Decolourization of the azo dye Orange G in aqueous

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Decolourization of the azo dye Orange G in aqueous solution via a heterogeneous Fenton-like reaction catalysed by goethite a

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Honghai Wu , Xiaowen Dou , Dayi Deng , Yufeng Guan , Liguo Zhang & Guangping He a

School of Chemistry and Environment, South China Normal University, Guangzhou, China

Accepted author version posted online: 09 Dec 2011. Version of record first published: 11 Jan 2012

To cite this article: Honghai Wu, Xiaowen Dou, Dayi Deng, Yufeng Guan, Liguo Zhang & Guangping He (2012): Decolourization of the azo dye Orange G in aqueous solution via a heterogeneous Fenton-like reaction catalysed by goethite, Environmental Technology, 33:14, 1545-1552 To link to this article: http://dx.doi.org/10.1080/09593330.2011.635709

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Environmental Technology Vol. 33, No. 14, July 2012, 1545–1552

Decolourization of the azo dye Orange G in aqueous solution via a heterogeneous Fenton-like reaction catalysed by goethite Honghai Wu∗ , Xiaowen Dou, Dayi Deng, Yufeng Guan, Liguo Zhang and Guangping He School of Chemistry and Environment, South China Normal University, Guangzhou, China

Downloaded by [Honghai Wu] at 20:12 01 August 2012

(Received 18 October 2011; final version received 21 October 2011 ) Decolourization of the azo dye Orange G (OG) was investigated by using goethite/H2 O2 as a heterogeneous Fenton-like reagent. Five principle operational parameters, namely pH, ion strength, concentrations of goethite (α-FeOOH) and hydrogen peroxide (H2 O2 ), and reaction temperature, were taken into account to investigate how these controlling factors mediated OG decolourization. Goethite surfaces catalysed a Fenton-like reaction responsible for decolourizing OG following pseudo-firstorder kinetics (R2 > 0.964). This process was effective but seriously impacted by the medium pH and the dosages of both α-FeOOH and H2 O2 . The decolourization efficiencies of OG increased with the decrease of solution pH and NaCl (chloride ion) concentration and/or the increase of H2 O2 . The acidic aqueous medium conditions were likely favourable due to the surface adsorption of the negatively charged OG leading to the promotion of decolourizing OG. The apparent activation energy (E) for this reaction was 42.18 kJ mol−1 , a relatively low value. This is consistent with the OG decolourization being enhanced with the reaction temperature increase. Keywords: azo dye, decolourization, heterogeneous Fenton-like reaction, Goethite, Orange G

1. Introduction There are more than 100,000 commercially available dyes, with an estimated annual production of over 700,000 t worldwide [1]. Azo dyes are the largest group of dyes, constituting approximately 70% of the yield of all dyestuffs used worldwide, and it is estimated that 280,000 t of textile dyes are discharged into industrial effluents every year worldwide [2]. Azo dyes generally absorb light in the visible spectrum due to their chemical structure, which is characterized by one or more azo groups (–N=N–). Azo dyes are non-biodegradable and their disposal in streams and rivers poses an important environmental threat [3]. Many azo dyes are visible in water at concentrations as low as 1 mg L−1 and the presence of even trace concentrations of the dyes in industrial effluent is highly visible and undesirable. Dyeing industries are often unable to meet stringent regulations concerning coloured wastewater discharge. Moreover, azo dyes are generally known to be toxic to aquatic and terrestrial organisms, and recalcitrant to traditional physical, chemical and biological wastewater treatment processes. Therefore, special treatment of industrial effluents containing azo dyes and their metabolites is necessary prior to their final discharge to the environment. As a result, there has been great deal of effort to develop a process to efficiently partially or totally eliminate these dangerous pollutants [4–6]. Among the most investigated advanced oxidation processes in wastewater treatment is the very promising Fenton ∗ Corresponding

author. Email: [email protected]

