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chlorine process for ammonia removal and disinfection by-product reduction: Comparison with chlorination
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UV/chlorine process for ammonia removal and disinfection by-product reduction: Comparison with chlorination Xinran Zhang a, Weiguang Li a,b,*, Ernest R. Blatchley IIIc,d, Xiaoju Wang a, Pengfei Ren a a
School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, PR China State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology), Harbin 150090, PR China c Lyles School of Civil Engineering, Purdue University, West Lafayette, IN 47907-2051, USA d Division of Environmental & Ecological Engineering, Purdue University, West Lafayette, IN 47907-2051, USA b
article info
abstract
Article history:
The combined application of UV irradiation at 254 nm and chlorination (UV/chlorine
Received 10 July 2014
process) was investigated for ammonia removal in water treatment. The UV/chlorine
Received in revised form
process led to higher ammonia removal with less chlorine demand, as compared to
14 October 2014
breakpoint chlorination. Chlorination of NH3 led to NH2Cl formation in the first step. The
Accepted 20 October 2014
photolysis of NH2Cl and radical- mediated oxidation of ammonia appeared to represent the
Available online 11 November 2014
main pathways for ammonia removal. The trivalent nitrogen of ammonia was oxidized, presumably by reactions with aminyl radicals and chlorine radicals. Measured products
Keywords:
included NO 3 and NO2 ; it is likely that N2 and N2O were also generated. In addition, UV
Ammonia
irradiation appeared to have altered the reactivity of NOM toward free chlorine. The UV/
Ultraviolet
chlorine process had lower chlorine demand, less C-DBPs (THMs and HAAs), but more
Chlorination
HANs than chlorination. These results indicate that the UV/chlorine process could repre-
Natural organic matter
sent an alternative to conventional breakpoint chlorination for ammonia-containing
Disinfection by-products
water, with several advantages in terms of simplicity, short reaction time, and reduced chemical dosage. © 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
Chlorination is well-developed and commonly used in water treatment process (Wang et al., 2005; Deborde and Gunten, 2008). In disinfection process, chlorination could limit the growth of micro-organisms and remove the majority of
planktonic bacteria (Hoff and Geldreich, 1981). However, amino nitrogen and natural organic matter (NOM) in water sources can react with chlorine-based disinfectants to form chloramines and halogenated disinfection by-products (DBPs), respectively (Kim et al., 2003; Kulkarni and Chellam, 2010). Halogenated DBPs are known to express toxicity toward humans (Muellner et al., 2007; Chang et al., 2013).
* Corresponding author. School of Municipal and Environmental Engineering, Harbin Institute of Technology, Box No. 2602, 73 Huanghe Road, Harbin, Heilongjiang 150090, PR China. Tel.: þ86 13904512510; fax: þ86 0451 86283003. E-mail address: [email protected] (W. Li). http://dx.doi.org/10.1016/j.watres.2014.10.044 0043-1354/© 2014 Elsevier Ltd. All rights reserved.
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Moreover, chloramines are intermediates to yield nitrogenbased disinfection by-products (N-DBPs), which were more toxic than regulated carbon-based DBPs (C-DBPs) (Shah and Mitch, 2011; Wang et al., 2007). Therefore, the presence of ammonia and NOM could reduce disinfection efficacy and promote DBPs formation. Alternative water treatment methods that are effective for both ammonia degradation and NOM removal may yield benefits. The UV/chlorine process has been considered as a novel water treatment method. Under UVC irradiation, free chlorine photo-decomposes to form a hydroxyl radical (OH) and a , 1992). The quantum chlorine radical (Cl) (Nowell and Hoigne yield of the UV/chlorine process depends on the chlorine concentration, the wavelength(s) of the UV source, as well as water quality (Feng et al., 2007); however, the quantum yield for this process is close to 1 mol Es1 for most germicidal UV radiation. Watts and Linden (2007) reported that the UV254/ chlorine process has greater potential to generate $OH than the UV254/H2O2 process because the quantum yield and molar absorptivity for HOCl are higher than those of H2O2. However, molar absorptivities for HOCl and OCl are still relatively low, ranging from 20 to 350 M1 cm1 for wavelengths ranging from 200 nm l 350 nm (Feng et al., 2007). The UV/chlorine process offers potential improvements over other existing treatment processes. It has been demonstrated that the UV/chlorine process is effective for degradation of many pollutants, including methanol, chlorobenzoic acid, trichloroethylene, as well as some emerging contaminants (Wang et al., 2012; Jin et al., 2011; Sichel et al., 2011; Zhang et al., 2013). A solar-driven UV/chlorine advanced oxidation process also has been studied (Chan et al., 2012). Moreover, the UV/chlorine process can inactivate waterborne microorganisms through several parallel pathways (Jin et al., 2011). However, only limited information is available to describe the potential of the UV/chlorine process for ammonia and DBP removal, simultaneously. The goal of this research was to evaluate the effectiveness of the UV/chlorine process from the perspective of two treatment objectives: ammonia removal and reduction of DBP yield. To address these issues, experiments were conducted to examine the degradation of ammonia and DBP precursors, with particular attention to the production of the common CDBPs and N-DBPs including: trihalomethanes (THMs), haloaceticacids (HAAs), and haloacetonitriles (HANs). The effects of chlorine to ammonia molar ratio (Cl2/N) and chlorination contact time for ammonia removal and DBPs formation on the UV/chlorine process were investigated. Furthermore, the above two factors were compared with those of chlorination.
2.
Materials and methods
2.1.
Experimental procedures
All reagents used in the experiments were analytical grade purchased from SigmaeAldrich Ltd., (Ontario, Canada). Free chlorine stock solutions (100 mg L1 as Cl2) were prepared by dilution of a 10% (by weight) sodium hypochlorite solution with deionized water. Free chlorine solutions were freshly prepared before each experiment. 0.02 M phosphate buffer
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solution was added into samples and the pH value was adjusted to 7.20 with 1.00 M NaOH or 0.50 M H2SO4. Samples used in this study included filtered water collected from a municipal drinking water treatment facility (the seventh WTP, Harbin, China), and all samples were used within 24 h of collection. Treatment processes located upstream of the sample collection point included aluminum sulfate coagulation, inclined plate sedimentation, and (quartz) sand filtration. To avoid the effect of large particles, water samples were passed through 0.45 mm fiber filters before the experiments. After NH4Cl addition (as needed), water quality characteristics of the samples were ammonia-N 1.00 mg L1 (7.00 102 mM) and TOC 3.20 mg L1. Chemical and photochemical processes were conducted in a 1.50 L UV reactor (stainless steel, L ¼ 61.00 cm, d ¼ 8.00 cm) as shown in Fig. 1. The UV reactor was equipped with a 25 W low pressure Hg lamp (253.7 nm, HNG, Germany), which was placed into a quartz sleeve (L ¼ 61.00 cm, d ¼ 2.50 cm) along the longitudinal axis of the reactor. The lamp was operated before use for 20 min to promote stability of lamp output. The nominal fluence delivered by the UV reactor was estimated by application of NH2Cl as a chemical actinometer. In this experiment, NH2Cl was applied at the influent of the reactor at a concentration of 1.00 mg/L (as N). The steady-state NH2Cl concentration in the effluent from the reactor was also measured. Similarly, the NH2Cl solution was subjected to a range of UV254 doses using a collimated beam. By comparing NH2Cl photodecay from the collimated beam with the steadystate performance of the reactor, an effective UV254 dose was estimated. This method is similar to the work of Bolton and Stefan (2002), except that their work involved free chlorine as the actinometer, while NH2Cl was used here.
2.2.
Analytical methods
Ammonia concentrations were measured by the ammoniaselective electrode method (4500-NH3 D, APHA, 2005) using an ammonia selective electrode (Orion Co., 95-12, USA) and pH/ISE meter (Thermo Orion Co., 720A, USA). The concentrations of inorganic chloramines (mono-, di- and tri-) were
Fig. 1 e Schematic illustration of photochemical reactor (1) influent tank; (2) valves; (3) sample pump; (4) chemical tank; (5) valves; (6) chemical pump; (7) mixer; (8) flow meter; (9) sample inlet; (10) UV reactor body; (11) quartz sleeve; (12) low-pressure mercury lamp; (13) sample outlet; (14) effluent tank.
