陳美娟，朱 威，WONG Yuktung

1.西安交通大學(xué) 人居環(huán)境與建筑工程學(xué)院，西安 710049 2.香港理工大學(xué) 土木及環(huán)境工程學(xué)系，香港 999077

1 Introduction

Water pollution is already a global problem and causes some countries facing water shortage problem.One of water pollution sources is synthetic organic compounds. The application of synthetic organic compounds has major concern on the environmental and human health risk although they have employed as important role in the development of modern society, since they are nonbiodegradable, highly toxic and may be carcinogenic and mutagenic to organisms(Wilkinson et al, 2017). Synthetic dyes have been widely used in various fields such as textile, leather treatment, paper, food technology, pharmaceutical,agriculture research, photoelectrochemical cells,hair coloring (Shukla and Oturan, 2015). The water containing synthetic dyes is undesirable because the coloring agents are obvious and may be harmful to the aquatic environment. Dyes can reflect and absorb sunlight to the water that they can obstruct the population growth rate of bacteria and inhibit photosynthesis of aquatic plants and algae (Salleh et al,2011).

Rhodamine B (RhB) is an important representative of xanthene-based dyes. It is extensively used as a colorant in various textile-processing industries and food stuffs, including dyeing silk, wool, jute, leather,cotton and so on. The LD50for RhB in oral mouse is 887 mg · kg?1. Human exposure to RhB can cause irritation in contact with skin, eyes and respiratory tract (Palphramand et al, 2011). In California, USA, it is required that products containing RhB must contain a warning on its label.

Fenton reaction is proven to be an efficient oxidation process for organic pollutants in previous literatures, due to the fact that ferrous salt is widely available, non-toxic and relative cheap and that the end product is environmentally benign (Klavarioti et al,2009). Besides, it is easy to operate and maintain as compared to other AOPs (advanced oxidation processes). The hydroxyl radical (·OH) is the active species to degrade target compounds, according to Eq. (1) (Epold et al, 2015).

Fe2++ H2O2→ Fe3++ OH?+ ·OH (1)

However, the rapid depletion and slow regeneration of ferrous ions throughout the reaction is one of the main drawbacks in classical Fenton process (Rahim Pouran et al, 2015). The alternative to the Fenton's reagent is the photo-Fenton and electro-Fenton process, in which the catalytic reaction is propagated by Fe(Ⅱ) regeneration via equation (2)and (3), respectively (Rahim Pouran et al, 2015).

FeⅢ(OH)2++ hν → Fe2++ ·OH (2)

Fe3++ e?→ Fe2+(Cathode) (3)

In this study, a novel electric photo-Fenton process was employed, in which Fenton's reagents of ferrous ions (Fe2+) and H2O2were used in the electrochemical cell to produce hydroxyl radicals,the iron steel worked as sacrificial anode for the generation of supplementary Fe2+, and Fe2+were also regenerated via the reduction of ferric ions by the cathode and UV light. The dye RhB was chosen as a target pollutant to evaluate the performance in aqueous solution. Three types of Fenton reactions (i.e.sole-Fenton, photo-Fenton and electro photo-Fenton)were studied and compared. The effect parameters such as pH, hydrogen peroxide concentration, ferrous ion dosage, and electric current were evaluated and optimized.

2 Experiment

The dye RhB (Rhodamine B, C28H31N2O3Cl,N-[9-(ortho-carboxyphenyl)-6-(diethyl-amino)-3H-xanthen-3-yli-dene] diethyl ammonium chloride) was purchased from Sigma-Aldrich Inc (USA). Fenton reagents, including hydrogen peroxide (35%) and Iron(Ⅱ) sulfate heptahydrate (99.0%) were of analytic reagent grade and obtained from Sigma-Aldrich Inc.(USA). Sulfuric acid was employed to adjust pH values. Sodium thiosulfate was used to quench the Fenton reaction. Sodium sulfate anhydrous at the concentration of 0.05 M (mol · L?1) was employed as background electrolyte for the electric photo-Fenton process. The deionized and distilled water with an 18.2 MΩ resistivity was generated by a Bamstead NANO pure water treatment system (USA).

