- Email: [email protected]
Electrochemically assisted decomposition of ozone for degradation and mineralization of Diuron
Journal Pre-proof Electrochemically assisted decomposition of ozone for degradation and mineralization of Diuron Irene Bavasso, Daniele Montanaro, Luca Di Palma, Elisabetta Petrucci PII:
S0013-4686(19)32295-9
DOI:
https://doi.org/10.1016/j.electacta.2019.135423
Reference:
EA 135423
To appear in:
Electrochimica Acta
Received Date: 6 August 2019 Revised Date:
8 November 2019
Accepted Date: 29 November 2019
Please cite this article as: I. Bavasso, D. Montanaro, L. Di Palma, E. Petrucci, Electrochemically assisted decomposition of ozone for degradation and mineralization of Diuron, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2019.135423. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
generator
OZONE
Absorbance
3
AIR
2
1
anode
0 190
220
250
280
Wave length/nm
Diuron
310
340
cathode porous stone
(3-(3,4-Dichlorophenyl)-1,1-dimethylurea)
O3 + HO2- OH + O2- + O2
O3 + e- O3O3- ↔ O2- + OO3- +H2O OH +OH-
Electrochemically assisted decomposition of ozone for degradation and mineralization of Diuron Irene Bavasso, Daniele Montanaro, Luca Di Palma, Elisabetta Petrucci Chemical Engineering Materials & Environment Department Sapienza University of Rome Via Eudossiana, 18 – 00184, Roma, Italy
Corresponding author: [email protected]
Abstract In this work, we explore the possibility of enhancing the ozonation of a solution of Diuron by combination with electrochemical processes. To this aim, the ozonation was performed in a membrane free electrolyzer where a reticulated vitreous carbon (RVC) and a stainless steel (SS) cathode have been alternatively tested and compared. The effect of pH, current density and ozone flow on degradation and mineralization of Diuron has been investigated. The involvement of radical species has been verified by means of test conducted with selective scavenging agents. The results show that both cathodes can promote the decomposition of ozone and therefore the degradation of Diuron. The use of scavengers has shown that the removal of Diuron is driven by means of radical species. Depending on the adopted materials, different mechanisms occur and different operating conditions are required. In particular, the use of a carbon-based cathode implies the production of hydrogen peroxide and therefore the occurrence of an Electro-peroxone process, which is favored at alkaline pH and low current values. On the other hand, the adoption of a metal cathode, whose efficiency is highly promoted by acidic pH and high currents, implies the direct reduction of ozone with the production of hydroxyl radicals.
1
Keywords: Diuron; ozone; stainless steel cathode; reticulated vitreous carbon cathode; electroperoxone.
1. Introduction Ozone is a powerful oxidant, widely used for a variety of applications from disinfection, to the removal of color and odor and sludge reduction [1]. It exhibits high selectivity towards specific functional groups such as unsaturated bonds and deprotonated amines. However, in spite of its high redox potential (2.07 V vs SHE) [2], the direct oxidation by ozone (Eq.1), results in low degradation rate and negligible mineralization [3] of organic compounds. O3 + H2O → 2O2 + 2H+
(1)
To overcome this issue, different strategies have been adopted. First, ozonation can be promoted by selecting appropriate pH values. In fact, as known, alkaline conditions determine fast ozone decomposition. Different routes initiated by hydroxyl ions have been proposed and the possible production of hydroxyl radicals, which is a non-selective highly powerful oxidant (2.8 V vs SHE), has been also hypothesized.
Route 1 [4] O3 + OH– → O2•– + HO2• HO2• ↔ H+ + O2•–
(2) (pKa=4.8)
(3)
O3 + O2•– → O3•– + O2
(4)
O3•– + H2O → •OH + OH– + O2
(5)
Route 2 [4] O3 + OH– → HO2– + O2 O3 + HO2– → •OH + O2•– + O2
(k=40 L mol s-1)
(6) (7)
2
followed by reactions (4) and (5)
Route 3 [5] O3 + OH– → HO2– + O2
(k=40 L mol s-1)
(8)
O3 + HO2– → O3•– + HO2•
(9)
HO2• + OH– ↔ O2•– + H2O
(10)
followed by reactions (4) and (5).
A further acceleration in ozone decomposition and therefore in hydroxyl radical production can be obtained by addition of hydrogen peroxide, at alkaline conditions, in the so-called peroxone process. In fact, in the presence of hydrogen peroxide, high pH values shift the equilibrium of Eq. 11 towards the species HO2- [4], thus avoiding the limitation due to the occurrence of Eq. 6 and 8 and resulting in a quicker evolution in accordance with route 2 and route 3. H2O2 ↔ HO2– + H+
(pKa=11.6)
(11)
To justify the reduced production of hydroxyl radicals, approximately half of what is expected, an alternative mechanism has been proposed [6] which provides for the formation of an adduct (Eq. 12) HO2− + O3 → HO5−
(∆G° = −39.8 kJ mol−1)
(12)
whose fate includes two competing reactions only one of which leads to the formation of active radical species (Eq. 13) while the other releases oxygen and hydroxide ions (Eq. 14). HO5− → HO2• + O3•−
(∆G0 = 13.2 kJ mol−1)
(13)
HO5− → 2O2 + OH−
(∆G0 = -197 kJ mol−1)
(14)
An increasing number of papers deal with the catalytic decomposition of ozone. A great variety of materials have been proposed as catalysts, mostly of metallic nature [7] either at the nano-scale [8] 3
or supported at the meso-scale [9]. However, in many cases, the actual mechanism, the health and environmental impact, the scale-up costs, the stability of the catalyst and its possible reuse remain uncertain, thus limiting the feasibility and viability of this process [10]. Another approach to achieving higher efficiency and faster removal rates is the synergistic integration of ozonation with other methods such as photolysis [11] and sonolysis [12,13], which are only slightly effective if applied alone. In recent years, the possibility of obtaining electricity from renewable sources has led to renewed interest in electrochemical technologies, which are appreciated for environmental compatibility, effectiveness and ease of control. In the field of environmental treatments in the liquid phase, much of the research has been focused on the development of anodic treatments, while less attention has been paid to cathodic treatments, with the exception of Fenton-type processes or electrochemical dechlorination. However, with respect to anodic treatments, the cathodic treatments present the advantage of working at lower potentials. This, on one hand, reduces significantly the wear and degradation of the electrode materials and, on the other hand, allows greater selectivity thus limiting the parasitic reactions and the production of harmful intermediate species. This paper investigates the possibility of enhancing the ozonation of a model pollutant by combination with a cathodic electrochemical process. To this aim, the ozone has been sparged into a membrane-free electrolyzer where a reticulated vitreous carbon cathode (RVC- O3) and a stainless steel cathode (SS- O3) have been alternatively tested and compared. Due to the direct reduction on the cathode surface, the adoption of a metal electrode seems to promote the ozone decomposition to •OH according to the following reactions (Eq. 15-16) [14]. O3 + e- → O3•-
(15)
O3•- + H2O ↔ •OH + OH- + O2
(16)
While, the adoption of a carbon-based cathode enables the in-situ production of hydrogen peroxide by cathodic reduction of oxygen (Eq.17). This process, known as electro-peroxone, represents an 4
improvement of the peroxone treatment since avoids the external addition of H2O2 that is unsafe to handle due to its high reactivity [15]. O2 + 2H+ + 2e- → H2O2
(17)
Both electrochemically-aided ozonation processes have been tested in the oxidation and mineralization of Diuron as the model compound. The effect of the main operative conditions on the treatment efficiency has been investigated and discussed. Diuron (3-(3,4-Dichlorophenyl)-1,1-dimethylurea) is an herbicide exhibiting toxic and teratogenic effects to mammals and other living organisms and also suspected of being an endocrine disruptor [16]. For this reason, it has been included in the “Candidate Contaminant List” of EPA agency (CCL3). Different treatments have been tested for the removal of this pollutant but few of them have proved effective [17]. In particular, only partial mineralization has been achieved. This aspect is of particular concern since several studies have verified that the compounds formed by degradation of Diuron may exhibit significantly higher toxicity than Diuron itself [18,19].
2. Materials and Methods
2.1 Chemicals Diuron (UN 3077- Lamirsa) stock solution (40 mg L-1) was prepared with distilled water and 0.05 M of Na2SO4 as background electrolyte. The initial pH of the solution was about 6.5±0.5 while the total organic content was approximately 17.4± 0.9 mg L-1. When required, the pH of the solution was adjusted using H2SO4 (1 M) and NaOH (2 M). For the detection of hydroxide and oxide radicals tert-butyl alcohol and L-Histidine were used as radical scavengers. All reagents were purchased from Sigma Aldrich and used without any further purification.
2.2 Electrochemical tests
5
Experiments were performed in a cylindrical glass reactor (4 cm inner diameter and 15 cm long) filled with 100 mL of Diuron solution and equipped with a ceramic micro-porous ozone diffuser. Two different air-fed corona discharge ozone generators have been used. An INO3MAX-04 of 100 mg h-1 maximum production capacity, to obtain a low flow rate of ozone (17.4 mgO3 L-1h-1, indicated as LOR in the text) and a Steril 1000 SEPRA of 1 g h-1 maximum production capacity, to obtain a high flow rate of ozone (260 mgO3 L-1h-1, indicated as HOR in the text). The concentration of provided ozone was evaluated by using the indigo colorimetric method [20]. The electrochemically assisted ozone treatment was conducted by using two different cathodic materials. For the electro-peroxone process (RVC-O3), we adopted a 100 ppi RVC electrode (ERG Aerospace Corporation, Oakland CA) with a submerged geometric surface of 1 cm x 1 cm x 2.5 cm. No oxygen flow was supplied and the dissolved oxygen present in solution derived from the ozone generator that insufflated a gaseous mixture of ozone and air. For the ozone decomposition (SS-O3), we adopted a 304 stainless steel plate electrode, with a submerged geometric surface of 2 cm x 2.5 cm. A platinum (Pt) wire electrode was used as the anode. The electrodes were placed vertically at a distance of about 1 cm. Galvanostatic tests were conducted at different current intensity values from 25 mA to 250 mA by using an Amel 2051 potentiostat/galvanostat (Amel, Italy). All tests were performed at 24±1 °C. The occurrence of diuron stripping by ozone bubbling was excluded by sparging air instead of ozone under the same operative conditions. The H2O2 electrogeneration accumulation tests were performed either on a two-chamber cell (each having a volume of 100 mL, separated by a Nafion 324 cation exchange membrane and stirred with a magnetic bar) or in the undivided reactor adopted for the ozone-based experiments, by supplying a 200 mL min-1 oxygen flow.
