Abstract

A modification method that combines thermal and oxidation treatments was used to improve catalytic activity of activated carbon (AC) which catalyzed ozonation of the aquatic contaminant humic acid (HA). As a consequence of modifying virgin AC, modified AC (ACN2O2N2) had good catalytic performance for ozonation of HA. Apparent first-order rate constants (kapp) were ACN2O2N2 (7.88 × 10−3 s−1) > virgin AC (3.28 × 10−3 s−1). This change was discussed in terms of three factors: textural property, graphitization degree, and surface chemical property. From analysis results, it was deduced that the surface chemical property (the concentration of surface groups) was the main factor that influenced catalytic activity. An increase in the concentration of hydroxyl groups on AC enhanced catalytic activity of AC in ozonation of HA. Effects of phosphate (both a ligand and a strong Lewis base) further confirmed that Lewis acid sites (hydroxyl groups) were the active centers for free radical reaction in catalytic ozonation of AC.

INTRODUCTION

Activated carbon (AC) by itself has been reported to be a catalyst for decomposition of O3 into highly oxidative species such as hydroxyl radicals (·OH) (Kaptijn 1997). Many researchers have studied catalytic ozonation of organics over AC, and it has been proved to achieve a high degradation rate and mineralization degree of organic pollutants together with improved utilization of ozone (Rivera-Utrilla & Sanchez-Polo 2002; Cao et al. 2014; Dai et al. 2014). Therefore, it is significant to study the catalytic mechanism and to enhance catalytic activity of the AC catalyst.

Based on research regarding influences of physical and chemical properties on catalytic ozonation, researchers have proposed that basic groups on AC influence the process of catalytic ozonation (Kasprzyk-Hordern et al. 2003; Sanchez-Polo et al. 2005; Alvarez et al. 2006; Faria et al. 2007). It has been reported that an AC catalyst with a higher pHpzc (pH at the point of zero charge) usually has better performance in ozonation of organics in water (Faria et al. 2006; Valdes & Zaror 2006). One is beneficial to more effective transformations of ozone and some intermediates (such as H2O2) into ·OH. The other one favours the adsorption of anion species of organics in aqueous solution when pH of solution (pHaq) is lower than pHpzc of AC and is higher than the dissociation constant of the organics (pKa).

The surface of AC also has a large number of acidic groups, including carboxyl, lactone, hydroxyl, and carbonyl groups. Cycloaddition reaction and electrophilic attack of ozone on AC can oxidize carbons, which are linked lactone/carbonyl groups, to yield carboxylic acid and H2O2 as by products (Alvarez et al. 2006; Pocostales et al. 2010). The catalytic mechanism of AC hydroxyls on ozonation is not clear yet. Sun et al. (2014) proposed a catalytic mechanism for surface hydroxyls of MnOx/SBA-15, and this mechanism can be taken into consideration in the study of AC hydroxyls. On the basis of general knowledge and extensive research regarding the ozone decomposition in water (Yamamoto et al. 1979; Pi et al. 2005), a possible reaction mechanism can be tentatively proposed. A fraction of dissolved ozone would be transferred from the liquid bulk to the external surface (Sex), and from there to the internal surface (Sin) of the catalyst. The surface of catalyst is mainly in the protonated form when pHaq is lower than pHpzc (Beltran et al. 2006). O3 is bound to this site due to the electrostatic forces or/and hydrogen bonding followed by the initiation of radical chain reaction. Steps of the mechanisms are the following.

Mass transfer of O3:  
formula
(1)
pHpzc > pHaq:  
formula
(2)
Electrostatic forces or/and hydrogen bonding:  
formula
(3)
Formation of radicals:  
formula
(4)
 
formula
(5)

In the present work, we are committed to find a modification method for enhancing the catalytic activity of AC. In an inert atmosphere, some surface groups are thermally less stable and decompose on heat treatment, evolving CO2. However, some surface groups form when carbon is treated with oxygen at temperature of 400 °С (Roop & Meenakshi 2005). Therefore, a modification method that combines thermal and oxidation treatments was used to change the concentration of AC surface groups and to provide the material basis for studying the active center. AC has been shown to have a catalytic effect on removing organics in landfill leachate (Liu et al. 2012; Li et al. 2017). Therefore, humic acid from leachate was selected as a model pollutant.

