Electrochemical Sensor Mediated Assessment of Carbonate Moieties in Soil Matrix
Authors: Dhamu Vikram Narayanan , Muthukumar Sriram , Prasad Shalini 
Published: November 2022

Carbon sequestration is a major global phenomenon that is particularly relevant now with the effect of climate change monitoring and regenerative agriculture. It refers to different practices that contribute to the absorption and storage of carbon in the soil. While the impact of sequestration of carbon is majorly from organic sources, the effect from soil inorganic carbon accumulation is not trivial especially in arid and semi-arid regions where the inorganic carbon (IC) share in sequestration is significant. Movement of Atmospheric Carbon Dioxide (CO2) into soil resources via photosynthesis and root respiration with the subsequent formation of bicarbonate in soil, which exists as bicarbonate in solution phase in groundwater sources or precipitates into CaCO3 in soil phase. Current soil carbon monitoring is either based on extensive soil sampling for analyzing in the laboratory or using proximal sensing methods, which are less accurate. Thus, implementation of novel soil sensors for accurate in-situ measurements could fundamentally improve soil carbon assessment and monitoring. Comparatively, the level and depth of assessing inorganic carbon in soil is significantly lower than that of soil organic carbon (SOC) monitoring. Electrochemistry as a transduction mode is highly viable in this regard due to its ease of applicability in various environments including the complex-soil matrix is a vital component that holds together the Earth’s ecosystem. The key mechanism of releasing the inorganic carbon relies on capturing the different pools that encompass the inorganic/ mineral carbon in soil including- carbonate ions in soil from Calcium (Calcite) which is the major pool of the IC in soil contributing to around 90% of this source and Magnesium (Dolomite). The other minor fraction that is present in the IC pool arises from Bicarbonates (HCO3-).
In this sensor development study, we have designed a modified 3-electrode system which is functionalized with a correlated ion-capture film that is functional to changes in carbonate moieties in the soil electrolyte test system. Then this composite sensor was introduced into test soil setups with varying carbonate content to measure the sensor response- wherein, cyclic voltammetry (DC based) and Electrochemical Impedance Spectroscopy was used to determine signal output as a function of increasing dose. It was seen that the peak current around 0.4-0.5V on the positive axis was modulated to increase with carbonate content (% or ppm) and correspondingly- changes in the Rct (charge-transfer resistance) and Zw (Warburg impedance) along with the capacitive element shift was noticeable in the EIS spectra. The experimental cycle was performed using an in-house designed portable potentiostat device which was integrated into a probe head setup that could be inserted into soil for testing. The results from these experiments yielded linearity metrics from regression fits as R2 > 0.97 and measurable sensing range from 0.01% (100 ppm) to 1% (10,000 ppm). Therefore, a first-of-a-kind sensor system was developed for determining carbonate content in real soil samples using an electrochemical mode which will be subsequently tested in-field to survey field deployable and point-of-use capability of the system.

Figure 1