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Al2O3 catalysts
Catalysis Communications 9 (2008) 2386–2391
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Degradation of oxalic acid from aqueous solutions by ozonation in presence of Ni/Al2O3 catalysts Sorin Marius Avramescu *, Corina Bradu, Ion Udrea, Nicoleta Mihalache, Florin Ruta University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry, Bd. Regina Elisabeta, 4–12, Bucharest, Romania
a r t i c l e
i n f o
Article history: Received 26 February 2008 Received in revised form 27 May 2008 Accepted 1 June 2008 Available online 6 June 2008 Keywords: Catalytic ozonation Oxalic acid Nickel oxide Kinetic Reaction mechanism
a b s t r a c t The ozonation of oxalic acid has been carried out in an agitated slurry semi-batch reactor using NiO/Al2O3 as catalysts. The presence of catalysts in the ozonation processes significantly improves the oxalic acid removal rate, in comparison to the catalytic process. The influence of calcination temperature on the catalyst activity and on leaching of the active component was assessed. At the experimental conditions applied, the catalytic process develops under complete mineralization of organic substrate. The proposed mechanism justifies the first order kinetic experimental for oxalic acid and ozone. This kinetics also complies with experimental results obtained for different temperatures, ozone partial pressure and different mass of catalyst per slurry volume. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Ozonation processes in water and wastewater treatment are becoming very popular in the last years due to their high efficiency in the removal of different pollutants and pathogens [1,2]. However, there is a large range of refractory pollutants that cannot be eliminated by single ozonation and therefore require advanced oxidation processes (AOP’s). This type of processes involves production of hydroxyl radicals in sufficient quantity to perform water purification. Some of the refractory compound such as carboxylic acids (oxalic, acetic, pyruvic etc) result from oxidation processes of other organic molecules (e.g. phenols). In comparison with other organic acids the oxalic acid is very resistant to oxidation even in the presence of hydroxyl radicals and therefore was chosen as target pollutants in several studies [2–4]. Different types of AOP’s based on ozone activation (O3/OH, O3/UV, O3/H2O2, O3/H2O2/UV) are very efficient in removal of recalcitrant compounds from wastewaters however they present some drawbacks related to the turbidity of water, capital and maintenance costs and presence of hydroxyl radical scavengers [5–9]. As a result, in the last years, in order to increase the mineralization of the organic substrate from aqueous effluents new ozonation processes based on solid catalysts has been studied [10,11]. The large range of supported or unsupported materials has been used to remove aqueous pollutants [1,3–4]. Some of these materials such us activated carbon were selected * Corresponding author. Tel.: +40 10745172435. E-mail address: [email protected] (S.M. Avramescu). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.06.001
due to their good adsorptive properties and the presence of surface basic sites. Zaror [12] and McKay [13] were among the first who evaluated the catalytic properties of activated carbon in the removal of chlorophenol, pyrocathecol and others compounds. Jans and Hoigne [14] have been published a study on the aqueous ozone decomposition catalyzed by activated carbon. In the study they found that hydroxyl radicals are formed in ozone–carbon interactions. Furthermore, Rivera-Utrilla and Sanchez-Polo [15] studied the influence of chemical and textural properties in oxidation of 1,3,6-naphthalene-trisulphonic acid. Some authors [16,17] remarked a severe decrease in the catalytic activity after repeated use of some of the active carbon samples probably as an effect of basic sites deactivation. Hence, this behavior limits the use of this material in ozonation processes. Concerning the metallic oxides, there have been several studies that emphasize their catalytic activity in the ozonation processes. Yunrui et al. [18] uses Ru/Al2O3 for dimethyl phthalate removal, Alvarez et al. [19] tested Co/Al2O3 catalysts in the oxidation process of pyruvic acid, Cooper and Burch [20] proved the efficiency of TiO2 and Fe2O3/Al2O3 in ozonation of chloroethanol and chlorophenol from aqueous solutions and Kasprzyk-Hordern et al. [21] uses alumina covered with perfluorooctanoic acid for MTBE removal from drinking water. Also Udrea et al. were tested a series of catalysts based on Ni and Co oxides for 2-nitro phenol [22] and cyanide [23] removal. The objective of this paper was the evaluation of NiO/Al2O3 catalysts efficiency in ozonation process of oxalic acid. Moreover the influence of operational parameters on the oxidation process was emphasized.
S.M. Avramescu et al. / Catalysis Communications 9 (2008) 2386–2391
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initial aqueous concentration and the residual concentration of the filtrates.
2. Experimental 2.1. Catalytic systems
2.3. Analytical methods Nickel based catalysts (Table 1) were prepared by impregnating c-Al2O3 with nickel nitrate aqueous solutions until the metal concentration of the catalyst reach 10%. After impregnation samples were dried and calcined at a temperature range of 550–950 °C. Phase composition was determined by X-ray diffraction with a Shimadzu XRD 6000 and the specific surface area of the supported catalysts has been determined by nitrogen adsorption using a Sorbty 1750 (Carlo Erba). In all diffractograms of prepared catalysts (Fig. 1) peaks of cAl2O3 can be seen. Characteristic peaks for NiO are less emphasized which suggest that nickel oxide are highly dispersed on the alumina surface. As the calcination temperature increase NiO characteristic peaks become more pronounced due to the formation of crystallites higher in size and in the same time for NiO(950) the characteristic peaks for nickel aluminate can be observed. The formation of nickel aluminate on NiO(750) surface also occur even it is less pronounced and it is in concordance with Chokkaram work [24]. Surface area for prepared catalytic systems is lower comparing with those of alumina and decrease significantly when calcination temperature increase (from 18% to 65%) probably due to sintering processes.
