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New insights on degradation of methylene blue using thermocatalytic reactions catalyzed by low-temperature excitation
Journal of Hazardous Materials 260 (2013) 112–121
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
New insights on degradation of methylene blue using thermocatalytic reactions catalyzed by low-temperature excitation Xuegang Luo a,b,∗ , Sizhao Zhang a,b , Xiaoyan Lin a,b a b
School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, Sichuan, China Engineering Research Center of Biomass Materials, Ministry of Education, Mianyang 621010, Sichuan, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Thermocatalytic • • • •
reaction using room-temperature excitation was demonstrated. Results imply that thermal catalyst adopted can be feasible. An excellent contact between contaminant molecule and catalyst was notably required. The optimal removal of 82.07% was reached in total removal efficiency. The mechanism of thermocatalytic process was proposed.
a r t i c l e
i n f o
Article history: Received 19 January 2013 Received in revised form 11 April 2013 Accepted 4 May 2013 Available online 13 May 2013 Keywords: Thermocatalytic degradation Thermal sensitizer Methylene blue Adsorption Mechanism
a b s t r a c t Although photocatalysis has been actively surveyed on removing organic pollutants in ultraviolet (UV) environment, because of lacking UV in solar exposure, photodegradation is difficult to be considerably degraded in conventional exposure condition. In this work, an innovative approach was proposed to compensate for it, which was developed in model wastewater using thermal sensitizer at room temperature. At the optimal component condition, the removal rate of adsorption and thermocatalytic degradation processes can reach the highest level of 82.07% solely response to temperature in the dark. Moreover, the kinetics of degradation rate was modeled considering that it was found similarly to Langmuir–Hinshelwood behavior, and a tentative mechanism was objectively established, describing reasonably well in line with the experimental results. On the other hand, it was found that high amount of methylene blue (MB) adsorbed onto thermal sensitizer was of unambiguous importance to subsequent thermocatalytic performance. Briefly, all above suggest that the feasibility to the thermodegradation route has been successfully verified under room temperature excitation. Herein the insight into degradation pattern of dye over thermal excitation may further enlarge applications for wastewater treatment. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Clean water is substantially essential to survive for the living world in our natural systems. Nevertheless, global warming and
∗ Corresponding author at: Engineering Research Center of Biomass Materials, Ministry of Education, Mianyang 621010, Sichuan, China. Tel.: +86 816 6089009; fax: +86 816 6089009. E-mail addresses: [email protected], [email protected] (X. Luo), [email protected] (S. Zhang). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.05.005
contamination from industrialization continue to strain the already scarce supply of clean water [1–4]. To date, among all the settlement to these problems, photocatalysis is one of the most available routes for degradation of organic pollutants to innoxious substances (such as H2 O and CO2 , or other species) in wastewater [5–7]. In particular, to use semiconductor materials as its photocatalyst, such as TiO2 , ZnO, CdS, and Ag particles, are in response to ultraviolet (UV) light and attract considerable attentions for applications [8–16]. However, little progress has been made in industrial application because its quantum efficiency in photocatalyst reaction is generally not high enough from application point of view [17–19].
X. Luo et al. / Journal of Hazardous Materials 260 (2013) 112–121 Table 1 Data obtained by XRF analysis.
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correlation coefficients were higher than 0.999, it could be permitted to apply in subsequent tests.
Main ingredients for thermal sensitizer SiO2 /%
CaO/%
MnO/%
NiO/%
CuO/%
18.14
17.75
38.58
9.61
11.68
Therefore, it is a pity that the degradation processes would take place difficultly when without or lacking UV light in solar exposure. To overcome aforementioned disadvantage, it is imperative to explore more effective ways to solve currently arisen problems. As we know, any physical or chemical change in substances as a result of corresponding environmental factors, such as light, heat, moisture, chemical conditions or biological activity [20]. Recently, by means of the synergistic effect, between photocatalytic and thermocatalytic reactions, benzene has been degraded using Pt-loaded TiO2 /ZrO2 catalyst [21]. It exhibits that there is a reasonable approach to achieve thermocatalytic degradation. No data on degrading model pollutants were reported through heat excitation, however. To the best of our knowledge, a comprehensive analysis of the existing literature exhibits no previous studies have been conducted at understanding the destruction of the substrate using thermal sensitizer excited by room-temperature excitation. Herein the aims of the study are to explore a novel material which can be viewed as thermal catalyst, and then demonstrate that it is feasible to thermocatalytic reactions. Thus, in continuation of the work on surveying the effect of reaction temperature, reaction time, catalyst dosage, initial concentration and initial pH of the solution. Besides the adsorption process was also studied prior to the degradable reaction [22]. A possible systemic mechanism for thermocatalytic degradation was proposed, and the dependence of degradation rate on reactional time was also modeled. 2. Experimental 2.1. Chemicals and reaction apparatus All of the chemicals were of analytical grade or reagent grade without further purification except thermal sensitizer (Mn–Si–Ca–Cu–Ni complex oxides synthesized by high temperature calcination, considered as thermal catalyst, details in Table 1). Methylene blue (MB) dye was performed to the corresponding solutions of desired concentrations. Deionized and doubly distilled water was employed throughout the experiment. The different pH values of the reaction solutions were adjusted by adding diluted HCl or NaOH solutions (both 0.05 M) at room temperature. Experiments were carried out in a reaction apparatus. The schematic diagram is shown in Fig. 1. The absorbances of various concentrations for MB solutions were recorded by using Ultraviolet Visible (UV–vis) spectrometer. In all cases, when the measured
Fig. 1. Schematic diagram of reaction apparatus: (1) heat, (2) reaction solution, (3) water environment, (4) conical flask for reaction site, (5) stirring rod, and (6) magnetic agitation and heater.
2.2. Tests for adsorption and degradation In this study, all experiments were implemented at atm air pressure. Tests for adsorption and degradation were both conducted in darkness. Thermal sensitizers were processed into different particle sizes. The dosage of thermal sensitizer was suspended at 50 mg in all solutions, and it was added to glass vessels containing 50 mL dye solution of different concentrations (including 10, 20 30, 40 and 50 mg/L), and the temperatures of reaction processes were varied at 30, 35, 40, and 45 ◦ C. In order to disperse the thermal sensitizer powders, a magnetic stirring device was applied to the reaction system. When the adsorption process was achieved to an adsorption–desorption equilibrium after stirring the suspension for 2 h, the amount of adsorption was recorded, and it could be identified by changes of the concentrations of MB solutions. MB solution of 5 mL was taken out after the same intervals and centrifuged immediately at 5500 rpm for 10 min. Then, the centrifuged solution was filtered to remove the suspended catalyst agglomerates. Additionally, in order to confirm the reproducibility of the results, duplicated processes were performed for each condition for averaging results, and the experiment error was found to be within ±5%. Thermocatalytic reaction was controlled in 50 mL glass vessel containing 50 mL of different dye solutions, and thermal sensitizer of 50 mg was added in it. Each solution was kept under constant stirring and heated with a heater system. The reaction solutions were carried out for 30 h, sampling was in step of 6 h, and the absorbances of MB solutions were recorded after each step. In addition, blank experiments were also employed under dark following the same procedures. To optimize the design of the process, an orthogonal test was applied to the whole tests, concerning the initial concentration, catalyst dosage, pH value and temperature. The decomposition of the dyes is estimated by the following equation: Degradation(%) =
c0 − c1 − c2 × 100 c0
(1)
where c0 represents the concentration of the dye without any treatment, c1 denotes the concentration of the dye after adsorption–desorption equilibrium process, c2 exhibits the concentration of the dye which was affected by bank experiment, respectively. 2.3. Characterizations The detected elements of thermal sensitizer were measured by X-ray Fluorescence spectrometer (XRF, Rigaku Primini, Japan) equipped with 50 W end window which was used with Rh anode X-ray tube, operated at a maximum voltage of 40 kV and current of 1.25 mA, control temperature of 36.5 ± 0.5 ◦ C, gas flow rate 5–7 mL/min. The optical spectra of all samples were obtained using Ultraviolet Visible (UV–vis) spectrometer (UV3900, Hitachi Corporation, Japan). The analysis of particle size was carried out on a particle size analyzer (90Plus Brookhaven Instruments Corporation, USA). Infrared (IR) spectra of all dried materials were measured at 400–4000 cm−1 by infrared spectrophotometer (Nicolet 6700, Nicolet Instruments Corporation, USA) using KBr pellets. The surface structure and morphology of thermal sensitizer were characterized using Scanning Electron Microscopy (SEM, Ultra 55, Zeiss Corporation, Germany). Thermal analysis was performed on a Thermogravimetric Analyzer (TGA, Q500, TA Instruments Corporation, USA).
