Preparation and characterization of a biochar from pistachio hull biomass and its catalytic potential for ozonation of water recalcitrant contaminants

Bioresource Technology 119 (2012) 66–71

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Preparation and characterization of a biochar from pistachio hull biomass and its catalytic potential for ozonation of water recalcitrant contaminants Gholamreza Moussavi ⇑, Rasoul Khosravi Department of Environmental Health Engineering, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran

h i g h l i g h t s " This work demonstrates the preparation of a biochar from pistachio hull biomass. " The prepared catalyst was very active in catalyzing the dye ozonation process. " The COP mineralized RR198 to a much greater extent than does single ozonation.

a r t i c l e

i n f o

Article history: Received 5 March 2012 Received in revised form 19 May 2012 Accepted 22 May 2012 Available online 29 May 2012 Keywords: Biochar Ozonation Catalyst Decolorization Mineralization

a b s t r a c t This work introduces a biochar as novel catalyst prepared from the pistachio hull, and demonstrates its catalytic potential for degrading the reactive red 198 (RR198) dye in catalytic ozonation processes (COPs). The prepared pistachio hull biochar (PHB) was a macroporous, basic material with low specific surface area. PHB had the greatest catalytic potential at an optimal alkaline pH of 10. Significant catalytic potential was observed when PHB was added to the ozonation reactor; a 58.4% catalytic potential was obtained in the decolorization of RR198 in the COP with 0.2 g of catalyst after a reaction time of 60 min. A 71% mineralization (TOC reduction) of the dye solution was observed in the COP after a reaction time of 60 min. Overall, it can be concluded from the experimental results that the PHB is a promising and affordable catalyst for use in COPs for treatment of resistant organic compounds. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Several classes of synthetic organic chemicals (SOCs), which are increasingly produced and used worldwide, are present in the industrial (chemical, petrochemical, pharmaceutical, textile, etc.) effluents and municipal wastewaters that end up in receiving waters. Many of the SOCs are toxic to human and aquatic life (Crittenden et al., 2005). Moreover, most of these SOCs are biorecalcitrant and resistant to biodegradation, and they may be toxic to microorganisms. This means that conventional biological treatment systems are inefficient in removing recalcitrants. To exploit the unique feature of biological processes for treating biorecalcitrants, the biological processes must be supplemented with a process capable of efficiently degrading these compounds into simple and biodegradable intermediates (Tabrizi and Mehrvar, 2004). Advanced oxidation processes (AOPs) are among the most attractive and promising techniques considered for the degradation of biorecalcitrant compounds. Many studies are available ⇑ Corresponding author. Tel.: +98 21 82883827; fax: +98 21 82883825. E-mail address: [email protected] (G. Moussavi). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.05.101

regarding the successful use of AOPs for the degradation and mineralization of different classes of organic compounds (e.g., Klamerth et al., 2010; Byun et al., 2011; Méndez-Arriaga et al., 2011). Among the AOPs, those associated with ozone are more attractive than UV-based processes for full-scale wastewater treatment because there is less interference from turbidity and colorant with O3 than with UV. A recent ozone-based AOP is known as the catalytic ozonation process (COP). In the COP, a solid material is added to the ozonation process as a catalyst for the decomposition of O3 and thereby generates reactive radicals (Lv et al., 2010). These radicals result in more degradation and mineralization of the organic contaminants compared to single ozonation (Moussavi et al., 2010). Based on the type of catalyst used, the COPs are divided into homogeneous and heterogeneous processes. The heterogeneous COPs have a higher efficacy for the degradation and mineralization of recalcitrants, and the catalyst can be separated more easily at the end of the reaction resulting in lower residuals in the treated stream (Yang et al., 2010). According to the literature, which has been recently reviewed by Nawrocki and Kasprzyk-Hordern (2010) the catalytic effectiveness of several types of materials, including activated carbon, me-

