Radiation-induced decomposition of small amounts of perchloroethylene in water

Appl. Radial. hf. Vol. 38, No. 11, pp. 911-919, Int. J. Radiat. Appl. Instrum. Parr A

1987 Copyright

0

0883-2889/87 63.00 + 0.00 1987 Pergamon Journals Ltd

Printed in Great Britain. All rights reserved

Radiation-induced Decomposition of Small Amounts of Perchloroethylene in Water EMIL

Austrian

PROKSCH,

Research

Center

PETER GEHRINGER, WALTER and HELMUT ESCHWEILER

SZINOVATZ

Seibersdorf,

Seibersdorf,

Department

of Chemistry,

A-2444

Austria

(Received 21 January 1987; in revised form 4 May 1987) In air-saturated reagent grade water and at concentrations between 5 and 10 ppm, perchloroethylene is decomposed by y radiation with a constant G-value of 4.4. At lower concentrations, and in drinking waters, the G-value decreases with decreasing perchloroethylene concentration and with increasing concentration of inorganic (especially HCO; and NO;) and organic solutes present. Below 1 ppm perchloroethylene, the decomposition in most cases follows first-order kinetics. Nearly 100% of the organic chlorine degraded is converted to chloride ions. A tentative reaction scheme is given.

1. Introduction The contamination of drinking water with small amounts of chlorinated hydrocarbons-inter alia of perchloroethylene-is becoming an increasingly severe problem in certain geographical areas. The conventional methods for removal of these contaminants, i.e. adsorption onto activated carbon, or stripping, suffer from certain disadvantages. Both methods do remove the contaminants but they do not destroy them. Therefore, this might often result merely in a displacement of the environmental problem from the water to the atmosphere. Since there are indications (see e.g. Frank and Frank, 1986; Frank and Frank, 1985; Frank, 1985) for chlorinated hydrocarbons being involved in the causes for the “new forest decline” this should be avoided in future. In the case of perchloroethylene polluted water, radiation-induced decomposition could serve as a welcome alternative to the conventional methods in use. Since perchloroethylene is a good scavenger to the primary products of water radiolysis, a radiation treatment should result in a very efficient decomposition of this contaminant despite the host of other solutes present in ground waters. However, up to now, this has not yet been verified experimentally. Even information about the radiolytic decomposition of perchloroethylene in pure water is scarce. Kiister and Asmus (1972) has measured the rate constants for the reaction of perchloroethylene and other chlorinated ethylenes with hydrated electrons and hydroxyl radicals. A recent paper by Getoff (1986) mentions the chloride ion production from-inter alin-perchloroethylene solutions in pure water in the ppm region. Very little from these results is directly applicable to the problem in question.

It was felt worthwhile, therefore, to investigate the perchloroethylene/water system in a way which is more related to practice, i.e. down to sufficiently low contaminant concentration, taking into due consideration the influence of the inorganic and organic solutes present in ground waters, and looking at the contaminant decomposition itself as well as at the formation of products. This paper summarizes the result of extensive investigations made using y irradiations. Waters of different grade have been used as the solvent for perchloroethylene, including reagent grade water and two local drinking water types. Perchloroethylene concentration was varied between 10 ppb and 10 ppm. Part of these results have already been published elsewhere (Gehringer et al., 1986a; 1985). 2. Experimental 2.1. MateriaIs used -Reagent grade water (RG water): produced by (millipore Corporation; the “Milli-Q-process” deionized water is fed through a prefilter, an activated carbon adsorber bed and two mixed bed ion exchangers). Specific conductivity: 0.05 pSS/cm. -Drinking water A: fresh water taken from the Vienna City water supply system (originating from karst springs; treated by chlorination). Analysis: 4 m-equiv/L total hardness: pH 6.8; 195 ppm bicarbonate; 6.5 ppm nitrate; 4.5 ppm chloride; 30.5 ppm sulfate; 0.6 ppm DOC: 1-2 ppb chloroform; abt 0.2 ppb dichlorobromomethane; abt 0.1 ppb dibromochloromethane. -Drinking water B: fresh water taken from the water supply system of the Austrian Research Centre

912

et al.

