~ Pergamon 0043-1354(94)E0073-F Wat. Res. Vol. 28, No. 12, pp. 2499-2506, 1994

Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved

0043-1354/94 $7.00 + 0.00

DEGRADATION OF SULPHUR CONTAINING S-TRIAZINES DURING WATER CHLORINATION

GIUSEPPE MASCOLO l, ANTONIO LOPEZ l, ROBERTO PASSINO 2., GIUSEPPINA RICCO l and GIOVANNI TIRAVANT! 1 {~

tC.N.R.-Istituto di Ricerca Sulle Acque, Via F. De Blasio 5, 70123 Bari and 2C.N.R.-Istituto di Ricerca Sulle Acque, Via Reno l, 00198 Roma, Italy

(First received February 1993; accepted in revised form March 1994)

Abstract--The reactions of four sulphur containing s-triazines (prometryne, terbutryne, ametryne and desmetryne) with hypochlorous acid (HC10) and chlorine dioxide (C102) have been investigated using an 11 ppm/3 ppm oxidant/herbicide ratio. The main objective of the study was the identification of by-products. Additionally, to study the effect of oxidant concentration on the reaction rate, two more oxidant/herbicide ratios (3 ppm/3 ppb and I 1 ppb/3 ppb) have been investigated only for prometryne. Oxidation reactions were monitored by high performance liquid chromatography (HPLC), while, the identification of by-products was initially carried out by low resolution HPLC-mass spectrometry (HPLC-MS) and confirmed by accurate mass measurement. Under the experimental conditions (T = 20°C, pH = 8, reaction time = 48 h), the results indicate that all the investigated triazines react in the same way with each oxidant. The reactions with HC10 occur much faster than those with C102 and give rise to three identified oxidation by-products: the sulfoxide, the sulfone and the sulfone's hydrolysis product. The reactions with C102, instead, give rise to a sole oxidation by-product: the sulfoxide. With both oxidants, as expected, the lower the oxidant concentration the slower the oxidation rate.

Based on the obtained results, a general pathway for the oxidation of sulphur containing s-triazines is proposed.

Key words--prometryne, terbutryne, desmetryne, ametryne, triazines, water, chlorination, chlorine dioxide, HPLC, mass-spectrometry, thermospray

INTRODUCTION

The occurrence of herbicides in groundwater is mainly due to the massive and prolonged use of these chemicals in agriculture. Concern about such water resource contaminat ion is increasing, particularly regarding those areas characterized by permeable soils, and whose primary drinking water sources are groundwaters (Research Triangle Institute, 1988; Hormann, 1979; Galassi, 1990).

Technologically, ignoring those water treatment techniques which are not yet totally reliable such as electrochemical (Patermarakis, 1990; Kirmaier, 1984) and photocatalytic processes (Pelizzetti, 1990; Ollis, 1991), microbial (Speitel, 1987) and photolytic degra- dat ion (Stein, 1980; Muller, 1986), the removal of herbicides from drinking water can be achieved only by advanced physico chemical treatments such as adsorption onto activated carbon (Graese, 1987; Miltner, 1989), reverse osmosis (Baier, 1987) or ozonation alone or coupled with ultraviolet radiation or hydrogen peroxide (Glaze, 1987).

In many drinking water treatment plants operating in potentially contaminated areas, however, such

*Author to whom all correspondence should be addressed.

advanced treatments are absent as information on the occurrence of herbicides in groundwater is often not available because of both the practical difficulty of analyzing for all of the contaminants present and the fact that, usually, herbicides are not monitored very frequently.

In such plants, contaminated water undergoes con- ventional treatments such as clarification, filtration, disinfection, among which the only one that could significantly reduce herbicides concentration by oxi- dation is disinfection (EI-Dib, 1977; Dennis, 1979; Lamley, 1984). However, organic micropollutants oxidation usually produces by-products which may result less or more toxic than their parent compounds and hence their identification assumes a determinant importance.

Among the available disinfectants (chlorine, chlor- ine, dioxide, chloramines, ozone, etc.), in spite of the concern over their potential hazardous by-products, currently, those mostly used are chlorine, gaseous (C12) or in aqueous solution (HCIO), and chlorine dioxide (C102) (Lykins, 1986 and 1990).

Whether such disinfectants react with sulphur con- taining herbicides and the identities of the resulting oxidation by-products are still open questions.

