Wet Air Oxidation of Desizing Wastewater From the Textile Industry

8/12/2019 Wet Air Oxidation of Desizing Wastewater From the Textile Industry

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Wet Air Oxidation of Desizing Wastewater from the Textile Industry

Lecheng Lei, Xijun Hu,* Guohua Chen, J ohn F. Porter, and Po Lock Yue

Depart m ent of Chemi cal E ngi neeri ng, H ong K ong U ni versi t y of S ci ence and T echnol ogy,

Cl ear Wat er B ay, K owl oon, Hong K ong

We t a ir o xid atio n (WAO) wa s a p plie d to tre at th e d e siz in g wa ste wa te r f ro m n at u ral an d m an -

ma de fiber processing in a 2-L a utoclave react or. The ra nge of operat ing tempera tur es exa minedwa s betw een 150 and 290 C . The part ial pressure of oxygen ra nged from 0.375 to 2.25 MPa a ta r eference tempera tur e of 25 C . Va ria tions in chemical oxygen deman d (COD ) a nd tota l orga niccarb on we re m o n itore d d u rin g e ach e xpe rim e n t an d u sed to e valu at e t h e p e rform a n ce of t h ereaction process. Experimenta l results showed th a t WAO is an efficient meth od for th e trea tmentof desizing w ast ewa ter. A higher COD remova l wa s a chieved under gentler rea ction conditionswith th e a id of a cat alyst . A tw o-p ara m e ter m a th e m at ical m od e l in volvin g a fast re action a n da slow reaction was used to describe the WAO reaction kinetics and to calculate the reactiona ctivat ion energies.

Introduction

Wast ewa ter discha rged from the desizing processesof the textile industry is chara cterized by its very high

chemical oxygen demand (COD) and is one of the mainpollution sources in the textile industry. The desizingwa s t e wa t e r f rom n a t u ra l f ibe r p roce ss in g op era t io n sma in ly c o n t a in s s t a rc h , g lu c o s e , a n d y e a s t a n d h a s aCOD value of 10000-20000 mg/L a nd b iologica l oxygendema nd (B OD) from 5000 to 10 000 m g/L. Ma n-ma def iber de sizin g wa s t e wa t e rs t y p ica l ly h a v e p oly (v iny lalcohol) (PVA) as the major content, at COD levels ofbetw een 10 000 and 40 000 mg/L, a s w ell as a rela tivelysmall amount of BOD (500-1000 mg/L). The l ow B OD /CO D ra t io mea n s t h a t t h is k in d o f wa s t e wa t e r is v erydifficult to biodegra de. To comply wit h th e curr ent H ongKong discharge regulat ions, factories have to reducet h e ir CO D a n d B O D v a lu es t o 2000 a n d 800 mg /L ,respectively, before discharging to sewers.

I n g en er a l , t h er e a r e f ou r m et h od s a v a i la b l e f orwastewater treatment: chemical oxidation, biochemicaloxidat ion, incinerat ion, an d wet a ir oxidat ion. Chemicaloxidat ion is to oxidize the pollutants in the w ast ewa terin t o c a rbon diox ide a n d wa t e r wit h s t ron g ox idizers ,such as ozone and hydrogen peroxide. The chemicaloxidation process is relatively simple to operate but isonly cost-effective for polluta nts a t low concentra tions.I t would become expensive for high COD wastewaters u ch a s t h a t disc ha rg e d f rom t h e de sizin g p roce ss .Biological oxidation is a commonly applied method inwa s t e wa t e r t re a t me n t . H o we ve r , i t is n o t s u i t a ble f orwa stewa ter with COD above 10 000 mg/L. The la rgequantity of sludge produced by this method also imposes

further problems. Incineration may convert the pollut-ants effectively to innocuous end products at tempera-t u re s a bov e 700 C. I t is n o t e con omica l ly a t t ra c t iveunless the COD of the wa ste is a bove 100 000 mg/Lbe c a u s e a la rg e a mo u n t o f e n e rg y wil l be wa s t e d inbo il in g a n d h e a t in g t h e wa t e r ( s t e a m) i f t h e o rg a n icconcentra t ion is not high enough. Wet air oxidat ion(WAO), which mineralizes the pollutants at elevatedtempera tures (200-350 C ) a n d p re ss u re s, h a s t h e

ability to convert most organic compounds into carbondiox ide a n d wa t e r .1,2 Co mpa re d wit h t ra di t ion a l t e ch -nologies, the wet air oxidat ion process has the advan-tage of high react ion rate, small quantity of secondarypolluta nts, a nd less spa ce requirement. Thus t he WAOprocess might be an economically feasible method fort h e t re a t me n t of de s izin g wa s t e wa t e r .

