Article pubs.acs.org/IECR
Photooxidative Degradation of Aromatic Carboxylic Acids in Water: Influence of Hydroxyl Substituents Nikolina Milovac,† Daria Juretic,† Hrvoje Kusic,*,† Jasna Dermadi,‡ and Ana Loncaric Bozic*,† †
Faculty of Chemical Engineering and Technology, University of Zagreb, Marulicev trg 19, Zagreb 10000, Croatia TAPI R&D, Pliva Croatia Ltd., TAPI Croatia, Prilaz baruna Filipovica 25, Zagreb 10000, Croatia
‡
S Supporting Information *
ABSTRACT: The UV/H2O2 process was applied for the degradation of aromatic carboxylic acids (ACAs). Benzoic acid, salicylic acid, gentisic acid, and gallic acid are chosen as typical water pollutants representing ACAs with the increased number of hydroxyl groups. The effect of the structural characteristics of ACAs and UV/H2O2 process parameters on their degradation kinetics was investigated by a statistical/empirical approach employing the design of experiments and response surface methodology. Different optimal conditions for maximal degradation rates were established, while degradation rates followed the decreasing order benzoic acid > salicylic acid > gentisic acid > gallic acid with the increasing number of hydroxyl groups. The inversed order was established in the case of mineralization kinetics. Structurally influenced degradation pathways of studied ACAs, with the shared sequence related to the preferable hydroxylation position at the benzene ring, are reflected in the observed changes in the biodegradability and toxicity toward Vibrio fischeri during UV/H2O2 treatment. with respect to the oxidant concentration.16 The reactor geometry determines the effective path length, which is inversely proportional to the yield of generated radicals and the flow regime, whereas turbulent flow is preferable in order to overcome the mass-transfer limitations.16,17 The characteristics of wastewater, i.e., the pollutant structure, presence of HO• scavengers, and suspended solids, play an important role because of scavenging reactions and photon-shielding effects.12,13,17 The UV/H2O2 process is based on the application of a UV−C irradiation source, typically operating at 254 nm, and the addition of H2O2, a strong oxidant with good operational features (e.g., easy dissolution in water, which could be limiting in the application of S2O82−),18 low operational costs (e.g., UV/O3 requires a unit for ozone generation),6 and rather low environmental impact.6 Because it is demonstrated that in well-mixed reactor systems and the absence of suspended solids, besides the operating pH and [H2O2], the pollutant structure may play an important role because of the competitive kinetics for generated HO•,12,13 the intention of this study was to investigate the degradation of ACAs from the structural aspect. Hence, four typical ACAs (benzoic acid, salicylic acid, gentisic acid and gallic acid) were chosen as model pollutants with increasing number of hydroxyl groups in their structure to be degraded by the UV/H2O2 process. Besides the structural influence on degradation kinetics, which was studied along with the influence of key operational parameters (pH and [H2O2]) using the response surface methodology (RSM) approach, we also investigated how structural features of studied ACAs influenced the
1. INTRODUCTION The aromatic carboxylic acids (ACAs) represent a group of chemicals widely used as raw materials or produced in various industries such as dyeing, pharmaceutical, food, cosmetics, and paper milling. Hydroxy derivatives of ACAs are common constituents of agroindustrial effluents like olive mill wastewaters or can even be found as constituents of landfill leachate.1−4 ACAs are stable and initially nonbiodegradable water pollutants and need to be removed by appropriate treatment technology in order to avoid hazardous effects to the aquatic environment.2,5 Advanced oxidation technologies (AOTs) are based on the generation and activity of hydroxyl radicals (HO•). Because of their capability of degrading/mineralizing the majority of organics present in water, AOTs have attracted increased interest when the treatment of recalcitrant organics is considered.6 Although the complete abatement of organic load in wastewater by AOTs might be costly, these treatment processes can be efficiently applied for biodegradability enhancement.7 Among AOTs, the processes combining the application of UV irradiation and strong oxidants (H2O2, S2O82−, and O3), so-called photochemical AOTs (PC-AOTs), showed high efficiency in treating a vast array of recalcitrant pollutants, including ACAs.8−13 The generation of radical species in PC-AOTs, primarily HO•, is based on photolysis of the present oxidant. The PC-AOT efficiency depends on several key process parameters. Hence, the operating pH determines the yield and the type of radicals generated because of (i) undesired dissociation of the oxidant in the case of H2O2, (ii) a preferable degradation mechanism in the case of O3, and/ or (iii) radical dissociation equilibrium in the case of S2O82−.14 The concentration of the oxidant (regardless of its type) determines the yield of radicals generated, taking into account the scavenging effect at excess oxidant.15 The radiant power at wavelength applied determines the yield of generated radicals © 2014 American Chemical Society