2.3 Analysis
6
Cyclic voltammetry (CV) experiments were conducted using a bio-Logic VSP 300 instrument. The tests were performed in 50 mL of 0.5 M H2SO4 solution, at room temperature using a threeelectrode cell consisting of the SS cathode, a Pt sheet as the counter electrode and a saturated calomel electrode (SCE) as reference. The scan rate was 100 mV s-1 and the potential ranged from 1 to +1.4 V. The solution pH was measured using a Crison GLP 421. Hydrogen peroxide concentration was determined through the Merck reflectometric kit based on the use of a specific peroxidase reagent (detection limit 0.5 mg L-1). The Diuron removal was monitored by absorbance measurement at λ= 248 nm (Fig.S1) using a PG Instruments T80+ UV/Vis spectrophotometer (using a quartz cell of 1 cm path length). The mineralization extent was quantified using a Shimadzu TOC-L CSH/CSN analyzer. Prior to analysis, samples were acidified to pH 3 by adding a few drops of 37% HCl and then sparged with air for 5 min to remove the dissolved carbon dioxide. Diuron degradation (D%) and mineralization (M%) were calculated as: (%) =
( − ) ∙ 100
(18)
(%) =
( − ) ∙ 100
(19)
where A0 and TOC0 are the absorbance (measured at λ= 248 nm) and total organic carbon values at initial time, while At and TOCt are the corresponding values at a generic t. The mineralization current efficiency was calculated as [21]: (%) =
( − ) ∗ 100 4.32 ∙ 10
(20)
where n is the number of electrons exchanged in the molecule mineralization according to eq. 21 [22,23], F is the Faraday constant (96485 C mol−1), V is the sample volume (L), TOC0 and TOCt 7
are the total organic carbon values at initial and at a generic t time, 4.32 × 107 is a conversion factor for units homogenization (3600 s h−1 × 12,000 C mol−1), m is the number of carbon atoms in the molecule, I is the applied current (A) and t is the electrolysis time (h). C9H10Cl2N2O+ 17H2O → 9CO2 + 2NH4+ + 2Cl− + 36H+ + 36e−
(21)
All tests were repeated at least three times and the results are reported as mean values with coefficient variation less than 7 %.
3. Results and discussion 3.1 Ozonation experiments The effect of ozone alone as a function of pH was investigated under both LOR and HOR regimes (Fig. 1) The effect of ozone alone as a function of pH was investigated under both LOR and HOR regimes (Fig. 1). It is worth noting that in the entire investigated pH range Diuron shows no tendency either to protonation nor to deprotonation (pKa1=13.55, pKa2=-1.09, pKa3=-2.48) thus maintaining its molecular structure substantially unaltered [24].
(Figure 1) As can be seen (Fig. 1a), after a two-hour reaction, the degradation of Diuron at pH 3 was only 20%. A significant improvement was observed in the test conducted at pH 10. Nonetheless, the herbicide removal after two hours barely reached 80%. The behavior of the solution where pH was unadjusted can be explained by considering the pH evolution. In fact, as ozone reacted rapidly with the available OH-, the solution experienced fast acidification. Therefore, as long as the pH remained at circumneutral values, the removal of Diuron did not differ much from that in the test conducted at pH 10. When, after about 30 minutes of reaction, the pH value decreased, the reaction slowed down and only partial degradation was achieved at the end of the reaction (60% after 2 hours).
8
The UV spectra recorded after 120 minutes of reaction (Fig. 2Sa) show a reduction of the peak at 248 nm depending on the applied pH but a substantial persistence of the peak at 190 nm, which is related to the aromatic ring, and the presence of a further peak at about 290 nanometers probably ascribable to a reaction intermediate. At higher ozone rate (HOR), the degradation efficiency improved considerably (Fig. 1b). In particular, even at acidic pH, after only 30 minutes of reaction, the degradation of the compound was over 75%. At pH 10 almost total removal was achieved in only 20 minutes. Unlike what has been observed previously, the test conducted at free pH showed efficiencies low and similar to those found at pH 3. This behavior can be easily explained by considering the evolution of pH that dropped to very acidic values after only 5 minutes. Despite the rapid degradation, UV spectra recorded after 30 minutes (Fig. 2Sb) show that the aromatic rings remained intact thus suggesting poor mineralization. This was confirmed by TOC measurements that revealed appreciable mineralization only for the test conducted at alkaline pH (39.2%) (Table 1). However, as the mineralization rate tended to slow down, no significant improvement was obtained by extended treatments. In fact, although a 20% mineralization was reached in the first hour of treatment, thus suggesting a moderate reactivity of the molecule, only 11% and 8 % were removed during the second and the third hour, respectively, thus indicating the production of less reactive intermediates. The low reactivity at low pH is generally explained considering that under these conditions the ozone reacts in its molecular form, according to the direct mechanism, and therefore with great selectivity but mild oxidizing power. At alkaline pH conditions, higher mineralization efficiencies were expected due to the production of hydroxyl radicals according to the mechanism proposed by several authors [4- 25,26]. Instead, the results reported in Table 1 seem to denote a low production of hydroxyl radicals.
9
Table 1. Effect of pH on mineralization efficiency M(%) during LOR and HOR ozonation. Ozone rate
pH
Time (min)
M (%)
LOR LOR LOR HOR HOR HOR HOR HOR
3 Free 10 3 Free 10 10 10
120 120 120 180 180 30 120 180
<5 <5 <5 <5 <5 10.08 31.41 39.18
To clarify this particular aspect, further ozonation tests have been carried out at pH 10 with the addition of tert-buthyl alcohol, which is a compound known for its scavenging activity of hydroxyl radicals (Fig. 2a) and therefore it is expected to significantly impair the Diuron degradation in case ●
OH radicals were the oxidizing agents. (Figure 2)
Interestingly, the curves reported in Fig. 2a differ little from each other, thus suggesting only a partial contribution of the hydroxyl radical to the process and the likely production and involvement of other radical species that successfully degrade Diuron but are substantially ineffective or slowly reactive in its mineralization.
3.2 RVC-O3 experiments Preliminary tests were conducted to quantify the hydrogen peroxide production and its depletion due to concomitant side reactions. According to our previous investigation [27], a 25 mA current was adopted as best compromise between the production rate and the current efficiency. Fig. 3 shows that in a two-chamber reactor, the concentration of hydrogen peroxide increased linearly with time and was about 150 mg L-1 after 90 minutes. A substantial reduction (- 80%) in its accumulation was observed in the undivided reactor adopted in this research. The detrimental effect can be attributed to the simultaneous decomposition to water and oxygen of the electrogenerated 10
hydrogen peroxide on the anode surface. In the presence of ozone, a further H2O2 depletion is expected. Firstly, because, due to its high solubility, ozone sparging may negatively affect the amount of dissolved oxygen and secondly, because the copresence of ozone and hydrogen peroxide gives rise to the peroxone reaction. However, the results in Fig. 3 show two different responses of the system depending on the ozone flow rate. Under LOR conditions, ozone was slowly transferred into solution (17 mg L-1h-1) and therefore either limited stripping of oxygen by ozone turbulence or H2O2 consumption in the direct reaction, occurred. Instead, under HOR conditions, both effects were amplified by the large availability of dissolved ozone (260 mg L-1 h-1) and only a small residual peroxide concentration was detected (less than 10 mg L-1). (Figure 3) Considering that the peroxone treatment benefits from a O3 to H2O2 ratio higher than 1 [28,29] with optimal performance when such a ratio is 2 [30], experiments under LOR conditions were expected to be slightly effective in the degradation and mineralization of Diuron. By comparing the production values of hydrogen peroxide (data reported in Fig.3 in divided cell and under oxygen supply) and the ozone supply in the two adopted configurations (LOR and HOR), it is possible to estimate an average molar ratio of the two reagents equal to 0.13 and 1.97, respectively. Fig.4a shows that even under the mild adopted conditions (LOR at 25 mA), the RVC-O3 resulted in a significant improvement in the herbicide degradation. However, depending on the pH conditions, quite different behaviours were observed. At pH 10 the combined treatment produced a prompt beneficial effect due to the occurrence of Eq. 11 that fastened the production of oxidants thus reducing the reaction time. The disappearance of Diuron with H2O2 was completed in about 2 hours while ozonation alone, in the same time span, provided only 80% decay. Unexpectedly, a notable acceleration was observed at acidic pH in comparison with the tests conducted with ozone alone. In particular, the degradation curve (Fig. 4a) showed a quasi-linear 11
trend as well as in ozonation alone, thus suggesting the occurrence of mechanisms different from those determined by higher pH values. The increased slope indicated that efficiencies had more than doubled. In fact, after two hours of treatment, Diuron degradation rose up from about 27% to more than 60%. (Figure 4) This enhancement can be attributed to different phenomena. Firstly, a significant role was played by the local alkalinisation [31,32] occurring on the cathode surface as a consequence of the water reduction, even in a very acidic electrolyte, (Eq.17) that promoted the indirect ozone decomposition mechanism (Eq. 2-10). Moreover, the electrogeneration of hydrogen peroxide by molecular oxygen reduction is a complex and still unclear reaction that involves during its multiple steps different radical species and among them O2●-, the so called superoxide anion [27], which at acidic conditions could reduce the ozone and successively promote its decomposition to hydroxyl radicals (Eq. 4 and Eq. 5). Finally, it has been reported that, in the presence of both ozone and oxygen, carbon-based electrodes may favour the ozone reduction. This is particularly evident for those cathodes with high specific surface area, such as RVC, as they promote ozone mass transfer, which is the limiting factor of the process [33]. Whilst the prevalent reduction of ozone impairs the production of hydrogen peroxide at the same time it promotes the occurrence of Eq. 15 that might evolve into hydroxyl radicals (Eq. 16). At circumneutral pH, the system experienced the beneficial action of the combined treatment only at times higher than 60 minutes when the pH of the solution dropped at acidic values mainly due to the anodic reaction of water discharge and also due to the production of carboxylic acids. Therefore, the combined treatment only minimally reduced the Diuron half-life but accelerated the subsequent degradation that, at the end of the two hours, was about 85% instead of 65%. Despite the improved efficiencies and the possible involvement of the hydroxyl radical in the entire investigated pH range, after two hours of treatment, the mineralization of the molecule has not yet 12
begun as indicated by the TOC values reported in Table 2. This can be explained by considering that in the presence of hydrogen peroxide some parasitic reactions may occur determining the reduction of the hydroxyl radical concentration in favor of the formation of less powerful species (Eq. 22) [34]:
H2O2 +•OH → HO2• + H2O
k=3.3x109 M-1 s-1
(22)
In this case, the standard potential drops from 2.73 V to 1.46 V [35], which are the values exhibited by the hydroxyl radical and the hydroperoxyl radical, respectively. Further, hydroperoxyl radical is endowed with a significantly reduced reactivity [36] that results in slower mineralization rate [37].
