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Decolorization and Detoxification of Acid Orange 7 by Zero-Valent Iron with UV Light.

Byline: Huiyuan Li, Juanjuan Qiu, Xue Wang and Hui Zhang

Summary: The decolorization and detoxification of Acid Orange 7 (AO7), a commonly used azo dye, by zero-valent iron (Fe0) and UV light was investigated. The influence of important reaction parameters, such as initial pH and the Fe0 dosage, on the color removal and effluent toxicity, was elucidated. Results indicated that the decolorization of AO7 was enhanced in the UV/Fe0 system, as compared to that with only UV or Fe0, in which the decolorization efficiency in 60 min increased with increasing Fe0 dosage and decreasing solution pH. The kinetics of the AO7 removal conformed to the Langmuir-Hinshelwood model with high correlation coefficient (R2 = 0.9962).

Under the optimum conditions, AO7 was decolorized rapidly within 60 min (over 97% decolorization efficiency), with 34.8% of TOC reduction. Fe0 could be used for repeated cycles to decolorize AO7 effectively. Furthermore, nearly no Daphnia magna immobilization was found after the treatment, suggesting that the method was effective to reduce the AO7 toxicity.

Keywords: Acute toxicity, Azo dye, Fe0 reuse, Langmuir-Hinshelwood model, Zero-valent iron

Introduction

Various synthetic dyes are extensively used in textile, paper/pulp, plastic, color-photography, pharmaceutical, and cosmetic industries [1]. During the dyeing process, large quantities of unused dyes are released into the aquatic environment. Decolorization of various dye-containing wastewaters is one of the important problems since the wastewater may be noticeably colored even at low dye levels, which will create aesthetic problems [2]. Furthermore, the dyes may absorb sunlight and initiate various chemical reactions, forming mutagenic or carcinogenic intermediates; such toxicities greatly affect the water quality and the efficiency in biological wastewater treatment [3]. Therefore, the degradation of dye wastewater has become a subject of worldwide concern. However, since these dyes resist to biological degradation because of their complex structure and stability, it is ineffective to remove them by conventional biological treatments (mainly based on activated sludge process) [3, 4].

During the past decades, the use of zero-valent iron (Fe0) as a reactive medium has shown great promise for wastewater treatment because Fe0 is readily available at low costs, is easy to be removed from solutions, and has low toxicity [5, 6]. Moreover, Fe0 can also be applied as a pretreatment method prior to biological treatment because it can effectively detoxify the hazardous contaminants [7-10]. In this respect, Fe0 has been widely studied to effectively degrade dye-containing wastewater [1, 6,11, 12].

To improve the degradation of dye wastewater, UV light is often introduced into the system (UV/Fe0). The removal of azo dyes, including Reactive Red 2, Reactive Black 5, and Acid Orange 7 (AO7), in the UV/Fe0 process was reported previously [13, 14]. However, only the operating parameter effects on the dye decolorization was investigated in these studies. More importantly, the mechanism of the dye removal by UV/Fe0 process was not fully understood. Deng et al. [13] observed that the introduction of UV light enhanced the decolorization of Reactive Red 2 by Fe0 with no mechanism proposed. Rahmani et al. [14] attributed AO7 removal to a Fenton-like mechanism, in which H2O2 was produced by H2O under UV irradiation and Fe2+ was generated by the corrosion of Fe0.

The formed Fenton's reagent in turn oxidized AO7. Nevertheless, the proposed Fenton-like mechanism was not verified by measuring the production of either H2O2 or *OH.

As far as the toxicity is concerned, it is believed that the reaction intermediates may exhibit higher toxicity than the parent compounds during the degradation process. Therefore, the evaluation of the effluent toxicity is significant in order to avoid the adverse effect on the aquatic environment. For the above reasons, the decolorization of AO7 with a combined UV/Fe0 process was investigated in the present study. AO7 was chosen as the target contaminant because of its low price and wide use in the textile and paper industries [15]. The effect of hydroxyl radical scavenger (tert-butyl alcohol) on the decolorization of AO7 was firstly investigated to verify the reaction mechanism in this process. To achieve a rapid decolorization as well as to reduce the effluent toxicity, the effects of initial pH and Fe0 dosage were examined to determine the optimum reaction parameters. Furthermore, the reaction kinetics was proposed and the reusability of Fe0 was evaluated.

Experimental

Chemicals

AO7 was purchased from Shanghai No. 3 Reagent Factory (China) and used as received. The zero-valent iron powder (Fe0) was obtained from Shanghai No. 2 Metallurgical Plant and used without any pretreatment. The radical scavenger, tert-butyl alcohol (TBA), was obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Other chemicals, such as H3BO3, MnCl2A*4H2O, ZnSO4A*7H2O, CuSO4A*5H2O, (NH4)6Mo7O24A*4H2O, FeCl3A*4H2O, and EDTA, were supplied by Sinopharm Chemical Reagent Co., Ltd. (China) and used for Daphnia magna (D. magna) cultivation. All chemicals were of analytical grade and used without further purification. All solutions were prepared with deionized water.

