AIMS Environmental Science, 9(4): 505–525.
DOI: 10.3934/environsci.2022030
Received: 18 April 2022
Revised: 24 June 2022
Accepted: 28 June 2022
Published: 09 August 2022
http://www.aimspress.com/journal/environmental

Research article

Effect of electrode modification on the production of electrical energy
and degradation of Cr (VI) waste using tubular microbial fuel cell
Raden Darmawan1,*, Sri Rachmania Juliastuti1, Nuniek Hendrianie1, Orchidea Rachmaniah1,
Nadila Shafira Kusnadi1, Ghassani Salsabila Ramadhani1, Yawo Serge Marcel1, Simpliste
Dusabe1 and Masato Tominaga2,3
1

Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember, Campus ITS, KeputihSukolilo, Surabaya, 60111, Indonesia
2
Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan
3
Graduate School of Science and Engineering, Saga University, Saga 840-8502, Japan* Correspondence: Email: rdarmawan@chem-eng.its.ac.id; Tel: +6282311459899.
Abstract: Carcinogenic hexavalent chromium is increasing worldwide due to the increased
electroplating, welding and textile industry. On the other hand, molasses, the sugar factory’s byproductwith high organic compounds (sugars), may pollute the environment if it is not processed. However,microbial fuel cell (MFC) seems to be a promising technology due to its ability to produce electricalenergy from pollutant degradation using microbes while reducing hexavalent chromium to trivalentchromium with less toxicity. Carbon felt was used at both electrodes. This research aimed to determine
the effect of modifying the anode with rice bran and cathode with Cu catalyst towards electricity
generation and pollutant removal in molasses and reducing Cr (VI) into Cr (III) using tubular microbial
fuel cells. Moreover, the effect of mixing Sidoarjo mud and Shewanella oneidensis MR-1 aselectricigen bacteria toward electrical energy production and pollutant removal was determined.
Experiments revealed that the S/CM/AM variable, which only used Shewanella oneidensis MR-1 asan electricigen bacteria with both modified electrodes, produced the highest total power density of
530.42 mW/m2 and the highest percentage of Cr (VI) reduction of 98.87%. In contrast, the highest
microbial population of 66.5 × 1010 cells/mL, 61.28% of Biological Oxygen Demand (BOD5) removal
and 59.49% of Chemical Oxygen Demand (COD) were achieved by SSi/CM/AM variable, mixing
Shewanella oneidensis MR-1 and Sidoarjo mud as an electricigen bacteria with both modified
electrodes. Therefore, this study indicates that double chamber tubular microbial fuel cells may be asustainable solution for managing molasses and carcinogen hexavalent chromium.

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Keywords: hexavalent chromium; electrode modification; Sidoarjo mud; Shewanella oneidensis
MR-1; carbon felt; tubular microbial fuel cell

1.

Introduction

Energy is the basis of sustainable development. Currently, most human activities are supported
by fossil fuels [1]. Energy demand will continue to increase every year while the availability of fossil
fuels is limited. In addition, the ongoing use of fossil fuels is not sustainable due to greenhouse gas
emissions [2]. Replacing non-renewable fossil fuels with something more sustainable is a universal
issue affecting communities and the environment. The extensive use of fossil fuels harms theenvironment due to carbon dioxide emissions, which result in air pollution, global warming, and healthissues [3]. The depletion of energy reserves on earth requires humans to develop alternative energy.
Several technologies have been proposed, such as solar photovoltaic cells, wind turbines, fuel cells,
and geothermal energy to replace fossil fuels [4]. Microbial fuel cells (MFCs) seem to be newtechnology, one type of fuel cell currently being developed.
MFC has attracted significant research interest due to its potential for generating energy in anenvironmentally sustainable manner. Recently, researchers reported MFCs as a new technology fordirectly converting chemical energy stored in organic substances to electricity through catalytic
reactions of exoelectrogenic bacteria [5]. The basic principle behind the MFC technology is that thechemical energy compounds are converted into electrical energy during the decomposition of organicand inorganic matters with the help of catalytic action of microorganisms and electron transfer
efficiency [6]. MFC has several advantages over conventional processing technology. It is a highpotential green energy technology because of its easy operation and excellent waste treatment
capability with zero pollution emission since it is environmentally friendly [7,8]. However, MFC
presents several challenges for their wide applications due to low power generation and costly
fabrication materials of MFCs.
The performance of the MFC in generating electrical energy is influenced by several factors suchas the type of microbe used [9], the type of contaminants degraded [10], types of the electrode [11],electrode modification [12], the type of configuration [3], type of separator used and the operating
conditions of the MFC, including but not limited to pH and temperature [5]. In MFCs, thebiodegradation of organic matter and electron transfer efficiency determine the generation ofbioelectric energy [11]. Moreover, the internal resistance of the system and sustainability of electrontransfer are also determining factors for a bio-electricity generation [13,14]. The exoelectrogenic
bacteria produce free electrons to be collected by the electrode for power generation and are essential
in MFC to remove pollutants while producing electrical energy [9,15]. Researchers revealed that the
mixture of exoelectrogenic bacteria might boost electricity production [5]. The authors used
Shewanella oneidensis MR-1, an exoelectrogenic bacteria with high productivity [16], and Sidoarjo
mud-containing bacteria that might play a key role in MFC electron transfer. Darmawan et al. revealed
that Sidoarjo mud could be utilised as an alternative source for bioelectricity production [17]. Sidoarjo
mudflow is due to the mud volcano eruption in May 2016 in Sidoarjo, East Java, Indonesia. It consistsof 70% solids and 30% water, with a 32%–40% salinity, pH 6.6–7, cation exchange capacity (CEC)
3.89–35.42 (meq/100 g), and total organic carbon 54.75%–55.47% [18].
The Author used hexavalent chromium waste as a catholyte and molasses as an anolyte in thisAIMS Environmental Science