ISSN 0959-3330 print/ISSN 1479-487X online © 2012 Taylor & Francis http://dx.doi.org/10.1080/09593330.2011.635709 http://www.tandfonline.com

process, first discovered by Fenton in the 1890s, which is extensively used in the decolourization and degradation of various dyes [7]. However, the homogeneous Fenton system that uses ferrous iron salt is inappropriate because it produces a significant amount of ferric hydroxide sludge which requires further separation and disposal. More recently, research has been oriented to the immobilization of iron compounds on different supports to facilitate iron separation, employing mainly solid supporters to avoid more complex post-treatments [8]. For instance, much effort has made to find efficient heterogeneous iron-bearing solid catalysts [8–17], including iron oxides such as goethite (α-FeOOH) [8,9], ferrihydrite (Fe5 HO8 ·4H2 O) [13], lepidocrocite (γ -FeOOH) [14], hematite [15] and magnetite (Fe3 O4 ) [8], and other clay catalysts such as Fe-pillared interlayered clays [16] and Fe-containing zeolites [17]. Environmentally friendly goethite as a solid catalyst has received considerable recognition due to being inexpensive and widely available [18]. In the last two decades, the use of hydrogen peroxide catalysed by goethite (α-FeOOH) was found to effectively oxidize organic compounds [18–23]. For example, Khan and Watts [20] reported that adding an appropriate amount of H2 O2 into contaminated soils in the presence of goethite can produce the highly reactive • OH radicals, which can degrade most organic pollutants. Lu [22] also determined that 2-chlorophenol can be decomposed with H2 O2 catalysed by goethite. Due to three possible iron

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sources including FeOOH, Fe2+ and Fe3+ existing in the aforementioned goethite/H2 O2 process, there are at least two pathways with respect to the main possible mechanisms for generation of • OH radicals in the goethite/H2 O2 Fenton-like process. The first is the pathway similar to homogeneous Fenton reactions due to the presence of Fe ions leached from the catalyst through the reductive dissolution of goethite, and then mixed with hydrogen peroxide to produce hydroxyl radicals (• OH) [22]. The second is the pathway due to surface adsorption along with red-oxidation of hydrogen peroxide. The main heterogeneous generation process of OH taking place directly at the interface of water/goethite, is shown as follows [23]: ≡ Fe(III) + H2 O2 →≡ Fe(HO2 )2+ + H+ ≡ Fe(HO )


2 2+

•

→≡ Fe(II) + HO2

≡ Fe(II) + H2 O2 → Fe(III) + • OH + OH−

(1) (2) (3)

Hence, goethite was chosen due to its proven and excellent catalytic nature. Moreover, goethite is one of the most widespread forms of iron oxy-hydroxides in terrestrial soils, sediments, and ore deposits. Goethite is very stable with a very small solubility (pKsp = 14.7). Sun et al. [4] reported a study on decolourizing Orange G (OG) using the homogeneous Fenton process. Hsueh et al. [24] have even reported that OG could be effectively degraded by the Fenton-like (H2 O2 /Fe3+ ) system. However, to our knowledge, the effects of various conditions on the decolourization of OG via the heterogeneous goethite-catalysed Fenton-like process have not yet been well studied, even in the abovementioned studies. The goal of this work is to evaluate if goethite-catalysed Fenton processes are capable of decolourizing OG dye. In this study, batch experiments were carried out to investigate the main impacting factors that mediate OG decolourization in the goethite-catalysed Fenton-like process. The obtained results can be used as fundamental knowledge for reclaiming OG-contaminated soil and ground water, and polluted industrial effluents using the heterogeneous Fenton-like process. This is a very important treatment process for

Table 1. Dye

Orange G

environmental pollutants, since both H2 O2 and iron oxides are constituents of natural and atmospheric waters [25]. 2.