Among the pesticides potentially occurring in drinking waters resources, triazines represent for

2499

2500 GIUSEPPE MASCOLO et al.

sure a class of herbicides worthy of being con- sidered (Worthing, 1991). In Europe, over the period 1986-1992, after atrazine, the most used chlorine containing triazine, has been banned because of its toxicity, the sale of sulphur containing triazines, as expected, is progressively increased, over 150%, to the detriment of that of chlorine containing homolo- gous herbicides (Mandl, 1992). F rom the chemical point of view, an interesting difference between chlor- ine and sulphur containing triazines is their reaction with hypochlorous acid. In fact, the former are not significantly oxidized by hypochlorous acid, while the latter react very quickly with it (Cremisini, 1990). However, there is no information on by-product identification, nor a comparison between the oxi- dative effectiveness of chlorine and chlorine dioxide for this class of triazines.

To date investigations of the degradation of sul- phur containing pesticides have been mainly focused on carbamates and carbamoyloximes (Lamley, 1984; Mason, 1990; Miles, 1991).

The present paper reports the results of an extensive investigation undertaken to study the oxidation reactions of four sulphur containing s- triazines (prometryne, terbutryne, ametryne and desmetryne) with hypochlorous acid and chlorine dioxide and to identify the resulting by-products.

EXPERIMENTAL

Chemicals

Triazines of 99% purity (Polyscience-Niles, IL, U.S.A.) were used without further purification. Water and methanol used during liquid chromatography were HPLC-grade (Carlo Erba-Milan-Italy). Methanol and methylene chlorine used for solid-liquid extractions were pesticide-grade from Carlo Erba. Hypochlorite solutions were prepared from Carlo Erba stock solutions after proper dilutions and used after iodometric titration to check their exact concen- trations (Standard Methods, 1989). All other chemicals were analytical grade from Carlo Erba. Stock solutions of chlorine dioxide were prepared according to the pro- cedure of Masschelein (Masschelein, 1967) which provides aqueous chlorine dioxide without traces of active chlorine (Rav-Acha, 1987). All the stock solutions were stored in dark bottles at 4°C and, before using, checked for chlorine dioxide, chlorite, chlorate and chlorine content (Aieta, 1984). High purity water from a Millipore Milli Q-Water System was used for preparing all aqueous solutions.

Chlorination experiments

The appropriate volume of herbicide methanolic sol- ution (250 ppm) was introduced in a 250 ml flask. After methanol evaporation under vacuum, a phosphate (1 mM) buffer solution (pH = 6 or 8) was added. The solution obtained was stirred to dissolve the herbicides and then the appropriate amount of hypochlorite or chlorine dioxide solution was added. In the resulting solution (200 ml) the initial concentrations of herbicide and hypochlorite or chlorine dioxide were 3 ppm and 11 ppm respectively. Oxi- dant concentrations were both measured as CI. Samples of the reaction mixture were withdrawn at scheduled times, quenched with sodium thiosulfate, and then analyzed by HPLC.

For the chlorination experiments at low herbicide con- centration (3 ppb), the concentration of the starting herbi- cide stock solution was 0.1 ppm. Four replicate reactions

were carried out in four 250 ml flasks. In each case, at the scheduled time, the reaction was quenched and the solution volume reduced to 0.2 ml by solid-liquid extraction using 500 mg ENVI-18 cartridges (Supelco, Bellefonte, PA, USA). The extraction procedure consisted of a cartridge con- ditioning step (first methylene chloride/methanol I/1 v/v, then methanol, then water), a sample introduction step, and a sample elution step using methanol which was reduced in volume by a gentle stream of purified air.

HPLC conditions

A Perkin-Elmer (Norwalk, CN, U.S.A.) Series 10 pump module equipped with a Rheodyne (Cotati, CA, U.S.A.) 7125 injection valve and a (250 x 4 mm i.d.) stainless steel column packed with 5/zm spherex C-8 silica solid phase from Labservice Analytica (Bologna, Italy) was used as the chromatographic system. Compound detection was car- ded out by a Dionex Model VDM-I u.v. detector (Dionex Co., Sunnyvale, CA, U.S.A.) set at 254 nm. Samples were injected by a 10#1 Rheodyne loop. The mobile phase composition was methanol/pH =9 ammonium acetate solution (0.I M) 70/30 v/v with a flow rate of 0.9 ml/min. For desmetryne only, the flow rate was 0.7 ml/min.

Mass spectrometric analysis

A VG (Manchester, U.K.) TS-250 trisector mass spec- trometer equipped with a thermospray interface and a PDP-11 microcomputer (Digital, Maynard, MA, U.S.A.) for data acquisition, operating both in thermospray and plasmaspray modes was used (Brown 1990, VG Analytical, 1987). The samples to be analyzed by HPLC-MS were concentrated 15 times by the solid-liquid extraction pro- cedure reported above.

