Patent Application
20210053027
A major challenge for environmental engineering is the development of sustainable water treatment systems with high rates of micropollutant removal. Today, activated carbon (AC) is widely used in water treatment plants as it has proven to be an effective adsorbent for removing organic compounds from water.1-3 This is due to its high surface area, its internal microporosity, and the presence of large amounts of various surface functional groups.4
However, it is a simple separation step: the organic pollutants are not degraded after this step. The AC is loaded/saturated with organic pollutants and becomes waste that must be treated. The treatment must lead both to the regeneration/reuse of the AC (in order to improve the sustainability and profitability of the AC process) and to the degradation of the organic pollutants (in order to avoid any environmental contamination).
While the efficiency and adsorption mechanisms of a wide range of organic compounds on various AC materials have already been widely reported in the literature,3,5 there remains a need to develop innovative and efficient processes for regenerating spent/saturated/loaded AC.
Thermal regeneration is the most widely used process. The efficiency depends closely on the nature of the adsorbed organic compounds and the nature of the interactions with the AC surface. Thermal regeneration with an inert atmosphere often leads to poor recovery of the initial adsorption capacity due to insufficient desorption of the chemisorbed compounds.6 In addition, additional treatment is necessary for the degradation of the desorbed pollutants. Higher removal rates are achieved during thermal treatment under oxidizing conditions but the microporous structure of the AC is then strongly affected by the process, and the adsorption capacity is then reduced during reuse.6,7
Chemical regeneration by oxidation, using for example ozone or the Fenton reaction, limits the oxidation of AC but can also strongly affect its chemical and textural features. In addition, low regeneration efficiency is often observed for microporous AC and chemical regeneration is therefore often applied only to mesoporous or non-porous materials.8-10
Recently, the electro-Fenton (EF) process has emerged as a promising solution for regenerating AC. The continuous electro-generation of H2O2 from the 2-electron reduction of O2 at the surface of the AC combined with the supply of a catalytic amount of iron (II) continuously regenerated at the cathode allows the formation of hydroxyl radicals (.OH) (eq 1).11-13 A platinum (Pt) anode is used as counter-electrode.
Fe2++H2O2→Fe3++.OH+OH− (k=63 M−1 s−1) (1)
It has been observed that a wide range of organic pollutants are completely mineralized using the EF process.11,13,14 It has also been shown that .OH are capable of oxidizing organic compounds adsorbed on granular AC and thus participate in AC regeneration and pollutant degradation.15,36 In addition, Banuelos et al. (2015) observed that cathodic polarization of granular AC during the EF process protects the surface from oxidation and can thus avoid alteration of the material and loss of adsorption capacity.7 However, the development, improvement and scale-up of the EF process for regenerating AC is still hindered by technical aspects when using AC as cathode7,15, mainly due to ohmic drops within the granular AC bed and a lack of interconnection at the microstructure level leading to a very heterogeneous potential distribution in the granular AC beds used as cathode.
Porous fibers have unique features compared with AC in grain or powder form16. The thin fiber shape and the open pore structure reduce the resistance to intraparticle diffusion of organic compounds from solution to active adsorption sites. This shape also gives the material mechanical and geometrical features suitable for the design of electrochemical reactors. Compared with AC grain beds, porous AC fibers provide a better level of interconnection at the microstructure level and thus reduce ohmic drops as well as dead zones (non-electroactive zones). Thus, AC fiber is an efficient material for adsorption of organic compounds and generation of H2O2 during water treatment.17,18
The inventors studied a technology based on electro-Fenton (EF), using AC fiber as cathode and a boron-doped diamond (BDD) coated anode for both regeneration of AC and mineralization of desorbed organic pollutants. The large specific surface area, open pore structure and low resistance to intraparticle diffusion of the porous AC fibers resulted in a high maximum phenol (PH) adsorption capacity (3.7 mmol g−1) and rapid adsorption kinetics.
