Patent Application
20230097537
The current invention relates to a reactor suitable for use to adsorb pollutants from a wastewater onto an activated carbon bed and subsequent regeneration of the activated carbon by electrochemical means. Also disclosed is a method of using the reactor.
The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.
Industrial contamination of fresh water supplies is a growing problem with serious environmental consequences in both developed and developing countries. Indeed, the toxic and recalcitrant nature of industrial pollutants renders conventional biological wastewater treatment methods challenging. Adsorption onto activated carbon (AC) has proven well-suited for the removal of non-biodegradable compounds from industrial wastewaters and is a simple, effective and inexpensive process. However, adsorption onto AC still suffers from being strictly a separation technology with no catalytic degradation activity towards the adsorbed recalcitrant compounds. Furthermore, the disposal of the exhausted material is expensive and presents a threat to the environment with risks of leaching. Thus, there is a need for AC to be regenerated upon saturation.
Current AC regeneration technologies like chemical, thermal and microbiological processes are limited by factors such as high energy requirement and cost, limited number of regeneration cycles, porosity, deterioration of AC, weight loss in AC, etc. Electrochemical regeneration methods appear promising to overcome these limitations because they can be carried out in situ under ambient conditions of pressure and temperature and do not cause apparent detrimental modification in the structure or mass loss of the AC, allowing its reuse after multiple cycles of adsorption and regeneration (Bañuelos, J. A. et al. J. Appl. Electrochem. 2015, 45, 523-531).
Most electrochemical regeneration approaches are based on the electro-desorption of the pollutant from the AC promoted by local changes in pH (Zanella, O. et al., Environ. Technol. 2016, 38, 549-557; Karabacakoğlu, B. et al., Ind. Eng. Chem. Res. 2014, 53, 13171-13179; and Hou, P. et al., Carbon 2014, 79, 46-57). However, the degradation of the accumulated contaminants poses a challenge. To overcome this issue, a few studies have proposed in situ electrochemical AC regeneration by the hydroxyl radical (.OH)—a strong and non-selective oxidant—generated either by electro-peroxone (Zhan, J. et al., Carbon 2016, 109, 321-330) or electro-Fenton (EF) (Ba
O2+2H++2e−→H2O2 (1)
Fe2++H2O2→Fe3++.OH+−OH (2)
Fe3++e−→Fe2+ (3)
BDD+H2O→BDD(.OH)+H++e− (4)
The combination of high chemical resistance and large surface area make carbonaceous materials the best cathodes to carry out the reduction of oxygen to H2O2 (Garcia-Rodriguez, O. et al., Electrochim. Acta 2018, 276, 12-20), making AC a promising material not just for adsorption but also for electrochemical processes (Bañuelos, J. A. et al., Electrochim. Acta 2014, 140, 412-418). However, most reports carried out the saturation and regeneration of AC in separate vessels (Karabacakoğlu, B. et al., Ind. Eng. Chem. Res. 2014, 53, 13171-13179), implying additional logistic costs and constraints. In the few instances where the regeneration and adsorption took place in the same reactor, it was done in batch mode (Bañuelos, J. A. et al., Environ. Sci. Technol. 2013, 47, 7927-7933; and Zhou, W. et al., Electrochim. Acta 2019, 296, 317-326), yet operation in continuous-flow systems is a requirement prior to industrial applications.
Therefore, there is a need to develop a novel reactor design that allows continuous wastewater treatment through adsorption and in situ electrochemical regeneration of AC produced from organic wastes.
Aspects and embodiments of the current invention will now be described by reference to the following numbered clauses.
1. A wastewater treatment reactor for use in electrochemical advanced oxidation processes, the reactor comprising:
2. The reactor according to Clause 1, wherein each of the one or more fixed bed compartments has a height/diameter ratio of from 8 to 12, such as 10.
3. The reactor according to Clause 1 or Clause 2, wherein the activated carbon is provided in the form of granules.
4. The reactor according to any one of the preceding clauses, wherein the activated carbon is provided in an amount of from 0.05 to 5 g (such as from 0.1 to 1 g, such as 0.2 g) per cubic centimetre of volume in each of the one or more fixed bed compartments.
