Anodic oxidation of nitrobenzene on bdd electrode: variable effects and mechanisms of degradation

Contents lists available at Separation and Purification Technology Anodic oxidation of nitrobenzene on BDD electrode: Variable effectsand mechanisms of degradation Nejmeddine Rabaaoui ,Younes Moussaoui ,Mohamed Salah Allagui , Bedoui Ahmed ,Elimame Elaloui a Materials, Environment and Energy Laboratory, Science Faculty of Gafsa, University of Gafsa, Gafsa 2112, Tunisiab Science Faculty of Gafsa, University of Gafsa, Tunisiac Science Faculty of Gabes, University of Gabes, Gabes 6072, Tunisiad Physical Organic Chemistry Laboratory, Science Faculty of Sfax, University of Sfax, Tunisia The electrochemical oxidation of pesticide, nitrobenzene (NB) as one kind of pesticide that is potentially Received 5 December 2012 dangerous and biorefractory, was studied by galvanostatic electrolysis using boron-doped diamond Received in revised form 27 January 2013 (BDD) as anode. The influence of several operating parameters, such as applied current density, support- Accepted 28 January 2013 ing electrolyte, and initial pH value, was investigated. The best degradation occurred in the presence of Available online 5 February 2013 Na2SO4 (0.05 M) as conductive electrolyte. After 8 h, nearly complete degradation of nitrobenzene wasachieved (88%) using BDD electrodes at pH = 3 and at current density equals 60 mA cm2. The decay kinetics of nitrobenzene follows a pseudo-first-order reaction. Aromatic intermediates such as phenol, catechol, resorcinol, 1,2,4-trihydroxybenzene, hydroquinone and benzoquinone and carboxylic acids such as maleic glycolic, malonic, glyoxilic and oxalic, have been identified and followed during the nitro- benzene treatment by chromatographic techniques. From these anodic oxidation by-products, a plausible Wastewater treatment reaction sequence for NB mineralization on BDD anodes is proposed.
Ó 2013 Elsevier B.V. All rights reserved.
H2O2/UV and H2O2/Fe2+/UV . The great effectiveness of AOPsis due to the production of hydroxyl radical (OH), which is a non- Nitrobenzene belong to nitroaromatic compounds which are selective, very powerful oxidizing agent able to react with organics deemed to be the exclusive pollutants from anthropogenic sources giving dehydrogenated or hydroxylated derivatives, up to their . These chemicals are widely spread across the environment complete mineralization (conversion into CO2, water and inorganic due to their extensive use as raw materials or synthetic intermedi- ates in the manufacture of pharmaceuticals, wood preservatives, In the last years, effective electrochemical treatments for the rubber chemicals, pigments, dyes, plastics, pesticides and fungi- destruction of biorefractory organics in waters are being devel- cidal agents, explosives and industrial solvents o-Nitrophe- oped. The most usual technique is anodic oxidation, where solu- nols pose potential risks to both the human health and the tions are decontaminated during electrolysis by the direct ecosystem since they are toxic to plants, fish and many other reaction of pollutants with adsorbed OH formed at the anode sur- organisms and can accumulate in the food chain The US Envi- face from oxidation either of water in acid and neutral media or ronmental Protection Agency (USEPA) has classified o-nitrophenol hydroxide ion at pH P 10 as a pollutant of group C (possible human carcinogens) . Purifi-cation of wastewater polluted by nitrobenzene is a very difficult H2O ! OHads þ Hþ þ e task. The presence of nitro group in the aromatic ring enhancesthe stability of these molecules to chemical and biological degrada- OH ! OHads þ e tion . In order to treat these toxic effluents, several papershave reported the rapid removal of nitro-aromatic from waters However, most aromatics in acid and alkaline media treated by by some advanced oxidation processes (AOPs), such as O anodic oxidation with conventional anodes such as Pt, PbO PbO2, doped SnO2 and IrO2, are slowly depolluted due to the gen-eration of difficulty oxidizable carboxylic acids The recent ⇑ Corresponding author at: Materials, Environment and Energy Laboratory, use of a boron-doped diamond thin film anode has shown that it Science Faculty of Gafsa, University of Gafsa, Gafsa 2112, Tunisia. Tel.: +216 28153 135; fax: +216 76 677 441.
