Diethyl phenylene diamine (DPD) oxidation to measure low concentration permanganate in environmental systems

Permanganate has been used traditionally in drinking water treatment for its oxidation properties and ease of use. The concentration of permanganate in treatment conditions is low and difficult to detect. A colorimetric method using diethylphenylene diamine (DPD) oxidation to measure low levels (i.e., less than 6 μM) of permanganate in water was developed and applied to quantify permanganate scavenging by dissolved organic matter (DOM). Manganese dioxide (MnO₂) particles were shown to interfere with DPD oxidation; however, particles were removed effectively using 0.1 μm PVDF filters prior to reaction with DPD. DOM and complexed-Mn(III) were concluded to not interfere with the DPD reaction. The DPD method was validated by obtaining the second-order rate constant for permanganate reaction with phenol (1.7 ± 0.2 M^-1 s^-1), and comparing to the rate constant obtained independently by monitoring phenol degradation (i.e., UPLC-UV) (1.6 ± 0.2 M^-1 s^-1). Permanganate reaction with DOM isolate samples did not follow pseudo-first order kinetics. Faster reaction rates were observed with higher ionic strength (1 mM versus 5 mM carbonate). No change in reaction rates with pH was observed at lower ionic strength (1 mM); while at higher ionic strength, the reaction rate was faster at pH 7 than at pH 10. In contrast, linear kinetics were observed for permanganate reaction with DOM in filtered whole water samples. These samples showed similar trends with pH and ionic strength as for DOM isolates. The presented method is valid to quantify permanganate reaction rates with organic contaminants or with natural scavengers.