EMIL PROKSCH

Seibersdorf (orginating from a local groundwater well; untreated). Analysis: 8.6 m-equiv/L total hardness; pH 7.4; 252 ppm bicarbonate; 60 ppm nitrate; 42 ppm chloride; 112 ppm sulfate; 1 ppm DOC. -Perchloroethylene, 99% purity (Merck). -Sodium bicarbonate, sodium nitrate, sodium chloride, sodium sulfate: p.a. (Merck). -Humic acid, MW 600-1000 (Fluka). -Sodium formate, pure (99%) (Merck).

---_

-_ w--__

--c-___ *-

P

E -

2.2. Irradiation An AECL “Gammacell 220” @‘Co-irradiation source was used throughout. Average dose-rate: 1.5 Gy/s. Irradiation temperature (except where stated otherwise): 20°C. Special care was taken to minimize perchloroethylene losses by evaporation during irradiation and pre-irradiation handling (Gehringer et al., 1986b). 2.3 Analysis

methods

Perchloroethylene contents were measured by GC using a Carlo Erba MEGA 5340 gas chromatograph equipped with a cold on-column injector and a 6”Ni-electron capture detector. Column: 30 m, 0.32 mm i.d. quartz capillary coated with 1 pm DB-5 (95% dimethyl/5% diphenyl-polysiloxane). Column temperature: 70°C. Detector temperature: 300°C. Carrier gas: 2 mL/min H,. Make-up: 15 mL/min N,. Injected sample volume: 1 pL. Chloride contents were measured in general by titrimetry using a 0.001 M Hg(II) nitrate solution, and 1,5 diphenylcarbazone as end point indicator (Corliss and Miller, 1963). Ion chromatography (DIONEX system 2010 i; with conductivity detector) was used for some of the chloride measurements (separation column: HPIC-AS 1; pre-column MPIC-NG 1; suppressor P/N 035691; eluent 2mL/min of 3 mM NaHCO,/ 2.4mM Na,CO,) as well as for the detection of organic acids (separation column: HPICE-AS 1; no pre-column; suppressor P/N 30956; eluent: 1 ml/mm 0.005 M HCl).

50

100 DOSS

150

200

(GY)

Fig. 1. Decomposition of perchloroethylene in reagent grade water (linear scale). (0) values measured by gas chromatography; (m) values measured by titrimetry. Dashed line: results after addition of 6 x lo-‘M sodium formate.

(4.4 molecules per 100 eV) but the zero order range is only marginal. At still lower initial concentrations the reaction orders tend more and more towards a first

3. Results and Discussion 3.1. Solutions

in reagent grade water

Results obtained for RG water with different amounts (50 ppb to 10ppm; i.e. 3.0 x lo-’ to 6.0 x lo-’ M) of perchloroethylene, are summarized in two different ways of presentation in Figs 1 and 2. For the highest concentration used, lOppm, the decomposition is of zero order in perchloroethylene concentration down to about 5 ppm (Fig. 1). The G( - perchloroethylene)-abbreviated in the following as G( -P)--resulting for that region is 4.4 molecules per 100 eV (see also Table 1). Below that region the decomposition curve starts to deviate from zero reaction order in a progressive manner, towards lower rates. For 5 ppm initial concentration the initial G-value, G,(-P) is still the same as at 10ppm

I

200

100 Dose

300

(Gy)

Fig. 2. Decomposition of perchloroethylene in reagent grade water (semi-log scale). Open symbols (0, 0): values measured by gas chromatography; full symbols (0): values measured by titrimetry.

