Characteristics of C-, N-DBPs formation from algal organic matter: Role of molecular weight fractions and impacts of pre-ozonation

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Characteristics of C-, N-DBPs formation from algal organic matter: Role of molecular weight fractions and impacts of pre-ozonation Shiqing Zhou a, Shumin Zhu a, Yisheng Shao a,b,*, Naiyun Gao a a b

State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092, China China Academy of Urban Planning & Design, Beijing 100037, China

article info

abstract

Article history:

Extracellular organic matter (EOM) and intracellular organic matter (IOM) of Microcystis

Received 1 June 2014

aeruginosa have been reported to contribute to the formation of carbonaceous disinfection

Received in revised form

by-products (C-DBPs) and nitrogenous disinfection by-products (N-DBPs). Little is known

30 October 2014

about DBPs formation from different molecular weight (MW) fractions, especially for N-

Accepted 15 November 2014

nitrosodimethylamine (NDMA). This study fractionated EOM and IOM into several MW

Available online 26 November 2014

fractions using a series of ultrafiltration membranes and is the first to report on the C-DBPs and N-DBPs formation from chlorination and chloramination of different MW fractions.

Keywords:

Results showed that EOM and IOM were mainly distributed in low-MW (<1 KDa) and high-

Extracellular organic matter

MW (>100 KDa) fractions. Additionally, the low-MW and high-MW fractions of EOM and

Intracellular organic matter

IOM generally took an important part in forming C-DBPs and N-DBPs, either in chlorination

Disinfection byproducts

or in chloramination. Furthermore, the effects of pre-ozonation on the formation of DBPs

Pre-ozonation

in subsequent chlorination and chloramination were also investigated. It was found that

N-nitrosodimethlamine (NDMA)

ozone shifted the high-MW fractions of EOM and IOM into lower MW fractions and increased the C-DBPs and N-DBPs yields to different degrees. As low-MW fractions are more difficult to remove than high-MW fractions by conventional treatment processes, therefore, activated carbon adsorption, nanofiltration (NF) and biological treatment processes can be ideal to remove the low-MW fractions and minimize the formation potential of C-DBPs and N-DBPs. Moreover, the use of ozone should be carefully considered in the treatment of algal-rich water. © 2014 Elsevier Ltd. All rights reserved.

1.

Introduction

Blooms of cyanobacteria are ubiquitous in lakes and reservoirs and pose a great challenge for drinking water supplies (Xie et al., 2013; Yang et al., 2008; Zamyadi et al., 2013). Specific

cyanobacteria species (e.g., Microcystis aeruginosa) generate a variety of algal organic matter (AOM), including extracellular organic matter (EOM) and intracellular organic matter (IOM). EOM are the excreted metabolities of algal cells during exponential and stationary growth phases, whereas IOM result from cell lysis by aging of algae population and pre-oxidation

* Corresponding author. State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092, China. Tel./ fax: þ86 21 65982691. E-mail address: [email protected] (Y. Shao). http://dx.doi.org/10.1016/j.watres.2014.11.023 0043-1354/© 2014 Elsevier Ltd. All rights reserved.

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in water treatment process (Coral et al., 2013; Henderson et al., 2008; Pivokonsky et al., 2006). These organic substances are poorly removed by coagulation or pre-oxidation enhanced coagulation, and contribute to carbonaceous disinfection byproducts (C-DBPs) or nitrogenous disinfection by-products (N-DBPs) formation during chlor(am)ination due to the dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) content (Fang et al., 2010a, 2010b; Li et al., 2012; Wert and Rosario-Ortiz, 2013). The characterization of EOM and IOM has been reported in the literature, with respect to aromaticity (SUVA), fluorescence, hydrophobicity, proteins, carbohydrates, and molecular weight (MW) distribution (Henderson et al., 2008; Her et al., 2004; Li et al., 2012; Nguyen et al., 2005). In general, EOM and IOM are considered to contain more organic nitrogen (low DOC/DON), more hydrophilic content, and less aromatic content (low SUVA) (Fang et al., 2010b; Henderson et al., 2008; Li et al., 2012). Henderson et al. (2008) found that the AOM of Microcystis aeruginosa was dominated by proteins and polysaccharides, and had bimodal MW distribution with 55% greater than 30 KDa and 38% less than 1 KDa. In another study, Li et al. (2012) observed that IOM's MW fractions in <1 KDa, 40e800 KDa, and >800 KDa were ~27%, 42%, and 31%, respectively, while MW of the primary EOM molecules ranged within 1e100 KDa. Furthermore, size fractionation results of Qu et al. (2012) showed that the DOC fractions of high-MW (>100 KDa) and low-MW (<1 KDa) in the EOM of Microcystis aeruginosa accounted for 42.91% and 24.9% of total DOC. Meanwhile, 27.70% and 31.79% of total proteins, and 27.73% and 40.44% of total polysaccharides were distributed in the high-MW and low-MW fractions, respectively. Cyanobacteria-derived organic matter have been identified for decades as potential precursors for various C-DBPs and NDBPs (e.g., trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs), trichloronitromethane (TCNM), and N-nitrosamines) (Bond et al., 2011, 2012; Hoehn, 1980; Hong et al., 2008; Huang et al., 2009). Fang et al. (2010b) reported that the EOM of Microcystis aeruginosa formed smaller quantities of C-DBPs and N-DBPs than did IOM and algal cell, in chlorination and chloramination. Li et al. (2012) showed that the specific yields of chloroform and chloroacetic acid were 21.46 and 68.29 mg/mg C for IOM, and 32.44 and 54.58 mg/mg C for EOM, respectively. Moreover, N-nitrosodimethylamine (NDMA) formation has also been reported from cyanobacteria-derived organic matter after chlorination and chloramination (Fang et al., 2010b; Li et al., 2012; Zamyadi et al., 2012). While there have been several studies investigating the DBPs formation of EOM and IOM, little is known about the DBPs formation potential (DBPFP) from different molecular weight fractions, especially for the formation of NDMA. Pre-ozonation prior to disinfection has often been used as a pre-treatment ahead of the conventional process in drinking water treatment (Coral et al., 2013; Hua and Reckhow, 2013). Previous studies showed that ozone can react with some DBP precursors and affect the formation of DBPs depending on the water qualities and DBP species. For example, Plummer and Edzwald (2001) reported that pre-ozonation increased the chloroform formation to different degrees during chlorination of two algae species. Hua and Reckhow (2007) showed that

pre-ozonation decreased THMs, HAAs and total organic halogen (TOX) for most natural waters, while increased these DBPs for a water of low humic content. Wert and Rosario-Ortiz (2011) exhibited that pre-ozonation reduced the formation of THMs by up to 10 mg/L and the sum of five HAAs by up to 5 mg/L at two full-scale drinking water facilities. Therefore, a sequential ozone-chlorination (O3eCl2) or ozone-chlorami nation (O3eNH2Cl) process would be necessary to investigate the impact of pre-ozonation on C-DBPs and N-DBPs formation from EOM and IOM. The objective of this study were: (1) to compare different molecular weight characteristics of EOM and IOM from Microcystis aeruginosa; (2) to evaluate the contributions of different molecular weight fractions on the formation of CDBPs and N-DBPs; (3) to investigate the effects of preozonation on the formation of DBPs during subsequent chlorination and chloramination.

2.

Materials and methods

2.1.

Algae cultivation and AOM extraction

Microcystis aeruginosa (FACHB-912) was obtained from the Institute of Hydrobiology, Chinese Academy of Sciences, and cultured in BG11 media. The cultures were incubated at 25  C in 1 L conical flasks, under a 12-h diurnal cycle every day. M. aeruginosa cells in late exponential growth phase were harvested and centrifuged at 10,000 g for 10 min. The supernatants were subsequently filtered through 0.7 mm GF/F glassfiber filters (Whatman), and the filtrates were referred as EOM. Thereafter, the remaining cell pellets in glass-fiber filters together with centrifugal sediments were washed three times and then re-suspended in ultrapure water (Milli-Q, USA). The cells were then exposed to three freeze/thawing cycles to release the intracellular materials, followed by centrifuging and filtration. The filtrates were used as IOM.

2.2.

