Solar photocatalytic degradation of Zn2+ using graphene based TiO2

Separation and Purification Technology 168 (2016) 294–301

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Solar photocatalytic degradation of Zn2+ using graphene based TiO2 Gloria Kumordzi a, Ghodsieh Malekshoar a, Ernest K. Yanful b, Ajay K. Ray a,⇑ a b

Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada Department of Civil and Environmental Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada

a r t i c l e

i n f o

Article history: Received 11 December 2015 Received in revised form 5 May 2016 Accepted 28 May 2016 Available online 28 May 2016 Keywords: Adsorption Photocatalysis Photo-reduction Solar light Graphene TiO2 Heavy metal

a b s t r a c t The improvement of photocatalytic efficiency under an abundant natural resource, sun light, presents the next step in the large-scale application of photocatalysis for the treatment of dissolved organic and inorganic pollutants in wastewater. In this study, a composite catalyst of TiO2 and Graphene synthesised by a hydrothermal treatment method is used to photo-reduce Zn2+, the most abundant heavy metal found in combined sewer overflows (CSOs). The performance of this composite photo-catalyst was assessed under various process conditions such as pH, light intensity, catalyst loading and light source. The TiO2-Graphene composite photo-catalyst showed a 20.3 ± 0.04% increase in the photo-reduction of Zn2+ under solar light compared to un-doped TiO2 when reaction rate constants are compared. This enhancement is a result of the availability of more sorption sites, decrease in band-gap of the TiO2, and effectiveness of the charge separation in the TiO2-G composite catalyst. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction The application of heterogeneous photocatalytic technology for degradation of various dissolved organic and inorganic pollutants has been widely studied in recent years because of its potential in environmental remediation [7,8,17,21,24,37,41,43,54,57,68]. One such application, for which solar photocatalytic treatment can be extended to, is the removal of dissolved components in Combined Sewer Overflows (CSOs) in wastewater treatment plants. CSOs comprising industrial, domestic wastewater and surface run-offs commonly occur during wet weather events in most urban areas using aged combined sewer systems. Although storm water run-off pollutant concentration is greatly influenced by the land use in a particular area, CSOs may generally contain significant levels of suspended solids, heavy metals, nutrients, oxygen demanding organic material, pathogens, suspended solids, oil and grease [16,26]. At present, CSOs discharge requirement are not stringent. For example, in Ontario Canada, the minimum level of treatment is primary treatment or equivalent corresponding to 30% CBOD5 and 50% TSS removal [73]. For these reasons there is increased concern over their occurrence because overflows are detrimental to the overall receiving surface water quality [48,64] and this becomes even more critical when the same water body is the source of ⇑ Corresponding author. E-mail address: [email protected] (A.K. Ray). http://dx.doi.org/10.1016/j.seppur.2016.05.040 1383-5866/Ó 2016 Elsevier B.V. All rights reserved.

drinking water [12]. To this effect, the need to develop high-rate treatment technologies with the ability to also treat the dissolved components in anticipation of future requirements is laudable. Heavy metals as a major constituent of overflows find their way into wastewater streams by a number of means such as domestic activities, industrial wastewater, storm water runoff etc. [22]. Focusing on dissolved metal ion pollutants, some studies conducted on CSOs found aluminium (Al), chromium (Cr), copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn) concentrations exceeded that for tertiary treated wastewater. In addition, surface run-off contributes high concentrations of Cu and Zn to the wastewater while Fe and Al are also present in significant quantities [58]. These metal ions are non-biodegradable and they accumulate in concentration over time, which pose a long term effect [47]. However, for the purposes of this study, Zn2+ (
0.76 V) being the most abundant metal ion in most surface run-offs was used as the model compound. More so, studies showing severe environmentally unfriendly conditions arise when other heavy metals such as cadmium are present in significant quantities due to synergistic effects [60]. Photocatalytic reactions occur when electron-hole pairs are generated as a result of the irradiation of a photocatalyst with light, providing energy equal to or greater than the band-gap. These photo-generated electrons may react with electron acceptors such as oxygen and/or metal cations (photo-reduction), while the photo-generated holes may react with electron donors such as organic materials (photo-oxidation) [9]. In recent times, the goal

