Activated Carbon Assisted Fenton-like Treatment of Wastewater Containing Acid Red G

: The Fenton reaction as an effective advanced oxidation technology has been widely used in wastewater treatment for its stable efﬂuent quality, simple operation, mild condition, and higher organic degradation with non-selectivity. However, the traditional Fenton reaction is limited by the sluggish regeneration of Fe 2+ , resulting in a slower reaction rate, and it is necessary to further increase the dosage of Fe 2+ , which will increase the production of iron sludge. Activated carbon (AC) has a strong adsorption property, and it cannot be ignored that it also can reduce Fe 3+ . In this study, the degradation of acid red G (ARG) by adding AC to the Fe 3+ /H 2 O 2 system, the role of the reducing ability, and the reason why AC can reduce Fe 3+ were studied. By adding three kinds of ACs, including coconut shell-activated carbon (CSPAC), wood-activated carbon (WPAC), and coal-activated carbon (CPAC), the ability of ACs to assist the Fe 3+ /H 2 O 2 Fenton-like system to degrade ARG was clariﬁed. Through the ﬁnal treatment effect and the ability to reduce Fe 3+ , the type of AC with the best promotion effect was CSPAC. The different inﬂuence factors of particle size, the concentration of CSPAC, concentration of H 2 O 2 , concentration of Fe 3+ , and pH value were further observed. The best reaction conditions were determined as CSPAC powder with a particle size of 75 µ m and dosage of 0.6 g/L, initial H 2 O 2 concentration of 0.4 mmol/L, Fe 3+ concentration of 0.1 mmol/L, and pH = 3. By reducing the adsorption effect of CSPAC, it was further observed that CSPAC could accelerate the early reaction rate of the degradation process of ARG by the Fe 3+ /H 2 O 2 system. FT-IR and XPS conﬁrmed that the C-O-H group on the surface of CSPAC could reduce Fe 3+ to Fe 2+ . This study can improve the understanding and role of AC in the Fenton reaction, and further promote the application of the Fenton reaction in sewage treatment.


Introduction
The Fenton reaction as one of the effective advanced oxidation technologies (AOPs) has been widely used in wastewater treatment for its stable effluent quality, simple operation, higher organic loading, and mild condition [1][2][3].For instance, the Fenton process can dispose of wastewater with a large chemical oxygen demand (COD) ranging from 100,000 to 150,000 mg/L [4,5].Meanwhile, the Fenton reaction can degrade not only conventional organic pollutants but also bioaccumulated and refractory pollutants, such as high-concentration nitrogenous dye [6][7][8][9][10], estrogens, and antibiotics.Fenton process can also be applied to soil remediation [11][12][13], biomedicine [14], treating deep bacteria-infected diseases [15], and water treatment due to its lower side effects and lack of equipment restrictions.
However, the Fenton reaction mainly relies on the core reaction between Fe 2+ and H 2 O 2 at low pH conditions (pH of 2-4) to generate hydroxyl radicals (•OH) for efficient

Characterization
The specific surface areas (SSAs) of the ACs were determined on an SSA analyzer (SSA-4300, Beijing Builder Electronic Technology Co., Ltd., Beijing, China) using the nitrogen adsorption-desorption method at 77 K (Brunauer-Emmett-Teller method), and the pore volumes were calculated by the Barrett-Joyner-Halenda method offered by the equipment.Fourier transform infrared spectroscopy (FT-IR) was used to characterize the chemical groups in the samples, and the FT-IR spectra in this study were acquired on a Bruker Tensor 37 infrared spectrometer (Bruker TENSOR 37 FT-IR) in the region from 400 to 4000 cm −1 by the KBr pellet method.X-ray photoelectron spectroscopy (XPS, Thermo Scientific EscaLab 250Xi) with the emission source of Al Kα (1486.71eV) was used to analyze the composition of the surface elements of the ACs and the valence state of the samples.The C 1s electron binding energy (284.8eV) was used as the standard to correct all the binding energies (BEs) [56].

Experimental Procedures
All experiments were conducted in a 250 mL glass beaker.The reaction was initiated with the addition of desired dosages of 200 mL of 100 mg/L Acid Red G (ARG), 0.01 mol/L FeCl 3 , AC, and 0.1 mol/L H 2 O 2 .The beaker was placed in a constant temperature water bath controlled at 25 ± 1 • C. At each time interval, a 2 mL sample was withdrawn using a 2.5 mL syringe and filtered immediately using the cellulose acetate membrane with a pore diameter of 0.45 µm, and quenched by the addition of methanol.The desired initial pH was adjusted with 0.1 mol/L NaOH and 0.1 mol/L HCl before the reaction.
According to the test data, ARG removal rate (η COLOR %) and COD removal rate (η COD %) were analyzed, and the calculation by Formula (1) and (2) [3,58]: where A 0 and A t are the absorbance of ARG at the initial moment and the time of t, respectively, and COD 0 (mg/L) and COD t (mg/L) are COD concentration in water at the initial time and time t.