Spent/saturated porous AC fibers were then used as cathode during the EF process. After 6 h of treatment at 300 mA, 70% of PH was removed from the surface of the porous AC fibers. The inventors surprisingly observed a high efficiency of the process attributed to (i) direct oxidation of the PH adsorbed by hydroxyl radicals generated by the electro-Fenton reaction, (ii) continuous displacement of the adsorption equilibrium due to the oxidation of organic compounds in solution by electro-Fenton reaction and on the surface of the anode by anodic oxidation, (iii) local increase in pH at the cathode leading to repulsive electrostatic interactions, (iv) high electroactive surface area and the good level of interconnection at the microstructure resulting from the use of AC in fiber form, (v) involvement of the BDD anode in the formation of oxidizing species. It is remarkable that 91% of the PH removed from the AC was completely mineralized by the electro-Fenton reaction and anodic oxidation, thus avoiding the adsorption of degradation by-products and the accumulation of toxic compounds such as benzoquinone. The morphological and chemical features of the AC were not affected due to the effect of protection by cathodic polarization. The porous AC fibers were successfully reused for 10 adsorption/regeneration cycles with a regeneration efficiency ranging from 65 to 78%, consistent with the amount of PH removed from the surface of the AC fibers at the end of each regeneration cycle.
Again surprisingly, the inventors successfully combined the EF process with anodic oxidation using BDD as anode. This both promotes the oxidation of adsorbed compounds by mediated oxidation (production of ozone, persulfate, sulfate radical, species that can oxidize compounds on the surface of AC)13,19 and increases the mineralization of desorbed pollutants and degradation by-products due to their oxidation by hydroxyl radicals generated on the surface of the BDD anode by water discharge (eq 2 where M is the anode material).19
M+H2O→M(.OH)+H++e− (2)
The main disadvantages of conventional regeneration methods are thus avoided by using the EF process according to the invention. Compared with chemical oxidation, a much higher regeneration efficiency of a microporous adsorbent can be achieved. This process according to the invention can also completely mineralize organic molecules, whereas thermal regeneration under inert conditions only leads to the desorption of pollutants. Moreover, the adsorption capacity of porous AC fibers is not affected, unlike chemical oxidation and heat treatment under oxidizing conditions. The choice of using AC in fiber form plays a crucial role on the efficiency of the process because this material has suitable features for both adsorption and regeneration steps.
A first subject matter the invention relates to a device for regenerating activated carbon (AC), comprising at least one electrochemical cell comprising:
the device making it possible to create, during the electro-Fenton reaction, oxidizing species at the cathode and the anode, the oxidizing species created at the anode by the anodic oxidation being at least: .OH, O3, preferably: .OH, O3, SO4.− and S2O82−
these oxidizing species mineralizing the organic pollutants at the anode, at the cathode and in the electrolyte solution.
In one embodiment, the electroactive surface of the cathode comprises at least 90% of the porous activated carbon fibers allowing the generation of H2O2 at their surface.
In another embodiment, the electroactive surface of the cathode comprises only porous activated carbon fibers allowing the generation of H2O2 at their surface.
Advantageously, the device is a continuous filtration column reactor of a flow for which the porous fibers are used in situ in the reactor to both filter the pollutants of the flow and be regenerated in situ in the same reactor; several sets of cathode (porous AC fibers) with anode (BDD or sub-stoichiometric titanium oxide) in the flow can be placed in series or in parallel so that the cathode of one set can be regenerated while continuing to filter the flow with the other sets of cathode (porous AC fibers) with anode (BDD or sub-stoichiometric titanium oxide).
A second subject matter the invention relates to a process for regenerating activated carbon loaded with organic pollutants using the device according to the invention.
A third subject matter the invention relates to the use of a filter composed of porous activated carbon fibers as electroactive cathode surface for the electro-Fenton reaction, the porous fibers generating H2O2 at their surface during the electro-Fenton reaction, in the device according to one of claims 1 to 14, for regenerating the porous activated carbon fibers loaded with organic pollutants, said filter having been previously loaded with organic pollutants by filtration of polluted water or polluted air.