5. The reactor according to any one of the preceding clauses, wherein the separator fixes the carbon brushes and activated carbon in the one or more fixed bed compartments.
6. The reactor according to any one of the preceding clauses, wherein the separator comprises a frame and a carbon cloth or a metal mesh (e.g. a stainless steel mesh) disposed within the frame, such that the carbon cloth or metal mesh contacts the cathode and anode.
7. The reactor according to any one of the preceding clauses, wherein the anode is formed of boron doped diamond.
8. The reactor according to any one of the preceding clauses, wherein each inlet of the one or more fixed bed compartments of the cathode has a height of 20 cm, a width of 2.5 cm and a depth of 1 cm.
9. The reactor according to any one of the preceding clauses, wherein the one or more fixed bed compartments of the cathode are three fixed bed compartments.
10. The reactor according to any one of the preceding clauses, wherein the reactor comprises part of a wastewater treatment apparatus, the apparatus further comprising a wastewater source in fluid communication with each inlet of the one or more fixed bed compartments of the cathode, a power supply connected to the cathode and anode and a treated water receptacle in fluid communication with each outlet of the one or more fixed bed compartments of the cathode.
11. A method of wastewater treatment comprising the steps of:
(a) a decontamination stage, where a wastewater comprising at least one contaminant is continuously supplied to a wastewater treatment apparatus as described in Clause 10, comprising a reactor according to any one of Clauses 1 to 9, such that the wastewater enters through an inlet of one of the one or more fixed bed compartments of the cathode and is subjected to an electrochemical advanced oxidation process to remove the contaminant, such that the water passing through an outlet of the one or more fixed bed compartments of the cathode is a decontaminated water that has substantially none of the at least one contaminant present;
(b) a regeneration step, where the decontamination process of step (a) is stopped when a breakthrough amount of the at least one contaminant is detected and the reactor of any one of Clauses 1 to 9 is then placed into an electrochemical regeneration cycle; and
(c) repeating steps (a) and (b).
12. The method according to Clause 11, wherein the, optionally wherein these flow rates relate to a reactor as described in Clause 8 and the flow rate used in a reactor according to any one of Clauses 1 to 7, Clause 9 as dependent upon any one of Clauses 1 to 7 and Clause 10 as dependent upon Clauses 1 to 7 and 9 as dependent upon any one of Clauses 1 to 7 with a differing dimension is pro-rated accordingly.
13. The method according to Clause 11 or Clause 12, wherein the current applied in the electrochemical advanced oxidation process is from 1 to 30 mA/g, such as from 10 to 25 mA/g, such as from 15 to 18 mA/g, such as 16.6 mA/g.
14. The method according to any one of Clauses 11 to 13, wherein the electrochemical advanced oxidation process is an electro-Fenton process.
15. The method according to any one of Clauses 11 to 14, wherein the electrochemical regeneration cycle of the regeneration step is conducted for a period of from 10 to 180 minutes, such as from 30 to 140 minutes, such as from 60 to 130 minutes, such as 120 minutes.
16. The method according to Clause 15, wherein the electrochemical regeneration cycle of the regeneration step is conducted on activated that has reached from 18 to 50% of its theoretical loading capacity, such as from 30 to 40% of its theoretical loading capacity, such as about 38% of its theoretical loading capacity.
17. The method according to any one of Clauses 11 to 16, wherein steps (a) and (b) can be conducted from 10 to 10,000 times, such as from 10 to 500 times, such as from 10 to 100 times, such as 10 times.
In view of the issues and limitations mentioned above, it has been surprisingly found that a continuous flow reactor can be designed and operated that allows continuous wastewater treatment through adsorption and in-situ electrochemical regeneration of activated carbon produced from organic waste. Thus, in a first aspect of the invention, there is provided a wastewater treatment reactor for use in electrochemical advanced oxidation processes, the reactor comprising:
In embodiments herein, the word “comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components/features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of” or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of” or the phrase “consists essentially of” or synonyms thereof and vice versa.
