Application of bioelectrochemical systems for carbon dioxide sequestration and concomitant valuable recovery: A review

Journal Pre-proofs Application of bioelectrochemical systems for carbon dioxide sequestration and concomitant valuable recovery: A review Sovik Das, Swati Das, Indrasis Das, M.M. Ghangrekar PII: DOI: Reference:

S2589-2991(19)30097-7 https://doi.org/10.1016/j.mset.2019.08.003 MSET 104

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Please cite this article as: S. Das, S. Das, I. Das, M.M. Ghangrekar, Application of bioelectrochemical systems for carbon dioxide sequestration and concomitant valuable recovery: A review, Materials Science for Energy Technologies (2019), doi: https://doi.org/10.1016/j.mset.2019.08.003

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Application of bioelectrochemical systems for carbon dioxide sequestration and concomitant valuable recovery: A review Sovik Das1, Swati Das2, Indrasis Das1, M. M. Ghangrekar1, 2 1 Department

of Civil Engineering, Indian Institute of Technology, Kharagpur – 721302, India

2 PK

Sinha Centre for Bioenergy & Renewables, Indian Institute of Technology, Kharagpur – 721302, India

* Corresponding author Tel.: +91 3222 283440; Fax: +91 3222 282254. E-mail address: [email protected] Abstract The rise in global atmospheric temperature due to increase in the atmospheric carbon dioxide concentration needs to be tackled immediately before it reaches the point of no return. The application of innovative technologies based on the concepts of bioelectrochemical systems (BESs) can contribute in this direction by simultaneously sequestrating CO2 and producing value-added products in the process. Wastewater treatment with simultaneous bioenergy and biofuel recovery is also one of the added advantage of employing BESs for CO2 fixation. This review focuses on the potential of employing BES-based technologies like microbial carbon capture, plant-microbial fuel cell and microbial electrosynthesis cell for the concomitant production of valuables and CO2 sequestration. Also, various parameters affecting performance of BES that need to be optimized for the proper field-scale demonstration of these technologies are discussed.

1. Introduction Atmospheric carbon dioxide is one of the most important and primary pollutant contributing to the increase in the concentration of greenhouse gases. In the 21st century, CO2 emission, mainly due to manmade activities, is leading to the rise in atmospheric temperature as a reason of global warming. These irreversible manmade activities can change the climate, and the global 1

sea level will increase from 0.4 m to 1.0 m, if atmospheric CO2 concentration in 21st century exceeds 600 parts per million by volume [1]. In order to mitigate this global problem of CO2 emission, development of various CO2 fixation technologies that contribute in the sequestration of atmospheric CO2 with long-term storage and energy recovery is the need of the hour [2]. Carbon sequestration techniques employing several chemical processes such as chemical absorption, membrane separation, physical adsorption and cryogenic methods produce secondary pollutants and are also expensive [3, 4]. Additionally, several biological methods, such as soil carbon sequestration and phytosequestration, through several photosynthetic mechanisms like C3, C4 and crassulacean acid metabolism pathways in plants using carboxysomes in cyanobacteria and pyrenoids in microalgae, also contribute in the fixation of atmospheric carbon [5]. It has been already reported that the efficiency of CO2 fixation and the growth rates of photosynthetic micro-organism, such as microalgae and cyanobacteria, are much higher than the plants cultivated on land [6]. Various bioelectrochemical systems (BESs) like microbial carbon-capture cell (MCC), plant-microbial fuel cell (P-MFC) and microbial electrosynthesis (MES) cell can be employed for the simultaneous production of value-added products and carbon sequestration. Recently, photosynthetic MCC, which combines CO2 fixation through photosynthetic microorganism with a bio-electrochemical system, is viewed as a promising and environmentally friendly approach for CO2 sequestration and energy production [7]. In the anodic chamber of MCC, substrate i.e. organic matter present in the wastewater is oxidized by special class of microorganisms named as exoelectrogens (generally anaerobic microbiota), thus degrading organic compounds producing protons, electrons and CO2. The photosynthetic microorganism (e.g., algae, cyanobacteria) present in the cathodic chamber converts CO2 (from the anodic chamber or atmospheric air) to biomass with light illumination and releases oxygen, which acts as an electron acceptor for supporting the oxygen reduction reaction (ORR) [8, 9]. Research efforts on MCC are mainly focused on the enhancement of the performance of MCC and by-product synthesis by optimizing the cathodic configuration and operating conditions, making favorable environment to support higher growth of biomass and enhancing cathodic reaction kinetics. Microbial fuel cell (MFC) employ the concepts of BESs for the production of bioelectricity with the simultaneous treatment of wastewater [10, 11]. A MFC can be hybridized by introducing plants in the anodic chamber of a MFC, thus developing a new concept of plant 2