The invention relates to a device for regenerating activated carbon, comprising at least one electrochemical cell comprising:
the device making it possible to create, during the electro-Fenton reaction, oxidizing species at the cathode and the anode, the oxidizing species created at the anode by the anodic oxidation being at least: .OH, O3, preferably: .OH, O3, SO4.− and S2O82−
these oxidizing species mineralizing the organic pollutants at the anode, at the cathode and in the electrolyte solution.
The advantage of the EF reaction is the simultaneous promotion of oxidation of organic compounds both in solution and adsorbed on the AC fabric.
The device according to the invention is particularly advantageous because it makes it possible to reach a degradation kinetics faster than the adsorption kinetics. Thus, the re-adsorption of oxidation by-products on the AC fabric is avoided. The formation of more hydrophilic by-products as well as electrostatic interactions due to the locally high pH at the surface of the AC fabric also help to prevent the adsorption of degradation by-products onto the AC fabric.
The total mineralization of pollutants avoids the accumulation of toxic by-products.
The electrochemical cell has any shape making it possible to delimit a suitable container for the electrodes and the electrolyte solution, for example cylindrical or parallelepipedal.
The electrochemical cell is made of any material making it possible to delimit a suitable container for the electrodes and the electrolyte solution.
It can be open or closed, divided or not. Preferably, it is open and undivided.
Preferably, the activated carbon in the form of porous fibers having served as a filter for organic pollutants is saturated with organic pollutants.
The porous activated carbon fibers loaded with organic pollutants serve as cathode. They come in the form of fabric (woven ordered porous fibers) or felt (non-woven disordered porous fibers), preferably in the form of fabric. The fabric consists of thousands of thin porous fibers with a very high specific surface area.
Advantageously, the cathode consists of activated carbon in the form of porous fibers. They come from or are a filter used previously to filter pollutants from water and/or air.
Preferably, the diameter of the porous fibers is greater than 0.1 micrometer and less than 1 000 micrometers, even more preferable is greater than 1 micrometer and less than 100 micrometers.
The specific surface area (SBET) of the porous fibers is preferably greater than 100 m2·g−1, even more preferably greater than 600 m2·g−1.
Advantageously, the porous fibers have a porosity such that more than 30% of the pore volume of each of the porous fibers is made up of pores smaller than 2 nm, even more preferably more than 80%.
According to one embodiment, the anode consists of a non-active anode material.
According to another embodiment, the anode consists of a substrate at least partially covered with a non-active anode material.
According to the model proposed by Comninellis44,45, the materials used as anode in the electro-oxidation of organic pollutants in aqueous media can be divided into two groups: active and non-active anodes.
In the case of active anodes, the hydroxyl radical (.OH) formed is chemically adsorbed and only slightly available for oxidation of the organic compounds in solution. Rather, these materials promote the O2 release reaction.
In the case of non-active anodes (such as BDD), the overvoltage for O2 release is higher (compared with active anodes) and the .OH radicals formed are physically adsorbed. In this case they are more available and react directly with organic compounds.44,45,13,19
The non-active anode material is defined as a material with an oxygen release overvoltage greater than 0.4 V, preferably greater than 0.6 V.
Preferably, the non-active anode material is chosen so that the oxidizing species created are at least: .OH, O3, preferably .OH, O3, SO4.− and S2O82− if sulfate ions are present in the solution.
The .OH ions attack the pollutants adsorbed at the cathode until they are mineralized. Advantageously, the cathodic polarization preserves the surface of the porous activated carbon fibers. The total mineralization of the pollutants avoids the accumulation of toxic by-products.
The non-active anode material is boron-doped diamond (BDD) or a sub-stoichiometric titanium oxide (properties close to BDD in terms of oxygen release overvoltage). Preferably, the non-active anode material is boron-doped diamond (BDD).
The device of the invention may comprise several anodes, in particular several BDD anodes.
By way of example, the device according to the invention may comprise an anode as defined above and two cathodes on either side of the anode as defined above.