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tal ions, metal oxides, metal or metal oxides on supports, and natural minerals, have been investigated. Nevertheless, the main challenges in this field are the complexity and high cost of production, and the leaching of metals into the liquid phase (for metal and metal supported catalysts). These challenges technically and economically limit their full-scale application. Therefore, the main concern facing COPs is the development of a more active catalyst with a simple and low-cost production method. This type of catalyst would lead to a higher fraction of ozone transferred and decomposed in the reactor and enhance the rate of degradation of the target contaminant(s) at a lower cost. Therefore, research is ongoing to find novel materials with the high catalytic activities that are also easy and cheap to produce. Accordingly, the present work focuses on preparing biochar from agricultural waste materials, to investigate its catalytic activity in a COP and efficiency for degrading a model biorecalcitrant. The reactive red 198 (RR198) azo dye was selected as the model SOC to examine the catalytic activity of the prepared biochar. Azo dyes, a main class of synthetic dyes used to colorize various products, are complex organic compounds that are toxic and resistant in the environment (Brown and DeVito, 1993). Indeed, the presence of azo group(s) bound to aromatic rings make azo dyes a very complex and biorecalcitrant class of synthetic compounds (Dong et al., 2007); therefore, they are an appropriate candidate to examine the catalytic activity of pistachio hull biochar (PHB) and its ability to degrade recalcitrant compounds in the COP. We first focus on the preparation and characterization of a biochar from the pistachio hull. Subsequently, the catalytic activity of the prepared PHB was investigated as a heterogeneous catalyst for the ozonation of RR198. 2. Methods 2.1. Preparation and characterization of catalyst The PHB was prepared from pistachio hull waste materials as follows. First, the pistachio hull waste materials, collected from a local farm in Iran, were air-dried for three consecutive days and then powdered (mesh 200) using a grinder. The prepared powder was then transferred to an oven at a temperature of 500 °C under air and kept at this temperature for 2 h. Thereafter, the oven was turned off and allowed to cool. The resulting black powder, the PHB, was stored in a capped-glass container for use as a catalyst as needed. The prepared PHB was characterized for its surface functional groups, specific surface area, and pore size.

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procedure was used in the single ozonation experiments as required, except that no catalyst was added to the solution; for the adsorption experiments, a similar procedure was used, except that no ozone injected to the solution. Furthermore, the catalytic ozonation of the RR198 with standard activated carbon (Merck Co.) powder (mesh 200), an established catalyst (Nawrocki and Kasprzyk-Hordern, 2010), was conducted in the COP to highlight the potential catalytic activity of the prepared catalyst. According to the supplier, the activated carbon had a specific surface area of 920 m2/g, pore volume and size of 0.42 cm3/g and 1.8 nm, respectively, and pHpzc determined to be 8.9. All experiments were conducted at room temperature (25 ± 2 °C). All chemicals used in this study were of analytical grade and used as received. All solutions were prepared with distilled water. The effect of the solution pH (3–10) and PHB dose (1–5 g/L) on the decolorization of RR198 by the PHB catalyst in the COP was investigated. The overall reaction kinetics of RR198 decolorization was determined by fitting the experimental data with the pseudo-first order reaction model with respect to the dye concentration. 2.3. Analytical methods The pH of point of zero charge (pHpzc) of the PHB was determined by the procedure detailed by Altenor et al. (2009). The surface functional groups of PHB were determined using Fourier transform infrared spectroscopy (FTIR-KBr) at wave numbers ranging from 400 to 4000 cm1 with cm1 resolution. The BET specific surface area and the pore size of the PHB was determined using the N2 adsorption/desorption method at 196 °C. The decolorization of the RR198 in the solution samples was analyzed by measuring the light absorbance of the sample at a wavelength of 518 nm using a Unico-UV 2100 UV–Vis spectrophotometer. The concentration was then calculated from a calibration curve. It should be noted that the amount of dye retained in the filter was determined to be ignorable. The degree of RR198 mineralization was evaluated by measuring the total organic carbon (TOC) using a Shimadzu Co. TOC analyzer. The pH was measured using a specific electrode (Jenway 3505 pH meter). The concentration of ozone in the gas stream was determined by sparging air into a 2% KI solution and analyzing the solution using iodometric titration (APHA, 2005). The concentration of ozone in the water (in the absence of dye) that was used for the investigation of ozone decomposition was measured by direct UV absorbance at a wavelength of 258 nm (Langlais et al., 1991). The concentration of ozone in the dye solution was measured by the Indigo reagent method described in Standard Methods (APHA, 2005).

