Is it time to start moving soil microbial fuel cell research out of the lab and into the field?

Soil microbial fuel cells (SMFCs) function as bioelectrochemical energy harvesters that convert electrons stored in soil organic matter into useful electrical energy. Broadly, an SMFC comprises three essential components: an anode buried in the soil (the negative terminal), a colony of exoelectrogenic microorganisms residing on this anode, and a cathode (the positive terminal). As the exoelectrogens respire, they release electrons to the anode, which acts as an external receptor. These released electrons then flow through a load (e.g. a resistor), connecting the anode and cathode. Though minuscule, the electrical power produced by SMFCs has a number of potential applications such as sustaining low-power embedded electronics, pollutant remediation, or as a bio-sensing proxy for soil qualities and microbial activity. This discussion aims to emphasize the potential of SMFCs in addressing real-world environmental issues and to generate interest in the larger scientific community for broad interdisciplinary research efforts, particularly in field deployments.

Effect of electron acceptors on electricity production and desalination of caspian sea water using microbial desalination cells.

The Microbial Desalination Cell (MDC) stands out as an innovative and a sustainable technology for both renewable energy generation and water treatment. The choice of electron acceptor significantly influences the efficiency of electricity flow. This study focuses on exploring the MDC performance under different conditions, including variations in cathode electron acceptors, initial pH levels, and hydraulic retention time (HRT). The investigation assesses simultaneous reduction of TDS and power generation from Caspian Sea water, a prominent saline water source in northern Iran, in both open-circuit (OC) and closed-circuit (CC) modes. The findings reveal that sodium hypochlorite, potassium permanganate, and potassium bromate as catholyte achieved TDS reduction rates of 84%, 77%, and 72%, respectively, under CC conditions at pH 5. Furthermore, it was observed that increasing HRT and pH levels lead to a decrease in desalination efficiency and power generation. Notably, the study highlights that the maximum power density was attained using permanganate, hypochlorite, and bromate as catholyte in both OC and CC configurations. By showcasing the adaptability of MDC performance with different cathode electron acceptors under varying conditions, this research offers valuable insights for optimizing MDC efficiency when treating real saline water sources.

Advancement in constructed wetland microbial fuel cell process for wastewater treatment and electricity generation: a review.

The constructed wetland coupled with a microbial fuel cell (CW-MFC) is a wastewater treatment process that combines contaminant removal with electricity production, making it an environmentally friendly option. This hybrid system primarily relies on anaerobic bioprocesses for wastewater treatment, although other processes such as aerobic bioprocesses, plant uptake, and chemical oxidation also contribute to the removal of organic matter and nutrients. CW-MFCs have been successfully used to treat various types of wastewater, including urban, pharmaceutical, paper and pulp industry, metal-contaminated, and swine wastewater. In CW-MFC, macrophytes such as rice plants, Spartina angalica, Canna indica, and Phragmites australis are used. The treatment process can achieve a chemical oxygen demand removal rate of between 80 and 100%. Initially, research focused on enhancing power generation from CW-MFC, but recent studies have shifted towards resource recovery from wastewater. This review paper provides an overview of the development of constructed wetland microbial fuel cell technology, from its early stages to its current applications. The paper also highlights research gaps and potential directions for future research.

Microbial fuel cell in long-term operation and providing electricity for intermittent aeration to remove contaminants from sewage.

Microbial fuel cells (MFCs) show promise in sewage treatment because they can directly convert organic matter (OM) into electricity. This study aimed to demonstrate MFCs stability over 750 days of operation and efficient removal of OM and nitrogenous compounds from sewage. To enhance contaminant removal, oxygen was provided into the anode chamber via a mini air pump. This pump was powered by the MFCs' output voltage, which was boosted using a DC-DC converter. The experimental system consisted of 12 sets of cylindrical MFCs within a 246L-scale reactor. The boosted voltage reached 4.7 V. This voltage was first collected in capacitors every 5 min and then dispensed intermittently to the air pump for the MFCs reactor in 4 s. This corresponds to receiving average DO concentration reaching 0.34 ± 0.44 mg/L at 10 cm above the air-stone. Consequently, the degradation rate constants (k) for chemical oxygen demand (COD) and biological oxygen demand (BOD) in the presence of oxygen were 0.048 and 0.069, respectively, which surpassed those without oxygen by 0.039 and 0.044, respectively. Aeration also marginally improved the removal of ammonia because of its potential to create a favorable environment for the growth of anammox and ammonia-oxidizing bacteria such as Candidatus brocadia and Nitrospira. The findings of this study offer in-depth insight into the benefits of boosted voltage in MFCs, highlighting its potential to enhance contaminant degradation. This serves as a foundation for future research focused on improving MFCs performance, particularly for the removal of contaminants from wastewater.

