The role played by local pH and pore size distribution in the electrochemical regeneration of carbon fabrics loaded with bentazon

CARBON 94 (2015) 816–825

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CARBON journal homepage: www.elsevier.com/locate/carbon

The role played by local pH and pore size distribution in the electrochemical regeneration of carbon fabrics loaded with bentazon Sandrine Delpeux-Ouldriane ⇑, Mickaël Gineys, Nathalie Cohaut, François Béguin ICMN, CNRS-Université, 1B rue de la Férollerie, 45071 Orléans, France

a r t i c l e

i n f o

Article history: Received 12 March 2015 Received in revised form 23 June 2015 Accepted 2 July 2015 Available online 3 July 2015 Keywords: Adsorption Carbon fabrics Pesticide Regeneration

a b s t r a c t Adsorption of the herbicide bentazon on activated carbon fabrics was found to be enhanced in acidic medium, showing that bentazon is mainly adsorbed through dispersive interactions. Bentazon was reversibly desorbed by applying a cathodic polarization of the cloth, leading to its fast regeneration. The mechanisms involved were carefully examined in light of the nanoporous texture of carbon and surface functionality, using various electrolytes at different pH values. Results showed that bentazon was strongly adsorbed and that the negative charges on the carbon surface together with the local electrical field were not sufficient to induce desorption of anionic bentazon through electrostatic interactions. The pH increase inside the porosity, especially due to water decomposition, was crucial in favoring the dissociation of surface groups, thereby strengthening electrostatic repulsions between the negatively charged carbon surface and the anionic bentazon molecule. This mechanism occurred in the micropores and mesopores, the latter contributing to the desorption kinetics through transportation of the adsorbate from the carbon surface to the electrolyte. Additionally, the presence of acidic surface groups on the adsorbent or the use of a basic electrolyte improved the desorption process thanks to the increase in negatively charged functionalities and the promotion of electrostatic repulsions. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, a great effort has been made in the water sector to develop technologies aiming to improve water quality. Adsorption has become a well-established technique to remove organic pollutants. Among the various adsorbents used to remove organic compounds in waste water treatment processes, activated carbons are the most prevalent and competitive, especially at low pollutant concentrations. The major disadvantage encountered is their short lifetime due to their low and expensive regeneration capacities. Generally, the loaded carbon adsorbent can be regenerated ex situ through high energy-consuming processes such as thermal treatments or steaming, which involve a huge loss of activated carbon [1]. In most cases, when an exhausted carbon adsorbent reaches its saturation limit, it is stored in a landfill and later incinerated, therefore becoming a secondary pollutant [2]. Carbon cloths have many advantages compared to powder or granules, such as their easy handling, high mechanical integrity and regeneration potential. Due to their microtexture and their small fiber diameters (around 10 lm), they have minimal diffusion limitation and greater adsorption rates, which makes them ideal ⇑ Corresponding author. E-mail address: [email protected] (S. Delpeux-Ouldriane). http://dx.doi.org/10.1016/j.carbon.2015.07.010 0008-6223/Ó 2015 Elsevier Ltd. All rights reserved.

candidates for adsorption purposes [3]. As a result, intensive research involving carbon cloths or felts has been conducted in recent years and has demonstrated the high adsorption efficiency of carbon cloths towards noxious organic pollutants. The main pollutants concerned are charged organic adsorbates such as phenol derivatives [4], phthalates [5], dyes [6], surfactants [7], and pesticides [8–12] but also inorganic species such as ethyl xanthate, thiocyanate [13] and transition or heavy metals [14]. Additionally, the cost of adsorptive separation processes is largely affected by the adsorption rate and capacity together with the regeneration efficiency of the adsorbent. Electrochemical techniques involving high specific surface area carbon cloths are innovative and environmentally friendly processes allowing a better control of pollutant removal in aqueous solutions and especially a fast regeneration of the adsorbent. Electrochemical polarization has been used, for instance, to increase adsorption capacity and kinetics [15], but also to accomplish the reversible desorption of ions or organic compounds, depending on the selected polarization charge and potential in order to restore adsorption sites and therefore prolong the service life of the adsorbents [16]. Hence, for water treatment applications the use of carbon fabrics combined with electrochemical regeneration could contribute to reducing the reactor size. So far, extensive research has been carried out on the reversible electrosorption of inorganic ions [17–19] and

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more recently on organic ionizable molecules such as phenol, aniline or pesticides [20–26]. However, only a few studies have paid attention to the mechanism involved and to the impact of polarization on the characteristics of the activated carbon electrode material [27,28]. Among toxic organic pollutants, the increasing occurrence of pesticides in wastewater because of their heavy use in intensive agriculture has become a matter of public concern in view of their negative environmental and health impact [29]. In particular, their ability to diffuse rapidly in the environment, their long-term persistence (in some cases over 30 years), their high toxicity, endocrine disrupting effect and carcinogenic properties, make them targeted pollutants to be eliminated. Numerous pesticides are therefore considered as priority pollutants by the European Union which fixed the maximum limit for each pesticide at 0.1 mg/L [30]. Year after year the list of regulated contaminants grows and legislative texts are upgraded. Recently, the implementation of the 76/464/EEC directive defined new targeted pollutants including pesticides such as Aclonifen, Bifenox, Cybutryne, Cypermethrine, and Terbutryne. Other classes of pollutants such as chlorinated hydrocarbons and polymer additives are also concerned; these molecules have to be eliminated selectively in water, which implies the development of new and more efficient treatment techniques [31]. Since being banned in Europe in 2003, the popular herbicide Atrazine has been replaced by the bentazon molecule or by Alachlor which has the same spectrum of action [32]. These molecules belong to the triazine family whose activity is based upon their ability to inhibit photosynthesis in plants and are used as selective herbicides to control weeds in many cereal crops (wheat, corn, rice, sorghum, etc.). This class of pesticides is particularly worrying because of their resistance to the biological degradation processes commonly used in water treatment plants. The possibility of performing the adsorption (under anodic polarization) or the desorption (under cathodic polarization) of bentazon on a carbon textile was already demonstrated in a previous study [33,34]. However, the data need to be completed using different adsorbents in order to explain the role of the porous texture and of the surface functionality on the electrochemical regeneration mechanism. In the present study, we first investigated the adsorption of the herbicide bentazon on different activated carbon cloths having various nanotexture and surface chemical characteristics, and as a function of the pH medium. After chemical adsorption of bentazon, the reversible desorption of bentazon and regeneration of the porosity were investigated through cathodic polarization of the carbon adsorbent. Depending on pH and electrochemical conditions, the variations in adsorption/regeneration ability are explained in terms of pore size distribution and interactions between the surface and the adsorbate (electrostatic, dispersive). On the basis of these experiments, the adsorption/desorption mechanism is proposed by comparing the electrochemical results with the physico-chemical parameters of the different carbon textiles.

