Decomposition of 2-naphthalenesulfonate in aqueous solution
by ozonation with UV radiation
Y.H. Chen
a, C.Y. Chang
a,*, S.F. Huang
a, C.Y. Chiu
b, D. Ji
a, N.C. Shang
a,
Y.H. Yu
a, P.C. Chiang
a, Y. Ku
c, J.N. Chen
daGraduate Institute of Environmental Engineering, National Taiwan University, Taipei 106, Taiwan bDepartment of Environmental Engineering, Lan-Yang Institute of Technology, I-Lan 261, Taiwan cDepartment of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
dGraduate Institute of Environmental Engineering, National Chiao-Tung University, Hsin-Chu 300, Taiwan
Abstract
This study investigates the ozonation of 2-naphthalenesulfonate (2-NS) combined with ultraviolet (UV) radiation. Naphthalenesulfonic acids are of importance as dye intermediates for the dye and textile auxiliary industries. Its derivatives, such as 2-NS, have been found in rivers and tannery effluents causing pollution problems. Thus, the 2-NS is of concern for the aquatic pollution control especially in the surface and waste waters. Ozonation combined with UV radiation is employed for the removal of 2-NS in the aqueous solution. Semibatch ozonation experiments were proceeded under different reaction conditions to study the effects of ozone dosage and UV radiation on the oxidation of 2-NS. The concentrations of 2-NS and sulfate are analyzed at specified time intervals to elucidate the decomposition of 2-NS. In addition, values of pH and oxidation reduction potential are continuously measured in the course of experiments. Total organic carbon is chosen as a mineralization index of the ozonation of 2-NS. The mineralization of 2-NS via the ozonation is remarkably enhanced by the UV radiation. These results can provide useful information for the proper removal of 2-NS in the aqueous solution by the ozonation with UV radiation. r 2002 Published by Elsevier Science Ltd.
Keywords: Ozone; Ozonation; UV radiation; 2-Naphthalenesulfonate; Wastewater
1. Introduction
Aromatic sulfonates, which are produced in large amounts in chemical industry since the end of 19th century, have been widely applied in many industrial processes, including the various steps of industrial textile procedure [1]. Among them, naphthalenesulfonic acids (NSAs) are of importance as dye intermediates and commonly used in the textile auxiliary industry employ-ing many azo dyes and pigments, which are the major outlets of naphthalenesulfonates. In addition, the uses of 2-naphthalenesulfonic acid (2-NSA) include flotation collector, initiator for the catalytic polymerization of
caprolactam, stabilizer of maleic anhydride-olefin copo-lymers, condensation with formaldehyde and alcohol product to form surface-active agents, and tanning materials [2]. Its salt is also used as a brightening and stabilization agent in the electroplating solution of printed wiring board industry.
The aquatic toxicity of aromatic sulfonates appears to be small and the risk of bioaccumulation is limited since the octanol-water partition coefficients (log Kow values)
typically range below 2 [3]. Low log Kow values are
indication of high mobility within the aquatic system. Furthermore, many of them were reported to be persistent to microbial degradation [4]. Consequently, aromatic sulfonates would be expected to be discharged from wastewater treatments, and have been found in wastewater as well as surface water. For instance,
*Corresponding author. Tel./fax: +886-2-2363-8994. E-mail address:cychang3@ccms.ntu.edu.tw (C.Y. Chang).
0043-1354/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 0 4 3 - 1 3 5 4 ( 0 2 ) 0 0 1 3 5 - 5
Alonson and Barcel!o [2] detected that the values of 2-NSA concentration were 50 mg/L in Llobregat River in Spain, and varied from several mg/L to several mg/L in different untreated tannery effluents in Sweden, Spain, and Portugal. Accordingly, 2-NSA is thqs of concern as aqueous pollutant in wastewater treatment.
Ozonation is an effective way to reduce the chemical oxidation demand (COD) and total organic carbons (TOCs) by oxidizing the stream solutions with ozone [5]. Ozone may attack on the pollutants via two different reaction pathways: (1) the direct ozonation by the ozone molecule, and (2) the radical ozonation by the highly oxidative free radicals such as hydroxyl free radicals, which are formed by the decomposition of ozone in the aqueous solution [6,7]. The radical ozonation is non-selective and vigorous. The ozonation process in the acid condition mainly takes place through the direct oxida-tion reacoxida-tion, which is selective [8,9]. Ozonaoxida-tion combined with ultraviolet (UV) radiation is deemed as a more effective process to remove organics comparing to the sole ozonation. UV radiation is commonly employed to enhance the ozone decomposition yielding more free radicals resulting in a higher ozonation rate [10]. The degradation of less reactive compounds may be accelerated with the presence of free radicals.
