1. Introduction
The sewage design flow rate that flows into the sewage treatment plant through the sewerage system is based on the planned sewage generation rate (Q) of 1Q, and it is possible to treat all of the sewage generated on clear days since the amount of sewage generated is less than 1Q. However, during rainfall, the amount of sewage generated exceeds the design capacity, and the excess sewage is discharged untreated into the river, which is called Combined Sewer Overflows (CSOs). The amount of pollution caused by CSOs in Korea is reported to account for 31% of the total pollution load in the entire watershed [1–3].
Treatment facilities for CSOs are classified into natural and engineered systems. Natural systems that use soil and vegetation can simultaneously treat high levels of suspended solids and dissolved organics, but they require large areas [4]. In contrast, engineered systems that use filtration and coagulation-flocculation technologies have a compact facility size and good efficiency in treating suspended solids, but they have the disadvantage of being unable to effectively treat dissolved organics such as soluble BOD and soluble COD [5, 6]. A hydrodynamic vortex separator can remove suspended solids and sediments simultaneously with lower head loss than common coagulation-flocculation treatment. However, the removal efficiency of vortex separators is not enough for CSO treatments [7, 8].
Generally, biological treatment processes are suitable for the treatment of dissolved organic matter, but due to the characteristics of CSOs treatment facilities that are generally operated without permanent staff, physical treatment that can treat dissolved organic matter are more favorable [9]. Therefore, adsorption could be considered as an efficient way to treat dissolved organic matter as a non-biological treatment technology. Adsorption is a surface process that leads to transfer of a molecule from a fluid bulk to solid surface. This can occur because of physical forces or by chemical bonds. Adsorption properties result in the manufacturing of high quality and economically feasible products for various applications. Mesoporous materials like SBA-15 [10], MCM-41 [11], and MCM-48 [12] have been extensively studied and used in various fields due to their high surface area, well-defined pore structures, and narrow pore size distributions. Atiyah et al. examined the potential of SBA-15 as a carrier for drug delivery, specifically focusing on the loading of curcumin onto the surface of SBA-15 [10]. Li et al. studied the MCM-48, as a typical pure cubic mesoporous silica material, has the characteristics of a large specific surface area [12], and these characteristics are different from two-dimensional mesoporous materials such as MCM-41 and SBA-15. MCM-48 is widely used in the field of adsorption and separation of macromolecules in water.
Until now, the GAC adsorption process has been used for advanced water treatment processes [13], tertiary treatment processes in sewage treatment, and the removal of refractory organic carbon (ROC) or color substances in wastewater [14, 15]. As the adsorption progresses, the adsorption capacity of the GAC gradually decreases and finally reaches a breakthrough state. The spent GAC is typically recovered and reused by thermal regeneration [16]. Due to the high energy cost of this process, the economics of the GAC process is dependent on the utilization rate. Therefore, it is necessary to regenerate it in an economical way rather than replacing it with new activated carbon or thermal regeneration [17–20].
It is expected that the denitrification and removal of labile organic carbon (LOC), the organic matter adsorbed during the denitrification reaction, will differ depending on the dissolved oxygen conditions of the GAC adsorption column, the concentration of nitrate nitrogen, and the mass transfer characteristics of the GAC pore structure [21–24]. Since it is difficult to find proper examples of biological regeneration utilizing denitrification properties in the field of GAC regeneration, this study is expected to provide interesting insights.
Several advanced oxidation process (AOP) technologies such as UV photocatalysis, Fenton processes, electrochemical oxidation processes and ozonation were investigated to be effective for organics removal in ROC [25–29]. The Fenton oxidation reaction shows a strong oxidative degradation of refractory organic matter under high H2O2/Fe2+ ratio conditions because the chemical oxidation reaction is dominant. However, under low H2O2/Fe2+ ratio conditions, the coagulation reaction is dominant in the Fenton reaction [30–32]. Thus, the high cost for the hydrogen peroxide reagent used and the disposal of the coagulation precipitate generated as a reaction byproduct might be weakness.