Promotion of Fenton-Like Reaction by Different Kinds of ACs
The degradation of ARG by the Fe 3+ /H 2 O 2 system, introducing commercially available coconut shell-activated carbon (CSPAC), wood-activated carbon (WPAC), and coalactivated carbon (CPAC) respectively, was investigated.Through characterization, it was observed that the three kinds of ACs have different pore size distributions, specific surface areas, pore volumes, and average pore size (Figure S1 and Table S1).It can be seen from Figure 1a that ARG in the Fe 3+ /H 2 O 2 system without AC was slowly degraded in the early stage of the reaction, only a decolorization rate of 30% was achieved in the first 10 min, and a decolorization rate of 63% was achieved within 60 min.Nevertheless, when AC was added, the degradation rate of ARG significantly increased in the early stage.Decolorization rates of 84%, 65%, and 57% could be achieved in Fe 3+ /H 2 O 2 /CSPAC, Fe 3+ /H 2 O 2 /WPAC, and Fe 3+ /H 2 O 2 /CPAC systems within 10 min, respectively.The final decolorization rates of 90%, 73%, and 73% could be achieved in the three reaction systems, indicating that the addition of AC can improve the initial decolorization rate of the Fe 3+ /H 2 O 2 system and the final decolorization.At the same time, the removal rate of COD after the addition of AC was also observed and is shown in Figure 1b.The addition of AC also improved the COD removal efficiency in the system to varying degrees.The promoting effect of AC may be due to its adsorption effect, which can improve the removal effect of ARG, and it is worth noting that AC may also interact with Fe 3+ to promote the formation of Fe 2+ to increase the decolorization rate of ARG.To further illustrate this speculation, adsorption experiments were performed on the three kinds of ACs, as well as the determination of Fe 2+ concentration in the Fe 3+ /AC system (Figure 2).It can be seen from Figure 2a that all three kinds of ACs have the ability to adsorb ARG 61%, 21%, and 15% decolorization rates at first 10 min, and 74%, 25%, and 17% decolorization rates within 60 min were achieved in CSPAC, WPAC, and CPAC systems, respectively, by adsorption.It is shown in Figure 2b that the three kinds of ACs were also able to reduce Fe 3+ to Fe 2+ .At 5 min, the concentration of Fe 2+ in the solution including CSPAC, WPAC, and CPAC were 0.05 mmol/L, 0.01 mmol/L, and 0.005 mmol/L, respectively, which indicates that AC can improve the treatment effect of ARG by using its adsorption and reduction properties.By comparison, the adsorption capacity, the ability to reduce Fe 3+ , and the final degradation effect of CSPAC were the best, so CSPAC was chosen as the object of study in the subsequent experiments.To avoid the effect of leaching iron ions on the experimental results, the concentration of iron ions in the solution after the addition of AC treated and untreated by acid solution was studied (Figure S2).The results illustrate that the acid treatment is conducive to avoiding interference caused by iron ion dissolution from AC.
the effect of leaching iron ions on the experimental results, the concentration of iron ions in the solution after the addition of AC treated and untreated by acid solution was studied (Figure S2).The results illustrate that the acid treatment is conducive to avoiding interference caused by iron ion dissolution from AC.

Influencing Factors of Fe 3+ /H2O2/CSPAC System
The different particle sizes and concentrations of CSPAC affect the adsorption and reduction properties of CSPAC, which will have the effect of CSPAC in a Fenton-like reaction.As the oxidant and the source of hydroxyl radical in the reaction, the concentration of H2O2 will affect the efficiency of Fenton and Fenton-like reactions.The different concentrations of Fe 3+ affect the decomposition rate of H2O2, so the concentration of Fe 3+ is also a factor affecting the efficiency of Fenton and Fenton-like reactions.The value of pH is also an important factor in Fenton and Fenton-like reactions.So the effects of particle size and dosages of CSPAC, H2O2 concentration, Fe 3+ concentration, and pH on the degradation of ARG were systematically investigated.
Figure 3 shows the decolorization of ARG, COD removal rate, and Fe 3+ reduction performance in the Fe 3+ /H2O2/CSPAC system under different CSPAC particle sizes.As can be seen in Figure 3a, the decolorization rate of ARG had been accelerated in the early stage  the effect of leaching iron ions on the experimental results, the concentration of iron ions in the solution after the addition of AC treated and untreated by acid solution was studied (Figure S2).The results illustrate that the acid treatment is conducive to avoiding interference caused by iron ion dissolution from AC.

Influencing Factors of Fe 3+ /H2O2/CSPAC System
The different particle sizes and concentrations of CSPAC affect the adsorption and reduction properties of CSPAC, which will have the effect of CSPAC in a Fenton-like reaction.As the oxidant and the source of hydroxyl radical in the reaction, the concentration of H2O2 will affect the efficiency of Fenton and Fenton-like reactions.The different concentrations of Fe 3+ affect the decomposition rate of H2O2, so the concentration of Fe 3+ is also a factor affecting the efficiency of Fenton and Fenton-like reactions.The value of pH is also an important factor in Fenton and Fenton-like reactions.So the effects of particle size and dosages of CSPAC, H2O2 concentration, Fe 3+ concentration, and pH on the degradation of ARG were systematically investigated.
Figure 3 shows the decolorization of ARG, COD removal rate, and Fe 3+ reduction performance in the Fe 3+ /H2O2/CSPAC system under different CSPAC particle sizes.As can be seen in Figure 3a, the decolorization rate of ARG had been accelerated in the early stage

