Résumés
Résumé
Cette étude présente les principaux résultats se rapportant d'une part à l'évolution des consommations en chlore par des composés organiques en solutions synthétiques de minéralisation variable. D'autre part, quelques essais sont consacrés aux possibilités de formation de sous- produits de la chloration tels les trihalométhanes.
Les différences de réactivité entre les composés simples (phénol, résorcinol, aniline) et les substances humiques, déjà observées en eau distillée, ont pu également être mises en évidence lors de leur dilution dans des milieux minéralisés (eaux de sources ou de forages). L'influence de la minéralisation, appréhendée par le paramètre "conductivité" ou par des teneurs variables en éléments minéraux spécifiques, s'est concrétisée par des écarts souvent considérables entre les potentiels de consommation en chlore en eau distillée et ceux en eaux minéralisées.
La présence d'ions bromures dans les eaux les plus minéralisées induit en particulier une formation plus importante des THM bromés. L'intervention d'ions chlorures et sulfates, en provoquant une diminution des potentiels de consommation en chlore, peut rendre plus complexe le schéma réactionnel du chlore avec les différentes entités réactives présentes dans une eau naturelle.
L'application du procédé de chloration à des eaux de surface algériennes a permis de distinguer entre les eaux très minéralisées du sud algérien et celles moins minéralisées du nord du pays.
Leur réactivité vis-à-vis du chlore s'est trouvée directement liée à leurs caractéristiques physico-chimiques.
Les différences de réactivité relevées peuvent être attribuées d'une part à la variabilité de la proportion en matériel humique et d'autre part aux teneurs relatives en éléments minéraux promoteurs ou inhibiteurs de la réaction de chloration.
Mots-clés:
- Minéralisation,
- chloration,
- substances organiques,
- eaux de surface algériennes,
- consommations en chlore,
- trihalométhanes
Abstract
The organic compounds studied during this work are simple aromatic structures (phenol, resorcinol, aniline), as well as macromolecular structures (humic substances). These compounds are representative of natural organic matter in surface waters. Various studies (De Laat et al., 1982; Norwood and Christman, 1987; Legube et al., 1990) showed that chlorination of these organic substances led to a strong chlorine consumption and the formation of volatile and non-volatile organohalogenated compounds. However, the majority of these experiments were conducted in aqueous media at very low ionic strength or in distilled water. The aim of the present study was to observe the reactivity of organic compounds in mineralised media. Indeed, Algerian waters are often very mineralised, excessively hard with high amounts of chloride and sulphate (Achour, 2001). In addition, humic substances can account for 60 to 90% of the total organic carbon (TOC) in most surface waters used to produce drinking water (Achour and Moussaoui, 1993).
The first part of this study involved the investigation of the impact of the overall parameter "total mineralisation", which can be directly related to conductivity measurements in the dilution waters. In addition to distilled water, dilution media for the organic compounds included four ground waters initially free of organic matter and residual chlorine. Figure 1 shows the geographic location of sampling sites for the waters of Drauh and Oued Biskra (Biskra) as well as those of waters of Ifri (Bedjaïa) and Youkous (Tebessa). Their main physico-chemical characteristics are presented in Table 1. The influence of certain specific inorganic constituents such as ammonia, bromide, chloride and sulphate is also observed on the chlorination of the organic compounds tested. The objective was to determine and to compare chlorine consumption potentials respectively for the simple organic compounds (D) and the humic substances (PCCl2) in media with differing degrees of mineralisation and under controlled conditions (pH, contact time and chlorine dose applied). Chlorination ratio is expressed for the simple organic compounds by r=moles of introduced chlorine/moles of organic compound and for the humic substances by m=mg of introduced chlorine/mg of humic substances.
Tests were performed to evaluate the formation of trihalomethanes (chloroform and brominated trihalomethanes). Analysis was done by gas chromatography using a head space method. The main experimental chlorination conditions for the synthetic solutions of organic compounds are summarised in Table 2. The results obtained (Table 3) show that the organic compounds tested remain very reactive towards chlorine, whatever the mineralisation of the dilution media. Potentials of chlorine consumption in distilled water and in mineralised waters are different (E), showing the obvious effect of the degree of mineralisation (Table 4). The reactivity of humic substances seems intermediary between the structures of resorcinol and phenol or aniline. The highest variations (E) between distilled water and the natural mineralised waters were observed in the case of the most highly mineralised water (water of Oued Biskra); generally the gaps decreased in the order E (Oued Biskra) > E (Ifri) > E (Youkous) > E (Drauh). In studies at different pH values, the oxidation capacity of chlorine was exerted more vigorously at circumneutral pH values (Table 5). For phenol and humic substances, gaps are more meaningful within five minutes of contact time, during the first step of the chlorination reaction (Table 6).
Total mineralisation does not seem to be the only parameter able to affect the reactivity of organic matter. The composition of the inorganic matrix must be considered too. Thus, according to the relative proportions of ammonia and bromides, competitive reactions (chlorine/organic matter and chlorine/inorganics) will condition chlorine consumption and the formation of trihalomethanes (THMs). For waters with a low ammonia/bromide ratio, an increase of the bromide concentration can induce a significant degradation of the chlorine consumption potentials, even for resorcinol, which is habitually more reactive with chlorine than with bromide (Figure 2). In waters with high levels of bromides (waters of Drauh and Oued Biskra), bromoform may be formed (Table 7). The presence of high concentrations of chloride and sulphate in aqueous solutions with humic substances seems to induce a reduction of chlorine consumption potentials (Figures 3 and 4). Several hypotheses can be advanced to explain this phenomenon, especially the decrease of activity of the chlorinating element by the increase of the ionic strength of the aqueous media. At the same time, it can be assumed that chlorinated entities may be formed with less reactivity than hypochlorous acid. The reduction of chlorine consumption is less significant in the presence of sulphates than chlorides. Moreover, the cation associated with sulphate seems to have an effect on the reactivity of the humic substances towards to chlorine.
In the second part of the work, chlorination was applied to four surface water samples. The sampling sites (Figure 1) relate to reservoir waters of Souk El Djemâa (Tizi-Ouzou), Aïn Zada (Sétif), Foum El Gherza (Biskra) and Fontaine des gazelles (Biskra). Amounts of humic material are relatively important and constitute about 60% of the TOC found in the raw waters. The mineralisation in these waters is considerable, particularly for waters in the southern part of the country (Table 8). Chlorine consumption and THMs formed seem to be correlated with the nature and concentration of organic matter, as well as with the inorganic component of these waters (Table 9). Chlorination of water samples taken from the north of Algeria (Aïn Zada and Souk El Djemâa) leads to higher values of chlorine consumption potentials than for water samples taken from the south of the country. Taking into account the results obtained on synthetic solutions, this trend can be explained by the appreciable contents of humic substances as well as by a lower degree of mineralisation of waters in the north. It also interesting to note that proportion of brominated trihalomethanes compared to the total trihalomethanes increases not only with the initial concentration of bromides but also with increasing levels of chlorides and sulphates. Highly mineralised waters, containing humic material, seem therefore to favour numerous competitive reactions during chlorination.
Keywords:
- Mineralisation,
- chlorination,
- organic compounds,
- Algerian surface waters,
- chlorine consumption,
- trihalomethanes
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