Experimental materials and methods

2.1. Chemicals and reagents The azo dye OG used for this study was purchased from Shanghai Chemicals Co., China, and was used without further purification. The chemical characteristics of OG are illustrated in Table 1 [4]. All other chemicals used in this study, including hydrogen peroxide (H2 O2 , 30%), sulphuric acid (H2 SO4 , 96%), ferric nitrate (Fe(NO3 )3 , p.a.), ferrous sulfates (FeSO4 ·7H2 O, p.a.), potassium hydroxide (KOH, p.a.) and sodium hydroxide (NaOH, p.a.), were supplied by Guangzhou Chemicals Co., China. Doubly deionized water was used to make dye solutions with the desired concentrations. The goethite was prepared as follows: 180 mL of potassium hydroxide solution of concentration at 5 mol L−1 was first slowly added into 100 mL of ferric nitrate solution (1 mol L−1 ) in a plastic container, stirring vigorously for 5 min at room temperature, and then adding deionized water. The mixed solution was aged for 60 h at 70◦ C. Next, the product was washed several times with deionized water, and then freeze-dried for 24 h at −48◦ C under vacuum. 2.2.

Characterization of materials

The specific surface area (Brunauer-Emmett-Teller, BET), pore size (Barrett-Joyner-Halenda, BJH), and pore volume (V p) of the catalysts were determined by the N2 adsorption– desorption method at liquid nitrogen temperature (at 77 K) using an ASAP 2020M apparatus (Micromeritics Instrument, USA). The structure and phase identification were characterized by X-ray powder diffraction (XRD). The XRD patterns of the samples were recorded on a Y2000 X-ray diffractometer equipped with Cu Kα radiation (Dandong Aulong X-ray diffraction Co., China). The size and morphology of the mineral samples were characterized with transmission electron microscopy (TEM, H-3000, Hitachi, Japan). After dispersion by an ultrasonicator, several droplets of the minerals/ethanol suspension solution were deposited on a carbon-coated Cu-grid. After

Physiochemical characteristics of an azo dye Orange G [4]. Chemical structure

Molecular formula

MW (g·mol−1 )

λmax (nm)

C16 H10 N2 Na2 O7 S2

452

478

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the ethanol had completely evaporated, the samples were introduced into a vacuum chamber.


2.3.

Experimental procedure

All experiments were conducted in a double glass cylindrical jacket reactor with a total volume of 0.5 L, in which water can cycle to maintain a constant temperature. Before the decolourization reactions, we adjusted of a series of OG solutions to the desired pH. Then, mineral power of a given amount (catalyst dosage 0.2–1.2 g) was added into 0.4 L of OG solution. The resulting suspension was stirred for about 30 min by a magnetic stirring bar, and then the decolourization reaction was initiated by the addition of hydrogen peroxide into the OG solution. The concentrations of OG and H2 O2 were 50 mg L−1 and 7.5–60 mmol L−1 , respectively. Temperatures of 25, 30, 40 and 50◦ C were obtained through a thermostat using a magnetic stirrer to stir the reaction suspension solutions. The desired initial pH (3.0, 3.5, 4.0, 4.5 and 6.5, respectively) of the suspension solution was adjusted with the addition of NaOH or H2 SO4 of 0.1 mol L−1 , not controlled during the course of the reaction, and adding goethite caused negligible pH change. 2.4.

Figure 1. X-ray diffraction (XRD) analysis of as-synthesized goethite samples (a) in comparison with the reference diffractogram for α-FeOOH (b).

Analytical methods

The concentration of OG in aqueous solutions was determined by UV–vis spectrophotometry (Unico 3802; Shanghai instrument Co.) at a maximum absorption wavelength of 478 nm to evaluate the decolourization of OG. The efficiency of the decolourization of OG was determined based on a calibration curve, obtained by a batch of standard OG solutions with defined concentrations. At regular time intervals, samples were withdrawn from the reactor using a pipette and filtered with a 0.45μm membrane to retain the solid phase substances, and then analysed by a UV–Vis spectrophotometer. The standard deviations (P) for examination of decolourization of OG are very small, ranging from 0.08–0.003, and most P < 0.05. The efficiency of decolourization of OG was determined with the following expression: Decolourization efficiency (E%) = (C0 -Ct )/C0 × 100 (4) where Ct is the concentration of OG at time t; and C0 is the initial concentration of OG. The initial and final pH values of aqueous solutions were measured by a PHS-3C pH meter. The initial and final solution concentration for overall iron ion was detected by using analysis of atomic absorption spectroscopy (AAS). 3. Results and discussion 3.1. Characterization of synthetic goethite The XRD pattern of the goethite used in this study is shown in Figure 1. The reflections correspond well with the

