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TiO2 system in the presence of the electron acceptors Na2S2O8 and H2O2
Journal Pre-proofs Photocatalytic Degradation of Aniline by Solar/TiO2 System in the Presence of the Electron Acceptors Na2S2O8 and H2O2 J.M. Monteagudo, A. Durán, I. San Martín, B. Vellón PII: DOI: Reference:
S1383-5866(19)34641-6 https://doi.org/10.1016/j.seppur.2019.116456 SEPPUR 116456
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Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
10 October 2019 18 December 2019 18 December 2019
Please cite this article as: J.M. Monteagudo, A. Durán, I. San Martín, B. Vellón, Photocatalytic Degradation of Aniline by Solar/TiO2 System in the Presence of the Electron Acceptors Na2S2O8 and H2O2, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116456
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Photocatalytic Degradation of Aniline by Solar/TiO2 System in the Presence of the Electron Acceptors Na2S2O8 and H2O2
J.M. Monteagudo*, A. Durán, I. San Martín, B. Vellón a
Department of Chemical Engineering, Grupo IMAES, Escuela Técnica Superior de
Ingenieros Industriales, Instituto de Investigaciones Energéticas y Aplicaciones Industriales (INEI), University of Castilla-La Mancha, Avda. Camilo José Cela 3, 13071 Ciudad Real (Spain).
* To whom correspondence should be addressed Department of Chemical Engineering, Grupo IMAES Escuela Técnica Superior de Ingenieros Industriales, Instituto de Investigaciones Energéticas y Aplicaciones Industriales (INEI) University of Castilla-La Mancha, Avda. Camilo José Cela 3, 13071 Ciudad Real (Spain). Fax: 0034 926295361. Phone: 0034 926295300, ext: 3888 Email: [email protected]
ABSTRACT
Aniline aqueous solution degradation by solar/TiO2 system in combination with electron acceptors such as Na2S2O8 and/or H2O2 was studied. The effects of TiO2 initial concentration, dose of persulfate and hydrogen peroxide, and pH were evaluated. The solar/TiO2/Na2S2O8 process was found to be more efficient than the solar/TiO2/H2O2 process. Under optimal conditions, the percentage of removal of solution TOC was 91.5% in approximately t30W= 30 min (reaction time= 60 min). The addition of persulfate, until to an optimum value, to the solar/TiO2 system inhibited the electron/hole pair recombination, increasing the availability of holes and SO4- and consequently HO radicals. H2O2 also inhibited the electron/hole recombination and generated HO radicals although an excess reduced the degradation efficiency. The roles played by reactive intermediate species, such as SO4-, HO•, O2•- and holes in both solar/TiO2/Na2S2O8 and solar/TiO2/H2O2 processes were determined using appropriated quenchers. SO4- radicals and holes were found to be the main responsible species in the solar/TiO2/Na2S2O8 system.
Keywords: aniline; solar; TiO2; persulfate; H2O2; electron acceptor.
1
1. INTRODUCTION
Aniline is an organic compound mainly used in the manufacture of precursors to pharmaceuticals [1]. In the last few years, the presence of this pollutant refractory to biodegradation in industrial wastewater efluents has increased. Besides, aniline causes toxicity in living organisms [2, 3]. Therefore, an effective degradation and mineralization of aniline should be studied. Table 1 shows different types of aniline removal treatment previously published in the literature [4-26]. The use of titanium dioxide has become of great interest due to its low cost, high chemical stability and its activation with solar light [27]. It is well-known that different Reactive Oxygen Species, ROS, such as HO and O2, generated in the catalyst surface from reactions between electron/hole pair and O2 and H2O2, are responsible for contaminant degradation. Nevertheless, the efficiency of this TiO2 photocatalytic system decreases considerably when the recombination of photoinduced electrons and holes takes place. In this sense, the addition of electron acceptors such as S2O82- or H2O2 prevents this recombination since they trap photogenerated electrons, increasing the availability of holes in the medium and facilitating their participation in the generation of ROS [28-30]. However, to the best of our knowledge, the addition of S2O82- or H2O2 to the solar/TiO2 system for aniline degradation has not yet been studied.
The aim of this work was to evaluate the removal of aniline and reaction intermediates by a solar process based on TiO2 combined with persulfate and/or hydrogen peroxide as electron acceptors. The effect of several variables such as initial concentration of TiO2, dose of H2O2 or S2O82- and pH were studied. Finally, the roles of HO•, SO4-, O2•- and holes, hVB+, in both solar/TiO2/electron acceptor-based systems were investigated.
2
2. EXPERIMENT
2.1. Materials
TiO2 P25 (Aeroxide, Degussa Corporation) was obtained from Evonik Industries. Aniline, C6H7N (99%), p-benzoquinone, tert-butyl alcohol (99.7%), TiOSO4, methanol (99.8%) and glycerol solution (86-89%) were supplied by Sigma-Aldrich. Sodium persulfate (Na2S2O8, 98%) was purchased from Panreac, and hydrogen peroxide (30% w/v) was obtained from Merck.
2.2 TiO2 Characterization
Characterization of TiO2 including XRD, TEM, UV-Vis reflectance spectra, Raman spectroscopy, Z-potential measurements, Tauc plots and BET analysis were previously publicated by the authors [27, 31].
2.3. Solar CPC reactor
A solar CPC reactor (Ecosystem, S.A.) was employed for the degradation experiments. It was well characterized in our previous work [27]. The total volume of the equipment (tubes + tank) was 3.5 L. A centrifugal pump was used to recirculate the water flow (flow rate: 30 L min-1) through the CPC. 3
2.4. Solar/TiO2/electron acceptor Process
First, aniline and TiO2 catalyst were mixed in the tank for 30 min until adsorption equilibrium was attained. Then, the mixture in the presence or absence of electron acceptors (S2O82- or H2O2) was pumped through the CPC and irradiated with solar light. Different samples along the reaction were collected to analyze the concentrations of aniline, Total Organic Carbon (TOC), persulfate, H2O2 and dissolved oxygen.
Several quenchers of radicals and holes were used to quantify the roles played by the reactive intermediate species, as will be explained below.
2.5. Analytic methods
2.5.1. Analysis of Aniline and TOC
The concentration of aniline was analyzed by HPLC (Agilent Technologies 1100 HPLCUV; column Eclipse XDB-C18; = 205 nm; flow rate= 0.8 ml min-1; mobile phase= 30:70 (v/v) acetonitrile/(water/1% acetic acid). Mineralization degree (TOC) was obtained by using a TOC-5050 Shimadzu analyzer.
2.5.2. H2O2, S2O82- and dissolved O2 measurement
4
H2O2 concentration was analyzed spectrophotometrically by using TiOSO4 [32, 33]. The residual S2O82- concentration was measured by reaction with sodium carbonate and potassium iodide [34]. Dissolved O2 was obtained using a Jenway 9200 DO2 meter.
2.5.3. Solar Radiation Intensity Determination
UV-A solar radiation intensity (W m-2) was measured by a radiometer (Ecosystem, model ACADUS-85) [35].
2.5.4. Normalized illumination time
When the intensity of solar radiation varied considerably from one experiment to another, a widely accepted equation in the literature (Eq. (1)) was used to normalize the data for comparative purposes [36]:
𝑡30𝑊,𝑛 = 𝑡30𝑊,𝑛−1 + ∆𝑡𝑛
𝑈𝑉 𝑉𝑖 30 𝑉𝑇
(1)
where tn is the reaction time for each sample, UV is the average ultraviolet radiation (<400 nm) measured between tn-1 and tn, and t30W is the normalized illumination time refered to a constant solar power of 30 W m-2 (typical solar radiation on a perfectly sunny day around noon). VT is the total volume and Vi is the irradiated volume.
