Abstract

The TiO2 nanoparticles synthesized from the extract of Coffea arabica L. (or TiO2/C) were used to remove paraquat from contaminated water in heterogeneous photocatalysis process. In this work, the sol-gel process using Coffea arabica L. as the solvent chemical were performed to obtain the TiO2 nano-catalyst. The value of pHpzc of TiO2/C was 2.9 which caused a highly acidic surface of catalyst. The paraquat is effectively removed in alkaline medium due to the adsorption ability of paraquat on the surface of TiO2/C. The paraquat degradation followed the pseudo-first-order model with the apparent rate constants of 5.84 × 10−2, 4.08 × 10−2, and 2.28 × 10−2 min−1 for TiO2/C, TiO2, and without TiO2, respectively, under the presence of ultraviolet (UV) and H2O2. The combined TiO2/C with UV and H2O2 was the most efficient process, exhibiting a maximum 66.3% degradation of 50 mg/L over 90 min at pH 10.

INTRODUCTION

Titanium dioxide (TiO2) has proved to be a highly useful and active photocatalytic material to remove or degrade recalcitrant organic contaminants through the chemical oxidation reaction for water and air purification. Due to its physical and chemical properties, the application of TiO2 with ultraviolet (UV) has been widely used, and it has been considered as one type of advanced oxidation processes (Chen et al. 2016). The organic pollutants can be degraded by photocatalytic oxidation in which photo-induced holes in TiO2 oxidize OH or water molecules adsorbed on the surface of the particles to produce HO and O2−• which subsequently attack adsorbed organic molecules (Hoffman et al. 1995). Photocatalytic reactions have been applied to degrade several pesticides including atrazine, bentazon, monuron, dichlorvos, propyzamide, dicloran and triadimefon (Pelizzetti et al. 1989; Pelizzetti et al. 1993; Pramuro et al. 1993; Minero et al. 1997).

The emerging of ‘green chemistry’ leads to the new synthesizing method of nanoparticles that can improve the physico-chemical properties of the materials. These green nanoparticles are expected to provide higher efficiency in contaminant removal (Humayun et al. 2017). However, few works reported in detail about the kinetics and degradation efficiency of the pollutant using those green nanomaterials, especially the TiO2. In this area, there has also been increasing interest in identifying environmentally friendly materials that are multifunctional and provide good properties of materials. In this work, coffee extract was chosen to be a solvent for TiO2 formation following the green chemistry synthesis method. Caffeine/polyphenols from coffee extracts can form complexes with metal ions in solution and reduce them to the corresponding metals (Nadagouda et al. 2010). The caffeine/polyphenols contain molecules bearing alcoholic functional groups which can be exploited for reduction, as well as stabilization of the nanoparticles. Thus, these chemicals act as reducing agent and stabilizing agent in TiO2 synthesis. The chemicals from coffee extracts are also the capping agent or dispersing agent of TiO2 that can prevent the agglomeration of the nanoparticles and enhance the photocatalytic efficiency of the nanomaterials (Nadagouda et al. 2010; Varma 2012).

This work aims to synthesize the TiO2 using green methods by applying the green solvent obtained from the extract of Coffea arabica L. (or TiO2/C) and applying it in removal of paraquat from aqueous solution. Paraquat (1,1′-dimethyl-4,4′-bipyridinium dichloride) is a widely used commercial herbicide applied in many agricultural activities. This chemical has a high toxicity to mammals and is recognized as a potent human poison. The U.S. EPA has classified paraquat as a possible carcinogen and has recommended the maximum concentration of paraquat in drinking water at 200 μg/L (Rossi 1997). The removal of paraquat using TiO2 has been published previously (Zahedi et al. 2015). However, the application of green photocatalytic to remove paraquat is not reported elsewhere.

In this work, the details in kinetics and paraquat degradation efficiency were calculated and reported. The paraquat degradation performance using this new type of TiO2/C in comparison with that using the conventional TiO2 was also discussed. This approach addresses several factors that play an important role in paraquat removal enhancement by this new type of green TiO2 nanomaterial.

