Environ Chem Lett (2005) 3:1–5 DOI 10.1007/s10311-005-0103-1
ORIGINAL PAPER
Gulnara M. Shaydullina · Natalya A. Sinikova · Albert T. Lebedev
Reaction of ortho-methoxybenzoic acid with the water disinfecting agents ozone, chlorine and sodium hypochlorite Received: 14 January 2005 / Accepted: 2 March 2005 / Published online: 30 April 2005 Springer-Verlag 2005
Abstract Ozone, chlorine and sodium hypochlorite are commonly used as disinfecting agents for drinking water production. The reaction pathways of ozonation and chlorination of o-methoxybenzoic acid in aqueous solution were studied using gas chromatography-mass spectrometry (GC-MS) and high pressure liquid chromatography (HPLC). The results show that less than 1% of omethoxybenzoic acid remains in reaction. The final major products using ozone oxidation are oxalic and glyoxalic acids. Phenols appear only at insufficient ozone levels. Sodium hypochlorite leads to higher levels of primary products. Molecular chlorine leads to the formation of higher amounts of polychlorinated derivatives. Model experiments allow to propose schemes of o-methoxybenzoic acid transformation under the conditions simulating water treatment processes. Keywords ortho-methoxybenzoic acid · Ozone · Chlorine · Sodium hypochlorite · Mass spectrometry · GC/MS · HPLC · Water treatment · Disinfecting agents · Disinfection by-products
Introduction The preparation of water for human consumption is a major industry. Chlorination is an effective method of water disinfection. Chlorine and sodium hypochlorite are rather efficient disinfectants and stable enough to remain in water distribution systems for a long time, thus preventing further water reinfection. In spite of these positive features, chlorination of drinking water has been identified as contributing to the formation of wide variety of disinfection by-products with adverse health effects G. M. Shaydullina · N. A. Sinikova · A. T. Lebedev ()) Department of Organic Chemistry, Moscow State University, Leninskie gory-1/3, 119992 Moscow, Russia e-mail:
[email protected] Tel.: +7-095-939-1407 Fax: +7-095-939-1407
(Rook 1974). These concerns led to an increased interest in alternative disinfectants such as ozone. Currently, ozone applications for water treatment are numerous and widespread all over the world in spite of their two major technological drawbacks (Hoigne 1982). Firstly, under conditions common to water and wastewater treatment, ozone is an unstable gas and it must, therefore, be generated and applied at its point of use; secondly, as ozone is only partially soluble in water, its introduction into water or wastewater requires a very efficient gas-liquid contact to maximize the mass transfer of ozone from the gas phase to the liquid phase. Since the detection of carcinogenic disinfection byproducts in treated drinking water much effort has gone into determination of their precursors. The reaction of aquatic humic substances, composing the largest fraction of organic material in natural water, is believed to be the major source of undesirable halomethanes and other chlorination by-products (Rook 1974). Formation of wide array of hazardous organic impurities identified in water after ozonation was also attributed to be as result of decay of natural organic matter (Richardson et al. 2000). However, the complexity and variability of the composition and structures of natural humic substances hardly allow proposing reliable schemes of their transformation. Therefore, to study the mechanisms of these reactions model organic compounds representing structural fragments of humic acids have been used (Rook 1977; Yamamoto et al. 1979). Some processes realized upon aqueous chlorination of various model compounds have been reported by our laboratory (Tretyakova et al. 1994; Lebedev et al. 1997, 2004). Having been detected in natural water o-methoxybenzoic acid was selected as a model organic compound to investigate its reactions with ozone, chlorine and sodium hypochlorite in aqueous solutions. GC-MS and HPLC-UV were used to identify and quantify the resulting products (UV: ultraviolet). The main goal involved identification of all the ozonation and chlorination byproducts and proposal of the mechanisms of these compounds formation.