MATERIALS AND METHODS

Materials and catalysts preparation

Powder humic acid (HA) was extracted from landfill leachate using the method proposed by Thurman & Malcolm (1981). Landfill leachate was collected from a landfill in Sichuan, China. (Its pH was 7.48 and chemical oxygen demand (COD) was 3,104 mg/L.) HA solutions (500 mg/L) were prepared using this HA and deionized water. Solutions were adjusted to pHaq 5 using 0.5 mol/L NaOH solution.

Wood-derived AC was purchased from Xinsen Carbon Industry Co., Ltd, China. ACN2O2N2 was prepared by a three-step method. (1) Initial thermal treatment: dried AC powder was calcined at 1,000 °С for 5 h in a tubular furnace under a N2 current of 100 mL/min. The obtained carbon was denoted as ACN2. (2) Oxidation treatment: ACN2 was calcined in a muffle furnace (exposed to air) at 400 °С for 5 h. The obtained carbon was denoted as ACN2O2. (3) Further thermal treatment: ACN2O2 was calcined at 950 °С for 5 h in a tubular furnace under a N2 current of 100 mL/min. The obtained carbon from this three-step treatment was the final modified AC and was denoted as ACN2O2N2.

Ozonation reaction

Experiments were carried out in a slurry reactor with the carbon powder suspended in 1 L of aqueous medium at 20 °С, with agitation of 150 r/min, oxygen flow of 12 L/h, ozone flow of 50 mg/h, and AC dosage of 0.50 g. Ozone was produced from pure O2 by an ozone generator (Model CFG-1P, Jinan Sankang Co., Ltd, China). The experimental apparatus is shown in Supplementary Figure S1 (available with the online version of this paper). At regular time intervals, samples were withdrawn from the reactor using a dispenser and analyzed for the organics concentration. Samples were filtered by a 0.45 μm filter at the sampling point. The reaction was quenched by Na2S2O3 solution (0.1 mol/L). In addition to ozonation in the presence of catalysts, ozonation alone and adsorption on catalysts were conducted under the same conditions for comparison. All AC samples were done in triplicate, and the values were averaged. Experimental errors less than 5% were taken throughout the whole paper.

Analytical techniques

HA solutions were analyzed of COD, total organic carbon (TOC), and concentration of HA. COD is an index for the amount of reduced organic compounds, and it was analyzed by the fast digestion spectrophotometric method (HJ/T 399-2007, China) using a heater and spectrometer (Model DB200 and D1010, Hach Co., Ltd, USA). The blank was Na2S2O3 solution that was the same concentration as samples. HA concentration was followed by a spectrophotometer (Model 2600A, Unico Co., Ltd, USA) equipped with 1-cm pathlength quartz cells. Moreover, the dimensionless concentration of HA was equal to the ratio of absorbance after and absorbance before reaction. The test for HA concentration is described in detail (supplementary materials). As ozonation proceeds, HA degrades into some carboxylic acids such as oxalic acid, acetic acid, and pyruvate (Qiang et al. 2013; Wang et al. 2013). Mineralization of HA was monitored via TOC removal by a TOC analyzer (Model P/N638-91062-33, Shimadzu Co., Japan).

Catalyst characterization

Specific Brunauer–Emmett–Teller (BET) surface area and pore volume of catalysts were determined from the adsorption–desorption isotherms of N2 at −196 °С using a Builder SSA-420 instrument. Samples were outgassed at 300 °С for 2.5 h in a vacuum environment before measurements. Adsorption isotherms were used to calculate the surface area of each sample using the BET equation. The iodine value and methylene blue value are traditional indicators used to evaluate AC adsorption. These indicators represent the degree of developed pore in AC and the adsorption capacity for typical substances (iodine and methylene blue).

Boehm's titration was used to analyze the concentrations of acidic and basic groups on the AC surface. The three bases used in titration are regarded as approximate probes of acidic groups according to the following scheme: 0.05 N NaHCO3 (carboxyl), 0.05 N Na2CO3 (carboxyl and lactone), 0.05 N NaOH (carboxyl, lactone and hydroxyl), and 0.25 N NaOH (carboxyl, lactone, hydroxyl and carbonyl). HCl neutralization result represents the total concentration of basic groups (Boehm 1966; Vicdic et al. 1997). The pHpzc of AC was determined by the mass titration method (Noh & Schwarz 1990). X-ray photoelectron spectra (XPS) measurements were performed with a Kratos XSAM800 using an Al Kα (1486.6 eV) radiation source. XPS data corresponding to C spectra were fitted using the software CasaXPS.