MW fractionation of EOM and IOM

EOM and IOM were fractionated into different fractions using a series of ultrafiltration (UF) membranes (polyethersulfone; Sartorius, Germany) with MW cut-offs of 100, 30, 10, 5, and 1 KDa, respectively. The fractionation experiment was conducted in a 400 mL stirring cell (Amicon 8400, Millipore Corp., USA) under a constant nitrogen gas pressure of 0.1 MPa. Prior to the operation, ultrapure water was filtered through the membranes to remove any possible leached organics until the DOC of the effluent was less than 0.1 mg-C/L. After UF separation, the filtrates were analyzed for their organic contents and tested for DBP formation potentials (DBPFP). Finally, the DOC, DON, UV254 and DBPFP of EOM and IOM fractions within the MW ranges of <1, 1e5, 5e10, 10e30, 30e100 and >100 KDa were determined.

2.3.

DBP formation potential experiments

The C- and N-DBPFP experiments during chlorination and chloramination were carried out using sealed 300 mL amber bottles at 25  C in the dark for 3 days. The doses of chlorine

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and monochloramine (NH2Cl) were determined as follows: Cl2 (mg/L) ¼ 3  DOC (mg-C/L) þ 7.6  NH3eN (mg-N/L) þ 10 (mg/L) and NH2Cl (mg/L) ¼ 3  DOC (mg-C/L), which can allow the reaction to approach completion (Krasner et al., 2007). The stock solution of free chlorine (50 g/L as Cl2) was prepared using a 5% liquid sodium hypochlorite (Sinopharm Chemical Reagent Co., Ltd., China) solution. A NH2Cl solution was freshly prepared by mixing ammonium chloride and sodium hypochlorite with a Cl2/N molar ratio of 0.8 M/M at pH 8.5. After 3-d reaction time, free chlorine and total chlorine residuals were measured using the N,N-dethyl-p-phenylenediamine (DPD) colorimetric method (Moberg and Karlberg, 2000). Prior to analysis, the residual chlorine or monochloramine was quenched by ascorbic acid and then extracted for DBP analysis. The analyzed C-DBPs include four THMs (i.e., chloroform (CHCl3), chlorodibromomethane (CHBr2Cl), bromodichloromethane (CHBrCl2), and bromoform (CHBr3)) and five HAAs (i.e., monochloroacetic acid (CAA), dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), monobromoacetic acid (BAA), and dibromoacetic acid (DBAA)). The analyzed N-DBPs include HANs (i.e., dichloroacetonitrile (DCAN), trichloroacetonitrile (TCAN), dibromoacetonitrile (DBAN), bromochloroacetonitrile (BCAN)), trichloronitromethane (TCNM) and N-nitrosodimethylamine (NDMA).

2.4.

Pre-ozonation experiments

Additional disinfection experiments were performed to evaluate the effects of pre-ozonation on the formation of DBPs. Ozone stock solutions were made by dissolving a high concentration of gaseous ozone (prepared from bottled oxygen) into deionized water at 4  C. Ozonation was conducted by adding an aliquot of ozone stock solution to achieve the desired dose based on an O3: DOC ratio of 1.0. The concentration of ozone stock solution and the dissolved ozone residual in samples were measured by indigo trisulfonate colorimetric method (Bader and Hoigne, 1981). The ozonated samples were stored at 4  C in the dark for no more than 24 h before the chlorination or chloramination experiments.

2.5.

Analytical methods

2.5.1.

DOC, DON and SUVA analysis

DOC and total dissolved nitrogen (TN) were measured using a TOC/TN analyzer (TOC-VCPH, Shimazu, Japan). The DON concentration was the difference between the TN and the total dissolved inorganic nitrogen (ammonia, nitrite, and nitrate). UV254 absorbance was measured with a UV/Vis spectrophotometer (HACH DR/2000, USA). SUVA was calculated as the ratio of UV254 to DOC.

2.5.2.

Analysis of DBPs

THMs, HANs and TCNM were quantified by liquid/liquid extraction with methyl-tertiary-butyl-ether (MTBE) followed by a Thermo TSQ Quantum XLS Triple Quadruple GCeMS/MS (Thermo Fisher Scientific, CA, USA), based on USEPA Method 551.1 (U.S.EPA, 1995). HAAs were analyzed by liquid/liquid extraction with MTBE followed by derivatization with acidic methanol and by GCeMS/MS according to USEPA Method 552.2 (U.S.EPA, 2003). The column was a Thermo Scientific TG-

383

5MS capillary column (30 m  0.25 mm inner diameter with 0.25 mm film thickness). NDMA was extracted from the samples by solid-phase extraction (SPE) and analyzed by a Thermo TSQ Quantum Access Max LC-MS/MS (Thermo Fisher Scientific, CA, USA) with a Basic-C18 HPLC column (100 mm  2.1 mm, 5 mm, Thermo, USA) (Zhou et al., 2012). Packed cartridges with activated charcoal (Sigma, USA) were used in the extraction process with a SPE Vacuum Manifolds (Supelco, PA, USA). Deuterated NDMA-d6 was used as surrogate standard and added to each sample prior to analysis (Xu et al., 2011).