2.2. Catalytic ozonation experiments 3. Results and discussion The catalytic ozonation experiments were conducted in a labscale glass reactor, equipped with a sintered-glass diffuser at the bottom for diffusing ozone into the solution and an ozone-destruction system (KI solution) to destruct the ozone in the off-gas of the reactor. The ozone was generated in a commercial ozone-generator (Arda Co.) fed with clean compressed air. The ozone was regulated at a fixed mass flow rate of 1.5 mg/min throughout the study. For each catalytic ozonation test, a 100-mL solution of RR198 with a concentration 100 mg/L was transferred into the reactor, the pH was adjusted to the desired value using 0.1 N HCl and NaOH solutions, the required mass of catalyst was added to the solution, and the ozonation was started. To completely mix the reactants, the contents of the reactor were stirred with a magnetic stirrer. The ozonation of the prepared suspension was carried out for a specified time. At the end of each test, the suspension was filtered through paper filter (0.2 lm pore size) to separate the catalyst, and the filtrate was analyzed for residual RR198. A similar

3.1. Characteristics of the prepared catalyst The pHpzc of the prepared powder was determined from the titration curve to be approximately 11.5, demonstrating that the PHB has a strongly basic surface. This basic surface makes PHB a strong ozonation catalyst (Moussavi et al., 2010), allowing it to induce ozone decomposition and generation of reactive radical species. The specific BET surface area and the total pore volume (at P/Po = 0.990) of the fresh PHB were 2.85 m2/g and 0.12 cm3/g, respectively. The mean diameter of the pores in the fresh catalyst was found to be 168.4 nm (macroporous powder). It is clear from these data that the ozonation process changed the texture of the PHB; the specific surface area of the pores increased, and total volume and the average diameter of the pores decreased significantly. Qu et al. (2007) also found that the number of micropores increased when activated carbon fiber was ozonated, resulting in

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an increase in surface area. However, in another study, the specific surface area of commercial activated carbon slightly decreased, and the volume of the pores was unchanged when it was ozonated (Sánchez-Polo et al., 2005). It is inferred, therefore, that the influence of ozone on the structure of a carbon-based catalyst depends on its origin and nature. The FTIR spectra of fresh and used PHB catalyst were determined. The FTIR spectra of fresh PHB demonstrated the presence of a number of important functional groups. A wide absorption band at wave numbers between 3000 and 3500 cm1 (with a maximum at 3210 cm1), which is representative of free and bonded hydroxyl groups (Garg et al., 2007; Nunes et al., 2009) bonded to the surface of fresh PHB, was observed. Moreover, a broad absorption band at wave numbers ranging from 1300 to 1500 cm1 was observed in the spectra of fresh PHB, indicating that phenolic groups (Garg et al., 2007) are also present on the surface of PHB. Absorption at wave numbers of 1130 and 2350 cm1 indicates the presence of a surface –OCH3 group and C@O bond, respectively (Garg et al., 2007). Absorption at wave numbers of 700 and 880 cm1 are attributed to C–H bending vibrations (Wahab et al., 2010). Accordingly, the hydroxyl and phenolic groups are the main functional groups on the surface of fresh PHB, imparting a basic character to it and explaining the high value of pHpzc. These basic groups contribute to ozone adsorption and catalyze the decomposition of ozone on the surface of the PHB, resulting in the initiation of reactive radical species generation. The FTIR spectra of the used PHB indicated a number of changes in the surface functionality of the catalyst, specifically the disappearance of the absorption peaks associated with the hydroxyl and phenolic bands (1300– 1500 cm1). This finding confirms that the hydroxyl and phenolic surface functional groups were the most active in catalyzing the ozonation.

3.2. Effect of solution pH and catalytic potential of PHB The effect of the initial solution pH in the range of 3–10 on the decolorization of the RR198 by PHB in the COP was investigated under conditions of PHB dose of 2 g/L, and reaction time of 10 min. Fig. 1 presents the decolorization of RR198 in the COP as a function of initial pH, showing a considerable effect of solution pH on the dye decolorization rate. As observed in Fig. 1, the rate of dye decolorization increased from 65% at a pH of 3 almost linearly to 84% at a pH of 10 during a 10-min reaction time under the selected conditions. The increase in the RR198 decolorization as a