2. Experimental 2.1. Materials The activated carbon cloths (ACC) referenced CT-13, CT-27, and CT-71 prepared from a viscose precursor, were supplied by the Mast Carbon company (UK). The ACC sample referenced Ky originating from a phenol–formaldehyde resin was supplied by the KYNOL company (Japan). Prior to adsorption and electrochemical

817

investigations, the adsorbents were washed with boiling distilled water using a Soxhlet extractor under nitrogen and then dried under vacuum at 393 K. The herbicide, bentazon, of Pestanal grade was purchased from Riedel de Haen. Its complex molecular structure (Fig. 1) shows an aromatic ring together with polar functions able to give a keto-enolic equilibrium, and therefore leading to alternating neutral or anionic forms. Initial solutions of bentazon were prepared at a concentration of 20 mg/L. The molecule dimensions are reported in Fig. 1. The concentration of bentazon was monitored during kinetic studies and at adsorption equilibrium by UV spectroscopy (Uvikon xs, Bio-Tek), at 212 nm (pH = 2) or 224 nm (pH > 6.5). An HPLC system (DIONEX, ultimate 3000) equipped with a Photo Diode Array detector (THERMOFISHER, Accela 80 Hz PDA detector, cell of 5 cm) was also used to track the bentazon concentration but mainly to determine the amount of bentazon submitted to degradation under polarization and the presence and amount of by-products. The column was a C18 (Hypersil gold, 100  2.1 mm, particle size 3 lm). The pH of the aqueous phase was adjusted to 2.9 using orthophosphoric acid 0.01% and a gradient was applied using a mixture of water and CH3CN, from 5% to 95% CH3CN within 18 min. Analyses were conducted at a flow rate of 0.25 ml/min with an operating pressure in the range of 75–90 bar, and injection volumes of 25 ll. 2.2. Textural and chemical surface properties of ACC After outgassing overnight under vacuum up to 10 mtorr at 393 K, the carbon cloths’ porosity was characterized by N2 and CO2 adsorption at 77 K and 273 K, respectively, using an Autosorb-1 (Quantachrome). The total pore volume was estimated from the amount adsorbed at P/P0 = 0.95. The micro- and meso-pore volumes were determined by applying the density functional theory method (DFT) to the N2 isotherms assuming slit pore shape. The Dubinin–Radushkevich theory (DR) was applied to the CO2 adsorption isotherm at 0 °C to estimate the volume of ultramicropores. Gas adsorption was also used before and after chemical adsorption and also after electrochemical desorption of bentazon to determine the nature of the pores involved in the adsorption process. The pH value corresponding to an ACC net charge of zero, namely pHPZC was measured at constant ionic strength in NaNO3 (0.01 mol/L) using the method previously described by Noh and Schwarz [35]. Surface chemical functionalities were measured by potentiometric titration (PT) according to the pKa distribution method based on a titration with NaOH (0.1 mol/L) using a very low incremental volume (0.01 ml) in a wide range of pH (3–11). The method consisted in dispersing 100 mg of ACC in 75 ml of NaNO3 0.01 mol L1, previously degassed by nitrogen bubbling during 48 h. The pH of the solution was then adjusted to 3 using HCl 0.1 mol/L and titrated by NaOH. The pKa values and amounts of the acidic functionalities were determined using the SAIEUS program applied on the proton affinity versus pH curves [36]. Temperature programmed desorption (TPD) was performed on a NETZSCH apparatus, based on the coupling of a STA449C microbalance with a MTA 445C mass spectrometer. This technique, based on the thermal decomposition of surface groups and the detection of evolving gases with mass spectrometry, provides information about the nature and the amount of acidic functionalities on the carbon surface. Samples were heat treated up to 1000 °C with a ramp of 10 °C/min under helium. Quantitative analysis was carried out using calcium oxalate as external standard and on the assumption that CO2 evolution comes from the decomposition of carboxylic and lactone groups, CO from ether and H2O from phenolic functionalities [37].

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Fig. 1. Molecular structure and dissociated form of bentazon.

