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Enhanced WWTP effluent organic matter removal in hybrid ozonation-coagulation (HOC) process catalyzed by Al-based coagulant
Accepted Manuscript Title: Enhanced WWTP effluent organic matter removal in hybrid ozonation-coagulation (HOC) process catalyzed by Al-based coagulant Author: Xin Jin Pengkang Jin Rui Hou Lei Yang Xiaochang C. Wang PII: DOI: Reference:
S0304-3894(16)31184-0 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.12.043 HAZMAT 18275
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
6-9-2016 21-12-2016 23-12-2016
Please cite this article as: Xin Jin, Pengkang Jin, Rui Hou, Lei Yang, Xiaochang C.Wang, Enhanced WWTP effluent organic matter removal in hybrid ozonation-coagulation (HOC) process catalyzed by Al-based coagulant, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.12.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced WWTP effluent organic matter removal in hybrid ozonation-coagulation (HOC) process catalyzed by Al-based coagulant Xin Jin 1, Pengkang Jin 1*, Rui Hou 1, Lei Yang 2, Xiaochang C. Wang 1
1. School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an, Shaanxi Province, 710055, China 2. Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800, Australia
Corresponding author: Pengkang Jin, [email protected], NO.13, Yanta Road, Beilin District, Xi’an, China, 710055
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GRAPHICAL ABSTRACT
2
HIGHLIGHTS • • • •
A novel HOC process was firstly put forward to apply in wastewater reclamation. Interactions between ozone and Al-based coagulants was found in the HOC process. Ozonation can be catalyzed and enhanced by Al-based coagulants in the HOC process. HOC process showed better organics removal than pre-ozonation-coagulation process.
Abstract: A novel hybrid ozonation-coagulation (HOC) process was developed for application in wastewater reclamation. In this process, ozonation and coagulation occurred simultaneously within a single unit. Compared with the conventional pre-ozonation-coagulation process, the HOC process exhibited much better performance in removing dissolved organic matters. In particular, the maximal organic matters removal efficiency was obtained at the ozone dosage of 1 mgO3/mg DOC at each pH value (pH 5, 7 and 9). In order to interpret the mechanism of the HOC process, ozone decomposition was monitored. The results indicated that ozone decomposed much faster in the HOC process. Moreover, by using the reagent of O3-resistant hydroxyl radical (•OH) probe compound, para-chlorobenzoic acid (pCBA), and electron paramagnetic resonance (EPR) analysis, it was observed that the HOC process generated higher content of •OH compared with pre-ozonation process. This indicates that the •OH oxidation reaction as the key step can be catalyzed and enhanced by Al-based coagulants and their hydrolyzed products in this developed process. Thus, based on the catalytic effects of Al-based coagulants on 3
ozonation, the HOC process provides a promising alternative to the conventional technology for wastewater reclamation in terms of higher efficiency.
Keywords: Hybrid ozonation-coagulation (HOC) process; Al-based coagulants; Ozonation; Ozone decomposition; Hydroxyl radical generation
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1. Introduction Ozone has traditionally been applied in drinking water treatment plants for disinfection and oxidation (e.g., decolouration, taste and odour control, and elimination of micropollutants) [1]. In addition, effluents from municipal wastewater treatment plants (WWTPs) have been identified as a major source of micropollutants, such as hormones, pharmaceuticals and personal care products [2]. Ozone has been shown to have a favourable effect on microbial disinfection and oxidation of trace contaminants during wastewater treatment and reclamation [3-8]. The coagulation process is an important treatment technology with a wide range of applications in water and wastewater treatment facilities [9, 10]. Moreover, coagulation is the most commonly employed advanced wastewater treatment process for economic reasons [11]. Polyaluminum chloride (PAC) is widely used as the coagulant, which was superior to traditional aluminum coagulants in removing particulate and/or dissolved organic matter (DOM) [12-15]. For DOM in WWTP effluent from biological processes, it contains soluble microbial products (SMPs) which is from substrate metabolism and biomass decay [16, 17]. These dissolved compounds probably have a detrimental effect on the efficiency of the downstream coagulation process due to their hydrophilic characteristics [18, 19]. It was reported previously that the DOM from WWTP effluent were poorly removed by either AlCl3 or PAC coagulation [11]. Ozone has often been used as a pre-oxidant in conventional water treatment process 5
(coagulation-sedimentation-filtration) [20]. Pre-ozonation has been demonstrated to facilitate coagulation for decreasing coagulant dosage, destabilizing the aggregation of particles and increasing the length of filter runs [21]. There are several mechanisms to interpret the enhancing effect of pre-ozonation on coagulation including: increasing concentration of oxygenated functional groups, reduction of the stabilized organic coatings on particles and polymerization of meta-stable organics leading to particle aggregation via bridging reactions [22]. However, pre-ozonation-coagulation process showed limited dissolved organic matter removal efficiency [23, 24]. In addition, the enhancing effect of pre-ozonation on coagulation highly rely on the ozone dose, coagulation conditions and raw water characteristics [25, 26], i.e., pre-ozonation no longer facilitates or even retards coagulation in terms of dissolved organic matter removal efficiency when the proper operation conditions were not found [23, 27]. Ozone reacts with organic contaminants through both direct reactions and hydroxyl radical (•OH) reactions [1]. •OH is quite different from other oxidants in terms of its high reactivity and low selectivity [28]. Ozonation efficiency can be improved by enhanced •OH generation via the combination of ultraviolet radiation, H2O2, metal ions, metal oxides, etc. [29]. Metal coagulants may function as catalyst to facilitate •OH generation and to improve dissolved organic matter removal efficiency, providing that coagulation and ozonation could simultaneously occur in a single unit. Therefore, for the first time, the hybrid ozonation-coagulation (HOC) process was developed based on the interactions between ozone and metal coagulants, where ozonation and coagulation are designed to occur simultaneously in a single reactor. 6