MFC (P-MFC), which can also sequester atmospheric carbon dioxide [12]. Biodegradable matter present in wastewater along with toxic contaminants, nitrates and phosphates can be consumed by plants as a substrate in P-MFC, thus offering effective treatment to the wastewater [13]. Plants present in the anodic chamber of a P-MFC utilizes carbon dioxide and sun ray during photosynthesis and store the resultant product in their root zone as rhizodeposits [14]. This rhizodiposites and organic matter from wastewater present in the anodic chamber of PMFC can be anaerobically oxidized by electrogenic microorganisms to generate electrons, protons, new microbial biomass and other oxidized by-products. Electrons migrate towards the anode and protons towards the cathode in the presence or absence of proton exchange membrane and develop potential difference. Usage of anaerobic oxidation in the anodic chamber of P-MFC produce CO2 instead of methane, thus reducing methane production and recover electricity as a by-product. Therefore, the excellent applicability P-MFC in agricultural lands like paddy field was demonstrated which would also reduce methane emission considerably [15]. Plants present in the anodic chamber can also consume toxic compounds, remove heavy metals from wastewater and reduce nitrate and phosphate level to restrict eutrophication [16]. Microbial electrosynthesis (MES) is another approach, where the concepts of BES are used for the production of multi-carbon organic compounds by the sequestration of CO2in the presence of biocatalysts [17]. It generally comprises of a biotic cathode, where the reduction of CO2 takes place by delivering electrons through an external power source. The anode could be abiotic or biotic depending on the configuration of the setup. In abiotic anodes, water is disintegrated into protons, electrons and oxygen electrochemically. For biotic anodes, various electron donors like sulphide can be used [18]. Mostly, the biotic anodes in MES are not sustainable for a longer period of operation, and the energy produced is also less. Thus, the use of abiotic anodes is prevalent in this field of research. The electrons generated in the anodic chamber are transferred to the cathodic chamber by applying an external potential through a potentiostat, where they are used along with protons transported through the proton exchange membrane, separating the two chambers, to produce organic molecules. If the electrical energy required for MES is supplied from renewable sources, then the process can be called as artificial photosynthesis [19]. A wide range of multi-carbon organic products ranging from C1 to C4 have been synthesised using MES; however, acetate is the most widely reported product of MES [20].

3

In this review article, the potential of various BES like P-MFC, MCC and MES for the sequestration of CO2 has been discussed. The valuable products synthesized using these novel techniques and their applications in day to day usage have been explained. Emphasize has also been given on the factors affecting the performance of these BESs.

2. Plant MFC Application of plants in the anodic chamber of MFC exposed a research field called P-MFC [21]. Plants present in the anodic chamber of MFC fix carbon in the form of CO2 during photosynthesis in presence of solar energy and release rhizodeposits (mostly carbohydrates) into the soil in the form of root exudates (Fig 1). The plants receive necessary nutrients from the wastewater, which gets simultaneously treated in the anodic chamber of P-MFC. The electrogens present in the anodic chamber oxidize dissolve organics and rhizodeposits and generate electrons and protons. Electrons reduce anode and protons migrate towards the cathode, which generates potential difference and thus convert solar energy to electricity with the application of plants in a P-MFC [22]. The power generated employing P-MFC can be utilized for the operation of low power electronic devices or sensors [23-25]. In the year of 2008, the concept of P-MFC was presented for the first time in the scientific literature [12]. Within a short period of time, this technology gained significant attractions from the researchers around the globe, and several attempts lead to the discovery of green rooftops [26], floating water bodies [27], marshy wetlands [28], and paddy fields [29] etc. Valuable biomass production by carbon fixation is another excellent advantage of P-MFC. Potentially hazardous contaminant removal is also possible using P-MFC, and several investigations have been reported in this context [30-32]. Abundance of sunlight is an important factor governing the performance of P-MFC and subsequent plant biomass production [12, 15]. However, this is not the only factor in P-MFC. Photosynthetic pathways (C3 or C4) and efficiency, salt resistance, root morphology, the quantity of root exudates and plant-microbes relationship also play a significant role in the performance of P-MFC [13]. The C4 plants were observed to have higher efficiency of conversion of solar energy to bioelectricity [33]. However, some C3 plants like Arundo donax also demonstrated better energy recovery in comparison to C4 plants [14]. Mostly salt-tolerant plants like Glyceria maxima, Oryza sativa (paddy) were observed to be used in P-MFC because of higher biomass production and salinity tolerance. 4

Fig 1. Schematic of a typical P-MFC setup 2.1 Types of plants used Wastewater treatment and electricity recovery are common features of MFC, however P-MFC has an added advantage of valuable biomass recovery from the plants and simultaneous carbon sequestration [34]. Initially, plants selected for P-MFC were not based on the in-depth knowledge and rather these were chosen on the availability of plants at the local vicinity and salinity tolerance of the plants [22]. Decrease in methane emission from paddy field because of the installation of P-MFC is motivating researchers further to pursue in this field [15]. Paddy or Oryza sativa was commonly chosen for initial studies pertaining to P-MFC because of its excellent anaerobic environment generation in the rhizospheric region, which favours anaerobic oxidation reaction [15, 28]. Spartina anglica [14], Arundinella anomala [14], Acorus calamus [35], and Lolium perenne [36] are several other plants that have been observed to be an excellent plant species applicable during the operation of P-MFC (Table 1).