Figure 2. Transmission electron microscopy (TEM) image of the synthetic goethite used in this study.

standard pattern for α-FeOOH (goethite, JCPDS No. 290713) [26], and the main reflections locate at 2θ = 21.23◦ , 33.28◦ and 36.67◦ . This indicated that a highly purified α-FeOOH was obtained. The BET surface area of the goethite was 87.84 m2 g−1 , and the pore volume and average pore width were 0.086 cm3 g−1 and 3.94 nm, respectively. A TEM image of the goethite is shown in Figure 2; the needle-like goethite exhibited typical lengths in the range of 1–4μm. The surface charge properties of oxides are of crucial importance to their chemical behaviour in aqueous systems, influencing sorption, dissolution and precipitation processes as well as redox reactions [27]. Previous studies have shown that goethite generally has a pHpzc (the point of zero charge) of 7–9.5 [28], and thus the goethite surface is presumably positively charged in acidic solution (pH < pHpzc ) and negatively charged in alkaline solution (pH Table 2. Equilibrium constants describing the acid–base properties of goethite from Stumm and Morgan [29]. > FeOH + H+ ⇔> FeOH2+ > FeOH ⇔> FeO− + H+ Site density Point of zero charge BET surface area (this work)

log Ka1 : 7.47 log Ka2 : −9.5 Ns: 1.7 sites·nm−2 pHzpc :7–9.5 87.84 m2 ·g−1

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> pHpzc ). The surface parameters listed in Table 2 [29], when combined with the α-FeOOH loading of 1.0 g L−1 , can be used to calculate a surface-site loading (ST ) of 0.255 mmol L−1 . Due to strong protonation, goethite possesses a high surface-site density of Bronsted acid sites at pHs in the range from 3.0–7.0. Therefore, it is predicted to be favoured for the adsorption of OG.

3.2. Decolourization of the azo dye OG The process of decolourization of OG using synthetic goethite as a catalyst is shown in Figure 3, in which the UV– vis spectra of the OG decolourization evolution with the goethite/H2 O2 reagent versus reaction time are presented. The absorption spectra of OG were scanned in the range of 250–600 nm. Xu and Li [30] reported that the spectrum of light absorption by the OG solution before reaction consists of three main peaks at 259, 328, and 478 nm, plus a shoulder peak at 421 nm. The peak at 478 nm was attributed to the absorption of the π → π ∗ transition related to the –N=N– group, while additional bands at 259 and 328 nm were attributed to the π → π ∗ transition of the naphthalene ring in the OG molecule. Interestingly, similar decolourization relative to Xu and Li [30] using another method was also obtained in our study. As shown in Figure 3, as the reaction proceeded, the two characteristic absorption peaks at 328 and 478 nm decreased rapidly and essentially disappeared after 180 min, showing that the chromophore and conjugated π ∗ systems were completely destroyed. On the other hand, the zone around 259 nm declined slowly, indicating that the aromatic rings were still present. Apparently the OH radicals produced in the heterogeneous Fenton process can attack the azo groups and break down the –N=N– bonds, followed by the destruction of the long conjugated π ∗ systems. However, there was strong absorbance in the range of 259 nm, which meant that some intermediates with aromatic rings existed after decolourization of OG in the heterogeneous Fenton-like process. The variation of the absorption peak

Figure 3. UV–vis spectral changes of azo dye Orange G during decolourization process as a function of reaction time (C0 = 50 mg L−1 of OG, 30 mmol L−1 of H2 O2 , 1.0 g L−1 of α-FeOOH, 400 mL, pH 3.0, T = 25◦ C).

at 478 nm was therefore applied to evaluate the extent of OG decolourization, while the degradation of OG (total organic carbon, i.e. TOC removal) was not tested in this study due to its incomplete decomposition.

