Degradation of Phenol and Naphthalene Derivatives in Tanning´s Wastewater Process Using Ozonation as a Pretreatment in Biological Plants T.Poznyak and B. Araiza G.

Superior School of Chemical Engineering. National Polytechnic Institute of Mexico (ESIQIE - IPN). Edif. 7 UPALM, C.P. 07708, Mexico D.F., Mexico.

Abstract

Ozonation of phenol and naphthalene derivatives (2- naphthalenesulfonic acid and p-phenolsulfonic acid) is studied. The elimination of the high concentration of these contaminants (the total COD is around 200g L-1) by the direct reaction with ozone is shown to be successfully realized in aqueous media. The dependence of the obtained kinetics on the pH variation has been studied. The optimal conditions of ozonation, corresponding to maximum elimination for the shortest time, nave been obtained by the numerical simulation of this reaction for several compounds and model mixtures.

Introduction

The tanning´s production as well as its derivatives obtained from phenol and naphthalene has an important impact in water pollution due to high levels of toxic organic matter in the process wastewater (1). Tanning production plants usually deal with effluents containing the phenol concentration around 35 g L-1 (COD is as 200 g L-1) and the total volume of 4 m3 day-1 (2). It should be diluted after biodegradation process, since the concentration at the inlet wastewater treatment is subjected to essential variations. The dilution provides respectively good elimination results (around 70 – 80 %), but leads to higher enough cost of the treatment. By this reason, in the case of the high phenol and naphthalene derivatives concentration in wastewater, the ozonation can be suggested as a good pre-treatment of a biodegradation (3,4,6-12). In addition, the use of ozone increases the biodegradability of the treated wastewater (5) reducing its organic matter content. In view of this, ozone might be used as a pre-treatment step for biological digestion of contaminated effluent stream (13). It also provides the feasibility to obtain other chemicals with less toxicity and simple substances that seems to be much more easily than biodegradation making more efficient the biological treatment itself (14).

In this work, the ozonation effect is studied by numerical simulation for several compounds and model mixtures. The main contribution of this paper consists in finding the optimal conditions of the ozonation reaction including such parameters as initial ozone concentration, gas flow, ozonation time and the reactor volume. The applied mathematical model describes the mass transfer (15,19) as well as the chemical reactions in a semi-batch reactor (16-18). The use of this model helps essentially to optimize the conditions and the number of experiments. Based on the experimental and simulation results and varying the pH value, the best conditions for the elimination of the organics have been derived.

Experimental Reaction apparatus Figure 1 presents the block-diagram of the experimental system for the model aqueous solution of phenol, 2-naphthalenesulfonic acid (2-NSA) and p-phenolsulfonic acid (p-PSA) ozonation. The reactor is the semi-batch type (0.5 L). The ozonation conditions are as follows: the initial ozone concentration of 2.7 x 10-4 mole L-1, the initial organics concentration is approximated as 1.25 x 10-3 mole⋅L-1; maintained a pollutant and ozone relation as 20:1 to facility the analysis. The ozone-oxygen mixture flow is of 0.5 L min-1. The reaction temperature is 20ºC. Agitation is provided by means of bubbling of an ozone-oxygen mixture. To obtain the best ozone distribution the magnetic agitation (usually operated at 120 rpm) in water is realized. During the experiments, the samples were withdrawn regularly from the reactor for the current analysis. Ozone is generated from oxygen by the ozone generator (corona discharge type) HTU500G (“AZCO” INDUSTRIES LIMITED– Canada,) with the ozone concentration and the oxygen flow regulation.

Oxygen Tank

Ozone Generator

Reactor

UV Analysis

Ozone Analyzer Figure 1. The block-diagram of the experimental system for the phenol and naphthalene derivatives ozonation.

Analysis Ozone in the gas phase is analyzed in the outlet of the reactor by the Ozone Analyzer BMT 963 (BMT Messtechnik, Berlin). In the beginning, some preliminary experiments were realized to carry out a sweeping with flow of oxygen through the ozone generator of 0.5 L⋅min-1. To make successfully the residual ozone measurement, the reactor and the analyzer of ozone in the gas phase were specially tuned to obtain so-named “the line background”. The data were captured and kept in the PC memory each second.

