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Sonolytic degradation of pesticide metazachlor in water: The role of dissolved oxygen and ferric sludge in the process intensification
Journal of Environmental Chemical Engineering 7 (2019) 103095
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Sonolytic degradation of pesticide metazachlor in water: The role of dissolved oxygen and ferric sludge in the process intensification
T
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M. Kask , M. Krichevskaya, J. Bolobajev Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086, Tallinn, Estonia
A R T I C LE I N FO
A B S T R A C T
Keywords: Sonolysis Fenton-like oxidation Metazachlor Pesticide Advanced oxidation process
The study of sonolytic degradation of metazachlor (MZC), which was activated by either oxygen or insoluble Fe (III) was performed. Herbicide MZC was selected as a target compound due to its wide use in agriculture. The results indicated the positive effect of oxygen used as undoubtedly the most harmless H%-scavenger promoting the penetration of HO% from the gaseous phase of the cavitation bubble into the liquid phase and the interphase region. Another approach in activation of sonolysis was the application of Fe2O3 used as a source of iron in order to utilize H2O2 generated as a result of combination of HO∙-s in the interphase region of the cavitation bubble. The present study revealed the synergistic effect of the combination of sonolysis and Fenton-like oxidation. The pseudo-first order rate constant of MZC sonolytic degradation increased from 1.11 × 10−2 min-1 for conventional sonolysis to 1.79 × 10−2 and 2.88 × 10−2 min-1 for O2-saturated and Fe2O3-added solutions, respectively. The application of ultrasound demonstrated the amplifying effect on the detachment of Fe(III) from insoluble ferric oxyhydroxide (Fe2O3·nH2O) in acidic environment into the aqueous solution, thereby improving the oxidation of organic compounds. MZC followed two degradation pathways, which are the cleavage of the aliphatic sites of MZC with the initial hydroxylation reactions and oxidation of pyrazole group.
1. Introduction Over the last few decades, the occurrence of organic micropollutants (MPs) in different environmental matrices has become a worldwide issue [1]. Less or more MPs could be found in water (e.g. surface water, drinking water, groundwater, sewage) and solid matrices (e.g. soil and sediments), where the concentrations vary from ng L−1 to μg L−1 [2] and from μg kg−1 to mg kg−1 [3,4], respectively. MPs may reach the environment as initial compound or metabolites, thereby exerting toxic effect on living organisms including humans via food and drinking water. Due to the growth of population, the use of various products (e.g., plant protection products) has increased [5]. One emerging organic MP is a chloroacetamide herbicide metazachlor (MZC) [6–8], which belongs to the group of endocrine disrupting chemicals [9]. It is widely used in Europe, e.g., the consumption of MZC on arable crop was 177,550 kg in 2003 in the UK and ca. 200,000 kg in Czech Republic in 2010 [8,9]. In 2016, the quantity of MZC delivered to Estonia was 31,515 kg, which is slightly more than one-third higher compared to the year 2011 [10]. The widespread use of this herbicide inevitably leads to the pollution of ground and surface waters. For example, in Estonia it has been found in river Navesti’s two tributaries at concentrations of 0.6 ng L−1 in both places [11]. In Europe the ⁎
concentration of MZC in water bodies varies from 0.1 up to 100 μg L−1, thus requiring the effective water treatment methods to minimize the harmful impact of MZC to living organisms [8]. For instance, it has been found that MZC exposes moderate toxicity to aqueous organisms, mainly daphnia, bluegill, sunfish and carp. The highest sensitivity was found in trout, where it can damage kidneys and liver [9]. For the decomposition of most MPs, the advanced oxidation processes (AOPs) are usually found to be effective. For example, MZC (10 mg L−1) was totally degraded during the photocatalytic oxidation on lanthanide-doped anatase titanium dioxide photocatalysts [9]. Another promising method for the oxidative degradation of organic pollutants is sonolysis [12]. The application of ultrasound at the frequencies of around 20 KHz produces the sequence of compression and rarefaction cycles in liquid phase forming the cavitation bubbles. The adiabatic character of the bubble collapse creates the conditions at which radical species are formed via the homolytic cleavage of H2O and O2. Several factors influence the efficiency of this process, e.g., the input energy of ultrasound, dissolved gases and the composition of organic matter in aqueous media. In addition, the frequency also affects the performance of sonolytic oxidation process [13–16]. This study is focused on the intensification of the sonolytic degradation of chloroacetamide herbicide MZC and the potential of
Corresponding author. E-mail address: [email protected] (M. Kask).