The phrase, “consists essentially of” and its pseudonyms may be interpreted herein to refer to a material where minor impurities may be present. For example, the material may be greater than or equal to 90% pure, such as greater than 95% pure, such as greater than 97% pure, such as greater than 99% pure, such as greater than 99.9% pure, such as greater than 99.99% pure, such as greater than 99.999% pure, such as 100% pure.
The fixed bed compartments may have any suitable size or dimension. While any suitable shape may also be used for the fixed bed compartments, it is noted that, as the compartments are intended for continuous flow application, they may be cylindrical or, more particularly, cuboidal in shape (e.g. cylindrical). For example, in embodiments that may be mentioned herein, each of the one or more fixed bed compartments may have a height/diameter ratio of from 8 to 12, such as 10. As will be appreciated, when the compartments are cylindrical, the diameter is simply the inner diameter of the compartment. In embodiments where the compartment is cuboidal, the diameter may instead refer to the largest distance between any two edges of a cross sectional place of the cuboid. A reactor according to the current invention may have any suitable number of fixed bed compartments. For example, the reactor may have from 1 to 10 compartments, such as from 2 to 5 compartments, such as 3 compartments.
The activated carbon may be provided in any suitable form within the fixed bed compartments. For example, in embodiments that may be mentioned herein, the activated carbon may be in the form of granules. Any suitable size of granules may be used, for example the activate carbon granules may have a size of greater than 0.9 mm. Other forms of activated carbon that may be suitable include powdered activated carbon.
The amount of activated carbon used will depend on the volume of the fixed bed container. For example, activated carbon may be provided in an amount of from 0.05 to 5 g (such as from 0.1 to 1 g, such as 0.2 g) per cubic centimetre of volume in each of the one or more fixed bed compartments. Thus, in the example fixed bed compartment mentioned hereinbefore (having a height of 20 cm, a width of 2.5 cm and a depth of 1 cm), each fixed bed compartment may contain 10 g of activated carbon, along with a carbon brush (where the carbon brush may be 20 cm in length and 2 cm in diameter).
In order to ensure a reaction occurs, the fixed bed compartments may have an open face that faces towards the anode. As such, the activated carbon (and perhaps the carbon brushes) might be dislodged from the fixed bed compartments when used. To prevent this, the separator may fix the carbon brushes and activated carbon in the one or more fixed bed compartments. Any suitable material (or combination of materials) may be used as a separator in the current invention. For example, the separator may comprise a frame (e.g. a rubber frame) and a carbon cloth or a metal mesh (e.g. a stainless steel mesh) disposed within the frame, such that the carbon cloth or metal mesh contacts the cathode and anode. It will be appreciated that the carbon cloth or metal mesh can be replaced by any other suitable conductive material that provides the desired conductivity, while also serving to prevent dislodgement of the activated carbon and/or the carbon brushes.
Any suitable material may be used as the anode. For example, the anode may be formed from boron doped diamond.
A reactor according to the current invention will now be described by reference to
To prevent spillage and leaching of the activated carbon and the carbon brushes from the fixed bed compartments in the cathode, a separator 120 is used in this embodiment. In this embodiment, the separator takes the form of a frame 121 (e.g. a rubber frame) and a carbon cloth 122 disposed within the frame 121, such that the carbon cloth contacts the cathode and anode when in use. The anode 130 is formed from boron doped diamond (BDD), where two BDD plates with a total area of 200 cm2 are placed parallel to the cathode with a gap of 0.5 cm. A backplate 140 that can be secured onto the cathode through the rubber frame is also used to secure all of the components together.
The reactor described hereinbefore may be useful as part of a wastewater treatment apparatus. As such, and as depicted in
As will be appreciated, there may be multiple reactors in a wastewater treatment apparatus, such that the system allows for a reactor to be regenerated, while other reactors are used in the desired decontamination reaction.
As will be appreciated, the reactor described hereinbefore is intended for use in wastewater treatment. As such, a further aspect of the current invention is the provision of a method of wastewater treatment comprising the steps of:
(a) a decontamination stage, where a wastewater comprising at least one contaminant is continuously supplied to a wastewater treatment apparatus as described herein, comprising a reactor as described herein, such that the wastewater enters through an inlet of one of the one or more fixed bed compartments of the cathode and is subjected to an electrochemical advanced oxidation process to remove the contaminant, such that the water passing through an outlet of the one or more fixed bed compartments of the cathode is a decontaminated water that has substantially none of the at least one contaminant present;
(b) a regeneration step, where the decontamination process of step (a) is stopped when a breakthrough amount of the at least one contaminant is detected and the reactor as described herein is then placed into an electrochemical regeneration cycle; and
(c) repeating steps (a) and (b).
