Introduction

After a brief introduction to some ultraviolet basics' the

upfli"ution of UV-driven Advanced Oxidation Technologies for

th; treatnent of organic pollutants, such as pesticides'

herbicides, MTBE' NDMA etc., in contaminated drinking waters

will be discussed.

Ultraviolet Basics

The absorption of ultraviolet light usually stimulates chemical

reactions (a process called "photochemistry) that otherwise

*orla ,rot'o"c*.',0 Photochemical reactions are common in

almost all areas of the environm€nt, such as photosynthesis in

g"* pf*", sun tanning and sun b*Tg::T fading of textiles'

fgh,;;*t;, UV curiig of coatings, UV photodegradation of

piUrrt*tt in contaminated watsrs, photochemical smog and UV

disinfection.

The initiation of photochemistry usually involves light with

wavelengths n tni tlttraviolet ranges' The-divrsion into three

sub-ranjes is connected with the human skin's sensitivity to

Jtravioiet light. The UVA ran ge (320 - 400rrm) causes changes

in the skin that lead to sun tanning' The UVB range (280 - 320

"*l "* cause sun burning and is known-eventually to induce

skin cancer. The UVC range (200 - 280 nm) is extremely

;;d;, since it is absorbed by proteins, fNA and DNA and

can"lead to cell mutations, cancer and/or cell death' The UVC

" An excellent textbook on Photochemistry is: R' P' Wayne'

Prineiples and Applications of Photochemis-try,.MordUniversrty Press, Oford, U.K., 1988' Another important

refersnce is the Report of the IUPAC PhotochernistryCommission, "Glossary of Terms in Photochemisty" J'

Verhoeven, Pure Appi. C hem. 68, 2223-2286 (1 996) (available

on the Web at http:7/www.unibas.ch/epafuelcome'htrnl)'

b Some of the "basiC'material on UV has been exhacted from

"IJltraviolet Applications Handbook', 2"d Ed' J' R' Bolton'

2001, Bolton Pliotosciences Inc., 628 Cheriton Crescent'

Edmonton, AB, Canada T6R 2M5'

range is sometimes called the germicidal .range, s:ncn it is very

effe"ctive in inactivating bacteria and viruses' T\e Vacuum

iliaviolet range (lOd- 200 nm) is absorbed by aknost flsubstances lincludurg water and air)' Thus it can only be

transmitted in a vacuum. The absorption of a VUV photon

caus€s one or more bond breaks.

The First Law of Photochemistry states that no photochemical

reaction can occur without the absorption of photons' Hence' it

is important to determine the fraction of light absorbed from an

incident beam. For monochromatic or narrow band light in the

absence of reflection, the fraction of light absorbed f 7 n he

medium is give,n bY:

The fraction 76, of the absorbed light that is absorbed by a

specific component i with conceirtration c, and molar absorption

coefficient e, is:t rCt

I r=-T

. f . t= l -T, l -19-e*r(la)

(1b)

where a is the total absorption coe'ficient (=A/l) of the solution'

The rate R" (M"s t) of the photochemical reaction of a

component C is given bY:

^"=91* (2a)

where G is the photon flow (einstein s-1) into the reactor' F" (=

fr z^) is the fraction of the photon flow that is absorbed by

6-p**t C, rD is the quantum yield (defrned as the fraction of

ttt" uU*tU"a photons that result in a photochemical reaction of

C) and l/is the volume (L) of the reactor'

If F. is near unity, then the photochemical reaction will exhibit

)ui-ordu kinetics (i.e., that rate does not depend on the

concentration of the contaminant)' In the special case where the

total absorbance is srnall, such that Fc < 0 'l , Equation (2a) muy

be expanded in a Taylor series, so that the rate expression

becomes:

t6

(3b)

Thus in this case, the kinetics become first orfur in theconoentration of C. The UV photolysis of N-nitosodimethylamine (NDMA) (see Figure l) illustates anexample of this kinetic behavior. Initially, where the startingconcentation is I mM Qa mgL), the kinetics are zero orderHowever, once the concentation has dropped to about 0.02 mMQ.a mgL),the kinetics shift tofrut ordei,whqe tne fust-oraerrate constant is given by:

such as trichloroethylene, perchloroethylene, 1,4-dioxane, methyltert-butyl ether (MTBE), acetone, phenols, N-nitrosodimethylamine (NDMA), BTEX (berzene, toluene,ethylberzene and xylenes found in waters contaminated withgasoline), and many others, in contaminated grormd waters andiqdustrial effluents.