MATERIALS AND METHODS

Materials

Titanium tetraisopropoxide (TTiP), paraquat, and glacial acetic acid (CH3COOH) were purchased from Aldrich Chemicals, Thailand. Nitric acid (HNO3), sulphuric acid (H2SO4) and sodium hydroxide (NaOH) were obtained from Merck Chemicals, Thailand. All chemicals were used as received for preparing the nanocatalysts and conducting photocatalytic experiments. The 18 MΩ deionised water (H2O) was used for the preparation of all the solutions. All reagents used were of analytical grade and employed as received.

Tio2 catalyst synthesis

All experiments were conducted at the Center of Excellence on Environmental Research and Innovation, Naresuan University. The typical sol–gel synthesis of TiO2-loaded nanoparticles was used in this work. The method is as follows: TTIP (4.83 mL), glacial acetic acid (15.35 mL) and water were used in the molar ratio of 1:10:300.16 TTIP was mixed with glacial acetic acid in an ice bath. When this mixture turned to a sol, it was stirred at room temperature to form a gel before undergoing the drying process. After drying at 100 °C for 90 min, the powder was collected and calcined at 600 °C. The nanoparticles obtained from this process were shortly denoted as ‘TiO2’.

For the green TiO2 nanoparticles synthesized from Coffea arabica L., 1 g of coffee was boiled in 50 mL of water and filtered through a filter paper (Whatman, No. 41). Then 5 mL of coffee extract was added to the mixture solution of TTiP and glacial acetic acid. The drying process was the same procedure as above. The obtained TiO2 nanoparticles were denoted as ‘TiO2/C’.

Adsorption and photocatalytic of paraquat

Photocatalysis experiments were performed in a batch reactor. The reactor was cylindrical with a volume of 1.1 L made from quartz glass (ACE Glass Co. 7841-06; Vineland, NJ, USA). The UV light source was a 10 W (Philips) with 254 nm wavelength. The initial concentration of paraquat was in the range of 5–50 mg/L, and the catalyst was 0.6 g/L. In this photocatalytic experiment, the paraquat solution was injected into the reactor and treated in batch operation mode with a steady temperature maintained at 25 ± 2 °C throughout the test in an oxygen atmosphere. The liquid was allowed to equilibrate in the dark for 30 min. The paraquat adsorption data were collected. After reaching equilibrium, the reaction was started by switching on the light at t = 0, and the initial concentration of paraquat was designated as C0. The initial pH of solution was 7. It was adjusted to pH 3 by adding 36.8 N H2SO4 and to pH 10 by adding NaOH before the photoreaction experiment unless otherwise specified. At a chosen interval of irradiation time, aliquots of the reaction mixture were withdrawn and filtered through a membrane filter (0.1 μm). Paraquat concentration was detected by a colorimetric method using UV-Vis spectrophotometer (Model Lambda 650, Perkin Elmer, USA) by reducing paraquat to its blue radical. A solution sample in the test tube was added with 0.1% sodium dithionite in 0.1 M sodium hydroxide. The mixture was gently mixed and measured for light absorbance within 1 min. The detection was done at 600 nm wavelength using a spectrophotometer. The paraquat concentrations in the final solution for each experiment were also confirmed by liquid chromatography–mass spectrometry (Model Single Quadrupole LC/MS, Agilent, USA). Total organic carbon (TOC) analysis was done by TOC carbon analyzers (Model TOC-L, Shimadzu, Japan). The Brunauer–Emmett–Teller (BET) surface area for all nanoparticles were done by BET surface area analyzer (Model SA-9600, Horiba, Japan). Analysis of pH at point of zero charge (pHpzc) for all nanoparticles was measured by Zetasizer (Model ZS90, Malvern, UK).