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Fig. 2. Effect of methylene blue adsorbed onto thermal sensitizer in the conditions of different particle sizes of thermal sensitizer (a) and different concentrations of methylene blue solutions (b) on adsorption capacity.
3. Results and discussion 3.1. Adsorption of methylene blue In order to achieve an excellent degradation efficiency, it is essential to gain a good adsorption capacity. In this regard, to select the catalyst with appropriate particular particle size is very critical. As shown in Fig. 2a, the absorbances of various state solutions were measured (see Fig. S1 and S2, Supplementary Material), it is considerably obvious that, the adsorption of methylene blue (MB) onto catalyst surface was gradually approaching to an adsorption equilibrium after 80 min, whereas that of 100 mesh size remained hardly unchanged, until adsorption–desorption equilibrium was noticeably established at 100 min. On the other hand, the adsorption capacity increased rapidly within the previous 80 min, and increased with the increase of the number of particle mesh at the initial stage, especially, that of 200 mesh size was nearly approaching to the final adsorption equilibrium at the early stage. It is strongly obvious that the effect of adsorption capacity fundamentally depends upon the particle size, and namely the appropriate specific surface is of importance for catalysis. To further confirm the predicted insight into the influence of adsorption, as shown in Fig. 2b, the absolute amount of adsorption increased with the concentrations of MB solutions, although the higher concentrations owned a lower rate of adsorption, owing
Fig. 3. The particle size and particle distribution of thermal sensitizer obtained.
to their high concentrations, by calculating, the solutions of orderly increasing concentrations possessed in turn corresponding increasing capacity. Therefore, it is clear that there still exist a vast number of available adsorption sites in a relative diluted solution. Since agitation speed was constant, and the diffusion of dye molecules through the solution to the surface of catalyst was affected by the dye concentration; the dye concentration with an increase accelerates the diffusion of dyes, because of the increase in the driving force of the concentration gradient [23,24]. In other word, the activity of adsorption would be enhanced with the rise of concentration. Fig. 3 shows the size of thermal sensitizer with the effective diameter up to 442 nm, and the dispersibility as a function of particles was approximately 0.269 a.u., and the dispersibility was in accordance with a normal distribution. In view of the outstanding property in adsorption capacity, the chosen catalyst was in the 200 mesh size, which was applied to the subsequent tests. 3.2. Thermocatalytic degradation processes To exclude the negative impact on the experimental materials caused by thermal aspect, we should fully understand the thermal properties of all substances adopted. From Fig. 4a, at first step, weight loss of MB observed is due to the existence of water molecules in MB, when temperature ranging from initial point to 257.9 ◦ C. Subsequently, at second step, the loss of 23.7 wt% is probably on the ground of some weak bonds of MB molecules broken, and 17.87 wt% loss occurred because of the chemolysis of MB molecules, which happens chemical decomposition reactions on account of heat effect. It shows that the complete decomposed temperature has been achieved in this time. As indicated in Fig. 4b, a similar tendency was found in the process, it was divided into two stages. Loss occurred at first step was even up to 342.2 ◦ C. That is to say, the possible influence resulted from thermal aspect plays an extremely weak role in MB molecules and catalysts at room temperature. In Fig. 5, it is apparent that no change was observed in terms of maximum absorbance wavelength of MB solutions, and there were only some slight differences in absorbance intensities of solutions as the concentration increasing. By now, the current studies have confirmed that the temperature to all substances adopted, and the concentration of solution to maximum absorbance wavelength in experiments, both have insignificant influence on the thermocatalytic degradation [25]. The relationships of thermocatalytic degradation rate, experimental temperatures, solution concentrations, and reaction time have been investigated [26]. It is worth mentioning, the degradation rates calculated have removed the parts which have been
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Fig. 4. Thermogravimetric analysis of methylene blue (a) and catalyst (b).
affected by the adsorption capacity and the blanks. The results obtained are seen in Fig. 6. From Fig. 6a and 6b, it is found that the temperature of the best value for thermocatalytic degradation rate was given at 45 ◦ C regardless of 10 or 20 mg/L in experiments. Additionally, the case of 35 ◦ C was higher than that of 30 ◦ C, it elucidates that the accumulating heat excitation is probably responsible for the thermocatalytic degradation in a way. However, the case of 40 ◦ C disagreed completely with the above explanation. This possible reason may come from the effect of adsorption thermodynamics on the surface of catalyst. But the general trend is rational, which is that the degradation rates increase with the increase of the reaction time in all cases, at least it exhibits that the accumulating heat is significantly important within the limited time interval before the degradation process happens. In light of the concentrations of Fig. 6a and 6b are both at relative lower concentration in comparison to that of Fig.6c, 6d, and 6e. The degradation rate of high concentrations was noticeably enhanced compared with the lower ones (Fig. 6a and 6b), simultaneously, the case of 35 ◦ C for the highest concentrations of 30, 40 and 50 mg/L also gained a better degradation rate than that for the temperatures of 30, 40 and 45 ◦ C [26]. The rate difference seems to be due to the increase in total surface area, and many more active sites formed, which are available to the degradation reaction, it has been
Fig. 5. Effect of different initial concentrations of methylene blue solutions on maximum absorbance wavelength.
confirmed again that there still exist many vacant active sites, which are in well agreement with the case of adsorption test above. Interestingly, the transformation happened in the concentration of 30 mg/L under 35 ◦ C, it manifests that a critical point is existed between the adsorption property and the adsorption thermodynamics. Reactions herein are highly dependent on chemical potentials of species, temperature and pressure, understanding the surface structure and stability of catalyst, according to ab initio atomistic thermodynamics [27]. Specifically, active adsorption sites are directly determined by adsorption property prior to the critical point, thus to further control the degradation rate, and whereas the degradation process has pasted the adsorption equilibrium, in contrast to the former, the adsorption thermodynamics currently plays a crucial role in the process. To sum up, it is concluded that the premised condition of degradation reactions is eventually controlled by the amount of adsorption to some extent. However, a significant drop of degradation rate was observed with the increase of the temperature in Fig. 6d and 6e, , partly because it is likely to associate with an excessive intensive collision with molecules. MB molecules attached to active sites are too unstable to fix to react with catalyst, partly due to much heat is absorbed by MB molecules rather than by catalyst particles. In addition, a plausible explanation is that the intermediate substances formed from degradation process competes with MB molecules for the limited adsorption active sites on the surface of catalyst, and then reduces the degradation rate. By means of the results observed, the optimal values of tests were 24.61, 41.68 and 32.71% purely concerning degradation processes, respectively, wherein the concentrations were 30, 40, and 50 mg/L, respectively. The optimal rate in abatement of organic contaminants was 82.07% in the concentration of 40 mg/L, including the thermocatalytic degradation and physical adsorption (Fig. 7), because the less than 40 mg/L cases no longer provide enough available actives sites, bringing about the lower rate, and the more (50 mg/L only) than 40 mg/L case cannot contribute more available interactional sites because of the competition of MB molecules and others species, consequently, giving rise to the lower rate only. Herein the concentration of 40 mg/L achieves the best rate. The two solutions (placed in one beaker and one cuvette, respectively. owing the initial concentration of 40 mg/L) of left side achieving the adsorption equilibrium, and that of right side with the treatment of degradation after reaction time of 30 h, are presented in Fig. 8. Transparently, two ones above have largely appeared change in color, thus, it is no doubt that the validation of thermocatalytic degradation, and we believe again that this study would be a meaningful attempt at tackling an important issue for the removal technology of water treatment.