2.2 Application of P-MFCs Wastewater treatment and concurrent electricity recovery are the primary advantages of MFC [37]. Presence of plants in the anodic chamber of a MFC adds several application possibilities of P-MFC. Plants consume biodegradable waste and nutrients present in wastewater as substrate and sequesters carbon dioxide from the atmosphere during photosynthesis. Through these steps, plants reduce organic load, nutrients concentration (nitrate and phosphate) and fix

5

carbon [38]. Plants can also uptake heavy metals and emerging contaminants [31, 39] as bioremediation and thus offer treatment to the wastewater in the anodic chamber of P-MFC. Valuable plant biomass recovery is also an added advantage of P-MFC [31]. In situ harvesting of electrical energy for powering sensor based applications is also other added advantage of PMFCs [24]. Table: 1 Application of different plants in P-MFC Plant used Ardisia pusilla

Application Electrical response from MFC used as an indicator of plant health conditions

Reference [40]

Pennusetum alopecuroides

Chromium removal

[39]

Phragmites communits

Chromium removal

[39]

Biomass and renewable energy recovery

[31]

Phytoremediation of Ni2+

[31]

Water spinach (Ipomoea aquatica) and water lettuce (Pistia stratiotes) Water hyacinth (Eichhornia crassipes)

2.2.1 Contaminant removal The presence of toxic non-biodegradable contaminants or heavy metals in surface or ground water is a severe concern in the present age. Partially treated wastewater discharged from industries is the main reason behind the contamination of fresh water sources from these toxic compounds. These contaminants can be properly managed by P-MFC based system and research efforts were focused in this direction in recent past. During a P-MFC’s performance investigation, a 99 % of Cr6+ removal was reported with a first-order rate constant of 0.058 per h with an initial Cr6+ concentration of 19 mg/L in the feed water [36]. Hexavalent chromium removal in P-MFC was also reported by other researchers [30, 39]. Textile effluent containing approximately 150 mg/L scarlet RR dye was degraded by 89 % and 87 % by Chrysopogon zizanioides and Typha angustifolia, respectively, within 60 h of retention time in a P-MFC [41].

6

Cyperus alternifolius and short leaf Cyperus malaccensis based P-MFC efficiently stabilized and immobilized As, Zn and Cd in the sediments [42]. Phytoremediation of Ni2+ using Eichhornia crassipes based P-MFC was also successfully demonstrated in P-MFC [31]. Efficient degradation of polycyclic aromatic hydrocarbons (phenanthrene and pyrene) and oil with simultaneous bioelectricity generation was also demonstrated in a P-MFC using Aglaonema commutatum [32].

2.2.2 Powering biosensors Biosensor based applications powered by P-MFC have a serious scope of utilizing the recovered electricity while treating wastewater. Most of the sensor-based applications of PMFC were found to be developed in the last five years and notable improvement was noticed within a short period of time. A biosensor was developed by Brunelli et al. (2016) to monitor floral health by determining the change in cell potential of plant-MFC over a period of time [43]. Ultra-low power wake-up receiver was also coupled with P-MFC, which offered a step toward large-scale wireless sensor applications for long-range communication [24, 44]. According to another study, electricity supply to the low power electronic devices, like an intermittent light emitting diode and a buzzer with simultaneous transmission of remote data at low speed (three messages of 12 bites each, in 6 s), was piloted by Schievano et al. [27]. Bench and pilot scale operations were also conducted to estimate soil water content in green roofs growing in semi-arid climates [25]. In a recent investigation, internet of things based wireless sensor networks were proposed and successfully implemented by using P-MFC [23].

2.2.3 Nutrient removal Eutrophication is a serious issue for ponds or lakes and untreated nutrients present in water bodies is majorly responsible for this. This phenomenon subsequently causes anaerobicity in water bodies and hampers the ecology of ponds or lakes. Reverse osmosis, ion exchange processes, electrodialysis, chemical denitrification, and adsorption were previously considered for controlling eutrophication. Nitrate removal can be efficiently controlled by the use of plant sediment microbial fuel cell (P-SMFC). Aquatic plant Ceratophyllum demersum based PSMFC was used for this purpose and its performance was compared with control P-SMFC. Almost 80 % of nitrate removal efficiency was observed by the means of P-SMFC during 18 days of operation. Nitrate removal was efficiently achieved by means of nitrate uptake by 7

plants, present in the anodic chamber of P-SMFC, which indicates that the application of plants can efficiently enhance nitrate removal in P-SMFC [45]. Similarly, phosphate and other micronutrient removal is also possible by employing P-SMFC. Therefore, the application of PSMFC has a potential of cost-effective reduction of nutrient concentrations in lakes but significant research for proper field-scale demonstration is required towards making this technology ready for field applications.

