Concomitantly to the appearance of the protonated monochloramine signal, in pure monochloramine solutions, a well-marked signal of NH
4+ is measured. The values of the latter signal are as high as the NH
3Cl
+ signal (
Figure 4a). Moreover, it presents a steady increase with increase of monochloramine concentration in the feed.
The presence of the NH
4+ ion was not expected as earlier monochloramine MIMS experiments did not show this feature. Electron ionization (EI) is common in MIMS and monochloramine has been studied mainly using that ionization method. In EI-MIMS, NH
2Cl
+ was generally the only product formed at 70 eV. Moreover, the EI product NH
2Cl
+ is quite stable as CID measurement did not exhibit strong fragmentation [
12]. NH
2+ is formed but with a low branching ratio (7%).
These hypotheses are successively studied below.
3.1. Is It Due to Permeation of NH3 from the Feed Solution?
As demonstrated previously (
Figure 2), permeation of substances is dependent on the form of the species (neutral or ionic). The pH of the solution is then established to a value for which ammonia is represented by its acid form NH
4+. The pH of the feed solutions were then all set to values below the pKa (pH 8.1). Using the same concentration of ammonia, we compared solutions of chloramine and ammonia at pH 8.1 (red squares on
Figure 4a,b). No
m/
z 18.034 signal could be evidenced when analyzing the ammonia feed solution. On the contrary, the monochloramine solution exhibited a strong
m/
z 18.034 signal. The ammonia that may be present in the feed solution is then not bound to permeate through the membrane at the pH of the solution.
Another interpretation would be to consider that at the vicinity of the membrane, as monochloramine is consumed by solubilization in the PDMS, a disequilibrium would occur. The difference of solubilization should then modify the pH in the limit layer. The consumption of NH
2Cl increases HO
− formation, pH may then increase locally as we didn’t use a buffer solution to set the pH.
Figure 4 presents the time response of a chloramine solution (
Figure 4a) and ammonia solution (
Figure 4b) at the same concentration for a basic pH 10.6. The two solutions exhibit significantly different time responses for ammonia: the NH
4+ signal from monochloramine analysis is at least three times shorter than the NH
4+ signal from ammonia analysis. If ammonia were formed locally in the feed water solution, the time response of the
m/
z 18.034 from monochloramine solutions should be of the same order of magnitude or may be delayed compared to ammonia solution response. On the contrary, the NH
4+ signal observed from monochloramine solution appears rapidly.
Therefore, it is probable that the NH4+ signal observed is not due to ammonia permeation itself through the membrane.
3.2. Is It Due to Ion-Molecule Reactivity?
When looking to MIMS response of NH
3Cl
+ and NH
4+ for a monochloramine solution (
Figure 4a), it appears that permeation times of both ions are quite similar. It is then possible that the observed NH
4+ is due to a phenomenon occurring in the vacuum chamber after monochloramine pervaporation.
Figure 5 shows the dependency of the transient and stable response of the MIMS signal to monochloramine solutions of increasing concentrations with the H
3O
+ signal. The NH
4+ signal presents a pattern consistent with a reaction scheme such as:
All three reactions are exothermic [
23,
24]. Reactions (4) and (5) are parallel, as no delay for NH
4+ formation was highlighted: at low reaction extent both ion products are observed. Reaction (6) is a secondary reaction that would explain the increase of NH
4+ signal compared to NH
3Cl
+ signal for higher reaction extent.
As the permeate is mainly made of water, a reaction with water may be considered:
From formation enthalpies [
23,
25], we calculated a reaction enthalpy of 0.5 kJ·mol
−1. The reaction is thermoneutral, reaction rate coefficient is then likely to be slow. The yield of reaction (7) is then bound to be much lower than the amount of ammonia signal.