D a t i ng b a ck t o t h e ea r ly 20t h cen t ur y , w et a i roxidat ion has been used for treat ing spent sulfite liquorfrom paper mills.3 And th ere a re over 200 Zimpro WAOunits in operation at more than 160 locations through-out the world.4 However, its application has been limiteddue to its severe operat ion condit ions; i .e. , i t usuallyruns under high pressure and temperature. The purposeo f t h is wo rk is t o in v e s t ig a t e t h e W A O t re a t me n t o fdesizing wa stewa ter, to lower t he react ion temperaturea n d p r e s s u r e b y a d d i n g a c a t a l y s t , a n d t o s t u d y t h ereaction kinetics.

Experimental Section

A schematic diagram of the wet air oxidation (WAO)s ys t em , w h ich h a s b een u sed t o t r ea t d y ei ng a n dprinting wastewater in our previous studies,5,6 is shownin Figure 1. The WAO system has a gas supply, a 2-Lre a c t o r , a wa s t e wa t e r c h a rg in g s y s t e m, a n d a l iq u idsampling point . The operat ion procedures of WAOp roce ss a re de scribed below. F irs t t h e re a ct o r wa spreheated to 60-70 C. The wa stewa ter (1.4 L) wa s thenc h a rg e d in t o t h e re a c t o r u s in g t h e wa t e r p u mp . T h espace occupied by gas inside the rea ctor wa s 600 mL.P u r e n i t r o g e n w a s u s e d t o p u r g e t h e a i r i n s i d e t h ere a c t o r a t a t o t a l p re s s u re o f 1 M P a f o r 2 min a f t e r

which the system w as isolated. The reactor w as heatedto th e desired react ion tempera ture (betw een 150 a nd290 C ) w hich took between 1 a nd 2 h. Once the selectedre a c t io n t e mp e ra t u re wa s re a c h e d, p u re o x y g e n wa sin t ro du c e d in t o t h e re a c t o r a n d t h e re a c t io n wa s a s -sumed to start (time t ) 0). At designated time intervalsthereafter, l iquid samples were ta ken from t he reactorand analyzed for COD and total organic carbon (TOC)con t e n t s . Th e s y s t em p res s u re is k ep t con s t a n t bysupplying oxygen to ba lance the pressure drop causedby sa mpling. At t he end of the reaction time, the heat ingjacket w a s turned off and t he reactor wa s cooled for ha lf

* To whom all correspondence should be addressed. Email:kexhu @ust .hk. F a x: (852) 23580054. Tel: (852) 2358 7134.

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an hour. Ta p wa ter w as t hen passed through the coolingcoil t o f u rt h e r cool t h e re a ct o r t o 60-7 0 C . P u r enitrogen was passed through the reactor to discharget h e t re a t e d wa s t e wa t e r . T h e re a c t o r wa s f i l le d wit hdeionized water, and the st irrer was operated at 1000rpm for 5 min. This f inal step was repeated three orfour times until the dischar ged wa ter wa s visually clean.

The COD value is assayed with an HACH-DR2000COD an alyzer. The an alyt ical agent , w hich consists of0.05 g of HgSO 4, 2.5 mL of 98% H 2S O4, 0 . 5 mL o f 1mol /L K 2C r 2O5, i s m i x e d w i t h 2 m L o f w a s t e w a t e rs a mp le a n d in cu ba t e d a t 150 C f or 2 h . C O D is t h e na s s a y e d a f t er cool in g w it h t h e CO D a n a ly zer . All t h echemicals used are in analyt ical purity grade.

The desizing wastewaters investigated in this studywer e provided by t w o Hong Kong textile companies. Thefirst w as from a cotton desizing process a nd containedprimar ily sta rch an d glucose. The COD, B OD, a nd TOCvalues for the cotton d esizing w ast ewa ter w ere 10280,3754, an d 3558 mg/L, r espectively . The second w a s froma ma n -ma de f iber de sizin g p roce s s, a n d p oly (v iny lalcohol) (P VA) wa s its ma jor component with COD,

B OD, a nd TOC values of 12600, 620, a nd 3824 mg/L,respectively.

