saturation zone with ZVI amendment further promoted NO3- removal: removal was 1.8 times greater relative to the control in the first infiltration test, but was minimal in following tests. The reduction in performance of the ZVI amendment in subsequent tests might be due to diminished NO3-N input to the saturation zone after treatment by the biochar-amended vadose zone. The redox potential and dissolved oxygen content at the vadose/saturation zone interface also indicated more favorable denitrification conditions in the Biochar/ZVI cell. Biochar amendment demonstrated significant promise for increasing nitrate removal in bioretention systems.

New methods for assessing electron storage capacity and redox reversibility of biochar.

Black carbon such as biochar has been shown to support microbial redox transformation by accepting and/or donating electrons. Electron storage capacity (ESC) is an important property that determines the capacity of a biochar to mediate redox processes in natural and engineered systems. However, it remained unclear whether a biochar's ESC is constant and reversible and if so to what extent, over what redox potential range ESC is distributed, and what fraction of the ESC is microbially accessible. In this study, we developed chemical methods that employed combinations of reductants and oxidants of different potentials - Ti(III) citrate, ferricyanide, dithionite, and dissolved O2 - to measure the ESC of Soil Reef biochar, a wood-derived biochar that can serve as an electron donor or acceptor for Geobacter metallireducens. For a given oxidant-reductant pair, the ESC obtained over multiple redox cycles was constant and fully reversible, though lower than that of the virgin biochar. Pore diffusion within biochar particles was rate-limiting and controlled the timescale for redox equilibrium. Results suggest that redox-facile functional groups in biochar were distributed over a broad range of potentials. The ESC measured using dithionite indicates approximately 22% of the biochar's reversible ESC was accessible to G. metallireducens. We propose that reversible ESC may be regarded as a constant and quantifiable property of black carbon.

Chemical methods for determining the electron storage capacity of black carbon.

Electron storage capacity (ESC) is a new and important property that determines the capacity of a black carbon to mediate abiotic and microbial electron transfer reactions in natural and engineered systems. It is necessary to develop accurate and reproducible methods to measure black carbon's ESC in order to understand its redox behavior and to predict its capacity to support redox transformation of contaminants in subsurface environments. In this study, we developed chemical methods that employed combinations of reductants and oxidants of different redox potentials - Ti(III) citrate or dithionite as reductant, and ferricyanide or dissolved O2 as oxidant - to measure the ESC of a wood-derived biochar. Pore diffusion within biochar particles was rate-limiting and controlled the timescale for redox equilibrium and ESC measurements. •The new methods can handle sample mass on the order of a gram•Sample pretreatment (e.g., oxidation via aeration) is necessary to produce consistent results•For a given reductant-oxidant pair, colorimetric (or potentiometric) measurements gave constant and reproducible ESC.

Nutrient release and ammonium sorption by poultry litter and wood biochars in stormwater treatment.

The feasibility of using biochar as a filter medium in stormwater treatment facilities was evaluated with a focus on ammonium retention. Successive batch extractions and batch ammonium sorption experiments were conducted in both deionized (DI) water and artificial stormwater using poultry litter (PL) and hardwood (HW) biochars pyrolyzed at 400°C and 500°C. No measureable nitrogen leached from HW biochars except 0.07 µmol/g of org-N from 400°C HW biochar. PL biochar pyrolyzed at 400°C leached 120-127 µmol/g of nitrogen but only 7.1-8.6 µmol/g of nitrogen when pyrolyzed at 500°C. Ammonium sorption was significant for all biochars. At a typical ammonium concentration of 2mg/L in stormwater, the maximum sorption was 150 mg/kg for PL biochar pryolyzed at 400°C. In stormwater, ion competition (e.g. Ca(2+)) suppressed ammonium sorption compared to DI water. Surprisingly, ammonium sorption was negatively correlated to the BET surface area of the tested biochars, but increased linearly with cation exchange capacity. Cation exchange capacity was the primary mechanism controlling ammonium sorption and was enhanced by pyrolysis at 400°C, while BET surface area was enhanced by pyrolysis at 500°C. The optimal properties (BET surface area, CEC, etc.) of biochar as a sorbent are not fixed but depend on the target pollutant. Stormwater infiltration column experiments in sand with 10% biochar removed over 90% of ammonium with influent ammonium concentration of 2mg/L, compared to only 1.7% removal in a sand-only column, indicating that kinetic limitations on sorption were minor for the storm conditions studied. Hardwood and poultry litter biochar pyrolyzed at 500°C and presumably higher temperature may be viable filter media for stormwater treatment facilities, as they showed limited release of organic and inorganic nutrients and acceptable ammonium sorption.

Phosphorus release behaviors of poultry litter biochar as a soil amendment.

Phosphorus (P) may be immobilized and consequently the runoff loss risks be reduced if poultry litter (PL) is converted into biochar prior to land application. Laboratory studies were conducted to examine the water extractability of P in PL biochar and its release kinetics in amended soils. Raw PL and its biochar produced through 400°C pyrolysis were extracted with deionized water under various programs and measured for water extractable P species and contents. The materials were further incubated with a sandy loam at 20 g kg(-1) soil and intermittently leached with water for 30 days. The P release kinetics were determined from the P recovery patterns in the water phase. Pyrolysis elevated the total P content from 13.7 g kg(-1) in raw PL to 27.1 g kg(-1) in PL biochar while reduced the water-soluble P level from 2.95 g kg(-1) in the former to 0.17 g kg(-1) in the latter. The thermal treatment transformed labile P in raw PL to putatively Mg/Ca phosphate minerals in biochar that were water-unextractable yet proton-releasable. Orthophosphate was the predominant form of water-soluble P in PL biochar, with condensed phosphate (e.g., pyrophosphate) as a minor form and organic phosphate in null. Release of P from PL biochar in both water and neutral soils was at a slower and steadier rate over a longer time period than from raw PL. Nevertheless, release of P from biochar was acid-driven and could be greatly promoted by the media acidity. Land application of PL biochar at soil pH-incorporated rates and frequency will potentially reduce P losses to runoffs and minimize the adverse impact of waste application on aquatic environments.