Letter pubs.acs.org/journal/estlcu
Diketone-Mediated Photochemical Processes for Target-Selective Degradation of Dye Pollutants Shujuan Zhang,*,† Xitong Liu,† Mengshu Wang,† Bingdang Wu,† Bingcai Pan,† Hua Yang,† and Han-Qing Yu‡ †
State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, P. R. China ‡ Department of Chemistry, University of Science and Technology of China, Hefei 230026, China S Supporting Information *
ABSTRACT: Free radical-mediated advanced photochemical oxidation processes are extensively studied for the degradation of variant pollutants. However, their practical application in dyerich wastewater treatment is limited because of the poor efficiency and the slow degradtion rate caused by the poor reaction selectivity. Here we report that two small molecular diketones could act as effective photoactivators for dye degradation through a non-free radical pathway. Under ultraviolet (UV) irradiation, diketones form charge-transfer complexes with dyes, which might turn the target dyes from shields (inner filters in some photochemical processes, such as UV with H2O2 and UV with TiO2) into spears. As a result, the diketone-mediated process is much more efficient and target-selective. Moreover, diketones and their degradation products have good biocompatibility, making this novel process potentially suitable as a pretreatment step in sequential chemical−biological wastewater treatment.
■
INTRODUCTION With the potential utilization of inexpensive and inexhaustible solar radiation, photochemical advanced oxidation processes (PAOPs) have attracted extensive interest because of the potential for destroying chemically stable or biorefractory contaminants.1−3 The activators used in most of the PAOPs can be categorized into three groups: inorganic oxidants (e.g., ozone, hydrogen peroxide, and persulfate), metal or semiconductor photocatalysts, and organic−inorganic hybrid catalysts.3−8 The aforementioned methods are usually effective for targeting substrates in model solutions, but their practical application in wastewater is still limited because of the poor solar efficiency and the slow degradation rate caused by the poor reaction selectivity. Direct chemical oxidation for complete mineralization is generally expensive and energyconsuming. An attractive potential strategy is to combine PAOPs with the conventional biological treatment.9 Chemical oxidation can be applied as a pretreatment to convert the initially persistent organic pollutants to more biodegradable intermediates, which can then be treated in a biological treatment process at a considerably low cost. One concern in sequential chemical and biological oxidation is that the chemicals used in most of PAOPs are normally toxic to microorganisms or need postrecovery before proceeding to the next process. Thus, the development of eco-friendly methods for the removal of recalcitrant organic pollutants is strongly desired. © XXXX American Chemical Society
Small molecular ketones, such as acetone and diacetyl [2,3butanedione (BD), an α-diketone], have been studied as sensitizers for the photodegradation of dyes.10,11 It is reported that acetone could enhance the photodecay of disperse dyes as a cosolvent through energy transfer.10 Oxciplex formed by addition of molecular oxygen to triplet BD and the consequent acyl and acylperoxyl radicals were proposed as the main oxidative species in the photofading of some azo dyes.11 Considering the nearly unity quantum yield of hydroxyl radicals (•OH) in the UV/H2O2 process and the stronger oxidation potential of •OH compared to those of acyl and acylperoxyl radicals,12 the UV/BD process is not expected to be more efficient than the UV/H2O2 process. Acetylacetone [2,4-pentanedione (AA)] is a β-diketone with two interesting structural isomers, diketo and enol, the population of which is quite sensitive to solvent.13,14 The enolic form of AA is more stable than the keto form in the gas phase because of the strong intramolecular hydrogen bonding in the cyclic conjugated system. The equilibrium shifts toward keto in solution as the solvent polarity increases. In water, the intramolecular H-bond in AA is replaced by the intermolecular H-bonds between water and AA, which better stabilize the keto form. Because of the peculiar structure and activity, AA has attracted extensive interest in many fields of science Received: July 4, 2013 Revised: September 4, 2013
A
dx.doi.org/10.1021/ez400024b | Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Environmental Science & Technology Letters
Letter
(Supporting Information).15,16 However, information about the photochemistry of AA in aqueous solutions is still very limited. Herein, we, for the first time, report a novel photochemical approach with AA or acetonylacetone [2,5-hexanedione (HD), a γ-diketone] as potent photoactivators for dye decoloration. The underlying mechanisms were proposed on the basis of factorial experiments and product analysis.
■
MATERIALS AND METHODS Materials. AA, BD, HD, and H2O2 of analytical grade were purchased and used as received. Three azo dyes [acid orange 7 (AO7), methyl orange (MO), and Congo red (CR)] and one anthraquinone wastewater (denoted AQ301) were tested as model compounds. The molecular structures of the studied diketones and dyes are illustrated in Charts S1 and S2 of the Supporting Information. Irradiation Experiments. UV irradiation was conducted with a medium-pressure mercury lamp as the light source, whose output spectrum is listed in Table S1 of the Supporting Information. More experimental details are provided in Text S1 of the Supporting Information.