G. Kumordzi et al. / Separation and Purification Technology 168 (2016) 294–301

295

Nomenclature C I Kads k1 k2 r q NHE SHE T

concentration (ppm) light intensity (mW/cm2) adsorption equilibrium constant (ppm
1) pseudo-first-order rate constant (min
1) pseudo-second-order rate constant (g/mg min) reaction rate (mg/L min) adsorption capacity (mg/g) normal hydrogen electrode standard hydrogen electrode temperature (K)

of photocatalysis is to utilize direct sunlight, an enormous renewable energy source, for the photo-degradation process due to its potential of becoming cost effective and reliable in augmenting currently expensive and complicated conventional treatment methods [7]. However, Titanium Dioxide (TiO2) as the most widely used semiconductor material, which has proven to be biologically and chemically stable, nontoxic, insoluble under most conditions and inexpensive, has a major drawback with respect of its wide band-gap (3.2 eV) [8,42]. This limits the light energy sufficient for the activation and continuity of reaction to the ultraviolet (280–390 nm) range of the solar spectrum. The sum of ultraviolet (UV) wavelengths makes up only approximately 4% of the entire solar energy spectrum [1] while the visible light reaching the surface of the earth represents about 43–46% of the solar radiation. This translates to reduced efficiency and the need to concentrate the energy in this small window and hence, the broad study of photo-reduction of metal ions using this UV range [11,29,62]. The three factors that are crucial to the effectiveness of photoreduction are; pollutant adsorption onto the catalyst surface, light absorption of semiconductor photocatalyst and the charge transportation and separation [70]. For these reasons, improving the ability of the semiconductor to absorb light in the visible light region while hindering photo-generated charge recombination has proved to directly translate into the improvement of overall quantum efficiency of the process and also reduce future cost for larger-scale implementation. Therefore, modification to TiO2 is necessary. Semiconductor catalyst enhancement can be achieved either by augmenting the band-gap (band-gap engineering) or photo-sensitisation technique [3,18,19,52]. Even though transition metal particle doping and other material combination with TiO2 has been studied widely as a form of increasing the catalyst activity in the solar spectrum [52], carbon based TiO2 composite materials are becoming of great interest [65]. At the center of these carbonbased material, is the use of graphene [20,23,27,38,41,51,56] due to its unique characteristics such as chemical inertness, stability in both acidic and basic mediums, relatively large theoretical surface area (2630 m2/g), its abundance, flexible structure, high transparency, and good electrical and thermal conductivity (2000–5000 W/m/K) [65]. It is made up of a single layer of graphite and is recognised as the thinnest and hardest material known currently (mechanical strength of 2.4 ± 0.4 TPa) [2,5,25,70]. Another interesting finding in recent studies indicates that graphene and its derived materials exhibit anti-microbial activity, which is essential for the disinfection of wastewater [34]. Graphene-semiconductor composites, synthesised by different methods have been used for the degradation of various dyes and other recalcitrant organic [45,55]. However, its application in photo-reduction reactions has not yet been systematically studied to the best of our knowledge. Liu et al. Liu et al. [39] reported the enhancement in photocatalytic activity of a ZnO-TiO2-RGO (10 wt% graphene) composite for the reduction of Cr(VI) to Cr(III) as