https://doi.org/10.1016/j.jece.2019.103095 Received 18 February 2019; Received in revised form 1 April 2019; Accepted 13 April 2019 Available online 16 April 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103095
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was reduced to 0% and remained isocratic. Mass spectra were scanned over a m/z range of 50–500 with a scan speed of 938 amu s−1. Ionization was performed using an electrospray ionization positive-ion mode, a probe voltage of 4.5 kV and nebulizer and drying gas flow rates of 1.5 and 15 L min−1, respectively. The desolvation line heater and heated block temperatures were 250 and 400 °C, respectively. The detection limit was 0.1 μM MZC. Samples for LC–MS analysis were filtered using 0.45 μm cartridge filters (Millipore). The presence of hydroxyl radicals (HO%) in the reaction mixture was estimated using the deoxyribose method [17]. This method is based on the selective degradation of deoxyribose (DR) by HO% followed by the formation of malondialdehyde (MDA), which then reacts with 2-thiobarbituric acid (TBA), thereby forming a pink chromophore (the reaction product of TBA and MDA). A 2.8-mM DR solution was oxidized by HO% generated by sonolysis or Fenton’s reagent at similar treatment conditions to that of MZA degradation. At specified time intervals, 1 mL of the sample was taken from the reactor and the radical oxidation process was terminated by addition of 1.25 mL of a 2.8% (w/v) trichloroacetic acid solution. After that TBA (1.25 mL, 1% w/v) was added and the reaction mixture was heated up to 90–100 °C and kept for 20 min at this temperature. The light absorbance of TBA and MDA adduct was determined photometrically in a 1-cm-pathlength cuvette at 532 nm. The quantification of formed HO% was performed using MDA standards prepared via hydrolysis of 1,1,3,3-tetraethoxypropane in a presence of 0.1-N HCl. H2O2 in the reaction mixture was analysed photometrically at 410 nm as a complex of H2O2 with Ti4+ [18]. All the spectrophotometric analyses were performed using a Helios ultravioletvisible spectrophotometer (Thermo Fisher Scientific). The preparation of samples for identification of oxidation intermediates was performed by liquid-liquid extraction using dichloromethane. Obtained extracts were dried with Na2SO4. 10 mL of each extract were concentrated by evaporation to 1 mL for the further analysis by a gas chromatography with mass spectrometry (GC–MS, GC2010, Shimadzu). Gas chromatography was equipped with a Phenomenex Zebron capillary column ZB-5MS (30 m x 0.32 mm inner diameter with film thickness of 0.25 μm). The quadrupole mass-spectrometer (Shimadzu, GCMS-QP2010) was used. Extracts (2 μL) were injected splitless to the GC with injection port temperature of 250 °C. The GC column oven program started from the initial temperature holdup at 80 °C for 2 min with the following increase from 80 to 180 °C at the rate of 20 °C min−1. The next temperature increase was to 280 °C at the rate of 5 °C min−1 with the final holdup for 3 min. Mass spectra of oxidation intermediates were obtained using an electron-impact ionization (EI) mode by scanning over a m/z range of 40–340 with a scan speed of 1666 amu s−1. The ion source and interface temperatures were 230 and 250 °C, respectively. All the experiments were duplicated. The standard deviation of the results was less than 5%.
combination of sonolysis with another AOP. The intensification of the sonolysis was achieved by either addition of insoluble ferric oxyhydroxide (Fe2O3·nH2O) or saturation of water with oxygen. Both approaches allowed reducing the treatment time and the energy consumption. As insoluble ferric oxyhydroxides are considered to be the by-products from Fenton water treatment, their utilization as additives in the sonolysis serves as the basis for the smart integration of both AOPs. 2. Experimental 2.1. Materials and reagents MZC (C14H16ClN3O, ≥ 99.6%), 2-deoxy-D-ribose (C5H10O4, 99%), 2-thiobarbituric acid (C4H4N2O2S, 98%), trichloroacetic acid (C2HCl3O2, 99%), 1,1,3,3-tetraethoxypropane ((C2H5O)2CHCH2CH (OC2H5)2, 96%), ferric sulphate nonahydrate (Fe2(SO4)3·9H2O, ≥ 99%) and sodium hydroxide were purchased from Sigma-Aldrich. Oxygen and nitrogen with technical purity were provided by Elme Messer Gaas AS. All other chemicals were of analytical grade. All the aqueous solutions were prepared using ultrapure water from ultrapure water UV-system (Simplicity®, Merck Millipore). Ferric oxyhydroxide, which served as a catalyst for the Fenton-like mediated oxidation was synthesized by mixing sodium hydroxide and ferric sulphate solutions followed by filtration and washing out of sodium sulphate from precipitate which was then dried at 105 °C for 2 h. 2.2. Procedure and experimental conditions Sonochemical degradation of MZC was conducted using Bandelin Sonopuls HD 3100 (100 W) ultrasound generator with frequency of 20 kHz. Generator was equipped with titanium alloy (TiAl6V4) probe VS-70 T (diameter of 13 mm) that was immersed into 200 mL of 10 μM MZC-spiked solution at the depth of 20 mm. The spiral cooler made of stainless steel was provided to maintain the constant temperature of 22 ± 2 °C. The temperature was measured with ultrasound sensor integrated into the generator. The schematic illustration of the experimental setup is shown in Fig. 1. The experiments were conducted at 70% of maximum ultrasound generator power in the presence and in the absence of dissolved oxygen under atmospheric conditions. For the enrichment of solutions of MZC or water with oxygen, the pure oxygen was introduced into the reactor through the mounted glass bubbler; the oxygen flow rate of 2.0 L min−1 was maintained by means of rotameter. The experiments without oxygen were conducted by bubbling the solution with nitrogen. The total treatment time was 120 min. The pH of MZC solution was adjusted to 3.0 using a 0.5-M sulphuric acid solution and it did not change in the course of the experiments. In the experimental series on combination of sonolysis with insoluble ferric iron, the powdery ferric oxyhydroxide was added to the reactor immediately after turning on the ultrasound generator. The initial concentration of ferric oxyhydroxide in all experiments was 50 mg L−1. The Fenton oxidation process was terminated by adding 1 mL of freshly prepared 0.1-M Na2SO3 solution to 2 mL of sample.
3. Results and discussion 3.1. Effect of oxygen It is particularly important to promote the formation of HO% in the sonolysis of non-volatile compounds, such as MZC (vapour pressure of 1 × 10−7 kPa [19]), since it allows the maximum utilization of the oxidation potential of hydroxyl radicals and other reactive oxygen species (e.g., superoxide radical anion, O2∙−) in the sonochemical processes. The sonolytic treatment process was characterized by the MZC degradation reaction rate. The integral analysis of the dependence of MZC concentration on treatment time (t) indicated that the decomposition of MZC by sonolysis (and its modifications including the experiments on HO%-scavenging) fit in pseudo-first order kinetics. The pseudo-first order reaction rate constant k’ (Table 1) was calculated from Eq. (1).
