Sensing of chemical oxygen demand (COD) by amperometric detection—dependence of current signal on concentration and type of organic species

Environ Monit Assess (2023) 195:630 https://doi.org/10.1007/s10661-023-11228-3 RESEARCH Sensing of chemical oxygen demand (COD) by amperometric detection—dependence of current signal on concentration and type of organic species Samira Lambertz · Marcus Franke · Michael Stelter · Patrick Braeutigam Received: 15 December 2022 / Accepted: 6 April 2023 © The Author(s) 2023 Abstract The standard method to determine chemical oxygen demand (COD) with K 2Cr2O6 uses harmful chemicals, has a long analysis time, and cannot be used for on-site online monitoring. It is therefore necessary to find a fast, cheap, and harmless alternative. The amperometric determination of COD on boron-doped diamond (BDD) electrodes is a promising approach. However, to be a suitable alternative, the electrochemical method must at least be able to determine the COD of water samples independently of the contained substances. Therefore, the current signal as a function of various organic materials was investigated for the first time. It was shown that the height of the signal current depended on the type of Supplementary Information The online version contains supplementary material available at https://doi. org/10.1007/s10661-023-11228-3. S. Lambertz · M. Franke · M. Stelter · P. Braeutigam (*) Institute for Technical Chemistry and Environmental Chemistry, Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany e-mail: S. Lambertz · M. Franke · M. Stelter · P. Braeutigam Center for Energy and Environmental Chemistry, CEEC Jena), Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany M. Stelter · P. Braeutigam Fraunhofer IKTS, Fraunhofer Institute for Ceramic Technologies and Systems, Michael‑Faraday‑Straße 1, 07629 Hermsdorf, Germany organic matter in single-substance solutions and that this substance dependency increases with the amount of COD. Those findings could be explained by the mechanism proposed for this reaction, showing that the selectivity of the reaction depends on the ratio of the concentration of hydroxyl radicals and organic species. We give an outlook on how to improve the method in order to increase the linear working range and avoid signal variance and how to further explain the signal variance. Keywords Chemical oxygen demand (COD) · Sensor · Amperometric detection · Oxidation of organics · Wastewater treatment Abbreviations COD Chemical oxygen demand BDD Boron-doped diamond ANOVA Analysis of variance AOP Advanced oxidation process Introduction The ongoing industrialization, increased use of chemical products, and climate crisis are all factors potentially leading to water-related conflicts in the long run and pose a threat to natural water bodies (Goel, 2006). One way to face this potential danger and protect water as a natural resource is water monitoring and treatment (Quevauviller et al., 2007; Shannon Vol.: (0123456789) 13 630 Page 2 of 15 et al., 2008). However, many classical methods of wastewater analysis use toxic chemicals, which leads to an increase in waste generation, analysis costs, and specialized personnel required for performing the analytical procedure. An important sum parameter that is widely used in water monitoring is COD (Pisarevsky et al., 2005). COD measures the amount of oxygen that is needed to mineralize the organic content of a given water sample defined by standard reaction conditions. It can be used to monitor the quality of water bodies, access the degree of contamination in wastewater, and control wastewater treatment plants (Awe et al., 2016). The standard method to determine COD uses potassium dichromate (K2Cr2O6) or potassium permanganate (KMnO4) as an oxidant (Boyles, 1997). Both are toxic and harmful to the person using it and the environment. The usage of K 2Cr2O6 is likely to disappear in the future, since it is damaging to the environment and therefore the subject of several regulations (ECHA, 2015). In addition, the standard method uses other toxic chemicals (HgSO4, H2SO4, Ag2SO4), consumes high amounts of energy due to a long heating time, and is time-consuming (~4 h), making it unsuitable for in-time measurements and also resulting in a higher negative impact on climate change (Li et al., 2018). COD is mentioned as a mandatory parameter in various laws and is therefore part of standard procedures in many countries. The German federal law states that COD must be measured whenever wastewater is discharged into surface waters (Ordinance on Requirements for the Discharge of Wastewater into Watrs, German Federal Law, 1997). Similar laws can be found in various other states (e.g., USA, EU) (Urban Waste Water Treatment Directive, Annex I: Discharge requirements, 1991; US EPA The Water Quality Standards Regulation, 1983). Therefore, it is necessary to find a sustainable alternative to the standard method that can also be used in other applications like realtime monitoring and control of sewage treatment plants (Geerdink et al., 2017). One way to avoid the use of toxic chemicals is to switch to methods that work without toxic chemicals, such as optical or electrochemical methods (Su et al., 2007). Among the research that has been conducted in finding alternative methods for the determination of COD, there have been non-oxidative and oxidative methods. A non-oxidative method is the spectrophotometric method which correlates the absorbance of Vol:. (1234567890) 13 Environ Monit Assess (2023) 195:630 water samples with COD. However, in this case, COD is only determined indirectly, the method has to be calibrated and no conclusion about the actual oxidizability can be drawn. The spectrophotometric method depends on the correlation between the adsorbance at a certain wavelength and COD (Mrkva, 1983). While a good correlation between UV absorbance and COD is shown for certain substances (lignin and humic substances, phenolic wastes), no conclusion on the actual oxidizability of the water samples can be made. The wide range of oxidation-based methods that have been investigated includes some that are based on advanced oxidation processes (AOPs): the photocatalytic method, the photoelectrocatalytic method, and the electrocatalytic method (Li et al., 2018). For the electrocatalytic method, research is mainly focused on finding suitable electrode materials (GutierrezCapitan et al., 2015; Cheng et al., 2011; Zhou et al., 2012) and establishing an electrochemical method. The most promising electrode material for the determination of COD is BDD, which has been investigated by several groups (Bogdanowicz et al., 2012) (Kondo et al., 2016; Wang et al., 2012a, b; Yu et al., 2007, 2009; Kondo et al., 2014). BDD has the advantage of a higher overpotential for oxygen evolution than other conventional electrode materials. (Panizza & Cerisola, 2009) Its potential for oxygen evolution reaction is 2.3 V vs SHE as compared to 1.9 V vs SHE for PbO2, which has the second highest potential. The electrochemical deter (...truncated)