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Wat. Sci. Tech. Vol. 37, No.3. pp. 195-201, 1998.© 1998 IAWQ. Published by Elsevier Science Ltd

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RADIATION INDUCED POLLUTANTDECOMPOSITION IN WATER

P. Gehringer* and H. Matschiner**

*Austrian Research Centre Seibersdorf, A-2444 Seibersdorf, Austria** ECH Elektrochemie Halle GmbH, Weinbergweg 23, D-06120 Halle, Germany

ABSTRACT

Two sets of experiments are described, one dealing with the radiation induced decomposition of somevolatile chlorinated solvents in deionized water using electron beam irradiation with and without addition ofozone. the other dealing with y-irradiation of aqueous peroxodisulfate solutions for OR free radicalgeneration. It was found that the radiation induced decomposition of 10 mgIL 1,1,I-trichloroethane indeionized water is not improved by addition of ozone. Accordingly the resulting dose requirement fordecomposition is too high for a technical application. No difference was recorded for the radiation induceddecomposition of 10 mgIL trichloroethylene (TeE) as well as 10 mglL perchloroethylene (PeE) in deionizedwater with and without addition of ozone. These results deviate from previous results obtained ingroundwater. Ozone addition reduced the dose requirements in a way that could be of interest for technicalapplication. Based on the assumption of a 25 kW mobile electron beam accelerator a technical design isdiscussed that would result in a capacity for the destruction of lO t PeE/a. The results of some experimentsperformed with the combination peroxodisulfate/y-irradiation to decompose benzene as well as IA-dioxanedissolved in tap water did not really recommend this combination for a technical use. © 1998 IAWQ.Published by Elsevier Science Ltd

KEYWORDS

Chlorinated solvents; electron beam accelerator; gamma irradiation; ozone; peroxodisulfate; water.

INTRODUCTION

The action of ionizing radiation to water is known to result in the formation of ions, molecular and freeradical species (so-called water radiolysis). For pollutant decomposition only the free radical species (OHfree radicals and solvated electrons e-aqu' in particular) are of interest. OH free radicals are the strongestoxidants known to occur in water (Eo = 2.8 V) the same is valid for the solvated electrons as reductants (Eo= 2.77 V). Since both species are formed in almost equal amounts water radiolysis represents a 'hybrid'process and the way in which the pollutants are decomposed can be both oxidation and reduction. Which ofboth ways is dominant depends on many parameters but mainly on concentration and chemical structure ofthe pollutants as well as on the water matrix.

It has been demonstrated (Proksch et al., 1987; Gehringer et aI., 1988) that the decomposition of chlorinatedethylenes such as trichloro- (TCE) and perchloroethylene (PCE), respectively is mostly carried out byoxidation if the pollutants are in groundwater in sub-ppm quantities only. For chlorinated alkanes containedin low concentrations in groundwater the decomposition was mainly done by reduction (Gehringer et aI.,1997). In both cases most of the free radicals species are scavenged by natural solutes contained in

195

196 P. GEHRINGER and H. MATSCHINER

groundwater i.e. bicarbonate scavenges the OH free radicals, solvated electrons are scavenged by oxygen aswell as by nitrate. Thus, the overall efficiency of the irradiation process is rather low.

Most of the chlorinated compounds known to occur frequently in groundwater are rather volatile. A transferof such compounds from groundwater (after stripping) or from soil (after pumping) into deionized water istherefore quite easy. An irradiation treatment after dissolution of the pollutants in deionized water must bevery effective with regard to pollutant decomposition because there are almost no competitors for the freeradical species formed by water radiolysis. In groundwater contaminated with trace amounts of chlorinatedethylenes pollutant decomposition by electron beam irradiation was considerably improved by ozoneaddition before or during irradiation (Gehringer et ai., 1992, 1995).

However, experiments performed with an aqueous solution of 50 mg PCEIL dissolved in deionized waterhave shown that addition of ozone during electron beam irradiation does not at all effect PCE decomposition(Gehringer et ai., 1997).

Nevertheless, ozone addition before or during irradiation may be useful in some cases to improve pollutantdecomposition. However, ozone is expensive and its handling i.e. the transfer from the gas phase into thewater is a demanding task from the technical point of view. Accordingly other possibilities to improve theefficiency of an irradiation process are still of interest. Peroxodisulfate is another potential OH free radicalsource (Matschiner et ai., 1994). They are highly water soluble and non toxic but they need activation togenerate OH free radicals. Activation has been successfully performed by means of non-ionizing UVradiation. However, the use of UV radiation in water treatment is sometimes limited especially when wastewater is considered. If the water to be treated is coloured or turbid or contains particles or gas bubbles theefficiency of non-ionizing UV radiation is drastically reduced. The efficacy of ionizing radiation, however,is almost not affected by such conditions.