has much larger O2 overvoltage than the above anodes, giving a E-mail address: (N. Rabaaoui).
much higher concentration of adsorbed OH and a quicker oxida- 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
N. Rabaaoui et al. / Separation and Purification Technology 107 (2013) 318–323 tion of pollutants. Anodic oxidation with BDD then seems a suit- potentiostat–galvanostat. The solution pH was measured with a able method for degrading organics up to their total mineraliza- Crison 2000 pH-meter. Total Organic Carbon (TOC) amounts in tion, as found for HClO4 aqueous solutions containing carboxylic aqueous solutions were obtained with a Shimadzu VCSH carbon acids such as acetic, malic, formic and oxalic indigo carmine analyzer. During all experiments, samples were collected and phenol and 2-(4-chloro-2-methylphenoxy) propionic immediately measured without using any filter. Chemical oxygen acid aromatic amines and for amarantha dyestuff in Na2- demand (COD) data were obtained with a Merck Model SQ118 spectrophotometer after digestion of samples in a Merck Model Our research focused on studying the degradation rate of nitro- TR-300 thermoreactor. nitrobenzene and all other intermediates benzene effluents using BDD electrode. A concentrated (2 mM) were identified by reversed-phase chromatography with a water solution of this pollutant was treated to explain the role of aro- system composed of a Waters 600 HPLC liquid chromatography fit- matic intermediates and carboxylic acids formed on the degrada- ted with a Spherisorb ODS2 5 lm, 150  4.6 mm column, at room tion process. The decay kinetics of nitrobenzene and the temperature, and coupled with a waters 996 photodiode array evolution of by-products for nitrobenzene were then followed by detector selected at 256 nm, controlled through a Millennium-32 chromatographic techniques, allowing the proposal of a reaction programÒ. These analyses were made by injecting 20 lL aliquots pathway for nitrobenzene mineralization.
into the chromatograph and circulating a 39/59/2 (v/v/v) metha-nol/water/acetic acid mixture at 0.7 mL min1 as mobile phase.
Generated carboxylic acids were followed by ion-exclusion chro- 2. Materials and methods matography by injecting 20 lL samples into the above HPLC sys-tem with a SUPELCOGEL C-610H, 250  4.6 mm column, at 35 °C 2.1. Electrodes preparation from Bio-Rad. For these measurements, the photodiode detector(L-2400) was selected at 210 nm and the mobile phase was PbO2 was deposited galvanostatically on the pretreated lead substrate by electrochemical anodization of lead in oxalic acid 2SO4 at 0.2 mL min1. Inorganic ions were analyzed by ion chromatography using a Dionex ICS-1000 Basic Ion Chromatog- solution (100 g L1) at 25 °C. This acid solution was electrolyzed raphy system fitted with an IonPac AS4A-SC, 25 cm  4 mm anion- galvanostatically for 30 min at ambient temperature using an ano- exchange column. These measurements were conducted by inject- dic current density of 100 mA cm2. The cathode was stainless ing 25 lL samples and using a mobile phase composed of 1.8 mM steel (austenitic type), the two electrodes were concentric with of sodium carbonate and 1.7 mM of sodium bicarbonate at the flow the lead electrode as axial. This arrangement gave the formation rate of 0.8 mL min1.
of a regular and uniform deposit .
BDD films were provided by CSEM and synthesized on a con- ductive p-Si substrate (1 mm, Siltronix) via a hot filament, chemi- 3. Results and discussion cal vapor deposition technique (HF-CVD). The temperature of thefilament was from 2440 to 2560 °C and that of the substrate was 3.1. Effect of experimental parameters on nitrobenzene mineralization monitored at 830 °C. The reactive gas used was 1% methane inhydrogen containing 1–3 ppm of trimethylboron. The gas mixture 3.1.1. Effect of the anode material was supplied to the reaction chamber at a flow rate of 5 L min1 to A series of comparative electrolyses for nitrobenzene was per- give a growth rate of 0.24 lm h1 for the diamond layer. This pro- formed using the BDD anode. Solutions containing 256 mg L1 cedure gave a columnar, randomly textured, polycrystalline dia- COD of each nitrophenol of pH 3.0 were treated at 60 mA cm2 mond film, with a thickness of about 1 lm and a resistivity of and at 20 °C by prolonging the electrolysis time to attain almost 15 mX cm (±30%) onto the conductive p-Si substrate .
