Effects of sodium sulfite on the concentration of organohalogenated compounds and on the mutagenic activity of chlorinated solutions of humic substances
If is a well known tact that mimerous organohalogenated compounds are formed during the chlorination (preoxidation or final disinfection) of drinking water. Some of these compounds have been shown to be mutagenic. Recent studies have suggested that a treatment with oxygenated derivatives of SIV (SO2, NaHSO3 and Na2SO3) could reduce the genotoxicity of chlorinated drinking water. The general aim of Ibis study was to determine the effect of dechlorination treatments on the mutagenic activity of chlorinated drinking water. The following experiments were carried out in order to point out the effect of a treatment with sodium sulfite on the concentration of total organohalogenated compounds (TOX) and on the mutagenic activity of chlorinated dilute solutions of Aquatic Humic Substances (AHS). At first, the affects of pH, sodium sulfite dose and contact time on TOX concentration were investigated. Then, the importance of the dechlorination rate (partial or complete) on TOX concentration and also on the mutagenic activity could be studied. Experimental Aquatic Humic Substances (natural mixture of fulvic and humic acids) were dissolved in phosphate-buffered ultra-pure water at 5 and 15 mg l-1 concentrations (pH 6.1 and 6.9 respectively). Stock solutions of chlorine were prepared in the laboratory and titrated by iodometry. Chlorination and dechlorination treatments were carried out in headspace-free baffles, at 20± 1 °C in the dark. Residual chlorine was determined by spectrophotometric measurements at 510 nm, following the calorimetric method using N,N-diethylphenylene-1,4-diamine (DPD). To avoid the slow oxidation of Slv into Svl by dissolved oxygen, the sodium sulfite solutions were prepared freshly before use. TOX concentrations were measured using a DOHRMAN DX-20 TOX analyser equipped with a MC-1 microcoulometric cell and with an AD-2 adsorption module. Before analysis, the residual chlorine was neutralized with sodium thiosulfate and samples were acidified to pH 1.4. The mutagenic activity was determined using acetone-dichloromethane extracts (AMBERLITE XAD-8 and XAD-2 resins) of the aqueous samples of chlorinated and dechlorinated solutions of AHS, acidified to pH 2.0 before extraction. The mutagenicity tests were carried out on TA 98 and TA 100 tester strains, following the method described by MARON and AMES (1983). Results -Effect of pH, addition of sodium sulfite and storage time on the TOX concentration The experiments carried out with dilute solutions of AHS ([AHS] = 5 mg 1-1; DOC = 2.5 mg Cl-1; pH = 6.1) showed a linear relationship between TOX production and chlorine consumption in the range 0-2.0 mg Cl2 l-1 (fig. 2).15 % of the chlorine demand was incorporated as organic chlorine in molecules. Experiments performed on solutions containing no residual free chlorine showed that organohatogenated compounds could be partially destroyed upon storage at neutral or basic pH (table 1). Reductions in TOX concentrations of 10 % at pH 6.1-8.5 in 24 hours and of 20 % at pH 11.5 in 2 hours were observed. This was enhanced by increasing the storage time. The addition of sodium sulfite (100 μmol l-1) in solutions containing no residual free chlorine significantly reduced the TOX concentration (10 % in 2 hours at pH 6.1-8.5; table 1). This reduction was enhanced by increasing sulfite dose and storage time and by increasing pH (30 % in 2 hours at pH 11.5). Furthermore, at a given pH value and for a reaction time of 2 hours, the decrease in TOX concentration was larger in presence of sulfite. - Effect of a dechlorination treatment on the TOX concentration As shown in figure 3, a dechlorination treatment (reduction of the residual free chlorine concentration) with sodium sulfite could significantly reduce the TOX concentration of the dilute solutions of AHS at pH 6.1 only if an excess of the dechlorinating agent was added. This effect was enhanced by increasing the excess of sulfite but nevertheless seemed to be limited (less than 15 % of reduction for the highest doses used; table 2). The free chlorine residuals measured after a 2 hours partial dechlorination confirmed the stoichiometric factor of 1 mole/mole for the reaction between chlorine and sodium sulfite. - Effect of a dechlorination treatment on the mutagenic activity and on the TOX concentration The dechlorination treatments were carried out on chlorinated dilute solutions of AHS ([AHS] = 15 mg l-1; DOC 7.5 mg C l-1; pH = 6.9). The TOX concentrations were measured on aqueous solutions and mutagenicity tests were performed on the corresponding acetone-dichloromethane extracts following a solvent exchange (dimethylsulfoxide). The results obtained showed again that only a total dechlorination treatment could reduce the TOX concentration of the aqueous chlorinated solutions and was able to destroy a significant part of the mutagenic activity of the extracts (table 3 and fig. 4). Although the effect of sulfite on TOX concentration seemed limited (less than 7 % reduction for the highest sulfite dose tested), the reduction in the genotoxicity was more important when the excess of sulfite was increased. No correlation between the TOX concentration and the mutagenic activity could be established. The mutagenic compounds destroyed by sodium sulfite do not appear to be organohalogenated ones. If they are, they are present at trace levels and thus are extremely patent and account for a very little part of the TOX concentration.