Elsevier

Biochemical Engineering Journal

Volume 133, 15 May 2018, Pages 205-213
Biochemical Engineering Journal

Regular article
Enhancement of bioelectricity generation and algal productivity in microbial carbon-capture cell using low cost coconut shell as membrane separator

https://doi.org/10.1016/j.bej.2018.02.014Get rights and content

Highlights

  • Use of coconut shell (CS) as proton exchange membrane is proposed.

  • Presence of hygroscopic oxides and pore structure in CS favour transfer of protons.

  • MCC with CS membrane gave better power density and CE than MCC with Nafion.

  • Enhanced algal growth observed in the cathodic chamber of MCC using CS separator.

Abstract

Proton exchange membranes (PEMs) are the most prominently used separator in microbial fuel cell (MFC) and microbial carbon capture cell (MCC). This study aims at evaluating the characteristics of coconut shell (CS) to explore its potential as a PEM. The CS exhibited superior water absorption (32%), which can stimulate the proton transmission through water molecules to the cathodic chamber. The proton conductivity of CS separator was comparable to Nafion 117; however, the oxygen mass transfer coefficient of CS separator was lower than Nafion 117, indicating it as superior separator. These separators were used in MCC with Chlorella sorokiniana grown in cathodic chamber. The maximum power density (MPD) and coulombic efficiency (CE) of MCC with CS separator were 3.2 W/m3 and 16.53%, respectively, whereas the MCC with Nafion 117 membrane showed a MPD of 1.8 W/m3 and CE of 8.42%. Although the COD removal efficiency in the anodic chamber of Nafion-MCC (72.14 ± 0.15%) was superior to CS-MCC (65.97 ± 0.83%), the algal specific growth rate at cathode was found better in CS-MCC (2.64 day−1) than Nafion-MCC (2.16 day−1). This study reveals the feasibility of using CS as low cost as well as energy efficient membrane separator for the application in MCC.

Introduction

Microbial fuel cell (MFC) technology has been considered as a wastewater treatment process having ability to offer long term solution for the increasing clean and reliable energy demand all over the world. As a key component of MFC, proton exchange membranes (PEMs) are gaining extensive attention in recent years because of its selective permeability towards protons to run the MFC with highly efficient way. However, the Nafion membrane, which is most commonly used in MFC, is associated with several limitations such as oxygen diffusion, cation accumulation, substrate loss, durability due to fouling, high cost, etc. [1]. These limitations have led to immense efforts in the development of alternative membrane materials that can effectively serve as low-cost PEM.

Over the past decade, researchers have focused on modification of Nafion membrane by pre-treatment of the membrane [2], developing composite membranes like Nafion embedded with silicon micro wire [3], Nano-fiber/Nafion PEM [4] etc. In 2010, Behera et al. [5] proved that ceramic separator can be a substitute for costly PEMs, which demonstrated a power output of 16.8 W/m3 without employing any cathodic mediator. Similarly, a few studies were also done on ceramic separator with special emphasis on ceramic thickness [6], ceramic types [[7], [8]], addition of cation exchangers such as montmorillonite and kaolinite [9], economic analysis [10] and certain challenges such as salt deposition and electrolyte evaporation on membrane surface [11].

As reflected from the scientific explorations about PEM till date, the performance of membrane system is largely decided by the membrane material and its composition. Biological membranes have very high water permeability and selectivity, which can improve the performance of membrane in terms of proton conductivity [12]. Considering characteristics such as chemical composition, stability and micro-level structure; coconut shell (CS) can be a low-cost alternative to the existing PEMs. A review of CS properties by Ting et al. [13] has demonstrated the presence of the following oxides by XRD analysis: SiO2 (45%), Al2O3 (15.6%), Fe2O3 (12.4%) and other oxides like MgO, CaO, Na2O, K2O etc. in trace amounts. Hygroscopic oxides such as SiO2 can improve hydration properties of CS separator along with strength. Achaw and Afrane [14] have reported that at micro-level, CS pore structure is made up of cylinder like tubes which can facilitate the passage for water molecules, the proton carriers [15]. Aside from chemical composition and structure of CS, its low cost, good thermal properties, high toughness and reduced wear increase the scope of using it as a membrane material for efficient performance of MFC.

A terminal electron acceptor (TEA) get reduced by consuming electrons and protons, produced through the oxidation of organic compound by micro-organism at anode, that reach the cathode through external circuit and PEM, respectively. Thus an efficient but easily available TEA [[16], [17]] along with low-cost PEM are a pre-requisite for good performance of an MFC. As a source of oxygen, micro-algae can replace costly and energy consuming aeration system [18], thus converting MFC into a microbial carbon-capture cell (MCC). Chlorella sorokiniana, known for its high photosynthetic productivity and lipid content, is a good feedstock for biodiesel production along with its application in MCC [19].

The present work aims to study the feasibility of bioelectricity generation and algal production in an MCC employing CS as novel low-cost separator. A comparative performance evaluation with respect to wastewater treatment, power generation and algal production of two different MCCs employing different separator, namely CS and Nafion, and using microalgae C. sorokiniana in cathodic chamber was accomplished to achieve this objective. This work was done with an intention to shed light on the application of CS as a membrane separator that can replace the commercially available expensive PEMs.

Section snippets

Collection and preparation of separator

Shell of coconuts discarded by a local coconut exporting firm in Kharagpur, India was used in this study. The shell was dried and surface was cleaned by scraping off the fibrous layer on the shell surface. Since this was the first study in which application of CS as membrane separator material was investigated, no pre-treatment was done to the CS. The separator used in this study had an effective surface area of 50 cm2 and thickness of 4 mm and it was assembled with the help of non-toxic

Oxygen transfer coefficient

A difference in DO concentration between anode and cathode chamber is a prerequisite for measuring the oxygen transfer rate across the membrane. In one chamber, the tap water was initially spurged with nitrogen to remove DO; whereas, the second chamber was continuously aerated in order to maintain saturated DO conditions. A DO probe was immersed in the first chamber for monitoring the DO change after every 2 min for a total duration of 90 min. The mass transfer coefficient of oxygen for the

Fourier transform infrared spectroscopy analysis

In FTIR spectra (Fig. 3) it was found that finely powdered and dried coconut shell contains molecular segment similar to Nafion [36]. A strong absorption at 1030 cm−1, which corresponds to sulphonic acid group (single bondSO3H) indicates that CS contains single bondSO3H, which improves the proton conductivity of the separator. Absorption bands observed at 1370 cm−1 and 3390 cm−1 attributed to the bending and stretching vibration of hydroxyl (single bondOH) group, that comes from the sulphonic acid group (-SO3H), which verifies

Conclusion

Proton exchange membranes impulsively play a significant role in performance of MCC. It is worth to mention that CS can be directly used as low cost cation exchange membrane without the need of costly membranes like Nafion. Presence of hygroscopic oxides in CS improves the performance of MCC by enhancing the cation transport ability and its compact structure reduced the oxygen diffusion. The CS used as PEM in this study showed good proton conductivity but still has some problems like biofouling

Acknowledgements

The research project was supported by Department of Biotechnology, Government of India (BT/EB/PAN IIT/2012) providing the financial assistance. The authors would like to thank Mr. Sovik Das and Mr. Chhattulal Maity for the support and assistance provided in completing the work.

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