Table 2. Effect of pH on mineralization efficiency M (%) during LOR and HOR RVC-O3 at 25 mA. Ozone rate
pH
Time (min)
M (%)
LOR LOR LOR HOR HOR HOR
3 Free 10 3 Free 10
120 120 120 180 180 180
<5 <5 <5 34.03 34.65 58.87
The results indicate that the current supply is crucial for obtaining high extent of Diuron degradation and mineralization and that the main limitation remains the low availability of ozone. A significant improvement in the performance at all investigated pH values was thus expected by increasing the ozone to hydrogen peroxide ratio in the tests conducted under HOR conditions (HOR-RVC-O3). However, Fig. 4b shows that a clear contribution of the combined treatment was detected only at acidic pH values. Under these conditions, the Diuron removal experienced a significant acceleration with respect to LOR RVC-O3 tests, whose efficiency after 30 minutes was increased by over 650%
13
and a weak improvement with respect to those conducted with HOR-O3 tests, whose efficiency after the same time was increased of about 10%. This improvement can be explained considering that the ozone concentration no longer represents the limiting stage of the process thus contributing to increase the production of oxidizing species. Although to a lesser extent, this effect was also found in the tests conducted without pH adjustment where the increase in efficiency after 30 minutes of reaction between HOR RVC-O3 and HOR O3 was limited to a few percentage points. Interestingly, the adoption of a higher flow of ozone in the electro-peroxone treatment at alkaline pH slowed down the removal of the Diuron. The delay was weak and was almost completely recovered after 30 minutes of treatment when the final achieved efficiency differed little from HOR-O3 (93.8% by HOR RVC-O3 vs 96.0% by HOR-O3). Consistently with a previous paper [31], we attribute this particular behavior to the depletion of dissolved ozone determined by the reaction with the produced hydrogen peroxide and whose presence is pivotal for the fast herbicide attack due to its high selectivity. Although not particularly beneficial for the degradation of Diuron, the HOR RVC-O3 resulted in significant but sill uncomplete mineralization. The best result was obtained at pH 10 where 58.8% of mineralization was reached after 3 hours of reaction while lower, similar to each other, yields were achieved at free and acidic pH with, respectively, 34.6 % and 34.0 %. The similar behavior of these two tests can be explained by considering that when unadjusted the pH tended to acidic values from the first minutes of reaction. The results obtained show that the electro-assisted ozonation on RVC cathodes produces an effective synergy only at alkaline pH. In an attempt to improve the performance of electro-peroxone, we investigated the effect of the current in the range 25-50 mA. The results obtained show that Diuron degradation was unaffected by an increase in this parameter thus confirming the driving role of ozone alone (Fig. S3). This statement was confirmed by comparing tests conducted in the presence and in the absence of tert14
butyl alcohol (Fig. 2b) at 50 mA where only partial inhibition of Diuron degradation was observed (about 20% reduction). Although limited, this effect implies the formation of •OH radicals that are expected to play an important role in the mineralization of the molecule. In fact, the current intensity greatly affected the mineralization extent (Fig.5a) and after a 3 hours treatment 58.37%, 72.22% and 92.20% efficiency values were found when, respectively, 25 mA, 35 mA and 50 mA were provided. A first kinetic order well described the experimental results and kinetic constants of 2.4 x 10-3 min-1, 3.9 x 10-3 min-1 and 1.1 x 10-2 min-1 were calculated when 25 mA, 35 mA and 50 mA, respectively, were applied.
(Figure 5)
The evolution of MCE with the applied current (Fig. 5b) showed similar values and trends in the whole range investigated. As expected the curve obtained with the lowest current outperformed the others since in these conditions the occurrence of the main side reactions was reduced (i.e. H2O2 decomposition at the anode and 4-electron reduction of oxygen at the cathode). However, considered the high MCE values, the impact of these reactions appears to be limited even at the highest current (50 mA) which, therefore, can be regarded as an acceptable compromise between energy consumption, high mineralization efficiency and treatment duration. The fast decrease of MCE values can be attributed to mass transfer limitations due to the organic depletion and formation of by-product less promptly oxidizable. It was not considered worthwhile to extend the range of current investigation since it has been verified that excessive current values affect the productions of hydrogen peroxide and the durability of the cathode [27].
15
3.3 SS-O3 experiments As the synergistic contribution of the electrogenerated peroxide occurs only at alkaline pH, in the treatment of neutral or acid effluents there is no need to resort to a cathode capable of producing a reagent that remains unused and, due to its accumulation, must be removed before disposal. Hence, in the subsequent tests, we have replaced the RVC with a cheaper and more easily available stainless steel cathode on the assumption that this material provided electron transfer, useful for ozone reduction while avoiding the production of hydrogen peroxide. Preliminary tests have excluded the formation of hydrogen peroxide suggesting, in the absence of ozone, the prevalence of the water discharge reaction. Fig. 6 shows cyclic voltammograms recorded at 100 mV s-1 sweep rate in sulfuric acid. As can be seen, in tests conducted in the absence of ozone, Diuron alone did not undergo direct reduction on the steel cathode (cycle A), while a well-defined peak, presumably corresponding to the ozone reduction (cycle B), was found at about -0.5 V in ozonized solutions [38]. The cathodic response of the electrode increased with increasing concentration of ozone.
(Figure 6)
According to equations 15 and 16, the ozone reduction may evolve to hydroxyl radicals’ production. Equation 16, in particular, suggests that acidic pH values can shift the reaction towards the products thus resulting in enhanced •OH electrogeneration with great influence on the molecule decay. Consistently with the electro-peroxone, the initial experiments were conducted at 25 mA by testing the system ability in the degradation both at LOR (Fig. 7a) and HOR (Fig. 7b).
(Figure 7)
16
Surprisingly, at LOR conditions, the expected effect of pH on degradation was not observed, and within two hours of treatment, the molecule experienced only partial degradation in the entire pH range. In general, the tests conducted at alkaline pH, both at LOR and HOR, were more rapid than those conducted at lower pH but completely similar to the unassisted ones (LOR pH 10 and HOR pH 10). This result can be explained considering that the effect of ozone alone by indirect reaction provided a rapid removal of the molecule which was overpassed, under LOR conditions, only by the synergistic effect obtained with the electro-peroxone. This also accounts for the slowing down of the SS LOR free pH test, initially similar to that conducted at alkaline pH up to the time when acidification occurred, about 60 minutes after the start of electrolysis. The experiments conducted at pH 3 resulted in the slow decay of Diuron whose content was reduced only by about 60% at the end of the treatment. Interestingly, a comparison between the SSO3 tests (Fig. 7a) and RVC-O3 tests (Fig. 4a) at LOR and pH 3 conditions, reveals a substantial overlap of the curves supporting the hypothesis of an occurrence of ozone electro-reduction also on RVC cathode surface. At LOR, irrelevant mineralization was found being the only appreciable yield obtained after a 2hour treatment at acidic pH (about 5%). This is a consequence of the limited availability of ozone that results in a limited production of radical species with a negative effect on TOC decay. When the ozone concentration in the liquid phase was increased (Fig. 7b) a certain acceleration of the process was observed in comparison to the tests conducted at LOR. Due to the pH evolution, the tests conducted at free and acid pH degraded Diuron at a similar rate and after 30 minutes allowed for a Diuron removal of about 80%. However, the improvement achieved with respect to tests conducted with only ozone at HOR is almost negligible (+ 5%). Considering also that TOC removal was only slightly improved with a maximum value of 25% (Fig. 8b) after 3 hours at pH 3, further experiments were conducted at higher current in the range 25-250 mA under HOR conditions and pH 3.
17
The results show a limited effect of this parameter on Diuron degradation that after 30 min of reaction exhibited a removal efficiency of 80-87% in the entire range of current (Fig. 8a) thus confirming that the Diuron degradation can be successfully driven by a wide variety of oxidizing species including ozone alone. However, the increase in current was crucial for the achievement of a high mineralization extent. Figure 8b shows a very characteristic trend since after an initial low and similar, for all the adopted current intensities, mineralization value (< 20 %), the trends of the curves strongly depended on the current value. This particular behaviour was previously observed also in another study [22] where the same molecule was treated with a boron doped diamond anode, which is an electrode capable of producing high amounts of physo-sorbed hydroxyl radicals. In particular, the authors identified a mineralization limiting step in the formation of insoluble compounds that had to be dissolved prior to their complete oxidation. Figure 8b shows that at the lowest adopted currents this limiting step was not (25 mA) or hardly overcome (50 mA, after 120 minutes of reaction). A further increase in the applied current resulted in a quasi-linear mineralization trend (100 mA) thus indicating that the reaction was under current control rather than under mass transport control. A 200 mA current intensity was sufficient to accelerate the mineralization that was completed in 120 minutes.
(Figure 8)
A further increase above 200 mA did not show to speed up either degradation or mineralization. The analysis of mineralization current efficiency data (Fig.8c) shows low values for all the investigated currents thus supporting the consideration previously reported about the presence of a limiting step in the first part of the process as already reported also by other authors [21]. On the
18
other hand, the persistence of such low values in the last part of the treatment can be ascribed to the mass transport limitation due to the disappearance of the intermediate compounds. Despite low efficiency values are normally attributed to an energy waste due to concomitant side reactions, the herbicide mineralization proceeded quickly especially at high currents presumably due to the formation of highly oxidizing species such as the hydroxyl radical. To identify the involved radicals and their contribution to the Diuron degradation some experiments were performed by the addition of tert-butyl alcohol (TBA) as an •OH -specific radical scavenger and also of L-Histidine (L-H) used as universal radical scavenger [39]. The tests were conducted at HOR, pH 3 and 100 mA (Fig.2c). The addition of L-histidine to the electrolyte resulted in a sharp reduction in the degradation of Diuron, thus indicating that the removal of the molecule depended only limitedly on the oxidation of the direct ozone and had to be attributed mainly to the action of the radical species. In fact, after 30 minutes of reaction, the disappearance of Diuron decreased from 80% to 21.67%. The comparison of the results with those obtained in the test in which TBA was added as a scavenger of only hydroxyl radicals, confirms that during ozone electrolysis at acid pH, approximately 64% of Diuron removal could be attributed to the •OH-driven oxidation while only 9% to the action of other radical species. The remaining 7% oxidation was related to non-radical species (presumably direct ozonation or non-radical route promoted by local alkalization).
4. Conclusions In this work, the possibility of enhancing ozonation by combination with cathodic processes was evaluated. The following conclusion can be drawn: •
the ozonation conducted at a high ozone flow rate and alkaline pH provides fast degradation of Diuron.
19
•
The combination of ozonation with electrochemical methods promotes ozone decomposition and fastens the mineralization of Diuron which is strongly pH-dependent.
•
Under alkaline pH conditions, due to the synergy produced by the electro-generated hydrogen peroxide, electro-peroxone outperforms electro-reduction of ozone thus implying the use of carbon-based electrode materials. The scavenging tests indicate the cooperation of different oxidizing species of which the hydroxyl radical represents only a minority. The performance benefited by increasing current.
•
Under acidic pH, the •OH-driven mechanism of ozone electro-reduction enabled the achievement of fast degradation and almost complete mineralization in two hours treatment by adopting 200 mA of current on a stainless steel cathode.
References
[1] D. Gardoni, A. Vailati, R. Canziani, Decay of ozone in water: a review. Ozone Sci. Eng. 34 (2012) 233. [2] D. Shahidi, R. Roy, A. Azzouz, Advances in catalytic oxidation of organic pollutants Prospects for thorough mineralization by natural clay catalysts. Appl. Catal. B: Environ. 174-175 (2015) 277. [3] F.J. Beltran. Ozone Reaction Kinetics for Water and Wastewater Systems. Lewis Publisher, Boca Raton (Fla), 2003. [4] J. Staehelin, J. Holgné, Decomposition of Ozone in Water: Rate of Initiation by Hydroxide Ions and Hydrogen Peroxide, Environ. Sci. Technol. 16 (1982) 676. [5] S. Khuntia, S.K. Majumder, P. Ghosh, Quantitative prediction of generation of hydroxyl radicals from ozone microbubbles, Chem. Eng. Res. Des. 98 (2015) 231.