t V k

time (min) volume (L) wavelength

Subscript/superscript 0 initial app apparent cat catalyst m maximum

compared with pure ZnO-RGO. Zhang et al. Zhang et al. [71] reported that TiO2-RGO (5 wt% graphene) composite enhance reduction of Cr(VI) under natural sunlight. The enhanced performance of TiO2-RGO for the visible light removal of Cr(VI) to Cr(III) was discussed by Zhao et al. Zhao et al. [72] while Lee and Yang [32] investigated the adsorption capacity of TiO2-G based on the hydrothermal treatment period of the composite semiconductor for the adsorption of Zn2+, Cd2+ and Pb2+. Until date, there is extensive research on photocatalytic heavy metal ion reduction efficiencies and viabilities under various conditions. Metals such as Cr, Se, Ag, Pb and Cu that are more easily photo-reduced due to the position of their standard reduction potential, have been widely studied [13,14,30,31,33,35,53,63,66,7 2]. Ku and Jung [31] studied the kinetics of Cr(VI) photoreduction, while Canterino et al. Canterino et al. [6] studied that of Cu(II). However, the application of photocatalytic treatment method for other metal ions such as Cd, Zn, Mn and Al, which are thermodynamically less favoured has been scarcely studied [10,44]. Therefore, further studies is required in this area due to the coexistence of metal ions in polluted wastewater streams such as CSOs [58]. So far, small concentrations of Zn2+ has been photo-reduced in the presence of TiO2 under UV light irradiation [10] and also in the presence of natural sunlight (65–80 mW/cm2) and hydrogen peroxide [28,61]. This study investigates and reports a systematic study of the effects of various process parameters such as pH, light intensity, light source and initial metal ion concentration on the photo-reduction of Zn2+ (the most abundant heavy metal ion found in CSOs) in suspended TiO2 and TiO2-G composite catalyst for the application in CSO treatment. It also reports the effect of the catalyst modification using graphene on the photo-reduction of the selected model compound Zn2+. 2. Experimental section 2.1. Materials Degussa P25 titanium dioxide (Evonic Degussa Corporation) was used. Zinc nitrate [Zn(NO3)26H2O - 99%], trizma base, sodium hydroxide, hydrochloric acid, formic acid and ethanol (99.5%) were purchased from Sigma-Aldrich Canada Ltd. All chemicals were used without further purification. 2.2. Photocatalyst preparation and characterization TiO2-G composite was prepared using a hydrothermal process as reported by Malekshoar et al. [41]. X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV spectra analysis, transmission electron microscope (TEM) and the scanning electron microscope (SEM) for structural and morphological analysis of the composite TiO2-G 1% (mass ratio) were carried out and

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discussed in an earlier work [41]. In this work, a diffraction peak was observed for GO at 2h = 11.3°, similar to other reported works, however the XRD pattern for TiO2-G showed peaks associated with TiO2 but no peak was observed for GO. This indicated that GO was completely reduced to graphene during the hydrothermal treatment process. UV–vis spectroscopy was used in the determination of the optical properties of the composite material. Measurements revealed that after the introduction of graphene the absorption edged shifted to the visible light region. The band gap of TiO2-G showed a noticeable decrease from 3.1 eV to 2.1 eV when the Tauc plot generated from the measurements were analysed. It was concluded that there was a strong interaction between graphene and TiO2. In addition, the XPS analysis of GO as compared to TiO2-G showed a significant decrease of the intensity of oxygenated carbon species (CAOH, CAOAC, C@O). Therefore it was concluded that during the hydrothermal process, there was effective reduction of GO to graphene sheets. The oxygenated carbon species on GO thus provide active sites for deposition of TiO2. Another reliable method used to characterize carbonaceous material known at Raman spectroscopy was used. This was used to assess the changes in structure of GO to graphene. The Raman spectra characteristic peaks assigned to carbon materials known as the D and G bands occurred around 1344 cm
1 and 1599 cm
1 respectively. The degree of functionalization indicated by D/G ratio was found to have increased from 0.86 for GO to 0.95 for TiO2-G confirming a good attachment of TiO2 with graphene sheets. SEM and TEM images were also used to verify the loading of TiO2 onto graphene sheets. AOH and ACOOH functional groups of GO aids the loading [41]. 2.3. Photo-reduction experimental procedure After the required pH adjustment, the catalysts were added in appropriate quantity to 150 mL of the prepared simulated wastewater comprising 20 mg/L metal ion concentration and 250 mg/L formic acid unless otherwise stated. This was followed by 5 min of ultrasonic dispersion performed in a Fisher Scientific FS60H bath. The resulting suspension was carefully transferred to a Pyrex glass photocatalytic reactor of 600 mL maximum capacity (6.3 cm height and 11 cm diameter). Prior to light irradiation for each experiment, metal ion TiO2 suspension were equilibrated in the dark for 30 min. Solar light was generated by a solar simulator 1000 W Xe arc Lamp with a 100 mW/cm2 intensity at full power equipped with an AM 1.5 G filter (model SS1KW, science tech ON Canada). Ultraviolet light (k between 300 and 388 nm) only conditions were obtained from the solar simulator by placing a long pass or visible light cut-off filter at the light exit by the help of an external attachment and the visible light (k between 420 and 650 nm) only condition were obtained by placing a UV cut-off filter inside the simulator, which eliminated wavelengths lower than 338 nm from the incident radiation. All experiments were conducted with continuous stirring using a magnetic stirrer while purging continuously with ultrapure nitrogen. Liquid samples were collected regularly and filtered through a polypropylene 0.45 lm, 25 mm diameter polypropylene syringe filter before analysis. Zn2+ concentration was measured using an axially configured Vista-Pro inductively coupled plasma optical emission spectroscopy (ICP-OES). 3. Results and discussion

