Environ. Sci. Technol. 2009, 43, 3784–3788
Iodide-Mediated Photooxidation of Arsenite under 254 nm Irradiation
I-+ H2O + hν(254 nm) f I-H2O*
(1)
I-H2O* f (I • , e-) + H2O
(2)
-
-
(I • , e ) f I • + e JIMAN YEO AND WONYONG CHOI* School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea
(3b)
I • + I • f I2
(4)
I • + I- f I2-
-
-
I 2 + I 2 T I3 + I
Introduction Arsenic is one of the most significant water contaminants of natural origin, and a large amount of the population in developing countries are at risk because of widespread arsenic pollution of groundwater (1, 2). Arsenic contamination originates mainly from natural weathering or dissolution of As-containing minerals that contain arsenite [As(III)] and/ or arsenate [As(V)] (3). Among the two commonly occurring arsenic compounds, arsenite As(III), is more toxic, more mobile, and more difficult to remove by coagulation or precipitation because of its lower affinity for adsorbents than that of arsenate As(V) (4). Therefore, the preoxidation of As(III) to As(V) is a highly desirable process to increase the removal efficiency of arsenic in water treatment. Various treatment methods for the oxidation of As(III) have been reported in the literature; As(III) can be oxidized by ozone (5), Fenton reaction (6, 7), manganese dioxide (8-11), UV/iron (12-14), and TiO2/UV (4, 15-19). In this work, we propose the photooxidation of As(III) in the presence of iodide and 254 nm irradiation as a new preoxidation method. Iodides are photochemically active under 254 nm irradiation (20, 21) * Corresponding author e-mail:
[email protected]; fax: +8254-279-8299. 3784
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(3a)
(I • , e-) + O2 f I • + O2•-
Received September 5, 2008. Accepted March 24, 2009.
The preoxidation of As(III) to As(V) is a desirable process to increase the removal efficiency of arsenic in water treatment. In this work, the photooxidation of As(III) under 254 nm irradiation was investigated in the concentration range of 1-1000 µM in the presence of potassium iodide (typically 100 µM). Although the direct photooxidation of As(III) in water was negligible, the presence of iodide dramatically enhanced the oxidation rate. The quantitative conversion of As(III) to As(V) was achieved. The quantum yields of As(III) photooxidation ranged from 0.08 to 0.6, depending on the concentration of iodide and As(III). The excitation of iodides under 254 nm irradiation led to the generation of iodine atoms and triiodides, which seem to be involved in the oxidation process of As(III). Because the efficiency of iodine atom generation is highly dependent on the presence of suitable electron acceptors, the photooxidation of As(III) was efficient in an air- or N2O-saturated solution but markedly reduced in the N2-saturated solution. The production of H2O2 was also accompanied by the generation of As(V). The addition of excess methanol (OH radical scavenger) did not reduce the photooxidation rate at all, which ruled out the possibility of hydroxyl radical involvement. It was found that the in situ photogenerated triiodides oxidize As(III) with regenerating iodides by completing a cycle. The proposed UV254/KI/As(III) process is essentially an iodide-mediated photocatalysis.
aq
(5) -
(6)
I2+ I- T I3- (K ) 700)(22)
(7)
Light absorption by iodide (reaction 1) in water generates a caged complex, containing an iodine atom and an electron (I•, e-) through a charge transfer (reaction 2). The caged complex may dissociate into an aqueous electron and an iodine atom (reaction 3a), preferably in the presence of an electron acceptor (reaction 3b). Reactions 4-7 of iodine atoms lead to the generation of iodine, an iodine anion radical, and triiodide. The spectrophotometric determination of the triiodide at 352 nm is the basis of the iodide chemical actinometer for 254 nm irradiation (20). Such a unique photochemistry of iodide can be coupled with the oxidation of As(III). In this study, the role of iodide in the photooxidation of As(III) and the mechanism of the newly proposed process were investigated in detail. We propose that the UV254/KI/ As(III) process is essentially an iodide-mediated photocatalytic process.