The thermospray/plasmaspray conditions (positive ions) were: source temperature = 275°C, vapour temperature = 130°C, scan speed = 1 s, resolution = 500, mass range 100 and 400 a.m.u.

For accurate mass measurements the mobile phase con- tained, as internal standard, a mixture (50 ppm + 50 ppm) of two Poly-Ethylene-Glycols: PEG 200 MW and PEG 300 MW from Aldrich (Milwaukee, WI, U.S.A.). Addition- ally as reported by Harbach (Harbach, 1989), a resol- ution = 1000 was set, and the amount of each compound injected through a post-column injection valve was about 500 ng.

RESULTS AND DISCUSSION

The chemical structures of the investigated sul- phur containing s-triazines (prometryne, terbutryne, ametryne and desmetryne) are shown in Fig. 1.

H H

I I / N . N, N ~

SCH 3

R 1 R 2

Prometryne CH(CH3) 2 CH(CH3) 2

Terbutryne C(CH3) 3 CH2CH 3

Ametryne CH(CH3) 2 CH2CH 3

Desmetryne CH(CH3) 2 CH 3

Fig. 1. Chemical structures of investigated triazines.

Degradation of s-triazines during chlorination 2501

The oxidation of the triazines by HCIO and CIO 2 were carried out for 48 h, at T = 20°C, in buffered (pH = 8) aqueous solutions, at 11 ppm/3 ppm oxi- dant/herbicide ratio. This ratio was chosen for analytical convenience. The reaction time, 48h, was chosen as a time representative of an aver- age drinking water residence time in distribution systems.

It should first be pointed out that the investigated herbicides all showed the same chemical behaviour with respect to each oxidant. Therefore, in each case, instead of discussing the behaviour of each single triazines, only one will be discussed as representative of the others.

Under the chosen experimental conditions, all the investigated herbicides behave qualitatively as curves in Fig. 2 show for prometryne, i.e. they react very quickly with HCIO and much more slowly with CIO 2 •

In particular, the time necessary for a complete (100%) oxidation by HC10 was found to be 2, 3, 8 and l0 min and the residual amounts after reacting for 48h with ClO2, were 60, 79, 68 and 62%,

respectively for prometryne, terbutryne, desmetryne and ametryne.

Figure 2 also shows the effect of the concen- tration of both oxidants on the prometryne oxidation rate. The results indicate that, with both oxidants, the reaction rate increases with increasing oxidant concentration. In particular, with HCIO, a complete prometryne oxidation is achieved within 3min, 5 min and 8 h at decreasing oxidant concentrations of 11 ppm, 3 ppm and 3 ppb respectively. Conversely, with CIO2, after 48 h, unreacted prometryne amounts are 90, 87 and 56% at increasing C102 concentrations of 3 ppb, 3 ppm and 11 ppm.

Additionally Fig. 2 shows that to obtain com- plete prometryne oxidation within 48h, a CIO2/ prometryne ratio as high as l l0ppm/3ppm is necessary.

The different reaction rates with HCIO and C102 can be qualitatively explained by assuming two differ- ent oxidation mechanisms, each one characterized by its own kinetics, very fast with HCIO, slower with C102. According to other authors (Rosenblatt, 1975; Bailey, 1982), triazine oxidation should occur via the following mechanisms:

CH3 CH 3

R R

CIO2/H20

_H +

CH3 CH 3 [ /o \ -.ClO: I

: ~----O i \ o / C l ~ O H = : R R

CH3 CH 3 I I e

:S: + HOCI ~- : S " ~ O ~ H

I I R R

_H + CH3 I

:S '---O

I R

, 00

80 x - e.- - -- _ _ _ _-~ I b ' ,

60 sth,lkx\ x.,

~-- l / \XXxx ~ • •

2040 ~ e ] / ] ~- " ~

._d I 1 I ~ ' ~ . .A

2 4 6 0 12 24 36 48

Time (min) Time (h) Fig. 2. Prometryne chlorination decay with HCIO (full lines) and CIO 2 (dashed lines). Initial (ppm oxidant)/(ppm herbi- cide) ratios: &=0.011/0.003, O=3/0.003, B = I I / 3 ,

A=I10/3, pH=8.

Considering oxidation by-products, for all the investigated triazines, as qualitatively shown in Fig. 3 for prometryne, the reaction with HCIO gives rise to the sequential formation of three by-products (a), (b) and (c). To assess if (c) is really the end by-product, the prometryne oxidation has been monitored for two weeks, confirming that once formed, by-product (c) remains stable and does not react further on.