photo-generated electrons from the conduction band of a semiconductor during the photocatalytic process is mainly dependent on the disposition of the standard reduction potential of the metal ion relative to that of the conduction band edge of the semiconductor [8]. Meanwhile, aqueous systems comprising various metal ions will possess a wide variety of complex metal ion species, with different standard reduction potentials, depending on the pH of the wastewater under consideration [28,29]. Some examples of metal ion and ion complexes with varying standard reduction potentials which might be abundant in an aqueous system containing zinc are shown below [50].

Zn2þ ðaqÞ þ 2e
! ZnðSÞ;

E0 ¼
0:76 V

½ZnðOHÞ4  ðaqÞ þ 2e ! ZnðsÞ þ 4OH
;


2








ð1Þ E0 ¼
1:22 V

ð2Þ

0

ZnðOHÞ2 ðsÞ þ 2e ! ZnðsÞ þ 2OH ;

E ¼
1:25 V

ð3Þ

In addition to this, the position of valence and conduction bands of TiO2 are changed when they come into contact with electrolyte solutions of different ionic strengths and pH [8,49,69] according to these equations

Ecb ¼
0:3
0:59 pHðat 25  CÞ

ð4Þ

Evb ¼ 2:9
0:059 pHðat 25 CÞ

ð5Þ

With the aid of Visual MINTEQ 3.1 chemical equilibrium software, zinc metal speciation over pH range 0–14 was modeled for the synthetic wastewater composition. Fig. 1 shows the speciation of zinc in this synthetic wastewater system. The major species in 2
 solution are Zn2+, Zn(OH)02 Zn(OH)
3 and Zn(OH)4 . Species of significant abundance are shown. Below pH 8, the major species recorded was in the form of Zn2+. However, the surface charge density of the semiconductor is highly affected by pH according to the equations;

pH < PZC : TiOH þ Hþ $ TiOH2

þ

ð6Þ




pH > PZC : TiOH þ OH $ TiO þ H2 O ð½59Þ

ð7Þ




Theoretically, higher pH should favor the photo-reduction of zinc according to Eq. (4), However it is observed from Fig. 1 that as pH increases from 0 to 8 there is a steady increase in the activity of Zn(OH)2, which reaches a plateau between pH 8 and 12. Within the same range of pH (8–12), Zn2+ activity begins to decline. This gives an indication that as the Zn2+ activity decreases, there is simultaneous precipitation of zinc as Zn(OH)2 from solution. This was physically observed as a white gelatinous precipitate, that settles at the bottom of the reactor prior to addition of TiO2. This precipitate may be retained on a filter paper as residue. In addition to

pH 0

1

2

3

4

5

6

7

8

9

10

11

12

13

0

-10

Log Activity

296

-20

-30 Zn(OH)2 (aq)

3.1. The effect of initial pH on the adsorption and photo-reduction of Zn2+ on TiO2 and TiO2-G One of the major process parameters that affects the effective photo-reduction of metal ion in aqueous solution is pH [28]. The feasibility of reducing metal ions to their zero valent form using

[Zn(OH)4 ]2-

-40

Zn 2+ Zn-Formate+

-50 Fig. 1. Zinc species distribution profile as a function of pH.