Radiation decomposition Table 1. (molecules

Water

Perchloroethylene decomposition: per IOOeV) obtained in different c0 = initial concentration (ppb)

tvx

Reagent water

Cn

grade

G,(-P)

50 500 1000 5000

10,000 10,000 Drinking

Drinking *After

water A

water B addition

10 50 100 500 5000 50 of 6 x lo-‘M

sodium

G,( -P) types of

values water.

perchloroethylene enged completely

3.6 2.8 2.6 0.88 0.44

0.0053 0.029 0.051 0.25

I .08

0.53 0.58 0.51 0.50 0.22

0.020

0.41

+ OH’ -

should

be scav-

HOCCl,&CI,

(1)

k(1) = 1.7 x lo9 M-‘s-i (Kiister and Asmus, 1972). A sequence of reaction follows which, in principle, are qualitatively known: HOCCl,-CCI, COCl-CCl,

-

+ 0, -

2cocl-ccl,oo’

COCILCCI,

+ H+ + Cl - (2)

COCI-CCI,

00’

(3)

-

02 + 2COCI-ccl,

c0c1LCc1, 0’ -

‘COCl + CCI, 0

(5a)

Ccl,0

CO2 + 2H+ + 2Cl-

(5b)

+ H,O-

‘COCl + H,O ----+ ‘COO-

formate

order reaction (Fig. 2), the G,( -P) values decrease rapidly (Table l), and at the same time the G,,/cO ratios (Table 1) seem to converge towards a constant value of about 0.004 (1OOeV))’ ppb-‘. At 1, 5 and IO ppm initial concentration the decomposition was followed not only via the gas chromatographic determination of the remaining perchloroethylene but also by the titrimetric determination of the amounts of chloride ions formed (at 1 ppm only after an evaporative concentration step). All these values are also included in Figs 1 and 2 as well. It can be seen clearly that there are no systematic differences, on average the results of both methods agree within the mutual error margins, i.e. within a few percent. This strongly indicates that inorganic chloride is the only chlorine containing reaction product. Moreover, by careful analysis of the gas chromatograms the formation of volatile chlorine containing side products up to hexachlorobutadiene could be ruled out. In view of the high experimental expenditure connected with measuring the expected main carbon containing end product CO*, its determination was dispensed with. For the same reason, a search for the possible product CO was not attempted. However, a thorough search was made for organic acids to be expected as side products. This was done at 10 ppm initial perchloroethylene concentration by ion chromatography of samples preconcentrated again by evaporation (after being made weakly alkaline). After 55% conversion, 3.4 and 1.2% of the organic carbon degraded were found as oxalic and formic acid, respectively. Aftei 95% conversion these figures decreased to 2% oxalic acid and 0.3% formic acid. Besides these two acids three unidentified peaks are present in the chromatograms whose total concentration is in the order of the formic acid concentration (at 95% conversion) or even much lower (at 55% conversion). A large carbonate peak is present in all chromatograms. Qualitatively, all these results are in good agreement with the expected reaction mechanism. The main oxidizing agents are certainly the hydroxyl radicals OH’ which, at sufficiently high

concentrations, by the latter:

CCl,=CCl,

G,( - P)/c, x IO’

0.18 1.4 2.6 4.4 4.4 0.30.

913

of C,Cl, in water

-coo-

+ 0, -

‘0,

0’

+ 2H+ + Cl-

+ co*.

(4)

(6) (7)

Carbon-carbon bond rupture (reaction 5a) seems to be faster than chlorine atom hydrolysis (reaction 5b); the final products should also be the same in the reverse case. Whether reaction (5a) is indeed a unimolecular or rather a bimolecular reaction, cannot be decided; the final products are the same anyhow. The superoxide radical anion formed in reaction (7), according to the literature, should not be able to interfere with the scheme given above; its most probable fate is its disproportionation to H,O, and 0,. According to this scheme each hydroxyl radical leads to the decomposition of one perchloroethylene molecule and to the formation of 2C02 and 4C1-. This explains a G( -P), at high perchloroethylene concentrations, of about 2.8, i.e. of about 64% of the decomposition found experimentally. The assumption of total scavening also explains the zero order concentration dependence found. An additional contribution to perchloroethylene decomposition should be initiated by the hydrated electrons. Their main fate is scavenging by oxygen (whose concentration in air saturated water at 20°C is about 2.9 x 10m4M (D’Ans-Lax, 1967) and formation of superoxide radical anions: eeq + 0, = ‘0;