Towards environmentally-friendly solutions for real textile-dyeing wastewater with energy recovery: Effluent recirculation and Rubia plant-derived purpurin electron mediator in microbial fuel cell.

Escalating global water pollution exacerbated by textile-dyeing wastewater (TDW) poses significant environmental and health concerns due to the insufficient treatment methods being utilized. Thus, it is imperative to implement more effective treatment solutions to address such issues. In this research, different environmentally-friendly strategies involving effluent recirculation (ER) and Rubia cordifolia plant-derived purpurin electron mediator (EM) were introduced to enhance the treatment of real TDW and bioelectricity generation performance of an anti-gravity flow microbial fuel cell (AGF-MFC). The results revealed that optimum performance was achieved with a combination of hydraulic retention time (HRT) of 48 h with a recirculation ratio of 1, where the reduction efficiency of biochemical oxygen demand (BOD5), chemical oxygen demand (COD), ammonium (NH4+), nitrate (NO3-), sulphate (SO42-), ammonia nitrogen (NH3-N), colour and turbidity were 82.17 %, 82.15 %, 85.10 %, 80.52 %, 75.91 %, 59.52 %, 71.02 % and 93.10 %, respectively. In terms of bioelectricity generation performance, AGF-MFC showed a maximum output voltage and power density of 404.72 mV and 65.16 mW/m2, respectively. Moreover, the results also signified that higher treatment performance of TDW was obtained with natural purpurin from Rubia cordifolia plant than synthetic purpurin as EM. The reduced reactivity of highly stable synthetic purpurin EM for mediating the electron transfer was a contributing factor to the outperformance of plant-derived purpurin. Additionally, detailed electron-mediating mechanisms of purpurin were proposed to unravel the underlying electron transfer pathway involved in AGF-MFC. This research offers insight into the development of more sustainable solutions for managing TDW, and consequently reducing environmental pollution.

Effects of Co3O4 modified with MoS2 on microbial fuel cells performance.

In this study, Co3O4@MoS2 is prepared as anodic catalytic material for microbial fuel cells (MFCs). As the mass fraction of MoS2 is 20%, the best performance of Co3O4@MoS2 composite catalytic material is achieved, and the addition of MoS2 enhances both the electrical conductivity and catalytic performance of the composite catalyst. Through the structural characterization of Co3O4@MoS2 composite catalytic material, nanorod-like Co3O4 and lamellar MoS2 interweaved and stacked each other, and the agglomeration of Co3O4 is weakened. Among the four groups of single-chamber MFCs constructed, the Co3O4@MoS2-MFC shows the best power production performance with a maximum stable output voltage of to 539 mV and a maximum power density of up to 2221 mW/m2. Additionally, the ammonia nitrogen removal rate of the MFCs loaded with catalysts is enhanced by about 10% compared with the blank carbon cloth MFC. Overall, the findings suggest that Co3O4@MoS2 composite catalysts can significantly improve the performance of MFCs, making them more effective for both energy production and wastewater treatment.

Electrode-based floating treatment wetlands: Insights into design operation factors influencing bioenergy generation and treatment performance.