2.3. Determination of adsorption kinetics and isotherms For the adsorption isotherms, pieces of carbon cloth of different mass were allowed to equilibrate at 25 °C for 72 h with 50 ml of a 20 mg/L (initial concentration) bentazon solution. The effect of the solution pH on the adsorption mechanism was investigated using 0.1 mol L1 KCl/HCl (pH = 2) and a phosphate buffer (pH = 8). The concentration of bentazon was measured by UV spectroscopy and the equilibrium data were fitted according to the Langmuir– Freundlich (1) single solute equation, where Ce is the equilibrium concentration per mass unit of adsorbent, qe is the amount of solute adsorbed per unit gram of adsorbent and qm is its maximum adsorption in mg g1. K is the Langmuir-type constant as described in the Van’t Hoff equation, related to the heat of adsorption. The heterogeneity parameter n reflects the nature and the adsorption interaction force but also the adsorption sites heterogeneity [38]:

qe ðKC e Þn ¼ qm 1 þ ðKC e Þn

ð1Þ

In order to monitor the kinetics of adsorption in open circuit and of desorption under polarization, 2 ml of the solution was extracted after a predefined time interval for pH measurement and UV spectroscopy analysis and was then re-injected in the reacting mixture. The carbon surface cover level of ACC (h) at equilibrium was calculated from the amount of bentazon adsorbed taking into account the surface area of the ACC and the number of carbon atoms per square meter [10]. 2.4. Electrochemical polarization procedures Electrochemical experiments were performed using a classical three-electrode system connected to a galvanostat/potentiostat (VMP-1, Biologic). A 102 mol L1 Na2SO4 (pH = 6.5) or H2SO4 (pH = 2) solution was selected as supporting electrolyte; a platinum basket was used as counter electrode and Hg/Hg2SO4 (E = 0.649 V/NHE) as reference electrode. Electrolytes were not

degassed prior to electrochemical regeneration. In basic medium a 102 mol L1 NaOH solution (pH = 12) was used and Hg/HgO (E = 0.098 V/NHE) as reference electrode. Disks of ACC were cut (Ø = 14 mm, from 15 to 20 mg depending on ACC density) and fixed on a gold collector to determine both the adsorption and the electro-desorption kinetics. After adsorption in open-circuit, the electro-desorption of the loaded bentazon was studied through the cathodic polarization of the carbon cloth electrode in different pH conditions. The kinetic curves show C/C0 as a function of time (adsorption time or polarization time), where C0 is the initial pesticide concentration and C the pesticide concentration measured at a defined time. Adsorption without polarization and electro-assisted desorption kinetics were performed with the same mass of carbon textile and keeping a constant solution concentration and volume (20 mg/L, 40 ml).

3. Results and discussion 3.1. ACC textural and chemical characteristics In sorption processes, porous network and structure are of great importance since surface area, defined by graphene domains, is the reactive site where physisorption and/or chemisorption occur. The chemical functionalities and pHPZC value of an activated carbon are also crucial to predict and understand its adsorption ability towards a target pollutant depending on its chemical structure, solubility and the characteristics of the solution (pH, ionic strength, temperature). The pH value in particular affects the charge of the carbon surface in relation with its pHPZC and also the dissociation state of the adsorbate depending on the pKa value. As seen from the data on gas adsorption, the activated carbon cloths CT-13 and CT-71 have a relatively high BET specific surface area of 1350–1400 m2/g and a pore size distribution showing a large number of narrow micropores together with a large proportion of mesopores, which represent 30–50% of the total pore volume (Table 1). In addition, the DR micropore volumes calculated from N2 adsorption, i.e. 0.52 and 0.48 cm3/g for CT-13 and CT-71

Table 1 Textural characteristics of ACC determined by N2 and CO2 adsorption. ACC

SBET (m2/g)

Vtotal (cm3/g)

Vmicro (N2, DFT) (cm3/g)

Vmeso (N2, DFT) (cm3/g)

Vultra (CO2, DR) (cm3/g)

CT-71 CT-13 CT-27 Ky CT13ox12 CT13ox16

1400 1350 1130 1170 1249 934

1.27 0.85 0.75 0.66 0.87 0.60

0.46 0.56 0.43 0.55 0.49 0.41

0.76 0.26 0.20 0.07 0.33 0.14

0.41 0.48 0.45 0.63 0.47 0.43

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are slightly larger than those detected by CO2 adsorption, thus suggesting that porosity is composed of a wide pore size distribution (ultramicropores narrower than 0.7 nm together with micropores between 0.7 and 2 nm). For CT-27, mesopores are present but do not exceed 30% of the total pore volume. The Kynol ACC shows a completely different nanoporous architecture, composed exclusively of narrow micropores with diameters below 1.5 nm (Table 1). In addition, its ultramicropore volume determined from CO2 adsorption (VDR(CO2) = 0.63 cm2/g) is higher than that determined from N2 adsorption (VDR(N2) = 0.56 cm2/g), showing that the microporosity is mainly made up of ultramicropores. It is quite complex to characterize the acidity of the different functions existing at the surface of an activated carbon as they can be numerous and influence the neighboring polar groups. TPD and potentiometric titration results show that Kynol ACC is the most hydrophobic and contains the lowest amount of oxygen (<3%) (Table 2). Its pHpzc is rather basic (ffi8) while CT-27 is neutral and CT-13 and CT-71 are acidic. Based on the correlation between pHpzc, PT and TPD, the four ACC used in this work were classified by increasing acidity, i.e. Kynol < CT-27 < CT-13 < CT-71. Quantitatively, the amount of oxygen functionalities at the surface of CT-71 was twice that encountered on Kynol ACC. According to evolved CO2, CO and H2O respectively, CT-71 had the largest amount of phenolic groups (1.15 mmol/g) and CT-13 the largest amount of carboxylic groups (0.73 mmol/g). 3.2. ACC adsorption properties Prior to exploring the regeneration ability of ACC loaded with bentazon using electrochemical polarization, it was crucial to determine the key parameters governing the adsorption of this pollutant on ACC. As reported in previous studies, the absorbability of bentazon is strongly dependent on pH conditions [10,33,34,39]. With a pKa value of 3.3, the bentazon molecule is a weak acid and its dissociation is directly dictated by pH (Fig. 1). Therefore neutral and/or anionic forms may exist in acidic or neutral media, respectively. In the adsorption process, the possible interactions between activated carbon and bentazon are either dispersive interactions between the aromatic ring and the p electrons of the graphitic structure, or electrostatic interactions, either attractive or repulsive when localized coulomb charges are present. By modulating pH it was therefore possible to identify the adsorption mechanism of bentazon and to determine the adsorption sites and the role of pore size. From the experimental point of view, the residual bentazon concentrations in the solution were measured at equilibrium by UV spectroscopy and the amount of adsorbed pesticide per unit of carbon mass was calculated from the following equation:

Q e ðmg=gÞ ¼

ðC 0  C e Þ  V m

where V is the volume of the bentazon solution, and m is the mass of ACC in g. C0 and Ce are initial and equilibrium bentazon concentrations respectively in mg/L.