However, there are few data available to support this hypothesis. In order to unveil the mechanism and to further improve dissolved organic matter removal efficiency, the HOC process was performed in the bench scale at pH 5, 7 and 9 with polyaluminium chloride (PAC), using either ultrapure water or WWTP effluent in comparison with the conventional pre-ozonation-coagulation process. Interactions between ozone and coagulants were systematically studied using •OH probe and electron paramagnetic resonance (EPR) analysis. The results indicate that the •OH oxidation reaction can be catalyzed and enhanced by Al-based coagulants and their hydrolyzed products in this developed process. Hence, based on the catalytic effects of Al-based coagulants on ozonation, the HOC process provides a promising alternative to the conventional technology for wastewater reclamation in terms of higher efficiency. 2. Materials and methods 2.1 Raw water The raw water used in this study was collected before the disinfection step at the effluent of the sedimentation tank in a municipal wastewater treatment plant (WWTP) in Xi’an, China. The WWTP mainly treats domestic wastewater with a biological anaerobic-anoxic-oxic (AAO) treatment process. The raw water was filtered by a 0.45-μm filter (Shanghai Xinya, China) to remove particles prior to use. The filtered raw water possessed the following characteristics: DOC = 7.032±1.012 mg/L, UV 254 = 0.145±1.012 cm-1, color = 1.65±0.03 c.u., pH 7.55±0.21, alkalinity = 4.505±0.210 mM as HCO3-, and NO3-N = 0.177±0.017 mgN/L. 7
2.2 Reagents All chemicals used in this study were of reagent grade. Potassium indigo trisulfonate, para-chlorobenzoic acid (pCBA), tert-butanol, and dimethyl pyridine N-oxide (DMPO) were purchased from Sigma-Aldrich. PAC was obtained from Kermel, China. All stock solutions were freshly prepared using ultrapure water from a Millipore ultrapure water system. The concentrated ozone stock solutions (≥60 mg/L) were produced by bubbling ozone gas through a flask of Milli-Q water (pH=3) cooled with ice bath. The ozone concentration of the stock solution was determined by its UV absorbance with ε258nm = 3000 M-1cm-1. The gaseous ozone was generated by a Sankang ozone generator (Model: SK-CFQ-3P, China). 2.3 Ozonation and coagulation experiments Coagulation experiments were conducted using jar tests in a 500-mL batch reactor. Coagulation conditions were rapid mix at 300 rpm (G=124.2 s-1) for 1 min, and slow mix at 60 rpm (G=14.3 s-1) for 30 min respectively. Dissolved organic carbon (DOC) was measured after 30 min of sedimentation. PAC was chosen as the coagulant, and the dosage was 6, 10 and 14 mg Al/L at each corresponding pH 5, 7 and 9 respectively, which was the optimal dosage at each corresponding pH. Ozonation experiments were performed in the same 500-mL batch reactor as in the coagulation experiments. The reagents were spiked to the following final concentrations: 2 mM borate (pH 9 experiments) or phosphate buffer (pH 5 and 7 experiments) and 0.5 μM of pCBA. The pH of the solution was adjusted using 0.1 M H3PO4 or 0.1 M NaOH. Ozone stock solution was added to the proper concentration 8
(O3/DOC=0.5, 1 and 1.5) via an inlet tube, and samples were taken at predetermined time intervals for ozone and pCBA analysis. Samples were quenched with sodium thiosulfate for pCBA analysis. Raw water was diluted five times for analyzing ozone and pCBA decomposition. 10 mM tert-butanol (k•OH,
pCBA=6×10
8
M-1s-1) was
employed as the •OH scavenger. 2.4 HOC experiments To perform the HOC process, ozone stock solution was added at the beginning of slow-mix step during coagulation. This allows that ozonation and coagulation occurred at the same time. The interactions between ozone and coagulants were analyzed in this process. Rct value, defined as the ratio of •OH exposure to O3 exposure (∫[•OH]dt/∫[O3]dt), was calculated for each test to determine whether the yield of •OH varies. All experiments were performed at ambient temperature and repeated at least three times for averaged results. 2.5 Analytical methods DOC was measured using a Shimadzu TOCVCPH analyzer with infrared detection. The DOC analyzer was calibrated with potassium hydrogen phthalate standard solutions before each run. All samples were filtered with a 0.45-μm filter, acidified with H2SO4 and purged with nitrogen to remove inorganic carbon before measurement. UV254 was measured at 254 nm by a UV-VIS spectrophotometer (UV-2102C UNICTM, China) using 1-cm path length quartz cells. The pH was determined with a pH meter (Dapu, Shanghai, China). The indigo method was used for aqueous ozone analysis [30]. The O3 decomposition that occurs within the initial 9
15 s of ozonation can be characterized by instantaneous O3 demand (IOD), which can be calculated as the difference between applied O3 dose and dissolved residual O3 observed after 15 s [7, 31]. The concentration of hydroxyl radicals in the solution was indirectly monitored by the depletion of the •OH-probe compound (pCBA), which was analyzed by high performance liquid chromatography (HPLC) with mobile phase acetonitrile: H2O (adjusted to pH 2 with phosphoric acid) = 50:50. The flow rate was kept at 1 mL/min. pCBA was selected as •OH-probe not only because it displays slow reactions with ozone (kO3< 1 M-1s-1) while rapid oxidation kinetics with the •OH radical (k •OH, pCBA=5×10
9
M-1 s-1), i.e., the reaction between pCBA and ozone is negligible compared
with that between pCBA and •OH [32], but also because it was hardly removed by coagulation process (Fig. S1). EPR experiments were conducted on a Bruker EMXmicro spectrometer (Germany) at room temperature. The basic parameters include resonance frequency of 9.77 GHz, microwave power of 25.18 mW, modulation frequency of 100 kHz, modulation amplitude of 1.0 G, sweep width of 700 G, time constant of 81.92 ms, sweep time of 35 s, and receiver gain 30 dB. 100 mM DMPO (pH 7.4, 10 mM phosphate buffer) was added to the reaction system at different pH values. 3. Results and discussion 3.1 Comparison between the HOC process and pre-ozonation-coagulation process The comparison among the three processes at different ozone dosages and three pH levels (5, 7 and 9) is shown in Fig. 1. As can be seen, the HOC process effectively 10
enhanced DOC removal efficiency in general compared with the conventional coagulation process and pre-ozonation-coagulation process. The averaged DOC removal efficiency increased by 16.6%, 17.0% and 19.9% respectively at pH 5, 7 and 9. In contrast, the pre-ozonation-coagulation process hardly improved the treatment efficiency at ozone dosage 0.5 mgO3/mg DOC, and the DOC removal at 1.5 mgO3/mg DOC still showed no large difference compared with coagulation alone. The results are highly in accordance with previous study that there was no significant difference in DOC removal with or without pre-ozonation at pH 5, 7, 9 and at ozone dosages of 0.15, 0.45 and 0.85 mgO3/mg DOC [33]. This is because pre-ozonation, especially at a higher ozone dosage, inhibited floc formation, i.e., the massive production of low-molecular-weight organic matters highly inhibited the adsorption of organic matters onto metal hydroxides formed during coagulation [34]. Therefore, in practice, to assist coagulation, the ozone dose of pre-ozonation in conventional water treatment was generally remained quite low, and only low DOC removal efficiencies were achieved [20, 21]. Previous study already indicated that pre-ozonation only showed a minor improvement in coagulation for DOC removal in most of waters at ozone dose of 1 mgO3/mg TOC [35]. Only around 22% total organic carbon (TOC) removal efficiency were obtained by pre-ozonation-coagulation process with PAC and alum [24]. Some studies even showed that the DOC removal during coagulation can be retarded by pre-ozonation [23, 27]. Furthermore, independent on pH values, at least 30% DOC removal efficiency was achieved via the HOC process in this study, which indicated a remarkable enhancing effect of ozonation on coagulation in the HOC 11
process. In particular, the DOC removal efficiency reached 46.6% at 1 mgO3/mg DOC at pH 9, and it was even doubled by the HOC process under certain conditions.