3. Microbial carbon-capture cell Microbial carbon-capture cell (MCC) is a budding technology, which sequesters carbon using photosynthetic microorganism and it can recover electrical energy during wastewater treatment (Fig 2). In a MCC, the CO2 generated during oxidative degradation of organic matter using anaerobic electrogenic microorganisms in the anodic chamber of MCC can be simultaneously used by photosynthetic microorganisms for electricity generation, CO2 sequestration and biomass synthesis in the cathodic chamber [7]. The complete biochemical reactions that occur in the anodic and cathodic chambers of a MCC are demonstrated in Eq. (1) through Eq. (4). In the anodic chamber: CH3COO- + 2H2O → 2CO2 + 7H+ + 8e-

(1)

Two types of reactions, namely light dependent and light independent reactions occur in the cathodic chamber of a MCC, which are illustrated in Eq. (2), (3) and (4), respectively. Light dependent reaction in the cathodic chamber: nCO2 + nH2O → (CH2O)n + nO2

(2)

2O2 + 8e− + 8H+ → 4H2O

(3)

Light independent reaction at the cathodic chamber: C2H4O2 + 2O2 → 2CO2 + 2H2O

(4)

8

Fig 2. Schematic indicating working of a microbial carbon-capture cell 3.1 Microbial CO2 sequestration using MCC Many microorganisms especially cyanobacteria and algae have the ability to sequester atmospheric CO2. Photosynthesis is an important biochemical process that converts solar energy into biomass and other valuable products, with simultaneous CO2 sequestration by photosynthetic bacteria both through Calvin-Benson and reductive tricarboxylic acid cycle. On the other hand, algal cells follow only Calvin-Benson cycle during photosynthesis [46]. The advantages of using microorganisms for CO2 sequestration are as follows: (i) rapid production of microbial biomass, (ii) high photosynthetic conversion efficiency, (iii) high capability of environmental bioremediation, such as CO2 bio-fixation from the atmosphere or flue gases, and (iv) capacity to produce a wide variety of value-added products. Different organisms having ability of CO2 fixation were also explored along with the prominent biochemical pathways involved in CO2 biofixation and their performance in MCC are listed (Table 2). Also, the power density was enhanced by 31% when nitrate was added into the catholyte externally [47]. The power density of 57.8 mW/m2 was also achieved in the same investigation for Anabaena sp. cultivated in the cathodic chamber grown with a CO2–air mixture. Various other oxygenic and anoxygenic phototrophic bacteria and alga have been cultured in the cathodic and anodic chambers, which simultaneously improved the power density of MCCs (Table 2).

9

Table 2: Performance comparison of MCCs using different photosynthetic microorganisms System configuration

Power density

Reference

Cyanobacteria

Dual chamber MCC

100 mW/m2

[47]

Rhodopseudomonas palustris

Dual chamber MFC

2.78 W/m2

[48]

750 mW/m2

[49]

2.7 W/ m3

[50]

6.4 W/m3

[51]

64.2 mW/m2

[52]

58.4 mW/m3

[53]

3.2 W/m3

[54]

2.48 W/m3

[55]

263 mW/m2

[56]

Micro-organism Oxygenic bacteria Anoxygenic phototrophic bacteria

Phototrophic sludge Chlorella vulgaris Chlorella pyrenoidosa Desmodesmus sp.

Algae

Microcystis aeruginosa Chlorella sorokiniana Immobilized Chlorella vulgaris Chlorella sp. and blue green Phormidium sp.

Double chamber MFC Double chamber MFC Double chamber MCC Double chamber MFC Single chamber MFC Dual chamber MCC Double chamber MCC Dual chamber MFC

3.2 Factor affecting the performance of MCCs and cultivation of photosynthetic microorganism The electricity generated in MCC depends upon many physiological conditions that favor bacterial growth in the anodic chamber, which include pH of the anolyte and electrode materials as well as cathodic conditions such as the growth of photosynthetic microorganisms, CO2 concentration, etc. Generally, algal species are chosen to be cultivated in the cathodic chamber of MCC because of higher photosynthetic efficiency to capture CO2 and higher saturated and unsaturated lipid content, which can be further harvested to produce algal-based biodiesel [57]. Several other environmental factors, like carbon source, light intensity etc., also affect the growth of microorganism, in turn influencing the quantity of biomass and other valuable product recovered from the microorganism (Table 3). The maximum biomass concentration and CO2 biofixation rate of 1.84 g/L and 0.288 g/L.d for Scenedesmus obliquus, and 1.55 g/L and 0.260 g/L.d for Chlorella pyrenoidosa, respectively, at 10 % CO2 concentration were reported by Tang et al. [58]. 10

Table 3: Environmental factor affecting the performance of MCC and recovery of valuable products Environmental

Photosynthetic

factor

microorganism

Culture conditions

Valuable product recovery

Scenedesmus

CO2 concentration increased

Maximum biomass concentration of 1.84 g/L at

obliquus

from 5 % to 20 %

10 % CO2 concentration

Chlorella

5 - 20 g/L of glucose

Maximum specific growth rate of 0.031 per h

Zofingiensis

concentration

at 20 g/L of glucose concentration

Chlorella

Sodium alginate was used as

vulgaris

polymer matrix

Chlorella

Light intensity from 2.4 to

Maximum power density of 972.5 mW/m3 with

vulgaris

11.4 W/m2

the optimum light intensity of 8.9 W/m2

Desmodesmus

Light intensity was increased

sp.

from 1500 lux to 3500 lux

Nitrogen

Chlorella

Nitrate concentration was

Power generation was increased with increase

concentration

sorokiniana

increased from 0.5 to 2.0 g/L

in nitrate concentration

Carbon source

Immobilized

Light

Maximum power density of 2.48 W/m3

References [58]

[59]

[55]

[60]

Illumination enhanced the output of the photoMFC by six-times from 11.3 mW/m-2 to 64.2

[52]

mW/m-2

11

[54]

3.3 Applications of MCC 3.3.1 Power generation Carbon and energy neutral wastewater treatment with minimum energy consumption is the major goal focusing on the attainment of environmental sustainability. However, the current technologies like chemical absorption, membrane separation, physical adsorption and cryogenic methods only reduce CO2 emission rather than using wastewater as a resource for energy recovery. Jadhav et al. used Anabaena ambigua and C. pyrenoidosa in the cathodic chamber of a MCC, demonstrating the power density of 4.29 W/m3 and 6.36 W/m3, respectively [51]. According to Rajesh et al., anaerobic sludge pre-treated with marine algae Chaetoceros inhibited methanogenic archaea and enhanced the coulombic efficiency (CE) of the MFC by 2.73 times and demonstrated a power density of 21.43 W/m3 [61]. Also, the power density of a MCC with low-cost coconut shell as separator produced maximum power of 3.2 W/m3, which was almost 2-fold higher than the MCC using Nafion 117 as separator [54].

3.3.2 Scope of valuable by-product recovery using MCC Photosynthetic microorganism, specially microalgal biomass contain high concentration of carbohydrates, proteins and lipids, which can be utilized as a potential feedstock for antibiotic, biofuel production (e.g., biodiesel, bioethanol, biobutanol, biohydrogen) and other applications such as animal feed (e.g., pigments, polyunsaturated fatty acids, antioxidants) [62, 63]. Dunaliella salina contains β-Carotene and other photosynthetic pigments, which can be used as a vitamin C supplement and as food colouring agent for the preparation of orange juices [64, 65]. Brown algae such as Fucus vesiculosus and Turbinaria conoides generate several polysaccharides (e.g., laminaran, fucoidan and alginates, etc.), which possesses antioxidant compounds those are used in anti-ageing cream to prevent skin disorders [66]. Other valuable products recovered from several other algal species such as Botryococcus sp., Chlorella sp., Dunaliella sp., Haematococcus sp., Phaeodactylum sp., Porphyridium sp. and Spirulina sp. etc. are being utilized in cosmetic industries [67, 68]. In microalgae bio-refinery applications, MCC was developed for generating feedstock for bioethanol production with simultaneous bioelectricity generation using fermented beer yeast as substrate in the anodic chamber [69]. Additionally, bio-hydrogen production from algal biomass employing Scenedesmus obliquus and Chlamydomonas reinhardtii in the cathodic chamber of MCC is well documented in the literature [70]. These specific algal species can be cultured in the 12

cathodic chamber of MCC with simultaneous wastewater treatment, which upon harvesting would produce value-added products.

3.3.3 Wastewater treatment Organic pollutants containing wastewater can be effectively treated in the anodic chamber of a MCC by degrading the organic matter present in it anaerobically, with the application of electrogenic microorganisms as biocatalysts. The septic tank effluent, waste activated sludge, municipal wastewater, agricultural wastewater, domestic wastewater, effluent from foodprocessing industries, pharmaceutical waste, ligno-cellulosic waste etc. can be successfully treated in MCC along with the simultaneous generation of bioelectricity and value-added product recovery from the harvested biomass. Lu et al. (2015) used photosynthetic microorganisms in the cathodic chamber of MCC and 80 - 93 % of the CO2 was recovered along with nearly 100 % chemical oxygen demand (COD) removal from artificial wastewater and around 56 % of COD removal in the real wastewater [71]. According to Hou et al. (2016), power density of 6.3 W/m3 was accomplished in MCC along with 44 % of COD removal using Golenkinia sp in the cathodic chamber. [72]. Heavy metal recovery from wastewater mediated through algal cells has also been reported, thus producing reusable quality water [73]. Thus, the application of MCC for wastewater treatment and value-added product synthesis can be a potential futuristic solution, which would simultaneously help in mitigating the problem of higher atmospheric CO2 concentration.

4. Microbial electrosynthesis Microbial electrosynthesis (MES) is a multidisciplinary technique, where the fundamentals of BESs are applied to sequester carbon and generate multi-carbon organic compounds with the aid of biocatalysts [17]. For this novel technology, mainly CO2 is used as a substrate and the same is sequestered by the microbes to produce organic compounds. In a typical MES setup (Fig 3), the cathodic chamber is biotic where anaerobic condition is maintained, and it is separated from the anodic chamber using a proton exchange membrane [74]. The anodic chamber is generally abiotic, where water splitting takes place producing protons (H+), electrons and oxygen. The protons are transferred to the cathodic chamber through the proton exchange membrane, where they are utilised by the microbes for the production of valuables.