Studies on the ozonation of NSA have been quite few among aromatic compounds. Shin and Lim [11] found that the biodegradability (BOD5/COD) of oxidation products can be improved by preozonation of NSA. It was shown that the rate of NSA elimination at pH 3 is faster than that at pH 7, while the yield of sulfate is higher at pH 7 than that at pH 3. However, the effect of pH is insignificant on the rate of COD reduction and on the improvement of biodegradability. In general, ozona-tion of aromatic compounds may proceed with either the aromatic ring via decyclization or one of the side-chain substituents [12]. The ozonation of naphthalene
investigated by Legube et al. [13] indicated 1,3-dipolar cyclo-addition of ozone on the 1,2 bond of naphthalene. The intermediates of naphthalene in ozonation were found to contain cyclic peroxide, oxalic acid, oxoma-lonic acid, formic acid, hydrogen peroxide, ortho-phthaldialdehyde, phthalic acid, and phthaladehydic acid. The above results provide referable information about the destruction of aromatic ring. However, the role of sulfonic substituent and the effect of UV radiation on the ozonation of 2-naphthalenesulfonate (2-NS) still need to be elucidated.
The objective of this study is to investigate the ozonation of 2-NS with UV radiation. Semibatch ozonation experiments are proceeded under different reaction conditions. The concentrations of 2-NS and sulfate are analyzed at specified time intervals to study the decomposition of 2-NS. TOC is chosen as a mineralization index of the ozonation of 2-NS, while the values of pH and oxidation reduction potential (ORP) are measured continuously in the course of experiments. The decomposition of 2-NS accompanies the production of sulfate, diminution of TOC, and variations of pH and ORP. These phenomena during the ozonation of 2-NS are studied. All these results can provide useful information about removing 2-NS in the aqueous solution by the ozonation with UV radiation.
2. Experimental 2.1. Chemicals
The concentration of 2-NS as the sole organic target is 200 mg/L according to the prescription of electroplating solution [14]. The 2-NS with chemical formula as C10H7SO3Na, which is purchased from Aldrich (Mil-wankee, WI, USA) and used without any further Nomenclature
CAGi0 gas concentration of ozone of inlet gas (mg/L)
CBLb concentration of 2-NS in bulk liquid (M or
mg/L)
CBLb0 initial concentration of 2-NS in bulk liquid (M
or mg/L)
Ce experimental ORP data (mV)
Ce average value of all experimental ORP data
(mV)
Cp predicted ORP values (mV)
CSO2
4 concentration of sulfate in bulk liquid (M or
mg/L)
CTOC concentration of total organic carbon (TOC)
(M or mg/L)
CTOC0 initial concentration of TOC (M or mg/L)
½Iuv light intensity of UV lamp (W/m2)
NL volume of gas in liter at 273 K, 1 atm (L) R2 determination coefficient, 1 ½SðC
e
CpÞ2=SðCe CeÞ2
t ozonation time (min)
tf ;NS ozonation time for complete elimination of
CBLb(min)
tf ;TOC ozonation time for complete elimination of
CTOC (min)
ZTOC removal efficiency of TOC defined by Eq. (2) (%)
purification, has molecular weight of 230.22. The molecular structure of 2-NS is shown as Fig. 1. All experimental solutions are prepared with deionized water without other buffers. The initial values of pH and TOC of experimental solution are about 5.20 and 104 mg/L, respectively.