3.3. Effect of initial pH As seen earlier, the catalytic degradation efficiency is affected by the surface charge properties of goethite, the charge of the pollutant molecule, adsorption of the pollutant molecule on to goethite surface, and hydroxyl radical concentration [8,9]. Figure 4 shows the decolourization efficiency of batch experiments run at an initial solution pH ranging from 3.0–6.5. It is observed that the reaction efficiency increases at lower pH values, and the final efficiency of decolourization of OG in aqueous solution reaches 99.5% at an initial pH of 3.0 after 180 min. These observations can be explained as follows: goethite exhibits a positive zeta potential at pH values below pHpzc (7–9.5) due to the protonation of surface ≡Fe–OH groups to form positively charged ≡Fe–OH+ 2 groups whose population increases as pH decreases. On the other hand, the OG is generally negatively charged as the sulphonated group (–SO− 3 ) in its structure is hydrolysed. Due to electrostatic attraction, the acidic solution condition favours adsorption of OG onto the goethite surface, and thus the decolourization efficiency of OG increases accordingly. This model also explains why neutral compounds such as nitrobenzene, which can be oxidized by goethite-catalysed Fenton reactions, are influenced little by pH change [31]. At an initial pH of 3, more Fe ions are expected to be released from the goethite surface, and this may benefit the generation rate of OH radicals. However, by further lowering the initial pH, the concentration of H+ may be in excess. The H+ ions may interact with the azo linkage (– N=N–), leading to a decrease in the electrophilic reactivity of OH radical to the azo group [32]. However, decolourizing OG in the homogeneous Fenton reaction is favoured at initial pH of 4, not at pH 3 (not shown). This has been attributed to the formation of Fe(OH)2 , which is about 10

Figure 4. Effect of initial pH values on the decolourization of OG (C0 = 50 mg L−1 of OG, 30 mmol L−1 of H2 O2 , 1.0 g L−1 of α-FeOOH, 400 mL, T = 25◦ C).

Environmental Technology


times more reactive than the Fe2+ ion in aqueous solution. It has been proven that H2 O2 may favour the more negatively charged oxide surface because it is able to form strong complexes with compounds possessing weak base properties [33]. Liang et al. [5] reported that the decolourization rate of Acid Orange II through homogeneous Fenton reactions was below 10% after 180 min using a magnetite dissolved at pH 2.88–2.96. It can be derived that at pH 3.0, heterogeneous Fenton-like oxidation should contribute much more importance to OG decolourization due to OG adsorption being enhanced on goethite surfaces. It should also be noted that dissolved Fe ion concentrations are below the detection limit of AAS at pH above 4. 3.4. Effect of the catalyst concentration The effect of the goethite concentration on decolourization of OG was investigated in catalyst dosages that varied from 0.2–1.2 g L−1 . The decolourization efficiency increased with the increase of goethite catalyst dosage up to 1.0 g L−1 , and then slightly decreased upon further addition of the goethite catalyst, as shown in Figure 5. Because a heterogeneous goethite-catalysed Fenton-like reaction depends on the specific area of the goethite catalyst, an increase of goethite dosage corresponds to having a higher total surface-site concentration, i.e. increasing active sites for adsorption–decomposition of both H2 O2 and OG leads to an increase in the OG decolourization rate. However, the highest catalyst dosage diminished the efficiency of decolourization of the OG dye. There are two possible reasons for this decrease. One possibility is the aggregation state of the goethite, which obviously affects the surface availability for solution interaction. However, the goethite is observed to disperse very well in this study. Another possibility is that as the concentration of the catalyst rises, the H2 O2 begins to react excessively with the goethite not in the presence of OG, resulting in an ‘unproductive destruction of the reactant’ [34]. It has been verified that the catalytic action of goethite in the heterogeneous Fenton reaction encounters a highly undesired competing reaction, leading

Figure 5. Effect of goethite dosage on the decolourization of OG (C0 = 50 mg L−1 of OG, 30 mmol L−1 of H2 O2 , 400 mL, pH 3.0, T = 25◦ C).