During the electrochemical treatment of GAC, oxidation and decomposition reactions may occur through i) direct electron transfer, ii) indirect oxidation via electrogenerated hydroxyl radicals, and iii) indirect oxidation via electrogenerated oxidizing species such as active chlorine species (HOCl, −OCl, Cl2) and hydrogen peroxides [33]. Direct oxidation is a process in which contaminant species first migrate to and are adsorbed onto the anode surface, where direct electron transfer results in an oxidized compound [34, 35]. During indirect oxidation, agents such as active chlorine, hydrogen peroxide, and hydroxyl radicals are generated on-site at the electrode surface and are immediately utilized to degrade pollutant species within the primary solution [36–40].
Electrochemical advanced oxidation processes (EAOPs) based on Fenton’s reaction chemistry are eco-friendly methods that have recently garnered significant attention for water remediation, with the electro-Fenton process being the most popular EAOP [41–44].
Ultrasound (US) is an advanced oxidation technology, in which hydroxyl radical species (OH•) are generated after the formation and implosion of cavitation bubbles [45–46]. In recent decades, researchers have discovered that the application of US in conjunction with other oxidants, such as US-O3 [47], US-Fenton [48], US-H2O2 AUV [49], etc., has garnered considerable attention and been extensively investigated. Numerous efforts have recently been dedicated to primarily emphasizing the enhancement of available •OH in the solution by utilizing sonochemically produced H2O2 during sonication for aromatic compounds [50], pesticides [46], and other refractory organic compounds [49].
According to Feng et al., Fenton treatment of effluent from a biological wastewater treatment process using MBR showed 39.3% TOC removal and 69.5% color removal, and the Fenton reaction is dominated by flocculation reaction at low H2O2/Fe2+ ratio and chemical oxidation at high H2O2/Fe2+ ratio [51]. According to Guo et al., when H2O2 was 1.0 M and Fe2+ concentration was 0.36 M, COD removal rate of 85.29% and TOC removal rate of 75.23% were obtained after Fenton treatment of benzene wastewater [52]. Guilanea et al. used ultrasound as a method to regenerate GAC saturated with p-chlorophenol, and the regeneration efficiency improved with increasing sound intensity at a frequency of 21 kHz, and the desorption rate of adsorbed p-chlorophenol was proportional to the increase in temperature and the intensity of ultrasonic power [53].
In a study by Parsa & Jafari, a regeneration experiment of GAC used for the adsorption of Rhodamine B resulted in a regeneration efficiency of 30% for ultrasound alone and 87.8% for the fusion of 20 kHz ultrasound and Fenton treatment [54]. Lu et al. investigated the regeneration of GACs adsorbing low-concentration sewage under three frequency conditions below 100 kHz and found a regeneration efficiency of 60–80% [55]. However, the difference is that the GACs used for regeneration were not operated for a long period of time until breakthrough, but were regenerated using GACs adsorbing sewage once.
This study investigated the characteristics of GAC treatment in CSOs with a high concentration of biodegradable organic matter, which is different from the general application of GAC, and applied three regeneration methods considering economics and efficiency to increase the regeneration efficiency of spent GAC generated after the operation. The feasibility of the idea of utilizing the spent GAC generated during CSOs treatment as a source of denitrification carbon was evaluated, and the regeneration efficiency of Electro-Fenton, which combines Fenton oxidation and electrolysis, and Sono-Fenton, which combines frequency-specific ultrasound and Fenton oxidation, was evaluated and compared. The type of electrode material, current intensity, and ultrasonic frequency (40 kHz, 750 kHz, respectively) were investigated as driving factors of the regeneration methods, and the changes in specific surface area and micropore volume of GAC were analyzed using BET, and the micro-surface characteristics were observed using FE-SEM and EDS. Previous studies have rarely reported on GAC adsorption processes for removing organic matter from CSOs or sewage. However, this study that evaluated and compared four technologies for the regeneration of spent GAC at the same time can be considered as an originality of this study. And, although there have been many previous studies on AOP methods such as Fenton, ultrasonic, UV, and Sono-Fenton, they were mainly performed on the oxidation of recalcitrant dissolved organic matter as a removal target and focused on liquid-liquid reactions, whereas this study focused on solid-liquid reactions that cause desorption or oxidative degradation of organic matter adsorbed in the spent-GAC micropores, which shows a difference from previous studies.
