Indian Journal of Chemical Technology Voi.IO, May 2003 , pp. 305-310

Articles

Catalytic wet air oxidation of pulp and paper mill effluent

Anurag Garg, Somen Saha, Yinayak Rastogi & Shri Chand*

Department of Chemical Engineeri ng, Indi an Institute of Technology , Roorkee 247 667, India

Received II January 2002 ; revised received 28 October 2002; accepted 22 No vember 2002

The catalytic wet air oxidation of pu lp and paper mill effluent was studied using CuS04 , perovskite based oxides, zeo­lites and bagasse tlyash as catalysts at atmospheric pressure using air as the source of oxygen . Among the catalysts studied the ac tivity of CuS04 was the best, having a reduction of 36.5 and 68.1 % in BOD and COD values respectively. Among the insoluble catalysts, Lao.3Ce0.7Co03 was the best with a marginally lower activity in comparison to CuS04. The performance of tlyash was also closer to CuS04 and Lao.3Ce0.7Co03. At a higher catalyst concentration of 5 g/L tlyash (initial concentra­tion 1 giL), the reduction in BOD and COD values of the pulp and paper mill effluent were 48.9 and 77.4% respectively, whereas the BOD/COD value was increased to 0.56, a value much closer to 0.63, which is an indicator for the complete biodegradability of the effluent. The reactivity of the catalyst was found to be a strong function of the pH of the effluent solution, and a pH of 10 was found to be the best.

The effluent from the pulping mill contains organic matter in the form of organic acids, resin acids, phe­nolics, unsaturated fatty acids, terpines, etc. Among these organics, the refractory organics are most haz­ardous as they are resistant to conventional biological oxidation. The quantum of effluent generated per ton of pulp manufactured is of the order of 300 m3

. The organics present in the pulp and paper mill effluent are mostly non-biodegradable in nature and wet air oxidation (W AO) is the on ly promising treatment method. Wet air oxidation of various compounds and industrial effluents has been critically reviewed by Mishra et al. 1. Matatov-Meytal and Sheintuch2 have compiled an extensive review of literature related to solid catalyzed oxidation and reduction processes for the treatment of wastewaters containing small con­centrations of toxic compounds. The application of various catalysts have been suggested to reduce the severity of temperature and pressure conditions for W AO. The most prominent catalysts used for the treatment of phenol was CuS0/ 4

• Mallik and Chaud­hari5 observed the potential of coal ash as catalyst for the oxidation of aqueous sodium sulphite solutions. The degradation of pulp and paper mill effluent was studied by Sonnen et al. 6 using water soluble salt of polyoxometalate. In the present work, the compara­tive activities of various catalysts such as CuS04, ba-

*For correspondence (E-mail : schanfch @iitr.ernet.in ; Fax: 91-1332-73560)

gasse tlyash, ion-exchanged zeolites and perovskite based oxides have been tested. Perovskite based ox­ides such as LaCo03 have been reported to be a good oxidation catalyst for the oxidation of carbon­monoxide and low molecular weight hydrocarbons during the treatment of automotive exhause·8

. Zeolites have also been reported as good oxidation catalyst in ion-exchanged forms9

.

Experimental Procedure The pulp and paper mill effluent was obtained from

the pulping unit of Star Paper Mills, Saharanpur. CuS04 used was of minimum 99% purity and ob­tained in powder form from Loba Chemi, India. The bagasse tlyash was obtained from a sugar mill, which was washed, dried and calcined at 400°C for 5 h in a current of air. LaCo03 was prepared by ceramic method. Stoichiometric amounts of lanthanum oxide and cobalt nitrate were mixed thoroughly in a pastle and mortar in presence of acetone. The mixture was then heated at a regulated rate of 2°C/min up to 800°C, where it was calcined for 10-12 h. The perov­skite phase formation was confirmed by X-ray pow­der diffraction. The process of calcination was re­peated in cases where the phase formation was in­complete. Lao.3Ceo.7Co03, LaFe03 and BaCo03 were prepared by similar method using the oxides/nitrates of required elements in their stoichiometric propor­tions. The BET surface area of these perovskite based oxides were quite low and ranged between 3-5 m2/g.

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The sampl es of commercially ava ilabl e 13X and HZSM-5 zeolites were ion-exchanged with Ce and Co respectively using 4% respective salt solutions. The ion-exchanged zeolites were pelletized at 5 t/cm2 and the size range -22+25 B.S . mesh was used for the ac­tivity tes ts. The BET surface area of CoZSM5 zeolite was 290 m2/g, whereas, it was around 160 m2/g for CeX.