AOTs for Drinking Water Treatment

AOTs important for drinking water treatnent are those thatinvolve absorption of UV and/or visible ligbt in a homoge,neousaqueous solution. Photochemical processes in solution lead tothe ge,neration of oOH radicals, which initiate the oxidation anddegradation of the organic pollutants. An impodant exception isthe use of UV to photolyze N-nitrosodimethylamine (NDMA) incontarninated waters.

The IIV/O, Process

The photolysis of ozone (Or) in the UVC region (200-280 nm)can lead to the generation of oOH radioals through the reactions:

O, + hv ---) Oz + O('D) (4a)

O('D) + IlO ---+ [HO'..."OFI]..---+ HrO, (4b)

H,O, + hv -----+ 2eOH (4c)

where the squme brackets in reaction (4b) represent a solventcage in which almost all the hydroxyl radical pairs combine toform FlO, within the cage.

The UV/O, process has been used commercially, particularly inthe treatrnent of clear ground waters containing contaminantssuch as hichloroethylene (TCE), perchloroethylene @CE) ortrinitrotoluene (TNT); however, for most applications it is notconsidered economical when compared to the UVftO, process.

TheVIIV lVater Photolysis Process

Water absorbs UV in the "vacuum ultaviolef' (VUV) region(100 - 200 nm) to undergo the photochemical reaction:

HrO + hv -----+ Ho + oOH <D = 0.4 (5)

The molar absorption coefficient of IlO increases sharply aswavelengths < 190 nm, so alnost all of the UV is absorbedwithin a few Fm. The advantage of this process is that noadditional chemicals are required. For this reason, it is used inthe treatment of ultrapure water in the semiconductor rndustry.Common light sopurces for this process are "ozone-producing"low pressure mercury lamps (emitting at 186 nm) and the Xeexcimer lamp (emittin g at 17 2 nm)..

^. *?ln(lo)e"c"/

*, "ffttn(to)e"/ (3c)

Note that G, I and Z are known (e.g., in a collimated beamapparatus), eq. 3c provides a convenient way to determine thequantumyield @..

Advanced Oxidation Technologies

Advanced Oxidation Technologies (AOTs) are those that utilizepowerful oxidizing intermediates (e.g., the hydroxyl radical"OFI) to oxidize primarily organic pollutants from contarninatedair and water. The term "advanced" is used because thechemical reactions involved are essentially the same (exceptbillions of times faster) as those that take place very slowly ifthese organic pollutants are dispersed into the envircnment.Most of the commercially viable AOTs use ultraviolet andvisible light to generate "OH radicals. These can be zubdividedinto homogeneous and heterogeneous technologies. A veryusefirl Handbook on Advanced Oxidation Technologies isavailable for free." Two other reviews may be conzulted forfurther coverage.d,"

Treatrreirt with AOTs leads not only to the destruction of thetarget pollutants, but also, given sufficient teatnent time, to thecomplete mineralization (i.e., the only products me COr, FIOand mineral acids for any Cl, N S, eto., present in the pollutants)of the pollutants and their byproducts. AOTs have proven to bevery effective in treating a wide variety of organic contaminants,

" "Advanced Oxidation Tecbnologies - A HandbooK', CalgonCarbon Corporation, P.O. Box 717, Pittsburgh, PA 15230-0717.

d "Homogeneous photodegradation of pollutants incontaminated water: An introduction', J. R. Bolton and S. R.Cater.InAquatic and Surface Photochemistry, G. R. Helz, R.G. Zepp and D. G. Crosby, Eds., tewis, Boca Ratorq FL,1994,pp. 467 -49O.