overall decontamination. In these trials the solution pH remainedpractically constant and the starting colorless solutions changed 2.2. Electrolysis of nitrobenzene solutions to a clear brown color after 1 h of electrolysis, becoming colorlessagain at the end of treatment. This change in solution color can be Galvanostatic electrolyses were carried out at BDD and PbO related to the formation of benzoquinone intermediates, as will be electrodes, with current density ranging from 0 to 60 mA cm2.
discussed below.
Runs were performed at 20 °C. Solutions of 256 mg L1 of pure The COD abatement as function of the consumed specific charge nitrobenzene were used. Electrolysis was done with 0.05 M of dif- obtained for the above experiments is depicted in . A fast and ferent types of electrolytes NaCl, KCl, Na quite COD decay can be observed for our solution, yielding more 3PO4 and Na2SO4 with pH around 3.0–10.0. All electrolyses were conducted in an open, one-compartment and thermostated cylindrical cell containing a 150 ml solution stirred with a magnetic bar. The anode was a nitrobenzene by PbO2"
42 cm2 BDD. For comparative purposes, a 42 cm2 PbO2 was alsoemployed as anode. The cathode was always a 42 cm2 graphite nitrobenzene by BDD
bar from Sofacel. The interelectrode gap was about 3 cm. The cur-rent and potential measurements were carried out using digital COD / mgL 100
2.3. Analytical techniques The used compounds were either reagent or analytical grade from Sigma–Aldrich. Anhydrous sodium sulfate used as back- ground electrolyte was analytical grade from Fluka. All solutions were prepared with water from a Millipore Milli-Q system (con- Fig. 1. Chemical oxygen demand removal for the anodic oxidation of 100 mL of ductivity <6  108 S cm1). The temperature of the electrolyte solution with 2 mM of nitrobenzene in 0.05 M Na2SO4 of pH 3.0. (h) BDD or (j) was fixed by using a water thermostat. The current density for PbO2 anode and a graphite cathode (all them of 42 cm2 area) operating at the electrolysis was kept at the desired level with an Amel 2053 60 mA cm2; pH = 3 and at T = 20 °C.
N. Rabaaoui et al. / Separation and Purification Technology 107 (2013) 318–323 than 92% mineralization after consumption of 20 A h L1 for 8 h.
ven charge loading. It was found that the amount of polymeric These findings evidence that nitrobenzene degradation process products increased with a rise in current density.
on BDD since BDD (OH) formed from water oxidation by reaction The experimental results above suggest that the usage of differ- is able to completely mineralize these compounds at similar ent current densities at different oxidation stages is a good practice in industrial applications.
In order to compare the oxidation ability of BDD and PbO2, a commercially and more available material, we decided to investi- 3.1.3. Effect of pH value gate the degradation of nitrobenzene using the latter anode. In this Solution pH is an important factor for wastewater treatment. In case, the effluent acquired an intense dark brown color and a black anodic oxidation, there are many reports on the influence of solu- precipitate, probably composed of polymeric products, was slowly tion pH, but the results are diverse and even contradictory due to formed during the first hour of electrolysis. As illustrated in different organic structures and electrode materials Some the use of PbO2 leads to a slow COD decay, only attaining 71% authors reported that the oxidation process is more favorable in decontamination of the nitrobenzene effluent 20 A h L1 for 8 h.
acidic media . In contrast, others indicated that the efficiencyof the process was increased in alkaline media . According to 3.1.2. Effect of current density this literature, it can be concluded that the effect of pH strongly de- Applied current density is an important factor affecting the pends on the nature of the investigated organics and of the sup- electrolysis kinetics and process economics. It corresponds to the porting electrolyte. Therefore, the effect of pH on the degradation ratio between the applied current and the surface of the working rate of NB was studied at large pH range from acidic to basic. Aque- electrode. Therefore, the current density can be altered by chang- ous solutions of NB (246 mg L1) were electrolyzed at pH values of ing the current and/or the surface of the working electrode. The 3, 6, and 10 (). As can be seen from this figure, the degradation anodic oxidation process is an electrochemical process ruled by of NB in the acid medium is more efficient than the degradation in the following equation: alkaline and neutral medium. This effect may be due to the exten-sive oxidation and/or chemical modification of the electrode sur- BDD ! ðH2OÞ ! BDD ðOHÞ þ Hþ þ e face, which suggests a change in the surface properties.
Moreover, the solution was not buffered at the working pH value, So, the current increases and improves the degradation rate by the latter progressively decreased during the reaction presumably the rise of the OH production rate, as shows. This latter dem- due to the formation of oxidized by-products. In fact, COD reduc- onstrates two different regions: the first is when the charge load- tion attains more than 92% at pH = 3.0 with a total disappearance ing is below 3 A h L1. In this case, the COD, decreasing linearly, of the color but it does not exceed 60% at pH = 10.0. This is very is always more rapidly reduced when current density rises from important because industrial wastewater may have different pH 20 to 60 mA cm2 and there is no substantial effect of the current density on the oxidation efficiency. Note that the current efficiency In pH = 10.0, we observe that the solutions become a little bit (CE) is close to 100%, independent of the current densities used.
turbid after oxidation. This indicates the formation of polymeric The second is when the charge loading is above 3 A h L1 In this intermediate products by the hydroxyl group of NB that makes case, the COD decreases when current density rises from 20 to the degradation much more difficult The reactions of anodic oxidation of pollutants occur heteroge- neously considering that pollutants must be transported to the 3.1.4. Effect of type of electrolyte electrode surface first, and then be oxidized there. The pollutant shows the influence of nature of supporting electrolyte degradation may be subjected to current control or mass transfer (NaCl, KCl, Na2SO4 and Na3PO4) on the evolution with Q of COD control. Initially, the COD concentration was relatively large, and during galvanostatic electrolyses (j = 60 mA cm2) of 2 mM of accordingly its reduction rate was subjected to the current control.
nitrobenzene at 20 °C. It can be seen that the complete mineraliza- In this case, 100% CE could be obtained and COD decreased linearly tion of organic matter is achieved in the presence of the four elec- with the charge loading based on the mathematical model devel- trolytes. The graph shows that the abatement of the COD of oped by Comninellis and coworkers In the case of the nitrobenzene in the presence of Na2SO4 or Na3PO4 is more rapid COD reduction rate that was subjected to the mass transfer control, than in the presence of NaCl and KCl. This suggests that the oxida- only part of current supplied was used to oxidize pollutants, while tion of NB should be carried out by both direct electro-oxidation the rest current was wasted for generation of oxygen. Therefore, and mediated oxidation by hydroxyl radicals and other strong oxi- the residual COD increased as the current density increased at a gi- dants electro-generated from the oxidation of the supporting elec- Fig. 2. Influence of applied current density on the evolution of COD with specific Fig. 3. Influence of pH values on the evolution of COD with specific electrical charge electrical charge passed during electrolysis of nitrobenzene on BDD anode: (d) passed during electrolysis of nitrobenzene on BDD anode: (j) pH = 3, (N) pH = 6, j = 20 mA cm2, (N) 40 mA cm2, and (j) 60 mA cm2. Conditions: initial concen- and (d) pH = 10. Conditions: initial concentration 2 mM; current density: tration 2 mM; electrolyte 0.05 M Na2SO4; pH = 3 and at T = 20 °C.
j = 60 mA cm2; Electrolyte 0.05 M Na2SO4 and at T = 20 °C.
N. Rabaaoui et al. / Separation and Purification Technology 107 (2013) 318–323 sorbed on the PbO2 surface, where it reacts selectively and more slowly with adsorbed organics According to this behavior, several authors recently reported that phenol, o-substituted phe- nols and pesticide methamidophos are more rapidly destroyed with BDD than with PbO 2 using small conventional electrolytic The above concentration decays were well-fitted to a pseudo first-order kinetic equation and the excellent straight lines thus ob-tained are depicted in the inset panel of . This suggests the production of a constant OH concentration at each anode during electrolysis, which is much higher than that of the initial pollutant reacting with it either on the PbO2 surface or in the vicinity of the Fig. 4. Influence of supporting electrolyte on the evolution of COD with specific electrical charge passed during galvanostatic electrolyses of nitrobenzene on BDDanode: (r) KCl 0.05 M, (d) NaCl 0.05 M, (j) Na2SO4 0.05 M, and (N) Na3PO4 0.05 M; 3.3. Identification and evolution of intermediates conditions: initial concentration: 2 mM; pH = 3; current density: j = 60 mA cm2and at T = 20 °C.