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Fig. 6. Degradation efficiencies of methylene blue solutions catalyzed by thermal sensitizer within 30 h. 10 mg/L (a); 20 mg/L (b); 30 mg/L (c); 40 mg/L (d); 50 mg/L (e). In all cases, catalyst loading was 50 mg and the volume of each system was 50 mL.
3.3. Degradation kinetics We found that MB thermodegradation follows a pseudo first order kinetic expression, as presented in Table 2, to list the pseudo first order rate constant, kobs , for degrading MB at different initial concentrations. The kinetics of thermodegradation of MB, paralleled to the heterogeneous catalysis, has been modeled with a simple Langmuir–Hinshelwood equation (Eq. (2)) [28] r=−
kK[MB] d[MB] = = kobs [MB] 1 + K[MB] dt
(2)
Table 2 Pseudo first order rate constants kobs for the thermocatalytic degradation of methylene blue. Initial MB concentration (mg/L)
kobs (h−1 )
10 20 30 40 50
0.06964 0.05944 0.05278 0.04487 0.03677
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Fig. 9. Plot of 1/kobs versus initial concentrations of methylene blue.
Fig. 7. Variation of total removal rate in dependence of the concentration of methylene blue solution.
1 kobs
=
[MB] 1 + kK k
(3)
Table 3 Coupling state affects factor and level. Levels
Integration of Eq. (3) leads to Eq. (4):
ln
[MB] 0
[MB]
= kobs t
(4)
where r is the disappearance of reagent, [MB] represents the reagent concentration (mg/L), [MB]0 is the initial concentration. k exhibits the rate constant of the surface reaction (mg/(L h)), K is the Langmuir–Hinshelwood adsorption equilibrium constant (L/mg), and kobs denotes the pseudo first order rate constant (h−1 ). When to plot ln([MB]0 /[MB]) versus t we could achieve the corresponding kobs values (Table 2). Based on Eq. (3), 1/kobs versus [MB] is a straight line and k = 3.21 mg/(L ·h), and K = 0.029 L/mg calculated from Fig. 9. The obtained R-square is up to 0.9598, demonstrating the expression for thermodegradation fits well with the model above in the kinetics aspect. 3.4. Orthogonal test An orthogonal test design is used for optimizing the degradation conditions. The key parameters that influenced the rate of degradation were analyzed, including reaction temperature, catalyst dosage, initial concentration and initial pH of solution. Hence L9 (34 ) orthogonal array with four factors and three levels was established. In this section, 9 runs labeled S1–S9 were conducted,
Fig. 8. Color of thermocatalytic degradation of methylene blue solutions before and after the process. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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1 2 3
Factors A (mg)
B (mg/L)
C
D (◦ C)
40 50 60
30 40 50
4.4 6.4 8.4
35 40 45
Note: A, dosage; B, concentration; C, pH; D, temperature.
and the removal rate of degradation was determined for each run. The detailed values on levels are revealed in regard to the relevant fators in Table 3, and factors and levels organized, including results and range analysis of orthogonal test, are orderly presented in Table 4. As seen from Fig. 10 and references in Table 4, it is observed that the maximum degradation rate was experienced with the case of S4, and the lowest one was contributed by S6. The rate of degradation varied from even −0.54% to 26.53%. Considering the influences of multiple factors, range analysis was carried out in Table 4, we could find that the influence on the rate of degradation in decreasing order: B > D > A > C according to the R values. The
Fig. 10. The results of orthogonal test in particular conditions designed.
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Table 4 Results and range analysis of L9 (34 ) orthogonal test. Processing number
Dosage A (mg)
Concentration B (mg/L)
Initial pH C
Temperature D (◦ C)
Removal RD (%)
S1 S2 S3 S4 S5 S6 S7 S8 S9
A1(40) A1(40) A1(40) A2(50) A2(50) A2(50) A3(60) A3(60) A3(60)
B1(30) B2(40) B3(50) B1(30) B2(40) B3(50) B1(30) B2(40) B3(50)
C1(4.4) C2(6.4) C3(8.4) C2(4.4) C3(8.4) C1(4.4) C3(8.4) C1(4.4) C2(4.4)
D1(35) D2(40) D3(45) D3(45) D1(35) D2(40) D2(40) D3(45) D1(35)
13.02 15.61 15.51 26.53 13.52 −0.54 25.06 23.42 11.39
K1 K2 K3 R
44.14 39.51 59.87 20.36
62.97 52.55 26.36 36.61
35.90 53.53 54.09 18.19
37.91 40.13 65.46 27.55
Note: Si represents Serial i test. Ki is obtained by adding any number of columns corresponding to i factor. R is the difference between the maximum value and the minimum value of Ki of any columns. RD denotes the removal rate of degradation.
initial concentration to organic matter was found to be the most important determinant of the rate. Fig. 11 presents the effects of various factors on removal rate of degradation. We found that although the factors mentioned play a role in boosting the degradation, considering all kinds of levels, the difference among influences was not considerable obvious, similar observations were also obtained from range analysis. Furthermore, all values in the first part of pareto 80% were relative approaching, causing many more factors belonged to the pareto 80%. Therefore, in case facing complex factors in practical wastewater treatment, the findings could benefit in establishing a control strategy. The effect of MB concentration on degradation rate was dramatically revealed through orthogonal test, it is mainly because the available active sites form with the concentration rising, and they are used to degrade many more MB molecules. But the decrease of the degradation rate was achieved in a certain substrate concentration, which is thought to be due to the combination of direct and reverse reaction [29]. Heat excitation is depended upon changes of temperature, the band gap of catalyst is excited by the accumulating heat. Hence the results obtained in this study seemed to really do not be very appropriate that the irradiation was deemed to be the primary source of electron–hole pairs at ambient temperature, because the band gap was too high to overcome by thermal excitation [30]. We think that heat excitation, and UV light excitation which has been widely studied, are both different ways of energy sources, thus there is a possibility in existence of the mechanism of electron–hole
excitation, more importantly, this validity of thermocatalytic degradation has been confirmed. Regarding the catalyst dosage, it is due to the increasing number of catalytic sites on the surface of catalyst, but too much catalysts may result in the falling amount of MB molecules adsorbed onto the catalyst surface, meanwhile, particle aggregation may also reduce the catalytic activity. It is shown that the influence of different pH values on the absorbance wavelength of MB solutions was very negligible in Fig. 12 [31]. The pH of the wastewater in industry is 6–8, whether pH values had positive or negative effect on degradation rate for organic pollutants, we adjusted the pH of MB solution by ∼2 units down (pH 4.4) and up (pH 8.4) from the initial value (pH 6.4). We found that it did not cause a dramatic increase, yet a bit of decrease in degradation rate, some activated groups in MB molecules is possibly believed to be the main reason for it.
Fig. 11. Pareto chart showing the effect of dependent variables on the removal efficiency. Ki is obtained by adding any number of columns corresponding to i factor.
Fig. 12. Effect of different pH values of methylene blue solutions on maximum absorbance wavelength.