As shown in Fig. 1, in general, the enhancing effect for DOC removal appeared greater at 1 mgO3/mg DOC at each pH value in the HOC process. The averaged DOC removal efficiency was around 41.5% at 1 mgO3/mg DOC. While at the higher ozone dosage of 1.5 mgO3/mg DOC, the removal efficiency decreased. This can be attributed to that the over-oxidation would rapidly decompose macromolecular organics into small-molecule organic matters, which are therefore more difficult to be removed by coagulation due to their high hydrophilicity and low adsorption capability [22, 25, 36]. Hence, 1 mgO3/mg DOC was selected as the optimum ozonation dosage in the further analyses for this study. 3.2 Ozone decomposition in the HOC process To investigate the mechanism of enhancing effect of ozonation on coagulation in the HOC process, ozone decomposition in pre-ozonation and the HOC process in either effluent from WWTP or ultrapure water was monitored at ozone dosage of 1 mgO3/mg DOC. The results are shown in Fig. 2. In general, it is well agreed that ozone can decompose in natural water or wastewater due to its reaction with hydroxide ions (OH-), which leads to the formation of •OH through a series of chain reactions [37-40]. As expected, the ozone depletion occurred more quickly as pH value increased with higher OH- concentration. Moreover, ozone decomposition was observed to become much faster in the HOC process which was in the presence of 12
coagulant, suggesting that coagulant promoted ozone decomposition probably due to its enhancing effect on •OH production. In order to verify this hypothesis, the effect of tert-butanol, a well-known •OH scavenger, on ozone decay was investigated. As shown in Fig. 2, tert-butanol inhibited the ozone decomposition either in the HOC or pre-ozonation process, which indicated that ozone was decomposed into •OH radicals through chain reactions [41].
In addition, the instantaneous O3 demand (IOD) without tert-butanol was calculated and summarized in Table 1. Previous studies determined the IOD values typically between 3~5.5 mg/L at pH around 7 during ozonation of different WWTP effluents [31, 42]. Therefore, the IOD values measured in this study are highly consistent with those in the literatures. The results in Table 1 indicated that IOD significantly depended on the pH values. A higher pH value led to a higher IOD at the same applied ozone dose. Approximately 70% of the ozone was consumed in IOD phase at pH 9 in ultrapure water. In contrast, only 9.3% of the ozone was decomposed at pH 5 in ultrapure water. Moreover, bimolecular reactions between ozone and the specific moieties of dissolved organic matters may also contribute to initiating the ozone decomposition and •OH generation during the IOD phase in WWTP effluent [43]. This probably explains that the IOD of WWTP effluent was generally higher than that of ultrapure water at a given pH value. In particular, it is worth noting that for both ultrapure water and WWTP effluent at all different pH values, the IOD values were always higher in the HOC process than that in pre-ozonation (Table 1). Previous 13
studies also indicated that the IOD phase features remarkably high •OH concentration and rapid ozone decomposition [44, 45]. Therefore, the fact that coagulant in the HOC process always enhanced IOD values could potentially be due to the catalytic enhancing effect of coagulant on •OH generation.
3.3 Effect of coagulant on •OH generation in the HOC process The •OH exposure was indirectly measured via the probe compound pCBA. pCBA decomposition was examined during the HOC process either with or without tert-butanol. The results are shown in Fig. 3. The significant inhibition of pCBA removal by tert-butanol clearly proved that the HOC process involved •OH reaction since the majority of •OH were scavenged by tert-butanol [46]. As the pH value increased, the removal efficiency of pCBA became higher. At pH 9, more than 60% of pCBA was removed with PAC in ultrapure water, which indicated more •OH generation at higher pH value. Previous study also showed that ozone decomposition into •OH was enhanced by high OH- concentration at high pH value [47]. The low pCBA removal in WWTP effluent implied that •OH preferentially reacted with the scavengers in WWTP effluent, such as effluent organic matter (EfOM), bicarbonate and occasionally nitrite [43]. In the IOD phase (<15 s), pCBA exhibited an obvious removal, which implied significant •OH exposure in the IOD phase, especially at high pH. Several studies have previously revealed that significant •OH exposure can occur in the IOD phase [7, 43-45]. In particular, it is worth noting that for both ultrapure water and WWTP effluent at all different pH values, coagulant in the HOC process 14
always promoted the pCBA removal (Fig. 3), which indicated that •OH generation were enhanced by coagulant in the HOC process [48].
In order to further prove the existence of •OH in the HOC process, EPR was used to monitor •OH generation, where •OH was spin-trapped by DMPO to form stable adduct. Fig. 4 shows the EPR spectra of DMPO-OH adduct during obtained from the HOC process and pre-ozonation process at pH 7. The EPR spectra at pH 5 and 9 can be seen in supplementary data (Fig. S2a and S2b). The typical EPR spectral peak of DMPO-OH adduct appears as a 1:2:2:1 quartet due to hyperfine coupling by αN and αH [49, 50]. As shown in Fig. 4, for pre-ozonation process, a weak quartet peak of DMPO-OH centred at 3360 G was observed in EPR spectrum indicating that only trace amount •OH was generated in ultrapure water, while this characteristic quartet disappeared in WWTP effluent probably due to the elimination of trace amount •OH by scavengers in WWTP effluent [43]. However, in contrast, when PAC was used as coagulant during the HOC process, the EPR intensity of DMPO-OH quartet became remarkably strong in both ultrapure water and WWTP effluent indicating that PAC indeed functioned as the catalyst to promote the massive generation of •OH. Previous studies showed that PAC can form several hydrolyzed aluminium species detected by electrospray ionization mass spectrometry [51]. In addition, it was demonstrated that metal oxides can catalytically enhance •OH generation from aqueous ozone [52, 53]. Therefore, it can be inferred that PAC-catalyzed ozonation afforded high •OH yield to enhance the DOC oxidation via radical chain reactions during the HOC process. 15
As the •OH reaction was involved, the steady state parameter Rct was utilized to characterize the HOC process for each condition. Rct is defined as the ratio of •OH radical exposure to ozone exposure, i.e., ∫[•OH]dt/∫[O3]dt [32]. To determine Rct, the oxidation of pCBA as a function of ozone exposure (∫[O3]dt) was measured based on Fig. 2 and Fig. 3 [32]. However, only during the second phase (>15s) rather than the IOD phase (<15s), ozone decomposition is controlled by the radical chain reaction [43] following an apparent first-order rate law [7, 42-45]. Therefore, based on the data from the second phase of ozone decay [31, 38], as shown in Table 2, the calculated Rct values lie in the magnitude of 10-8~10-9 under each condition. Previous studies also suggested that the calculated Rct values showed the magnitude of 10-8~10-9 in natural waters at pH 8 [32, 47]. Another study obtained Rct values of 10-8 magnitude as well for three different WWTP secondary treated effluent samples [31]. According to Table 2, Rct value increased with increasing pH value, indicating an increasing •OH exposure per unit of ozone exposure. As reported previously, this is due to that ozone decomposition into •OH is enhanced by higher OH- concentration at higher pH value [46, 47]. Moreover, Rct values in the HOC process were always higher in both ultrapure water and WWTP effluent at each pH than those in pre-ozonation process, indicating the catalytic effect of PAC on •OH generation in the second phase of ozonation during the HOC process.
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Based on the Rct value (Table 2) and ozone decomposition curve (Fig. 2), the •OH exposure was calculated under each condition shown in Fig. 5. According to Fig. 5, higher •OH exposure was obtained in the HOC process in both ultrapure water and WWTP effluent, which is consistent with the observation in Fig. 4. Hence, based on the above discussion, it can be inferred that the HOC process benefited from the enhanced •OH generation catalyzed by coagulant resulting in a better DOC removal efficiency compared with the conventional pre-ozonation-coagulation process.