The electric photo-Fenton experiments were conducted in an electrochemical cell, composing of a single-compartment quartz beaker, a steel sheet with total surface area of 12.5 cm2as sacrif i cial anode, and a platinum sheet as the cathode. An Agilent E3641A DC potentionstat-galvanostat power supply was employed to provide the constant current. Two 8 W fl uorescent lamps with emitting wavelength at 350 nm were placed on the top of the cell to supply UV light.Experiments were carried out in 100 mL 0.1 mM RhB aqueous solution containing 0.05 M Na2SO4as background electrolyte with pH adjustment. The reaction was initialized by pipetting a suitable amount of H2O2and Fe2+into the reactor and turning on the UV lamps simultaneously when a specified constant electrical current was supplied. For photo-Fenton process, the anode, electrolyte and current were disposed. For sole-Fenton process, the UV lamps were disposed also.

The solution was mixed by a magnetic stirrer to maintain a complete homogeneity solution throughout the reaction. At determined time intervals, the def i ned portion of sample was taken from the reactor. The portion sample was mixed immediately with specif i c amount of sodium thiosulfate to quench the reaction.After that, the solution was then analyzed by a UVVIS spectrophotometer at the wavelength of 553 nm to quantify the remaining dye. All the experiments were carried out at (23 ± 1)℃. Parts of test were examined in triplicate. The relative standard deviations of the determination were less than 10%.

3 Results and discussion

3.1 Comparison of different Fenton processes

Three types of Fenton processes (i.e. sole-Fenton, photo-Fenton and electro photo-Fenton) were conducted to identify the RhB degradation eff i ciency.As shown in Fig.1, the sole-Fenton process showed about 30% of RhB decay in 30 min, while 55% of RhB decay was observed in photo-Fenton process.Among the three processes, the electro photo-Fenton was the optimum one with 95% RhB decay. The best performance of electro photo-Fenton should be ascribed to its different Fenton reactions. Apparently,the sole-Fenton process in the dark involved the generation of hydroxyl radicals as stated in Eq. (1).When in the photo-Fenton process, the utilization of UVA (λ = 350 nm) contributed a positive effect of photo-reduction of ferric ions to ferrous ions and hydroxyl radicals (·OH), via Eq. (2). Besides, the direct photolysis of Fenton reagent H2O2also contributes to the production of ·OH radicals according to Eq. (4).

As a result, the photo-Fenton process showed a faster RhB decay than sole-Fenton. The outstanding performance of electro photo-Fenton was achieved by an extra application of electrochemical method, where both Fenton reagents of H2O2and Fe2+can be yielded.Theoretically, H2O2can be electro-generated via the reduction of dissolved oxygen at the cathode (Eq. (5))(?zcan et al, 2008), while Fe2+ions can be electrically generated on a sacrificial anode through iron oxidation(Eq. (6)) and electro-regenerated on the cathode (Eq. (3)).

Fig.1 RhB degradation at different Fenton processes

3.2 Effect of Fenton reagents

In the electro photo-Fenton process, the effect of H2O2/Fe(Ⅱ) molar ratio was determined in the range from 1∶10 (5∶50) to 10∶1 with [H2O2]0fi xed at 0.5 mM and 1.0 mM, respectively. The results were shown in Fig.2a and Fig.2b. The RhB showed a two-stage decay with a rapid degradation in the first several minutes,and then a retarded slow second reaction. The rapid first-stage was caused by the abundant amount of Fenton reagents, contributing to the fast yield of hydroxyl radical (Eq. (1)). The slow second-stage resulted from its lower concentration of hydroxyl radicals because of the fast depleting of Fenton reagents Fe (Ⅱ) and H2O2, and the slow regeneration rate of Fe (Ⅱ) and H2O2(Qiang et al, 2003). Besides,after the first stage, the RhB molecule is almost completely degraded, the byproducts should be the competitor for radicals and retard the RhB decay in the second stage (Chen and Chu, 2014).

Fig.3 summarized the alternation trends of remaining RhB as H2O2/Fe(Ⅱ) molar ratio varied.It can be found that as the [H2O2]0fi xed at 0.5 mM or 1.0 mM, the decay eff i ciency were both increased with the decrement of Fe(Ⅱ). The optimum RhB decay was achieved at H2O2/Fe(Ⅱ) molar ratio = 1, after which the decay performance levelled off. On the other hand, the increment of [H2O2]0always brought the performance improvement as shown in Fig.3. In Masomboon and coworker's study (Masomboon et al,2010), both the increment of Fe(Ⅱ) dosage and [H2O2]0benefitted the performance of electro photo-Fenton process. Differently, in this study, the decreament of Fe(Ⅱ) dosage improved the decay performance.This property is a great advantage for the practical application of Fenton process, because the generation of less ferric hydroxide sludge would reduce the workload for additional separation and disposal (Chou et al, 1999).

