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Medium-chain carboxylic acids (MCCAs) are valuable platform chemicals and can be produced from waste biomass sources or syngas fermentation effluent through microbial chain elongation. We have previously demonstrated successful approaches to separate >90% purity oil with different MCCAs (MCCA oil) by integrating the anaerobic bioprocess with membrane-based liquid-liquid extraction (pertraction) and membrane electrolysis. However, two-compartment membrane electrolysis unit without pertraction was not able to separate MCCA oil. Therefore, we developed a five-compartment electrodialysis/phase separation cell (ED/PS). First, we tested an ED/PS cell in series with pertraction and achieved a maximum MCCA-oil flux of 1.7 × 103 g d-1 per projected area (m2) (19 mL oil d-1) and MCCA-oil transfer efficiency [100% × moles MCCA-oil moles electrons-1] of 74% at 15 A m-2. This extraction system at 15 A m-2 demonstrated a ∼10 times lower electric-power consumption (1.1 kWh kg-1 MCCA oil) than membrane electrolysis in series with pertraction (9.9 kWh kg-1 MCCA oil). Second, we evaluated our ED/PS as a stand-alone unit when integrated with the anaerobic bioprocess and demonstrated that we can selectively extract and separate MCCA oil directly from chain-elongating bioreactor broth with just an abiotic electrochemical cell. However, the electric-power consumption increased considerably due to the lower MCCA concentrations in the bioreactor broth compared to the pertraction broth.

The pH is a critical parameter in chain-elongating bioreactors, affecting: (1) the concentration of inhibitory undissociated carboxylic acids, which in turn affects the efficiency of product extraction; (2) the thermodynamics; and (3) the kinetics. Here, we examined the effect of five different pH levels (5.5 to 7.0) on n-caprylate (C8) production using an anaerobic sequencing batch reactor (ASBR) with continuous membrane-based liquid-liquid extraction (pertraction). We found that the product spectrum was directed by pH: mildly acidic pH (5-6) led to n-caproate (C6) production, while near-neutral and neutral pH (6.75-7) favored n-caprylate production. In particular, the pH of 6.75 led to the maximum values of volumetric n-caprylate production rate (75.6 ± 0.6 mmol C L−1 d−1; 0.06 g L−1 d−1) and n-caprylate concentration in the fermentation broth (420 mM C; 7.57 g L−1). Given that methane production remained low at near-neutral and neutral pH, we theorized that the high concentration of undissociated n-caprylic acid (5.71 mM C) inhibited methanogenesis. We then demonstrated such an inhibitory effect at neutral pH in: (1) microcosm experiments; and (2) the continuous bioreactor by adding methanogenic sludge. Furthermore, 16S rRNA gene sequencing analysis revealed that near-neutral and neutral pH led to more diverse microbial communities than at mildly-acidic pH. For the first time, we report predominant n-caprylate production in a microbiome at near-neutral and neutral pH conditions where methanogenesis was controlled by the inhibitory effects of undissociated n-caprylic acid. At the same time, extraction of this species occurred even at near-neutral and neutral pH.

Northern peatlands are experiencing more frequent and severe fire events as a result of changing climate conditions. Recent studies show that such a fire-regime change imposes a direct climate-warming impact by emitting large amounts of carbon into the atmosphere. However, the fires also convert parts of the burnt biomass into pyrogenic carbon. Here, we show a potential climate-cooling impact induced by fire-derived pyrogenic carbon in laboratory incubations. We found that the accumulation of pyrogenic carbon reduced post-fire methane production from warm (32 °C) incubated peatland soils by 13–24%. The redox-cycling, capacitive, and conductive electron transfer mechanisms in pyrogenic carbon functioned as an electron snorkel, which facilitated extracellular electron transfer and stimulated soil alternative microbial respiration to suppress methane production. Our results highlight an important, but overlooked, function of pyrogenic carbon in neutralizing forest fire emissions and call for its consideration in the global carbon budget estimation., Warmer and drier conditions are increasing the frequency of forest fires, which in turn produce pyrogenic carbon. Here the authors show that accumulation of pyrogenic carbon can suppress post-fire methane production in northern peatlands and can effectively buffer fire-derived greenhouse gas emissions.