Procedure and Analysis

All experiments were performed at room temperature. The degradation reactions were carried out in a 500-ml beaker containing 300 ml of the test solution stirred continuously. The UV lamp (I>> = 254 nm, 20 W, Liang Xing Electrical, Zhongshan) was placed vertically in the middle of the reactor. Before each run, a stock solution of AO7 was prepared in deionized water and the initial concentration was fixed at 100 mg L-1. Sodium hydroxide and sulfuric acid were used to adjust the initial dye pH (FE20 pH meter, Mettler-Toledo Instruments Co., Ltd., Shanghai). The required amount of Fe0 powder (0.3-2.0 g L-1) was added into the stirred solution and the UV lamp was switched on when the reaction began. Samples (1 mL) were withdrawn by syringe at selected time intervals and immediately filtered through 0.45 um membranes to determine the absorbance using a Rayleigh UV-9100 spectrophotometer (Rayleigh Co., China) at I>>max = 485 nm.

The UV-Vis spectra from 200 to 800 nm were recorded with a UV-1700 spectrophotometer (Shimadzu). Total organic carbon (TOC) was measured by a Jena multi N/C 3100 total organic carbon analyzer. The decolorization efficiency was calculated according to the following equation:

Decolorization efficiency (%) = A0 - A/A0 X 100 -------- (1)

Where A0 and A were the absorbance of AO7 at I>>max = 485 nm at time 0 and t, respectively.

The acute toxicity was determined by D. magna immobilization test [16], in which D. magna was cultured in the laboratory for more than three generations. D. magna was selected due to its high sensitivity and ease of maintenance [16-18]. The acute toxicity experiments were carried out in 50-ml-capacity test beakers using 25 24-h-old D. magna which were divided into five groups. Four groups were used for the test while one group for the blank; they were incubated at 20 AdegC in a 16 h light - 8 h dark cycle. No foods were given during the acute toxicity tests. The experiments were repeated four times, and the survived and mobile D. magna were counted after 24 h.

Results and Discussion

Decolorization of AO7 in Different Systems

Experiments were conducted to investigate the decolorization efficiency of AO7 in different systems (UV, Fe0 and UV/Fe0), with initial pH of 3 and the Fe0 dosage of 2.0 g L-1. As seen in Fig. 1, there was a slight color removal (10.4%) under UV irradiation alone, indicating that UV irradiation did not pose a significant effect on AO7 removal. In the presence of Fe0, the decolorization efficiency was much higher (84.2%) than that by UV alone. When UV light was introduced into the Fe0/H2O system, the decolorization efficiency reached 97.4% after 60 min treatment, indicating that the UV irradiation and Fe0 had a synergistic effect on the AO7 removal.

To investigate the contribution of hydroxyl radical (*OH) to the decolorization of AO7, tert-butyl alcohol (TBA), a typical hydroxyl radical scavenger, was added into the UV/Fe0 system. It can be seen from Fig. 2 that the presence of TBA had very little effect on AO7 removal, indicating hydroxyl radical played an unimportant role in the decolorization of AO7 under the conditions in this work.

There are many works proposing that the degradation mechanism with Fe0 comprises of heterogeneous reactions. It is reported that the reactions between reactants and Fe0 occur on the Fe0 surface. The dye molecule is first adsorbed onto the Fe0 surface, and then the electrons are transferred from the Fe0 surface to the adsorbed dye molecule, resulting in chemical reduction. Finally, the AO7 reaction products are desorbed from the metal surface and diffused to the bulk solution [19, 20]. When an effective collision takes place between a dye molecule and iron, the iron, as an electron donor, loses electrons, the dye, as an electron acceptor, accepts electrons and combines with H+ to form a transitional product. This product captures electrons and combines with H+ again to form the terminal product, resulting in the cleavage of the azo bond and decolorization of the dye, which were shown in Eqs. (2) and (3) [6, 21]:

Fe0 a Fe2+ + 2e- -------- (2)

(Equation)

In addition, at the initial stage of the reaction involving Fe0 (Fe0 and UV/Fe0), the pH value increased to 5 in a few minutes and finally remained at 5.8. According to Eq. (3), the reduction of AO7 by Fe0 consumed a large amount of H+, resulting in the decrease of the H+ concentration. Therefore, the pH value would increase. In other words, the amount of OH- would increase. Then the oxidized Fe species would form such products as Fe(OH)2 and Fe(OH)3 (Eqs. (4)-(6)) [22]:

Fe2+ a Fe3+ + e- ----------- (4)

Fe2+ + 2OH- a Fe(OH)2 ----------- (5)

Fe3+ + 3OH- a Fe(OH)3 ----------- (6)

As a result, AO7 or its intermediate products would co-precipitate with the oxidized iron [23]. It should be noted that the corrosion of Fe0 would be improved by UV irradiation (Eq. (7)) [24]:

Fe0 + h a Fe2+ + 2e- ----------- (7)

In this way, with more electrons generated with UV, the decolorization efficiency of AO7 will be improved. Moreover, with the enhanced oxidation of Fe0, the contaminant co-precipitation should be improved. Therefore, the decolorization efficiency of AO7 in the UV/Fe0 process was higher than that in the Fe0 process.

To elucidate the changes of molecular and structural characteristics of AO7 during the UV/Fe0 process, solution samples were taken at 0, 10, 20, 40 and 60 min and changes of the representative UV-Vis spectra of the solution as a function of reaction time were followed. Fig. 3 shows the spectra and three absorbance peaks at 229, 310 and 485 nm of AO7 were observed. In the visible region, there was one main peak located at 485 nm, which was characteristic of the azo structure. Meanwhile, in the ultraviolet region, the absorbance peaks at 229 and 310 nm were attributed to benzene ring and naphthalene ring, respectively [21, 25, 26]. As the reaction proceeded, the visible band disappeared with time, indicating the breakage of the azo links and thus the rapid decolorization. This leads to the formation of sulfanilic acid and 1-amino-2-naphthol [27-29].

Although 1-amino-2-naphthol is shown to be cytotoxic, it undergoes further decomposition without forming any aromatic species [28]. The decay of the absorbance at 229 and 310 nm is due to aromatic degradation of the dye molecule and its intermediates. On the other hand, a new peak appeared at 250 nm and its absorbance increased with time, indicating a new structure unit was being formed during the dye degradation.

The new peak at 250 nm may probably be attributed to sulfanilic acid [30-32], which is persistent against further removal by UVC irradiation [33].

Effect of Initial pH on AO7 Decolorization and Effluent Toxicity

To determine the influence of initial dye pH on the degradation of AO7, experiments were carried out at a pH range of 2-5, with a Fe0 dosage of 2.0 g L-1. As seen in Fig. 4(a), the initial solution pH remarkably influenced the efficiency of color removal in the UV/Fe0 process, in which a higher pH value led to a lower decolorization efficiency.

After 60 min of reaction, the color removal efficiencies were 98.1, 97.4, 44.7, and 21.3% at initial pH 2, 3, 4, and 5, respectively. On one hand, by reaction (3), a low pH would provide more H+ to accelerate the cleavage of azo bond and thus the decolorization of AO7. On the other hand, at high pH, the amount of H+ would decrease. Therefore, the oxidation of Fe0, which involved H+, would occur more slowly, producing less oxidized Fe species (namely Fe2+ and Fe3+). As a result, the influence of co-precipitation would be weakened (Eqs. (5) and (6)). Consequently, at initial pH 2 and 3, the color removal rate was much higher compared to those at pH of 4 and 5.

The toxicity of the treated dye solution was evaluated by a D. magna inhibition test and presented in terms of the percentage of the immobilizing individuals. As shown in Fig. 4(b), the immobilization rates of D. magna were not higher than 10% for all treated effluents, compared with an immobilization rate of 40% for the untreated solution. Therefore, although the color removal varied considerably with the initial pH, the variation in acute toxicity was relatively small after the treatment. Considering the color removal rates of AO7 under different initial pHs, pH 3 was chosen as the optimum initial pH and used for the sequential experiments.

Effect of Zero-Valent Iron Dosage on AO7 Decolorization and Effluent Toxicity

Since the Fe0 concentration is an important factor on the reaction, the effect of Fe0 dosage on dye decolorization with UV/Fe0 process was investigated. Different amounts of Fe0 (0.3, 0.5, 1.0, 1.5, and 2.0 g L-1) were added to the solution separately. The results are depicted in Fig. 5(a). It can be seen that the color removal efficiency increased from 65.5% with 0.3 g L-1 of Fe0 to 97.4% with 2.0 g L-1 of Fe0. In the UV/Fe0 system, Fe0 provides electrons to decolorize the dye, and thus more electrons will be produced with more Fe0, leading to a higher decolorization efficiency. Moreover, as the decolorization of AO7 by UV/Fe0 occurs at the Fe0/H2O interface, the available iron surface area affects the decolorization rate [3]. An increasing dosage of iron particles can provide more active surface sites to accelerate the initial reaction by colliding with dye molecules [34].