3. RESULTS AND DISCUSSION 5
3.1. Aniline photocatalytic Degradation
3.1.1. Solar/TiO2 process
Effect of TiO2 initial concentration
Different experiments were done to minimize the consumption of catalyst in suspension and maximize the light absorption. In this sense, the effect of the TiO2 concentration (from 75 to 500 mg L-1) on the photocatalytic degradation of aniline was studied. First, two control tests [i) individual photolysis without TiO2 and ii) TiO2 without solar light] were carried out to find out the influence of the catalyst on the degradation of 10 mg L-1 aniline. Subsequently, several trials were performed under the following operating conditions: [aniline]o = 10 mg L-1; TiO2 load = 75, 125, 250 and 500 mg L-1; pH = 4; solar radiation intensity in these experiments= UVA: 34-38 W m-2; average temperature= 33 ° C; reaction time= 90 min. Figs. 1a and 1b show the evolution of the concentrations of aniline and Total Organic Carbon (TOC) versus reaction time, respectively, for the control experiments and using different initial concentrations of TiO2. As shown in Fig. 1a, the reduction in the concentration of aniline was only 6% when the reaction was carried out using TiO2 alone in the dark. In this case, the removal of aniline in solution was due to its adsorption on the catalyst surface since the catalyst did not generate oxidizing radical in the dark. With respect to the experiments carried out only by direct photolysis (sunlight only), the percentages of aniline removal and mineralization were 50% and 4%, respectively, in 90 min. The low efficiency can be justified because aniline 6
only absorbs a small fraction of solar radiation above 290 nm, as shown in its absorption spectrum (Fig. 1c).
As can be seen in Figs. 1a and 1b, the presence of the TiO2 catalyst irradiated by sunlight improved the elimination efficiencies of both aniline and TOC. The effect of TiO2 concentration was positive until a concentration of 125 mg L-1 was used. Above this value, a reaction inhibitory effect took place and the efficiencies of degradation and mineralization decreased. The increase in TiO2 increased the concentration of reactive oxygen species, ROS, (HO•, O2•-, H2O2) photocatalytically generated under UV A (λ <390 nm) solar radiation according to reactions (2) to (7) [37, 38].
TiO2 + hυ → hVB + + eCB -
(2)
H2 O + hVB + → HO + H+
(3)
HO + HO → H2 O2
(4)
H2 O2 + HO → HO2 + H2 O
(5)
O2 + eCB - → O2 -
(6)
O2 - + HO2 + H+ → H2 O2 + O2
7
(7)
However, when 250 and 500 mg L-1 TiO2 were used, a slight decrease in degradation efficiency was observed possibly due to an accumulation of TiO2 particles at the bottom of the reactor exerting a screen effect. For this reason, the optimal concentration of 125 mg L-1 TiO2 was selected in order to avoid excessive and unnecessary use of catalyst.
Fig. 2 shows the dissolved O2 values along the reaction of aniline degradation by the solar/TiO2 process. It can be seen that the curves were similar in all tests. In a first stage (until 30 min), dissolved oxygen decreased rapidly, probably due to the reaction with photoexcited electrons in the conduction band of the catalyst according to Eq. (6). During this period of time, aniline was almost completely eliminated (see Fig. 1a). Later, in a second stage, after 30-35 min, the oxygen concentration remained practically constant, possibly due to the fact that oxygen consumption (Eq. (6)) was compensated with oxygen formation by reaction between O2•- and HO2 (Eq. (7)).
3.1.2. Solar/TiO2/S2O82- system
Effect of S2O82- initial dose
Different doses of persulfate were added to solar/ TiO2 process to evaluate the effect of S2O82- on aniline removal. First, two control experiments [i) S2O82- alone in the dark and ii) solar/S2O82- system] were carried out to study the individual effect of persulfate in the reaction. Afterwards, several experiments were carried out under the operating conditions: [aniline]o = 10 mg L-1; TiO2 load= 125 mg L-1; pH = 4; [S2O82-]o= 150, 300 and 450 mg L-1; reaction time= 90 min. In these tests, the intensity of solar radiation 8
varied considerably from one experiment to another, between 30 and 43 W m-2, as well as their average temperature, between 25 and 35 ° C, and t30W was used, as indicated previously.
Figs 3a and 3b show the evolution of the concentrations of aniline and TOC as a function of t30W, respectively. The percentages of aniline removal and mineralization reached in the dark/S2O82--(300 mg L-1) process were 13% and 1.5%, respectively. This indicated that persulfate did not act as an aniline oxidant when it was used individually. Likewise, it can be observed that it was possible to eliminate 56% of aniline by the solar/S2O82system for a t30W of 70 min (Fig. 3a). Taking into account the result of degradation of aniline obtained by the individual application of sunlight (50%, Fig. 3a) it can be deduced that there was no synergy in this combined solar/S2O82- system with respect to the individual solar and persulfate processes. In addition, the mineralization degree reached in the solar/S2O82- process was only 7% (Fig. 3b). It can be concluded, based on these results, that persulfate in the presence of solar light generated an insignificant amount of reactive oxygen species, responsible for the oxidation of both aniline and the intermediate compounds formed in the photocatalytic degradation reaction. As shown in the UVVisible absorption spectrum of a 250 mg L-1 persulfate aqueous solution (Fig. 3c), only a slight absorption of sunlight by S2O82- occurred between 290 and 340 nm. So, the generation of sulfate radical by solar activation of persulfate was minimal. However, it was observed that the TiO2/S2O82- combined system irradiated with sunlight favored the elimination of aniline and intermediate organic compounds. The increase of initial S2O82increased the mineralization efficiency until an optimal value of 300 mg L-1 S2O82- was used. Under these optimal conditions, the TOC removal percentage was 91.5% in approximately t30W= 35 min (illumination real time = 60 min). As it is shown in Fig. S1 9
(Supplementary Material), the remaining persulfate in solution under the optimal operating conditions ([S2O82-]o= 300 mg L-1) practically disappeared in t30W= 30 min. In this period of time, the maximal TOC removal was achieved (Fig. 3b).
In this reaction system, the persulfate anion, S2O82-, reacted with the electrons generated in the conduction band of the TiO2 particles exposed to solar radiation according to Eq. (8) generating sulfate radicals, SO4•-. This reaction inhibited the recombination of the electron/hole pairs, increasing their availability in the aqueous medium. In adition, available sulfate radical can be converted to hydroxyl radical according to Eqs. (9) and (10) [39]. The increase of available hVB+ could increase the concentration of HO• according to Eq. (3).
− 𝑆2 𝑂82− + 𝑒𝐶𝐵 → 𝑆𝑂4− + 𝑆𝑂42−
(8)
𝑆2 𝑂82− + ℎ → 2𝑆𝑂4−
(9)
𝑆𝑂4− + 𝐻2 𝑂 → 𝑆𝑂42− + 𝐻𝑂 + 𝐻 +
(10)
However, no improvement in the efficiency of degradation of aniline was observed at persulfate concentrations higher than 300 mg L-1. This could be due to the entrapment of SO4- and HO radicals with excess persulfate by Eqs. (11) and (12):
𝑆2 𝑂82− + 𝐻𝑂 → 𝑂𝐻 − + 𝑆2 𝑂8− 10
(11)
𝑆2 𝑂82− + 𝑆𝑂4− → 𝑆𝑂42− + 𝑆2 𝑂8−
(12)
On the other hand, when excess radicals were generated, they could react with each other according to reactions (4), (13) and (14), decreasing their availability in the reaction medium [40]. In addition, excess persulfate molecules could adsorb light photons which did not activate the TiO2 catalyst.
𝑆𝑂4− + 𝑆𝑂4− → 𝑆2 𝑂82−
(13)
1
𝑆𝑂4− + 𝐻𝑂 → 𝐻𝑆𝑂4− + 2 𝑂2
(14)
Effect of pH
The influence of pH on aniline degradation and mineralization by solar/TiO2/S2O82process was studied. The operating conditions of three tests were the following: [aniline]o= 10 mg L-1; load of TiO2= 125 mg L-1; [S2O82-]= 300 mg L-1; pH= 4, 7 and 10; intensity of solar radiation= 41-42 W m2; temperature= 32-33 ºC. Table 2 includes the degradation and mineralization results obtained for the reactions carried out at the three studied different pH values. It can be concluded that the elimination of aniline and its reaction intermediates was favored at pH 4, below TiO2 isoelectric point (pHzpc (TiO2): 6). The TiO2 surface was positively charged at acidic pH (Eq. (15)) while its charge was negative at pH values above its pHzpc (Eq. (16)):
11
pH < pHzpc
TiOH + H+ TiOH2+
(15)
pH > pHzpc
TiOH + OH- TiO- + H2O
(16)
With respect to aniline, it is mainly in its cationic form at pH < pKa (pKa (aniline) = 4.6), while, at higher pH values, it predominates in its molecular form, without charge. Therefore, at pH 4, a slight electrostatic repulsion could occur between the aniline cationic form and the TiO2 positive charge. This justified the small degree of adsorption of aniline on the catalyst surface, as discussed above. However, pH significantly affected the roles of different radicals in the solar/TiO2/S2O82- system. The conversion of sulfate radical to hydroxyl radical by Eq (17) depends on pH as follows: at pH <7, SO4•- radical was the predominant species; at pH 9, both SO4•- and HO• radicals were present in the reaction medium, and at pH> 9, HO• radical predominated [41]. So, SO4•- played a role more significant than HO in the removal of the intermediates present in the water in this reaction system, as will be explained below. So, pH 4 was chosen as optimal value for this degradation reaction.