Exponential increases in energy consumption and wastewater have often irreversible environmental impacts. As a result, bio-electrochemical devices like microbial fuel cells (MFCs), which convert chemical energy in organic matter to electricity using exoelectrogenic bacteria, have gained interest. However, operational factors affecting efficiency and energy output need further study. This research investigated bioenergy production and COD, TN, and TP removal in mesoscale floating treatment wetlands (FTW-MFC) using Phragmites australis, Iris pseudacorus, and a mix of both. The Iris FTW-MFC achieved a high voltage peak of 2100 mV. The maximum power densities of 484 mW/m2, 1196 mW/m2, and 441 mW/m2 were observed for Phragmites, Iris, and mixed FTW-MFCs, respectively. Despite promising bioenergy yields, pollutant removal was unsatisfactory. A low area/height ratio (0.38 m2/0.8 m) and high loading rate (18.1 g/m2·d COD) boosted bioenergy output but hindered treatment performance and stressed plants, causing root decay. No significant pollutant removal differences were found between FTW-MFC and FTW. Higher relative plant growth rates occurred in the FTW-MFC. Microbial analysis shown that representatives of Pseudomonas and Clostridium species were consistently found across all samples, involved in both organic compound transformation and electricity generation, contributed to successful microscale results. A supporting microscale MFC experiment showed wastewater composition's impact on bioenergy yield and pollutant removal. Pre-inoculated reactors improved organic matter transformation and electricity generation, while aeration increased voltage and treatment performance. The role of plants requires further verification in future experiments.

Experimental Proof of Principle of 3D-Printed Microfluidic Benthic Microbial Fuel Cells (MBMFCs) with Inbuilt Biocompatible Carbon-Fiber Electrodes.

Microbial fuel cells (MFCs) represent a promising avenue for sustainable energy production by harnessing the metabolic activity of microorganisms. In this study, a novel design of MFC-a Microfluidic Benthic Microbial Fuel Cell (MBMFC)-was developed, fabricated, and tested to evaluate its electrical energy generation. The design focused on balancing microfluidic architecture and wiring procedures with microbial community dynamics to maximize power output and allow for upscaling and thus practical implementation. The testing phase involved experimentation to evaluate the performance of the MBMFC. Microbial feedstock was varied to assess its impact on power generation. The designed MBMFC represents a promising advancement in the field of bioenergy generation. By integrating innovative design principles with advanced fabrication techniques, this study demonstrates a systematic approach to optimizing MFC performance for sustainable and clean energy production.

Integrated tea polyphenols and polydopamine functionalized graphene anode for improved bioelectricity generation and Cr(VI) reduction in microbial fuel cells.

Microbial fuel cells (MFCs) have the dual advantage of mitigating Cr(Ⅵ) wastewater ecological threats while generating electricity. However, the low electron transfer efficiency and the limited enrichment of active electrogens are barriers to MFCs advancement. This study describes the synthesis of the TP-PDA-RGO@CC negative electrode using tea polyphenol as a reducing agent and polydopamine-doped graphene, significantly enhances the roughness and hydrophilicity of the anode. The charge transfer resistance was reduced by 94%, and the peak MFC power was 1375.80 mW·m-2. Under acidic conditions, the Cr(Ⅵ) reduction rate reached 92% within 24 h, with a 52% increase in coulombic efficiency. Biodiversity analysis shows that the TP-PDA-RGO@CC anode could enrich electrogens, thereby boosting the electron generation mechanism at the anode and enhancing the reduction efficiency of Cr(Ⅵ) in the cathode chamber. This work emphasizes high-performance anode materials for efficient pollutant removal, energy conversion, and biomass reuse.

Multi-metal ferrite as a promising catalyst for oxygen reduction reaction in microbial fuel cell.

Improving catalytic activity of cathode with noble metal-free catalysts can significantly establish microbial fuel cells (MFCs) as a sustainable and economically affordable technology. This investigation aimed to assess the viability of utilizing tri-metal ferrite (Co0.5Cu0.5 Bi0.1Fe1.9O4) as an oxygen reduction reaction (ORR) catalyst to enhance the performance of cathode in MFCs. Trimetallic ferrite was synthesized using a sol-gel auto-combustion process. Electrochemical evaluations were conducted to assess the efficacy of as-synthesized composite as an ORR catalyst, employing electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). This evaluation revealed that the impregnation of bismuth in the Co-Cu-ferrite structure improves the reduction current response and reduces the charge transfer resistance. Further experiments were conducted to test the performance of this catalyst in an MFC. The MFC with tri-metal ferrite catalyst generated a power density of 11.44 W/m3 with 21.4% coulombic efficiency (CE), which was found to be comparable with commercially available 10% Pt/C used as cathode catalyst in MFC (power density of 12.14 W/m3 and CE of 23.1%) and substantially greater than MFC having bare carbon felt cathode without any catalyst (power density of 2.49 W/m3 and CE of 7.39%). This exceptionally inexpensive ORR catalyst has adequate merit to replace commercial costlier platinum-based cathode catalysts for upscaling MFCs.

