INTRODUCTION:
Ozone is a well known oxidizing reagent for the decolourization and partial oxidation of effluents released from pulp and paper industries1-5.It is capable of decomposing colouring substances, humicacid materials, tannins, lignin degraded products etc. at a faster rate without generating any toxic byproduct2.The application of ozone in pulp and paper industries has become more popular after the development of technology in producing low cost ozone which can be safely used for the production of total chlorine free(T.C.F) or elementally chlorine free(E.C.F) pulp. Although ozonization is a very good choice for the physical and chemical treatment of effluents released from pulp and paper industry, the exact mechanism and kinetics of ozone induced decolourization have not yet studied systematically. In the present work an attempt has been made to study the kinetics of ozone induced decolourization of post oxygen (PO) stage effluent generated from a pulp and paper industry producing pulp with RDH cooking and OC/DEopD bleaching process to have an idea regarding the mechanism of degradation. The degradation kinetics is also monitored in presence and absence of catalyst to know whether the degradation process is further enhanced in presence of a catalyst.
MATERIALS AND METHODS:
The reagents used are of anal-R grade and are recrystallised or redistilled before use. For Ozone determination, the iodometric method as described in standard method has been followed6. The sample is collected in a 250 ml volumetric flask containing 10 ml of 0.1 N H2SO4 and 20 ml of 20 % KI solution. A blank is also set up and the titrations are carried out using 0.1M sodium thio-sulphate solution.
The ozone concentration applied has been calculated from the graph of standard concentration at different flow rate. The kinetic runs are performed by following the disappearance of colour in the effluent at different reaction time after passing the ozone at fixed flow rate. The reactions are quenched by putting ice. The reduction in colour (in Platinum Cobalt Unit) concentration are determined spectrophotometrically at 465 nm after centrifuging the solution. The pseudo first order rate constants are calculated from the slope of log (conc.) vs. time plots. The rate constants are reproducible within ± 5%. Only a representative set of average values of kinetic data at 300C (±0.10C) are presented.Table-1 gives a summary of various characteristics of effluent water employed in order to determine kinetic data. The values of reaction rate constants for the variation in each parameter such as [colour], [O3], [Salt], temperature are studied and order has been calculated for uncatalysed reaction (Table-2). Similar experiments are carried out with catalyzed reaction by using Fe2(SO4)3 as catalyst. The kinetic runs are done by varying the parameters like [Colour], [O3], [Catalyst] and temperature (Table-3). In all cases, more than 95% of level of confidence was obtained.
Kinetic experiments are carried out by passing the O3 at a fixed flow rate into a reaction flask of one liter capacity with 500 ml of effluent after adjusting the pH to 9.5.The amount of O3 passing to the solution was calculated from a standard curve done earlier. Aliquots are withdrawn at regular intervals and the reaction has been quenched by ice and the intensity of colour has then determined after centrifuging. The measurement of the colour of the supernatant liquid is done by a UV-VIS spectrophotometer at 465 nm. All the kinetic runs were carried out in duplicate and the results were found to be reproducible within ± 5%.
Stoichiometry and product analysis:
The results of stoichiometric experiments conducted with effluent containing an excess of O3 over that of [Colour] at a fixed pH of 9.5 reveals that 1.0 mg dm-3 of [O3] is required for the reduction of 1.0 mg dm-3 of colour to give the colourless product. The stoichiometric runs is also done in presence of catalyst and it is found that 0.5 mg dm-3 of [O3] is required for the reduction of 1.0 mol of colour.
The ozonation is carried out by generating ozone from a laboratory ozone generator supplied by the Aquazone systems and Engg. Ahmadabad (Model CD 500). The gas feed to the ozone generator is oxygen, producing 2.4 g/hr. of ozone at a flow rate of O2 of 1 lit/min. The ozone produced is passed through the effluent in a closed beaker with a porous plug at the outlet.