As will be appreciated, the current invention may relate to any wastewater that contains a waste product suitable to be treated by the methods disclosed herein. For example, the wastewater may be a domestic wastewater or, more particularly, the wastewater may be an industrial wastewater. Examples of domestic and industrial wastewaters that may be mentioned herein may be ones in which the contaminant may be an organic compound, such as phenol or derivatives thereof.
While the reactor described herein is intended to be used for wastewater treatment, it may also be configured for other uses. For example, for the trapping of contaminants in a fridge. In this arrangement, the activated carbon may act as a filter and an adsorbent of a fluid (in this case the air in the fridge), thereby trapping contaminants that may give rise to unpleasant odours. As discussed herein, the activated carbon in this application may be regenerated using the methods described herein. It will be appreciated that the configuration of the reactor used in this application may be identical to that described for used in wastewater and in this case, the contaminated air may be seen as the wastewater, as it is a fluid that passes through the reactor.
When used herein, the term “substantially none” is intended to refer to a reduction of the at least contaminant that is at least a 95% reduction, such as at least a 96.5% reduction, such as at least a 97% reduction, such as at least a 99% reduction, such as at least a 99.5% reduction, such as at least a 99.9% reduction, such as a 100% reduction of the contaminant compared to the original value within the wastewater.
When used herein, the term “breakthrough amount” refers to the level when the amount of the at least one contaminant is considered to be over a desired level as determined by the contaminant and the end use of the water. For example, the breakthrough amount may refer to the present of 0.1% of the at least one contaminant, such as 0.5%, such as 1%, such as 2% etc. As will be appreciated, the breakthrough amount will depend in part on the contaminant(s) in question and the desired end use of the water and can be readily determined by the person skilled in the art.
Any suitable flow rate of the wastewater may be used in the method described above. For example, the flow rate of the wastewater continuously supplied to the wastewater treatment apparatus is from 5 to 40 mL/min, such as from 8 to 14 ml/min, such as from 8 to 10 mL/min. In particular, these flow rates may be suited to the reactor discussed in relation to
As will be appreciated, the exact flow rate of the wastewater may be changed to match the dimensions of fixed bed compartments used. When this is the case, the change in flow rate may be pro-rated based on the flow rates mentioned above in relation to the reactor discussed in relation to
Any suitable current may be used to conduct the electrochemical advanced oxidation process. For example, the current may be from 1 to 30 mA/g, such as from 10 to 25 mA/g, such as from 15 to 18 mA/g, such as 16.6 mA/g. In embodiments of the invention that may be mentioned herein, the electrochemical advanced oxidation process may be an electro-Fenton process.
As noted the current reactor may be regenerated. This electrochemical regeneration cycle may be conducted for any suitable period of time. For example, the wherein the electrochemical regeneration cycle of the regeneration step may be conducted for a period of from 10 to 180 minutes, such as from 30 to 140 minutes, such as from 60 to 130 minutes, such as 120 minutes. While not wishing to be bound by theory, it is believed that the electrochemical regeneration cycle may be more sustainable (in that more cycles may be conducted) if the electrochemical regeneration cycle of the regeneration step is conducted on activated carbon that has reached only up to about 50% of its theoretical loading capacity. For example, the activated carbon may have reach from 18 to 50% of its theoretical loading capacity, such as from 30 to 40% of its theoretical loading capacity, such as about 38% of its theoretical loading capacity.
The method described herein may be conducted as many times as possible—that is, up to the point where the activated carbon has been exhausted. For example, steps (a) and (b) of the method may be conducted from 10 to 10,000 times, such as from 10 to 500 times, such as from 10 to 100 times, such as 10 times. When used herein, “exhausted” may take its normal meaning in the art. Additionally or alternatively, the term “exhausted” when used herein may refer to the point where the activated carbon cannot be regenerated to provide a desired level of adsorption anymore.