function of pH can be attributed to the influence of pH on the transfer of ozone from the gas to the liquid phase, on the ozone decomposition reaction and on the properties of the catalyst surface (Leitner and Fu, 2005; Valdés et al., 2009). In effect, the increase in pH enhanced the decomposition of ozone through both homogeneous (due to an increase in the quantity of the hydroxyl anions, which favor the decomposition of ozone in the solution) and heterogeneous (catalytic) reactions (Beltrán et al., 2002). These reactions resulted in improvement of the ozone mass transfer rate from gas to the liquid phase and in increase of the reactive oxidizing radical species, and therefore, a higher decolorization rate was attained. The effects of solution pH on the degradation of organics in COPs have been shown in the literature. For example, Zhao et al. (2008) observed an increase in the degradation rate of nitrobenzene in a COP with a Mn-honeycomb catalyst with an increase in the solution pH from 3 to 11. On the other hand, Martins and Quinta-Ferreira (2009) found a reduction in the mineralization of phenolic acids in a catalytic ozonation over Mn–Ce–O as a function of solution pH between 3 and 10. The decolorization of RR198 in a COP with an MgO nanocrystal was found to be optimal at an alkaline pH over 8 (Moussavi and Mahmoudi, 2009). It can be deduced, therefore, that the way in which the solution pH affects the degradation of a contaminant in the COP depends on both the type and structure of the reacting compound and on the type of the catalyst. Therefore, the optimal pH of the COP must be determined for each specific condition. The influence of solution pH on removal of RR198 (100 mg/L) by adsorption onto the PHB (2 g/L) and by single ozonation during a 10-min reaction time was also determined (Fig. 1). Maximum around 21% of the dye was removed by adsorption on the PHB at pH 10. This finding suggests that PHB is an inefficient adsorbent for the adsorption of reactive dyes, revealing that RR198 is degraded primarily by oxidation processes. The low adsorption capacity is likely related to the low specific surface area of the PHB. Additionally, the rate of RR198 decolorization in a single ozonation process (SOP) was determined; the results are included in Fig. 1. The decolorization of the dye was very low at acidic pH. Generally, ozone can directly oxidize organic molecules at a pH of approximately 2. However, the low RR198 decolorization under this pH condition might be due to the small amount of ozone dosed into the reactor (1.5 mg/min). Nonetheless, decolorization increased with an increase in the solution pH over 7 and reached 17% at a pH of 10. This pH-related increase is due to the improvement of ozone decomposition under alkaline conditions and thus generation of OH radicals (Valdés et al., 2009), which have a greater oxidation potential than ozone molecules. Overall, the results obtained in this step of the study (Fig. 1) confirm that the prepared PHB considerably enhances the degradation rate of the model recalcitrant and thus has significant catalytic potential and is promising for use in catalyzing the ozonation of recalcitrant compounds. After determining the effect of pH on the decolorization of RR198 in the COP and establishing the potential of PHB to catalyze ozonation of the target compound, we attempted to quantify this potential. To accomplish this, the catalytic potential of the PHB on the ozonation of RR198 was calculated using the following equation from the data obtained at the pH at which the greatest decolorization rate was attained:

Catalytic potential ¼ ½removal in COP  ðremoval in SOP þ adsorption on PHBÞ

Fig. 1. The effect of solution pH on the removal of RR198 in the COP, during single ozonation and during adsorption.

ð1Þ

The results in Fig. 2 demonstrate a significant catalytic potential for PHB on ozonation in the decolorization of RR198. As seen in Fig. 2, the extents of dye removal by adsorption onto PHB and by single ozonation were each less than 20%. The catalytic potential

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COP

SOP

Adsorption

Catalytic potential

100

80

Degradation of RR198 (%)

Degradation of RR198 (%)

100

60 40 20

Ozone/PHB

80

Ozone/AC Difference

60

40

20

0 0

10

20

30

40

50

60

Reaction time (min) Fig. 2. The effect of the PHB catalytic potential on the efficacy of ozonation in the decolorization of RR198.