Medium-chain carboxylic acids (MCCAs) are valuable platform chemicals with numerous industrial-scale applications. These MCCAs can be produced from waste biomass sources or syngas fermentation effluent through an anaerobic fermentation process called chain elongation. We have previously demonstrated successful approaches to separate >90%-purity oil with several MCCAs by integrating the anaerobic bioprocess with membrane-based liquid-liquid extraction (pertraction) and membrane electrolysis. However, membrane electrolysis without pertraction was not able to separate MCCA oil. Therefore, we developed an electrodialysis/phase separation cell (ED/PS) and evaluated whether it can function as a stand-alone extraction and separation unit. First, we tested an ED/PS cell, which, when evaluated in series with pertraction, achieved a maximum MCCA-oil flux of 1,665 g d-1 per projected area (m2) (19.3 mL oil d-1) and a MCCA-oil transfer efficiency [100%*moles MCCA-oil moles electrons-1] of 74% at 15 A m-2. This extraction system demonstrated a ∼10 times lower electric-power consumption of 1.05 kWh kg-1 MCCA oil when compared to membrane electrolysis in series with pertration (11.1 kWh kg-1 MCCA oil) at 15 A m-2. Second, we evaluated our ED/PS as a stand-alone unit when integrated with the anaerobic bioprocess (without pertraction), and demonstrated that we can selectively extract and separate MCCA oil directly from chain-elongating bioreactor broth with just an abiotic electrochemical cell. We assumed that such a stand-alone unit would reduce capital and operating costs, but electric-power consumption increased considerably due to the lower MCCA concentrations in the bioreactor broth compared to the pertraction broth. Only a full techno-economic analysis will be able to determine whether the use of the ED/PS cell should be as a stand-alone unit or after pertraction.

Northern peatlands are experiencing more frequent fire events as a result of changing climate conditions. Forest fires naturally result in a direct and negative climate impact by emitting large amounts of carbon into the atmosphere. Recent studies show that this extensive emission may shift the soil carbon regime from a sink to a source. However, the fires also convert parts of the burnt biomass into pyrogenic carbon. Here, we show an indirect, but positive, climate impact induced by fire-derived pyrogenic carbon. We found that the accumulation of pyrogenic carbon reduced post-fire methane production from peatland soils by 13-24%. The conductive, capacitive, and redox-cycling electron transfer mechanisms enabled pyrogenic carbon to function as an electron snorkel, which redirected soil electron fluxes to facilitate alternative microbial respiration and reduced the rate of methane production by 50%. Given the fact that methane has a 34-fold greater warming potential than carbon dioxide, we estimate that global greenhouse gas emissions are reduced by 35 Tg CO2e annually through the electron snorkeling of pyrogenic carbon in peatlands. Our results highlight an important, but overlooked, function of pyrogenic carbon in neutralizing forest fire emissions and call for its consideration in the global carbon budget estimation.

Summary Acid whey is a Greek-yogurt waste stream and can be a resource to produce biofuel precursors. Our objective was to convert acid whey into medium-chain carboxylic acid (MCCA) oil with the open-culture carboxylate platform. Here, we developed a temperature-phased bioprocess with different anaerobic reactor microbiomes, performing thermophilic lactic acid production and mesophilic chain elongation, to produce MCCAs (C6–C9) from acid whey via lactic acid as an intermediate. For the lactic acid-producing bioreactor, we achieved a volumetric lactic acid production rate of 1,230 mmol C L −1 day −1 (1.54 g L −1 hr −1 ). For the chain-elongating bioreactor, we achieved a volumetric MCCA production rate of 111 mmol C L −1 day −1 and a volumetric n -caproic acid (C6) production rate of 81 mmol C L −1 day −1 (0.07 g L −1 hr −1 ). We converted a real waste stream into mainly MCCAs without the external addition of an electron donor.

Bioelectrochemical systems (BESs) employ extracellular electron transfer from bacteria that grow at electrodes. Due to biofilm and electrode limitations, industrial-scale applications require large electrode areas, and thus inexpensive electrode materials. Here, electro-spun polyacrylonitrile (PAN) and carbon nanofiber (CNF) were manufactured. In addition, the conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) was applied as a coating to these materials and to carbon cloth (CC). We tested these materials as electrodes by using physicochemical measurements, cyclic voltammetry, and bioelectrochemical growth-studies with Geobacter sulfurreducens. PAN is a nonconductive material without capacitance, but with PEDOT coating the conductivity and capacitance became sufficient to support electric current production in our BES. CNF outperformed CC in capacitance, but behaved similarly in our BES when normalized to projected surface area. With the PEDOT coating, CNF increased electric current production by 38% in our BES, while this was 64% for CC. When applied to a gold microfluidic electrode, electric current with G. sulfurreducens increased almost three-fold. PEDOT added considerable specific surface area to electrodes possessing a low surface area, but not with a high surface area such as CNF. This work demonstrates that electro-spun electrodes and PEDOT coating are a promising electrode alternative that can be readily implemented into existing BESs.