Influencing Factors of Fe 3+ /H 2 O 2 /CSPAC System
The different particle sizes and concentrations of CSPAC affect the adsorption and reduction properties of CSPAC, which will have the effect of CSPAC in a Fenton-like reaction.As the oxidant and the source of hydroxyl radical in the reaction, the concentration of H 2 O 2 will affect the efficiency of Fenton and Fenton-like reactions.The different concentrations of Fe 3+ affect the decomposition rate of H 2 O 2 , so the concentration of Fe 3+ is also a factor affecting the efficiency of Fenton and Fenton-like reactions.The value of pH is also an important factor in Fenton and Fenton-like reactions.So the effects of particle size and dosages of CSPAC, H 2 O 2 concentration, Fe 3+ concentration, and pH on the degradation of ARG were systematically investigated.
Figure 3 shows the decolorization of ARG, COD removal rate, and Fe 3+ reduction performance in the Fe 3+ /H 2 O 2 /CSPAC system under different CSPAC particle sizes.As can be seen in Figure 3a, the decolorization rate of ARG had been accelerated in the early stage than that of the Fe 3+ /H 2 O 2 system with the decrease in particle size, and the overall treatment efficiency was also improved.The COD removal rate also increased with the decrease in CSPAC particle size (Figure 3b).This is because the adsorption performance of CSPAC increases as the particle size decreases, while the reduction performance of Fe 3+ was also increased (Figure 3c).However, when the particle size of CSPAC was too small, the improvement of decolorization rate and COD removal were not obvious, and the ability to reduce ferric ions would be slightly reduced, which may be because the particle size is too small to float on water when stirring, and it is not easy to stir evenly and the reaction is not sufficient.At the same time, the small particle size of activated carbon is not conducive to the settlement after the reaction and will increase the operation cost.Therefore, 75 µm was selected as the best CSPAC particle size for the subsequent study.
than that of the Fe 3+ /H2O2 system with the decrease in particle size, and the overall treatment efficiency was also improved.The COD removal rate also increased with the decrease in CSPAC particle size (Figure 3b).This is because the adsorption performance of CSPAC increases as the particle size decreases, while the reduction performance of Fe 3+ was also increased (Figure 3c).However, when the particle size of CSPAC was too small, the improvement of decolorization rate and COD removal were not obvious, and the ability to reduce ferric ions would be slightly reduced, which may be because the particle size is too small to float on water when stirring, and it is not easy to stir evenly and the reaction is not sufficient.At the same time, the small particle size of activated carbon is not conducive to the settlement after the reaction and will increase the operation cost.Therefore, 75 μm was selected as the best CSPAC particle size for the subsequent study.The decolorization of ARG and COD removal rate in the Fe 3+ /H2O2/CSPAC system at different CSPAC concentrations and the reduction of Fe 3+ at different CSPAC concentrations are depicted in Figure 4.It can be seen that in the range of CSPAC concentration of 0-0.6 g/L, the adsorption of ARG and reduction of Fe 3+ can be promoted by the increase in CSPAC concentration, which further enhanced both the decolorization rate and COD removal rate.When the CSPAC concentration exceeded 0.6 g/L and increased to 0.8 g/L, the COD removal rate decreased.Therefore, 0.6 g/L was chosen as the optimum concentration of CSPAC for the subsequent experiments.The ARG decolorization rate and the COD removal rate in the Fe 3+ /H2O2/CSPAC system at different H2O2 concentrations are depicted in Figure 5.It can be seen that the COD removal rates at different H2O2 concentrations are all high, which is because the adsorption performance of CSPAC can improve COD removal and maintain COD removal even The decolorization of ARG and COD removal rate in the Fe 3+ /H 2 O 2 /CSPAC system at different CSPAC concentrations and the reduction of Fe 3+ at different CSPAC concentrations are depicted in Figure 4.It can be seen that in the range of CSPAC concentration of 0-0.6 g/L, the adsorption of ARG and reduction of Fe 3+ can be promoted by the increase in CSPAC concentration, which further enhanced both the decolorization rate and COD removal rate.When the CSPAC concentration exceeded 0.6 g/L and increased to 0.8 g/L, the COD removal rate decreased.Therefore, 0.6 g/L was chosen as the optimum concentration of CSPAC for the subsequent experiments.
CSPAC increases as the particle size decreases, while the reduction performance of was also increased (Figure 3c).However, when the particle size of CSPAC was too s the improvement of decolorization rate and COD removal were not obvious, and the ity to reduce ferric ions would be slightly reduced, which may be because the particle is too small to float on water when stirring, and it is not easy to stir evenly and the rea is not sufficient.At the same time, the small particle size of activated carbon is not co cive to the settlement after the reaction and will increase the operation cost.Therefor μm was selected as the best CSPAC particle size for the subsequent study.The decolorization of ARG and COD removal rate in the Fe 3+ /H2O2/CSPAC syste different CSPAC concentrations and the reduction of Fe 3+ at different CSPAC conce tions are depicted in Figure 4.It can be seen that in the range of CSPAC concentratio 0-0.6 g/L, the adsorption of ARG and reduction of Fe 3+ can be promoted by the increa CSPAC concentration, which further enhanced both the decolorization rate and COD moval rate.When the CSPAC concentration exceeded 0.6 g/L and increased to 0.8 g/L COD removal rate decreased.Therefore, 0.6 g/L was chosen as the optimum concentra of CSPAC for the subsequent experiments.The ARG decolorization rate and the COD removal rate in the Fe 3+ /H2O2/CSPAC tem at different H2O2 concentrations are depicted in Figure 5.It can be seen that the C removal rates at different H2O2 concentrations are all high, which is because the ads tion performance of CSPAC can improve COD removal and maintain COD removal at very low H2O2 concentrations.However, too low of a H2O2 concentration would reduce the decolorization rate of ARG.When the H2O2 concentration was over 0.4 mmol/L, the decolorization rate of ARG did not increase much when increasing H2O2 concentration.Therefore, 0.4 mmol/L was selected as the optimal H2O2 concentration for the subsequent experiments in terms of the ARG decolorization rate, COD removal rate and economy.The ARG decolorization rate and the COD removal rate in the Fe 3+ /H2O2/CSPAC system at different Fe 3+ concentrations are depicted in Figure 6.As can be seen in Figure 6, after the addition of Fe 3+ , the decolorization rate is improved to a certain extent, but the COD removal rate is decreased, compared with the situation where the adsorption capacity of activated carbon is used to remove pollutants without Fe 3+ , indicating that the addition of Fenton oxidation does not promote the degradation of COD.With the increase in Fe 3+ concentration, the oxidation capacity was enhanced, and the decolorization rate and COD began to increase.When the concentration of Fe 3+ rose to 0.1 mmol/L, the decolorization rate and COD removal rate did not change significantly with the increase in the concentration of Fe 3+ , which may be due to the limitation of hydrogen peroxide.Considering the decolorization rate, COD removal rate, and economic benefits, 0.1 mmol/L was selected as the optimal Fe 3+ concentration for subsequent experiments.The ARG decolorization rate and the COD removal rate in the Fe 3+ /H 2 O 2 /CSPAC system at different Fe 3+ concentrations are depicted in Figure 6.As can be seen in Figure 6, after the addition of Fe 3+ , the decolorization rate is improved to a certain extent, but the COD removal rate is decreased, compared with the situation where the adsorption capacity of activated carbon is used to remove pollutants without Fe 3+ , indicating that the addition of Fenton oxidation does not promote the degradation of COD.With the increase in Fe 3+ concentration, the oxidation capacity was enhanced, and the decolorization rate and COD began to increase.When the concentration of Fe 3+ rose to 0.1 mmol/L, the decolorization rate and COD removal rate did not change significantly with the increase in the concentration of Fe 3+ , which may be due to the limitation of hydrogen peroxide.Considering the decolorization rate, COD removal rate, and economic benefits, 0.1 mmol/L was selected as the optimal Fe 3+ concentration for subsequent experiments.
Catalysts 2022, 12, x FOR PEER REVIEW 7 of 15 at very low H2O2 concentrations.However, too low of a H2O2 concentration would reduce the decolorization rate of ARG.When the H2O2 concentration was over 0.4 mmol/L, the decolorization rate of ARG did not increase much when increasing H2O2 concentration.Therefore, 0.4 mmol/L was selected as the optimal H2O2 concentration for the subsequent experiments in terms of the ARG decolorization rate, COD removal rate and economy.The ARG decolorization rate and the COD removal rate in the Fe 3+ /H2O2/CSPAC system at different Fe 3+ concentrations are depicted in Figure 6.As can be seen in Figure 6, after the addition of Fe 3+ , the decolorization rate is improved to a certain extent, but the COD removal rate is decreased, compared with the situation where the adsorption capacity of activated carbon is used to remove pollutants without Fe 3+ , indicating that the addition of Fenton oxidation does not promote the degradation of COD.With the increase in Fe 3+ concentration, the oxidation capacity was enhanced, and the decolorization rate and COD began to increase.When the concentration of Fe 3+ rose to 0.1 mmol/L, the decolorization rate and COD removal rate did not change significantly with the increase in the concentration of Fe 3+ , which may be due to the limitation of hydrogen peroxide.Considering the decolorization rate, COD removal rate, and economic benefits, 0.1 mmol/L was selected as the optimal Fe 3+ concentration for subsequent experiments.The ARG decolorization rate and the COD removal rate in the Fe 3+ /H 2 O 2 /CSPAC system at different pH conditions are depicted in Figure 7.It can be seen that the decolorization rate and COD removal rate at pH 2.5 are smaller than those at pH 3, because with a lower pH, too much H+ in the solution destroyed the balance between Fe 2+ and Fe 3+ , affecting the generation of the hydroxyl radicals, thereby reducing the oxidation ability of Fenton reaction, and affecting the removal of pollutants by the Fe 3+ /H 2 O 2 /CSPAC system [59,60].When pH > 3, this promoted Fe 2+ and Fe 3+ to convert to hydrogenate precipitation.The oxidation properties of the Fenton reaction will be reduced [59,60], so the decolorization rate will decrease.However, the adsorption capacity of CSPAC is strong, and the COD removal rate remains very high.Because pH > 3 is not conducive to the subsequent study of the Fenton reaction in the system, pH = 3 is finally selected as the best condition for subsequent experiments.The ARG decolorization rate and the COD removal rate in the Fe 3+ /H2O2/CSPAC system at different pH conditions are depicted in Figure 7.It can be seen that the decolorization rate and COD removal rate at pH 2.5 are smaller than those at pH 3, because with a lower pH, too much H+ in the solution destroyed the balance between Fe 2+ and Fe 3+ , affecting the generation of the hydroxyl radicals, thereby reducing the oxidation ability of Fenton reaction, and affecting the removal of pollutants by the Fe 3+ /H2O2/CSPAC system [59,60].When pH > 3, this promoted Fe 2+ and Fe 3+ to convert to hydrogenate precipitation.The oxidation properties of the Fenton reaction will be reduced [59,60], so the decolorization rate will decrease.However, the adsorption capacity of CSPAC is strong, and the COD removal rate remains very high.Because pH > 3 is not conducive to the subsequent study of the Fenton reaction in the system, pH = 3 is finally selected as the best condition for subsequent experiments.