Experimental Section Chemicals. NaAsO2 [As(III), 96.7%, Aldrich, U.S.A.] and Na2HAsO4 · 7H2O [As(V), 99%, Kanto] were used as the source of arsenic. KI (99.5%, Samchun Chemical) was used as the iodide source. FeSO4 · 7H2O [Fe(II), 99.0%, Aldrich, USA] was used as the Fe(II) source. I2 (99.8%, Shinyo), KIO3 (99.96%, Yanagishima), KIO4 (99.8%, Aldrich), Na2B4O7 · 10H2O (99%, Junsei), and methanol (99.8%, Kanto) were of reagent grade and used as received. N2 (BOC Gases, 99.999% purity) and N2O (Sinan Gases, Korea, 99.999% purity) gases were used when the effect of the dissolved gas was investigated. Ultrapure deionized water (18 MΩ cm) prepared by a Barnstead purification system was used in all experiments. Photolyses and Analyses. The initial arsenite concentrations employed in this work were 1, 10, 50, 100, 500, and 1000 µM. The pH of the solutions was adjusted with HClO4 or NaOH solution to a desired value. The photooxidation experiments were carried out in a 4 mL quartz cell reactor without stirring. All experiments were performed in a duplicate or triplicate set to ensure reproducibility. Photoirradiation at λ ) 254 nm employed a 15 W germicidal lamp (GLD15MQ, Sankyo, Japan) as the light source. The distance between the lamp and the photoreactor was 11 cm. The incident light intensity was measured by iodide/iodate actinometry (21). A standard solution consisting of 0.6 M KI, 0.1 M KIO3, and 0.01 M borate buffer (pH 9.25) was prepared, and the following irradiation of the actinometer solution resulted in the formation of triiodide, which was quantified by measuring its absorbance at 352 nm (ε ) 26400 M-1cm-1). The absorbance-time curve showed a good linearity (R2 ∼0.99) with the present photoreactor setup, and the incident light intensity (Ii) was determined to be 3.70((0.25) × 10-6 Einstein L-1 sec-1. On the basis of the molar absorptivity of iodide at 254 nm (ε ) 192.5 M-1cm-1), the light intensity absorbed (Ia) by the iodide solution of 10, 100, and 1000 µM should be 1.6 × 10-8, 1.6 × 10-7, and 1.3 × 10-6 Einstein 10.1021/es900602n CCC: $40.75
2009 American Chemical Society
Published on Web 04/09/2009
L-1 sec-1, respectively. Light absorption by arsenites at 254 nm is insignificant compared with the light absorption by iodides in the present experimental condition. When the effect of the irradiation of λ > 300 nm on the photooxidation of As(III) was tested as a control, a 300 W Xe arc lamp (Oriel) was used as an alternative light source. The incident light passed through a 10 cm infrared (IR) water filter and a ultraviolet (UV) cutoff filter (λ > 300 nm), and then the filtered light was focused onto the quartz cell reactor. For the photooxidation experiments in the absence of dissolved oxygen, the reactor was purged with nitrogen or nitrous oxide gas for 20 min prior to irradiation and continuously during irradiation. When needed, methanol (20 mM) was added as a hydroxyl radical scavenger. Identification and quantification of photogenerated arsenate As(V), when using [As(III)]0 of 100 to 1000 µM, were performed using an ion chromatograph (IC, Dionex DX-120), with a detection limit of ∼5 µM, that was equipped with a Dionex IonPac AS 14 (4 mm × 250 mm) column and a conductivity detector (16, 18). The eluent was a mixture of 3.5 mM Na2CO3 and 1 mM NaHCO3 solutions. The IC calibration for [As(V)] was done using the standard aqueous solutions of Na2HAsO4 in the concentration range up to 2000 µM with a satisfactory linearity (R2 ∼0.98). The lower As concentrations (from 1 to 50 µM) were analyzed by using a graphite furnace atomic absorption spectrometry (GFAAS, PerkinElmer 5100 spectrophotometer), within the detection limit of ∼5 ppb (23). The AAS calibration for arsenic was carried out in a 10-100 ppb range using commercial standard arsenic solutions. For the As speciation analysis, the sample solutions were allowed to pass through a silica-based anion exchange cartridge (LC-SAX SPE Tube, Supelco) that retained As(V); As(III) was collected in the effluent solution. The concentrations of As(III) were analyzed by GFAAS using this effluent solution. The concentrations of in situ-generated As(V) were calculated from the difference of the total arsenic concentration (the concentration before passing the cartridge tube) and [As(III)]. The concentration of photogenerated triiodide (I3-) was determined by measuring the absorbance at 352 nm (ε ) 26400 M-1cm-1) using a UV/visible (vis) spectrophotometer (Agilent 8453) (24). The concentration of photogenerated H2O2 was measured by the DPD (N,Ndiethyl-p-phenylenedimine) method, with the detection limit of 0.8 µM (25, 26). The analyses of triiodide and H2O2 were always carried out immediately after the sampling to minimize analysis error (from dark decay) and done within a minute. All the analyses were done at least twice with the same sample and most were within (15% uncertainty.