" "Photochemical processes for water teatnent", O. Legrini,E. Oliveros and A. M. Braun, Chem. Rev.,93,671-698 (1993).

t7

ooor- /<o=0' le lnMnin-" ' .^ 4=131 nint

t ta

aa

aa

\\\

- \\

\I

in the overall teatonent costs. Thus it is understandable thatFigures-of-Merit have been developed that are based on theefficiency in the use of electrical en€rgy in driving thedegradation processes.

The Photochemistry Commission of the Intemational Union ofFure and Applied Chemistry GUPAC) has reconrmended the useof the Figure-of-Merit,r the Electrical Energt per Order (Er),defined as the electrical energt in kilowatt hours (kWh) requiredto bring about the degradation ofa contaminant C by one orderof magnitude in I m3 (1000 L) of contaminatedwater or air. TheE o can be calculated from the expressions:

F"EO -

EED68

tine/nin

Figure l. UV photolysis of NDMA at pH 3 illustrating thekinetic behavior at high concenkation (zero order) and lowconcentration (first order). [Data from M.I. Stefan and J.R.Bolton, Helv. Chim. Acta(in press)f.

The IIV/I[O, Process

This is by far the most important commercial AOT. It is basedon the direct photolysis ofadded hydrogen peroxide.

H,O, + hv -----+ z"OH (6)

The quanturn yield for generation of oOH radicals is 1.0, andmost organic pollutants can be degraded rapidly. Since the molarabsorption coeffrcients of FlO, are low, sufficieirt FlO, must beadded (uzually > l0 mg/L) so that a significant fraction of theUV between 200 and 300 nm is absorbed. It is important to havea UV lamp (e.g., a medium pressure mercury lanrp) that emitsstrongly below 250 nm. This process is the one most likely to beapplied for the treatrnent of drinking water.

Concept of Electrical Energy Dose

Most AOTs are driven by processes (e.g., UV lamps) thatconsume electrical en€f,gy. It is usefirl to defure the ElectricalEnergt Dose (EED) as the electrical energlt (kWh) consumed perunitvolume (".g., I m3 or 1000 gal) of water treated. The EEDmay be calculated from:

where P is the electrical power (W), r is the time (min) and Z isthe volume (L) of water treated. The units of EED in Equation7 are kWh/rrr.^r. If the reference volume used is 1000 US gallons,then the volume should have units of US gallons. Figures-of-Merit for Advanced Oxidation Technolosies

Almost all AOTs involve u "ondO"ruUle

urput of electricalenergy, such that the cost of electricity becomes a major factor

1.0

0.8

=0.6 :

El!

0.4 PE

0.2 E

E 0'1

€ o.ol

E om1

log(c,/c 7)

Pr;"Eo -

6OF log(c,/cr)(8b)

where Equatron (8a) is used for a batch reactor and Equation (8b)for a flow-tbrough reactor. F is the flow rate (L/min or gpm) andthe other symbols have the same meanings as for Equation (7).Note that for a batch reactor, rf log(c/c,) is plotted vs the EED,the.Euo is obtained from the negative inverse of the slope. Notethat the E o becomes smaller as the e,ffcieircy of the processincreases.

The most efficient UV-based AOTs have E"o values in the rangeof 0.1 - 1.0 kWh/order/ur-3 or 0.4 - 4 kWh/order/l0OO gal. Asthe total organic carbon level increases, so does ttr" E o becauseof increased competition for "OH radicals from scavengers. Alsoas the overall absorbance in the 200 - 300 nm region increases,so does tlre Euo because of increasing competition for photonsfrom absorbing species other than the primary absorber (e.g.,HrOr). For pollutants with small "OH radical rate constants (e.9.,CHCI3), the E"o will be large.

