Electrochemical Ammonia Recovery from Anaerobic Centrate Using a Nickel-Functionalized Activated Carbon Membrane Electrode

Eco-Friendly Ammonia Adsorption from Aqueous Solutions Using Activated Madhuca indica Leaves Charcoal and Optimization of Parameters by RSM-BBD: A Sustainable Zero-Waste Approach

Ammonia separation from industrial effluents is a big concern for aquatic, plant, and human lives. Adsorption is best suited for separating ammonia from effluents due to its high effectiveness. In this paper, the activated Madhuca indica leaves charcoal (AMLC) adsorbent was synthesized and characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX) to know the characteristics of the adsorbent before and after adsorption. Further, thermal gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET) analysis were performed on the AMLC adsorbent, and a very high surface area of 964.252 m2/g was observed. The AMLC was utilized for adsorption experiments, and the adsorption parameters were optimized using response surface methodology-Box-Behnken design (RSM-BBD). The optimized parameters reported using RSM-BBD are a contact time of 123.19 min, an adsorbent dosage of 29.1 g/L, a pH of 10.33, an initial ammonia concentration of 312.28 mg/L, and a shaker speed of 82.70 rpm that gives the maximum ammonia removal efficiency of 100%. The adsorption equilibrium studies were analyzed using Langmuir, Freundlich, and Dubinin isotherms, with the Langmuir isotherm being the best fit for the data. The adsorption kinetics were modeled with pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion (IPD) models, indicating that PSO best describes the rate of kinetics. In addition, adsorption thermodynamics is also studied. The change in enthalpy (ΔH) was positive, suggesting that the adsorption is an endothermic process. The negative energy of entropy (ΔS) indicated complex formation with the AMLC adsorbent. In contrast, the positive change in Gibb's free energy showed that adsorption was not spontaneous. The present study supports UN SDG Goal 6 by providing sustainable and cost-effective solutions to water treatment. It also promotes biomass use and reduces chemical usage toward an eco-friendly water purification process.

Gas permeable membrane electrode assembly with in situ utilization of authigenic acid and base for transmembrane electro-chemisorption to enhance ammonia recovery from wastewater

Ammonia recovery from wastewater is of great significance for aquatic ecology safety, human health and carbon emissions reduction. Electrochemical methods have gained increasing attention since the authigenic base and acid of electrochemical systems can be used as stripper and absorbent for transmembrane chemisorption of ammonia, respectively. However, the separation of electrodes and gas permeable membrane (GPM) significantly restricts the ammonia transfer-transformation process and the authigenic acid-base utilization. To break the restrictions, this study developed a gas permeable membrane electrode assembly (GPMEA), which innovatively integrated anode and cathode on each side of GPM through easy phase inversion of polyvinylidene fluoride binder, respectively. With the GPMEA assembled in a stacked transmembrane electro-chemisorption (sTMECS) system, in situ utilization of authigenic acid and base for transmembrane electro-chemisorption of ammonia was achieved to enhance the ammonia recovery from wastewater. At current density of 60 A/m2, the transmembrane ammonia flux of the GPMEA was 693.0 ± 15.0 g N/(m2·d), which was 86 % and 28 % higher than those of separate GPM and membrane cathode, respectively. The specific energy consumption of the GPMEA was 9.7∼16.1 kWh/kg N, which were about 50 % and 25 % lower than that of separate GPM and membrane cathode, respectively. Moreover, the application of GPMEA in the ammonia recovery from wastewater is easy to scale up in the sTMECS system. Accordingly, with the features of excellent performance, energy saving and easy scale-up, the GPMEA showed good prospects in electrochemical ammonia recovery from wastewater.