Results and Discussion

COD and TOC Removals Caused by Heating.Since heating the reactor to the desired react ion tem-per a t u re t ook f rom 1 t o 2 h , t h e C O D a n d TO Cvaria t ions during the hea ting period in t he a bsence ofoxygen were examined first . Figure 2 shows the CODand TOC reductions of the cotton desizing wa stewa terwh e n i t wa s h e a t e d f ro m a mbien t t e mpe ra t u re t o 290C. The reduction or removal is defined as the differencebetween the init ial COD or TOC va lue and its va lue ata given t ime divided by its init ial amount. I t was seen

that there was lit t le change in COD or TOC when thetemperature was below 240 C. When the wastewaterwa s f u rt h e r h e a t e d f ro m 240 t o 290 C, t h e re wa s asignificant COD reduction. A COD reduction of 24%anda TO C re mov a l of 8% cou ld be a c h iev ed by s implethermal decomposit ion at 290 C.7 This is the reasonthe C OD or TOC reduction a t zero react ion t ime ha s afinite number instead of zero in the following figures.The reduction of COD is more prominent t han tha t ofTOC during thermal decomposition. This is due to the

pyrolysis producing hydrogen gas that might be lostfrom t he solution at high temperatur e, which does notaffect the TOC value.

Effect of Oxygen on the WAO Process. However,by keeping the reactor a t 290 C for a longer t imesupto 150 minslittle further r eduction in COD or TOC w asobserved in the absence of oxygen, as shown in Figure3. Figure 3 also shows t ha t a significant increase in bothCOD a nd TOC r emova l (over 75%) w a s observed in th epresence of oxygen at a partial pressure of 1.5 MPa (ata re fe ren ce t e mpe ra t u re o f 25 C )swh ic h is j u s t t h erequired st oichiometric am ount to completely minera lizethe organic carbon in the cotton desizing wastewater.

Five different levels of initia l oxygen pa rtia l pressure,0.375, 0.75, 1.125, 1.5, and 2.25 MPa, all at a referencetempera tur e of 25 C, w hich correspond to 25%, 50%,75%, 100%, a nd 150%of th e th eoretica l req uirem ent ofoxygen, were tested t o study th e effect of oxygen part ialpressure on the performa nce of th e WAO process. Thestoichiometric (theoretical) amount is defined as theam ount of oxygen needed to chemically mineralize a llthe pollutant s in th e wa stewa ter. The WAO trea tmento f n a t u ra l f ibe r de s izin g wa s t e wa t e r u n de r di f f e re n tinit ial oxygen part ial pressures is shown in Figure 4.The COD a nd TOC removal ra tes were not surprisinglyfound to increase with oxygen part ial pressure as theconcentra tion of dissolved oxygen in t he liquid pha se isproportional to the corresponding oxygen partial pres-sure.

Figure 1. Schematic diagram of the wet a ir oxidat ion system.

Figure 2. COD and TOC reductions during the reactor heat ingperiod for cot ton desizing w astew ater.

Figure 3. E f f ec t of ox y g en on t h e WAO of c ot t on d e siz in gw a s t e w a t e r a t 2 9 0 C.

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In Figure 4 we noted that the COD removal exceededthe oxygen supplied at low oxygen part ial pressure. Forexample, 48%of COD removal is achieved with 25%ofoxygen supplied. The rea son for this phenomenon is t ha tthe removal of COD is the overall result of oxidat ionre a ct ion a n d t h e rmoly s is .7-9 An ot h e r re a s on is t h ere gu la r a ddit ion s of ox y g en t o ma in t a in t h e re a ct ionpressure aft er sampling so tha t in fact th e COD remova lis not in excess of the actual oxygen supply; i .e. , theactua l oxygen supply is m ore tha n 25%. This phenom-

enon begins t o disappear when the oxygen supplied ismore than 75%of the theoretical amount.