Received: Revised: Accepted: Published: 10590
March 31, 2014 June 3, 2014 June 9, 2014 June 9, 2014 dx.doi.org/10.1021/ie501338q | Ind. Eng. Chem. Res. 2014, 53, 10590−10598
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Discovery C18 column, with mobile phase H2O/CH3OH/oH3PO4 (ratios dependent on the type of pollutant) at flow 1.0 mL min−1. The mineralization of model solutions was monitored by a total organic carbon analyzer, TOC-VCPN (Shimadzu, Japan). A Handylab pH/LF portable pH-meter (Schott Instruments GmbH, Mainz, Germany) was used for pH measurements. COD and BOD5 values were determined by colorimetric methods using a HACH DR2800 spectrophotometer (Hach-Lange, USA). In COD analysis, the results are corrected for interference by H2O2. For BOD5 and toxicity measurements, samples were treated prior to analysis with Na2SO3 in excess to remove H2O2.19 The toxicity on V. f ischeri was examined using a BioFix Lumi-10 toxicity analyzer (Macherey-Nagel, Düren, Germany) according to ISO 113483.20 The results were expressed as effective concentrations causing 50% reduction of bioluminescence (EC50) and toxicity units (TU = 100/EC50%). A Lambda EZ 201 UV/vis spectrophotometer (PerkinElmer, USA) was used for measuring the absorbance of aromatic structures in model wastewater during the treatment at two typical wavelengths: 254 and 280 nm. 2.4. Calculations. Modified miscellaneous full factorial design and RSM were employed in this study. UV/H2O2 process parameters and structural characteristic of studied ACAs are represented by independent variables: pH (X1), [H2O2] (X2), and number of hydroxyl groups (X3). Their combined influence on the degradation kinetics (i.e., dependent variable; Y) is described by the polynomial equation
mineralization kinetics and changes in the biodegradability and toxicity during the treatment.
2. MATERIALS AND METHODS 2.1. Chemicals. Four ACAsbenzoic acid (BenzAc), ≥99.5; 2-hydroxybenzoic acid (salicylic acid, SalAc), ≥99.0; 2,5-dihydroxybenzoic acid (gentisic acid, GenAc), ≥99.0; 3,4,5trihydroxybenzoic acid (gallic acid, GalAc), ≥99.0, all purchased from Sigma-Aldrichwere used as model pollutants in the study. Their chemical structures, formulas, physical properties, and activities toward HO• and Vibrio f ischeri are summarized in Table S1 (Supporting Information, SI). Methanol (CH3OH, HPLC grade) and o-phosphoric acid (oH3PO4, >85%), used in the mobile phase for HPLC analyses, were purchased from Sigma-Aldrich. The inorganic chemicalshydrogen peroxide (H2O2, w = 30%), sodium hydroxide (NaOH, p.a.), sodium chloride (NaCl, p.a.), sodium sulfite (Na2SO3, p.a.), and sulfuric acid (H2SO4, >96%)were purchased from Kemika, Croatia. All experiments were performed with deionized water with conductivity of less than 1 μS cm−1. 2.2. Procedure. All experiments were performed with the model solutions (c0 = 1 mM and V = 1.4 L) in a glass waterjacketed (T = 25.0 ± 0.2 °C) batch photoreactor equipped with a UV−C lamp (I0 = 5.12 × 10−6 einstein s−1) and a magnetic stirrer. Initial pH values and hydrogen peroxide concentrations ranged from 3 to 11 and from 10 mM to 200 mM, respectively (Table 1). The conversion of parent pollutants was monitored
Y = β0 + β1X1 + β11X12 + β2X 2 + β22X 2 2 + β3X3
Table 1. Applied Experimental Range and the Levels of Independent Variables Used in Miscellaneous (Two Numerical and One Categorical Variables) Modified 33 Full Factorial Design process parameter numerical pH [H2O2], (mM) categorical no. of hydroxyl substituents
coded value X1 X2 X3
+ β12X1X 2 + β13X1X3 + β23X 2X3
where β0 is the intercept, β1, β2, and β3 are linear, β11 and β22 are quadratic, and β12, β13, and β23 are interaction coefficients. It should be pointed out that the categorical parameter (in our case X3) cannot be represented by the quadratic term in eq 1; it has only mathematical but not physical meaning. The values of independent variables (summarized in Table 1) were transformed in dimensionless coded values: three levels for X1 and X2 and four levels for X3. The full experimental matrix is summarized in Table S2 in the SI, along with the obtained results and values of responses. The fitting of models was evaluated using the coefficient of determination (R2) and the analysis of variance (ANOVA). Calculations and analyses were performed using STATISTICA 10.0 (StatSoft, USA), DesignExpert 8.0 (StatEase, USA), and Mathematica 9.0 (Wolfram Research, USA).