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research. Hexavalent chromium is a heavy metal mainly used in electroplating, welding, woodpreservation and textile dyes. It is well-known for its natural and negative health effects due to its
poisonous quality and mutagens [19]. The United States Environmental Protection Agency (US EPA)
identified Cr (VI) as one of seventeen chemicals that endanger human health [20]. Costello et al. [21]revealed that Cr (VI) is around 100 times more poisonous compared to Cr (III). Intensive research wasdeveloped to reduce Cr (VI) to Cr (III). Different pre-treatment methods were used, including but notlimited to membrane filtration, ion exchange and chemical precipitation. However, these methods
aren’t sustainable since they require much energy and mainly generate hazardous waste [19]. So, MFC
seems to be a promising technology that may reduce Cr (VI) to Cr (III) environmentally sustainable.
On the other hand, molasses, the sugar factory’s most valuable by-product [22], was used as an anolyte
since it contains high organic compounds (sugars) that might be used as a carbon source in MFC.
Molasses could also be used in yeast fermentation, pharmaceutical, animal feed industries, feedstock
for fuel ethanol production, paddy soil to absorb cadmium and arsenic, and feedstock for anaerobic
digestion to produce biogas [23,24]. In this research, molasses was first sterilised by autoclave to kill
any microorganisms that may live in it before its use.
Electrode materials play a significant role in MFC Performance [11]. Different materials may be
used to make a perfect anode for MFCs, which require a large surface area to enhance the electrontransfer rate. The large electrode surface area can capture more electrons and allow microorganisms tomove effectively on the electrode’s surface [25]. Anodic materials are also necessary because they
help microbes oxidise organic substrate by increasing their metabolic rate [26]. This research selectedcarbon felt at both electrodes since it has several advantages. These include enormous porosity value,chemical and thermal stability, higher specific area and mechanical strength with high electricalconductivity [27]. The research conducted by Yaqoob et al. [28] showed that the electrode’s
modification to achieve a high surface area, good electron transferability, and bacterial adhesion isneeded to enhance electricity production. In this study, the anode was modified by rice bran. The rice
milling process resulted in rice bran as a byproduct. Annually, 29.3 million tons are supplied
globally [29]. It contains about 11.3%–14.9% protein, 34.0%–62.0% carbohydrates and 15.0%–19.7%
oil [30]. Rice bran nutrients with high organic content may increase bacterial growth, resulting in highelectricity generation. Moreover, the rice bran contains a high silica concentration, increasing surface
area [31] for bacterial attachment. High silica concentration can stick around the anode surface,modifying the anode to increase the surface area for bacterial attachment [31]. This study used ricebran to increase the contact area between microbes and the electrode. On the other hand, A highly
active catalyst Cu modified cathode to improve the performance of the MFC. A catalyst at the cathodecauses the cathodic reaction’s activation energy to be lower and increases the reaction rate [12]. Cu
was selected due to its perfect Oxygen Reduction Reaction. Pt metal with the highest ORR activity
compared to other metals wasn’t selected since it is costly and its scarcity limit its large scale
application. Moreover, Pt has a poor stability in long-term use [32]. Therefore, Cu was used in thisstudy for the cost and availability compared to Pt. Fan et al. [33] revealed that the MFC’s power density
was increased by 51.91% for a modified electrode with MnO2, more than that of an unmodified
electrode. The power density were 6.8 and 10.33 mW/m2 before and after modification, respectively.
In this study, a double chamber Tubular MFC configuration was used. Researchers revealed thatusing Tubular MFC has advantages such as low cost of construction, high power output and waste
degradation, and ease of construction [34]. A double chamber is an MFC configuration with two
chambers, namely the anode chamber and the cathode chamber, and two chambers are connected
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through a membrane separator [35,36]. Li et al.[37] revealed that the absence of a separator increases
oxygen and substrate diffusion, lowering the anode microorganism’s electrocatalytic activity.
However, one of the difficulties in separator applications is the slow transfer of protons from the anodeto the cathode chamber, leading to pH splitting and decreasing MFC performance. In this study,
Gore-Tex® was selected since it is the most efficient and effective separator with affordable cost,
simple construction, and low internal resistance. Gore-Tex® was able to become an efficient andeffective separator in MFC applications due to its characteristics of being nonselective to ion transportand microporous structure, as well as its thin thickness. It enhanced the proton transfer rate and oxygen
diffusion compared to ion-exchange membranes [38,39]. Three layers of Gore-Tex® with a 1 cm
distance between the separator and the cathode were used. Therefore, this research aimed to determine
the effect of modifying the anode with rice bran and cathode with active catalyst Cu towards
bioelectricity generation and pollutant removal in molasses and reduction of carcinogen Cr (VI) into
Cr (III) using tubular microbial fuel cells. Moreover, the effect of mixing Sidoarjo mud and Shewanella
oneidensis MR-1 as electricigen bacteria toward bioelectricity production was determined.
2.

