Autonomous Silicate Electrochemical Sensor in Seawater

Abstract

Long term monitoring of dissolved inorganic nutrient concentrations such as silicate is the key to improve our understanding of biogeochemical cycles in the ocean. Electrochemical methods are proposed to measure in situ silicate concentration as they allow developing miniaturized, reagentless sensor. We report here on the development and the optimization of the in situ version of our silicate sensor.Silicate is a non-electroactive specie and its determination is performed by measuring its corresponding silicomolybdic complex formed in situ at acidic pH after the oxidation of a molybdenum electrode while the counter electrode is isolated behind a Nafion® membrane [1].The mechanical design of the sensor has been adapted from the laboratory prototype containing two connected cells: one to form the complexe and one for its detection on a gold electrode (Φ=2mm) by cyclic voltammetry. All the electrochemical and chemical parameters, such as the Mo oxidation, the complexation time, the silicomolybdic complex detection as well as the complex transfer from one cell to the other and the cleaning protocol have been optimized to detect the smallest silicate concentration possible with the shortest time. Reproducible calibrations performed with this sensor provided linear regressions between 1.63 µmol L-1 and 140 µmol L-1 and a limit of quantification of 1.63 µmol L-1. Repeatability is achieved over 53 tests on the same silicate concentration with less than 4% variation. The study of temperature effect revealed no influence on the electrochemical signal between 7° and 21°C.The very first time series of silicate concentration was obtained after a successful deployment of our silicate sensor at 55 meters depth on a mooring off Coquimbo, Chili in 2017 [2].More recently, two synchronized silicate sensors have been deployed in May 2019 off-shore Easter Island (Rapa Nui) at 100 meters depth in the Pacific Ocean. Several seawater samples were also collected at the same depth of sensor deployment to analyse the silicate concentration by colorimetric measurement in order to inter-compare the results obtained by the sensors and make corrections of any drift.Also, one silicate electrochemical sensor has been deployed in Thau pond (Mediterranean Sea), France at 1.5 meters depth during 19 days (July 2019). Water samplings were carried out at the same depth, twice a day (one in the morning and one in the afternoon) during the week and will be analysed by colorimetric measurement at the Marine Station in order to inter-compare the results obtained with silicate sensors.The sensors were measuring silicate concentration every 45 minutes. All the results obtained during these deployments will be presented and discussed.[1] Lacombe et al., Talanta 77 (2008) 744-750.[2] Barus/Chen Legrand et al., Frontier in Marine Science (2018) 5, 60