(8)

k(8) = 1.9 x 10” M-’ SK’ (Anbar et al., 1973). Comparatively little is known from the literature about the reaction possibilities of ‘0;. Most probably, however, it will not add to double bonds. It is also known that it does not transfer electrons to CCL, (Miinig et al., 1983a) or CF,CHClBr (M&rig et al., 1983b) and it is thus very unlikely that it would be able to reduce CCl,=CCl,. Therefore only the small fraction of electrons unscavenged by 0, (14% at 10 ppm, i.e. at the highest perchloroethylene concentration used) will contribute to the overall decomposition: CC12=CCl,

+ e& -

Cl- + CCl,=CCI

k(9) = 1.3 x 10” M-’ s-’ (Kiister

and Asmus,

(9) 1972).

EMIL F'ROKSCH et al.

914

A sequence of reactions follows which through oxygen addition, elimination of peroxidic oxygen, hydrolysis of chlorine atoms, enol-keto rearrangement, elimination of CO* and further 0, addition tentatively results in the formation of the dichloromethylperoxi-radical CHCl,OO’ which, according to Asmus et al. (1985), disintegrates under the formation of carbon monoxide. Altogether, one electron should result in the destruction of one C,Cl, and in the formation of one CO,, one CO, and four Cl-. At 10 ppm C,Cl,, this reaction path should be responsible for another 0.38 molecules per 100 eV, i.e. for an additional 9% of the G,( - P) found experimentally. Altogether, about 73% of the observed decomposition is accounted for by both routes together. The first order concentration dependence of the electron initiated part should not significantly alter the zero-order dependence of the main part. The origin of the missing 27% decomposition is not clear. Addition of 6 x 10e3 M HCOONa (which should convert > 99% of the OH’ radicals into ‘0; ) reduces G,,( - P) from 4.4 to 0.3 (Fig. 1). This corresponds sufficiently with the above mentioned value of 0.38 originating from ea; attack. It can be excluded definitely therefore, that ‘0; still takes any part in the decomposition. The same is valid for H’ which is converted almost exclusively to ‘0, in air-saturated solutions. Most likely, the missing share is due to some side reaction leading to a chain reaction. Acidic side products are to be expected mainly from bimolecular reactions of radicals escaping to some extent the oxygen scavenging reactions (3) and (7): 2 COCl-Ccl,

+ COCl-c,

CI,COCl

with the back-reaction of these radicals operative in y-irradiated water, as well as with their reaction with the CO,, HCO, (and possibly other trace constituents) present in air-saturated reagent grade water. As a consequence, the decomposition G-values should drop and the rate-law should be shifted towards a first-order reaction-as was indeed observed (Table 1, Figs 1 and 2). However, this should change neither the reaction mechanism nor the product spectrum to a significant extent. The increases in the O,/radicals ratio occurring at the same time should cause an even further reduction in the formation of acidic side products and of CO. At higher perchloroethylene concentrations than investigated in the present paper, a larger fraction of the electrons available should be scavenged by the perchloroethylene. On the other hand, up to 50 ppm perchloroethylene sufficient oxygen should be still available for scavenging all the CCl&Cl radicals thus formed. Therefore the G( -P) values should increase further to the values measured in the present investigation. Recent measurements made by Getoff (1986) using air-saturated solutions of 17 ppm perchloroethylene resulted in a G,(Cl-) value of 16.9 which corresponds to a G,( - P) of 4.2-in excellent agreement with the values of 4.4 found for 5 and 10ppm in the present study. At 170ppm, G,(Cl-) was found to increase to 36.3 corresponding to a G,( - P) as high as 9.1. This is principally in agreement with the expectation expressed above. 3.2. Solutions

in drinking waters

Results obtained for solutions of lo-5000ppb in drinking water A are summarized in Fig. 3 and 10‘?i

= COCI-Ccl,

HOOCC,Cl,-COOH + ‘COO- -!?