Mineralization of aqueous NB solution was then monitored by measuring the TOC values during treatment. The decay of the trolyte. It seems that the mediated oxidation by the electro-gener- TOC as a function of specific electric charge is shown in ated reagents from the anodic oxidation of salt plays an important The overall mineralization was attained after 480 min showing role in the efficiency of the electrochemical process. To explain that electrogenerated hydroxyl radicals efficiently destroy aro- these results, it has to be taken into account that BDD-anodic oxi- matic hydroxylated intermediates and all final carboxylic acids as dation of aqueous solutions containing chloride, sulfate and phos- phate anions promotes the formation of hypochlorites, persulfates The degradation of the toxic organic compounds by the anodic and perphosphates respectively, and that these chemical species oxidation process can produce some aromatic intermediates that are very powerful oxidant with high standard reduction potentials can be much more toxic than the initial product and that can pol- This can be explained assuming that the oxidation with per- lute not only the water but also the environment. Thus the identi- sulfates or perphosphates seems to be more severe in the final fication of these intermediates is necessary to be able to propose a stages and it promotes the formation of carbon dioxide but the oxi- mechanism of mineralization by the attack of hydroxyl radicals dation with hypochlorites lead to a less effective oxidation of NB and in this case only mediated oxidation by hydroxyl radicals During anodic oxidation of NB, main reactions are successively should be considered in the mechanism of electrochemical process electrophilic addition of hydroxyl radical on the aromatic ring leading to the formation of polyhydroxylated benzene derivatives,such as phenol, catechol, resorcinol, 1,2,4-trihydroxybenzene, 3.2. Decay kinetics of nitrobenzene hydroquinone and benzoquinone. The evolution of these sub-stances in aqueous medium is represented in in that these The kinetics of the reaction of initial pollutants with BDD (OH) compounds are accumulated to the maximum between 120 min and 180 min of treatment with BDD, further being slowly removed 2 (OH) under the above operating conditions was followed by reversed-phase HPLC. The change of concentration of nitroben- to disappear in 480 min, that is a time similar to that needed for zene with electrolysis time is given in . The time required for the total removal of the initial pollutant (see total removal of nitrobenzene with BDD is very similar to that The evolution of carboxylic acids which are identified during needed for their overall mineralization, indicating that nitroben- the mineralization of NB (malonic, glyoxilic and oxalic) is repre- zene persist in solution up to the end of the combustion process sented in where they start to form at the beginning of the and they are degraded in parallel to their by-products by BDD electrolysis and they attain maximum concentration between 60 (OH). However, shows that nitrobenzene appears to be de- and 120 min with a similar kinetic. Maleic and glycolic acids were stroyed at slower rate using a PbO detected in low concentrations The higher concentration is attrib- 2 anode and completely removed in a time much more than 480 min. It is well known that OH uted to glyoxilic acid: 3.2 mg L1 at 60 min. Then the concentration formed at BDD remains mainly free on its surface where it attacks of these acids decreases to be eliminated after 480 min which is non-selectively and directly organics, whereas this radical is ad- proved in where the TOC is removed in the same period.
60 120 180 240 300 360 420 Time / min
120 180 240 300 360 420 480 Time/ min
Time/ min
Fig. 5. Decay of nitrobenzene concentrations with electrolysis time for the trials Fig. 6. TOC removal with time during anodic oxidation of nitrobenzene aqueous reported in The inset panel shows the kinetic analysis for the corresponding solution on BDD anode: conditions: initial concentration: 2 mM, current density: experiments assuming a pseudo-first-order reaction for each initial compound.
j = 60 mA cm2; electrolyte 0.05 M Na2SO4; pH = 3 and at T = 20 °C.