Ozone concentration at inlet and outlet of the reactor was measured by iodometric method and dissolved ozone in aqueous phase was determined using the indigo method [25]. Oxalic acid concentration was determined by means of a HPLC system (Pro Star, Varian) with a Inertsil 5 C8 column and UV detector (k = 210 nm). A NaH2PO4 aqueous solution (0.1 M) was used as mobile phase with a flow rate of 1.3 ml min1. Dissolved total organic carbon (TOC) was measured using a (HiPerTOC, Thermo Electron) apparatus. Catalyst composition in terms of nickel content and nickel ions from aqueous samples withdrawn from the reactor were determined with an atomic absorption spectrophotometer (Solaar M5, Thermo Electron). Previous to nickel analysis the catalysts samples were digested using a microwave digester (Ethos Sel, Milestone).
2.2. Experimental Experimental set-up presented in a previous publication [23] consists of an ozone generator (COM-AD-01, Anseros) and a thermostated semi-batch slurry reactor provided with magnetic stirrer. Excess ozone was retained in a bubble vessel filled with KI solution. Ozone was obtained using high purity air (99.99%). All other reagents were of analytical purity (Merck). During the reaction tests samples were withdrawn at certain intervals then filtered through 0.45 lm membranes and analyzed. Operational parameters were: Oxalic acid concentrations (M): 10–3. Catalyst concentration (g l–1): 1; 2; 3; 4. Ozonated air flow rate (l h–1): 10; 20; 30. Temperature (°C): 12; 22; 32. pH: 2.4.
In order to obtain adsorption isotherms for oxalic acid on NiO(550) batch adsorption measurements were performed. Different amounts of NiO(550) catalyst weighted from 0.1 g to 2 g were contacted with 100 ml of aqueous solution of oxalic acid (102 M). Flasks were sealed and left in a temperature controlled shaking bath for 2 days to before reaching equilibrium. After equilibration all the solutions were filtered through 0.45 lm membranes. The amounts of oxalic acid adsorbed onto NiO(550) has been determined by calculating the difference between the
Table 1 Structural and textural properties of prepared catalysts Sample
(%) Ni a
A12O3 Ni(55O) Ni(750) Ni(950) a
S (mV)
S(m2g1)
Phase composition
247,0 203,1 161,3 93,7
g(c)–A12O3 NiO, g(c)–AbO3 NiO > NiAli04, g(c)–Al2O3 NiO, NiAl2O4, g(c)–Al2O3
a
T
E
– 10 10 10
– 9.70 9.55 9.42
T, theoretic; E, experimental.
Fig. 1. XRD diffractograms of prepared Ni(550), Ni(750), Ni(950) catalysts.
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3. Results and discussion 3.1. Influence of the thermal treatment on catalytic activity. Catalysts stability Experimental tests showed a significant decrease of the activity for NiO(750) (60%) and for NiO(950) (70%) in comparison with NiO(550) (Fig. 2). In our opinion this behavior can be attributed to the decrease of the surface area as well as to the formation of aluminate. Complete oxidation of oxalic acid occur in 80 min in presence of Ni(550) catalyst in comparison with single ozonation. Since the direct reaction of ozone with oxalic acid has a low rate constant (<0.04 M1 s1 at pH 2) the lack of reactivity of oxalic acid in single ozonation process can be attributed to the acidic pH of the solution which does not allow ozone transformation in hydroxyl radicals. An important characteristic of the oxidation process in aqueous solutions consist in the possibility of active component leaching. This phenomenon has two implications: (i) a decrease in catalyst activity at further reuse and (ii) metallic ions can play a catalytic role in the process. The leaching degree of active component was evaluated by measurement of Ni2+ ions concentration. From Fig. 3 it can be observed that nickel ions concentration increase during the first minutes of reaction and rapidly reach a plateau. This suggest that the release of active component in bulk
Fig. 2. Oxalic acid oxidation in the presence and absence of catalytic systems (pH = 2.4; T = 295 K,Ccat = 2 g l1, Qgas = 20 L/h).
solution is not a continuous process and only a small amount of nickel oxide leave the catalyst surface. As the calcination temperature increase the level of metallic ion concentration decrease probably due to NiO transformation in aluminate (Fig. 3a). However for all catalytic systems nickel solubilisation remain at a low level (<0.75%) comparing with entire quantity of supported active component. Moreover, after repeated use of the Ni(550), the catalytic activity is maintained at the same level and the leaching of active component decreases (Fig. 3b). Due to the leaching process it is necessary to emphasize whether the catalytic reaction occur in homogeneous or heterogeneous phase. Therefore some experiments were carried out in presence of 10 ppm Ni2+ solution (all other conditions remain the same as above). It was found that the removal of oxalic acid was similar with non catalytic process which confirmed the heterogeneous nature of catalytic reaction. As it is well known that catalyst stability and leaching process depends strongly on the pH level of the solution, several tests at different pH values were performed. From Fig. 4 it can be observed that as the solution pH increases the leaching effect decreases significantly hence the catalytic system can be used on a large pH domain. 3.2. Influence of operational parameters Due to the complexity of the reaction system (gas–liquid–solid) it is important to establish the conditions for chemical regime
Fig. 4. Evolution of Ni2+ ion concentration during the oxidation of oxalic acid in aqueous solutions (Ni(550), T = 295 K, Ccat = 2 g l1, Qgas = 20 L/h.).