Effect of Reaction Temperature on the WAOProcess. The WAO trea tment of natu ra l fiber desizingwa stewat er wa s further examined at f ive temperat ures,with the supply of oxygen fixed at 1.5 MPa, which isequivalent to the stoichiometric amount of 100%CODremoval. The rates of WAO reaction and COD removalin c re a s e wit h t h e re a c t io n t e mp e ra t u re , a s s h o wn inFigure 5. There is no significant COD or TOC removalat a t empera ture of 150 C. A COD r emova l of near 80%is obta ined after 100 min of react ion at 290 C.

Catalytic WAO Process. Although the WAO r eac-tion is very successful in the treatment of natural fiberde sizin g w a s t e wa t e r , i t mu s t be op era t e d u n der re la -tively severe conditions in order to achieve a high C ODremoval r at e. I t is necessary to improve the process toreduce the reaction temperature and pressure. Addingsome catalyst in the WAO process is one of the mostpopular options.5,10-12 I n t h is s t u dy , t ra n s i t ion me t a lsin t h e f o rm o f s u l f a t e s a n d n i t ra t e s we re u s e d a s t h ecata lysts. The am ount of cat alyst added int o the reactionsystem wa s 100 mg/L in t erms of meta l ion concentra -tion. Again, t he oxygen supply w as fixed a t t he theoreti-cal requirement, 1.5 MPa at a reference temperatureof 25 C. The react ion temperature was set at 240 C.The catalyt ic wet air oxidat ion (CWAO) treatment ofcotton desizing wastewater is shown in Figures 6 and7. The a ddit ion of cat alysts significant ly enha nces t he

removal ra tes of COD a nd TOC. The cata lyt ic act ivityof nitrates is slightly higher than that of sulfates, andCu(NO3)2is the best cata lyst a chieving a COD r emovalof 90%.

WAO of Desizing Wastewater from a Man-MadeFiber Process. Ha ving shown WAO to be a useful t ooli n t h e t r ea t m e n t of cot t on d es iz in g w a s t e w a t e r , i t sa p p lica bi l it y t o t h e t re a t me n t of de sizin g wa s t e wa t e rfrom a m an -ma de fiber processing operation w as exam-in ed. Th e in i t ia l C O D , B O D , a n d TO C v a lu es of t h is

Figure 4. Effect of oxygen part ial pressur e on the WAO of cottond e siz in g w a s t e w a t e r a t 2 40 C.

Figure 5. Effect of reaction temperature on the WAO of cottondesizing wastewater a t 1.5 MPa part ia l oxygen pressure.

Figure 6. E ffect of meta l sulfat es on the WAO of cott on desizingw a s t e w a t e r a t 2 40 C a n d 1 .5 M P a p a r t ia l ox y ge n p r es s u r e.

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wa stew at er w ere 12 600, 620, a nd 3824 mg/L, r espec-tively. The extremely low B OD/COD ra tio (0.05) impliest h a t t h is wa s t e wa t e r is v ery di f f ic ul t t o be t re a t e d bybiological methods. Figure 8 shows the effect of tem-perature on the WAO treatment of this wastewater atan oxygen part ial pressure of 2 MPa an d four differentreact ion temperatures. A higher oxygen pressure (2MPa ) was used here beca use the chemical fiber desizingwa stewat er had a larger COD value so more oxygen wa srequired to completely oxidize the waste content. The

ch oi ce of 2 M P a w a s a g a i n b a s ed on p rov id in g t h es y s t em wit h t h e re q u ire d s t o ich iome t ric a mou n t t omineralize the organic car bon in th e wa stewa ter. Simi-la r t o c ot t o n de s izin g wa s t e wa t e r , t h e C O D a n d TO Cremovals were improved by increasing react ion tem-perat ure. At 270 C , 90%COD reduction a nd 80%TOCremoval were achieved after 2 h.

Figure 8 a lso shows the biodegrada bility change forthe man-made fiber desizing wastewater. The biode-gradability is defined as the BOD to COD rat io. Afterthe WAO treatment, the biodegradability was signifi-cantly enhancedsm or e t h a n 80% w a s ob t a i n ed a t areaction temperature of 270 C for 2 h. An increase inthe react ion temperature helps to enhance the biode-gradability of wastewater. The increase in BOD to CODratio is due to the decomposition of the large relativelybiologically stable molecule (PVA) into smaller morebiodegradable molecules. This suggests that WAO shouldbe a suitable pretreatment step before biological deg-ra da t io n .