14

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2.5

2.0 TiO2 TiO2-G

1.5

qe , mg/g

this, the neutral Zn(OH)2 does not have an affinity for the charged catalyst surface. As a result of its relatively more negative redox potential (Eq. (2)) as well as its neutral nature, this inhibits adsorption and photo-reduction reactions. Therefore, the theoretical pH limit for this system is at pH 8, which is also observed by others. The pH of CSOs has typically been measured to be within the range of 6.9–7.0, reduction by photocatalysis, is therefore, a competitive technology for treatment, since pH adjustment is not required as it is for other treatment methods such as coagulation and precipitation. Other metals have also been shown to be effectively photoreduced at neutral pH [4,40,67].

1.0

0.5

3.2. The Iso-electric point for TiO2-G The Iso-electric point for the composite TiO2-G was determined using a pH vs zeta potential (i.e. the electrostatic potential at the interfacial boundary layer of a particle) plot as shown in Fig. 2. The (pHzpc) is defined as the pH at which the concentration of the protonated and deprotonated surface groups are equal. The measured pHpzc for this new catalyst material was found to be at pH 4.9. Morales-Torres et al. Morales-Torres et al. [46] measured the pHZPc of 1 wt% TiO2-G photocatalyst to be at pH 4.1. As it was discussed in earlier sections that a net negative charge enhanced by an increase in pH is favorable for adsorption and photo-reduction, finding the threshold pH value for this favorability is necessary. The effect of the difference in the pHpzc of TiO2 (6.2–6.9 [15] as compared to TiO2-G (4.9) is shown in Fig. 3. The adsorption capacities of 2 g/L TiO2 and TiO2-G was studied using different initial pH values. 2 g/L catalyst loading was determined experimentally as the optimum beyond which there is no increase in adsorption capacity. Below the pHpzc of both TiO2 and TiO2-G at pH 4, the maximum amount of Zn2+ adsorbed is less than 0.5 mg/g. TiO2-G at this pH adsorbed 16.5% more Zn2+ than TiO2. As pH approached 7, a resulting increase in Zn2+ adsorption occurs with TiO2-G recording 36%, 25% and 24% more than TiO2 at pH 5, 6, and 7 respectively. The net surface charge of the composite catalyst material as compared to pure TiO2 is advantageous because it aids the adsorption of the cations at more acidic pH values. The graphene present contributes to the formation of various complexes with the metal ions thereby increasing the overall sorption capacity of the material. This result, therefore, shows that the adsorption of metal ions onto TiO2-G is favoured over a wider pH range than that of TiO2 only. The equilibrium adsorbed Zn2+ per unit adsorbent mass qe (mg/g) was determined as shown in Eq. (8).

0.0 4.0

5.0

6.0

7.0

pH Fig. 3. Equilibrium Zn2+ adsorbed (mg/g) at different pH. [Zn2+] = 21 mg/L, Vr = 50 ml each, [Catalyst] = 2 g/L, [FA] = 250 ppm.

qe ¼

ðC0
Ce ÞV m

ð8Þ

where C0 (mg/L) is the initial metal ion concentration, Ce (mg/L) is the equilibrium concentration of Zn2+ remaining in solution as measured using the ICP-OES while m (g) is the mass of catalyst and V is the initial volume of the solution (L). The effect of solution pH on photo-reduction of Zn2+ was also studied as shown in Fig. 4. After 60 min of solar irradiation, the removal of Zn2+ using TiO2 was 4.3%, 55.0%, 74.2% and 94.6% for pH 4, 5, 6 and 7 respectively. Also, that for TiO2-G was 6.7%, 74.2%, 99.0% and 99.4% over the same pH range. When the reaction rate constants for both catalysts are compared, TiO2-G outperformed that of pure TiO2. The calculated reaction rate constants located in Fig. 4 shows that TiO2-G performs 51.4%, 58.6%, 56.1% and 25.5% better than TiO2 at pH 4, 5, 6 and 7 respectively. Mechanism of photo-reduction Step 1 - Catalyst activation hv >Eg

