Assessment of an MnCe-GAC Treatment Process for Tetramethylammonium-Contaminated Wastewater from Optoelectronic Industries

Featured Application: The process proposed in this study could be an e ﬀ ective method for the treatment of high-tech industrial wastewater to meet the new tetramethylammonium hydroxide (TMAH) discharge limit. Abstract: Nitrogen-containing wastewater is an important issue in optoelectronic and semiconductor industries. Wastewater containing nitrogen compounds such as ammonium, monoethanolamine (MEA), and tetramethylammonium hydroxide (TMAH) must be properly treated due to concerns about health and environmental e ﬀ ects. MnCe-GAC (granular activated carbon) processes were developed in this study for the treatment of TMAH-contaminated wastewater in high-tech industries. The MnCe-GAC processes could e ﬀ ectively remove ammonium, MEA, and TMAH from aqueous solutions. The removal e ﬃ ciencies of ammonium and MEA by these processes were better than observed for TMAH. Parameters a ﬀ ecting TMAH removal such as type of process, type of wastewater (synthetic or real), pH, salts, and t-butanol were investigated. In general, removal e ﬃ ciencies of TMAH by various processes were in the following order: MnCe-GAC / O 3 / H 2 O 2 > MnCe-GAC / O 3 > MnCe-GAC / H 2 O 2 > MnCe-GAC > GAC. The negative e ﬀ ect of sulfate and nitrate on pollutant removal might be due to the salting-out e ﬀ ect. Based on t-butanol experiments, the main degradation mechanisms of TMAH by the MnCe-GAC / O 3 / H 2 O 2 process likely involved hydroxyl radicals. The process proposed in this study could be an e ﬀ ective alternative method for the treatment of high-tech industrial wastewater to meet the new TMAH discharge limit.


Introduction
Nitrogen-containing wastewater is an important issue in high-tech industries such as optoelectronic and semiconductor manufacturing industries. For example, in thin-film transistor liquid crystal display (TFT-LCD) manufacturing processes, wastewater containing high concentrations of nitrogen compounds (such as ammonium (NH 4 + ), monoethanolamine (MEA), or tetramethylammonium hydroxide (TMAH)) is generated [1,2]. TMAH is a caustic developing fluid, widely used in the manufacture of TFT-LCD and light emitting diodes (TFT-LED) and in semiconductor industries as a developer or etchant [2][3][4]. For example, a TFT-LCD factory (sixth generation) could generate 30,000 cubic meter per day (CMD) of TMAH-containing wastewater [5]. In Japan, one factory could

Materials and Methods
Samples of wastewaters discharged from four optoelectronic (Plants 1-4) and six semiconductor (Plants 5-10) plants in Central Taiwan Science Park, Taiwan, were collected and analyzed in this research, and their temperature, conductivity, suspended solids (SS), chemical oxygen demand (COD), biological oxygen demand (BOD), nitrate, sulfate, ammonium, and TMAH are summarized in Table 1. Since salts might be present in wastewaters, the effect of three model salts (including sulfate, nitrate, and carbonate) on the processes was investigated. Both synthetic (ammonium, MEA, or TMAH) and real wastewater (Plant 3) samples were treated by the above processes. A known amount of solid (1.0 g of GAC or MnCe-GAC) was packed in a column (internal diameter 1.1 cm, column height 3.9 cm, flow rate 100 mL/min). A vessel (500 mL) was filled with known concentrations of a target solution (ammonium, MEA, or TMAH) with pH adjusted to the desired values (3, 7, or 11), and the solution was circulated through the column as indicated in Figure 1. A known amount of O 3 and/or H 2 O 2 was added into the system as the process required. Samples were collected, filtered, and analyzed in triplicate within acceptable analytical error (±5%). TMAH, monomethylamine (MH, CH 6 ClN), dimethylamine (DH, C 2 H 8 ClN), and trimethylamine (TH, C 3 H 10 ClN) concentrations were analyzed by Dionex ICS-1100 ion chromatography system with Ion Pac CS19A IC columns and Dionex CERSTM 300.  MnCe-GAC was prepared by impregnating Mn and Ce oxides onto GAC (F400, Calgon Carbon Corporation) surfaces. For example, known amounts of Mn(NO3)2·4H2O and Ce(NO3)3·6H2O solids (molar ratio of Mn:Ce = 6:4) were dissolved in deionized water. A known amount of GAC was immersed in the above solution and sonicated for 30 min before being dried in an oven at 90 °C for 12 h. The resulting mixture was heated at 200 °C for 120 min and then washed several times with deionized water to remove detachable Mn and Ce oxides. The resulting composite (MnCe-GAC) was dried at 105 °C and stored at room temperature until needed. Apparent bulk density, particle density, and pore volume fraction of GAC were 0.54 kg/L, 0.78 kg/L, and 0.82 kg/L, respectively. Prior to being used in this study, GAC was sieved so that the particle size was between 0.425 and 0.600 mm, washed several times with deionized water, and baked in an oven at 200 °C .The BET-pecific surface areas and surface morphology of MnCe-GAC and GAC were examined by ASAP 2010 accelerated surface area and porosimetry system (Micromeritics, Norcross, GA, USA) and field emission scanning electron microscope (FE-SEM) (SEM, ABT-150S, Topcon, Japan), respectively.
The amount of Mn and Ce impregnated on MnCe-GAC was measured by extracting MnCe-GAC in a HNO3 (10%) solution for 3 h at 180 °C. Total Mn or Ce in the extracted solution was measured by inductively coupled plasma (ICP-AES, Kontron, S-35). The amount of Mn and Ce on the spent MnCe-GAC (after treatment) was measured as well. The loss of Mn and Ce on MnCe-GAC after the treatment processes was determined by comparing the amount of Mn and Ce between newly prepared MnCe-GAC and spent MnCe-GAC. All labware were acid washed and thoroughly rinsed with deionized water. Chemicals used in this study were reagent grade, unless specified. Hydrogen peroxide (35%) was purchased from Sigma-Aldrich Chemical Co. H2O2 was added into the reactor by a syringe pump at a constant feed rate. Ozone was produced by the ozone generator OW-K2/O (Taiwan) and was introduced into the reactor via a fine bubble diffusor. A standard iodometric titration method was used to determine the concentration of ozone [21]. MnCe-GAC was prepared by impregnating Mn and Ce oxides onto GAC (F400, Calgon Carbon Corporation) surfaces. For example, known amounts of Mn(NO 3 ) 2 ·4H 2 O and Ce(NO 3 ) 3 ·6H 2 O solids (molar ratio of Mn:Ce = 6:4) were dissolved in deionized water. A known amount of GAC was immersed in the above solution and sonicated for 30 min before being dried in an oven at 90 • C for 12 h. The resulting mixture was heated at 200 • C for 120 min and then washed several times with deionized water to remove detachable Mn and Ce oxides. The resulting composite (MnCe-GAC) was dried at 105 • C and stored at room temperature until needed. Apparent bulk density, particle density, and pore volume fraction of GAC were 0.54 kg/L, 0.78 kg/L, and 0.82 kg/L, respectively. Prior to being used in this study, GAC was sieved so that the particle size was between 0.425 and 0.600 mm, washed several times with deionized water, and baked in an oven at 200 • C. The BET-pecific surface areas and surface morphology of MnCe-GAC and GAC were examined by ASAP 2010 accelerated surface area and porosimetry system (Micromeritics, Norcross, GA, USA) and field emission scanning electron microscope (FE-SEM) (SEM, ABT-150S, Topcon, Japan), respectively.
The amount of Mn and Ce impregnated on MnCe-GAC was measured by extracting MnCe-GAC in a HNO 3 (10%) solution for 3 h at 180 • C. Total Mn or Ce in the extracted solution was measured by inductively coupled plasma (ICP-AES, Kontron, S-35). The amount of Mn and Ce on the spent MnCe-GAC (after treatment) was measured as well. The loss of Mn and Ce on MnCe-GAC after the treatment processes was determined by comparing the amount of Mn and Ce between newly prepared MnCe-GAC and spent MnCe-GAC. All labware were acid washed and thoroughly rinsed with deionized water. Chemicals used in this study were reagent grade, unless specified. Hydrogen peroxide (35%) was purchased from Sigma-Aldrich Chemical Co. H 2 O 2 was added into the reactor by a syringe pump at a constant feed rate. Ozone was produced by the ozone generator OW-K2/O (Taiwan) and was introduced into the reactor via a fine bubble diffusor. A standard iodometric titration method was used to determine the concentration of ozone [21].

Characteristics of Wastewater Composition from Photoelectric and Semiconductor Factories
Wastewaters from four photoelectric (Plants1-4) and six semiconductor (Plants 5-10) plants were shown in Table 1. Most of these plants (except Plant 10) had higher TMAH concentrations than the discharge limit (30 mg/L).

Characteristics of MnCe-GAC
The BET-specific surface areas for GAC and MnCe-GAC were approximately 811 and 590 m 2 /g, respectively. The lower surface area of MnCe-GAC indicated that some of the pores of GAC might be blocked by the impregnation of Mn and Ce on the GAC surface. Surface characteristics of GAC and MnCe-GAC were further examined by SEM as shown in Figure 2a and b, respectively. The amount of Mn and Ce on MnCe-GAC surfaces was approximately 65.1 mg Mn/g (SD = 1.5 mg Mn/g) and 27.0 mg Ce/g (SD = 1.1 mg Ce/g), respectively. The loss of Mn and Ce on MnCe-GAC was determined by comparing the amount of Mn and Ce on MnCe-GAC before and after the treatment processes. In these tests, the total loss of Mn and Ce was less than 6%. This result indicated that MnCe-GAC has the potential to be reused.

Characteristics of Wastewater Composition from Photoelectric and Semiconductor Factories
Wastewaters from four photoelectric (Plants1-4) and six semiconductor (Plants 5-10) plants were shown in Table 1. Most of these plants (except Plant 10) had higher TMAH concentrations than the discharge limit (30 mg/L).

Characteristics of MnCe-GAC
The BET-specific surface areas for GAC and MnCe-GAC were approximately 811 and 590 m 2 /g, respectively. The lower surface area of MnCe-GAC indicated that some of the pores of GAC might be blocked by the impregnation of Mn and Ce on the GAC surface. Surface characteristics of GAC and MnCe-GAC were further examined by SEM as shown in Figure 2a and b, respectively. The amount of Mn and Ce on MnCe-GAC surfaces was approximately 65.1 mg Mn/g (SD = 1.5 mg Mn/g) and 27.0 mg Ce/g (SD = 1.1 mg Ce/g), respectively. The loss of Mn and Ce on MnCe-GAC was determined by comparing the amount of Mn and Ce on MnCe-GAC before and after the treatment processes. In these tests, the total loss of Mn and Ce was less than 6%. This result indicated that MnCe-GAC has the potential to be reused.

Removal of Ammonium, MEA, and TMAH by Various Processes (Synthetic Wastewater)
Removal results for ammonium, MEA, and TMAH by the processes examined in this study (GAC, Figure 3 (pH 11, C o = 100 mg/L, V = 0.5 L, solid 1g, 0.25 mmol O 3 /min, 0.125 mmol H 2 O 2 /min, 30 min). In general, the removal efficiencies of ammonium and MEA by these processes were better than that of TMAH. For example, the removal efficiencies of ammonium, MEA, and TMAH by MnCe-GAC/O 3 /H 2 O 2 were 92.3, 71.6, and 42.0%, respectively. This result is in agreement with Lei et al. [11]. They reported that the degradation rate of MEA was significantly higher than that of TMAH. In addition, the ozone processes (O 3 , GAC/O 3 , and MnCe-GAC/O 3 ) had slightly better removal efficiencies (40,50, and 64% for ammonium removal, respectively) than the H 2 O 2 processes (H 2 O 2 , GAC/H 2 O 2 , and MnCe-GAC/H 2 O 2 ; 28, 38, and 61% ammonium removal, respectively) ( Figure 3). The removal of ammonium and MEA by the MnCe-GAC/O 3 /H 2 O 2 process was slightly better than that observed for the GAC/O 3 /H 2 O 2 process. However, the removal of TMAH by the MnCe-GAC/O 3 /H 2 O 2 process (42%) was significantly greater than that observed for the GAC/O 3 /H 2 O 2 process (29%) (Figure 3). The addition of H 2 O 2 could improve the removal of pollutants in wastewater in ozonation processes and it might be due to the formation of •OH radicals by the conjugated base of H 2 O 2 with ozone [16,[22][23][24][25]. In addition, an overdose of H 2 O 2 might act as a scavenger and react with •OH instead of ozone. Therefore, the overall removal efficiency might be hindered by the overdose of H 2 O 2 . Therefore, the ratio of H 2 O 2 /O 3 applied in the process is important. The stoichiometric molar ratio of H 2 O 2 /O 3 being 1/2 [16] was applied in the study to ensure high efficiency of the treatment processes. As shown in Figure 3, MnCe-GAC/O 3 /H 2 O 2 had the highest removal efficiencies among these processes for ammonium, MEA, and TMAH removal. This result indicated that the removal mechanism of the process is a combination of adsorption and degradation via catalysis/oxidation. The addition of H 2 O 2 could improve the removal of pollutants in wastewater in ozonation processes. Therefore, this process was used in the subsequent experiments.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 16 efficiency might be hindered by the overdose of H2O2. Therefore, the ratio of H2O2/O3 applied in the process is important. The stoichiometric molar ratio of H2O2/O3 being 1/2 [16] was applied in the study to ensure high efficiency of the treatment processes. As shown in Figure 3, MnCe-GAC/O3/H2O2 had the highest removal efficiencies among these processes for ammonium, MEA, and TMAH removal. This result indicated that the removal mechanism of the process is a combination of adsorption and degradation via catalysis/oxidation. The addition of H2O2 could improve the removal of pollutants in wastewater in ozonation processes. Therefore, this process was used in the subsequent experiments. The solution pH played a significant role in these processes. A higher solution pH (pH 11) had a better ammonium removal than those of pH 3 and pH 7 (Figure 4). For example, the removal of ammonium at pH 3, 7, and 11 by the MnCe-GAC/O3/H2O2 process were 34, 61, and 92%, respectively (Co = 100 mg/L, V = 0.5 L, solid 1g, 0.25 mmol O3/min, 0.125 mmol H2O2/min, 30 min). Similar results The solution pH played a significant role in these processes. A higher solution pH (pH 11) had a better ammonium removal than those of pH 3 and pH 7 (Figure 4). For example, the removal of ammonium at pH 3, 7, and 11 by the MnCe-GAC/O 3 /H 2 O 2 process were 34, 61, and 92%, respectively (C o = 100 mg/L, V = 0.5 L, solid 1g, 0.25 mmol O 3 /min, 0.125 mmol H 2 O 2 /min, 30 min). Similar results were obtained for the removal of TMAH by these processes (Figure 5). The solution pH had a significant impact on the removal of TMAH. In general, the order of removal efficiencies of TMAH is as follows: pH 11 > pH 7 > pH 3. At high pH values, ozone decomposes into nonselective hydroxyl radicals which oxidize organic pollutants. Similar pH effect was observed in the findings reported by Lu et al. [26] and the authors investigated using activated carbon/silver catalytic combined with ozone and hydrogen peroxide for the removal of TMAH. On the other hand, under acidic or neutral conditions, the direct attack on organic pollutants by molecular ozone occurs. In general, hydroxyl radicals could destroy organic compounds more effectively than ozone [16,27].
Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 16 were obtained for the removal of TMAH by these processes ( Figure 5). The solution pH had a significant impact on the removal of TMAH. In general, the order of removal efficiencies of TMAH is as follows: pH 11 > pH 7 > pH 3. At high pH values, ozone decomposes into nonselective hydroxyl radicals which oxidize organic pollutants. Similar pH effect was observed in the findings reported by Lu et al. [26] and the authors investigated using activated carbon/silver catalytic combined with ozone and hydrogen peroxide for the removal of TMAH. On the other hand, under acidic or neutral conditions, the direct attack on organic pollutants by molecular ozone occurs. In general, hydroxyl radicals could destroy organic compounds more effectively than ozone [16,27].  Since significant amounts of salts might be present in the wastewater, the effect of these salts on the processes has to be investigated in order to be able to apply the processes effectively in practice. Three types of salts such as sulfates, nitrates, and carbonates were chosen in this study as model compounds. As shown in Figure 6, these salts had a negative effect ( Figure 6) on the removal of these Since significant amounts of salts might be present in the wastewater, the effect of these salts on the processes has to be investigated in order to be able to apply the processes effectively in practice. Three types of salts such as sulfates, nitrates, and carbonates were chosen in this study as model compounds. As shown in Figure 6, these salts had a negative effect ( Figure 6) on the removal of these pollutants at the tested pH values (3, 7, and 11). For example, ammonium removal decreased from 61 to 33, 45, and 36% due to the presence of sulfates (100 mg/L), nitrates (500 mg/L), and carbonates (500 mg/L) in solution, respectively ( Figure 6). TMAH removal decreased from 80 to 46, 41, and 46% in aqueous solutions containing sulfates (100 mg/L), nitrates (250 mg/L), and carbonates (250 mg/L), respectively (Figure 7). Salts can affect the ozone oxidation process in several ways. For example, the presence of a salt generally lowers the solubility of a gas (salting out). A change in ozone solubility can be observed in the salt-containing solutions [28,29]. Rischbieter et al. [28] reported that sulfates and nitrates could reduce the solubility of ozone significantly with Henry constants about 15.3 and 12.1 kPa m 3 mol −1 , respectively (salt concentration 0.5 kmol m −3 ). Salts can also influence the size distribution of gas bubbles in a bubble column. They might inhibit bubble coalescence, resulting in a lower average bubble diameter. As a result, mass transfer rates might increase [30,31]. Carbonate ions are well-known species for scavenging the radicals involved in ozone decomposition [28,29,32]. On the other hand, sulfate, phosphate, nitrate, and chloride ions are extremely slow in reacting with •OH. Their scavenging effect can generally be neglected [33]. Therefore, the negative effect of sulfates and nitrates on pollutants removal might due to the salting-out effect. pollutants at the tested pH values (3, 7, and 11). For example, ammonium removal decreased from 61 to 33, 45, and 36% due to the presence of sulfates (100 mg/L), nitrates (500 mg/L), and carbonates (500 mg/L) in solution, respectively ( Figure 6). TMAH removal decreased from 80 to 46, 41, and 46% in aqueous solutions containing sulfates (100 mg/L), nitrates (250 mg/L), and carbonates (250 mg/L), respectively (Figure 7). Salts can affect the ozone oxidation process in several ways. For example, the presence of a salt generally lowers the solubility of a gas (salting out). A change in ozone solubility can be observed in the salt-containing solutions [28,29]. Rischbieter et al. [28] reported that sulfates and nitrates could reduce the solubility of ozone significantly with Henry constants about 15.3 and 12.1 kPa m 3 mol −1 , respectively (salt concentration 0.5 kmol m −3 ). Salts can also influence the size distribution of gas bubbles in a bubble column. They might inhibit bubble coalescence, resulting in a lower average bubble diameter. As a result, mass transfer rates might increase [30,31]. Carbonate ions are well-known species for scavenging the radicals involved in ozone decomposition [28,29,32]. On the other hand, sulfate, phosphate, nitrate, and chloride ions are extremely slow in reacting with •OH. Their scavenging effect can generally be neglected [33]. Therefore, the negative effect of sulfates and nitrates on pollutants removal might due to the salting-out effect.  This study proposed that MnCe-GAC could enhance the formation of free radicals (such as hydroxyl radicals) in the MnCe-GAC/O3/H2O2 process and subsequent oxidation of adsorbed pollutants [34,35]. This assumption was tested by the addition of t-butanol. t-Butanol has a high rate constant with •OH as shown in Equation (1) t-Butanol could be used as a hydroxyl radical scavenger [37] and served as an •OH-scavenger in this study. If the removal of pollutants were significantly hindered by the addition of t-butanol, it indicated that the hydroxyl radical reaction might play an important role in the removal processes. The effect of t-butanol on the removal of ammonium and TMAH by the MnCe-GAC/O3/H2O2 process is shown in Figure 8a and b, respectively. As indicated in the figure, the removal of ammonium as well as TMAH was significantly reduced due to the presence of t-butanol. For examples, the removal of ammonium decreased from 92 to 46% and the removal of TMAH decreased from 80 to 51%. The removal efficiencies of ammonium and TMAH were reduced to 46 and 29%, respectively, due to the presence of t-butanol. In agreement with the findings of Wang et al. [38], this result indicated that the hydroxyl radical reaction might play an important role in the removal processes. Kim and Choi [39]  This study proposed that MnCe-GAC could enhance the formation of free radicals (such as hydroxyl radicals) in the MnCe-GAC/O 3 /H 2 O 2 process and subsequent oxidation of adsorbed pollutants [34,35]. This assumption was tested by the addition of t-butanol. t-Butanol has a high rate constant with •OH as shown in Equation (1) [36].
t-Butanol could be used as a hydroxyl radical scavenger [37] and served as an •OH-scavenger in this study. If the removal of pollutants were significantly hindered by the addition of t-butanol, it indicated that the hydroxyl radical reaction might play an important role in the removal processes. The effect of t-butanol on the removal of ammonium and TMAH by the MnCe-GAC/O 3 /H 2 O 2 process is shown in Figure 8a,b, respectively. As indicated in the figure, the removal of ammonium as well as TMAH was significantly reduced due to the presence of t-butanol. For examples, the removal of ammonium decreased from 92 to 46% and the removal of TMAH decreased from 80 to 51%. The removal efficiencies of ammonium and TMAH were reduced to 46 and 29%, respectively, due to the presence of t-butanol. In agreement with the findings of Wang et al. [38], this result indicated that the hydroxyl radical reaction might play an important role in the removal processes. Kim and Choi [39] reported that •OH could initiate the degradation of TMAH by either abstracting H-atoms or adding hydrocarbon molecules. Chen et al. [40] reported that metal oxide catalysts (such as Mn-Ce-O composites) were efficient catalysts and they possessed many oxidation states to facilitate electron transfer processes in the radical-producing step of wet oxidation.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 10 of 16 reported that •OH could initiate the degradation of TMAH by either abstracting H-atoms or adding hydrocarbon molecules. Chen et al. [40] reported that metal oxide catalysts (such as Mn-Ce-O composites) were efficient catalysts and they possessed many oxidation states to facilitate electron transfer processes in the radical-producing step of wet oxidation.

Real Wastewater
The intermediate products produced during TMAH degradation [12] by the MnCe-GAC/O3/H2O2 process, including NH4 + , MH, DH, and TH are presented in Figure 9. More than 80 and 73% of TMAH were removed in synthetic and real wastewaters, respectively. For example, in real wastewater, the concentrations of NH4 + , MH, DH, TH, and TMAH were under detection limit (ND), 3.38, 1.17, ND, and 26.89 mg/L, respectively (120 min). Both NH4 + and TH were not observed in the aqueous solution. A small amount of MH and DH (less than 3.5 mg/L) was observed in the process. This result indicated that the MnCe-GAC/O3/H2O2 process could remove TMAH efficiently to meet the discharge limit of the Science Park (30 mg/L) for both synthetic and real wastewaters.

Real Wastewater
The intermediate products produced during TMAH degradation [12] by the MnCe-GAC/O 3 /H 2 O 2 process, including NH 4 + , MH, DH, and TH are presented in Figure 9. More than 80 and 73% of TMAH were removed in synthetic and real wastewaters, respectively. For example, in real wastewater, the concentrations of NH 4 + , MH, DH, TH, and TMAH were under detection limit (ND), 3.38, 1.17, ND, and 26.89 mg/L, respectively (120 min). Both NH 4 + and TH were not observed in the aqueous solution.
A small amount of MH and DH (less than 3.5 mg/L) was observed in the process. This result indicated that the MnCe-GAC/O 3 /H 2 O 2 process could remove TMAH efficiently to meet the discharge limit of the Science Park (30 mg/L) for both synthetic and real wastewaters.

Reaction Kinetics
Experimental results indicated that the MnCe-GAC processes could effectively remove ammonium, MEA, and TMAH from aqueous solutions. In general, the removal of ammonium and MEA by these processes were better than that of TMAH as shown in Figure 10. Since the MnCe-GAC/O3/H2O2 process had the highest removal efficiency among these processes, reaction kinetics of the process was investigated. A suitable model could help in future design and implementation of the process. Many sophisticated models such as pore diffusion, homogeneous surface diffusion, and heterogeneous diffusion models have been developed to describe various kinetics processes. However, these models might not be convenient for practical use due to the mathematical complexity of these models [41,42]. Therefore, four simplified models (pseudo-first-order equation or generalized first order equation [41][42][43], pseudo-second-order equation [2], intra-particle diffusion model [2,38,[43][44][45][46], and liquid film diffusion [46]) were applied in this research.

Reaction Kinetics
Experimental results indicated that the MnCe-GAC processes could effectively remove ammonium, MEA, and TMAH from aqueous solutions. In general, the removal of ammonium and MEA by these processes were better than that of TMAH as shown in Figure 10. Since the MnCe-GAC/O 3 /H 2 O 2 process had the highest removal efficiency among these processes, reaction kinetics of the process was investigated. A suitable model could help in future design and implementation of the process. Many sophisticated models such as pore diffusion, homogeneous surface diffusion, and heterogeneous diffusion models have been developed to describe various kinetics processes. However, these models might not be convenient for practical use due to the mathematical complexity of these models [41,42]. Therefore, four simplified models (pseudo-first-order equation or generalized first order equation [41][42][43], pseudo-second-order equation [2], intra-particle diffusion model [2,38,[43][44][45][46], and liquid film diffusion [46]) were applied in this research. Pseudo-first-order equation: , (2) .
In the above equations, qt is the amount of adsorbate adsorbed (mg g −1 ) at time t (min), qe is the maximum adsorption capacity (mg g −1 ), and K1 is the pseudo-first-order rate constant (min −1 ) for the process.
In the above equations, K2 is the pseudo-second-order rate constant (g mg −1 min −1 ). Intra-particle diffusion model (or Weber and Morris' method): In the above equation, Cp is the intercept (mg g −1 ) and Kp is the intra-particle diffusion rate constant (mg min -0.5 g −1 ). Liquid film diffusion model: In the above equation, KL is the liquid film diffusion rate constant (min -1 ) for the adsorption process.
Experimental and simulation results are presented in Figure 11 and Table 2. To evaluate these models, root mean square error (RMSE) was calculated as follows: In the above equation, qexp and qpre are model experimental and predicted values, respectively, and N is the number of data points. Pseudo-first-order equation: In the above equations, q t is the amount of adsorbate adsorbed (mg g −1 ) at time t (min), q e is the maximum adsorption capacity (mg g −1 ), and K 1 is the pseudo-first-order rate constant (min −1 ) for the process.
Pseudo-second-order equation: In the above equations, K 2 is the pseudo-second-order rate constant (g mg −1 min −1 ). Intra-particle diffusion model (or Weber and Morris' method): In the above equation, C p is the intercept (mg g −1 ) and K p is the intra-particle diffusion rate constant (mg min −0.5 g −1 ). Liquid film diffusion model: In the above equation, K L is the liquid film diffusion rate constant (min −1 ) for the adsorption process. Experimental and simulation results are presented in Figure 11 and Table 2. To evaluate these models, root mean square error (RMSE) was calculated as follows: ammonium and MEA probably transported from the bulk of the solution into the solid phase through the liquid film. For the removal of TMAH, it seemed that intra-particle diffusion played an important role in the process. TMAH was first adsorbed onto the MnCe-GAC surface, and then oxidized via catalytic reactions. This result is similar to the results presented by Fan et al. (2006), who reported that the intra-particle diffusion model was the best fitted model for GAC/H2O2 and FeGAC/H2O2 processes for the removal of acid black 24.   In the above equation, q exp and q pre are model experimental and predicted values, respectively, and N is the number of data points.
In general, a model with a smaller RMSE value is a better fit model. According to RMSE results (Table 2), both pseudo-first-order equation and liquid film diffusion models seem to fit the removal process of ammonium and MEA well with smaller RMSE values (1.31 and 0.43 for ammonium and MEA, respectively). This result is similar to the findings reported by Wang et al. [38], who reported that liquid film diffusion was probably the rate limiting step in an MnCe-AC/O 3 process for the removal of humic acid in solution. On the other hand, the intra-particle diffusion model can better describe the removal process of TMAH (with lower RMSE = 0.38). The pseudo-second order model was unable to fit these systems as indicated by a larger RMSE value. Based on the above simulations, liquid film diffusion is probably the rate limiting step for the removal of ammonium and MEA. Both ammonium and MEA probably transported from the bulk of the solution into the solid phase through the liquid film. For the removal of TMAH, it seemed that intra-particle diffusion played an important role in the process. TMAH was first adsorbed onto the MnCe-GAC surface, and then oxidized via catalytic reactions. This result is similar to the results presented by Fan et al. (2006), who reported that the intra-particle diffusion model was the best fitted model for GAC/H 2 O 2 and FeGAC/H 2 O 2 processes for the removal of acid black 24.

Conclusions
An MnCe-GAC process combining O 3 and H 2 O 2 was evaluated in this research for the removal of nitrogen-containing wastewater, typical of high-tech industries. Experimental results indicated that the MnCe-GAC/O 3 /H 2 O 2 process could effectively lower ammonium, MEA, and TMAH concentrations to the desired levels. The presence of salts (carbonates, nitrates, or sulfates) interfered with the removal of these pollutants. Model simulation results indicated that the rate limiting step for the removal of ammonium and MEA is liquid film diffusion. On the other hand, intra-particle diffusion is probably the rate limiting step for the removal of TMAH by the MnCe-GAC/O 3 /H 2 O 2 process. The process proposed in this study could be an effective alternative method for the treatment of high-tech industrial wastewater.