Herbicide disappearance as well as by-product formation have been monitored by HPLC as reported in the experimental section. Table 1 reports the retention times of both the investigated herbicides and their corresponding by-products.

In Fig. 3, it is qualitatively evident that the first by-product (a) forms quickly, the second (b) and the third (c) more slowly. As for by-products result- ing from herbicide oxidation by C102, for each

2502 GIUSEPPE MASCOLO et al.

10 -- /~u~'INN ~ • a

* b 8 - - • ~ c

~ 4

2

I ~ ~ l ~ . * ~ k j I • l 0 1 10 100 1000 10,000

Time (min) Fig. 3. By-product formation during prometryne oxidation by HCIO. Initial oxidant /herbic ide ratio: I I ppm/3 ppm, p H = 8. Measu red peak areas are repor ted on an a rb i t r a ry

scale.

triazine, after 48 h a sole by-product is detectable. For each triazine the resulting by-product has the same retention time as by-product (a) reported in Table 1. Furthermore, it is formed rather slowly, according to the herbicide disappearance rate quali- tatively shown in Fig. 2 for prometryne. After 48 h, only using an oxidant concentration of l l 0 p p m it is possible to observe the formation of a small amount of by-product (b).

The identification of by-products has been carried out by HPLC-MS only for the reactions with 11 ppm of HCIO. It has been assumed, on the basis of the retention times, that the corresponding by-products (a) from the reactions with C102 are the same.

For all the investigated herbicides the identified by-products were found to be structurally similar.

Table I. HPLC retention times (minutes) of investigated herbicides and corresponding by- products resulting from their oxidation by HCIO. Chromatographic conditions are reported in the

text

By-product Herbicide a b c

Prometryne 9.7 6.2 5.5 5 Terbutryne 10.3 6.2 5.6 5.2 Ametryne 7.3 5.1 4.6 4.3 Desmetryne 7.8 5.7 5.3 5

For each herbicide, in fact, by-product (a) shows a molecular ion 16a.m.u. higher than its own molecular ion [M + H] ÷. This means that (a) is the starting herbicide with one additional oxygen atom (16 a.m.u.), i.e. it is the corresponding sulfoxide. The molecular ion of by-product (b) was found to be 16 a.m.u, higher than (a). This means that a further oxidation occurs giving rise to the corresponding sulfone. Finally, by-product (c) has a molecular ion 62 a.m.u, lower than (b), and this could mean that during this third step hydrolysis of the methyl-sulfone group takes place, giving rise presumably to the correspondent hydroxy-herbicide. Figure 4 shows as an example, the plasmaspray mass spectra of ter- butryne and its by-products. The thermospray spec- tra have not been reported as they show just the molecular ion without any fragmentation.

If the hypothesis of the formation of a hydrolysis product is correct, the third step (b) ~ (c) should be, as for all hydrolysis reactions, pH dependent.

To substantiate this hypothesis, a set of oxidation reactions with HC10 has been carried out for all the herbicides at pH = 6 buffered conditions. Comparing Fig. 5(a) and (b) which show the resulting trends for ametryne chlorination at pH = 8 and pH = 6, at least qualitatively, it seems that among the three steps:

Table 2. MH + accurate mass measurements for investigated triazines and their oxidation by-products: a, b, c

Compound

Most probable Postulated Calculated Measured elemental composition Mass relative elemental mass mass from computer outputs error. 106

composition (A) (B) C H N S O [(A )-(B )]/(A )

Desmetryne C8HI6NsS 214.1126 214.1129 8 16 5 1 0 - 1 . 4 a CsHI6NsSO 246.1076 246.1079 8 16 5 1 1 - 1.3 b CsHt6N~SO 2 246.1025 246.1029 8 16 5 1 2 - 1.6 c C7HI4N50 184.1198 184.1204 7 14 5 0 1 -3 .3

Ametryne C 9HIsN~S 228.1283 228.1264 9 18 5 1 0 8.3 a CgHIsNsSO 244.1232 244.1228 9 18 5 I 1 1.6 b CgHIsNsSO 2 260.1181 260.1177 9 18 5 1 2 1.5 c CsHI6NsO 198.1355 198.1353 8 16 5 0 1 1.0

Prometryne Cj0H20N~S 242.1439 242.1433 10 20 5 1 0 2.5 a CIoH20NsSO 258.1389 258.1392 10 20 5 1 1 - 1.2 b CIoH20N~SO: 274.1338 274.1329 10 20 5 1 2 3.3 c C9HIsN50 212.1511 * * *

Terbutryne CIoH20NsS 242.1439 242.1443 10 20 5 I 0 - 1.6 a C~0H20NsSO 258.1389 258.1393 10 20 5 I 1 - 1.5 b Ct0H20N~SO: 274.1338 274.1341 10 20 5 I 2 - 1.0 c CgHI~NsO 212.1511 * * *

*Accurate mass measurement of by-product (c) for prometryne and terbutryne is not possible because of the interference of the ion fragment of mass 212.150 of the internal standard PEG (PolyEthylenGlycol). Instrumental conditions are reported in the text.