Fig.2 Effect of molar ratios of H2O2/Fe(Ⅱ) at (a) [H2O2]0 = 0.5 mM, and (b) [H2O2]0 = 1.0 mM

Fig.3 The remaining RhB at 15 min reaction as a function of H2O2/Fe(Ⅱ) molar ratio

3.3 Effect of pH level

The inf l uence of solution pH on RhB degradation was examined with the pH value in the range from 2.0 to 4.0. The results were depicted in Fig.4. The highest RhB degradation efficacy was reached at an initial pH value around 3.0. The degradation performances of above or below this pH value were worse. In an aqueous solution, the generated Fe(Ⅲ)underwent spontaneous-hydrolysis with water to form four species Fe(Ⅲ)-hydroxo complexes of FeⅢOH2+,FeⅢ(OH)2+, FeⅢ2(OH)24+, and FeⅢ(OH)30(Martin et al, 1998). At the investigated pH level of 3.0, the predominant specie is the monohydroxy complex,FeⅢOH2+, which is the most photosensitive species of the four (Flynn, 1984). Such a complex is capable of producing hydroxyl radicals directly through photosensitization reaction, as well as the re-generation of ferrous reagent (Eq. (7)).

At pH 2.0 and 4.0, the photolysis of FeⅢOH2+in Eq. (7) was restrained because the lower amount of FeⅢOH2+. Besides, the precipitant of FeⅢ(OH)30is formed at pH 4.0 (Ensing et al, 2003), which is known as an adsorbent for organic pollutant (Peng et al,2006). The RhB could be adsorbed on the FeⅢ(OH)03and precipited from the aqueous solution. Therefore,the RhB removal efficacy at pH 4.0 is much better than 2.0.

Fig.4 Effect of initial solution pH

3.4 Effect of electric current

The influence of the electric current (I ) on the RhB degradation was studied in the range from 0.002 A to 0.030 A at current-controlled conditions. Fig.5 showed that the electric current presented a certain effect on the RhB decay. The insert of Fig.5 depicted the variation of the decay rate in the fi rst two minutes with I changed, where a signif i cant jump was observed when I increased from 0.002 A to 0.005 A. However,the decay rate leveled off as the electric current further increased. The increment of decay rate at higher current(from 0.002 A to 0.005 A) was likely attributed to the faster release of H2O2on the cathode (see Eq. (4)).When the current further increased, excess Fe(Ⅱ)produced nearby the anode via Eq. (6) and the cathode through Eq. (3), which may become ahydroxyl radical scavenger as shown in Eq. (8) (Buxton et al, 1988).

The energy consumption for the photoelectro-Fenton process was also examined. The energy consumpiton (E, Wh·m?3) was calculated via the following Eq. (9),

where, U is the voltage measured during the reaction(volt), I is the electric current (A), t is the electrolysis time(h), and V is the volume of reaction solution (m3). The energy consumptions for 90% RhB removal at different electric current were listed in Tab.1. It deserved to note that both the lowest energy consumption and the shortest reaction time were achieved at I = 0.005 A. Therefore,the electric current of 0.005 A is the optimal condition in consideration of the RhB decay efficiency, hydraulic retention time and energy consumption.

Fig.5 Effect of electric current

Tab.1 The energy consumption for 90% of 0.1 mM RhB degradation at different electric current

4 Conclusions

The degradation of RhB was studied by using various Fenton reactions (i.e. sole-Fenton, photo-Fenton and electric photo-Fenton). The electric photo-Fenton with a sacrificed anode showed the optimum performance for RhB degradation. The degradation curve can be defined as a two-stage reaction comprised of a rapid fi rst stage and a retarded second stage. The effect of various parameters such as the Fenton reagent H2O2/Fe(Ⅱ) molar ratio, initial solution pH value, and electric current was further investigated and optimized. The optimal molar ratio of H2O2/Fe(Ⅱ) was 1∶1 where the higher [H2O2]0could benef i t the RhB decay. The optimal pH for RhB degradation was determined at pH 3.0 in the solution.Moreover, the electric current had a great effect on the RhB degradation process. In consideration of both the RhB decay rate and energy consumption, the optimal electric current is examined as 0.005 A in our electric photo-Fenton process.