2. Materials and Methods
2.1. GAC Preparation
Coal-based GAC with a size of 1 to 2 mm, an average micro-pore diameter of 20 to 25 Å, and a moisture content of 5% or less was used as the activated carbon in this study. It was immersed in the effluent line of the primary sedimentation tank of the S city sewage treatment plant, and after being removed through a continuous adsorption reaction, it was washed once with tap water and dried at room temperature to prepare the spent-GAC for the GAC regeneration experiment.
2.2. Continuous Operation of a GAC Adsorption Column
To experiment with the dissolved organic matter treatment process of CSO using GAC, a simple upflow GAC adsorption tower was designed and built for continuous operation. As a result of the GAC adsorption experiment according to the type of organic matter, the GAC adsorption tank was continuously operated using synthetic sewage using sucrose and starch, which showed the most similar adsorption characteristics to sewage, and then denitrification, Electro-Fenton and Sono-Fenton treatments were evaluated using real sewage.
The synthetic influent supplied to the reactor was made with sucrose to have a theoretical oxygen demand of 100 mg/L. The flow rate of the influent supplied to the reactor was 33.3 mL/min, and the reactor’s hydraulic retention time was 0.5 hr. The volume of GAC inside the reactor for filtration was set to 1 L.
To find organic matter that shows similar adsorption efficiency to the actual sewage, batch adsorption experiments were conducted using Jar-test. GAC was injected at concentrations of 1, 2, 4, and 8 g/L, and adsorption experiments were conducted for different reaction times for actual sewage, glucose, sodium acetate, methanol, LB broth, and sucrose. The mixing speed was maintained at 150 rpm, and the organic matter concentration was analyzed based on COD. Organic matter was selected for use in this experiment by comparing the adsorption efficiency of each type of organic matter and selecting the one that had similar characteristics to the actual sewage adsorption. The selected organic matter was then used to make synthetic sewage for this experiment.
2.3. Denitrification-Driven Regeneration of Spent GAC
Fig. 1 is a schematic of the regeneration mechanism of spent GAC, with denitrification regeneration on the left and Sono-Fenton chemistry on the right. After the operation of the GAC was terminated, the adsorbed organic matter of the spent GAC was used as an organic carbon source for denitrification microorganisms to examine the possibility of simultaneously removing nitrate nitrogen in the sewage and adsorbed organic matter of the spent GAC.
The denitrification efficiency and adsorption capacity of GAC were evaluated by inducing a denitrification reaction for 48 hours under a stirring intensity of 50 rpm, using activated sludge at a concentration of 1,000 mg/L with 5 g of spent-GAC as the denitrification carbon source. NO3-N solution was prepared using KNO3 and tested at concentrations of 8.5, 99, and 490 mg/L.
2.4. Electro-Fenton and Sono-Fenton-Driven Regenerations of Spent GAC
In the Electro-Fenton study, the regeneration efficiency was tested based on 20 g of spent GAC. The concentration of Fe2+ was 2.5 mmol/L, and H2O2 was 250 mmol/L, which are the operational factors used in the Fenton oxidation reaction. For the electrolysis, the distance between the anode and cathode was 5.5 cm, and the size of the electrode in contact with the electrolyte was 4.5 cm × 3.0 cm. The experiments were conducted based on the reaction time, electrode type, and current intensity, which are the main factors of the Electro-Fenton oxidation reaction. The materials of the anode and cathode varied from SUS-SUS, Ti/Pt-Ti/Pt, and SUS-Ti/Pt. A small amount of NaCl was added as the electrolyte, and the constant current conditions were fixed at 0.25, 0.5, 0.75, and 1 A.