Fluorine Modification Promoted Water Dissociation into Atomic Hydrogen on a Copper Electrode for Efficient Neutral Nitrate Reduction and Ammonia Recovery

Electrocatalytic nitrate reduction to ammonia (NITRR) offers an attractive solution for alleviating environmental concerns, yet in neutral media, it is challenging as a result of the reliance on the atomic hydrogen (H*) supply by breaking the stubborn HO-H bond (∼492 kJ/mol) of H2O. Herein, we demonstrate that fluorine modification on a Cu electrode (F-NFs/CF) favors the formation of an O-H···F hydrogen bond at the Cu-H2O interface, remarkably stretching the O-H bond of H2O from 0.98 to 1.01 Å and lowering the energy barrier of water dissociation into H* from 0.64 to 0.35 eV at neutral pH. As a benefit from these advantages, F-NFs/CF could rapidly reduce NO3- to NH3 with a rate constant of 0.055 min-1 and a NH3 selectivity of ∼100%, far higher than those (0.004 min-1 and 9.2%) of the Cu counterpart. More importantly, we constructed a flow-through coupled device consisting of a NITRR electrolyzer and a NH3 recovery unit, realizing 98.1% of total nitrogen removal with 99.3% of NH3 recovery and reducing the denitrification cost to $5.1/kg of N. This study offers an effective strategy to manipulate the generation of H* from water dissociation for efficient NO3--to-NH3 conversion and sheds light on the importance of surface modification on a Cu electrode toward electrochemical reactions.

Extreme accumulation of ammonia on electroreduced mackinawite: An abiotic ammonia storage mechanism in early ocean hydrothermal systems

An increasing amount of evidence suggests that early ocean hydrothermal systems were sustained sources of ammonia, an essential nitrogen species for prebiotic synthesis of life's building blocks. However, it remains a riddle how the abiotically generated ammonia was retained at the vent-ocean interface for the subsequent chemical evolution. Here, we demonstrate that, under simulated geoelectrochemical conditions in early ocean hydrothermal systems ([Formula: see text][Formula: see text] V versus the standard hydrogen electrode), mackinawite gradually reduces to zero-valent iron ([Formula: see text]), generating interlayer [Formula: see text] sites. This reductive conversion leads to an up to 55-fold increase in the solid/liquid partition coefficient for ammonia, enabling over 90% adsorption of 1 mM ammonia in 1 M NaCl at neutral pH. A coordinative binding of ammonia on the interlayer [Formula: see text] sites was computed to be the major mechanism of selective ammonia adsorption. Mackinawite is a ubiquitous sulfide precipitate in submarine hydrothermal systems. Given its reported catalytic function in amination, the extreme accumulation of ammonia on electroreduced mackinawite should have been a crucial initial step for prebiotic nitrogen assimilation, paving the way to the origin of life.

Enhanced etching terminal wastewater treatment and H2 production by in-situ deposited heavy metals on carbon dots/g-C3N4 photocathode microbial electrolysis cells

Sustainable and cost-effective semiconducting photocathodes of microbial electrolysis cells (MECs) are attractively promising for efficient treatment of actual industrial wastewaters containing complex recalcitrant organics and multiple heavy metals. Herein carbon dots/graphitic carbon nitride (CDs/g-C3N4) photocathodes were employed to achieve efficient treatment of actual etching terminal wastewater (ETW) with simultaneous H2 production in MECs, allowing the effluent meeting national discharge standards (GB39731-2020). The progressively in-situ deposited heavy metals on the CDs/g-C3N4 photocathodes, formed as metal oxides/CDs/g-C3N4 after simple calcinations, further enhanced the ETW treatment (recalcitrant organics mineralization: 42.2 mg/L/h vs. 35.5 mg/L/h; heavy metal removal: Cu(II): 9.9 mg/L/h vs. 7.4 mg/L/h, Ni(II): 4.7 mg/L/h vs. 3.5 mg/L/h, Zn(II): 0.7 mg/L/h vs. 0.5 mg/L/h) and H2 production (0.1138 m3/m3/d vs. 0.0662 m3/m3/d). The importation of heavy metals, formed as metal oxides/CDs/g-C3N4 altered the proportion of reactive oxidative species and thus promoted mineralization of recalcitrant organics, besides offering additional electrochemical removal of heavy metals with simultaneous more H2 production. This study demonstrates a new feasible protocol for achieving efficient ETW treatment, and gives a comprehensive appreciation of the effect of in-situ deposited heavy metals on the CDs/g-C3N4 photocathodes, which has a profound effect on subsequent ETW treatment with simultaneous H2 production.