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analyzed by the membrane introduction mass spectrometry (MIMS), as described previously (Shang and Blatchley, 1999; Weng et al., 2012). Free chlorine and total chlorine were monitored with the DPD colorimetric method (4500-Cl G) (APHA, 2005). Solution pH was measured with a pH meter (Thermo Orion Co., 720A, USA). UV254 absorbance and total organic carbon (TOC) were measured with a UVeVis spectrophotometer (Hitachi Co., U-3010, Japan) and a TOC analyzer (Shimadzu Co., TOC-VCPN, Japan), respectively. Analysis of DBP concentrations, including THMs, HAAs and HANs, were performed by a GC-6890N (Agilent, America), based on US EPA method series (US EPA Method 524.2, 1995; US EPA Method 552.2, 1995; US EPA Method 551.1, 1995).
3.
Results and discussion
3.1.
Comparison of negative trivalent nitrogen removal
3.1.1. UV absorption spectra of chlorine, monochloramine and ammonia The molar absorptivities of chlorine, monochloramine and ammonia at pH 7.2 and pH 10.0 are illustrated in Fig. 2. Since þ the pKa for NHþ 4 is 9.25, NH4 is the dominant form of total ammonia at pH 7.2, whereas NH3 becomes the dominant form at pH 10.0. As illustrated in Fig. 2, the maximum molar absorptivity of monochloramine was observed at 243 nm, with a value of is 431 M1 cm1 at pH 10.0; the value at pH 7.2 was slightly lower. The molar absorptivities of chlorine varied with pH, and maximum absorbance was observed at 292 nm. At pH 7.2, the molar absorptivities of chlorine and monochloramine at 254 nm were 60 M1 cm1 and 354 M1 cm1, respectively, which are comparable with previously reported values (Li and Blatchley, 2009; De Laat et al., 2010). These data indicated that NH2Cl is a stronger absorber of UVC radiation than either HOCl or OCl. Ammonia has a weak absorption band at wavelengths greater than 220 nm, regardless of the form: NHþ 4 -N (pH at 7.2) or NH3-N (pH at 10.0). Therefore, UV irradiation (alone) could promote photodecomposition of NH2Cl (and
Fig. 2 e Molar absorptivity of monochloramine, free chlorine and ammonia at pH ¼ 7.20 and pH ¼ 10.0 in aqueous solution.
other inorganic chloramines) and free chlorine, but had essentially no effect on ammonia degradation.
3.1.2.
Comparison of negative trivalent nitrogen removal
In this study, the sum of the concentrations of chloramines and ammonia-N is referred to negative trivalent nitrogen, expressed as N (-III). Inorganic chloramines and ammonia can interconvert by substitution, hydrolysis, and monochloramine disproportionation (Vikesland et al., 1998; Jafvert and Valentine, 1992; Vikesland et al., 2001). Fig. 3 illustrates timecourse and UV254 dose-dependent changes of N (-III) concentration resulting from UV254 irradiation, chlorination, and the UV/chlorine process at pH 7.20. When the samples were exposed to UV254 irradiation alone, the concentration of ammonia-N showed essentially no change, which was consistent with the results described in Section 3.1.1. At a Cl2/ N molar ratio 0.80, chlorine substitution occurred to form NH2Cl. The concentration of N (-III) decreased only about 10%. In the UV/chlorine process, the concentration of negative trivalent nitrogen decreased with increasing UV254 dose. 50% of N (-III) was oxidized after exposure to UV254 at dose 800 mJ cm2 with Cl2/N molar ratio 0.80. Therefore, an apparent synergistic effect in the UV/chlorine process was observed to promote N (-III) oxidization. At Cl2/N molar ratio 0.8, NH2Cl was the dominant for of N (-III) in the system due to the large rate constant for N-chlorination of ammonia (Deborde and Gunten, 2008), although some residual ammonia also existed in the system. UV254 irradiation promotes cleavage of the NeCl bond of monochloramine to yield an aminyl radical and chlorine radical (Eq. (1)) (Li and Blatchley, 2009). It has been reported that the chlorine radicals will promote ammonia decomposition (Xiao et al., 2009), as expressed in Eq. (2). Moreover, NH2 radicals will be oxidized to form NH2O2 radicals in the presence of oxygen (De Laat et al., 2010), as indicated by Eq. (3).
Fig. 3 e Negative trivalent nitrogen removal as a function of UV254 irradiation, chlorination, and the UV/Chlorine process ([Ammonia-N]0 ¼ 0.07 mM, Cl2/N molar ratio ¼ 0.80, pH ¼ 7.20). The secondary horizontal axis refers to the dark decay experiment for ammonia chlorination. Symbols are data points and lines are model calculations.
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The final nitrogenous products formed in the UV/chlorine process were nitrite and nitrate, as summarized by Eq. (4). As shown in Table 1, the concentration of nitrate and nitrite increased steadily with increasing UV254 dose. Moreover, the decrease of total nitrogen concentration indicated that detectable nitrogen was lost from the system. It is hypothesized that some N2 also formed during this process. Nitrous oxide also formed during the photodegradation of monochloramine (Eq. (5) and (6)) (Schreiber and Mitch, 2007; Bazylinski and Hollocher, 1985). hv
NH2 Cl!NH2 $ þ $Cl
(1)
NH3 þ $Cl/NH2 $ þ Hþ þ Cl
(2)
$NH2 þ O2 /NH2 OO$
(3) O2
O2
NH2 OO$/NO$ þ H2 O!NO 2 !NO3
(4)
NH2 OO$/HNO þ H2 O2
(5)
2HNO/N2 O þ H2 O
(6)
3.1.3.
Effect of Cl2/N ratio
Fig 4(a) illustrates the concentration of total N (-III) and ammonia at different Cl2/N molar ratios resulting from the UV/chlorine process and chlorination process. The samples were exposed to UV254 irradiation for 200 s in the UV/chlorine process, and the same contact time (200 s) was applied in chlorination. In chlorination process, total N (-III) declined slightly at Cl2/N molar ratios less than 1.0. At the Cl2/N molar ratio in range of 1.0e1.6, oxidation of N (-III) was opened up and the concentration of N (-III) were decrease. At a Cl2/N molar ratio of 1.60 or more, negative trivalent nitrogen was totally oxidized to stable nitrogen compounds, predominantly N2 and NO 3 , as defined by the chemistry of breakpoint chlorination (Jafvert and Valentine, 1992). Comparably, the UV/chlorine process exhibited higher N (-III) removal efficiency than chlorination process. The concentration of N (-III) dropped sharply and linearly with increasing Cl2/N molar ratio (up to 1.6). In the UV/chlorine process, oxidation of N (-III) was even observed at Cl2/N molar ratios less than 1.0. Therefore, the UV/chlorine process could significantly reduce chlorine dosage in nitrogen containing water treatment. It was also worth noting that the UV/chlorine process improved ammonia-N removal (15%e20%)
Table 1 e Nitrogenous product in the UV/chlorine process (Cl2/N molar ratio 0.80, pH ¼ 7.2, background nitrate 1.41 mg L¡1). Nominal UV254 dose (mJ cm2) 0 100 200 400 600 800
NHþ NO NO Total N 4 -N 2 -N 3 -N 1 1 (mg L ) (mg L ) (mg L1) (mg L1) 1.04 0.74 0.67 0.59 0.56 0.51
0.00 0.00 0.00 0.02 0.03 0.04
1.41 1.47 1.50 1.53 1.54 1.66
2.60 2.35 2.34 2.32 2.32 2.31
Fig. 4 e Comparison of (a) nitrogen removal and (b) residual chlorine concentration as a function of Cl2/N molar ratio for the UV/chlorine process and chlorination process ([Ammonia-N]0 ¼ 0.07 mM, pH ¼ 7.20, UV dose 800 mJ/cm2 for UV/Chlorine process, reaction time 200 s for chlorination).
