The use of chloramine as a disinfectant in water treatment plants is becoming popular due to its lower reactivity and higher stability than free chlorine.^1–3 Chloramines are produced by the reaction of free chlorine (HOCl, OCl^-) with nitrogen compounds to form monochloramine (NH₂Cl), dichloramine (NHCl₂) or nitrogen trichloride (NCl₃), depending on pH and N/Cl ratio.⁴ Dichloramine and nitrogen trichloride tend to create odour and taste problems in drinking water. Thus, only monochloramine is preferred for disinfection. Typically, 0.5-2 mg/L of monochloramine is maintained in the water distribution system.⁵ Maintaining the concentration level of monochloramine is crucial to prevent pathogen growth in the drinking water. Currently, there is no direct method to measure chloramine. However, U.S. EPA-approved amperometric titration and colorimetric methods are available which can be used to measure total and free chlorine in aqueous media.² An amperometric titration method (SM 4500-Cl D) is capable of distinguishing 3 common forms of chlorine: Cl₂ / HOCl / OCl^-, NH₂Cl, and NHCl₂. However, it fails at concentrations greater than 2 mg/L (as Cl₂).^2,3 Even though this method is not affected by common oxidizing agents, temperature changes, turbidity, and colour, it does require a greater degree of skill. Operationally simpler, N,N-diethyl-p-phenylenediamine (DPD) methods (ferrous and colorimetric) are used to measure free and total chlorine and then their subtraction gives the concentration of monochloramine, assuming no NHCl₂ and NCl₃ are present. DPD methods are subjected to interferences like copper, manganese (oxidized), iodide and chromate.⁶ Additionally, the DPD method is not suitable for continuous monitoring of monochloramine which is essential in water distribution plants to maintain the appropriate concentration of disinfectant.^2,3,7

Here we demonstrate a chemiresistive sensor array for the continuous monitoring of chloramine in the water. Chemiresistive sensors are cheap, robust and use low power. These sensors detect an analyte through changes in the electronic properties of the transducing element. A nanocarbon network was airbrushed onto the frosted side of a microscope glass slide as the transducing element between two pencil trace contact patches. Copper tapes were placed on top of the pencil patches and then covered with a dielectric. 10 mV voltage was applied for the measurements, and the changes in resistance were measured as the analyte interacted with the transducing element. The surface of the nanocarbon network is functionalized with suitable dopant molecules by submerging the sensor in the molecule solution. This array of molecules will be able to capture the parameters to be able to classify the type of chloramine present in water. Fresh chloramine solution is prepared before each experiment by adding NH₄Cl and NaOCl in Phosphate Buffered Saline (PBS). Sensor responses are recorded as positive current change with increasing concentrations of monochloramine. Here the hole density of the inherently p-doped substrate increases when exposed to monochloramine, and thereby resulting in increasing current. Sensors can be reset with ascorbic acid or water. Sensors were tested with 0.054 ppm to 1.437 ppm of monochloramine in pH 7.5 and 8.5. Functionalized sensor devices showed a considerably higher response than the unfunctionalized ones. The tap water sample was tested with the calibrated devices. We have therefore demonstrated a robust sensor array capable of continuously monitoring chloramine in aqueous media.

References:

1. T. L. Engelhardt and V. B. Malkov, Chlorination, chloramination and chlorine measurement, p. 1–67, (2015).
2. US Environmental Protection Agency - Office of Water, Alternative disinfectants and oxidants Guidance manual, 1st Ed., p. 1–328, (Washington, DC) US Environmental Agency, (1999).
3. S. H. Jenkins, Water Res., 16, 1495–1496 (1982).
4. T. H. Nguyen et al., Sensors Actuators, B Chem., 187, 622–629 (2013).
5. T. H. Nguyen et al., Sensors Actuators, B Chem., 208, 622–627 (2015).
6. Health Canada, Chloramines in drinking water (2019).
7. World Health Organization, Guidelines for drinking-water quality: fourth edition incorporating the first addendum, 4th Ed + 1., Geneva: World Health Organization, (2017).