Ozone in the liquid phase was determined by the absorbency at 254 nm on a spectrophotometer (Perkin Elmer UV/VIS LAMBDA 2S). The extinction coefficient of 1830 L mol –1 s –1 was used to calculate the ozone concentration in water.

The organic compound analysis in water was realized by means of absorbency at 190 – 320 nm on a spectrophotometer. The preliminary calibration by the model mixtures of the organic compounds in distilled water was realized. The corresponding absorbency for the NSA was at 274 nm, for the PSA at 224 nm (see Figure 2 for structures) and for the phenol at 260 nm. OH SO3H

SO3H p-Phenolsulfonic acid

2-Naphtalenesulfonic acid

Figure 2. Molecular structures of the compounds investigated.

The dynamic of the organics decomposition was fixed at 1, 3, 5, 10, 15, 20, 30, 45, 60, 75 and 90 minutes. The samplers were taken from a reactor sample valve and quartz cell. The specters were processed by special specter software in a PC. Figure 3, 4 and 5 present the current variation of the UV specters for the NSA, PSA and phenol, respectively, during the ozonation.

The pH variation and for the pH meter calibration were realized with sulfuric acid and sodium hydroxide. A series of ozonation experiments with NSA and PSA (excluded phenol) were carried out by the pH varying of the solution from 2 to 12.

4 3.5 3 2.5

A

2 1.5 1 0.5

321

311

301

291

281

271

261

251

241

0

nm

Figure 3. The NSA UV specter variation during ozonation at 0, 1, 3, 5, 10, 15, 20, 30, 45, 60, 75, 90, 105 and 120 min. The NSA initial concentration is 1.3064 x 10 –3 mol⋅L-1 at pH of 10.

2.5

2

1.5

A 1

0.5

245

235

225

215

205

0

nm

Figure 4. The PSA UV specter variation during ozonation: at 0, 1, 3, 5, 10, 15, 20, 30, 45, 60 and 90 min. The PSA initial concentration of 1.25 x 10 –3 mol⋅L-1 at pH of 3.30.

Phenol Elimination Phenol Elimination (0 min)

Phenol Elimination (5 min)

Phenol Elimination (15 min)

Phenol Elimination (20 min)

Phenol Elimination (10 min)

3.5

3

Absorption (A)

2.5

2

1.5

1

0.5

0 197

217

237

257 Wave Length (nm)

277

297

Figure 5. The phenol UV specter variation during ozonation: 0, 5, 10, 15 and 20 min.

Simulation of the Phenol, NSA and PSA Ozonation Mathematical Model We propose the modified mathematical model with a stoichiometric coefficient (ni ). It presents the system of the fife differential equations describing the ozone concentration variation in water and in gas phase as well as the chemical reaction of organic with ozone (16, 19):

dcg (t) 1 k = Wg(c0g −cg(t)) − sat (Qmax−Q(t)) dt Vgas Vgas

[1]

N dQ(t) = ksat[Qmax − Q(t)] − Q(t)∑ni kiCi (t) dt i=1

[2]

 Q (t )  dc PH (t )  = − k PH c 1PH ( t )  V  dt  liq 

n PH

 Q(t )  dc NSA (t )  = −k NSAc 1NSA (t )  Vliq  dt  

n NSA

 Q (t )  dc PSA (t )  = −k PSAc 1PSA (t )  Vliq  dt  

[3]

[4]

n PSA

[5]

where

c0g , cg (t ) are the initial and current ozone concentrations in the gas phase of the reactor (mol L-1 ); Wg is the gas flow-rate, (L s-1); V g is the volume of the gas phase, (L); is the current ozone amount in liquid phase, (mole),

Q (t )

k sat is the saturation constant of

-1

ozone in water, (s ); ci (0), ci (t) are the initial and current concentration of the organic in the reactor,(mol L-1), ki is the reaction rate constant of the organic with ozone, (L mol-1 s-1),

n i is the stoichiometric coefficient of the reaction; Q max is the maximum ozone amount in a saturation state in the liquid phase, (mol). The value of

Q max

can be calculated as follows:

Qmax = Hc 0gVliq

[6]

where −1 Vliq is the volume of the liquid phase, (L): H is Henry’s law constant, (mol Lliq / mol 1 L−gas ).