3.4 Proposed mechanism Using Al oxides/hydroxides and Al-based composites as catalysts has been extensively studied. When Al-based catalysts were introduced into aqueous solution, water molecules were strongly adsorbed onto the oxides surface, and dissociated into OH− and H+ to form the surface hydroxyl groups [54]. Surface hydroxyl groups were found to be the active site to catalyze ozone decomposition [55-57]. Ozone in aqueous solution tends to interact with the surface hydroxyl groups on the catalysts driven by two basic attractive forces: electrostatic force or/and hydrogen bonding [58]. Furthermore, the surface hydroxyl groups on metal catalysts initiated catalytic ozone decomposition to form •O2 -, •O2H, •O3H and •O4H to further generate •OH through chain reactions [58, 59]. In this study, the feasible mechanism of •OH generation in the HOC process can be summarized in the following two pathways (Fig. 6): 1) Direct •OH generation initialized by OH- in aqueous phase. 2) PAC-catalyzed •OH generation via three steps: (i) hydroxyl groups forms on PAC surface via hydration 17
(protonation/deprotonation) [60]; (ii) ozone adsorbs to and interacts with these surface hydroxyl groups to form six-membered ring due to electrostatic force or/and hydrogen bonding; (iii) •OH is generated due to the dissociation of the ring structure. Based on the above mechanism, more •OH can be generated in the HOC process leading to a better dissolved organic matter removal performance than the conventional pre-ozonation-coagulation process.
4. Conclusions For the first time, the HOC process was developed, where ozonation and coagulation occurred simultaneously within a single unit under slow mixing. Benefiting from the interactions between ozone and coagulant, this process showed much better DOC removal efficiency than the conventional pre-ozonation-coagulation process. It was proven that coagulant (PAC) in this process functions as catalyst to promote •OH generation during both IOD and second phases of ozone decomposition, which eventually improved the DOC removal efficiency. Hence, based on the catalytic effects of Al-based coagulants on ozonation, the HOC process provides a promising pathway for wastewater reclamation in terms of high efficiency.
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Acknowledgement This study was financially supported by National Key Technology Support Program (2014BAC13B06), National Natural Science Foundation of China (51378414, 51178376), the Program for Innovative Research Team in Shaanxi (2013KCT-13).
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27
Fig. 1 DOC removal comparison between the HOC and pre-ozonation-coagulation process Fig. 2 Ozone depletion at different pH values in pre-ozonation and the HOC process Fig. 3 pCBA decomposition in the pre-ozonation and HOC process at different pH values Fig. 4 EPR spectra of DMPO-OH adduct obtained from pre-ozonation and the HOC process at pH 7. a: ultrapure water, b: WWTP effluent Fig. 5 Comparison of •OH exposure between the HOC process and pre-ozonation process at different pH. a: ultrapure water, b: WWTP effluent Fig. 6 Proposed mechanism of •OH generation in the HOC process
28
Figure 1
60
Removal percentage (%)
50 40 30 20
Removal percentage (%)
60 50 40
20
5.576
5.203 5.810
6.299
pH 5
pH 7
pH 9
coagulation HOC process 4.007 pre-ozonation-coagulation 4.574
30 6.111
5.697
5.637
5.079
3.757
b
5.203
10 0
60
Removal percentage (%)
5.637
6.111
4.435
4.519
4.868
10 0
a
coagulation HOC process pre-ozonation-coagulation
50 40 30 20
pH 5
pH 7
coagulation HOC process pre-ozonation-coagulation 4.825
6.111
5.779
5.637
pH 9
4.015 4.266
5.714
5.203
c
5.338
10 0
pH 5
pH 7
pH 9
Fig. 1 DOC removal comparison between the HOC and pre-ozonation-coagulation process a: 0.5 mgO3/mgDOC, b: 1 mgO3/mgDOC, c: 1.5 mgO3/mgDOC Actual DOC value was shown inside each figure
Figure 2
1.2
[O3]/[O3]0
1
a
pH=5
b
pH=5
a
pH=7
b
pH=7
a
pH=9
b
pH=9
0.8 0.6 0.4 0.2 0 1.2 1
[O3]/[O3]0
0.8 0.6 0.4 0.2
0 1.2
[O3]/[O3]0
1 0.8 0.6 0.4 0.2 0
0
2
4
6
Time (min)
8
a: ultrapure water
10
pre-ozonation+tert-butanol pre-ozonation
12 0
2
4
6
Time (min)
b: WWTP effluent
8
10
12
HOC process+tert-butanol HOC process
Fig. 2 Ozone depletion at different pH values in pre-ozonation and the HOC process
Figure 3
1.1
[pCBA]/[pCBA]0
1
0.9
a
pH=5
b
pH=5
a
pH=7
b
pH=7
a
pH=9
b
pH=9
0.8 0.7 0.6 0.5 0.4
0.3 1.1
1
[pCBA]/[pCBA]0
0.9 0.8 0.7 0.6 0.5 0.4
0.3 1.1
[pCBA]/[pCBA]0
1
0.9 0.8 0.7 0.6 0.5 0.4 0.3
0
1
2
3
Time (min)
a: ultrapure water
4
pre-ozonation+tert-butanol pre-ozonation
5 0
1
2
3
Time (min)
b: WWTP effluent
4
5
HOC process+tert-butanol HOC process
Fig. 3 pCBA decomposition in the pre-ozonation and HOC process at different pH values
Figure 4
0.04
0.02
0
0.4 -0.02
-0.04 0.3 3300 0.2
a
0.1
3320
3340
3360
3380
pre-ozonation
3400
3420
0
-0.1
HOC process
-0.2
-0.3 3300
3320
3340
3320
3340
3360
3380
3400
3360
3380
3400
Magnetic value (G)
3420
0.05
0 0.4 -0.05 0.33300 0.2
b
pre-ozonation
3420
0.1 0
-0.1
HOC process
-0.2
-0.3 3300
3320
3340
3360
Magnetic value (G)
3380
3400
3420
Fig. 4 EPR spectra of DMPO-OH adduct obtained from pre-ozonation and the HOC process at pH 7. a: ultrapure water, b: WWTP effluent
Figure 5
3.0
a
2.5
pre-ozonation HOC process
∫[•OH]dt (M.min) ×10-12
∫[•OH]dt (M.min) ×10-12
4.0
2.0 1.0 0.0
pH 5
pH 7
pH 9
2.0
b
pre-ozonation HOC process
1.5 1.0 0.5 0.0
pH 5
pH 7
pH 9
Fig. 5 Comparison of •OH exposure between the HOC process and pre-ozonation process at different pH. a: ultrapure water, b: WWTP effluent
Figure 6
OH2+ pathway 2 (i)
-O
PAC
O-
H2O
•O2H
•O3H
•OH
pathway 2 (iii)
OH
PAC HO
O+
•O2-
Hydrogen bond
O-
O3
O H
O+ O
pathway 2 (ii)
OH2+
-O
O
H+
O-
Electrostatic forces
H+
OH
PAC HO
pathway 1
O3
O-
OH2+
Fig. 6 Proposed mechanism of •OH generation in the HOC process
Table 1 Summary of IOD values for each testing condition Table 1 Summary of IOD values for each testing condition IOD (mg/L)
Ultrapure water
WWTP effluent
Pre-ozonation
HOC process
Pre-ozonation
HOC process
pH=5
0.651
1.821
3.000
4.857
pH=7
1.940
2.952
4.333
5.762
pH=9
4.905
5.571
5.619
6.190
Table 2 Summary of Rct values for each testing condition Table 2 Summary of Rct values for each testing condition Rct (×10-8)
Ultrapure water
WWTP effluent
Pre-ozonation
HOC process
Pre-ozonation
HOC process
pH=5
0.498
0.611
0.469
0.985
pH=7
0.839
1.450
1.449
4.429
pH=9
1.277
4.054
3.351
7.595
29
S0304-3894(16)31184-0 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.12.043 HAZMAT 18275
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
6-9-2016 21-12-2016 23-12-2016
Please cite this article as: Xin Jin, Pengkang Jin, Rui Hou, Lei Yang, Xiaochang C.Wang, Enhanced WWTP effluent organic matter removal in hybrid ozonation-coagulation (HOC) process catalyzed by Al-based coagulant, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.12.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced WWTP effluent organic matter removal in hybrid ozonation-coagulation (HOC) process catalyzed by Al-based coagulant Xin Jin 1, Pengkang Jin 1*, Rui Hou 1, Lei Yang 2, Xiaochang C. Wang 1
1. School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an, Shaanxi Province, 710055, China 2. Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800, Australia
Corresponding author: Pengkang Jin, [email protected], NO.13, Yanta Road, Beilin District, Xi’an, China, 710055
1
GRAPHICAL ABSTRACT
2
HIGHLIGHTS • • • •
A novel HOC process was firstly put forward to apply in wastewater reclamation. Interactions between ozone and Al-based coagulants was found in the HOC process. Ozonation can be catalyzed and enhanced by Al-based coagulants in the HOC process. HOC process showed better organics removal than pre-ozonation-coagulation process.
