Electrochemistry of Free Chlorine and Monochloramine and its Relevance to the Presence of Pb in Drinking Water

Abstract

The commonly used disinfectants in drinking water are free chlorine (in the form of HOCl/OCl-) and monochloramine (NH2Cl). While free chlorine reacts with natural organic matter in water to produce chlorinated hydrocarbon byproducts, there is also concern that NH2Cl may react with Pbto produce soluble Pb(II) products--leading to elevated Pb levels in drinking water. In this study, electrochemical methods are used to compare the thermodynamics and kinetics of the reduction of these two disinfectants. The standard reduction potential for NH2Cl/Cl- was estimated to be +1.45 V in acidic media and +0.74 V in alkaline media versus NHE using thermodynamic cycles. The kinetics of electroreduction of the two disinfectants was studied using an Au rotating disk electrode. The exchange current densities estimated from Koutecky-Levich plots were 8.2 x 10(-5) and 4.1 x 10(-5) A/cm2, and by low overpotential experiments were 7.5 +/- 0.3 x 10(-5) and 3.7 +/- 0.4 x 10(-5) A/cm2 for free chlorine and NH2Cl, respectively. The rate constantforthe electrochemical reduction of free chlorine at equilibrium is approximately twice as large as that for the reduction of NH2Cl. Equilibrium potential measurements show that free chlorine will oxidize Pb to PbO2 above pH 1.7, whereas NH2Cl will oxidize Pb to PbO2 only above about pH 9.5, if the total dissolved inorganic carbon (DIC) is 18 ppm. Hence, NH2Cl is not capable of producing a passivating PbO2 layer on Pb, and could lead to elevated levels of dissolved Pb in drinking water.

Effects of Changing disinfectants on lead and copper release

“…Research indicates the importance of Pb(IV) scale formation in the presence of highly oxidizing waters. The oxidation reduction potential (ORP) of water, which can be largely affected by the disinfectant, controls the formation and dominance of specific lead species (Rajasekharan et al, 2007; Schock et al, 2001, 1996; Schock, 1990; Schock & Wagner, 1985). Scale analysis of utility pipe samples provided the background for the current understanding of lead scales and the importance of Pb(IV) (Schock et al, 2005; 2001).…”

Kinetics and Mechanisms of Cr(VI) Formation via the Oxidation of Cr(III) Solid Phases by Chlorine in Drinking Water

“…HOCl is more oxidative and more electrophilic than OCl − . 56,57 Second, pH affects speciation of Cr(III) surface 7,62,63 Because each surface hydroxo species possesses distinct reactivity with chlorine, the resultant reactivities of Cr(III) solids changes with pH.…”

Oxidation of cefazolin by potassium permanganate: Transformation products and plausible pathways

“…Products m/z 471a and m/z 471b were identified as sulfoxide compounds in which oxidation of thioether occurred at S2 and S1 atoms, respectively. Likewise, product m/z 487 was the di-sulfoxide in which both thioethers of S1 and S2 (Dash et al, 2009;Xie et al, 2010;Rajasekharan et al, 2007), indicating that the oxidizing ability of KMnO 4 is comparable to that of HClO. Therefore, it was reasonable that these three products that were formed with HClO could also be produced after reaction with Mn(VII).…”