Role of membrane fouling layer in microbial fuel cell-membrane bioreactor (MFC-MBR) for controlling sulfamethoxazole and corresponding resistance genes.

Integrated MFC-MBR systems effectively remove antibiotics and control the release of antibiotic resistance genes (ARGs). However, the fouling layers on membranes can potentially act as reservoirs for ARGs. This study aims to elucidate the roles of membrane fouling layers and levels in influencing sulfamethoxazole (SMX) removal and ARGs control within an MFC-MBR system. Our findings demonstrate that low-intensity bioelectricity (400-500 mV) mitigates membrane fouling rates. The membrane fouling layer significantly contributes (39%-47%) to SMX removal compared to the cathode/anode zones. Higher extracellular polymeric substance (EPS) content and a lower protein/polysaccharide (PN/PS) ratio favor SMX removal by the membrane fouling layer. Across different levels of membrane fouling, the PN/PS ratio rather than EPS concentration plays a crucial role in SMX removal efficiency. The MFC-MBR with low fouling achieved superior SMX removal (69.1%) compared to medium (54.3%) and high fouling conditions (46.8%). The presence of ARGs in the membrane fouling layer increases with fouling formation, with intrinsic ARGs prevailing. Dense membrane fouling layers effectively retain ARGs, thereby reducing the risk of extracellular ARGs (eARGs) diffusion in effluents. These results provide insights into controlling ARGs in MFC-MBR systems and underscore the significant role of membrane fouling layers in antibiotics and ARGs removal.

Salinity-driven nitrogen removal and bacteria community compositions in microbial fuel cell-integrated constructed wetlands.

The effects of salinity gradients (500-4000 mg·L-1 NaCl) on electricity generation, nitrogen removal, and microbial community were investigated in a constructed wetland-microbial fuel cell (CW-MFC) system. The result showed that power density significantly increased from 7.77 mW m-2 to a peak of 34.27 mW m-2 as salinity rose, indicating enhanced electron transfer capabilities under saline conditions. At a moderate salinity level of 2000 mg·L-1 NaCl, the removal efficiencies of NH4+-N and TN reached their maximum at 77.34 ± 7.61% and 48.45 ± 8.14%, respectively. This could be attributed to increased microbial activity and the presence of critical nitrogen-removal organisms, such as Nitrospira and unclassified Betaproteobacteria at the anode, as well as Bacillus, unclassified Rhizobiales, Sphingobium, and Simplicispira at the cathode. Additionally, this salinity corresponded with the highest abundance of Exiguobacterium (3.92%), a potential electrogenic bacterium, particularly at the cathode. Other microorganisms, including Geobacter, unclassified Planctomycetaceae, and Thauera, adapted well to elevated salinity, thereby enhancing both electricity generation and nitrogen removal.

Electroactive biofilm communities in microbial fuel cells for the synergistic treatment of wastewater and bioelectricity generation.

Increasing industrialization and urbanization have contributed to a significant rise in wastewater discharge and exerted extensive pressure on the existing natural energy resources. Microbial fuel cell (MFC) is a sustainable technology that utilizes wastewater for electricity generation. MFC comprises a bioelectrochemical system employing electroactive biofilms of several aerobic and anaerobic bacteria, such as Geobacter sulfurreducens, Shewanella oneidensis, Pseudomonas aeruginosa, and Ochrobacterum pseudiintermedium. Since the electroactive biofilms constitute a vital part of the MFC, it is crucial to understand the biofilm-mediated pollutant metabolism and electron transfer mechanisms. Engineering electroactive biofilm communities for improved biofilm formation and extracellular polymeric substances (EPS) secretion can positively impact the bioelectrochemical system and improve fuel cell performance. This review article summarizes the role of electroactive bacterial communities in MFC for wastewater treatment and bioelectricity generation. A significant focus has been laid on understanding the composition, structure, and function of electroactive biofilms in MFC. Various electron transport mechanisms, including direct electron transfer (DET), indirect electron transfer (IET), and long-distance electron transfer (LDET), have been discussed. A detailed summary of the optimization of process parameters and genetic engineering strategies for improving the performance of MFC has been provided. Lastly, the applications of MFC for wastewater treatment, bioelectricity generation, and biosensor development have been reviewed.

