Cinetiques et mecanismes de la degradation de la creatinine sous l'action de l'hypochloriteKinetics and mechanisms of hypochlorite oxidation of creatinine

An area of substantial interest in current research on chlorination is the formation, stability and nature of chloramines formed by the interaction of chlorine with nitrogen organic compounds of biological origin in natural water or swimming pool water. It is desirable to be able to predict the lifetime of these harmful compounds under various conditions. The research described here constitutes an effort to gather important baseline data regarding the rate of formation of creatinine chloramines, the stabilities of these products and their identities.

Some researchers have studied the effect of the presence of chlorinated creatinine compounds in swimming pool water. Lomas (1967), showed that the presence of urine in water allowed the formation of compounds which reacted with DPD like dichloramine. He reported that the presence of this apparent dichloramine could be due to a chlorine derivative of creatine and creatinine derived from urine. Hamence (1980) confirmed this work and found that urine and particularly creatinine were responsible for the apparent nitrogen trichloride. As a result of this work it was concluded that the DPD-fast titrimetric method of analysis did not determine nitrogen trichloride but other chlorine compounds, particularly those of chlorinated creatinine and creatine. We found it interesting to examine in this study, for a range of hypochlorite creatinine ratios and pHs, the kinetics and mechanisms of formation and decomposition of N-chlorocreatinines.

The hypochlorite oxidation of creatinine in aqueous solution has been investigated in the dark. The following of creatinine and chloramines concentrations by the DPD-fast titrimetric method and by their u.v. spectra confirmed Lomas' and Hamence's works. However we observed dichloramine formation (Fig. 4) when the molar ratio of hypochlorite and creatinine was sufficient to decompose all chlorinated creatinine forms. The creatinine determination (HPLC method) suggested that N-chlorocreatinines were formed rapidly at an initial stage. Then they were decomposed by an apparent first order reaction at pH 8. With equimolar (1:1 mmol) amounts of hypochlorite and creatinine at pH 8, it appeared that N-chlorocreatinines were decomposed by hydrolysis to regenerate creatinine. We observed then the formation of creatine, 1-methylhydantoin, chlorocreatinines and NH₂Cl (Fig. 3). When the molar ratio was greater, the N-chlorocreatinines decomposed completely to form carbon dioxide, chlorite ion and mineral chloramines (see Table 1).

The reaction in the initial stage should be considered as an electrophile substitution followed slowly by hydrolysis when pH remained around 8 (Scheme 2). If the addition of hypochlorite affects the amine group of the molecule, 1-methylhydantoin is produced (Scheme 3) with NH₂Cl. Reaction yield was about 10% of initial creatinine.

In acid aqueous solution, with a molar ratio of 3, we also obtained trichlorocreatinine. This reaction is due to the various form of creatinine after addition of proton on amino of N-H groups of the molecule. In these conditions N-chlororcreatinines remained stable in aqueous solution for many days. However in the presence of free chlorine, we observed the production of carbon dioxide and mineral chloramines. After 4 days the residual concentration of N-chlorocreatinines was half the initial value.

It appears that N-chlorocreatinines formed during the chlorination of natural or swimming pool water were relatively stable, leading to the increase of the combined chlorine level. This stability was a function of the molar ratio of hypochlorite and creatinine, and pH. However, since most of the difference types of water had a pH in the range of 6–9, there would be little effect of pH at ambient temperature.