0 0

10

20

30

40

50

60

Reaction time (min) Fig. 3. Comparison of PHB and commercial activated carbon (AC) catalytic potential in the decolorization of RR198 in the COP.

of 2 g/L of PHB increased from 17% to 58.4% with an increase in reaction time from 2 to 60 min, suggesting that PHB significantly accelerates the rate of dye oxidation and is thus an excellent catalyst to be used for catalyzing the ozonation of recalcitrant compounds. The catalytic contribution of PHB in the degradation process may arise from the presence of a high density of basic functional groups (Fig. 1), which promote the decomposition of ozone in the presence of PHB. To determine if this is the case, the decomposition of ozone was investigated in distilled water in the absence and presence of PHB (2 g/L) at the reaction time between 5 and 30 min. The results of these ozone decomposition experiments clearly showed that the rate of ozone decomposition promoted from 4.7% in the absence of PHB (SOP with distilled water) to 41.1% in the presence of PHB (COP with distilled water), confirming the catalytic role of the PHB. The pHpzc of PHB (approximately 11.5) and the pH of the solution (10) suggest that the surface of the PHB is positively charged and therefore electrophilic. Because of the nucleophilic nature of ozone, it is attracted to the functional groups on the surface of PHB, and this effect therefore initiates and promotes the catalytic decomposition of ozone (Muruganandham and Wu, 2007). It was also observed that the ozone consumption during processing of the dye solution increased at a higher rate in the COP (75.8%) than in the SOP (11.3%). This increase in ozone consumption might be caused by the interaction of the generated radical species and the ozone itself with the dye molecules, thereby enhancing the mass transfer rate of the ozone from the gas phase to liquid–solid interface. To highlight and better demonstrate the catalytic capability of the PHB catalyst, the decolorization of RR198 in the COP with the PHB catalyst was compared to the degradation with commercial activated carbon in the same reactor under similar conditions (dye concentration of 100 mg/L, pH of 10, catalyst dose of 2 g/L, and reaction time between 2 and 60 min). Activated carbon is one of the most common and efficient heterogeneous catalysts used in COPs to destroy toxic compounds (Nawrocki and Kasprzyk-Hordern, 2010). As shown in Fig. 3, a greater decolorization of RR198 occurred in the COP with PHB than with the activated carbon under the same experimental conditions. With activated carbon, the RR198 decolorization efficiency ranged from 19% to 44% with an increase in reaction time from 2 to 60 min, and with PHB, the decolorization efficiency increased from 43% to 98% at the same reaction times. In other words, the use of PHB results in a 54% greater removal than the use of activated carbon at a reaction time of 60 min. The difference in the dye removal attained using PHB and activated carbon can be related particularly to the structural and the surface characteristics of these materials. The

kinetic of decolorization rate of RR198 in the COP with either catalyst under the optimum experimental conditions was fit with a pseudo-first-order reaction (R2 = 0.996), with reaction rate constants of 6.66 and 0.86 min1 for PHB and activated carbon, respectively. The rate of RR198 decolorization in the COP with PHB was 7.7 times that with activated carbon, which suggests that the use of PHB requires a shorter time to attain the treatment goal than does activated carbon. This result may be related with the textural properties of the activated carbon used; since the activated carbon used was a microporous material (see Section 2.2), it may not possibly be suitable for catalyzing the decolorization of RR198 which is a high molecular size dye. Other properties of the catalyst surface, including the presence of functional groups, the pHpzc, and the surface porosity might also play a role in this context. This catalytic potential of the PHB suggests that it may be a novel and promising highly active catalyst with a simple production method to be used in catalyzing the degradation of recalcitrant compounds that are present in water and wastewater. Meanwhile, the capability of used catalyst in the ozonation process of RR198 was also tested. It was observed that the decolorization of the model dye decreased by around 25% in the COP when adding the used powder compared to that of fresh powder under similar operational conditions. This revealed that the PHB has little potential to be reused. However, since the PHB can be prepared from the agricultural waste (available at no cost) using a simple procedure, this is not a significant concern and does not impart its considerable catalytic features.