compared with chlorination process at same Cl2/N molar ratios. These results indicate that ammonia oxidation can be promoted by chloramine photodecay. Therefore, the decrease of N (-III) concentration was presumed to be the combination result from not only photodecomposition of chloramine but also from the advanced oxidation reaction of ammonia. Li and Blatchley (2009) reported that the apparent quantum yield (ɸ254) for aqueous NH2Cl, at a Cl2/N molar ratio 1.05:1, was 0.62 ± 0.049 mol Es1. In this study, ɸ254 was 0.29 ± 0.02 mol Es1, at the Cl2/N molar ratio 0.80. This result was in agreement with results previously at similar Cl2/N molar ratios reported by Cooper et al. (2007) (0.26 mol Es1, Cl2/ N molar ratio 0.33:1, pH 8.5) and by Watts and Linden (2007) (0.3 mol Es1, Cl2/N molar ratio 0.78:1, pH 9). The decrease of the apparent NH2Cl quantum yield in the presence of ammonia was presumed to be the result of radical-mediated reactions with NH3. As can be seen from Fig. 4(b), in both chlorination process and UV/chlorine process, the variation of free chlorine and total chlorine concentration at different Cl2/ N molar ratios were consistent with conventional breakpoint chlorination curve (Jafvert and Valentine, 1992). At Cl2/N
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molar ratios of 1.0 or less, monochloramine was the dominant inorganic chloramine. For Cl2/N molar ratios from 1.0 to 1.6, dichloramine and trichloramine were formed. When the Cl2/N molar ratio exceeded 1.6, free chlorine was the dominant form of residual chlorine, with smaller quantities of NCl3. The concentrations of both total chlorine and free chlorine in UV/ chlorine process were lower than that in chlorination process at all Cl2/N molar ratios, presumably due to the photodecomposition of chloramine and free chlorine by UV254 irradiation (Watts and Linden, 2007).
3.2.
Comparison of DBP precursor removal
Since natural organic matter (NOM) has been identified as an important DBP precursor, control of NOM is important for control of DBP formation. Fig. 5 illustrates the effects of the Cl2/N molar ratio on NOM removal in chlorination and the UV/ chlorine process, while the hydraulic loading (residence time), the initial concentrations of NOM, as well as initial ammonia concentration remained the same. The results of these experiments indicate that TOC removal rate was appreciably higher in the UV/chlorine process than in the chlorination process. For example, at Cl2/N molar ratio 0.8 (mass ratio 4), TOC removal was almost 47% by the UV/chlorine process, whereas only 22% TOC was removed by chlorination. Previous investigations have indicated that NOM can be degraded by advanced oxidation processes (AOPs) through abstraction or addition reactions (Wang et al., 2006; Toor and Mohseni, 2007). Moreover, Chan et al. (2012) reported that the combined application of UV/chlorine represents an advanced oxidation process that can promote degradation of a wide variety of organic compounds. Fig. 5 presents the UV254 absorption changes in the two processes with different Cl2/N molar ratios. During chlorination, UV254 absorbance increased up to Cl2/N molar ratio at 0.8 (mass ratio 4), and then a steady decline was observed as the Cl2/N ratio further increased. The precise cause(s) of the initial increase of UV254 absorbance is (are) not known, but was probably due to production of intermediate compounds that
Fig. 5 e Comparison of TOC and UV254 absorbance as a function of Cl2/N molar ratio for chlorination and the UV/ chlorine process ([TOC]0 ¼ 3.20 mg L¡1, UV dose ¼ 800 mJ/ cm2 for UV/Chlorine process, pH ¼ 7.20).
were stronger absorbers than their parent compounds. However, the UV/chlorine process yielded a steady decline of UV254 absorbance with increasing Cl2/N molar ratio. UV254 absorbance is largely attributable to unsaturated bonds in organic lu and molecules, such as aromatic groups (Akmehmet Balcıog € Otker, 2003); however, N-chlorination is also known to increase UV254 absorbance (see Fig. 2). The UV/chlorine process may be effective for opening of aromatic ring structures, thereby yielding products that are characterized by lower UV254 absorbance, such as carbonyl and carboxylic functional groups. These results suggest that the UV/chlorine process yielded similar behavior to other AOPs, such as UV/H2O2 and UV/TiO2, which have been shown to oxidize organic pollutants by powerful, non-selective radicals (Sanly et al., 2007; Lin and Wang, 2011).
3.3.
Comparison of DBP formation
3.3.1.
THM and HAA formation
Fig. 6 illustrates a comparison of trihalomethane (THM) and haloacetic acid (HAA) formation in the chlorination process
Fig. 6 e Comparison of (a) THM formation and (b) HAA formation as a function of dark incubation period (chlorination contact time) by the UV/chlorine process (left bar) and chlorination process (right bar) ([TOC]0 ¼ 3.20 mg L¡1, [Residual chlorine]0 ¼ 1.50 mg L¡1 as Cl2, UV dose ¼ 800 mJ/cm2 for UV/Chlorine process, pH ¼ 7.20).
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and the UV/chlorine process. In both processes, the initial chlorine dosages were chosen to satisfy the requirement of Chinese drinking water standard, in terms of negative trivalent nitrogen (less than 0.5 mg L1) and total chlorine (0.50e3.00 mg L1). Since the water samples contained 0.07 mM ammonia-N, the Cl2/N molar ratio was 1.6 (chlorine dosage 0.11 mM) in the chlorination process to achieve the breakpoint. In the UV/chlorine process, the Cl2/N molar ratio was fixed at 0.8 (chlorine dosage 0.06 mM) and UV dose was 800 mJ cm2. The residual chlorine concentrations were fixed at 0.02 mM (1.50 mg L1 as Cl2) in both processes and the samples were placed in a dark incubator at 25 C. After a range of dark incubation periods (30 mine72 h), the reaction was quenched with a Na2SO3 solution for DBP detection. As shown in Fig 6(a), there were lower concentrations of THMs generated in the UV/chlorine process than the chlorination process. Chloroform (TCM) was the most abundant THM found in both processes. The concentration of TCM ranged from 15 to 19 mg L1 and the amounts were similar in the two processes. Moreover, the TCM concentration did not change appreciably during the 72 h of the experiments. Bromodichloromethane (BDCM) was the other THM species produced in chlorination, and accounted for 59% of the total THMs after 72 h of chlorination. However, the concentration of BDCM was below the detection limit in the combined process (limit of detection was 0.50 mg L1). In addition, chlorodibromomethane (CDBM) and bromoform (TBM) were not detected in either process. It has been previously reported that CeBr bonds tend to more sensitive to UV254 irradiation than CeCl bond (Cimetiere and De Laat, 2014), which was consistent with these results. Therefore, the UV/chlorine process could reduce the formation of brominated THMs, thereby decreasing the total concentration of THMs. The yields of HAAs rose gradually with the increase of the incubation period in both processes, as presented in Fig. 6(b). HAAs produced in the UV/chlorine process ranged from 9.27 to 18.84 mg L1, which was lower than in the chlorination process (from 22.30 to 38.03 mg L1). Dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA) were the two dominant HAAs formed in both processes, whereas monochloroacetic acid (MCAA), monobromoacetic acid (BCAA) and dibromoacetic acid (DBAA) were not detected in samples because their concentrations were below the limit of detection (0.50 mg L1). Therefore, the application of UV254 irradiation did not affect HAA speciation, but did appear to reduce overall production of HAAs. The experimental results indicated that the UV/chlorine process produced less C-DBPs (THMs and HAAs) than chlorination. These results consist with the recent study reported that the low pressure UV did not improve DBP formation (Reckhow et al., 2010). In ammonia-containing water treatment, the chlorine dosage in the UV/chlorine process is less than that in chlorination process, which could reduce chlorinated DBP formation. Simultaneously, the species of residual chlorine in the UV/chlorine process and chlorination process were monochloramine and free chlorine, respectively. It has been reported that monochloramine resulted in lower DBP production, in terms of TCM, DBCM, TCAA and DCAA, than free chlorine (Bougeard et al., 2010). In addition, as mentioned in section 3.2, the UV/chlorine process could change the
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structures of NOM by opening aromatic ring structures, thereby reducing the concentration of DBP precursors and subsequent THM formation. These results is consisted with studies reported that UV irradiation subsequent to chlorination could change the NOM structure (Liu et al., 2006; Dotson et al., 2010). Moreover, the UV irradiation at 254 nm could also destruct DBPs, as reported by Cimetiere and De Laat (2014).
3.3.2.
HAN formation
Fig. 7 shows a comparison of HAN production in the two processes, in terms of dichloroacetonitrile (DCAN), trichloroacetonitrile (TCAN), dibromoacetonitrile (DBAN), bromochloroacetonitrile (BCAN). The experimental conditions were same as the experiment described in section 3.3.1. Total HAN concentrations formed in the UV/chlorine process and chlorination process were 39.30 and 20.52 mg L1, respectively. DCAN accounted for 79% and TCAN for 21% to total HANs in the combined process and the proportions were DCAN 86% and TCAN 14% in chlorination process. DBAN and BCAN were not detected in any samples (detection limit 1.00 mg L1). Similar results were reported by Chu et al. (2012) in an investigation of tyrosine-containing water treatment by combined chlor(am)ination and UV irradiation. It has been reported that a high concentration of chlorine could accelerate DCAN decomposition (Reckhow et al., 2001), which may have contributed to lower DCAN concentration in chlorination than in UV/chlorine process. However, UV254 irradiation of some N-chloro compounds has also been demonstrated to promote halonitrile formation, especially when conducted in the presence of free chlorine (Weng et al., 2012).
4.
Conclusion
This work compared and evaluated the efficiency of chlorination and UV/chlorine for ammonia nitrogen (negative
Fig. 7 e Comparison of HAN formation as a function of dark incubation period (chlorination contact time) by the UV/ chlorine process (left bar) and chlorination process (right bar) ([TOC]0 ¼ 3.20 mg L¡1, [Residual chlorine]0 ¼ 1.50 mg L¡1 as Cl2, UV dose ¼ 800 mJ/cm2 for UV/Chlorine process, pH ¼ 7.20).
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trivalent nitrogen) removal and DBP formation in ammonia containing water treatment. The experimental results demonstrated that the UV/chlorine process is more effective for N (-III) removal than chlorination, with lower chlorine dosage and higher removal rate. In the chlorination process, N-chlorination was the dominant reaction with little N (-III) oxidation when Cl2/N molar ratio was less than 1.0. However, in the UV/chlorine process, the oxidation of N (-III) was achieved at Cl2/N ratios ranging from 0 to 1.6 through the UV254 photolysis of chloramines and advanced oxidation reactions. 50% N (-III) removal was achieved with the condition that Cl2/ N molar ratio 0.8 and UV dose 800 mJ/cm2. The required UV dose could achieve by adjusting the UV254 intensity and UV254 exposure time. In addition, the higher molar ratio of chlorine and nitrogen, the less UV dose needed. Moreover, the UV/ chlorine process was more effective for decomposition of NOM than chlorination, as measured by changes of TOC and UV254 absorption. Furthermore, the UV/chlorine process generated less THMs and HAAs, but more HANs compared with chlorination. The results suggest that the UV/chlorine process may represent an effective option for ammonia-N containing water treatment. However, the enhanced formation of HANs in the combined process should be taken into consideration.
Acknowledgments This work was financially supported by Heilongjiang Province Funds for Distinguished Young Scientists (JC200708).
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Chu, W., Gao, N., Krasner, S.W., Templeton, M.R., Yin, D., 2012. Formation of halogenated C-, N-DBPs from chlor (am) ination and UV irradiation of tyrosine in drinking water. Environ. Pollut. 161, 8e14. Cimetiere, N., De Laat, J., 2014. Effects of UV-dechloramination of swimming pool water on the formation of disinfection byproducts: a lab-scale study. Microchem. J. 112, 34e41. Cooper, W.J., Jones, A.C., Whitehead, R.F., Zika, R.G., 2007. Sunlight-induced photochemical decay of oxidants in natural waters: implications in ballast water treatment. Environ. Sci. Technol. 41 (10), 3728e3733. De Laat, J., Boudiaf, N., Dossier-Berne, F., 2010. Effect of dissolved oxygen on the photodecomposition of monochloramine and dichloramine in aqueous solution by UV irradiation at 253.7 nm. Water Res. 44 (10), 3261e3269. Deborde, M., Gunten, U.V., 2008. Reactions of chlorine with inorganic and organic compounds during water treatmentkinetics and mechanisms: a critical review. Water Res. 42 (1), 13e51. Dotson, A.D., Keen, V.O.S., Metz, D., Linden, K.G., 2010. UV/H2O2 treatment of drinking water increases post-chlorination DBP formation. Water Res. 44 (12), 3703e3713. Feng, Y., Smith, D.W., Bolton, J.R., 2007. Photolysis of aqueous free chlorine species (HOCl and OCl ) with 254 nm ultraviolet light. J. Environ. Eng. Sci. 6 (3), 277e284. Hoff, J.C., Geldreich, E.E., 1981. Comparison of the biocidal efficiency of alternative disinfectants. J. Am. Water Works Assoc. 73, 40e44. Jafvert, C.T., Valentine, R.L., 1992. Reaction scheme for the chlorination of ammoniacal water. Environ. Sci. Technol. 26 (3), 577e586. Jin, J., El-Din, M.G., Bolton, J.R., 2011. Assessment of the UV/ Chlorine process as an advanced oxidation process. Water Res. 45 (4), 1890e1896. Kim, J., Chung, Y., Shin, D., Kim, M., Lee, Y., Lim, Y., Lee, D., 2003. Chlorination by-products in surface water treatment process. Desalination 151 (1), 1e9. Kulkarni, P., Chellam, S., 2010. Disinfection by-product formation following chlorination of drinking water: artificial neural network models and changes in speciation with treatment. Sci. Total Environ. 408 (19), 4202e4210. Li, J., Blatchley III, E.R., 2009. UV photodegradation of inorganic chloramines. Environ. Sci. Technol. 43 (1), 60e65. Lin, H.C., Wang, G.S., 2011. Effects of UV/H2O2 on NOM fractionation and corresponding DBPs formation. Desalination 270, 221e226. Liu, W., Cheung, L.M., Yang, X., Shang, C., 2006. THM, HAA and CNCl formation from UV irradiation and chlor (am) ination of selected organic waters. Water Res. 40 (10), 2033e2043. Muellner, M.G., Wagner, E.D., McCalla, K., Richardson, S.D., Woo, Y.T., Plewa, M.J., 2007. Haloacetonitriles vs. regulated haloacetic acids: are nitrogen-containing DBPs more toxic? Environ. Sci. Technol. 41 (2), 645e651. , J., 1992. Photolysis of aqueous chlorine at Nowell, L.H., Hoigne sunlight and ultraviolet wavelengths- II. Hydroxyl radical production. Water Res. 26 (5), 599e605. Reckhow, D.A., Platt, T.L., MacNeill, A., McClellan, J.N., 2001. Formation and degradation of DCAN in drinking waters. J. Water Supply Res. Technol- Aqua 50, 1e13. Reckhow, D.A., Linden, K.G., Kim, J., Shemer, H., Makdissy, G., 2010. Effect of UV treatment on DBP formation. J. Am. Water Works Assoc. 102 (6), 100e113. Sanly, L.M., Chiang, K., Amal, R., Fabris, R., Chow, C., Drikas, M., 2007. A study on the removal of humic acid using advanced oxidation processes. Sep. Sci. Technol. 42 (7), 1391e1404. Schreiber, I.M., Mitch, W.A., 2007. Enhanced nitrogenous disinfection byproduct formation near the breakpoint:
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implications for nitrification control. Environ. Sci. Technol. 41, 7039e7046. Shah, A.D., Mitch, W.A., 2011. Halonitroalkanes, halonitriles, Haloamides, and N-Nitrosamines: a critical review of nitrogenous disinfection byproduct formation pathways. Environ. Sci. Technol. 46 (1), 119e131. Shang, C., Blatchley III, E.R., 1999. Differentiation and quantification of free chlorine and inorganic chloramines in aqueous solution by MIMS. Environ. Sci. Technol. 33 (13), 2218e2223. Sichel, C., Garcia, C., Andre, K., 2011. Feasibility studies: UV/ chlorine advanced oxidation treatment for the removal of emerging contaminants. Water Res. 45 (19), 6371e6380. Toor, R., Mohseni, M., 2007. UV-H2O2 based AOP and its integration with biological activated carbon treatment for DBP reduction in drinking water. Chemosphere 66 (11), 2087e2095. US EPA Method 524.2, 1995. Measurement of Purgeable Organic Compounds in Water by Capillary Column Gas Chromatography/Mass Spectrometry. National Exposure Research Laboratory Office of Research and Development, Cincinnati, OHIO, USA. US EPA Method 551.1, 1995. Determination of Chlorination Disinfection Byprod-ucts, Chlorinated Solvents, and Halogenated Pesticides/Herbicides in Drinking Water by Liquid- Liquid Extraction and Gas Chromatography with Electron-capture Detection. National Exposure Research Laboratory Office of Research and Development, Cincinnati, OHIO, USA. US EPA Method 552.2, 1995. Determination of Haloacetic Acids and Dalapon in Drinking Water by Liquideliquid Extraction, Derivatization and Gas Chromatog-raphy with Electron Capture Detection. National Exposure Research Laboratory Office of Research and Development, Cincinnati, OHIO, USA. Vikesland, P.J., Ozekin, K., Valentine, R.L., 1998. Effect of natural organic matter on monochloramine decomposition: pathway
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elucidation through the use of mass and redox balances. Environ. Sci. Technol. 32 (10), 1409e1416. Vikesland, P.J., Ozekin, K., Valentine, R.L., 2001. Monochloramine decay in model and distribution system waters. Water Res. 35 (7), 1766e1776. Wang, L.K., Hung, Y.T., Shammas, N.K., 2005. Halogenation and disinfection. In: Wang, L.K., Yuan, P.C., Hung, Y.T. (Eds.), Handbook of Environmental Engineering, Physicochemical Treatment Processes, vol. 3. Humana Press Inc, Totowa, NJ, USA, pp. 288e293. Wang, G.S., Liao, C.H., Chen, H.W., Yang, H.C., 2006. Characteristics of natural organic matter degradation in water by UV/H2O2 treatment. Environ. Technol. 27 (3), 277e287. Wang, L.S., Hu, H.Y., Wang, C., 2007. Effect of ammonia nitrogen and dissolved organic matter fractions on the genotoxicity of wastewater effluent during chlorine disinfection. Environ. Sci. Technol. 41 (1), 160e165. Wang, D., Bolton, J.R., Hofmann, R., 2012. Medium pressure UV combined with chlorine advanced oxidation for trichloroethylene destruction in a model water. Water Res. 46, 4677e4686. Watts, M.J., Linden, K.G., 2007. Chlorine photolysis and subsequent OH radical production during UV treatment of chlorinated water. Water Res. 41 (13), 2871e2878. Weng, S., Li, J., Blatchley III, E.R., 2012. Effects of UV254 irradiation on residual chlorine and DBPs in chlorination of model organic-N precursors in swimming pools. Water Res. 46 (8), 2674e2682. Xiao, S., Qu, J., Zhao, X., Liu, H., Wan, D., 2009. Electrochemical process combined with UV light irradiation for synergistic degradation of ammonia in chloride-containing solutions. Water Res. 43 (5), 1432e1440. Zhang, X.R., Li, W.G., Ren, P.F., 2013. Natural organic matter removal by UV/chlorine process: modeling and optimization. Adv. Mater. Res. 807-809, 466e471.
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UV/chlorine process for ammonia removal and disinfection by-product reduction: Comparison with chlorination Xinran Zhang a, Weiguang Li a,b,*, Ernest R. Blatchley IIIc,d, Xiaoju Wang a, Pengfei Ren a a
School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, PR China State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology), Harbin 150090, PR China c Lyles School of Civil Engineering, Purdue University, West Lafayette, IN 47907-2051, USA d Division of Environmental & Ecological Engineering, Purdue University, West Lafayette, IN 47907-2051, USA b
article info
abstract
Article history:
The combined application of UV irradiation at 254 nm and chlorination (UV/chlorine
Received 10 July 2014
process) was investigated for ammonia removal in water treatment. The UV/chlorine
Received in revised form
process led to higher ammonia removal with less chlorine demand, as compared to
14 October 2014
breakpoint chlorination. Chlorination of NH3 led to NH2Cl formation in the first step. The
Accepted 20 October 2014
photolysis of NH2Cl and radical- mediated oxidation of ammonia appeared to represent the
Available online 11 November 2014
main pathways for ammonia removal. The trivalent nitrogen of ammonia was oxidized, presumably by reactions with aminyl radicals and chlorine radicals. Measured products
Keywords:
included NO 3 and NO2 ; it is likely that N2 and N2O were also generated. In addition, UV
Ammonia
irradiation appeared to have altered the reactivity of NOM toward free chlorine. The UV/
Ultraviolet
chlorine process had lower chlorine demand, less C-DBPs (THMs and HAAs), but more
Chlorination
HANs than chlorination. These results indicate that the UV/chlorine process could repre-
Natural organic matter
sent an alternative to conventional breakpoint chlorination for ammonia-containing
Disinfection by-products
water, with several advantages in terms of simplicity, short reaction time, and reduced chemical dosage. © 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
Chlorination is well-developed and commonly used in water treatment process (Wang et al., 2005; Deborde and Gunten, 2008). In disinfection process, chlorination could limit the growth of micro-organisms and remove the majority of
planktonic bacteria (Hoff and Geldreich, 1981). However, amino nitrogen and natural organic matter (NOM) in water sources can react with chlorine-based disinfectants to form chloramines and halogenated disinfection by-products (DBPs), respectively (Kim et al., 2003; Kulkarni and Chellam, 2010). Halogenated DBPs are known to express toxicity toward humans (Muellner et al., 2007; Chang et al., 2013).
* Corresponding author. School of Municipal and Environmental Engineering, Harbin Institute of Technology, Box No. 2602, 73 Huanghe Road, Harbin, Heilongjiang 150090, PR China. Tel.: þ86 13904512510; fax: þ86 0451 86283003. E-mail address: [email protected] (W. Li). http://dx.doi.org/10.1016/j.watres.2014.10.044 0043-1354/© 2014 Elsevier Ltd. All rights reserved.