Performance of CSPAC Enhanced Fe 3+ /H2O2 Oxidation
To further understand the oxidation performance enhancement of the Fe 3+ /H2O2 system by the addition of CSPAC, the degradation of ARG in the single CSPAC system, Fe 3+ /CSPAC, H2O2/CSPAC, Fe 3+ /H2O2, and Fe 3+ /H2O2/CSPAC systems were investigated, respectively.As shown in Figure 8a, only about 77% of the decolorization rate was achieved in the Fe 3+ /H2O2 system, but the reaction rate was slow in the early stage.However, the decolorization rate of ARG in the single CSPAC system, Fe 3+ /CSPAC system, and H2O2/CSPAC system could reach 99% after adding CSPAC, which is due to the strong adsorption ability of CSPAC.It can be seen from Figure 8a that the treatment effects of CSPAC, Fe 3+ /CSPAC, and H2O2/CSPAC systems are the same, indicating that adding Fe 3+ and H2O2 systems alone to CSPAC does not promote the removal of pollutants.Since the strong adsorption performance of CSPAC would mask other effects of CSPAC, the decolorization in Fe 3+ /H2O2 and Fe 3+ /H2O2/CSPAC systems was investigated after reducing the adsorption performance of CSPAC.As shown in Figure 8b, it can be seen that a decolorization rate of 61.8% was achieved at the first 5 min in the Fe 3+ /H2O2/CSPAC system, which is much greater than that of the Fe 3+ /H2O2 system (only 26.0%) and single CSPAC adsorption (only 21.7%), indicating that the addition of CSPAC exhibits a certain degree of acceleration in the Fe 3+ /H2O2 system decolorization rate of ARG in the early reaction stage.Then the final decolorization effect is basically the same due to the limited hydrogen peroxide.Additionally, as can be seen in Figure 8, the CSPAC with reduced adsorption properties and untreated CSPAC were added to the Fe 3+ /H2O2 system.Through the contrast of the decolorization curves, it can be seen that untreated CSPAC in the initial stages has a faster decolorization rate and a better decolorization effect.This indicates that the adsorption