Materials and methods

2.1. Materials
2.1.1.

Type of materials used

Materials used in this study are molasses, K2Cr2O7, Sidoarjo mud, Shewanella oneidensis MR-1,
Copper wire, Carbon powder, carbon felt 3F, Gore-Tex® separator, rice bran and Copper (II) sulfate.
Sidoarjo mud was collected 30–45 cm below the mud’s outer surface at coordinates 7o31′45.6″S and
112o42′43.6″E in Porong, Sidoarjo, East Java (Indonesia). Molasses were collected at P.T Energi Agro
Nusantara, located in East Java (Indonesia) and K2Cr2O7 from Ferak Berlin GmbH (German). The
carbon powder (KB 600) was obtained from Gemmy Industrial Corp (Taiwan), whereas the carbon felt
(GF-20, Nippon carbon) 3F with a size of 500 cm × 1000 cm and a density of 320 g/m2 was purchasedfrom Japan. Gore-Tex® separator and Shewanella oneidensis MR-1 were found in the Laboratory ofIndustrial Wastewater Treatment & Biomass, Institut Teknologi Sepuluh Nopember (ITS). On the other
hand, all the other chemicals not listed used in this research are from Merck (Darmstadt, Germany).
2.1.2.

MFCs reactors

A double chamber Tubular MFC reactor was used in this research. The anode and cathodechamber dimensions were 24 cm × 5.08 cm and 24 cm × 7.62 cm, respectively. Carbon felt was usedat both electrodes with 12.5 cm × 20 cm for the anode and 20 cm × 24 cm for the cathode. The
distance between the separator and cathode was 1 cm, as shown in Figure 3. Three layers of theGore-Tex® (60 cm × 15.5 cm in size) separate both chambers. As shown in Figure 2, the external
circuit was connected to the cathode and anode by Copper wire, with an external resistance of 1 kΩsoldered to the Printed Circuit Board (PCB). The digital multimeter monitored the Voltage output andelectrical current.

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Figure 1. Prototype tubular microbial fuel cell.
2.2. Methods
2.2.1.

Media preparation and sterilisation

Liquid Nutrient Broth media was made with 0.6 grams of Nutrient Broth Powder and 0.225 gramsof D-glucose mixed in 75 mL distilled water. The media was then autoclaved at 121 °C for 20 minutes
at 15 psi to sterilise it.
2.2.2.

Microorganism growth

A spore of Shewanella oneidensis MR-1 has grown aseptically on a nutrient broth fresh medium.
The incubation was carried out at 30 °C. A hemocytometer was used to monitor the growth phase ofbacterial growth [40].
2.2.3.