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Experimental Ozonation experiments Ozonation experiments were conducted in a semi-batch reactor at ambient temperature. Ozone was generated from dried oxygen by electric discharge ozone generator. An ozonized oxygen stream with ozone concentration 20 mg/l or 80 mg/l, was bubbled at a flow rate 6 l/h to phosphate-buffered (pH 7) aqueous solution of orthomethoxybenzoic acid (103 M). The molar ratios of the studied acid to ozone were determined by taking into account ozone concentration, flow rates and time of gas mixture bubbling. The molar ratios of organic substrate (S) to ozone [S]:[O3] were 2:1, 1:1, 1:2, 1:5, 1:10 and 1:50. Chlorination experiments Reactions of ortho-methoxybenzoic acid with chlorine or sodium hypochlorite were conducted in the dark at 20C in 0.2 M phosphate buffer solution (pH 7) prepared from reagent grade monobasic potassium phosphate and dibasic sodium phosphate. A total of 50 ml of buffer and calculated amount of chlorinating agents was added to substrate solution and the volume was adjusted to 100 ml with distilled water. Active chlorine content was determined by iodometric titration before chlorination. In all experiments, the concentration of ortho-methoxybenzoic acid was 103 M. The molar ratios of substrate to active chlorine [S]:[Clact] were 1:1, 1:5 and 1:50. After 24 h, unreacted chlorinating agents were reduced by addition of an excess of reagent grade sodium sulphite. Analytical methods Analysis of volatile products was carried out with HP5973 GC-MS system equipped with 50 m DB-5 fused silica capillary column (50 m length, 0.25 mm i.d.). Aliquot of 5 ml of a sample was injected into a “purge and trap” concentrator and purged by helium for 10 min. Then trap sorbent was heated to 170C allowing organic molecules into the gas chromatograph which operating conditions were as follows: carrier gas—helium, flow rate 1 ml/min, initial temperature 40C (2 min), programming rate 10C/min up to 250C, then isothermal for 1 min. Electron ionization (EI) mass spectra were obtained at 70 eV electron energy with the ion source at 180C. Bromofluorobenzene was used as an internal standard to quantify the products. Semi volatile products were consequently extracted at neutral and acidic pH with three sequential aliquots of freshly distilled dichloromethane. Sodium chloride was added to aqueous solutions to enhance the extraction of organic by-products. Extracts were dried over anhydrous sodium sulphate and concentrated by evaporation. Analysis was carried out with SSQ-7000 GC-MS instrument
(Finnigan) using HP-5MS fused silica capillary column (30 m length, 0.25 mm i.d.). The operating conditions were as follows: carrier gas-helium, flow rate 1 ml/min, initial temperature 50C (2 min), programming rate 10C/min up to 280C, then isothermal for 3 min. EI mass spectra were obtained at 70 eV electron energy with the ion source at 180C. The background-subtracted mass spectra were matched against those in the NIST mass spectra library and interpreted on the basis of the observed fragmentation. The quantities of products in the reaction mixture were estimated using naphthalene-d8 and phenanthrene-d10 as internal standards. The concentration of non volatile products were monitored by HPLC-UV with a 1100 chromatographic system (Agilent Technologies) equipped with a Spherisorb C-18 (2504.6 mm i.d.) column and a multiple wavelength detector, set at 196 nm. Injected samples (10 ml) were eluted with water acidified with TFA (pH = 1.5) at a flow rate of 1 ml/min.
Results and discussion Background From the chemical point of view, molecular ozone in aqueous solution remains as O3, or decomposes to form very reactive hydroxyl radicals. The latter is a stronger oxidizing agent (Eo=2.80 V) than the molecular ozone itself (Eo=2.07 V): O3 þ H2 O ! 2OH þ O2 Ozone and hydroxyl radicals react with organics by the following principal mechanisms: ozonolysis of unsaturated bonds, hydroxyl radical attack of aromatic rings and oxidation (Yamamoto et al. 1979). The term “aquatic chlorination” has been commonly used to name transformation of organic substrates by chlorine as well as by sodium hypochlorite in aquatic solution. However, there are some differences between reaction behavior of these chlorinating agents. Both reagents exist in aquatic solution as mixtures of different species due to equilibrium reactions with the solvent. Thus, aquatic chlorine represents a mixture of molecular chlorine, hydrochloric and hypochlorous acids with the corresponding anions: Cl2 þ H2 O > HClO þ Hþ þ Cl HClO þ H2 O > H3 Oþ þ ClO while sodium hypochlorite exists in aquatic solution mainly as hypochlorous acid and hypochlorite anion due to equilibrium reactions: NaOCl > Naþ þ OCl ClO þ H2 O > HClO þ OH Molecular chlorine is a stronger oxidizing agent (Eo=1.59) than hypochlorous acid (Eo=1.50) or hypo-
3 Fig. 1 Degradation pathways of ortho-methoxybenzoic acid during ozonation in aqueous solution in a semi-batch reactor at ambient temperature and neutral pH at various molar ratios of investigated acid to bubbled ozone: (a): hydroxyl radical attack of aromatic ring with concurrent decarboxylation; (b): radical hydroxylation of aromatic ring; (c): fast ozonolysis of unsaturated bonds; (d): formation of oxalic acid as alienated off-fragment during ozonolysis of double bonds at different intermediate stages; (e): dehydration; (f): oxidation. m/z refers to mass spectrum data
Table 1 Products of orthomethoxybenzoic acid ozonation in aqueous solution in a semi-batch reactor at ambient temperature and neutral pH at various molar ratios of investigated acid (S) to bubbled ozone (O3) (mg)
No
1 2 3 4 5 6 7 8 9 10 11 12 13 14
[S]:[O3] 2:1
1:1
1:2
1:5
1:10
7630 – 0.23 – 0.27 – – – 0.11 – 40.0 – – –
6510 – 0.45 – 0.62 – – – 0.21 – 180 – – –
5560 0.36 0.69 0.08 2.22 0.05 0.19 0.36 0.40 – 280 – 0.28 –
4610 0.39 1.18 0.09 2.18 0.07 – 0.40 0.13 – 360 – 0.26 0.04
3900 0.46 1.22 0.18 2.15 0.21 – 1.52 0.19 – 620 30.0 0.23 0.15
chlorite anion (Eo=0.89). The mentioned species react with organic compounds by addition, substitution or oxidation (Boyce and Horning 1983). Moreover according to ab initio calculations a complex of hydroxonium ion with HOCl molecule is a reactive particle in the reaction of aqueous chlorination (Lebedev et al 2004).
