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Development and characterization of solid-state, internet of things-based pH sensors for in-situ monitoring of soil and groundwater




VanTilburg, Charles Henry, IV, author
Scalia, Joseph, advisor
Sale, Thomas, advisor
Ham, Jay, committee member

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Herein I test and examine a new solid state pH sensor design for use in soils and groundwater monitoring. The concept presented here is intended to expand the capabilities for monitoring geochemical parameters in the subsurface by combining a durable, solid-state pH sensor for subsurface deployment with an automated 'internet of things' (IoT) based pH meter that allows the collection of near-real-time continuous data streams for monitoring biogeochemical processes in hydrologic systems. Tests performed in this work were intended to provide a benchmark for further refinement of the design and yielded promising results, including hydrogeologically useful response times (on the order of hours), durability (stresses >1,000 kPa), and reproducible behaviors with multiple sensors. These results support that this technology is promising for future work. The pH sensor design combines a titanium mixed-metal-oxide electrode (TiMMO), solid epoxy body, and a proton-selective Nafion™ ionomer coating to yield a durable solid-state sensor that is sensitive to aqueous proton activities. As the sensor is exposed to water, the diffusion of aqueous protons through the selective Nafion™ coating causes an increase in voltage on the electrode as compared to a reference electrode. The Nafion™ coating reduces the influence of other ions in the system, creating a proton selective sensor. Because of the durable solid-state construction, the sensor can likely be deployed in-situ in challenging environments such as in soils where common glass pH sensors are too fragile for use. This unique advantage allows the pursuit of new biogeochemical monitoring strategies that leverage a high volume of discrete in-situ measurements for near-real-time continuous datastreams. This new strategy, powered by IoT systems, can integrate with smart networks of multiple components and generate large amounts of data for use in artificial intelligence and machine learning systems while also providing insight into processes that occur at smaller spatial and temporal scales than those understood with current subsurface monitoring strategies. pH is a master variable in aqueous and soil chemistry, both an indicator and controller of most chemical reactions and many physical processes that take place in soil and groundwater. pH is important for understanding chemical speciation, mobility, and stability in the soil, while also influencing soil physical properties like soil structure. pH is a parameter of interest to many industries and fields of study including, but not limited to, agriculture, mining, water resources, and engineering. As this work was intended to be a first approximation for studying this technology, multiple promising results and points of improvement were discovered. This work identifies a clear voltage response by the sensor to pH changes (-29 mV/pH) while also demonstrating the change behavior during stepwise pH changes to be approximately logarithmic (Δvolt=3.85ln[t], where Δvolt is the change in millivolts and t is time in minutes). Furthermore, this work demonstrated that these sensors can be used with an IoT monitoring system in the intended application. However, more work is needed to remove variability in the data, explore further designs and processes for coating and treating the sensors, analyze the long-term use, drift, and standardization of the sensors, and employ the data in analytics. Future work should include further lab testing to compare alternative design features and to evaluate stressors such as non-target ions and dehydration. After refinement in the lab, the sensors should be installed in pilot scale studies and in the field to evaluate their performance in real world conditions.


Includes bibliographical references.
2022 Fall.

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Embargo Expires: 01/09/2025


internet of things
pH sensor


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