The resulting isotherm is the result of superposing the interactions between the solute and solvent, surface, other solute molecules but also between surface and solvent. In the case of aqueous solutions, water can be reactive and may be present in different forms (H2O, H3O+ and OH) and at different concentrations depending on pH. As seen from the adsorption isotherm data on ACC with different characteristics, at three pH values, the adsorption capacity decreased at higher pH when the anionic bentazon form was predominant in solution (Table 3). In the case of adsorption at pH = 6.5 and 8, the qm values were comparable because bentazon is anionic and presents a good solubility in water. In addition, part of the ACC acidic functionalities may be dissociated, leading to electrostatic repulsion interactions. At acidic pH, the qm values were much higher because the neutral form of bentazon has a lower water affinity and spontaneous adsorption occurs through dispersive interactions. It was demonstrated that, as far as dissociated bentazon is concerned, the ultramicropores played a huge role in the adsorption process and adsorption capacities were proportional to the ultramicropore volume (Table 4). Additionally, for neutral bentazon, the lowering of solubility was predominant, leading to a spontaneous adsorption and consequently adsorption capacities were proportional to the total pore volume including mesopores. The oxygen level of Kynol ACC was two times lower than that of the other ones and its adsorption capacities showed only slight variations as a function of pH. Despite its moderate porous volume, it has a pronounced hydrophobic character and a high microporous volume allowing an efficient adsorption affinity towards a small aromatic organic molecule such as bentazon. The mean micropore size Lo calculated for Kynol was 0.99 nm, i.e. slightly higher than the length of bentazon, leading to an efficient adsorption through high interactions with the carbon surface whatever the speciation of bentazon. The same tendency was observed for CT-27 which has a higher amount of oxygenated surface groups (1.3 mmol/g). This material contains mainly phenolic groups which are only partially

Table 3 Langmuir–Freundlich isotherm parameters for bentazon removal at various pH. ACC

pH

qm* (mg/g) (±10)

K* (L/mg) (±0.006)

n* (±0.08)

h

CT-71

2 6.5 8

383 121 92

0.035 0.339 0.173

0.76 0.48 0.45

1.44 0.45 0.32

CT-13

2 6.5 8

293 116 71

0.018 0.020 0.009

0.45 0.44 0.40

0.86 0.30 0.33

CT-27

2 6.5 8

236 226 185

0.008 0.039 0.012

0.34 0.30 0.30

0.75 0.56 0.69

Kynol

2 6.5 8

266 169 215

0.051 0.056 0.047

0.62 0.59 0.49

1.22 0.83 0.76

* qm, K and n variables are described in Section 2.3, determination of adsorption kinetics and isotherms in relation with the Langmuir–Freundlich Eq. (1).

Table 2 Chemical characteristics of ACC determined by potentiometric titration and TPD. ACC

CT-71 CT-13 CT-27 Ky CT13ox12 CT13ox16

pHPZC

5.8 6.6 6.9 8 6.3 5.9

Oxygen evolution from TPD

Functional groups pKa distribution (mmol/g)

CO2 (lmol/g)

CO (lmol/g)

H2O (lmol/g)

O (w/w %)

3 < pKa < 7 carboxylic

7 < pKa < 11 phenolic

Total

483 505 280 220 – –

2001 1088 1400 1110 – –

973 393 340 200 – –

6.3 3.9 3.7 2.8 – –

0.16 0.73 0.47 0.52 0.16 0.11

1.15 0.36 0.53 0.18 1.38 1.99

1.31 1.09 1.00 0.70 1.54 2.10

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Table 4 Porous nanotexture characteristics before and after bentazon adsorption at pH 6 (50 mg/L). ACC

SBET (m2/g)

Vmicro (cm3/g)

Vmeso (cm3/g)

Vultra (cm3/g)

