ADVANCED OXIDATION PROCESSES APPLIED TO THE TREATMENT OF ORGANIC AND INORGANIC CONTAMINANTS
Alberici, R.M.1; Nogueira, R.P.3; Canela, M.C. 1; Guimarães, J.R. 2; Jardim, W.F.1
1 2
Laboratório de Química Ambiental, Instituto de Química, CP 6154 Laboratório de Saneamento, Faculdade de Engenharia Civil, CP 6021 UNICAMP, Campinas-SP, Brazil 3
Instituto de Química de Araraquara – CP 355 UNESP, Araraquara-SP, Brazil e-mail:
[email protected]
KEY
WORDS:
Advanced
photocatalysis, Fenton's reagent.
oxidation
processes,
Heterogeneous
1
INTRODUCTION
Advanced Oxidation Processes (AOP) are of special interest for the treatment of water and wastewater due to their efficiency in mineralize a great variety of pollutants, including recalcitrant compounds, through the oxidation by generated hydroxyl radicals. AOPs are divided into homogeneous and heterogeneous systems in which hydroxyl radicals are generated with or without UV irradiation. Due to its high reactivity, hydroxyl radicals can react with a great variety of organic compounds [1,2]. Among the AOP, photocatalysis using mainly titanium dioxide (TiO2) as photocatalyst, has been extensively studied in the last years. This process consists on activation of a semiconductor by near-UV radiation. A semiconductor is characterized by valence and conduction bands (VB and CB) and the region in between is called bandgap. The semiconductor is activated by the absorption of photons with energy above that of the bandgap, which results in the transference of an electron from the valence band to the conduction band, generating a hole at the valence band. The schematic representation of a semiconductor particle is shown in Figure 1. Water molecules adsorbed at the TiO2 surface can be then oxidized and generate hydroxyl radicals which can subsequently oxidize the pollutants [3,4]: TiO2 + hν ↔ TiO2 + (e- + h+)
(1)
.
TiO2(h+) + H2Oads. → TiO2 + OH + H+
(2)
Figure 1- Schematic representation of electron/hole pair generation at semiconductor particle. D- electron donor; A- electron acceptor.
2
Oxygen is the main electron acceptor in the photocatalytic process. In the absence of an electron acceptor or donor in the medium, recombination of electron/hole pair occurs, hindering the photodegradation process. The photocatalytic process using TiO2/UV offers several advantages when compared to the others processes. Since the destruction is made at room temperature and ambient pressure, it is not necessary to use additional reagents and the final products, mainly CO2 and H2O, show little or no toxicity, compared with to parent compound. The possibility of application of photocatalysis for the decontamination of water and wastewaters was demonstrated for the first time by Pruden and Ollis [5] who achieved the total oxidation of chloroform and trichloroethylene (TCE)
to inorganic ions during the irradiation of TiO2 suspension. Since then,
heterogeneous photocatalysis has attracted great interest of different research groups in the world due to the potential application of the method to the destruction of a variety of organic and inorganic contaminants. Recently, photocatalysis has been considered a promising technology to destruction of contaminants in gas-phase. Photocatalytic oxidation in gas-phase can be applied to remediation of contaminated soils and groundwater, treatment of industrial process vents and treatment of indoor air. This technology using TiO2 was first explored by Dibble and Raupp [6]. They reported high levels of destruction of TCE when TiO2 was irradiated with UV light. Other classes of organic air contaminants, such as ketones, alcohols, aldehydes, aromatics, ethers and chlorinated organics have also been studied [7], as well as the phodestruction of inorganic compounds in gas-phase [8]. Fenton’s reagent, how used AOP, is know since last century, and consists in the decomposition of H2O2 in the presence of Fe(II). This oxidizing power was latter attributed to the generation of hydroxyl radicals in acid media (eq. 1). This process has gained great attention in the last years as an alternative for the treatment of wastewaters, since many classes of compounds can be oxidized [9,10]. The process is specially interesting when used in combination with UV-Vis light (photo-Fenton reaction) due to considerable enhancement of the oxidation efficiency. This effect has been attributed mainly to the photoreduction of Fe (III) generated in reaction 3, regenerating Fe (II) which participates again in reaction 1, keeping the cycle. Fe (II) + H2O2 → Fe (III) + OH- + HO.
(3)
The combination of different sources of hydroxyl radicals has been proposed as an alternative to increase the efficiency of the process. The influence of Fe (III) on illuminated TiO2 has been investigated by Wei and co-workers [11] and Sclafani and co-workers [12] for the induction of photo-Fenton reaction. The present work shows the applicability of photocatalysis using TiO2 for the destruction of organic and inorganic contaminants present in water, wastewaters and air. The combination of photocatalysis using TiO2 and Fenton's reagents was studied during irradiation of the wastewater containing HCB.
