HETEROGENEOUS CATALYTIC OXIDATION of

HETEROGENEOUS CATALYTIC OXIDATION of PHENOL for WASTEWATER TREATMENT USING RUTHENIUM CATALYST 1,2

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Syaifullah Muhammad , Pradeep R. Shukla , Shaobin Wang , Moses O. Tadé

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Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, WA 6845, Australia 2 Department of Chemical Engineering, Syiah Kuala University, Banda Aceh, Indonesia Email: [email protected] ABSTRACT Heterogeneous catalysts of ruthenium were prepared by impregnation on activated carbon and ZSM-5 and were used to degrade phenol in the presence of peroxymonosulphate (PMS). The ruthenium based catalysts were characterised by several techniques such as XRD, SEM, and N2 adsorption. It was found that Ru-AC is highly effective in heterogeneous activation of peroxymonosulphate to produce sulphate radicals resulting in higher reaction rate compared with Ru-ZSM5. Degradation efficiency of phenol follows the order of Ru-AC > Ru-ZSM5 at all reaction conditions due to higher surface area and pore volume of Ru-AC. Degradation efficiency of phenol could be achieved at 100% of phenol degradation and 70% of TOC removal in 1 hour. It was also found that phenol degradation is strongly influenced by amount of catalyst loading, phenol concentration, oxidant concentration and temperature. Kinetic study proved that the pseudo firts order kinetics would fit to phenol oxidation. Key words : Heterogeneous catalyst, impregnation, sulphate radical and phenol degradation

Introduction Phenol is one of wastewater pollutants needed to be removed due to its effect on the environment even in small concentration. Phenol is widely used as a raw material in many industries such as petrochemical, chemical and pharmaceutical industries (Fortuny et al, 1998). Moreover, phenol is a water pollutant which can not be degraded with primary and secondary treatment process so that a tertiary treatment of wastewater becomes important to be adopted. These tertiary treatments include thermal oxidation, chemical oxidation, wet air oxidation, catalytic oxidation including heterogeneous oxidation etc. which are generally known as Advance Oxidation Processes. In principle, a tertiary treatment process is to reduce the contaminants to harmless products such as CO2 and H2O (Chiron et al, 2000). Among the methods, heterogeneouos catalytic oxidation usually has some advantages such as operation at room temperature and normal pressure and high energy efficiency. Further, heterogeneous catalysts can be synthesised by using cheap material supports such as activated carbon, zeolite, silica, alumina etc., and can be regenerated for reuseable treatment process (Camporro et al, 1994). The most popular method to degrade organic compounds in wastewater is Fenton reagent which involves of hydrogen peroxide and Fe ion to generate hydroxyl radical in the solution (Wang, 2008). Nowday, this principle has been developed with many other oxidants such as peroxymonosulphate and persulphate which are found effective in chemically mineralising various organic pollutants (Chan & Chu, 2009). Many researchers have proved that some

Syaifullah Muhammad, Pradeep R. Shukla, Shaobin Wang and Moses O. Tadé

heavy metals such as cobalt can activate peroxymonosulphate (PMS) to produce sulphate radical for oxidation of organic pollutants to harmless end products. The following reactions show the formation of sulphate radicals (Anipsitakis et al, 2003). (1) Co2+ + HSO5− → Co3+ + SO4−• + OH− − −• 3+ 2+ + Co + HSO5 → Co + SO5 + H (2) It was reported, in comparison with conventional Fenton reagent, the rate of organic oxidation by sulphate radical is very reasonable to be implemented. Moreover, the sulphate radical is less dependent on pH in the reaction, providing an alternative oxidant to degrade organic contaminants (Anipsitakis et al, 2003). However, the major problem for using heavy metal ions in wastewater as a catalyst is the toxicity caused by the presence of the heavy metals in the treatment system. It can generate many health problems such as asthma and pneumonia. Therefore, heterogeneous catalytic oxidation has to be conducted. For this purpose, the heavy metal should be loaded into many catalyst supports such as activated carbon (AC), zeolite (ZSM5), silica, alumina etc., with many methods including impregnation and ion exchange. Ruthenium (Ru) is one of the popular noble metals which are usually used as a catalyst in chemical degradation of organic compounds. Pirkanniemi and Sillanp (2002) reported that Ruthenium has traditionally been used as a heterogeneous catalyst. Cavallo et al. (2007) used Ruthenium metal for heterogeneous hydrogenation of substituted phenols over Al2O3 with very good activity. Oliviero et al. (2000) used activated carbon supported Ru as a catalyst in catalytic wet air oxidation of phenol and acrylic acid. It was found that the catalyst was very reactive to oxidise phenol. Further, Cybulski and Trawczyński (2004) studied ruthenium catalyst loaded on carbon black in catalytic wet air oxidation to degrade phenol solution, and also concluded that this catalyst was very reactive for phenol removal. A similar study in mineralising organic contaminants by using ruthenium based catalyst also has been done by Kowalczyk et al. (1996), Liu et al. (2010) and Gallezot et al. (1997). All of them reported that this heavy metal had very good performance. However, the use of ruthenium based catalysts with the presence of pereoximonosulphate (PMS) to generate sulphate radicals for phenol oxidation is less developed.

