Article pubs.acs.org/est
Photooxidation of Arsenite under 254 nm Irradiation with a Quantum Yield Higher than Unity Jungho Ryu,† Damián Monllor-Satoca,‡ Dong-hyo Kim,‡ Jiman Yeo,‡ and Wonyong Choi‡,* †
Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea
‡
S Supporting Information *
ABSTRACT: Arsenite (As(III)) in water was demonstrated to be efficiently oxidized to arsenate (As(V)) under 254 nm UV irradiation without needing any chemical reagents. Although the molar absorption coefficient of As(III) at 254 nm is very low (2.49 ± 0.1 M−1cm−1), the photooxidation proceeded with a quantum yield over 1.0, which implies a chain of propagating oxidation cycles. The rate of As(III) photooxidation was highly enhanced in the presence of dissolved oxygen, which can be ascribed to its dual role as an electron acceptor of photoexcited As(III) and a precursor of oxidizing radicals. The in situ production of H2O2 was observed during the photooxidation of As(III) and its subsequent photolysis under UV irradiation produced OH radicals. The addition of tert-butyl alcohol as OH radical scavenger significantly reduced (but not completely inhibited) the oxidation rate, which indicates that OH radicals as well as superoxide serve as an oxidant of As(III). Superoxide, H2O2, and OH radicals were all in situ generated from the irradiated solution of As(III) in the presence of dissolved O2 and their subsequent reactions with As(III) induce the regeneration of some oxidants, which makes the overall quantum yield higher than 1. The homogeneous photolysis of arsenite under 254 nm irradiation can be also proposed as a new method of generating OH radicals.
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INTRODUCTION Groundwater contamination with arsenic has been recognized as a great threat to the water supply and public health in many countries.1−3 In particular, Bangladesh and West Bengal in India have the largest population that is under a serious risk of chronic arsenic poisoning. Recently, the WHO and EPA lowered the maximum contaminant level (MCL) of arsenic in drinking water to 10 μg/L.4 The increased concern about arsenic poisoning and the stricter regulation of arsenic pollution have motivated many researchers to develop cost-effective methods for removing and transforming arsenic species in drinking water.5−7 Arsenic contamination can occur both from anthropogenic sources (e.g., industrial, agricultural, and coal burning activities) and from natural weathering or dissolution of arsenic bearing minerals.2 Arsenic exists in groundwater commonly as As(III) and As(V), with As(III) species prevalent under anoxic conditions.8 The oxidation state of arsenic critically affects its toxicity and mobility in natural systems.8 Since As(III) is more toxic and mobile than As(V), As(III) needs to be preoxidized both to reduce its toxicity and to enhance its immobilization in arsenic removal processes. In this regard, it has been reported that As(III) can be oxidized to As(V) by various methods using O3,9 H2O2,10 Mn-oxides,11,12 UV/iron-containing species,13−17 ultrasound,18,19 TiO2/UV,20−28 and KI/UV254.29,30 Photochemical treatment technologies have been extensively studied to remediate contaminated water and air31,32 and © 2013 American Chemical Society
frequently applied to the arsenic contaminated systems. The heterogeneous photocatalytic oxidation of As(III) to As(V) was successfully demonstrated in UV-illuminated (air-equilibrated) suspensions and on thin films of TiO 2 . 21,22,26−28 In homogeneous photooxidation systems, UV-induced oxidation of As(III) usually requires reagents such as iron species and H2O2.14,15 Yeo and Choi29 reported that the photooxidation of As(III) to As(V) in the presence of iodide and 254 nm irradiation is mediated by in situ generated triiodide as the main oxidant. Recently, Buschmann et al.33 observed that dissolved humic acid induces As(III) oxidation under UV-A and visible light irradiation. The purely photochemical oxidation of As(III) without any reagents requires UV wavelengths shorter than 200 nm, while common low-pressure mercury lamps with their main wavelength at 254 nm are not efficient for photooxidizing As(III).34 Bissen et al.35 observed As(III) oxidation under a solar UV simulator and explained that this might be due to residual emission at wavelengths below 260 nm or possibly very weak absorbance of As(III) at wavelengths over 280 nm. Yoon et al.36 reported that As(III) oxidation proceeded rapidly under VUV irradiation whereas the rate was negligible under UV-C irradiation. Until now, the direct photooxidation mechanism of Received: Revised: Accepted: Published: 9381
May 6, 2013 July 13, 2013 July 23, 2013 July 23, 2013 dx.doi.org/10.1021/es402011g | Environ. Sci. Technol. 2013, 47, 9381−9387
Environmental Science & Technology
Article
calculated from the difference between the total arsenic concentration (before passing the cartridge tube) and [As(III)]. The light absorbance of arsenic species and the concentration of benzoic acid were monitored by using a UV/visible spectrophotometer (Agilent 8453). The formation of H2O2 as a byproduct of arsenite photooxidation was also monitored. The concentration of photogenerated H2O2 was measured by DMP (2,9-dimethyl-1,10-phenanthroline) method.21
As(III) under 254 nm irradiation without any reagents has not been investigated in detail because of its low efficiency. In this study, we explored the purely photochemical oxidation mechanism of As(III), which has not been well established so far. Although the measured molar absorption coefficient of arsenite was markedly low (2.49 ± 0.1 M−1cm−1) at 254 nm, the photooxidation proceeded with an estimated quantum yield over 1, which implies the presence of unknown oxidation cycles. The photooxidation reaction system was investigated in detail and a new mechanism is proposed.