þ

TiO2
G ! e
CB þ hVB

ð9Þ

Step 2 - Oxidation of water and organic compounds present 1.0

60 0.8 K app , min-1

40

TiO2 TiO2 -G pH 4 0.0011 0.0023

C/C 0

Zeta Potential, mV

0.6 20

pH 5 0.0124 0.0299 pH 6 0.0310 0.0706

0.4

pH 7 0.0654 0.0878

pH 4, Ti O2 pH 4, Ti O2-G pH 5, Ti O2 pH 5, Ti O2 -G pH 6, Ti O2 pH 6 Ti O2 -G pH 7, Ti O2 pH 7 Ti O2 -G

0 0.2

-20

0.0

-40

0

10

20

30

40

50

60

70

80

90

100

Time, min -60 2

3

4

5

6

7

8

9

pH Fig. 2. The isoelectric point of TiO2-G.

10

11

12

Fig. 4. Effect of pH and catalyst type on the removal of Zn2+. Inset: Apparent reaction rate constants at different pH values for TiO2 and TiO2-G. Experimental conditions: [Zn2+] = 21 mg/L, Vr = 150 ml each, [Catalyst] = 2 g/L, N2-saturated I = 100 mW/cm2.

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þ

hVB þ H2 Oads ! HOads þ Hþ

ð10Þ

þ hVB

ð11Þ

!

HOads

HOads þ H2 O2 þ H2 O ! CO2 þ H2

(a)

ð12Þ

HOads þ CH2 O2 ! CO2 þ H2

0

-1

ð13Þ

Step 3 - Reduction of metals present

Zn2þ þ 2e
! Zn0

ð14Þ

3.3. Zinc photo-degradation kinetics: determination of reaction order for Zn2+ photo-reduction using TiO2 and TiO2-G

Ln (C/C0 )

þ

HO
ads

-2

-3 10 mg/L 20 mg/L 28 mg/L 38 mg/L 50 mg/L

-4

lnðC=C0 Þ ¼ kapp t

ð15Þ 2+

where C0 is the initial Zn concentration, C is concentration at reaction time t, and kapp (min
1) is the apparent rate constant. Fig. 6 shows the pseudo first-order kinetic model fit to experimental data. Comparing the rate constants, TiO2-G performed 31%, 14%, 59%, 24% and 7% better than TiO2 with increasing initial concentration of Zn2+. As the initial Zn2+ concentration at a fixed catalyst concentration increases, the active sites on the surface of the catalyst become more and more saturated, limiting the instantaneous availability of conduction band electrons. Therefore, more ions would remain in solution at the specified time as compared to a catalyst surface which is less saturated with pollutant ions. The rate of photo-reduction of Zn2+ was observed to follow a Langmuir-Hinshelwood type kinetic model with respect to the initial Zn2+ concentration as shown in Eq. (10) [11].




dCZn2þ kred Kads C0 ¼ r0 ¼ dt 1 þ K ads C0

ð16Þ

0.6

0.7

0.5

0.6 0.5

0.4

0.4 0.3 0.3 0.2

r1, mg/gcat . min

r0 mg/L.min

where r0 is the initial reaction rate, Kads (ppm
1) is the adsorption constant and kred is the reaction rate constant, C0 is the initial concentration of Zn2+. The apparent reaction rate constant is given as;

0.2 0.1

0.1

0.0

0.0 0.0

1.0

2.0

3.0

4.0

5.0

Catalyst Loading g/L Fig. 5. Effect of catalyst dosage on the photo-degradation rate. Experimental conditions: [Zn2+] = 21 ± 1 mg/L, pH = 7.1 ± 0.1, Vr = 150 ml, 250 ppm FA, I = 100 mW/cm2, N2 saturated.