Pyrogenic carbon contains redox-active functional groups and polyaromatic carbon matrices that are both capable of transferring electrons. Several techniques have been explored to characterize the individual electron transfer process of either functional groups or carbon matrices individually. However, simultaneous analysis of both processes remains challenging. Using an approach that employs a four-electrode configuration and dual-interface electron transfer detection, we distinguished the electron transfer by functional groups from the electron transfer by carbon matrices and simultaneously quantified their relative contribution to the total electron transfer to and from pyrogenic carbon. Results show that at low to intermediate pyrolysis temperatures (400-500 °C), redox cycling of functional groups is the major mechanism with a contribution of 100-78% to the total electron transfer; whereas at high temperatures (650-800 °C), direct electron transfer of carbon matrices dominates electron transfer with a contribution of 87-100%. Spectroscopic and diffraction analyses of pyrogenic carbon support the electrochemical measurements by showing a molecular-level structural transition from an enrichment in functional groups to an enrichment in nanosized graphene domains with increasing pyrolysis temperatures. The method described in this study provides a new analytical approach to separately quantify the relative importance of different electron transfer pathways in natural pyrogenic carbon and has potential applications for engineered carbon materials such as graphene oxides.

Acid whey is a Greek-yogurt waste stream and should be considered a resource to produce chemicals that can be used as biofuel precursors. Our objective was to convert acid whey into valuable medium-chain carboxylic acid (MCCA) oil with the open-culture carboxylate platform. Here, we developed a two-phase bioprocess with different anaerobic reactor microbiomes, performing thermophilic acidogenesis and mesophilic chain elongation, to produce MCCAs from acid whey via lactic acid as an intermediate. We achieved an MCCA production rate of 4.8 g chemical oxygen demand (COD) Lreactor-1 day-1 based on the volume of the chain-elongating bioreactor. The overall COD conversion efficiency for acid whey to MCCAs (C6-C9) was 53.1% with an MCCA specificity of 68.5% based on COD. n-Caproic acid (n-hexanoic acid [C6]) was the main product with a COD specificity of 49.8%. We converted a real waste stream into mainly MCCAs by chain elongation without the addition of electron donors.

We had extracted n-caproate from bioreactor broth. Here, we introduced in-line membrane electrolysis that utilized a pH gradient between two chambers to transfer the product into undissociated n-caproic acid without chemical addition. Due to the low maximum solubility of this acid, selective phase separation occurred, allowing simple product separation into an oily liquid containing ∼90% n-caproic and n-caprylic acid.

Surface functional groups constitute major electroactive components in pyrogenic carbon. However, the electrochemical properties of pyrogenic carbon matrices and the kinetic preference of functional groups or carbon matrices for electron transfer remain unknown. Here we show that environmentally relevant pyrogenic carbon with average H/C and O/C ratios of less than 0.35 and 0.09 can directly transfer electrons more than three times faster than the charging and discharging cycles of surface functional groups and have a 1.5 V potential range for biogeochemical reactions that invoke electron transfer processes. Surface functional groups contribute to the overall electron flux of pyrogenic carbon to a lesser extent with greater pyrolysis temperature due to lower charging and discharging capacities, although the charging and discharging kinetics remain unchanged. This study could spur the development of a new generation of biogeochemical electron flux models that focus on the bacteria–carbon–mineral conductive network., Electron transfer reactions govern most biogeochemical processes, yet we have a limited knowledge of the electrochemistry of pyrogenic carbon, a major component of organic matter. Here, the authors quantify electron transfers between pyrogenic carbon and mineral phases under different pyrolysis temperatures.

Azo dyes are the most widely used coloring agents in the textile industry, but are difficult to treat. When textile effluents are discharged into waterways, azo dyes and their degradation products are known to be environmentally toxic. An electrochemical system consisting of a graphite-plate anode and a stainless-steel mesh cathode was placed into a lab-scale anaerobic bioreactor to evaluate the removal of an azo dye (Direct Black 22) from synthetic textile wastewater. At applied potentials of 2.5 and 3.0 V when water electrolysis occurs, no improvement in azo dye removal efficiency was observed compared to the control reactor (an integrated system with electrodes but without an applied potential). However, applying such electric potentials produces oxygen via electrolysis and promoted the aerobic degradation of aromatic amines, which are toxic, intermediate products of anaerobic azo dye degradation. The removal of these amines indicates a decrease in overall toxicity of the effluent from a single-stage anaerobic bioreactor, which warrants further optimization in anaerobic digestion.