levels/ranges/types −1 3 10 1 0
0 7 105 2 1
1 11 200 3 2
(1)
4 3
within 60 min of UV/H2O2 treatment, and the obtained data were used to estimate the kinetic order and apparent degradation rates. The changes in the total organic carbon (TOC) values were monitored through an extended treatment period to determine the reference treatment times [RTTs; t1/4(OC) and t1/2(OC)] when the organic content of model wastewater is decreased for one-quarter and half of the initial amount. The duration of the experiments ranged from 218 to 301 min depending on the number of hydroxyl groups in the studied ACAs. For determined RTTs, samples were taken for chemical oxygen demand (COD), biochemical oxygen demand (BOD5), and toxicity analysis to estimate the quality of the treated water. All experiments were performed in triplicate, and averages were reported; the reproducibility of the experiments was >95%. 2.3. Analyses. The conversion of studied ACAs (BenzAc, SalAc, GenAc, and GalAc) was monitored by a highperformance liquid chromatograph (Shimadzu, Japan), equipped with a diode-array UV detector, and SPD-M10AVP (Shimadzu, Japan) using a 5 μm, 25.0 cm × 4.6 mm, Supelco
3. RESULTS AND DISCUSSION 3.1. Influence of the ACA Structure on Degradation. The degradation rate constant (kobs) was chosen as the response in RSM modeling. In such a manner, the influence of the treatment time was indirectly considered as well. Because the reactions and their rates can be compared only when they can be fitted to the same order,21 we compared the obtained experimental results to the apparent kinetics order and found out that the degradation of studied aromatics obeys first-order kinetics; RLR2 for the employed first-order integrated equation displaying a functional dependence of the concentration on time ranged from 0.961 to 0.999 (Figure S1 in the SI). The RSM methodology suggests that if the ratio of maximum and minimum values of responses is higher than 10, a certain transformation of the response may improve the model 10591
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accuracy.22 The calculated kobs ranged from (0.20 to 7.17) × 10−4 s−1 (ratio 35.57). Employed graphical analysis over the Box−Cox plot identified the sqrt function as the appropriate mathematical transformation for our responses. Applied multiple regression analysis on the design matrix and transformed responses (Y′) yielded polynomial equation (2), presenting RSM model M1, which predicts the behavior of the studied system depending on the structural (type of substituent of the studied aromatics) and UV/H2O2 process parameters (pH and [H2O2]): Y′ =
UV/H2O2 process.17 Accordingly, the corresponding model terms (X1, X12, X2, and X22) are shown to be significant. Furthermore, all model terms related with the structure of studied ACAs, i.e., the number of hydroxyl groups, are found to be highly significant [p(X3), p(X1X3), p(X2X3) < 0.0001; Table 2]. The influence of X3, the term representing the number of hydroxyl groups, can be plausibly explained by the different degradation rate constants of the studied ACAs with the main reactive species, HO• (Table S1 in the SI), and structurally influenced degradation mechanisms. The interaction terms X1X3 and X2X3 represent the combined influences of the number of hydroxyl groups and the initial pH and [H2O2], respectively. Their significance within M1 indicates on a certain pattern in the mutual effects between the considered process and structural parameters on the degradation kinetics of the studied ACAs. The observed overall degradation rate of ACAs by the UV/H2O2 process is influenced by the rates of direct photolysis and HO•-driven reactions. According to our previous study, the degradation of ACAs by direct photolysis proceeds rather slowly. In the same reactor system, conversion of 6.5−8.5% was