20
[6] G. Merényi, J. Lind, S. Naumov, C.V. Sonntag, Reaction of ozone with hydrogen peroxide (peroxone process): a revision of current mechanistic concepts based on thermokinetic and quantum-chemical considerations, Environ. Sci. Technol. 44 (2010) 3505. [7] G. Boczkaj, A. Fernandes, Wastewater treatment by means of advanced oxidation processes at basic pH conditions: a review, Chem. Eng. J. 320 (2017) 608. [8] Z. Cai, A.D. Dwivedi, W.-N. Lee, X. Zhao, W. Liu, M. Sillanpää, D. Zhao, C.-H. Huang, J. Fu, Application of nanotechnologies for removing pharmaceutically active compounds from water: Development and future trends, Environ. Sci.: Nano 5 (2018) 27. [9] S.P Ghuge, A.K. Saroha, Catalytic ozonation for the treatment of synthetic and industrial effluents - Application of mesoporous materials: a review, J. Environ. Manage. 211 (2018) 83. [10] J. Nawrocki, B. Kasprzyk-Hordern, The efficiency and mechanisms of catalytic ozonation, Appl. Catal. B: Environmental, 99 (2010), 27. [11] N. Bensalah, A. Bedoui, Enhancing the performance of electro-peroxone by incorporation of UV irradiation and BDD anodes, Environ. Technol. 38 (2017) 2979. [12] N.H. Ince. Ultrasound-assisted advanced oxidation processes for water decontamination, Ultrason. Sonochem. 40 (2018) 97. [13] X. Xiong, B. Wang, W. Zhu, K. Tian, H. Zhang, A review on ultrasonic catalytic microbubbles ozonation processes: Properties, hydroxyl radicals generation pathway and potential in application Catalysts, 9 (2019), 10. [14] N. Kishimoto, Y. Morita, H. Tsuno, T. Oomura, H. Mizutani, Advanced oxidation effect of ozonation combined with electrolysis, Water Res. 39 (2005) 4661. [15] S. Yuan, Z. Li, Y. Wang, Effective degradation of methylene blue by a novel electrochemically driven process, Electrochem. Commun. 29 (2013) 48.
21
[16] A. Wong, M.R. De Vasconcelos Lanza, M.D.P.T. Sotomayor, Sensor for diuron quantitation based on the P450 biomimetic catalyst nickel(II) 1,4,8,11,15,18,22,25octabutoxy-29H,31H-phthalocyanine, J. Electroanal. Chem. 690(2013) 83. [17] K. Kovács, J. Farkas, G. Veréb, E. Arany, G. Simon, K. Schrantz, A. Dombi, K. Hernádi, T. Alapi, Comparison of various advanced oxidation processes for the degradation of phenylurea herbicides, J. Environ. Sci. Health - B 51 (2016) 205. [18] T. Alapi, G. Simon, G. Veréb, K. Kovács, E. Arany, K. Schrantz, A. Dombi, K. Hernádi, Toxicology Aspects of the Decomposition of Diuron by advanced Oxidation Processes, Hung. J. Ind. Chem. 43(2015) 25. [19] C. Tixier, M. Sancelme, F. Bonnemoy, A. Cuer, H. Veschambre, Degradation products of a phenylurea herbicide, diuron: Synthesis, ecotoxicity, and biotransformation, Environ.Toxicol. Chem. 20 (2001) 1381. [20] H. Hoigné, J. Bader, Determination of ozone in water by the indigo method, Water Res. 15 (1981) 449. [21] E. Brillas, I. Sirés, M.A. Oturan, Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry, Chem. Rev. 109 (2009) 6570. [22] A.M Polcaro, M. Mascia, S. Palmas, A. Vacca, Electrochemical degradation of diuron and dichloroaniline at BDD electrode, Electrochim. Acta 49 (2004) 649. [23] A.R.F. Pipi, I. Sirés, A.R. De Andrade, E. Brillas, Application of electrochemical advanced oxidation processes to the mineralization of the herbicide diuron, Chemosphere 109 (2014) 49. [24] Environment Canada, Screening Assessment for the Challenge Urea, N'-(3,4dichlorophenyl)-N,N-dimethyl- (Diuron), https://www.ec.gc.ca/ese-ees/6BC4E5D3-6E964EB2-938F-1BF7F2CC5701/batch10_330-54-1_en.pdf (2011) 8. [25] U. Von Gunten, Ozonation of drinking water: Part I. Oxidation kinetics and product formation, Water Research 37 (2003) 1443. 22
[26] M.E. Mutseyekwa, Ş. Doǧan, S. Pirgalioǧlu, Ozonation for the removal of bisphenol A, Water Science and Technology 76 (2017) 2764-2775. [27] E. Petrucci, A. Da Pozzo, L. Di Palma, On the ability to electrogenerate hydrogen peroxide and to regenerate ferrous ions of three selected carbon-based cathodes for electroFenton processes, Chem. Eng. J. 283(2016) 750. [28] Nélieu, S., Kerhoas, L., Einhorn, J. Degradation of atrazine into ammeline by combined ozone/hydrogen peroxide treatment in water. (2000) Environmental Science and Technology, 34 (3), pp. 430-437. DOI 10.1021/es980540k. [29] Kepa, U., Stanczyk-Mazanek, E., Stepniak, L. The use of the advanced oxidation process in the ozone + hydrogen peroxide system for the removal of cyanide from water. (2008) Desalination, 223 (1-3), pp. 187-193. DOI 10.1016/j.desal.2007.01.215 [30] J.P. Pocostales, M.M. Sein, W. Knolle, C. Von Sonntag, T.C. Schmidt, Degradation of ozone-refractory organic phosphates in wastewater by ozone and ozone/hydrogen peroxide (peroxone): the role of ozone consumption by dissolved organic matter, Environ. Sci. Tecnhnol. 44 (2010) 8248. [31] H. Wang, B. Bakheet, S. Yuan, X. Li, G. Yu, G., S. Murayama, Y. Wang, Kinetics and energy efficiency for the degradation of 1,4-dioxane by electro-peroxone process, J. Hazard. Mater. 294 (2015) 90. [32] A. Da Pozzo, E. Petrucci, C. Merli, Electrogeneration of hydrogen peroxide in seawater and application to disinfection, J. Appl. Electrochem. 38 (2008) 997. [33] G. Xia, Y. Wang, B. Wang, J. Huang, S. Deng, G. Yu, The competition between cathodic oxygen and ozone reduction and its role in dictating the reaction mechanisms of an electroperoxone process, Water Res. 118 (2017) 26. [34] A. Da Pozzo, P. Ferrantelli, C. Merli, E. Petrucci, Oxidation efficiency in the electroFenton process, J. Appl. Electrochem. 35 (2005) 391.
23
[35] D.A.,Armstrong, R.E. Huie, S. Lymar, W.H. Koppenol, G. Merényi P. Neta, D.M. Stanbury, S. Steenken, P.Wardman, Standard electrode potentials involving radicals in aqueous solution: Inorganic radicals, Bioinorg. React. Mech. 9 (2013) 59. [36] H. Shemer, K.G.Linden, Photolysis, oxidation and subsequent toxicity of a mixture of polycyclic aromatic hydrocarbons in natural waters, J. Photochem. Photobiol. A Chem. 187 (2007) 186. [37] N. Oturan, E. Brillas, M.A. Oturan, Unprecedented total mineralization of atrazine and cyanuric acid by anodic oxidation and electro-Fenton with a boron-doped diamond anode, Environ. Chem. Lett. 10 (2012) 165. [38] Y. Ishii, T. A. Ivandini, K. Murata, Y. Einaga, Development of electrolyte-free ozone sensors using boron-doped diamond electrodes, Anal. Chem. 85 (2013) 4284. [39] Zhu, J., Chen, C., Li, Y., Zhou, L., Lan, Y. Rapid degradation of aniline by peroxydisulfate activated with copper-nickel binary oxysulfide. (2019) Separation and Purification Technology, 209, pp. 1007-1015.
24
Captions
Fig. 1. Diuron degradation by LOR (a) and HOR (b) ozone flow rate. Effect of pH: 3 (∆), 10 () and free pH () with its evolution during the unadjusted test (◆). Conditions: [Diuron]=40 mg/L; [Na2SO4]=0.05 M ; T=24±2 °C. Fig. 2. Effect of scavenger on Diuron degradation under high ozone flow rate by: O3 (a) RVCO3 (b) and SS-O3 (c) with 100 mM tert-butyl alcohol (), with 6.4 mM L-Histidine (∆) and without scavenger (). Conditions: [Diuron]=40 mg/L; [Na2SO4]=0.05 M ; T=24±2 °C; pH=10.
Fig. 3. H2O2 accumulation: in two-chamber reactor under oxygen flow (), two-chamber under ozone flow at LOR (), two-chamber under ozone flow at HOR (∆) and single-chamber reactor under oxygen flow (◆). Conditions: [Na2SO4]=0.05 M ; T=24±2 °C; I= 25 mA.
Fig. 4. Diuron degradation by RVC-O3 at LOR (a) and HOR (b). Effect of pH: 3 (∆), 10 () and free pH () with its evolution during the unadjusted test (◆). Conditions: [Diuron]=40 mg/L; [Na2SO4]=0.05 M ; T=24±2 °C; I= 25 mA. Fig. 5. Diuron (a) mineralization and (b) mineralization current efficiency (MCE%) by RVC-O3 at HOR and 25 mA (∆), 35 mA () and 50 mA (). Conditions: [Diuron]=40 mg/L; [Na2SO4]=0.05 M ; T=24±2 °C; pH=10.
Fig. 6. Cyclic voltammeters of Diuron (dotted line), Ozone at LOR (dashed line) and Ozone at HOR (solid line). Scan rate = 100 mVs-1
Fig. 7. Diuron degradation by SS-O3 at LOR (a) and HOR (b) ozone flow rate. Effect of pH: 3 (∆), 10 () and free pH () with its evolution during the unadjusted test (◆). Conditions: [Diuron]=40 mg/L; [Na2SO4]=0.05 M ; T=24±2 °C; I= 25 mA.
Fig. 8. Effect of 25mA (), 50mA (●), 100 mA (), 200mA () and 250 mA () as current intensity on Diuron (a) degradation, (b) mineralization and (c) mineralization current efficiency 25
during SS-O3 at HOR treatment. Conditions: [Diuron]=40 mg/L; [Na2SO4]=0.05 M ; T=24±2 °C; pH=3.
26
Figure 1 100
8
a)
7
7
80
6
5
60
4 40
3 2
20
D/%
6
pH
D/%
80
8
b)
5
60
4 40
3 2
20
1
1 0
0 0
30
60
t/min
90
120
0
0 0
5
10
15
t/min
20
25
30
pH
100
Figure 2 a)
100 80
D/%
60 40 20 0 0
5
10
15
20
25
30
20
25
30
20
25
30
t/min
b)b)
100
D/%
80 60 40 20 0 0
5
10
15
t/min
c)
100
D /%
80 60
40 20 0 0
5
10
15 t/min
Figure 3
200
H2O2 /mg L-1
160 120 80 40 0 0
20
40
60
t/min
80
100
Figure 4
100
8
7
b)
7 6
4 40
3 2
20
D/%
5
5
60
6
80
pH
D/%
80
60
4
40
3 2
20
1
1 0
0 0
30
60
t/min
90
120
0
0 0
5
10
15
t/min
20
25
30
pH
a)
100
Figure 5
100
50
a)
40
MCE/%
80
M/%
b)
60
30
40
20
20
10 0
0 0
30
60
90
t/min
120
150
180
0
30
60
90
t/min
120
150
180
Figure 6
1.5
1
I/mA
0.5
0
-0.5
-1 -0.6
-0.1
0.4
E/mV
0.9
Figure 7
100
8
a)
7
6
80
6
5
4 40
3 2
20
D/%
5
60
pH
D/%
80
7
b)
60
4
40
3 2
20
1
1 0
0 0
30
60
t/min
90
120
0
0 0
5
10
15
t/min
20
25
30
pH
100
Figure 8
a)
100
D/%
80 60 40 20 0 0
5
10
15
20
25
30
t/min
b)
100
M/%
80 60 40 20 0 0
30
60
90
120
150
180
t/min
c)
50
MCE/%
40
30 20 10 0 0
50
100
t/min
150
200
Highlights
Electrochemistry promotes ozone decomposition and Diuron mineralization
RVC and SS cathode are compared
Diuron degradation enhanced by providing high ozone flow rate and alkaline pH condition.