According to one embodiment, the anode consists of a substrate at least partially covered with a non-active anode material. Preferably, the anode then consists of a substrate entirely covered with a non-active anode material. Suitable substrates can be cited: Ti, Nb or Si. The thickness of the non-active anode material on the substrate varies from 0.1 to 0.5 mm depending on the overall size of the electrode.
The electrochemical solution has a continuous supply of oxygen for the production of hydrogen peroxide. The oxygen supply is achieved by an inlet of oxygen bubbles or air bubbles into the electrolyte solution, preferably by an inlet of air bubbles into the electrolyte solution. Bubbling helps mix the electrochemical solution.
Preferably, the initial supply of Fe2+ ions has a catalytic concentration in the electrolyte solution greater than 10−5 M and less than 10−2 M, even more preferably comprised between 3*10−5M and 10−3 M. The initial supply of Fe2+ ions is advantageously low since these ions are regenerated at the cathode throughout the process (
The electrodes are separated by a few centimeters, preferably less than 10 cm.
The electrolyte will be chosen appropriately by the person skilled in the art. The presence of salt is necessary to ensure the conductivity of the solution, for example, Na2SO4., Na Cl, etc. The conductivity of the solution is greater than 0.01 S m−1.
The electrolyte concentration is between 10−3 and 10−1 M.
The electrochemical solution is stirred by magnetic or mechanical stirring, for example.
The pH is adjusted, preferably between 2 and 5, even more preferably between 2.6 and 3.6.
The electrochemical cell is supplied with constant current. The current density is preferably adjusted between 0.1 and 100 mA/cm2, preferably between 1 and 30 mA/cm2, of activated carbon surface as soon as the spent/saturated AC cathode has been immersed in the electrolyte.
The current density is determined to optimize the production of H2O2 and .OH and minimize secondary reactions such as oxygen and hydrogen evolution.
Another subject matter the invention relates to a process for regenerating activated carbon loaded with organic pollutants using the device according to the invention.
Any type of activated carbon filter made from porous activated carbon fibers may be used as cathode of the device according to the invention in order to be regenerated after its use as a filter of organic air and/or water pollutants and may thus be reused again as a filter of organic air and/or water pollutants. This use/regeneration cycle can be repeated several times.
Another subject matter the invention relates to the use of a filter composed of porous activated carbon fibers as electroactive cathode surface for the electro-Fenton reaction, the porous fibers generating H2O2 at their surface during the electro-Fenton reaction, in the device according to one of claims 1 to 14, for regenerating the porous activated carbon fibers loaded with organic pollutants, said filter having been previously loaded with organic pollutants by filtration of polluted water or polluted air.
The following study aims to evaluate the regeneration efficiency of the AC fiber during the EF process using the BDD anode and the AC fiber loaded with organic pollutant as cathode. Choosing phenol (PH) as the model organic pollutant, the objectives of this study were to evaluate (i) the adsorption capacity and adsorption kinetics of PH and major aromatic oxidation by-products on the porous AC fibers (ii) the removal of PH from the surface of the AC fiber loaded with organic pollutant by the EF process (iii) the release of PH and degradation by-products into the solution and their subsequent mineralization (iv) the adsorption capacity and characteristics of the regenerated material after 1 and 10 adsorption/regeneration cycles.
All chemicals are reagent grade purchased from Acros Organics (PH and iron (II) sulfate heptahydrate), Sigma Aldrich (hydroquinone (HQ), benzoquinone (BQ), catechol (CAT), methanol, sodium sulfate) or Fluka (sulfuric acid). All solutions are prepared using ultrapure water (resistivity>18.2 MQ cm) from a Millipore Milli-Q system (Molsheim, France).
Microporous AC fabric (Dacarb, France), prepared from a phenolic resin, was used as adsorption material. N2 adsorption isotherms were performed for the determination of BET surface area, total pore volume and pore size distribution (using the two-dimensional non-local density functional theory method). The main characteristics of the material are presented in Table 1. Some experiments were also performed using AC felt prepared from phenolic resin (Dacarb, France) with different morphological features but similar surface area and microporosity.