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to the rapid disappearance of H2 O2 [8]. Our results in this study suggested that the specific catalytic surface area is an important impacting factor. 3.5. Effect of hydrogen peroxide concentration The influence of hydrogen peroxide concentrations on decolourizing OG in the Fenton-like process was investigated by varying its initial concentration from 7.5 mmol L−1 and 60 mmol L−1 (Figure 6). With increasing hydrogen peroxide initial concentration from 7.5 mmol L−1 to 30 mmol L−1 , the efficiency of decolourization of OG increased from 53.9% to 83.8% within 80 min; at the same time, the decolourization process was significantly accelerated, suggesting that more hydroxyl radicals were formed. However, it should be noted that if the initial concentration of hydrogen peroxide was more than 30 mmol L−1 , the efficiency of decolourization of OG decreased slightly due to the competition of H2 O2 with the hydroxyl radical, explained by the well-known hydroxyl radicals scavenging effect [35]: H2 O2 + • OH → H2 O + • O2 H •

•

O2 H + OH → H2 O + O2 •

H2 O2 + 2 OH → 2H2 O + O2

(5) (6) (7)

Although (• O2 H) was produced, its oxidation potential was relatively weak compared with that of the • OH species [5]. Moreover, the • O2 H can interact with • OH, reducing the amount of hydroxyl radicals. Hence, an optimum H2 O2 dosage of 30 mmol L−1 was determined in this study, which could be used in subsequent tests for decolourization of OG dye. 3.6.

Influence of sodium chloride concentration

The influence of the presence of chloride ions on OG dye decolourization was investigated (C0 = 50 mg L−1 of OG, 30 mmol L−1 of H2 O2 , 1.0 g L−1 of FeOOH, 400 mL, pH = 3.0, T = 40◦ C) and the obtained results are shown in

Figure 6. Effect of H2 O2 dosage on the decolourization of OG (C0 = 50 mg L−1 of OG, 1.0 g L−1 of α-FeOOH, 400 mL, pH 3.0, T = 25◦ C).


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Figure 7. Influence of sodium chloride concentration on the degradation of OG (C0 = 50 mg L−1 of OG, 30 mmol L−1 of H2 O2 , 1.0 g L−1 of α-FeOOH, 400 mL, pH 3.0, T = 40◦ C).

Figure 8. Influence of temperature on the decolourization of OG (C0 = 50 mg L−1 of OG, 30 mmol L−1 of H2 O2 , 1.0 g L−1 of α-FeOOH, 400 mL, pH 3.0).

Figure 7. Under these conditions, it can be seen that chloride ion had a negative impact on the decolourization of OG in the heterogeneous Fenton-like process. The decolourization efficiency within 60 min dropped from 99.1% to 84.6% as a consequence of increasing the concentration of sodium chloride (NaCl) from 0 to 15 mg L−1 . The reason is likely that the negatively charged chloride ion may compete with + –SO− 3 in OG for ≡Fe–OH2 sites on the goethite surface, and thus decrease the adsorbed OG leading to a decrease in efficiency of OG decolourization. The inhibitive effect of chloride ions on the decolourization of OG can also be explained by the scavenging effect of chloride ion for OH according to the following reactions [36]:

Figure 9. The pseudo-first-order linear relationship for reaction kinetic curve (C0 = 50 mg L−1 of OG, 30 mmol L−1 of H2 O2 , 1.0 g L−1 of FeOOH, 400 mL, pH 3.0, T = 25◦ C).

Cl− + • OH → ClOH•− •−

ClOH

•−

ClOH

3.7.

+ Fe

2+

−

(8) −

→ Cl + OH + Fe

+

+ H → Cl + H2 O

3+

(9)

(Figure 9), indicating that the OG degradation follows the pseudo-first-order kinetic model below:

(10)

ln(C0 /Ct ) = kapp t

Influence of temperature

The influences of temperature at 25, 30, 40 and 50◦ C on OG decolourization were also investigated in this study. It can be seen from Figure 8 that increasing temperature has an advantage on the decolourization of OG, and the time period required for decolourizing OG was much shorter at higher temperature. For example, the efficiency of decolourization of OG was increased from 49.3% to 98.7% within 40 min while the temperature increased from 25 to 50◦ C. The generation rate of OH promoted by higher temperatures will accelerate the heterogeneous Fenton-like oxidation reaction and can therefore enhance the OG decolourization [4]. In addition, it is possible that higher temperatures may enhance the adsorption rate of OG and H2 O2 through improving their diffusion rate at the solid/water interface. 3.8. Kinetic model of the process The plot of ln(C0 /Ct ) versus reaction time (t) shows a linear relationship with good correlation coefficient (R2 > 0.988)