-5 0

20

40

60

80

100

Time, min

(b)

0

-2

Ln(C/C o )

In order to prevent the initial catalyst concentration and/or the h+ scavenger from becoming the rate limiting steps for the reaction, the optimum concentrations of catalyst loading (Fig. 5) and formic acid (h+ scavenger) were determined first and subsequent experiments were performed at optimum concentration of catalysts loading and formic acid to determine the effect of the initial Zn2+ concentration, light intensity and light source on photo-reduction of Zn2+. The optimum formic acid, catalyst concentration and pH were recorded as 250 ppm and 2 g/L and neutral respectively. At neutral pH, the photo-reduction was observed to follow a pseudo first-order reaction when TiO2 and TiO2-G are used as catalyst. A good fit was observed when the data was fitted to Eq. (9).

-4

-6 10 mg/L 22 mg/L 29 mg/L 41 mg/L 47 mg/L

-8

-10 0

20

40

60

80

100

Time, min Fig. 6. Effect of initial metal ion concentration on the reaction rate constant using (a) TiO2 and (b) TiO2-G. Experimental conditions: Vr = 150 ml each, pH = 7.1 ± 0.1, [TiO2] = 2 g/L, [FA] = 250 ppm, I = 100 mW/cm2, N2 = saturated.

kapp ¼ kred Kads

ð17Þ 2+

After photo-irradiation of Zn the initial reaction rate increased with increase in the initial metal ion concentration while the apparent rate constant reduced with increase in initial concentration. At high Zn2+ concentrations, a zero-order rate is described. Kads C0  1, indicating that the reaction rate did not depend on the initial Zn2+ concentration present. The reaction rate is maximum and may be equated to the value of kred. Conversely, at low concentrations of Zn2+ when Kads C0  1, the reaction rate is firstorder. The reaction rate constants kred and Kads was determined as 0.530 ± 0.012 ppm/min and 0.107 ± 0.008 ppm
1 for TiO2 and that for TiO2-G was determined as 0.671 ± 0.055 ppm/min and 0.081 ± 0.021 ppm
1. 3.4. Effect of solar light intensity One of the most important reaction rate controlling parameters which may affect photocatalytic reactions is the intensity of the incident light on the photocatalyst surface, since it is the source of the photons required for electron-hole separation [41]. Natural sunlight is however highly depended on geographical location and time of the day. It is therefore important to establish how the light intensity influences the efficiency of the system. Under normal weather conditions, light intensity does not exceed 1300 W/m2 on any part of the earth surface. The solar light intensity was varied between 25 and 100 mW/cm2 in this study as

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shown in Fig. 7. When the light intensity is decreased, there is corresponding decrease in the apparent reaction rate constant due to the reduced availability of photons required to initiate the charge separation. The reaction rate constant (kapp) was shown to relate to the light intensity by a power law:

kapp ¼ a Ibs

3.5. Effect of light source on reduction of Zn2+ In this study, experiments were conducted using 100 mW/cm2 light intensity under full solar radiation (wavelength 300– 650 nm) and ultraviolet light only (300–388 nm). This was done to evaluate the performance of the activity of the TiO2-G composite catalyst. The threshold wavelength required in order to activate any semiconductor can be calculated using Plank’s equation as shown below;

1240 Ebg ðeVÞ

ð19Þ

where k is the wavelength and Ebg is the band energy. According to the measurement of the synthesised TiO2-G, the band-gap reduced from 3.1 (pure TiO2) to 2.2 eV [41]. The theoretical threshold wavelength required for e
/h+ separations is approximately 563 nm as compared to 400 nm for pure P-25 TiO2. This is because combining graphene with TiO2, reduces the band-gap of TiO2 through energy favoured hybridization of O2p and C2p atomic orbital enabling the formation of a new valence band [34]. Fig. 8 shows the kinetics of Zn2+ photoreduction under solar light and UV light at optimum conditions. The rate of photo-reduction of Zn2+ on TiO2 was 62% higher for full solar spectrum (kapp = 0.067 ± 0.006 min
1) than that of only UV light (kapp = 0.025 ± 0.007 min
1) when the reaction rate constants were