The Sono-Fenton experiment was performed at the optimum Fe2+ concentration of 5/10 mmol/L obtained through the previous Fenton experiment, and the concentration of H2O2 was in the range of 10, 40, 100, and 1000 mmol/L, with ultrasonic frequencies of 40, 80, 132, 168, and 750 kHz, and the regenerated GAC was treated for 2 hours. After removing the Fenton solution using a mesh, the raw sewage was added to the solution diluted to a concentration of 100 mg COD/L and adsorbed for 30 min.
2.5. Chemical Analytical Methods
FE-SEM was used to observe the microstructure and pore changes of virgin GAC, spent GAC, and Sono-Fenton-regenerated GAC through images obtained by shooting an electron beam onto the sample surface and then reflecting the beam back from the surface. At the same time, EDS (X-flash 4010, BRUKER, USA) analysis was performed to observe the elemental distribution, which is a chemical characteristic of the granular activated carbon surface. COD, and NO3-N tests were conducted for synthetic sewage and actual sewage samples in accordance with APHA standard method (5220D, 5210B, 1998). 2.5–3 times dilution with distilled water was performed for each actual sewage sample to make similar COD concentration with CSOs.
3. Results and Discussion
3.1. Effect of Types of Organic Compounds on GAC Adsorption
Fig. 2-a) evaluates the adsorption capacity of GAC according to the type of organic matter. The types of organic matter used were glucose, sodium acetate, methanol, LB broth, sucrose, and sewage. The purpose of this experiment was to select the organic matter whose adsorption capacity for GAC is most similar to sewage and use it for continuous adsorption operation by concentration. The results showed that sewage and sucrose showed the highest adsorption capacity among the six organics, 75.7% and 68%, respectively, after 2 hours of adsorption at an initial COD of 169 mg/L. On the other hand, glucose, sodium acetate, methanol, and LB broth showed poor adsorption capacities of 6%, 11%, 16%, and 28% when used as organic matter.
GAC is a well-known adsorbent used to remove various organic and inorganic compounds from water due to its high surface area and porosity. In general, GAC is more effective at adsorbing hydrophobic compounds due to its predominantly hydrophobic surface [13]. Given that both glucose, methanol and sucrose are hydrophilic, their adsorption capacity on GAC may be limited. The glucose, methanol, sodium acetate, and sucrose used in the experiment are hydrophilic in nature, despite their different molecular weights, and therefore do not match the chemical adsorption properties of GAC. It is interesting to note that despite the above chemical characteristics of organic matter, sucrose showed a greater adsorption capacity than other substances on GAC, and the adsorption capacity was similar to that of real sewage. Therefore, it may be premature and misleading to judge the adsorption of organic matter on activated carbon, GAC, based on whether it is hydrophilic or hydrophobic. Therefore, the specific adsorption capacities can be influenced by factors such as GAC’s pore size distribution, surface functional groups, and the presence of other competing compounds in the solution besides weather the organic matter is hydrophobic or hydrophilic.
The reason for the large adsorption capacity of sucrose can be considered to be due to the difference in bond structure and functional groups that affect the binding to GAC. For example, glucose and sucrose differ in their functional groups, which are chemical groups within a molecule that can participate in chemical reactions. Glucose is a monosaccharide, and its functional group is an aldehyde (−CHO) or a ketone (−CO-) group, depending on the form of glucose. On the other hand, sucrose is a disaccharide, made up of glucose and fructose molecules chemically bonded together. The functional group of sucrose is an acetal (−OR) group. These differences in functional groups can affect the way the molecules interact with other substances, including GAC, and thus contribute to differences in their adsorption capacities.