3.5. Structural analysis After the thermocatalytic degradation, in order to confirm the degradation of MB molecules catalyzed by catalyst, Infrared (IR) spectra of before and after degradation process are presented. As is shown in Fig. 13a, previous to degradation, the MB adsorbed onto the catalyst exhibits the prominent bands of characteristic C N ring stretching vibrations at 1599 cm−1 , C C side ring
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Fig. 13. IR spectra of methylene blue adsorbed onto the catalyst (a), catalyst (b) and catalyst after thermocatalytic degradation (c).
stretching vibrations at 1537 and 1482 cm−1 , and N CH3 stretching vibrations at 1240 and 1184 cm−1 [32]. Fig. 13b is the situation that the catalyst was without any treatment. From Fig. 13c, after thermocatalytic degradation happened, the disappearance of the related characteristic bands illustrates the decomposition of MB molecules. Through the comparison above, it elucidates that MB molecules adsorbed onto the surface of catalyst are completely degraded into some small products. In addition, the MB solutions were measured and calculated before and after the tests by UV–vis spectrometer after 30 h, also, a large drop was found in terms of the absorbances of the solutions (also see in Fig. 8), which strongly justified the degradation occurrence from another perspective. According to the Scanning Electron Microscopy (SEM) images in Fig. 14a, the catalyst surface consisted mainly of large agglomerates and blocks substances without any treatment. From Fig. 14b, the adsorption equilibrium was observed in this state, it was found that the catalyst is well compatible with a large of MB molecules. There were some something in common between Fig. 14a and c, as expected, they both remained the paralleled structures on the surfaces of catalyst. It could suggest that MB molecules are exfoliated because of chemical reactions, namely MB molecules are degraded to regain the same structure of the catalyst. The occurrence of thermocatalytic degradation is strongly testified once again. 3.6. Mechanism for thermocatalytic degradation With a series of experiments performed previously, a mechanistic pathway by heat excitation was investigated. The requirements of thermocatalytic degradation are listed as follows: (1) Strong adherence between catalyst and pollutant molecule; (2) Offer a specific surface area as high as possible; (3) Provide sites that can effectively react with pollutant molecules as many as possible. The previous group reported that the acetaldehyde of organic compounds that the thermo-photocatalytic oxidation be a partial oxidation of acetaldehyde by Pt thermocatalyst, and then be an oxidation of intermediates by TiO2 photocatalyst [33]. But our study elucidated that the thermocatalytic reaction on thermal catalyst were more likely to occur during the degradation test under no light.
Fig. 14. SEM images of surfaces of catalysts: without methylene blue (a), adsorption equilibrium with methylene blue (b) and after thermocatalytic degradation (c). Magnification in images is 50 k.
The mechanism of thermocatalytic degradation of pollutants is shown in Fig. 15 under heat excitation (Eq. (5)). The initial reaction happens in the electron cloud of the catalyst cores, and a series of the subsequent reactions are activated by this to achieve the degradation. Upon exposure to the particular temperature, when the heat energy equivalent to or greater than the band gap of catalyst, electrons in the valence band are excited into the conduction band, meanwhile, leaving holes in the valence band. These
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more attention in reduced recombination [45]. For this reason, we will search for more alternative materials and possible techniques to settle it in the aspect of thermocatalytic degradation. h+ + e− → energy
(11)
4. Conclusions
Fig. 15. Schematic illustration of thermocatalytic degradation mechanism of thermal sensitizer.
generated electrons and holes will strongly cause reduction (Eq. (6)) and oxidation (Eq. (7)) reactions, respectively. Catalyst + heatexcitation → h+ + e−
(5)
O2 + e− → O2 −
(6)
H2 O + h+ → H+ + • OH
(7)
A hole can migrate to the surface and oxidize an electron donor, in turn, while at the surface, the catalyst can donate electrons to reduce an electron acceptor [34]. Electrons in the conductor band can be rapidly trapped by molecular oxygen adsorbed on the catalyst surface, which is reduced to form superoxide radical anion (Eq. (6)) that may further react with H+ to generate hydroperoxyl radical (Eq. (8)) and further electrochemical reduction yields H2 O2 (Eq. (9)). O2 − + H+ → • OOH
(8)
• OOH
(9)
+ • OOH → H2 O2 + O2
Besides, intensive active hydroxyl radicals are formed from holes reacting with either H2 O or OH− adsorbed on the catalyst surface [35–38]. The • OH and O2 2− are widely accepted as primary oxidants in heterogeneous catalysis. The oxidizing power of the • OH radicals is strong enough to completely oxidize organic pollutants [39]. These reactive oxygen species may also contribute to the oxidative pathways such as the degradation of organic pollutants (Eq. (10)) [40,41,35]. Pollutants + O2 − + • OH → intermediates → → CO2 + H2 O
(10)
The mechanism of thermocatalytic degradation can be categorized into five steps: (1) transfer of reactants in the fluid phase to the surface; (2) adsorption of the reactants; (3) reaction in the adsorbed phase; (4) desorption of the products; and (5) removal of products from the interface region [42]. In Fig. 15, the recombination of thermocatalytic degradation including the internal and external carriers is the major limitation as it reduces the total quantum efficiency [36]. When the recombination (Eq. (11)) occurs, the excited electron reverts to the valence band without reacting with adsorbed species, non-radiatively or radiatively, dissipating the energy as light or heat [43,44]. Therefore, to enhance separation of generated electrons and holes is paid
Thermocatalytic degradation of MB, considering a variety of factors, such as reaction temperature, reaction time, catalyst dosage, initial concentration and initial pH of the solution, has been explored under room-temperature excitation. In the past, it was originally viewed as impossibility [30], while it ultimately exhibited that the degradation occurred could be conducted. At optimum condition, the abatement rate, containing adsorption and thermocatalytic degradation processes, can reach the highest level of 82.07% solely response to temperature in darkness. Based on experimental results, the dependence of degradation rate on reactional time was modeled considering that it was found similarly to Langmuir–Hinshelwood behavior, and the reactional mechanism for thermocatalytic degradation was assumed and proposed elaborately. Moreover, the content of MB adsorbed onto catalyst was discovered to be crucial in degradation performance. We expect that this degradable route will be extendable to deal with more deleterious pollutants from industrial wastewater and provide a practical guidance on environmental treatment domain. Acknowledgement The authors acknowledge the financial support from the National Key Technology R&D Program, Minister of Science and Technology, People’s Republic of China (Grant No. 2007BAE42B04). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2013.05.005. References [1] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712–717. [2] S. Sarmah, A. Kumar, Photocatalytic activity of polyaniline–TiO2 nanocomposites, Indian J. Phys. 85 (2011) 713–726. [3] M. Fathinia, A.R. Khataee, M. Zarei, S. Aber, Comparative photocatalytic degradation of two dyes on immobilized TiO2 nanoparticles: effect of dye molecular structure and response surface approach, J. Mol. Catal. A: Chem. 333 (2010) 73–84. [4] A. Demirbas, Agricultural based activated carbons for the removal of dyes from aqueous solution: a review, J. Hazard. Mater. 167 (2009) 1–9. [5] W.G. Kuo, Decolorizing dye wastewater with Fenton’s reagent, Water Res. 26 (1992) 881–886. [6] A. El-Nemar, O. Abdel Wahab, A. El-Sikaily, A. Khaled, Removal of direct blue-86 from aqueous solution by new activated carbon developed from orange peel, J. Hazard. Mater. 161 (2009) 102–110. [7] J.H. Sun, S.P. Sun, M.H. Fan, H.Q. Guo, L.P. Qiao, R.X. Sun, A kinetic study on the degradation of p-nitroaniline by Fenton oxidation process, J. Hazard. Mater. 148 (2007) 172–177. [8] L. Lagunas-Allué, M.T. Martinez-Soria, J.S. Asensio, A. Salvador, C. Ferronato, J.M. Chovelon, Photocatalytic degradation of imazalil in an aqueous suspension of TiO2 and influence of alcohols on the degradation, Appl. Catal. B: Environ. 115/116 (2012) 285–293. [9] D.A. Lambropoulou, I.K. Konstantinou, T.A. Albanis, A.R. Fernández-Alba, Photocatalytic degradation of the fungicide Fenhexamid in aqueous TiO2 suspensions: identification of intermediates products and reaction pathways, Chemosphere 83 (2011) 367–378. [10] J. Liu, L. Liu, H. Bai, Y. Wang, D.D. Sun, Gram-scale production of grapheme-oxide TiO2 nanorod composites: towards high-activity photocatalytic materials, Appl. Catal. B: Environ. 106 (2011) 76–82. [11] S.H. Hwang, J. Song, Y. Jung, O.Y. Kweon, H. Song, J. Jang, Electrospun ZnO/TiO2 composite nanofibers as a bactericidal agent, Chem. Commun. 47 (2011) 9164–9166.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
New insights on degradation of methylene blue using thermocatalytic reactions catalyzed by low-temperature excitation Xuegang Luo a,b,∗ , Sizhao Zhang a,b , Xiaoyan Lin a,b a b
School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, Sichuan, China Engineering Research Center of Biomass Materials, Ministry of Education, Mianyang 621010, Sichuan, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Thermocatalytic • • • •
reaction using room-temperature excitation was demonstrated. Results imply that thermal catalyst adopted can be feasible. An excellent contact between contaminant molecule and catalyst was notably required. The optimal removal of 82.07% was reached in total removal efficiency. The mechanism of thermocatalytic process was proposed.