Performance of CSPAC Enhanced Fe 3+ /H 2 O 2 Oxidation
To further understand the oxidation performance enhancement of the Fe 3+ /H 2 O 2 system by the addition of CSPAC, the degradation of ARG in the single CSPAC system, Fe 3+ /CSPAC, H 2 O 2 /CSPAC, Fe 3+ /H 2 O 2 , and Fe 3+ /H 2 O 2 /CSPAC systems were investigated, respectively.As shown in Figure 8a, only about 77% of the decolorization rate was achieved in the Fe 3+ /H 2 O 2 system, but the reaction rate was slow in the early stage.However, the decolorization rate of ARG in the single CSPAC system, Fe 3+ /CSPAC system, and H 2 O 2 /CSPAC system could reach 99% after adding CSPAC, which is due to the strong adsorption ability of CSPAC.It can be seen from Figure 8a that the treatment effects of CSPAC, Fe 3+ /CSPAC, and H 2 O 2 /CSPAC systems are the same, indicating that adding Fe 3+ and H 2 O 2 systems alone to CSPAC does not promote the removal of pollutants.Since the strong adsorption performance of CSPAC would mask other effects of CSPAC, the decolorization in Fe 3+ /H 2 O 2 and Fe 3+ /H 2 O 2 /CSPAC systems was investigated after reducing the adsorption performance of CSPAC.As shown in Figure 8b, it can be seen that a decolorization rate of 61.8% was achieved at the first 5 min in the Fe 3+ /H 2 O 2 /CSPAC system, which is much greater than that of the Fe 3+ /H 2 O 2 system (only 26.0%) and single CSPAC adsorption (only 21.7%), indicating that the addition of CSPAC exhibits a certain degree of acceleration in the Fe 3+ /H 2 O 2 system decolorization rate of ARG in the early reaction stage.Then the final decolorization effect is basically the same due to the limited hydrogen peroxide.Additionally, as can be seen in Figure 8, the CSPAC with reduced adsorption properties and untreated CSPAC were added to the Fe 3+ /H 2 O 2 system.Through the contrast of the decolorization curves, it can be seen that untreated CSPAC in the initial stages has a faster decolorization rate and a better decolorization effect.This indicates that the adsorption property of CSPAC can promote the decolorization rate of ARG at the initial stage and can improve the decolorization effect of ARG in the Fe 3+ /H 2 O 2 /CSPAC system.To further study the performance of CSPAC to accelerate the degradation of ARG in the Fe 3+ /H2O2/CSPAC system, Fe 2+ and H2O2 were monitored during the reaction.From Figure 9a, it can be seen that the final concentration of Fe 2+ in the Fe 3+ /H2O2 system was 0.007 mmol/L, and the final concentration of Fe 2+ in the Fe 3+ /CSPAC system increased to 0.076 mmol/L, up to 10 times higher, which illustrates the ability of CSPAC reduce Fe 3+ to Fe 2+ .After the addition of CSPAC to the Fe 3+ /H2O2 system, the concentration of Fe 2+ decreased rapidly compared with the Fe 3+ /CSPAC system due to the rapid reaction between H2O2 and the generated Fe 2+ .To verify this, the different systems were compared in terms of their ability to degrade H2O2.From Figure 9b, only about 8% of H2O2 was consumed in the Fe 3+ /H2O2 system, and about 35% of H2O2 was degraded in the H2O2/CSPAC system, while the H2O2 consumption in the Fe 3+ /H2O2/CSPAC system was significantly increased by more than 95%.It can also be observed that the concentration of Fe 2+ started to increase after 20 min in the Fe 3+ /H2O2/CSPAC system (Figure 9a) when the H2O2 concentration in the system almost reduced to zero (Figure 9b).These results indicate the positive role of activated carbon in Fenton and Fenton-like reactions.To further study the performance of CSPAC to accelerate the degradation of ARG in the Fe 3+ /H 2 O 2 /CSPAC system, Fe 2+ and H 2 O 2 were monitored during the reaction.From Figure 9a, it can be seen that the final concentration of Fe 2+ in the Fe 3+ /H 2 O 2 system was 0.007 mmol/L, and the final concentration of Fe 2+ in the Fe 3+ /CSPAC system increased to 0.076 mmol/L, up to 10 times higher, which illustrates the ability of CSPAC reduce Fe 3+ to Fe 2+ .After the addition of CSPAC to the Fe 3+ /H 2 O 2 system, the concentration of Fe 2+ decreased rapidly compared with the Fe 3+ /CSPAC system due to the rapid reaction between H 2 O 2 and the generated Fe 2+ .To verify this, the different systems were compared in terms of their ability to degrade H 2 O 2 .From Figure 9b, only about 8% of H 2 O 2 was consumed in the Fe 3+ /H 2 O 2 system, and about 35% of H 2 O 2 was degraded in the H 2 O 2 /CSPAC system, while the H 2 O 2 consumption in the Fe 3+ /H 2 O 2 /CSPAC system was significantly increased by more than 95%.It can also be observed that the concentration of Fe 2+ started to increase after 20 min in the Fe 3+ /H 2 O 2 /CSPAC system (Figure 9a) when the H 2 O 2 concentration in the system almost reduced to zero (Figure 9b).These results indicate the positive role of activated carbon in Fenton and Fenton-like reactions.To further study the performance of CSPAC to accelerate the degradation of ARG in the Fe 3+ /H2O2/CSPAC system, Fe 2+ and H2O2 were monitored during the reaction.From Figure 9a, it can be seen that the final concentration of Fe 2+ in the Fe 3+ /H2O2 system was 0.007 mmol/L, and the final concentration of Fe 2+ in the Fe 3+ /CSPAC system increased to 0.076 mmol/L, up to 10 times higher, which illustrates the ability of CSPAC reduce Fe 3+ to Fe 2+ .After the addition of CSPAC to the Fe 3+ /H2O2 system, the concentration of Fe 2+ decreased rapidly compared with the Fe 3+ /CSPAC system due to the rapid reaction between H2O2 and the generated Fe 2+ .To verify this, the different systems were compared in terms of their ability to degrade H2O2.From Figure 9b, only about 8% of H2O2 was consumed in the Fe 3+ /H2O2 system, and about 35% of H2O2 was degraded in the H2O2/CSPAC system, while the H2O2 consumption in the Fe 3+ /H2O2/CSPAC system was significantly increased by more than 95%.It can also be observed that the concentration of Fe 2+ started to increase after 20 min in the Fe 3+ /H2O2/CSPAC system (Figure 9a) when the H2O2 concentration in the system almost reduced to zero (Figure 9b).These results indicate the positive role of activated carbon in Fenton and Fenton-like reactions.