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The electrons, on the other hand, are drawn through a conducting wire using an external potentiostat to the biocathode. It has been found that acetogens mainly dominate the microbial consortia present in the cathodic chamber. Methanogens were also reported to be present in the cathodic chamber, but suppressors were used to render them inactive during the operation [75]. Both the group of bacteria are active under anaerobic conditions and the same is maintained in the cathodic chamber of MES. Biotic anode has also been used in MES, where electron donors are used to generate electrons, but the yield for this case is reported to be lesser [18]. The feedstock, CO2 is purged into the cathodic chamber either periodically or intermittently depending on the scope of the study. MES can also be termed as artificial photosynthesis if the electrical energy required for the process is supplied using a renewable source of energy like solar cell [19]. Various organic compounds starting from methane to butyrate have been reported to be synthesised using MES [76].

Fig 3. Schematic of a microbial electrosynthesis cell Products synthesised using MES can be termed as electro-biocommodities or electro-fuels [77]. The fuels that are produced with the help of electrical energy are known as electro-fuels. During the production of electro-fuels, electrical energy is converted into chemical energy and it is stored in the C-C bonds of the chemicals produced. The raw material used for MES is CO2, which has an added advantage regarding the production of biofuels. Mainly it doesn’t require cultivable land for its production and CO2 is available in abundance in the atmosphere. Also, 14

the process itself doesn’t need the external supply of nutrients, thus making it cost effective [78]. This route of production of fuels is carbon neutral and therefore can be used for the production of various fuels sustainably.

4.1 Value-added products synthesised using different biocatalysts Various species can be cultured in MES depending on the product intended to be synthesised (Table 4). When mixed anaerobic culture is used as inoculum for MES, acetogens and methanogens, tend to dominate the cathodic chamber. As acetogens dominate in MES, the most abundantly found organic compound is acetate in the catholyte. Other multi-carbon organic compounds can also be synthesised by manipulating the cathodic conditions and operating parameters of the setup. Both pure culture and mixed culture have been reported to be implemented in MES for carbon sequestration and concomitant generation of organic compounds. Pure cultures like Sporomusa species and genetically modified Clostridium have been reported to be used in MES for the production of biofuels [19, 79]. Use of mixed culture inoculum in MES has also been widely reported in the literature [80]. The use of mixed culture is more feasible as it can be quite easily implemented in the field scale application of MES. The ease of scalability of MES increases as one shifts from pure culture biocathodes towards mixed culture as it is difficult to maintain aseptic conditions in the field. Also, mixed culture is easier to maintain in the field conditions and it is less affected by various operating stresses like changes in the environmental conditions [81]. The syntrophic association of different species in a mixed culture makes the culture more robust and self-sustainable. Recently, the application of thermophiles in MES for synthesis of organic compounds has also gained an attention from researchers round the globe. In an investigation with thermophilic Moorella thermoautotrophica immobilized cathode the production rate of acetate and formate at 55 °C was reported to enhanced by 2.8 and 23.2 times as compared with production at 25 °C using same strain [82]. In an another investigation with thermophiles namely, Moorella thermoacetica and M. thermoautotrophica the CE of 79 % and 72 %, and the rate of acetate production of 6.9 mM/m2.d and 11.6 mM/m2.d, respectively were obtained; these respective values were 2.1 mM/m2.d and 3.5 mM/m2.d for operation under mesophilic conditions [83]. ]. According to Bibra et al. (2018), thermophilic bacteria, like Geobacillus sp., can produce thermostable enzyme xylanase, thus utilizing lignocellulosic biomass as substrate for the ethanol production [84]. Xylanase enzyme can hold more than 60 % enzyme activity at high 15

temperature (50 °C – 80 °C). When Geobacillus sp. strain DUSELR13 was co-cultivated with Geobacillus thermoglucosidasius, it produced 3.53 and 3.72 g/L ethanol, respectively, by utilising lignocellulosic substrate prairie cord grass and corn stover [84]. Table 4. A brief review of microbial electrosynthesis

Feed gas

CO2

CO2

Products

Rate of

Type of culture/

production

Enzyme

Acetate

0.18 mM/d

Formate

1 mM/d

CH4

1 mM/d

Butanol

0.21 mM/d 2.75 mM/d

Ethanol

0.91 mM/d

Acetate, CO2, N2, H2

Acetobacterium &

(V)*

- 0.59

[85]

Clostridium

- 0.80

[76]

- 0.40

[86]

- 1.0

[87]

- 1.1

[74]

- 0.85

[88]