a r t i c l e
i n f o
Article history: Received 19 January 2013 Received in revised form 11 April 2013 Accepted 4 May 2013 Available online 13 May 2013 Keywords: Thermocatalytic degradation Thermal sensitizer Methylene blue Adsorption Mechanism
a b s t r a c t Although photocatalysis has been actively surveyed on removing organic pollutants in ultraviolet (UV) environment, because of lacking UV in solar exposure, photodegradation is difficult to be considerably degraded in conventional exposure condition. In this work, an innovative approach was proposed to compensate for it, which was developed in model wastewater using thermal sensitizer at room temperature. At the optimal component condition, the removal rate of adsorption and thermocatalytic degradation processes can reach the highest level of 82.07% solely response to temperature in the dark. Moreover, the kinetics of degradation rate was modeled considering that it was found similarly to Langmuir–Hinshelwood behavior, and a tentative mechanism was objectively established, describing reasonably well in line with the experimental results. On the other hand, it was found that high amount of methylene blue (MB) adsorbed onto thermal sensitizer was of unambiguous importance to subsequent thermocatalytic performance. Briefly, all above suggest that the feasibility to the thermodegradation route has been successfully verified under room temperature excitation. Herein the insight into degradation pattern of dye over thermal excitation may further enlarge applications for wastewater treatment. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Clean water is substantially essential to survive for the living world in our natural systems. Nevertheless, global warming and
∗ Corresponding author at: Engineering Research Center of Biomass Materials, Ministry of Education, Mianyang 621010, Sichuan, China. Tel.: +86 816 6089009; fax: +86 816 6089009. E-mail addresses: [email protected], [email protected] (X. Luo), [email protected] (S. Zhang). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.05.005
contamination from industrialization continue to strain the already scarce supply of clean water [1–4]. To date, among all the settlement to these problems, photocatalysis is one of the most available routes for degradation of organic pollutants to innoxious substances (such as H2 O and CO2 , or other species) in wastewater [5–7]. In particular, to use semiconductor materials as its photocatalyst, such as TiO2 , ZnO, CdS, and Ag particles, are in response to ultraviolet (UV) light and attract considerable attentions for applications [8–16]. However, little progress has been made in industrial application because its quantum efficiency in photocatalyst reaction is generally not high enough from application point of view [17–19].
X. Luo et al. / Journal of Hazardous Materials 260 (2013) 112–121 Table 1 Data obtained by XRF analysis.
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correlation coefficients were higher than 0.999, it could be permitted to apply in subsequent tests.
Main ingredients for thermal sensitizer SiO2 /%
CaO/%
MnO/%
NiO/%
CuO/%
18.14
17.75
38.58
9.61
11.68
Therefore, it is a pity that the degradation processes would take place difficultly when without or lacking UV light in solar exposure. To overcome aforementioned disadvantage, it is imperative to explore more effective ways to solve currently arisen problems. As we know, any physical or chemical change in substances as a result of corresponding environmental factors, such as light, heat, moisture, chemical conditions or biological activity [20]. Recently, by means of the synergistic effect, between photocatalytic and thermocatalytic reactions, benzene has been degraded using Pt-loaded TiO2 /ZrO2 catalyst [21]. It exhibits that there is a reasonable approach to achieve thermocatalytic degradation. No data on degrading model pollutants were reported through heat excitation, however. To the best of our knowledge, a comprehensive analysis of the existing literature exhibits no previous studies have been conducted at understanding the destruction of the substrate using thermal sensitizer excited by room-temperature excitation. Herein the aims of the study are to explore a novel material which can be viewed as thermal catalyst, and then demonstrate that it is feasible to thermocatalytic reactions. Thus, in continuation of the work on surveying the effect of reaction temperature, reaction time, catalyst dosage, initial concentration and initial pH of the solution. Besides the adsorption process was also studied prior to the degradable reaction [22]. A possible systemic mechanism for thermocatalytic degradation was proposed, and the dependence of degradation rate on reactional time was also modeled. 2. Experimental 2.1. Chemicals and reaction apparatus All of the chemicals were of analytical grade or reagent grade without further purification except thermal sensitizer (Mn–Si–Ca–Cu–Ni complex oxides synthesized by high temperature calcination, considered as thermal catalyst, details in Table 1). Methylene blue (MB) dye was performed to the corresponding solutions of desired concentrations. Deionized and doubly distilled water was employed throughout the experiment. The different pH values of the reaction solutions were adjusted by adding diluted HCl or NaOH solutions (both 0.05 M) at room temperature. Experiments were carried out in a reaction apparatus. The schematic diagram is shown in Fig. 1. The absorbances of various concentrations for MB solutions were recorded by using Ultraviolet Visible (UV–vis) spectrometer. In all cases, when the measured
Fig. 1. Schematic diagram of reaction apparatus: (1) heat, (2) reaction solution, (3) water environment, (4) conical flask for reaction site, (5) stirring rod, and (6) magnetic agitation and heater.
2.2. Tests for adsorption and degradation In this study, all experiments were implemented at atm air pressure. Tests for adsorption and degradation were both conducted in darkness. Thermal sensitizers were processed into different particle sizes. The dosage of thermal sensitizer was suspended at 50 mg in all solutions, and it was added to glass vessels containing 50 mL dye solution of different concentrations (including 10, 20 30, 40 and 50 mg/L), and the temperatures of reaction processes were varied at 30, 35, 40, and 45 ◦ C. In order to disperse the thermal sensitizer powders, a magnetic stirring device was applied to the reaction system. When the adsorption process was achieved to an adsorption–desorption equilibrium after stirring the suspension for 2 h, the amount of adsorption was recorded, and it could be identified by changes of the concentrations of MB solutions. MB solution of 5 mL was taken out after the same intervals and centrifuged immediately at 5500 rpm for 10 min. Then, the centrifuged solution was filtered to remove the suspended catalyst agglomerates. Additionally, in order to confirm the reproducibility of the results, duplicated processes were performed for each condition for averaging results, and the experiment error was found to be within ±5%. Thermocatalytic reaction was controlled in 50 mL glass vessel containing 50 mL of different dye solutions, and thermal sensitizer of 50 mg was added in it. Each solution was kept under constant stirring and heated with a heater system. The reaction solutions were carried out for 30 h, sampling was in step of 6 h, and the absorbances of MB solutions were recorded after each step. In addition, blank experiments were also employed under dark following the same procedures. To optimize the design of the process, an orthogonal test was applied to the whole tests, concerning the initial concentration, catalyst dosage, pH value and temperature. The decomposition of the dyes is estimated by the following equation: Degradation(%) =
c0 − c1 − c2 × 100 c0
(1)
where c0 represents the concentration of the dye without any treatment, c1 denotes the concentration of the dye after adsorption–desorption equilibrium process, c2 exhibits the concentration of the dye which was affected by bank experiment, respectively. 2.3. Characterizations The detected elements of thermal sensitizer were measured by X-ray Fluorescence spectrometer (XRF, Rigaku Primini, Japan) equipped with 50 W end window which was used with Rh anode X-ray tube, operated at a maximum voltage of 40 kV and current of 1.25 mA, control temperature of 36.5 ± 0.5 ◦ C, gas flow rate 5–7 mL/min. The optical spectra of all samples were obtained using Ultraviolet Visible (UV–vis) spectrometer (UV3900, Hitachi Corporation, Japan). The analysis of particle size was carried out on a particle size analyzer (90Plus Brookhaven Instruments Corporation, USA). Infrared (IR) spectra of all dried materials were measured at 400–4000 cm−1 by infrared spectrophotometer (Nicolet 6700, Nicolet Instruments Corporation, USA) using KBr pellets. The surface structure and morphology of thermal sensitizer were characterized using Scanning Electron Microscopy (SEM, Ultra 55, Zeiss Corporation, Germany). Thermal analysis was performed on a Thermogravimetric Analyzer (TGA, Q500, TA Instruments Corporation, USA).