2 ‘COO-

(10)

cocl-ccl,-coo~

HOOC-Ccl,-COOH

2

+ 2Cl-

HOOC-COOH

+ Cl

(11)

(12)

According to this scheme, dichloromalonic and tetrachlorosuccinic acid should be formed beside the main side product found, oxalic acid. The two former acids may well be identical to two of the three unidentified chromatographic peaks. Formic acid is formed most possible by reaction of carboxyl ion radicals ‘COO- with some hydrogen donor (possibly a carboxyl radical, or an H’ radical escaping HO; formation). The last unidentified acid probably could be dichloroacetic acid CHCl,COOH, formed by an analogous reaction of the COCI-CC& radical. POSSibly, some additional oxalate is produced from COCl-CCl,O’ radicals undergoing homolytic chlorine atom cleavage. At low concentrations, perchloroethylene as a scavenger for OH’ and e, can no longer compete

200 Dose

Fig. 3. Decomposition

400

600

(Gy)

of perchloroethylene water A.

in drinking

Radiation decomposition Table I. Up to at least 500 ppb the decomposition is strictly first order, the initial Go values being almost exactly proportional to the initial concentrations q,(G,/c, = 0.00053 (100 eV)-’ ppb-‘). At 5000 ppb Initial concentration the rate law is no longer of first order and G,/c, already deviates significantly from the value given above. Compared to RG water the G-values are lower by a factor of about 4-7. Due to :he high background in the content of chlorides and other ionic species, neither a chloride nor an organic acid analysis could be performed. The reduction in G-values must be attributed to the high scavenging ability of several of the solutes contained in the drinking water. It is well known that the presence of especially NO;, Cl- or CO:- greatly reduces the ability to degrade organic substances of irradiated water (Gilbert and G&ten, 1977; Sakumoto and Miyata, 1984). It is also known from the zxcellent work of Hoigne and Bader (1979; 1977) on water ozonation that CO:-, HCO; , free NH, and humic material are effective scavengers to OH’ radicals in natural waters. Since Cl-, CO:-, humic material (DOC) and NH, are present only in negligible to low concentration, NO; and HCO; should become the most important scavengers in the present zase, possibly together with SO:- which is present in relatively high concentration. If NO,, HCO;, and SO:- together are added to RG water, at the same concentrations as present in drinking water A, this drastically reduces the rate

lo4

_$

Table

2. Perchloroethylene decomposition in “synthetic” mixtures. Basis: RG water containing 5 ppm C,CI, Added solutes

Water type

260 Dose

360

(Gy )

Fig. 4. Decomposition of perchloroethylene in reagent grade water after addition of 195 ppm HCO,; 30.5 ppm SO; and 6.5 ppm NO;. Results obtained for drinking water A (upper dashed line) and reagent grade water (lower dashed line) are included for comparison.

water

G&--P)

Synthetic water A

drinking

195 ppm HCO; + 6.5 ppm NO; + 30.5 ppm SO:-

2.0

Synthetic water B

drinking

252 ppm HCO, + 60 ppm NO, + 112ppm SO:-

1.6

of perchloroethylene decomposition indeed (Fig. 4, Table 2). The resulting curve is already very near to that found for drinking water A itself. Under the conditions given for drinking water A the scavenging of hydrated electrons by nitrate ions should result in a rather small direct contribution to the drop in G,( -P). From the published rate constant of 1.1 x 10” M-’ SK’ (Anbar et al., 1973) it can be calculated that about 17% of the electrons are scavenged by 6.5 ppm NO;. At 5 ppm C,Cl, concentration this reduces the electron contribution to C,Cl, decomposition only slightly from 0.19 to 0.15 molecules per 100eV. However, this reaction should be responsible for the production of nitrite which, due to its toxicity, might become a serious obstacle to the practical application of the radiation decomposition process. Suitable counter-measures have to be taken therefore. Theoretically, a G(NO,) of about 0.22 is to be expected. Preliminary measurements of the actual NO, formation (Gehringer et al., 1986a) resulted in an initial value of roughly 0.2, in good agreement with expectations. However, G(N0;) decreases fairly rapidly with increasing dose. The major share to the drop in G,(-P) should result from scavenging of OH’ radicals by bicarbonate, under the formation of the carbonate radical anion CO;: HCO;

100

915

of C,CI, in water

+ OH’-CO,

+ H,O

(13)