This research is investigating the use of ruthenium based catalysts supported on activated carbon (AC) and ZSM5 (zeolite) by impregnation in heterogeneous catalytic oxidation process with the presence of peroxymonosulphte (oxone) as an oxidant to generate sulphate radical for chemical mineralising of phenol in the solution. Several key parameters in the kinetic study such as phenol concentration, catalyst loading, oxone concentration and temperature, were also investigated. Experimental Synthesis of Ruthenium impregnated activated carbon and ZSM-5

Catalyst synthesis was carried out following the general procedure reported by Shukla et al. (2010). Ruthenium (Ru)-Activated Carbon (AC) was synthesised using an impregnation method. At first, a fix amount of Ruthenium Chlorate (Sigma-Aldrich) was added into 200 ml ultrapure water until the ruthenium compound was dissolved. Next, AC (Picactif) with particle size of 60-100 µm was added into the solution and kept stirring for 24 hours. After

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that, the suspension was evaporated in a rotary evaporator at temperature of 500C under vacuum. The solid was then recovered and dried in an oven at 1200C for 6 hours. Calcination of catalyst was conducted in a tube furnace at 5500C for 6 hours in the presence of inert gas of nitrogen. The catalyst was stored in a desiccator until used. The same method was also implemented for Ru-ZSM5 synthesis with a different burning process. The Ru-ZSM5 was calcined in the presence of air. Characterisation of Catalyst

The synthesised catalysts were characterised by XRD, SEM and N2 adsorption. The XRD (Siemen, D501, Diffractometer) was used to identify the structural features and the mineralogy of the catalyst. The XRD pattern was obtained using filtered CuKα radiation with accererating voltage of 40 kV and cuurent of 30 mA. The samples were scanned at 2θ from 5-1000. SEM (Philips XL30) with secondary and backscatter electron detector was used to obtain a visual image of the samples to show the texture and morphology of the catalysts with magnification more than 20,000. The catalysts were also characterised by nitrogen adsorption-desorption (Autosorb-1) to identify the pore volume and surface area (BET). Prior to the analysis, the catalyst samples were degassed under vacuum at 2000C for 12 hours Kinetic study of phenol oxidation

Catalytic oxidation of phenol was conducted in 500 ml phenol solution with concentration of 25, 50, 75 and 100 ppm. A reactor attached to a stand was dipped into a water bath with temperature control. The solution was stirred constantly at 400 rpm to maintain homogeneous solution. Next, a fixed amount of oxidant of peroxymonosulphate (oxone, DuPont’s triple salt 2KHSO5.KHSO4.K2SO4, Aldrich) was added to the mixture until completely dissolved. Then, a fixed amount of catalysts (Ru-AC or Ru-ZSM5) was added into the reactor for starting the oxidation of phenol. The reaction was run for 2 hours and at the fixed interval time, 0.5 ml of sample was witdrawn from the solution and filtered using HPLC standard filter of 0.45 µm and mixed with 0.5 ml methanol as a quenching reagent to stop the reaction. Phenol was analysed on a HPLC with a UV detector at wavelength of 270 nm. The column is C18 with mobile phase of 80% acenonitrile and 20% ultrapure water. For some selected samples, total organic content (TOC) was determined by a Shimadzu TOX5000 CE analyser where 0.5 ml sample was diluted to 20 ml by ultrapure water and examined within 1 hour. Result and discussion Characterisation of ruthenium impregnated activated carbon and ZSM-5 catalysts

