Modeling drinking water chlorination at the breakpoint: I. Derivation of breakpoint reactions

Abstract

A B S T R A C TFrom a bibliographic synthesis on the reactions that can occur during breakpoint chlorination. We carry out the stability study of produced entities under the applied conditions in the potable water production, where the pH range 6.5-8.5, and the initial weight ratio Cl/N including in 10/1-15/1 at ambient temperature. The choice of dominant species and reactions that have been taken into account must be based on data of the thermodynamics and the kinetics governed by pH and temperature. Through this selection appears a large disagreement in results of researches between the rate expressions of the kinetics and observed and even with the empirical results, especially for reaction the monochloramine formation and for equilibrium the formation-hydrolysis of trichloramine. Then, the nature of identified intermediate and the observed rate of its formation reaction were not agree in different published works, as well the possibility of destruction of the free chlorine residual by this intermediate is not confirmed yet. Through this study, we obtain a representative selected group of reactions during breakpoint chlorination process, this identified group of reactions is compared with other models of breakpoint reactions proposed in the literature, where some differences appear.

Trichloramine and Hydroxyl Radical Contributions to Dichloroacetonitrile Formation Following Breakpoint Chlorination

“…Besides • NO and • NO 2 , other reactive nitrogen species, such as NOH and ONOOH/ONOO – may also be present. NOH is a proposed intermediate, but it has not been observed directly in the reactions. ,, ONOOH/ONOO – could be the nitrating reagent if present . We had tried to use uric acid as a scavenger for ONOO – . , However, uric acid is reactive to free chlorine, and the addition of uric acid at 5 μM changed the breakpoint chlorination greatly due to its fast consumption of a large amount of free chlorine (Table S7).…”

“…NOH is a proposed intermediate, but it has not been observed directly in the reactions. 9,15,16 ONOOH/ONOO − could be the nitrating reagent if present. 20 We had tried to use uric acid as a scavenger for ONOO − .…”

“…If excessive chlorine is applied, ammonia is oxidized to nitrogen gas or nitrate (eqs and ), and free residual chlorine is produced. The speciation of chloramines and free chlorine mostly depends on the molar ratio of free chlorine to ammonia (Cl 2 /NH 3 ) and also pHs, and it is described well by the “breakpoint chlorination curve”. − When the Cl 2 /NH 3 molar ratio is ≤1:1 and at pHs commonly found in drinking water (i.e., pH 7–9), NH 2 Cl is predominantly produced and “chloramination” is applied. NH 2 Cl is transformed into NHCl 2 and then NCl 3 , leading to a loss of total residual chlorine when the Cl 2 /NH 3 ratio exceeds 1:1.…”

“…The point at which the total residual chlorine is minimized is called the “breakpoint,” and the breakpoint theoretically occurs at a Cl 2 /NH 3 molar ratio of 1.5:1 to 1.7:1. Further increasing the Cl 2 /NH 3 ratio past the breakpoint (i.e., Cl 2 /NH 3 molar ratios >1.5–1.7) leads to the presence of free chlorine, and at this region, the process is called “breakpoint chlorination.” − …”

Dominant Dissolved Oxygen-Independent Pathway to Form Hydroxyl Radicals and the Generation of Reactive Chlorine and Nitrogen Species in Breakpoint Chlorination

“…Within a reaction time of 30 min, the exposures of reactive species increased from <1.00 × 10 −12 M•s to >2.77 × 10 −11 M•s with an increasing Cl/N ratio from 9.5 to 15 (Figure S13(a)). These results indicate that in water with lower pH (e.g., reverse osmosis permeate, pH 5.0− 5.5), 39,40 a higher dosage of free chlorine and a longer reaction time are needed to generate reactive species compared to the neutral condition. Besides, it is interesting that the generations of reactive species were observed only until free chlorine was detectable (i.e., at a Cl/N mass ratio of ≥10, Figure S13), suggesting that free chlorine was likely involved in the decomposition of NHCl 2 in generating reactive species or their precursors (e.g., HNO).…”