3
EXPERIMENTAL
Chemical
Titanium dioxide (P25 , Degussa), 70:30 anatase form, BET surface area 50 m2/g, and 30 nm mean particle size, was used as a photocatalyst. All reagents used were analytical grade. As hydrogen sulfide source was used H2S generator solution. This solution was made with 0.6 g of Na2S.9H2O and 1.44 g of Na2HPO4 in 100 mL of distilled water, and the pH was initially fixed at 8.1 ± 0.1 using 1 mol L-1 orthophosphoric acid [8].
Photodestruction in aqueous phase
Photodegradation experiments were made using a 400 mL batch reactor (Figure 2) and a 125 W mercury lamp as source of irradiation. Titanium dioxide was used at the concentration of 0.01 wt % and the suspension was stirred magnetically during the experiments. The temperature was maintained at 25°C by a water cooling jacket. Fenton’s reagent was prepared in situ by the addition of Fe (II) (5 ppm) and H2O2 (200 ppm) in the sample at pH 3.7. The solution was stirred during 30 minutes in the dark before irraditation. The wastewater composition was: 22 µg L-1 of (TOC) and pH 3.7.
hexachlorobenzene (HCB); 2.0 mg L-1 Total Organic Carbon
4
Figure 2- Typical well-mixed batch reactor. Solar photodegradation experiments were made in a TiO2-fixed-bed reactor consisting of a glass plate placed on a wooden support with 22° inclination, as can be seen in Figure 3. The reactor was operated in a single pass mode, at a flow rate of 2.7 L h-1 [13]. The solution was pumped to the top of the plate and irradiated during flow by gravity over the catalyst surface. The solution was collected at the bottom and analyzed. Volume corrections for losses due to water evaporation were made for each experiment. All the experiments were performed under clear sky conditions between 10 a.m. and 2 p.m. at the University campus located in Campinas, Brazil (23° S).
Figure 3- Schematic view of TiO2-fixed bed solar reactor.
Photodegradation in gas phase
An annular plug flow photoreactor consisting of a glass cylinder measuring 855 mm with a 35 mm internal diameter and a total volume of 405 mL was used. The TiO2 was coated onto the internal glass surface using an aqueous slurry, followed by drying with hot air. Illumination was provided by a 30 W blacklight lamp with maximum light intensity output at 365 nm. The lamp was fixed at the center of the reactor. The basic experimental setup used in this study is shown in Figure 4. It consists of synthetic air (21 % oxygen and 23 % relative humidity) used as carrier gas contaminated with VOC. The concentration of VOC in the atmosphere was obtained by vaporization of organic compounds using pre-determined values of flow rate.
5
Figure 4- experimental setup used in the photocatalytic destruction of VOCs.
Analysis
Conversion rates for HCB, VOCs and malodorous compounds were monitored using a gas chomatograph (SHIMADZU GC-14B) equipped with a DB-624 (30 m x 0.54 mm x 3 µm J&W) fused silica megabore column. For the analysis of HCB, quantitative extraction (yields > 90 %) from aqueous phase was made with toluene and quantified using electron capture detector (ECD). A flame ionization detector (FID) was used in the analysis of VOCs and malodorous compounds in the gas phase. Total Organic Carbon (TOC) was determined for phenol, pentachlorophenol, dichloroacetic acid and wastewaters samples using a total organic carbon analyzer (TOC 5000 SHIMADZU). Cyanide concentrations were determined by potenciometric titration with a AgNO3 (0.025 mol L-1) solution, using a silver electrode and a double junction electrode as reference. Methylene Blue concentrations were determined spectrophotometrically by measuring the absorbance at 660 nm and interpolation using a calibration curve. The H2S concentration was measured by trapping the carried gas in NaOH solution, using a washing bottle with a glass diffuser. Colorimetric determination of H2S was carried in the trapped solution using the Standard Methylene Blue Method.
6
RESULTS AND DISCUSSIONS
Photodegradation in aqueous phase
The photodegradation of HCB was studied in two different pH values (3.7 and 10.7) and the results showed that the photodegradation was favored in acidic medium as can be seen in Figure 5. After 480 minutes of irradiation, 85 % of HCB degradation were achieved in pH 3.7, while 40 % were observed in pH 10.7. On the other side, when Fenton’s reagent was added to TiO2 process, the photodegradation efficiency was enhanced resulting in 100 % destruction of HCB in only 180 minutes of irradiation. This effect can be explained in terms of additional search of hydroxyl radicals. Furthermore, ferric ions generated by Fenton’s reaction, can act as electron acceptor at TiO2 surface avoiding electron/hole recombination. This result demonstrates the advantage to use AOP combination to increase the efficiency of the process.