XRD patterns of Ru-AC and Ru-ZSM5 are presented in Figure 1. It can be seen, ruthenium species were found in the form of RuO2 on Ru-ZSM5 at 2θ coordinate of 28, 35, 40 dan 54.30. On the other hand, ruthenium oxide was found at 2θ angle of 280 and Ru at 38.4, 42.2, 44 and 69.40 on Ru-AC. The differences of ruthenium species was produced from different calcination processes. For Ru-ZSM5, the calcinations were done in air. Whereas the calcination of Ru-AC was carried in nitrogen gas. Ru ion would be reduced by carbon in inert gas to form Ru metal.

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Figure 1. XRD Spectra of Ru-ZSM5 and Ru-AC

Figure 2. SEM Image of Ru-AC, (A) SE Detector and (B) BSE Detector The texture and morphology of Ru-AC is shown in Figure 2. Fig.2A shows SEM image of the sample analysed by secondary electron detector (SE). It can be seen that the milled sample has different particle shape and size in a range of 5-60 µm. At the same area, the catalyst sample was also analysed using a backscattered detector (BSE) as shown in Fig. 2B. The presence of ruthenium specks is seen at the brighter area in the catalyst particles. It implies that ruthenium is well coated in the activated carbon samples. Further, in the SEM images, it was not observed some individual small bright particles spread out around the AC particles. It means that the assimilated ruthenium in the AC is not in the form of RuO2. It is also confirmed by XRD examination that Ru is the major species in the catalyst sample. The catalyst samples were also characterised by N2 adsorption to identify pore size distribution and surface area (BET). As seen in Table 1, Ru-AC (1177.8 m2/g) catalys has higher surface area than Ru-ZSM5 (386.0 m2/g). The higher surface area of Ru-AC than RuZSM5 can also be predicted by XRD pattern above. From XRD pattern, it is seen that RuAC sample has an amorphous character exhibiting higher surface area compared to RuZSM5. It is well known that the more amorphous of material structure, the higher surface area of the material is (Pradhan and Parida, 2010). Ru-AC (0.108 cc/g) also has higher pore volume than Ru-ZSM5 (0.085 cc/g). However, both Ru-AC and Ru-ZSM5 have similar pore radius of 15.6 Å and 15.7 Å. Both catalysts have the pore radius less than 20 Å, which means they are microporous materials (Mintova, 2003). Detail of pore size distribution and N2 isotherm analysis can be seen in Figure 3. 4


Tabel 1. Surface area, pore volume and pore radius of Ru-AC and Ru-ZSM5 Catalyst Ru-AC Ru-ZSM5

Surface Area (BET)(m2/g) 1177.8 386.0

Pore Volume (cc/g) 0.108 0.085

Pore Radius (Å) 15.6 15.7

Figure 3. Pore size distribution and N2 isoterm of Ru-AC and Ru-ZSM5 calc. at 5500C. Preliminary study of phenol oxidation

Preliminary tests including adsorption and phenol degradation from aqueous solution on RuAC and Ru-ZSM5 are presented in Figure 4. The test conditions are 0.2 g catalyst loading, 1 g oxone in 500 ml phenol solution of 50 ppm, temperature of 250C and stirring speed of 400 rpm. Generally, all the samples of AC, ZSM5, Ru-AC and Ru-ZSM5 can adsorp phenol compound despite at low efficiency. Among them, AC has the highest efficiency in adsorption of phenol with 34% removal in 2 hours prior to equilibrium. Lower adsorption efficiency of 10% for 2 hours can be seen for ZSM5. AC has much higher surface area and pore volume than ZSM5, resulting in higher adsorption.

