Elsevier

Chemosphere

Volume 287, Part 3, January 2022, 132277
Chemosphere

Alizarin-graphene nanocomposite for calibration-free and online pH monitoring of microbial fuel cell

https://doi.org/10.1016/j.chemosphere.2021.132277Get rights and content

Highlights

  • Graphene oxide (GO) interface realized all-solid-state voltammetric pH quantification.

  • Nernstain response of GO enhanced by surface doping of alizarin reaching pH range over 4.0–9.0.

  • Stable pH quantification was illustrated for 7 continuous days.

  • Calibration-free and stable pH monitoring for MFC was realized.

Abstract

Microbial fuel cells (MFCs) are sensitive to acidity variations in both bioelectricity generation and biochemical digestion aspects, therefore online pH monitoring is of necessity to guarantee optimal function of MFCs. Present pH meters hardly fulfill this special need. In this work, we designed a novel voltammetric pH sensor based on electrochemically reduced graphene oxide (rGO) modified screen printed electrode. By surface doping of alizarin, good linearity of pH sensing over the range of 4.0–9.0 can be realized. Fast readout can be acquired within 15 s for each test. pH monitoring for artificial wastewater with inoculum of granular activated sludge in a MFC was successfully illustrated. Specially, it was verified that the performance was improved with alizarin doping due to the enhanced rGO surface proton diffusion. This approach provides an online, calibration-free and long stable pH monitoring method for the future MFC development.

Introduction

Encountering global challenge of ongoing depletion of fossil fuels, climate change and contaminant discharges, summoning of green energy sources and non-toxic waste treatments is becoming louder (Ahammad et al., 2014; Xiao et al., 2021a; Xiao et al., 2021c; Singh et al., 2018; Singh et al., 2015; Pophali et al., 2021; Pophali et al., 2020; Singh et al., 2021). In this regard, microbial fuel cells (MFCs) are attracting intense focus due to its great potential in bio-electrocatalytic digestion of bio-/chemo-mass in wastewaters, and extracting bio-electricity thereby (Choi et al., 2017; Xiao et al., 2019; Kondaveeti et al., 2019; Jadhav et al., 2020). MFCs utilized in removal of organic matter, metal and dyes have become a trend of future wastewater treatment as well as green energy production (Nancharaiah et al., 2015; Neethu et al., 2017; Ilamathi et al., 2018; Xiao et al., 2021b). In order to maximize the benefit from a wastewater MFC model, electricity yield and bio-electrocatalytic activity are the foremost two performances need to exploit. Optimizations in reactor design, electrode material, electrolyte composition, etc., are often involved in the former aspect. However in the later, choice of microorganism as well as working environment stabilization are emphasized especially (Choudhury et al., 2021; Xiao et al., 2021b).

Bio-electrocatalytic activity of the microorganisms used in MFC directly related to the matter removal efficiency in wastewater and, indirectly related to electricity generation efficiency (Xiao et al., 2021b; Bhembe et al., 2020; Dianey et al., 2021; Le et al., 2021). Among the factors, acidity or pH value of the electrolyte is the predominant one determining the bio-electrocatalytic activity. Glass bulb based pH meter is presently the standard method quantifying aqueous pH. However, glass bulb shows many disadvantages in online monitoring in industrial applications, ranging from fragility, instability, alkali error to inevitability of re-calibrations (Streeter et al., 2004; Kakiuchi, 2011; Skoog et al., 2014; Stredansky et al., 2000). MFC processes are often anaerobic, ionicity changing and time costing, which is challenging to the conventional methodology. In comparison, all-solid-state pH sensors are of great promise to satisfy this need, due to dispensation of internal reference system (Dong et al., 2018). However, alkali error and need of re-calibrations are still inevitable problem to those of the potentiometric designs (Webster and Eren, 2014; Lu and Compton, 2014a, b; Michalak et al., 2015). In comparison, voltammetric based all-solid-state pH sensors presented obvious advantages in overcoming the above shortages. Different to the potentiometric ones, voltammetric pH sensors rely on kinetic processes on the electrode surface. Therefore, voltammetric sensors avert re-activations and re-calibrations. Considering calibration-free pH sensing mode is a primary need for MFC monitoring, voltammetric pH sensor is a preferred choice for online pH detection.