Buxton G V, Greenstock C L, Helman W P, et al. 1988. Critical review of rate constants for reactions of hydrated electrons,hydrogen atoms and hydroxyl radicals (·OH/·O?) in aqueous solution [J]. Journal of Physical and Chemical Reference Data, 17(2): 513 – 886.

Chen M, Chu W. 2014. Photo-oxidation of an endocrine disrupting chemical o-chloroaniline with the assistance of TiO2and iodate: Reaction parameters and kinetic models [J].Chemical Engineering Journal, 248: 273 – 279.

Chou S, Huang Y H, Lee S N, et al. 1999. Treatment of high strength hexamine-containing wastewater by electro-Fenton method [J]. Water Research, 33(3): 751 – 759.

Ensing B, Buda F, Baerends E J. 2003. Fenton-like chemistry in water: oxidation catalysis by Fe(Ⅲ) and H2O2[J]. The Journal of Physical Chemistry A, 107(30): 5722 – 5731.

Epold I, Trapido M, Dulova N. 2015. Degradation of levof l oxacin in aqueous solutions by Fenton, ferrous ion-activated persulfate and combined Fenton/persulfate systems [J].Chemical Engineering Journal, 279: 452 – 462.

Flynn C M. 1984. Hydrolysis of inorganic iron(Ⅲ) salts [J].Chemical Reviews, 84(1): 31 – 41.

Klavarioti M, Mantzavinos D, Kassinos D. 2009. Removal of residual pharmaceuticals from aqueous systems by advanced oxidation processes [J]. Environment International, 35(2):402 – 417.

Martin R L, Hay P J, Pratt L R. 1998. Hydrolysis of ferric ion in water and conformational equilibrium [J]. The Journal of Physical Chemistry A, 102(20): 3565 – 3573.

Masomboon N, Ratanatamskul C, Lu M C. 2010.Mineralization of 2,6-dimethylaniline by photoelectro-Fenton process [J]. Applied Catalysis A: General, 384(1):128 – 135.

?zcan A, ?ahin Y, Sava? Koparal A, et al. 2008. Carbon sponge as a new cathode material for the electro-Fenton process:Comparison with carbon felt cathode and application to degradation of synthetic dye basic blue 3 in aqueous medium [J]. Journal of Electroanalytical Chemistry,616(1): 71 – 78.

Palphramand K L, Walker N, McDonald R A, et al. 2011.Evaluating seasonal bait delivery to badgers using rhodamine B [J]. European Journal of Wildlife Research,57(1): 35 – 43.

Peng X, Luan Z, Zhang H. 2006. Montmorillonite-Cu(Ⅱ)/Fe(Ⅲ) oxides magnetic material as adsorbent for removal of humic acid and its thermal regeneration [J].Chemosphere, 63(2): 300 – 306.

Qiang Z, Chang J H, Huang C P. 2003. Electrochemical regeneration of Fe2+in Fenton oxidation processes [J].Water Research, 37(6): 1308 – 1319.

Rahim Pouran S, Abdul Aziz A R, Wan Daud W M A. 2015.Review on the main advances in photo-Fenton oxidation system for recalcitrant wastewaters [J]. Journal of Industrial and Engineering Chemistry, 21: 53 – 69.

Salleh M A M, Mahmoud D K, Karim W A W A, et al. 2011.Cationic and anionic dye adsorption by agricultural solid wastes: A comprehensive review [J]. Desalination, 280(1):1 – 13.

Shukla S, Oturan M A. 2015. Dye removal using electrochemistry and semiconductor oxide nanotubes [J].Environmental Chemistry Letters, 13(2): 157 – 172.

Wilkinson J L, Hooda P S, Swinden J, et al. 2017. Spatial distribution of organic contaminants in three rivers of Southern England bound to suspended particulate material and dissolved in water [J]. Science of the Total Environment, 593: 487 – 497.