Unfortunately, the reactivity of NH
2Cl and H
3O
+ is difficult to study as NH
2Cl is nearly never formed as a pure product. To overcome this limitation, we proposed to study the products obtained using different precursors: 2 proton donor precursors (para-difluorobenzene F
2C
6H
4 [
26] and 1,3,5-trimethylbenzene (CH
3)
3C
6H
3) and a potential hydride or chloride abstraction precursor CF
3+ [
27]. Proton affinity (PA) of monochloramine is 797.05 kJ·mol
−1 [
23], it should then react by PTR with H
3O
+ as observed, and with F
2C
6H
5+. On the contrary, PA of 1,3,5-trimethylbenzene is higher. The reactivity with that ion should then be limited. The reactivity of CF
3+ is only known for NH
3 [
28]:
The kinetic rate coefficient is 2.1 × 10−9 cm3·s−1.
Interestingly, the formation of NH
4+ is not prevented by a change of the precursor to higher PA (
Table 1). Moreover, the observed products fit with a reactivity of ammonia with the PTR precursors, as the chosen precursors have all a PA below ammonia (AP(NH
3) = 853.6 kJ·mol
−1). Using CF
3+, two products are also observed. The CF
2NH
2+ ion is very probably due to Reaction (9). Reaction of chloramine and CF
3+ gives rise to a similar reaction:
From those experiments, it appears that the ammonium signal is probably due to direct ionization of ammonia and not from Reaction (5).
Occurrence of Reaction (6) was also tested. First, NH
3Cl
+ ion is isolated in the ICR cell by the excitation and the ejection of the other ions present in the cell. Then, a pulse of permeate is injected in the ICR chamber. Time is left for the reaction to occur. Finally, no NH
4+ was detected. Ricci and Rosi [
23], studying the reactivity of NH
3Cl
+ produced by CI/CH
4 in a FTICR apparatus, observed the ion NH
2Cl
2+ as a product of a secondary reaction (formed by Cl
+ transfer). Likewise, they did not report NH
4+ formation.
The observed ammonia signal is then probably not due to a secondary reaction of ionized NH2Cl. Those results tend to point to an alternate scenario: ammonia is present in the cell before ionization.
3.3. Is Neutral Monochloramine Reacting in the Vacuum Chamber?
From the previous experiments, we could not explain the presence of NH
4+ peak from ammonia in the solution or from the ionization of monochloramine. Ammonia is then probably formed in-between from a neutral reaction:
The reactivity of neutrals on surfaces may take place inside the vacuum cell. However, probability of encountering a surface or a molecule is low in a low pressure FTICR. If such a phenomenon occurred, it would not explain the high proportion of m/z 18.034 peak. Moreover, one could argue that the problem is due to an inner problem of our instrument. That is partially true only. We tested our membrane system on two different FTICR instruments. Despite being similar, their internal structure and in particular the cell is different in terms of material used. Both exhibited a strong NH4+ signal proportional to NH2Cl concentration.
The probable reason for the presence of ammonia in the vacuum chamber is a reaction occurring in the membrane itself. As explained, ammonia is not permeating through the membrane as it is mainly under its ammonium form. This induces a disequilibrium inside the PDMS membrane with an excess of monochloramine compared to ammonia and hypochlorite. NH
2Cl may then react with water inside the membrane to form NH
3 and HOCl as in Equation (10). HOCl has a low proton affinity [
30], it cannot react with the proton transfer precursors used in this study.
An in-membrane reactivity may explain why the ammonia signal appears at the same time as the monochloramine signal (
Figure 4a). Besides, it appears that the
m/
z 18.034 signal generally keeps on steadily increasing even after monochloramine signal reached the steady state. This feature would be due to transfer of ammonia in the membrane from where it was formed originally. Diffusion of ammonia being slower than monochloramine, response of
m/
z 18.034 signal would involve two diffusion parameters, one due to monochloramine diffusion and the second due to ammonia diffusion in the membrane. The appearance of the ammonia signal is then a two-step process.
Finally, experiments of Provin and Fujii [
31] in a microdevice suggested that reactivity inside a PDMS membrane between different species is possible with no chemical alteration of the membrane.