Typically, voltammetric pH sensors can be realized via a proton coupled electron transfer (PCET) process (Lu and Compton, 2014a, 2014b). Quinonoid compounds are well studied with a classic 2e/2H+ electrochemical process described by “square-mechanism” and pH quantifications can be therefore achieved and described by Nernst equation (Jacq, 1971; Batchelor-McAuley et al., 2011). Anthraquinones, catechols and N-/O- heterocyclic derivatives were reported to be successful voltammetric pH sensing indicator candidates (Gao et al., 2018; Zuaznabar-Gardona and Fragoso, 2018; Casimero et al., 2018; Read et al., 2019; Tham et al., 2019). The most common challenge is the PCET kinetics modulation under weakly buffered conditions (Dai et al., 2015; Cobb et al., 2019). Quinoid kinetics could be maintained by introducing inter-/intra-molecular hydrogen bonds under unbuffered conditions. The possible mechanism was the enhancement of proton diffusion brought by the extra introduced hydrogen bonds. However, quinoids based voltammetric sensors often suffer from stability decay, activity loss and electrode poisoning. This requires complicated chemical engineering and surface modification which may cause Nernstain response distortions due to the susceptibility of the complex electrochemical kinetics.

In this work, we demonstrated a carbon nanomaterial based interface for all-solid-state voltammetric pH sensor design, facing online monitoring of MFC anodic electrolyte. We selected graphene as the pH sensitive electrochemical indicator, exploiting the electrochemically active functionalities. It was demonstrated that graphene interface underwent a classic quinonoid PCET process, facilitating pH quantification via voltammetric measurements. A typical quinoid, alizarin, was used to enhance the PCET process of graphene interface. More importantly, on-time pH changes of the MFC was realized by this new designed sensor. This sensor did not rely on exogenous pH indicator molecules, avoiding of complex surface modifications. Stable pH sensing was achieved by utilizing the chemical stability of graphene surface. Meanwhile, the preparation of this sensor is extremely easy, labour-saving and cost-effective.

Section snippets

Reagents and instruments

Chemicals and reagents: Monopotassium phosphate (KH2PO4, 99.0%), potassium hydroxide (KOH), hydrochloric acid (HCl, 37%), potassium ferricyanide (K3[Fe(CN)6], 99.5%), potassium ferricyanide trihydrate (K4[Fe(CN)6]·H2O, 99.0%), phosphoric acid (H3PO4, 99.0%), graphite powder (C, 800 mesh, 99.95%), ammonium chloride (NH4Cl, 99.5%), sodium bicarbonate (NaHCO3, 99.8%), calcium chloride dihydrate (CaCl2·H2O, 99.0%), magnesium sulfate (MgSO4·7H2O, 99.5%), potassium phosphate dibasic (K2HPO4·3H2O,

Physicochemical and electrochemical characteristics of electrode

To investigate the interfacial changes, we first captured morphological state of the electrodes. Scanning electronic microscope (SEM) image showed a typical graphite composite topography on SPE (Fig. 1a). For the rGO-SPE (Fig. 1b), laminar structure was observed with the disappearance of the composite substrate. It indicated surface attachment of graphene film onto the SPE. GO is a highly water miscible material, ready to be removed from the SPE surface and, its high electrochemical impedance

Conclusion

Herein we proposed an all-solid-state voltammetric pH sensor using electrochemically reduced graphene oxide as the sensitive element. Based on this, alizarin was used to acquire ideal sensing linearity. The sensor can fulfill fast pH monitoring over the range of pH 4.0–9.0, which would covers the most common cases of MFC applications. It was also verified that this improvement from alizarin was out of its capability in enhancing the proton transfer process on the sensing interface. Specially,

Credit author statement

Y.M., L.X., Y.L. and H.Z. designed the research. Y.L., and Y.W. performed experiments. Y.L., Y.M., L.X., S.K and Y.T. analyzed the data, Y.L., L.X. and Y.M. wrote the article.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was financially supported by the Key Research and Development Program of Shandong Province (Major Science and Technology Innovation Project) (2020CXGC010602), Youth Innovation Promotion Association CAS (2021213), and the National Natural Science Foundation of China (no. 42077025). Science, Education and Industry Integration Innovation Pilot Project of Qilu University of Technology (Shandong Academy of Sciences) (2020KJC-ZD15).

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