The Catalytic Process of CSPAC during the Reaction
The changes of CSPAC surface groups before and after the reaction of different systems were observed by FT-IR and XPS to reveal the mechanism of the reduction of Fe 3+ by CSPAC during the Fenton reaction.Figure 10 shows the FT-IR spectra of CSPAC before and after the reaction in the Fe 3+ /H 2 O 2 /CSPAC system as well as after the reaction in the Fe 3+ /CSPAC system.The peak appearing near 3400 cm −1 corresponds to the stretching band of the hydroxyl functional group O-H [61], the peak at 1737 cm −1 corresponds to COOH [62], the peak at 1600-1400 cm −1 corresponds to the C=C bond of the aromatic ring in AC structure [63], and the peak at 1088 cm −1 corresponds to the C-O stretching [63,64].The CSPAC before the reaction contains O-H as well as C-O functional groups, while the peak at 1737 cm −1 is very weak and almost absent, while after the reaction, a more obvious peak at 1737 cm −1 appears, which indicates that CSPAC was oxidized and carboxyl groups are produced during the reaction.To further investigate the role of Fe 3+ in the reaction process, the FT-IR spectrum of CSPAC after degradation of ARG in the Fe 3+ /CSPAC system was also tested, and it can be seen that CSPAC also showed a more obvious peak at 1737 cm −1 after the reaction, which indicates that Fe 3+ can oxidize a portion of organic groups on CSPAC surface during the reaction.So a portion of carbons the surface of CSPAC was oxidized to form carboxyl groups by Fe 3+ and thus Fe 3+ was reduced to Fe 2+ at the same time.

