MnO₂-resistant ABTS spectrophotometry for trace permanganate

Abstract

Permanganate (Mn(VII)) is a commonly used oxidant for water treatment. However, manganese dioxide (MnO₂) produced in situ during permanganate oxidation reactions significantly interferes with the spectrophotometric measurement of Mn(VII) when using the previously reported spectrophotometry. In this study, a convenient, low-cost, and reliable spectrophotometry method was firstly established for measuring trace Mn(VII) (μmol/L−level) during the permanganate oxidation reactions without the need to filter out MnO₂. The method was based on the oxidation of 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) by Mn(VII) to produce a stable green radical (ABTS^•+) under mildly alkaline conditions (pH ≥ 8), with detection at long wavelengths (i.e., 732 nm and 820 nm). The strong resistance to interference from MnO₂ of the modified ABTS method was attributed to the inertness of MnO₂ toward ABTS under mildly alkaline conditions and the relatively low molar absorptivity of MnO₂ at long wavelengths compared to ABTS^•+. Using the characteristic peak at 732 nm as the detection wavelength, the sensitivity of the modified ABTS method for the Mn(VII) measurement was determined to be 4.84 × 10⁴ L/(mol·cm). Additionally, the modified ABTS method exhibited high tolerance towards common environmental background matrices and was reliable in measuring the Mn(VII) concentration in natural water samples. The accuracy of the modified ABTS method for the measurement of Mn(VII) in the presence of MnO₂ was also validated by comparing different detection methods. Furthermore, the modified ABTS method was successfully applied to detect the variation of Mn(VII) concentration during diclofenac removal with the humic acid-enhanced Mn(VII) process.

Graphical abstract

Keywords

Permanganate / 2,2-Azino-bis(3-ethylbenzothiazoline-6-sulfonate) / Manganese dioxide / Spectrophotometry

Highlight

	● The proposed method was based on the oxidative coloration of ABTS by Mn(VII) at pH ≥ 8.
	● The long visible wavelength of 732 nm was chosen as the determination wavelength.
	● The modified ABTS method possessed well anti-interference capacity toward MnO2.
	● The modified ABTS method was convenient, rapid, and sensitive to measure trace Mn(VII).
	● The proposed method was successfully used to monitor Mn(VII) in practical application.

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