Ammonia recovery from organic nitrogen in synthetic dairy manure with a microbial fuel cell

Increasing pressures on the animal and cropland agriculture sectors have led to the realization of problems with animal waste management and ammonia-based fertilizer supply. Bioelectrochemical systems (BES) are a new-age technology that offer a way to address these problems. Microbial fuel cells (MFCs), one type of BES, are traditionally used for electricity generation from microbial degradation of organic matters, but can also be used to recover nutrients from wastes simultaneous with treatment. This research investigated an MFC for ammonia recovery from the organic nitrogen (orgN) fraction of synthetic dairy manure, using the simple amino acid glycine as the orgN source. We used five different synthetic manure compositions to determine their effects on MFC performance, and found minimal sacrifices in performance under orgN conditions when compared to the base condition without orgN. The MFC achieved greater than 90% COD removal in all orgN conditions. Nitrogen (N) removal efficiencies of between 40% and 60% were achieved in orgN conditions, indicating that organic nitrogen can be used as the substrate for ammonia mineralization and further recovery as fertilizer. In addition, we found the MFC was largely populated by electrogenic organisms from the phyla Bacteroidota, Firmicutes, Proteobacteria, and Halobacterota, with organisms in both Bacteroidota and Firmicutes capable of N mineralization present. Lastly, we found that in conditions where orgN is scarce and the only N source provided, microbes preferentially degraded organic matter from other dead organisms, especially as an N source. This increases the concentration of N in the MFC system and introduces important operational constraints for MFCs operated for ammonia recovery from orgN.

Effective nutrient recovery from digester centrate assisted by in situ production of acid/base in a novel electrochemical membrane system

Nutrient recovery from wastewater is important to the circular economy and requires technological advancements. Herein, a novel electrochemical membrane system (EMS) was developed to recover both phosphorus and nitrogen from real digester centrate. The EMS synergistically coupled electrodialysis with membrane contactor to facilitate the selective recovery of individual nutrient. Under a constant current density of 10 mA cm^-2, the EMS recovered more than 95% of PO₄^3--P and 80% of NH₄⁺-N, at energy consumption of 670 ± 48 kWh kg^-1 P and 52 ± 2 kWh kg^-1 N. It should be noted that the same energy was used to recover two nutrients. When the acid produced from the anodic reaction was directly reused for N absorption, the final concentrations of PO₄^3--P and NH₄⁺-N reached 144 ± 3 and 1232 ± 130 mg L^-1, respectively. Adding extra acid did not affect phosphorus recovery but significantly enhance nitrogen recovery to 1797 ± 83 mg L^-1. The results of this study have demonstrated the feasibility of the proposed EMS and encouraged further investigation to reduce its energy consumption and improve nutrient recovery.

Anode passivation mitigation by homogenizing current density distribution in electrocoagulation

Electrode passivation is the most challenging technical problem in electrocoagulation (EC) water treatment process, but research on understanding and mitigating passivation evolution are still lacking. Herein, homogenization of current density (CD) distribution was found to be a critical factor in alleviating the anode passivation during EC process. Decreasing electrode area decelerated the growth of passivation layer on anode through homogenizing CD distribution, which was quantified by the ratios of CD distributed at the electrode edges and centers. When aluminum anode area decreased from 8 cm² to 2 cm² with a constant CD, the homogenization degree increased by 24.0%, and passivation was reduced by 24.3%. The depth profiles of passivated anodes confirmed the inhomogeneity of the anode passivation. Thicker passivation layers were observed at edges due to high CD distributions, which originated from the "edge effect" of electric field distribution between parallel plate electrodes. A facile strategy to homogenize CD distribution by splitting electrodes into smaller electrodes is then proposed for passivation mitigation, which can save energy consumption by 21.8% with unchanged removal efficiency. This study provides a unique insight into anode passivation mitigation and a feasible electrode design in EC.