Substrate preparation for anode chamber

Molasses and distilled water were sterilised by autoclave to kill any microorganisms that may livein them. Then, the pH of the acidic molasses was neutralised with a 5M KOH solution to pH 7, and aphosphate buffer solution of pH 7 was added. Neutral buffers were used to stabilise the electrode pH
at an optimal level to get a good performance of MFC [5].
2.2.4.

Cathode solution preparation

Cr (VI) solution was prepared by dissolving 30 mg of potassium dichromate (K 2Cr2O7) in
300 mL distilled water. This reference is according to Indonesia Government Rule Number 22
of 2021 on class II river water quality standards [41]. Then, Buffer citrate was used to maintain thecathode acidity at pH 3.

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2.2.5.

Electrode modifications

2.2.5.1. Cathode modification
The impact of electrode modifications on MFC performance was analysed in this research. Cu
modified cathode since a highly active catalyst causes the cathodic reaction’s activation energy tolower and increase the reaction rate [12]. The CuSO4 was dissolved into 87.8 wt% distilled water. Then,
added 20 mL of ethylene glycol. Wang et al. revealed that 87.8 wt% Pt/C catalysts synthesised with a
20 mL Ethylene Glycol/23 mL H2O ratio exhibit comparable electrocatalytic properties such as
Oxygen Reduction Reaction (ORR) and stability. It was concluded that the controlled addition of water
to the Ethylene Glycol solvent during the catalyst synthesis process promotes not only smaller particle
sizes and more uniform size distribution but also favourable electrochemical properties of the catalyst
in terms of mass activity and durability. Because of the synthesis process’s simplicity and ease offiltering when water is added to Ethylene Glycol, these positive impacts have significant implications
for applications in the large-scale production of Pt/C catalysts for PEM fuel cells [42]. These changes
were applicable when the CuSO4 catalyst was used in this research. On the other side, carbon felt was
put into the mixture of 25 mL of ethylene glycol and 23 mL of distilled water. Then, CuSO4 solution
was added dropwise into the carbon felt container. The pH was adjusted to 7 using 1M NaOH solution.
The mixture was refluxed at 72 °C within 3 hours. Then, the modified carbon felt was dried in an ovenat 110 °C.
2.2.5.2. Anode modification
Rice bran, containing high silica [31] able to stick around the anode surface, modified the anodeto increase the surface area for bacterial attachment. Anode modification was prepared by spreading
the rice bran-carbon paste on the carbon felt surface. The modification paste was made bydissolving 8 grams rice bran with distilled water, and then 25 mL of rice bran solution was mixed
with 0.125 grams carbon powder.
2.3. MFC experimentThis study used double tubular MFC reactors, as shown in Figure 1. The cathode and anodechambers were separated using a Gore-Tex® cloth separator, as shown in Figures 2 and 3. We used 3
layers of the Gore-Tex® in each experiment. 6 variables were analysed, as seen in Table 1. The cathodechamber was filled with Cr (VI) solution. The anode chamber was filled with sterilised molasses and1011 cells of Shewanella oneidensis MR-1 at their log phases mixed with/without 15 mL of Sidoarjo
mud as electricigen bacteria. Carbon felt was used at both electrodes. The carbon felt was cut into
12.5 cm × 20 cm for the anode and 20 cm × 24 cm for the cathode and sewn with copper wire. Then,the anode is varied with/without modification of rice bran, and the cathode is varied with/without
modification of Cu catalyst. The distance between the separator and cathode is 1.0 cm, as shown in
Figure 3.
The MFC chamber was connected to an external electrical circuit and observed for 12 days. A
digital multimeter was used to analyse the electrical current and voltage produced daily. The power
density was calculated as shown in Eq 1, where V is voltage (V), I is current (mA) and A is electrodearea (m2) [43]. The BOD5 (biochemical oxygen demand) analysis was carried out every four days,
referring to SNI 6989.72: 2009. SNI stands for Indonesian National Standard. COD analysis was
carried out on the first and last days of observation, referring to SNI 6989.2: 2009 and Cr (VI) analysis
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every four days, referring to SNI 6989.71: 2009. The BOD5, COD and Cr (VI) removal percentage canbe calculated using Eq 2.
𝑝𝑜𝑤𝑒𝑟 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 (𝑚𝑊/𝑚2 ) =%𝑅𝑒𝑚𝑜𝑣𝑎𝑙 =𝐼 (𝑚𝐴) 𝑥 𝑉 (𝑣𝑜𝑙𝑡)
𝐴 (𝑚2 )

(1)

𝐼𝑛𝑖𝑡𝑖𝑎𝑙 – 𝑂𝑢𝑡𝑙𝑒𝑡
𝑥 100%
𝐼𝑛𝑖𝑡𝑖𝑎𝑙(2)

Figure 2. Tubular microbial fuel cell diagram.