Abstract: A novel hybrid ozonation-coagulation (HOC) process was developed for application in wastewater reclamation. In this process, ozonation and coagulation occurred simultaneously within a single unit. Compared with the conventional pre-ozonation-coagulation process, the HOC process exhibited much better performance in removing dissolved organic matters. In particular, the maximal organic matters removal efficiency was obtained at the ozone dosage of 1 mgO3/mg DOC at each pH value (pH 5, 7 and 9). In order to interpret the mechanism of the HOC process, ozone decomposition was monitored. The results indicated that ozone decomposed much faster in the HOC process. Moreover, by using the reagent of O3-resistant hydroxyl radical (•OH) probe compound, para-chlorobenzoic acid (pCBA), and electron paramagnetic resonance (EPR) analysis, it was observed that the HOC process generated higher content of •OH compared with pre-ozonation process. This indicates that the •OH oxidation reaction as the key step can be catalyzed and enhanced by Al-based coagulants and their hydrolyzed products in this developed process. Thus, based on the catalytic effects of Al-based coagulants on 3
ozonation, the HOC process provides a promising alternative to the conventional technology for wastewater reclamation in terms of higher efficiency.
Keywords: Hybrid ozonation-coagulation (HOC) process; Al-based coagulants; Ozonation; Ozone decomposition; Hydroxyl radical generation
4
1. Introduction Ozone has traditionally been applied in drinking water treatment plants for disinfection and oxidation (e.g., decolouration, taste and odour control, and elimination of micropollutants) [1]. In addition, effluents from municipal wastewater treatment plants (WWTPs) have been identified as a major source of micropollutants, such as hormones, pharmaceuticals and personal care products [2]. Ozone has been shown to have a favourable effect on microbial disinfection and oxidation of trace contaminants during wastewater treatment and reclamation [3-8]. The coagulation process is an important treatment technology with a wide range of applications in water and wastewater treatment facilities [9, 10]. Moreover, coagulation is the most commonly employed advanced wastewater treatment process for economic reasons [11]. Polyaluminum chloride (PAC) is widely used as the coagulant, which was superior to traditional aluminum coagulants in removing particulate and/or dissolved organic matter (DOM) [12-15]. For DOM in WWTP effluent from biological processes, it contains soluble microbial products (SMPs) which is from substrate metabolism and biomass decay [16, 17]. These dissolved compounds probably have a detrimental effect on the efficiency of the downstream coagulation process due to their hydrophilic characteristics [18, 19]. It was reported previously that the DOM from WWTP effluent were poorly removed by either AlCl3 or PAC coagulation [11]. Ozone has often been used as a pre-oxidant in conventional water treatment process 5
(coagulation-sedimentation-filtration) [20]. Pre-ozonation has been demonstrated to facilitate coagulation for decreasing coagulant dosage, destabilizing the aggregation of particles and increasing the length of filter runs [21]. There are several mechanisms to interpret the enhancing effect of pre-ozonation on coagulation including: increasing concentration of oxygenated functional groups, reduction of the stabilized organic coatings on particles and polymerization of meta-stable organics leading to particle aggregation via bridging reactions [22]. However, pre-ozonation-coagulation process showed limited dissolved organic matter removal efficiency [23, 24]. In addition, the enhancing effect of pre-ozonation on coagulation highly rely on the ozone dose, coagulation conditions and raw water characteristics [25, 26], i.e., pre-ozonation no longer facilitates or even retards coagulation in terms of dissolved organic matter removal efficiency when the proper operation conditions were not found [23, 27]. Ozone reacts with organic contaminants through both direct reactions and hydroxyl radical (•OH) reactions [1]. •OH is quite different from other oxidants in terms of its high reactivity and low selectivity [28]. Ozonation efficiency can be improved by enhanced •OH generation via the combination of ultraviolet radiation, H2O2, metal ions, metal oxides, etc. [29]. Metal coagulants may function as catalyst to facilitate •OH generation and to improve dissolved organic matter removal efficiency, providing that coagulation and ozonation could simultaneously occur in a single unit. Therefore, for the first time, the hybrid ozonation-coagulation (HOC) process was developed based on the interactions between ozone and metal coagulants, where ozonation and coagulation are designed to occur simultaneously in a single reactor. 6
However, there are few data available to support this hypothesis. In order to unveil the mechanism and to further improve dissolved organic matter removal efficiency, the HOC process was performed in the bench scale at pH 5, 7 and 9 with polyaluminium chloride (PAC), using either ultrapure water or WWTP effluent in comparison with the conventional pre-ozonation-coagulation process. Interactions between ozone and coagulants were systematically studied using •OH probe and electron paramagnetic resonance (EPR) analysis. The results indicate that the •OH oxidation reaction can be catalyzed and enhanced by Al-based coagulants and their hydrolyzed products in this developed process. Hence, based on the catalytic effects of Al-based coagulants on ozonation, the HOC process provides a promising alternative to the conventional technology for wastewater reclamation in terms of higher efficiency. 2. Materials and methods 2.1 Raw water The raw water used in this study was collected before the disinfection step at the effluent of the sedimentation tank in a municipal wastewater treatment plant (WWTP) in Xi’an, China. The WWTP mainly treats domestic wastewater with a biological anaerobic-anoxic-oxic (AAO) treatment process. The raw water was filtered by a 0.45-μm filter (Shanghai Xinya, China) to remove particles prior to use. The filtered raw water possessed the following characteristics: DOC = 7.032±1.012 mg/L, UV 254 = 0.145±1.012 cm-1, color = 1.65±0.03 c.u., pH 7.55±0.21, alkalinity = 4.505±0.210 mM as HCO3-, and NO3-N = 0.177±0.017 mgN/L. 7
2.2 Reagents All chemicals used in this study were of reagent grade. Potassium indigo trisulfonate, para-chlorobenzoic acid (pCBA), tert-butanol, and dimethyl pyridine N-oxide (DMPO) were purchased from Sigma-Aldrich. PAC was obtained from Kermel, China. All stock solutions were freshly prepared using ultrapure water from a Millipore ultrapure water system. The concentrated ozone stock solutions (≥60 mg/L) were produced by bubbling ozone gas through a flask of Milli-Q water (pH=3) cooled with ice bath. The ozone concentration of the stock solution was determined by its UV absorbance with ε258nm = 3000 M-1cm-1. The gaseous ozone was generated by a Sankang ozone generator (Model: SK-CFQ-3P, China). 2.3 Ozonation and coagulation experiments Coagulation experiments were conducted using jar tests in a 500-mL batch reactor. Coagulation conditions were rapid mix at 300 rpm (G=124.2 s-1) for 1 min, and slow mix at 60 rpm (G=14.3 s-1) for 30 min respectively. Dissolved organic carbon (DOC) was measured after 30 min of sedimentation. PAC was chosen as the coagulant, and the dosage was 6, 10 and 14 mg Al/L at each corresponding pH 5, 7 and 9 respectively, which was the optimal dosage at each corresponding pH. Ozonation experiments were performed in the same 500-mL batch reactor as in the coagulation experiments. The reagents were spiked to the following final concentrations: 2 mM borate (pH 9 experiments) or phosphate buffer (pH 5 and 7 experiments) and 0.5 μM of pCBA. The pH of the solution was adjusted using 0.1 M H3PO4 or 0.1 M NaOH. Ozone stock solution was added to the proper concentration 8
