3.1 The bio-catalytic devices modification characterization and its electrocatalytic activities.
The surface images of anode modification by immobilization of yeast within alginate polymer and PNR layer on the anode surface have been observed by SEM and EDX as displayed in Fig. 3.
Images found that there are several spherical shapes around 4 nm, spread homogenous within the alginate polymer film and it was confirmed as yeast cells. The thickness of alginate polymer film was varied from 4 to 16 nm and has a good roughness is due to the presence of yeast inside and ready to be used for electro-catalytic process. Fig 3c and 3d shows the morphology of the PNR film deposited on the CF support refers from EDX spectrum. Results found that 5.04% of N has been contained in CF modified and it was confirmed that the PNR has been presented at the CF surface. The present of C and N atoms was completed expected since they come from NR and CF, meanwhile potassium and oxygen atoms from KNO3 solution which used as the supported electrolyte solution during the synthesis of polymer and sodium could delivered from phosphate buffer as supporting pH solution.
The observation of current density delivered from bio-devices modification has been recorded and All the experiments result as seen in Fig 4.
The chronoamperometry observation has been performed, electrodes were polarized at 0.3 V/SCE to ensure glucose oxidation. All surface modifications have been tested as working electrode and Pt as counter electrode. Results repot that CF/PNR has the highest current density delivered compared with others. Meanwhile CF without modification has been examined also using 2 % of yeast and 10 mM NR in PB pH 6 solution. Fig 4 a reports that compared with conventional method, by employing surface modification through the formation of PNR layer gives the best performance delivered 64% of current density value against to CF/Y-NR in solution. While, CF/Immob Y-NR and CF/PNR–Immob have been resulted enhancement 43% and 35% of current density value compared with conventional method i.e anode without modification. Besides that, CF/Immob-Y without involving NR has been act as base line of measurement. All current density values generated have been monitored during 2 hours after a 30 minutes of stabilization time.
The next electro-catalytic observation was continued through MFC and dual-chambers of MFC have been prepared. All the surface modification electrodes have been applied as anode, there were CF/Immob Y-NR; CF/PNR; CF/PNR-Immob Y. However, a conventional MFC has been carried out also using yeast and NR in solution where CF without modification plays as anode. The power density produced from MFC has been recorded using digital multimeter voltcraft (VC 850) and 1 kΩ of load has been fixed. Results confirmed that the utilization of PNR as surface modification method provides good effectiveness of MFC process during 10 days of observation as displayed in Fig. 4b. According to the results, PNR definitely leads to a higher power density followed with CF/Immob Y-NR then CF/PNR-Immob Y if we compare the results with applying conventional MFC. In our case, after 10 days of experimental observation, the maximum power density delivered from CF/PNR was 5.02 + 0,25 W m-2. It is difficult to compare this value with previous research that has been studied, because MFC operate under a large variety conditions such as temperature, pH, the availability of mediator, material electrode, size of reactor and time of operation. However, the maintenance of pH is important to provide the optimum operation conditions. In this present study, the measurement have been done at pH 7 as a good alternative to get high performance of MFC [32]
Meanwhile, the scheme of electron transfers from glucose to the electrode has been explained in Fig. 4c. MFCs working with surface modification shows a good stability of current especially stable by using NR as mediator entrapped within surface of anode. During perform as anode in MFC, the biological activity of yeast within glucose oxidation has been facilitated by PNR. The PNR oxidized form captures electrons generated and PNR reduced form will directly transfer electron to anode. The recycling of NaDH to NAD+ is important to keep the sustainability the glycolysis process in fermentation pathways. The use of redox mediator, substituting between oxidized state (PNRox) and reduced state (PNRred) is obligatory in order to touch the electron transfer chain which is found at the mitochondria within cytoplasm [46].
3.2 Microbial desalination cell Characterization.
As preliminary work, the determination of the best configuration has been done using bio-devices non modification. The composition of each chamber compose of, 2% of yeast in 0.1 M glucose; 0.1 M KNO3; 0.1 M NaCl and 0.02 M PF as solution in anode, concentrate, dilute and cathode chambers, respectively. The observation of current density has been recorded during 10 days delivered from each variation of configuration model design and results can be seen in Fig 5a.
Fig 5a illustrations that from model 2, the % transport of salt ion was lower than model 1. Refers to model 1, 31% of salt ion has successfully migrated from dilute to concentrate chamber, meanwhile 20% of NaCl have been removed from initial concentration. Cl- has migrated from dilute to anode and passing AEM, in contrast Na+ has moved to concentrate chamber after passing through CEM. Moreover, the measurement of pH on anolyte has been investigated also, the results was obtained the fact that the pH of anolyte has decreased from 6.88 as initially to 4.78. It was assumed that protons could not pass through the AEM membrane, so they remain in anode chamber. In the same time, the present of Cl- ion will react with proton and produced HCl. The evolution of pH value on anolyte could affect the stability of biocatalyst performance, then for the next MDC appearance, model 2 is no longer recommended for used.