Fig. 3. Evolution of the Ni2+concentration in the acid oxalic oxidation process. (a) Variation of catalyst type and (b) reuse of catalyst (pH = 2.4; T = 295 K, Ccat = 2 g l1, Qgas = 20 L/h).
S.M. Avramescu et al. / Catalysis Communications 9 (2008) 2386–2391
control. Operational parameters studied were: agitation speed (100–300 rpm), gas flow rate (10–30 l h1), temperature, ozone concentration in gas phase and catalyst dose. As the concentration profile of the oxalic acid do not vary significantly as a result of speed or and gas flow rate (Fig. 5a and b) and the catalytic system is in powder form it can be assumed that there is no external or internal mass transfer limitation. The temperature effect was investigated in the range of 285– 305 K (Fig. 5c). From this variation two opposite phenomena occur. At lower temperature ozone solubilisation increase and therefore ozonation process can be improved. In our case, due to the resistance to oxidation of the oxalic acid, a supplementary amount of ozone may be detrimental because ozone could become a sink for hydroxyl radicals. At higher temperature the oxidation process should be normally improved as the process is chemically controlled. Instead the results show that variation of oxalic acid concentration is almost similar for the runs at 295 K and 305 K respectively. This can be attributed to the significant decrease of ozone concentration in aqueous solutions. These observations are in accordance with other studies reported in literature [10]. As expected the conversion of oxalic acid significantly increase with ozone concentration (Fig. 5d). In the case where a catalyst dose over 2 g l1 is used the increase in removal rate is not significant. Nevertheless from Fig. 5e it can be observed that for the first 40 min the oxalic acid removal rate is higher for 3 g l1 and 4 g l1 comparing with 2 g l1 probably due to rapid adsorption of organic substrate on catalyst surface as a result of large number of active sites. After that, the removal rate of oxalic acid are comparable for all three catalyst doses and therefore it can be considered that the optimal dose for the catalyst is 2 g l1. 3.3. Kinetic and mechanistic consideration Catalytic ozonation implies a series of phenomena such as: mass transfer, surface reactions, reactions between chemical spe-
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cies present in solutions, oxalic acid adsorption on the catalyst surface which must be taken into account in order to asses overall rate equation. 3.4. Ozone interaction with catalyst surface In order to evaluate the capacity of catalytic systems to decompose ozone Ni(550) was tested due to the higher activity among the others catalysts. From the resulted variation of ozone concentration it can be assumed that ozone decomposition follow a first order kinetics regardless the presence of catalysts which is in accordance with other studies [26]. This kinetic equation can be expressed as:
dCO3 ¼ k1;oz CO3 dt
ð1Þ
wherek1,oz is the first order kinetic constant of ozone decomposition in bulk solutions and CO3 is the ozone concentration in aqueous solutions. Taking into account that in the catalytic process ozone decomposition occur in the bulk solution and on catalyst surface the kinetic equation can be written as:
d½O3 ¼ kt;oz CO3 ¼ ðk1;oz þ k2;oz ÞCO3 dt
ð2Þ
where kt,oz is the overall kinetic constant for ozone decomposition in aqueous solution, k1,oz is the first order kinetic constant of ozone decomposition in bulk solutions and k2,oz is the first order kinetic constant of ozone decomposition on catalyst surface. By applying the linear regression analysis method, firstorder rate constants were determined: kt,oz = 0.151 min1, k1,oz = 0.0024 min1, k2,oz = 0.1486 min1. Higher conversions obtained in the in catalytic processes lead to the assumption that these catalysts favored hydroxyl radicals formation due to the semiconductor properties of nickel oxide and according to the following reactions:
Fig. 5. Oxalic acid oxidation in presence of Ni(550) catalyst: (a) variation of flow rate, (b) variation of agitation speed, (c) variation of temperature, (d) variation of ozone concentration in gas phase and (e) variation of catalyst concentration (pH = 2.4; Qgas = 20 L/h).