𝑆𝑂4− + 𝑂𝐻 − → 𝑆𝑂42− + 𝐻𝑂
3.1.3. Solar/TiO2/H2O2 system
Effect of H2O2 initial dose
12
(17)
First, a control experiment (single H2O2 process in the dark) was carried out. Then, several tests were evaluated by using the following conditions: [aniline]o = 10 mg L-1; TiO2 load= 125 mg L-1; pH = 4; [H2O2]o= 20, 40, 50, 250 and 500 mg L-1; reaction time: 90 min; UVA solar radiation intensity= 40- 43 W m-2; average temperature= 34 ° C. It can be seen in Fig. 4 that hydrogen peroxide alone in the dark did not act as an aniline oxidant, reaching only 3.22% mineralization in 90 min. On the other hand, it can be concluded that H2O2 positively affected the mineralization reaction until a concentration of 40 mg L-1 in the solar/TiO2/H2O2 system. TOC removal was increased from 82.54% (solar/TiO2 system) to 85.3% (solar/TiO2/40 mg L-1 H2O2) due to the increase in the concentration of HO• radicals generated from its reaction with electrons of the conduction band according to Eq. (18), which inhibited the electron-hole pairs recombination and increased the availability of holes. However, higher concentrations of hydrogen peroxide (above 40 mg L-1) negatively affected the photocatalytic activity due to its possible reaction with hydroxyl radicals according to Eq. (5) generating perhydroxyl radicals, HO2, less oxidants, and which can consume radicals hydroxyl according to Eq. (19) [42]. The degree of mineralization was reduced to 70.47%, 65.69% or 56.89% for doses of 50, 250 or 500 mg L-1 of H2O2, respectively.
− 𝑒𝐶𝐵 + 𝐻2 𝑂2 → 𝑂𝐻 − + 𝐻𝑂
(18)
𝐻𝑂2 + 𝐻𝑂 → 𝐻2 𝑂 + 𝑂2
(19)
In addition, excess HO• and HO2• radicals can recombine with each other to form hydrogen peroxide, according to Eqs. (4) and (20), respectively, decreasing their availability in the reaction medium. 13
𝐻𝑂2 + 𝐻𝑂2 → 𝐻2 𝑂2 + 𝑂2
(20)
Fig. S2 (Supplementary Material) shows the evolution of the remaining H2O2 concentration along the reaction carried out under optimal conditions. It can be seen that peroxide was practically consumed during the reaction.
3.1.4. Solar/TiO2/S2O82-/H2O2 system
The combination of persulfate and hydrogen in the solar/TiO2 system under the best operating conditions previously obtained was studied. The experimental conditions were the following: [aniline]o = 10 mg L-1; TiO2 load= 125 mg L-1; pH = 4; [S2O82-]o= 300 mg L-1; [H2O2]o= 40 mg L-1; reaction time: 90 min; intensity of solar radiation= 32- 41 W m2
; temperature= 27- 33 °C. Fig. 5 shows the evolution of the concentration of aniline
versus t30W for the three optimal experiments performed using the solar/TiO2, solar/TiO2/S2O82- and solar/TiO2/S2O82-/H2O2 systems. It can be concluded that the efficiency of degradation of aniline was greater for the solar/TiO2/S2O82- system based on the arguments presented in the previous section. The combination of the two electron acceptors, S2O82- and H2O2, inhibited the reaction mainly attributed to an excess of radicals generated as indicated above and due to a decrease in the active sites of TiO2 surface. The degree of mineralization achieved for the aniline solution was maximum (91.54%) by using the solar/TiO2/S2O82- system, being only 68.21% for the solar/TiO2/S2O82-/H2O2 (see inset of Fig. 5).
3.1.5 Quencher study 14
The degradation of aniline in aqueous solution was carried out with or without reactive intermediate species quenchers in order to determine their contribution to the global reaction in the two main systems used in this work: solar/TiO2/S2O82- and solar/TiO2/H2O2.
In order to quantify the levels of oxidation by means of ROS and holes, the inhibition of the reaction was carried out using the following quenchers: tert-butyl alcohol and methanol to distinguish the contributions of HO• and SO4•- radicals, 1,4-benzoquinone (it has a great capacity to trap O2•- (k= 0.9-1 109 M-1s-1) [43] and to react with HO (k= 6.6 109 M-1 s-1) [44]) and glycerol (hole (hVB+) quencher). Tert-butyl alcohol reacts 1000 times faster with HO• radical (k = (3.8-7.6) × 108 M-1 s-1) than with SO4- radical (k = (4.0-9.1) × 105 M-1 s-1) being an effective HO• inhibitor. On the other hand, methanol reacts similarly with both oxidizing species, HO• and SO4•- (k = 9.7 x 108 M-1 s-1 and k = 1.1 x 107 M-1 s-1, respectively) [45, 46].
Roles of ROS and holes in the solar/TiO2/S2O82- system
Fig. 6a shows the aniline degradation efficiency when the solar/TiO2/S2O82- system was used in the presence and absence of inhibitors of reactive intermediate species. Operating parameters: [aniline]o= 10 mg L-1; pH = 4; average temperature = 30 ° C; load of TiO2= 125 mg L-1; [S2O82-]o=: 300 mg L-1; [t-butOH]o = 1 M; [MeOH]o = 1 M; [glycerol]o = 1 M; [p-benzoquinone]o = 1M; average solar power: 36 W m-2; reaction time = 90 min.
15
It is shown in Fig. 6a that the greatest inhibition of aniline degradation occurred in the presence of methanol or glycerol. The percentage of degradation decreased from 99.9% (solar/TiO2/S2O82- process without scavengers) to only 9% or 11% in the presence of methanol or glycerol, respectively. With respect to the role played by both HO• and SO4•radicals, it can be seen that the degradation efficiency decreased from 99.9%, obtained in the scavengers free reaction, to only 9% or 29% when methanol or tert-butyl alcohol were used, respectively. This indicated that sulfate radical was found to be more significant than HO• because the reaction inhibition was lower when tert-butyl alcohol (mainly HO • inhibitor) was used. Regarding the role played by the holes generated in TiO2, the inhibition of the reaction in the presence of glycerol (11% degradation), which acts primarily as a hole scavenger [47] although it can also react with HO [48], suggest that the holes also played an important role in the aniline degradation reaction. The holes can react with aniline or with the intermediates adsorbed on the surface of TiO 2, and on the other hand, they can initiate the HO• formation reaction according to Eqs. (3) and (21).
hVB + +O𝐻 − → HO
(21)
The presence of 1,4-p-benzoquinone (O2- and HO quencher) reduced the percentage of aniline removal from 99.9% obtained in the scavenger free reaction to only 23%. In this case, the reaction inhibition was slightly higher than with tert-butyl alcohol (29% aniline removal). This indicated that 1,4-p-benzoquinone trapped HO and also reacted with O2 generated in reactions (6) and (22) decreasing its availability in the reaction medium. So,
16
superoxide radical anion had a moderate role in the aniline degradation reaction under the solar/TiO2/S2O82- system.
𝐻𝑂2 ↔ 𝐻 + 𝑂2−
(22)
Roles of ROS and holes in the solar/TiO2/H2O2 system
Fig. 6b shows the results of aniline degradation obtained when the solar/TiO2/H2O2 process was used in the presence and absence of radical and holes quenchers. The experimental conditions were the following: [aniline]o= 10 mg L-1; pH = 4; average temperature = 26 °C; load of TiO2= 125 mg L-1; [H2O2]= 40 mg L-1; [t-butOH]o = 1 M; [MeOH]o = 1 M; [glycerol]o = 1 M; [p-benzoquinone]o = 1 M; average solar power = 36 W m-2; reaction time: 90 min.