2.2. Instrumentation
The airtight reactor of 17.2 cm inside diameter is made of Pyrex glass with an effective volume of 5.5 L, and equipped with water jacket to maintain a constant solution temperature at 251C in all experiments. The design of reactor is based on the criteria of the shape factors of a standard six-blade turbine [15]. The gas diffuser in cylindrical shape with pore size of 10 mm is located at the bottom of reactor. Two quartz tubes of 3.8 cm outside diameter symmetrically installed inside the reactor are used to house the UV lamps. The low-pressure mercury lamps emitted principally at 254 nm supply the UV radiation. The radiation intensity is measured by a digital radiometer (ultra-violet products (UVP), Upland, CA, USA) with a model UVP-25 radiation sensor. About 3.705 L solution is used in each experiment, while the total sampling volume is within 5% of solution. The stirred speed is as high as 800 rpm to ensure the complete mixing of liquid and gas phases according to previous tests [16,17]. The generation of ozone is controlled by the power input of ozone generator (model SG-01A, Sumitomo, Tokyo, Japan) with constant gas pressure (1 kgf/cm2). The ozone generator used in this research employs two steel plate electrodes and ceramic dielectric. Ozone-containing gas generated by pure oxygen is introduced into the reactor with a gas flow rate of 1.78 NLPM. A circulation pump is used to transport the liquid from the reactor to the sensors of monitor with a flow rate of 0.18 L/min, and to reflow it back during the ozonation. Samples are drawn out from the reactor at desired intervals in the course of experiments. The residual dissolved ozone in the sample is removed by stripping with nitrogen.
The concentrations of 2-NS ðCBLbÞ are analyzed using
high performance liquid chromatography system with 250 4.6 mm2model BDS C18 (5 mm) column (Thermo Hypersil-keystone, Bellfonte, PA, USA), and UV/ Visible detector (model 1706, Bio-Rad, Hercules, CA, USA) at 254 nm. The effluent with flow rate of 1.0 mL/ min has the composition with water: CH3CN of 83:17.
The injection volume of analytic solution is 20 mL. The ionic chromatography employed to analyze the concen-tration of sulfate ðCSO2
4 Þ is equipped with 150 5.5 mm
AN300 column (MetaChem, Lake forest, CA, USA) and model conductivity series IV detector (LabAlliance, Lemont, PA, USA). The effluent with flow rate of 2.0 mL/min has the composition with NaHCO3/Na2CO3 of 1.7/1.8 mM. The TOC concentration ðCTOCÞ of
samples is analyzed by a TOC analyzer (model 700, OI Corporation, TX, USA). The instrument utilizes the UV-persulfate technique to convert the organic carbon for the subsequent analysis by an infrared carbon dioxide analyzer calibrated with the potassium hydrogen phthalate standard. The pH (model 300 T, Suntex, Taipei, Taiwan) and ORP (model 900C, Apogee, Taipei, Taiwan) meters with sensors are used to measure the values of pH and ORP of solution. All fittings, tubings and bottles are made of stainless steel, Teflon, or glass. The experimental apparatus employed in this work is shown in Fig. 2.
2.3. Experimental procedures
The semibatch experiments of 2-NS ozonation are performed to examine the variations of CBLb; CSO2
4 ;
CTOC; pH, and ORP. Before starting the ozonation
experiments, the ozone-containing gas is bypassed to the photometric analyzer (model SOZ-6004, Seki, Tokyo, Japan) to assure the stability and ozone concentration. Light intensity (½Iuv) of 60.35 W/m2is employed to test
the effect of UV radiation on the ozonation. A part of gas stream at preset flow rate is directed into the reactor when reaching the set conditions.
3. Results and discussion 3.1. Variations of CBLb; CSO2
4 ; CTOC; pH, and ORP in
ozonation of 2-NS
The mineralization reaction between O3and 2-NS can be stoichiometrically expressed as follows.
C10H7SO3Na þ 8O3-10CO2þ SO24 þ H þ
þ 3H2O þ Naþ: ð1Þ
The variations of CBLb; CSO2
4 ; CTOC; pH, and ORP
with ozonation time under experimental conditions of sole O3and O3/UV are shown in Fig. 3. There are two important characteristic times of concern during the ozonation of 2-NS. One is the time for the complete elimination of 2-NS signed as tf ;NS; the other is that
for the complete mineralization of TOCs denoted as tf ;TOC: Comparison of ozonation results at tf ;NS and
tf ;TOCunder different experimental conditions is given in
Table 1 for four cases. The differences of the values of
S O
-Na+
O O
CSO2
4 =CBLb0under the conditions of sole O3and O3/UV
at tf ;NSare remarkable. The yield of sulfate (about 43%,
average of Cases 3 and 4) with O3/UV is greater than two times of that (about 19%, average of Cases 1 and 2) with O3alone. This may be contributed to the cause that the aromatic compound with the sulfonic substituent, which is electron withdrawing, has low reactivity with O3 molecules in ozonation [12]. Therefore, the initial attack of ozone on 2-NS is not mainly toward the generation of sulfate [11]. However, all the decomposi-tion of 2-NS accompanies with moderate diminudecomposi-tion of TOC. The removal efficiencies of TOC (ZTOC) defined by Eq. (2) at tf ;NSrange from 12.61% to 15.98%, indicating
only slight difference. Thus the intermediates produced from the decomposition of 2-NS still contribute over 84% TOC relative to the initial value.