Figure 4. Preliminari test of catalytic phenol oxidation The adsorption performance of AC and ZSM5 was decreased when the materials were loaded by heavy metal of Ruthenium. The phenol removal efficiency of Ru-AC is 27% and 5


Ru-ZSM5 is 6% in 2 hours. The decrease in removal efficiency of both catalysts is caused by the decrease of surface area and pore volume when ruthenium covered the catalyst surfaces. In oxidation tests, addition of oxone (PMS) without a catalyst did not make oxidation reaction occur. AC-oxone could degrade phenol up to 48% and ZSM5-oxone could decrease phenol concentration of 38% in 2 hours. These results indicate that both catalysts can slightly activate oxone to generate sulphate radicals (SO5-•) for removing phenol from solution. The AC-oxone system could increase removal efficiency of 14% and ZSM5-oxone by 28%. Comparing both systems, it seems, ZSM5 has better performance to activate oxone to produce sulphate radical than AC-oxone. This is probably caused by the presence of alkali metal contained in ZSM5 such as sodium. The best result for phenol removal is that the catalysts (Ru-AC and Ru-ZSM5) and oxidant (PMS/oxone) are presented in the process. In comparison between Ru-AC-Oxone and RuAC-Oxone systems in phenol oxidation, the Ru-AC-Oxone gived better result which can completely remove phenol in 20 minutes while Ru-ZSM5-Oxone can completely remove phenol in 50 minutes. Based on the results, both catalysts can activate Oxone to produce sulpate radical. According to XRD examination, the major species of ruthenium assimilated into AC is Ru and only small amount of RuO2 contained in Ru-AC sample.

Figure 5. Phenol removal in multiple use of (A) Ru-AC and (B) Ru-ZSM5 at 50 ppm, at 1 g oxone, 0.2 g catalyst, 250C Meanwhile, the ruthenium species in ZSM5 is RuO2. Thus, it is believed that they are the active sites for activation of peroxymonosulphate (PMS) to sulphate radical in phenol oxidation system. This research confirms that supporting noble metal on metal oxide or activated carbon surface will increase activity the catalysts as reported by many researchers ((Matatov-Meytal and Sheintuch, 1998). TOC removal in Ru-AC-Oxone and Ru-AC-Oxone systems was also examined and the results showed that about 70% and 60% of TOC removal were obtained for Ru-AC-Oxone and Ru-ZSM5-Oxone, repectively within 1 hour. Both the catalysts were also regenerated for multiple use. It can be seen in Fig. 5, catalyst deactivation occurred in the second and third runs, however, the deactivation is not significant. Complete removal of phenol could still be achieved within 1 h for Ru-AC and 2 hours for Ru-ZSM5. The deactivation occurs presumably because there are a small portion of loose ruthenium metal (leaching) from the support of AC and ZSM5.

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Effects of reaction parameters on phenol removal

The first parameter measured in this study was phenol concentration in 25 - 100 ppm, as shown in Figure 6. Removal efficiency of phenol decreases with increasing phenol concentration. At phenol concentrations of 25 and 50 ppm, 100% removal of phenol is achieved within 1 hour. For 75 ppm, complete phenol removal achieved at time over 90 minutes and for 100 ppm phenol, removal efficiency obtained after 2 hours is about 83%. A similar trend can also be seen in Fig. 6B using Ru-ZSM5. For phenol concentration of 25 ppm, complete removal occurs within 2 hours, but for phenol concentration of 100 ppm, within 2 hours, removal efficiency is only 52%. Meanwhile, for phenol concentration of 50 and 75 ppm, efficiency of 90% and 70% can be reached in 2 hours, respectively.