Figure 3. Illustration of tubular microbial fuel cell variables.
Note: a is the anode chamber diameter; b is the cathode chamber diameter; ∆x is the number of layers of the GoreTex® separator (3); ∆y Cathode-separator distance (1 cm); Size for anode = 12.5 cm × 20 cm; Size for cathode = 20cm × 24 cm; Separator Size = 60 cm × 15.5 cm (3 layers).

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Table 1. Sample code and different variations.
Code

Substrate

Types of electricigen Bacteria usedcathodeAnode

S/CWM/AWM

Molasses + hexavalent

Shewanella oneidensis MR-1

Cathode without

Anode without

modification

modification

Cathode modified

Anode withoutwith Cu Catalyst

modification

Cathode modified

Anode modified with

with Cu Catalyst

rice branchromium
S/CM/AWM

Molasses + hexavalent

Shewanella oneidensis MR-1

chromium
S/CM/AM

Molasses + hexavalent

Shewanella oneidensis MR-1

chromium
SSi/CWM/AWM
SSi/CM/AWM
SSi/CM/AM

Molasses + hexavalent

Shewanella oneidensis MR-1 +Cathode without

Anode without

chromiumSidoarjo mud

modification

modification

Molasses + hexavalent

Shewanella oneidensis MR-1 +Cathode modified

Anode without

chromiumSidoarjo mudwith Cu Catalyst

modification

Molasses + hexavalent

Shewanella oneidensis MR-1 +Cathode modified

Anode modified withchromiumSidoarjo mudwith Cu Catalyst

rice bran

Note: S: Shewanella oneidensis MR-1; Si: Sidoarjo mud; CM: cathode modified with Cu Catalyst; CWM: cathode without modification;
AWM: anode without modification; AM: anode modified with rice bran.

3.

Results and discussion

3.1. Growth curve of Shewanella oneidensis MR-1
A Nutrient broth Agar (NBA) fresh medium was used aseptically to grow a spore of Shewanella
oneidensis MR-1. The bacteria were used in their log phase. The log phase is when the number of cells
increases exponentially with time [44], and cells in this phase are the healthiest [45]. Therefore, before
the bacteria Shewanella oneidensis MR-1 was added to the reactor for electricity production, it wasnecessary to know the log phase of this bacteria and the optimum time. The optimum time for the log
phase is when the highest bacterial population growth is achieved, after which the bacteria undergo adeath phase marked by a significant decrease in bacterial populations [46]. The growth curve for
Shewanella oneidensis MR-1 was observed every hour during 24 hours of incubation to determine theoptimum time. Regularly counting the bacterial population in the culture media was performed usinga hemocytometer.

Figure 4. Growth curve of Shewanella Oneidensis MR-1.

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As shown in Figure 4, it was noticed that there was a considerable increase in the number of cells
at the log phase [47]. The log phase for Shewanella oneidensis MR-1 is between the 11–13th hour.
After the 13th hour, the bacteria experienced a death phase. Therefore, the bacterial culture wastransferred to the MFC chamber at the 13th hour.
3.2. pH control during MFCs operation
MFC utilises exoelectrogenic bacteria to reduce pollutants and directly produce electrons andprotons then converted into electricity [48]. The rate of microbial growth is influenced by pH [49].
Since microorganisms play an important role in an MFC reactor, the pH of an electrolyte is among theparameters that may affect the efficiency and performance of MFC. The transport of cationic ions (Na+,K+, etc.) in the cathodic chamber could raise the pH and conductivity. Because protons are consumed
in the cathode reaction, transport of cation ions (other than H+) results in an increased pH in the cathodechamber, which reduces MFC performance [5]. Hence, the pH of both electrodes should be kept at an
optimal level to get a good performance of MFC. Neutral buffers are techniques that may be used tostabilise the electrode pH.