As previously shown in Figure 2 for the natural fiberdesizing wastewater, some degree of thermal decompo-sit ion was also observed for this system. Once again,different initial COD and TOC reductions were observedat different react ion temperatures. However, the bio-degradability could not be improved by simply heatingt h e w a s t e wa t e r . Th is is e viden t f rom F ig u re 8 w h e rethe BOD to COD ra t ios have the same init ial value fordifferent react ion tempera tures.

Reaction Kinetics. The reactions during WAO arev er y com p le x i n t e rm s of m a n y i n t er m ed ia t e a n dult imat e products.13 N o a t t e mpt is ma de t o p rov ide a

detailed rea ct ion ana lysis on individual compounds.Instead a brief discussion on the kinetics study of CODre du c t io n wil l be g iv e n . Sin c e t h e re a c t o r wa s we lls t irre d a n d t h e o x y g e n s u p p ly wa s ma in t a in e d a t t h estoichiometric requirement a nd more oxygen w as givenafter each sam pling, the ma ss tra nsfer controlled regimeis elimina ted a nd th e dissolved oxygen concentra tion isassumed constant for a given pa rt ia l oxygen pressure.The rate data were modeled by f irst-order kinetics asin the following equa tion, with t he reactant concentra -t ion given as COD

wh e re t is react ion t ime and k is the specific reactionr a t e con s t a n t w h i ch h a s t h e f ol low i n g t e mp er a t u r edependency

wh e re k0 is the pre-exponential factor, E i s t h e a ct iv a -tion energy, R i s t h e u n ive rsa l g a s con s t a n t , a n d T isthe temperatur e in kelvin. Integra t ing eq 1 gives

w h e r e C O D 0 i s t h e i n it i a l C O D v a l ue . B y p lot t i n gln(COD 0/COD) versus t ime, t he slope is the specific

Figure 7. E ffect of meta l nitra tes on the WAO of cotton desizingw a s t e w a t e r a t 2 40 C a n d 1 .5 M P a p a r t ia l ox y ge n p r es s u r e.

Figure 8. Effect of react ion temperature on the WAO of man-made fiber desizing w astewa ter a t 2 MPa part ia l oxygen pressure.

-

d COD

d t ) kC OD (1)

k ) k0

exp(-E/R T) (2)

ln(COD 0/C OD ) ) k t (3)



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re a ct ion ra t e con s t a n t k. A typical plot of the WAOt r ea t m e n t of cot t on d es iz in g w a s t ew a t e r a t a f ix ed

part ial oxygen pressure of 1.5 MPa and four differentreaction temperatures (Figure 4) is shown in Figure 9.Th e da t a f it we l l in t o t wo s t ra ig h t l in e s f or a g ive ntempera ture, indicat ing tha t oxidat ion proceeds in t wodistinct steps: a fa st initia l reaction of lar ge moleculesdecomposed into intermediate products, carbon dioxide,a n d w a t e r , f ol low e d b y a s low r ea c t ion of f ur t h eroxidizing the intermediate products into end productsof car bon dioxide and wa ter.

To calculate the act ivated energies, eq 2 was trans-formed into a logarithmic form, which was plotted inFigure 10. As expected, the ra te consta nt increases w ithincreasing t empera ture, but this t empera ture effect ismore significant for the fast reaction step. As the slow

react ion, the react ion rate is not very sensit ive to thec h a n g e o f t e mp e ra t u re . T h is in dic a t e s t h a t t h e f a s treact ion has a high act ivat ion energy, while the slowreact ion has a small act ivated energy. The calculatedactivated energies ar e Efast ) 30 kJ /mol a nd Eslow ) 9.2kJ /mol.

I t should be remembered that the ra te consta nt in eq1 is the product of the true specific rate constant andthe dissolved oxygen concentration in water which is afunction of the partial oxygen pressure, i.e.

T h e ra t e c o n s t a n t k c a n b e r e a s o n a b l y a s s u m e d a s

constant for a given oxygen partial pressure only whenoxygen is in excess. Otherwise as the part ial oxygenpressure changes, the rate constant k may vary. Thisis shown in Figure 11 for the WAO of cotton desizingwa s t e wa t e r a t 240 C (F ig u re 4). Th e p a rt ia l ox y g enpressure va ries from 0.375 to 2.25 MP a . The extra ctedra te consta nts versus partia l oxygen pressure are shownin Figure 12.