Some AOTs Treatment Examples

Direct Photolyris of N-nitrosodimethylanine (NDMA)

NDMA first emerged as a pollution problem in Elmira" Ontario,Canada, where the drinking water sourc€s for the to\ryn w€recontaminated with NDMA as a result of discharges from a localcherrical plant. Over the past two years, reports of relativelyhigh (in excess ofthe crurent California control level of20 ppt)levels of NDMA in drinking water and wastewater haveincreased significantly in number and in the water diskicts

rJ. R. Bolton, K. G. Bircher, W. Tumas and C. A. Tolman,"Figures-of-Merit for the technical dwelopment andapplication of Advanced Oxidation Processes",J. Adv. Oxid.Technol., l, 13-17 (1996).

(8a)

FjED = Pt

601/ (7)

18

affected. This has caused considerable concern amongpublic, the California Department of Health Services andWater Dist-icts affected.

The removal and treatrnent of NDMA is difflrcult. NDMA doesnot biodegrade sigrificantly, so it persists in the environment.NDMA is very soluble in water and so it does not air strip norstick to activated carbon. The only keatment that has been forurdeffective is ultraviolet photolysis. The reason is that theabsorption of ultraviolet light (even the UV portion of sunlight)by NDMA causes the dissociation of the molecule into harmlessfragments.

Figure 2 shows the results for the UV treatment of NDMA in abatch reactor with a 1 kW medium pressure UV lamp.(water pathlength 14 cm). Note that the treatnent follows first orderkinetics very well. The.Euo is 0.43 kWh/order/I000 ga1

00 01 02 03 04 05EdidEqrlb/(l{ff'tr0

Figure2. Direct photolysis UV treatment ofNDMAindrinking water.

W lWO, Treatment of 1 r4-Dioxane

Most commercial AOT treatnents utilize the UV/HzC.2process,in which a certain concentration of HrO, is metered into thewater upstream of the UV reactor. Figure 3 illustrates thetreatmsnt of 100 mg/L l,4-dioxane in the presence of 200 mg/Lof HrOr. Clearly, the kinetics are first order. Generally, as theconcentration of l,4-dioxane decreases, the E"o also decreases;the reason is that at lower concentrations, there is a lowerconcentration of degradation byproducts that can act asscavengers of oOH radicals. Although this example is notrelevant to drinking water, the UV/FI,O, process has been usedto treat small concentrations (<0. I mg/L) in drinking water. Inthis case, the concentration of HrO, would be about 10-25 mg/L.

I

=1L7t{trtl@rtgd

051015n25 SEdid EErg/rbefid/[flUngd)

Figrne 3. UV/HzAr treatrnent of l,4-dioxane in tap water inthe presence of 2OO mglL of I!Or. @ata courtesyof Calgon Carbon Corporation.)

UVfitO, Treatment of Pesticides and Herbicides

There have been many studies of thq. UV/I!O, treatment ofpesticides and herbicides that demonskate the efficiency of thisprocess. -Euo values are in the range of 0.4 - 1.0 kWh/order/m3.For one order of magnitude removal, the treatment costs are$0.05 - $0.12 per ur-3, including the costs of FlO, and lampreplacement. These costs are low enough that some utilities areplarming to install major treatment facilities. Indeed a plannedinstallation in Ernope wrll probably be the largest UV drinkingwater treatment system in the world.

Other Pollutants

Although there have been very few experimental studies, thereis reason to believe that UV-driven AOTs could be effective inremoving endocrine disruptors and hormone mimickers that haverecent$ been shown to be present at v€ry low concentations indrinking water.

Conclusions

Although the focus of the Intemational Ultraviolet Associationhas been primarily in the mea of Ulbaviolet Disinfection ofdrinking water and wastewat€r, it is important to realize that themajority of industrial applications of UV lie outside this area.Hopefirlly tlis overview has provided a usefirl survey ofone ofthese UV applications, namely the UV treatment of organicpollutants in drinking water.

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