Figure 5. pH in the anodic chamber for all variables within 12 days of MFC operation. a:
pH in the anodic chamber using only Shewanella oneidensis MR-1 as an electricigen
bacteria; b: pH in the anodic chamber using Shewanella oneidensis MR-1 mixed withbacteria in Sidoarjo mud as electricigen bacteria.
Both Shewanella oneidensis MR-1 and electricigen bacteria in Sidoarjo Mud have the optimum
pH growth of 7 [50]. The slow transfer of protons from anode to cathode and the accumulation of H+
protons in the anode chamber decreased the pH value. Therefore, a strong base, 5M KOH, kept the pH
neutral. Here, the neutral buffer wouldn’t be sufficient since the rate of H+ transferred was slower than
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the rate of H+ formed. Figure 5 shows that the pH control was done every 3 hours from 0 to the 3rd
day of MFC operation for the variables with no cathode modification. From 0 to the 2nd day of MFC
operation for the variables with the modification of cathodes with actively Cu catalysts. Therefore, theelectroneutrality for electrodes modified by Cu catalysts is faster than for electrodes without
modification in a tubular MFC reactor. Kaul et al.[51] revealed that modified cathode using Cu metal
increases the reaction rate. From the 3rd to the rest of the days, the anode chamber’s pH change wasinsignificant. Therefore, pH control wasn’t necessary. There was a balance between the rate of transfer
of electrons and the rate of proton transfer. This condition is called electroneutrality[12].
3.3. Effect of electrode modification on the production of power density
MFC removes pollutants while simultaneously producing electricity [52]. MFC works byoxidising the organic substrate into smaller molecules using the microorganism and producing
electrons and proton H+. Then, the produced electrons are transported to the cathode through anexternal electrical circuit and converted into electrical energy while the protons migrate into thecathode surface. The potential difference between both electrodes causes the flow of electrons. The
cathode’s protons and electrons are consumed by reducing soluble electron acceptors such as oxygen.
In brief, the reactions that occur in MFC are [53]:𝑎𝑛𝑜𝑑𝑒: 𝑜𝑟𝑔𝑎𝑛𝑖𝑐 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 → 𝐻 + + 𝑒 − + 𝑜𝑥𝑖𝑑𝑖𝑧𝑒𝑑 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒(3)

𝑐𝑎𝑡ℎ𝑜𝑑𝑒 ∶ 4𝐻 + + 4𝑒 − + 𝑂2 → 2𝐻2 𝑂(4)This study used a double tubular MFC with hexavalent chromium waste as a catholyte andmolasses as an anolyte. Shewanella oneidensis MR-1 mixed with bacteria in Sidoarjo mud was utilised
as an electricigen bacteria that oxidised the molasses and transferred the produced electrons to theanode. The flow of produced electrons streamed through an external electrical circuit and copper wire
while the protons migrated into the Gore-Tex® separator from the anode to the cathode chamber. The
cathode’s protons and electrons are consumed by reducing hexavalent dichromate ions (Cr2O72-) into
Cr3+ ions. The following reactions took place[54]:
𝐶𝑟2 𝑂7 2− + 14𝐻 + + 6𝑒− → 2𝐶𝑟 3+ + 7𝐻2 𝑂(5)The power density generated in the first days was low due to the adaptation of electricigen bacteria
to the new environment. In this study, we had a continuous reactor, so by adding the substrate, thepower density increased and decreased at a certain time. As seen in Figure 6a, we only used Shewanella
oneidensis MR-1 as an electricigen bacteria. The power density was high on the 5th to 6th day of MFC
operation, then declined until the 12th day of MFC operation. In contrast, by mixing Shewanella
oneidensis MR-1 and Sidoarjo mud from days 10–12, we continue to experience the power density
increase as seen in Figure 6b. The authors suspect that the electricigen bacteria competed with nonelectricigen bacteria until the 10th day, even though no microbial community analysis was performed.
Therefore, from the 0–10th day, the produced bacteria were mostly non-electricigen bacteria.
S/CM/AM, which only used Shewanella oneidensis MR-1 as an electricigen bacteria with bothmodified electrodes, achieved the highest power density of 3.13 mW/m2 on the 6th day of MFC
operation.

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Figure 6. Power density produced using a double chamber Tubular MFC within 12 days.
a: power density produced using only Shewanella oneidensis MR-1 as an electricigen
bacteria; b: power density produced using Shewanella oneidensis MR-1 mixed withbacteria in Sidoarjo mud as electricigen bacteria.

Figure 7. Total power density on each variable produced within 12 days of MFC operation.