Further aspects and embodiments of the invention will now be discussed by reference to the following non-limiting examples.
Materials
Potassium sulfate (K2SO4), titanium (IV) oxysulfate-sulfuric acid solution (TiOSO4.(H2SO4)x), sulfuric acid (H2SO4), phenol (C6H5OH), diethyl ether ((CH3CH2)2O) and iron (II) sulfate heptahydrate (FeSO4.7H2O) were purchased from Sigma-Aldrich (Singapore) and used without any further modification. All solutions were prepared with high-purity water from a Millipore Milli-Q system (resistivity>18 MΩ cm at room temperature). Boron doped diamond (BDD) electrodes were obtained from Condias (Germany) and carbon cloth made of graphitized spun yarn with a count of 38×38 yarns in−1 from Fuel Cell Earth (USA). Carbon brushes were made of PAN-carbon fibers (SGL group, USA) with a stainless steel wire as current collector.
Analytical Techniques
The charge transfer resistance of the activated carbon (AC) was evaluated by electrochemical impedance spectroscopy (EIS) in an electrochemical cell with a three-electrode set-up using a potentiostat/galvanostat Autolab PGSTAT204 equipped with an EIS module FRA32 M (Metrohm Ltd, Switzerland). Carbon paste electrodes were prepared with the AC following the method of Banuelos et al. (Bañuelos, J. A. et al., Environ. Sot. Technol. 2013, 47, 7927-7933) and used as working electrodes. Ag/AgCl (3 M NaCl) and BDD were used as reference and counter electrodes, respectively. The electrolytic solution consisted of 50 mM K2SO4 at pH 3.
The specific surface area was determined using the Brunauere-Emmette-Teller (BET) method under N2 adsorption/desorption isotherms at 77 K. The Horvath-Kawazoe and Barrett-Joyner Halenda methods (Sing, K. S. W. et al., Pure Appl. Chem. 1982, 54, 2201-2218) were used to characterize the microporosity and mesoporosity, respectively. The analysis was done using an ASAP 2010 Micromeritics Analyzer (Micromeritics Instrument Corp., USA). Samples were degassed at 623 K for 48 h prior to adsorption. A field emission scanning electron microscope (FESEM, JEOL JSM-6701F, USA) was used to characterize the surface morphology of the AC. The surface elemental composition was obtained by X-ray photoelectron spectroscopy (XPS) using a Kratos AXIS UltraDLD spectrometer (UK) with a monochromatic radiation Al Kα (hv=1486.7 eV). The peaks of the XPS spectra were fitted with OriginPro 9.0 software. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry of the extracted compounds from the AC after electrolysis was performed using an Autoflex II mass spectrometer (Brucker-Daltonics GmbH, Bremen, Germany). The samples were prepared by extraction with diethyl ether as solvent, following the procedure described by Cooney et al. (Cooney, D. O. et al., Water Res. 1983, 17, 403-410).
AC was prepared by direct activation with steam, using biochar from yard trimmings waste as raw material. For each batch, 100 g of biochar were placed in a semi-rotating quartz tube under a nitrogen flux (0.5 L min−1) with a ramp of temperature of 10° C. min−1 until 800° C. At this constant temperature, steam was added (1.2 mL min−1) for 80 min to the nitrogen flux.
Then, cooling was allowed under nitrogen flux until ambient temperature. The obtained AC was rinsed with deionized water until a constant pH was attained and dried at 105° C. Finally, AC was sieved with a mesh to only retain particles with size above 0.9 mm, which were used in this study.
General Procedure 1
The reactor design and experimental set-ups are described below.