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NiO þ O3 ! NiOðhþÞ þ O 3
ð3Þ
Table 2 Overall global rates constants for oxalic acid oxidation for different reaction condition
þ O 3 þ H ! HO3
ð4Þ
Parameter
Value
kg, g1L min1103
HO3 ! O2 þ OH
ð5Þ
Ozone dose (mg/l)
22.5 105 5.1
42.81 26.57 14.24
NiOðhþÞ þ H2 O ! NiO þ OH þ Hþ
ð6Þ
Catalyst concentration (g l1)
1 2 3 4
18.74 42.81 46.75 48.57
Reaction cycle
1 2 3
42.31 40.22 36.81
This mechanism is sustained by several studies [27–29] and is plausible since O3 a highly electrophile molecule can interact with p-type oxide semiconductors (nickel, cobalt etc.) forming on the catalyst surface different oxygenated radicals: O 3 , O2 , O . Furthermore these species interact with protons from aqueous solution resulting in other radical species such us: HO3 , HO2 , HO . HO3 and HO2 which can be transformed into HO radicals according to the following reactions [2]:
dC B ¼ k1 C B C c C O3 dt 1
BðadsÞ þ O3 ¢ þCO2 þ H2 O
HO3 ! HO þ O2
ð7Þ
2HO2 ! H2 O2 þ O2
ð8Þ
– Reaction of OH with adsorbed oxalic acid:
H2 O2 þ O 2 ! HO þ HO þ O2
ð9Þ
BðadsÞ þ OH ¢ þCO2 þ H2 O
Hydroxyl radicals are capable to react with oxalic acid adsorbed on catalyst surface or in bulk solution. 3.5. Oxalic acid interaction with catalyst surface
ln qc ¼ ln K F þ
1 ln C e n
ð10Þ
where qe is the oxalic acid adsorbed on catalyst surface (mmol/g), Ce is the equilibrium oxalic acid concentration (mmol/L), KF is the Freundlich isotherm constant (mmol1n g1 Ln), n is the Freundlich isotherm exponent (dimensionless). From the value of KF = 1.2 103 it was assumed that the catalytic system has a high adsorption capacity of catalytic system and since the value of n value is less than one unit (0.81) this suggests a strong adsorption on catalyst surface. 3.6. Interaction of oxalic acid and O3 on catalyst surface
– Adsorption of oxalic acid on catalyst surface:
þ B ¢ BðadsÞ – Reaction of O3 with adsorbed oxalic acid:
ð11Þ
dC B dt
¼ k3 C B C OH
ð14Þ
3
The overall rate of organic substrate disappearance ozonation process can be represented by a linear combination of contributing terms:
dC B ¼ k1 C B C c CO3 þ k2 C B C c C OH þ k3 C B C OH dt total ¼ k1 C c C O3 þ k2 C c C OH þ k3 C OH C B ¼ kt C OX
ð15Þ
where B is the oxalic acid, * is the active site on catalyst surface, CB is the oxalic acid concentration, C O3 is the ozone concentration in aqueous solution, C OH is the concentration of hydroxyl radicals in aqueous solution, k1, k2, k3, k4 is the rate constants for competitive processes (adsorption, heterogeneous reactions, and homogeneous reactions), kt is the overall rate constant for the oxidation process of oxalic acid oxidation.By separating variables and integrating, Eq. 4 becomes:
ln
From the variation of the oxalic acid concentration in correspondence to reaction time (Fig. 4) it can be assumed that catalytic ozonation process follow a first-order kinetic which results from linear regression of the semi logarithmic plots. Oxalic acid oxidations occur by a series of competitive processes which implies: oxalic acid and ozone adsorption on catalyst surface, ozone transformation into radicals, the reaction of oxalic acid with bulk ozone or with radicals. These suppositions are based on the capacity of the catalytic system to produce hydroxyl radicals (emphasized in a previous work [23]) and other similar results obtained by other authors [19] in similar conditions which state that adsorbed oxalic acid can react with bulk ozone. Experimental results corroborated with data reported in the literature lead us to a possible mechanistic approach for the oxalic acid oxidation process:
ð13Þ
– Reaction of OH with oxalic acid from bulk aqueous phase:
B þ OH ! CO2 þ H2 O
Oxalic acid interaction with catalyst surface was estimated by means of adsorption isotherms. A strong adsorption of the organic substrate on the catalyst surface was observed and isotherm constants, K, n, were estimated by fitting a set of experimental data to Freundlich isotherm and by using linear regression methods:
dC B ¼ k2 C B C c C OH dt 2
ð12Þ
C Bt ¼ ðk1 C c þ k2 C c C O3 þ k3 C c C OH þ k3 C OH Þt ¼ kt t C Bo
ð16Þ
C
Plotting ln C BBt as a function of reaction time t for different conditions o a series o straight lines (regression coefficient between 0.98 and 1) were obtained and the rate constants obtained from theirs slopes are presented in Table 2. Linear dependence of TOC values obtained from analyzer versus TOC values calculated from the corresponding concentrations of oxalic acid determined from HPLC (not shown) suggest that in the catalytic ozonation process oxalic acid is completely mineralized. 4. Conclusions Based on experimental data the following conclusions can be drawn: The use of catalysts in oxalic acid oxidation significantly improve the removal rate of organic substrate in comparison to the with non-catalytic process. Catalyst activity can be explained by radicals formation after interaction of ozone with the catalyst surface.
S.M. Avramescu et al. / Catalysis Communications 9 (2008) 2386–2391
Among studied catalysts the most active seem to be Ni(550). When calcination temperature increase a considerable decrease of catalyst activity occur as a result of structural transformation and specific surface reduction. It was stressed out that catalytic activity of Ni(550) remain almost the same after several runs and the leaching of active component is insignificant regardless the pH value of aqueous solution. Also the leaching of the active component decrease with the calcinations temperature probably due to the formation of NiAlO4. Oxidation rate of oxalic acid considerably increases when increasing the ozone dose used. The same evolution is observed for the catalyst concentration. However, an optimal catalyst concentration of 2 g L1 was found above which the reaction rate does not increase. At temperatures above 22 °C the positive kinetic effect is hindered by the decrease of ozone concentration due to a decrease of solubility. The kinetic study led to an overall first order kinetic with respect to oxalic acid and the proposed mechanism takes into account the of heterogeneous and homogeneous processes which may occur.
References [1] P.R. Gogate, A.B. Pandit, Adv. Env. Res. 8 (2004) 501. [2] F.J. Beltran, Ozone Reaction Kinetics for Water and Wastewater Systems, Lewis, Publishers, Florida, 2003.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Degradation of oxalic acid from aqueous solutions by ozonation in presence of Ni/Al2O3 catalysts Sorin Marius Avramescu *, Corina Bradu, Ion Udrea, Nicoleta Mihalache, Florin Ruta University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry, Bd. Regina Elisabeta, 4–12, Bucharest, Romania
a r t i c l e
i n f o
Article history: Received 26 February 2008 Received in revised form 27 May 2008 Accepted 1 June 2008 Available online 6 June 2008 Keywords: Catalytic ozonation Oxalic acid Nickel oxide Kinetic Reaction mechanism
a b s t r a c t The ozonation of oxalic acid has been carried out in an agitated slurry semi-batch reactor using NiO/Al2O3 as catalysts. The presence of catalysts in the ozonation processes significantly improves the oxalic acid removal rate, in comparison to the catalytic process. The influence of calcination temperature on the catalyst activity and on leaching of the active component was assessed. At the experimental conditions applied, the catalytic process develops under complete mineralization of organic substrate. The proposed mechanism justifies the first order kinetic experimental for oxalic acid and ozone. This kinetics also complies with experimental results obtained for different temperatures, ozone partial pressure and different mass of catalyst per slurry volume. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Ozonation processes in water and wastewater treatment are becoming very popular in the last years due to their high efficiency in the removal of different pollutants and pathogens [1,2]. However, there is a large range of refractory pollutants that cannot be eliminated by single ozonation and therefore require advanced oxidation processes (AOP’s). This type of processes involves production of hydroxyl radicals in sufficient quantity to perform water purification. Some of the refractory compound such as carboxylic acids (oxalic, acetic, pyruvic etc) result from oxidation processes of other organic molecules (e.g. phenols). In comparison with other organic acids the oxalic acid is very resistant to oxidation even in the presence of hydroxyl radicals and therefore was chosen as target pollutants in several studies [2–4]. Different types of AOP’s based on ozone activation (O3/OH, O3/UV, O3/H2O2, O3/H2O2/UV) are very efficient in removal of recalcitrant compounds from wastewaters however they present some drawbacks related to the turbidity of water, capital and maintenance costs and presence of hydroxyl radical scavengers [5–9]. As a result, in the last years, in order to increase the mineralization of the organic substrate from aqueous effluents new ozonation processes based on solid catalysts has been studied [10,11]. The large range of supported or unsupported materials has been used to remove aqueous pollutants [1,3–4]. Some of these materials such us activated carbon were selected * Corresponding author. Tel.: +40 10745172435. E-mail address: [email protected] (S.M. Avramescu). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.06.001
due to their good adsorptive properties and the presence of surface basic sites. Zaror [12] and McKay [13] were among the first who evaluated the catalytic properties of activated carbon in the removal of chlorophenol, pyrocathecol and others compounds. Jans and Hoigne [14] have been published a study on the aqueous ozone decomposition catalyzed by activated carbon. In the study they found that hydroxyl radicals are formed in ozone–carbon interactions. Furthermore, Rivera-Utrilla and Sanchez-Polo [15] studied the influence of chemical and textural properties in oxidation of 1,3,6-naphthalene-trisulphonic acid. Some authors [16,17] remarked a severe decrease in the catalytic activity after repeated use of some of the active carbon samples probably as an effect of basic sites deactivation. Hence, this behavior limits the use of this material in ozonation processes. Concerning the metallic oxides, there have been several studies that emphasize their catalytic activity in the ozonation processes. Yunrui et al. [18] uses Ru/Al2O3 for dimethyl phthalate removal, Alvarez et al. [19] tested Co/Al2O3 catalysts in the oxidation process of pyruvic acid, Cooper and Burch [20] proved the efficiency of TiO2 and Fe2O3/Al2O3 in ozonation of chloroethanol and chlorophenol from aqueous solutions and Kasprzyk-Hordern et al. [21] uses alumina covered with perfluorooctanoic acid for MTBE removal from drinking water. Also Udrea et al. were tested a series of catalysts based on Ni and Co oxides for 2-nitro phenol [22] and cyanide [23] removal. The objective of this paper was the evaluation of NiO/Al2O3 catalysts efficiency in ozonation process of oxalic acid. Moreover the influence of operational parameters on the oxidation process was emphasized.
S.M. Avramescu et al. / Catalysis Communications 9 (2008) 2386–2391
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initial aqueous concentration and the residual concentration of the filtrates.
2. Experimental 2.1. Catalytic systems
2.3. Analytical methods Nickel based catalysts (Table 1) were prepared by impregnating c-Al2O3 with nickel nitrate aqueous solutions until the metal concentration of the catalyst reach 10%. After impregnation samples were dried and calcined at a temperature range of 550–950 °C. Phase composition was determined by X-ray diffraction with a Shimadzu XRD 6000 and the specific surface area of the supported catalysts has been determined by nitrogen adsorption using a Sorbty 1750 (Carlo Erba). In all diffractograms of prepared catalysts (Fig. 1) peaks of cAl2O3 can be seen. Characteristic peaks for NiO are less emphasized which suggest that nickel oxide are highly dispersed on the alumina surface. As the calcination temperature increase NiO characteristic peaks become more pronounced due to the formation of crystallites higher in size and in the same time for NiO(950) the characteristic peaks for nickel aluminate can be observed. The formation of nickel aluminate on NiO(750) surface also occur even it is less pronounced and it is in concordance with Chokkaram work [24]. Surface area for prepared catalytic systems is lower comparing with those of alumina and decrease significantly when calcination temperature increase (from 18% to 65%) probably due to sintering processes.