2.5.3.

Polysaccharide and protein analysis

Polysaccharide and protein were measured using the phenolsulfuric acid method (Zhang et al., 1999) and a modified Lowry method (Frolund et al., 1995), respectively. Glucose was used for calibration at 490 nm and bovine serum albumin (BSA) was used for calibration at 562 nm with a HACH UV/Vis spectrophotometer. Bicinchoninic acid (BCA) was purchased from Shanghai Sangon Biological Engineering Technology & Services Co., Ltd.

2.5.4. High-performance size-exclusion chromatography (HPSEC) analysis Molecular weight distributions were determined by a HighPerformance Size-Exclusion Chromatography method. A high-performance liquid chromatography (HPLC, Waters e2695) was used with UVA (Waters 2489) and on-line DOC (Sievers 900 turbo TOC) detectors following size separation by a TSK-GEL G3000PWXL column (7.8 mm  300 mm, TOSOU Corporation, Japan). Polystyrene sulfonate sodium (PSS) and polyethylene glycols (PEGs) were used for calibration of the relationship between MW and retention time.

2.5.5. 3D fluorescence excitationeemission matrix (EEM) spectroscopy 3D EEM spectra of all samples were acquired by collecting excitation and emission spectra over a range of 250e550 nm using a fluorescence spectrophotometer (F-4500, Hitachi, Japan). Data were analyzed with Surfer Software and expressed by contour lines.

3.

Results and discussion

3.1.

Molecular weight fractions of EOM and IOM

The characteristics of different molecular weight fractions of EOM and IOM are summarized in Table 1. Based on the DOC data, the molecular weight fractions <1, 1e5, 5e10, 10e30, 30e100 and >100 KDa accounted for 21.5%, 11.6%, 2.6%, 4.7%, 7.8% and 51.8% of EOM, and for 28.5%, 4.7%, 4.7%, 5.8%, 10.0% and 46.3% of IOM, respectively. UV254 values followed the similar trends among different MW fractions and relatively more aromatic moieties were presented in low-MW fractions (MW < 1 KDa) and high-MW fractions (MW > 100 KDa). Furthermore, the average DOC/DON of IOM were lower than that of EOM, and much lower than that of natural organic matter (NOM) (Fang et al., 2010b; Xu et al., 2011), indicating that IOM possessed more organic N than EOM and NOM.

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Table 1 e Water characteristics of various EOM and IOM fractions. MW DOC DON DOC/ UV254 SUVA fraction (mg/L)a (mg/L) DON (cm1) (L/mg m)b EOM

IOM

<1 K 1e5 K 5e10 K 10e30 K 30e100 K >100 K <1 K 1e5 K 5e10 K 10e30 K 30e100 K >100 K

4.30 2.33 0.52 0.95 1.55 10.35 5.70 0.95 0.95 1.16 2.00 9.25

1.05 0.48 0.11 0.17 0.26 1.68 2.12 0.26 0.16 0.17 0.47 3.05

4.09 4.82 4.62 5.50 6.00 6.15 2.69 3.60 6.00 6.88 4.22 3.03

0.075 0.056 0.007 0.022 0.019 0.079 0.060 0.042 0.037 0.028 0.023 0.150

1.75 2.41 1.33 2.36 1.22 0.77 1.05 4.44 3.89 2.45 1.16 1.63

a

The total DOC of EOM or IOM was normalized to 20.00 mg/L. DOC, DON and UV254 in each fraction were calculated using the method described by Lee et al. (2006). b SUVA was calculated from UV absorbance at 254 nm (UV254) divided by the DOC.