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Moreover, chloramines are intermediates to yield nitrogenbased disinfection by-products (N-DBPs), which were more toxic than regulated carbon-based DBPs (C-DBPs) (Shah and Mitch, 2011; Wang et al., 2007). Therefore, the presence of ammonia and NOM could reduce disinfection efficacy and promote DBPs formation. Alternative water treatment methods that are effective for both ammonia degradation and NOM removal may yield benefits. The UV/chlorine process has been considered as a novel water treatment method. Under UVC irradiation, free chlorine photo-decomposes to form a hydroxyl radical (OH) and a , 1992). The quantum chlorine radical (Cl) (Nowell and Hoigne yield of the UV/chlorine process depends on the chlorine concentration, the wavelength(s) of the UV source, as well as water quality (Feng et al., 2007); however, the quantum yield for this process is close to 1 mol Es1 for most germicidal UV radiation. Watts and Linden (2007) reported that the UV254/ chlorine process has greater potential to generate $OH than the UV254/H2O2 process because the quantum yield and molar absorptivity for HOCl are higher than those of H2O2. However, molar absorptivities for HOCl and OCl are still relatively low, ranging from 20 to 350 M1 cm1 for wavelengths ranging from 200 nm l 350 nm (Feng et al., 2007). The UV/chlorine process offers potential improvements over other existing treatment processes. It has been demonstrated that the UV/chlorine process is effective for degradation of many pollutants, including methanol, chlorobenzoic acid, trichloroethylene, as well as some emerging contaminants (Wang et al., 2012; Jin et al., 2011; Sichel et al., 2011; Zhang et al., 2013). A solar-driven UV/chlorine advanced oxidation process also has been studied (Chan et al., 2012). Moreover, the UV/chlorine process can inactivate waterborne microorganisms through several parallel pathways (Jin et al., 2011). However, only limited information is available to describe the potential of the UV/chlorine process for ammonia and DBP removal, simultaneously. The goal of this research was to evaluate the effectiveness of the UV/chlorine process from the perspective of two treatment objectives: ammonia removal and reduction of DBP yield. To address these issues, experiments were conducted to examine the degradation of ammonia and DBP precursors, with particular attention to the production of the common CDBPs and N-DBPs including: trihalomethanes (THMs), haloaceticacids (HAAs), and haloacetonitriles (HANs). The effects of chlorine to ammonia molar ratio (Cl2/N) and chlorination contact time for ammonia removal and DBPs formation on the UV/chlorine process were investigated. Furthermore, the above two factors were compared with those of chlorination.
2.
Materials and methods
2.1.
Experimental procedures
All reagents used in the experiments were analytical grade purchased from SigmaeAldrich Ltd., (Ontario, Canada). Free chlorine stock solutions (100 mg L1 as Cl2) were prepared by dilution of a 10% (by weight) sodium hypochlorite solution with deionized water. Free chlorine solutions were freshly prepared before each experiment. 0.02 M phosphate buffer
805
solution was added into samples and the pH value was adjusted to 7.20 with 1.00 M NaOH or 0.50 M H2SO4. Samples used in this study included filtered water collected from a municipal drinking water treatment facility (the seventh WTP, Harbin, China), and all samples were used within 24 h of collection. Treatment processes located upstream of the sample collection point included aluminum sulfate coagulation, inclined plate sedimentation, and (quartz) sand filtration. To avoid the effect of large particles, water samples were passed through 0.45 mm fiber filters before the experiments. After NH4Cl addition (as needed), water quality characteristics of the samples were ammonia-N 1.00 mg L1 (7.00 102 mM) and TOC 3.20 mg L1. Chemical and photochemical processes were conducted in a 1.50 L UV reactor (stainless steel, L ¼ 61.00 cm, d ¼ 8.00 cm) as shown in Fig. 1. The UV reactor was equipped with a 25 W low pressure Hg lamp (253.7 nm, HNG, Germany), which was placed into a quartz sleeve (L ¼ 61.00 cm, d ¼ 2.50 cm) along the longitudinal axis of the reactor. The lamp was operated before use for 20 min to promote stability of lamp output. The nominal fluence delivered by the UV reactor was estimated by application of NH2Cl as a chemical actinometer. In this experiment, NH2Cl was applied at the influent of the reactor at a concentration of 1.00 mg/L (as N). The steady-state NH2Cl concentration in the effluent from the reactor was also measured. Similarly, the NH2Cl solution was subjected to a range of UV254 doses using a collimated beam. By comparing NH2Cl photodecay from the collimated beam with the steadystate performance of the reactor, an effective UV254 dose was estimated. This method is similar to the work of Bolton and Stefan (2002), except that their work involved free chlorine as the actinometer, while NH2Cl was used here.
2.2.
Analytical methods
Ammonia concentrations were measured by the ammoniaselective electrode method (4500-NH3 D, APHA, 2005) using an ammonia selective electrode (Orion Co., 95-12, USA) and pH/ISE meter (Thermo Orion Co., 720A, USA). The concentrations of inorganic chloramines (mono-, di- and tri-) were
Fig. 1 e Schematic illustration of photochemical reactor (1) influent tank; (2) valves; (3) sample pump; (4) chemical tank; (5) valves; (6) chemical pump; (7) mixer; (8) flow meter; (9) sample inlet; (10) UV reactor body; (11) quartz sleeve; (12) low-pressure mercury lamp; (13) sample outlet; (14) effluent tank.
806
w a t e r r e s e a r c h 6 8 ( 2 0 1 5 ) 8 0 4 e8 1 1
analyzed by the membrane introduction mass spectrometry (MIMS), as described previously (Shang and Blatchley, 1999; Weng et al., 2012). Free chlorine and total chlorine were monitored with the DPD colorimetric method (4500-Cl G) (APHA, 2005). Solution pH was measured with a pH meter (Thermo Orion Co., 720A, USA). UV254 absorbance and total organic carbon (TOC) were measured with a UVeVis spectrophotometer (Hitachi Co., U-3010, Japan) and a TOC analyzer (Shimadzu Co., TOC-VCPN, Japan), respectively. Analysis of DBP concentrations, including THMs, HAAs and HANs, were performed by a GC-6890N (Agilent, America), based on US EPA method series (US EPA Method 524.2, 1995; US EPA Method 552.2, 1995; US EPA Method 551.1, 1995).
3.
Results and discussion
3.1.
Comparison of negative trivalent nitrogen removal
3.1.1. UV absorption spectra of chlorine, monochloramine and ammonia The molar absorptivities of chlorine, monochloramine and ammonia at pH 7.2 and pH 10.0 are illustrated in Fig. 2. Since þ the pKa for NHþ 4 is 9.25, NH4 is the dominant form of total ammonia at pH 7.2, whereas NH3 becomes the dominant form at pH 10.0. As illustrated in Fig. 2, the maximum molar absorptivity of monochloramine was observed at 243 nm, with a value of is 431 M1 cm1 at pH 10.0; the value at pH 7.2 was slightly lower. The molar absorptivities of chlorine varied with pH, and maximum absorbance was observed at 292 nm. At pH 7.2, the molar absorptivities of chlorine and monochloramine at 254 nm were 60 M1 cm1 and 354 M1 cm1, respectively, which are comparable with previously reported values (Li and Blatchley, 2009; De Laat et al., 2010). These data indicated that NH2Cl is a stronger absorber of UVC radiation than either HOCl or OCl. Ammonia has a weak absorption band at wavelengths greater than 220 nm, regardless of the form: NHþ 4 -N (pH at 7.2) or NH3-N (pH at 10.0). Therefore, UV irradiation (alone) could promote photodecomposition of NH2Cl (and
Fig. 2 e Molar absorptivity of monochloramine, free chlorine and ammonia at pH ¼ 7.20 and pH ¼ 10.0 in aqueous solution.
other inorganic chloramines) and free chlorine, but had essentially no effect on ammonia degradation.
3.1.2.
Comparison of negative trivalent nitrogen removal
In this study, the sum of the concentrations of chloramines and ammonia-N is referred to negative trivalent nitrogen, expressed as N (-III). Inorganic chloramines and ammonia can interconvert by substitution, hydrolysis, and monochloramine disproportionation (Vikesland et al., 1998; Jafvert and Valentine, 1992; Vikesland et al., 2001). Fig. 3 illustrates timecourse and UV254 dose-dependent changes of N (-III) concentration resulting from UV254 irradiation, chlorination, and the UV/chlorine process at pH 7.20. When the samples were exposed to UV254 irradiation alone, the concentration of ammonia-N showed essentially no change, which was consistent with the results described in Section 3.1.1. At a Cl2/ N molar ratio 0.80, chlorine substitution occurred to form NH2Cl. The concentration of N (-III) decreased only about 10%. In the UV/chlorine process, the concentration of negative trivalent nitrogen decreased with increasing UV254 dose. 50% of N (-III) was oxidized after exposure to UV254 at dose 800 mJ cm2 with Cl2/N molar ratio 0.80. Therefore, an apparent synergistic effect in the UV/chlorine process was observed to promote N (-III) oxidization. At Cl2/N molar ratio 0.8, NH2Cl was the dominant for of N (-III) in the system due to the large rate constant for N-chlorination of ammonia (Deborde and Gunten, 2008), although some residual ammonia also existed in the system. UV254 irradiation promotes cleavage of the NeCl bond of monochloramine to yield an aminyl radical and chlorine radical (Eq. (1)) (Li and Blatchley, 2009). It has been reported that the chlorine radicals will promote ammonia decomposition (Xiao et al., 2009), as expressed in Eq. (2). Moreover, NH2 radicals will be oxidized to form NH2O2 radicals in the presence of oxygen (De Laat et al., 2010), as indicated by Eq. (3).