In addition, Fig 5b describes the diffusion process during MDC where the concentration gradient across to ion exchange membrane (IEM) can also induce ion diffusion processes. In our study, this phenomenon has been monitored refers to model 1 of MDC during 14 days operation and without the presence of yeast in anode. Results confirm that 9.3 % of Cl- ion concentration has been decrease from dilute compartment. This can be interpreted that diffusion can contribute to the MDC process. Meanwhile, as seen in Fig 5c, the conductivity from dilute and concentrate chamber have been observed also and result mention that there has been an increase the conductivity value from concentrate chamber up to 9% from the initial 50.8 mS s-1 into 55.2 mS s-1 respectively. The concentration gradient across to IEM also induce the osmotic water transport. It was suggested that the osmotic water transport has been delivered from cathode and compartment chamber to dilute chamber. During diffusion process, 8.4 mL of water has been moved into dilute chamber for 14 days of MDC perform and resulting up to 9.3% of reduction of salinity (initially 3.5 g L-1 NaCl).
In the meantime, the electrodialysis process has been monitored using Chronopotentiometry by current employing of 50 µA, refers to the average value of the current generated from model 1. Fig 5d. reports that Cl- ions have been migrated from dilute to concentrate chamber during electrodialysis and produced 16% of ion transport during 14 days of electrodialysis process. However, it should be noted that the most important as driving force in MDC is the electric current generated by exoelectrogenic microbes.
3.3 Microbial desalination cell using bio-devices modification.
First MDC observation has been made by employing bio-devices non modification, 43% of NaCl transport and 73.3 mA m-2 of maximum current density have been obtained from 30 days of running. The observation of salt ion concentration has been measured from dilute chamber. The maintenance of all MDCs perform have been conducted by the addition of 1 g glucose in anolyte and refreshing of 0.02 M PF in catholyte every 13th day of perform. As described in Fig 6d. initially the currents generated were an increase then gradually decreased until 13th days, but after refreshment of anolyte and catholyte composition, the improvement of the current value does not appear until at the end of perform. It can be assumed that the lifetime of yeast has been reached.
Hereinafter, studies were performed by varying the bio-devices modification as anode to understand the effect of modification and its MDC behavior on desalination process during 30 days perform and bioelectricity generation. CF/Immob Y-NR has been tested in MDC and resulted the 83 mA m-2 of max current density and 55% of NaCl transport. As seen in Fig 6a at 11th day of running, the current has been decrease slightly and at 20th day, there has been declined until the end of process. Then, the CF/PNR electro-kinetic behavior has been investigated and delivered 92.5 of max current density and 61 % of NaCl transport. Compared with others, this value is highest both for current density and % of NaCl transport. The reduction of electrochemical signal has been occurred at 12th day and slightly becomes drop from 19th day until completed the process as reported in Fig 6b. It was suggested that the presence of PNR layer has been successfully plays as redox mediator at surface of anode and provides the effectiveness of electron transport process.
Moreover, at the end of investigation the CF/PNR-Immob Y has been used as anode. According to the results in Fig 6c. The evolution of current density has been delivered 80.2 mA m-2 of maximum current density and 51.53% of NaCl transport. After 11th day although glucose and PF have been added, it seems there was no significantly affect to the signal produced. It was assumed that the existence of alginate layer inhibits for glucose to penetrate within yeast alginate layer and this condition causes the slightly decrease current generated until MDC accomplishment. All experiments result can be seen in Fig 6d.
Prior used, the composition of sea water has been determined and the large concentration was coming from sodium and chloride ( i.e 12.14 g L-1 and 22.85 g L-1). While the other ions such as nitrate (1.35 g L-1); nitrite (0.99 g L-1); sulfate (6.30 g L-1) potassium (2.05 g L-1); calcium (3.05 g L-1); magnesium (2.05 g L-1) and phosphate (0.98 g L-1). During MDC process, the electrical potential gradient was created by the electrode reactions, and commonly responsible for the salt ion migration. Besides that, the IEM junction potential and water transport could affect also to the desalination rate. In a four-chamber MDC, these factors work can play also as additional driving forces for desalination.
The current efficiency from this experiment has been measured refers to eq 2 [47].
The current efficiency (ηi) is the amount of ion divided by the amount of electrons transferred at the bio-devices modification. Where ∆c is reduction of salt water concentration, V is the volume desalinated, Ncp is the number of cell pairs and i is the current. All the current efficiency values are displayed in Table 1. However, the current efficiency is also dependent on the existence of mediator in anolyte, because the electron transport has been facilitated by mediator. Tabel 1 also describe the efficiency of bio-devices modification expressed per availability mass of mediator from each bio-devices.
Table 1 explains that the highest efficiency value was obtained from CF/PNR followed with CF/ImmobY-NR and CF/PNR-Immob Y. It was assumed that the manifestation of PNR layer at the anode surface is more effective than the monomer (NR) entrapped in yeast immobilization layer. Hence, the thickness of the alginate layer can also be one of the obstacles in the process of entry of glucose into the surface of layer, where the electricity generated mostly dependent of glucose oxidation.
We can estimate the amount of PNR deposited on surface refers to Equation 3:
where b is the number of electrons exchanged per mole involved in the redox couple of PNR (b=2); w (g) is the weight of PNR; F is Faraday constant (96.500 C) ; M is NR molecular weight ; and v represents the scan rate. According to equation 3, the amount of PNR is 14.2 µg cm-2 from 10.2 cm2 of surface area.