Ozone concentration at inlet and outlet of the reactor was measured by iodometric method and dissolved ozone in aqueous phase was determined using the indigo method [25]. Oxalic acid concentration was determined by means of a HPLC system (Pro Star, Varian) with a Inertsil 5 C8 column and UV detector (k = 210 nm). A NaH2PO4 aqueous solution (0.1 M) was used as mobile phase with a flow rate of 1.3 ml min1. Dissolved total organic carbon (TOC) was measured using a (HiPerTOC, Thermo Electron) apparatus. Catalyst composition in terms of nickel content and nickel ions from aqueous samples withdrawn from the reactor were determined with an atomic absorption spectrophotometer (Solaar M5, Thermo Electron). Previous to nickel analysis the catalysts samples were digested using a microwave digester (Ethos Sel, Milestone).
2.2. Experimental Experimental set-up presented in a previous publication [23] consists of an ozone generator (COM-AD-01, Anseros) and a thermostated semi-batch slurry reactor provided with magnetic stirrer. Excess ozone was retained in a bubble vessel filled with KI solution. Ozone was obtained using high purity air (99.99%). All other reagents were of analytical purity (Merck). During the reaction tests samples were withdrawn at certain intervals then filtered through 0.45 lm membranes and analyzed. Operational parameters were: Oxalic acid concentrations (M): 10–3. Catalyst concentration (g l–1): 1; 2; 3; 4. Ozonated air flow rate (l h–1): 10; 20; 30. Temperature (°C): 12; 22; 32. pH: 2.4.
In order to obtain adsorption isotherms for oxalic acid on NiO(550) batch adsorption measurements were performed. Different amounts of NiO(550) catalyst weighted from 0.1 g to 2 g were contacted with 100 ml of aqueous solution of oxalic acid (102 M). Flasks were sealed and left in a temperature controlled shaking bath for 2 days to before reaching equilibrium. After equilibration all the solutions were filtered through 0.45 lm membranes. The amounts of oxalic acid adsorbed onto NiO(550) has been determined by calculating the difference between the
Table 1 Structural and textural properties of prepared catalysts Sample
(%) Ni a
A12O3 Ni(55O) Ni(750) Ni(950) a
S (mV)
S(m2g1)
Phase composition
247,0 203,1 161,3 93,7
g(c)–A12O3 NiO, g(c)–AbO3 NiO > NiAli04, g(c)–Al2O3 NiO, NiAl2O4, g(c)–Al2O3
a
T
E
– 10 10 10
– 9.70 9.55 9.42
T, theoretic; E, experimental.
Fig. 1. XRD diffractograms of prepared Ni(550), Ni(750), Ni(950) catalysts.
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3. Results and discussion 3.1. Influence of the thermal treatment on catalytic activity. Catalysts stability Experimental tests showed a significant decrease of the activity for NiO(750) (60%) and for NiO(950) (70%) in comparison with NiO(550) (Fig. 2). In our opinion this behavior can be attributed to the decrease of the surface area as well as to the formation of aluminate. Complete oxidation of oxalic acid occur in 80 min in presence of Ni(550) catalyst in comparison with single ozonation. Since the direct reaction of ozone with oxalic acid has a low rate constant (<0.04 M1 s1 at pH 2) the lack of reactivity of oxalic acid in single ozonation process can be attributed to the acidic pH of the solution which does not allow ozone transformation in hydroxyl radicals. An important characteristic of the oxidation process in aqueous solutions consist in the possibility of active component leaching. This phenomenon has two implications: (i) a decrease in catalyst activity at further reuse and (ii) metallic ions can play a catalytic role in the process. The leaching degree of active component was evaluated by measurement of Ni2+ ions concentration. From Fig. 3 it can be observed that nickel ions concentration increase during the first minutes of reaction and rapidly reach a plateau. This suggest that the release of active component in bulk
Fig. 2. Oxalic acid oxidation in the presence and absence of catalytic systems (pH = 2.4; T = 295 K,Ccat = 2 g l1, Qgas = 20 L/h).
solution is not a continuous process and only a small amount of nickel oxide leave the catalyst surface. As the calcination temperature increase the level of metallic ion concentration decrease probably due to NiO transformation in aluminate (Fig. 3a). However for all catalytic systems nickel solubilisation remain at a low level (<0.75%) comparing with entire quantity of supported active component. Moreover, after repeated use of the Ni(550), the catalytic activity is maintained at the same level and the leaching of active component decreases (Fig. 3b). Due to the leaching process it is necessary to emphasize whether the catalytic reaction occur in homogeneous or heterogeneous phase. Therefore some experiments were carried out in presence of 10 ppm Ni2+ solution (all other conditions remain the same as above). It was found that the removal of oxalic acid was similar with non catalytic process which confirmed the heterogeneous nature of catalytic reaction. As it is well known that catalyst stability and leaching process depends strongly on the pH level of the solution, several tests at different pH values were performed. From Fig. 4 it can be observed that as the solution pH increases the leaching effect decreases significantly hence the catalytic system can be used on a large pH domain. 3.2. Influence of operational parameters Due to the complexity of the reaction system (gas–liquid–solid) it is important to establish the conditions for chemical regime
Fig. 4. Evolution of Ni2+ ion concentration during the oxidation of oxalic acid in aqueous solutions (Ni(550), T = 295 K, Ccat = 2 g l1, Qgas = 20 L/h.).