RESULTS AND DISCUSSION

Adsorption of paraquat onto TiO2 surface

Preliminary adsorption experiments revealed that in the absence of TiO2, no noticeable change in paraquat concentration occurred during a 60-min experimental period. Adsorption of paraquat using TiO2/C at different pH was evaluated as shown in Figure 1. The adsorption was rapid in the first few minutes and reached equilibrium within 20 min for all pH conditions. The paraquat adsorption was highest at pH 10 with the maximum adsorption capacity of 20 mg/g. At pH 3 and pH 7, the maximum adsorption capacities were 16.8 and 12.2 mg/g, respectively. Our results are in good agreement with previous work. Florencio et al. (2004) reported that the degradation of diquat and paraquat did not take place in acid medium but was pronounced in an alkaline solution. Less adsorption of paraquat on the surface of TiO2 resulted in low degradation of the chemicals in acidic and moderate degradation in neutral pH medium (Florencio et al. 2004).

Figure 1

Adsorption of paraquat using TiO2/C at different pHs. Experimental condition: [TiO2] = 0.6 g/L, and [PQ] = 50 ppm.

Figure 1

Adsorption of paraquat using TiO2/C at different pHs. Experimental condition: [TiO2] = 0.6 g/L, and [PQ] = 50 ppm.

The paraquat adsorption of TiO2/C was compared with that of TiO2 from chemical-based process as shown in Figure 2. The BET surface areas of TiO2/C (75.8 m2/g) was slightly changed when compared with TiO2 (73.2 m2/g). While the values of pHpzc were 2.9 and 4.6 for TiO2/C, and TiO2, respectively. The lower the pHpzc causes the higher affinity for cations to adsorb on the surface of nanoparticles. Thus, using the Coffea Arabic L. for TiO2 synthesis mainly affected on the surface charge rather than the surface area and, consequently, resulting in higher adsorption ability of TiO2/C over TiO2. At pH 10, the surface charges of the catalyst were negative, favouring the adsorption of cations like paraquat. This finding also occurred for other types of catalyst such as TiO2/SBA-15 and Cu-TiO2/SBA-15 (Sorolla II et al. 2012).

Figure 2

Comparison of paraquat adsorption using TiO2/C, and TiO2 at pH 10. Experimental condition: [TiO2] = 0.6 g/L, [PQ] = 50 ppm and pH 10.

Figure 2

Comparison of paraquat adsorption using TiO2/C, and TiO2 at pH 10. Experimental condition: [TiO2] = 0.6 g/L, [PQ] = 50 ppm and pH 10.

Paraquat removal by TiO2/C photocatalytic reaction

Paraquat degradation using TiO2/C with UV in different concentrations of paraquat is shown in Figure 3. The removal percentages of paraquat with initial concentrations of 5, 10, 20, 35, and 50 mg/L were 76.3, 52.1, 31.7, 27.9, and 4.3%, respectively. These results clearly indicate that low concentration paraquat solutions were easily degraded by photocatalytic reactions. However, as the initial concentration of paraquat increased (higher than 35 ppm), the TiO2/C with UV hardly degraded paraquat within 1 h. It was previously reported that the paraquat removal using TiO2 was feasible for the concentration lower than 30 mg/L since it is chemically stable in nature (Moctezuma et al. 1999). The paraquat solutions remained stable in the temperature range of 20–40 °C and after standing for 23 days (Florencio et al. 2004).

Figure 3

Paraquat degradation by photocatalytic process using different concentration of TiO2/C. Experimental condition: [TiO2] = 0.6 g/L, UV 10 watt and pH 10.

Figure 3

Paraquat degradation by photocatalytic process using different concentration of TiO2/C. Experimental condition: [TiO2] = 0.6 g/L, UV 10 watt and pH 10.

To enhance the paraquat degradation, H2O2 was added to the experiment. Solutions of 50 ppm paraquat were irradiated under four different conditions: H2O2/UV, TiO2/C with UV (without H2O2), TiO2/C with UV + H2O2, TiO2 with UV (without H2O2), and TiO2 with UV + H2O2 systems. Figure 4 shows the changes in the degree of degradation of aqueous paraquat versus irradiation time. It is noted that there was no paraquat degradation by the direct photolysis with a 10 W UV lamp during 90 min. As shown previously in Figure 3, the 50 mg/L of paraquat was hardly decreased using TiO2/C with UV irradiation and the removal efficiency was only 4.3%. The paraquat removal increased up to 36.2% in the presence of 10 mM of H2O2 under UV irradiation (without TiO2/C). Approximately 66.3% of paraquat was degraded within 90 min when 0.6 g/L TiO2/C was added with UV and 10 mM H2O2.