Fig. 2-b) shows the results of continuous operation of the GAC adsorption tower using synthetic wastewater simulating the peak, average, and low concentrations of CSOs using sucrose. Based on the point where the difference between the COD concentration of the treated water and the influent concentration of the GAC adsorption tower falls below 30%, the adsorption period required to reach that point was 32 days for low concentration influent (64 mg COD/L), 28 days for average concentration (96 mg COD/L), and 21 days for high concentration (128 mg COD/L).
As the average rainfall days in Korea are about 46–54 days, it is concluded that the GAC adsorption tower cannot be operated for one year without replacing the new activated carbon if evaluated based on the average concentration of CSOs for the period when the removal rate is maintained above 30%. Therefore, it is necessary to replace the new activated carbon or try to extend the operation period through regeneration.
3.2. Denitrification-Driven Regeneration of Spent GAC
Fig. 3 shows the results of an experiment that confirms whether the organic matter adsorbed on GAC used for CSOs treatment can be used as an external carbon source for denitrification reactions, and verifies the regeneration ability of the spent GAC as an external carbon source. The experiment was divided into three parts according to the injected NO3-N concentration, and denitrification was conducted under constant conditions with an GAC injection amount of 5 g and an MLSS injection concentration of 1000 mg/L. When the initial NO3-N concentration was 8.5 mg/L, it decreased to 1.4 mg/L after 2 hours of operation and to 0.9 mg/L after 48 hours. When the NO3-N concentration was 99 mg/L, it decreased to 93 mg/L after 2 hours of operation and to 8 mg/L after 48 hours, showing the highest denitrification amount. Finally, when the NO3-N concentration was 490 mg/L, the result showed a decrease to 440 mg/L after 48 hours of operation, and the reason for the decrease from 490 mg/L without following a first-order reaction with respect to the nitrate nitrogen concentration is considered to be due to the acidic pH caused by the high influent nitrate nitrogen.
3.3. Electro-Fenton Regeneration of Spent GAC
Dissolved Fe2+ and H2O2 in Fenton reagent are efficient for the degradation of recalcitrant organic pollutants in wastewaters owing to the formation of nonselective hydroxyl radicals(•OH). Disadvantages include the high cost of H2O2, which is used as an oxidant in the Fenton reaction, and the generation of sludge when Fe3+ ions generated by the oxidation of Fe2+ react with OH−.
Electro-Fenton are mediated electrochemical treatments based on the destruction of ROC at the anode or using the Fenton’s reagent partially generated from electrode reactions [41]. And the efficiency of H2O2 generation is dependent on the type of electrode material like a cathode. In the GAC regeneration experiment using Electro-Fenton oxidation, Fig. 4-a) evaluates the regeneration efficiency of GAC according to the change of current intensity and Fig. 4-b) tested for the type of material used as anode and cathode.
As shown in Fig. 4-a), the results of the Electro-Fenton reaction at current intensities ranging from 0.25 to 1.0 A showed a linear increase in the regeneration efficiency of spent-GAC from 0.5 to 1.0 A, with a regeneration efficiency of 22% at 1.0 A. However, the lowest current intensity, 0.25 A, showed an interesting result, with an Electro-Fenton regeneration efficiency of 17%, indicating a secondary efficiency.
To investigate the effect of electrode materials in the Electro-Fenton experiment, the regeneration efficiency of spent GAC is shown in Fig. 4-b) for the three cases of SUS-SUS, Ti/Pt-Ti/Pt, and SUS-Ti/Pt as anode and cathode.
Prior to the Electro-Fenton experiment using the above three types of electrode materials, iron was tested as the electrode material, but the iron electrode was difficult to use repeatedly due to corrosion and small holes after the reaction. The SUS-SUS electrode, a stainless steel material, was more durable than the iron electrode and could be used repeatedly, but it could not completely prevent corrosion. Finally, the Ti/Pt electrode with Pt coating on TiO2 substrate showed stable results with no corrosion despite repeated use.