Fig. 3. Evolution of the Ni2+concentration in the acid oxalic oxidation process. (a) Variation of catalyst type and (b) reuse of catalyst (pH = 2.4; T = 295 K, Ccat = 2 g l1, Qgas = 20 L/h).
S.M. Avramescu et al. / Catalysis Communications 9 (2008) 2386–2391
control. Operational parameters studied were: agitation speed (100–300 rpm), gas flow rate (10–30 l h1), temperature, ozone concentration in gas phase and catalyst dose. As the concentration profile of the oxalic acid do not vary significantly as a result of speed or and gas flow rate (Fig. 5a and b) and the catalytic system is in powder form it can be assumed that there is no external or internal mass transfer limitation. The temperature effect was investigated in the range of 285– 305 K (Fig. 5c). From this variation two opposite phenomena occur. At lower temperature ozone solubilisation increase and therefore ozonation process can be improved. In our case, due to the resistance to oxidation of the oxalic acid, a supplementary amount of ozone may be detrimental because ozone could become a sink for hydroxyl radicals. At higher temperature the oxidation process should be normally improved as the process is chemically controlled. Instead the results show that variation of oxalic acid concentration is almost similar for the runs at 295 K and 305 K respectively. This can be attributed to the significant decrease of ozone concentration in aqueous solutions. These observations are in accordance with other studies reported in literature [10]. As expected the conversion of oxalic acid significantly increase with ozone concentration (Fig. 5d). In the case where a catalyst dose over 2 g l1 is used the increase in removal rate is not significant. Nevertheless from Fig. 5e it can be observed that for the first 40 min the oxalic acid removal rate is higher for 3 g l1 and 4 g l1 comparing with 2 g l1 probably due to rapid adsorption of organic substrate on catalyst surface as a result of large number of active sites. After that, the removal rate of oxalic acid are comparable for all three catalyst doses and therefore it can be considered that the optimal dose for the catalyst is 2 g l1. 3.3. Kinetic and mechanistic consideration Catalytic ozonation implies a series of phenomena such as: mass transfer, surface reactions, reactions between chemical spe-
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cies present in solutions, oxalic acid adsorption on the catalyst surface which must be taken into account in order to asses overall rate equation. 3.4. Ozone interaction with catalyst surface In order to evaluate the capacity of catalytic systems to decompose ozone Ni(550) was tested due to the higher activity among the others catalysts. From the resulted variation of ozone concentration it can be assumed that ozone decomposition follow a first order kinetics regardless the presence of catalysts which is in accordance with other studies [26]. This kinetic equation can be expressed as:
dCO3 ¼ k1;oz CO3 dt
ð1Þ
wherek1,oz is the first order kinetic constant of ozone decomposition in bulk solutions and CO3 is the ozone concentration in aqueous solutions. Taking into account that in the catalytic process ozone decomposition occur in the bulk solution and on catalyst surface the kinetic equation can be written as:
d½O3 ¼ kt;oz CO3 ¼ ðk1;oz þ k2;oz ÞCO3 dt
ð2Þ
where kt,oz is the overall kinetic constant for ozone decomposition in aqueous solution, k1,oz is the first order kinetic constant of ozone decomposition in bulk solutions and k2,oz is the first order kinetic constant of ozone decomposition on catalyst surface. By applying the linear regression analysis method, firstorder rate constants were determined: kt,oz = 0.151 min1, k1,oz = 0.0024 min1, k2,oz = 0.1486 min1. Higher conversions obtained in the in catalytic processes lead to the assumption that these catalysts favored hydroxyl radicals formation due to the semiconductor properties of nickel oxide and according to the following reactions:
Fig. 5. Oxalic acid oxidation in presence of Ni(550) catalyst: (a) variation of flow rate, (b) variation of agitation speed, (c) variation of temperature, (d) variation of ozone concentration in gas phase and (e) variation of catalyst concentration (pH = 2.4; Qgas = 20 L/h).