Before use, the AC was washed several times in deionized water and dried at 70° C. The pH value was set to 3 for all adsorption experiments, the pH value required for the EF regeneration step. Control tests without AC showed that less than 3% PH was lost by volatilization or adsorption to glass after 24 h.
Equilibrium adsorption experiments were performed at room temperature (20° C.) with single compounds in 500 mL glass bottles stirred continuously for 24 h in a rotary shaker set at 20 rpm. For the isothermal experiments, 250 mL of PH (1 mM), BQ (0.5 mM), CAT (0.8 mM) or HQ (0.9 mM) were mixed with various AC concentrations from 0.08 to 1 g L-1. The most widely used models, Langmuir (eq 3) and Freundlich (eq 4), were used to model the experimental data.
where qe is the amount of solute adsorbed per unit weight of AC at equilibrium (mmol g-1), qm is the maximum adsorption capacity (mmol g-1), KL is a constant related to the free energy of adsorption (L mmol-1) and Ce is the concentration of solute in the stock solution at equilibrium (mmol L-1).
q
e
=K
F
C
e
1/n (4)
where KF and n are constants related to adsorption capacity and adsorption intensity, respectively.
The organic pollutant-loaded CAs used for the EF regeneration experiments were obtained by mixing 250 mL PH at 11 mM with 500 mg AC (2 g L-1).
Dynamic adsorption experiments were performed with unique compounds and using a configuration similar to electrochemical regeneration to ensure the same hydrodynamic conditions. The initial concentrations of AC (2 g L-1) and organic compounds ([PH]=1 mM; [HQ]=[CAT]=0.1 mM; [BQ]=0.05 mM) were chosen according to the experimental conditions observed during the EF regeneration step. The data were analyzed using both pseudo-first order (eq 5) and pseudo-second order (eq 6).
ln(qe−qt)=ln qe−k1t (5)
where qt is the amount of solute adsorbed per unit weight of AC at time t and k1 is the first-order velocity constant.
where k2 is the second order velocity constant.
3. EF Regeneration of Porous AC Fibers Loaded with Organic Pollutant
The electrochemical regeneration of porous AC fibers loaded with organic pollutant was carried out in batch mode using an open, cylindrical and undivided electrochemical cell, similar to the configuration previously described by Trellu et al. (2016).20 500 mg of spent AC (55 cm2×0.5 mm) was used as cathode. The anode consisted of a thin film of BDD deposited on a Nb substrate (24 cm2×0.2 cm, Condias Gmbh, Itzehoe, Germany). For comparison, some experiments were performed with a platinum grid as anode. The electrodes were placed face to face with a space of 3 cm between the anode and the cathode. The AC cathode was fixed in the electrochemical cell using a Teflon grid. Oxygen supply for the production of hydrogen peroxide was provided by continuous air bubbling through sintered glass.
0.05 M Na2SO4 (electrolyte) was dissolved in Milli-Q water, the pH was adjusted to 3.0 with H2SO4 and 0.1 mM Fe2+ (catalyst) was added to the solution.21 Continuous stirring was provided by magnetic stirring and air bubbling. The constant current supply was provided by a power supply (HAMEG, model 7042-5, Germany) adjusted to 300 mA as soon as the AC cathode loaded with organic pollutant was immersed in the electrolyte. This corresponds to 5.5 or 12.5 mA cm−2 as current density considering either the surface of the AC cathode or the surface of the BDD anode, respectively. The current density was determined to optimize the production of H2O2 and .OH and to minimize secondary reactions such as oxygen and hydrogen evolution.