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The Trapezoidal Rule was used to calculate the area under the curve and obtain the total power
density within 12 days of MFC operation. As shown in Figure 7, all the variables with electrodemodifications produce an amount of power density than the unmodified electrode. Modifying theanode with rice bran increased the contact area [31] between electricigens bacteria and the anoderesulting in a higher power density production. Moreover, rice bran contains a high organic compound,which increases bacteria growth. On the other hand, the cathode modification with Cu causes thecathodic reaction’s activation energy to lower and increase the reaction rate [12]. The total power
density produced by S/CM/AM, which only used Shewanella oneidensis MR-1 as an electricigen
bacteria with both modified electrodes, was 1.6 times higher than S/CWM/AWM only used Shewanella
oneidensis MR-1 as an electricigen bacteria with both unmodified electrodes.
Gul et al. [5] revealed that the mixture of exoelectrogenic bacteria might boost electricity
production. This research used Shewanella oneidensis MR-1 and Sidoarjo mud as electricigens
bacteria. As seen in Figure 6, the total power density produced by mixing Sidoarjo mud with
Shewanella oneidensis MR-1 was low compared to the power density produced using only Shewanella
oneidensis MR-1. The total power density produced by SSi/CM/AM, mixing Shewanella oneidensis
MR-1 and Sidoarjo mud as an electricigen bacteria with both modified electrodes, was 221 mW/m2,
while the total power density produced by S/CM/AM, which only used Shewanella oneidensis MR-1
as an electricigen bacteria with both modified electrodes, was 530 mW/m2. The result wasn’t inaccordance with Gul et al. [5] that a mixed culture of electricigen bacteria in MFC might enhance thepower density produced. It was concluded that the bacteria in Sidoarjo mud inhibit the growth of
Shewanella oneidensis MR-1, resulting in lower power density production.
3.4. The growth of microbial population in MFC
Researchers revealed that microbial growth depends on different factors, including but not limited
to substrate concentration, pH and temperature [15]. As seen in Figure 8, the microbial population was
low in the first days due to their adaptation to the new environment. The microbial population increases
at a certain time and declines for the rest of the days since they have entered the death phase. It was
attributed to the low availability of organic compounds over time. The highest microbial population(66.5 × 1010 cells/mL), as seen in Figure 8b, was achieved by SSi/CM/AM, mixing Shewanella
oneidensis MR-1 and Sidoarjo mud as an electricigen bacteria with both modified electrodes variable,at the 6th day of MFC operation. In this study, the highest microbial population growth was seen whereis a mixed culture of electricigen bacteria. It was due to the large size of inoculation in the mixed
culture. In addition, an organic compound [18] in Sidoarjo mud may favour microbial growth.
However, in Sidoarjo mud, there are the electricigen and non-electricigen bacteria. As seen in
Figure 7, the highest total power density was achieved by the S/CM/AM variable even though thehighest number of microbial growth was achieved by the SSi/CM/AM variable since the bacteria in
Sidoarjo mud inhibit the growth of Shewanella oneidensis MR-1. On the other hand, the microbial
population for variables with modified electrodes is higher than for variables without electrodes’
modification. It shows that rice bran’s organic content is essential to microbial growth.

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Figure 8. Microbial population growth in MFC within 12 days. a: Microbial population
growth in variables using only Shewanella oneidensis MR-1 as an electricigen bacteria; b:
Microbial population growth in variables using Shewanella oneidensis MR-1 mixed withbacteria in Sidoarjo mud as electricigen bacteria.
3.5. The effect of electrode modification into BOD5 and COD removal
The highest BOD5 and COD indicate pollution in water [55]. Gul et al. [5] revealed that thedecrease of pollutant removal in MFC indicates that active bacteria oxidise the large molecules into
smaller molecules while producing electrical energy. In this study, Shewanella oneidensis MR-1 andbacteria in Sidoarjo mud influenced the pollutant removal in organic molasses waste.
Figures 9 and 10 show that SSi/CM/AM, mixing Shewanella oneidensis MR-1 and Sidoarjo mud
as an electricigen bacteria with both modified electrodes, achieved 61.28% and 59.49% of BOD5 and
COD removal, respectively, on the 12th day of MFC operation. The bacteria population mainly
influences pollutants removal [50]. As shown in Figures 9 and 10, the most effective removal of BOD5
and COD occurs between the 4th to 8th day of MFC operation; this was directly proportional to thehighest microbial population present in the reactor, as seen in Figure 8. Moreover, the variables withmodified electrodes could produce pollutant removal higher than for variables without electrodes’
modification. It was due to the increased microbial population in modified electrodes, as shown in
Figure 8. On the other hand, variables with a mixed culture of electricigen bacteria produce highpollutant removal than variables that used only Shewanella oneidensis MR-1, as shown in Figure 9 but
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produce lower total power density, as shown in Figure 7. It was because non-electricigen bacteria were
dominant in mixed culture, resulting in lower total power density.

Figure 9. BOD5 removal of molasses substrate in a double chamber Tubular MFC reactor
within 12 days of MFC operation.