The ozone saturation constant in water ( ksat ) is the character system parameter, which can be estimated using experimental dates (without the preliminary diffusion coefficient calculation) according to the equation (15,19):

ksat =

.

Wg (c0g − c g (t )) − Vgas Qmax

dcg (t) dt e k satt

[7]

The proposed parameter k sat can characterize the ozone dissolution in any solvent (15,18,19). Its estimate depends on some parameters such as the initial and current ozone g concentration ( c 0 , c g (t ) ), the gas flow rate (Wg ), and the gas phase volume (Vgas ). Therefore, it provides the simple estimation of the constant value for each concrete experiment. In this study the constant value was equal to 6.33 x 10-5 s-1. The simulation of the ozonation was realized to find the optimal conditions to perform the decomposition of the real initial concentrations of the organic (PH and NSA of 1.30 x 10–3 mol⋅L-1, PSA of 1,25 x 10–3 mol⋅L-1 ). Results and Discussion As the simulation results of the organic ozonation, the following experimental conditions have been selected: • • • •

The initial ozone concentration of 2.7 x 10-4 mol L-1 The ozone-oxygen mixture flow of 0.5 L min-1 The reactor volumen of 0.5 L The reaction temperature was 20ºC.

The NSA spectrum for the initial step of the reaction is presented in the figure 3. It is worth to notice that above 260 nm no absorption of NSA is seen. The maximum absorbency of this specter moved from 274 nm to 260 nm during of the ozonation time. That why, the absorbency increase observed in the region of 260 nm, is most probably due to the byproduct formation. But, by the following reaction with ozone, it finally disappears. This means that for the ozone with the unsaturated by-product formation one of two aromatic rings has been broken The PSA spectrum for the initial step of the reaction and during the ozonation is presented in the figure 4. These specters have a constant reduction. In this case, the by-product formation, corresponding to (9,10), also happens, but the spectrum of by-products cannot separate. In the case of the phenol decomposition, the absorbance increasing, observed in the region of 245 nm during the ozonation, also probably take place due to the by-product formation (Fig. 5). As one can see from the figures 3 and 5, the by-products creation always takes place during the ozonation. But, by the sequent reaction with ozone they finally disappear. In the next figures the decomposition dynamics of NSA, PSA and phenol under the selected experimental conditions are represented. Figures 6, 7 and 8 show the aqueous normalized

remaining concentration of NSA, PSA and phenol, respectively, with time during the ozonation. Varying the pH of the solution from 2 to12, a series of ozonation experiments with the compounds studied (excluded phenol) has been carried out. These figures show the results of the ozonation for the organic at different pH, that justifies the assumption that organics have a relation between pH and the initial ozonation rate.

1 0.9 0.8

pH = 2.5 pH =7.5 pH=9.8 pH= 10.01 pH= 11.5

0.7

C i /C0

0.6 0.5 0.4 0.3 0.2 0.1

7000

6000

5000

4000

3000

2000

1000

0

0

( t ) seg.

Figure 6. The aqueous normalized remaining concentration of NSA with time during the ozonation to different pH

The figure 6 shows the NSA rate reaction behavior in the pH region of 2.5 to 11.5. The major rate reaction is at pH about 7. However, we cannot confirm for sure this result, since this effect can be observed only in the initial reaction time period. This effect may be masqueraded by the reaction of a by-product, which has a competition reaction with ozone. In the case of the PSA ozonation, the behavior of this process is clearer than for NSA in the pH region within 3.30 -12.00. It can be observed from the figure 7.