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Fig. 2. Effect of methylene blue adsorbed onto thermal sensitizer in the conditions of different particle sizes of thermal sensitizer (a) and different concentrations of methylene blue solutions (b) on adsorption capacity.
3. Results and discussion 3.1. Adsorption of methylene blue In order to achieve an excellent degradation efficiency, it is essential to gain a good adsorption capacity. In this regard, to select the catalyst with appropriate particular particle size is very critical. As shown in Fig. 2a, the absorbances of various state solutions were measured (see Fig. S1 and S2, Supplementary Material), it is considerably obvious that, the adsorption of methylene blue (MB) onto catalyst surface was gradually approaching to an adsorption equilibrium after 80 min, whereas that of 100 mesh size remained hardly unchanged, until adsorption–desorption equilibrium was noticeably established at 100 min. On the other hand, the adsorption capacity increased rapidly within the previous 80 min, and increased with the increase of the number of particle mesh at the initial stage, especially, that of 200 mesh size was nearly approaching to the final adsorption equilibrium at the early stage. It is strongly obvious that the effect of adsorption capacity fundamentally depends upon the particle size, and namely the appropriate specific surface is of importance for catalysis. To further confirm the predicted insight into the influence of adsorption, as shown in Fig. 2b, the absolute amount of adsorption increased with the concentrations of MB solutions, although the higher concentrations owned a lower rate of adsorption, owing
Fig. 3. The particle size and particle distribution of thermal sensitizer obtained.
to their high concentrations, by calculating, the solutions of orderly increasing concentrations possessed in turn corresponding increasing capacity. Therefore, it is clear that there still exist a vast number of available adsorption sites in a relative diluted solution. Since agitation speed was constant, and the diffusion of dye molecules through the solution to the surface of catalyst was affected by the dye concentration; the dye concentration with an increase accelerates the diffusion of dyes, because of the increase in the driving force of the concentration gradient [23,24]. In other word, the activity of adsorption would be enhanced with the rise of concentration. Fig. 3 shows the size of thermal sensitizer with the effective diameter up to 442 nm, and the dispersibility as a function of particles was approximately 0.269 a.u., and the dispersibility was in accordance with a normal distribution. In view of the outstanding property in adsorption capacity, the chosen catalyst was in the 200 mesh size, which was applied to the subsequent tests. 3.2. Thermocatalytic degradation processes To exclude the negative impact on the experimental materials caused by thermal aspect, we should fully understand the thermal properties of all substances adopted. From Fig. 4a, at first step, weight loss of MB observed is due to the existence of water molecules in MB, when temperature ranging from initial point to 257.9 ◦ C. Subsequently, at second step, the loss of 23.7 wt% is probably on the ground of some weak bonds of MB molecules broken, and 17.87 wt% loss occurred because of the chemolysis of MB molecules, which happens chemical decomposition reactions on account of heat effect. It shows that the complete decomposed temperature has been achieved in this time. As indicated in Fig. 4b, a similar tendency was found in the process, it was divided into two stages. Loss occurred at first step was even up to 342.2 ◦ C. That is to say, the possible influence resulted from thermal aspect plays an extremely weak role in MB molecules and catalysts at room temperature. In Fig. 5, it is apparent that no change was observed in terms of maximum absorbance wavelength of MB solutions, and there were only some slight differences in absorbance intensities of solutions as the concentration increasing. By now, the current studies have confirmed that the temperature to all substances adopted, and the concentration of solution to maximum absorbance wavelength in experiments, both have insignificant influence on the thermocatalytic degradation [25]. The relationships of thermocatalytic degradation rate, experimental temperatures, solution concentrations, and reaction time have been investigated [26]. It is worth mentioning, the degradation rates calculated have removed the parts which have been
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Fig. 4. Thermogravimetric analysis of methylene blue (a) and catalyst (b).
affected by the adsorption capacity and the blanks. The results obtained are seen in Fig. 6. From Fig. 6a and 6b, it is found that the temperature of the best value for thermocatalytic degradation rate was given at 45 ◦ C regardless of 10 or 20 mg/L in experiments. Additionally, the case of 35 ◦ C was higher than that of 30 ◦ C, it elucidates that the accumulating heat excitation is probably responsible for the thermocatalytic degradation in a way. However, the case of 40 ◦ C disagreed completely with the above explanation. This possible reason may come from the effect of adsorption thermodynamics on the surface of catalyst. But the general trend is rational, which is that the degradation rates increase with the increase of the reaction time in all cases, at least it exhibits that the accumulating heat is significantly important within the limited time interval before the degradation process happens. In light of the concentrations of Fig. 6a and 6b are both at relative lower concentration in comparison to that of Fig.6c, 6d, and 6e. The degradation rate of high concentrations was noticeably enhanced compared with the lower ones (Fig. 6a and 6b), simultaneously, the case of 35 ◦ C for the highest concentrations of 30, 40 and 50 mg/L also gained a better degradation rate than that for the temperatures of 30, 40 and 45 ◦ C [26]. The rate difference seems to be due to the increase in total surface area, and many more active sites formed, which are available to the degradation reaction, it has been
Fig. 5. Effect of different initial concentrations of methylene blue solutions on maximum absorbance wavelength.
confirmed again that there still exist many vacant active sites, which are in well agreement with the case of adsorption test above. Interestingly, the transformation happened in the concentration of 30 mg/L under 35 ◦ C, it manifests that a critical point is existed between the adsorption property and the adsorption thermodynamics. Reactions herein are highly dependent on chemical potentials of species, temperature and pressure, understanding the surface structure and stability of catalyst, according to ab initio atomistic thermodynamics [27]. Specifically, active adsorption sites are directly determined by adsorption property prior to the critical point, thus to further control the degradation rate, and whereas the degradation process has pasted the adsorption equilibrium, in contrast to the former, the adsorption thermodynamics currently plays a crucial role in the process. To sum up, it is concluded that the premised condition of degradation reactions is eventually controlled by the amount of adsorption to some extent. However, a significant drop of degradation rate was observed with the increase of the temperature in Fig. 6d and 6e, , partly because it is likely to associate with an excessive intensive collision with molecules. MB molecules attached to active sites are too unstable to fix to react with catalyst, partly due to much heat is absorbed by MB molecules rather than by catalyst particles. In addition, a plausible explanation is that the intermediate substances formed from degradation process competes with MB molecules for the limited adsorption active sites on the surface of catalyst, and then reduces the degradation rate. By means of the results observed, the optimal values of tests were 24.61, 41.68 and 32.71% purely concerning degradation processes, respectively, wherein the concentrations were 30, 40, and 50 mg/L, respectively. The optimal rate in abatement of organic contaminants was 82.07% in the concentration of 40 mg/L, including the thermocatalytic degradation and physical adsorption (Fig. 7), because the less than 40 mg/L cases no longer provide enough available actives sites, bringing about the lower rate, and the more (50 mg/L only) than 40 mg/L case cannot contribute more available interactional sites because of the competition of MB molecules and others species, consequently, giving rise to the lower rate only. Herein the concentration of 40 mg/L achieves the best rate. The two solutions (placed in one beaker and one cuvette, respectively. owing the initial concentration of 40 mg/L) of left side achieving the adsorption equilibrium, and that of right side with the treatment of degradation after reaction time of 30 h, are presented in Fig. 8. Transparently, two ones above have largely appeared change in color, thus, it is no doubt that the validation of thermocatalytic degradation, and we believe again that this study would be a meaningful attempt at tackling an important issue for the removal technology of water treatment.
