Oxidative degradation of TMAH solution with UV persulfate activation

Chemical Engineering Journal 254 (2014) 472–478

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Oxidative degradation of TMAH solution with UV persulfate activation Chi-Wei Wang, Chenju Liang ⇑ Department of Environmental Engineering, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 402, Taiwan

h i g h l i g h t s 2

 The UV/S2O8

is an effective method for degrading TMAH at acidic condition. 2 dose, UV intensity, and temperature.  The demethylation mechanism is the main TMAH degradation pathway.  TMAH degradation byproducts are identified and transformed to nitrate and ammonium.

 The TMAH degradation is affected by pH, S2O8

a r t i c l e

i n f o

Article history: Received 20 March 2014 Received in revised form 26 May 2014 Accepted 27 May 2014 Available online 4 June 2014 Keywords: TMAH Sodium persulfate Remediation Wastewater treatment Sulfate radical Advanced oxidation process

a b s t r a c t Tetramethylammonium hydroxide (TMAH) is an alkaline, neuronal toxic, and chemically stable compound; furthermore, it is widely used in the high-tech industry as a developing agent. Disposal of TMAH wastewater from an industrial plant is a difficult and costly problem. Ultraviolet light (UV) activated persulfate (S2O2 8 ) is an advanced oxidation process proven to be effective in destroying a variety of organic process to treat pollutants. This bench-scale study investigated the feasibility of using the UV/S2O2 8 TMAH. The effects of various operational parameters, including pH conditions, dosages of persulfate, UV intensities, and system temperatures were evaluated. The results revealed that pH 2 exhibited higher decay rate of TMAH (kobs = 0.0331 ± 0.0031 min1) than other pH conditions (pH 7 and 11) at 20 °C. In general, the TMAH decay increased with increasing persulfate dosage; however, the highest TMAH degradation rate was observed with a persulfate concentration of 50 mM. Also, higher reaction temperature and stronger UV irradiation can increase the degradation of TMAH. TMAH degradation byproducts were identified and finally transformed to nitrate and ammonium, which suggested that the demethylation mechanism was the main degradation pathway. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction In recent years, due to increasing demand for electronic products, the semiconductor and thin-film-transistor liquid-crystal display (TFT-LCD) industries are rapidly growing. Their manufacturing processes involve a variety of highly complex and delicate units, such as photolithography, stripping, etching, and cleaning, etc., and various organic solvents are used for cleaning the wafers and panels. Hence, a large amount of organic solvent containing wastewater is generated [1]. Tetramethylammonium hydroxide (TMAH, (CH3)4NOH) is typically one of several ingredients in commercial etching/stripping mixtures or alkali washing liquid [1,2]. TMAH is a high alkaline, neuronal toxic, and chemically stable compound [3,4]. Shibata et al. [5] reported about 2500 tons of TMAH per year being discharged from a factory in Japan. Moreover, ⇑ Corresponding author. Tel.: +886 4 22856610; fax: +886 4 22862587. E-mail address: [email protected] (C. Liang). http://dx.doi.org/10.1016/j.cej.2014.05.116 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.

Chang et al. [1] also indicated that up to about 30,000 cubic meters of TMAH-containing wastewater can be discharged each day from a six-generation TFT-LCD factory in Taiwan. TMAH concentrations in raw wastewater can be high as about 1000 mg/L, which is suspected of being harmful to aquatic life, and is much greater than the reported median lethal concentration value of 55.6 mg/L/48 h (based on Daphnia magna (water flea) bioassays) [6]. TMAH-containing wastewater can be treated with aerobic biodegradation [7], anaerobic biodegradation [1,4], catalytic oxidation [8], photocatalytic degradation [3], or ion exchange [5] technologies. Among them, biodegradation is the most common process applied for treating TMAH-containing wastewater in the semiconductor and TFT-LCD industries. Anthony [7] and Urakami et al. [9] reported that TMAH could be degraded by some aerobic microbes, such as Methylotroph and Paracoccus, etc. Aerobic TMAH degradation occurs mainly via the demethylation pathway (i.e., resulting in the removal of a methyl group (-CH3) from a molecule). TMAH is initially decomposed to trimethylamine (TMA, (CH3)3N), and

C.-W. Wang, C. Liang / Chemical Engineering Journal 254 (2014) 472–478

then progressively degraded to dimethylamine (DMA, (CH3)2NH), methylamine (MoMA, CH3NH2) and ammonia (NH3) [3,7,9]. Additionally, Chang et al. [1] and Hu et al. [2] reported that TMAH-containing wastewater can be successfully treated by the up-flow anaerobic sludge blanket technique. However, their results indicated that high concentrations of TMAH (>4500 mg/L) [2] or the presence of inhibitory chemicals (e.g., surfactants and sulfate) in wastewater would constrain the microbial growth and restrict biological TMAH degradation. TMAH-containing wastewater alternatively could also be treated by catalytic oxidation technology [8]. This process involves the reaction mechanism of pyrolyzing TMAH to TMA and further decomposing TMA to N2, H2O, and CO2 through a base metal series or Pt series oxidation catalyst. Although 1% TMAH-containing wastewater could be completely decomposed during the process, it seems too expensive to be applied to industrial scale wastewater treatment. It is noteworthy that, ion exchange is also a potential technology [5], which recovers TMAH from the wastewater by three steps, i.e. cation exchange, elution and conversion. However, if the chemical composition of the wastewater is complicated, the functionality of ion exchange resin would be rapidly depleted. Due to TMAH recalcitrant characteristics, TMAH might not be effectively removed by conventional wastewater treatment processes; therefore, a TMAH wastewater treatment alternative is needed. Persulfate anion (S2O2 8 ) is a strong oxidant (e.g., in the salt form of sodium persulfate, SPS) with a redox potential of 2.01 V (Eq. (1)) [10], which is recognized as effective in degrading a variety of organic contaminants present in soil and groundwater [11,12]. Persulfate reaction mechanism is based on the generation of the  strong sulfate radical (SO 4 ) and/or hydroxyl radical (HO ) through various persulfate activations for degrading organic contaminants [13]. Common persulfate activations include heating [11,14–16], transitional metals induced electron transfer [17–19], and alkaline pH adjustment [20–22]. Additionally, UV irradiation as an effective method of water disinfection can be considered as a form of energy for persulfate activation. It has been reported that one mole of persulfate can be activated to generate two mole of sulfate radical under UV irradiation in accordance with Eq. (2) [23–26] for degrading organic contaminants in wastewater. The research group of Dionysiou has reported the promising degradation of the selected emerging organic contaminants (e.g., cyanobaterial toxin cylindrospermopsin, endosulfan, and microcystin-LR) by UV photolysis and advanced oxidation processes (AOPs) (such as UV/TiO2, UV/H2O2,  UV/S2O2 8 , and UV/HSO5 ) [27–29]. More specifically, among UV/ peroxide systems evaluated, UV/S2O2 usually shows the most 8 effective process for the removal of contaminants at either the same molar or mass concentrations of peroxide oxidants. Results of these studies suggest that UV based AOPs exhibits a feasible approach to remove recalcitrant organic contaminants from water resources. Lin et al. [26] found that the UV/S2O2 8 process achieved complete degradation of phenol (0.5 mM) within 20 min with a UV wavelength of 254 nm irradiating a 84 mM S2O2 8 solution. Moreover, Fang and Shang [24] discovered that 100% of the bromine atoms in dibromoacetamide can be converted to bromate by the UV/S2O2 8 process, and Gao et al. [30] demonstrated that the UV/ S2O2 8 process at pH 3, 6.5, and 11 can effectively decompose the sulfamethazine in aqueous phase within 45 min with a UV wavelength of 254 nm and a S2O2 8 concentration of 0.2 mM. 2  S2 O2 8 þ 2e ! 2SO4

UV

 S2 O2 8 ! 2SO4

Eo ¼ 2:01 V

ð1Þ ð2Þ

Based on the oxidative potential of the UV/S2O2 8 process, the objective of this study was to evaluate utilization of UV irradiation to activate S2O2 8 to degrade TMAH in water. Therefore, the effects

473

of solution pHs, dosages of S2O2 8 , UV intensities, and temperatures on the TMAH degradation were explored in this study. Additionally, the reaction intermediates were identified, and the oxidation pathways were proposed. 2. Materials and methods 2.1. Chemicals The water was purified by a Millipore reverse osmosis (RO) purification system. Dimethylamine 2 M in tetrahydrofuran solution ((CH3)2NH) was purchased from Acros; sodium persulfate (Na2S2O8, >99.0%) was purchased from Merck; tetramethylammonium hydroxide 25% w/w aq. solution ((CH3)4NOH, >99.9%), methylamine hydrochloride (CH3NH2HCl, 99% min), and trimethylamine hydrochloride ((CH3)3NHCl, 98% min) were purchased from Alfa Aesar; sulfuric acid (H2SO4, 95% min), potassium hydrogen phthalate (KHC8H4O4, 99.5% min), and 2,6-pyridinedicarboxylic acid (C7H5NO4, 99%) were purchased from Aldrich; acetone (CH3COCH3, 99.5% min), sodium nitrite (NaNO2, 99.7% min), phosphoric acid (H3PO4, 86.5% min), and acetic acid (CH3COOH, >80%) were purchased from J.T. Baker; starch ((C6H10O5)n), sodium carbonate (Na2CO3, 99.5% min), sodium thiosulfate (Na2S2O35H2O, 99.5% min), sodium bicarbonate (NaHCO3, 99.7% min), potassium dichromate (K2Cr2O7, >99.8%), and sodium hydroxide (NaOH, >99%) were purchased from Riedel-dehaën; ammonium chloride (NH4Cl, 98.5% min), sodium nitrate (NaNO3, 99% min), potassium iodide (KI, 99.5% min) were purchased from Union Chemical Works. 2.2. Experimental procedure Experiments were performed in a 3 L reaction flask. The reaction solution (2.5 L) was mixed by a magnetic stirrer to ensure homogeneity during the reaction. A mercury lamp emitting 254 nm monochromatic UV light (8 or 15 W UV irradiance to be 1.3 or 4.5 mW/cm2, respectively, Sparkie GLQ-D287 supplied by Biddy Photronic Co., Taiwan) with quartz tube protection was placed in the center of the reaction flask. In the first phase of experiments, the feasibility of an UV/S2O2 8 process (i.e., 15 W UV/100 mM SPS) in degrading the 1.1 mM TMAH at various pH conditions (i.e., 2, 7, and 11) was investigated. Solution pH was adjusted by a pH controller (pH/ORP controller PC-310, Suntex Instruments), which pumped either 1 N H2SO4 or NaOH solutions; in addition, the solution pH was maintained within a pH unit ±0.2 of the target pH during the course of reaction. At each designated sampling time, 10 mL of aqueous sample was collected using a glass pipette from a sampling port on the flask cover for analyzing TMAH, associated byproducts, SPS, and total organic carbon (TOC). When the optimum pH was determined, which resulted in the highest TMAH degradation rate in the first phase of experiments, further experiments were conducted to investigate the influence of SPS doses (i.e., 10, 50 and 100 mM), UV intensities (i.e., 8 and 15 W), and temperatures (i.e. 10, 20, and 30 °C) on the TMAH degradation rate. The procedure in these experiments was similar to that described earlier. Control tests were conducted in parallel. All tests were performed in duplicate and averaged data were reported. 2.3. Analysis Analysis of TMAH and its ionic intermediates were performed using an ion chromatograph (IC, Metrohm 790 Personal), which was equipped a conductivity detector and separation was done

474

C.-W. Wang, C. Liang / Chemical Engineering Journal 254 (2014) 472–478

with either a Metrosep A Supp 4 (4 mm  250 mm) for measuring anions or a Metrosep C4 (4 mm  250 mm) for measuring cations. The eluent solutions were 1.8 mM Na2CO3/1.7 mM NaHCO3 for anion analysis or 1:9 (v:v) of acetone and 3 mM HNO3 mixture for cation analysis. A TMAH standard curve with a R2 value greater than 0.995 was used for quantitative analysis and the retention time of TMAH appeared at 15.6 min in the IC chromatograph. The persulfate anion was measured by an iodometric titration method with sodium thiosulfate titrant [31]. 1 mL of aqueous sample was removed and filtered by a syringe (a polytetrafluorethylene filter (0.2 lm) placed within a stainless syringe holder (Advantec, KS-13)) and then 20 lL of the filtered solution was injected into a total organic carbon (TOC) analyzer (Aurora 1030 W, O.I. Analytical) for determining remaining TOC concentrations in solution during the course of reaction. A portable pH/ORP meter (Suntex, TS-100) equipped with a pH electrode and a temperature probe was used to monitor the solution pH and temperature in this study. 3. Results and discussion 3.1. Effect of pH The investigation of the effect of varying pH conditions on the degradation of TMAH by the UV/S2O2 process was carried out 8 for pH 2, 7, and 11. Fig. 1 shows the degradation profile of TMAH, TOC and persulfate by the UV/S2O2 process, and the pseudo8 first-order kinetic rate constants (kobs) at different pH levels were 0

30

60

90

Time (min) 30 60 90

120 0

120 0

30

60

90

120

0

-1

ln(C/C0)

-2

-3

pH 2 pH 7 pH 11

-4

-5

(b)

(a) 0

10000 20000 30000 0

(c) 10000 20000 30000 0

10000 20000 30000

2

UV Fluence (mJ/cm ) Fig. 1. The degradation profile of (a) TMAH, (b) TOC and (c) persulfate in the UV/ S2O2 process at different pH conditions. ([TMAH]0 = 1.1 mM, [SPS]0 = 100 mM, 8 UV = 15 W).

determined as presented in Table 1. Persulfate degradation rate constants are also listed in Table 1. Persulfate activated with a fixed UV intensity produced a slightly faster degradation rate at acidic pH than other pHs due to acid-catalyzed persulfate effect [32]. The TMAH degradation reaction was obviously dependent on the pH. The TMAH removal (at 130 min) was 100% at pH 2, while approximately 50% removal was observed as the pH increased to between 7 and 11. The kobs at acidic pH 2 (i.e., 0.0331 ± 0.0031 min1) was 1–2 orders of magnitude faster than those determined at neutral or basic pHs. Additionally, TOC removals at pH 2, 7 and 11 were 70%, 23% and 6%, respectively, which are comparable to the TMAH degradations. However, TOC data exhibited incomplete mineralization upon TMAH degradation and indicated the presence of reaction intermediates during the UV/S2O2 8 oxidation process. Further exploration regarding the TMAH degradation pathway will be elucidated in later section. Nevertheless, it is speculated that increasing the reaction time may achieve complete TOC removal. The results of four sets of control tests, i.e., (a) SPS 100 mM, pH uncontrolled, and no UV; (b) pH controlled at pH 2, 7, and 11, no UV and no SPS; (c) pH controlled at pH 2, 7, and 11, no SPS and 15 W UV; (d) UV 15 W, no SPS and pH uncontrolled, each exhibited generally less than 10% of TMAH variation during the course of 120 h reaction time (see Fig. S1, Supporting Information). Although (CH3)nNH4n+ are present in solution as protonated or deprotonated species according to pH, 95 and 98% are present as (CH3)4N+ at pH 3.45 and 11.05 [3]. Hence, it can be seen that the molecular charge of TMAH is not significantly affected by pH. However, its degradation by UV/S2O2 8 oxidation process is strongly pH dependent. Liang et al. [12,33] demonstrated that upon persulfate activation, sulfate radicals are the predominant radical species at acidic pH while hydroxyl radicals (HO) dominate at basic pH in accordance with Eq. (3). Free radicals react with organic compounds primarily by three mechanisms: hydrogen abstraction, hydrogen addition, and electron transfer [22]. However, SO 4 preferably undergoes hydrogen abstraction and electron transfer and hence more selectively reacts with organics than HO. Based on these results, it appears that a lower degree of TMAH removal at pH 11 may be due to the fact that although HO (E0 = 2.7 V) has a 0  higher redox potential than SO 4 (E = 2.4 V), HO is less prone to TMAH degradation. For the N-containing compounds, HO could initiate degradation in hydrocarbon molecules by the above mentioned three general mechanisms and also attack the lone-pair electron in nitrogenous compounds. However, the electrophilic HO favorably attacks the lone-pair electron on the N-atom, but (CH3)4N+ has no lone-pair electron and thereby exhibited much slower degradation rate by the predominant HO in the UV/S2O2 8 oxidation process at basic pH. In contrast, hydrogen abstraction and electron transfer are the major TMAH degradation paths [34– 36], which are favored by SO 4 attack on the C-H bond at acidic pH.

Table 1 Pseudo-first-order degradation rate constants of TMAH, TOC and persulfate at different operating conditions. Objective

pH

SPS (mM)

Temp. (°C)

UV (W)

kobs,TMAH (R2) (102 min1)

kobs,TOC (R2) (102 min1)

kobs,PS (R2) (102 min1)

kobs,TOC/kobs,TMAH

pH effect

2 7 11 2

100

20

15

20

15

20 10 20 30

8 15 15

3.31 ± 0.31 (0.99) 0.47 ± 0.01 (0.95) 0.50 ± 0.01 (0.93) 3.25 ± 0.10 (1.00) 3.89 ± 0.43 (0.98) 3.31 ± 0.31 (0.99) 1.77 ± 0.24 (0.99) 3.89 ± 0.43 (0.98) 2.02 ± 0.11 (0.99) 3.89 ± 0.43 (0.98) 6.05 ± 0.99 (1.00)

0.88 ± 0.02 0.20 ± 0.01 – 0.74 ± 0.02 1.06 ± 0.05 0.88 ± 0.02 0.49 ± 0.04 1.06 ± 0.05 0.49 ± 0.05 1.06 ± 0.05 1.56 ± 0.02

0.08 ± 0.00 0.11 ± 0.01 0.10 ± 0.01 1.45 ± 0.07 0.23 ± 0.02 0.08 ± 0.00 0.11 ± 0.00 0.23 ± 0.02 0.12 ± 0.01 0.23 ± 0.02 0.31 ± 0.02

0.27 0.43 – 0.23 0.27 0.27 0.28 0.27 0.24 0.27 0.26

Persulfate effect

UV strength effect

2

10 50 100 50

Temp. effect

2

50

(–) The values were not determined due to limited extent of TOC degradation and a low correlation coefficient.

(0.96) (0.99) (0.99) (0.97) (0.96) (0.99) (0.97) (0.99) (0.97) (0.99)

(0.99) (0.99) (1.00) (0.98) (1.00) (0.99) (0.99) (1.00) (1.00) (1.00) (0.97)

475

C.-W. Wang, C. Liang / Chemical Engineering Journal 254 (2014) 472–478  2  Alkaline pH : SO 4 þ OH ! SO4 þ HO

The lower initial persulfate concentration resulted in the higher persulfate degradation rate as seen in Table 1. However, consumptions of persulfate are 8.5, 12.8 and 10.3 mM at the end of 130 min reaction time for initial persulfate concentration of 10, 50, and 100 mM, respectively. These results also indicated that the net competing reactions (i.e., between Eqs. (4) and (5) and TMAH degradation) resulted in higher kobs,TOC/kobs,TMAH ratios for the 50 and 100 mM persulfate experiments than that occurred in the lowest 10 mM persulfate experiment (see Table 1). Additionally, regarding the similar kobs,TOC/kobs,TMAH ratio of 0.27 in the 50 and 100 mM persulfate experiments, a higher persulfate concentration (i.e., 100 mM) did not promote greater TMAH and TOC degradations. Therefore, a 50 mM initial persulfate concentration was selected in subsequent experiments examining the effect of UV intensity and temperature. As can be seen in Fig. 2, when the UV light intensity increased from 8 W to 15 W, the rate constant of TMAH degradation increased from 0.0117 ± 0.0024 to 0.0389 ± 0.0043 min1, and the TOC degradation rate also increased from 0.0049 ± 0.0004 to 0.0106 ± 0.0005 min1 (see Table 1). Hence, it is evident that the persulfate degradation is highly dependent upon the intensity of the UV light source. This is in agreement with the literature regarding UV/S2O2 application [26,37] which 8 reported accelerated degradations of contaminants when a higher intensity of UV source was used. It should also be noted that the increased SO 4 generation (i.e., by 15 W UV) accelerates both TMAH and TOC degradations to a similar degree. Therefore, the kobs,TOC/kobs,TMAH ratio remains the same.

ð3Þ

Additionally, ratios between TMAH and TOC degradation rate constants (kobs,TOC/kobs,TMAH) are presented in Table 1. It can be seen that although acidic condition facilitated TMAH degradation, its kobs,TOC/kobs,TMAH ratio (0.27) is lower than the neutral condition (0.43). The SO 4 is prone to undergo H-atom abstraction [34–36], which is the only possible path for degrading (CH3)4N+ that does not have the lone-pair electron [3]. When pH is raised and OH  is increased in solution, SO 4 is gradually converted to HO , which  is a more electrophilic radical than SO 4 [12], and hence HO may afterward favorably attack the lone-pair electron on the nitrogen atom of intermediate products (e.g., tri-, di-, and methylamine) upon TMAH degradation. Therefore, due to the presence of both  SO 4 /HO at neutral pH as opposed the presence of predominately SO at acidic pH [12,33], TMAH appeared to more rapidly degrade 4 in acidic solution than in the neutral solution, while TOC degradation by HO is more pronounced in the neutral solution. It should be noted that due to the predominance of HO in the basic solution, TMAH has no lone-electron and exhibits much lower TMAH and TOC degradations by HO. 3.2. Effect of S2O2 8 concentration and UV intensity Based on the results of the initial pH experiments, acidic pH (i.e., pH 2) was selected as a favorable experimental condition for subsequent experiments. The effects of persulfate concentration on the TMAH, TOC, and persulfate degradations are shown in Fig. 2. Persulfate concentrations 10, 50, and 100 mM resulted in nearly complete TMAH degradations within 130 min and the pseudo-first-order reaction rate constants are presented in Table 1. The TMAH degradation rate constant increased with the increase of persulfate concentration from 10 to 50 mM, but a slightly negative effect was observed when persulfate was increased to 100 mM. It should be noted that if higher persulfate dose is employed and  excess SO 4 is generated, excessive SO4 may work as scavengers  for S2O2 8 or SO4 in accordance with Eqs. (4) and (5), respectively [36,37]: 2 2  SO 4 þ S2 O8 ! SO4 þ S2 O8  2 SO 4 þ SO4 ! S2 O8

k ¼ 6:1  105 M1 s1

30

60

Degradations of TMAH with 50 mM persulfate and a UV intensity of 15 W under initial pH 2 at 10, 20, and 30 °C system temperatures were evaluated. The results of the degradation profile of TMAH, TOC and persulfate in the UV/S2O2 8 process are shown in Fig. S2 (Supporting Information). Changing the magnitude of thermal activation is one of the ways to control the generation of SO 4 [11,14,22,38]; therefore, the kobs,TMAH values can be seen to increase with increased system temperatures (see Table 1). TMAH can be oxidized almost completely at low temperatures (e.g., 10 °C) over an extended reaction time. Moreover, the temperature dependency of the rate constants was further evaluated using the Arrhenius equation (see Fig. 3). The activation energies (Ea) obtained by

ð4Þ

k ¼ 4  108 M1 s1

0

3.3. Effect of reaction temperature

ð5Þ

90

120 0

30

Time (min) 60 90

120 0

30

60

90

120

0

-1

ln(C/C0)

-2

-3 SPS 10 mM SPS 50 mM (UV = 15 W) SPS 100 mM SPS 50 mM (UV = 8 W)

-4

-5

(a)

(c)

(b)

(UV = 15 W)

0 0

15000 3000

6000

30000 0 9000 0

15000 3000

6000

30000 0 9000 0

15000 3000

6000

30000

(UV = 8 W)

9000

2

UV Fluence (mJ/cm ) Fig. 2. The degradation profile of (a) TMAH, (b) TOC and (c) persulfate by the UV/S2O2 8 process at different persulfate concentrations ([SPS]0 = 10, 50 and 100 mM) and UV strength (UV = 8 and 15 W). ([TMAH]0 = 1.1 mM, pH = 2).

476

C.-W. Wang, C. Liang / Chemical Engineering Journal 254 (2014) 472–478

-2.0

y = (-4.659 ± 0.439) x + (12.587 ± 1.500)

-2.5

-1

ln Kobs (s )

R2 = 0.9957 -3.0

-3.5

-4.0

-4.5 3.3

3.4

3.5 3

-1

3.6

-1

10 T (K ) Fig. 3. Arrhenius plot for TMAH degradation by the UV/S2O2 process. 8 ([TMAH]0 = 1.1 mM, [SPS]0 = 50 mM, UV = 15 W, pH = 2).

analysis of experimental data were determined to be 39.19 ± 0.09 kJ/mole. The Ea value obtained for TMAH is generally lower than those obtained for ibuprofen (168 kJ/mole) [15], methylene blue (107.4–145.3 kJ/mole) [16], chloramphenicol (103.1 kJ/ mole) [39], and diuron (166.7 kJ/mole) [38] by thermally activated persulfate. The results suggest that exposure to UV can be used as an activation process and a source of energy and hence to reduce the required activation energy of the reaction. 3.4. TMAH degradation pathway Intermediates during the course of the UV/S2O2 8 degradation of TMAH were monitored with experimental conditions of 50 mM SPS, 15 W UV, pH 2 and 20 °C. Fig. 4 shows the degradation of TMAH and the formation of intermediates as a function of reaction time. Six intermediates including TMA, DMA, MoMA, NH+4, NO 2 and NO 3 during the degradation of TMAH were detected and the calculation of N-mass balance among these compounds was conducted. It can be seen in Fig. 4 that as soon as the TMAH and the sequential demethylation degradation processes begin TMA and DMA are accumulated at the initial stage of TMAH degradation, and MoMA is also formed as a minor intermediate. As the reaction proceeded, both TMA and DMA are degraded subsequently, and NH+4 and NO 3 are increasingly accumulated. These results may indicate conversions of TMAH to TMA and DMA, and consequently to NH+4 and Time (min) 0

30

60

90

120

Total N mass balance

Conc. (mM)

+

(CH3)3NH (TMA)

-

NO2

-

NO3

60

+

0.6

(CH3)2NH2 (DMA) +

CH3NH3 (MMA) 0.4

40

+

NH4

N mass balance (C/C0)

80 (CH3)4N (TMAH)

20

0.2 0.0

0 0

5000

10000

15000

20000

25000

30000

ð6Þ

ðCH3 Þ3 Nþ CH2 þ H3 Oþ ! ðCH3 Þ3 Nþ CHO þ 4Hþ

ð7Þ

ðCH3 Þ3 Nþ CHO þ H3 Oþ ! ðCH3 Þ3 Nþ þ CH2 O þ H2 O

ð8Þ

 2 þ þ ðCH3 Þ3 Nþ CHO þ SO 4 ! ðCH3 Þ3 N CO þ H þ SO4

ð9Þ

ðCH3 Þ3 Nþ CO ! ðCH3 Þ3 Nþ þ CO

ð10Þ

ðCH3 Þ3 Nþ þ e þ Hþ ! ðCH3 Þ3 Nþ H

ð11Þ

Additionally, a TMA radical generated from the demethylation pathway may undergo acid-catalyzed hydrolysis reaction forming a trimethylamine N-oxide ((CH3)3N+O) (Eq. (12)), which proceeds sequential demethylation pathways to generate dimethyl nitroxide ((CH3)2N+O) and CH3N+O (Eq. (13)). Further acid-catalyzed hydrolysis CH3N+O reaction (Eq. (14)) and demethylation (Eq. (15)) lead ultimately to NO 3 (Eq. (16)). These degradation routes confirm observations of TMA and NO 3.

ð12Þ

100

1.0 0.8

 2 þ þ ðCH3 Þ4 Nþ þ SO 4 ! ðCH3 Þ3 N CH2 þ H þ SO4

ðCH3 Þ3 Nþ þ H3 Oþ ! ðCH3 Þ3 Nþ O þ 3Hþ

1.2

+

NO 3 . Furthermore, the overall N-mass balance calculated based on these detected compounds by the IC analysis exhibited an average of nearly 100% N-recovery (88–106%) during the course of reaction. These results indicate that TMAH can be transformed to intermediate products and may suggest that given sufficient reaction time, TMAH can be degraded completely by the UV/S2O2 8 pro+ cess reaching complete degradations with NO 3 and NH4 as final products. Based on the experimental data, combined with literature survey, schematic pathways of the UV/S2O2 degradation of TMAH 8 are illustrated in Fig. 5. It has been well demonstrated that one mole of persulfate activated by UV irradiation under acidic conditions (e.g., pH < 3) mainly produce two mole of SO 4 (Eq. (2)). As previously described, in general, SO 4 is most likely to participate in electron transfer and hydrogen abstraction in this case of UV/ S2O2 oxidation of quaternary ammonium salt such as TMAH, 8 rather than a radical addition reaction. Therefore, the first step in the destruction of TMAH could occur through hydrogen abstraction by the attack of SO 4 at any methyl groups (Eq. (6)), leading to carbon centered radical, which may undergo acid-catalyzed hydrolysis reaction forming an aldehyde product ((CH3)3N+CHO) (Eq. (7)). (CH3)3N+CHO can subsequently undergo either hydrolysis (Eq. (8)) or additional SO 4 attack (Eqs. (9) and (10)) with the formation of (CH3)3N+ (TMA radical), which stabilizes to form TMA (Eq. (11)). The degradation of TMAH in accordance with Eqs. (6)– (10) is a demethylation pathway, as noted in Fig. 5.

35000

2

UV Fluence (mJ/cm ) Fig. 4. The degradation of TMAH and the formation of intermediates during the reaction time. ([TMAH]0 = 1.1 mM, [SPS]0 = 10 mM, UV = 15 W, pH = 2).

ðCH3 Þ3 Nþ O

demethylation

!

ðCH3 Þ2 Nþ O

demethylation

!

ðCH3 ÞNþ O þ H3 Oþ ! ðCH3 ÞNþ O2 þ 3Hþ ðCH3 ÞNþ O2

demethylation

!

þ  NO 2 þ H3 O ! NO3

NO 2

ðCH3 ÞNþ O

ð13Þ ð14Þ ð15Þ ð16Þ

On the other hand, TMA generated can again be initiated by a stepwise demethylation path as shown in Fig. 5 to generate DMA, MoMA, and ammonium. Wherein, the DMA radical formed in the process of generating the DMA can be degraded by acid-catalyzed hydrolysis (Eqs. (17) and (18)) and SO 4 attack (Eq. (19)) to form dimethyl nitroxide ((CH3)2N+O), which would proceed further to generate NO 3 in accordance with Eqs. (13)–(16).

477

C.-W. Wang, C. Liang / Chemical Engineering Journal 254 (2014) 472–478

Demethylation pathway HCHO

CH3

N+

Eq. 8

H+/H2O

(TMAH) CH3 CH3

CH3

Eq. 6 SO4-• H++SO42-

CH3 CH3

N+

CH2•

Eq. 7 H+/H2O

CH3 N+

CH3

4H+

CH3

O

Eq. 9 SO4-•

CH

O

N+

C•

CH3

H++SO42-

CH3

CH3

CH3

Eq. 10 SO4-•

N+ •

CH3

H++SO42-

CH3

Eq. 11 eH+

(TMA) CH3 N+

CH3

CH3

H

CH3

Eq. 12 H+/H2O

Eq. 13 O O-

N+ O-

Eq. 16 H+/H2O 3H+

O • N+ O-

O

Eq. 15 Demethylation

CH3

N+ O-

Eq. 14 H+/H2O 3H+

O

O CH3

Demethylation

N+ •

CH3

N+

3H+

CH3 Demethylation

N+

CH3

CH3

O-

Demethylation

CH3 H+/H2O Eq. 17 4H+

Eq. 19 CH3 H+/H2O

SO4-•

Eq. 24

CH3

H++SO42-

3H+

N+ H

O-

CH3

3H+

• N+

O

H++SO42-

H

N+

Eq. 23

SO4-• O

H+

CH3

H

H

H++SO42Eq. 21

N+ H

O-

N+

H

H H+/H2O

Demethylation

4H+

CH3

CH3

SO4-•

e-

N+ •

Eq. 22

CH3

(DMA) CH3

CH3

Eq. 18 H+/H2O

H+/H2O 3H+

(MoMA) CH3

CH3 H

N+ •

eH+

H

N+

H

H

H

Eq. 20

Demethylation

(Ammonium) H

H H

N+ • H

eH+

H

N+

H

H

2 Fig. 5. Proposed pathways of TMAH degradation by SO 4 attack in UV/S2O8 process.

ðCH3 Þ2 Nþ H þ H3 Oþ ! ðCH3 Þ2 NO þ 4Hþ

ð17Þ

Acknowledgement

ðCH3 Þ2 Nþ H þ H3 Oþ ! ðCH3 Þ2 Nþ HO þ 3Hþ

ð18Þ

This study was funded by the Top University Plan Office at the National Chung Hsing University, Taiwan.

2 þ þ ðCH3 Þ2 Nþ HO þ SO 4 ! ðCH3 Þ2 N O þ H þ SO4

ð19Þ

Moreover, the degradation of DMA is through either stepwise demethylation to MoMA and ammonium or coupled acid-cata+  lyzed hydrolysis and SO 4 attack to nitromethane ((CH3)N O2 ) (Eqs. (20)–(24)), then eventually to nitrate.

ðCH3 ÞNþ H2 þ H3 Oþ ! ðCH3 ÞNþ H2 O þ 3Hþ

ð20Þ

2 þ þ ðCH3 ÞNþ H2 O þ SO 4 ! ðCH3 ÞN HO þ H þ SO4

ð21Þ

ðCH3 ÞNþ H2 þ H3 Oþ ! ðCH3 ÞNþ HO þ 4Hþ

ð22Þ

2 þ  þ ðCH3 ÞNþ HO þ SO 4 ! ðCH3 ÞN O þ H þ SO4

ð23Þ

ðCH3 ÞNþ O þ H3 Oþ ! ðCH3 ÞNþ O2 þ 3Hþ

ð24Þ

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.05.116. References

4. Conclusions Application of UV was successfully used to activate persulfate for degrading TMAH in solution at acidic condition. The TMAH degradation efficiencies appeared to be influenced by factors such as pH, persulfate concentration, UV intensity, and reaction temperature. Based on the analysis of intermediate products (i.e., TMA,  DMA, MoMA, NH+4, NO 2 , and NO3 ), the reaction mechanism pro posed in this study for a SO4 driven oxidation process indicates that the destruction of TMAH most likely happens through the demethylation process via the attack of SO 4 at the methyl group. The results of this study may serve as a reference for TMAH treatment in the high-tech industry using the UV activated persulfate oxidation process.

[1] K.F. Chang, S.Y. Yang, H.S. You, J.R. Pan, Anaerobic treatment of tetra-methyl ammonium hydroxide (TMAH) containing wastewater, semiconductor manufacturing, IEEE Trans. Semicond. Manuf. 21 (2008) 486–491. [2] T.H. Hu, L.M. Whang, P.W.G. Liu, Y.C. Hung, H.W. Chen, L.B. Lin, C.F. Chen, S.K. Chen, S.F. Hsu, W. Shen, R. Fu, R. Hsu, Biological treatment of TMAH (tetramethyl ammonium hydroxide) in a full-scale TFT-LCD wastewater treatment plant, Bioresour. Technol. 113 (2012) 303–310. [3] S. Kim, W. Choi, Kinetics and mechanisms of photocatalytic degradation of (CH3)nNH4n+ (0 6 n 6 4) in TiO2 suspension: the role of OH radicals, Environ. Sci. Technol. 36 (2002) 2019–2025. [4] C.N. Lei, L.M. Whang, P.C. Chen, Biological treatment of thin-film transistor liquid crystal display (TFT-LCD) wastewater using aerobic and anoxic/oxic sequencing batch reactors, Chemosphere 81 (2010) 57–64. [5] J. Shibata, N. Murayama, S. Matsumot, Recovery of tetra-methyl ammonium hydroxide from waste solution by ion exchange resin, Resour. Process. 53 (2006) 199–203. [6] Transene Company Inc., Material safety data sheet of tetramethylammonium hydroxide, (2012). [7] C. Anthony, The Biochemistry of Methylotrophs, Academic Press, New York, 1982. [8] K. Hirano, J. Okamura, T. Taira, K. Sano, A. Toyoda, M. Ikeda, An efficient treatment technique for TMAH wastewater by catalytic oxidation, IEEE Trans. Semicond. Manuf. 14 (2001) 202–206. [9] T. Urakami, H. Araki, H. Oyanagi, K. Suzuki, K. Komagata, Paracoccus aminophilus sp. nov. and paracoccus aminovorans sp. nov., which utilize N, N-dimethylformamide, Int. J. Syst. Bacteriol. 40 (1990) 287–291. [10] W.M. Latimer, Oxidation Potentials, Prentice-Hall Inc., Englewood Cliffs, New Jersey, 1952. [11] C.J. Liang, J.B. Clifford, C.M. Michael, L.S. Kenneth, Thermally activated persulfate oxidation of trichloroethylene (TCE) and 1,1,1-trichloroethane

478

[12]

[13]

[14] [15] [16] [17]

[18] [19]

[20] [21] [22] [23]

[24] [25] [26]

C.-W. Wang, C. Liang / Chemical Engineering Journal 254 (2014) 472–478 (TCA) in aqueous systems and soil slurries, Soil Sediment Contam. 12 (2003) 207–228. C. Liang, Y.Y. Guo, Y.R. Pan, A study of the applicability of various activated persulfate processes for the treatment of 2,4-dichlorophenoxyacetic acid, Int. J. Environ. Sci. Technol. (2013), http://dx.doi.org/10.1007/s13762-013-0212-5. C. Liang, Z.S. Wang, N. Mohanty, Influences of carbonate and chloride ions on persulfate oxidation of trichloroethylene at 20 °C, Sci. Total Environ. 370 (2006) 271–277. Y.C. Lee, S.L. Lo, J. Kuo, Y.L. Lin, Persulfate oxidation of perfluorooctanoic acid under the temperatures of 20–40 °C, Chem. Eng. J. 198–199 (2012) 27–32. A. Ghauch, A.M. Tuqan, N. Kibbi, Ibuprofen removal by heated persulfate in aqueous solution: a kinetics study, Chem. Eng. J. 197 (2012) 483–492. A. Ghauch, A.M. Tuqan, N. Kibbi, S. Geryes, Methylene blue discoloration by heated persulfate in aqueous solution, Chem. Eng. J. 213 (2012) 259–271. C. Liang, I.L. Lee, In situ iron activated persulfate oxidative fluid sparging treatment of TCE contamination – a proof of concept study, J. Contam. Hydrol. 100 (2008) 91–100. 2+ 2+ G.V. Buxton, T.N. Malone, G.A. Salmon, Reaction of SO and 4 with Fe , Mn Cu2+ in aqueous solution, J. Chem. Soc. Faraday Trans. 93 (1997) 2893–2897. D.Y.S. Yan, I.M.C. Lo, Removal effectiveness and mechanisms of naphthalene and heavy metals from artificially contaminated soil by iron chelate-activated persulfate, Environ. Pollut. 178 (2013) 15–22. C. Liang, Y.Y. Guo, Remediation of diesel-contaminated soils using persulfate under alkaline condition, Water Air Soil Pollut. 223 (2012) 4605–4614. O.S. Furman, A.L. Teel, R.J. Watts, Mechanism of base activation of persulfate, Environ. Sci. Technol. 44 (2010) 6423–6428. C. Liang, Z.S. Wang, C.J. Bruell, Influence of pH on persulfate oxidation of TCE at ambient temperatures, Chemosphere 66 (2007) 106–113. L. Dogliotti, E. Hayon, Flash photolysis of per[oxydi]sulfate ions in aqueous solutions. The sulfate and ozonide radical anions, J. Phys. Chem. 71 (1967) 2511–2516. J.Y. Fang, C. Shang, Bromate formation from bromide oxidation by the UV/ persulfate process, Environ. Sci. Technol. 46 (2012) 8976–8983. C.C. Lin, L.T. Lee, L.J. Hsu, Performance of UV/S2O2 process in degrading 8 polyvinyl alcohol in aqueous solutions, Photochem. Photobiol. 252 (2013) 1–7. Y.T. Lin, C. Liang, J.H. Chen, Feasibility study of ultraviolet activated persulfate oxidation of phenol, Chemosphere 82 (2011) 1168–1172.

[27] N.S. Shah, X. He, H.M. Khan, J.A. Khan, K.E. O’Shea, D.L. Boccelli, D.D. Dionysiou, Efficient removal of endosulfan from aqueous solution by UV-C/peroxides: a comparative study, J. Hazard. Mater. 263 Part 2 (2013) 584–592. [28] X. He, A.A. de la Cruz, D.D. Dionysiou, Destruction of cyanobacterial toxin cylindrospermopsin by hydroxyl radicals and sulfate radicals using UV-254 nm activation of hydrogen peroxide, persulfate and peroxymonosulfate, J. Photochem. Photobiol. A Chem. 251 (2013) 160–166. [29] M.G. Antoniou, P.A. Nicolaou, J.A. Shoemaker, A.A. de la Cruz, D.D. Dionysiou, Impact of the morphological properties of thin TiO2 photocatalytic films on the detoxification of water contaminated with the cyanotoxin, microcystin-LR, Appl. Catal. B Environ. 91 (2009) 165–173. [30] Y.Q. Gao, N.Y. Gao, Y. Deng, Y.Q. Yang, Y. Ma, Ultraviolet (UV) light-activated persulfate oxidation of sulfamethazine in water, Chem. Eng. J. 195–196 (2012) 248–253. [31] J.J. Lingane, Volumetric analysis. Volume III. Titration methods: oxidation– reduction reactions, J. Am. Chem. Soc. 80 (1958) 2599–2600. [32] D.A. House, Kinetics and mechanism of oxidations by peroxydisulfate, Chem. Rev. 62 (1962) 185–203. [33] C. Liang, H.W. Su, Identification of sulfate and hydroxyl radicals in thermally activated persulfate, Ind. Eng. Chem. Res. 48 (2009) 5558–5562. [34] S.L. Khursan, D.G. Semes’ko, R.L. Safiullin, Quantum-chemical modeling of the detachment of hydrogen atoms by the sulfate radical anion, Russ. J. Phys. Chem. 80 (2006) 366–371. [35] C. Liang, Y.Y. Guo, Y.C. Chien, Y.J. Wu, Oxidative degradation of MTBE by pyriteactivated persulfate: proposed reaction pathways, Ind. Eng. Chem. Res. 49 (2010) 8858–8864. [36] G.R. Peyton, The free-radical chemistry of persulfate-based total organic carbon analyzers, Mar. Chem. 41 (1993) 91–103. [37] W. Chu, Y.R. Wang, H.F. Leung, Synergy of sulfate and hydroxyl radicals in UV/ S2O2 8 /H2O2 oxidation of iodinated X-ray contrast medium iopromide, Chem. Eng. J. 178 (2011) 154–160. [38] C. Tan, N. Gao, Y. Deng, N. An, J. Deng, Heat-activated persulfate oxidation of diuron in water, Chem. Eng. J. 203 (2012) 294–300. [39] M. Nie, Y. Yang, Z. Zhang, C. Yan, X. Wang, H. Li, W. Dong, Degradation of chloramphenicol by thermally activated persulfate in aqueous solution, Chem. Eng. J. 246 (2014) 373–382.