Kynol Saturated Kynol CT-71 Saturated CT-71

1170 35 1400 424

0.55 0.06 0.46 0.07

0.07 0.04 0.76 0.37

0.63 0.12 0.41 0.21

dissociated at pH 8 thus minimizing electrostatic repulsions. On the contrary for CT-13 and CT-71 adsorption capacities were strongly affected when increasing pH and capacities at pH 2 were three to five times those measured at pH 8. These ACC present low pHPZC values, suggesting the presence of a higher amount of carboxylic groups which start to dissociate from pH 3. In addition these fabrics contain a large amount of mesopores that play a great role in the adsorption process of neutral adsorbate. The coverage level (h) which reflects the number of occupied hydrophobic sites was over one for CT-71 at pH 2, showing that several layers of bentazon may be adsorbed in mesopores (Table 3). Additionally, one should keep in mind that there is a tendency for sulfones to form molecular association which could explain the adsorption tendency in mesopores in acidic medium. For Kynol, h remained high whatever the pH conditions. Moreover, the presence of surface groups, especially when dissociated, increases competing adsorption of water molecules, reducing the possibility of dispersive interactions. In order to clarify the role played by the pore size in bentazon adsorption behavior, gas adsorption experiments were conducted before and after bentazon adsorption. Adsorption experiments were performed until saturation of the porosity on the highly microporous (Kynol) and on the highly mesoporous (CT71) ACCs, at a concentration of 50 ppm at pH 6 by repeated renewal of the solution. For Kynol ACC, the results showed an almost complete filling of the micropores and a clear decrease in the ultramicropore volume (Table 4). In the case of the micro-meso ACC CT-71, micropores were totally filled and mesopores partially. These data demonstrate that bentazon, even in the dissociated form, is preferentially adsorbed in the micropores and ultramicropores. Taking into account bentazon’s dimensions (Fig. 1), it confirms that it penetrates with its aromatic ring parallel to the pore surface via p–p dispersive attractions. These features suggest that the adsorption mechanism is dominated by dispersive interactions between the neutral form of bentazon and the carbon surface, as compared to the weaker interaction of carbon surface with anionic bentazon molecules. These results are in good agreement with those reported in previous studies establishing a strong adsorption pH dependence [14,15,18,19].

3.3. Desorption under polarization 3.3.1. Polarization effects on ACC texture and chemistry As bentazon can be considered an anionic adsorbate at neutral and basic pH, a cathodic (negative) polarization was applied to the carbon cloth samples pre-loaded with bentazon in order to study the adsorbents’ regeneration potentialities. Firstly the behavior and stability of the carbon porous texture under cathodic polarization was investigated. For this purpose, CT13 ACC was selected and submitted to a polarization of 600 mA/g during 5 h in Na2SO4 0.01 mol/L. The chemical surface and porous characteristics were characterized using TPD and gas adsorption respectively. Results are presented in Tables 5 and 6, where the polarized samples are indicated by the symbol (). It can be seen that neither the chemical functionality nor the porous nanotexture of the carbon cloth was modified to a large extent upon prolonged negative polarization. Only a slight oxidation of CT13 ACC was observed under negative polarization, shown by the introduction of phenolic functionalities. Oxidation may come from direct or secondary reactions with in situ produced peroxide radical species at the working electrode [28]. This negative polarization did not alter the porous volumes, whatever the type of ACC, whether highly microporous (Kynol) or rather mesoporous (CT13) (Table 5). The slight variations in the porous volumes measured are in the range of analytical error (5–10%) or may result from the introduction of a small amount of phenolic group through oxidation. These results are in good agreement with those described in the literature on activated carbon powders, with a tendency for ACC to show a better stability than activated powders [28]. However, the consequences of anodic or cathodic polarization on the surface functionality of activated carbon have so far been poorly documented. Nevertheless, it is now quite well established that under positive polarization carbon can be readily oxidized [27,28]. As adsorption kinetics on activated carbon fibers are very quick, the aging of the carbon material through oxidation explains why the adsorption of anionic species under anodic (+) polarization is of very limited interest [20,33]. Additionally, the potential window available to ensure ACC stability was determined using cyclic voltammetry in Na2SO4 0.5 mol/L (r = 79 mS/cm) in a

Table 5 Porous nanotexture characteristics of CT13 before and after cathodic polarization (600 mA/g). ACC

SBET (m2/g)

Vmicro (N2, DFT) (cm3/g)

Vmeso (N2, DFT) (cm3/g)

Vultra (CO2, DR) (cm3/g)

Vtotal (P/P0 = 0.95) (cm3/g)

CT13 CT13 () Kynol Kynol ()

1330 1210 1170 1268

0.48 0.45 0.55 0.56

0.29 0.28 0.07 0.07

0.49 0.51 0.63 0.64

0.84 0.82 0.66 0.68

Table 6 Chemical characteristics of CT13 before and after cathodic polarization (600 mA/g) determined by TPD. ACC

CT-13 CT13()

Mass loss (%)

7.1 7.4

Oxygen evolution from TPD CO2 (lmol/g)

CO (lmol/g)

H2O (lmol/g)

O (w/w%)

505 520

1088 1046

393 619

3.9 4.3

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-1.6

-1.2 -0.8 -0.4 E (V vs Hg/Hg2SO4)

8 6

0.0

b

I (mA)

4 2 0 -2 -4

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

E (V vs Hg/Hg2SO4) Fig. 2. Voltamograms performed on CT-13 (16 mg), in Na2SO4, 0.5 mol/L, at 1 mV/s: (a) reduction mode in the negative range of potential, (b) oxidation mode in the positive range of potential.

three-electrode cell using Hg/H2SO4 as reference electrode. The three-electrode system was cycled from a small negative potential (0.6 V) to the more negative side (to 1.8 V vs. Hg/H2SO4). It can be seen that at EWE > 0.8 V there was no remarkable hydrogen evolution process and only at EWE < 0.8 V was a large increase in current density observed. When cycling from 0.6 V to the more positive side (+0.6 V) we observed the positive potential limit of 0.3 V, where probably some surface oxidation or oxygen evolution already starts to take place as the current density starts to increase exponentially (Fig. 2). 3.3.2. ACC regeneration under cathodic (-) polarization Adsorption was first performed under open circuit at constant carbon cloth mass and constant concentration and volume of adsorbate (20 mg/L initial concentration of bentazon in Na2SO4 0.01 mol/L, pH = 6.5, 40 ml). After an equilibrium time of 24 h, the concentration of bentazon decreased from 20 to 0.88 mg/L showing that 0.795 mg out of 0.8 mg had been trapped (more than 99%). Cathodic polarization was then applied on the carbon fabric electrode by either the galvanostatic or the potentiostatic method, and the current or potential evolution was measured as a function of the polarization time. Under galvanostatic polarization conditions, the bentazon concentration in the solution, measured by UV spectroscopy, increased quickly leading to fast and almost complete desorption (up to 75% during the first cycle) (Fig. 3).