Figure 4

Paraquat degradation using different processes, (a) UV/H2O2, (b) TiO2+ UV (without H2O2), (c) TiO2/C+ UV (without H2O2), (d) TiO2/C + UV + H2O2, and (e) TiO2 + UV + H2O2, Experimental condition: [TiO2] = 0.6 g/L, [PQ] = 50 ppm, [H2O2] = 10 mM, UV = 10 W, and pH 10.

Figure 4

Paraquat degradation using different processes, (a) UV/H2O2, (b) TiO2+ UV (without H2O2), (c) TiO2/C+ UV (without H2O2), (d) TiO2/C + UV + H2O2, and (e) TiO2 + UV + H2O2, Experimental condition: [TiO2] = 0.6 g/L, [PQ] = 50 ppm, [H2O2] = 10 mM, UV = 10 W, and pH 10.

In comparison, the paraquat degradation using TiO2/C was compared with TiO2 in the presence of H2O2 under UV irradiation as shown in the same figure. The TiO2/C provided the higher photocatalytic efficiency for paraquat degradation than the TiO2. The high efficiency in paraquat removal using TiO2/C partially due to the high adsorption of paraquat onto its surface as explained earlier, enhancing more paraquat to react with OH radicals. The least amount of degradation was observed for chemical based TiO2, which has slightly lower surface area, higher value of pHpzc and the lower adsorption of paraquat on its surface as seen in Figure 2. The overall percentages of degradation for paraquat after 90 min of irradiation were 66.3, 48.7, 36.2, and 4% for TiO2/C with UV + H2O2, TiO2 with UV + H2O2, UV/H2O2, and TiO2/C +UV (without H2O2), respectively.

The reaction mechanisms during the TiO2 in the presence of H2O2 under UV irradiation are summarized as follows (Barakat et al. 2005; Gao et al. 2015):
formula
(1)
formula
(2)
formula
(3)
formula
(4)
formula
(5)
formula
(6)
formula
(7)
where RH refers to the paraquat compound.

In the system that contained only TiO2, the electron (e) and hole (h+) on the TiO2 surface were generated upon UV irradiation (Equation (1)). Equations (3), (4), and (7) express that the hydroxyl radicals were formed under light excitation when the positive holes reacted with OH on the TiO2 surface. When the H2O2 was presented in the system, the UV/H2O2 reaction can generate OH (Equation (2)), enhancing the rate of photocatalytic reaction. H2O2 was a better electron acceptor than oxygen in other mechanisms (Equation (5)). Thus, the amount of electron recombination decreased, and the paraquat degradation using this photocatalytic process was enhanced as shown in Figure 4.

To describe the kinetics of this paraquat degradation, the Langmuir–Hinshelwood model (or L–H model) was selected as it demonstrated a good fit in photocatalysis process in many previous works (Rajeshwar et al. 2008). The development of this model to obtain a kinetic equation depends on the limiting step (adsorption, surface reaction, or desorption). The L–H model relates the degradation rate (r) to the concentration of the organic reactant (C), as shown in Equation (8).
formula
(8)
where kr is the rate constant, and Kad is the adsorption equilibrium constant. When the adsorption is relatively weak, or the concentration of the reactant is low, Equation (8) can be simplified to be the first-order kinetics with an apparent rate constant kapp (Equation (9)):
formula
(9)

Plotting −ln(C/C0) vs. reaction time (t) generates a straight line with a slope equal to kapp (Chiou et al. 2008). The apparent rate constants (kapp), initial degradation rates (r), and correlation coefficients (R2) for paraquat degradation using different types of TiO2 are shown in Table 1.