The experiment with phenol was realized at pH of 7. The corresponding ozonation dynamics is given in the figure 8. We can conclude that the pH effect has significantly influence under the initial rate reaction for both compounds (the ozonation time corresponds to 50% of the compound decomposition). But this dependence for both organics within the studied region of pH is quit different. As one see from the figure 9, this dependence for NSA has minimum of the ozonation time at pH of 8, and them the optimal pH range is from 7.5 to 9.

1 0.9

pH = 3.30

0.8

pH=7.05 pH=11.1

0.7

pH= 12.00

C i /C0

0.6 0.5 0.4 0.3 0.2 0.1

4500

4000

3500

3000

2500

2000

1500

1000

500

0

0

( t ) seg.

Figure 7. The aqueous normalized remaining concentration of PSA with time during the ozonation to different pH

For PSA the pH tendency is practically a line function. So, the pH increasing reduces the initial reaction rate.

Phenol elimination 1 0.9 0.8

Ci / C0

0.7 0.6 0.5 0.4 0.3 0.2 0.1 2100

1800

1500

1200

933

780

600

420

266

120

0

0

( t ) Seg

Figure 8. The aqueous normalized remaining concentration of phenol with time during the ozonation at pH of 7.

2000 1800 1639

1600

1615

1536

t i [seg]

1400 1259

1200

1178

1134

1000 915 800 716 600

PSA NSA

400

554

200 0 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

pH

Figure 9. Effect of the pH variation on the initial ozonation rate of NSA and PSA.

1

0.8

0.6

% 0.4

NSA PSA PH

0.2

0 0

60

180

300

600

900

1200

1800

(seg)

Figure 10. The by-product dynamics during the NSA, PSA and phenol ozonation.

Conclusions The biological treatment of wastewater effluents of tanning production plants, being applied alone, is a non-viable technology if take into account the high concentration of the organics such as 2- naphthalenesulfonic acid and p-phenolsulfonic acid. As is it shown in this paper, the ozonation of phenol and naphthalene derivatives can be considered as a feasible complementary technology to biological treatment significantly improving this process. Ozonation of the compounds mentioned above is carried out direct reactions with ozone with the by-products formation. It is important to emphasize that in the considered case the stoichiometric coefficients of the reaction are more than 1 (around 3-4). The mathematical model of the ozonation considered in this paper includes several parameters and one of them is the saturation constant (k sat ) which can be calculated directly using the measurements of the current ozone concentration in the gas phase (without the preliminary diffusion coefficient estimation) for any concrete experimental

conditions. In this study the constant value is. Applying the MATLAB 5.2 – SIMULINK software and based on the calculation scheme developed in (15,19), this constant was found to be equal to 6.33 x 10-5 s-1. The important observation, which can be done from the kinetic study, is as follows: there is the dependence of the ozone reactivity to pH for different organics. So, this dependence is, practically, lineal for PSA, but for NSA the pH influence is more essential compared to PSA and has the extreme character with the maximal initial reaction rate at pH of 8.

Acknowledgement The authors thank the DEPI of the National Polytechnic Institute of Mexico for the economic support (Research Project DEPI 970124).