Raman spectra of carbon have two bands named the D-band (∼1,360 cm−1) and G-band (∼1,580 cm−1). The D-band is identified with defective or disorganized carbon, and the G-band with graphitic carbon. The intensity ratio of the G-to-D-bands (IG/ID) indicates the graphitization degree of AC (Tuinstra & Koenig 1970; Nakamizo et al. 1978; Nemanich & Solin 1979). A Renishaw-RM 2000 micro confocal Raman spectrometer was used in this research.

RESULTS AND DISCUSSION

Catalytic performance

Catalytic ozonation of HA solutions was carried out in the presence of virgin AC, ACN2, ACN2O2, and ACN2O2N2. As seen in Figure 1, virgin AC catalyzed the HA degradation with a faster decrease in COD compared to the single ozonation. However, HA concentration in ozonation catalyzed by virgin AC was significantly higher than that in single ozonation. This was because that ozone slightly oxidized AC and produced new organics (Pocostales et al. 2010). As seen in Supplementary Figures S3 and S4 (available with the online version of this paper), the TOC and absorbance of water increased during ozonation of AC in pure water. This reaction consumed the ozone in water, and thus reduced the degradation rate of HA. Hence, virgin AC was not a sufficient catalyst for HA ozonation.

Figure 1

Catalytic performances of samples: (a) evolution of COD and (b) evolution of HA concentration. (Conditions: 1 L of HA solution, initial pHaq of 5, initial HA concentration of 0.50 g/L, and catalyst dose of 0.50 g.)

Figure 1

Catalytic performances of samples: (a) evolution of COD and (b) evolution of HA concentration. (Conditions: 1 L of HA solution, initial pHaq of 5, initial HA concentration of 0.50 g/L, and catalyst dose of 0.50 g.)

ACN2, which was obtained by treating virgin AC at 1,000 °С in N2, catalyzed ozonation of HA with faster decreases in COD and HA concentration than virgin AC. This was because partial oxidation of ACN2 was much less than that of virgin AC and yielded fewer organics (Figures S3 and S4). The initial thermal treatment removed the free carbon sites and reactive unsaturated carbons (graphitic carbons linked to carbonyl and lactone) (Alvárez et al. 2006; Liu et al. 2010). After oxidizing ACN2 in air at 400 °С, the catalytic activity of obtained ACN2O2 decreased significantly and there was a slower consumption of COD and HA than with ACN2. It corresponded to the more byproducts during ozonation of ACN2O2 in pure water than that of ACN2 (Figures S3 and S4). Accordingly, the further thermal treatment was necessary for ACN2O2. ACN2O2N2, which was obtained from further thermal treatment of ACN2O2, had the best catalytic performance with a faster consumption of COD and HA. Essential reasons for the improved catalytic activity will be discussed in terms of three factors: textural property, graphitization degree, and surface chemical property.

Degradation of an organic compound in ozonation was considered a second-order reaction, and the reaction rate was proportional to the concentrations of organics and ozone. The ozone concentration was constant in the reactor in which ozone was added by continuous bubbling. In this case, the degradation was considered an apparent first-order reaction, and the reaction rate was proportional to TOC (Qin et al. 2015). Accordingly, the kinetic equation can be described by the following equation:  
formula
(6)
where TOC0 and TOC are the concentration indices of organics in aqueous solutions at the beginning of reaction and at t min of reaction. When ln(TOC/TOC0) was plotted versus time, the data fitted a straight line and kapp (apparent first-order rate constant) corresponded to the slope of the line. The kapp and corresponding R-square are listed in Table 1, and kapp of ozonation catalyzed by samples are (in descending sequence): ACN2O2N2 > ACN2 > ACN2O2 ≈ virgin AC.
Table 1

Apparent first-order rate constant (kapp) in TOC evolution and corresponding R-square