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RESULTS AND DISCUSSION As(III) Photooxidation under 254 nm Irradiation. Photochemical reactions are initiated by absorbing photons. Since UV absorption by As(III) is essential for initiating its photooxidation, the absorption spectral profiles of arsenite solution are compared at different concentrations. Figure 1
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EXPERIMENTAL SECTION Materials and Chemicals. NaAsO2 (As(III), 96.7%, Aldrich) and Na2HAsO4·7H2O (As(V), 99%, Kanto) were used as the arsenic source. Other chemicals used include benzoic acid (Aldrich), Na2Cr2O7 (Cr(VI), Aldrich), 4chlorophenol (4-CP, Sigma), KH2PO4 (Kanto), 2,9-dimethyl1,10-phenanthroline (DMP, Aldrich), CuSO4·5H2O (Shinyo), Nitrotetrazolium Blue (NBT, Aldrich), Superoxide dismutase (SOD) from Escherichia coli (Sigma, manganese-containing enzyme, lyophilized), and tert-butyl alcohol (TBA, Shinyo), all of which were of reagent grade and used as received. N2 (BOCGases, 99.999% purity) and O2 (Sinan-Gases, Korea, 99.999% purity) gases were used when the effect of the dissolved gas was investigated. Deionized ultrapure water (18 MΩ·cm) was prepared with a Barnstead purification system. Photolyses and Analyses. The initial concentration of arsenite [As(III)]0 in most experiments was fixed at 100 μM and the solution pH was adjusted to 3.0, 7.0, or 9.0 with HCl or NaOH standard solutions. For the photolysis experiments in the absence of dissolved oxygen, the solution was purged with nitrogen gas for 30 min before irradiation to remove residual oxygen and was continuously purged during the photoreaction. A double-tube germicidal lamp (GLD 15MQ (15W), Sankyo, Japan) that mainly emits UV of 253.7 nm was used as a light source and was immersed in a cylindrical-type quartz reactor (diameter 3.5 cm, height 37 cm, and total volume 200 mL). The reactor allows a UV path length of ca. 1 cm. The photon flux at 254 nm was determined using two chemical actinometers. The incident photon flux (Einstein/L·s) was measured to be 8.2 × 10−6 and 1.0 × 10−5 with the iodide/iodate actinometer37 and monochloroacetic acid actinometer,38 respectively. The reactor was open to the ambient air (airequilibrated condition) or sealed with Parafilm under continuous purging with nitrogen (N2-saturated condition) or oxygen gas (O2-saturated condition), and stirred magnetically during irradiation. Sample aliquots were intermittently withdrawn with a syringe during the photoreaction. Quantitative analysis of As(V), Cr(VI), and Cl− was performed using an ion chromatograph (IC, Dionex DX120), which was equipped with a Dionex IonPac AS 14 (4 × 250 mm) column and a conductivity detector.21,22 The eluent solution was 3.5 mM Na2CO3/1 mM NaHCO3. The lower As concentrations (10 μM) were analyzed by using graphite furnace atomic absorption spectrometry (GFAAS, PerkinElmer 5100 spectrophotometer, ∼5 ppb detection limit).29 The AAS calibration for arsenic was carried out in 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 selectively retained As(V); As(III) was collected in the effluent solution, which was analyzed by GFAAS. The concentrations of in situ generated As(V) were
Figure 1. UV absorption spectra of arsenite solutions with different [As(III)]. The inset shows the calibration plot of the absorbance at 254 nm vs [As(III)]. All spectra were recorded with using a 5 cm-path cell.
shows that the absorbance at 254 nm was noticeable only at [As(III)] > 1 mM. The concentration calibration plot (Figure 1 inset) estimates the arsenite absorption coefficient at 254 nm to be 2.49 ± 0.1 M−1cm−1. Despite the very weak absorption of 254 nm light by arsenite, the photooxidation of As(III) proceeded with a significant rate as shown in Figure 2. Under air-equilibrated condition, the photooxidation of 10 and 100 μM As(III) was completed within 1.5 and 2.5 h of irradiation, respectively. The removal of As(III) was accompanied by the concurrent and quantitative production of As(V) with meeting the total As balance (Figure 2a), which indicates that the photoconversion from As(III) to As(V) proceeds stoichiometrically. The homogeneous photooxidation of As(III) sensitively depends on the presence of dissolved O2 (Figure 2b): the oxidation rate was the fastest under the O2 saturation, moderately retarded under the air saturation, and markedly inhibited under the N2 saturation. This implies a crucial role of dissolved oxygen in the photochemical oxidation mechanism. Although the photooxidation of As(III) was not completely inhibited under N2-saturated condition, it might be ascribed to the low level of dissolved O2 which cannot be removed by simple N2 purging. The concentration of dissolved O2 under N2 purging condition was maintained at 0.1 ppm or lower (