Degradation of s-triazines during chlorination 2503

2504 GIUSEPPEMASCOLO el al.

15

12

15

(a)

1 100

(b)

I

10,000

12 S \ c

3 / , J *

0 1 100 10.000

Time (•in)

Fig. 5. (a) By-product formation during ametryne oxidation by HC10 at pH=8. Initial oxidant/herbicide ratio: 11 ppm/3 ppm. Measured peak areas are reported on an arbitrary scale. (b) By-product formation during ametryne oxidation by HCIO at pH = 6. Initial oxidant/herbicide ratio: 11 ppm/3 ppm. Measured peak areas are reported on

an arbitrary scale.

ametryne ---, (a) --~ (b) --} (c) the last one is the step whose rate is more affected by pH. In fact, its rate clearly results to be higher at higher pH. This is not surprising if it is assumed that OH- and/or CIO- (the conjugate bases of HCIO) concentrations affect positively the rate of the hydrolysis step: (b) --} (c). In any case, a quantitative kinetics investigation on sulphur containing s-triazines oxidation pathway is beyond the aim of the present investigation and is currently underway.

Finally, to confirm the structures of the proposed by-products, plasmaspray HPLC-MS analysis for accurate mass measurements have been carried out. Such analyses consisted of:

measuring the mass of MH + of both herbicides and corresponding by-products to four decimal places;

searching, via the MS-dedicated computer, the corresponding most probable elemental composition; comparing this composition with that hypoth- esized;

i H

S

N.J'.N I I

R 1 R 2

HCI O Fast

(a)

i H

S 0

~ N

H~N~ ~N 7H I I

R 1 R 2

HC10 slow

(b)

CH 3

O :----O

H ~ N L ~ ~ N ~ N 7 H

I I R 1 R 2

OH-

OH

(c) H ~ N L ~ l l ~ N

I I R 1 R 2

Fig. 6. Proposed pathway for sulphur-containing s-triazines oxidation by HC10. R~ and R 2 as in Fig. I.

Degradation of s-triazines during chlorination 2505

comparing the measured masses for each by-product with the corresponding values calculated on the basis of the hypothesized composition.

The results reported in Table 2 indicate the extremely good agreement between the postulated and measured mass values. On the basis of the above results, a proposed pathway for the oxi- dation of sulphur-containing s-triazines is presented in Fig. 6.

CONCLUSIONS

The reaction of four sulphur containing s-triazines with HC10 and CIO2 have been investigated.

Using the experimental conditions, T=20°C, pH = 8, reaction time = 4 8 h , the reactions with HCIO are very fast and give rise to the oxidation by-products identified by HPLC-MS: the sulfoxide, the sulfone and the sulfone's hydrolysis product.

The reactions with C102 are, in contrast, much slower and give rise to a sole by-product: the sulfoxide.

On the basis of these results, a general pathway for the oxidation of triazines has been proposed.

In addition, the results indicate that, with both oxidants, the herbicide oxidation rate is higher at higher oxidant concentrations.

From the technological standpoint, it is interest- ing to observe that if disinfection is carried out with HC10, as well as pathogen inactivation, an additional benefit such as triazine oxidation can be achieved. This is not possible using pure C102 as disinfectant.

It must pointed out, however, that at large scale plants chlorine dioxide is commonly obtained in situ by reduction of chlorates. The chlorine diox- ide obtained by this method, however, usually con- tains impurities of chlorine which, because its high reactivity, could be enough to oxidize traces of sulphur containing triazines. Furthermore, even the common practice of adding chlorine to maintain a residual in the distribution network and reduce the required dose of C102, might contribute to oxidize traces of sulphur containing triazines.

With the aim of demonstrating the oxidant potential of this mixture, and to assess the role of the matrix composition, further investigations are currently in progress on real groundwater samples.

Acknowledgements--The authors gratefully acknowledge Mr GianGiuseppe Lovecchio for his help during the exper- imental work. Authors also are very grateful to M. Fielding and H. James for their comments.

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