(11)

where the kapp is the observed reaction rate constant, which can be obtained from the slope of the line in the plots of ln(C0 /Ct ) versus t. In Figure 9, the rate constant of OG decolourization was found to be 0.027 min−1 under the reaction conditions given in the figure caption. In addition, similar reasonable results can also be obtained for other cases as listed in Table 3. The OG decolourization using goethite as a catalyst corresponded to the • OH heterogeneous mechanism [5]. Thus, H2 O2 was activated by the cations on the goethite surface via a Haber–Weiss mechanism to form radical • OH [37]: H2 O2 + S → • OH + OH− + S+

(12)

where S and S+ denote adsorption and oxidized location, respectively. According to the apparent kinetic rate constants at different temperatures, the value of apparent activation energy for the decolourization of OG can be computed with the Arrhenius equation. A good linear relationship was obtained in the Arrhenius plot of lnk versus 1/T . The value of the activation energy (E) was determined to be 42.18 kJ mol−1 with a constant

Environmental Technology Table 3.

Pseudo-first-order kinetic rate constants for OG decolourization at varied reaction conditions.

Operating parameters pH

Catalyst (g·L−1 )

H2 O2 (mmol·L−1 )

T (◦ C)


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C Variations

C Constant conditions

C k (×10−2 min−1 )

C R2

3.0 3.5 4.0 5.0 6.5 0.2 0.4 0.6 0.8 1.0 1.2 7.5 15 30 60 25 30 40 50

[FeOOH] = 1.0 g·L−1 [H2 O2 ] = 30 mmol·L−1 T = 25◦ C

2.7 1.2 0.9 0.8 0.7 0.5 0.6 0.8 1.1 2.7 1.5 1.3 1.5 2.7 1.7 2.7 3.5 6.9 9.6

0.988 0.982 0.964 0.971 0.968 0.978 0.977 0.992 0.994 0.988 0.997 0.979 0.991 0.988 0.997 0.988 0.994 0.986 0.997

[H2 O2 ] = 30 mmol·L−1 pH = 3.0 T = 25◦ C

[FeOOH] = 1.0 mg·L−1 pH = 3.0 T = 25◦ C [FeOOH] = 1.0 mg·L−1 [H2 O2 ] = 30 mmol L−1 pH = 3.0

item (A) value of 6.8 × 105 mol−1 min−1 . Generally, the activation energy of ordinary thermal reactions is between roughly 60 kJ mol−1 and 250 kJ mol−1 [1]. The obtained E in this study is relatively low, agreeing with the very high measured efficiency of this reaction.

4. Conclusion The decolourization of OG dye in aqueous solution can be efficiently driven by using a synthetic goethite as a heterogeneous Fenton-like catalyst. A decolourization efficiency as high as 99.6% at an initial dye concentration of 50 mg L−1 was achieved within 180 min under the conditions described above. The heterogeneous Fenton-like reaction during decolourizing OG obeyed pseudo-firstorder kinetics with a reasonable linear relationship (R2 > 0.964). Decolourizing OG in the Fenton-like reaction catalysed by goethite was effective, but strongly influenced by the solution pH and the concentrations of both FeOOH and H2 O2 under weakly acidic pH condition. The efficiencies of decolourization of OG dye increased with a decreasing solution pH and sodium chloride concentration and/or an increasing H2 O2 concentration. The apparent activation energy (E) measured for this reaction (42.18 kJ mol−1 ) is relatively low, but the result is consistent with the ease of OG decolourization enhanced by increasing of reaction temperature.

Acknowledgements This work was financed by research programs No. 40773080, No. 41072034, and No. 10151063101000028 under The National Natural Science Foundation of China, and The Provincial Natural Science Foundation of Guangdong province, China. Dr. Hochella

at Virginia Polytechnic Institute & State University, USA is gratefully acknowledged for linguistic corrections. We also appreciate the useful comments from two anonymous reviewers.

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