CP

8

0.8

7 6 Adsorption pH (open circuit)

0.4

4 0

10

1600 t = 40 min

1200 t = 80 min

800 400

t = 120 min

0 0

5

10

15

20

Retention time (min)

300

200

b Bentazon Peak at 12.5 min Peak at 13.2 min

100

5

0.2 0.0

2000 t = 0 min

pH

C/C0

Desorption (-)

0.6

a

Area (mAu)

OCV

1.0

Additionally, it is important to note that, during the electro-assisted desorption of bentazon, the UV spectrum was not modified, confirming that bentazon seems to be stable whatever the current density applied on the carbon fabric (from 100 to 1500 mA/g) and in a wide potential window from 2500 mV to +600 mV. The process is therefore reversible, preserving the whole integrity of the pesticide molecule and without generating side reactions giving rise to derivatives or fragments through oxidative phenomena. Additionally, the analysis of samples by HPLC measurements while applying polarization revealed that bentazon was recovered together with two major by-products, detected at 12.5 and 13.2 min in the chromatogram, the pristine bentazon molecule being eluted at 14.4 min. Using a calibration curve from 0.5 to 10 mg/L the quantitative analysis of unmodified bentazon was calculated and was found to be nearly 93%, indicating that the by-products represent 7% at most of the former quantity of bentazon. The by-products present slightly different UV spectra from bentazon, the two bands p ? p⁄ and n ? p⁄ being shifted to higher wavelengths due to the bathochromic effect (Fig. 4). These two degradation products may result from the oxidation of desorbed bentazon by hydroxyl radicals generated near the counter electrode. After a few minutes of negative polarization at 20 mA (1250 mA/g), the global pH of the solution decreased noticeably from 6.5 to 3, indicating that H3O+ ions produced at the counter electrode are not fully neutralized by OH- ions produced at the working electrode (Eqs. (2) and (3)). This shows that most of the hydroxyl anions were trapped in the ACC nano- and microporosity:

Area (mAu.min-1)

a

I (mA)

6 4 2 0 -2 -4 -6 -8 -10 -2.0

20

30

3

Time (h) Fig. 3. Bentazon kinetic adsorption curve at pH = 6.5 on CT13 under open circuit and kinetic desorption curve at 10 mA. C0 represents the initial concentration of bentazon. OCV: open circuit voltage, CP: chronopotentiometry.

0 200

300 Wavelength (nm)

Fig. 4. Chromatograms of a blank experiment performed on a bentazon solution upon negative polarization (a) and UV spectra extracted from the DAD detector of bentazon and of the two oxidized by-products (b).

S. Delpeux-Ouldriane et al. / CARBON 94 (2015) 816–825

1.0

1.0

0.8

0.8

0.6

0.6

C/C0

C/C0

822

0.4 0.2 0.0

- 20 mA

0.4 0.2

0

0.0

100 200 300 Polarization time (min) - 10 mA

- 1800 mV

- 1100 mV

- 900 mV

0

Open circuit

- 750 mV

Fig. 5. Bentazon desorption kinetics at pH = 6.5 using different polarization conditions on pre-loaded CT13 at pH = 6.5. C0 represents the initial concentration of bentazon. The potentials are expressed vs. Hg/Hg2SO4.

Working electrode 2H2 O þ 2e ! H2 þ 2OH

ð2Þ

Counter electrode 6H2 O ! O2 þ 4H3 Oþ þ 4e

ð3Þ

3.3.3. Role of water decomposition in the regeneration mechanism (potentiostatic mode) According to the Nernst law, the equilibrium value for water reduction at an initial pH of 6.5 is close to 1030 mV vs. Hg/Hg2SO4. In order to clarify the role of pH modifications related to water electro-decomposition, the working electrode potential was fixed at different values from 750 mV to 1800 mV. As shown by Fig. 5, bentazon desorption was negligible down to 900 mV, and it became observable at 1100 mV. Only 15% of the loaded bentazon was desorbed even if the polarization time at 1100 mV was maintained for 24 h. This result fits well with the hydrogen electrosorption measurements on nanoporous carbons, which show that the processes involving Eq. (2) occur with a relatively high overpotential [40]. The important role of water decomposition on the electrochemical desorption of aromatic species was already suggested in a previous work [25]. Using higher potential values, in the range of 1500 mV to 1800 mV, bubbles appeared near the working electrode due to evolving hydrogen and the desorption of bentazon became significant, with a rapid increase in the UV bands. Part of the OH reacts with bentazon to give its anionic form, enhancing its solubility and enabling its desorption from the carbon surface through electrostatic repulsions. The concentration of bentazon in the solution after reaching the desorption plateau was 13.23 mg/L showing that 0.53 mg of bentazon can be desorbed over the 0.795 mg initially adsorbed chemically; that is to say a regeneration level of over 70%. From 10 mA (10 and 20 mA) the regeneration regime tended to a limit. Potential values reached in galvanostatic mode were E = 1.75 V vs. Hg/Hg2SO4. To corroborate these results and to confirm the key role of OH production, responsible for a local pH effect in the desorption mechanism, kinetic experiments were performed without stirring the solution during electrochemical desorption, but with a short homogenization for a few seconds before each UV measurement. Without stirring, the desorption process was not favored and quickly reached a low stage of adsorbent regeneration (C/C0 < 0.2). This result suggests that rinsing the pores with the solution plays a role in the desorption process. One can assume that OH anions produced by reduction of water induce the ionization of adsorbed bentazon, giving rise to electrostatic repulsions with the negatively charged carbon surface. At the same time, if the electrolyte in the pores is not restored, the concentration of anionic bentazon within the pores increases and desorption is no longer favored. As a result, the renewal of the electrolytic solution during desorption concurs with the mechanism proposed to