Figure 6. Phenol removal at different oncentration of phenol (A) Ru-AC and (B) Ru-ZSM5 at 1 g oxone, 0.2 g catalyst, 250C For the reaction kinetics, a general equation of the pseudo first order kinetics was used, as shown in the following equation. (3) Where k is the first order rate constant of phenol removal, C is the concentration of phenol at various time, Co is the initial concentration of phenol. By integrating the equation above, the profile decrease in phenol concentration can be further elaborated in the following equation (Shukla et al. 2010). (4) From data fitting, it is obtained that this reaction can be represented by the pseudo first order kinetics. This can be validated from the values of R2, which are above 0.9 as shown in Table 2. The rate constants obtained at various concentrations of phenol are shown in Table 2.The rate constant (k) shows that in general the value of k for Ru-AC is higher than Ru-ZSM5, which means the AC is able to degrade phenol more rapidly. Table 2. The rate constant at various concentrations of phenol Catalyst Ru-AC

25 ppm R2 k 0.9289 0.998

Phenol Consentration 50 ppm 75 ppm R2 R2 k k 0.2706 0.9798 0.0306 0.967

100 ppm R2 k 0.0146 0.9879

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Ru-ZSM5

0.0495

0.9416

0.0482

0.9462

0.0067

0.9092

0.0057

0.9071

Figure 7 shows the effect of catalyst loading on phenol degradation. The greater of the amount of catalyst used, the higher of phenol reduction efficiency is. At 0.2 g Ru-AC (0.4 g/L), complete removal can be achieved within approximately 20 minutes. At Ru-AC 0.1 g (0.2 g/L) and 0.05g (0.1 g/L) complete removal was achived in about 60 minutes. From the data it can be concluded that the increased amount of catalyst from 0.1 g to 0.2 g will increase removal efficiency about 3 times. This phenomenon is reasonable, because increasing the amount of catalyst will increase the adsorption and also the availability of catalyst sites to activate oxone. Therefore, the addition of catalysts will increase reaction rate significantly. The same trend also occurred with Ru-ZSM5, as shown in Figure 7B. At the amount of 0.2 g and 0.3 g of Ru-ZSM5, complete removal could happen in 60 minutes. Instead Ru-ZSM5 under 0.2 g in solution did not produce a satisfactory removal of phenol. The removal efficiencies were only 40% and 20% in 2 hours for Ru-ZSM5 at 0.15 g and 0.1 g respectively. In this case, Ru-ZSM5 of 0.2 g (0.4 g/L) is optimal loading.

Figure 7. Effect of catalyst loading on phenol removal, (A) Ru-AC and (B) Ru-ZSM5 Figure 8A (Ru-AC) shows that increased concentration of oxone in a solution will accelerate phenol removal significantly. For example, at 0.5 g oxone, complete removal can be achieved in about 90 minutes. However, increasing in phenol degradation is very fast when 1 g oxone is used where the complete removal occurred within 20 minutes, an increase of phenol removal rate in 4 times high. A similar change is seen in Figure 8B for Ru-ZSM5. Complete removal was not obtained within 2 hours at 0.25 and 0.5 g oxone. In contrast at 1 g oxone in solution, complete removal can occur in about 60 minutes. The increase of reaction rate at the increased oxone concentration is caused by higher production of sulphate radical for reducing phenol. Effects of temperatures on phenol degradation are shown in Figure 9. As seen that temperature impacts quite significantly phenol oxidation process either using Ru-AC (Figure 9A) or Ru-ZSM5 (Figure 9B). The increase in temperature of 100 will enhance the reaction rate in average of two times. For example, the complete removal of phenol with Ru-AC at the temperature of 250C is achieved in about 20 minutes, and when the temperature was raised to 350C, complete removal can be achieved in about 10 minutes. Similarly, at temperatures of 450C, complete removal can be achieved in about 5 minutes. The same trend also occurred on Ru-ZSM5 as shown in Figure 9B. 8


Figure 8. Effect of oxone concentration on phenol removal, (A) Ru-AC and (B) Ru-ZSM5