The reactor was designed with two purposes in mind: the continuous adsorption of the model pollutant using AC and the electrochemical regeneration of the used adsorbent. The reactor scheme is shown in
Phenol Adsorption Experiments
The adsorption isotherms were first obtained through batch adsorption tests. 0.1 g of AC were added to 50 mL of phenol solutions with concentrations between 5 and 1000 mg L−1. The flasks were sealed and placed in an orbital shaking incubator (LM-450D Yihder Co. Ltd, Taiwan) at 160 rpm and 298 K for 48 h, allowing sufficient time to achieve equilibrium. The samples were filtered using a 0.45 mm PTFE membrane and their phenol concentration was determined by reverse-phase high-performance liquid chromatography (HPLC, Shimadzu SCL-10A, Japan) equipped with an Agilent extend-C18 column (150 mm×2.10 mm, 5 mm). Acetic acid (1%) and methanol (75:25, v/v) were used as the mobile phase with a flow rate of 0.25 mL min−1. The detection wavelength was set at 280 nm, controlled by a UV-absorbance detector (Shimadzu SPD-M 10A, Japan). The concentration of phenol in solution at equilibrium (Ce, mg L−1) and its initial concentration before adsorption (Co, mg L−1) were used to calculate the amount of phenol adsorbed per unit of AC at equilibrium (qe, mg g−1), following eq. (5), and the data was fitted to Langmuir and Freundlich isotherms.
q
e=(Co−Ce)V W−1 (5)
where V is the volume of solution (L) and W is the mass of AC (g).
The performance of the reactor in continuous flow was then evaluated by establishing breakthrough curves at different flow rates (8, 10, 14 and 20 mL min−1). A 10 mM phenol solution, within the typical concentration range of industrial phenolic wastewaters (Steevensz, A. et al., Enzym. Microb. Technol. 2014, 55, 65-71), flowed upwards through the reactor using a peristaltic pump (Masterflex L/S Cole-Parmer, USA) and samples were taken from the effluent at regular time intervals. Phenol concentration was then measured by HPLC, following the methodology described above. The amount of phenol adsorbed onto the AC was calculated through data integration.
Hydrogen Peroxide Production and AC Electrochemical Regeneration
The optimum current density (between 1 and 25 mA g−1) was assessed through the monitoring of electrochemical H2O2 generation. For this purpose, electrolysis tests were carried out in the electrochemical reactor in recirculation mode, with 400 mL of electrolyte (50 mM K2SO4 at pH 3) and continuous air bubbling, pumped through the reactor at a flow rate of 10 mL min−1. Samples were withdrawn every 5 min for a period of half an hour and H2O2 was quantified using a photospectrometric method based on the addition of titanium oxysulfate to the solution sample to form a complex whose color intensity was measured at a wavelength of 405 nm (Garcia-Rodriguez, O. et al., Electrochim. Acta 2018, 276, 12-20). The current efficiency (CE) for H2O2 generation was determined by eq. (6) (Brillas, E. et al., Chem. Rev. 2009, 109, 6570-6631).
where F is the Faraday's constant (96 487 C mol−1), n refers to the stoichiometric number of electrons transferred in the oxygen reduction, c(H2O2) is the accumulated H2O2 concentration (mg L−1), V corresponds to the volume of the electrolyte (L), M(H2O2) refers to the molecular weight of H2O2 (34 g mol−1), 1000 is a conversion factor and Q stands for the charge that was used during the electrolysis.
Electrochemical regeneration experiments of AC were carried out in recirculation mode (flow rate of 10 mL min−1) at a constant current of 16.6 mA g−1. The source of ferrous ions consisted of iron (II) sulfate heptahydrate added to the electrolyte (50 mM K2SO4 at pH 3) to obtain a 0.2 mM iron (II) concentration. The regeneration efficiency was evaluated by varying the electrolysis time (impacting on the amount of AC saturated in the column), as well as the number of adsorption-regeneration cycles. First, the adsorption process was carried out to obtain different loadings of saturated AC in the reactor. Then, the electrochemical regeneration process took place by varying the electrolysis time from 60 to 120 min. Finally, the adsorption process was carried out again and the concentration of phenol in the effluent was compared to that of the first adsorption cycle to assess the regeneration efficiency. The energy consumption (EC) was calculated using equation (7), where Ecell corresponds to the potential difference through the regeneration (V), I stands for the applied current (A), t refers to the treatment time (h), ACmass is the mass of AC in the electrochemical reactor (kg) and 1000 is a conversion factor:
In order to get an overall understanding of its properties in terms of adsorption capacity, a preliminary batch adsorption characterization of the new adsorption material (prepared in Example 1) was conducted as described in the phenol adsorption experimental section in General Procedure 1.