As shown in Fig. 6b, the aniline removal curves versus t30W in the reactions carried out in the presence of tert-butyl alcohol or methanol are similar because SO4•- was not generated in this solar/TiO2/H2O2 system. The degradation efficiency of aniline decreased from 99.9% (in t30W = 40 min) to approximately 28% in t30W = 60 min. Similar degradation result was achieved when the reaction was carried out in the presence of glycerol (hole scavenger and to a lesser extent of HO•), which indicated that HO• radical and hVB+ were primarily responsible for aniline degradation in the solar/TiO2/H2O2 system.
17
The percentage of degradation achieved in the presence of p-benzoquinone (26%) was slightly smaller than with tert-butyl alcohol (28%), indicating that O2•- played a role less significant in the solar/TiO2/H2O2 system than in the solar/TiO2/S2O82- process.
If both solar/TiO2/S2O82- and solar/TiO2/H2O2 systems are compared, it can be concluded the following (see Fig. 7): S2O82- was more efficient as an electron acceptor than H2O2 since the holes, hVB+, had a more significant effect in the solar/TiO2/S2O82- system (11% or 28% degradation in the presence of glycerol in the S2O82- or H2O2 systems, respectively) due to a greater inhibition of the electron-hole pairs recombination; the superoxide radical anion, O2•-, played a more significant role in the solar/TiO2/S2O82system than in the solar/TiO2/H2O2 process. This could be due to the lower availability of perhydroxyl radicals, HO2, in the solar/TiO2/H2O2 system to form O2•- through Eq. (22) due to its consumption in reactions (19) and (20); the results of degradation obtained in the solar/TiO2/S2O82- confirmed the main role of sulfate radicals and holes as oxidizing agents under the studied operating conditions.
4. CONCLUSIONS
In this work, the degradation of aniline in aqueous solution has been studied by means of a solar photocatalytic system using the TiO2 catalyst in suspension with the addition of two electron acceptors to the system, S2O82- and/or H2O2. It was found that the degradation of aniline by direct photolysis with sunlight was negligible (4%). An excess of TiO2 decreased the degradation efficiency possibly due to a decrease in sunlight penetration. Persulfate and hydrogen peroxide individually did not act as aniline oxidants. 18
On the other hand, it was observed that the combined system of persulfate and titanium dioxide irradiated with sunlight favored the elimination of the contaminant and of the intermediates. Under optimal conditions, the percentage of TOC removal was 91.5%. The addition of persulfate inhibited the recombination of electron/hole pairs, increasing their availability in the aqueous medium. The presence of H2O2 in the solar/TiO2 system above 40 mg L-1 decreased the effectiveness of aniline removal due to its possible reaction with hydroxyl radicals. Persulfate had a higher efficiency as an electron acceptor (greater availability of holes) than H2O2 in the solar/TiO2 system. The superoxide radical anion, O2•-, played a more significant role in the solar/TiO2/S2O82- system than in the solar/TiO2/H2O2. The degradation results obtained in the solar/TiO2/S2O82- system confirmed the participation of sulfate radicals as oxidizing agents more influential than hydroxyl radicals. The solar/TiO2/S2O82- system can be a potential alternative to degrade emerging pollutants such as aniline in wastewater.
5. ACKNOWLEDGMENTS
Financial support from Ayuda a grupos from UCLM is gratefully acknowledged.
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25
Figure captions:
Figure 1: Effect of the concentration of TiO2 on a) degradation of aniline and b) mineralization. Solar/TiO2 process; c) absorption spectrum of a 10 mg L-1 aniline aqueous solution.
Figure 2: Evolution of the concentration of dissolved oxygen in the degradation and mineralization of an aqueous solution of 10 mg L-1 aniline by the solar/TiO2 process at different TiO2 concentrations.
Figure 3: Effect of S2O82- dose on a) degradation of aniline and b) mineralization. Solar TiO2/S2O82- system; c) absorption spectrum of a 250 mg L-1 S2O82- aqueous solution.
Figure 4: a) Effect of H2O2 dose on the mineralization of a 10 mg L-1 aniline aqueous solution by solar/TiO2/H2O2 process.
Figure 5: Comparative study of degradation and mineralization of aniline by different solar/TiO2/S2O82- and/or H2O2 systems.
26
Figure 6: Degradation of an aniline aqueous solution in the absence or presence of reactive intermediate species quenchers. a) Solar/TiO2/S2O82- system; b) Solar/TiO2/H2O2 system
Figure 7: Comparative study of the roles of reactive intermediate species in the degradation and mineralization of aniline aqueous solution by solar/TiO2 process in the `presence of the electron acceptors persulfate or hydrogen peroxide.
27
1.00
Aniline, C/Co
0.80
0.60
125 mg/L TiO2 dark
0.40
solar light alone, no catalyst solar/75 mg/L TiO2 solar/125 mg/L TiO2
0.20
solar/250 mg/L TiO2 solar/500 mg/L TiO2
0.00 0
10
20
30
40
50
60
70
80
90
100
60
70
80
90
100
Time (min)
a) 100.00 solar light alone, no catalyst solar/75 mg/L TiO2 solar/125 mg/L TiO2
80.00
solar/250 mg/L TiO2
TOC removal, %
solar/500 mg/L TiO2
60.00
40.00
20.00
0.00 0
10
20
30
40
50
Time (min)
b)
28
Absorbance
2
1
0 200
220
240
260
280
wavelenght, nm
c)
29
300
320
340
8.00
6.00 5.00 4.00 3.00 2.00 1.00
75 mg/L TiO2
125 mg/L TiO2
250 mg/L TiO2
500 mg/L TiO2
0.00
0
10
20
30
40
50
60
70
80
Time (min)
1
0.8
Aniline, C/Co
Dissolved oxygen (mg L-1)
7.00
0.6
0.4 300 mg/L PS, no light solar/300 mg/L PS solar/125 mg/L TiO2 alone solar/125 mg/L TiO2/150 mg/L PS
0.2
solar/125 mg/L TiO2/300 mg/L PS solar/125 mg/L TiO2/450 mg/L PS
0 0
10
20
30
40
50
t30W (min)
a)
30
60
70
80
90
90
100
% TOC removal
80
60 300 mg/L PS, no light
solar/300 mg/L PS
40
solar/125 mg/L TiO2 solar/125 mg/L TiO2/150 mg/L PS solar/125 mg/L TiO2/300 mg/L PS
20
solar/125 mg/L TiO2/450 mg/L PS
0 0
10
20
30
40
50
60
70
80
90
t30W (min)
b)
Absorbance
0.03
0.02
0.01
0.00
240
290 340 Wavelenght (nm) c)
31
390
100 82.54
85.3
84.1
%TOC removal
80
70.47
65.69 56.89
60
40
20 3.22 0
32
1.0 solar/125 mg/L TiO2 solar/125 mg/L TiO2/300 mg/L PS solar/125 mg/L TiO2/300 mg/L PS/40 mg/L H2O2
0.8
100
91.54 85.3
0.6 %TOC removal
Aniline, C/Co
82.54 80
0.4
68.21 60
40
20
0.2 0
solar/TiO2/PS
solar/TiO2
solar/TiO2/H2O2
solar/TiO2/PS/H2O2
0.0 0
10
20
70
60
50
40
30
80
t30W (min)
1.00
0.80
Aniline, C/Co
0.60
0.40
0.20
0.00 0
10
20
30
40
50
60
t30W(min) without scavengers
tert-butyl alcohol
glycerol
p-benzoquinone
33
methanol
70
90
a) 1.00
0.80
Aniline, C/Co
0.60
0.40
0.20
0.00 0
10
20
30
40
50
60
70
t30W (min) without scavengers
tert-butyl alcohol
glycerol
p-Benzoquinone
methanol
b)
NO DEGRADATION Dark
H2O2 TiO2
+
+
TiO2/H2O2 Dark
PRODUCTS 99.9 % AN removal in 30 min 91.5 % TOC removal in 70 min
PS TiO2 TiO2/PS
Aniline
+ + NO DEGRADATION
Role -
Role
hVB+ HO
-
O2-
PRODUCTS 99.9 % AN removal in 60 min 85.3 % TOC removal in 90 min
34
hVB+ HO SO4O2-
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
35
1. We have studied the removal of aniline by solar/TiO2/electron acceptor 2. 91.5% TOC removal was achieved in optimal conditions 3. Persulfate and hydrogen peroxide inhibited the e-/h+ recombination 4. Persulfate was a more effective electron acceptor than H2O2 5. SO4- and holes were found to be the main reactive intermediate species
36
S1383-5866(19)34641-6 https://doi.org/10.1016/j.seppur.2019.116456 SEPPUR 116456
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
10 October 2019 18 December 2019 18 December 2019
Please cite this article as: J.M. Monteagudo, A. Durán, I. San Martín, B. Vellón, Photocatalytic Degradation of Aniline by Solar/TiO2 System in the Presence of the Electron Acceptors Na2S2O8 and H2O2, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116456
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© 2019 Elsevier B.V. All rights reserved.