Synergistic effect of RVC-O3 at alkaline pH values
Complete mineralization obtained by SS-O3 in 2 h at pH 3 and 200 mA
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0013-4686(19)32295-9
DOI:
https://doi.org/10.1016/j.electacta.2019.135423
Reference:
EA 135423
To appear in:
Electrochimica Acta
Received Date: 6 August 2019 Revised Date:
8 November 2019
Accepted Date: 29 November 2019
Please cite this article as: I. Bavasso, D. Montanaro, L. Di Palma, E. Petrucci, Electrochemically assisted decomposition of ozone for degradation and mineralization of Diuron, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2019.135423. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
generator
OZONE
Absorbance
3
AIR
2
1
anode
0 190
220
250
280
Wave length/nm
Diuron
310
340
cathode porous stone
(3-(3,4-Dichlorophenyl)-1,1-dimethylurea)
O3 + HO2- OH + O2- + O2
O3 + e- O3O3- ↔ O2- + OO3- +H2O OH +OH-
Electrochemically assisted decomposition of ozone for degradation and mineralization of Diuron Irene Bavasso, Daniele Montanaro, Luca Di Palma, Elisabetta Petrucci Chemical Engineering Materials & Environment Department Sapienza University of Rome Via Eudossiana, 18 – 00184, Roma, Italy
Corresponding author: [email protected]
Abstract In this work, we explore the possibility of enhancing the ozonation of a solution of Diuron by combination with electrochemical processes. To this aim, the ozonation was performed in a membrane free electrolyzer where a reticulated vitreous carbon (RVC) and a stainless steel (SS) cathode have been alternatively tested and compared. The effect of pH, current density and ozone flow on degradation and mineralization of Diuron has been investigated. The involvement of radical species has been verified by means of test conducted with selective scavenging agents. The results show that both cathodes can promote the decomposition of ozone and therefore the degradation of Diuron. The use of scavengers has shown that the removal of Diuron is driven by means of radical species. Depending on the adopted materials, different mechanisms occur and different operating conditions are required. In particular, the use of a carbon-based cathode implies the production of hydrogen peroxide and therefore the occurrence of an Electro-peroxone process, which is favored at alkaline pH and low current values. On the other hand, the adoption of a metal cathode, whose efficiency is highly promoted by acidic pH and high currents, implies the direct reduction of ozone with the production of hydroxyl radicals.
1
Keywords: Diuron; ozone; stainless steel cathode; reticulated vitreous carbon cathode; electroperoxone.
1. Introduction Ozone is a powerful oxidant, widely used for a variety of applications from disinfection, to the removal of color and odor and sludge reduction [1]. It exhibits high selectivity towards specific functional groups such as unsaturated bonds and deprotonated amines. However, in spite of its high redox potential (2.07 V vs SHE) [2], the direct oxidation by ozone (Eq.1), results in low degradation rate and negligible mineralization [3] of organic compounds. O3 + H2O → 2O2 + 2H+
(1)
To overcome this issue, different strategies have been adopted. First, ozonation can be promoted by selecting appropriate pH values. In fact, as known, alkaline conditions determine fast ozone decomposition. Different routes initiated by hydroxyl ions have been proposed and the possible production of hydroxyl radicals, which is a non-selective highly powerful oxidant (2.8 V vs SHE), has been also hypothesized.
Route 1 [4] O3 + OH– → O2•– + HO2• HO2• ↔ H+ + O2•–
(2) (pKa=4.8)
(3)
O3 + O2•– → O3•– + O2
(4)
O3•– + H2O → •OH + OH– + O2
(5)
Route 2 [4] O3 + OH– → HO2– + O2 O3 + HO2– → •OH + O2•– + O2
(k=40 L mol s-1)
(6) (7)
2
followed by reactions (4) and (5)
Route 3 [5] O3 + OH– → HO2– + O2
(k=40 L mol s-1)
(8)
O3 + HO2– → O3•– + HO2•
(9)
HO2• + OH– ↔ O2•– + H2O
(10)
followed by reactions (4) and (5).
A further acceleration in ozone decomposition and therefore in hydroxyl radical production can be obtained by addition of hydrogen peroxide, at alkaline conditions, in the so-called peroxone process. In fact, in the presence of hydrogen peroxide, high pH values shift the equilibrium of Eq. 11 towards the species HO2- [4], thus avoiding the limitation due to the occurrence of Eq. 6 and 8 and resulting in a quicker evolution in accordance with route 2 and route 3. H2O2 ↔ HO2– + H+
(pKa=11.6)
(11)
To justify the reduced production of hydroxyl radicals, approximately half of what is expected, an alternative mechanism has been proposed [6] which provides for the formation of an adduct (Eq. 12) HO2− + O3 → HO5−
(∆G° = −39.8 kJ mol−1)
(12)
whose fate includes two competing reactions only one of which leads to the formation of active radical species (Eq. 13) while the other releases oxygen and hydroxide ions (Eq. 14). HO5− → HO2• + O3•−
(∆G0 = 13.2 kJ mol−1)
(13)
HO5− → 2O2 + OH−
(∆G0 = -197 kJ mol−1)
(14)
An increasing number of papers deal with the catalytic decomposition of ozone. A great variety of materials have been proposed as catalysts, mostly of metallic nature [7] either at the nano-scale [8] 3
or supported at the meso-scale [9]. However, in many cases, the actual mechanism, the health and environmental impact, the scale-up costs, the stability of the catalyst and its possible reuse remain uncertain, thus limiting the feasibility and viability of this process [10]. Another approach to achieving higher efficiency and faster removal rates is the synergistic integration of ozonation with other methods such as photolysis [11] and sonolysis [12,13], which are only slightly effective if applied alone. In recent years, the possibility of obtaining electricity from renewable sources has led to renewed interest in electrochemical technologies, which are appreciated for environmental compatibility, effectiveness and ease of control. In the field of environmental treatments in the liquid phase, much of the research has been focused on the development of anodic treatments, while less attention has been paid to cathodic treatments, with the exception of Fenton-type processes or electrochemical dechlorination. However, with respect to anodic treatments, the cathodic treatments present the advantage of working at lower potentials. This, on one hand, reduces significantly the wear and degradation of the electrode materials and, on the other hand, allows greater selectivity thus limiting the parasitic reactions and the production of harmful intermediate species. This paper investigates the possibility of enhancing the ozonation of a model pollutant by combination with a cathodic electrochemical process. To this aim, the ozone has been sparged into a membrane-free electrolyzer where a reticulated vitreous carbon cathode (RVC- O3) and a stainless steel cathode (SS- O3) have been alternatively tested and compared. Due to the direct reduction on the cathode surface, the adoption of a metal electrode seems to promote the ozone decomposition to •OH according to the following reactions (Eq. 15-16) [14]. O3 + e- → O3•-
(15)
O3•- + H2O ↔ •OH + OH- + O2
(16)
While, the adoption of a carbon-based cathode enables the in-situ production of hydrogen peroxide by cathodic reduction of oxygen (Eq.17). This process, known as electro-peroxone, represents an 4
improvement of the peroxone treatment since avoids the external addition of H2O2 that is unsafe to handle due to its high reactivity [15]. O2 + 2H+ + 2e- → H2O2
(17)
Both electrochemically-aided ozonation processes have been tested in the oxidation and mineralization of Diuron as the model compound. The effect of the main operative conditions on the treatment efficiency has been investigated and discussed. Diuron (3-(3,4-Dichlorophenyl)-1,1-dimethylurea) is an herbicide exhibiting toxic and teratogenic effects to mammals and other living organisms and also suspected of being an endocrine disruptor [16]. For this reason, it has been included in the “Candidate Contaminant List” of EPA agency (CCL3). Different treatments have been tested for the removal of this pollutant but few of them have proved effective [17]. In particular, only partial mineralization has been achieved. This aspect is of particular concern since several studies have verified that the compounds formed by degradation of Diuron may exhibit significantly higher toxicity than Diuron itself [18,19].
2. Materials and Methods
2.1 Chemicals Diuron (UN 3077- Lamirsa) stock solution (40 mg L-1) was prepared with distilled water and 0.05 M of Na2SO4 as background electrolyte. The initial pH of the solution was about 6.5±0.5 while the total organic content was approximately 17.4± 0.9 mg L-1. When required, the pH of the solution was adjusted using H2SO4 (1 M) and NaOH (2 M). For the detection of hydroxide and oxide radicals tert-butyl alcohol and L-Histidine were used as radical scavengers. All reagents were purchased from Sigma Aldrich and used without any further purification.
2.2 Electrochemical tests
5
Experiments were performed in a cylindrical glass reactor (4 cm inner diameter and 15 cm long) filled with 100 mL of Diuron solution and equipped with a ceramic micro-porous ozone diffuser. Two different air-fed corona discharge ozone generators have been used. An INO3MAX-04 of 100 mg h-1 maximum production capacity, to obtain a low flow rate of ozone (17.4 mgO3 L-1h-1, indicated as LOR in the text) and a Steril 1000 SEPRA of 1 g h-1 maximum production capacity, to obtain a high flow rate of ozone (260 mgO3 L-1h-1, indicated as HOR in the text). The concentration of provided ozone was evaluated by using the indigo colorimetric method [20]. The electrochemically assisted ozone treatment was conducted by using two different cathodic materials. For the electro-peroxone process (RVC-O3), we adopted a 100 ppi RVC electrode (ERG Aerospace Corporation, Oakland CA) with a submerged geometric surface of 1 cm x 1 cm x 2.5 cm. No oxygen flow was supplied and the dissolved oxygen present in solution derived from the ozone generator that insufflated a gaseous mixture of ozone and air. For the ozone decomposition (SS-O3), we adopted a 304 stainless steel plate electrode, with a submerged geometric surface of 2 cm x 2.5 cm. A platinum (Pt) wire electrode was used as the anode. The electrodes were placed vertically at a distance of about 1 cm. Galvanostatic tests were conducted at different current intensity values from 25 mA to 250 mA by using an Amel 2051 potentiostat/galvanostat (Amel, Italy). All tests were performed at 24±1 °C. The occurrence of diuron stripping by ozone bubbling was excluded by sparging air instead of ozone under the same operative conditions. The H2O2 electrogeneration accumulation tests were performed either on a two-chamber cell (each having a volume of 100 mL, separated by a Nafion 324 cation exchange membrane and stirred with a magnetic bar) or in the undivided reactor adopted for the ozone-based experiments, by supplying a 200 mL min-1 oxygen flow.
2.3 Analysis
6
Cyclic voltammetry (CV) experiments were conducted using a bio-Logic VSP 300 instrument. The tests were performed in 50 mL of 0.5 M H2SO4 solution, at room temperature using a threeelectrode cell consisting of the SS cathode, a Pt sheet as the counter electrode and a saturated calomel electrode (SCE) as reference. The scan rate was 100 mV s-1 and the potential ranged from 1 to +1.4 V. The solution pH was measured using a Crison GLP 421. Hydrogen peroxide concentration was determined through the Merck reflectometric kit based on the use of a specific peroxidase reagent (detection limit 0.5 mg L-1). The Diuron removal was monitored by absorbance measurement at λ= 248 nm (Fig.S1) using a PG Instruments T80+ UV/Vis spectrophotometer (using a quartz cell of 1 cm path length). The mineralization extent was quantified using a Shimadzu TOC-L CSH/CSN analyzer. Prior to analysis, samples were acidified to pH 3 by adding a few drops of 37% HCl and then sparged with air for 5 min to remove the dissolved carbon dioxide. Diuron degradation (D%) and mineralization (M%) were calculated as: (%) =
( − ) ∙ 100
(18)
(%) =
( − ) ∙ 100
(19)
where A0 and TOC0 are the absorbance (measured at λ= 248 nm) and total organic carbon values at initial time, while At and TOCt are the corresponding values at a generic t. The mineralization current efficiency was calculated as [21]: (%) =
( − ) ∗ 100 4.32 ∙ 10
(20)
where n is the number of electrons exchanged in the molecule mineralization according to eq. 21 [22,23], F is the Faraday constant (96485 C mol−1), V is the sample volume (L), TOC0 and TOCt 7
are the total organic carbon values at initial and at a generic t time, 4.32 × 107 is a conversion factor for units homogenization (3600 s h−1 × 12,000 C mol−1), m is the number of carbon atoms in the molecule, I is the applied current (A) and t is the electrolysis time (h). C9H10Cl2N2O+ 17H2O → 9CO2 + 2NH4+ + 2Cl− + 36H+ + 36e−
(21)
All tests were repeated at least three times and the results are reported as mean values with coefficient variation less than 7 %.