Figure 10. COD removal of molasses substrate in a double chamber Tubular MFC reactor
within 12 days of MFC operation.
3.6. The effect of microbial population and electrode’s modification for Cr6+ removal
In MFC, the potential difference between anode and cathode causes the flow of electrons andprotons from reducing organic substrate by electricigen bacteria towards the cathode chamber. The
protons and electrons at the cathode chamber reacted with hexavalent dichromate ions into trivalentchromium [56]. Reducing hexavalent chromium to trivalent chromium with less toxicity [19] is highly
preferred for a sustainable future.

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Figure 11. Cr6+ removal in a double chamber Tubular MFC reactor within 12 days of MFC
operation.
Figure 11 shows that the highest percentage of hexavalent chromium reduction of 99.38% wasachieved by S/CM/AM, which only used Shewanella oneidensis MR-1 as an electricigen bacteria withboth modified electrodes. It was due to the highest electrons and protons produced, which reacted tochromium hexavalent in the cathodic chamber. The highest percentage of hexavalent chromium
removal and total power density, as seen in Figure 7, were higher when we used only Shewanella
oneidensis MR-1 as an electricigen bacteria than in mixed culture since the bacteria in Sidoarjo mud
inhibit the growth of Shewanella oneidensis MR-1. On the other side, the highest percentage ofhexavalent chromium was higher with modified electrodes than without modification. Cu metal
facilitates the movement of electrons from the anode to the cathode through the electrical circuit.
Therefore, the rate of chromium reduction reaction is faster. In addition, Cu metal causes the cathodic
reaction’s activation energy to lower and increase the reaction rate [12], resulting in higher power
density and hexavalent chromium removal.
Table 2. Different variables on MFC performance and pollutant removal.
Sample code

BOD5 removal (%)

COD removal (%)

Cr6+ removal (%)

Total power density (mW/m2)

S/CWM/AWM

26.92

38.46

96.49

329

S/CM/AWM

40.36

52.84

98.92

450

S/CM/AM

59.41

56.53

99.38

530

SSi/CWM/AWM

40.82

52.21

97.85

184

SSi/CM/AWM

52.47

53.88

98.87

221

SSi/CM/AM

61.28

59.49

99.26

298

3.7. Uncertainly analysis
The results of the analysis of the experiments obtained are strongly influenced by several factors,
such as: Sidoarjo Mud, molasses, and electrode modification with rice bran. With all of these factors,
the Sidoarjo mud is one of the most significant uncertainties. This is due to the condition of the Sidoarjo
mud which since 2006 had been gushing onto the earth until now and it is still releasing hot mud fromAIMS Environmental Science

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the earth. The Lapindo mud is a hot mudflow event located in Sidoarjo, East Java—Indonesia on
May 29, 2006 [57]. So that, the microbial community used in the study, from year to year also
experiences differences. This is shown by research related to the analysis of microbial communities
that has been carried out in 2018 (Figure 12) and 2022 (Figure 13), as shown in the following figures.

Figure 12. Microbial community analysis from Sidoarjo mud collected in 2018.

Figure 13. Microbial community analysis from Sidoarjo mud collected in 2022.
4.

Conclusions

This research proposed an innovative and environmentally friendly method for waste
management. The electric energy was produced from the organic compound in molasses whileremoving pollutants, and carcinogen hexavalent chromium was reduced to trivalent chromium using a
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double chamber tubular microbial fuel cell. Shewanella oneidensis MR-1 and Sidoarjo mud mixture
as electricigen bacteria achieved the highest BOD5 and COD removal of 61.28% and 59.49%,
respectively. But, only Shewanella oneidensis MR-1 achieved higher power density production andhexavalent chromium removal of 530.42 mW/m2 and 98.87%, respectively. It was due to the higher
microbial population in mixed culture due to the large size of inoculation and organic compound in
Sidoarjo mud that may favour microbial growth. But, non-electricigen bacteria were dominant inmixed culture, resulting in lower total power density and hexavalent chromium reduction. On the other
hand, modified anode with rice bran and cathode with Cu metal resulted in higher power density
production, pollutant removal and hexavalent reduction than unmodified electrodes. Therefore, thecurrent finding indicates that double chamber tubular microbial fuel cells with modified electrodes
may be a better solution for managing molasses and carcinogen hexavalent chromium.
Acknowledgements
The authors gratefully acknowledge financial support from the Institut Teknologi Sepuluh
Nopember for this work, under the project scheme of the Publication Writing and IPR Incentive
Program (PPHKI) 2022.
Conflict of interest
The authors declare no conflict of interest.
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