Catalysts kapp (×10−3 s−1R2 
None 1.02 ± 0.06 0.98 
Virgin AC 3.28 ± 0.12 0.92 
ACN2 4.83 ± 0.10 0.96 
ACN2O2 3.46 ± 0.12 0.94 
ACN2O2N2 7.88 ± 0.16 0.97 
ACN2O2N2a 7.37 ± 0.14 0.95 
ACN2O2N2b 6.04 ± 0.12 0.96 
Catalysts kapp (×10−3 s−1R2 
None 1.02 ± 0.06 0.98 
Virgin AC 3.28 ± 0.12 0.92 
ACN2 4.83 ± 0.10 0.96 
ACN2O2 3.46 ± 0.12 0.94 
ACN2O2N2 7.88 ± 0.16 0.97 
ACN2O2N2a 7.37 ± 0.14 0.95 
ACN2O2N2b 6.04 ± 0.12 0.96 

aUsed one time.

bUsed three times.

Influences of textural properties

Textural properties of virgin AC and modified AC were obtained from N2 adsorption/desorption analysis and adsorption capacity test. Table 2 summarizes the BET surface area and pore volume of the samples. Compared to virgin AC, ACN2O2N2 had a larger surface area and pore volume, and a smaller pore diameter. This indicates that ACN2O2N2 had more porous structure after the modification which combined thermal and oxidation treatments. Table 2 also shows the adsorption capacities of samples for iodine and methylene blue. After initial thermal treatment, the adsorption capacities of AC decreased, and this might be because a few pores in AC collapsed from the thermal stress. Oxidation of obstructions then regenerated these pores and formed some new pores. Thus, ACN2O2N2 had better adsorption capacities than other samples. HA removal (at pHaq 5) via adsorption for 30 min is also shown in Table 2, and the descending sequence is: ACN2O2 (3.66%) ≈ ACN2O2N2 (3.62%) > virgin AC (3.42%) >ACN2 (2.84%).

Table 2

N2 adsorption/desorption results and adsorption test results of samples

Samples N2 adsorption/desorption
 
Adsorption test
 
SBET
(m2/g) 
Vmeso+macro
(cm3/g) 
Vmicro
(cm3/g) 
Dp
(nm) 
Iodine
(mg/g) 
Methylene blue (mg/g) HA (mg/g) 
Virgin AC 1,184 0.74 0.54 1.79 947 183 34.2 
ACN2 1,170 0.68 0.56 1.75 952 160 28.4 
ACN2O2 1,400 0.81 0.62 1.61 983 194 36.6 
ACN2O2N2 1,399 0.83 0.63 1.62 1,002 188 36.2 
Samples N2 adsorption/desorption
 
Adsorption test
 
SBET
(m2/g) 
Vmeso+macro
(cm3/g) 
Vmicro
(cm3/g) 
Dp
(nm) 
Iodine
(mg/g) 
Methylene blue (mg/g) HA (mg/g) 
Virgin AC 1,184 0.74 0.54 1.79 947 183 34.2 
ACN2 1,170 0.68 0.56 1.75 952 160 28.4 
ACN2O2 1,400 0.81 0.62 1.61 983 194 36.6 
ACN2O2N2 1,399 0.83 0.63 1.62 1,002 188 36.2 

Alvárez et al. (2006) proposed that there was no direct relationship between ozone decomposition and the surface area of AC in catalytic ozonation of AC. Also, Sanchez-Polo et al. (2005) studied the relationship between ozone decomposition and volume of meso- and macropores and proposed that catalytic ozonation was controlled by pore diffusion. In this work, we studied the relationship between catalytic effects and pore character to assess the influences of textural properties of AC on catalytic activity. In Figure 2, TOC removal of solution was plotted against the volume of meso- and macropores. The results show that pore volume had no direct influence on TOC removal, and this is likely because each of the samples had a pore volume large enough to facilitate the diffusion of ozone and organics through the pore network. Based on the pathways and mechanism of catalytic reaction, it can be expected that, if no serious diffusion limitations develop, the ozone can be adsorbed onto AC and produce surface free radicals.

Figure 2

Influences of textural properties on TOC removal: (a) meso- and macropore volumes and (b) micropore volume. (Conditions: 1 L of HA solution, initial pHaq of 5, initial HA concentration of 0.50 g/L, and catalyst dose of 0.50 g.)