Table 1

Apparent rate constants (kapp), initial degradation rates (r), and correlation coefficients (R2) for paraquat degradation using different types of TiO2

TiO2Photocatalytic conditionkapp (min−1)r (mM/min)R2
– UV + 10 mM H2O2 0.0228 0.0044 0.9990 
TiO2 UV + 10 mM H2O2 0.0408 0.0106 0.8391 
TiO2/C UV only 0.0014 0.0004 0.9080 
TiO2/C UV + 10 mM H2O2 0.0584 0.0130 0.9203 
TiO2Photocatalytic conditionkapp (min−1)r (mM/min)R2
– UV + 10 mM H2O2 0.0228 0.0044 0.9990 
TiO2 UV + 10 mM H2O2 0.0408 0.0106 0.8391 
TiO2/C UV only 0.0014 0.0004 0.9080 
TiO2/C UV + 10 mM H2O2 0.0584 0.0130 0.9203 

The initial reaction rate of the TiO2/C with UV + H2O2 (5.84 × 10−2 mM/min) in paraquat removal was nearly forty-two fold that of the TiO2/C with UV but without the H2O2 system (1.4 × 10−3 mM/min). This difference is attributed to the contributions of the hydroxyl radicals produced by H2O2 during the TiO2/C with the UV + H2O2 process.

The degradation percentage of paraquat regarding mineralization of TOC in the presence of UV + H2O2, TiO2 with UV and TiO2 with UV + H2O2 was also investigated. The initial concentration of paraquat was 50 ppm with an initial TOC value of 32.08 ppm. The degradation percentage and mineralization to the TOC of paraquat using different processes are shown in Figure 5. All cases were expressed as mean ± standard deviation. An analysis of variance (ANOVA) was used to test the significance of the results with p < 0.05 and F = 4.34 was statistically significant. From the results, mineralization of paraquat for TiO2/C in the presence of UV with H2O2 showed the highest yield. By contrast, the TiO2 from chemical-based synthesis provided lowest TOC percentage removal.

Figure 5

TOC concentration of paraquat in photocatalytic process using different processes. Experimental condition: [TiO2] = 0.6 g/L, [PQ] = 50 ppm, [H2O2] = 10 mM, UV = 10 W, and pH 10.

Figure 5

TOC concentration of paraquat in photocatalytic process using different processes. Experimental condition: [TiO2] = 0.6 g/L, [PQ] = 50 ppm, [H2O2] = 10 mM, UV = 10 W, and pH 10.

CONCLUSIONS

In this work, a green, low-cost, and reproducible of TiO2 nanoparticles were used to remove paraquat from contaminated water in heterogeneous photocatalysis process. The novel ‘green’ nanoparticles were obtained by the sol-gel process using coffee as the solvent chemical. The paraquat degradation using the TiO2/C was conducted in comparison with the TiO2 from chemical-based synthesis. Both TiO2 with UV and TiO2 with UV/H2O2 have been investigated. From overall experiments, the combined TiO2/C with UV and H2O2 was the most effective photocatalyst, exhibiting a maximum 66.3% degradation of 50 mg/L over 90 min at pH 10. The paraquat removal depended on the pH of the medium. The TiO2/C provided the highest adsorption percentage of paraquat due to its dominated negative charge on the surface. The BET surface areas of TiO2/C (75.8 m2/g) was slightly changed when compared with TiO2 (73.2 m2/g); however, the values of pHpzc of both materials were pronouncedly different. The pHpzc of TiO2 were 2.9, and 4.6 for TiO2/C, and TiO2, respectively. Thus, the surface charge of particles played a major role for the adsorption of positive ions of paraquat in this work. The high adsorption of paraquat on TiO2/C led to high efficiency in paraquat removal. The Langmuir–Hinshelwood model could well describe the photodegradation of paraquat during all three catalytic oxidation processes. The apparent first-order rate constants were 5.84 × 10−2, 4.08 × 10−2, and 2.28 × 10−2 min−1 for TiO2/C, TiO2, and UV, respectively, under the presence of UV and H2O2. Results from this work offer the benefit of using the green nanoparticles (such as TiO2/C) in pollutant removal for water and wastewater treatment.