From the published rate constant of k = 8.5 x lo6 M-‘s-l (Buxton and Elliot, 1986) it can be calculated that indeed even at 5 ppm C&l, concentration 195 ppm HCO; already scavenges about 34% of the OH’ radicals formed. At 100 ppb C,C&, 96% of the OH’ radicals are scavenged. Taking into account only direct e; and OH’ attack to perchloroethylene, G,( -P) values of about 2.0 and 0.25 should result for 5 ppm 100 ppb CzCl, in drinking water A. A possible contribution from chain reactions should push these values (especially the former one) slightly upwards. The experimental values (Table 1) however are somewhat (at 5 ppm) to considerably (at 100 ppb) smaller than those figures. Most of this difference should be due to back reactions of e, and OH’ to H,O still going on, possibly supported by the following phenomena: NO; formed by the action of electrons onto NO; is converted back very efficiently to NO, by OH’ radicals (k = 4 x lo9 M-‘s-l (Anbar et al., 1973). This results in the observed sharp drop in G(N0;) with dose. Already at very low doses NO; not only acts as an electron scavenger but also as an indirect

916

EMIL

hOKSCH

OH’ scavenger and to some degree “catalyzes” the back reaction of e.; and OH’ to water. The reaction product of HCO; with OH’, i.e. the CO;, reacts very fast with ‘0, (k = 4 x ~O*M-‘S~~ (Anbar et al., 1973; Behar et al., 1970) most probably to Cy:; (Behar et al., 1970; Chen et al., 1973) which eqmhbrates back to HCO, again. Since ‘0; does not participate in the main reaction scheme this back reaction has no influence on the overall decomposition rate. At sub-ppm perchloroethylene concentrations the scavenging actions of HCO, and NO; onto OH’ and ea; become predominant and the C,Cl, decomposition competing therewith becomes a first order reaction. At 5 ppm this is obviously no longer the case. As the scavenging reactions are less dominant here, the difference in G,( - P) between reagent grade water and drinking water A is much smaller than in the sub-ppm region, in accordance with what had been found experimentally. Although the presence of NO; and HCO, reduces the G (- P) values it should neither affect the reaction scheme nor change the product spectrum. This also means that in drinking water HCI and CO* should be the principal reaction products. Organic acids should also be formed in drinking water only in the range of a few percent of the total carbon degraded. Drinking water A has also been used to study the influence of reaction temperature on C,Cl, decomposition. At 100 ppb initial concentration G,( -P) showed an increase of about 25% when decreasing the irradiation temperature from 20°C to 10°C. This is a very important fact because 10°C is certainly more typical for a ground-water source temperature than 20°C. Some additional measurements have also been made using drinking water B which contains considerably more inorganic and organic solutes than drinking water A. The most significant difference relates to the nitrate content which is higher by a factor of about 9. At 50 ppb initial perchloroethylene concentration G,( - P) is 0.020 which is 30% lower than for drinking water A (Table 1). At higher doses the decomposition progressively slows down even more and the concentration dependence deviates towards a higher reaction order (Fig. 5). The main reason for both decrease in G,( - P) and increase in reaction order, is obviously the very high nitrate content. Preliminary measurements (Gehringer et al., 1986a) of NO, formation in drinking water B resulted in a G,(NO;) of about 1.0 which is in good agreement with a theoretical value of about 0.9 calculated by competition kinetics. Although G(N0;) decreases with dose, there is a steady increase with dose of NO; present, and consequently an increasingly larger fraction of OH’ radicals becomes ‘Scavenged, on top of those already scavenged by HCO;. Therefore, the degradation of perchloroethylene decreases with dose not only due to the decreasing concentration of C,Cl, but also due

et al.

lo* 1

I

200

400 Dose

600

(‘3~)

Fig. 5. Decomposition of perchloroethylene in drinking water B. Results obtained for drinking water A CuDner dashed line) and reagent grade water (lower dashed l&j ire included

for comparison.

to the decreasing concentration partner, the OH’ radicals.