Since the stoichiometric oxygen requirement for com-plete oxidation of the cotton desizing wastewater is 1.5M P a a t 2 5 C , i t h a s a s i g n i f i c a n t e f f e c t o n t h e r a t econ s t a n t of t h e f a s t r ea c t ion i f t h e ox yg en p a r t ia lp re ss u re i s l es s t h a n t h i s a m o un t . H ow e ve r, w h e noxygen is in excess (higher t han the stoichiometricoxygen dema nd), the ra te const a nt becomes independentof the partial pressure of oxygen because the dissolvedoxygen concentration can be considered as constant in

this case. The slow react ion sta ge has a wea ker depen-de n c y o n t h e o x y g e n p a rt ia l p re s s u re f o r t h e wh o lepressure region considered indicat ing t ha t the oxygenrequirement of the slow rea ction step is much less thant h a t of t h e f a s t r ea c t ion . I t a l s o i nd ica t e s t h a t t h eintermediate products are difficult to be oxidized evenat a large excess of oxygen.

Conclusions

Wet air oxidation is found to be an effective methodfor treat ing the desizing wastewater from both cottonand ma n-ma de fiber processing operat ions, one of themajor COD contributors in the textile industry waste-wa ter. More tha n 90% COD reduction a nd 80% TOC

Figure 9. WAO of cot ton desizing wast ewat er a t 1.5 MPa part ia loxygen pressure.

Figure 10. Ef f e c t of t e m p e r a t u r e on r a t e c on s t a n t s of c ot t ondesizing wastewater a t 1.5 MPa part ia l oxygen pressure.

k ) k[PO2]n (4)

Figure 11. WAO of cot ton desizing wastewater a t 240 C.

Figure 12. Effect of oxygen concentrat ion on rate constants ofcot ton desizing wastewater a t 240 C.

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re mov a l ca n be obt a in e d by p rop erly ch oos in g t h ereaction conditions. Increasing the reaction temperatureand pressure helps to enhance the WAO efficiency.However, a certain port ion of the organic pollutantscould not be removed even at elevat ed temperat ure a ndprolonged reaction time. The performa nce of t he WAOp roce ss ca n b e i m pr ov ed b y a d d i ng s om e s u it a b l ecatalyst . WAO was also demonstrated to be a suitablepretreatm ent method before biodegrada tion because t heBOD /COD r at io of desizing w ast ewa ter w as significan tlyincreased by WAO treatment. A preliminary investiga-tion of reaction kinetics shows that wet air oxidation ofdesizing wastewater follows two steps, a fast react ionfollowed by a slow react ion st age.

Acknowledgment

The a uthors a re gra teful to the Indust ry a nd Technol-ogy Development C ouncil of Hong Kong for their finan -cial support.

Nomenclature

C OD )chemica l oxygen dema nd, a pa ra met er for w a s t e-wa ter char acteriza tion, mg/L

C OD 0 ) initial COD value, m g/L

E )act iva tion energy , kJ /mol

k ) reaction constant

k0 ) pre-exponential factor in the Arrhenius equation

R )ga s const an t, 8.314 J /(mol K)

t ) reaction time, min

T ) rea ct ion t empera t ure, C or K

TOC ) t ot a l orga nic ca rbon, a pa ra met er for w a s t ew a t erchara cterization, mg/L

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(11) Hu, X. ; Lei, L. ; C hu, H . P . ; Yue, P. L. C opper/ActivatedCa r b on a s C a t a ly s t f or O r g a n ic Wa s t e w a t e r Tr e a t m e n t . Carbon1999, 37, 631-637.

(1 2) Ch u , H . P . ; L e i, L . ; H u , X .; Y u e, P . L . M e t a l lo-O r g a n icChemical Vapor Deposit ion (MOCVD) for the Development ofHeterogeneous Catalyst . Energy Fuels1998, 12, 1108-1113.

(13) Ha o, O. J . ; P hull, K. K. Wet oxidat ion of Nitrotoluene-s u lf on ic Ac id : S om e I n t e r m e dia t e s , R e a c t ion P a t h w a y s , a n dBy products Toxicity. En vir on. Sci. T echnol. 1993, 2 7, 1650-1658.

Receiv ed for r evi ew August 9, 1999Revised ma nu scri pt received March 30, 2000

Accepted April 18, 2000

IE990607S


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