The Catalytic Process of CSPAC during the Reaction
The changes of CSPAC surface groups before and after the reaction of different systems were observed by FT-IR and XPS to reveal the mechanism of the reduction of Fe 3+ by CSPAC during the Fenton reaction.Figure 10 shows the FT-IR spectra of CSPAC before and after the reaction in the Fe 3+ /H2O2/CSPAC system as well as after the reaction in the Fe 3+ /CSPAC system.The peak appearing near 3400 cm −1 corresponds to the stretching band of the hydroxyl functional group O-H [61], the peak at 1737 cm −1 corresponds to COOH [62], the peak at 1600-1400 cm −1 corresponds to the C=C bond of the aromatic ring in AC structure [63], and the peak at 1088 cm −1 corresponds to the C-O stretching [63,64].The CSPAC before the reaction contains O-H as well as C-O functional groups, while the peak at 1737 cm −1 is very weak and almost absent, while after the reaction, a more obvious peak at 1737 cm −1 appears, which indicates that CSPAC was oxidized and carboxyl groups are produced during the reaction.To further investigate the role of Fe 3+ in the reaction process, the FT-IR spectrum of CSPAC after degradation of ARG in the Fe 3+ /CSPAC system was also tested, and it can be seen that CSPAC also showed a more obvious peak at 1737 cm −1 after the reaction, which indicates that Fe 3+ can oxidize a portion of organic groups on CSPAC surface during the reaction.So a portion of carbons on the surface of CSPAC was oxidized to form carboxyl groups by Fe 3+ and thus Fe 3+ was reduced to Fe 2+ at the same time.The chemical states of C and O elements in CSPAC were further analyzed using Xray photoelectron spectroscopy.By comparing the full-scan spectra of CSPAC before and after the reaction (Figure S3), the percentage of O 1s increased from 15.8% to 17.4%, while the percentage of C 1s decreased from 84.2% to 82.6% due to the oxidation of the system, and the Fe species did not appear in CSPAC.Additionally, it can be seen from Figure S4 that CSPAC did not adsorb Fe 3+ and Fe 2+ , which indicated that CSPAC had almost no chelation with Fe 3+ or Fe 2+ .Figure 11 shows high-resolution C 1s spectra of CSPAC before and after the reaction, and after the reaction in the Fe 3+ /CSPAC system.The values of 284.0 eV, 284.8 eV, 286 eV, and 288.9 eV, represent C=C, C-C, C-O, and C=O in the presence of carboxylate structure, respectively [65,66]. Figure 12 shows the high-resolution O 1s spectra of CSPAC before and after the reaction, and after the reaction in the Fe 3+ /CSPAC system.The chemical states of C and O elements in CSPAC were further analyzed using X-ray photoelectron spectroscopy.By comparing the full-scan spectra of CSPAC before and after the reaction (Figure S3), the percentage of O 1s increased from 15.8% to 17.4%, while the percentage of C 1s decreased from 84.2% to 82.6% due to the oxidation of the system, and the Fe species did not appear in CSPAC.Additionally, it can be seen from Figure S4 that CSPAC did not adsorb Fe 3+ and Fe 2+ , which indicated that CSPAC had almost no chelation with Fe 3+ or Fe 2+ .Figure 11 shows high-resolution C 1s spectra of CSPAC before and after the reaction, and after the reaction in the Fe 3+ /CSPAC system.The values of 284.0 eV, 284.8 eV, 286 eV, and 288.9 eV, represent C=C, C-C, C-O, and C=O in the presence of carboxylate structure, respectively [65,66]. Figure 12 shows the high-resolution O 1s spectra of CSPAC before and after the reaction, and after the reaction in the Fe 3+ /CSPAC system.The values of 532.3 eV, 533.5 eV, and 534.7 eV represent O-H in the hydroxyl group, C-O, and C=O in the carboxyl group, respectively [65].The results illustrate that the surface of CSPAC before the reaction contains C-O as well as O-H functional groups, which is consistent with the FT-IR characterization, and the presence of C=O in the carboxyl structure after the reaction indicates that CSPAC was oxidized after the reaction, which further indicates that CSPAC can reduce Fe 3+ to Fe 2+ while the reducing functional groups on the surface of CSPAC were oxidized to generate carboxyl groups.The values of 532.3 eV, 533.5 eV, and 534.7 eV represent O-H in the hydroxyl group, C-O, and C=O in the carboxyl group, respectively [65].The results illustrate that the surface of CSPAC before reaction contains C-O as well as O-H functional groups, which is consistent with the FT-IR characterization, and the presence of C=O in the carboxyl structure after the reaction indicates that CSPAC was oxidized after the reaction, which further indicates that CSPAC can reduce Fe 3+ to Fe 2+ while the reducing functional groups on the surface of CSPAC were oxidized to generate carboxyl groups.Through the above characterization, it was observed that CSPAC has an O-H functional group, and it is speculated that the C-O-H group on the surface of CSPAC transfers electrons to Fe 3+ and is oxidized to form the carboxyl group.To further prove that it was the C-O-H functional group on the surface of CSPAC that reduced Fe 3+ , CSPAC was treated with 1 mol/L NaOH at 60 °C to increase the O-H on its surface.The reduction ability of CSPAC was compared with that of untreated CSPAC, and the results are shown in Figure 13.It can be seen from FT-IR spectra that, after NaOH treatment, the O-H content on CSPAC increased, which proved that O-H functional groups were successfully added on the surface of NaOH-CSPAC.Then, the Fe 3+ reduction ability of the two CSPACs with or without NaOH treatment was compared, and the results are shown in Figure 13b.NaOH-CSPAC can generate 0.086 mmol/L of Fe 2+ , which is higher than that of untreated CSPAC, 0.076mmol/L, indicating that the C-O-H group on the surface of CSPAC can reduce Fe 3+ to Fe 2+ and itself was oxidized into the carboxyl group.The values of 532.3 eV, 533.5 eV, and 534.7 eV represent O-H in the hydroxyl group, C-O, and C=O in the carboxyl group, respectively [65].The results illustrate that the surface of CSPAC before the reaction contains C-O as well as O-H functional groups, which is consistent with the FT-IR characterization, and the presence of C=O in the carboxyl structure after the reaction indicates that CSPAC was oxidized after the reaction, which further indicates that CSPAC can reduce Fe 3+ to Fe 2+ while the reducing functional groups on the surface of CSPAC were oxidized to generate carboxyl groups.Through the above characterization, it was observed that CSPAC has an O-H functional group, and it is speculated that the C-O-H group on the surface of CSPAC transfers electrons to Fe 3+ and is oxidized to form the carboxyl group.To further prove that it was the C-O-H functional group on the surface of CSPAC that reduced Fe 3+ , CSPAC was treated with 1 mol/L NaOH at 60 °C to increase the O-H on its surface.The reduction ability of CSPAC was compared with that of untreated CSPAC, and the results are shown in Figure 13.It can be seen from FT-IR spectra that, after NaOH treatment, the O-H content on CSPAC increased, which proved that O-H functional groups were successfully added on the surface of NaOH-CSPAC.Then, the Fe 3+ reduction ability of the two CSPACs with or without NaOH treatment was compared, and the results are shown in Figure 13b.NaOH-CSPAC can generate 0.086 mmol/L of Fe 2+ , which is higher than that of untreated CSPAC, 0.076mmol/L, indicating that the C-O-H group on the surface of CSPAC can reduce Fe 3+ to Fe 2+ and itself was oxidized into the carboxyl group.Through the above characterization, it was observed that CSPAC has an O-H functional group, and it is speculated that the C-O-H group on the surface of CSPAC transfers electrons to Fe 3+ and is oxidized to form the carboxyl group.To further prove that it was the C-O-H functional group on the surface of CSPAC that reduced Fe 3+ , CSPAC was treated with 1 mol/L NaOH at 60 • C to increase the O-H on its surface.The reduction ability of CSPAC was compared with that of untreated CSPAC, and the results are shown in Figure 13.It can be seen from FT-IR spectra that, after NaOH treatment, the O-H content on CSPAC increased, which proved that O-H functional groups were successfully added on the surface of NaOH-CSPAC.Then, the Fe 3+ reduction ability of the two CSPACs with or without NaOH treatment was compared, and the results are shown in Figure 13b.NaOH-CSPAC can generate 0.086 mmol/L of Fe 2+ , which is higher than that of untreated CSPAC, 0.076mmol/L, indicating that the C-O-H group on the surface of CSPAC can reduce Fe 3+ to Fe 2+ and itself was oxidized into the carboxyl group.