100 200 Polarization time (min) H2SO4 (pH = 2)

Na2SO4 (pH = 6,5)

300 NaOH (pH = 12)

Fig. 6. Desorption kinetics curves of bentazon under open circuit and under cathodic polarization in potentiostatic mode at different pH = 2 (E = 700 mV vs. Hg/Hg2SO4); pH = 6.5 (E = 900 mV vs. Hg/Hg2SO4), and pH = 12 (E = 600 mV vs. Hg/HgO). CT13 (16 mg) was pre-loaded with bentazon (20 ppm, 40 ml) at pH = 6.5.

interpret the desorption phenomenon. It must also be remembered that in water solution, complex adsorption competition between adsorbate and solvent molecule always occurs. Hence, under stirring, the substitution of bentazon by water molecules on the adsorption sites might accelerate the desorption step and enhance the desorption rate. Additionally, we studied the regeneration of C13 ACC under cathodic polarization at different pH values in order to clarify the role of water electrolysis. Fig. 6 shows the bentazon desorption results in neutral (Na2SO4 0.01 mol/L), basic (NaOH 102 mol/L) and acidic pH (H2SO4 102 mol/L). These experiments were performed using the potentiostatic mode respectively at E = 0.9 V vs. Hg/Hg2SO4, E = 0.6 V vs. Hg/HgO and E = 0.7 V vs. Hg/Hg2SO4. Let us recall that at pH = 2 and pH = 12 the equilibrium potentials for water reduction are respectively 0.799 V vs. Hg/Hg2SO4 and 0.61 vs. (Hg/HgO). The potentials applied were always under the potential needed to achieve water hydrolysis in order to quantify the amount of desorbed bentazon without the help of water electrolysis and pH changes. For the blank regeneration experiment conducted without any applied potential, no bentazon was observed. In acidic medium, the regeneration level was almost negligible. At neutral pH, desorption was in the range of 10% showing that water electrolysis is needed to achieve a pH increase at the working electrode responsible for dissociation of the bentazon molecule and its desorption through electrostatic repulsions. Regeneration is clearly enhanced in basic medium where anionic bentazon can be desorbed easily thanks to the electrostatic field. 3.3.4. Role of current density on desorption The role of current density (2, 5, 20 mA) was investigated on CT13, the effect of texture and chemistry on the four pristine and oxidized cloths at a fixed current density (5 mA) (Table 7, Fig. 7).

Table 7 Characteristics of the desorption kinetics of bentazon preloaded on ACC. ACC

Amount of loaded bentazon

I (mA)

Qdes (mg/ g)

k1  103 (min1)

CT71 CT13 CT27 Kynol CT13

Qads = 46 mg/g

5

Qads = 46 mg/g k1 = 6.9  103 min1

2 5 20 5

38 36 20 15 28 36 35 36.3 41.1

41.7 17.6 22.9 25.8 9.2 17.6 40.5 19.5 22.3

CT13ox12 CT13ox16

Qads = 43 mg/g Qads = 40 mg/g

823

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50 40

30

Qdesorbed (mg/g)

Qdesorbed (mg/g)

40

20 10 0

0

- 310 mA/g

10 0

- 125 mA/g

Fig. 7. Desorption kinetics curves at pH = 6.5 using different polarization conditions on CT13 (16 mg) was pre-loaded with bentazon (20 ppm, 40 ml) at pH = 6.5.

The desorption kinetics was readily influenced by the current density applied on the carbon electrode. The desorption regime was quicker than the adsorption one, even at very low current density, showing that bentazon desorption operates very easily under cathodic polarization. The first order kinetic k1 value for bentazon adsorption on CT13 (6.9  103 min1) is given for comparison, showing that the desorption rate was five times higher than the adsorption rate (Table 7). All the curves can be fitted with pseudo first order kinetic models. The higher the current density applied, the higher the rate of bentazon desorption. For low current density (from 0 to 600 mA/g), the constant k1 calculated for first order kinetic was rather proportional to the applied current density, whereas over 600 mA/g, no gain in k1 was observed. Nevertheless, desorption reached a limit whatever the current density applied. The electrochemical desorption process was similarly applied for every ACC used in this study, but desorption levels varied from 40% to 70% depending on the ACC characteristics (Fig. 8). Additionally, for the same polarization conditions, desorption kinetics differed significantly depending on the selected ACC. In particular, for CT71, the most mesoporous ACC, the desorption kinetic was twice that observed for Kynol material which has a highly microporous nanotexture (Table 7). The mesopores play a positive role in desorption kinetics by increasing the transport of bentazon from the carbon surface, in its adsorbed state, to the electrolyte.

3.3.5. Role of surface chemistry on desorption In order to clarify the role of surface functionalities in the desorption mechanism, the CT13 ACC was chemically modified through chemical oxidation using various oxidants such as nitric acid (CT13–12%) and hydrogen peroxide (CT13–16%). CT13 contains 6.8 at.% of oxygen and the two oxidized samples contain respectively 12 and 16 at.% of oxygen, determined from XPS analysis. The amount of surface groups was measured by

100 200 Polarization time (min) CT-13

CT-13 Ox12%

300

CT-13 Ox16%

Fig. 9. Desorption kinetics curves at pH = 6.5 at 300 mA/g on CT13 and on oxidized CT13 by HNO3 acid treatments: CT13-Ox12% and CT13-Ox16%. ACC samples (16 mg) were pre-loaded with bentazon (20 mg/L, 40 ml) at pH = 6.5.