2.3. Analytical methods The concentration of MZC in spiked water was measured using a high-performance liquid chromatography (HPLC) with mass spectrometry (LC-MS2020, Shimadzu). HPLC was equipped with a Phenomenex Gemini (150 mm x 2.0 mm inner diameter) NX-C18 (110 Å pore size, 5 μm particle size) column. The total flow rate, the analysis time, and the injected sample volume were 0.2 mL min−1, 45 min, and 20 μL, respectively. Gradient elution was achieved with eluent A (0.1% CH3COOH in ultrapure water) and eluent B (pure acetonitrile). The gradient elution started at 0% of eluent B with a continuous increment to 100% in 30 min. After 5 min at this level, the percentage of eluent B 2
Journal of Environmental Chemical Engineering 7 (2019) 103095
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Fig. 1. The principle scheme of ultrasound assembly for degradation of MZC. 1-ultrasonic transducer, 2-probe VS-70 T, 3-temperature sensor, 4-inner spiral cooler, 5glass bubbler, 6-glass reactor, 7-temperature sensor port, 8-ultrasound generator, 9-rotameter, 10-gas flow switch, 11-gas flow regulator.
the basis for the following explanation of the phenomenon obtained: presumptive scavenging of H% by oxygen hinders the recombination reactions between hydrogen and hydroxyl radicals thus promoting the overall oxidation process. This assumption is described in more detail below. The present hypothesis on the positive influence of oxygen in the sonolytic degradation relies on the data obtained in the previous studies [20,21], where CCl4, C6F14, and KIO3 were used as a scavenger of H% in the sonochemical treatment of organic pollutants. It is reasonable to assume here that oxygen could behave in a similar manner as it was observed with CCl4 and other H% scavengers. Since CCl4 is toxic compound and persistent environmental pollutant there is no doubt that saturation of water with oxygen has exceptional benefit compared to that of CCl4. This effect might be due to the competitive reactions (Eqs. (2)–(4))
Table 1 Pseudo-first rate constant of MZC sonolytic degradation. Process
k’ x 102, min−1
Conventional sonolysis Sonolysis in water saturated with O2 Sonolysis in water in the absence of O2 Sonolysis in water with addition of Fe2O3·nH2O
1.11 1.79 0.22 2.88
ln ([MZC ]0 /[MZC ]) = −k ′t
(1)
The experiments demonstrated that saturation of water with oxygen favours the sonolytic degradation of MZC (Fig. 2), whereas the similar experiments in the absence of oxygen showed the opposite results with the degradation being substantially delayed. These results put forward
Fig. 2. Sonolytic degradation of aqueous MZC solution: [MZC]0 = 10 μM, pH = 3.0. 3
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Fig. 3. Formation of HO% in water by sonolysis in the presence and in the absence of O2.
allows additional HO% entering the aqueous phase. Evidently, the pyrolysis of oxygen takes place as well (Eq. (5)). The resultant oxygen atoms form the oxidative species via the reaction with water (Eq. (6)), thus producing the additional amount of oxidative species including HO%.
in the cavitation bubble in gaseous phase, where oxygen reacts with hydrogen radical instead of HO%. The hydroxyl radicals emerge during a reaction (Eq. (2)), where water pyrolysis occurs at high temperatures resulting in a release of HO% and H% to the gaseous medium. H2O +))) → H% + HO%
(2)
H% + HO% → H2O
(3)
H% + O2 → HO2%
(4)
O2 → 2O% %
(5) %
O + H2O → 2HO
(6)
Both reaction mechanisms highlight the necessity of intentional water saturation with oxygen.
These radicals tend to bind by recombination reaction (Eq. (3)) forming water molecule. This process is known as a non-productive in terms of advanced oxidation processes, since a certain amount of the resulting HO% participates in a reversible reaction with H%. If oxygen penetrates to the gaseous phase of cavitation bubble and reacts with formed hydrogen radicals, then a higher amount of HO% could potentially reach the interphase environment and be finally released to the liquid phase. The experiments on the presence of HO% in water confirmed the stated assumption (Fig. 3). The amount of HO% in the bulk solution tends to increase by saturation of water with oxygen, whereas the opposite results were observed in the absence of oxygen. This phenomenon could also be described in terms of reaction kinetics. The reaction between O2 and H% results in the formation of hydroxyperoxyl radical (Eq. (4)) and proceeds with a rate constant of 2.1 × 109 L (mol s)−1. Whereas, the non-productive H% and HO% recombination reaction (Eq. (3)) has a rate constant of 7.0 × 109 L (mol s)−1 [22]. It can be assumed that oxygen is prone to scavenge H%, which
3.2. Effect of ferric iron-containing solid compound Another approach to improve the sonolytic oxidation performance was the application of Fe2O3∙nH2O used as a source of iron in order to utilize H2O2 generated as a result of combination of HO%-s in the interphase region of the cavitation bubble (Eq. (7)). 2HO% → H2O2
(7)
Fe2O3∙nH2O is the by-product of the Fenton wastewater treatment technology and it is considered as a solid waste that needs the additional utilization and treatment. It was presumed that ferric waste could serve as the iron source in the combination of the Fenton-like oxidation with sonolysis. This approach allows reducing the expenses on the chemicals in the Fenton process.
Fig. 4. Formation of H2O2 in aqueous phase in the course of sonolysis of distilled water. 4
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degradation rate and almost all MZC (97%) was degraded within 120 min (Fig. 6). That is in conformity with the calculated degradation rate constants, where k’ (Table 1) was more than twice as high as in mere sonolysis.