1:50 40.0 – – – – – – – 270 3150 800 – –
Ozonation by-products The results on o-methoxybenzoic acid ozonation are summarized in Fig. 1 and Table 1. The transformation of o-methoxybenzoic acid during ozonation starts with hydroxyl radical attack accompanied by decarboxylation giving methoxyphenols 2 and 3. Comparison of yields of the isomers proves that ipso-substitution of carboxylic group is preferable. Consequent stages result in formation of methoxybenzenediols 4, 5 and 8.
4 Table 2 Products of orthomethoxybenzoic acid chlorination with Cl2 and NaClO in aqueous solution at various molar ratios of substrate (S) to active chlorine (Clact) at ambient temperature and neutral pH (mg)
No
[S]:[Clact] Cl2
1 15 16 17 18 19 20 21 22 23 CHCl3
NaClO
1:1
1:5
14100 50.4 – – 2.46 – – – 166 – –
1180 712 – – 59.8 – 9.15 – 4060 – –
1:50 37.4 184 – 6.32 14.9 29 880 26.6 1650 2820 21.0
1:1 12100 171 – – 10.1 – – – 657 – –
1:5 790 946 – – 62.4 1,4 15.3 – 4130 – –
1:50 31.5 1022 8.4 19.6 58.5 – 4.31 – 4270 – 6.70
Ozonolysis of ring C-C bonds adjacent to electronodonating methoxy-group led to the cleavage of the aromatic ring and formation of compounds with linear structures. Concentration of linear C6 compounds in reaction mixtures was low in all the experiments due to fast ozonation of double bonds. Concentration of unsaturated dialdehyde 6 was higher than that of ester 7. Following reactions involving double bonds of the intermediates 6 and 7 bring to several short-chain products. There were few primary products detected at the highest molar ratio of ozone to substrate. Only oxalic (11), glyoxalic (12) and maleic (10) acids were observed with rather high yields. These products originate at different intermediate stages of ozonation. A decrease of other products concentration at the highest ratios of ozone to substrate is accounted for their decay at the advanced stages of conversion with final formation of CO2 and H2O. Chlorination by-products Reactions of o-methoxybenzoic acid with chlorine and sodium hypochlorite in aqueous solutions are presented in Fig. 2. Monochloro derivatives 15 and 22 are the major products (Table 2). Electrophilic substitution of two hydrogen atoms for chlorine takes place by coordinated control of both groups. Another primary reaction— chlorodecarboxylation—leads to the formation of orthochloromethoxybenzene (18). Comparison of the assortment and relative amounts of reaction products demonstrates higher chlorination activity of sodium hypochlorite at the initial stages. The extent of conversion of o-methoxybenzoic acid with equimolar ratios of investigated agents was 5.6% in case of chlorine and 14.9% in case of sodium hypochlorite. Further deceleration of electrophilic substitution occurs due to insertion of chlorine atom into aromatic ring. It is worth mentioning the fact that only chloroform was detected in “purge-and-trap” experiments. Since CHCl3 is a dominant product of aqueous chlorination of a reactive organic molecule, transformation of orthomethoxybenzoic acid stops mainly at initial stages (Fig. 2)
Fig. 2 Transformation pathways of ortho-methoxybenzoic acid during chlorination with sodium hypochlorite and chlorine in aqueous solution at various molar ratios of substrate (S) to active chlorine (Clact) at ambient temperature and neutral pH: electrophilic substitution of hydrogen atoms for chlorine (a) or bromine (b) and electrophilic substitution in aromatic ring with concurrent decarboxylation - chlorodecarboxylation (c). m/z refers to mass spectrum data
Aromatic ring cleavage is a minor process. Brominated derivatives (16, 17) appeared due to reactions of substrate with bromine impurities in chlorinating agent solutions. An increase of a chlorinating agent concentration increased a variety of resulting organochlorines. It is to be noted that the assortment of chlorinated by-products was higher in case of chlorine in comparison with sodium hypochlorite.
Conclusions Ozonation of o-methoxybenzoic acid in aqueous solution led to its nearly complete disappearance, while oxalic and glyoxalic acids are the dominant by-products. Formation
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of reaction products in conditions of chlorination depends on molar ratios of chlorinating reagents to the organic substrate. An increase of the active chlorine concentration increased variety of resulting products. Cl2 and NaOCl react similarly. However, the yields of monochlorinated derivatives were higher with sodium hypochlorite. On the contrary the levels of polychlorinated derivatives were higher in case of molecular chlorine. The ozonation products are definitely less toxic than the chlorination ones. Nevertheless formation of phenols at pre-ozonation stage of water treatment process could be a source of hazardous chlorophenols, haloforms, and other toxic chemicals during final chlorination.
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