3.3. Mechanism of RR198 degradation The degradation of the organic molecules in a heterogeneous COP can occur either on the catalyst surface or in the bulk solution by radicals or molecular ozone (Moussavi and Mahmoudi, 2009). As shown in the previous section, PHB enhances the decomposition of ozone, resulting in acceleration of the dye decolorization rate compared to the rate in the SOP. Based on the literature (Faria et al., 2008; Valdés and Zaror, 2006), a pathway was proposed to be involved in the ozone decomposition and dye decolorization enhancement in the COP with PHB: ozone molecules are adsorbed on the surface functional groups of the PHB and subsequently are decomposed, resulting in the generation of surface radical species. To test the possible mechanism, the decolorization of RR198 (100 mg/L) was investigated in the COP with 2 g/L PHB in the presence of a well-known OH radical scavenger: tert-butanol (pH of 10

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Decolorization of RR198 (%)

and reaction time between 2 and 60 min). The results were compared with the decolorization of the dye in the COP without the tert-butanol in Fig. 4. As seen in Fig. 4a, the decolorization of the dye in the COP in the presence of tert-butanol was similar to that obtained in the absence of the scavenger under the selected conditions. The point that the decolorization rate is not affected by the presence of radical scavenger suggests that the catalyst surface reactions are dominant in the oxidation of RR198. This is an important feature of this process because the radical scavenger species that may be present in the contaminated stream will not interfere in the degradation of the target contaminant(s) using the developed COP. Although no report could be found in the literature reporting PHB as an ozone catalyst, other researchers observed a similar finding in the destruction of different types of contaminants in a COP with different catalysts (Moussavi et al., 2010; Dong et al., 2007; Martins and Quinta-Ferreira, 2009; Ma et al., 2005). To further examine this mechanism of RR198 decolorization in the COP, an experimental run was conducted with the addition of phosphate, which strongly bonds with the surface functional groups (Lv et al., 2010; Sui et al., 2010). As indicated in Fig. 4b, phosphate (5 mM) depressed the decolorization rate of RR198 by 50%, from 95% to 45% after a 30-min reaction. The kinetics of RR198 decolorization in the COP in the absence and presence of phosphate was analyzed by fitting the data obtained under the optimum experimental conditions with pseudo-first order reaction model. The pseudo-first order model was the best fit of the experimental data (R2 > 0.99). The values of the pseudo-first order reaction constants for the decolorization of RR198 in the absence dye dye (koverall ) and presence of phosphate (kPO3 ) were calculated to be 4 3.94 and 0.233 min1, respectively.

a

100

2 dye 3 dye koverall  kPO3 4 5 4  100 Heterogeneous degradation ð%Þ ¼ dye koverall

ð2Þ

Homogeneous degradation ð%Þ ¼ ½100  heterogeneous degradation ð%Þ dye koverall

ð3Þ

dye kPO3 4

and represent the pseudo-second order reaction constants for the decolorization of RR198 in the absence and presence of phosphate, respectively. Using Eqs. (2) and (3), the contributions of degradation that occurred in the solution phase and on the surface of the PHB were found to be 6% and 94%, respectively. It is therefore clear that the oxidation reactions occurring on the surface of the PHB (heterogeneous degradation) were the dominant mechanism of RR198 decolorization, reconfirming its catalytic potential. 3.4. Decolorization and mineralization of RR198 in the COP over reaction time

80 60 40 COP

COP+ 0.3 g t-But.

20 0 0

10

20 30 40 Reaction time (min)

50

60

Fig. 5 shows the degree of decolorization and mineralization (TOC reduction) of RR198 in the developed COP with 2 g/L of PHB catalyst. The model dye was approximately 82% degraded after 10 min of reaction, and the decolorization was complete (100% dye removal) after 50 min of reaction. In contrast, TOC had a slow reduction trend, particularly at the initial steps of reaction. Approximately 13% of the TOC of RR198 was removed after 20 min of reaction, at which point the dye removal was 96%. However, continuing the reaction resulted in an increase in the

b

100 Decolorization of RR198 (%)

Hence, the catalyst surface reactions, with both oxy-radicals (mainly OH radicals) that were generated from ozone decomposition (see Section 3.2) and undecomposed ozone molecules that were bonded to the surface functional groups, are likely the main mechanism involved in the oxidation of RR198 molecules (Moussavi et al., 2009a,b). In addition, the PHB provided a contact surface for the adsorption of the dye molecules and subsequent oxidation by ozone molecules or surface radical species. Moreover, some decolorization of RR198 likely occurred through oxidation in the solution bulk by molecular ozone and radical species that were released into the solution phase. Because the phosphate bonded strongly with the functional groups on the catalyst surface and thus inhibited heterogeneous oxidation, the contribution of the degradation reaction on the surface catalyst (heterogeneous) and in the solution phase (homogeneous) on the overall degradation rate was quantified using the following expressions derived from Valdés and Zaror (2006):

80 60 40 20

COP

COP/phosphate

0 0

10

20

30

Reaction time (min) Fig. 4. The effect of the OH radical scavenger tert-butanol (a) and phosphate (b) on the decolorization of RR198 in the COP.