The sonolytic generation of H2O2, which is a prerequisite compound for the Fenton reaction to occur, has been studied. In Fig. 4 it could be seen that during 120 min of sonolysis the concentration of H2O2 grows up to 25 μM. Although H2O2 is an oxidizing agent and is hypothetically able to participate in the oxidation of MZC, the present reaction is usually considered as non-productive one. In the absence of activator or catalyst, the oxidation of H2O2 occurs through the ion-molecule mechanism. This process is characterized by a slow oxidation rate, where the main limiting factor is the release of hydroxide ion from the H2O2 molecule, and therefore, the oxidizing capacity of H2O2 is significantly limited [20]. H2O2 homolysis with subsequent HO% formation can be initiated efficiently by transition metals. The use of various Fe(II) forms with H2O2 is known as Fenton reagent (Eq. (8)). This method is widely used to produce HO% from H2O2. Fe(II) + H2O2 → Fe(III) + OH− + HO%
3.3. By-products MZC degradation by-products were identified by means of GC–MS. The mechanism of the degradation is shown in Fig. 7. The compounds indicated in blue were not detected but might conceivably be considered as the precursors of some by-products. The molecular structure of intermediates referred to two general degradation pathways, which are the cleavage of the aliphatic sites of MZC with the initial hydroxylation reactions and oxidation of pyrazole group. One of the degradation pathways is initiated by the hydroxyl radical attack on the methyl group attached to the aromatic ring, thereby resulting in a formation of hydroxyl group (substance A). The corresponding oxidation path results in demethylation of aromatic ring (substance C). The present by-product of MZC was found in a study of Fuerst et al. [24] where the degradation occurred in soil. Consequently, the second methyl group is exposed to hydroxylation similarly (substance D). This sequence of transformations serves as an example of decarboxylation of organic molecule where inorganic CO2 is released and MZC is partially mineralized. This type of reactions could proceed through the formation of corresponding carboxylic acid such as substance B that is the probable precursor of substance C. However, the compounds with a carboxylic acid functional group were not identified. Another decomposition pathway involves the oxidation and detachment of the pyrazole moiety in MZC. Unfortunately, no oxidation intermediates were detected that could indicate the denitrification process. Therefore, it is assumed that the second degradation pathway starts from the substance E. Subsequent decomposition could proceed in three different ways, e.g. the demethylation that results in a formation of product G followed by the formation of aldehyde H, or formation of the aldehyde on methyl group of either acetamide (substance J) or aromatic ring (substance F). Interestingly, the substances J and H are structural isomers of each other and therefore possessed close retention times (Table 2). The present study allowed identifying the primary by-products. The following degradation might proceed through the dechlorination, the subsequent stepwise decarboxylation, ring-opening reactions etc.
(8)
It might apparently seem reasonable to add the soluble Fe(II) in order to trigger immediately the Fenton oxidation process. However, due to gradual generation of H2O2 during distilled water sonolysis (Fig. 4) the presence of iron in aqueous medium would result in unproductive scavenging of HO% through the reaction (Eq. (9)), thus reducing the overall degradation of target compound. Therefore, gradual release of Fe(III) into the solution is favourable. That was the reason why the use of insoluble ferric iron was appropriate under the conditions studied. Fe(II) + HO% → Fe(III) + OH−
(9)
The gradual increase of iron concentration in aqueous phase was achieved by leaching process from the insoluble ferric oxyhydroxide under acidic conditions (pH = 3.0) (Fig. 5). The aptitude of ferric oxyhydroxide utilization in the Fentonmediated oxidation of water pollutants has been previously confirmed [23]. For instance, acidic conditions facilitated the release of Fe(III) from the surface of ferric oxyhydroxide to the aqueous medium, causing the oxidation of organic pollutant trichlorophenol via the Fenton process. The ultrasound promotes the leaching of ferric iron ions into aqueous phase through disintegration of ferric oxyhydroxide solid particles, i.e. ultrasound produces the iron-containing powder with larger specific surface area, thus allowing better contact between liquid and solid phases for the protonation of ferric oxyhydroxide. As it could be seen in Fig. 5, application of ultrasound made the iron leaching process up to three times faster if compared to conventional mechanical stirring. The presence of ferric oxyhydroxide led to increase in the
4. Conclusions The present study was aimed to facilitate novel and simple
Fig. 5. Effect of conventional stirring and ultrasound on the leaching of ferric iron into aqueous phase [20]. 5
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Fig. 6. Degradation of MZC by conventional sonolysis and by sonolysis in the presence of ferric oxyhydroxide.
Fig. 7. Proposed mechanism of MZC degradation by sonolysis.
degradation of MZC: the rate constants were more than twice as high compared to mere sonolysis. Ultrasonic vibration facilitated the leaching of ferric iron into the aqueous phase, where it participated in the reactions with H2O2 formed as a result of ultrasound-mediated processes contributing to the Fenton-like oxidation process. The degradation products of MZC followed two degradation pathways. In the first one the aromatic rings were subjected to the demethylation as a result of sequence of oxidative reactions involving HO%. The second
approaches for enhancement of sonolytic degradation of widely used toxic emerging pollutant MZC. The saturation of water with oxygen resulted in more rapid degradation of studied pesticide. The obtained effect relied on the reaction of oxygen with H% in gaseous phase of cavitation bubble. This process is known as H% scavenging that might hinder the unproductive recombination of H% with HO% and result in the liberation of additional amount of HO% into the bulk solution. The combination of sonolysis with insoluble ferric iron also accelerated the
Table 2 The list of substances identified by GC–MS. Substance
Retention time, min
IUPAC name
metazachlor A C D F G H J
10.7 14.8 13.0 17.5 6.8 6.4 7.1 7.4
2‐chloro‐N‐(2,6‐dimethylphenyl)‐N‐[(1H‐pyrazol‐1-yl)methyl]acetamide 2‐chloro‐N‐[2‐(hydroxymethyl)‐6‐methylphenyl]‐N‐[(1H‐pyrazol‐1-yl)methyl]acetamide 2‐chloro‐N‐(2‐methylphenyl)‐N‐[(1H‐pyrazol‐1-yl)methyl]acetamide 2‐chloro‐N‐[2‐(hydroxymethyl)phenyl]‐N‐[(1H‐pyrazol‐1‐yl)methyl]acetamide 2‐chloro‐N‐(2‐formyl‐6‐methylphenyl)‐N-methylacetamide 2‐chloro‐N‐(2,6‐dimethylphenyl)acetamide 2‐chloro‐N‐(2‐formyl‐6‐methylphenyl)acetamide N‐(2‐chloroacetyl)‐N‐(2‐methylphenyl)formamide
6
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degradation pathway involved the detachment of the pyrazole moiety with subsequent stepwise attack of reactive oxygen species on the methyl groups of formed intermediates.