Photocatalytic Degradation of Aniline by Solar/TiO2 System in the Presence of the Electron Acceptors Na2S2O8 and H2O2
J.M. Monteagudo*, A. Durán, I. San Martín, B. Vellón a
Department of Chemical Engineering, Grupo IMAES, Escuela Técnica Superior de
Ingenieros Industriales, Instituto de Investigaciones Energéticas y Aplicaciones Industriales (INEI), University of Castilla-La Mancha, Avda. Camilo José Cela 3, 13071 Ciudad Real (Spain).
* To whom correspondence should be addressed Department of Chemical Engineering, Grupo IMAES Escuela Técnica Superior de Ingenieros Industriales, Instituto de Investigaciones Energéticas y Aplicaciones Industriales (INEI) University of Castilla-La Mancha, Avda. Camilo José Cela 3, 13071 Ciudad Real (Spain). Fax: 0034 926295361. Phone: 0034 926295300, ext: 3888 Email: [email protected]
ABSTRACT
Aniline aqueous solution degradation by solar/TiO2 system in combination with electron acceptors such as Na2S2O8 and/or H2O2 was studied. The effects of TiO2 initial concentration, dose of persulfate and hydrogen peroxide, and pH were evaluated. The solar/TiO2/Na2S2O8 process was found to be more efficient than the solar/TiO2/H2O2 process. Under optimal conditions, the percentage of removal of solution TOC was 91.5% in approximately t30W= 30 min (reaction time= 60 min). The addition of persulfate, until to an optimum value, to the solar/TiO2 system inhibited the electron/hole pair recombination, increasing the availability of holes and SO4- and consequently HO radicals. H2O2 also inhibited the electron/hole recombination and generated HO radicals although an excess reduced the degradation efficiency. The roles played by reactive intermediate species, such as SO4-, HO•, O2•- and holes in both solar/TiO2/Na2S2O8 and solar/TiO2/H2O2 processes were determined using appropriated quenchers. SO4- radicals and holes were found to be the main responsible species in the solar/TiO2/Na2S2O8 system.
Keywords: aniline; solar; TiO2; persulfate; H2O2; electron acceptor.
1
1. INTRODUCTION
Aniline is an organic compound mainly used in the manufacture of precursors to pharmaceuticals [1]. In the last few years, the presence of this pollutant refractory to biodegradation in industrial wastewater efluents has increased. Besides, aniline causes toxicity in living organisms [2, 3]. Therefore, an effective degradation and mineralization of aniline should be studied. Table 1 shows different types of aniline removal treatment previously published in the literature [4-26]. The use of titanium dioxide has become of great interest due to its low cost, high chemical stability and its activation with solar light [27]. It is well-known that different Reactive Oxygen Species, ROS, such as HO and O2, generated in the catalyst surface from reactions between electron/hole pair and O2 and H2O2, are responsible for contaminant degradation. Nevertheless, the efficiency of this TiO2 photocatalytic system decreases considerably when the recombination of photoinduced electrons and holes takes place. In this sense, the addition of electron acceptors such as S2O82- or H2O2 prevents this recombination since they trap photogenerated electrons, increasing the availability of holes in the medium and facilitating their participation in the generation of ROS [28-30]. However, to the best of our knowledge, the addition of S2O82- or H2O2 to the solar/TiO2 system for aniline degradation has not yet been studied.
The aim of this work was to evaluate the removal of aniline and reaction intermediates by a solar process based on TiO2 combined with persulfate and/or hydrogen peroxide as electron acceptors. The effect of several variables such as initial concentration of TiO2, dose of H2O2 or S2O82- and pH were studied. Finally, the roles of HO•, SO4-, O2•- and holes, hVB+, in both solar/TiO2/electron acceptor-based systems were investigated.
2
2. EXPERIMENT
2.1. Materials
TiO2 P25 (Aeroxide, Degussa Corporation) was obtained from Evonik Industries. Aniline, C6H7N (99%), p-benzoquinone, tert-butyl alcohol (99.7%), TiOSO4, methanol (99.8%) and glycerol solution (86-89%) were supplied by Sigma-Aldrich. Sodium persulfate (Na2S2O8, 98%) was purchased from Panreac, and hydrogen peroxide (30% w/v) was obtained from Merck.
2.2 TiO2 Characterization
Characterization of TiO2 including XRD, TEM, UV-Vis reflectance spectra, Raman spectroscopy, Z-potential measurements, Tauc plots and BET analysis were previously publicated by the authors [27, 31].
2.3. Solar CPC reactor
A solar CPC reactor (Ecosystem, S.A.) was employed for the degradation experiments. It was well characterized in our previous work [27]. The total volume of the equipment (tubes + tank) was 3.5 L. A centrifugal pump was used to recirculate the water flow (flow rate: 30 L min-1) through the CPC. 3
2.4. Solar/TiO2/electron acceptor Process
First, aniline and TiO2 catalyst were mixed in the tank for 30 min until adsorption equilibrium was attained. Then, the mixture in the presence or absence of electron acceptors (S2O82- or H2O2) was pumped through the CPC and irradiated with solar light. Different samples along the reaction were collected to analyze the concentrations of aniline, Total Organic Carbon (TOC), persulfate, H2O2 and dissolved oxygen.
Several quenchers of radicals and holes were used to quantify the roles played by the reactive intermediate species, as will be explained below.
2.5. Analytic methods
2.5.1. Analysis of Aniline and TOC
The concentration of aniline was analyzed by HPLC (Agilent Technologies 1100 HPLCUV; column Eclipse XDB-C18; = 205 nm; flow rate= 0.8 ml min-1; mobile phase= 30:70 (v/v) acetonitrile/(water/1% acetic acid). Mineralization degree (TOC) was obtained by using a TOC-5050 Shimadzu analyzer.
2.5.2. H2O2, S2O82- and dissolved O2 measurement
4
H2O2 concentration was analyzed spectrophotometrically by using TiOSO4 [32, 33]. The residual S2O82- concentration was measured by reaction with sodium carbonate and potassium iodide [34]. Dissolved O2 was obtained using a Jenway 9200 DO2 meter.
2.5.3. Solar Radiation Intensity Determination
UV-A solar radiation intensity (W m-2) was measured by a radiometer (Ecosystem, model ACADUS-85) [35].
2.5.4. Normalized illumination time
When the intensity of solar radiation varied considerably from one experiment to another, a widely accepted equation in the literature (Eq. (1)) was used to normalize the data for comparative purposes [36]:
𝑡30𝑊,𝑛 = 𝑡30𝑊,𝑛−1 + ∆𝑡𝑛
𝑈𝑉 𝑉𝑖 30 𝑉𝑇
(1)
where tn is the reaction time for each sample, UV is the average ultraviolet radiation (<400 nm) measured between tn-1 and tn, and t30W is the normalized illumination time refered to a constant solar power of 30 W m-2 (typical solar radiation on a perfectly sunny day around noon). VT is the total volume and Vi is the irradiated volume.