The proteins and polysaccharides analyses for different molecular weight fractions are presented in Fig. S1 of the Supporting Information. It would be noted that 33.9% of total proteins for EOM were distributed in the low-MW fractions (MW < 1 KDa) and 44.9% of total proteins for IOM were distributed in the high-MW fractions (MW > 100 KDa),

respectively. However, it was completely different in the distribution of polysaccharides for EOM and IOM. The most polysaccharides were found in the high-MW fractions (MW > 100 KDa) for EOM (49.2%) and in the low-MW fractions (MW < 1 KDa) for IOM (31.5%), respectively. These results were generally in agreement with the findings reported by Henderson et al. (2008) and Qu et al. (2012). In our study, for EOM and IOM, the low-MW fractions (MW < 1 KDa) were mainly comprised of proteins and polysaccharides, which had high potential in producing DBPs (Hong et al., 2008), and were less easily removed by traditional coagulation and filtration than high-MW fractions (MW > 100 KDa), thus leading to the increases of DBPs formation potentials during chlorination and chloramination within the water treatment plant.

3.2. Formation of C-DBPs and N-DBPs from different molecular weight fractions 3.2.1.

Total DBP formation

As bromide was not included in the solution, no brominatedDBPs were detected in this study. The formation of C-DBPs and N-DBPs from different molecular weight fractions of EOM and IOM are presented in Figs. 1 and 2, respectively. TCMFP were produced mostly from MW < 1 KDa and MW > 100 KDa fractions, which were the main groups of TCM precursors. The trends were generally consistent with the distribution of UV254 values. A good linear relationship was found between UV254 and TCMFP during chlorination and chloramination (see

Fig. 1 e Formation of C, N-DBP from different molecular weight fractions of EOM during chlorination and chloramination.

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Fig. 2 e Formation of C, N-DBP from different molecular weight fractions of IOM during chlorination and chloramination.

Fig. S2, Supporting Information), which was also found in the work of White et al. (2003). Meanwhile, somewhat of similar slopes between EOM and IOM were observed during chlorination and chloramination, which suggested the similar precursors and/or mechanisms of TCM formation. Furthermore, the percentages of TCM formation in MW fractions of <1, 1e5, 5e10, 10e30, 30e100 and >100 KDa of EOM were, respectively, 29.8%, 18.1%, 3.7%, 8.8%, 9.8% and 29.8% during chlorination and 26.5%, 14.0%, 4.9%, 3.3%, 13.1% and 38.2% during chloramination. While for IOM, the percentages of TCM formation in MW fractions of <1, 1e5, 5e10, 10e30, 30e100 and >100 KDa were, respectively, 15.8%, 6.5%, 1.6%, 2.5%, 9.2% and 64.4% during chlorination and 26.7%, 5.9%, 1.8%, 7.6%, 15.5% and 42.5% during chloramination. This observation indicated that the low-MW (<1 KDa) and high-MW (>100 KDa) fractions was important in the formation of TCM during either chlorination or chloramination. The species of HAA was dominated by DCAA and TCAA. Similar to the distribution of TCMFP, the MW < 1 KDa and MW > 100 KDa fractions contained the largest parts of HAAFP. Additionally, we also found somewhat of linear relationships (R2 ¼ 0.69e0.94) between the UV254 and HAA concentrations (see Fig. S3, Supporting Information), which implies the UV254 value as an indicator of HAA concentrations. Moreover, the percentages of HAA formation in MW fractions of <1, 1e5, 5e10, 10e30, 30e100 and >100 KDa of EOM were, respectively, 19.7%, 18.9%, 11.7%, 4.3%, 7.9% and 37.5% during chlorination and 20.5%, 21.1%, 2.7%, 6.4%, 16.8% and 32.5% during