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Sonolytic degradation of pesticide metazachlor in water: The role of dissolved oxygen and ferric sludge in the process intensification
T
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M. Kask , M. Krichevskaya, J. Bolobajev Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086, Tallinn, Estonia
A R T I C LE I N FO
A B S T R A C T
Keywords: Sonolysis Fenton-like oxidation Metazachlor Pesticide Advanced oxidation process
The study of sonolytic degradation of metazachlor (MZC), which was activated by either oxygen or insoluble Fe (III) was performed. Herbicide MZC was selected as a target compound due to its wide use in agriculture. The results indicated the positive effect of oxygen used as undoubtedly the most harmless H%-scavenger promoting the penetration of HO% from the gaseous phase of the cavitation bubble into the liquid phase and the interphase region. Another approach in activation of sonolysis was the application of Fe2O3 used as a source of iron in order to utilize H2O2 generated as a result of combination of HO∙-s in the interphase region of the cavitation bubble. The present study revealed the synergistic effect of the combination of sonolysis and Fenton-like oxidation. The pseudo-first order rate constant of MZC sonolytic degradation increased from 1.11 × 10−2 min-1 for conventional sonolysis to 1.79 × 10−2 and 2.88 × 10−2 min-1 for O2-saturated and Fe2O3-added solutions, respectively. The application of ultrasound demonstrated the amplifying effect on the detachment of Fe(III) from insoluble ferric oxyhydroxide (Fe2O3·nH2O) in acidic environment into the aqueous solution, thereby improving the oxidation of organic compounds. MZC followed two degradation pathways, which are the cleavage of the aliphatic sites of MZC with the initial hydroxylation reactions and oxidation of pyrazole group.
1. Introduction Over the last few decades, the occurrence of organic micropollutants (MPs) in different environmental matrices has become a worldwide issue [1]. Less or more MPs could be found in water (e.g. surface water, drinking water, groundwater, sewage) and solid matrices (e.g. soil and sediments), where the concentrations vary from ng L−1 to μg L−1 [2] and from μg kg−1 to mg kg−1 [3,4], respectively. MPs may reach the environment as initial compound or metabolites, thereby exerting toxic effect on living organisms including humans via food and drinking water. Due to the growth of population, the use of various products (e.g., plant protection products) has increased [5]. One emerging organic MP is a chloroacetamide herbicide metazachlor (MZC) [6–8], which belongs to the group of endocrine disrupting chemicals [9]. It is widely used in Europe, e.g., the consumption of MZC on arable crop was 177,550 kg in 2003 in the UK and ca. 200,000 kg in Czech Republic in 2010 [8,9]. In 2016, the quantity of MZC delivered to Estonia was 31,515 kg, which is slightly more than one-third higher compared to the year 2011 [10]. The widespread use of this herbicide inevitably leads to the pollution of ground and surface waters. For example, in Estonia it has been found in river Navesti’s two tributaries at concentrations of 0.6 ng L−1 in both places [11]. In Europe the ⁎
concentration of MZC in water bodies varies from 0.1 up to 100 μg L−1, thus requiring the effective water treatment methods to minimize the harmful impact of MZC to living organisms [8]. For instance, it has been found that MZC exposes moderate toxicity to aqueous organisms, mainly daphnia, bluegill, sunfish and carp. The highest sensitivity was found in trout, where it can damage kidneys and liver [9]. For the decomposition of most MPs, the advanced oxidation processes (AOPs) are usually found to be effective. For example, MZC (10 mg L−1) was totally degraded during the photocatalytic oxidation on lanthanide-doped anatase titanium dioxide photocatalysts [9]. Another promising method for the oxidative degradation of organic pollutants is sonolysis [12]. The application of ultrasound at the frequencies of around 20 KHz produces the sequence of compression and rarefaction cycles in liquid phase forming the cavitation bubbles. The adiabatic character of the bubble collapse creates the conditions at which radical species are formed via the homolytic cleavage of H2O and O2. Several factors influence the efficiency of this process, e.g., the input energy of ultrasound, dissolved gases and the composition of organic matter in aqueous media. In addition, the frequency also affects the performance of sonolytic oxidation process [13–16]. This study is focused on the intensification of the sonolytic degradation of chloroacetamide herbicide MZC and the potential of
Corresponding author. E-mail address: [email protected] (M. Kask).
