Method Development for the Determination of Diallyldimethylammonium Chloride at Trace Levels by Epoxidation Process

Domestic water is abstracted from its sources in raw form with a high content of dissolved and suspended material. Polydiallyldimethylammonium chloride (polyDADMAC) is a cationic polyelectrolyte used in the initial water clarification process. However, its residues in treated water pose a health risk as they react with chlorine to produce a carcinogenic compound. There is a need to determine the concentration of the polyelectrolyte cations that pass through the flocculation stage before the chlorine disinfection process in water treatment plants to ascertain the safety of water to consumers. The cationic polymer is UV inactive, and previously available methods for determining the concentrations of polyelectrolytes are unsatisfactory due to poor detection limits. This paper describes a UV-Visible (UV-vis) spectrophotometry method for the determination of residual polyDADMAC as an epoxide. The novelty method lies on the epoxidation of polyDADMAC using 20 % sodium hydroxide dissolved in 30 % hydrogen peroxide to produce a UV-Vis active compound. The epoxidation was confirmed by UV-Vis, FTIR and ¹H NMR techniques. Dilute solutions of varying concentrations of polyDADMAC (0.2-1.0 mg L^-1) were treated with a basic solution of hydrogen peroxide then analysed by UV-Vis spectrophotometry. The wavelength at maximum absorption (λ_max) was found to be 313 nm, and a linear calibration curve with a correlation coefficient (R²) of 0.993 was used for quantification purposes. The detection limit measured as three times the signal of the blank and was found to be 2.1 × 10^-4 mg L^-1. The method was applied to determine the concentration of polyDADMAC spiked in water samples collected from a pool as a model for environmental matrix. The results obtained agreed with the quantities spiked in the solution, thus qualified the method to be suitable for the determination of polyDADMAC in treated waters at trace levels. The method was also used to investigate the adsorption capacity of polyDADMAC on sand filters. The adsorption method was found to be in accordance with Langmuir with an adsorption capacity of 2.068 mg g^-1.