References

1. Garza, G.T. “Tratamiento de Efluentes de la Industria de la Curtiduría”, Reunión Conjunta en León Guanajuato, Sociedad Mexicana de Aguas, A. C., IMTA. 165- 207 (1985). 2. Campos Merino. “Experiencias sobre la Operación y Control de una Planta de Tratamiento de Aguas Residuales de una Tenería”, IPN-IMTA-CENCA. 55-57 (1987). 3. Beltrán, F.J., Garcia-Araya, J.F., Rivas, F.J., Álvarez, P. and Rodrígez E. “Kinetics of Competitive Ozonation of Some Phenolic Compounds Present in Wastewater from Food Processing Industries”. Ozone Sci. Eng. 22:167-183 (2000). 4. Beltrán, F.J., G. Ovejero, J.F., Garcia-Araya and J. Rivas. “Oxidation of polynuclear aromatic hydrocarbons in water. 2 UV radiation and ozonation in the presence of UV radiation”, Ind. Eng. Chem. Res. 34:1067-1615 (1995). 5. Beltrán, F.J., J.F. Garcia-Araya and P. Alvarez. “Impact of chemical oxidation on biological treatment of a primary municipal wastewater. 2 Effects of ozonation on kinetics of biological oxidation”, Ozone Sci. Eng. 19:513-526 (1997). 6. Gurol, M.D., and Singer, P.C. “Dynamics of the ozonation of phenol -II. Mathematical simulation”. Water Res. 17(9): 1173-1181 (1983). 7. Gurol, M.D. and S. Nekouinaini. “ Kinetic Behavior of ozone in aqueous solutions of substituted phenols”, Ind. Eng. Chem. Fundam. 23: 54-60 (1984). 8. Gould J.P. & Weber W. J. “Oxidation of phenols by ozone”. J. Wat. Pollut. Control Fed. 48, 47-60 (1976). 9. Razumovskii, S.D. “Reaction of Ozone with Phenols”. Neftekh. 12: 877-880 (1971). 10. Razumovskii, S.D. and Zaikov, G.E., “Ozone and it Reactions with Organic Compounds” (Amsterdam – Oxford – New York – Tokyo: Elsevier, 1984), p. 272. 11. Li, K.Y., Kuo, C.H. and Weeks, J.L. Jr. “A kinetic study of ozone-phenol reaction in aqueous solutions”. AlChE J., 25:583-591 (1979).

12. Hautaniemi, M., Kallas, J., Mumter, R., Trapido, M. and Laari, A. “Modeling of Chlorophenol Treatment in Aqueous Solutions. “. Ozonation under Basic Conditions”. Ozone Sci. Eng. 20:283-302 (1998). 13. Griffini, O., Bao, M., Bariere, K. , Burrini, D. , Santianni, D. and Pantani, F. “Formation and Removal of Biodegradable Ozonation By- Products during Ozonation – Biofiltration Treatment: Pilot Scale Evaluation”. Ozone Sci. Eng. 21:79-98 (1999). 14. Yordanov, R., Melvin, M., Law, S., Little John J. and Lamb, A. “Effect of Ozone PreTreatment of Colored Upland Water on Some Biological Parameters of sand Filters”. Ozone Sci. & Eng. 21:615-628 (1999). 15. Poznyak T.I., Vivero Escoto J.L.. “Modeling and Optimization of Ozone Mass Transfer in Semibatch Reactor”, Proceedings of the International Specialized Symposium of IOA/ EA3 G “Fundamental and Engineering Concepts for Ozone Reactor Design”, March 1-3, 2000, Toulouse, France, pp.133-136. 16. Poznyak, T. and Vivero Escoto, J.L. “Simulation and Optimization of Phenol and Chlorophenols Elimination from Wastewater”, Proceedings of the IOA PAG Conference, Vancouver, Canada, 18-21 October, 1998, pp. 615 – 628. 17. Poznyak, T.I. and Poznyak, A.S. “Application of dynamic Neural Networks for Parametric Identification of Ozonation Processes”, Proceedings of the 14th Ozone World Congress, Deaborn, Michigan, USA 22-26 August 1999, vol. 2, pp. 215-223. 18. Lisitsin, D.M., Poznyak, T.I. and Razumovski, D.M. “Mathematical modelling of the reaction of ozone with some organic compounds in a continuous-operation bubling reactor”. Kinet. Katal. 17:1049-1056 (1976). 19. Poznyak, T. and Vivero Escoto, J.L. “Kinetic Study of the Phenol and Chlorophenols Ozonation: The Phenols Elimination Optimization ”, Book of Abstracts of 3-rd Int. Simposium of the Ozone Applications, Havana, Cuba, 27-30 June, 2000.

Key Words Ozone; Ozonation Kinetics; 2- Naphthalenesulfonic Acid; p-Phenolsulfonic Acid; Phenol; Saturation Constante Estimation; Tanning Production Plants; Industrial Wastewater.