https://doi.org/10.1016/j.jece.2019.103095 Received 18 February 2019; Received in revised form 1 April 2019; Accepted 13 April 2019 Available online 16 April 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
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was reduced to 0% and remained isocratic. Mass spectra were scanned over a m/z range of 50–500 with a scan speed of 938 amu s−1. Ionization was performed using an electrospray ionization positive-ion mode, a probe voltage of 4.5 kV and nebulizer and drying gas flow rates of 1.5 and 15 L min−1, respectively. The desolvation line heater and heated block temperatures were 250 and 400 °C, respectively. The detection limit was 0.1 μM MZC. Samples for LC–MS analysis were filtered using 0.45 μm cartridge filters (Millipore). The presence of hydroxyl radicals (HO%) in the reaction mixture was estimated using the deoxyribose method [17]. This method is based on the selective degradation of deoxyribose (DR) by HO% followed by the formation of malondialdehyde (MDA), which then reacts with 2-thiobarbituric acid (TBA), thereby forming a pink chromophore (the reaction product of TBA and MDA). A 2.8-mM DR solution was oxidized by HO% generated by sonolysis or Fenton’s reagent at similar treatment conditions to that of MZA degradation. At specified time intervals, 1 mL of the sample was taken from the reactor and the radical oxidation process was terminated by addition of 1.25 mL of a 2.8% (w/v) trichloroacetic acid solution. After that TBA (1.25 mL, 1% w/v) was added and the reaction mixture was heated up to 90–100 °C and kept for 20 min at this temperature. The light absorbance of TBA and MDA adduct was determined photometrically in a 1-cm-pathlength cuvette at 532 nm. The quantification of formed HO% was performed using MDA standards prepared via hydrolysis of 1,1,3,3-tetraethoxypropane in a presence of 0.1-N HCl. H2O2 in the reaction mixture was analysed photometrically at 410 nm as a complex of H2O2 with Ti4+ [18]. All the spectrophotometric analyses were performed using a Helios ultravioletvisible spectrophotometer (Thermo Fisher Scientific). The preparation of samples for identification of oxidation intermediates was performed by liquid-liquid extraction using dichloromethane. Obtained extracts were dried with Na2SO4. 10 mL of each extract were concentrated by evaporation to 1 mL for the further analysis by a gas chromatography with mass spectrometry (GC–MS, GC2010, Shimadzu). Gas chromatography was equipped with a Phenomenex Zebron capillary column ZB-5MS (30 m x 0.32 mm inner diameter with film thickness of 0.25 μm). The quadrupole mass-spectrometer (Shimadzu, GCMS-QP2010) was used. Extracts (2 μL) were injected splitless to the GC with injection port temperature of 250 °C. The GC column oven program started from the initial temperature holdup at 80 °C for 2 min with the following increase from 80 to 180 °C at the rate of 20 °C min−1. The next temperature increase was to 280 °C at the rate of 5 °C min−1 with the final holdup for 3 min. Mass spectra of oxidation intermediates were obtained using an electron-impact ionization (EI) mode by scanning over a m/z range of 40–340 with a scan speed of 1666 amu s−1. The ion source and interface temperatures were 230 and 250 °C, respectively. All the experiments were duplicated. The standard deviation of the results was less than 5%.
combination of sonolysis with another AOP. The intensification of the sonolysis was achieved by either addition of insoluble ferric oxyhydroxide (Fe2O3·nH2O) or saturation of water with oxygen. Both approaches allowed reducing the treatment time and the energy consumption. As insoluble ferric oxyhydroxides are considered to be the by-products from Fenton water treatment, their utilization as additives in the sonolysis serves as the basis for the smart integration of both AOPs. 2. Experimental 2.1. Materials and reagents MZC (C14H16ClN3O, ≥ 99.6%), 2-deoxy-D-ribose (C5H10O4, 99%), 2-thiobarbituric acid (C4H4N2O2S, 98%), trichloroacetic acid (C2HCl3O2, 99%), 1,1,3,3-tetraethoxypropane ((C2H5O)2CHCH2CH (OC2H5)2, 96%), ferric sulphate nonahydrate (Fe2(SO4)3·9H2O, ≥ 99%) and sodium hydroxide were purchased from Sigma-Aldrich. Oxygen and nitrogen with technical purity were provided by Elme Messer Gaas AS. All other chemicals were of analytical grade. All the aqueous solutions were prepared using ultrapure water from ultrapure water UV-system (Simplicity®, Merck Millipore). Ferric oxyhydroxide, which served as a catalyst for the Fenton-like mediated oxidation was synthesized by mixing sodium hydroxide and ferric sulphate solutions followed by filtration and washing out of sodium sulphate from precipitate which was then dried at 105 °C for 2 h. 2.2. Procedure and experimental conditions Sonochemical degradation of MZC was conducted using Bandelin Sonopuls HD 3100 (100 W) ultrasound generator with frequency of 20 kHz. Generator was equipped with titanium alloy (TiAl6V4) probe VS-70 T (diameter of 13 mm) that was immersed into 200 mL of 10 μM MZC-spiked solution at the depth of 20 mm. The spiral cooler made of stainless steel was provided to maintain the constant temperature of 22 ± 2 °C. The temperature was measured with ultrasound sensor integrated into the generator. The schematic illustration of the experimental setup is shown in Fig. 1. The experiments were conducted at 70% of maximum ultrasound generator power in the presence and in the absence of dissolved oxygen under atmospheric conditions. For the enrichment of solutions of MZC or water with oxygen, the pure oxygen was introduced into the reactor through the mounted glass bubbler; the oxygen flow rate of 2.0 L min−1 was maintained by means of rotameter. The experiments without oxygen were conducted by bubbling the solution with nitrogen. The total treatment time was 120 min. The pH of MZC solution was adjusted to 3.0 using a 0.5-M sulphuric acid solution and it did not change in the course of the experiments. In the experimental series on combination of sonolysis with insoluble ferric iron, the powdery ferric oxyhydroxide was added to the reactor immediately after turning on the ultrasound generator. The initial concentration of ferric oxyhydroxide in all experiments was 50 mg L−1. The Fenton oxidation process was terminated by adding 1 mL of freshly prepared 0.1-M Na2SO3 solution to 2 mL of sample.
3. Results and discussion 3.1. Effect of oxygen It is particularly important to promote the formation of HO% in the sonolysis of non-volatile compounds, such as MZC (vapour pressure of 1 × 10−7 kPa [19]), since it allows the maximum utilization of the oxidation potential of hydroxyl radicals and other reactive oxygen species (e.g., superoxide radical anion, O2∙−) in the sonochemical processes. The sonolytic treatment process was characterized by the MZC degradation reaction rate. The integral analysis of the dependence of MZC concentration on treatment time (t) indicated that the decomposition of MZC by sonolysis (and its modifications including the experiments on HO%-scavenging) fit in pseudo-first order kinetics. The pseudo-first order reaction rate constant k’ (Table 1) was calculated from Eq. (1).