3. RESULTS AND DISCUSSION 5
3.1. Aniline photocatalytic Degradation
3.1.1. Solar/TiO2 process
Effect of TiO2 initial concentration
Different experiments were done to minimize the consumption of catalyst in suspension and maximize the light absorption. In this sense, the effect of the TiO2 concentration (from 75 to 500 mg L-1) on the photocatalytic degradation of aniline was studied. First, two control tests [i) individual photolysis without TiO2 and ii) TiO2 without solar light] were carried out to find out the influence of the catalyst on the degradation of 10 mg L-1 aniline. Subsequently, several trials were performed under the following operating conditions: [aniline]o = 10 mg L-1; TiO2 load = 75, 125, 250 and 500 mg L-1; pH = 4; solar radiation intensity in these experiments= UVA: 34-38 W m-2; average temperature= 33 ° C; reaction time= 90 min. Figs. 1a and 1b show the evolution of the concentrations of aniline and Total Organic Carbon (TOC) versus reaction time, respectively, for the control experiments and using different initial concentrations of TiO2. As shown in Fig. 1a, the reduction in the concentration of aniline was only 6% when the reaction was carried out using TiO2 alone in the dark. In this case, the removal of aniline in solution was due to its adsorption on the catalyst surface since the catalyst did not generate oxidizing radical in the dark. With respect to the experiments carried out only by direct photolysis (sunlight only), the percentages of aniline removal and mineralization were 50% and 4%, respectively, in 90 min. The low efficiency can be justified because aniline 6
only absorbs a small fraction of solar radiation above 290 nm, as shown in its absorption spectrum (Fig. 1c).
As can be seen in Figs. 1a and 1b, the presence of the TiO2 catalyst irradiated by sunlight improved the elimination efficiencies of both aniline and TOC. The effect of TiO2 concentration was positive until a concentration of 125 mg L-1 was used. Above this value, a reaction inhibitory effect took place and the efficiencies of degradation and mineralization decreased. The increase in TiO2 increased the concentration of reactive oxygen species, ROS, (HO•, O2•-, H2O2) photocatalytically generated under UV A (λ <390 nm) solar radiation according to reactions (2) to (7) [37, 38].
TiO2 + hυ → hVB + + eCB -
(2)
H2 O + hVB + → HO + H+
(3)
HO + HO → H2 O2
(4)
H2 O2 + HO → HO2 + H2 O
(5)
O2 + eCB - → O2 -
(6)
O2 - + HO2 + H+ → H2 O2 + O2
7
(7)
However, when 250 and 500 mg L-1 TiO2 were used, a slight decrease in degradation efficiency was observed possibly due to an accumulation of TiO2 particles at the bottom of the reactor exerting a screen effect. For this reason, the optimal concentration of 125 mg L-1 TiO2 was selected in order to avoid excessive and unnecessary use of catalyst.
Fig. 2 shows the dissolved O2 values along the reaction of aniline degradation by the solar/TiO2 process. It can be seen that the curves were similar in all tests. In a first stage (until 30 min), dissolved oxygen decreased rapidly, probably due to the reaction with photoexcited electrons in the conduction band of the catalyst according to Eq. (6). During this period of time, aniline was almost completely eliminated (see Fig. 1a). Later, in a second stage, after 30-35 min, the oxygen concentration remained practically constant, possibly due to the fact that oxygen consumption (Eq. (6)) was compensated with oxygen formation by reaction between O2•- and HO2 (Eq. (7)).
3.1.2. Solar/TiO2/S2O82- system
Effect of S2O82- initial dose
Different doses of persulfate were added to solar/ TiO2 process to evaluate the effect of S2O82- on aniline removal. First, two control experiments [i) S2O82- alone in the dark and ii) solar/S2O82- system] were carried out to study the individual effect of persulfate in the reaction. Afterwards, several experiments were carried out under the operating conditions: [aniline]o = 10 mg L-1; TiO2 load= 125 mg L-1; pH = 4; [S2O82-]o= 150, 300 and 450 mg L-1; reaction time= 90 min. In these tests, the intensity of solar radiation 8
varied considerably from one experiment to another, between 30 and 43 W m-2, as well as their average temperature, between 25 and 35 ° C, and t30W was used, as indicated previously.
Figs 3a and 3b show the evolution of the concentrations of aniline and TOC as a function of t30W, respectively. The percentages of aniline removal and mineralization reached in the dark/S2O82--(300 mg L-1) process were 13% and 1.5%, respectively. This indicated that persulfate did not act as an aniline oxidant when it was used individually. Likewise, it can be observed that it was possible to eliminate 56% of aniline by the solar/S2O82system for a t30W of 70 min (Fig. 3a). Taking into account the result of degradation of aniline obtained by the individual application of sunlight (50%, Fig. 3a) it can be deduced that there was no synergy in this combined solar/S2O82- system with respect to the individual solar and persulfate processes. In addition, the mineralization degree reached in the solar/S2O82- process was only 7% (Fig. 3b). It can be concluded, based on these results, that persulfate in the presence of solar light generated an insignificant amount of reactive oxygen species, responsible for the oxidation of both aniline and the intermediate compounds formed in the photocatalytic degradation reaction. As shown in the UVVisible absorption spectrum of a 250 mg L-1 persulfate aqueous solution (Fig. 3c), only a slight absorption of sunlight by S2O82- occurred between 290 and 340 nm. So, the generation of sulfate radical by solar activation of persulfate was minimal. However, it was observed that the TiO2/S2O82- combined system irradiated with sunlight favored the elimination of aniline and intermediate organic compounds. The increase of initial S2O82increased the mineralization efficiency until an optimal value of 300 mg L-1 S2O82- was used. Under these optimal conditions, the TOC removal percentage was 91.5% in approximately t30W= 35 min (illumination real time = 60 min). As it is shown in Fig. S1 9
(Supplementary Material), the remaining persulfate in solution under the optimal operating conditions ([S2O82-]o= 300 mg L-1) practically disappeared in t30W= 30 min. In this period of time, the maximal TOC removal was achieved (Fig. 3b).
In this reaction system, the persulfate anion, S2O82-, reacted with the electrons generated in the conduction band of the TiO2 particles exposed to solar radiation according to Eq. (8) generating sulfate radicals, SO4•-. This reaction inhibited the recombination of the electron/hole pairs, increasing their availability in the aqueous medium. In adition, available sulfate radical can be converted to hydroxyl radical according to Eqs. (9) and (10) [39]. The increase of available hVB+ could increase the concentration of HO• according to Eq. (3).
− 𝑆2 𝑂82− + 𝑒𝐶𝐵 → 𝑆𝑂4− + 𝑆𝑂42−
(8)
𝑆2 𝑂82− + ℎ → 2𝑆𝑂4−
(9)
𝑆𝑂4− + 𝐻2 𝑂 → 𝑆𝑂42− + 𝐻𝑂 + 𝐻 +
(10)
However, no improvement in the efficiency of degradation of aniline was observed at persulfate concentrations higher than 300 mg L-1. This could be due to the entrapment of SO4- and HO radicals with excess persulfate by Eqs. (11) and (12):
𝑆2 𝑂82− + 𝐻𝑂 → 𝑂𝐻 − + 𝑆2 𝑂8− 10
(11)
𝑆2 𝑂82− + 𝑆𝑂4− → 𝑆𝑂42− + 𝑆2 𝑂8−
(12)
On the other hand, when excess radicals were generated, they could react with each other according to reactions (4), (13) and (14), decreasing their availability in the reaction medium [40]. In addition, excess persulfate molecules could adsorb light photons which did not activate the TiO2 catalyst.
𝑆𝑂4− + 𝑆𝑂4− → 𝑆2 𝑂82−
(13)
1
𝑆𝑂4− + 𝐻𝑂 → 𝐻𝑆𝑂4− + 2 𝑂2
(14)
Effect of pH
The influence of pH on aniline degradation and mineralization by solar/TiO2/S2O82process was studied. The operating conditions of three tests were the following: [aniline]o= 10 mg L-1; load of TiO2= 125 mg L-1; [S2O82-]= 300 mg L-1; pH= 4, 7 and 10; intensity of solar radiation= 41-42 W m2; temperature= 32-33 ºC. Table 2 includes the degradation and mineralization results obtained for the reactions carried out at the three studied different pH values. It can be concluded that the elimination of aniline and its reaction intermediates was favored at pH 4, below TiO2 isoelectric point (pHzpc (TiO2): 6). The TiO2 surface was positively charged at acidic pH (Eq. (15)) while its charge was negative at pH values above its pHzpc (Eq. (16)):
11
pH < pHzpc
TiOH + H+ TiOH2+
(15)
pH > pHzpc
TiOH + OH- TiO- + H2O
(16)
With respect to aniline, it is mainly in its cationic form at pH < pKa (pKa (aniline) = 4.6), while, at higher pH values, it predominates in its molecular form, without charge. Therefore, at pH 4, a slight electrostatic repulsion could occur between the aniline cationic form and the TiO2 positive charge. This justified the small degree of adsorption of aniline on the catalyst surface, as discussed above. However, pH significantly affected the roles of different radicals in the solar/TiO2/S2O82- system. The conversion of sulfate radical to hydroxyl radical by Eq (17) depends on pH as follows: at pH <7, SO4•- radical was the predominant species; at pH 9, both SO4•- and HO• radicals were present in the reaction medium, and at pH> 9, HO• radical predominated [41]. So, SO4•- played a role more significant than HO in the removal of the intermediates present in the water in this reaction system, as will be explained below. So, pH 4 was chosen as optimal value for this degradation reaction.