Electroactive Brevundimonas diminuta consortium mediated selenite bioreduction, biogenesis of selenium nanoparticles and bio-electricity generation.

In this study, highly selenite-resistant strains belonging to Brevundimonas diminuta (OK287021, OK287022) genus were isolated from previously operated single chamber microbial fuel cell (SCMFC). The central composite design showed that the B. diminuta consortium could reduce selenite. Under optimum conditions, 15.38 Log CFU mL-1 microbial growth, 99.08% Se(IV) reduction, and 89.94% chemical oxygen demand (COD) removal were observed. Moreover, the UV-visible spectroscopy (UV) and Fourier transform infrared spectroscopy (FTIR) analyses confirmed the synthesis of elemental selenium nanoparticles (SeNPs). In addition, transmission electron microscopy (TEM) and scanning electron microscope (SEM) revealed the formation of SeNPs nano-spheres. Besides, the bioelectrochemical performance of B. diminuta in the SCMFC illustrated that the maximum power density was higher in the case of selenite SCMFCs than those of the sterile control SCMFCs. Additionally, the bioelectrochemical impedance spectroscopy and cyclic voltammetry characterization illustrated the production of definite extracellular redox mediators that might be involved in the electron transfer progression during the reduction of selenite. In conclusion, B. diminuta whose electrochemical activity has never previously been reported could be a suitable and robust biocatalyst for selenite bioreduction along with wastewater treatment, bioelectricity generation, and economical synthesis of SeNPs in MFCs.

Sustainable kitchen wastewater treatment with electricity generation using upflow biofilter-microbial fuel cell system.

The microbial fuel cell (MFC) is considered a modern technology used for treating wastewater and recovering electrical energy. In this study, a new dual technology combining MFC and a specialized biofilter was used. The anodic materials in the system were crushed graphite, either without coating (UFB-MFC) or coated with nanomaterials (nano-UFB-MFC). This biofilter served as a barrier to retain and remove turbidity and suspended solids, while also facilitating the role of bacteria in the removal of organic pollutants, phosphates, nitrates, sulfates, oil and greases. The results demonstrated that both systems exhibited high efficiency in treating kitchen wastewater, specifically greywater and dishwashing wastewater with high detergent concentrations. The removal efficiencies of COD, oil and grease, suspended solids, turbidity, nitrates, sulfates, and phosphates in first UFB-MFC were found to be 88, 95, 89, 86, 87, 75, and 94%, respectively, and in Nano-UFB-MFC were 86, 99, 95, 91, 81, 88, and 95%, respectively, with a high efficiency in recovering bioenergy reaching a value of 1.8 and 1.5 A m-3, respectively. The results of this study demonstrate the potential for developing MFC and utilizing it as a domestic system to mitigate pollution risks before discharging wastewater into the sewer network.

Heavy metals removal by algae and usage of activated metal-enriched biomass as cathode catalyst for improving performance of photosynthetic microbial fuel cell.

Cytotoxic, malignant, and mutagenic pollutants like heavy metals have emerged as a serious global threat to the ecosystem. Additionally, the quantity of noxious metals in water bodies has increased due to expanding industrial activities and the application of incompetent wastewater treatment techniques. Owing to the benefits of eco-friendly phytoremediation, the utilization of algae in photosynthetic microbial fuel cell (PMFC) for removal of heavy metals has attracted increasing attention among researchers. Therefore, a successful fabrication and operation of a modular PMFC for simultaneous algal biomass production was exhibited, thus resulting in significant removal efficiency of Cu(II) (94 %) and Co(II) (88 %). Moreover, Co(II)-accumulated algal biochar after thermal activation was utilized as a cathode catalyst for the first time and attained 64.2 mW/m2 of power density through PMFC. Hence, this easily synthesised green cathode catalyst proved its ability to enhance the overall performance of PMFC by attaining higher power output while treating wastewater.

Diversity in mechanisms of natural humic acid enhanced current production in soil bioelectrochemical systems.