of its main reaction

3.3 The influence of the main inorganic and organic solutes In order to determine the influence of the main inorganic and organic solutes contained in drinking water, two series of measurements have been made (Table 3). First, drinking water A containing 50ppb C&l, was modified by addition, one by one, of HCO,, NO;, SO:-, Cl- and humic acid (simulating DOC), in amounts necessary to reach the respective concentrations in type B drinking water. Humic acid addition (0.4ppm) resulted in no measurable change in decomposition; addition of sulfate (81.5 ppm) and chloride (37.5 ppm) lead to a decrease in G,( -P) of only 5 and 7%, which is near the limit of significance. Clearly significant effects could be observed when adding HCO; (57 ppm) or humic acid in tenfold amount (4 ppm); in both cases G,( - P) was lowered by 20% (Fig. 6). First order kinetics were strictly followed in all these cases. The highest effect was reached when adding 53.5 ppm NO;. The resulting decomposition curve was practically identical with that of drinking water B. Additionally, the two most important solutes, i.e. HCO; and NO;, have been added, one by one, to RG water in amounts to reach the respective concentrations in drinking water B (Table 2, Fig. 7). The

Radiation decomposition Table 3. The influence

917

of C,Cl, in water

of various

solute additions Perchloroethylene

PXk,

WI Water

Added solute S

type

Drinking water A

04)

57 ppm HCO; 4 ppmhumic acid 53.5 ppm NO,

0.93 x 10-3 5.0 x 10-61 0.86 x 10-s

60 ppm NO; 252 ppm HCO;

0.97 x lo-’ 4.1 x 10-3 -

50

3.0 x IO_

-

“.“-z

5000

0.7 0.8 2.7 500 5.2

-

,

3.7 1.6 4.4t

3.0 x 10-S

10”

Ga,k,

0.029 0.028 0.027 0.024 0.024 0.020 n nm+

5.0 x lo-” 0.85 x IO-’ 1.06 x lOma

0.4 ppm humic acid 8 1.5ppm SO:37.5 ppmcl-

Reagent grade water

G,(-P)

04)

(ppb)

1.3 2.9

*A molecular weight of 800 has been used for calculation. t Reference values taken from Table 1.

decomposition of 5 ppm C,Cl, was first order in both cases. A mathematical evaluation of these data is somewhat complicated due to the nonlinear behaviour of the nitrate. Additionally, there are at least two primary attacking species, OH’ and e;, perchloroethylene and solutes. The problem can be simplified if, as approximations, “effective” rate constants and “effective” G-values for the production of primary radicals in water are introduced. As long as the reaction rates of these primary radicals with perchloroethylene are small compared to their reaction rates with all other solutes (including 0,), i.e. as long as first-order kinetics are obeyed, the following

formula derived should hold:

from

simple

+$

competition

E

R

kinetics

1 x A[&]

(14)

P

[p] = initial molar perchloroethylene concentration; G, = initial G-value for perchloroethylene decomposition; CR = effective G-value for primary radical production in water; k, = effective rate constant for reaction of the primary radicals with the ith solute; k, = effective rate constant for reaction of the primary radicals with perchloroethylene;

P t

200

400 Dose

000 100

(0~)

Fig. 6 Influence of solute additions to the perchloroeth$ene decomposition in drinking water A. Lower dashed line: drinking water A without additions; (0) 57 ppm HCO; added; (A) 4 ppm humic acid added; (0) 53.5 ppm NO; added. Results for drinking water B (upper dashed line) are included for comparison.

200 Dose

300

(Gy)

Fig. 7. Influence of solute additions to the perchloroethylene decomposition in reagent grade water. Dashed line: reagent grade water without additions; (0) 252 ppm HCO; added; (0) 60 ppm NO, added; (A) 252 ppm HCO; + 60ppm NO,

+ 112 ppm SOi-

added.