As a consequence, the color removal efficiency increases with increasing Fe0 dosage. Fig. 5(b) presents the acute toxicity data, which show that the immobilization rates of UV/Fe0-treated effluents were quite low ( 15%) compared to that of the untreated solution. The results of the toxicity evaluation indicate that the use of UV/Fe0 can significantly reduce the biotoxicity of the treated dye. Consequently, the optimum Fe0 dosage was selected to be 2.0 g L-1.

Kinetic Modeling

The decolorization of AO7 in the UV/Fe0 combined system can be described by Langmuir-Hinshelwood (L-H) model, which considers the adsorption of AO7 on the Fe0 surface, surface reaction, and the desorption of the AO7 reaction products [35]. By assuming the L-H model for the reaction, the rate equation may be described as:

(Equation)

Where r0 represents the initial rate of reaction, C0 is the initial concentration of AO7, kr is the reaction rate constant between the adsorbed AO7 molecules and Fe0 powder, and KLH is the adsorption constant of AO7 on the Fe0 surface [36].

Therefore, from a plot of 1/r0 versus 1/C0 for AO7, the related parameters kr and KLH can be obtained from the respective slope and intercept values (Eq. (8)). In this study, the kinetics of AO7 destruction was studied using five different initial concentrations: 25, 30, 50, 75 and 100 mg L-1. As shown in Fig. 6, the kinetic data for the removal of AO7 in the UV/Fe0 reaction conformed to the L-H model with a R2 value of 0.9962. By a linear regression analysis, the kr and KLH values were found to be 21.79 mg L-1 min-1 and 0.006654 L mg-1, respectively.

The Reuse of Zero-Valent Iron

Repeating cycles for AO7 decolorization were performed to evaluate the reuse ability of the Fe0 powder. The operating conditions were AO7 concentration of 100 mg L, initial pH of 3 and Fe dosage of 1.5 g L-1. After each run, the solution was settled down, and the supernatant was removed.

Then 300 ml of freshly prepared AO7 with initial pH of 3 was added into the reactor. It should be noted that the Fe0 powder was easily separated because most of the powder was stick onto the magnetic stirring bar. As shown in Fig. 7, after three runs, the decolorization efficiency of AO7 still remained over 95%. At the end of each reaction, the surface of Fe0 was covered by the corrosion products. However, when adding AO7 solution under acidic condition, Fe0 could still dissolve and induce the decolorization process. The results implied that Fe0 could be used repeatedly for rapid decolorization of AO7 under UV irradiation, which could save the Fe0 consumption.

The TOC Removal of AO7

The removal efficiencies of AO7 and TOC upon the UV/Fe0 treatment under the optimum conditions were examined. The initial pH was 3 and the Fe0 dosage was 2.0 g L-1. The AO7 concentration changed from 100 mg L-1 to 2.6 mg L-1 while the TOC changed from 52 mg L-1 to 33.9 mg L-1 after 60 min of reaction. The results indicated that the decolorization efficiency (97.4%) was much higher than the TOC reduction (34.8%). As the color of AO7 is attributed to the azo bond [4], the decolorization would occur as long as the azo linkage is broken (Eq. (3)). The produced intermediates contribute to the TOC measurements, and thus during the reaction a small portion of TOC is reduced but the major part remains [34].

The results indicate that the UV/Fe0 process can partially reduce the TOC of pollutant though the TOC is more difficult to remove than the color under reductive conditions, which is consistent with the results of Chang et al. [34]. However, the toxicity test showed that the UV/Fe0- treated effluent presented relatively low D. magna inhibition (Fig. 4(b) and Fig. 5(b)), meaning that this method could efficiently reduce the acute toxicity of the dye.

Conclusions

In the UV/Fe0 process, the efficiency of AO7 color removal was improved by decreasing the initial pH and increasing the Fe0 concentration. UV enhanced Fe0 reduction was the main mechanism for color removal. The kinetics fitted the L-H model well and the kr and KLH values were calculated to be 21.79 mg L-1 min-1 and 0.006654 L mg-1, respectively. With an initial pH of 3, Fe0 dosage at 2.0 g L-1 and initial dye concentration of 100 mg L-1, the decolorization and TOC removal efficiency were 97.4% and 34.8%, respectively. The Fe0 powder could be used for three times, which could still remain its ability to remove over 90% AO7.

Remarkably, there was no D. magna immobilization observed after 60 min reaction, implying UV/Fe0 process is effective in decolorization and reducing the toxicity. The UV/Fe0 process can be applied as an effective method for the AO7 pretreatment before the subsequent biological process.

Acknowledgement

This work was supported by Natural Science Foundation of Hubei Province, China (Grant 2012FFA089). The generous help of Professor Cary T. Chiou in revising this manuscript is also greatly appreciated.

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