Chemical analyses were carried out to monitor the change in the concentration of PH and degradation by-products in solution and adsorbed on the porous AC fibers. Aqueous samples (1 mL) were periodically collected from the solution during treatment, while the analysis of the organic compounds adsorbed on AC required stopping the experiment in order to perform a desorption step. The porous AC fibers used as cathode were immersed in a 90%-10% 1 M NaOH ethanol solution and placed for 30 minutes in an ultrasonic bath. After mixing under magnetic stirring for an additional 30 minutes, an aliquot was collected and analyzed. Different authors have observed that such conditions effectively remove adsorbed organic compounds from the surface of AC.22,23 Preliminary experiments showed that more than 97% of the adsorbed PH was desorbed and recovered after repeating this procedure twice. The PH and aromatic by-products were analyzed by reversed-phase HPLC, while carboxylic acids were identified and quantified by ion-exclusion chromatography. Analytical conditions were similar to those of Pimentel et al. (2008).24 The PH mineralization rate was monitored by measurement of total organic carbon (TOC) with a Shimadzu TOC-V analyzer.
A scanning electron microscope (Phenom XL, PhenomWorld, The Netherlands) was used to analyze the surface morphology of the AC fabric. Since AC is conductive, no surface treatment was necessary prior to analysis.
Raman measurements were performed on an INVIA Renishaw spectrometer equipped with a microscope and CCD detector (LGE, France). The details are given below (see
A 532 nm green solid-state laser (Nd: YAG) was used with a maximum power of 50 mW. Acquisitions were performed using a Leica magnification objective (×50) after calibration on a silicon standard. With this configuration, the beam diameter did not exceed 2 microns. The component of Rayleigh diffusion was eliminated by an Edge filter, and the light diffused by Raman was dispersed by a holographic grating with 1 800 lines mm−1. The integration time was set at 2 min. The acquisitions were repeated at 3 different points of the material. The spectral analysis was carried out with the WIRE software.
1. Sorption of Phenol and Major Aromatic Oxidation by-Products on AC Tissues
The first step of this study consisted in determining the adsorption behavior of PH and the main aromatic oxidation by-products on AC fabric. The PH, BQ and CAT adsorption isotherms are presented in Table 2. As reported in previous studies, a classical L-shaped adsorption isotherm was obtained for all compounds.2,25,26 The Langmuir and Freundlich equations are applicable but slightly higher correlation coefficients were obtained using the Langmuir equation for all three compounds, indicating that the assumptions underlying the Langmuir model are appropriate for this material (adsorption of a monolayer of solutes on a homogeneous adsorbent surface with uniform adsorption energies). The maximum PH adsorption capacity (3.73 mmol g−1) is higher than the previous results reported using granular AC (2.32 mmol g−1). This is due to the larger BET surface area (1 326 vs. 929 m2 g−1) as well as the microporous structure of the AC fiber since the adsorption energy is improved in the smaller pore sizes. Furthermore, efficient adsorption requires that the average pore size (0.82 nm) be greater than 1.2 times27 or 1.7 times28 the second largest dimension of the adsorbed molecule (for PH 0.42 nm).29 A low steric hindrance effect is therefore expected in this study because this ratio reaches 2.0.27-29 Compared with PH, aromatic oxidation by-products (BQ and CAT) showed a much lower adsorption capacity on AC tissues (lower qm and KF values). This is consistent with the lower hydrophobicity of hydroxylated by-products, which are less likely to be adsorbed on the carbon surface. Physical adsorption probably plays the most important role for PH adsorption on porous AC fibers, especially π-π interactions.29 However, other factors may be involved in the adsorption mechanisms, including the formation of an electron donor/acceptor complex between the solute and the AC surface), electrostatic interactions (depending on the pH of the solution), molecular size and the effect of the solvent (competitive adsorption of water molecules).2,29
The kinetic study shows that a large amount of PH can be rapidly adsorbed onto the AC fabric. Similar to what has been demonstrated by several previous studies, the determining step in the process of PH adsorption on AC is intraparticle diffusion (linear relationship between qt and t1/2).29,30
The resistance to intraparticle diffusion is greatly reduced compared with granular AC due to the open pore structure.29 The AC fabric consists of thousands of thin porous fibers, which greatly increases the external surface area. Much better correlation coefficients were obtained using the pseudo-second order model compared with the pseudo-first order model. Such behavior is often observed for the adsorption of low molecular weight compounds on small adsorbent particles (adsorbent with a large external surface area).31 Adsorption processes also obey the pseudo-second order model when the initial solute concentration is sufficiently low.32 Experiments were performed using PH (1 mM), CAT (0.1 mM) and BQ (0.05 mM) concentrations corresponding to the maximum concentrations observed during the regeneration step. Therefore, the kinetic parameters could not be directly compared since pseudo-first order and pseudo-second order kinetic constants are complex functions of the initial solute concentration.32 However, Wu et al. (2009)31 showed that the parameter k2qe (eq 6) corresponds to the inverse of the half-life of the adsorption process and is a key parameter for the comparison of adsorption kinetics. Thus, from the observed k2qe values, it can be concluded that adsorption becomes faster in the following order: PH>BQ>CAT. Adsorption kinetics of PH on AC fabric were slower than in a previously reported study29, most likely due to the smaller average pore diameter affecting intraparticle diffusion. Steric hindrance may not affect the final amount of PH adsorbed but still reduce the adsorption kinetics.