ZTOC¼ ðCTOC0 CTOCÞ=CTOC0: ð2Þ
The pH value of solution decreases rapidly in the early period and then gradually approaches to a constant as ozonation time increases. The decrease of pH at tf ;NS
under the condition of O3/UV is more significant than that of sole O3, indicating that O3/UV induces the formation of intermediates with acidity higher than sole O3. The increase of acidity (decrease of pH) is consistent with the increase of the yield of sulfate as noted in the preceding paragraph. Correspondingly, the proportion of the intermediates with the substituent of sulfonic
group under the condition of sole O3is greater than that of O3/UV.
As the ozonation is further proceeded, the final pH values (at tf ;TOC) are close to the theoretical value of 3.06
calculated from Eq. (1). The values of ORP approach to constants of 990, 1070, 956, 1010 mV for the four cases in Table 1, which are close to that of organic-free solution, about the complete elimination of TOC.
Based on the above discussion on the variations of CBLb; CSO2
4 ; CTOC; pH, and ORP, and noting that
the reaction rates and intermediates of the ozonation of 2-NS are affected by the presence of UV radiation, one may then propose a simplified scheme of the decom-position pathways of the ozonation of 2-NS with UV radiation as shown in Fig. 4. The major contribution of UV is to generate OHd
via Reaction II.
3.2. Decomposition of 2-NS and formation of sulfate The times required for the complete decomposition of 2-NS (tf ;NS) in the present experiments as shown in
Table 1 are between 10 and 30 min. The effect of the ozone concentration of feed gas on the decomposition of 2-NS observed from Fig. 5 is remarkable. The value of tf ;NS with CAGi0¼ 44 mg/L is about one-third of that
with CAGi0¼ 11 mg/L under the condition of sole O3. That with CAGi0¼ 40 mg/L is about half of that with
CAGi0¼ 20 mg/L under the condition of O3/UV. Fig. 2. The experimental apparatus sketch. ——, — —, — - - —: ozone gas stream, experimental solution, isothermal water. Components: (1) oxygen cylinder, (2) drying tube, (3) ozone generator, (4) flow meter, (5) three-way valves, (6) stirrer, (7) ultra-violet (UV) lamps, (8) reactor, (9) sample port, (10) liquid ozone sensor, (11) pH sensor, (12) ORP sensor, (13) circulation pump, (14) thermostat, (15) gas ozone detector, (16) KI solution, (17) vent to hood.
However, comparing the results of CAGi0¼ 44 g/m3(O3 alone) and 40 g/m3 (O3/UV) indicates that the UV radiation affects the decomposition rate of 2-NS slightly. As depicted in the decomposition pathways of 2-NS by the ozonation with UV, Reaction II is enhanced by the presence of UV radiation. The pathways via hydroxyl free radicals noted as Reactions III and IV would then be promoted while the contribution of Reaction I would decrease due to the decrease of dissolved ozone concentration. As the result, the overall decomposition rate of 2-NS in ozonation is only slightly accelerated by the UV radiation.