Studying microbial fuel cells equipped with heterogeneous ion exchange membranes: Electrochemical performance and microbial community assessment of anodic and membrane-surface biofilms

In this study, microbial fuel cells deploying heterogeneous ion exchange membranes were assessed. The behavior of the cells as a function of the membrane applied was evaluated in terms of maximal current density, electron recovery efficiency and energy production rate (up to 427.5 mA, 47.7 % and 660 J m^-2h^-1, respectively) at different substrate (acetate) feedings (2.15 - 8.6 mM). System performance was characterized in the light of oxygen and acetate crossovers. The effect of membranes (in relation to the oxygen mass transfer coefficient, k_O) on the microbial diversity of anodic and membrane-surface biofilms was investigated. Based on the relative abundance of bacterial orders, the two populations could be distinguished and membranes with larger k_O tended to promote more the air-tolerant microbes in the biofouling layer. This indicates that membrane k_O has a direct effect on membrane foulant microbial composition, and thus, on the expected time-stability of the membrane.

Electrocatalytic Upcycling of Nitrate Wastewater into an Ammonia Fertilizer via an Electrified Membrane

Electrochemically upcycling wastewater nitrogen such as nitrate (NO₃^-) and nitrite (NO₂^-) into an ammonia fertilizer is a promising yet challenging research topic in resource recovery and wastewater treatment. This study presents an electrified membrane made of a CuO@Cu foam and a polytetrafluoroethylene (PTFE) membrane for reducing NO₃^- to ammonia (NH₃) and upcycling NH₃ into (NH₄)₂SO₄, a liquid fertilizer for ready-use. A paired electrolysis process without external acid/base consumption was achieved under a partial current density of 63.8 ± 4.4 mA·cm^-2 on the cathodic membrane, which removed 99.9% NO₃^- in the feed (150 mM NO₃^-) after a 5 h operation with an NH₃ recovery rate of 99.5%. A recovery rate and energy consumption of 3100 ± 91 g-(NH₄)₂SO₄·m^-2·d^-1 and 21.8 ± 3.8 kWh·kg^-1-(NH₄)₂SO₄, respectively, almost outcompete the industrial ammonia production cost in the Haber-Bosch process. Density functional theory (DFT) calculations unraveled that the in situ electrochemical conversion of Cu²⁺ into Cu¹⁺ provides highly dynamic active species for NO₃^- reduction to NH₃. This electrified membrane process was demonstrated to achieve synergistic nitrate decontamination and nutrient recovery with durable catalytic activity and stability.

Enhanced wettability improves catalytic activity of nickel-functionalized activated carbon cathode for hydrogen production in microbial electrolysis cells

A nickel-functionalized activated carbon (AC/Ni) was recently developed for microbial electrolysis cells (MECs) and showed a great potential for large-scale applications. In this study, the electroactivity of the AC/Ni cathode was significantly improved by increasing the oxygen (16.9%) and nitrogen (124%) containing species on the AC using nitric acid oxidation. The acid-treated AC (t-AC) showed 21% enhanced wettability that consequently reduced the ohmic resistance (6.7%) and the charge transfer resistance (33.3%). As a result, t-AC/Ni achieved peak values of hydrogen production rate (0.35 ± 0.02 L-H₂/L-d), energy yield (129 ± 8%), and cathodic hydrogen recovery (93 ± 6%) in MECs. The hydrogen production rate was 84% higher using t-AC/Ni cathode than the control, likely due to the enhanced wettability and a higher fraction of N on the t-AC. Also, the increases in polyvinylidene fluoride (PVDF) binder loadings (from 4.6 mg-PVDF/cm² to 7.3 mg-PVDF/cm²) demonstrated 47% higher hydrogen productions rates in MECs.