918

EMIL PROKSCH

A[&] = difference in initial molar concentration of the ith solute between sample and reference; rf = refers to a suitable reference solution to be chosen deliberately. Applying this formula to the one-by-one admixtures measured (Table 3) values for k,/G,k, can be obtained. They are also shown in Table 3. Somewhat surprisingly, although RG water as a reference does not fulfil the requirements of validity of equation (14), the value resulting from HCO; addition to RG water is in good agreement with the value obtained for drinking water A. For NO, addition the two values calculated do not agree. Taking average values for HCO; and NO,, the effective scavenging ability (i.e. the efficiency in lowering the G-value for C, Cl, decomposition) increases in the following order (numbers given are k,/G, k, x 103): SO:- (0.7) z Cl- (0.8) < HCO; zNO;

(2.8)

(3.3) < humic acid (500)

(15)

The value found for humic acid (based on a mean molecular weight of 800) can be used to a first approximation also for unknown organic matter present in water (DOC). The value found for HCO, is in remarkably good agreement with the true value calculable for OH’ attack only k,lG,k, (1.8 x 10-3). To test the validity of equations (14) and (15) G,( - P) values have been calculated for some rather extreme cases, i.e. for 50 ppb solutions in RG water and in drinking water B (Table l), as well as for 5 ppm solutions in “synthetic drinking water A” and “synthetic drinking water B” (Table 2). Drinking water A was used as the reference in all cases because its nitrate content seems to be representative for good drinking waters and because it fulfils all the requirements for validity of equation (14). Although all four solutions do not (or only approximately) follow a first order rate law, the calculated values agree with the experiments within about &20 to +30% over a range of about two powers of ten. Despite all the restrictions already mentioned, equation (14) seems to give quite reasonable estimates for the initial perchloroethylene decomposition rates over a quite broad range of perchloroethylene and sohtte concentrations. Based on these G,-values, doses necessary for reaching a certain degree of decomposition can be calculated easily, provided first order kinetics are still obeyed. The present work indicates up to what concentration limits (for perchloroethylene as well as for HCO;, NO;, and other solutes contained) this requirement is still fulfilled. The above mentioned doses, according to equation (14) obey an additivity rule with respect to solute additions, as was already reported in a previous paper (Gehringer et al., 1986a) 4. Conclusions The present investigation demonstrates amounts of perchloroethylene in drinking

that small water can

et

al.

be destroyed up to very high levels of conversion, by relatively small doses of y-irradiation. Almost 100% of the organic chlorine degraded is converted to chloride ions. At the level of a few ppm, a high percentage on the primary water radiolysis products reacts with perchloroethylene. At the sub-ppm level the perchloroethylene decomposition rates are considerably lower, due to the predominance of the primary attacking species becoming scavenged by the solutes present, mainly by HCO; and NO,. The decomposition is therefore highly dependent on the concentration of these substances. Nevertheless, at the sub-ppm level, the doses necessary for a 90% decomposition (which is roughly what is necessary for a typical practical application) are somewhere between 250 Gy (for water with low NO, content) and 600Gy (for water with high NO, content). This is a region where a technical realization seems to be still just economically feasible (Gehringer et al., 1986a). Due attention has to be payed to the nitrite formation accompanying the perchloroethylene decomposition. According to European Community recommendations (Richtlinie, 1980) drinking water should contain no nitrite at all; the maximum permissible level is only 100ppb. Although only l-1.5% of the nitrate present is converted to nitrate, at doses necessary for 90% C,Cl, conversion, this would lead to nitrite levels higher than permissible for initial nitrate concentrations higher than about 10 ppm. However, it should not be too difficult to oxidize the nitrite formed back to nitrate, or to modify the whole process in such a way that nitrite formation is being avoided. Work is already in progress in that direction. A technical realization of the radiation induced perchloroethylene decomposition is certainly feasible only when the radiation costs are kept as low as possible. The use of electron accelerators as radiation sources, delivering much higher dose-rates than the gamma source used in the present work, is therefore necessary. Possible dose rate effects although not very likely according to the reaction scheme proposed have to be looked after for that reason. ,&now/edgements-The authors are indebted to the “Ponds zur Foderung der wissenschaftlichen Forschung” for their financial support (Project P5130) and to the “Jubiliiumsfonds der osterreichischen Nationalbank” which supported this work by financing the Gammacell 220 used.

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