Results and Discussion Photooxidation of As(III) with Iodide. Figure 1 shows the time profiles of the removal of As(III) and the concurrent production of As(V) under 254 nm radiation in the absence or presence of iodide. Three different initial concentrations of [As(III)]0 ) 1, 10, and 50 µM at pH 7 that are relevant to the condition of natural water pollution were tested. In all cases, the direct photooxidation of As(III) in the absence of iodide was negligibly slow, whereas the presence of iodide (100 µM) drastically enhanced the removal rate of As(III) with the quantitative production of As(V) accompanied. The 254 nm irradiation with iodide quantitatively oxidized As(III) to As(V). Figure 1c shows the effect of the presence of Fe(II) (10 µM) on the photooxidation of As(III) because the ferrous ions are likely to be copresent under the anoxic reducing conditions where As(III) is found. The presence of Fe(II) had no effect on the oxidation rate, which implies that the presence of other inorganic ions may not significantly influence the iodide-enhanced photooxidation of As(III). The observed iodide effect on the photooxidation of As(III) needs to be studied systematically by varying the experimental parameters and monitoring the intermediate species
FIGURE 1. Time profiles of the removal of As(III) and the concurrent production of As(V) under 254 nm radiation in the absence or presence of iodide. Experimental conditions: air equilibrated; pHi ) 7.0; [As(III)]0 ) (a) 1 µM, (b) 10 µM, and (c) 50 µM; [KI]0 ) 100 µM; and [Fe(II)]0 ) 10 µM (when indicated). (e.g., I3- and H2O2) involved with the photooxidation process. However, the concentrations of such intermediates generated with the low concentration of arsenic were too low to be analyzed reliably. Therefore, detailed mechanistic investigations were carried out using a much higher initial concentration of As(III) at 1000 µM. Figure 2a compares the time profiles of As(V) generation from the photooxidation of As(III) in the presence of different initial concentrations of iodide. The iodide-enhanced effect was similarly observed in the high concentration region. The photooxidation of As(III) was insignificant in the absence of iodide but markedly enhanced with an increasing iodide concentration. The iodideenhanced effect was mostly saturated with [KI] g 500 µM. The quantum yield (Φ) of arsenite photooxidation at 254 nm radiation was determined by the following equation 2 Φ)
( d[As(V)] dt )
i
(8)
Ia
where Ia is the light intensity absorbed by the KI solution, and the production rate of As(V) is averaged for the initial 30 min. As we mentioned previously, Ia is strongly dependent on [KI]. Φ values determined with [As(III)]0 ) 1000 µM and [KI]0 of 100, 500, 1000, and 2000 µM are 0.61, 0.45, 0.44, and VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Time profiles of (a) the production of As(V) and (b) the accompanying in situ generation of H2O2 under different dissolved gas conditions (air, N2, and N2O). Experimental conditions: gas (air-, N2-, or N2O-saturated), pHi ) 3.0, [As(III)]0 ) 1000 µM, and [KI]0 ) 500 µM. reaction with triiodide (reaction 12), which is thermochemically spontaneous at acidic conditions (∆E ) 18 mV at pH 3).