Figure 1. (a) Evolution of the UV−vis spectra of AO7 solutions (0.16 mM) under UV irradiation. [AA] = [H2O2] = 1.0 mM. [P25] = 0.5 g/ L. The fluence rate was 3.6 mW/cm2. (b) Pseudo-first-order decoloration plots of AO7 solutions (0.16 mM) under UV irradiation. The fluence rates were 2.9 and 7.1 mW/cm2, and the activator concentrations were 1 and 5 mM. (c) Decoloration comparison of the three azo dyes with the four activators (0.5 mM for AO7 and MO and 0.3 mM for CR). (d) ESR spectra of solutions with 100 mM DMPO and 1.67 mM activators after irradiation under a high-pressure mercury lamp for 10 min. Abbreviations: AA, acetylacetone; P25, Degussa TiO2; BD, diacetyl; HD, acetonylacetone.
■
RESULTS AND DISCUSSION We found that although AA is similar to BD in chemical structure (Chart S1 of the Supporting Information), their roles in dye degradation were different. As illustrated in Scheme 1, Scheme 1. Degradation Patterns of Dye Pollutants in Free Radical-mediated PAOPs and the UV/AA Process
this band, which was also proven by the high-performance liquid chromatography (HPLC) analysis (Figure S1 of the Supporting Information) for both AO7 and MO solutions. The destruction of the molecular structure of AO7 was further proven by the proton nuclear magnetic resonance (1H NMR) spectra (Figure S2 of the Supporting Information). A self-acceleration phenomenon was observed for AO7 decoloration in the UV/AA process (Figure 1b), which consisted of two stages. The initial pseudo-first-order degradation rate constants (k1 values) of the UV/AA process were 4−6 times higher than those of the UV/H2O2 process, whereas the ratios of k1 were increased 20−25-fold in the latter stage. The inflection between the two stages depends on irradiation conditions. Within a certain range, a higher light intensity or a larger AA dosage led to a shorter slow stage and a higher decoloration rate. It should be noted that a larger AA dosage did not always result in a higher decoloration rate. For a 0.16 mM AO7 solution, a dose of 5 mM AA was sufficiently high for rapid decoloration. A further increase in the AA concentration would suppress the decoloration efficiency (Figure S3 of the Supporting Information), which is attributed to the attenuation of the light intensity available for AO7. A much larger H2O2 dosage was needed to achieve the same decoloration rate as in the UV/AA process. For the other two azo dyes, MO and CR, the UV/AA process was also much more efficient than the UV/H2O2 process (Figure 1c). The decoloration efficiency of the UV/AA process was 4.5 times higher than that of the UV/H2O2 process for the AQ301 wastewater under the given conditions (Figure S4a of the Supporting Information), indicating that the UV/AA process is highly efficient even in the presence of concentrated coexisting matter. However, the decoloration efficiency of the UV/AA process was suppressed by the presence of tannic acid or humic acid (Figure S4b of the Supporting Information). This
the UV/BD process, like the well-established UV/H2O2 and UV/TiO2 methods, suffered from poor solar efficiency (molar extinction coefficients for H2O2 and BD of 19.6 and 8.5 M−1 cm−1 at 254 nm, respectively, and for AA of 1800 M−1 cm−1 at 274 nm) and low selectivity. A direct charge-transfer mechanism was proposed for the UV/AA process based on the following experimental results. The UV/AA process was stoichiometrically efficient and target-selective with formation of few color intermediates. Photodegradation Efficiency. As illustrated in Figure 1a, the UV/AA process was the most efficient for AO7 decoloration followed by the UV/H 2 O 2 process. The heterogeneous UV/TiO2 process had the lowest decoloration efficiency, which could be partly attributed to mass-transfer limitations. Acetone, at the same concentration as AA, was found to be ineffective in decolorizing AO7 under UV irradiation. Either in the UV/H2O2 process (Figure 1a), in the catalytic oxidation by the combination of FeIII-TAML and H2O2,17 or in γ-ray radiolysis,18 an obvious absorbance increase at the valley of 325−375 nm was observed in the spectra of the irradiated AO7 solutions. Such an increase indicates the formation of intermediates with light absorbing abilities at B
dx.doi.org/10.1021/ez400024b | Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Environmental Science & Technology Letters
Letter
is true because these types of natural organic matter competed with AA for photons (Figure S4c of the Supporting Information). Compared to the rapid decoloration, the mineralization of dyes in the UV/AA process was rather slow. Under the given conditions, the UV/AA and UV/H2O2 processes showed similar changes in TOC (Figure S3 of the Supporting Information). After treatment, the chemical oxygen demand (COD) of the AO7 solution was decreased while the 5 day biological oxygen demand (BOD5) was increased. The BOD5/ COD value of the AO7 solution was increased from 0.07 (initial) to >0.3 (Figure S5 of the Supporting Information), suggesting that the solution would be apt for biological treatment. Toxicity Analysis. Toxicity is one of the most important acceptance criteria when a method is considered for water treatment. Like H2O2,19 AA was reported to have a low to moderate lethal toxicity, depending on the species.20,21 According to the decoloration experiments, for dye solutions at concentrations of 10−4 to 10−6 M,22