Figure 4 .
Figure 4. Effect of different CSPAC concentrations on the (a) decolorization rate of ARG, (b) COD removal rate in the Fe 3+ /H 2 O 2 /CSPAC system (0.1 mmol/L FeCl 3 , 0.2 mmol/L H 2 O 2 , 100 mg/L ARG, 0-0.8 g/L CSPAC (75 µm particle size), pH = 3), and (c) generation of Fe 2+ in the Fe 3+ /CSPAC system (0.1 mmol/L FeCl 3 , 0-0.8 g/L CSPAC (75 µm particle size), pH = 3).The ARG decolorization rate and the COD removal rate in the Fe 3+ /H 2 O 2 /CSPAC system at different H 2 O 2 concentrations are depicted in Figure 5.It can be seen that the COD removal rates at different H 2 O 2 concentrations are all high, which is because the adsorption performance of CSPAC can improve COD removal and maintain COD removal even at very low H 2 O 2 concentrations.However, too low of a H 2 O 2 concentration would

Catalysts 2022 ,
12,  x FOR PEER REVIEW 9 of 15 property of CSPAC can promote the decolorization rate of ARG at the initial stage and can improve the decolorization effect of ARG in the Fe 3+ /H2O2/CSPAC system.

Catalysts 2022 ,
12,  x FOR PEER REVIEW 9 of 15 property of CSPAC can promote the decolorization rate of ARG at the initial stage and can improve the decolorization effect of ARG in the Fe 3+ /H2O2/CSPAC system.

Figure 10 .
Figure10.FT-IR spectra of CSPAC before and after the reaction in the Fe 3+ /H2O2/CSPAC system, and after reaction in the Fe 3+ /CSPAC system.

Figure 10 .
Figure 10.FT-IR spectra of CSPAC before and after the reaction in the Fe 3+ /H 2 O 2 /CSPAC system, and after reaction in the Fe 3+ /CSPAC system.

Figure 11 .
Figure 11.High-resolution C 1s spectra in CSPAC (a) before and (b) after the reaction in the Fe 3+ /CSPAC/H2O2 system, and (c) after the reaction in the Fe 3+ /CSPAC system.

Figure 12 .
Figure 12.High-resolution O 1s spectra in CSPAC (a) before and (b) after the reaction in the Fe 3+ /CSPAC/H2O2 system, and (c) after the reaction in the Fe 3+ /CSPAC system.

Figure 11 .
Figure 11.High-resolution C 1s spectra in CSPAC (a) before and (b) after the reaction in the Fe 3+ /CSPAC/H 2 O 2 system, and (c) after the reaction in the Fe 3+ /CSPAC system.

Figure 11 .
Figure 11.High-resolution C 1s spectra in CSPAC (a) before and (b) after the reaction in the Fe 3+ /CSPAC/H2O2 system, and (c) after the reaction in the Fe 3+ /CSPAC system.

Figure 12 .
Figure 12.High-resolution O 1s spectra in CSPAC (a) before and (b) after the reaction in the Fe 3+ /CSPAC/H2O2 system, and (c) after the reaction in the Fe 3+ /CSPAC system.

Figure 12 .
Figure 12.High-resolution O 1s spectra in CSPAC (a) before and (b) after the reaction in the Fe 3+ /CSPAC/H 2 O 2 system, and (c) after the reaction in the Fe 3+ /CSPAC system.