Fig. 5. The degree of decolorization and mineralization of RR198 in the COP as a function of reaction time under optimal conditions.

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TOC removal rate to 71% after 60 min. The COP not only efficiently degraded the RR198 as a recalcitrant compound, but also attained a high degree of TOC reduction and thus mineralization of the degradation intermediates. This observation can be explained by considering the structure of RR198 which consists of an azo (–N@N–) bond, which is responsible for the dye color (chromophores), and aromatic rings (dos Santos et al., 2007). Because the destruction of azo bonds is easier than the destruction of aromatics (Wu et al., 2008), the oxidizing species first attack to the azo bonds in the structure of RR198, leading to its degradation and decolorization. Upon decolorization of the dye molecules, the degradation intermediates are then subjected to oxidation by oxidizing agents including molecular ozone and radical species, causing the subsequent decrease in TOC content (Moussavi and Mahmoudi, 2009). 4. Conclusion The PHB catalyst was prepared from pistachio hull wastes. Analysis indicated that PHB is a macroporous powder with a specific surface area of 2.85 m2/g. The findings indicated that the RR198 was mainly degraded through a series of reactions occurring on the surface of the PHB particles. Moreover, a significant degree of mineralization was observed for RR198 in the COP. Accordingly, it can be concluded that the developed PHB catalyst is an efficient and active catalyst in the decolorization and mineralization of reactive azo dyes using the catalytic ozonation technique. Acknowledgements We appreciate the Tarbiat Modares University for providing financial and instrumental support to conduct this work. References Altenor, S., Carene, B., Emmanuel, E., Lambert, J., Ehrhardt, J.J., Gaspard, S., 2009. Adsorption studies of methylene blue and phenol onto vetiver roots activated carbon prepared by chemical activation. Journal of Hazardous Materials 165, 1029–1039. APHA, AWWA, WEF, 2005. Standard Methods for the Examination of Water and Wastewater, 21st ed., American Public Health Association, Washington, DC. Beltrán, F.J., Rivas, J., Alvarez, P., Montero-de-Espinosa, R., 2002. Kinetics of heterogeneous catalytic ozone decomposition in water on an activated carbon. Ozone: Science and Engineering 24, 227–237. Brown, M.A., DeVito, S.C., 1993. Predicting azo dye toxicity. Critical Reviews in Environmental Science and Technology 23, 249–324. Byun, S., Davies, S.H., Alpatova, A.L., Corneal, L.M., Baumann, M.J., Tarabara, V.V., Masten, S.J., 2011. Mn oxide coated catalytic membranes for a hybrid ozonation–membrane filtration: comparison of Ti, Fe and Mn oxide coated membranes for water quality. Water Research 45, 163–170. Crittenden, C., Trussell, R.R., Hand, D.W., Howe, K.J., Tchobanoglous, G., 2005. Water Treatment: Principals and Design. John Wiley and Sons Inc.. Dong, Y., He, K., Zhao, B., Yin, Y., Yin, L., Zhang, A., 2007. Catalytic ozonation of azo dye active brilliant red X-3B in water with natural mineral brucite. Catalysis Communications 8, 1599–1603. dos Santos, A.B., Cervantes, F.J., van Lier, J.B., 2007. Review paper on current technologies for decolourisation of textile wastewaters: perspectives for anaerobic biotechnology. Bioresource Technology 98, 2369–2385. Faria, P.C.C., Órfão, J.J.M., Pereira, M.F.R., 2008. Activated carbon catalytic ozonation of oxamic and oxalic acids. Applied Catalysis B: Environmental 79, 237–243. Garg, U.K., Kaur, M.P., Garg, V.K., Sud, D., 2007. Removal of hexavalent chromium from aqueous solution by agricultural waste biomass. Journal of Hazardous Materials 140, 60–68.

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