The regeneration efficiency of spent GAC by electrode material was highest for SUS-SUS using stainless steel for both anode and cathode at 41.9%, followed by 32.9% for SUS-Ti/Pt electrode and 23.4% for Ti/Pt-Ti/Pt. Based on the regeneration efficiency and economic efficiency, SUS-SUS electrode is the most suitable electrode material for the regeneration of spent GAC. However, considering the maintenance and replacement cycle, Ti/Pt-Ti/Pt is a superior material. Therefore, it is necessary to develop dimensionally stable anode (DSA) such as Ti/Pt as an economical material in the long run.
3.4. Sono-Fenton-Driven Regeneration of Spent GAC
3.4.1. Ultrasonic waves
Ultrasonic waves refer to sound waves that have a frequency range of approximately 20 kHz to 1 GHz and are a form of high-frequency vibrational energy. By the ultrasonic waves, the medium undergoes repeated expansion and compression, creating sound waves and causing fluctuations in pressure. As a result of the pressure changes, the liquid is pulled and the nuclei of bubbles or air particles in the liquid gather or grow to create bubbles.
These bubbles develop further and eventually either disappear or remain as a group, and they continue to be compressed until they become small, hot, high-pressure microbubbles. When the bubbles are destroyed, they break down into their original molecular state and disperse, creating a shock wave of great momentary pressure at the moment of destruction. It is expected that this phenomenon can be utilized to remove organic materials adsorbed on the surface of GAC.
Fenton is based on production of various reactive species, often hydroxyl radical (HO•). The HO• is generated through the decomposition of H2O2 in the presence of Fe2+ as catalyst shown in Eq. (1). The generated reactive species are able to degrade and mineralize various organic pollutants. In Sono-Fenton process, the ultrasonic wave could promote Fenton reactions by the faster conversion of ferrous ions to ferric ion and the generation of more hydroxyl radicals from water without hydrogen peroxide consumption according to from Eq. (2) to Eq. (5).
[
3.4.2. Sono-Fenton process
Fig. 5 shows the results of ultrasound alone, Electro-Fenton, and Sono-Fenton treatment of spent GAC. During the treatment of spent GAC with ultrasound alone, three types of solvents were utilized: distilled water (DW), 20% EtOH, and 5% NaOH. The effectiveness of ultrasound in regenerating spent GAC was not significant, yielding recoveries of 8.7% in 5% NaOH, 6.9% in 20% ethanol, and 2.1% in distilled water after 4 hours of treatment.
The Sono-Fenton experiments were conducted using ultrasound frequencies of 40 kHz and 750 kHz, respectively. The H2O2 concentration in Sono-Fenton varied in four steps, from 10 mmol/L to 1000 mmol/L, while maintaining a consistent Fe2+ concentration of 10 mmol/L. The maximum regeneration efficiency of 68.5% was achieved at 40kHz and 67.8% at 750kHz in Sono-Fenton treatment. These results show that in the conventional Fenton regeneration method, the regeneration efficiency was halved when the H2O2 concentration exceeds 800mmol/L, whereas in Sono-Fenton, the regeneration efficiency tends to increase up to an H2O2 concentration of 1000mmol/L due to the reduction reaction of Fe3+ induced by ultrasonic treatment, which is inferred to have increased •OH.
Fenton oxidation using Fe2+ and H2O2 resulted in 63% recovery of GAC, Sono-Fenton fused with ultrasonic (40 kHz) Fenton oxidation resulted in 68% recovery, which was better than simple Fenton oxidation, but Electro-Fenton oxidation resulted in 43% recovery, which was lower than simple Fenton oxidation. The reduced regeneration effect of Electro-Fenton might be attributed to the unevenness of the electrochemical reaction and the possibility that the cathode reactant has an antagonistic relationship with the Fenton oxidation reaction.