Destruction (%)
100
80
60
40
TiO 2/ U V + H 2O 2 + Fe
20
2+
(pH 3.7)
TiO 2/ U V ( p H 3 . 7 ) TiO 2/ U V ( p H 1 0 . 7 )
0 0
100
200
300
400
500
Time of irradiation (min)
Figure 5 - Photodegradation of HCB in the wastewater. TiO2 (0.01 wt %); Fe2+ (5 mg L-1); H2O2 (200 mg L-1).
7
The photodegradation of different classes of compounds was evaluated using the fixed bed photoreactor and solar light as source of irradiation at concentrations ranging from 0.1 to 1.0 mmol L-1. The decay of TOC content of two real wastewaters was also observed during irradiation and the results are shown in Table I. The mineralization of methylene blue (C16H18N3SCl) by TiO2/UV, due its intense blue color and solubility in water, provides a simple visual demonstration of photodegradation as reported by Nogueira and Jardim (14). The mineralization of the all compounds tested showed a linear dependence of degradation with solar light intensity for clear sky conditions. Under cloudy skies, degradation also occurred, but to a smaller extent.
Table I - Photodegradation of contaminants using solar reactor.
Contaminant Cyanide Phenol Pentachlorophenol Dichloroacetic acid Methylene blue Wastewater Phenol Methyl amino phosforic acid
Initial Conc. (mmol L-1) 1.0 1.0 0.1 1.0 0.1 Initial TOC (mg C L-1) 150 140
Degradation (mg m -2 h-1) 352 268 47 470 192 Degradation (mgC m -2h-1) 215 88
Photodestruction in gas phase
Different classes of volatile organic compounds (VOCs) were destroyed with high efficiency as shown in the table II. For the majority of the compounds tested under the experimental conditions used in this study, conversions between 66.6 and 99.9 % were obtained after the steady-state was achieved. On the other hand, isopropylbenzene, methyl chloroform, pyridine and carbon tetrachloride showed much lower conversions. As benzene, toluene is also very recalcitrant to photooxidation, even using AOP. Carbon tetrachloride photoreduction is the absence of electrons donor (such as methanol), was not efficient. Malodorous compounds containing sulfur atoms were also studied in this system achieving high degradation rates. Compounds tested were trimethylene sulfide (C3H6S), propylene sulfide (C3H6S), tiophene (C4H4S), methyl disulfide (C2H6S2) in a range of concentration between 20 to 60 ppmv, in synthetic air. Concentration values were chosen considering the background levels in the environment, and the extremely low odor threshold of these compounds. The results showed in the table III demonstrate that for all sulfur containing compounds tested the degradation rate was nearly 100%.
8 Table II - Photocatalytic destruction of VOC. Q = 200 mL min-1; 21% oxygen and 23% relative humidity.
Compounds
Cin (ppmv)
Conversion (%)
Trichloroethylene
480
99.9
Isooctane
400
98.9
Acetone
467
98.5
Methanol
572
97.9
Methyl ethyl ketone
497
97.1
Tert-butyl methyl ether
587
96.1
Methylene chloride
574
90.4
Methyl isopropyl ketone
410
88.5
Toluene
506
87.2*
Isopropanol
560
79.7
Chloroform
572
69.5
Tetrachloroethylene
607
66.6
Isopropylbenzene
613
30.3
Methylchloroform
423
20.5
Pyridine
620
15.8
Carbon tetrachloride
600
0
* this value dropped to 20.9 % after 150 minutes of irradiation.
Table III - Malodorous compounds destruction by heterogeneous photocatalysis
Cin (ppmv)
Conversion (%)
Trimethylene sulfide
61
99
Propylene sulfide
86
99
Tiophene
54
99
Methyl disulfide
34
99
Hydrogen sulfide
579
99
Compounds
The general equations for the photocatalytic complete conversion of compounds may be described as bellow. The photocatalytic process proportionates the mineralization of the compounds according to eq 4-7, opposite to conventional processes where contaminants are only phase transfered.
9
Cx H y +
4x + y y O2 → xCO2 + H 2 O (4) 2 2
Cx H y O z +
4x + y + 2z y + 2z O2 → xCO2 + H 2O 4 2
Cx H y S z+
4x + y + 6z y − 2z O2 → xCO2 + 2 zH + + zSO42 − + H 2 O (6) 4 2
C x H y Cl z +
(5)
4x + y − z y + 2z O2 → xCO2 + zH + + zCl − + H 2 O (7) 4 2
CONCLUSION
These results demonstrated the efficiency of the advanced oxidation processes in the destruction of organic and inorganic contaminants in water and wastewater. Fenton's reagent combined with TiO2 can increase the photodegradation rate in aqueous phase. The photocatalysis using TiO2 is also applied in the destruction of contaminants in gas phase, what is a promising alternative for indoor air remediation as well as elimination of malodorous compounds present in wastewater treatment plants. The use of solar energy as source of irradiation is an interesting option, specially in tropical countries such as Brazil, to reduce the costs in the treatment of wastewaters.
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