3. Results and discussion 3.1 Ozonation experiments The effect of ozone alone as a function of pH was investigated under both LOR and HOR regimes (Fig. 1) The effect of ozone alone as a function of pH was investigated under both LOR and HOR regimes (Fig. 1). It is worth noting that in the entire investigated pH range Diuron shows no tendency either to protonation nor to deprotonation (pKa1=13.55, pKa2=-1.09, pKa3=-2.48) thus maintaining its molecular structure substantially unaltered [24].
(Figure 1) As can be seen (Fig. 1a), after a two-hour reaction, the degradation of Diuron at pH 3 was only 20%. A significant improvement was observed in the test conducted at pH 10. Nonetheless, the herbicide removal after two hours barely reached 80%. The behavior of the solution where pH was unadjusted can be explained by considering the pH evolution. In fact, as ozone reacted rapidly with the available OH-, the solution experienced fast acidification. Therefore, as long as the pH remained at circumneutral values, the removal of Diuron did not differ much from that in the test conducted at pH 10. When, after about 30 minutes of reaction, the pH value decreased, the reaction slowed down and only partial degradation was achieved at the end of the reaction (60% after 2 hours).
8
The UV spectra recorded after 120 minutes of reaction (Fig. 2Sa) show a reduction of the peak at 248 nm depending on the applied pH but a substantial persistence of the peak at 190 nm, which is related to the aromatic ring, and the presence of a further peak at about 290 nanometers probably ascribable to a reaction intermediate. At higher ozone rate (HOR), the degradation efficiency improved considerably (Fig. 1b). In particular, even at acidic pH, after only 30 minutes of reaction, the degradation of the compound was over 75%. At pH 10 almost total removal was achieved in only 20 minutes. Unlike what has been observed previously, the test conducted at free pH showed efficiencies low and similar to those found at pH 3. This behavior can be easily explained by considering the evolution of pH that dropped to very acidic values after only 5 minutes. Despite the rapid degradation, UV spectra recorded after 30 minutes (Fig. 2Sb) show that the aromatic rings remained intact thus suggesting poor mineralization. This was confirmed by TOC measurements that revealed appreciable mineralization only for the test conducted at alkaline pH (39.2%) (Table 1). However, as the mineralization rate tended to slow down, no significant improvement was obtained by extended treatments. In fact, although a 20% mineralization was reached in the first hour of treatment, thus suggesting a moderate reactivity of the molecule, only 11% and 8 % were removed during the second and the third hour, respectively, thus indicating the production of less reactive intermediates. The low reactivity at low pH is generally explained considering that under these conditions the ozone reacts in its molecular form, according to the direct mechanism, and therefore with great selectivity but mild oxidizing power. At alkaline pH conditions, higher mineralization efficiencies were expected due to the production of hydroxyl radicals according to the mechanism proposed by several authors [4- 25,26]. Instead, the results reported in Table 1 seem to denote a low production of hydroxyl radicals.
9
Table 1. Effect of pH on mineralization efficiency M(%) during LOR and HOR ozonation. Ozone rate
pH
Time (min)
M (%)
LOR LOR LOR HOR HOR HOR HOR HOR
3 Free 10 3 Free 10 10 10
120 120 120 180 180 30 120 180
<5 <5 <5 <5 <5 10.08 31.41 39.18
To clarify this particular aspect, further ozonation tests have been carried out at pH 10 with the addition of tert-buthyl alcohol, which is a compound known for its scavenging activity of hydroxyl radicals (Fig. 2a) and therefore it is expected to significantly impair the Diuron degradation in case ●
OH radicals were the oxidizing agents. (Figure 2)
Interestingly, the curves reported in Fig. 2a differ little from each other, thus suggesting only a partial contribution of the hydroxyl radical to the process and the likely production and involvement of other radical species that successfully degrade Diuron but are substantially ineffective or slowly reactive in its mineralization.
3.2 RVC-O3 experiments Preliminary tests were conducted to quantify the hydrogen peroxide production and its depletion due to concomitant side reactions. According to our previous investigation [27], a 25 mA current was adopted as best compromise between the production rate and the current efficiency. Fig. 3 shows that in a two-chamber reactor, the concentration of hydrogen peroxide increased linearly with time and was about 150 mg L-1 after 90 minutes. A substantial reduction (- 80%) in its accumulation was observed in the undivided reactor adopted in this research. The detrimental effect can be attributed to the simultaneous decomposition to water and oxygen of the electrogenerated 10
hydrogen peroxide on the anode surface. In the presence of ozone, a further H2O2 depletion is expected. Firstly, because, due to its high solubility, ozone sparging may negatively affect the amount of dissolved oxygen and secondly, because the copresence of ozone and hydrogen peroxide gives rise to the peroxone reaction. However, the results in Fig. 3 show two different responses of the system depending on the ozone flow rate. Under LOR conditions, ozone was slowly transferred into solution (17 mg L-1h-1) and therefore either limited stripping of oxygen by ozone turbulence or H2O2 consumption in the direct reaction, occurred. Instead, under HOR conditions, both effects were amplified by the large availability of dissolved ozone (260 mg L-1 h-1) and only a small residual peroxide concentration was detected (less than 10 mg L-1). (Figure 3) Considering that the peroxone treatment benefits from a O3 to H2O2 ratio higher than 1 [28,29] with optimal performance when such a ratio is 2 [30], experiments under LOR conditions were expected to be slightly effective in the degradation and mineralization of Diuron. By comparing the production values of hydrogen peroxide (data reported in Fig.3 in divided cell and under oxygen supply) and the ozone supply in the two adopted configurations (LOR and HOR), it is possible to estimate an average molar ratio of the two reagents equal to 0.13 and 1.97, respectively. Fig.4a shows that even under the mild adopted conditions (LOR at 25 mA), the RVC-O3 resulted in a significant improvement in the herbicide degradation. However, depending on the pH conditions, quite different behaviours were observed. At pH 10 the combined treatment produced a prompt beneficial effect due to the occurrence of Eq. 11 that fastened the production of oxidants thus reducing the reaction time. The disappearance of Diuron with H2O2 was completed in about 2 hours while ozonation alone, in the same time span, provided only 80% decay. Unexpectedly, a notable acceleration was observed at acidic pH in comparison with the tests conducted with ozone alone. In particular, the degradation curve (Fig. 4a) showed a quasi-linear 11
trend as well as in ozonation alone, thus suggesting the occurrence of mechanisms different from those determined by higher pH values. The increased slope indicated that efficiencies had more than doubled. In fact, after two hours of treatment, Diuron degradation rose up from about 27% to more than 60%. (Figure 4) This enhancement can be attributed to different phenomena. Firstly, a significant role was played by the local alkalinisation [31,32] occurring on the cathode surface as a consequence of the water reduction, even in a very acidic electrolyte, (Eq.17) that promoted the indirect ozone decomposition mechanism (Eq. 2-10). Moreover, the electrogeneration of hydrogen peroxide by molecular oxygen reduction is a complex and still unclear reaction that involves during its multiple steps different radical species and among them O2●-, the so called superoxide anion [27], which at acidic conditions could reduce the ozone and successively promote its decomposition to hydroxyl radicals (Eq. 4 and Eq. 5). Finally, it has been reported that, in the presence of both ozone and oxygen, carbon-based electrodes may favour the ozone reduction. This is particularly evident for those cathodes with high specific surface area, such as RVC, as they promote ozone mass transfer, which is the limiting factor of the process [33]. Whilst the prevalent reduction of ozone impairs the production of hydrogen peroxide at the same time it promotes the occurrence of Eq. 15 that might evolve into hydroxyl radicals (Eq. 16). At circumneutral pH, the system experienced the beneficial action of the combined treatment only at times higher than 60 minutes when the pH of the solution dropped at acidic values mainly due to the anodic reaction of water discharge and also due to the production of carboxylic acids. Therefore, the combined treatment only minimally reduced the Diuron half-life but accelerated the subsequent degradation that, at the end of the two hours, was about 85% instead of 65%. Despite the improved efficiencies and the possible involvement of the hydroxyl radical in the entire investigated pH range, after two hours of treatment, the mineralization of the molecule has not yet 12
begun as indicated by the TOC values reported in Table 2. This can be explained by considering that in the presence of hydrogen peroxide some parasitic reactions may occur determining the reduction of the hydroxyl radical concentration in favor of the formation of less powerful species (Eq. 22) [34]:
H2O2 +•OH → HO2• + H2O
k=3.3x109 M-1 s-1
(22)
In this case, the standard potential drops from 2.73 V to 1.46 V [35], which are the values exhibited by the hydroxyl radical and the hydroperoxyl radical, respectively. Further, hydroperoxyl radical is endowed with a significantly reduced reactivity [36] that results in slower mineralization rate [37].
Table 2. Effect of pH on mineralization efficiency M (%) during LOR and HOR RVC-O3 at 25 mA. Ozone rate
pH
Time (min)
M (%)
LOR LOR LOR HOR HOR HOR
3 Free 10 3 Free 10
120 120 120 180 180 180
<5 <5 <5 34.03 34.65 58.87
The results indicate that the current supply is crucial for obtaining high extent of Diuron degradation and mineralization and that the main limitation remains the low availability of ozone. A significant improvement in the performance at all investigated pH values was thus expected by increasing the ozone to hydrogen peroxide ratio in the tests conducted under HOR conditions (HOR-RVC-O3). However, Fig. 4b shows that a clear contribution of the combined treatment was detected only at acidic pH values. Under these conditions, the Diuron removal experienced a significant acceleration with respect to LOR RVC-O3 tests, whose efficiency after 30 minutes was increased by over 650%
13
and a weak improvement with respect to those conducted with HOR-O3 tests, whose efficiency after the same time was increased of about 10%. This improvement can be explained considering that the ozone concentration no longer represents the limiting stage of the process thus contributing to increase the production of oxidizing species. Although to a lesser extent, this effect was also found in the tests conducted without pH adjustment where the increase in efficiency after 30 minutes of reaction between HOR RVC-O3 and HOR O3 was limited to a few percentage points. Interestingly, the adoption of a higher flow of ozone in the electro-peroxone treatment at alkaline pH slowed down the removal of the Diuron. The delay was weak and was almost completely recovered after 30 minutes of treatment when the final achieved efficiency differed little from HOR-O3 (93.8% by HOR RVC-O3 vs 96.0% by HOR-O3). Consistently with a previous paper [31], we attribute this particular behavior to the depletion of dissolved ozone determined by the reaction with the produced hydrogen peroxide and whose presence is pivotal for the fast herbicide attack due to its high selectivity. Although not particularly beneficial for the degradation of Diuron, the HOR RVC-O3 resulted in significant but sill uncomplete mineralization. The best result was obtained at pH 10 where 58.8% of mineralization was reached after 3 hours of reaction while lower, similar to each other, yields were achieved at free and acidic pH with, respectively, 34.6 % and 34.0 %. The similar behavior of these two tests can be explained by considering that when unadjusted the pH tended to acidic values from the first minutes of reaction. The results obtained show that the electro-assisted ozonation on RVC cathodes produces an effective synergy only at alkaline pH. In an attempt to improve the performance of electro-peroxone, we investigated the effect of the current in the range 25-50 mA. The results obtained show that Diuron degradation was unaffected by an increase in this parameter thus confirming the driving role of ozone alone (Fig. S3). This statement was confirmed by comparing tests conducted in the presence and in the absence of tert14
butyl alcohol (Fig. 2b) at 50 mA where only partial inhibition of Diuron degradation was observed (about 20% reduction). Although limited, this effect implies the formation of •OH radicals that are expected to play an important role in the mineralization of the molecule. In fact, the current intensity greatly affected the mineralization extent (Fig.5a) and after a 3 hours treatment 58.37%, 72.22% and 92.20% efficiency values were found when, respectively, 25 mA, 35 mA and 50 mA were provided. A first kinetic order well described the experimental results and kinetic constants of 2.4 x 10-3 min-1, 3.9 x 10-3 min-1 and 1.1 x 10-2 min-1 were calculated when 25 mA, 35 mA and 50 mA, respectively, were applied.