(O3/DOC=0.5, 1 and 1.5) via an inlet tube, and samples were taken at predetermined time intervals for ozone and pCBA analysis. Samples were quenched with sodium thiosulfate for pCBA analysis. Raw water was diluted five times for analyzing ozone and pCBA decomposition. 10 mM tert-butanol (k•OH,
pCBA=6×10
8
M-1s-1) was
employed as the •OH scavenger. 2.4 HOC experiments To perform the HOC process, ozone stock solution was added at the beginning of slow-mix step during coagulation. This allows that ozonation and coagulation occurred at the same time. The interactions between ozone and coagulants were analyzed in this process. Rct value, defined as the ratio of •OH exposure to O3 exposure (∫[•OH]dt/∫[O3]dt), was calculated for each test to determine whether the yield of •OH varies. All experiments were performed at ambient temperature and repeated at least three times for averaged results. 2.5 Analytical methods DOC was measured using a Shimadzu TOCVCPH analyzer with infrared detection. The DOC analyzer was calibrated with potassium hydrogen phthalate standard solutions before each run. All samples were filtered with a 0.45-μm filter, acidified with H2SO4 and purged with nitrogen to remove inorganic carbon before measurement. UV254 was measured at 254 nm by a UV-VIS spectrophotometer (UV-2102C UNICTM, China) using 1-cm path length quartz cells. The pH was determined with a pH meter (Dapu, Shanghai, China). The indigo method was used for aqueous ozone analysis [30]. The O3 decomposition that occurs within the initial 9
15 s of ozonation can be characterized by instantaneous O3 demand (IOD), which can be calculated as the difference between applied O3 dose and dissolved residual O3 observed after 15 s [7, 31]. The concentration of hydroxyl radicals in the solution was indirectly monitored by the depletion of the •OH-probe compound (pCBA), which was analyzed by high performance liquid chromatography (HPLC) with mobile phase acetonitrile: H2O (adjusted to pH 2 with phosphoric acid) = 50:50. The flow rate was kept at 1 mL/min. pCBA was selected as •OH-probe not only because it displays slow reactions with ozone (kO3< 1 M-1s-1) while rapid oxidation kinetics with the •OH radical (k •OH, pCBA=5×10
9
M-1 s-1), i.e., the reaction between pCBA and ozone is negligible compared
with that between pCBA and •OH [32], but also because it was hardly removed by coagulation process (Fig. S1). EPR experiments were conducted on a Bruker EMXmicro spectrometer (Germany) at room temperature. The basic parameters include resonance frequency of 9.77 GHz, microwave power of 25.18 mW, modulation frequency of 100 kHz, modulation amplitude of 1.0 G, sweep width of 700 G, time constant of 81.92 ms, sweep time of 35 s, and receiver gain 30 dB. 100 mM DMPO (pH 7.4, 10 mM phosphate buffer) was added to the reaction system at different pH values. 3. Results and discussion 3.1 Comparison between the HOC process and pre-ozonation-coagulation process The comparison among the three processes at different ozone dosages and three pH levels (5, 7 and 9) is shown in Fig. 1. As can be seen, the HOC process effectively 10
enhanced DOC removal efficiency in general compared with the conventional coagulation process and pre-ozonation-coagulation process. The averaged DOC removal efficiency increased by 16.6%, 17.0% and 19.9% respectively at pH 5, 7 and 9. In contrast, the pre-ozonation-coagulation process hardly improved the treatment efficiency at ozone dosage 0.5 mgO3/mg DOC, and the DOC removal at 1.5 mgO3/mg DOC still showed no large difference compared with coagulation alone. The results are highly in accordance with previous study that there was no significant difference in DOC removal with or without pre-ozonation at pH 5, 7, 9 and at ozone dosages of 0.15, 0.45 and 0.85 mgO3/mg DOC [33]. This is because pre-ozonation, especially at a higher ozone dosage, inhibited floc formation, i.e., the massive production of low-molecular-weight organic matters highly inhibited the adsorption of organic matters onto metal hydroxides formed during coagulation [34]. Therefore, in practice, to assist coagulation, the ozone dose of pre-ozonation in conventional water treatment was generally remained quite low, and only low DOC removal efficiencies were achieved [20, 21]. Previous study already indicated that pre-ozonation only showed a minor improvement in coagulation for DOC removal in most of waters at ozone dose of 1 mgO3/mg TOC [35]. Only around 22% total organic carbon (TOC) removal efficiency were obtained by pre-ozonation-coagulation process with PAC and alum [24]. Some studies even showed that the DOC removal during coagulation can be retarded by pre-ozonation [23, 27]. Furthermore, independent on pH values, at least 30% DOC removal efficiency was achieved via the HOC process in this study, which indicated a remarkable enhancing effect of ozonation on coagulation in the HOC 11
process. In particular, the DOC removal efficiency reached 46.6% at 1 mgO3/mg DOC at pH 9, and it was even doubled by the HOC process under certain conditions.
As shown in Fig. 1, in general, the enhancing effect for DOC removal appeared greater at 1 mgO3/mg DOC at each pH value in the HOC process. The averaged DOC removal efficiency was around 41.5% at 1 mgO3/mg DOC. While at the higher ozone dosage of 1.5 mgO3/mg DOC, the removal efficiency decreased. This can be attributed to that the over-oxidation would rapidly decompose macromolecular organics into small-molecule organic matters, which are therefore more difficult to be removed by coagulation due to their high hydrophilicity and low adsorption capability [22, 25, 36]. Hence, 1 mgO3/mg DOC was selected as the optimum ozonation dosage in the further analyses for this study. 3.2 Ozone decomposition in the HOC process To investigate the mechanism of enhancing effect of ozonation on coagulation in the HOC process, ozone decomposition in pre-ozonation and the HOC process in either effluent from WWTP or ultrapure water was monitored at ozone dosage of 1 mgO3/mg DOC. The results are shown in Fig. 2. In general, it is well agreed that ozone can decompose in natural water or wastewater due to its reaction with hydroxide ions (OH-), which leads to the formation of •OH through a series of chain reactions [37-40]. As expected, the ozone depletion occurred more quickly as pH value increased with higher OH- concentration. Moreover, ozone decomposition was observed to become much faster in the HOC process which was in the presence of 12
coagulant, suggesting that coagulant promoted ozone decomposition probably due to its enhancing effect on •OH production. In order to verify this hypothesis, the effect of tert-butanol, a well-known •OH scavenger, on ozone decay was investigated. As shown in Fig. 2, tert-butanol inhibited the ozone decomposition either in the HOC or pre-ozonation process, which indicated that ozone was decomposed into •OH radicals through chain reactions [41].
In addition, the instantaneous O3 demand (IOD) without tert-butanol was calculated and summarized in Table 1. Previous studies determined the IOD values typically between 3~5.5 mg/L at pH around 7 during ozonation of different WWTP effluents [31, 42]. Therefore, the IOD values measured in this study are highly consistent with those in the literatures. The results in Table 1 indicated that IOD significantly depended on the pH values. A higher pH value led to a higher IOD at the same applied ozone dose. Approximately 70% of the ozone was consumed in IOD phase at pH 9 in ultrapure water. In contrast, only 9.3% of the ozone was decomposed at pH 5 in ultrapure water. Moreover, bimolecular reactions between ozone and the specific moieties of dissolved organic matters may also contribute to initiating the ozone decomposition and •OH generation during the IOD phase in WWTP effluent [43]. This probably explains that the IOD of WWTP effluent was generally higher than that of ultrapure water at a given pH value. In particular, it is worth noting that for both ultrapure water and WWTP effluent at all different pH values, the IOD values were always higher in the HOC process than that in pre-ozonation (Table 1). Previous 13
studies also indicated that the IOD phase features remarkably high •OH concentration and rapid ozone decomposition [44, 45]. Therefore, the fact that coagulant in the HOC process always enhanced IOD values could potentially be due to the catalytic enhancing effect of coagulant on •OH generation.
3.3 Effect of coagulant on •OH generation in the HOC process The •OH exposure was indirectly measured via the probe compound pCBA. pCBA decomposition was examined during the HOC process either with or without tert-butanol. The results are shown in Fig. 3. The significant inhibition of pCBA removal by tert-butanol clearly proved that the HOC process involved •OH reaction since the majority of •OH were scavenged by tert-butanol [46]. As the pH value increased, the removal efficiency of pCBA became higher. At pH 9, more than 60% of pCBA was removed with PAC in ultrapure water, which indicated more •OH generation at higher pH value. Previous study also showed that ozone decomposition into •OH was enhanced by high OH- concentration at high pH value [47]. The low pCBA removal in WWTP effluent implied that •OH preferentially reacted with the scavengers in WWTP effluent, such as effluent organic matter (EfOM), bicarbonate and occasionally nitrite [43]. In the IOD phase (<15 s), pCBA exhibited an obvious removal, which implied significant •OH exposure in the IOD phase, especially at high pH. Several studies have previously revealed that significant •OH exposure can occur in the IOD phase [7, 43-45]. In particular, it is worth noting that for both ultrapure water and WWTP effluent at all different pH values, coagulant in the HOC process 14
always promoted the pCBA removal (Fig. 3), which indicated that •OH generation were enhanced by coagulant in the HOC process [48].
In order to further prove the existence of •OH in the HOC process, EPR was used to monitor •OH generation, where •OH was spin-trapped by DMPO to form stable adduct. Fig. 4 shows the EPR spectra of DMPO-OH adduct during obtained from the HOC process and pre-ozonation process at pH 7. The EPR spectra at pH 5 and 9 can be seen in supplementary data (Fig. S2a and S2b). The typical EPR spectral peak of DMPO-OH adduct appears as a 1:2:2:1 quartet due to hyperfine coupling by αN and αH [49, 50]. As shown in Fig. 4, for pre-ozonation process, a weak quartet peak of DMPO-OH centred at 3360 G was observed in EPR spectrum indicating that only trace amount •OH was generated in ultrapure water, while this characteristic quartet disappeared in WWTP effluent probably due to the elimination of trace amount •OH by scavengers in WWTP effluent [43]. However, in contrast, when PAC was used as coagulant during the HOC process, the EPR intensity of DMPO-OH quartet became remarkably strong in both ultrapure water and WWTP effluent indicating that PAC indeed functioned as the catalyst to promote the massive generation of •OH. Previous studies showed that PAC can form several hydrolyzed aluminium species detected by electrospray ionization mass spectrometry [51]. In addition, it was demonstrated that metal oxides can catalytically enhance •OH generation from aqueous ozone [52, 53]. Therefore, it can be inferred that PAC-catalyzed ozonation afforded high •OH yield to enhance the DOC oxidation via radical chain reactions during the HOC process. 15
As the •OH reaction was involved, the steady state parameter Rct was utilized to characterize the HOC process for each condition. Rct is defined as the ratio of •OH radical exposure to ozone exposure, i.e., ∫[•OH]dt/∫[O3]dt [32]. To determine Rct, the oxidation of pCBA as a function of ozone exposure (∫[O3]dt) was measured based on Fig. 2 and Fig. 3 [32]. However, only during the second phase (>15s) rather than the IOD phase (<15s), ozone decomposition is controlled by the radical chain reaction [43] following an apparent first-order rate law [7, 42-45]. Therefore, based on the data from the second phase of ozone decay [31, 38], as shown in Table 2, the calculated Rct values lie in the magnitude of 10-8~10-9 under each condition. Previous studies also suggested that the calculated Rct values showed the magnitude of 10-8~10-9 in natural waters at pH 8 [32, 47]. Another study obtained Rct values of 10-8 magnitude as well for three different WWTP secondary treated effluent samples [31]. According to Table 2, Rct value increased with increasing pH value, indicating an increasing •OH exposure per unit of ozone exposure. As reported previously, this is due to that ozone decomposition into •OH is enhanced by higher OH- concentration at higher pH value [46, 47]. Moreover, Rct values in the HOC process were always higher in both ultrapure water and WWTP effluent at each pH than those in pre-ozonation process, indicating the catalytic effect of PAC on •OH generation in the second phase of ozonation during the HOC process.
16
Based on the Rct value (Table 2) and ozone decomposition curve (Fig. 2), the •OH exposure was calculated under each condition shown in Fig. 5. According to Fig. 5, higher •OH exposure was obtained in the HOC process in both ultrapure water and WWTP effluent, which is consistent with the observation in Fig. 4. Hence, based on the above discussion, it can be inferred that the HOC process benefited from the enhanced •OH generation catalyzed by coagulant resulting in a better DOC removal efficiency compared with the conventional pre-ozonation-coagulation process.
3.4 Proposed mechanism Using Al oxides/hydroxides and Al-based composites as catalysts has been extensively studied. When Al-based catalysts were introduced into aqueous solution, water molecules were strongly adsorbed onto the oxides surface, and dissociated into OH− and H+ to form the surface hydroxyl groups [54]. Surface hydroxyl groups were found to be the active site to catalyze ozone decomposition [55-57]. Ozone in aqueous solution tends to interact with the surface hydroxyl groups on the catalysts driven by two basic attractive forces: electrostatic force or/and hydrogen bonding [58]. Furthermore, the surface hydroxyl groups on metal catalysts initiated catalytic ozone decomposition to form •O2 -, •O2H, •O3H and •O4H to further generate •OH through chain reactions [58, 59]. In this study, the feasible mechanism of •OH generation in the HOC process can be summarized in the following two pathways (Fig. 6): 1) Direct •OH generation initialized by OH- in aqueous phase. 2) PAC-catalyzed •OH generation via three steps: (i) hydroxyl groups forms on PAC surface via hydration 17
(protonation/deprotonation) [60]; (ii) ozone adsorbs to and interacts with these surface hydroxyl groups to form six-membered ring due to electrostatic force or/and hydrogen bonding; (iii) •OH is generated due to the dissociation of the ring structure. Based on the above mechanism, more •OH can be generated in the HOC process leading to a better dissolved organic matter removal performance than the conventional pre-ozonation-coagulation process.
4. Conclusions For the first time, the HOC process was developed, where ozonation and coagulation occurred simultaneously within a single unit under slow mixing. Benefiting from the interactions between ozone and coagulant, this process showed much better DOC removal efficiency than the conventional pre-ozonation-coagulation process. It was proven that coagulant (PAC) in this process functions as catalyst to promote •OH generation during both IOD and second phases of ozone decomposition, which eventually improved the DOC removal efficiency. Hence, based on the catalytic effects of Al-based coagulants on ozonation, the HOC process provides a promising pathway for wastewater reclamation in terms of high efficiency.
18
Acknowledgement This study was financially supported by National Key Technology Support Program (2014BAC13B06), National Natural Science Foundation of China (51378414, 51178376), the Program for Innovative Research Team in Shaanxi (2013KCT-13).
19
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27
Fig. 1 DOC removal comparison between the HOC and pre-ozonation-coagulation process Fig. 2 Ozone depletion at different pH values in pre-ozonation and the HOC process Fig. 3 pCBA decomposition in the pre-ozonation and HOC process at different pH values Fig. 4 EPR spectra of DMPO-OH adduct obtained from pre-ozonation and the HOC process at pH 7. a: ultrapure water, b: WWTP effluent Fig. 5 Comparison of •OH exposure between the HOC process and pre-ozonation process at different pH. a: ultrapure water, b: WWTP effluent Fig. 6 Proposed mechanism of •OH generation in the HOC process
28
Figure 1
60
Removal percentage (%)
50 40 30 20
Removal percentage (%)
60 50 40
20
5.576
5.203 5.810
6.299
pH 5
pH 7
pH 9
coagulation HOC process 4.007 pre-ozonation-coagulation 4.574
30 6.111
5.697
5.637
5.079
3.757
b
5.203
10 0
60
Removal percentage (%)
5.637
6.111
4.435
4.519
4.868
10 0
a
coagulation HOC process pre-ozonation-coagulation
50 40 30 20
pH 5
pH 7
coagulation HOC process pre-ozonation-coagulation 4.825
6.111
5.779
5.637
pH 9
4.015 4.266
5.714
5.203
c
5.338
10 0
pH 5
pH 7
pH 9
Fig. 1 DOC removal comparison between the HOC and pre-ozonation-coagulation process a: 0.5 mgO3/mgDOC, b: 1 mgO3/mgDOC, c: 1.5 mgO3/mgDOC Actual DOC value was shown inside each figure
Figure 2
1.2
[O3]/[O3]0
1
a
pH=5
b
pH=5
a
pH=7
b
pH=7
a
pH=9
b
pH=9
0.8 0.6 0.4 0.2 0 1.2 1
[O3]/[O3]0
0.8 0.6 0.4 0.2
0 1.2
[O3]/[O3]0
1 0.8 0.6 0.4 0.2 0
0
2
4
6
Time (min)
8
a: ultrapure water
10
pre-ozonation+tert-butanol pre-ozonation
12 0
2
4
6
Time (min)
b: WWTP effluent
8
10
12
HOC process+tert-butanol HOC process
Fig. 2 Ozone depletion at different pH values in pre-ozonation and the HOC process
Figure 3
1.1
[pCBA]/[pCBA]0
1
0.9
a
pH=5
b
pH=5
a
pH=7
b
pH=7
a
pH=9
b
pH=9
0.8 0.7 0.6 0.5 0.4
0.3 1.1
1
[pCBA]/[pCBA]0
0.9 0.8 0.7 0.6 0.5 0.4
0.3 1.1
[pCBA]/[pCBA]0
1
0.9 0.8 0.7 0.6 0.5 0.4 0.3
0
1
2
3
Time (min)
a: ultrapure water
4
pre-ozonation+tert-butanol pre-ozonation
5 0
1
2
3
Time (min)
b: WWTP effluent
4
5
HOC process+tert-butanol HOC process
Fig. 3 pCBA decomposition in the pre-ozonation and HOC process at different pH values
Figure 4
0.04
0.02
0
0.4 -0.02
-0.04 0.3 3300 0.2
a
0.1
3320
3340
3360
3380
pre-ozonation
3400
3420
0
-0.1
HOC process
-0.2
-0.3 3300
3320
3340
3320
3340
3360
3380
3400
3360
3380
3400
Magnetic value (G)
3420
0.05
0 0.4 -0.05 0.33300 0.2
b
pre-ozonation
3420
0.1 0
-0.1
HOC process
-0.2
-0.3 3300
3320
3340
3360
Magnetic value (G)
3380
3400
3420
Fig. 4 EPR spectra of DMPO-OH adduct obtained from pre-ozonation and the HOC process at pH 7. a: ultrapure water, b: WWTP effluent
Figure 5
3.0
a
2.5
pre-ozonation HOC process
∫[•OH]dt (M.min) ×10-12
∫[•OH]dt (M.min) ×10-12
4.0
2.0 1.0 0.0
pH 5
pH 7
pH 9
2.0
b
pre-ozonation HOC process
1.5 1.0 0.5 0.0
pH 5
pH 7
pH 9
Fig. 5 Comparison of •OH exposure between the HOC process and pre-ozonation process at different pH. a: ultrapure water, b: WWTP effluent
Figure 6
OH2+ pathway 2 (i)
-O
PAC
O-
H2O
•O2H
•O3H
•OH
pathway 2 (iii)
OH
PAC HO
O+
•O2-
Hydrogen bond
O-
O3
O H
O+ O
pathway 2 (ii)
OH2+
-O
O
H+
O-
Electrostatic forces
H+
OH
PAC HO
pathway 1
O3
O-
OH2+
Fig. 6 Proposed mechanism of •OH generation in the HOC process
Table 1 Summary of IOD values for each testing condition Table 1 Summary of IOD values for each testing condition IOD (mg/L)
Ultrapure water
WWTP effluent
Pre-ozonation
HOC process
Pre-ozonation
HOC process
pH=5
0.651
1.821
3.000
4.857
pH=7
1.940
2.952
4.333
5.762
pH=9
4.905
5.571
5.619
6.190
Table 2 Summary of Rct values for each testing condition Table 2 Summary of Rct values for each testing condition Rct (×10-8)
Ultrapure water
WWTP effluent
Pre-ozonation
HOC process
Pre-ozonation
HOC process
pH=5
0.498
0.611
0.469
0.985
pH=7
0.839
1.450
1.449
4.429
pH=9
1.277
4.054
3.351
7.595
29