Fig. 3 e Negative trivalent nitrogen removal as a function of UV254 irradiation, chlorination, and the UV/Chlorine process ([Ammonia-N]0 ¼ 0.07 mM, Cl2/N molar ratio ¼ 0.80, pH ¼ 7.20). The secondary horizontal axis refers to the dark decay experiment for ammonia chlorination. Symbols are data points and lines are model calculations.
w a t e r r e s e a r c h 6 8 ( 2 0 1 5 ) 8 0 4 e8 1 1
807
The final nitrogenous products formed in the UV/chlorine process were nitrite and nitrate, as summarized by Eq. (4). As shown in Table 1, the concentration of nitrate and nitrite increased steadily with increasing UV254 dose. Moreover, the decrease of total nitrogen concentration indicated that detectable nitrogen was lost from the system. It is hypothesized that some N2 also formed during this process. Nitrous oxide also formed during the photodegradation of monochloramine (Eq. (5) and (6)) (Schreiber and Mitch, 2007; Bazylinski and Hollocher, 1985). hv
NH2 Cl!NH2 $ þ $Cl
(1)
NH3 þ $Cl/NH2 $ þ Hþ þ Cl
(2)
$NH2 þ O2 /NH2 OO$
(3) O2
O2
NH2 OO$/NO$ þ H2 O!NO 2 !NO3
(4)
NH2 OO$/HNO þ H2 O2
(5)
2HNO/N2 O þ H2 O
(6)
3.1.3.
Effect of Cl2/N ratio
Fig 4(a) illustrates the concentration of total N (-III) and ammonia at different Cl2/N molar ratios resulting from the UV/chlorine process and chlorination process. The samples were exposed to UV254 irradiation for 200 s in the UV/chlorine process, and the same contact time (200 s) was applied in chlorination. In chlorination process, total N (-III) declined slightly at Cl2/N molar ratios less than 1.0. At the Cl2/N molar ratio in range of 1.0e1.6, oxidation of N (-III) was opened up and the concentration of N (-III) were decrease. At a Cl2/N molar ratio of 1.60 or more, negative trivalent nitrogen was totally oxidized to stable nitrogen compounds, predominantly N2 and NO 3 , as defined by the chemistry of breakpoint chlorination (Jafvert and Valentine, 1992). Comparably, the UV/chlorine process exhibited higher N (-III) removal efficiency than chlorination process. The concentration of N (-III) dropped sharply and linearly with increasing Cl2/N molar ratio (up to 1.6). In the UV/chlorine process, oxidation of N (-III) was even observed at Cl2/N molar ratios less than 1.0. Therefore, the UV/chlorine process could significantly reduce chlorine dosage in nitrogen containing water treatment. It was also worth noting that the UV/chlorine process improved ammonia-N removal (15%e20%)
Table 1 e Nitrogenous product in the UV/chlorine process (Cl2/N molar ratio 0.80, pH ¼ 7.2, background nitrate 1.41 mg L¡1). Nominal UV254 dose (mJ cm2) 0 100 200 400 600 800
NHþ NO NO Total N 4 -N 2 -N 3 -N 1 1 (mg L ) (mg L ) (mg L1) (mg L1) 1.04 0.74 0.67 0.59 0.56 0.51
0.00 0.00 0.00 0.02 0.03 0.04
1.41 1.47 1.50 1.53 1.54 1.66
2.60 2.35 2.34 2.32 2.32 2.31
Fig. 4 e Comparison of (a) nitrogen removal and (b) residual chlorine concentration as a function of Cl2/N molar ratio for the UV/chlorine process and chlorination process ([Ammonia-N]0 ¼ 0.07 mM, pH ¼ 7.20, UV dose 800 mJ/cm2 for UV/Chlorine process, reaction time 200 s for chlorination).
compared with chlorination process at same Cl2/N molar ratios. These results indicate that ammonia oxidation can be promoted by chloramine photodecay. Therefore, the decrease of N (-III) concentration was presumed to be the combination result from not only photodecomposition of chloramine but also from the advanced oxidation reaction of ammonia. Li and Blatchley (2009) reported that the apparent quantum yield (ɸ254) for aqueous NH2Cl, at a Cl2/N molar ratio 1.05:1, was 0.62 ± 0.049 mol Es1. In this study, ɸ254 was 0.29 ± 0.02 mol Es1, at the Cl2/N molar ratio 0.80. This result was in agreement with results previously at similar Cl2/N molar ratios reported by Cooper et al. (2007) (0.26 mol Es1, Cl2/ N molar ratio 0.33:1, pH 8.5) and by Watts and Linden (2007) (0.3 mol Es1, Cl2/N molar ratio 0.78:1, pH 9). The decrease of the apparent NH2Cl quantum yield in the presence of ammonia was presumed to be the result of radical-mediated reactions with NH3. As can be seen from Fig. 4(b), in both chlorination process and UV/chlorine process, the variation of free chlorine and total chlorine concentration at different Cl2/ N molar ratios were consistent with conventional breakpoint chlorination curve (Jafvert and Valentine, 1992). At Cl2/N
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molar ratios of 1.0 or less, monochloramine was the dominant inorganic chloramine. For Cl2/N molar ratios from 1.0 to 1.6, dichloramine and trichloramine were formed. When the Cl2/N molar ratio exceeded 1.6, free chlorine was the dominant form of residual chlorine, with smaller quantities of NCl3. The concentrations of both total chlorine and free chlorine in UV/ chlorine process were lower than that in chlorination process at all Cl2/N molar ratios, presumably due to the photodecomposition of chloramine and free chlorine by UV254 irradiation (Watts and Linden, 2007).
3.2.
Comparison of DBP precursor removal
Since natural organic matter (NOM) has been identified as an important DBP precursor, control of NOM is important for control of DBP formation. Fig. 5 illustrates the effects of the Cl2/N molar ratio on NOM removal in chlorination and the UV/ chlorine process, while the hydraulic loading (residence time), the initial concentrations of NOM, as well as initial ammonia concentration remained the same. The results of these experiments indicate that TOC removal rate was appreciably higher in the UV/chlorine process than in the chlorination process. For example, at Cl2/N molar ratio 0.8 (mass ratio 4), TOC removal was almost 47% by the UV/chlorine process, whereas only 22% TOC was removed by chlorination. Previous investigations have indicated that NOM can be degraded by advanced oxidation processes (AOPs) through abstraction or addition reactions (Wang et al., 2006; Toor and Mohseni, 2007). Moreover, Chan et al. (2012) reported that the combined application of UV/chlorine represents an advanced oxidation process that can promote degradation of a wide variety of organic compounds. Fig. 5 presents the UV254 absorption changes in the two processes with different Cl2/N molar ratios. During chlorination, UV254 absorbance increased up to Cl2/N molar ratio at 0.8 (mass ratio 4), and then a steady decline was observed as the Cl2/N ratio further increased. The precise cause(s) of the initial increase of UV254 absorbance is (are) not known, but was probably due to production of intermediate compounds that
Fig. 5 e Comparison of TOC and UV254 absorbance as a function of Cl2/N molar ratio for chlorination and the UV/ chlorine process ([TOC]0 ¼ 3.20 mg L¡1, UV dose ¼ 800 mJ/ cm2 for UV/Chlorine process, pH ¼ 7.20).
were stronger absorbers than their parent compounds. However, the UV/chlorine process yielded a steady decline of UV254 absorbance with increasing Cl2/N molar ratio. UV254 absorbance is largely attributable to unsaturated bonds in organic lu and molecules, such as aromatic groups (Akmehmet Balcıog € Otker, 2003); however, N-chlorination is also known to increase UV254 absorbance (see Fig. 2). The UV/chlorine process may be effective for opening of aromatic ring structures, thereby yielding products that are characterized by lower UV254 absorbance, such as carbonyl and carboxylic functional groups. These results suggest that the UV/chlorine process yielded similar behavior to other AOPs, such as UV/H2O2 and UV/TiO2, which have been shown to oxidize organic pollutants by powerful, non-selective radicals (Sanly et al., 2007; Lin and Wang, 2011).