During the adsorption of HQ on AC fabric, the release of BQ into solution was observed simultaneously; subsequently, the adsorption of BQ was also observed (
2. Phenol Removal from Porous AC Fibers and Mineralization of Organic Compounds During EF Regeneration
The porous AC fibers are in fabric form. The PH-loaded AC fabric was regenerated using the EF process with a BDD anode and the PH-loaded AC fabric as cathode. Preliminary experiments have shown that the AC fabric is capable of producing more H2O2 than a conventional carbon felt usually used in the EF process (
After the adsorption step, the amount of phenol adsorbed on AC was 3.2 mmol g−1; this corresponds to a concentration in the electrochemical cell of 6.4 mM PH ([PH] 0) and a TOC concentration of 461 mg L−1 (TOC0). After 6 h of treatment at 300 mA, 70% of the initial adsorbed PH was removed from the surface of the AC fabric (
Various phenomena can contribute to the removal of PH from the AC surface. Firstly, a greater increase in the PH concentration in the solution was observed during the first minutes of electro-oxidation at 300 mA, compared with the control experiment without current supply (
The adsorbed organic compounds can react directly with oxidizing species such as —OH from the EF process and electrochemically generated redox reagents (H2O2, O3, persulfate, sulfate radical). Using conventional Fenton oxidation, a very low regeneration efficiency of microporous AC has been reported due to the limited availability of the adsorbed molecules in the micropores to the oxidizing species.8 During the EF process, H2O2 is generated on the surface of the AC pores, therefore, .OH can be produced in the vicinity of the target pollutants adsorbed on the surface of the AC according to the electrochemically supported Fenton reaction (eq 1). This increases the availability of the adsorbed pollutants for oxidation. Thus, a higher regeneration efficiency of microporous AC can be achieved by EF compared with conventional Fenton oxidation. In addition, the low intraparticle diffusion resistance of the porous AC fibers promotes the diffusion of oxidizing species into the microporosity of the AC, thus improving the availability of the adsorbed compounds to the oxidizing species. In addition, a high rate of PH degradation in solution implies a shift in the sorption equilibrium and the continuous release of PH from the AC fabric to the solution.
Similar experiments were also performed using porous AC fibers in the form of AC felt instead of AC fabric (
Whether using felt or AC fabric, the PH was primarily removed from the cathode for the first 3 hours, then the efficiency of the process decreased significantly. This could be related to the presence of physisorbed and chemisorbed pollutants and the slower removal of the chemisorbed PH. Furthermore, a lower availability (with respect to oxidizing species) of PH molecules adsorbed in the smaller pores of the porous AC fibers could also reduce the efficiency after the first 3 hours of treatment.
