Solid-State pH Sensors for Mine Water Monitoring
Authors: Güth Frederic , Selbmann Franz , Miersch Christian , Árki Pál , Joseph Yvonne 
Published: May 2020

Introduction

The monitoring of water quality parameters such as pH, redox potential, and conductivity is essential in order to investigate the variety of hydrochemical processes occurring in aqueous environments. For a better understanding of these processes, measurements with high temporal resolution are needed, which cannot be achieved with manual sampling procedures. Hence, alternative technical analysis methods, such as sensors, are used to monitor relevant parameters in-situ. Unfortunately conventional probes often experience harsh or detrimental conditions during on-site deployment. All-solid-state potentiometric sensors have been proposed as robust alternatives for demanding environments that deliver results with sufficient accuracy, at relatively low-cost, with experimental and instrumental simplicity [1].
This work focuses on the development of a solid-state pH sensor for the monitoring of mine water and acid mine drainage (AMD). AMD is the consequence of uncontrolled sulfidic mineral oxidation, typically as a result of ore or coal mining practices. The resulting outflow of acidic water, which can contain high concentrations of metal ions, is potentially disastrous to aquatic life and also poses a great threat to water supplies [2]. Our aim is to develop a robust and cost-effective sensor that enables in-situ AMD monitoring in mines and surrounding waterways, as well as pit lakes formed as the result of flooded open-pit mines. With increased spatial and temporal resolution in comparison to manual sampling, these sensors will help to predict hazardous conditions and increase the understanding of the complex underlying geochemical mechanisms.

Methods

The proposed sensor consists of a pH sensitive iridium oxide (IrOx) layer in combination with a solid-state reference electrode. During sensor fabrication structured titanium and gold electrodes were deposited on oxidised silicon substrates using standard cleanroom techniques. IrOx was subsequently electrochemically coated on these electrodes via cyclic voltammetry. The deposition solution was prepared according to the protocol published by Marzouk [3]. In a three electrode configuration (reference: 3 M KCl Ag/AgCl, counter: platinum gauze) the prepared metal electrodes functioned as the working electrodes. Homogenous IrOx layers with a thickness around 80 nm were obtained after 100 cycles at 0.1 V/s between -0.5 V and 0.85 V or -0.7 V and 0.9 V for the gold and titanium electrodes respectively (see Figures 1 and 2).
Several approaches for the construction of solid state reference electrodes (SSREs) have been presented in literature [4]. In this work, salt-saturated polymers were deposited on Ag/AgCl layers, similar to [5]. The selected polymers were either unsaturated polyester resins or UV-curing epoxy-based resins. These resins were mixed with up to 70 ma% of finely ground KCl. The resulting slurry was dropped on Ag/AgCl electrodes and cured to form the SSREs.
Data collection was performed with EZO-circuits from Atlas Scientific in connection with a RaspberryPi or an ESP32-based microcontroller using I2C or serial protocols.
The fabricated IrOx electrodes were characterised regarding their pH sensing performance. Their sensitivity was evaluated by recording the open circuit potential (OCP) vs. a commercial reference electrode (3 M KCl, Ag/AgCl) during a titration of a Briton-Robinson buffer with 0.5 M KOH at 25°C. Drift rates were measured in standard buffer solutions. In order to determine the SSRE’s electrochemical potential stability, OCP was recorded versus a conventional reference electrode in KCl solutions of varying concentration at 25°C.

Results and Conclusions

As shown in Figure 3, the IrOx electrodes exhibit a super-Nernstian pH sensitivity of around -73 mV/pH between pH 2 to 11.5. These values are comparable to published results [3]. Electrodes from several batches fabricated on different days show high reproducibility with respect to sensitivity and intercept, as indicated by the closely overlaying data points in Figure 3. These results suggest a high robustness of the coating process. The drift rate of the IrOx electrodes is less than 1 mV/h which is sufficient for the intended application. The functionality of the SSRE can be deducted from the data given in Figure 4: An uncoated Ag/AgCl electrode exhibits Nernstian behavior to changes in chloride ion concentration whereas the fabricated SSRE with a polymer coating shows a stable potential. The combination of SSRE and IrOx electrode results in a potentiometric pH sensor whose output changes with pH as shown in Figure 5.

Outlook

Future work will focus on improving the sensing characteristics as well as measurements in AMD samples. Eventually on-site deployment of the sensors is planned. Additionally, first experiments to transfer the electrodes to flexible and chemically inert substrates for ultimate robustness and potential applications in smart robotic skins have been conducted (see Figure 6).

Acknowledgments

This research has been funded by the European Social Fund (ESF) and the Free State of Saxony. (ARIDuA , project number 100310491).
References:
[1] M. Cuartero, All-solid-state potentiometric sensors: A new wave for in situ aquatic research, Current Opinion in Electrochemistry 10, 98-106 (2018); doi: 10.1016/j.coelec.2018.04.004
[2] D.K. Nordstrom, Hydrogeochemical processes governing the origin, transport and fate of major and trace elements from mine wastes and mineralized rock to surface waters, Applied Geochemistry 26, 1777-1791 (2011); doi: 10.1016/j.apgeochem.2011.06.002
[3] S.A.M. Marzouk, Improved Electrodeposited Iridium Oxide pH Sensor Fabricated on Etched Titanium Substrates, Analytic Chemistry 75, 1258-1266 (2003); doi: 10.1021/ac0261404
[4] U. Guth, Solid-state reference electrodes for potentiometric sensors, Journal Solid State Electrochemistry 13, 27-39 (2008); doi: 10.1007/s10008-008-0574-7
[5] F. Güth, Electrochemical Sensors Based on Printed Circuit Board Technologies, Procedia Engineering 168, 452-455 (2016); doi: 10.1016/j.proeng.2016.11.543

Figure 1