The quinoid component of humic acids (HAs) had been studied as exogenous electron mediators (EMs), but the redox-mediating abilities of other functional groups remained unclear. This study evaluated the effects of various HAs functional groups on cellular respiration and extracellular electron transfer. The three EMs increased the current density compared to the control. Current density increased significantly after adding ultraviolet-irradiated HAs (UV-HAs), suggesting that nitrogenous group-mediated redox reactions contributed to high-density current generation. Structural equation model (SEM) results indicated that the contribution of nitrogen-containing groups to electron transfer could exceed 20%. This study proposed a synergistic mechanism: in the soil microbial fuel cells (soil-MFCs), HAs accelerated their component evolution through irreversible redox reactions and promoted extracellular electron transfer. Additionally, HAs-induced high expression of c-Cyts could further enhance high-density current generation. This study demonstrates that humic acids enhance electron transfer and current in bioelectrochemical systems, aiding sustainable energy optimization.

The impact of bottom water light exposure on electrical and sediment remediation performance of sediment microbial fuel cells.

Sediment microbial fuel cells (SMFCs) generate bioelectricity from benthic sediments and thus providing both bioelectricity generation and sediment remediation. However, the high internal resistance of the cathode leads to a low power output, which requires research on cathode treatment. In this study, we explored the influence of light irradiation on bioelectricity production and nutrient removal in the SMFC system. The microcosm experiment of the SMFC system was designed with artificial illumination of 500 lux (light-SMFC) and compared with dark conditions of 15 lux (dark-SMFC), which showed that the current increases during photoperiods. The study reveals that light-illuminated SMFC consistently produced the highest voltage, with the highest voltage (553 mV) being 1.3 times higher than the dark-SMFC (440 mV). The polarization curves show a significant reduction in internal cathodic resistance under light condition, resulting in increased voltage generation. The light-SMFC exhibits the highest maximum power density of 35.93 mW/m2, surpassing the dark SMFC of 31.13 mW/m2. It was found that light illumination in the SMFC system increases oxygen availability in the cathodic region, which supports the oxygen reduction reaction (ORR) process. At the same time, the high bioelectricity output contributes to the highest sediment remediation by greatly reducing the chemical oxygen demand (COD) and phosphate (PO4-P) concentrations. The study highlights the potential of light illumination in mitigating cathodic limitation to improve SMFC performance and nutrient removal.

Preliminary evaluation of electricity recovery from palm oil mill effluent by anion exchange microbial fuel cell.

This study assessed the viability of an anion-exchange microbial fuel cell (MFC) for extracting electricity from palm oil mill effluent (POME), a major pollutant in palm-oil producing regions due to increasing demand. The MFC incorporated a tubular membrane electrode assembly (MEA) with an air core, featuring a carbon-painted carbon-cloth cathode, an anion exchange membrane (AEM), and a nonwoven graphite fabric (NWGF) anode. An additional carbon brush (CB) anode was placed adjacent to the tubular MEA. The MFC operated under semi-batch conditions with POME replacement every 7 days. Results showed superior performance of the AEM, with the highest power density (Pmax) observed in POME-treated MFCs. Current and power density increased with CB addition; the best chemical oxygen demand (COD) removal efficiency reached 73 %, decreasing from 1249 to 332 mg/L with three CBs. The Pmax was 0.18 W/m-2(-|-) with 1000 mg/L COD and three CBs, dropping to 0.0031 W/m-2(-|-) without CB and at 410 mg/L COD. Anode resistance, calculated using organic matter supplementation, COD, and anode surface area, decreased with increased COD or surface area, improving electricity production. AEM and CB compatibility synergistically enhanced MFC performance, offering potential for POME wastewater treatment and energy recovery.

Efficient anode material derived from nutshells for bio-energy production in microbial fuel cell.

Biochar is a carbonaceous solid that is prepared through thermo-chemical decomposition of biomass under an inert atmosphere. The present study compares the performance of biochar prepared from Peanut shell, coconut shell and walnut shell in dual chamber microbial fuel cell. The physicochemical and electrochemical analysis of biochar reveals that prepared biochar is macroporous, amorphous, biocompatible, and electrochemically conductive. Polarization studies show that Peanut shell biochar (PSB) exhibited a maximum power density of 165 mW/m2 followed by Coconut shell biochar (CSB) Activated Charcoal (AC) and Walnut shell biochar (WSB). Enhanced power density of PSB was attributed to its surface area and suitable pore size distribution which proved conducive for biofilm formation. Furthermore, the high electrical capacitance of PSB improved the electron transfer between microbes and anode.