This suggests that the electrochemical conditions at the anode and cathode generated by electrolysis are antagonistic, rather than synergistic, with the Fenton oxidation reaction. Examining the electrochemical reactions, the cathode shaft is characterized by the generation of hydrogen gas and OH− ions, which increase the pH of the solution. These alkaline conditions are likely to antagonize the Fenton oxidation reaction.
3.5. Characterization of the adsorbent
3.5.1. BET analysis
The BET method measures the amount of adsorption of a gas to derive a value for the “surface area” of a sample. Gas molecules can pass between particles and enter all pores and surface roughness, so the measurement provides the total micro-surface area of the sample. In the BET equation in Eq. (6), the number of adsorbed molecules is proportional to the gas concentration and inversely proportional to the difference between the saturation pressure and the gas pressure.
where P: Equilibrium pressure of adsorbates at the temperature of adsorption, P0: Saturation pressure of adsorbates at the temperature of adsorption, Va: Adsorbed gas quantity, Vm: Monolayer adsorbed gas quantity, C: BET constant
Table 1 compares the SSA, total pore volume, peak pore size, and peak and average pore size for samples of new GAC (virgin GAC), GAC adsorbed with CSOs (spent GAC), GAC treated with Fenton after adsorption (EF-GAC), and GAC simultaneously treated with ultrasound and Fenton (SF-GAC).
The SSA showed that Virgin-GAC had 1,004, Spent-GAC had 741, EF-treated-GAC had 912, and SF-treated-GAC had 856 m2/g. The specific surface area of GAC decreased by 26% for Spent-GAC compared to Virgin-GAC as CSOs adsorption treatment proceeded. In contrast, the specific surface area of EF-treated-GAC and SF-treated-GAC showed a decrease of 9.2% and 14.7%, respectively, compared to Virgin-GAC.
This implies that electro-Fenton and Sono-Fenton treatments have a significant effect on the specific surface area changes of spent-GAC. The changes in the size of GAC’s specific surface area show characteristics consistent with the behavior of the Vm value, which represents the gas adsorption capacity. Comparing the GAC pore size under different treatment conditions, Virgin-GAC exhibited average and peak pore sizes of 2.31 and 3.72 nm, respectively. In contrast, Spent GAC showed an average pore size of 1.93 nm and a peak pore size of 2.41 nm. This indicates that the size of GAC pores has become smaller through the organic matter adsorption process of CSOs. This tendency is also reflected in similar characteristics found in SSA and FE-SEM images.
Generally, pores that affect gas molecule adsorption are classified into three categories: micropores with pore sizes less than 2 nm, mesopores with pore sizes between 2 and 100 nm, and macropores with pore sizes larger than 100 nm [9]. Fig. 6 shows the volume of adsorbed gas (Va) for GAC samples after the three regeneration treatments and virgin GAC, as determined by BET analysis. Here, P represents the equilibrium pressure of adsorbates at the temperature of adsorption, P0 represents the pressure of adsorbates at the temperature of adsorption, and Va represents the adsorbed gas quantity. The shape of the graph in Fig. 6 provides information about the pore distribution of the GAC used. The shape of the graph in Fig. 6 is classified as Type 1, indicating that the pores of the GAC primarily exhibit micropore characteristics.
The pore size distribution (PSD) of GAC was obtained using the BJH method, and for GAC with uniformly sized pores, the size distribution of the pores is an important factor in determining the adsorption characteristics of GAC because it allows the movement of dissolved organic matter similar in size to the pores. BET analysis results showed that the virgin-GAC used in the experiment had well-developed micropores with pore sizes of 7 nm or less, while meso and macropores with pore sizes of 10 nm or larger accounted for about 10% of the total. Additionally, when comparing the pore volume of GAC according to the regeneration methods of spent GAC, the total pore volume decreased from 0.58 for Virgin-GAC to 0.37 for spent-GAC. After electro-Fenton and Sono-Fenton regeneration treatments, the pore volumes were 0.447 and 0.427 cm3/g, respectively.