FIGURE 2. Time profiles of (a) the production of As(V) and the accompanying in situ generation of (b) I3- (triiodide) and (c) H2O2 from the photooxidation of As(III) in the presence of different initial concentrations of iodide. Experimental conditions: air equilibrated, pHi ) 3.0, and [As(III)]0 ) 1000 µM. 0.36, respectively. Although the overall photooxidation rate of As(III) was higher with higher [KI], Φ decreased with increasing [KI]. At the fixed concentration of [KI]0 ) 100 µM, Φ with [As(III)]0 ) 1, 10, 50, 100, and 1000 µM was 0.08, 0.20, 0.30, 0.49, and 0.61, respectively. Along with the photogeneration of As(V), triiodide and hydrogen peroxide were also generated, and their concentration profiles are shown in panels b and c of Figure 2. The production of triiodides should proceed through reactions 1-7. However, it is noted that triiodides were produced after an induction period during which As(III) was converted to As(V). That is, the triiodides were not detected as long as As(III) remained. After the arsenites were fully converted, then the generation of triiodides increased with an increasing iodide concentration. Because the presence of triiodides can be easily detected visibly, the color appearance can be taken as a sign of the completion of arsenite oxidation. On the other hand, the photogeneration of H2O2 rapidly increased in the initial stage to reach a maximum, after which it gradually decreased (Figure 2c). In the absence of iodides, the production of H2O2 was negligible. The production of H2O2 should be initiated by the reaction of excited iodide with dioxygen (reactions 9 and 10). Therefore, the initial rate of H2O2 production increased with increasing [KI] and was saturated around [KI] ) 500 µM. As H2O2 accumulates in the solution, its photolysis (reaction 11) should decrease [H2O2] gradually. An alternative path of H2O2 decomposition is its 3786
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(I • , e-) + O2+ H+ f I • + HO2•
(9)
HO2•+ HO2• f H2O2+ O2
(10)
H2O2 + hν f 2HO•
(11)
H2O2+ I3- f O2+ 2H++ 3I-
(12)
The iodide-mediated photooxidation of As(III) was carried out under different dissolved gas (air, N2, and N2O) conditions and compared in Figure 3a. The concurrent production of H2O2 was also compared in Figure 3b. Incidentally, the iodidemediated photooxidation of As(III) under λ > 300 nm irradiation is negligible (Figure 3a) because the absorbance of iodide is essentially zero above 330 nm (21). The efficiency of the iodine atom generation via 254 nm excitation is highly influenced by the presence of suitable electron acceptors (e.g., O2). That is, the generation of an iodine atom via reaction 3b should be more efficient than that of reaction 3a. In a N2-saturated solution, where reactions 9 and 10 are absent, the production of As(V) was significantly retarded but not completely inhibited, while the photogeneration of H2O2 was completely absent. This indicates that reaction 3a, though much less efficient, enables the generation of iodine atoms that should be subsequently involved in the oxidation of As(III). On the other hand, when N2O was introduced as an alternative electron acceptor (reaction 13), the photooxidation of As(III) was not retarded at all compared with the air-equilibrated system. (I • , e-) + N2O + H2O f I • + HO•+ HO-+ N2
(13)
The production of H2O2 was significantly reduced in the N2O-saturated solution but not completely inhibited. H2O2
FIGURE 4. Effect of OH radical scavenger (methanol, MeOH) on the photooxidation of As(III). Experimental conditions: gas (air-, N2-, or N2O-saturated), pHi ) 3.0, [As(III)]0 ) 1000 µM, [KI]0 ) 500 µM, and [MeOH]0 ) 20 mM. in this case seems to be produced through the recombination of OH radicals generated from reaction 13. The fact that the photooxidation of As(III) was not hindered at all in the N2O-system implies that the main oxidant of As(III) does not seem to be a superoxide or H2O2 that could be generated through reactions 9 and 10. In a TiO2/UV system, it has been suggested that superoxides play the role of oxidants of As(III) (16, 18, 19, 27). An alternative oxidant is the OH radical that could be produced via the photolysis of H2O2 (reaction 11) or reaction 13 in the N2O system. To test the possible role of a OH radical as an oxidant, we investigated the effect of methanol (OH radical scavenger) addition on the photooxidation of As(III). Figure 4 shows that the presence of excess methanol had no influence on the oxidation rate under all tested conditions (air, N2, and N2O). This confirms that OH radicals should not be involved in the oxidation mechanism of As(III). Role of Triiodide in As(III) Oxidation. The photogeneration of triiodides in UV254/KI/As(III) and UV254/KI systems is compared under various conditions in Figure 5. In both systems, there was no production of triiodides in the N2-saturated solution. This indicates that the role of dioxygen as an electron scavenger (reaction 9) is critical in the overall process. On the other hand, the production of triiodide in the N2O-saturated solution was much enhanced compared with that in the airequilibrated solution in both systems. Because N2O is a better electron acceptor than O2, more iodine atoms should be produced with an accompanying higher concentration of triiodide. The addition of methanol reduced the photogeneration of triiodide probably because methanol scavenges iodine radical species. It is interesting to note that the photogenerated triiodides were rapidly removed from the N2O-saturated solution regardless of the presence of As(III), whereas those in the air-equilibrated solution remained. It appears that the triiodide and iodine species are scavenged by OH radicals generated through reaction 13. The most outstanding difference between UV254/KI/As(III) and UV254/KI systems shown in Figure 5 is that the photogeneration of triiodide immediately started without an induction period in the solution with KI alone, whereas photogeneration of triiodide was delayed in the presence of As(III). This suggests that the photogenerated I3- should be involved in As(III) photooxidation. To confirm the role of triiodide in As(III) oxidation, we tested As(III) oxidation in the presence of I3- under dark conditions. As shown in Figure 6, As(III) was immediately oxidized by triiodide in dark conditions (reaction 14). As(III) + I3- f As(V) + 3I-
(14)
According to the standard reduction potentials (reactions 15 and 16) and the Nernstian equation, the reduction
FIGURE 5. Time profiles of the in situ generation of I3- in the (a) UV254/As(III)/KI system and (b) UV254/KI system under various conditions. Experimental conditions: gas (air-, N2-, or N2O-saturated), pHi ) 3.0, [As(III)]0 ) 1000 µM, [KI]0 ) 500 µM, and [MeOH]0 ) 20 mM (when indicated).