2.3. Analytical methods The concentration of MZC in spiked water was measured using a high-performance liquid chromatography (HPLC) with mass spectrometry (LC-MS2020, Shimadzu). HPLC was equipped with a Phenomenex Gemini (150 mm x 2.0 mm inner diameter) NX-C18 (110 Å pore size, 5 μm particle size) column. The total flow rate, the analysis time, and the injected sample volume were 0.2 mL min−1, 45 min, and 20 μL, respectively. Gradient elution was achieved with eluent A (0.1% CH3COOH in ultrapure water) and eluent B (pure acetonitrile). The gradient elution started at 0% of eluent B with a continuous increment to 100% in 30 min. After 5 min at this level, the percentage of eluent B 2
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Fig. 1. The principle scheme of ultrasound assembly for degradation of MZC. 1-ultrasonic transducer, 2-probe VS-70 T, 3-temperature sensor, 4-inner spiral cooler, 5glass bubbler, 6-glass reactor, 7-temperature sensor port, 8-ultrasound generator, 9-rotameter, 10-gas flow switch, 11-gas flow regulator.
the basis for the following explanation of the phenomenon obtained: presumptive scavenging of H% by oxygen hinders the recombination reactions between hydrogen and hydroxyl radicals thus promoting the overall oxidation process. This assumption is described in more detail below. The present hypothesis on the positive influence of oxygen in the sonolytic degradation relies on the data obtained in the previous studies [20,21], where CCl4, C6F14, and KIO3 were used as a scavenger of H% in the sonochemical treatment of organic pollutants. It is reasonable to assume here that oxygen could behave in a similar manner as it was observed with CCl4 and other H% scavengers. Since CCl4 is toxic compound and persistent environmental pollutant there is no doubt that saturation of water with oxygen has exceptional benefit compared to that of CCl4. This effect might be due to the competitive reactions (Eqs. (2)–(4))
Table 1 Pseudo-first rate constant of MZC sonolytic degradation. Process
k’ x 102, min−1
Conventional sonolysis Sonolysis in water saturated with O2 Sonolysis in water in the absence of O2 Sonolysis in water with addition of Fe2O3·nH2O
1.11 1.79 0.22 2.88
ln ([MZC ]0 /[MZC ]) = −k ′t
(1)
The experiments demonstrated that saturation of water with oxygen favours the sonolytic degradation of MZC (Fig. 2), whereas the similar experiments in the absence of oxygen showed the opposite results with the degradation being substantially delayed. These results put forward
Fig. 2. Sonolytic degradation of aqueous MZC solution: [MZC]0 = 10 μM, pH = 3.0. 3
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Fig. 3. Formation of HO% in water by sonolysis in the presence and in the absence of O2.
allows additional HO% entering the aqueous phase. Evidently, the pyrolysis of oxygen takes place as well (Eq. (5)). The resultant oxygen atoms form the oxidative species via the reaction with water (Eq. (6)), thus producing the additional amount of oxidative species including HO%.
in the cavitation bubble in gaseous phase, where oxygen reacts with hydrogen radical instead of HO%. The hydroxyl radicals emerge during a reaction (Eq. (2)), where water pyrolysis occurs at high temperatures resulting in a release of HO% and H% to the gaseous medium. H2O +))) → H% + HO%
(2)
H% + HO% → H2O
(3)
H% + O2 → HO2%
(4)
O2 → 2O% %
(5) %
O + H2O → 2HO
(6)
Both reaction mechanisms highlight the necessity of intentional water saturation with oxygen.
These radicals tend to bind by recombination reaction (Eq. (3)) forming water molecule. This process is known as a non-productive in terms of advanced oxidation processes, since a certain amount of the resulting HO% participates in a reversible reaction with H%. If oxygen penetrates to the gaseous phase of cavitation bubble and reacts with formed hydrogen radicals, then a higher amount of HO% could potentially reach the interphase environment and be finally released to the liquid phase. The experiments on the presence of HO% in water confirmed the stated assumption (Fig. 3). The amount of HO% in the bulk solution tends to increase by saturation of water with oxygen, whereas the opposite results were observed in the absence of oxygen. This phenomenon could also be described in terms of reaction kinetics. The reaction between O2 and H% results in the formation of hydroxyperoxyl radical (Eq. (4)) and proceeds with a rate constant of 2.1 × 109 L (mol s)−1. Whereas, the non-productive H% and HO% recombination reaction (Eq. (3)) has a rate constant of 7.0 × 109 L (mol s)−1 [22]. It can be assumed that oxygen is prone to scavenge H%, which
3.2. Effect of ferric iron-containing solid compound Another approach to improve the sonolytic oxidation performance was the application of Fe2O3∙nH2O used as a source of iron in order to utilize H2O2 generated as a result of combination of HO%-s in the interphase region of the cavitation bubble (Eq. (7)). 2HO% → H2O2
(7)
Fe2O3∙nH2O is the by-product of the Fenton wastewater treatment technology and it is considered as a solid waste that needs the additional utilization and treatment. It was presumed that ferric waste could serve as the iron source in the combination of the Fenton-like oxidation with sonolysis. This approach allows reducing the expenses on the chemicals in the Fenton process.
Fig. 4. Formation of H2O2 in aqueous phase in the course of sonolysis of distilled water. 4
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degradation rate and almost all MZC (97%) was degraded within 120 min (Fig. 6). That is in conformity with the calculated degradation rate constants, where k’ (Table 1) was more than twice as high as in mere sonolysis.