3.5.2. FE-SEM analysis
Fig. 7 shows the results of analyzing the changes in micropore structure of virgin GAC, spent GAC, GAC after Sono-Fenton treatment, and GAC after electro-Fenton treatment using FE-SEM. All specimens used in the analysis were compared by capturing images at 5,000× and 20,000× magnification.
Fig. 7-a), representing the image of Virgin GAC before adsorption treatment, primarily shows the macropores of GAC due to the low magnification conditions, while Fig. 7-b) mainly shows the distribution of mesopores. The reason for this difference from the fact that over 90% of GAC’s BET analysis results were micropores is that micropores, with sizes smaller than 2 nm, could not be observed even at the 20,000× magnification used in the experiment. Moreover, despite not having undergone CSOs adsorption treatment, small fragments of around 1 μm were widely distributed on the GAC surface. Fig. 7-c) and 7-d) show the microstructure of spent GAC generated after continuous operation of CSOs adsorption. Most of the macropores in spent GAC are blocked, making it difficult to find voids on the GAC surface. Upon observing the 20,000× magnified images, viscous film-like substances filling the mesopores were found. Fig. 7-e) and 7-f) show images of GAC after Sono-Fenton treatment, while Fig. 7-g) and 7-h) represent images of GAC after electro-Fenton treatment. The surface characteristics of GAC after both Sono-Fenton and electro-Fenton treatments showed partial restoration of macropores and mesopores. Although no significant differences were found in the degree of pore recovery between the two regeneration treatments when observed, the Sono-Fenton treatment displayed a cleaner surface compared to electro-Fenton treatment. The surface of GAC after electro-Fenton treatment had substances resembling deposits, with sizes ranging from 100 to 200 nm, distributed across it.
As seen in the BET analysis results in Table 1, the SSA of spent GAC increased from 741 m2/g to 856 m2/g after Sono-Fenton treatment, and to 912 m2/g after electro-Fenton treatment. These results show a close correlation with the FE-SEM images.
4. Conclusions
Among the six organic compounds assessed for GAC adsorption, sewage and sucrose exhibited the highest adsorption capacities at 75.7% and 68%, respectively. Factors affecting GAC adsorption cannot be reliably determined by the hydrophobicity of the organic matter alone, but rather by an integrated consideration of GAC pore size, surface functional groups, and competitors in solution. The CSOs adsorption duration by GAC was contingent upon the influent concentration, extending from 21 to 32 days.
Denitrification regeneration experiments demonstrated that denitrifying microorganisms could utilize the organic matter adsorbed on the spent GAC, resulting in a reduction of the initial NO3-N concentration from 99 mg/L to 8 mg/L within 48 hours. In the Electro-Fenton treatment, SUS-SUS as the anode and cathode materials showed slightly better results than Ti/Pt-Ti/Pt, but Ti/Pt-Ti/Pt was superior in electrode stability for long-term use. As a combined strategy of ultrasound and Fenton treatment, 40 kHz showed a higher GAC regeneration efficiency of 68.5% than 67.8% at 750 kHz, and The Sono-Fenton method showed best regeneration efficiency among all the regeneration methods applied in this study.
Unlike conventional liquid-liquid oxidation, solid-liquid oxidation to remove organics adsorbed in the micropores of spent GAC was more of an obstacle due to additional interfacial resistance, which required a fusion oxidation technique to overcome. The results of this study confirm the feasibility of reusing CSOs adsorbed GAC without replacement through Sono-Fenton treatment under rainfall conditions in Korea.
PDF Links
PubReader
Full text via DOI
Download Citation 
Print 