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NiO þ O3 ! NiOðhþÞ þ O 3
ð3Þ
Table 2 Overall global rates constants for oxalic acid oxidation for different reaction condition
þ O 3 þ H ! HO3
ð4Þ
Parameter
Value
kg, g1L min1103
HO3 ! O2 þ OH
ð5Þ
Ozone dose (mg/l)
22.5 105 5.1
42.81 26.57 14.24
NiOðhþÞ þ H2 O ! NiO þ OH þ Hþ
ð6Þ
Catalyst concentration (g l1)
1 2 3 4
18.74 42.81 46.75 48.57
Reaction cycle
1 2 3
42.31 40.22 36.81
This mechanism is sustained by several studies [27–29] and is plausible since O3 a highly electrophile molecule can interact with p-type oxide semiconductors (nickel, cobalt etc.) forming on the catalyst surface different oxygenated radicals: O 3 , O2 , O . Furthermore these species interact with protons from aqueous solution resulting in other radical species such us: HO3 , HO2 , HO . HO3 and HO2 which can be transformed into HO radicals according to the following reactions [2]:
dC B ¼ k1 C B C c C O3 dt 1
BðadsÞ þ O3 ¢ þCO2 þ H2 O
HO3 ! HO þ O2
ð7Þ
2HO2 ! H2 O2 þ O2
ð8Þ
– Reaction of OH with adsorbed oxalic acid:
H2 O2 þ O 2 ! HO þ HO þ O2
ð9Þ
BðadsÞ þ OH ¢ þCO2 þ H2 O
Hydroxyl radicals are capable to react with oxalic acid adsorbed on catalyst surface or in bulk solution. 3.5. Oxalic acid interaction with catalyst surface
ln qc ¼ ln K F þ
1 ln C e n
ð10Þ
where qe is the oxalic acid adsorbed on catalyst surface (mmol/g), Ce is the equilibrium oxalic acid concentration (mmol/L), KF is the Freundlich isotherm constant (mmol1n g1 Ln), n is the Freundlich isotherm exponent (dimensionless). From the value of KF = 1.2 103 it was assumed that the catalytic system has a high adsorption capacity of catalytic system and since the value of n value is less than one unit (0.81) this suggests a strong adsorption on catalyst surface. 3.6. Interaction of oxalic acid and O3 on catalyst surface
– Adsorption of oxalic acid on catalyst surface:
þ B ¢ BðadsÞ – Reaction of O3 with adsorbed oxalic acid:
ð11Þ
dC B dt
¼ k3 C B C OH
ð14Þ
3
The overall rate of organic substrate disappearance ozonation process can be represented by a linear combination of contributing terms:
dC B ¼ k1 C B C c CO3 þ k2 C B C c C OH þ k3 C B C OH dt total ¼ k1 C c C O3 þ k2 C c C OH þ k3 C OH C B ¼ kt C OX
ð15Þ
where B is the oxalic acid, * is the active site on catalyst surface, CB is the oxalic acid concentration, C O3 is the ozone concentration in aqueous solution, C OH is the concentration of hydroxyl radicals in aqueous solution, k1, k2, k3, k4 is the rate constants for competitive processes (adsorption, heterogeneous reactions, and homogeneous reactions), kt is the overall rate constant for the oxidation process of oxalic acid oxidation.By separating variables and integrating, Eq. 4 becomes:
ln
From the variation of the oxalic acid concentration in correspondence to reaction time (Fig. 4) it can be assumed that catalytic ozonation process follow a first-order kinetic which results from linear regression of the semi logarithmic plots. Oxalic acid oxidations occur by a series of competitive processes which implies: oxalic acid and ozone adsorption on catalyst surface, ozone transformation into radicals, the reaction of oxalic acid with bulk ozone or with radicals. These suppositions are based on the capacity of the catalytic system to produce hydroxyl radicals (emphasized in a previous work [23]) and other similar results obtained by other authors [19] in similar conditions which state that adsorbed oxalic acid can react with bulk ozone. Experimental results corroborated with data reported in the literature lead us to a possible mechanistic approach for the oxalic acid oxidation process:
ð13Þ
– Reaction of OH with oxalic acid from bulk aqueous phase:
B þ OH ! CO2 þ H2 O
Oxalic acid interaction with catalyst surface was estimated by means of adsorption isotherms. A strong adsorption of the organic substrate on the catalyst surface was observed and isotherm constants, K, n, were estimated by fitting a set of experimental data to Freundlich isotherm and by using linear regression methods:
dC B ¼ k2 C B C c C OH dt 2
ð12Þ
C Bt ¼ ðk1 C c þ k2 C c C O3 þ k3 C c C OH þ k3 C OH Þt ¼ kt t C Bo
ð16Þ
C
Plotting ln C BBt as a function of reaction time t for different conditions o a series o straight lines (regression coefficient between 0.98 and 1) were obtained and the rate constants obtained from theirs slopes are presented in Table 2. Linear dependence of TOC values obtained from analyzer versus TOC values calculated from the corresponding concentrations of oxalic acid determined from HPLC (not shown) suggest that in the catalytic ozonation process oxalic acid is completely mineralized. 4. Conclusions Based on experimental data the following conclusions can be drawn: The use of catalysts in oxalic acid oxidation significantly improve the removal rate of organic substrate in comparison to the with non-catalytic process. Catalyst activity can be explained by radicals formation after interaction of ozone with the catalyst surface.
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Among studied catalysts the most active seem to be Ni(550). When calcination temperature increase a considerable decrease of catalyst activity occur as a result of structural transformation and specific surface reduction. It was stressed out that catalytic activity of Ni(550) remain almost the same after several runs and the leaching of active component is insignificant regardless the pH value of aqueous solution. Also the leaching of the active component decrease with the calcinations temperature probably due to the formation of NiAlO4. Oxidation rate of oxalic acid considerably increases when increasing the ozone dose used. The same evolution is observed for the catalyst concentration. However, an optimal catalyst concentration of 2 g L1 was found above which the reaction rate does not increase. At temperatures above 22 °C the positive kinetic effect is hindered by the decrease of ozone concentration due to a decrease of solubility. The kinetic study led to an overall first order kinetic with respect to oxalic acid and the proposed mechanism takes into account the of heterogeneous and homogeneous processes which may occur.
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