FIGURE 6. Oxidation of As(III) by I3- under dark conditions (air equilibrated, pHi ) 3.0, and [As(III)]0 ) 500 µM). The solution of triiodide was prepared by mixing iodide and iodine: [KI]0 ) 500 µM plus [I2]0 ) 100 µM to make [I3-]0 ) 100 µM; [KI]0 ) 500 µM plus [I2]0 ) 200 µM to make [I3-]0 ) 200 µM. potential of As(V) at pH 3 is calculated to be 0.40 VNHE. Therefore, the oxidation of As(III) by I3- should be thermochemically spontaneous (28) H2AsVO4-+ 3H++ 2e-)HAsIIIO2+ 2H2O Eo ) 0.67 VNHE (15) I3-(aq) + 2e-)3I- Eo ) 0.53 VNHE
(16)
Reaction 14 seems to proceed stoichiometrically. With [I3-]0 of 100 and 200 µM, [As(V)] reached in the plateau region close to 100 and 200 µM, respectively. When the photochemical reaction of iodides generates I3- in situ, the thermal oxidation of As(III) could be induced with regeneration of the iodides. In this respect, the iodide in the UV254/KI/As(III) system should function as a photocatalyst. The overall photocatalytic process is illustrated in Scheme 1. The iodides absorb photons and then are transformed into triiodides, which are subsequently reduced back to iodides upon oxidizing As(III) by completing a cycle. VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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SCHEME 1. Schematic Illustration of Iodide-Mediated Photocatalytic Oxidation of As(III) in a UV254/KI/As(III) System
Some Considerations of the UV254/KI/As(III) Process. In the practical treatment of groundwaters contaminated with arsenic, whose concentration typically ranges from 1 to 1000 µg/L (e10 µM) (29), the proposed UV254/KI/As(III) process can be successfully applied. Becasue the UV disinfection technology based on 254 nm radiation is well-established as a practical water treatment method, the proposed process should be easily integrated into the existing water treatment facilities employing germicidal lamps. When applied to the groundwater treatment, the water needs to be aerated prior to UV irradiation because the groundwaters containing As(III) are usually in the anaerobic condition. This process can be also applied to acid mine drainage that may contain much higher concentrations of arsenic (up to several hundred ppm) (30). Finally, the possible formation of oxo anions of iodine such as iodate (IO3-) and periodate (IO4-) from the photooxidation of iodides were studied because they could be harmful byproducts. No sign of such oxo anion (iodate and periodate) formation was seen from the IC analysis, within the detection limit of ∼5 µM, even with a 500 µM iodide solution that was irradiated for 4 h.
Acknowledgments This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean Ministry of Education, Science and Technology (MEST) (No. R0A-2008000-20068-0), the KOSEF EPB center (Grant R11-2008-05202002), and the Brain Korea 21 (BK21) program. We greatly appreciate the effort of Dr. Jungho Ryu and the kindly help from Prof. Kyoung-Woong Kim and Dr. Ju-Yong Kim at Gwangju Institute of Science and Technology (GIST) with the arsenic analysis.
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ES900602N