The activation of peroxodisulfate is based on the formation of sulfate radical anion S04- which is able toadapt an electron from hydroxide ions forming by that OH free radicals. Since solvated electrons formedduring water radiolysis react very fast with peroxodisulfate forming sulfate radical anion due to

it seemed worthwhile to study an activation of peroxodisulfate by solvated electrons generated by waterirradiation with ionizing radiation.

In this paper a detailed investigation of the radiation induced decomposition of some chlorinated solvents indeionized water with respect to a technical application is reported. Moreover, some experimental dataconcerning the use of y-irradiation to promote OH free radical generation from peroxodisulfate in aqueoussolution is presented, too.

METHODS

All experiments concerning the decomposition of chlorinated compounds in deionized water wereperformed in a pilot plant for continuous ozone/electron beam irradiation treatment of water describedelsewhere (Gehringer et ai., 1993). In these experiments perchloroethylene (99% purity, Merck),trichloroethylene (99.5% purity, Merck) and l,l, I-trichloroethane (reagent grade, Merck) were used.

To study the effect of y-irradiation as activator for peroxodisulfate solutions of about 10 mgIL benzene(reagent grade, Merck) and IA-dioxane (99% purity, Aldrich) as well in tap water (270 mglL bicarbonate,9 mg/L nitrate and < I mgIL DOC) have been prepared and irradiated after addition of various amounts ofsodium peroxodisulfate (reagent grade, Merck). A 'Gammacell 220' Cobalt-60 irradiation source with anaverage dose rate of 95 Gy/min was used for the y-irradiations. (The unit of the radiation dose is I Gray(Gy), I Gy = I J/kg).

Radiation induced pollutant decomposition

RESULTS AND DISCUSSION

197

Figure I shows the decomposition of TCE, PCE and 1,1, I-trichloroethane (in the following abbreviated withI, I, I-tri) in deionized water by electron beam irradiation as a function of radiation dose and initialconcentration. From this figure it is obvious that I, I, I-tri is more resistant against the attack of the speciesformed in water by electron beam irradiation than are the chlorinated ethylenes TCE as well as PCE.Concerning the dependence on the initial concentration 1,I,l-tri again shows a deviating behaviour. Whilethe slope of the decomposition curves for TCE and PCE as well becomes steeper with decreasing initialconcentration over the whole range investigated such an effect is recorded for I, I, I-tri only when goingfrom about 50 mg/L initial concentration to 10 mg/L. However, with an initial I, I,I-tri concentration ofI mg/L the same slope of the decomposition curve was obtained as was found with 10 mgIL initialconcentration. This is remarkable because the slope of the decomposition curves obtained for the chlorinatedethylenes becomes much more steeper when going from IO mg/L initial concentration to I mg/L than whengoing from 50 mg/L to 10 mg/L.

100

(,~

-U-..........A ..... ~~~ =u-

u~~Ia. -

-

-..: LJ -- ~-

A ~, ~i'..--c------ --- -_. -

-- I--- -

\ ..7:' .,.- --f---- ---...... --f-- --~

\ Q)\ -C::£I----\ "'Q

~ -- ---- -

_.-f---- f-------ll- -, .- -

..Y'\ t== -- ~"'-'------

--_._-~----

A y----- ----~-.. _-

""

10

~Obgc= 1'.cell.Qc~c=U

0,1

0,01o 500 1000 1500

Dose (Gy)

2000 2500

Figure I. Radiation induced decomposition of some chlorinated compounds in deionized water by electron beamirradiation. (L\. trichloroethylene; 0 perchloroethylene; 0 1,1, I-trichloroethane).

However, in deionized water the results found for the decomposition of chlorinated ethylenes deviateremarkably from results obtained with TCE and PCE in a natural groundwater. Electron beam irradiation ofnatural groundwater containing sub-ppm amounts of TCE and PCE, respectively resulted in half of the doserequirement for a certain decomposition of TCE as compared to PCE. In deionized water there is nodifference at all between TCE and PCE in the dose needed for a certain decomposition, even in the sub-ppmrange. This is a quite interesting result which clearly corroborates that solutes contained in water maystrongly influence a radiation induced decomposition of the target pollutant.