potentiometric titrations showing that CT13 contains 1 mmol/g of acidic functionalities (mainly carboxylic groups) whereas CT13ox12 and CT13ox16 possess respectively 1.6 and 2.1 mmol/g of acidic groups, mainly phenolic (Table 2). Their porous nanotextures differ slightly from the pristine CT13 and are characterized by a small loss of micropores (Table 1). As seen from Fig. 9, it appears clearly that the kinetics of bentazon desorption is favored on the oxidized ACC (Table 7). This result corroborates the desorption mechanism through electrostatic repulsions helped by a local pH increase. Indeed, following the pH increase in oxidized ACC porosity during cathodic polarization, the acidic groups become dissociated, giving rise to additional negative charges acting in favor of electrostatic repulsions with the bentazon adsorbate being also in its anionic form. 3.3.6. Porosity recovery after regeneration However, in the more favorable conditions, the regeneration level did not exceed C/C0 = 0.7, demonstrating that for the CT13 carbon cloth some irreversible trapping occurs (Figs. 7 and 8). Several phenomena have to be considered. The relatively high level of regeneration reached within a few minutes (70%) suggests that bentazon in its anionic form is weakly adsorbed. Nevertheless, there may be some strong adsorption sites and/or some pores where the pollutant could be blocked. The CO2 adsorption data on pre-loaded CT13 samples at different levels of electro-desorption show that when C/C0 reaches 0.65, the ultramicropores are fully emptied (Table 8). The micropore volume and the BET specific surface area of the regenerated samples are slightly lower than in the pristine CT13 carbon, suggesting that some ultramicropores remain occupied by bentazon. It can be speculated that graphitic units available for chemical adsorption, but not connected electrically to the carbon network, cannot participate in the electro-assisted desorption process. 3.3.7. Cycle-to-cycle regeneration In order to confirm this interpretation, bentazon was once more adsorbed on CT13 after the first electro-desorption stage. For this purpose, after desorbing 70% of pre-loaded bentazon, the sample

50 40 Qdesorbed (mg/g)

20

0

100 200 300 Polarization time (min) - 1250 mA/g

30

30 Table 8 Porous nanotexture characteristics after bentazon loading at pH 6 (20 mg/L) and after electrochemical regeneration.

20 10 0

0

100 200 Polarization time (min) CT-13

CT-71

CT-27

300 Kynol

Fig. 8. Bentazon desorption kinetics at pH = 6.5 at 300 mA/g on different types ACC. ACC samples (16 mg) were pre-loaded with bentazon (20 mg/L, 40 ml) at pH = 6.5.

ACC

SBET (m2/g)

Vmicro (cm3/g)

Vmeso (cm3/g)

Vultra (cm3/g)

CT13 CT13-Btz CT13-Btz () Kynol Kynol-Btz Kynol-Btz ()

1350 910 1187 1170 1193 1155

0.56 0.23 0.48 0.55 0.54 0.52

0.26 0.30 0.26 0.07 0.15 0.14

0.48 0.29 0.43 0.63 0.44 0.58

824

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It is clear that local pH effects related to OH production from water reduction facilitate bentazon desorption. The results presented here show that cathodic polarization is an effective tool for the in situ regeneration of adsorbents loaded with ionogenic pesticides.

1.0

C/C0

0.8 0.6 0.4 1st cycle

0.2 0.0

0

100 200 Polarization time (min)

2nd cycle

300

Fig. 10. Desorption kinetics curves at pH = 6.5 at 300 mA/g on CT13 preloaded with bentazon after a first adsorption cycle (1), after an second adsorption cycle (2).

was immersed in a fresh solution of bentazon and further submitted to a new cathodic polarization (Fig. 10). It can be clearly seen that the desorption kinetics is the same but that a higher stage of regeneration is reached. This result provides confirmation that some strong adsorption sites are occupied during the first adsorption process and that the regeneration level in the following cycles can reach values close to 100%. This feature illustrates the good cyclability of the process. After the second cycle, the regeneration level was in the range of 90–95% and this process continued through five additional cycles of adsorption followed by electrochemical regeneration stages. 4. Conclusion The adsorption rate and capacity of the pesticide bentazon was investigated on activated carbon textiles. The hypothesis that bentazon is mainly trapped via p–p dispersive interactions between its aromatic ring and the carbon surface was confirmed thanks to the establishment of strong adsorption pH dependence. It was demonstrated that, as far as dissociated bentazon is concerned, the ultramicropores play a major role in the adsorption process and adsorption capacities are proportional to the ultramicropore volume. For neutral bentazon, at acidic pH, the reduction in bentazon solubility is a predominant factor and as a result adsorption capacities are proportional to the total pore volume including mesopores. After applying cathodic () polarization to the pre-loaded carbon textile electrodes, the regeneration level reached at least 70% within a short period of time (1 h), showing that part of the bentazon was trapped irreversibly (30–50%). Desorption implies electrostatic repulsive forces between the negatively charged carbon surface and the negatively charged bentazon molecule, significantly reinforced by the presence of the electrostatic field. These effects are more pronounced when pH values increase. Water electrodecomposition plays a crucial role by causing a local pH increase in the porosity, and favoring the dissociation of bentazon, thereby promoting electrostatic repulsions. A gain in the regeneration level is even observed, reaching 95% over the cycles. The incomplete regeneration may come from a blockage of bentazon in the narrow micropores or on some strong adsorption sites. Using different ACC with different porous and chemical surface properties it has been possible to show that the ideal material for a regenerative process under polarization should combine the presence of micropores together with some mesopores. Micropores ensure a high adsorption kinetic and uptake level and mesopores allow the transportation of the solution and help in the desorption step. The establishment of an optimized surface chemistry is more complex. While a highly hydrophobic ACC is very efficient in the adsorption of aromatic species, the presence of oxygen functionalities alters adsorption performances but could help in the regeneration phase.

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