RESULTS AND DISCUSSION:
Reaction of colour with O3 (Uncatalysed):
The reaction rate constants data at varying [O3],[Colour], pH, [Salt] are recorded in Table-2. The plot of log [Colour] vs. time is linear (fig.1a). The kobs(sec-1) values are almost constant upto three half lives for any individual run. The pseudo-first order rate constants increase with the increase of [O3] upto 3.5 mg dm-3 with an apparent limiting tendency. The double reciprocal plot of kobs against [O3] i.e. 1/ kobs vs. 1/ [O3] is linear (fig.2) with a finite intercept in 1/ kobs axis at higher [O3] concentration, indicating complexation of the O3 with colouring compounds. The rate data at varying pH shows that the reaction rate is low at pH 7.0.
Table-1 : Initial Properties of PO stage effluent.
|
Sl No. |
Properties |
Value |
|
1 |
pH |
10.08 |
|
2 |
Colour(Pt.Co.U.), mg dm-3 |
3033 |
|
3 |
Turbidity, NTU |
5.7 |
|
4 |
Total suspended solid, mg dm-3 |
115.3 |
|
5 |
Total dissolved solid, mg dm-3 |
2850 |
|
6 |
COD, mg dm-3 |
1539 |
It is increased by going down pH to 4.5 and also increasing pH to 10.5. The effect of ionic strength is marginal on the reaction rate. The plot of log kobs versus pH gives two distinct lines. The rate is decreased while going from pH 10.5 to 7.0 and then increased from pH 7.0 to 4.5. The rate reaction of uncatalysed reaction carried out from pH 7 to 9.5, shows inverse fractional order dependence with respect to [H+] .The reaction rates are not effected by the addition of neutral salt i.e. NaCl, indicating the reaction to be of non ionic specific. To study the effect of temperature, rate constants at 30, 35, 40 and 50OC are determined and the values are given in the Table-2 and Arrhenius activation parameters computed at 30OC for the reaction of colour compound with O3 are: Ea = 17.35 Kg mol -1, ∆H+ = 14.83 JK-1mol -1 , ∆S+ = -211 JK-1mol -1 , Log A = 0.24.
Table-2 : Rate constants of reaction of Colour comounds of PO stage Effluent with Ozone :
|
[O3](mg /Lit. ) |
10-3x colour Pt Co U |
PH |
1011[H+] |
103 X kobs sec-1 |
|
0.944 |
2 |
9.5 |
31.6 |
0.461 |
|
1.78 |
2 |
9.5 |
31.6 |
1.79 |
|
2.66 |
2 |
9.5 |
31.6 |
2 |
|
3.5 |
2 |
9.5 |
31.6 |
2.648 |
|
1.78 |
1 |
9.5 |
31.6 |
1.919 |
|
1.78 |
1.5 |
9.5 |
31.6 |
1.85 |
|
1.78 |
2 |
9.5 |
31.6 |
1.79 |
|
1.78 |
2.5 |
9.5 |
31.6 |
2.09 |
|
1.78 |
2 |
10.5 |
3.16 |
2.159 |
|
1.78 |
2 |
9.5 |
31.6 |
1.79 |
|
1.78 |
2 |
8 |
1000 |
1.299 |
|
1.78 |
2 |
7 |
10000 |
1.599 |
|
1.78 |
2 |
5 |
1000000 |
1.645 |
|
1.78 |
2 |
4.5 |
31600000 |
2.399 |
|
1.78 |
2 |
9.5 |
31.6 |
1.951** |
|
1.78 |
2 |
9.5 |
31.6 |
1.79* |
|
1.78 |
2 |
9.5 |
31.6 |
1.88* |
|
1.78 |
2 |
9.5 |
31.6 |
2.33* |
|
1.78 |
2 |
9.5 |
31.6 |
2.76* |
* Reactions done at temperature; 30, 35,40 and 500C
** Reaction at [NaCl] = 1 mg dm-3
Fe(III) catalysed reaction of colour with O3 :
The kinetics of reaction is also studied in presence of Fe(III) catalyst in acidic pH of 5.0. The order with respect to reduction of [colour], [O3], pH and temperature on reaction rates are studied. The data clearly indicate that the order is unity with [Colour] [(as seen from linearity in plots of log[colour] vs. time ;(fig.1b)], fractional order with respect to [O3] and [Catalyst]. The effect of pH variation in acidic condition shows that the rates increase with the increase of acidity (i.e. towards lower pH). The order is found to be fractional. The rate constants are evaluated at 30, 40 and 500 C and the Arrhenius activation parameters computed at 300 C are:
Ea =10.94 Kg mol -1 , ∆H =8.42 JK-1mol -1 , ∆S =-215 JK-1mol -1, Log A = 0.034
Mechanism and rate law:-