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Fig. 6. Degradation efficiencies of methylene blue solutions catalyzed by thermal sensitizer within 30 h. 10 mg/L (a); 20 mg/L (b); 30 mg/L (c); 40 mg/L (d); 50 mg/L (e). In all cases, catalyst loading was 50 mg and the volume of each system was 50 mL.
3.3. Degradation kinetics We found that MB thermodegradation follows a pseudo first order kinetic expression, as presented in Table 2, to list the pseudo first order rate constant, kobs , for degrading MB at different initial concentrations. The kinetics of thermodegradation of MB, paralleled to the heterogeneous catalysis, has been modeled with a simple Langmuir–Hinshelwood equation (Eq. (2)) [28] r=−
kK[MB] d[MB] = = kobs [MB] 1 + K[MB] dt
(2)
Table 2 Pseudo first order rate constants kobs for the thermocatalytic degradation of methylene blue. Initial MB concentration (mg/L)
kobs (h−1 )
10 20 30 40 50
0.06964 0.05944 0.05278 0.04487 0.03677
X. Luo et al. / Journal of Hazardous Materials 260 (2013) 112–121
Fig. 9. Plot of 1/kobs versus initial concentrations of methylene blue.
Fig. 7. Variation of total removal rate in dependence of the concentration of methylene blue solution.
1 kobs
=
[MB] 1 + kK k
(3)
Table 3 Coupling state affects factor and level. Levels
Integration of Eq. (3) leads to Eq. (4):
ln
[MB] 0
[MB]
= kobs t
(4)
where r is the disappearance of reagent, [MB] represents the reagent concentration (mg/L), [MB]0 is the initial concentration. k exhibits the rate constant of the surface reaction (mg/(L h)), K is the Langmuir–Hinshelwood adsorption equilibrium constant (L/mg), and kobs denotes the pseudo first order rate constant (h−1 ). When to plot ln([MB]0 /[MB]) versus t we could achieve the corresponding kobs values (Table 2). Based on Eq. (3), 1/kobs versus [MB] is a straight line and k = 3.21 mg/(L ·h), and K = 0.029 L/mg calculated from Fig. 9. The obtained R-square is up to 0.9598, demonstrating the expression for thermodegradation fits well with the model above in the kinetics aspect. 3.4. Orthogonal test An orthogonal test design is used for optimizing the degradation conditions. The key parameters that influenced the rate of degradation were analyzed, including reaction temperature, catalyst dosage, initial concentration and initial pH of solution. Hence L9 (34 ) orthogonal array with four factors and three levels was established. In this section, 9 runs labeled S1–S9 were conducted,
Fig. 8. Color of thermocatalytic degradation of methylene blue solutions before and after the process. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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1 2 3
Factors A (mg)
B (mg/L)
C
D (◦ C)
40 50 60
30 40 50
4.4 6.4 8.4
35 40 45
Note: A, dosage; B, concentration; C, pH; D, temperature.
and the removal rate of degradation was determined for each run. The detailed values on levels are revealed in regard to the relevant fators in Table 3, and factors and levels organized, including results and range analysis of orthogonal test, are orderly presented in Table 4. As seen from Fig. 10 and references in Table 4, it is observed that the maximum degradation rate was experienced with the case of S4, and the lowest one was contributed by S6. The rate of degradation varied from even −0.54% to 26.53%. Considering the influences of multiple factors, range analysis was carried out in Table 4, we could find that the influence on the rate of degradation in decreasing order: B > D > A > C according to the R values. The
Fig. 10. The results of orthogonal test in particular conditions designed.
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Table 4 Results and range analysis of L9 (34 ) orthogonal test. Processing number
Dosage A (mg)
Concentration B (mg/L)
Initial pH C
Temperature D (◦ C)
Removal RD (%)
S1 S2 S3 S4 S5 S6 S7 S8 S9
A1(40) A1(40) A1(40) A2(50) A2(50) A2(50) A3(60) A3(60) A3(60)
B1(30) B2(40) B3(50) B1(30) B2(40) B3(50) B1(30) B2(40) B3(50)
C1(4.4) C2(6.4) C3(8.4) C2(4.4) C3(8.4) C1(4.4) C3(8.4) C1(4.4) C2(4.4)
D1(35) D2(40) D3(45) D3(45) D1(35) D2(40) D2(40) D3(45) D1(35)
13.02 15.61 15.51 26.53 13.52 −0.54 25.06 23.42 11.39
K1 K2 K3 R
44.14 39.51 59.87 20.36
62.97 52.55 26.36 36.61
35.90 53.53 54.09 18.19
37.91 40.13 65.46 27.55
Note: Si represents Serial i test. Ki is obtained by adding any number of columns corresponding to i factor. R is the difference between the maximum value and the minimum value of Ki of any columns. RD denotes the removal rate of degradation.
initial concentration to organic matter was found to be the most important determinant of the rate. Fig. 11 presents the effects of various factors on removal rate of degradation. We found that although the factors mentioned play a role in boosting the degradation, considering all kinds of levels, the difference among influences was not considerable obvious, similar observations were also obtained from range analysis. Furthermore, all values in the first part of pareto 80% were relative approaching, causing many more factors belonged to the pareto 80%. Therefore, in case facing complex factors in practical wastewater treatment, the findings could benefit in establishing a control strategy. The effect of MB concentration on degradation rate was dramatically revealed through orthogonal test, it is mainly because the available active sites form with the concentration rising, and they are used to degrade many more MB molecules. But the decrease of the degradation rate was achieved in a certain substrate concentration, which is thought to be due to the combination of direct and reverse reaction [29]. Heat excitation is depended upon changes of temperature, the band gap of catalyst is excited by the accumulating heat. Hence the results obtained in this study seemed to really do not be very appropriate that the irradiation was deemed to be the primary source of electron–hole pairs at ambient temperature, because the band gap was too high to overcome by thermal excitation [30]. We think that heat excitation, and UV light excitation which has been widely studied, are both different ways of energy sources, thus there is a possibility in existence of the mechanism of electron–hole
excitation, more importantly, this validity of thermocatalytic degradation has been confirmed. Regarding the catalyst dosage, it is due to the increasing number of catalytic sites on the surface of catalyst, but too much catalysts may result in the falling amount of MB molecules adsorbed onto the catalyst surface, meanwhile, particle aggregation may also reduce the catalytic activity. It is shown that the influence of different pH values on the absorbance wavelength of MB solutions was very negligible in Fig. 12 [31]. The pH of the wastewater in industry is 6–8, whether pH values had positive or negative effect on degradation rate for organic pollutants, we adjusted the pH of MB solution by ∼2 units down (pH 4.4) and up (pH 8.4) from the initial value (pH 6.4). We found that it did not cause a dramatic increase, yet a bit of decrease in degradation rate, some activated groups in MB molecules is possibly believed to be the main reason for it.
Fig. 11. Pareto chart showing the effect of dependent variables on the removal efficiency. Ki is obtained by adding any number of columns corresponding to i factor.
Fig. 12. Effect of different pH values of methylene blue solutions on maximum absorbance wavelength.
3.5. Structural analysis After the thermocatalytic degradation, in order to confirm the degradation of MB molecules catalyzed by catalyst, Infrared (IR) spectra of before and after degradation process are presented. As is shown in Fig. 13a, previous to degradation, the MB adsorbed onto the catalyst exhibits the prominent bands of characteristic C N ring stretching vibrations at 1599 cm−1 , C C side ring
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Fig. 13. IR spectra of methylene blue adsorbed onto the catalyst (a), catalyst (b) and catalyst after thermocatalytic degradation (c).
stretching vibrations at 1537 and 1482 cm−1 , and N CH3 stretching vibrations at 1240 and 1184 cm−1 [32]. Fig. 13b is the situation that the catalyst was without any treatment. From Fig. 13c, after thermocatalytic degradation happened, the disappearance of the related characteristic bands illustrates the decomposition of MB molecules. Through the comparison above, it elucidates that MB molecules adsorbed onto the surface of catalyst are completely degraded into some small products. In addition, the MB solutions were measured and calculated before and after the tests by UV–vis spectrometer after 30 h, also, a large drop was found in terms of the absorbances of the solutions (also see in Fig. 8), which strongly justified the degradation occurrence from another perspective. According to the Scanning Electron Microscopy (SEM) images in Fig. 14a, the catalyst surface consisted mainly of large agglomerates and blocks substances without any treatment. From Fig. 14b, the adsorption equilibrium was observed in this state, it was found that the catalyst is well compatible with a large of MB molecules. There were some something in common between Fig. 14a and c, as expected, they both remained the paralleled structures on the surfaces of catalyst. It could suggest that MB molecules are exfoliated because of chemical reactions, namely MB molecules are degraded to regain the same structure of the catalyst. The occurrence of thermocatalytic degradation is strongly testified once again. 3.6. Mechanism for thermocatalytic degradation With a series of experiments performed previously, a mechanistic pathway by heat excitation was investigated. The requirements of thermocatalytic degradation are listed as follows: (1) Strong adherence between catalyst and pollutant molecule; (2) Offer a specific surface area as high as possible; (3) Provide sites that can effectively react with pollutant molecules as many as possible. The previous group reported that the acetaldehyde of organic compounds that the thermo-photocatalytic oxidation be a partial oxidation of acetaldehyde by Pt thermocatalyst, and then be an oxidation of intermediates by TiO2 photocatalyst [33]. But our study elucidated that the thermocatalytic reaction on thermal catalyst were more likely to occur during the degradation test under no light.
Fig. 14. SEM images of surfaces of catalysts: without methylene blue (a), adsorption equilibrium with methylene blue (b) and after thermocatalytic degradation (c). Magnification in images is 50 k.
The mechanism of thermocatalytic degradation of pollutants is shown in Fig. 15 under heat excitation (Eq. (5)). The initial reaction happens in the electron cloud of the catalyst cores, and a series of the subsequent reactions are activated by this to achieve the degradation. Upon exposure to the particular temperature, when the heat energy equivalent to or greater than the band gap of catalyst, electrons in the valence band are excited into the conduction band, meanwhile, leaving holes in the valence band. These
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more attention in reduced recombination [45]. For this reason, we will search for more alternative materials and possible techniques to settle it in the aspect of thermocatalytic degradation. h+ + e− → energy
(11)
4. Conclusions
Fig. 15. Schematic illustration of thermocatalytic degradation mechanism of thermal sensitizer.
generated electrons and holes will strongly cause reduction (Eq. (6)) and oxidation (Eq. (7)) reactions, respectively. Catalyst + heatexcitation → h+ + e−
(5)
O2 + e− → O2 −
(6)
H2 O + h+ → H+ + • OH
(7)
A hole can migrate to the surface and oxidize an electron donor, in turn, while at the surface, the catalyst can donate electrons to reduce an electron acceptor [34]. Electrons in the conductor band can be rapidly trapped by molecular oxygen adsorbed on the catalyst surface, which is reduced to form superoxide radical anion (Eq. (6)) that may further react with H+ to generate hydroperoxyl radical (Eq. (8)) and further electrochemical reduction yields H2 O2 (Eq. (9)). O2 − + H+ → • OOH
(8)
• OOH
(9)
+ • OOH → H2 O2 + O2
Besides, intensive active hydroxyl radicals are formed from holes reacting with either H2 O or OH− adsorbed on the catalyst surface [35–38]. The • OH and O2 2− are widely accepted as primary oxidants in heterogeneous catalysis. The oxidizing power of the • OH radicals is strong enough to completely oxidize organic pollutants [39]. These reactive oxygen species may also contribute to the oxidative pathways such as the degradation of organic pollutants (Eq. (10)) [40,41,35]. Pollutants + O2 − + • OH → intermediates → → CO2 + H2 O
(10)
The mechanism of thermocatalytic degradation can be categorized into five steps: (1) transfer of reactants in the fluid phase to the surface; (2) adsorption of the reactants; (3) reaction in the adsorbed phase; (4) desorption of the products; and (5) removal of products from the interface region [42]. In Fig. 15, the recombination of thermocatalytic degradation including the internal and external carriers is the major limitation as it reduces the total quantum efficiency [36]. When the recombination (Eq. (11)) occurs, the excited electron reverts to the valence band without reacting with adsorbed species, non-radiatively or radiatively, dissipating the energy as light or heat [43,44]. Therefore, to enhance separation of generated electrons and holes is paid
Thermocatalytic degradation of MB, considering a variety of factors, such as reaction temperature, reaction time, catalyst dosage, initial concentration and initial pH of the solution, has been explored under room-temperature excitation. In the past, it was originally viewed as impossibility [30], while it ultimately exhibited that the degradation occurred could be conducted. At optimum condition, the abatement rate, containing adsorption and thermocatalytic degradation processes, can reach the highest level of 82.07% solely response to temperature in darkness. Based on experimental results, the dependence of degradation rate on reactional time was modeled considering that it was found similarly to Langmuir–Hinshelwood behavior, and the reactional mechanism for thermocatalytic degradation was assumed and proposed elaborately. Moreover, the content of MB adsorbed onto catalyst was discovered to be crucial in degradation performance. We expect that this degradable route will be extendable to deal with more deleterious pollutants from industrial wastewater and provide a practical guidance on environmental treatment domain. Acknowledgement The authors acknowledge the financial support from the National Key Technology R&D Program, Minister of Science and Technology, People’s Republic of China (Grant No. 2007BAE42B04). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2013.05.005. References [1] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712–717. [2] S. Sarmah, A. Kumar, Photocatalytic activity of polyaniline–TiO2 nanocomposites, Indian J. Phys. 85 (2011) 713–726. [3] M. Fathinia, A.R. Khataee, M. Zarei, S. Aber, Comparative photocatalytic degradation of two dyes on immobilized TiO2 nanoparticles: effect of dye molecular structure and response surface approach, J. Mol. Catal. A: Chem. 333 (2010) 73–84. [4] A. Demirbas, Agricultural based activated carbons for the removal of dyes from aqueous solution: a review, J. Hazard. Mater. 167 (2009) 1–9. [5] W.G. Kuo, Decolorizing dye wastewater with Fenton’s reagent, Water Res. 26 (1992) 881–886. [6] A. El-Nemar, O. Abdel Wahab, A. El-Sikaily, A. Khaled, Removal of direct blue-86 from aqueous solution by new activated carbon developed from orange peel, J. Hazard. Mater. 161 (2009) 102–110. [7] J.H. Sun, S.P. Sun, M.H. Fan, H.Q. Guo, L.P. Qiao, R.X. Sun, A kinetic study on the degradation of p-nitroaniline by Fenton oxidation process, J. Hazard. Mater. 148 (2007) 172–177. [8] L. Lagunas-Allué, M.T. Martinez-Soria, J.S. Asensio, A. Salvador, C. Ferronato, J.M. Chovelon, Photocatalytic degradation of imazalil in an aqueous suspension of TiO2 and influence of alcohols on the degradation, Appl. Catal. B: Environ. 115/116 (2012) 285–293. [9] D.A. Lambropoulou, I.K. Konstantinou, T.A. Albanis, A.R. Fernández-Alba, Photocatalytic degradation of the fungicide Fenhexamid in aqueous TiO2 suspensions: identification of intermediates products and reaction pathways, Chemosphere 83 (2011) 367–378. [10] J. Liu, L. Liu, H. Bai, Y. Wang, D.D. Sun, Gram-scale production of grapheme-oxide TiO2 nanorod composites: towards high-activity photocatalytic materials, Appl. Catal. B: Environ. 106 (2011) 76–82. [11] S.H. Hwang, J. Song, Y. Jung, O.Y. Kweon, H. Song, J. Jang, Electrospun ZnO/TiO2 composite nanofibers as a bactericidal agent, Chem. Commun. 47 (2011) 9164–9166.
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