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I know that a small amount of hydrogen peroxide will convert chlorine in drinking water into table salt, oxygen, and water:

$$\ce{NaOCl + H2O2 → NaCl + H2O + O2}$$

What I can't find is what effect it has on chloramine. Does it safely get rid of the chloramine, or does it produce undesirable byproducts?

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    $\begingroup$ See this publication and references therein. $\endgroup$ Jan 4, 2022 at 23:13

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Chloramine does react with hydrogen peroxide and the reaction kinetics has been studied in a paper linked by Todd Minehardt. Hydrogen peroxide is indeed used to remove chloramine disinfectants because it affects further water treatment (See the PDF linked below for more details). The proposed mechanism for the initial step of the reaction between monochloramine is as below:

enter image description here

The abstract says:

Rate constants for the mono- and dichloramine-peroxide reaction were on the order of $\pu{10^{-2}M^{-1}s^{-1}}$ and $\pu{10^{-5}M^{-1}s^{-1}}$, respectively. The reaction of trichloramine with peroxide was negligibly slow compared to its thermal and photolytically-induced decomposition. Arrhenius expressions of $\mathrm{ln(k_{H_{2}O_{2}-NH_{2}Cl})= \frac{(17.3±1.5)-(51500±3700)}{RT}}$ and $\mathrm{ln(k_{H_{2}O_{2}-NHCl_{2}})= \frac{(18.2±1.9)-(75800±5100)}{RT}}$ were obtained for the mono- and dichloramine peroxide reaction over the temperature ranges $\pu{11.4-37.9}$ and $\pu{35.0-55.0 ^\circ C}$, respectively.

Reference

  1. McKay G, Sjelin B, Chagnon M, Ishida KP, Mezyk SP. Kinetic study of the reactions between chloramine disinfectants and hydrogen peroxide: temperature dependence and reaction mechanism. Chemosphere. 2013 Sep;92(11):1417-22. doi: 10.1016/j.chemosphere.2013.03.045. (pdf)
  2. Photolysis of Mono- and Dichloramines in UV/Hydrogen Peroxide: Effects on 1,4-Dioxane Removal and Relevance in Water Reuse, Samuel Patton, Mariano Romano, Vincenzo Naddeo, Kenneth P. Ishida, and Haizhou Liu Environmental Science & Technology 2018 52 (20), 11720-11727 DOI: 10.1021/acs.est.8b01023
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The short answer is Yes, Hydrogen Peroxide will remove Chloramine from water.

There may, however, per my sourced review of the underlying chemistry, be also some immediately unfavorable by-products relating to chemical formations, and longer term storage/use of the treated Chloramine/$\ce{H2O2}$ may also present biological hazards as well.

To understand why, one must note that Chloramine ($\ce{NH2Cl)}$ in water undergoes hydrolysis forming Ammonia and Hypochlorous acid, the $\ce{HOCl }$ being the main disinfecting agent (paralleling the action of Chlorine in water) and the reputedly rationale of employing $\ce{NH2Cl}$ to disinfect water. Albeit, although Chloramine is decidedly weaker than aqueous Chlorine, it serves as a longer lasting stable provider of the active/unstable Hypochlorous acid in water:

$\ce{NH2Cl + H2O = NH3 + HOCl}$ (See Eq (1) here)

$\ce{Cl2 + H2O = H+ + Cl- + HOCl}$

The addition of a Hydrogen Peroxide to Hypochlorous acid is actually a fast reaction resulting in a powerful disinfectant action as a consequence of radicals and even transient Singlet Oxygen formation, but unfortunately, it completely destroys/removes the Hypochlorous acid.

$\ce{ H2O2 + HOCl = O2 + H2O + H+ + Cl- }$ (See Eq(4) here)

So if one adds Hydrogen Peroxide to both sides of the opening $\ce{NH2Cl}$ reaction cited above, one would expect the main residual product to include Ammonia with the removal of $\ce{HOCl}$. However, more recent chemistry, a 2008 article in the field of Atmospheric Science “Hydroxyl radical from the reaction between hypochlorite and hydrogen peroxide”, suggests that the above reaction proceeds first with the introduction of the powerful hydroxyl radical. Now, in the further presence of ammonia, this could introduce the amine radical and as a consequence similar unfriendly products as reported in the photolysis of such systems. For example, this work: “Photolysis of Mono- and Dichloramines in UV/Hydrogen Peroxide: Effects on 1,4-Dioxane Removal and Relevance in Water Reuse”, as to products, to quote:

photolysis of chloramines produces inorganic nitrogen breakdown products, including nitrate, ammonia, gaseous nitrogen, and organic nitrogen products (Figure 6)...Overall, ammonia accounted for 60−75% of the nitrogen products. The production of ammonia was likely via the breakdown of NH2• that was transformed into the intermediate N2H4 and then NH3 [38]. The initial formation of nitrite was possible via decomposition of NH2•; however, the subsequent oxidation of nitrite by chloramine generated nitrate [39].

where sunlight induced radicals (particularly, the hydroxyl and amine radical) can apparently foster unfriendly nitrite, nitrate and even traces of hydrazine ($\ce{N2H4}$).

To review what has occurred with the addition of Hydrogen Peroxide is the removal of the Hypochlorous acid and the introduction of Ammonia and transient powerful radicals. Now, if we consider the case of what occurs when we simply boil water containing dissolved Oxygen, it loses its disinfecting power and soon becomes contaminated with mold and bacteria. In the current case, similarly, one should likely expect the further action of Oxygen and bacteria acting on Ammonia, in the absence of disinfecting $\ce{HOCl}$, correspondingly allowing for such growth, which in the presence of Ammonia, may lead to nitrate. This problem has been well documented as occurring in well water where problematic high nitrate levels can kill cattle and, at low concentrations, even human infants appear to be very susceptible resulting in nitrate poisoning (see CDC’s “Nitrate and Drinking Water from Private Wells”).

My related recommended reading includes: "Occurrence, genotoxicity, and carcinogenicity of regulated and emerging disinfection by-products in drinking water: a review and roadmap for research", which notes: "Disinfection by-products (DBPs) are formed when disinfectants (chlorine, ozone, chlorine dioxide, or chloramines) react with naturally occurring organic matter, anthropogenic contaminants, bromide, and iodide during the production of drinking water."

So if even one wishes to dismiss/minimize potential problematic initial chemical compounds, there appears to be a good parallel between how to safely use freshly boiled water and water containing Chloramine that has been treated with Hydrogen Peroxide thereby removing the protective Hypochlorous acid.

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  • $\begingroup$ So the quick answer is: "For drinking water, boil it after; for plant-food water, use as is."? $\endgroup$ Jan 6, 2022 at 17:15
  • $\begingroup$ Yes, boiling after treating with H2O2 (but that is safe H2O2 for consumption as adding stabilizers to H2O2 can make it unfit for internal cosumption). On plants, a source: "Chloramines make the water acidic which over time can change soil pH. This may result in nutrient tie-up and create yellowing (chlorosis) problems in many plants. Chloramines prevent the absorption of other nutrients which also may lead to yellowing." So, H2O2 may make the NH2Cl water 'better' (not sure if I would eat those plants). Ref: natureswayresources.com/DocsPdfs/chloramine.pdf $\endgroup$
    – AJKOER
    Jan 6, 2022 at 17:28

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