0.14 TiO2 TiO2-G

0.12

0.10

K app , min -1

-2

ð18Þ

-4 1.0

0.8

TiO 2 -G Solar

-6

0.6

TiO2 Solar

C/C 0

ln (C/C o )

where b is the kinetic order and Is represents the solar light intensity (mW/cm2). The values of the constant obtained for TiO2 are a (the proportionality constant) = 3.7 10
5 ± 3.1 10
5 and b = 1.5 ± 0.2. The a and b values obtained with TiO2-G as catalyst was 1.3 10
5 ± 1.3 10
5 and 1.9 ± 0.2 respectively. When the apparent reaction rate constants were compared at optimum conditions (250 ppm formic acid, 2 g/L catalyst loading, 21 ppm initial Zn2+ concentration and 100 mw/cm2 light intensity and neutral pH), TiO2-G (kapp = 0.083 ± 0.005 min
1) showed a 20.3 ± 0.04% higher photo-activity than pure TiO2 (kapp = 0.066 ± 0.009 min
1) under solar light irradiation.

kðnmÞ ¼

0

0.4

-8

TiO 2 -G solar TiO 2 S olar

0.2

0.0 0

20

40

60

80

100

TiO 2 -G UV TiO 2 UV

Time, min

-10 0

20

40

60

80

100

Time, min Fig. 8. (a) Effect of catalyst type and light source (solar light and UV light) on the photo-reduction of Zn2+. Inset: Effect of catalyst type on the photo-reduction of Zn2+ under solar light conditions. Experimental condition: C0 = 21 ± 1 mg/L, pH = 7.1 ± 0.1, Vr = 150 ml each, Ccat = 2g/L, N2-saturated, I = 100 mW/cm2.

compared. Using TiO2-G, the photo-reduction rate under solar light (kapp = 0.083 ± 0.005 min
1) out-performed that of only UV light (kapp = 0.030 min
1) by 63.3% when the rate constants were compared. The reduction of Zn2+ showed significant improvement in the full solar spectrum. Graphene when combined with TiO2 reduces charge recombination. As the TiO2-G composite is irradiated with photons, electrons excited into the conduction band of the TiO2 from the valence band are transferred to the adjacent carbon films which then act as electron sinks and aid charge carrier separation in TiO2. In combination with the h+ scavenging action of the formic acid, photo-generated e
are therefore available for the reaction to occur thus increasing the reaction rate. This enhancement with the composite TiO2-G can be therefore be attributed to the efficiency of charge separation on the composite [65]. In addition to this, photoluminescence (PL) technique has often been employed to for further understanding of the efficiency of carrier charge trapping, immigration and transfer. Since PL signals are as a result of recombination of e
/h+, it is very useful in explain the efficiency of the composite semiconductor material TiO2-G as compared to bare TiO2. It has been reported that for most semiconductor materials, increase in photocatalytic activity is associated with lower PL intensities. PL values of bare TiO2 have been reported to range between 450 and 550 a.u. (300 nm) [36]. Zhang et al. Zhang et al. [71] observed that the PL intensity of P25-G (1%) was significantly lower than that of bare P25. This supports the theory that the rate of charge recombination in TiO2-G is considerably lower than that of pure TiO2. Also, the lower pHPZC of TiO2-G increased the adsorption capacity and this also translated into improved photo-reduction rate.

0.08

4. Conclusions 0.06

0.04

0.02

0.00 0

20

40

60

80

100

120

2

Light intensity, mW/cm

Fig. 7. Effect of light intensity on reaction rate using TiO2 and TiO2-G catalyst. Experimental conditions: [Zn2+] = 20 ± 1 mg/L, pH = 7.1 ± 0.1, Vr = 150 ml, [TiO2-G or TiO2] = 2 g/L, N2 = saturated.

The performance of TiO2-G synthesised by a hydrothermal treatment method was compared to pure TiO2 under solar radiation conditions. The systematic studies show that there is a significant improvement in the reaction rate when TiO2-G composite catalyst is used. Also, reaction conditions such as pH and light intensity significantly impact the reaction rate in both cases. The enhancement in reaction rate of TiO2-G system can be attributed mainly to effective electron-hole separation. An improvement in both adsorption and photo-reduction of Zn2+ under the solar spectrum was observed with TiO2-G.

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