N. Rabaaoui et al. / Separation and Purification Technology 107 (2013) 318–323 o
0 0 60 120 180 240 300 360 420 480
Time / min
Fig. 7. Time-course of nitrobenzene aromatic derivatives formed during anodic oxidation of nitrobenzene aqueous solution on BDD anode: conditions: initialconcentration: 2 mM, current density: j = 60 mA cm2; electrolyte 0.05 M Na2SO4; pH = 3 and at T = 20 °C.
Glyoxilic Acid
Oxalic Acid
Malonic Acid

Time / min
Fig. 8. Time-course of the concentration of relevant carboxylic acids detected during anodic oxidation of nitrobenzene aqueous solution on BDD anode: condi- tions: initial concentration: 2 mM, current density: j = 60 mA cm2; electrolyte0.05 M Na2SO4; pH = 3 and at T = 20 °C.
Fig. 10. Proposed reaction pathway for the mineralization of nitrobenzene in aqueous acid medium by hydroxyl radicals generated in anodic oxidation process.
Concentration / mgL
3.4. Reaction sequence for nitrobenzene degradation Time/ min
presents a plausible general pathway for the mineraliza- Fig. 9. Accumulation of nitrate (j) and ammonium (N) ions detected during anodic tion of nitrobenzene in acid medium by anodic oxidation. The oxidation of nitrobenzene aqueous solution on BDD anode: conditions: initial proposed sequence takes into account all products detected for concentration: 2 mM, current density: j = 60 mA cm2; electrolyte 0.05 M Na2SO4; nitrobenzene in this work and for sake of simplicity. This result pH = 3 and at T = 20 °C.
is significant, because there are few data in literature showingmechanisms for nitrobenzene degradation by hydroxyl radicals at-tack in acid medium. Then, the presented mechanism can help todetail nitrobenzene degradation mechanisms by others AOP's at Organic nitrogen of NB converted to mineral ions such as NHþ similar conditions.
and NO during its mineralization by anodic treatment was fol- lowed by ion chromatographic analyses. The concentrations ofNHþ and NO were 2.9 mg L1 and 0.8 mg L1, respectively, after 480 min of electrolysis (). This result proves the total transfor-mation of organic nitrogen into mineral ions. These results demon- The electrochemical oxidation of nitrobenzene, using the anodic strate clearly that the mineralization of NB by the process of anodic oxidation process with BDD anode, has been deeply investigated.
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A guide to the management of occupational asthma

Occupational Safety and Health Information Series A GUIDE TO THE This booklet was written by Dr Chris Walls with assistance from theOSH Departmental Medical Practitioners and Dr Julian Crane(Physician, Wellington), Dr Margaret Wilsher (Physician, Auckland)and Dr Colin Wong (Physician, Dunedin), members of the NODSAsthma Panel. Dr John Allen (Pukekohe), Dr Charles Skinner (Auckland) and DrRob Stewart (Auckland) provided a critique from a general practiceview point and their assistance is greatly appreciated.

Catalogo farg:layout

CATALOGO GENERALEGENERAL CATALOGUE made in Italy, made in F.A.R.G. Nei primi anni Sessanta ad Invorio, nella provincia di Novara, da sempre distretto di eccellenza nella produzione dell'industriadella rubinetteria, Giampiero Conton inizia la sua attività fondando la Rubinetteria Conton. Inizialmente l'azienda ebbe comescopo principale la commercializzazione di materiale idrosanitario; l'intuito del fondatore e alcuni segnali provenienti dallaclientela fecero capire le aperture del mercato e la possibilità di investire con ottimi risultati nella produzione di rubinetti agalleggiante con relative sfere in materiale plastico e in rame, senza dover fare i conti con una concorrenza troppo numerosa.E' nel 1996 che nasce F.A.R.G., naturale evoluzione di Rubinetteria Conton, che opera oggi su un'area di circa 15.000 mq dicui 5.000 mq coperti dedicati ai processi produttivi. Nel tempo la gamma dei prodotti si è ampliata con l'introduzione dialcuni componenti per impianti idrosanitari mantenendo la garanzia di qualità attestata da una produzione interamente ‘Made in Italy'. La costante attenzione della qualità, l'utilizzo di tecnologie avanzate e una rete di vendita che si avvale dellacollaborazione di agenti presenti sul territorio, hanno portato l'azienda a imporsi sul mercato nazionale e su quello estero.

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