3.3.
Comparison of DBP formation
3.3.1.
THM and HAA formation
Fig. 6 illustrates a comparison of trihalomethane (THM) and haloacetic acid (HAA) formation in the chlorination process
Fig. 6 e Comparison of (a) THM formation and (b) HAA formation as a function of dark incubation period (chlorination contact time) by the UV/chlorine process (left bar) and chlorination process (right bar) ([TOC]0 ¼ 3.20 mg L¡1, [Residual chlorine]0 ¼ 1.50 mg L¡1 as Cl2, UV dose ¼ 800 mJ/cm2 for UV/Chlorine process, pH ¼ 7.20).
w a t e r r e s e a r c h 6 8 ( 2 0 1 5 ) 8 0 4 e8 1 1
and the UV/chlorine process. In both processes, the initial chlorine dosages were chosen to satisfy the requirement of Chinese drinking water standard, in terms of negative trivalent nitrogen (less than 0.5 mg L1) and total chlorine (0.50e3.00 mg L1). Since the water samples contained 0.07 mM ammonia-N, the Cl2/N molar ratio was 1.6 (chlorine dosage 0.11 mM) in the chlorination process to achieve the breakpoint. In the UV/chlorine process, the Cl2/N molar ratio was fixed at 0.8 (chlorine dosage 0.06 mM) and UV dose was 800 mJ cm2. The residual chlorine concentrations were fixed at 0.02 mM (1.50 mg L1 as Cl2) in both processes and the samples were placed in a dark incubator at 25 C. After a range of dark incubation periods (30 mine72 h), the reaction was quenched with a Na2SO3 solution for DBP detection. As shown in Fig 6(a), there were lower concentrations of THMs generated in the UV/chlorine process than the chlorination process. Chloroform (TCM) was the most abundant THM found in both processes. The concentration of TCM ranged from 15 to 19 mg L1 and the amounts were similar in the two processes. Moreover, the TCM concentration did not change appreciably during the 72 h of the experiments. Bromodichloromethane (BDCM) was the other THM species produced in chlorination, and accounted for 59% of the total THMs after 72 h of chlorination. However, the concentration of BDCM was below the detection limit in the combined process (limit of detection was 0.50 mg L1). In addition, chlorodibromomethane (CDBM) and bromoform (TBM) were not detected in either process. It has been previously reported that CeBr bonds tend to more sensitive to UV254 irradiation than CeCl bond (Cimetiere and De Laat, 2014), which was consistent with these results. Therefore, the UV/chlorine process could reduce the formation of brominated THMs, thereby decreasing the total concentration of THMs. The yields of HAAs rose gradually with the increase of the incubation period in both processes, as presented in Fig. 6(b). HAAs produced in the UV/chlorine process ranged from 9.27 to 18.84 mg L1, which was lower than in the chlorination process (from 22.30 to 38.03 mg L1). Dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA) were the two dominant HAAs formed in both processes, whereas monochloroacetic acid (MCAA), monobromoacetic acid (BCAA) and dibromoacetic acid (DBAA) were not detected in samples because their concentrations were below the limit of detection (0.50 mg L1). Therefore, the application of UV254 irradiation did not affect HAA speciation, but did appear to reduce overall production of HAAs. The experimental results indicated that the UV/chlorine process produced less C-DBPs (THMs and HAAs) than chlorination. These results consist with the recent study reported that the low pressure UV did not improve DBP formation (Reckhow et al., 2010). In ammonia-containing water treatment, the chlorine dosage in the UV/chlorine process is less than that in chlorination process, which could reduce chlorinated DBP formation. Simultaneously, the species of residual chlorine in the UV/chlorine process and chlorination process were monochloramine and free chlorine, respectively. It has been reported that monochloramine resulted in lower DBP production, in terms of TCM, DBCM, TCAA and DCAA, than free chlorine (Bougeard et al., 2010). In addition, as mentioned in section 3.2, the UV/chlorine process could change the
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structures of NOM by opening aromatic ring structures, thereby reducing the concentration of DBP precursors and subsequent THM formation. These results is consisted with studies reported that UV irradiation subsequent to chlorination could change the NOM structure (Liu et al., 2006; Dotson et al., 2010). Moreover, the UV irradiation at 254 nm could also destruct DBPs, as reported by Cimetiere and De Laat (2014).
3.3.2.
HAN formation
Fig. 7 shows a comparison of HAN production in the two processes, in terms of dichloroacetonitrile (DCAN), trichloroacetonitrile (TCAN), dibromoacetonitrile (DBAN), bromochloroacetonitrile (BCAN). The experimental conditions were same as the experiment described in section 3.3.1. Total HAN concentrations formed in the UV/chlorine process and chlorination process were 39.30 and 20.52 mg L1, respectively. DCAN accounted for 79% and TCAN for 21% to total HANs in the combined process and the proportions were DCAN 86% and TCAN 14% in chlorination process. DBAN and BCAN were not detected in any samples (detection limit 1.00 mg L1). Similar results were reported by Chu et al. (2012) in an investigation of tyrosine-containing water treatment by combined chlor(am)ination and UV irradiation. It has been reported that a high concentration of chlorine could accelerate DCAN decomposition (Reckhow et al., 2001), which may have contributed to lower DCAN concentration in chlorination than in UV/chlorine process. However, UV254 irradiation of some N-chloro compounds has also been demonstrated to promote halonitrile formation, especially when conducted in the presence of free chlorine (Weng et al., 2012).
4.
Conclusion
This work compared and evaluated the efficiency of chlorination and UV/chlorine for ammonia nitrogen (negative
Fig. 7 e Comparison of HAN formation as a function of dark incubation period (chlorination contact time) by the UV/ chlorine process (left bar) and chlorination process (right bar) ([TOC]0 ¼ 3.20 mg L¡1, [Residual chlorine]0 ¼ 1.50 mg L¡1 as Cl2, UV dose ¼ 800 mJ/cm2 for UV/Chlorine process, pH ¼ 7.20).
810
w a t e r r e s e a r c h 6 8 ( 2 0 1 5 ) 8 0 4 e8 1 1
trivalent nitrogen) removal and DBP formation in ammonia containing water treatment. The experimental results demonstrated that the UV/chlorine process is more effective for N (-III) removal than chlorination, with lower chlorine dosage and higher removal rate. In the chlorination process, N-chlorination was the dominant reaction with little N (-III) oxidation when Cl2/N molar ratio was less than 1.0. However, in the UV/chlorine process, the oxidation of N (-III) was achieved at Cl2/N ratios ranging from 0 to 1.6 through the UV254 photolysis of chloramines and advanced oxidation reactions. 50% N (-III) removal was achieved with the condition that Cl2/ N molar ratio 0.8 and UV dose 800 mJ/cm2. The required UV dose could achieve by adjusting the UV254 intensity and UV254 exposure time. In addition, the higher molar ratio of chlorine and nitrogen, the less UV dose needed. Moreover, the UV/ chlorine process was more effective for decomposition of NOM than chlorination, as measured by changes of TOC and UV254 absorption. Furthermore, the UV/chlorine process generated less THMs and HAAs, but more HANs compared with chlorination. The results suggest that the UV/chlorine process may represent an effective option for ammonia-N containing water treatment. However, the enhanced formation of HANs in the combined process should be taken into consideration.
Acknowledgments This work was financially supported by Heilongjiang Province Funds for Distinguished Young Scientists (JC200708).
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