Results and Discussion
Langmuir and Freundlich isotherm adsorption models are simple and explicit, and are commonly employed in AC adsorption studies with phenol (Du, W. et al., RSC Adv. 2017, 7, 46629-46635), with the aim of obtaining a better understanding of the adsorption behavior of the adsorbate and determine important parameters such as the adsorption capacity of the material, inter alia. As reported widely, the Langmuir isotherm model is representative of a monolayer adsorption onto an adsorbent, assuming a lack of interaction between the adsorbed molecules on the surface of the adsorbent (Trellu, C. et al., Environ. Sci. Technol. 2018, 52, 7450-7457). On the other hand, the Freundlich model is applicable to a multilayer adsorption onto an adsorbent with a highly heterogeneous surface (Kundu, S. et al., J. Chem. Eng. Data 2018, 63, 559-573).
Experimental data for phenol adsorption after reaching equilibrium with different initial concentrations is represented by circles in
aqmax represents the maximum monolayer adsorption capacity of phenol, b is the Langmuir constant related to the energy of adsorption, K is the Freundlich constant associated to the adsorption capacity and n the intensity of adsorption, both empirical constants (Kim, Y. -S. et al., J. Chem. Therm. 2019, 130, 104-113; and Yuan, P. et al., Langmuir 2018, 34, 15708-15718).
It is obvious from
The maximum adsorption capacity of the AC according to Langmuir was 115 mg g−1, in the range of phenol adsorption for biomass-based AC reported by others, e.g. 85-160 mg g−1 (Nunell, G. V. et al., Adsorption 2016, 22, 347-356), 45 mg g−1 (Xiong, Q. et al., RSC Adv. 2018, 8, 7599-7605), 149 mg g−1 (Hameed, B. H. et al., J. Hazard Mater. 2008, 160, 576-581), 161 mg g−1 (Li, X. et al., Asia Pac. J. Chem. Eng. 2018, 13, e2240), among others. Following these preliminary batch experiments, the optimization of the reactor breakthrough dynamics will aim at approaching this maximum adsorption capacity in a reactor more relevant to practical applications.
Optimization of the operating conditions in the electrochemical reactor described in General Procedure 1 was carried out and the experimental results are provided below.
Results and Discussion
Flow Rate
Although there is a vast amount of new adsorbents generated from biomass, the study of these adsorbents is often limited to batch systems. However, column breakthrough dynamics is essential to determine their potential for real wastewater treatment applications. As mentioned above, the design of the reactor emulated three fixed-bed adsorption columns and their response to a continuous flow of a phenol solution was evaluated through breakthrough curves at different flow rates but with the same initial phenol concentration, as shown in
Indeed, the uptake capacity of an adsorbent in continuous flow is always lower than in batch, but it is counteracted by the necessity to exert a dynamic mode for real applications. Here, the adsorption capacity, obtained by integrating the breakthrough curve for each flow rate, remained high at lower flow rates of 8 and 10 mL min−1, reaching 104 and 102 mg g−1, respectively (only 10% lower than in the batch study). However, the removal performance dropped considerably at higher flow rates, reaching 90 mg g−1 at 14 mL min−1 (22% lower than in the batch study) and 53 mg g−1 at 20 mL min−1 (a 54% drop). The lower adsorption capacity at high flow rates could be attributed to a degradation of intraparticle mass transfer and formation of dead zones within the reactor and thus, the flow rate was set at 10 mL min−1 for further experiments, in order to maintain an effective mass transfer towards the AC material.
There are three special adsorption points that are easily identified in
Applied Current
Applied current is paramount because it determines both the efficiency of the regeneration process and its cost.
H2O2→HO2.+H++e− (8)
HO2.→O2(g)+H++e− (9)
AC+H2O2→AC++−OH+.OH (10)
AC++H2O2→AC+HO2.+H+ (11)
The maximum current efficiency for H2O2 electrogeneration (inset panel of
Regeneration Time
The electrolysis time is another key parameter in electrochemical processes, directly correlated to the energy consumption of the process. Thus, the electrochemical regeneration time was optimized in this section using electrolysis times between 60 and 120 min, applied after 1 h, 2 h and 8 h of adsorption, in order to assess the effect of different AC saturation levels within the reactor, as already explained above (cf.