𝑆𝑂4− + 𝑂𝐻 − → 𝑆𝑂42− + 𝐻𝑂
3.1.3. Solar/TiO2/H2O2 system
Effect of H2O2 initial dose
12
(17)
First, a control experiment (single H2O2 process in the dark) was carried out. Then, several tests were evaluated by using the following conditions: [aniline]o = 10 mg L-1; TiO2 load= 125 mg L-1; pH = 4; [H2O2]o= 20, 40, 50, 250 and 500 mg L-1; reaction time: 90 min; UVA solar radiation intensity= 40- 43 W m-2; average temperature= 34 ° C. It can be seen in Fig. 4 that hydrogen peroxide alone in the dark did not act as an aniline oxidant, reaching only 3.22% mineralization in 90 min. On the other hand, it can be concluded that H2O2 positively affected the mineralization reaction until a concentration of 40 mg L-1 in the solar/TiO2/H2O2 system. TOC removal was increased from 82.54% (solar/TiO2 system) to 85.3% (solar/TiO2/40 mg L-1 H2O2) due to the increase in the concentration of HO• radicals generated from its reaction with electrons of the conduction band according to Eq. (18), which inhibited the electron-hole pairs recombination and increased the availability of holes. However, higher concentrations of hydrogen peroxide (above 40 mg L-1) negatively affected the photocatalytic activity due to its possible reaction with hydroxyl radicals according to Eq. (5) generating perhydroxyl radicals, HO2, less oxidants, and which can consume radicals hydroxyl according to Eq. (19) [42]. The degree of mineralization was reduced to 70.47%, 65.69% or 56.89% for doses of 50, 250 or 500 mg L-1 of H2O2, respectively.
− 𝑒𝐶𝐵 + 𝐻2 𝑂2 → 𝑂𝐻 − + 𝐻𝑂
(18)
𝐻𝑂2 + 𝐻𝑂 → 𝐻2 𝑂 + 𝑂2
(19)
In addition, excess HO• and HO2• radicals can recombine with each other to form hydrogen peroxide, according to Eqs. (4) and (20), respectively, decreasing their availability in the reaction medium. 13
𝐻𝑂2 + 𝐻𝑂2 → 𝐻2 𝑂2 + 𝑂2
(20)
Fig. S2 (Supplementary Material) shows the evolution of the remaining H2O2 concentration along the reaction carried out under optimal conditions. It can be seen that peroxide was practically consumed during the reaction.
3.1.4. Solar/TiO2/S2O82-/H2O2 system
The combination of persulfate and hydrogen in the solar/TiO2 system under the best operating conditions previously obtained was studied. The experimental conditions were the following: [aniline]o = 10 mg L-1; TiO2 load= 125 mg L-1; pH = 4; [S2O82-]o= 300 mg L-1; [H2O2]o= 40 mg L-1; reaction time: 90 min; intensity of solar radiation= 32- 41 W m2
; temperature= 27- 33 °C. Fig. 5 shows the evolution of the concentration of aniline
versus t30W for the three optimal experiments performed using the solar/TiO2, solar/TiO2/S2O82- and solar/TiO2/S2O82-/H2O2 systems. It can be concluded that the efficiency of degradation of aniline was greater for the solar/TiO2/S2O82- system based on the arguments presented in the previous section. The combination of the two electron acceptors, S2O82- and H2O2, inhibited the reaction mainly attributed to an excess of radicals generated as indicated above and due to a decrease in the active sites of TiO2 surface. The degree of mineralization achieved for the aniline solution was maximum (91.54%) by using the solar/TiO2/S2O82- system, being only 68.21% for the solar/TiO2/S2O82-/H2O2 (see inset of Fig. 5).
3.1.5 Quencher study 14
The degradation of aniline in aqueous solution was carried out with or without reactive intermediate species quenchers in order to determine their contribution to the global reaction in the two main systems used in this work: solar/TiO2/S2O82- and solar/TiO2/H2O2.
In order to quantify the levels of oxidation by means of ROS and holes, the inhibition of the reaction was carried out using the following quenchers: tert-butyl alcohol and methanol to distinguish the contributions of HO• and SO4•- radicals, 1,4-benzoquinone (it has a great capacity to trap O2•- (k= 0.9-1 109 M-1s-1) [43] and to react with HO (k= 6.6 109 M-1 s-1) [44]) and glycerol (hole (hVB+) quencher). Tert-butyl alcohol reacts 1000 times faster with HO• radical (k = (3.8-7.6) × 108 M-1 s-1) than with SO4- radical (k = (4.0-9.1) × 105 M-1 s-1) being an effective HO• inhibitor. On the other hand, methanol reacts similarly with both oxidizing species, HO• and SO4•- (k = 9.7 x 108 M-1 s-1 and k = 1.1 x 107 M-1 s-1, respectively) [45, 46].
Roles of ROS and holes in the solar/TiO2/S2O82- system
Fig. 6a shows the aniline degradation efficiency when the solar/TiO2/S2O82- system was used in the presence and absence of inhibitors of reactive intermediate species. Operating parameters: [aniline]o= 10 mg L-1; pH = 4; average temperature = 30 ° C; load of TiO2= 125 mg L-1; [S2O82-]o=: 300 mg L-1; [t-butOH]o = 1 M; [MeOH]o = 1 M; [glycerol]o = 1 M; [p-benzoquinone]o = 1M; average solar power: 36 W m-2; reaction time = 90 min.
15
It is shown in Fig. 6a that the greatest inhibition of aniline degradation occurred in the presence of methanol or glycerol. The percentage of degradation decreased from 99.9% (solar/TiO2/S2O82- process without scavengers) to only 9% or 11% in the presence of methanol or glycerol, respectively. With respect to the role played by both HO• and SO4•radicals, it can be seen that the degradation efficiency decreased from 99.9%, obtained in the scavengers free reaction, to only 9% or 29% when methanol or tert-butyl alcohol were used, respectively. This indicated that sulfate radical was found to be more significant than HO• because the reaction inhibition was lower when tert-butyl alcohol (mainly HO • inhibitor) was used. Regarding the role played by the holes generated in TiO2, the inhibition of the reaction in the presence of glycerol (11% degradation), which acts primarily as a hole scavenger [47] although it can also react with HO [48], suggest that the holes also played an important role in the aniline degradation reaction. The holes can react with aniline or with the intermediates adsorbed on the surface of TiO 2, and on the other hand, they can initiate the HO• formation reaction according to Eqs. (3) and (21).
hVB + +O𝐻 − → HO
(21)
The presence of 1,4-p-benzoquinone (O2- and HO quencher) reduced the percentage of aniline removal from 99.9% obtained in the scavenger free reaction to only 23%. In this case, the reaction inhibition was slightly higher than with tert-butyl alcohol (29% aniline removal). This indicated that 1,4-p-benzoquinone trapped HO and also reacted with O2 generated in reactions (6) and (22) decreasing its availability in the reaction medium. So,
16
superoxide radical anion had a moderate role in the aniline degradation reaction under the solar/TiO2/S2O82- system.
𝐻𝑂2 ↔ 𝐻 + 𝑂2−
(22)
Roles of ROS and holes in the solar/TiO2/H2O2 system
Fig. 6b shows the results of aniline degradation obtained when the solar/TiO2/H2O2 process was used in the presence and absence of radical and holes quenchers. The experimental conditions were the following: [aniline]o= 10 mg L-1; pH = 4; average temperature = 26 °C; load of TiO2= 125 mg L-1; [H2O2]= 40 mg L-1; [t-butOH]o = 1 M; [MeOH]o = 1 M; [glycerol]o = 1 M; [p-benzoquinone]o = 1 M; average solar power = 36 W m-2; reaction time: 90 min.