Figure 9. Effect of temperature on phenol removal, (A) Ru-AC and (B) Ru-ZSM5 Conclusion This research proves that Ru-AC and Ru-ZSM5 are effective catalysts for degrading phenol in the presence of sulphate radical. Ru-AC has better ability of removing phenols than RuZSM5. Phenol removal is a combination of oxidation and adsorption. Both catalysts also showed good performance in the second and third runs. Therefore, it is quite promising for multiple use as heterogeneous catalysts for organic compound removal from wastewater. This research also confirmed that the concentration of phenol, catalyst loading, concentration of oxidant (oxone) and temperature are important parameters that affect the reaction rate in removing phenol. Kinetic studies show that phenol oxidation on the catalyst Ru-AC or RuZSM5 follows the first order reaction. REFERENCES Anipsitakis G.P., Dionysiou D.D. 2003, Degradation of Organic Contaminants in Water with Sulfate Radicals Generated by the Conjunction of Peroxymonosulfate with Cobalt, Environmental Science and Technonology 37, pp. 4790–4797. Camporro A., Camporro M.J., Coca J., Sastre H. 1994, Regeneration of an activated carbon bed exhausted by industrial phenolic wastewater, Journal of Hazardous Material 37, pp. 207-214.

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Cavallo-Solladié A., Barama A., Choucair E., Norouzi-Arasi H’, Schmitt M., Garin F. 2007, Heterogeneous hydrogenation of substituted phenols overAl2O3 supported ruthenium, Journal of Molecular Catalysis A: Chemical 273, pp. 92–98. Chan K.H., Chu W. 2009, Degradation of atrazine by cobalt-mediated activation of peroxymonosulfate: Different cobalt counteranions in homogenous process and cobalt oxide catalysts in photolytic heterogeneous process, Water Research 43, pp. 2513–2521. Chiron S., Fernandez-Alba A., Rodriguez A., Garcia-Calvo E. 2000, Pesticide chemical oxidation: state-of-the-art, Water Research 34, pp. 366–377. Cybulski A., Trawczyński J. 2004, Catalytic wet air oxidation of phenol over platinum and ruthenium catalysts, Applied Catalysis B: Environmental 47, pp. 1–13. Fortuny A., Font J., Fabregat A. 1998, Wet air oxidation of phenol using active carbon as catalyst, Journal of Applied Catalyst B: Envirenmental 19, pp. 165-173. Gallezot P., Chaumet S., Perrard A., Isnardy P. 1997, Catalytic Wet Air Oxidation of Acetic Acid on Carbon-Supported Ruthenium Catalysts, Journal of Catalysis 168, pp. 104-109. Kowalczyk Z., Jodzis S., Sentek J. 1996, Studies on kinetics of ammonia synthesis over ruthenium catalyst supported on active carbon, Applied Catalysis A: General 138, pp. 83-91. Liu W.M., Hu Y.Q., Tu S.T. 2010, Active carbon–ceramic sphere as support of ruthenium catalysts for catalytic wet air oxidation (CWAO) of resin effluent, Journal of Hazardous Materials 179, pp. 545–551. Matatov-Meytal Y.I., Sheintuch, M. 1998, Catalytic abatement of water pollutants, Ind. Eng. Chem. Resources. 37, pp. 309-326. Mintova S. 2003, Nanosized Molecular Sieves, Journal of Chem. Society, Chem. Comm. 68, pp. 2032-2054 Oliviero L., Barbier Jr., J., Duprez D., Guerrero-Ruiz A., 2000, Catalytic wet air oxidation of phenol and acrylic acid over Ru/C and Ru–CeO2/C catalysts, Applied Catalysis B: Environmental 25, pp. 267–275. Pirkanniemi K., Sillanp M. 2002, Heterogeneous water phase Catalysis as an environmental application: a review, Chemosphere 48, pp.1047–1060. Pradhan G.K., Parida K.M. 2010, Fabrication of iron-cerium mixed oxide: an efficient photocatalyst for dye degradation, International Journal of Engineering, Science and Technology 2, pp. 53-65. Shukla P.R., Wang S., Sun H., Ang H.M., Tadé M. 2010, Activated carbon supported cobalt catalysts for advanced oxidation of organic contaminants in aqueous solution, Applied Catalysis B: Environmental 100, pp. 529–534 Wang S., 2008, A Comparative study of Fenton and Fenton-like reaction kinetics in decolourisation of wastewater, Dyes and Pigments76, pp. 714-720

BRIEF BIOGRAPHY OF PRESENTER Syaifullah Muhammad is Lecturer at Chemical Engineering Department, Syiah Kuala University (Unsyiah) Banda Aceh Indonesia and Ph.D Student at Chemical Engineering Department, Curtin University Western Australia.

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