From the above results, we can conclude that the adsorption process can take place safely at least for 2 h (breakthrough capacity), corresponding to 38% of saturated AC, while still achieving high regeneration efficiency. Furthermore, it is not desirable to exceed the breakthrough capacity of the reactor in order to avoid concentrations of phenol higher than the permissible discharge limits.
EC is crucial to assess the feasibility of the regeneration process; at the industrial scale, thermal gasification methods suffer from high energy costs due to elevated temperatures (800-900° C.) and may cause AC strcture degradation (Salvador, F. et al., Microporous Mesoporous Mater. 2015, 202, 259-276). Alternative methods, including microwave regeneration, are also costly at 900 kWh kg−1 and 85% of RE (Pan, R. R. et al., RSC Adv. 2016, 6, 32960-32966). In contrast, the range of EC in our study (<1 kWh kg−1 of AC) largely outcompetes these processes. This low EC is further supported by other electrochemical regeneration technologies, including an electro-desorption study conducted by Alvarez-Pugliese et al. (Alvarez-Pugliese, C. E. et al., Diam. Relat. Mater. 2019, 93, 193-199), who achieved>80% regeneration efficiency at only 3.8 kWh kg−1 of AC (with more complex pollutants). It should be noted here that our proposed solution differs from that of Alvarez-Pugliese et al. (Alvarez-Pugliese, C. E. et al., Diam. Relat. Mater. 2019, 93, 193-199), because it relies on a combination of electro-Fenton and anodic oxidation, leading to mineralization of phenol, unlike desorption regeneration methods.
The stability of the optimized electrochemical regeneration process, as described in General Procedure 1 and Example 3, was evaluated.
Results and Discussion
The phenol removal efficiency was monitored during 10 consecutive cycles of adsorption and regeneration using AC-2 h and AC-8 h to study the stability of the regeneration process (
The decay in regeneration efficiency observed for AC-8 h could be attributed to a different phenol oxidation pathway, as mentioned in Example 3. It has been previously reported that phenol oxidation can lead to its mineralization and/or polymerization depending upon the experimental conditions such as concentration, electrode material, applied potential, etc. (Patra, S. et al., J. Electrochem. Soc. 2008, 155, F23-F30). The major steps of the possible phenol oxidation reaction pathways (
In order to get a better understanding of the pathway followed during the electrochemical regeneration (as described in General Procedure 1) of AC-8 h (prepared in Example 1) under optimized conditions (as described in Example 3), a mass spectra analysis of the extracted compounds from the AC was carried out and the organic extract was analyzed by MALDI-TOF.
Results and Discussion
To support the above observations, characterization through EIS, a nondestructive electrochemical method, allowed the following of the behavior of the ACs (prepared in Example 1), original AC, AC-8h and AC-2h, before and after the regeneration cycles performed under optimized conditions (as described in General Procedure 1 and Example 3). The EIS responses were interpreted by fitting the data to an equivalent circuit (inlet of
Results and Discussion
To get a better understanding of the effect of the electrochemical regeneration process (as described in General Procedure 1) on the surface of the AC (prepared in Example 1) within the optimal conditions (as described in Example 3), a physicochemical characterization was carried out before and after ten regeneration cycles of AC-2h as one of the main disadvantages of other regeneration methods (especially thermal processes) is the degradation of the AC structure caused by the harsh conditions undergone by the material.
Results and Discussion
The morphological structure of the AC before and after 10 cycles of electrochemical regeneration was analyzed using FESEM (
BET analysis and pore distribution size of AC and AC-2h are shown in
aAccording to IUPAC, micropore diameters < 2 nm; 2 nm < mesopore diameters < 50 nm.
The modification of surface chemistry and elemental composition of AC following electrochemical regeneration cycles was analyzed by XPS (
| Number | Date | Country | Kind |
|---|---|---|---|
| 10202001530U | Feb 2020 | SG | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2021/050084 | 2/22/2021 | WO |