Noting the deactivation of benzene ring toward electrophilic substitution caused by the sulfonic sub-stituent and steric hindrance of the sulfonic subsub-stituent [18] and referring to the mechanism of the sole ozonation of naphthalene reported by Legube et al. [13], one may propose the possible mechanism of Reaction I as shown in Fig. 6. The preferred site for electrophilic attack by ozone molecule is 1-position [19]. Indeed, Fig. 6 is similar to the ozonation pathways of naphthalene proposed by Legube et al. [13] who identified the ozonation products of naphthalene by GC/MS. However, 2-NS has the sulfonic substituent on
0 60 120 180 240
Ozonation time(min) 2 4 6 pH 400 800 1200 OR P ( mv ) 0 40 80 120 CTO C ( mg /L ) 0 40 80 CSO 4 2 -( mg /L ) 0 100 200 CBLb (mg /L ) 0 30 60 90 120
Ozonation time(min) 2 4 6 pH 400 800 1200 OR P ( mv ) 0 40 80 120 CTO C ( mg /L ) 0 40 80 CSO 4 2 -( mg /L ) 0 100 200 CBLb (mg /L ) (a) (b)
Fig. 3. Concentrations profile of 2-NS, SO42, and TOCs (CBLb; CSO2
4 ; CTOC), pH, and ORP when ozonation of 2-NS occurred in
the ring, which has deactivating effect by withdrawing electron density and directs incoming electrophiles to specific positions [20]. The byproducts of ozonated 2-NS were not analyzed in this study. Consequently, Fig. 6 is postulated simply to depict that ozone attacks the
substituted naphthalene with the consideration of the occurrence of subsequent possible reactions at preferred positions around the ring. The initial attacks of ozone on 2-NS may be proceeded via (a) ozone dipolar cycloaddition on the 1,2 bond or (b) an electrophilic substitution of ozone on carbon 1 [21]. The intermedi-ates include 4-sulfo-o-phthalaldehyde (denoted as A in Fig. 4), peroxycyclic benzenesulfonic acid (C), 4-sulfo-o-phthalaldehydic acid (E). Some peroxides such as hydroperoxy ethanal (B) and hydrogen peroxide (D) also appear during the ozonation. The decomposition rate of 2-NS decreases with ozonation time accounting for the generation of byproducts, which are also competitors for oxidation.
As shown in Fig. 7, which presents the variation of CSO2
4 =CBLb0 with ozonation time, the yield of sulfate
reaches nearly 100% in 60–180 min for the cases examined. It is found that the effects of the ozone concentration of feed gas and UV radiation are all important on the generation of sulfate. The generation of sulfate is consistent with the increase of ZTOCfor both sole O3and O3/UV treatments as shown in Fig. 8. It is seen that the more sulfates are formed via O3/UV than sole O3for ZTOCo40%: The enhancing effect of O3/UV over sole O3 on the generation of sulfate may be attributed to the cause that O3/UV treatments generates more hydroxyl free radicals than sole O3, which are non-selective and highly oxidative, thus increasing the opportunity of the release of sulfonic substituent via Reaction IV. As ZTOC increases to about 40–50%, the yields of sulfate for the four cases in Fig. 8 all approach to 100%.
3.3. Removal of TOC associated with variation of OPR During the decomposition of 2-NS, the solution still contains byproducts of high TOC contents as shown in Fig. 3. Fig. 9 shows the variation of TOC under different experimental conditions. The treatments with O3/UV take 60–120 min to completely eliminate TOCs. How-ever, it takes more than 240 min via the treatments with O3 Intermediates CO2 , SO42- , H 2O OH . I II II I IV V VI SO4 2-CO2 H2O SO3
-Fig. 4. Simplified scheme of the decomposition pathways of the ozonation of 2-NS with UV radiation.
0 5 10 15 20 25 30
Ozonation time (min) 0.0 0.2 0.4 0.6 0.8 1.0 CBL b / C BLb 0 (-)
Fig. 5. Variation of CBLb=CBLb0with time for the ozonation of 2-NS in semibatch system. J, n, & and K: CAGi0¼ 11 g/m3, 44 g/m3, 20 g/m3 with ½Iuv ¼ 60:35 W/m2, 40 g/m3 with ½Iuv ¼ 60:35 W/m2. CBLb¼ CBLb0at t ¼ 0:
Table 1
Comparison of ozonation results at times for complete eliminations of 2-NS ðtf ;NSÞ and mineralization of TOCs ðtf ;TOCÞ under different experimental conditions Experimental conditiona tf ;NS (min) CSO2 4 =CBLb0 b (%) ZTOC b (%) tf ;TOC (min) pHb pHc ORPb (mV) Case 1 CAGi0¼ 11 mg/L 30 15.36 13.70 NMd 3.88 NMd 534.4 Case 2 CAGi0¼ 44 mg/L 10 22.87 15.98 240 4.06 3.14 623.1
Case 3 CAGi0¼ 20 mg/L, ½Iuv ¼ 60:35 W/m2 20 40.82 12.61 120 3.26 3.01 596.9 Case 4 CAGi0¼ 40 mg/L, ½Iuv ¼ 60:35 W/m2 10 46.04 15.97 90 3.39 3.14 613.4
a
Initial values of pH, CBLb0are 5.20, 104 mg/L.
b Values at tf ;NS: c Values at tf ;TOC: d NM: not measured.
-SO3 O O -O H + -SO3 O3 + O 2 . . . O O O H H O3 H2O O3 H -SO 3 C O O H CH O -SO3 O H -SO3 H 2O -SO3 CH O CH CH O O H O H CH O 3 + H 2O2 + CH O CH CH CH O -SO3 CH O CH O -SO3 CH CH CH CH O -SO3 O O H O H 1 3 2 4 + H2O2 O H C + CH O (a) (b) CH CH O O H -SO3 O H O O H O H CH H O O H O CH O A B CH CH O O H -SO3 O O H C D E
Fig. 6. Ozonation mechanism of 2-NS with ozone molecule. The cases of initial attack by (a) ozone dipolar cyclo-addition on the 1, 2 bond, (b) an electrophilic substitution of ozone on carbon 1.
sole O3. Therefore, the introduction of UV radiation in 2-NS ozonation makes significant contribution for the mineralization. Moreover, the ozone concentration of feed gas also improves the removal of TOC for both sloe O3and O3/UV systems. For further illustrating the effects of CAGi0and UV radiation on the elimination
of TOC, Fig. 10 shows the variation of mean miner-alization rate, ðCTOC0 CTOCÞ=t; with ZTOC: It is seen
that the effect of UV radiation is not significant on enhancing the mean mineralization rate when
ZTOCo40%: Instead, CAGi0 affects the mineralization
rate remarkably. However, in the later ozonation stage (higher ZTOC), the values of ðCTOC0 CTOCÞ=t via
O3/UV are still high while those via sole O3 become low. This is because the intermediates in the later stage of the ozonation of 2-NS such as oxalic acid, phthalic acid, and formic acid, have low reactivity toward ozone molecule [13,22,23]. The occurrence of the acidic group causes the deactivation of the molecule. 0 20 40 60 80 100 ηTOC (%) 0.0 0.2 0.4 0.6 0.8 1.0 CSO 4 2 -/ CBLb 0 (
-) Fig. 8. CSO2
4 =CBLb0 vs. ZTOC for the ozonation of 2-NS in
semibatch system. ZTOC¼ ðCTOC0 CTOCÞ=CTOC0; CTOC¼ CTOC0 at t ¼ 0: Other notations are the same as specified in Fig. 5.
0 60 120 180 240
Ozonation time (min) 0.0 0.2 0.4 0.6 0.8 1.0 CTO C / CTO C0 ( -)
Fig. 9. Variation of CTOC=CTOC0with time for the ozonation of 2-NS in semibatch system. Notations are the same as specified in Fig. 5.
0 30 60 90 120 150 180
Ozonation time (min) 0.0 0.2 0.4 0.6 0.8 1.0 CSO 4 2 - / C BLb 0 (-) Fig. 7. Variation of CSO2
4 =CBLb0with time for the ozonation of
2-NS in semibatch system. Notations are the same as specified in Fig. 5. 0 20 40 60 80 100 ηTOC (%) 0.0 0.5 1.0 1.5 2.0 ( CTO C0 -CTO C ) / t (mg/L-min )
Fig. 10. ðCTOC0 CTOCÞ=t vs. ZTOCfor the ozonation of 2-NS in semibatch system. Notations are the same as specified in Fig. 5.
According to the previous studies on the oxidation of protocatechuic acid [24] and 4-chlorobenzaldehyde [25], the process of O3/UV has better performance for the degradation of protocatechuic acid and 4-chlorobenzaldehyde than ozonation alone. Thus, the oxidation reaction via hydroxyl free radicals is pre-dominant to proceed in the regime with higher ZTOC: These results then support the recommendation of employing O3/UV process in the treatment of 2-NS. Noting that the value of ðCTOC0 CTOCÞ=t in the early
stage of ozonation is low due to the cause that the ozone is mainly consumed for ring opening accompanying with the low diminish of TOCs.
Fig. 11(a) presents the variation of ORP of liquid with ZTOC: The use of ORP as a supplementary ozonation index of ozonation system has been introduced by Yu and Yu [26]. The experimental ORP data of the present study are further used to plot the average and smooth curve in Fig. 11(b) which shows the distinct variation of ORP with ZTOC: The value of ORP reveals slight variation in the initial period while increases signifi-cantly in the later period with higher ZTOC: For ZTOCo60%; the values of ORP are between 500 and 700 mV. In this stage, the organics of TOCs in the solution are predominant for controlling the ORP of system with small variation. While the concentration of TOCs (CTOC) decreases below 16 mg/L (ZTOC> 85%),
the ORP becomes >800 mV as shown in Fig. 11(b). As the ZTOC approaches to 100%, the value of ORP of system is close to the value of organic-free solution under the same condition. Thus, indeed, ORP is a useful index associated with the oxidation state of the ozonation of 2-NS in terms of ZTOC:
In summary, the difference between O3 and O3/UV treatments regarding the destruction rate of 2-NS is not significant, however, the condition of O3/UV gives significant contribution for the subsequent oxidation after the disappearance of 2-NS. Thus, the combination of ozone with UV radiation is recommended for treating the 2-NS solution as far as the TOC reduction is concerned, although the process with O3alone may be sufficient for removing the 2-NS.
It should be noted that, for treating the 2-NS in the wastewater containing other chemicals, the TOC value would be the lump of different organic sources. There-fore, the relations of CSO2
4 =CBLb0; ðCTOC0 CTOCÞ=t;
and ORP with ZTOC; and of CTOC=CTOC0 with time
obtained above for the case with 2-NS only may not be applicable to those with mixed chemicals. However, some comments may be noted. The removal percentage of TOC contributed by 2-NS in the wastewater during ozonation may be estimated by the value of CSO2
4 =CBLb0
according to Fig. 8 in the cases that the increasing CSO2
4 is mainly generated from the oxidation of 2-NS.
In addition, the trends of mineralization rates contributed by 2-NS under different cases as shown in
Fig. 9 may be referable for oxidizing the 2-NS in the wastewater. Of course, further work is needed for the detailed information of treating the 2-NS mixed with other chemicals by the ozonation in various waste-waters.
4. Conclusions
2-NS is an aqueous pollutant of environmental concern in the surface and wastewaters. Ozonation combined with UV radiation is employed as an effective way for the removal of 2-NS in the aqueous solution. The decomposition of 2-NS accompanies with the
0 20 40 60 80 100 ηTOC (%) 500 600 700 800 900 1000 1100 ORP (mv) 0 20 40 60 80 100 ηTOC (%) 500 600 700 800 900 1000 1100 ORP (mv) (a) (b)
Fig. 11. ORP vs. ZTOCfor the ozonation of 2-NS in semibatch system. Notations are the same as specified in Fig. 5. (a) Experimental data; (b) average and smooth curve of experi-mental data with R2¼ 0:867:
diminish of TOCs and generation of sulfate. The following conclusions may be drawn:
1. The decomposition rate of 2-NS increases with the ozone concentration of feed gas, while is not significantly enhanced by the presence of UV radiation. When the decomposition of 2-NS is completed, the removal efficiency of TOC (ZTOC) is between 12.61% and 15.98%.
2. Both the ozone concentration of feed gas and UV radiation can improve the generation rate of sulfate. When ZTOC is >50%, the yield of sulfate is nearly 100%.
3. In the early stage of ozonation with ZTOCo40%; the elimination of TOC is remarkably enhanced by the ozone concentration of feed gas while slightly by the UV radiation. However, in the later stage, the O3/UV system significantly gives higher mineraliza-tion rate than the sole O3system.
4. The ORP can be used as a supplementary index of oxidation state. Its value varies with the residual TOC concentration for the ozonation of 2-NS.
Acknowledgements
This study was supported by the National Science Council of Taiwan under Grant No. NSC 89-2211-E-002-107.
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