The sonolytic generation of H2O2, which is a prerequisite compound for the Fenton reaction to occur, has been studied. In Fig. 4 it could be seen that during 120 min of sonolysis the concentration of H2O2 grows up to 25 μM. Although H2O2 is an oxidizing agent and is hypothetically able to participate in the oxidation of MZC, the present reaction is usually considered as non-productive one. In the absence of activator or catalyst, the oxidation of H2O2 occurs through the ion-molecule mechanism. This process is characterized by a slow oxidation rate, where the main limiting factor is the release of hydroxide ion from the H2O2 molecule, and therefore, the oxidizing capacity of H2O2 is significantly limited [20]. H2O2 homolysis with subsequent HO% formation can be initiated efficiently by transition metals. The use of various Fe(II) forms with H2O2 is known as Fenton reagent (Eq. (8)). This method is widely used to produce HO% from H2O2. Fe(II) + H2O2 → Fe(III) + OH− + HO%
3.3. By-products MZC degradation by-products were identified by means of GC–MS. The mechanism of the degradation is shown in Fig. 7. The compounds indicated in blue were not detected but might conceivably be considered as the precursors of some by-products. The molecular structure of intermediates referred to two general degradation pathways, which are the cleavage of the aliphatic sites of MZC with the initial hydroxylation reactions and oxidation of pyrazole group. One of the degradation pathways is initiated by the hydroxyl radical attack on the methyl group attached to the aromatic ring, thereby resulting in a formation of hydroxyl group (substance A). The corresponding oxidation path results in demethylation of aromatic ring (substance C). The present by-product of MZC was found in a study of Fuerst et al. [24] where the degradation occurred in soil. Consequently, the second methyl group is exposed to hydroxylation similarly (substance D). This sequence of transformations serves as an example of decarboxylation of organic molecule where inorganic CO2 is released and MZC is partially mineralized. This type of reactions could proceed through the formation of corresponding carboxylic acid such as substance B that is the probable precursor of substance C. However, the compounds with a carboxylic acid functional group were not identified. Another decomposition pathway involves the oxidation and detachment of the pyrazole moiety in MZC. Unfortunately, no oxidation intermediates were detected that could indicate the denitrification process. Therefore, it is assumed that the second degradation pathway starts from the substance E. Subsequent decomposition could proceed in three different ways, e.g. the demethylation that results in a formation of product G followed by the formation of aldehyde H, or formation of the aldehyde on methyl group of either acetamide (substance J) or aromatic ring (substance F). Interestingly, the substances J and H are structural isomers of each other and therefore possessed close retention times (Table 2). The present study allowed identifying the primary by-products. The following degradation might proceed through the dechlorination, the subsequent stepwise decarboxylation, ring-opening reactions etc.
(8)
It might apparently seem reasonable to add the soluble Fe(II) in order to trigger immediately the Fenton oxidation process. However, due to gradual generation of H2O2 during distilled water sonolysis (Fig. 4) the presence of iron in aqueous medium would result in unproductive scavenging of HO% through the reaction (Eq. (9)), thus reducing the overall degradation of target compound. Therefore, gradual release of Fe(III) into the solution is favourable. That was the reason why the use of insoluble ferric iron was appropriate under the conditions studied. Fe(II) + HO% → Fe(III) + OH−
(9)
The gradual increase of iron concentration in aqueous phase was achieved by leaching process from the insoluble ferric oxyhydroxide under acidic conditions (pH = 3.0) (Fig. 5). The aptitude of ferric oxyhydroxide utilization in the Fentonmediated oxidation of water pollutants has been previously confirmed [23]. For instance, acidic conditions facilitated the release of Fe(III) from the surface of ferric oxyhydroxide to the aqueous medium, causing the oxidation of organic pollutant trichlorophenol via the Fenton process. The ultrasound promotes the leaching of ferric iron ions into aqueous phase through disintegration of ferric oxyhydroxide solid particles, i.e. ultrasound produces the iron-containing powder with larger specific surface area, thus allowing better contact between liquid and solid phases for the protonation of ferric oxyhydroxide. As it could be seen in Fig. 5, application of ultrasound made the iron leaching process up to three times faster if compared to conventional mechanical stirring. The presence of ferric oxyhydroxide led to increase in the
4. Conclusions The present study was aimed to facilitate novel and simple
Fig. 5. Effect of conventional stirring and ultrasound on the leaching of ferric iron into aqueous phase [20]. 5
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Fig. 6. Degradation of MZC by conventional sonolysis and by sonolysis in the presence of ferric oxyhydroxide.
Fig. 7. Proposed mechanism of MZC degradation by sonolysis.
degradation of MZC: the rate constants were more than twice as high compared to mere sonolysis. Ultrasonic vibration facilitated the leaching of ferric iron into the aqueous phase, where it participated in the reactions with H2O2 formed as a result of ultrasound-mediated processes contributing to the Fenton-like oxidation process. The degradation products of MZC followed two degradation pathways. In the first one the aromatic rings were subjected to the demethylation as a result of sequence of oxidative reactions involving HO%. The second
approaches for enhancement of sonolytic degradation of widely used toxic emerging pollutant MZC. The saturation of water with oxygen resulted in more rapid degradation of studied pesticide. The obtained effect relied on the reaction of oxygen with H% in gaseous phase of cavitation bubble. This process is known as H% scavenging that might hinder the unproductive recombination of H% with HO% and result in the liberation of additional amount of HO% into the bulk solution. The combination of sonolysis with insoluble ferric iron also accelerated the
Table 2 The list of substances identified by GC–MS. Substance
Retention time, min
IUPAC name
metazachlor A C D F G H J
10.7 14.8 13.0 17.5 6.8 6.4 7.1 7.4
2‐chloro‐N‐(2,6‐dimethylphenyl)‐N‐[(1H‐pyrazol‐1-yl)methyl]acetamide 2‐chloro‐N‐[2‐(hydroxymethyl)‐6‐methylphenyl]‐N‐[(1H‐pyrazol‐1-yl)methyl]acetamide 2‐chloro‐N‐(2‐methylphenyl)‐N‐[(1H‐pyrazol‐1-yl)methyl]acetamide 2‐chloro‐N‐[2‐(hydroxymethyl)phenyl]‐N‐[(1H‐pyrazol‐1‐yl)methyl]acetamide 2‐chloro‐N‐(2‐formyl‐6‐methylphenyl)‐N-methylacetamide 2‐chloro‐N‐(2,6‐dimethylphenyl)acetamide 2‐chloro‐N‐(2‐formyl‐6‐methylphenyl)acetamide N‐(2‐chloroacetyl)‐N‐(2‐methylphenyl)formamide
6
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degradation pathway involved the detachment of the pyrazole moiety with subsequent stepwise attack of reactive oxygen species on the methyl groups of formed intermediates.
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