The catalytic wet air oxidation react ion of the pulp and paper mill effluent was carried out in a glass re­actor of 500 mL capacity fitted with an agitation de­vice and an air distributor. 200 mL of wastewater was taken each time for the reaction studies, whereas , the catalyst amount was varied. The reaction vessel was immersed in a constant temperature oil bath and the temperature of the reaction mixture was maintained at

70 ±I oc. An air flow rate of 0 .3 Llmin with stirrer speed of 280 rpm was maintained for each test. The time of reaction in each run was kept constant at 2 h.

BOD and COD of the initial and final solution after ~he reaction were measured to estimate the extent of degradation of organic pollutants.

Results Catalytic W AO effects the complex feed in two

ways. One by e liminating non-biodegradables through its complete oxidation and the other by converting non-biodegradables to biodegradable ones. BOD/ COD ratio has been suggested as an indicator for the conversion of non-biodegradables to biodegradables. A value of BOD/COD ratio 0 .63 or more has been suggested for the sample to be biodegradable 10

. In the present work, therefore, the results have been reported in terms of the absolute values of BOD and COD as well as the BOD/COD ratio.

Effect of type of catalyst Fig. l shows BOD, COD and BOD/COD ratio of

the reaction product for various catalysts. The initial BOD and COD values of the pulp and paper mill ef­fluent used for the test were 370 and 1488 mg/L re­spectively . The catalyst concentration in all the runs were kept as 1.0 g/L. A maximum drop in the BOD value of the effluent was found to be up to 235 mg/L using CuS04 as catalyst, whereas LaFe03 had the least decrease of BOD value up to 345 mg/L. Similar results were found in COD reduction, giving the maximum decrease with CuS04 catalyst at a value of 475 mg/L and a minimum decrease with LaFe03 at I 189 mg/L. The performance of various catalysts to-

306

Indi an J. Che m. Techno!.. May 2003

wards BOD and COD removal had more or less an identical trend. The activities of various catalysts 111

terms of BOD reduction had the order:

CuS04 > Flyash > Ll(uCeru Co03 > CeX > LaCo0 3 > BaCo03 > CoZSM-5 > LaFe03

whereas, the COD reduction was in the order: C uS04 > Flyash > L<l(uCe0 7Co03 > CeX > LaCo03 > CoZSM-5 > BaCo03 > LaFe0 3

The maximum enhancement in the BOD/COD ra­tio, the parameter desirable for the bio-convertibility , was 0.49 in case of CuS04 cataiyst. LaFe03 had the minimum value of 0.29.

Effect of concentration of catalyst The effect of catalyst concentrati on was studied

using flyash as catalyst. The catalyst concentration in the effluent feed was varied at l , 3 and 5 giL. Figs 2 and 3 present the effect of flyash concentration on its performance towards BOD and COD reduction. It may be seen that with the increase in catalyst concen­tration , the reduction in BOD and COD values is in­creased, which, in turn , has resulted in a constant in­crease in the value of BOD/COD ratio . A maximum value of 0 .56 of BOD/COD ratio was found using 5 g/L of catalyst concentration, which is very close to the biodegradability limit of 0 .63 of the effluent.

Effect of pH The pH of the effluent solution was varied at 5, 7

and I 0 before the reaction. The activities of the fly ash catalyst at different pH values have been compared in Fig. 4 . The catalyst concentration in all these runs were kept constant at 3 g/L. The BOD and COD val­ues were found to decrease with increase in pH. At pH equal to I 0, the final BOD and COD values were 205 and 408 mg/L, respectively. The BOD/COD ratio was also found to increase constantly with the in­crease in pH and a maximum value of 0 .51 was no­ticed at pH I 0.

Discussion The selection of the catalysts tested in the present

work, namely, copper salt, perovsk ite based oxides, zeolite based catalysts and flyash were made, based on the literature survey of both aqueous organic po l­lutant treatment as well as removal of organic pollut­ants present in the vehicular exhaust. Perovskite based oxides (structure AB03), which are known to be ac­tive ox idation catalysts, consists La, Ba orCa as a

Garget a!.: Catalytic wet air oxidation of pulp and paper mill effl uent Articles

1600 -.-------------------------------,- 0.6

1400

1200

1000

:t 8 800 0

~ 600

400

200

Initial Etnuent cuso. Solution

m BOD (mg/L)

•coo (mg/L)

0 BOD/COD ratio

CoZSM-5 CeX Flyaah

0.5

0.4

.2 ~

0.3 8

~ 0.2

0.1

Catalyst

Fig. !- Comparison of BOD, COD values and BOD/COD ratio for different catalysts used during experimental runs (T = 70°C, P = I atm., Catalyst cone. = I g/L)

cation and Co or Fe as B cation . Substituted perovskite of the form A 1.xA;B03, where A' is usually Ce metal , has also been reported to be an efficient oxidation catalyst 1

1.12

. Noble metals have been widely used in catalytic converters as catalysts for cleaning vehicular exhaust gases. The activity of LaCo03, L<4l.3Ce0_7Co03 have shown 100% conversion of carbon-monoxide present in the exhaust at a much

lower temperature of 200°C. These results were found to be at par with the activity of noble metal based catalysts. The use of perovskite based oxides as an

alternative to noble metal based catalysts was proposed on account of it being inexpensive, easy availability and resistant to thermal and mechanical shocks.

NaCuX zeolite has been reported to be an effective catalyst for the oxidation of carbon-monoxide. A se­ries of Cu(ll) zeolites have shown that catalytic activ­ity for CO oxidation increases linearly with Cu(JI) concentration up to 36% exchange 13

. Kucherov et al. 14 have also reported the catalytic oxidation of methane using Cu(Il) containing HZSM-5 .

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Articles

700,------------------------------------,

600

500

::::; t 400

c 0 u g 300 CJ

200

100 .

~ ~

0~--~----~--~----~--~----~ 0 2 3 4 5 6

Concentration( giL)

Fig. 2-Effect of concentration of tlyash on BOD and COD val­ues of the final effluent (BOD;nitial = 370 mg/L, COD;nitial = 1488 mg/L, T = 70°C, P = I atm)

,g I!

0.6 ,--------------------------,

0.55

g 0.5

~ CJ

0.45 .

0.4 +-----~----~----~----~------,------l 0 2 3 4 5 6

Concentration of nyuh (gil)

Fig. 3- Variat ion of BOD/COD ratio with concentra ti on of tlyash in effluent

308

Indian J. Chem. Techno!. . May 2003

0 .6

0 .55 .

0

~ 0 0 0.5

~ 0 CJ

I 0.45 i

I

0.4 .l------~------.-----...-------1 4 6 8

pH

10 12

Fig. 4- Effect of pH of Effluent on BOD/COD ratio using tlyash as catalyst (Fly ash concentration = 3 g/L, T = 70°C, P = I atm)

Table !- Physico-chemical characterist ics of bagasse fl y ash

Particle size (J.t m)

> 600

600-500

500-425

425-355

355-250

250-212

212-180

180-150

<ISO

Bulk Composition

Moisture percent

Fixed carbon percent

Volatile matter percent

Ash percent

LOr percent

Si02

Al20 3 and Fe20 3

CaO

MgO

LOI-Loss on ignition

Weight percent

4.66

9.358

4. 604

8.723

6.766

3.876

4.670

26.980

30.70

5.28

13.44

6.88

74.40

6.88

4 1.90

4 .10

1.48

0.86

Garger al.: Catalytic wet air oxidation of pulp and paper mill effl uent

Flyash used in the present work as catalyst is a solid waste of bagasse fired thermal power plant, which contains a number of metallic and non-metallic oxides, like Si02, Ah03, Fe20 3, CaO, MgO and many others in traces, such as, Mn20 3, Ti02, along with the com­plexes of the above compounds, viz., ferrite, alumino­si licates, magnetite and some unburnt carbon. The characteristics of the flyash used in the present work is shown in Table I. Most of the ingradients present in the flyash have been frequently used as catalysts for the oxidation reactions. Recently, Chaudhuri and Sur15

and Mallik and Chaudhari5 have explored the potential of coal flyash as heterogeneous catalyst in peroxida­tive decolourization of aqueous solution of several reactive dyes and air oxidation of sodium sulphide solutions. The availability of bagasse flyash in abun­dance in the region had led to explore the possibilities of its prospective use as catalyst for the wet oxidation of organic pollutants. The negligible cost of flyash, its low catalytic deactivation, easy separation by simple gravity would be an added advantage, if the flyash is found to be a competitive catalyst for the oxidation of aqueous solution of organic compounds. The catalytic activity of coal flyash for the peroxidative decolouri­zation of reactive dyes using H20 2 have been reported to substantially intensify the rate of decolouization with only 2% flyash loading at pH 2.0, temperature 300 to 333 K and pressure I atm3.

Most of the research work on W AO reported in the literature is based on the use of soluble catalysts such as copper and zinc salts etc4'16

. It has been suggested that the soluble oxides should be incorporated in the lattice of the catalyst support for durabilit/ 6

. In the studies reported by Levec 16 when copper oxide was mixed with zinc oxide and alumina, it was observed that the catalysts are soluble in hot acidic water or could be transformed to a soluble state during reaction due to the presence of oxygen. However, when the catalyst was pre-calcined at 1133 K, the FTIR spectra and XRD pattern indicated that new phases (most probably CuAh04 and ZnAl20 4) were formed. The formation of compounds at the reported pretreatment conditions has three-fold advantages of (i) being resistant to thermal and mechanical shocks, (ii) insoluble in aqueous solution as well as (iii) the presence of highly active centres on the surface of the catalyst for the oxidation reaction. The selection of perovskite having a formula, a little different than the above compounds was made in the present work in line with the above observations.

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Conclusions The conclusions drawn from the present investiga­

tions are: (i) The comparative performance of the CuS04,

perovskite based oxides, zeolites (ZSM-5 and 13X) and bagasse flyash towards their catalytic activity for wet air oxidation of pulp and paper mill effluent showed CuS04 as the best. At 70°C reaction temperature and I g/L of catalyst con­centration the BOD and COD values reduced to 36.5 and 68.1% respectively. The BOO/COD ratio increased from an initial value of 0.248 to 0.495.

(ii) Among the insoluble catalysts, L<l(uCeo.7Co03 was the best. At the reaction conditions men­tioned above the reduction in BOD and COD values were 29.2 and 58.1% respectively .

(iii) The performance of flyash as catalyst was com­parable to CuS04 and Lao.3Ceo.1Co03 under identical reaction conditions. However, the ac­tivity increased with an increase in catalyst con­centration. At 5 giL flyash dose the reduction in BOD and COD values have been 48.9 and 77.4 percent respectively, whereas the BOD/COD value was elevated to a maximum of 0 .56 (a value much closer to 0.63, which is an indicator for the biodegradability of the effluent).

(iv) The reactivity of the catalyst is a strong function of the pH of the solution. The effect of variation of pH was studied using bagasse flyash as cata­lyst and a pH of 10 was found to be the best. The reason for thi s may be attributed to the pres­ence of OH- ions in the basic medium, which might have helped in dissociating oxygen mole­cule (of air) required for the oxidation reactions.

Acknowledgement The first author acknowledges the financial assis­

tance provided by Council of Scientific and Industrial Research (CSIR), New Delhi .

References l Mishra V S, Mahajani V V & Joshi 1 s; lnd Eng Chern Res,

34 (i995) 2. 2 Matatov-Meytal Y I & Sheintuch M, lnd Eng Chern Res, 37

(1998) 309. 3 Imamura S, Ind Eng Chem Res, 38 (1999) 1743. 4 Mitra P P & Pal T K, Indian Chem Eng Section A, 41 (1999)

T22. 5 Mallik D & Chaudhari S K, Warer Res, 33 ( 1999) 585. 6 Sonnen D M, Reiner R S, Atalla R H & Weinstock I A, Ind

Eng Chem Res, 36 ( 1997) 4134.

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7 Chand S, Sharma A K, Garg A & Mishra I M, J Sci lnd Res, 59 (2000) 944.

8 Grain F, Simonot L & Maire G, Appl Cataly 8: Environ. II ( 1997) 167.

9 Petunchi J 0 & Hall W K, J Cata/, 80 ( 1983) 403.

10 Garg S K & Garg R, Environmental Engineering (Vol. II):

3 10

Sewage disposal and air pollution engineering (Khan na Publi shers, Delhi), 1996, 199.

Indian J. Chem. Techno!., May 2003

II Chan K S, Ma J & Chuah G K, Appl Cataly A: General , 197 ( 1994) 20 I.

12 Garg A & Chand S, Indian J Chem Tecllllol, 8 (200 I ) 2 19. 13 Benn F R, Dwyer J , Esfahan i A, Ev merides N P &

Szczepura A K, J Catal, 48 ( 1977) 60. 14 Kucherove A V, Slinkin A A, Goryashenko S S & Slovet­

skaja K I, J Catal, 11 8 ( 1989) 459. 15 Chaudhuri S K & Sur B, J Environ Eng, 126 (2000) 583. 16 Levee J, Appl Catal, 63 (1990) Ll.