Pure culture of

formate,

--

2-

Formate

Reference

Methanobacterium

Sporomusa & Clostridium

oxobutyrate CO2, N2

potential

Pure culture of

Pure culture of Acetate

Applied

9.37 mg/L

Enzyme formate dehydrogenase Pure culture of

CO2

Acetate

1.3 mM/d

Clostridium ljungdahlii Mixed culture from

CO2

Acetate

1.3 mM/cm2.d

storm water pond sediment

* vs. standard hydrogen electrode In MES, the biocatalysts can either be in the form biofilm, i.e. attached to the cathode surface or the biomass can be planktonic, where the bacterial cells are in suspension. The cathodic 16

biofilm can accept electrons directly from the cathode surface, and this phenomenon is known as direct electron transfer [75]. For the planktonic cells, mediated electron transfer takes place where various mediators, like hydrogen, enzymes and external mediators, are used to transfer electrons from the electron donating cathode to the cells. Depending upon the scope of the study, the mode of electron transfer can be targeted in MES, and successively different products can be recovered in the process [20]. Generally, it is observed in MES that planktonic cells dominate the cathodic chamber. This is due to the electrostatic repulsion encountered by the cells when they are in the vicinity of the cathode, both being negatively charged [89]. Multiple species like Trichococcuspalustris, Desulfotomaculum, Oscillibacter, Clostridium celerecrescens, Clostridium propionicum, Tissierella, etc. have been reported to be used in MES for the sequestration of carbon [90]. Enriching the inoculum with predefined cultures can increase the yield of the system, and such a strategy was used by various researchers [91]. Not only acetate but the compounds like propionate and butyrate have also been reported to be produced using MES [92]. Butyrate was also reported to be produced as a by-product of microbial electrosynthesis of acetate from CO2 [76]. Other C3 compounds like propionate have also been reported to be produced from bicarbonate using MES [80]. The market price of alcohols is higher than other organic compounds, thus synthesis of the same through MES can make the process economically viable. These compounds can be synthesised in the cathodic chamber of MES by maintaining a reducing environment, i.e. by lowering the pH of the catholyte and increasing partial pressure of hydrogen. Production of polyol by the reduction of CO2 using Geobacter sulfurreducens in the presence of succinate has also been reported in the literature [93]. Other products like methane and hydrogen are also concomitantly produced during the synthesis of liquid organic compounds using MES. Methane is produced by the methanogens present in the mixed culture, which is generally suppressed to increase the effective yield of the system. On the other hand, hydrogen is abiotically produced, which helps in indirect electron transfer to the planktonic cells.

4.2 Cathode material used in MES In MES, the cathode is the working electrode and hence the performance of the system is significantly affected by its configuration. The cathodic chamber is biotic and therefore, the electrode material used as cathode should be biocompatible, non-toxic, chemically stable and highly conductive. The material should also be low-cost so that the technology becomes 17

economically feasible for successful scaling up of MES. Also, it should have a high specific surface area to encourage bacterial attachment. As explained in the previous section, electrostatic repulsion hampers bacterial attachment on the cathode surface. The solution to this could be the impregnation of positively charged particles onto the cathode. This would reduce the electrostatic repulsion and aid in the colonisation of the cathode surface. This strategy was used by various scientists to enhance the yield of organic compounds [89]. Metals like gold, palladium and nickel were used by Zhang et al. (2013) to create a layer of positive charge on the surface of the cathode [89]. Other chemicals like chitosan, polyaniline, melamine etc. were used by the same group to improve the yield of acetate in MES. Carbon cloth was modified by Chen et al. (2016) using reduced graphene oxide and positively charged tetraethylene pentamine nanoparticles, which lead to the formation of a highly structured biofilm and thus improved the overall performance of the setup [94]. Carbon-based electrode materials have generally been used as cathode materials for the production of electro-biocommodities using MES. These materials are biocompatible and offer a higher surface area, and hence they are preferred as compared to other materials. In the first reported study on MES, graphite cathode was used [86]. Various other materials like carbon rod [90], carbon stick [95], carbon plate and cloth [76] and reticulated vitreous carbon [96] have been reported to be used for the production of acetate using MES. Graphite electrodes possess less porosity and thus have less electroactive surface. To overcome this limitation, Marshall et al. (2013) used granular graphite, which enhanced the production of organic compounds [97]. Hydrogen, which is used as a mediator in the process of MES, also plays an essential role in the performance of the system. The use of stainless steel on carbon felt increased the production of hydrogen, which in turn enhanced the overall performance of the system [74]. Still, highly efficient and low-cost cathode materials are required to implement this technology in the field.

5. Comparing performance of P-MFC, MCC and MES The three types of BES, namely P-MFC, MCC and MES, which are employed for the sequestration of carbon dioxide can be compared on the basis of CE and carbon sequestration efficiency (Table 5). The CE of 16.53 % and 9.40 % was reported for MCCs operated with Chlorella sorokiniana and Chlorella vulgaris, respectively, in the cathodic chamber [54, 55]. For MES operated with mixed culture inoculum in the cathodic chamber, a very high CE of 50 18

% was reported for the production of acetate [74]. On the other hand, P-MFC with ryegrass (Lolium perenne) demonstrated the lowest CE of 2.05 % and 4.12 % in comparison to MCC, MES and P-MFC [36]. The highest CE was observed for MES in comparison to the other BES employed for carbon dioxide sequestration due to the plausible cause of employing cathodic imposed potential, thus compelling the microbes to follow only a specific pathway to enhance the CE. Whereas, P-MFC demonstrated the least CE, which is due to the fact that the organic matter present in wastewater is also simultaneously consumed by non-electrogenic microbes present in the rhizosphere, thus diminishing CE. Table 5. Efficiency comparison of MCC, P-MFC and MES

Type of BES

Coulombic efficiency (%)

Carbon sequestration efficiency (%)

Reference

16.53

NR

[54]

9.40

NR

[55]

NR

94 ± 1

[7]

NR

42 – 48

[98]

NR

[36]

MCC

P-MFC

MES

2.05 4.12 41.2

20.3

50

9.5

28.4

NR

[74] [100]

* NR - Not reported

Moreover, carbon sequestration efficiency of 94 % and 42 % was reported for MCC operated with C. vulgaris and sulphate reducing bacteria in the cathodic chambers, respectively [7, 98]. However, for MES using mixed culture inoculum in cathodic chamber only 20.3 % of carbon sequestration efficiency was observed [74]. Carbon sequestration efficiency for P-MFC has not been reported in the literature but the plants like Brassica juncea employed in P-MFC are reported to have fixed 8 to 10 mol CO2 kg-1 (fixed mass) s-1 [99]. In terms of energy efficiency, external potential needs to be applied for MES, which is not the case for MCC and P-MFC. 19

Furthermore, MCC and P-MFC are energy generating devices producing bioelectricity with simultaneous wastewater treatment. Maximum power density of 6.4 W/m3 (95 mW/m2, normalized to anode surface area) was reported to be produced by MCC operated with C. pyrenoidosa in the cathodic chamber [51]. The P-MFC with Pennisetum setaceum demonstrated a maximum power density of 163 mW/m2 [38]. Similarly, for the MES producing 1 mole of acetate from CO2, around 200 W-h of electrical energy was required [74].

6. BES in comparison with other CO2 sequestration technologies and future outlook Direct use of CO2 has been employed in various industrial processes like welding, production of foaming agent, food and soft drink preparation etc., and it has also been employed as a solvent for dry-cleaning, water treatment and in packaging industries [101]. However, the market demands of all the products of these processes are limited, and thus the present utilisation of CO2 might not provide complete solution for controlling global carbon emission. Various firms are also incorporating the techniques for CO2 utilization in their industrial processes. Companies like PRAXAIR and Great Point Energy are utilising CO2 directly and on the other hand Novomer, Newlight, Algenol etc. are converting CO2 to polypropylene carbonate, AirCarbon plastics and ethanol, respectively [102]. Biological routes for the production of biofuels from CO2 have also been adopted by various industries. PhycalTM is one such company that uses algae grown in ponds, supplied with CO2 and nutrients, to produce oils [103]. These technologies are advantageous over biomass-based biofuel production, as it doesn’t need arable land and fresh water for the process. The concepts and applications of BESs based technologies are still in the nascent phase owing to their inferior yield and unsuitability to handle field conditions [104]. The energy efficiency and carbon sequestration of these technologies need to be improved considerably before a successful field scale application of these technologies is possible. Scaling up of BES technology still remains a challenge to make it ready for large scale production. Thorough understanding of electrochemistry and related biochemistry need to be established further along with development of efficient yet low-cost long term stable materials for fabrication to achieve this. Also, the various parameters governing the performance of BES need to be optimized to facilitate suitable scale-up for a successful sustainable field scale application of this technology for harvesting CO2 from various sources and converting it into organo-chemicals and fuels.

20

7. Conclusion Various derivatives of BES like, MCC, P-MFC and MES can be considered as the plausible technologies to tackle the ever-increasing problem of global warming and increased CO2 concentration. On the other hand, valuable products like bioenergy, biofuels etc. can also be obtained with the simultaneous CO2 sequestration and wastewater treatment. However, proper field-scale demonstration of these BES-based technologies is still missing. The operating parameters affecting the yield and performance of the technologies need to be optimized before their successful field-scale application. Also, the cost associated with the operation and fabrication of BESs-based technologies needs to be reduced drastically by developing novel low-cost electrode materials and membranes. Low power output of these technologies is also a serious issue, which could be circumnavigated by suitably stacking multiple small-scale setups. Thus, research should be focused on these areas before the efficacious demonstration of BES-based technologies is possible.

Acknowledgement This work was financially supported by The Ministry of Drinking Water and Sanitation, Government of India (SAP17_IITKGP_05).

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Highlights  BESs are novel and budding techniques to tackle emerging environmental problems  Carbon sequestration is possible using various BES-based technologies  BES-based technologies treat wastewater with concomitant valuable product recovery  Capital cost of these BES need to be reduced considerably for practical applications

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