Reaction between ozone and colour compounds in the PO-stage effluent from pulp and paper industry are studied at temperatures ranging from 30 to 50 0C and pH values at 9.5. Absorbance are measured by spectrophotometer at 465nm for the disappearance of colour and rate equations are derived taking into account of depletion of ozone by decomposition and ozonation reaction, to determine orders of the reaction and rate constants. Ozone can be manufactured from dry air or oxygen. Its oxidation potential (-2.07V) is greater than that of chlorine (-1.36V) and hypochlorite acid (-1.49V). Ozone is thought to decompose as:
O3 + H2O → HO3* + OH*
HO3* + OH* → 2HO2*
O3 + HO2* → HO* + 2O2
HO* + HO2* → H2O + O2
The free radicals (HO2* and HO*) react with a variety of impurities such as metal salts, organic matter including microorganisms, hydrogen and hydroxide ion. The oxidation potential of common oxidants associated with waste water treatment are all of a lower oxidation potential than ozone. The colour compounds are in equilibrium form i.e. dissociated and undissociated form with H+. The undissociated form of species is the most reactive with ozone in alkaline conditions of pH 9.5.
[Colour] ç==è [Colour]- + H+
When the organic compounds present in PO-stage effluent originated from pulping are mixed with ozone, an intermediate product is formed in a fast step and further in slow reaction results the formation of degraded products. In addition, the reaction is accompanied by decomposition of ozone in aqueous solution. Mechanism consistent with all the experimental results is presented in Scheme-1.
Fast
2 O3 3 O2
Fast (K) k
O3 + [Coluor] [Complex]
Products + O2
SCHEME – 1
Reaction between ozone and colour can be represented by taking into consideration decomposition of ozone which is three half order with respect to ozone concentration7. And the rate law derived as follows:
Rate =-d [Colour] / dt = d [Complex] /dt = k [Complex]
= K.k [O3] [Colour] + ko [O3] 3/2 --------(i)
As colour compounds are taken in large excess in solution, its concentration can be considered to be constant and O3 reacts with colour very fast as compared to its decomposition and only a very small fraction of O3 is depleted during the reaction.Under this condition the magnitude of ko[O3] 3/2 is smaller than that of the 1st term in eq.( i ), and rate equation can be written as:
- d [Colour] / dt = K.k [O3] [Colour]
The catalytic ozonation by using Fe (III):
The ozonation also follows first order kinetics with respect to disappearance of colour regardless of pH values of an aqueous solution. But the rate constants are affected by changing the pH from 10.5 to 4.5. By changing the pH from 10.5 to 7.0 the rate constants are decreased and then there is an increasing trend observed from pH 7.0 to 4.5. These results suggest that the colour compounds are in equilibrium form i.e. dissociated and undissociated form with H+. The dissociated form of species is the most reactive at acid pH of 5.0 with ozone.
[Colour] ç==è [Colour]- + H+
Hence in the mechanism of Fe(III) catalyzed ozonization reaction can be postulated as the ozone molecules are adsorbed on the surface of the catalyst to form the ozone-catalyst complex. This complex will then form reactive oxygen species for oxidation. The most plausible mechanism of reaction pathway is:
K
O3 + [Catalyst] ç====è O3. Catalyst*
O3 . Catalyst* -------à O2* + O. Catalyst*
O3 . Catalyst* -------à O* + [O2.Catalyst*]
O2* or O*
O2* + [Colour] -----è [Complex-1] --------à Product + CO2 + H2O
O* or O2*
O* + [Colour] ----è [Complex-2] -- ------à Product + CO2 + H2O
O* + O* ç====è O2.