(Figure 5)
The evolution of MCE with the applied current (Fig. 5b) showed similar values and trends in the whole range investigated. As expected the curve obtained with the lowest current outperformed the others since in these conditions the occurrence of the main side reactions was reduced (i.e. H2O2 decomposition at the anode and 4-electron reduction of oxygen at the cathode). However, considered the high MCE values, the impact of these reactions appears to be limited even at the highest current (50 mA) which, therefore, can be regarded as an acceptable compromise between energy consumption, high mineralization efficiency and treatment duration. The fast decrease of MCE values can be attributed to mass transfer limitations due to the organic depletion and formation of by-product less promptly oxidizable. It was not considered worthwhile to extend the range of current investigation since it has been verified that excessive current values affect the productions of hydrogen peroxide and the durability of the cathode [27].
15
3.3 SS-O3 experiments As the synergistic contribution of the electrogenerated peroxide occurs only at alkaline pH, in the treatment of neutral or acid effluents there is no need to resort to a cathode capable of producing a reagent that remains unused and, due to its accumulation, must be removed before disposal. Hence, in the subsequent tests, we have replaced the RVC with a cheaper and more easily available stainless steel cathode on the assumption that this material provided electron transfer, useful for ozone reduction while avoiding the production of hydrogen peroxide. Preliminary tests have excluded the formation of hydrogen peroxide suggesting, in the absence of ozone, the prevalence of the water discharge reaction. Fig. 6 shows cyclic voltammograms recorded at 100 mV s-1 sweep rate in sulfuric acid. As can be seen, in tests conducted in the absence of ozone, Diuron alone did not undergo direct reduction on the steel cathode (cycle A), while a well-defined peak, presumably corresponding to the ozone reduction (cycle B), was found at about -0.5 V in ozonized solutions [38]. The cathodic response of the electrode increased with increasing concentration of ozone.
(Figure 6)
According to equations 15 and 16, the ozone reduction may evolve to hydroxyl radicals’ production. Equation 16, in particular, suggests that acidic pH values can shift the reaction towards the products thus resulting in enhanced •OH electrogeneration with great influence on the molecule decay. Consistently with the electro-peroxone, the initial experiments were conducted at 25 mA by testing the system ability in the degradation both at LOR (Fig. 7a) and HOR (Fig. 7b).
(Figure 7)
16
Surprisingly, at LOR conditions, the expected effect of pH on degradation was not observed, and within two hours of treatment, the molecule experienced only partial degradation in the entire pH range. In general, the tests conducted at alkaline pH, both at LOR and HOR, were more rapid than those conducted at lower pH but completely similar to the unassisted ones (LOR pH 10 and HOR pH 10). This result can be explained considering that the effect of ozone alone by indirect reaction provided a rapid removal of the molecule which was overpassed, under LOR conditions, only by the synergistic effect obtained with the electro-peroxone. This also accounts for the slowing down of the SS LOR free pH test, initially similar to that conducted at alkaline pH up to the time when acidification occurred, about 60 minutes after the start of electrolysis. The experiments conducted at pH 3 resulted in the slow decay of Diuron whose content was reduced only by about 60% at the end of the treatment. Interestingly, a comparison between the SSO3 tests (Fig. 7a) and RVC-O3 tests (Fig. 4a) at LOR and pH 3 conditions, reveals a substantial overlap of the curves supporting the hypothesis of an occurrence of ozone electro-reduction also on RVC cathode surface. At LOR, irrelevant mineralization was found being the only appreciable yield obtained after a 2hour treatment at acidic pH (about 5%). This is a consequence of the limited availability of ozone that results in a limited production of radical species with a negative effect on TOC decay. When the ozone concentration in the liquid phase was increased (Fig. 7b) a certain acceleration of the process was observed in comparison to the tests conducted at LOR. Due to the pH evolution, the tests conducted at free and acid pH degraded Diuron at a similar rate and after 30 minutes allowed for a Diuron removal of about 80%. However, the improvement achieved with respect to tests conducted with only ozone at HOR is almost negligible (+ 5%). Considering also that TOC removal was only slightly improved with a maximum value of 25% (Fig. 8b) after 3 hours at pH 3, further experiments were conducted at higher current in the range 25-250 mA under HOR conditions and pH 3.
17
The results show a limited effect of this parameter on Diuron degradation that after 30 min of reaction exhibited a removal efficiency of 80-87% in the entire range of current (Fig. 8a) thus confirming that the Diuron degradation can be successfully driven by a wide variety of oxidizing species including ozone alone. However, the increase in current was crucial for the achievement of a high mineralization extent. Figure 8b shows a very characteristic trend since after an initial low and similar, for all the adopted current intensities, mineralization value (< 20 %), the trends of the curves strongly depended on the current value. This particular behaviour was previously observed also in another study [22] where the same molecule was treated with a boron doped diamond anode, which is an electrode capable of producing high amounts of physo-sorbed hydroxyl radicals. In particular, the authors identified a mineralization limiting step in the formation of insoluble compounds that had to be dissolved prior to their complete oxidation. Figure 8b shows that at the lowest adopted currents this limiting step was not (25 mA) or hardly overcome (50 mA, after 120 minutes of reaction). A further increase in the applied current resulted in a quasi-linear mineralization trend (100 mA) thus indicating that the reaction was under current control rather than under mass transport control. A 200 mA current intensity was sufficient to accelerate the mineralization that was completed in 120 minutes.
(Figure 8)
A further increase above 200 mA did not show to speed up either degradation or mineralization. The analysis of mineralization current efficiency data (Fig.8c) shows low values for all the investigated currents thus supporting the consideration previously reported about the presence of a limiting step in the first part of the process as already reported also by other authors [21]. On the
18
other hand, the persistence of such low values in the last part of the treatment can be ascribed to the mass transport limitation due to the disappearance of the intermediate compounds. Despite low efficiency values are normally attributed to an energy waste due to concomitant side reactions, the herbicide mineralization proceeded quickly especially at high currents presumably due to the formation of highly oxidizing species such as the hydroxyl radical. To identify the involved radicals and their contribution to the Diuron degradation some experiments were performed by the addition of tert-butyl alcohol (TBA) as an •OH -specific radical scavenger and also of L-Histidine (L-H) used as universal radical scavenger [39]. The tests were conducted at HOR, pH 3 and 100 mA (Fig.2c). The addition of L-histidine to the electrolyte resulted in a sharp reduction in the degradation of Diuron, thus indicating that the removal of the molecule depended only limitedly on the oxidation of the direct ozone and had to be attributed mainly to the action of the radical species. In fact, after 30 minutes of reaction, the disappearance of Diuron decreased from 80% to 21.67%. The comparison of the results with those obtained in the test in which TBA was added as a scavenger of only hydroxyl radicals, confirms that during ozone electrolysis at acid pH, approximately 64% of Diuron removal could be attributed to the •OH-driven oxidation while only 9% to the action of other radical species. The remaining 7% oxidation was related to non-radical species (presumably direct ozonation or non-radical route promoted by local alkalization).
4. Conclusions In this work, the possibility of enhancing ozonation by combination with cathodic processes was evaluated. The following conclusion can be drawn: •
the ozonation conducted at a high ozone flow rate and alkaline pH provides fast degradation of Diuron.
19
•
The combination of ozonation with electrochemical methods promotes ozone decomposition and fastens the mineralization of Diuron which is strongly pH-dependent.
•
Under alkaline pH conditions, due to the synergy produced by the electro-generated hydrogen peroxide, electro-peroxone outperforms electro-reduction of ozone thus implying the use of carbon-based electrode materials. The scavenging tests indicate the cooperation of different oxidizing species of which the hydroxyl radical represents only a minority. The performance benefited by increasing current.
•
Under acidic pH, the •OH-driven mechanism of ozone electro-reduction enabled the achievement of fast degradation and almost complete mineralization in two hours treatment by adopting 200 mA of current on a stainless steel cathode.
References
[1] D. Gardoni, A. Vailati, R. Canziani, Decay of ozone in water: a review. Ozone Sci. Eng. 34 (2012) 233. [2] D. Shahidi, R. Roy, A. Azzouz, Advances in catalytic oxidation of organic pollutants Prospects for thorough mineralization by natural clay catalysts. Appl. Catal. B: Environ. 174-175 (2015) 277. [3] F.J. Beltran. Ozone Reaction Kinetics for Water and Wastewater Systems. Lewis Publisher, Boca Raton (Fla), 2003. [4] J. Staehelin, J. Holgné, Decomposition of Ozone in Water: Rate of Initiation by Hydroxide Ions and Hydrogen Peroxide, Environ. Sci. Technol. 16 (1982) 676. [5] S. Khuntia, S.K. Majumder, P. Ghosh, Quantitative prediction of generation of hydroxyl radicals from ozone microbubbles, Chem. Eng. Res. Des. 98 (2015) 231.
20
[6] G. Merényi, J. Lind, S. Naumov, C.V. Sonntag, Reaction of ozone with hydrogen peroxide (peroxone process): a revision of current mechanistic concepts based on thermokinetic and quantum-chemical considerations, Environ. Sci. Technol. 44 (2010) 3505. [7] G. Boczkaj, A. Fernandes, Wastewater treatment by means of advanced oxidation processes at basic pH conditions: a review, Chem. Eng. J. 320 (2017) 608. [8] Z. Cai, A.D. Dwivedi, W.-N. Lee, X. Zhao, W. Liu, M. Sillanpää, D. Zhao, C.-H. Huang, J. Fu, Application of nanotechnologies for removing pharmaceutically active compounds from water: Development and future trends, Environ. Sci.: Nano 5 (2018) 27. [9] S.P Ghuge, A.K. Saroha, Catalytic ozonation for the treatment of synthetic and industrial effluents - Application of mesoporous materials: a review, J. Environ. Manage. 211 (2018) 83. [10] J. Nawrocki, B. Kasprzyk-Hordern, The efficiency and mechanisms of catalytic ozonation, Appl. Catal. B: Environmental, 99 (2010), 27. [11] N. Bensalah, A. Bedoui, Enhancing the performance of electro-peroxone by incorporation of UV irradiation and BDD anodes, Environ. Technol. 38 (2017) 2979. [12] N.H. Ince. Ultrasound-assisted advanced oxidation processes for water decontamination, Ultrason. Sonochem. 40 (2018) 97. [13] X. Xiong, B. Wang, W. Zhu, K. Tian, H. Zhang, A review on ultrasonic catalytic microbubbles ozonation processes: Properties, hydroxyl radicals generation pathway and potential in application Catalysts, 9 (2019), 10. [14] N. Kishimoto, Y. Morita, H. Tsuno, T. Oomura, H. Mizutani, Advanced oxidation effect of ozonation combined with electrolysis, Water Res. 39 (2005) 4661. [15] S. Yuan, Z. Li, Y. Wang, Effective degradation of methylene blue by a novel electrochemically driven process, Electrochem. Commun. 29 (2013) 48.