As shown in Fig. 6b, the aniline removal curves versus t30W in the reactions carried out in the presence of tert-butyl alcohol or methanol are similar because SO4•- was not generated in this solar/TiO2/H2O2 system. The degradation efficiency of aniline decreased from 99.9% (in t30W = 40 min) to approximately 28% in t30W = 60 min. Similar degradation result was achieved when the reaction was carried out in the presence of glycerol (hole scavenger and to a lesser extent of HO•), which indicated that HO• radical and hVB+ were primarily responsible for aniline degradation in the solar/TiO2/H2O2 system.
17
The percentage of degradation achieved in the presence of p-benzoquinone (26%) was slightly smaller than with tert-butyl alcohol (28%), indicating that O2•- played a role less significant in the solar/TiO2/H2O2 system than in the solar/TiO2/S2O82- process.
If both solar/TiO2/S2O82- and solar/TiO2/H2O2 systems are compared, it can be concluded the following (see Fig. 7): S2O82- was more efficient as an electron acceptor than H2O2 since the holes, hVB+, had a more significant effect in the solar/TiO2/S2O82- system (11% or 28% degradation in the presence of glycerol in the S2O82- or H2O2 systems, respectively) due to a greater inhibition of the electron-hole pairs recombination; the superoxide radical anion, O2•-, played a more significant role in the solar/TiO2/S2O82system than in the solar/TiO2/H2O2 process. This could be due to the lower availability of perhydroxyl radicals, HO2, in the solar/TiO2/H2O2 system to form O2•- through Eq. (22) due to its consumption in reactions (19) and (20); the results of degradation obtained in the solar/TiO2/S2O82- confirmed the main role of sulfate radicals and holes as oxidizing agents under the studied operating conditions.
4. CONCLUSIONS
In this work, the degradation of aniline in aqueous solution has been studied by means of a solar photocatalytic system using the TiO2 catalyst in suspension with the addition of two electron acceptors to the system, S2O82- and/or H2O2. It was found that the degradation of aniline by direct photolysis with sunlight was negligible (4%). An excess of TiO2 decreased the degradation efficiency possibly due to a decrease in sunlight penetration. Persulfate and hydrogen peroxide individually did not act as aniline oxidants. 18
On the other hand, it was observed that the combined system of persulfate and titanium dioxide irradiated with sunlight favored the elimination of the contaminant and of the intermediates. Under optimal conditions, the percentage of TOC removal was 91.5%. The addition of persulfate inhibited the recombination of electron/hole pairs, increasing their availability in the aqueous medium. The presence of H2O2 in the solar/TiO2 system above 40 mg L-1 decreased the effectiveness of aniline removal due to its possible reaction with hydroxyl radicals. Persulfate had a higher efficiency as an electron acceptor (greater availability of holes) than H2O2 in the solar/TiO2 system. The superoxide radical anion, O2•-, played a more significant role in the solar/TiO2/S2O82- system than in the solar/TiO2/H2O2. The degradation results obtained in the solar/TiO2/S2O82- system confirmed the participation of sulfate radicals as oxidizing agents more influential than hydroxyl radicals. The solar/TiO2/S2O82- system can be a potential alternative to degrade emerging pollutants such as aniline in wastewater.
5. ACKNOWLEDGMENTS
Financial support from Ayuda a grupos from UCLM is gratefully acknowledged.
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Figure captions:
Figure 1: Effect of the concentration of TiO2 on a) degradation of aniline and b) mineralization. Solar/TiO2 process; c) absorption spectrum of a 10 mg L-1 aniline aqueous solution.
Figure 2: Evolution of the concentration of dissolved oxygen in the degradation and mineralization of an aqueous solution of 10 mg L-1 aniline by the solar/TiO2 process at different TiO2 concentrations.
Figure 3: Effect of S2O82- dose on a) degradation of aniline and b) mineralization. Solar TiO2/S2O82- system; c) absorption spectrum of a 250 mg L-1 S2O82- aqueous solution.
Figure 4: a) Effect of H2O2 dose on the mineralization of a 10 mg L-1 aniline aqueous solution by solar/TiO2/H2O2 process.
Figure 5: Comparative study of degradation and mineralization of aniline by different solar/TiO2/S2O82- and/or H2O2 systems.
26
Figure 6: Degradation of an aniline aqueous solution in the absence or presence of reactive intermediate species quenchers. a) Solar/TiO2/S2O82- system; b) Solar/TiO2/H2O2 system
Figure 7: Comparative study of the roles of reactive intermediate species in the degradation and mineralization of aniline aqueous solution by solar/TiO2 process in the `presence of the electron acceptors persulfate or hydrogen peroxide.
27
1.00
Aniline, C/Co
0.80
0.60
125 mg/L TiO2 dark
0.40
solar light alone, no catalyst solar/75 mg/L TiO2 solar/125 mg/L TiO2
0.20
solar/250 mg/L TiO2 solar/500 mg/L TiO2
0.00 0
10
20
30
40
50
60
70
80
90
100
60
70
80
90
100
Time (min)
a) 100.00 solar light alone, no catalyst solar/75 mg/L TiO2 solar/125 mg/L TiO2
80.00
solar/250 mg/L TiO2
TOC removal, %
solar/500 mg/L TiO2
60.00
40.00
20.00
0.00 0
10
20
30
40
50
Time (min)
b)
28
Absorbance
2
1
0 200
220
240
260
280
wavelenght, nm
c)
29
300
320
340
8.00
6.00 5.00 4.00 3.00 2.00 1.00
75 mg/L TiO2
125 mg/L TiO2
250 mg/L TiO2
500 mg/L TiO2
0.00
0
10
20
30
40
50
60
70
80
Time (min)
1
0.8
Aniline, C/Co
Dissolved oxygen (mg L-1)
7.00
0.6
0.4 300 mg/L PS, no light solar/300 mg/L PS solar/125 mg/L TiO2 alone solar/125 mg/L TiO2/150 mg/L PS
0.2
solar/125 mg/L TiO2/300 mg/L PS solar/125 mg/L TiO2/450 mg/L PS
0 0
10
20
30
40
50
t30W (min)
a)
30
60
70
80
90
90
100
% TOC removal
80
60 300 mg/L PS, no light
solar/300 mg/L PS
40
solar/125 mg/L TiO2 solar/125 mg/L TiO2/150 mg/L PS solar/125 mg/L TiO2/300 mg/L PS
20
solar/125 mg/L TiO2/450 mg/L PS
0 0
10
20
30
40
50
60
70
80
90
t30W (min)
b)
Absorbance
0.03
0.02
0.01
0.00
240
290 340 Wavelenght (nm) c)
31
390
100 82.54
85.3
84.1
%TOC removal
80
70.47
65.69 56.89
60
40
20 3.22 0
32
1.0 solar/125 mg/L TiO2 solar/125 mg/L TiO2/300 mg/L PS solar/125 mg/L TiO2/300 mg/L PS/40 mg/L H2O2
0.8
100
91.54 85.3
0.6 %TOC removal
Aniline, C/Co
82.54 80
0.4
68.21 60
40
20
0.2 0
solar/TiO2/PS
solar/TiO2
solar/TiO2/H2O2
solar/TiO2/PS/H2O2
0.0 0
10
20
70
60
50
40
30
80
t30W (min)
1.00
0.80
Aniline, C/Co
0.60
0.40
0.20
0.00 0
10
20
30
40
50
60
t30W(min) without scavengers
tert-butyl alcohol
glycerol
p-benzoquinone
33
methanol
70
90
a) 1.00
0.80
Aniline, C/Co
0.60
0.40
0.20
0.00 0
10
20
30
40
50
60
70
t30W (min) without scavengers
tert-butyl alcohol
glycerol
p-Benzoquinone
methanol
b)
NO DEGRADATION Dark
H2O2 TiO2
+
+
TiO2/H2O2 Dark
PRODUCTS 99.9 % AN removal in 30 min 91.5 % TOC removal in 70 min
PS TiO2 TiO2/PS
Aniline
+ + NO DEGRADATION
Role -
Role
hVB+ HO
-
O2-
PRODUCTS 99.9 % AN removal in 60 min 85.3 % TOC removal in 90 min
34
hVB+ HO SO4O2-
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
35
1. We have studied the removal of aniline by solar/TiO2/electron acceptor 2. 91.5% TOC removal was achieved in optimal conditions 3. Persulfate and hydrogen peroxide inhibited the e-/h+ recombination 4. Persulfate was a more effective electron acceptor than H2O2 5. SO4- and holes were found to be the main reactive intermediate species
36