ACKNOWLEDGEMENT

This research received funding support from National Research Council, Thailand (NRCT) through Naresuan University under grant No. R2562B021 and Thailand Research Fund under grant No. BRG6180009.

REFERENCES

Barakat
M. A.
,
Tseng
J. M.
&
Huang
C. P.
2005
Hydrogen peroxide-assisted photocatalytic oxidation of phenolic compounds
.
Applied Catalysis B: Environmental
59
,
99
104
.
Chen
J.
,
Cen
J.
,
Xu
X.
&
Li
X.
2016
The application of heterogeneous visible light photocatalysts in organic synthesis
.
Catalysis Science and Technology
6
,
349
362
.
Chiou
C. H.
,
Wu
C. Y.
&
Juang
R. S.
2008
Photocatalytic degradation of phenol and m-nitrophenol using irradiated TiO2 in aqueous solution
.
Separation and Purification Technology
62
(
3
),
559
564
.
Gao
M.
,
Zhu
L.
,
Ong
W. L.
,
Wang
J.
&
Ho
G. W.
2015
Structural design of TiO2-based photocatalyst for H2 production and degradation applications
.
Catalytic Science Technology
5
,
4703
4726
.
Hoffman
M. R.
,
Martin
S. T.
,
Choi
W.
&
Bahnemann
D. W.
1995
Environmental applications of semiconductor photocatalysis
.
Chemical Reviews
95
(
1
),
69
96
.
Humayun
M.
,
Raziq
F.
,
Khan
A.
&
Luo
W.
2017
Modification strategies of TiO2 for potential applications in photocatalysis: a critical review
.
Green Chemistry Letters and Reviews
11
,
86
102
.
Minero
C.
,
Maurino
V.
&
Pelizzetti
E.
1997
Heterogeneous photocatalytic transformations of s-triazine derivatives
.
Research on Chemical Intermediates
23
(
4
),
291
310
.
Moctezuma
E.
,
Leyva
E.
,
Monreal
E.
,
Villegas
N.
&
Infante
D.
1999
Photocatalytic degradation of the herbicide paraquat
.
Chemosphere
39
(
3
),
511
517
.
Nadagouda
M. N.
,
Castle
A. B.
,
Murdock
R. C.
,
Hussain
S. M.
&
Varma
R. S.
2010
In vitro biocompatibility of nanoscale zerovalent iron particles (NZVI) synthesized using tea polyphenols
.
Green Chemistry
12
,
114
122
.
Pelizzetti
E.
,
Maurino
V.
,
Minero
C.
,
Zerbinati
O.
&
Borgarello
E.
1989
Photocatalytic degradation of bentazon by TiO2 particles
.
Chemosphere
18
(
8
),
1437
1445
.
Pramuro
E.
,
Vincenti
M.
,
Augugliaro
V.
&
Palmisano
L.
1993
Photocatalytic degradation of monuron in aqueous TiO2 dispersions
.
Environmental Science & Technology
27
(
9
),
1790
1795
.
Rajeshwar
K.
,
Osugi
M. E.
,
Chanmanee
W.
,
Chenthamarakshan
C. R.
,
Zanoni
M. V. B.
,
Kajitvichyanukul
P.
&
Krishnan-Ayer
R.
2008
Heterogeneous photocatalytic treatment of organic dyes in air and aqueous media
.
Journal of Photochemistry and Photobiology C: Photochemistry Reviews
9
(
4
),
171
192
.
Rossi
L.
1997
Reregistration Eligibility Decision (RED): Paraquat Dichloride
.
Report EPA 738-F-96-018
,
United States Environmental Protection Agency
,
Washington, DC
,
USA
.
Sorolla
M. G.
II
,
Dalida
M. L.
,
Khemthong
P.
&
Grisdanurak
N.
2012
Photocatalytic degradation of paraquat using nano-sized Cu-TiO2/SBA-15 under UV and visible light
.
Journal of Environmental Sciences
24
(
6
),
1125
1132
.
Varma
R. S.
2012
Greener approach to nanomaterials and their sustainable applications
.
Current Opinion in Chemical Engineering
1
,
123
128
.