Figure 2

Influences of textural properties on TOC removal: (a) meso- and macropore volumes and (b) micropore volume. (Conditions: 1 L of HA solution, initial pHaq of 5, initial HA concentration of 0.50 g/L, and catalyst dose of 0.50 g.)

Influence of graphitization degree

Graphitic carbons possess significant amounts of mobile -electrons. Molecular orbital calculations indicate that the edges of graphite layers have a significant influence on the distribution of -electrons (Dai 2006; Serp & Figueiredo 2009). More chemically reactive zigzag edges induce peripheral electron localization, and less reactive armchair edge sites lead to a more uniform distribution of electron density. For example, physical activation of carbon is the gasification of carbon atoms from a graphite layer; this decreases the sizes of graphite layers and promotes the localization of -electrons.

To assess the influence of graphitization degree on catalytic ozonation, Raman spectra of AC and ACN2O2N2 were tested and are shown in Figure 3. The D and G bands, which are two characteristic peaks of carbon, are clearly seen in each spectrum. The intensity ratio of these two bands (IG/ID) for ACN2O2N2 (0.54) was lower than that for virgin AC (0.85), and this indicates that the graphitization degree decreased after modification. IG/ID of ACN2 (0.51) and ACN2O2 (0.53) were roughly the same as that of ACN2O2N2. Hence, the graphitization degree had no direct influence on the catalytic activity. This may be because there was little change in graphitization degree of the samples, and it was much lower than that of carbon nanotubes (Goncalves et al. 2010).

Figure 3

Raman spectra of virgin AC and ACN2O2N2.

Figure 3

Raman spectra of virgin AC and ACN2O2N2.

Influences of surface chemical properties

The concentration of surface groups on samples was carried out according to XPS and titration method. Table 3 listed the concentrations of surface groups on virgin AC and modified AC, and it can be observed that ACN2O2N2 had more hydroxyl groups and less carbonyl, lactone, carboxyl, and basic groups. As a consequence of the change in acidic/basic groups and graphitization degree, ACN2O2N2 was not changed greatly at pHpzc of ACN2O2N2 as compared to virgin AC. As seen in Table 3, ozonation changes pHpzc and the concentration of surface groups on ACN2O2N2. As stated, ozone reacts with some surface groups and ruptures some structures, and this might generate oxidizing species as reaction by products (Pocostales et al. 2010).

Table 3

Concentrations of surface groups on samples and pHpzc of samples

Samples Carboxyls (μeq/g) Lactones (μeq/g) Hydroxyls (μeq/g) Carbonyls (μeq/g) Total acid (μeq/g) Basic groups (μeq/g) pHPZC 
Virgin AC 76 45 136 99 356 74 5.8 
ACN2 12 10 88 20 90 35 6.7 
ACN2O2 123 47 189 104 463 31 5.1 
ACN2O2N2 48 12 212 51 323 28 6.3 
ACN2O2N2a 29 12 194 95 330 28 6.0 
ACN2O2N2b 56 18 168 104 346 24 5.9 
Samples Carboxyls (μeq/g) Lactones (μeq/g) Hydroxyls (μeq/g) Carbonyls (μeq/g) Total acid (μeq/g) Basic groups (μeq/g) pHPZC 
Virgin AC 76 45 136 99 356 74 5.8 
ACN2 12 10 88 20 90 35 6.7 
ACN2O2 123 47 189 104 463 31 5.1 
ACN2O2N2 48 12 212 51 323 28 6.3 
ACN2O2N2a 29 12 194 95 330 28 6.0 
ACN2O2N2b 56 18 168 104 346 24 5.9 

aUsed one time.

bUsed three times.

To assess the influences of surface chemical properties on catalytic ozonation, each of the following variables was analyzed as predictors: hydroxyl groups, basic groups and other acidic groups content. The kapp for TOC evolution in ozonation was used as a dependent variable to represent the catalytic performance. Then, a model Equation (7) with three variables was eventually obtained by the linear regression fitting procedure.  
formula
(7)
where C-OH and Cbasic are concentrations of hydroxyl groups and basic groups, respectively. Cother is the total concentration of other acidic groups, including carbonyls, lactones and carboxyl groups. The R-square of this model equation is 0.98, and the accuracy of this method was determined by verifying the linear relationship between the experimental values and the calculated ones (Supplementary Figure S5, available with the online version of this paper).