O2* ç====è O2
Table-3: Rate constants of Fe(III) catalyzed reaction of Colour compounds of PO-stage Effluent with Ozone :
|
[O3] (mg/ Lit) |
10-3x [Colour] Pt Co U |
PH |
1011[H+] |
[Fe2(SO4)3] (mg dm-3 ) |
103 X kobs sec-1 |
|
0.944 |
2 |
5 |
1000000 |
0.5 |
2.83 |
|
1.78 |
2 |
5 |
1000000 |
0.5 |
2.89 |
|
2.66 |
2 |
5 |
1000000 |
0.5 |
3.58 |
|
1.78 |
1 |
5 |
1000000 |
0.5 |
3.01 |
|
1.78 |
1.5 |
5 |
1000000 |
0.5 |
2.85 |
|
1.78 |
2 |
5 |
1000000 |
0.5 |
2.89 |
|
1.78 |
2 |
4 |
10000000 |
0.5 |
3.01 |
|
1.78 |
2 |
5 |
1000000 |
0.5 |
2.89 |
|
1.78 |
2 |
7 |
10000 |
0.5 |
2.77 |
|
1.78 |
2 |
9.5 |
31.6 |
0.5 |
2.87 |
|
1.78 |
2 |
5 |
1000000 |
0.2 |
2.71 |
|
1.78 |
2 |
5 |
1000000 |
0.5 |
2.89 |
|
1.78 |
2 |
5 |
1000000 |
1.0 |
2.95 |
|
1.78 |
2 |
5 |
1000000 |
0.5 |
2.89* |
|
1.78 |
2 |
5 |
1000000 |
0.5 |
3.41* |
|
1.78 |
2 |
5 |
1000000 |
0.5 |
3.84* |
* Reactions done at temperature; 30,40 and 500C
The above reaction sequence will be terminated when either CO2 molecule or stable O2 molecule are formed in the system or O3 decomposes to a stable oxygen molecule and a reactive oxygen atom in ozonation process. The reactive oxygen atom will then carry on the oxidation. Mechanism of catalyzed reaction is given below in scheme -2.
Ka
[Colour] ç==è [Colour]- + H+
K
O3 + Fe(III) ß====è O2* + O*
K1 k1
O* + [Colour] ç==è [Complex ] --------à Oxidation Products
Slow
K2 k2
or O2* + [Colour] ç==è [Complex ] ----à Oxidation Products
Slow
SCHEME-2
And the rate law is:
KK1 k1 [O3] [Colour] [Fe(III)] KK2 k2 [O3] [Colour] [Fe(III)]
-d [Colour] / dt = -------------------- OR -----------------------------
1+ KK1 [O3] [Colour][Fe(III)] 1 + KK2 [O3] Colour][Fe(III)]
The double reciprocal plots of 1/rate vs. 1/ [O3] (fig.3), 1/rate vs. 1/[colour] (fig.4) and 1/rate vs. 1/[Fe(III)] (fig.5) are linear giving a constant intercept (i.e. 1/k1) in each case indicating the consistent of the rate law for the reduction of colour of the effluent with the ozone catalyzed by Fe(III).
CONCLUSION:
1. The degree of colour reduction is primarily a function of amount of O3 applied, but influenced also by initial colour.
2. The colour of the PO-stage effluent can be easily decolourized by using O3, so it can be easily reused in the process. There will be also a considerable decrease in COD, Suspended solid with an increase in dissolved oxygen content in water which is more beneficial. Now a days O3 generation technology is also available at a lower cost.
3. The cost can also be further reduced by using the catalyst to enhance the reaction. The result of this research verified that at least two oxygen atoms were used for oxidation in an aqueous effluent and Fe(III) catalyst system.
ACKNOWLEDGEMENT:
One of the authors Mr. U. P. Tripathy is grateful to Management of J.K. Paper Mill, Rayagada (Orissa) and Pulp Paper Research Institute, Director Mr. A. K. Harichandan for giving permission and moral support to do some of the PhD work in the research Laboratory of Pulp and Paper Research Institute.
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Received on 19.11.2010 Modified on 27.11.2010
Accepted on 05.12.2010 © AJRC All right reserved
Asian J. Research Chem. 4(2): February 2011; Page 195-199