21
[16] A. Wong, M.R. De Vasconcelos Lanza, M.D.P.T. Sotomayor, Sensor for diuron quantitation based on the P450 biomimetic catalyst nickel(II) 1,4,8,11,15,18,22,25octabutoxy-29H,31H-phthalocyanine, J. Electroanal. Chem. 690(2013) 83. [17] K. Kovács, J. Farkas, G. Veréb, E. Arany, G. Simon, K. Schrantz, A. Dombi, K. Hernádi, T. Alapi, Comparison of various advanced oxidation processes for the degradation of phenylurea herbicides, J. Environ. Sci. Health - B 51 (2016) 205. [18] T. Alapi, G. Simon, G. Veréb, K. Kovács, E. Arany, K. Schrantz, A. Dombi, K. Hernádi, Toxicology Aspects of the Decomposition of Diuron by advanced Oxidation Processes, Hung. J. Ind. Chem. 43(2015) 25. [19] C. Tixier, M. Sancelme, F. Bonnemoy, A. Cuer, H. Veschambre, Degradation products of a phenylurea herbicide, diuron: Synthesis, ecotoxicity, and biotransformation, Environ.Toxicol. Chem. 20 (2001) 1381. [20] H. Hoigné, J. Bader, Determination of ozone in water by the indigo method, Water Res. 15 (1981) 449. [21] E. Brillas, I. Sirés, M.A. Oturan, Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry, Chem. Rev. 109 (2009) 6570. [22] A.M Polcaro, M. Mascia, S. Palmas, A. Vacca, Electrochemical degradation of diuron and dichloroaniline at BDD electrode, Electrochim. Acta 49 (2004) 649. [23] A.R.F. Pipi, I. Sirés, A.R. De Andrade, E. Brillas, Application of electrochemical advanced oxidation processes to the mineralization of the herbicide diuron, Chemosphere 109 (2014) 49. [24] Environment Canada, Screening Assessment for the Challenge Urea, N'-(3,4dichlorophenyl)-N,N-dimethyl- (Diuron), https://www.ec.gc.ca/ese-ees/6BC4E5D3-6E964EB2-938F-1BF7F2CC5701/batch10_330-54-1_en.pdf (2011) 8. [25] U. Von Gunten, Ozonation of drinking water: Part I. Oxidation kinetics and product formation, Water Research 37 (2003) 1443. 22
[26] M.E. Mutseyekwa, Ş. Doǧan, S. Pirgalioǧlu, Ozonation for the removal of bisphenol A, Water Science and Technology 76 (2017) 2764-2775. [27] E. Petrucci, A. Da Pozzo, L. Di Palma, On the ability to electrogenerate hydrogen peroxide and to regenerate ferrous ions of three selected carbon-based cathodes for electroFenton processes, Chem. Eng. J. 283(2016) 750. [28] Nélieu, S., Kerhoas, L., Einhorn, J. Degradation of atrazine into ammeline by combined ozone/hydrogen peroxide treatment in water. (2000) Environmental Science and Technology, 34 (3), pp. 430-437. DOI 10.1021/es980540k. [29] Kepa, U., Stanczyk-Mazanek, E., Stepniak, L. The use of the advanced oxidation process in the ozone + hydrogen peroxide system for the removal of cyanide from water. (2008) Desalination, 223 (1-3), pp. 187-193. DOI 10.1016/j.desal.2007.01.215 [30] J.P. Pocostales, M.M. Sein, W. Knolle, C. Von Sonntag, T.C. Schmidt, Degradation of ozone-refractory organic phosphates in wastewater by ozone and ozone/hydrogen peroxide (peroxone): the role of ozone consumption by dissolved organic matter, Environ. Sci. Tecnhnol. 44 (2010) 8248. [31] H. Wang, B. Bakheet, S. Yuan, X. Li, G. Yu, G., S. Murayama, Y. Wang, Kinetics and energy efficiency for the degradation of 1,4-dioxane by electro-peroxone process, J. Hazard. Mater. 294 (2015) 90. [32] A. Da Pozzo, E. Petrucci, C. Merli, Electrogeneration of hydrogen peroxide in seawater and application to disinfection, J. Appl. Electrochem. 38 (2008) 997. [33] G. Xia, Y. Wang, B. Wang, J. Huang, S. Deng, G. Yu, The competition between cathodic oxygen and ozone reduction and its role in dictating the reaction mechanisms of an electroperoxone process, Water Res. 118 (2017) 26. [34] A. Da Pozzo, P. Ferrantelli, C. Merli, E. Petrucci, Oxidation efficiency in the electroFenton process, J. Appl. Electrochem. 35 (2005) 391.
23
[35] D.A.,Armstrong, R.E. Huie, S. Lymar, W.H. Koppenol, G. Merényi P. Neta, D.M. Stanbury, S. Steenken, P.Wardman, Standard electrode potentials involving radicals in aqueous solution: Inorganic radicals, Bioinorg. React. Mech. 9 (2013) 59. [36] H. Shemer, K.G.Linden, Photolysis, oxidation and subsequent toxicity of a mixture of polycyclic aromatic hydrocarbons in natural waters, J. Photochem. Photobiol. A Chem. 187 (2007) 186. [37] N. Oturan, E. Brillas, M.A. Oturan, Unprecedented total mineralization of atrazine and cyanuric acid by anodic oxidation and electro-Fenton with a boron-doped diamond anode, Environ. Chem. Lett. 10 (2012) 165. [38] Y. Ishii, T. A. Ivandini, K. Murata, Y. Einaga, Development of electrolyte-free ozone sensors using boron-doped diamond electrodes, Anal. Chem. 85 (2013) 4284. [39] Zhu, J., Chen, C., Li, Y., Zhou, L., Lan, Y. Rapid degradation of aniline by peroxydisulfate activated with copper-nickel binary oxysulfide. (2019) Separation and Purification Technology, 209, pp. 1007-1015.
24
Captions
Fig. 1. Diuron degradation by LOR (a) and HOR (b) ozone flow rate. Effect of pH: 3 (∆), 10 () and free pH () with its evolution during the unadjusted test (◆). Conditions: [Diuron]=40 mg/L; [Na2SO4]=0.05 M ; T=24±2 °C. Fig. 2. Effect of scavenger on Diuron degradation under high ozone flow rate by: O3 (a) RVCO3 (b) and SS-O3 (c) with 100 mM tert-butyl alcohol (), with 6.4 mM L-Histidine (∆) and without scavenger (). Conditions: [Diuron]=40 mg/L; [Na2SO4]=0.05 M ; T=24±2 °C; pH=10.
Fig. 3. H2O2 accumulation: in two-chamber reactor under oxygen flow (), two-chamber under ozone flow at LOR (), two-chamber under ozone flow at HOR (∆) and single-chamber reactor under oxygen flow (◆). Conditions: [Na2SO4]=0.05 M ; T=24±2 °C; I= 25 mA.
Fig. 4. Diuron degradation by RVC-O3 at LOR (a) and HOR (b). Effect of pH: 3 (∆), 10 () and free pH () with its evolution during the unadjusted test (◆). Conditions: [Diuron]=40 mg/L; [Na2SO4]=0.05 M ; T=24±2 °C; I= 25 mA. Fig. 5. Diuron (a) mineralization and (b) mineralization current efficiency (MCE%) by RVC-O3 at HOR and 25 mA (∆), 35 mA () and 50 mA (). Conditions: [Diuron]=40 mg/L; [Na2SO4]=0.05 M ; T=24±2 °C; pH=10.
Fig. 6. Cyclic voltammeters of Diuron (dotted line), Ozone at LOR (dashed line) and Ozone at HOR (solid line). Scan rate = 100 mVs-1
Fig. 7. Diuron degradation by SS-O3 at LOR (a) and HOR (b) ozone flow rate. Effect of pH: 3 (∆), 10 () and free pH () with its evolution during the unadjusted test (◆). Conditions: [Diuron]=40 mg/L; [Na2SO4]=0.05 M ; T=24±2 °C; I= 25 mA.
Fig. 8. Effect of 25mA (), 50mA (●), 100 mA (), 200mA () and 250 mA () as current intensity on Diuron (a) degradation, (b) mineralization and (c) mineralization current efficiency 25
during SS-O3 at HOR treatment. Conditions: [Diuron]=40 mg/L; [Na2SO4]=0.05 M ; T=24±2 °C; pH=3.
26
Figure 1 100
8
a)
7
7
80
6
5
60
4 40
3 2
20
D/%
6
pH
D/%
80
8
b)
5
60
4 40
3 2
20
1
1 0
0 0
30
60
t/min
90
120
0
0 0
5
10
15
t/min
20
25
30
pH
100
Figure 2 a)
100 80
D/%
60 40 20 0 0
5
10
15
20
25
30
20
25
30
20
25
30
t/min
b)b)
100
D/%
80 60 40 20 0 0
5
10
15
t/min
c)
100
D /%
80 60
40 20 0 0
5
10
15 t/min
Figure 3
200
H2O2 /mg L-1
160 120 80 40 0 0
20
40
60
t/min
80
100
Figure 4
100
8
7
b)
7 6
4 40
3 2
20
D/%
5
5
60
6
80
pH
D/%
80
60
4
40
3 2
20
1
1 0
0 0
30
60
t/min
90
120
0
0 0
5
10
15
t/min
20
25
30
pH
a)
100
Figure 5
100
50
a)
40
MCE/%
80
M/%
b)
60
30
40
20
20
10 0
0 0
30
60
90
t/min
120
150
180
0
30
60
90
t/min
120
150
180
Figure 6
1.5
1
I/mA
0.5
0
-0.5
-1 -0.6
-0.1
0.4
E/mV
0.9
Figure 7
100
8
a)
7
6
80
6
5
4 40
3 2
20
D/%
5
60
pH
D/%
80
7
b)
60
4
40
3 2
20
1
1 0
0 0
30
60
t/min
90
120
0
0 0
5
10
15
t/min
20
25
30
pH
100
Figure 8
a)
100
D/%
80 60 40 20 0 0
5
10
15
20
25
30
t/min
b)
100
M/%
80 60 40 20 0 0
30
60
90
120
150
180
t/min
c)
50
MCE/%
40
30 20 10 0 0
50
100
t/min
150
200
Highlights
Electrochemistry promotes ozone decomposition and Diuron mineralization
RVC and SS cathode are compared
Diuron degradation enhanced by providing high ozone flow rate and alkaline pH condition.
Synergistic effect of RVC-O3 at alkaline pH values
Complete mineralization obtained by SS-O3 in 2 h at pH 3 and 200 mA
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: