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Photodegradation mechanism of sulfadiazine catalyzed by Fe(III), oxalate and algae under UV irradiation a
Junwei Zhang & Li Ma
a
a
The Key Laboratory of Food Colloids and Biotechnology , Ministry of Education, School of Chemical and Material Engineering, Jiangnan University , Wuxi , China Published online: 31 Jan 2013.
To cite this article: Junwei Zhang & Li Ma (2013) Photodegradation mechanism of sulfadiazine catalyzed by Fe(III), oxalate and algae under UV irradiation, Environmental Technology, 34:12, 1617-1623, DOI: 10.1080/09593330.2013.765915 To link to this article: http://dx.doi.org/10.1080/09593330.2013.765915
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Environmental Technology, 2013 Vol. 34, No. 12, 1617–1623, http://dx.doi.org/10.1080/09593330.2013.765915
Photodegradation mechanism of sulfadiazine catalyzed by Fe(III), oxalate and algae under UV irradiation Junwei Zhang∗ and Li Ma The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, China
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(Received 31 March 2012; final version received 24 December 2012 ) Photodegradation mechanism of sulfadiazine (SD) in a solution containing Fe(iii), oxalate and algae were investigated in this study. The results indicated that the degradation of SD was slow in a solution containing Fe(iii) or oxalate, whereas it was markedly enhanced when Fe(iii) and oxalate coexisted. The optimal pH for formation of ·OH was 4; a higher or lower pH resulted in a decrease in formation of ·OH. A moderate increase of oxalate concentration was beneficial to the formation of ·OH and the degradation of SD, and the algae enhanced the degradation rate of SD in a solution containing Fe(iii) and oxalate. Also, the degradation rate of SD rapidly decreased at low initial concentrations but slowly decreased at high initial concentrations, and pseudo-first order kinetics described the degradation process of SD well. A possible reaction mechanism in solution containing Fe(iii), oxalate and algae was proposed, and attack by ·OH was the main pathway of SD degradation in the photocatalytic reaction. Keywords: sulfadiazine; Fe(iii); oxalate; algae; photodegradation
1. Introduction Antibiotics, a subclass of pharmaceuticals, have been receiving increased attention due to their existence in aquatic environments. Therefore the development of antibiotic resistant bacteria on aquatic life in aquatic environments is becoming a problem [1–5]. Sulfadiazine (SD), a synthetic antibacterial agent of sulfonamides, is used in human and veterinary medicine, and has been used to treat a broad variety of Gram(+) and Gram(−) bacterial infections [6,7]. SD is a sulfonamide that is frequently detected in the environment in relatively high concentrations (ranging from micrograms to milligrams per litre), and the presence of sulfonamides have been reported in wastewater, surface water, ground water, and even drinking water [8,9], and the widespread occurrence of SD in different water types has led to increasing bacterial resistance [2,6]. Due to their high chemical stability and low biodegradability, biological or physicochemical techniques for the removal of antibiotics is not efficient enough, and they are therefore introduced into aquatic environments [2,4,10]. Thus, the development of new purification technologies for wastewater leading to the complete destruction of these antibiotics has both an important theoretical and practical value. Photocatalytic degradation is a promising treatment technology for the elimination micropollutants in aqueous solution, and these micropollutants can be decomposed by the participation of hydroxyl radicals generated by different ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
photocatalytic systems [11–15]. Over the past two decades, the photochemical activity of complexes of Fe(iii) and carboxylate anions has received consideration attention. Chen et al. [16], Wu et al. [17] and Guo et al. [18] reported that Fe(iii) and carboxylate anions could form stable complexes that exhibited high photoreactivity, and found that the organic compounds could be degraded gradually. The formation of Fe(ii) and hydroxyl radicals through a ligand-to-metal charge transfer path can be generated by Fe(iii)-carboxylate complexes, and the hydroxyl radicals can oxidize most organic compounds quickly and nonselectively due to their high oxidation potential value (E 0 = +2.80 V) [18,19]. Thus, Fe(iii) and carboxylate, two ubiquitous compounds in nature, can result in a photochemical reaction which plays an important role in the photodegradation of pollutants in natural water [17,18,20]. Some studies have indicated that Fe(iii)-carboxylate complexes have high photocatalytic efficiencies in the degradation of organic compounds [16–21]. Algae, important microorganisms in water, can secrete some lower molecular weight organic compound [22]. Thus, the degradation rate of an organic pollutant may be enhanced by the addition of algae in solution containing Fe(iii) and carboxylate. To the best of our knowledge, much is known about the effect of carboxylate anions on the photochemical reaction, however, the photodegradation of antibiotics by Fe(iii), carboxylate and algae has not been investigated.
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2. Experimental 2.1. Materials SD (purity ≥ 98%) purchased from Sigma-Aldrich Chemical Company (America) was used without further treatment. FeCl3 · 6H2 O and Na2 C2 O4 were of analytical grade and obtained from Shanghai Chemical Co. (China). The other chemicals used in this study were of an analytical grade (Shanghai Reagent Company, China). Deionized water was used to prepare the solutions in the experiments. All stock solutions were stored in a refrigerator at 4◦ C in the dark prior to use. The alga used in the experiments was Chlorella vulgaris, which obtained from the Wuhan Hydrobilology Institute of Chinese academy of sciences (China). The algae were grown in an axenic culture medium at 25◦ C using a 24 h light cycle in a culturing room equipped with a constant temperature air-conditioner. Under the condition of logarithmic growth, the algae were taken for use in experiments after being washed. The cell counting was carried out under inverted microscope at 400× and the density of algae (cell/L) was carefully calculated. The concentration of algae was gained by diluting washed algae with the deionized water. All glassware used in experiments was cleaned by soaking in 1 M HCl for 6 h and thoroughly rinsed with tap water and then deionized water.
dark, and the pH of the solutions were adjusted with dilute sulfuric acid and sodium hydroxide solution. Then, the reaction solutions were introduced into the photoreaction vessel and stirred with magnetic stirrers and sparged with air (0.5 L/min). At intervals, samples were collected and determined immediately by high performance liquid chromatography (HPLC) in order to avoid further reaction.
2.3. Analysis The concentration of SD was detected by HPLC (LC-10AT, Kromasil C18 column [150 × 4.6 mm, 5 μm], Shimadzu, Japan) with a flow rate of 1.5 mL/min at 277 nm. The mobile phase consisted of an 85/15 ratio of aqueous phase to acetonitrile (v/v), and the aqueous phase was 2.5 mM sodium-1-heptanesulfonate adjusted to pH 2 with H3 PO4 . A standard solution of phenol was used to calibrate the HPLC for the quantification of the formation yield of the hydroxyl radicals in the reaction since the aromatic hydroxylation was considered to be one typical reaction of hydroxyl radicals [15]. Moreover, it was assumed that the hydroxyl radicals-mediated oxidation of benzene formed phenol with nearly 100% yield and that phenol concentration represented the concentration of hydroxyl radicals.
3.
Results and discussion
3.1. Degradation of SD As shown in Figure 1, there was no chemical reaction between SD and the photocatalyst in the dark since the concentration of SD was stable. Under the irradiation of a 15 W medium pressure mercury lamp, the degradation rate of SD after 60 min was about 7%, this phenomenon could arise direct photolysis and chemical attenuation of SD. Moreover, degradation rate of SD was 17% in solution containing oxalate after 60 min
1.0 0.8
2.2. Procedures The photodegradation of SD was carried out in a cylindrical reactor (self-made, 500 mL capacity, with a diameter and length of 6 cm and 25 cm, respectively) equipped with a 15 W medium pressure mercury lamp (λmax ≥ 254 nm), which was positioned in the photoreactor. The lamp was surrounded with a quartz jacket and the tap water cooling circuit maintained the solution temperature at 25 ± 1◦ C. The incident of photo flux measured by the potassium ferrioxalate actinometry was 1.32 × 10−4 Einstein/(L·min) [23]. In a typical run, a mixture containing known concentration of SD, Fe3+ and C2 O2− 4 were prepared in the
SD (C/C0)
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In this study, the degradation process of SD catalyzed by Fe(iii), oxalate and algae was investigated under ultraviolet (UV) irradiation. The objectives of this study were as follows: (i) to examine the effect of several factors on the degradation of SD in an irradiated solution containing Fe(iii) and oxalate; (ii) to analyze the kinetics of SD catalyzed by Fe(iii) and oxalate; (iii) to elucidate the photochemical degradation potential and mechanism of SD in aqueous solutions containing Fe(iii), oxalate and algae. SD was selected as the model pollutant in this paper since it is a typical antibiotic that is widely used in aquaculture and animal husbandry. Fe(iii), oxalate and algae were applied since they are common environmental constituents.
0.6 0.4
Dark control Photolysis Oxalate Fe(III) Fe(III)+oxalate
0.2 0.0 0
10
20
30
40
50
60
Time (min)
Figure 1. Degradation of SD in different reaction systems where SD0 = 20 mg/L, Fe3+ 0 = 20 μmg/L, oxalate0 = 200 μmg/L, and pH = 4.0.
Environmental Technology irradiation. The formation of the oxidant generated by the photolysis of oxalate results in a small increase in the degradation rate of SD under acidic conditions [17,21,24]. The degradation rate of SD was about 39% in solution containing Fe(iii) after 60 min irradiation. This may be due to formation of hydroxyl radicals (·OH) that may absorb UV light to generate ·OH (Equations (1)–(2)) [17,19,20], and ·OH can oxidize most organic compounds non-selectively and mineralize them to carbon dioxide, water and other ions owing to its high oxidation potential (E 0 = +2.80 V) [17,18]. Fe(iii) + H2 O → Fe(OH)
2+
2+
+ hv → Fe(ii) + ·OH
(2)
Furthermore, the degradation rate of SD was enhanced by the addition of oxalate in a solution containing Fe(iii). After 60 min irradiation, the degradation rate of SD was about 80%, and the removal of SD was much more efficient in a solution containing Fe(iii) and oxalate. This phenomenon arises from the formation of Fe(iii)-carboxylate complexes, which are highly photocatalytic compounds, and ·OH can be generated by Fe(iii)-carboxylate complexes via a ligand-to-metal charge transfer [16,18]. 3.2. Formation of hydroxyl radicals The formation of ·OH could be induced and accumulated in different conditions with an increasing reaction time although dark control and photolysis of benzene showed negligible loss of benzene under irradiation (Figure 2A). The results also indicated that the formation yield of ·OH in a solution containing Fe(iii) or oxalate was smaller than in a solution containing Fe(iii) and oxalate, and the formation yield of ·OH could be significantly enhanced. Up to 2.54 μmol/L of ·OH was produced in a solution containing Fe(iii) and oxalate after 60 min irradiation. Moreover, the formation of ·OH is responsible for the degradation of SD in different conditions, and the formation of ·OH from photocatalysis may be the key to result in photodegradation (A) 3.0
of SD (Equations (3)–(9)). FeIII (Oxa) + hv → FeII -Oxa· → Fe(II) + Oxa− · Oxa− · + O2 → H+ +
O− 2·
O− 2·
+ CO2
↔ HO2 ·
(3) (4) (5)
HO2 ·/O− 2 · + Fe(ii) → Fe(iii) + H2 O2 HO2 ·/O− 2 · + Fe(iii) → Fe(ii) + O2 H2 O2 + Fe(ii) → Fe(iii) + ·OH + − ·OH
(8)
SD + ·OH → . . . → degradation products
(9)
(6) (7)
The formation yield of ·OH depends on the concentration of Fe(iii)-oxalate complexes which are influenced by pH of solution. As shown in Figure 2B, the formation yield of ·OH increased firstly and then decreased within a pH range of 3.0–6.0, indicating that it was unfavourable to the formation of ·OH at higher pH values, whereas it was beneficial to the formation of ·OH at lower pH values. The formation yield of the ·OH radicals dependence on pH is considered to mainly result from distribution of Fe(iii) and oxalate species. The distribution of Fe(iii) and oxalate species against pH was simulated by Yong et al. [16] with the MEDUSA software, and the fraction of Fe(OH)2+ was negligible and the ferrioxalate ions constituted of a main Fe(iii) species in the ferrioxalate system and Fe(C2 O4 )2− 2− and Fe(C2 O4 )3− 3 were the main species at pH 4, Fe(C2 O4 ) 3− and Fe(C2 O4 )3 displayed fairly strong photochemical activity. However, some of the photoreactive substances should be FeC2 O+ 4 and occur at pH 2, which was much less efficiently photolyzed than Fe(C2 O4 )2− and Fe(C2 O4 )3− 3 , moreover Fe2 O3 occurred when the pH was above 5. 3.3. Effect of Fe( III)/oxalate ratio The ratio of Fe(iii) to oxalate plays an important role in the degradation of SD since the Fe(iii)/oxalate ratio has an important effect on the formation of Fe(iii)-oxalate complexes and the yield of ·OH. As shown in Figure 3A, the concentration increase of oxalate ions could enhance the (B) 3.0
Dark control Photolysis Oxalate Fe(III) Fe(III)+oxalate
2.5 2.0
Hydroxyl radicals (µmol/L)
Hydroxyl radicals (µmol/L)
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Fe(OH)
(1)
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1.5 1.0 0.5
pH = 3.0 pH = 4.0 pH = 5.0 pH = 6.0
2.5 2.0 1.5 1.0 0.5 0.0
0.0 0
10
20
30
40
Time (min)
50
60
0
10
20
30
40
50
60
Time (min)
Figure 2. Formation of hydroxyl radicals under different conditions: (A) Fe3+ 0 = 20 μmg/L, oxalate0 = 200 μmg/L and pH = 4.0; (B) Fe3+ = 20 μmg/L, oxalate = 200 μmg/L within a pH range of 3.0–6.0. 0 0
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SD (C/C0)
0.8
Fe(III)/oxalate = 20µmol/L:100µmol/L
(B) 3.5
Fe(III)/oxalate = 20µmol/L:200µmol/L Fe(III)/oxalate = 20µmol/L:400µmol/L
3.0
Hydroxyl radicals (µmol/L)
(A)1.0
0.6 0.4 0.2
2.5 2.0 1.5 1.0 0.5 0.0
0
10
20
30
40
50
60
Time (min)
Fe(III)/oxalate = 20µmol/L:100µmol/L Fe(III)/oxalate = 20µmol/L:200µmol/L Fe(III)/oxalate = 20µmol/L:400µmol/L
0
10
20
30
40
50
60
Time (min)
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Figure 3. Effect of Fe(iii)/oxalate ratio on degradation of SD (A) and formation yield of hydroxyl radicals (B) where SD0 = 20 mg/L and pH = 4.0.
degradation rate of SD and greatly shorten the degradation time. At a given pH value, the Fe(iii)/oxalate ratio can determine the species distribution of Fe(iii)-oxalate complexes in solution. Ou et al. [20] noted that part of a carboxylic acid was deprotonated by Fe(iii) with ligand exchange and was ultimately transformed to carboxylate. An increase in citrate concentration easily formed these complexes for Fe(iii) and carboxyl groups and the complexes that formed at high citrate concentrations exhibited higher photoabsorption. Chen et al. [16] indicated that increase of logarithmic concentration of C2 O− 4 was beneficial for the formation of ferrioxalate ions which dominated in solution and increased the photoreactivity. However, an excessive increase of the oxalate concentration has negative impact on the degradation of SD since the superabundant Fe(iii)-oxalate complexes ion will oxidize the oxalate and the oxalate radicals, which reduce the formation of Fe(iii)-oxalate complexes and prevent the reduction of O2 and the formation of H2 O2 (Equations (4)– (6)). The results in Figure 3A also indicated that an increase in the oxalate concentration never led to a rapid increase in the degradation rate of SD. In order to explore the reason of the Fe(iii)/oxalate ratio effect, the yield of the hydroxyl radicals were determined under the same conditions. As shown in Figure 3B, the increase in the oxalate concentration enhanced the yield of hydroxyl radicals, and the increase in the yield of the hydroxyl radicals was not as noticeable than at the beginning when the Fe(iii)/oxalate ratio was 1/20. This indicates that the excessive increase in the oxalate concentration greatly affects the formation yield of the hydroxyl radicals. Therefore the formation of the hydroxyl radicals is in agreement with the degradation of SD, and the formation of the hydroxyl radicals has an important role in the degradation of SD. 3.4. Effect of algae Algae as an important type of microorganism in aqueous environments, as they can generate some smaller molecular
weight organics such as dissolved organic carbon (DOC) through secretion [25], and these DOC molecules may carry a huge variety of functional groups, the majority of which are carboxylic groups (humic and fulvic acid etc.) and phenolic hydroxyl groups [24], which are photosensitizers and lead to indirect photo-oxidative reactions. Rontani indicated that the photodegradation of different lipid compounds in suspensions of algae could form dicarboxylic acids [26], and the carboxylic acid could coordinate with Fe(iii) ions to produce more hydroxyl radicals after UV irradiation [27]. Thus, algae may accelerate the degradation process of organic contaminants in aqueous environments and it is interesting to investigate the effect of algae on the degradation of SD. As shown in Figure 4A, the solution containing algae which led to degradation of SD and degradation rate of SD increased with prolonging reaction time. After 60 min irradiation, up to 23% of SD was degraded in a solution containing algae. However, the degradation rate of SD was enhanced in the ternary photocatalytic system (Fe(iii), oxalate and algae), and the increased amplitude was not very high. Up to 92% of SD was removed in the ternary photocatalytic system after 60 min irradiation. This indicated that the occurrence of algae in aqueous environments can strengthen the degradation of organic contaminants. Fe(iii) can form Fe(iii)-carboxylate complexes with carboxylic anions, which are more photoreactive for the production of hydroxyl radicals [18]. Also, it has been reported that algae may release acidic dissolved organic matter (DOM), which contain low molecular weight carboxylic acids [28]. Liu et al. [25] reported that the degradation rate of 17 α-ethynylestradiol could be enhanced by the addition of algae in a solution containing Fe(iii). Peng et al. [24] studied the degradation of bisphenol A in simulated lake water containing algae, humic acid and ferric ions, and found that the algae could enhance the photodegradation of bisphenol A under a near UV light. The reason for the rapid degradation could arise from the secretion of algae, which could promote the formation of hydroxyl radicals. Deng et al.
Environmental Technology (B) 3.5 Hydroxyl radicals (µmol/L)
(A) 1.0 0.8 SD (C/C0)
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0.6 0.4 Algae Fe(III)+oxalate Fe(III)+oxalate+algae
0.2
Algae Fe(III)+oxalate Fe(III)+oxalate+algae
3.0 2.5 2.0 1.5 1.0 0.5 0.0
0.0 0
10
20
30
40
50
0
60
10
20
30
40
50
60
Time (min)
Time (min)
Figure 4. Effect of algae on degradation of SD (A) and formation yield of hydroxyl radicals (B) where SD0 = 20 mg/L, 8 Fe3+ 0 = 20 μmg/L, oxalate0 = 200 μmg/L, algae = 1.7 × 10 cell/L and pH = 4.0.
30 25 20
(B) 4 3 ln(CSD)
40 mg/L 30 mg/L 20 mg/L 10 mg/L 5 mg/L
35
CSD
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(A) 40
2 1 40mg/L 30mg/L 20mg/L 10mg/L 5mg/L
15 10
0
5
-1
0 0
10
20
30
40
50
60
Figure 5.
0
10
20
30
40
50
60
Time (min)
Time(min)
Effect of SD concentration on the degradation of SD where Fe3+ 0 = 20 μmg/L, oxalate0 = 200 μmg/L and pH = 4.0.
[22,28] indicated that the algae could give off DOM in the course of irradiation, and the majority of which were carboxylic groups. Therefore, the reason for the increase of the degradation rate of SD came from the combination effects of Fe(iii)-carboxylate complexes, Fe(iii) and the secretion of algae, which promotes the formation of hydroxyl radicals. In order to explore the reason of algae effect, the yield of hydroxyl radicals were determined under the same conditions. As shown in Figure 4B, the formation yield of hydroxyl radicals was enhanced by addition of algae in solution containing Fe(iii) and oxalate, indicating that occurrence of algae was benefit to promote the formation of hydroxyl radicals, which could strengthen the degradation of SD. The mechanism of formation of hydroxyl radical by Fe(iii) and algae are as followed:
Fe(iii)-org + hv → Fe(ii) + Org· Org· + O2 → Fe(ii) +
·O− 2
(10)
·O− 2
(11)
+ H+ → Fe(ii) + H2 O2
Fe(ii) + H2 O2 → Fe(iii) + ·OH + HO
−
(12) (13)
where Fe(iii)-org represents a Fe(iii)-organics complex and Org· represents active radicals, respectively.
3.5. Effect of SD concentration The effect of SD concentration in the range of 5–40 mg/L on the degradation rate of SD was investigated with 20 μmg/L Fe(iii) and 200 μmg/L oxalate at pH 4, and the results are given in Figure 5. The degradation rate of SD at low concentrations of SD rapidly decreased, whereas at high concentrations of SD the rate slowly decreased. Also, the degradation rate of SD decreased with an increase in the concentration of SD. At a higher concentration of SD, more SD molecules are oxidized by hydroxyl radicals generated by Fe(iii)-oxalate complexes. However, the amount of hydroxyl radicals formed is a constant amount at a fixed reaction condition, and the relative amount of hydroxyl radicals attacking the SD molecule decreases. Fe(iii)-oxalate complexes can produce hydroxyl radicals by a serial photochemical reaction under UV irradiation and the organic pollutant in solution is cracked by the participation of hydroxyl radicals (Equations (3)–(9)). Assuming that among the reactions taking place in the photocatalytic process, the reaction between the ·OH and organics is the rate-determining step, and the rate equation can be given by Equation (14). r=−
dC = k ∗ [·OH]C dt
(14)
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where C is the concentration of the organics and k ∗ is the reaction rate constant. Under the assumption that the hydroxyl radicals rapidly achieve a constant steady-state concentration, Equation (14) can be written as a pseudo-first-order reaction equation as follows (Equation (15)):
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r=−
dC = k ∗ [·OH]C = kC dt
(15)
where k is the apparent pseudo-first-order constant. The degradation rate constant, k (min−1 ), is determined from the slope of − ln(C/C0 ) = f (t), where C0 and C are the concentration of SD at 0 min and t min, respectively. The degradation rate constant of SD was calculated by analyzing the data of the degradation experiments with a linear-fit method. Also, the experimental data used in each reaction was from the first 15 min of the reactions in order to avoid variations as a result of competitive effects of intermediates and pH change of solution. The analytical result of SD degradation is shown in Figure 5B and is listed in Table 1. There was a good relationship between ln C/C0 and reaction time t under different concentrations of SD, and the correlation coefficients were above 0.99 (see Table 1), thus the experimental data appeared to fit well in the linear kinetic equation and the degradation of SD followed the pseudo-first-order kinetic. Also, the increase in concentration caused a decrease in the rate constant of SD according to the calculated results. This phenomenon could arise from the increase of SD molecules in solution, and the yield of hydroxyl radicals was relatively constant. 3.6.
Mechanism discussion
Algae can generate some smaller molecular weight organics such as DOC through secretion, and DOC may carry a huge variety of functionalities, the majority of which are carboxylic groups and phenolic hydroxyl groups [22,23]. Thus, based on the above discussions, a possible reaction mechanism in solution containing Fe(iii), oxalate and algae is proposed in Figure 6. Since attack by hydroxyl radicals is the main pathway of SD degradation in the reaction, the key intermediates are − Fe(ii), O− 2 ·, HO2 · /O2 · and H2 O2 . The absorption of a photon by Fe(iii)-oxalate complexes or Fe(iii)-org complexes initiate the formation of Fe(ii) and free oxalate radicals or Table 1.
The fitted equation and correlation coefficients.
C0 (mg/L) 5 10 20 30 40
Fitted equation ln C ln C ln C ln C ln C
= 0.04004t + 1.4327 = 0.03162t + 2.2781 = 0.02608t + 2.9272 = 0.01468t + 3.3946 = 0.01044t + 3.6515
k Correlation (min−1 ) coefficient, R2 0.04004 0.03162 0.02608 0.01468 0.01044
0.9914 0.9981 0.9954 0.9913 0.9918
Figure 6. Schematic diagram for Fe cycling and the main reactions in the degradation of SD.
org radical under irradiation. Then the reaction of free radical with O2 results in the formation of O− 2 ·, the reaction of + O− 2 · with H leads to HO2 ·, and then H2 O2 is the product of HO2 · /O− 2 · dismutation. Meanwhile, the simultaneous and rapid formation of Fe(ii) and H2 O2 under irradiation leads to the formation of hydroxyl radicals. Additionally, photocatalysis of Fe(iii)-oxalate complexes have an important influence to natural water since Fe(iii)-oxalate complexes are highly photochemically reactive in aqueous media and their role in mediation photochemical redox cycling in aqueous environment is important. Furthermore, the dissolved O2 possibly occur reduction since photocatalytic process need the participation of O2 , whereas the concentration of CO2 in solution could represent significant increase since CO2 is ultimate product through decarboxylation. The formation of hydroxyl radicals in the Fe(iii) cycling under irradiation, which results in the degradation of SD. The same mechanisms may also act in natural water since Fe(iii), oxalate and algae are commonly present in nature aquatic environment.
4. Conclusion The rapid degradation of SD in a solution containing Fe(iii) and oxalate occurred via oxidation of ·OH generated by Fe(iii)-oxalate complexes under irradiation. The formation of ·OH by Fe(iii)-oxalate complexes were pH-dependent, with an optimal pH for formation of ·OH being 4, and higher or lower pH values resulted in a decrease in the formation of ·OH. The moderate increase in concentration of oxalate was beneficial to formation of ·OH and formation of SD. Algae could enhance degradation rate of SD in solution containing Fe(iii) and oxalate, and the increased amplitude was not very high. The degradation rate of SD in low concentration of SD rapidly decreased, whereas that in high concentration of SD slowly decreased, the pseudo-first-order kinetic described well degradation process of SD. Possible reaction mechanism in solution containing Fe(iii), oxalate and algae
Environmental Technology was proposed, and the attack by ·OH was the main pathway of SD degradation in reaction.
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References [1] M.J. Benotti, R.A. Trenholm, B.J. Vanderford, J.C. Holady, B.D. Standford, and S.A. Snyder, Pharmaceuticals and endocrine disruption compounds in US drinking water, Environ. Sci. Technol. 43 (2009), pp. 597–603. [2] S. Esplugas, D.M. Bila, L.G.T. Krause, and M. Dezotti, Ozonation and advanced oxidation technologies to remove endocrine disrupting chemicals (EDCS) and pharmaceutical and personal care products (PPCPs) in water effluents, J. Hazard. Mater. 149 (2007), pp. 631–642. [3] M. El-Kemary, Y. Abdel-Moneam, M. Madkour, and I. El-Mehasseb, Enhanced photocatalytic degradation of Safranin-O by heterogeneous nanoparticles for environmental applications, J. Lumin. 130 (2010), pp. 2327–2331. [4] E.S. Elmolla and M. Chaudhuri, Degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution by the photo-Fenton process, J. Hazard. Mater. 172 (2009), pp. 1476–1478. [5] E. Hapeshi, A. Achilleos, M.I. Vasquez, C. Michael, N.P. Xekoukoulotakis, D. Mantzavinos, and D. Kassinos, Drugs degradation photocatalytically: Kinetics and mechanism of ofloxacin and atenolol removal on titania suspensions, Water Res. 44 (2010), pp. 1737–1746. [6] A.L. Batt, D.D. Snow, and D.S. Aga, Occurrence of sulfonamide antimicrobials in private water wells in Washington Conty, Idaho, USA, Chemosphere 63 (2006), pp. 1963–1971. [7] M. Masroor, M. Mehrab, and F. Ein-Mozaffari, Using an external-loop airlift sonophotoreactor to enhance the biodegradability of aqueous sulfadiazine solution, Sep. Purif. Technol. 90 (2012), pp. 173–181. [8] J. Raich-Montiu, J. Folch, R. Compañó, M. Granados, and M.D. Prat, Analysis of trace levels of sulfonamides in surface water and soil samples by liquid chromatographyfluorescence, J. Chromatogr. A 1172 (2007), pp. 186–193. [9] V.K. Sharma, S.K. Mishar, and N. Nesnas, Oxidation of sulfonamide antimicrobials by ferrate( vi) [FeVI O42− ], Environ. Sci. Technol. 40 (2006), pp. 7222–7227. [10] J.B. Ellis, Pharmaceutical and personal care products (PPCPs) in urban receiving waters, Environ. Pollut. 144 (2006), pp. 184–189. [11] J. Zhang, D. Fu, H. Gao, and L. Deng, Mechanism of enhanced photocatalytic of TiO2 by Fe3+ in suspensions, Appl. Surf. Sci. 258 (2011), pp. 1294–1299. [12] M.M. Haque and M. Muneer, Photodegradation of norfloxacin in aqueous suspensions of titanium dioxide, J. Hazard. Mater. 145 (2007), pp. 51–57. [13] D. Chebli, F. Fourcade, S. Brosillon, S. Nacef, and A. Amrane, Integration of photocatalysis and biological
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treatment for azo dye removal-application to AR183, Environ. Technol. 32 (2011), pp. 507–514. J. Marugán, R. van Grieken, and C. Pablos, Kinetics and influence of water composition on photocatalytic disinfection and photocatalytic oxidation of pollutants, Environ. Technol. 13 (2010), pp. 1435–1440. O.K. Mahadwad, P.A. Parkh, R.V. Jasra, and C. Patil, Photocatalytic degradation of reactive black-5 dye using TiO2 -impregnated activated carbon, Environ. Technol. 33 (2012), pp. 307–312. Y. Chen, F. Wu, Y. Lin, N. Deng, N. Bazhin, and E. Glebov, Photodegradation of glyphosate in the ferrioxalate system, J. Hazard. Mater. 148 (2007), pp. 360–365. F. Wu and N. Deng, Photochemistry of hydrolytic iron( iii) species and photoinduced degradation of organic compounds: A mini review, Chemosphere 41 (2000), pp. 1137– 1147. J. Guo, Y. Du, Y. Lan, and J. Mao, Photodegradation mechanism and kinetics of methyl orange catalyzed by Fe( iii) and citric acid, J. Hazard. Mater. 186 (2011), pp. 2083–2088. G. Liu, S. Zheng, X. Xing, Y. Li, D. Yin, Y. Ding, and W. Pang, Fe( iii)-oxalate complexes mediated photolysis of aqueous alkylphenol ethoxylates under simulated sunlight conditions, Chemosphere 78 (2010), pp. 402–408. X. Ou, X. Quan, S. Chen, F. Zhang, and Y. Zhao, Photocatalytic reaction by Fe( iii)-citrate complex and its effect on the photodegradation of atrazine in aqueous solution, J. Photochem. Photobiol. A 197 (2008), pp. 382–388. D. Zhou, F. Wu, and N. Deng, Fe( iii)-oxalate complexes induced photooxidation of diethylstilbestrol in water, Chemosphere 57 (2004), pp. 283–291. L. Deng, F. Wu, N. Deng, and Y. Zuo, Photoreduction of mercury( ii) in the presence of algae, Anabaena cylindrical, J. Photochem. Photobiol. B 91 (2008), pp. 117–124. A.M. Braun and M. Maurette, Photochemistry Technology, John Wiley, New York, 1991, p. 80. Z. Peng, F. Wu, and N. Deng, Photodegradation of bisphenol A in simulated lake water containing algae, humic acid and derric ions, Environ. Pollut. 144 (2006), pp. 840–846. X. Liu, F. Wu, Z. Liao, S. Li, and N. Deng, Photodegradation of 17 α-Ethynylestradiol in aqueous solution with Nitzschia hantzschiana or Chlorella vulgaris and Fe3+ , Wuhan Uni. J. Sci. 9 (2004), pp. 109–114. J.F. Rontani, Identification of a ‘pool’ of lipid photoproducts in senescent phytoplnktonic cells, Org. Geochem. 29 (1998), pp. 1215–1225. Y.G. Zuo and J. Hoigen, Formation of hydrogen peroxide and depletion of oxalic acid in atmospheric water by photolysis of iron( iii)-oxalato complexes, Environ. Sci. Technol. 43 (1992), pp. 1014–1022. L. Deng, N. Deng, L. Mou, and F. Zhou, Photo-induced transformations of Hg( ii) species in the presence of Nitzschia hantzschiana, ferric ion and humic acid, J. Environ. Sci. 22 (2010), pp. 76–83.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Photodegradation mechanism of sulfadiazine catalyzed by Fe(III), oxalate and algae under UV irradiation a
Junwei Zhang & Li Ma
a
a
The Key Laboratory of Food Colloids and Biotechnology , Ministry of Education, School of Chemical and Material Engineering, Jiangnan University , Wuxi , China Published online: 31 Jan 2013.
To cite this article: Junwei Zhang & Li Ma (2013) Photodegradation mechanism of sulfadiazine catalyzed by Fe(III), oxalate and algae under UV irradiation, Environmental Technology, 34:12, 1617-1623, DOI: 10.1080/09593330.2013.765915 To link to this article: http://dx.doi.org/10.1080/09593330.2013.765915
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Environmental Technology, 2013 Vol. 34, No. 12, 1617–1623, http://dx.doi.org/10.1080/09593330.2013.765915
Photodegradation mechanism of sulfadiazine catalyzed by Fe(III), oxalate and algae under UV irradiation Junwei Zhang∗ and Li Ma The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, China
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(Received 31 March 2012; final version received 24 December 2012 ) Photodegradation mechanism of sulfadiazine (SD) in a solution containing Fe(iii), oxalate and algae were investigated in this study. The results indicated that the degradation of SD was slow in a solution containing Fe(iii) or oxalate, whereas it was markedly enhanced when Fe(iii) and oxalate coexisted. The optimal pH for formation of ·OH was 4; a higher or lower pH resulted in a decrease in formation of ·OH. A moderate increase of oxalate concentration was beneficial to the formation of ·OH and the degradation of SD, and the algae enhanced the degradation rate of SD in a solution containing Fe(iii) and oxalate. Also, the degradation rate of SD rapidly decreased at low initial concentrations but slowly decreased at high initial concentrations, and pseudo-first order kinetics described the degradation process of SD well. A possible reaction mechanism in solution containing Fe(iii), oxalate and algae was proposed, and attack by ·OH was the main pathway of SD degradation in the photocatalytic reaction. Keywords: sulfadiazine; Fe(iii); oxalate; algae; photodegradation
1. Introduction Antibiotics, a subclass of pharmaceuticals, have been receiving increased attention due to their existence in aquatic environments. Therefore the development of antibiotic resistant bacteria on aquatic life in aquatic environments is becoming a problem [1–5]. Sulfadiazine (SD), a synthetic antibacterial agent of sulfonamides, is used in human and veterinary medicine, and has been used to treat a broad variety of Gram(+) and Gram(−) bacterial infections [6,7]. SD is a sulfonamide that is frequently detected in the environment in relatively high concentrations (ranging from micrograms to milligrams per litre), and the presence of sulfonamides have been reported in wastewater, surface water, ground water, and even drinking water [8,9], and the widespread occurrence of SD in different water types has led to increasing bacterial resistance [2,6]. Due to their high chemical stability and low biodegradability, biological or physicochemical techniques for the removal of antibiotics is not efficient enough, and they are therefore introduced into aquatic environments [2,4,10]. Thus, the development of new purification technologies for wastewater leading to the complete destruction of these antibiotics has both an important theoretical and practical value. Photocatalytic degradation is a promising treatment technology for the elimination micropollutants in aqueous solution, and these micropollutants can be decomposed by the participation of hydroxyl radicals generated by different ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
photocatalytic systems [11–15]. Over the past two decades, the photochemical activity of complexes of Fe(iii) and carboxylate anions has received consideration attention. Chen et al. [16], Wu et al. [17] and Guo et al. [18] reported that Fe(iii) and carboxylate anions could form stable complexes that exhibited high photoreactivity, and found that the organic compounds could be degraded gradually. The formation of Fe(ii) and hydroxyl radicals through a ligand-to-metal charge transfer path can be generated by Fe(iii)-carboxylate complexes, and the hydroxyl radicals can oxidize most organic compounds quickly and nonselectively due to their high oxidation potential value (E 0 = +2.80 V) [18,19]. Thus, Fe(iii) and carboxylate, two ubiquitous compounds in nature, can result in a photochemical reaction which plays an important role in the photodegradation of pollutants in natural water [17,18,20]. Some studies have indicated that Fe(iii)-carboxylate complexes have high photocatalytic efficiencies in the degradation of organic compounds [16–21]. Algae, important microorganisms in water, can secrete some lower molecular weight organic compound [22]. Thus, the degradation rate of an organic pollutant may be enhanced by the addition of algae in solution containing Fe(iii) and carboxylate. To the best of our knowledge, much is known about the effect of carboxylate anions on the photochemical reaction, however, the photodegradation of antibiotics by Fe(iii), carboxylate and algae has not been investigated.
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2. Experimental 2.1. Materials SD (purity ≥ 98%) purchased from Sigma-Aldrich Chemical Company (America) was used without further treatment. FeCl3 · 6H2 O and Na2 C2 O4 were of analytical grade and obtained from Shanghai Chemical Co. (China). The other chemicals used in this study were of an analytical grade (Shanghai Reagent Company, China). Deionized water was used to prepare the solutions in the experiments. All stock solutions were stored in a refrigerator at 4◦ C in the dark prior to use. The alga used in the experiments was Chlorella vulgaris, which obtained from the Wuhan Hydrobilology Institute of Chinese academy of sciences (China). The algae were grown in an axenic culture medium at 25◦ C using a 24 h light cycle in a culturing room equipped with a constant temperature air-conditioner. Under the condition of logarithmic growth, the algae were taken for use in experiments after being washed. The cell counting was carried out under inverted microscope at 400× and the density of algae (cell/L) was carefully calculated. The concentration of algae was gained by diluting washed algae with the deionized water. All glassware used in experiments was cleaned by soaking in 1 M HCl for 6 h and thoroughly rinsed with tap water and then deionized water.
dark, and the pH of the solutions were adjusted with dilute sulfuric acid and sodium hydroxide solution. Then, the reaction solutions were introduced into the photoreaction vessel and stirred with magnetic stirrers and sparged with air (0.5 L/min). At intervals, samples were collected and determined immediately by high performance liquid chromatography (HPLC) in order to avoid further reaction.
2.3. Analysis The concentration of SD was detected by HPLC (LC-10AT, Kromasil C18 column [150 × 4.6 mm, 5 μm], Shimadzu, Japan) with a flow rate of 1.5 mL/min at 277 nm. The mobile phase consisted of an 85/15 ratio of aqueous phase to acetonitrile (v/v), and the aqueous phase was 2.5 mM sodium-1-heptanesulfonate adjusted to pH 2 with H3 PO4 . A standard solution of phenol was used to calibrate the HPLC for the quantification of the formation yield of the hydroxyl radicals in the reaction since the aromatic hydroxylation was considered to be one typical reaction of hydroxyl radicals [15]. Moreover, it was assumed that the hydroxyl radicals-mediated oxidation of benzene formed phenol with nearly 100% yield and that phenol concentration represented the concentration of hydroxyl radicals.
3.
Results and discussion
3.1. Degradation of SD As shown in Figure 1, there was no chemical reaction between SD and the photocatalyst in the dark since the concentration of SD was stable. Under the irradiation of a 15 W medium pressure mercury lamp, the degradation rate of SD after 60 min was about 7%, this phenomenon could arise direct photolysis and chemical attenuation of SD. Moreover, degradation rate of SD was 17% in solution containing oxalate after 60 min
1.0 0.8
2.2. Procedures The photodegradation of SD was carried out in a cylindrical reactor (self-made, 500 mL capacity, with a diameter and length of 6 cm and 25 cm, respectively) equipped with a 15 W medium pressure mercury lamp (λmax ≥ 254 nm), which was positioned in the photoreactor. The lamp was surrounded with a quartz jacket and the tap water cooling circuit maintained the solution temperature at 25 ± 1◦ C. The incident of photo flux measured by the potassium ferrioxalate actinometry was 1.32 × 10−4 Einstein/(L·min) [23]. In a typical run, a mixture containing known concentration of SD, Fe3+ and C2 O2− 4 were prepared in the
SD (C/C0)
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In this study, the degradation process of SD catalyzed by Fe(iii), oxalate and algae was investigated under ultraviolet (UV) irradiation. The objectives of this study were as follows: (i) to examine the effect of several factors on the degradation of SD in an irradiated solution containing Fe(iii) and oxalate; (ii) to analyze the kinetics of SD catalyzed by Fe(iii) and oxalate; (iii) to elucidate the photochemical degradation potential and mechanism of SD in aqueous solutions containing Fe(iii), oxalate and algae. SD was selected as the model pollutant in this paper since it is a typical antibiotic that is widely used in aquaculture and animal husbandry. Fe(iii), oxalate and algae were applied since they are common environmental constituents.
0.6 0.4
Dark control Photolysis Oxalate Fe(III) Fe(III)+oxalate
0.2 0.0 0
10
20
30
40
50
60
Time (min)
Figure 1. Degradation of SD in different reaction systems where SD0 = 20 mg/L, Fe3+ 0 = 20 μmg/L, oxalate0 = 200 μmg/L, and pH = 4.0.
Environmental Technology irradiation. The formation of the oxidant generated by the photolysis of oxalate results in a small increase in the degradation rate of SD under acidic conditions [17,21,24]. The degradation rate of SD was about 39% in solution containing Fe(iii) after 60 min irradiation. This may be due to formation of hydroxyl radicals (·OH) that may absorb UV light to generate ·OH (Equations (1)–(2)) [17,19,20], and ·OH can oxidize most organic compounds non-selectively and mineralize them to carbon dioxide, water and other ions owing to its high oxidation potential (E 0 = +2.80 V) [17,18]. Fe(iii) + H2 O → Fe(OH)
2+
2+
+ hv → Fe(ii) + ·OH
(2)
Furthermore, the degradation rate of SD was enhanced by the addition of oxalate in a solution containing Fe(iii). After 60 min irradiation, the degradation rate of SD was about 80%, and the removal of SD was much more efficient in a solution containing Fe(iii) and oxalate. This phenomenon arises from the formation of Fe(iii)-carboxylate complexes, which are highly photocatalytic compounds, and ·OH can be generated by Fe(iii)-carboxylate complexes via a ligand-to-metal charge transfer [16,18]. 3.2. Formation of hydroxyl radicals The formation of ·OH could be induced and accumulated in different conditions with an increasing reaction time although dark control and photolysis of benzene showed negligible loss of benzene under irradiation (Figure 2A). The results also indicated that the formation yield of ·OH in a solution containing Fe(iii) or oxalate was smaller than in a solution containing Fe(iii) and oxalate, and the formation yield of ·OH could be significantly enhanced. Up to 2.54 μmol/L of ·OH was produced in a solution containing Fe(iii) and oxalate after 60 min irradiation. Moreover, the formation of ·OH is responsible for the degradation of SD in different conditions, and the formation of ·OH from photocatalysis may be the key to result in photodegradation (A) 3.0
of SD (Equations (3)–(9)). FeIII (Oxa) + hv → FeII -Oxa· → Fe(II) + Oxa− · Oxa− · + O2 → H+ +
O− 2·
O− 2·
+ CO2
↔ HO2 ·
(3) (4) (5)
HO2 ·/O− 2 · + Fe(ii) → Fe(iii) + H2 O2 HO2 ·/O− 2 · + Fe(iii) → Fe(ii) + O2 H2 O2 + Fe(ii) → Fe(iii) + ·OH + − ·OH
(8)
SD + ·OH → . . . → degradation products
(9)
(6) (7)
The formation yield of ·OH depends on the concentration of Fe(iii)-oxalate complexes which are influenced by pH of solution. As shown in Figure 2B, the formation yield of ·OH increased firstly and then decreased within a pH range of 3.0–6.0, indicating that it was unfavourable to the formation of ·OH at higher pH values, whereas it was beneficial to the formation of ·OH at lower pH values. The formation yield of the ·OH radicals dependence on pH is considered to mainly result from distribution of Fe(iii) and oxalate species. The distribution of Fe(iii) and oxalate species against pH was simulated by Yong et al. [16] with the MEDUSA software, and the fraction of Fe(OH)2+ was negligible and the ferrioxalate ions constituted of a main Fe(iii) species in the ferrioxalate system and Fe(C2 O4 )2− 2− and Fe(C2 O4 )3− 3 were the main species at pH 4, Fe(C2 O4 ) 3− and Fe(C2 O4 )3 displayed fairly strong photochemical activity. However, some of the photoreactive substances should be FeC2 O+ 4 and occur at pH 2, which was much less efficiently photolyzed than Fe(C2 O4 )2− and Fe(C2 O4 )3− 3 , moreover Fe2 O3 occurred when the pH was above 5. 3.3. Effect of Fe( III)/oxalate ratio The ratio of Fe(iii) to oxalate plays an important role in the degradation of SD since the Fe(iii)/oxalate ratio has an important effect on the formation of Fe(iii)-oxalate complexes and the yield of ·OH. As shown in Figure 3A, the concentration increase of oxalate ions could enhance the (B) 3.0
Dark control Photolysis Oxalate Fe(III) Fe(III)+oxalate
2.5 2.0
Hydroxyl radicals (µmol/L)
Hydroxyl radicals (µmol/L)
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Fe(OH)
(1)
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1.5 1.0 0.5
pH = 3.0 pH = 4.0 pH = 5.0 pH = 6.0
2.5 2.0 1.5 1.0 0.5 0.0
0.0 0
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40
Time (min)
50
60
0
10
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30
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50
60
Time (min)
Figure 2. Formation of hydroxyl radicals under different conditions: (A) Fe3+ 0 = 20 μmg/L, oxalate0 = 200 μmg/L and pH = 4.0; (B) Fe3+ = 20 μmg/L, oxalate = 200 μmg/L within a pH range of 3.0–6.0. 0 0
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SD (C/C0)
0.8
Fe(III)/oxalate = 20µmol/L:100µmol/L
(B) 3.5
Fe(III)/oxalate = 20µmol/L:200µmol/L Fe(III)/oxalate = 20µmol/L:400µmol/L
3.0
Hydroxyl radicals (µmol/L)
(A)1.0
0.6 0.4 0.2
2.5 2.0 1.5 1.0 0.5 0.0
0
10
20
30
40
50
60
Time (min)
Fe(III)/oxalate = 20µmol/L:100µmol/L Fe(III)/oxalate = 20µmol/L:200µmol/L Fe(III)/oxalate = 20µmol/L:400µmol/L
0
10
20
30
40
50
60
Time (min)
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Figure 3. Effect of Fe(iii)/oxalate ratio on degradation of SD (A) and formation yield of hydroxyl radicals (B) where SD0 = 20 mg/L and pH = 4.0.
degradation rate of SD and greatly shorten the degradation time. At a given pH value, the Fe(iii)/oxalate ratio can determine the species distribution of Fe(iii)-oxalate complexes in solution. Ou et al. [20] noted that part of a carboxylic acid was deprotonated by Fe(iii) with ligand exchange and was ultimately transformed to carboxylate. An increase in citrate concentration easily formed these complexes for Fe(iii) and carboxyl groups and the complexes that formed at high citrate concentrations exhibited higher photoabsorption. Chen et al. [16] indicated that increase of logarithmic concentration of C2 O− 4 was beneficial for the formation of ferrioxalate ions which dominated in solution and increased the photoreactivity. However, an excessive increase of the oxalate concentration has negative impact on the degradation of SD since the superabundant Fe(iii)-oxalate complexes ion will oxidize the oxalate and the oxalate radicals, which reduce the formation of Fe(iii)-oxalate complexes and prevent the reduction of O2 and the formation of H2 O2 (Equations (4)– (6)). The results in Figure 3A also indicated that an increase in the oxalate concentration never led to a rapid increase in the degradation rate of SD. In order to explore the reason of the Fe(iii)/oxalate ratio effect, the yield of the hydroxyl radicals were determined under the same conditions. As shown in Figure 3B, the increase in the oxalate concentration enhanced the yield of hydroxyl radicals, and the increase in the yield of the hydroxyl radicals was not as noticeable than at the beginning when the Fe(iii)/oxalate ratio was 1/20. This indicates that the excessive increase in the oxalate concentration greatly affects the formation yield of the hydroxyl radicals. Therefore the formation of the hydroxyl radicals is in agreement with the degradation of SD, and the formation of the hydroxyl radicals has an important role in the degradation of SD. 3.4. Effect of algae Algae as an important type of microorganism in aqueous environments, as they can generate some smaller molecular
weight organics such as dissolved organic carbon (DOC) through secretion [25], and these DOC molecules may carry a huge variety of functional groups, the majority of which are carboxylic groups (humic and fulvic acid etc.) and phenolic hydroxyl groups [24], which are photosensitizers and lead to indirect photo-oxidative reactions. Rontani indicated that the photodegradation of different lipid compounds in suspensions of algae could form dicarboxylic acids [26], and the carboxylic acid could coordinate with Fe(iii) ions to produce more hydroxyl radicals after UV irradiation [27]. Thus, algae may accelerate the degradation process of organic contaminants in aqueous environments and it is interesting to investigate the effect of algae on the degradation of SD. As shown in Figure 4A, the solution containing algae which led to degradation of SD and degradation rate of SD increased with prolonging reaction time. After 60 min irradiation, up to 23% of SD was degraded in a solution containing algae. However, the degradation rate of SD was enhanced in the ternary photocatalytic system (Fe(iii), oxalate and algae), and the increased amplitude was not very high. Up to 92% of SD was removed in the ternary photocatalytic system after 60 min irradiation. This indicated that the occurrence of algae in aqueous environments can strengthen the degradation of organic contaminants. Fe(iii) can form Fe(iii)-carboxylate complexes with carboxylic anions, which are more photoreactive for the production of hydroxyl radicals [18]. Also, it has been reported that algae may release acidic dissolved organic matter (DOM), which contain low molecular weight carboxylic acids [28]. Liu et al. [25] reported that the degradation rate of 17 α-ethynylestradiol could be enhanced by the addition of algae in a solution containing Fe(iii). Peng et al. [24] studied the degradation of bisphenol A in simulated lake water containing algae, humic acid and ferric ions, and found that the algae could enhance the photodegradation of bisphenol A under a near UV light. The reason for the rapid degradation could arise from the secretion of algae, which could promote the formation of hydroxyl radicals. Deng et al.
Environmental Technology (B) 3.5 Hydroxyl radicals (µmol/L)
(A) 1.0 0.8 SD (C/C0)
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0.6 0.4 Algae Fe(III)+oxalate Fe(III)+oxalate+algae
0.2
Algae Fe(III)+oxalate Fe(III)+oxalate+algae
3.0 2.5 2.0 1.5 1.0 0.5 0.0
0.0 0
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60
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Time (min)
Time (min)
Figure 4. Effect of algae on degradation of SD (A) and formation yield of hydroxyl radicals (B) where SD0 = 20 mg/L, 8 Fe3+ 0 = 20 μmg/L, oxalate0 = 200 μmg/L, algae = 1.7 × 10 cell/L and pH = 4.0.
30 25 20
(B) 4 3 ln(CSD)
40 mg/L 30 mg/L 20 mg/L 10 mg/L 5 mg/L
35
CSD
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(A) 40
2 1 40mg/L 30mg/L 20mg/L 10mg/L 5mg/L
15 10
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5
-1
0 0
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Figure 5.
0
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60
Time (min)
Time(min)
Effect of SD concentration on the degradation of SD where Fe3+ 0 = 20 μmg/L, oxalate0 = 200 μmg/L and pH = 4.0.
[22,28] indicated that the algae could give off DOM in the course of irradiation, and the majority of which were carboxylic groups. Therefore, the reason for the increase of the degradation rate of SD came from the combination effects of Fe(iii)-carboxylate complexes, Fe(iii) and the secretion of algae, which promotes the formation of hydroxyl radicals. In order to explore the reason of algae effect, the yield of hydroxyl radicals were determined under the same conditions. As shown in Figure 4B, the formation yield of hydroxyl radicals was enhanced by addition of algae in solution containing Fe(iii) and oxalate, indicating that occurrence of algae was benefit to promote the formation of hydroxyl radicals, which could strengthen the degradation of SD. The mechanism of formation of hydroxyl radical by Fe(iii) and algae are as followed:
Fe(iii)-org + hv → Fe(ii) + Org· Org· + O2 → Fe(ii) +
·O− 2
(10)
·O− 2
(11)
+ H+ → Fe(ii) + H2 O2
Fe(ii) + H2 O2 → Fe(iii) + ·OH + HO
−
(12) (13)
where Fe(iii)-org represents a Fe(iii)-organics complex and Org· represents active radicals, respectively.
3.5. Effect of SD concentration The effect of SD concentration in the range of 5–40 mg/L on the degradation rate of SD was investigated with 20 μmg/L Fe(iii) and 200 μmg/L oxalate at pH 4, and the results are given in Figure 5. The degradation rate of SD at low concentrations of SD rapidly decreased, whereas at high concentrations of SD the rate slowly decreased. Also, the degradation rate of SD decreased with an increase in the concentration of SD. At a higher concentration of SD, more SD molecules are oxidized by hydroxyl radicals generated by Fe(iii)-oxalate complexes. However, the amount of hydroxyl radicals formed is a constant amount at a fixed reaction condition, and the relative amount of hydroxyl radicals attacking the SD molecule decreases. Fe(iii)-oxalate complexes can produce hydroxyl radicals by a serial photochemical reaction under UV irradiation and the organic pollutant in solution is cracked by the participation of hydroxyl radicals (Equations (3)–(9)). Assuming that among the reactions taking place in the photocatalytic process, the reaction between the ·OH and organics is the rate-determining step, and the rate equation can be given by Equation (14). r=−
dC = k ∗ [·OH]C dt
(14)
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where C is the concentration of the organics and k ∗ is the reaction rate constant. Under the assumption that the hydroxyl radicals rapidly achieve a constant steady-state concentration, Equation (14) can be written as a pseudo-first-order reaction equation as follows (Equation (15)):
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r=−
dC = k ∗ [·OH]C = kC dt
(15)
where k is the apparent pseudo-first-order constant. The degradation rate constant, k (min−1 ), is determined from the slope of − ln(C/C0 ) = f (t), where C0 and C are the concentration of SD at 0 min and t min, respectively. The degradation rate constant of SD was calculated by analyzing the data of the degradation experiments with a linear-fit method. Also, the experimental data used in each reaction was from the first 15 min of the reactions in order to avoid variations as a result of competitive effects of intermediates and pH change of solution. The analytical result of SD degradation is shown in Figure 5B and is listed in Table 1. There was a good relationship between ln C/C0 and reaction time t under different concentrations of SD, and the correlation coefficients were above 0.99 (see Table 1), thus the experimental data appeared to fit well in the linear kinetic equation and the degradation of SD followed the pseudo-first-order kinetic. Also, the increase in concentration caused a decrease in the rate constant of SD according to the calculated results. This phenomenon could arise from the increase of SD molecules in solution, and the yield of hydroxyl radicals was relatively constant. 3.6.
Mechanism discussion
Algae can generate some smaller molecular weight organics such as DOC through secretion, and DOC may carry a huge variety of functionalities, the majority of which are carboxylic groups and phenolic hydroxyl groups [22,23]. Thus, based on the above discussions, a possible reaction mechanism in solution containing Fe(iii), oxalate and algae is proposed in Figure 6. Since attack by hydroxyl radicals is the main pathway of SD degradation in the reaction, the key intermediates are − Fe(ii), O− 2 ·, HO2 · /O2 · and H2 O2 . The absorption of a photon by Fe(iii)-oxalate complexes or Fe(iii)-org complexes initiate the formation of Fe(ii) and free oxalate radicals or Table 1.
The fitted equation and correlation coefficients.
C0 (mg/L) 5 10 20 30 40
Fitted equation ln C ln C ln C ln C ln C
= 0.04004t + 1.4327 = 0.03162t + 2.2781 = 0.02608t + 2.9272 = 0.01468t + 3.3946 = 0.01044t + 3.6515
k Correlation (min−1 ) coefficient, R2 0.04004 0.03162 0.02608 0.01468 0.01044
0.9914 0.9981 0.9954 0.9913 0.9918
Figure 6. Schematic diagram for Fe cycling and the main reactions in the degradation of SD.
org radical under irradiation. Then the reaction of free radical with O2 results in the formation of O− 2 ·, the reaction of + O− 2 · with H leads to HO2 ·, and then H2 O2 is the product of HO2 · /O− 2 · dismutation. Meanwhile, the simultaneous and rapid formation of Fe(ii) and H2 O2 under irradiation leads to the formation of hydroxyl radicals. Additionally, photocatalysis of Fe(iii)-oxalate complexes have an important influence to natural water since Fe(iii)-oxalate complexes are highly photochemically reactive in aqueous media and their role in mediation photochemical redox cycling in aqueous environment is important. Furthermore, the dissolved O2 possibly occur reduction since photocatalytic process need the participation of O2 , whereas the concentration of CO2 in solution could represent significant increase since CO2 is ultimate product through decarboxylation. The formation of hydroxyl radicals in the Fe(iii) cycling under irradiation, which results in the degradation of SD. The same mechanisms may also act in natural water since Fe(iii), oxalate and algae are commonly present in nature aquatic environment.
4. Conclusion The rapid degradation of SD in a solution containing Fe(iii) and oxalate occurred via oxidation of ·OH generated by Fe(iii)-oxalate complexes under irradiation. The formation of ·OH by Fe(iii)-oxalate complexes were pH-dependent, with an optimal pH for formation of ·OH being 4, and higher or lower pH values resulted in a decrease in the formation of ·OH. The moderate increase in concentration of oxalate was beneficial to formation of ·OH and formation of SD. Algae could enhance degradation rate of SD in solution containing Fe(iii) and oxalate, and the increased amplitude was not very high. The degradation rate of SD in low concentration of SD rapidly decreased, whereas that in high concentration of SD slowly decreased, the pseudo-first-order kinetic described well degradation process of SD. Possible reaction mechanism in solution containing Fe(iii), oxalate and algae
Environmental Technology was proposed, and the attack by ·OH was the main pathway of SD degradation in reaction.
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