The great advantage of this process is to avoid the accumulation of organic compounds in the solution. Only 6% of the initial adsorbed TOC is found in the solution after 6 h of treatment (
The total mineralization of pollutants avoids the accumulation of toxic by-products such as BQ (
Resorcinol was detected only in very low amounts since phenol hydroxylation is mainly promoted in the para (HQ) and ortho (CAT) positions.37 CAT quickly reached its maximum concentration at t=30 min (2.2% TOC0) because its production rate from PH oxidation is highest at the beginning of the experiment and then decreases continuously due to the lower PH concentration. The BQ concentration also peaked rapidly at t=20 min (1.8% TOC0) and then rapidly decreased below the detection limit at t=120 min. By comparison, the HQ concentration peaked later (t=90 min, 2.8% TOC0) and decreased much more slowly. Pimentel et al. (2008) observed a similar behavior during PH removal by EF with a conventional carbon cathode.24 As suggested in the literature, this could be explained by taking into account the equilibrium of the redox couple HQ/BQ (E°=0.70 V) and the possible reduction of BQ to HQ.37,38 In addition, Mousset et al. (2016) reported that the degradation kinetic constants of the oxidation of BQ to muconic and maleic acids is about an order of magnitude greater than those of the oxidation of HQ to the same degradation by-products.37 The aromatic by-products undergo aromatic ring-opening reactions to form short-chain carboxylic acids.11 Succinic, oxalic, and formic acids were the primary short-chain carboxylic acids detected and reached their maximum concentration at 90, 120, and 90 min of electrolysis, respectively. The change in the concentrations of oxidation by-products in solution depends (i) on the amount generated by PH degradation in solution or adsorbed on the AC fabric and (ii) on the degradation kinetics in solution and at the anode surface. Thus, the concentration of short-chain carboxylic acids decreased more slowly than that of aromatic by-products due to their slower reaction kinetics with .OH.37,39
It has been shown that the use of a non-active anode such as BDD plays an important role in the efficiency of the mineralization of desorbed pollutants. By using a platinum anode with porous AC fibers at the cathode, the increase in TOC in solution at the beginning of regeneration is greater, and then the decrease in TOC is much slower than by using a BDD anode (
Additional experiments were conducted to evaluate the potential of this technology for the reuse of regenerated AC. Porous AC fibers in the form of AC fabric were chosen as the most promising material, as AC felt showed insufficient mechanical properties for water treatment. Several adsorption cycles followed by EF regeneration were implemented in order to monitor the change in the efficiency of the regeneration process (
The regeneration efficiency (RE) was calculated by comparing the amount of PH that can be adsorbed on the regenerated AC (qreg) and the amount of PH adsorbed on the initial AC (qi) (eq 7)
RE was 78% after one cycle, while only 70% of the adsorbed PH was removed from the AC surface after 6 h of treatment. Thus, taking into account both the residual PH (30% of the initial AC adsorption capacity) and the new PH adsorbed on the AC (78%) after the first regeneration cycle, the adsorption capacity of the regenerated AC is higher than that of the initial material.
Raman analyses were performed to evaluate the change in the chemical composition of AC fabric. The spectra were analyzed using the following deconvolution procedure: a combination of three Lorentzian-shaped bands at about 1 600 cm−1 (G), 1 340 cm−1 (D1) and 1 185 cm−1 (D2) and one band of Gaussian shape at 1 545 cm−1 (D3) was used (an example is given in
A slight decrease in RE was observed during cycles 2 (74%) and 3 (70%). RE then reached a plateau, with a slight variation between 65% and 72%. The high RE obtained throughout the 10 regeneration cycles demonstrates the relevance of this treatment strategy. The cathodic polarization avoids damaging the AC surface. While the Raman analyses showed a cleaning effect of the AC fabric, comparison of the SEM images shows the absence of any change in the morphological texture of the AC fabric between the initial and regenerated samples (10 cycles) (
Breakdown products formed due to oxidation of adsorbed phenol by electrochemical regeneration of a graphite adsorbent. Electrochimica Acta 2013, 110, 550-559.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1852190 | Mar 2018 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/055803 | 3/8/2019 | WO | 00 |