As seen in Figure 4, the XPS C1 s spectrum of virgin AC has five peaks which correspond to the different carbon as follows: 284.6 eV (C = C), 285.6 eV (C-OH), 286.9 eV (C-O-C), 289.5 eV(C(O)O), and 291.5 eV ( satellite peak) (Mane et al. 2017; Yue et al. 1999a, 1999b). XPS C1 s of both virgin AC and ACN2O2N2 are dominated by a signal at ∼284.6 eV which corresponds to the sp2 and/or sp3 hybridized carbon of AC, a single at ∼285.6 eV which corresponds to the carbon of C-OH, and a single at ∼289.5 eV which corresponds to the carbon of C(O)O. (Chu & Li 2006). The deconvoluted spectrum of ACN2O2N2 had more C-OH (40.62%) than that of virgin AC (18.01%), and had no peaks of C-O-C and anymore. XPS spectra and surface compositions of ACN2 and ACN2O2 are shown in Supplementary Figure S6 and Table S1 (available with the online version of this paper). After thermal treatment, ACN2 had more graphitic carbon and a fewer oxygen-containing groups than AC. Subsequent oxidation treatment gave ACN2O2 more oxygen-containing groups. The XPS results are consistent with the results obtained via titration method.

Figure 4

XPS C1 s of (a) AC and (b) ACN2O2N2.

Figure 4

XPS C1 s of (a) AC and (b) ACN2O2N2.

Effect of phosphate

Phosphate, which is a common ligand and strong Lewis base, was used in the ozonation reactions to investigate whether ·OH initiation on catalytic surface was involved. Phosphate can inhibit the adsorption of O3 on catalysts and decrease the generation of surface hydroxyl radicals (Zhao et al. 2013). As seen in Figure 5, the addition of phosphate had little effect on TOC removal in single ozonation. In contrast, catalytic ozonation was suppressed in the presence of phosphate. When 0.01 mol/L phosphate was added into the solution, TOC removal decreased by 18% compared with that in catalytic ozonation without phosphate. With increasing amount of phosphate, TOC removal decreased accordingly. In the presence of 0.05 mol/L phosphate, TOC removal was almost identical with that in single ozonation, which implied that catalytic capacity of ACN2O2N2 was totally inhibited. The protonated form of hydroxyl (pHaq < pHpzc) was the Lewis acid sites of catalyst. Because phosphate covered Lewis acid sites and thus prevented the generation of radical reaction, it was further confirmed that surface hydroxyl groups were the active centers for AC in catalytic ozonation.

Figure 5

Effects of phosphate on TOC removal. (Conditions: 1 L of HA solution, initial pHaq of 5, initial HA concentration of 0.50 g/L, catalyst dose of 0.50 g, and phosphate concentrations of 0.01, 0.03, 0.05 mol/L.)

Figure 5

Effects of phosphate on TOC removal. (Conditions: 1 L of HA solution, initial pHaq of 5, initial HA concentration of 0.50 g/L, catalyst dose of 0.50 g, and phosphate concentrations of 0.01, 0.03, 0.05 mol/L.)

CONCLUSIONS

Thermal stress under high temperature led to the collapse of some pores in AC. Oxidation treatment regenerated these pores and formed some new pores. Thus, the combination of thermal and oxidation treatments increased the surface area and pore volume and increased the adsorption capacity. The combined treatments also led to a decrease in graphitization degree of AC, and the value of IG/ID for virgin AC, ACN2, ACN2O2, and ACN2O2N2 were 0.85, 0.51, 0.53, and 0.54, respectively. Both textural properties and graphitization degree had no direct influences on the degradation rate of HA by ozonation, and this was proved by the correlation analyses.

The combined treatments changed the concentrations of surface groups on AC, and this was proved to be the main reason for the change in catalytic activity. XPS, Boehm titration, and multiple regression analysis were used to study the influences of surface groups on catalytic activity. The results indicate that modification method enhanced the catalytic activity of AC by increasing the amount of hydroxyl groups and decreasing other acidic groups. Effects of phosphate further confirm that surface hydroxyl groups of AC were the active centers of catalytic ozonation in aqueous phase because phosphate prevented the generation of radical reactions by covering the Lewis acid sites.

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Supplementary data