From a technical point of view among the substances investigated PCE would be the most interesting one fortwo reasons: (I) in deionized water the radiation induced PCE decomposition can be achieved with the same

198 P. GEHRINGER and H. MATSCHINER

dose requirement as for TCE and (2) it is secured that PCE is totally mineralized by such a treatment(Gehringer et ai., 1992). However, that means that chloride ions and bicarbonate ions are formed andaccumulated in the water. Under the conditions given chloride ions do not influence the PCE decompositionbut bicarbonate ions are scavengers for OH free radicals. Lowering the pH of the water would shift thebicarbonate/carbon dioxide equilibrium towards carbon dioxide which is an excellent electron scavenger.Accordingly CO2 has to be removed from time to time to keep the water apt for use in a closed loop.Otherwise the conditions for PCE decomposition would become worse in a rather short time. However, arelease of CO2 into the atmosphere is possible only when the PCE residual concentration in the water is verylow. To realize such a low residual PCE concentration a rather high radiation dose is required. This wouldraise the cost to a level at which the process cannot compete with existing technologies. Therefore, it hasbeen decided to test again if ozone addition is able to improve the radiation induced pollutant decompositionalthough previous experiments with an initial PCE concentration of 50 mglL did not succeed.

Ozone addition to the water would increase simultaneously the oxygen content in the water and by thatfavour the electron transfer according to

Superoxide radical anion 02- is an effective promoter for the ozone decomposition into OH free radicals;accordingly CO2 could remain in the water because the electrons scavenged by CO2 are not lost butconverted into OH free radicals via the reaction of ozone with °2- (see Gehringer et ai., 1996).

----

--

-f------

'_=f-'

'-

1----

~

"'""~ h-. •''A~ 1!=.S -f •...

\=ell \u== 0,1U

Figure 2 summarizes the results of various experiments concerning the decomposition of 10 mgIL 1, I, I-tri,TCE and PCE in deionized water by electron beam irradiation with and without addition of ozone. It isobvious from that figure that in case of I, I, I-tri ozone addition does not improve the decomposition at all.Hence it can be definitely ruled out as a candidate for a radiation induced decomposition in deionized wateron a technical scale. On the other hand, decomposition of TCE as well as PCE is clearly improved by theaddition of ozone.

1-----1------ ----'----.---- 1------

I j0,01 L...-__..L....__-l...__--i.__--l__---I

o 500 1000 1500 2000 2500

Dosis (Gy)Figure 2. Radiation induced decomposition of some chlorinated compounds in deionized water by electron beamirradiation with (full symbols) and without (open symbols) addition of ozone. Symbols used correspond to those

given in Fig. I.

Radiation induced pollutant decomposition 199

Ozone was added as aqueous solution (corresponding to about 3 mg 031L in the water to be treated) whichlimited the range of decomposition to the doses shown in Fig. 2. However, this range may be extended whenozone is introduced gaseously and continuously during irradiation. In the following, considerations are givento apply the results obtained; they are based on the use of a 25 kW mobile electron beam accelerator unit. A20 kW mobile unit has been already operated successfully by High Voltage Environmental Application inthe USA as well as in Germany. If we take that for a reduction of 10 mg PCEIL down to about 0.1 mgPCEIL the dose requirement is 500 Gy together with about I kg ozonelh simultaneously applied duringirradiation a design as shown in Fig. 3 would be the result.

Under the conditions given about 126 m3/h deionized water can passed round which is used to purify a PCEcontaining air stream in a scrubber in such a way that about 10 mg PCEIL results in the water. Ozone isadded to the water under pressure in a static mixing unit. The excess oxygen which could not be dissolved inthe water is continuously removed in a gas separator working at the same pressure level as the mixing unitdoes. Under such conditions stripping of PCE in the gas separator is minimal only. Afterwards the waterflow is irradiated with a radiation dose of 500 Gy to reduce the PCE concentration to about 0.1 mg PCEIL.

deionized water (~126 m'/ h) pH ~ S.S < 0.1 mg peE / L

air

PeE-loaded air

deionized water(~10 mgPeE / L)

gas separator irradiationchamber

Figure 3. Flow diagram of a combined ozone/electron beam irradiation process for continuous PCE decompositionin deionized water.

The capacity of such a construction would be a decomposition of about 1.26 kg PCEIh or about 10 t1a. Sincemaximum concentration of PCE in water is 0.1 gIL the capacity of the construction considered correspondsto a clean-up of 108 m3/a of a water stream saturated with PCE. Such a capacity could be of interest for atechnical use. A cost evaluation is under way.

200 P. GEHRINGER and H. MATSCHINER

To study the effect of y-irradiation on aqueous peroxodisulfate solution the decomposition of 1,4-dioxaneand benzene dissolved in tap water has been investigated as a function of peroxodisulfate concentration andradiation dose as well (Fig. 4).

800600400Dose (Gy)

200

100 1

80 80

ti- 60 ti 60..= -OIl

~ ~~ 40 =~ ~....