achieved within 60 min.23 Such an observation speaks in favor of the prevalence of a HO•-driven mechanism in the degradation of ACAs by the UV/H2O2 process. kHO• values of nonhydroxylated ACAs, i.e., BenzAc, is 1 order of magnitude lower than those of hydroxylated ACAs (Table S1 in the SI). The values of kHO• range from (4.3 to 24) × 109 M−1 s−1 (Table S1 in the SI) and follow the decreasing order GenAc ≥ SalAc > GalAc ≫ BenzAc. Taking into account that in our experiments the oxidant was in excess toward organic pollutant (10 < oxidant/pollutant < 200), the degradation by the HO•-driven mechanism can be described by first-order kinetics.21 This was confirmed by the experimental results presented in the SI (Figure S2), where the first-order degradation rates (kobs) of ACAs treated by UV/H2O2 are calculated on the basis of the obtained results at conditions set by the MMFF used. When the results obtained are compared at the same operating conditions, it can be seen that kobs generally follows the decreasing order BenzAc > SalAc > GenAc > GalAc (Table S2 in the SI); with an increase of the number of hydroxyl groups bonded to the benzene ring, the overall degradation rate decreases. It should be noted that kobs summarizes the influence of (i) parent pollutants and (ii) formed byproducts on the overall degradation rate.12,13 Although both rate constants (kHO• and kobs) depend on the structural characteristics of the parent ACAs, their different decreasing orders indicate the important role of byproducts in the overall degradation rate. There are two main degradation routes determining the nature of the formed byproducts: (i) H-atom abstraction followed by hydroxylation at the aromatic ring and (ii) cleavage of the C−C bond at the carboxyl group followed by hydroxylation. The first route considers the formation of polyhydroxylated ACAs, while the second route results in the formation of benzenediols (e.g., catechol). According to the literature,1,24 the first route is predominant, resulting with the preferable formation of polyhydroxylated ACAs in the first step of the degradation pathway. However, regardless of the byproducts formed in the first step, aliphatic carboxylic acids are formed as open-ring byproducts upon cleavage of the aromatic ring.1,25 Hence, competition between various aromatic and aliphatic carboxylic acids for HO• occurs, influencing the overall rate of the studied ACAs. For instance, SalAc has 1 order of magnitude higher kHO• than BenzAc (Table S1 in the SI), while the overall degradation rates at the corresponding operating conditions of SalAc are
Y
= 1.93 + 0.04X1 − 0.19X12 + 0.19X 2 − 0.26X 2 2 + 0.79X3[1] + 0.049X3[2] − 0.20X3[3] − 0.042X1X 2 + 0.015X1X3[1] + 0.12X1X3[2] + 0.13X1X3[3] − 0.25X 2X3[1] + 0.16X 2X3[2] + 0.044X 2X3[3]
(2)
The significance of M1 (2) and variables (X1, X2, and X3) represented through M1 terms is performed by ANOVA (Table 2). The calculated Fisher F-test value accompanied by a Table 2. ANOVA of the Response Surface Model M1 for the Prediction of First-Order Degradation Rate Constants for the Studied Pollutants by the UV/H2O2 Process Depending on the Number of Hydroxy Substituent Groups Statistics factors (coded)
SS
df
MSS
F
p
model X1 X1 2 X2 X2 2 X3 X1X2 X1X3 X2X3 residual total
12.61 0.039 0.29 0.84 0.54 9.67 0.029 0.63 0.57 0.11 12.72
14 1 1 1 1 3 1 3 3 21 35
0.90 0.039 0.29 0.84 0.54 3.22 0.029 0.21 0.19 5.45 × 10−3
165.24 7.21 53.71 154.07 98.31 591.62 5.25 38.55 34.75
gentisic acid > gallic acid with the increasing number of hydroxyl groups. The structural effect was determined in the case of mineralization kinetics due to the preferable degradation pathway including hydroxylation of the benzene ring up to its saturation with hydroxyl groups and its subsequent cleavage, 10597
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