20

200 400 600 800Dose (Gy)

Figure 4. Radiation induced decomposition of IA-dioxane and benzene in tap water by y-irradiation in the presenceof various amounts of sodium peroxodisulfate (PODS). (e) without PODS; (0) 10-3 M. (~) 2.5 x 10-3 M. (D) 5 x

10-3 M of PODS added before irradiation.

Without addition of peroxodisulfate IA-dioxane was decomposed somewhat more efficient than benzenealthough the reaction rate constant for the reaction with OR free radicals k(OR + benzene) = 7.8 x 109 M-Is·1 is higher than that of 1,4-dioxane: k(OR + 1,4-dioxane) -1.1 x 109 - 2.4 x 109 M-I s·1 . lA-dioxaneperhaps reacts more efficiently with solvated electrons than benzene does: (e-aqu + CJi~ = 9 x 106 M-l sol;however, the reaction rate constant k(e-aqu + lA-dioxane) is not known yet. The results obtained afteraddition of peroxodisulfate seem to corroborate this interpretation. Benzene decomposition is moreimproved by peroxodisulfate addition than that for IA-dioxane. As already mentioned (see Introduction)peroxodisulfate effectively scavenges electrons and generate by that OR free radicals. Roughly speaking theonly active species in the combination peroxodisulfate/y-irradiation should be the OR free radical.Accordingly solvated electrons cannot contribute any longer to 1,4-dioxane decomposition. In consequence,with peroxodisulfate addition the resulting IA-dioxane decomposition is, indeed, less than that of benzenewhich ha~ to be expected for an advanced oxidation process according to the reaction rate constantspublished for the reactions of both substances with OR free radicals.

Regarding the dependence of the pollutant decomposition on peroxodisulfate concentration there is almostno improvement with increasing concentration. 10-3 M of peroxodisulfate should be enough to scavenge theelectrons efficiently. That means there is no other process involved than the scavenging of the electrons bythe peroxodisulfate. This makes the combined process not very attractive because it is essentially similar to ahydrogen peroxide addition. The only advantage of peroxodisulfate compared to hydrogen peroxide wouldbe a lower reactivity towards OR free radicals. To use this advantage would presuppose a restoring of theperoxodisulfate from sulfate at a price calculated from the corresponding hydrogen peroxide demand.

CONCLUSIONS

The radiation induced decomposition of PCE and TCE, respectively in deionized water by a combinedozone/electron beam irradiation process could be of interest for a technical application with regard to siteremediation. Considering a 25 kW mobile electron beam accelerator unit about 10 tla of PCE or TCE couldbe mineralized. 1, I ,I-trichloroethane is not apt for such a treatment process.

Radiation induced pollutant decomposition 201

The combination peroxodisulfatelionizing radiation to be used in water results in an Advanced OxidationProcess which seems to be not very attractive with regard to a technical application. Therefore, acontinuation of the experiments has not been considered yet.

REFERENCES

Gehringer, P.. Proksch, E., Szinovatz, W. and Eschweiler, H. (1988). Radiation-induced decomposition of aqueoustrichloroethylene solutions. Appl. Radiat. Isot., 39, 1227-1231.

Gehringer, P., Proksch, E., Eschweiler, H. and Szinovatz, W. (1992). Remediation of Groundwater Polluted with ChlorinatedEthylenes by Ozone-electron Beam Irradiation Treatment. Appl. Radiat.lsot., 43, 1107-1115.

Gehringer, P., Eschweiler, H., Szinovatz, W., Fiedler, H., Steiner, R. and Sonneck, G. (1993). Radiation-induced OH radicalgeneration and its use for groundwater remediation. Radiat. Phys. Chern., 42, 711-714.

Gehringer, P., Eschweiler, H. and Fiedler, H. (1995). Ozone-electron beam treatment for groundwater remediation. Radial. Phys.Chen!., 46, 1075-1078.

Gehringer, P. and Eschweiler, H. (1996). The use of radiation-induced advanced oxidation for water reclamation. Wat. Sci. Tech.,34(7-8),343-349.

Gehringer, P. and Eschweiler, H. (1997). Radiation Induced Clean-up of Water and Wastewater. In: Environmental Applicationsof Ionizing Radiation, W. J. Cooper, R. Curry and K. O'Shea (Eds.), J. Wiley & Sons, Inc., in press.

Matschiner, H., Liebau, A. and Thiele, W. (1994). Oxidativer Abbau von Wasserschadstoffen mit Peroxodisulfaten.Gewiisserschutz Wasser Abwasser, 143, 335-340.

Proksch, E., Gehringer, P., Szinovatz, W. and Eschweiler, H. (1987). Radiation-induced decomposition of small amounts ofperchloroethylene. Appl. Radiat. {sot., 38,911-919.