chloramination. While for IOM, the percentages of TCM formation in MW fractions of <1, 1e5, 5e10, 10e30, 30e100 and >100 KDa were, respectively, 18.1%, 8.1%, 6.5%, 4.2%, 12.5% and 50.6% during chlorination and 29.7%, 4.8%, 8.0%, 4.8%, 10.2% and 42.5% during chloramination. This observation further highlighted the importance of low-MW (<1 KDa) and high-MW (>100 KDa) fractions as the precursors of HAA during chlorination and chloramination. In addition, the highest C-DBPs concentrations (i.e., TCM, DCAA and TCAA) was observed in MW > 100 KDa fraction of IOM. It can be attributed to the more algal proteins in IOM > 100 KDa, which contained rich amino acid content (Becker, 2007), and thus had wideranging HAA formation potentials (Hong et al., 2009). The formation potentials of N-DBPs showed a similar trend with C-DBPs and the low-MW (<1 KDa) and high-MW (>100 KDa) fractions of EOM or IOM also take a more important part in the N-DBPs formation. The concentrations of DCAN ranged from 0.1 to 8.0 mg/L for EOM and from 0.2 to 28.8 mg/L for IOM, respectively. There were no measurable formation of TCAN from EOM and IOM. The lower concentration of DCAN in EOM than in IOM can be ascribed to the fact that IOM possessed more organic nitrogen than EOM. The MW < 1 KDa fraction contained the maximum concentrations of TCNM in EOM and IOM. Thibaud et al. (1988), who studied the size fraction using ultrafiltration membranes, also reported that 90% of TCNM precursors in surface waters were retained by 500 nominal molecular weight cutoff membranes. Furthermore, the concentrations of TCNM from chlor(am)

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ination of EOM was significantly greater than that of IOM, which may be attributed to the high concentration of nitrate and nitrite from BG11 media in the EOM solution (Fang et al., 2010b). The MW < 1 KDa fractions of EOM and IOM generally had a highest potential to form NDMA, either in chlorination or in chloramination, which was generally in agreement with the results reported during other studies (Kristiana et al., 2013; Mitch and Sedlak, 2004; Pehlivanoglu-Mantas and Sedlak, 2008). The second highest NDMA concentration was formed from the fractions with MW > 100 KDa. Moreover, the maximal NDMA concentration reached up to 149.8 and 252.8 ng/L for EOM, and 163.3 and 260.8 ng/L for IOM during chlorination and chloramination, respectively. In summary, similar trends were found for all tested DBPs from different molecular weight fractions of EOM and IOM, while the DBP yields varied and did not follow the same order in terms of molecular weight fractions (see Figs. S4 and S5, Supporting Information). The yields of C-DBPs and N-DBPs formed from low-MW (<1 KDa) and high-MW (>100 KDa) fractions were not of the highest among different molecular weight fractions. But due to the high DOC concentrations, the higher concentrations of C-DBPs and N-DBPs highlighted the importance of low-MW (<1 KDa) and high-MW (>100 KDa) fractions regarding to the formation of DBPs. Therefore, knowledge of the contributions of different molecular weight fractions on formation of C-DBPs and N-DBPs will provide useful information for effective water treatment process to reduce the DBP formation.

3.2.2.

formed in chloramination were up to two-fold higher than in chlorination. Since the most well-known organic nitrogen precursors are secondary and tertiary amines (Chen and Valentine, 2006; Mitch and Sedlak, 2002), inorganic nitrogen contained in monochloramine or dichloramine also plays an important role in the formation of NDMA (Choi and Valentine, 2003). Hence, the higher concentrations of inorganic nitrogen in chloramination than in chlorination enhanced the formation of NDMA in chloramination process.

3.3. Effect of pre-ozonation on chlorination and chloramination DBPs The effect of pre-ozonation on the EEM fluorescence spectra of EOM and IOM are presented in Fig. S7 of the Supporting Information. It is clear from the figure that specific peaks corresponding to humic-like (lex > 250 nm, lem > 380 nm) and/or protein-like (lex > 250 nm, lem < 380 nm) substances in EOM and IOM disappeared after pre-ozonation. The fluorescence variations were attributed to the fact that molecular ozone can react readily with carbonecarbon double bonds and activated aromatic moieties with high electron density (von Gunten, 2003), thus leading to the cleavage of aromatic rings and decrease in color and fluorescence.

Comparison of chlorination and chloramination

Overall, chlorination produced more DBPs than chloramination for the species examined in this study, except NDMA (Figs. 1 and 2). The average levels of total TCM, DCAA, TCAA, DCAN and TCNM were, respectively, 44.9, 27.6, 28.5, 3.0 and 0.7 mg/L for EOM and 45.3, 32.4, 36.2, 12.2 and 0.4 mg/L for IOM, after chlorination. In comparison, the total TCM, DCAA, TCAA, DCAN and TCNM were, respectively, 4.2, 12.5, 0.7, 0.6 and 0.3 mg/L for EOM and 5.4, 14.7, 0.7, 1.7 and 0.2 mg/L for IOM, after chloramination. The concentrations of TCM and HAA during chlorination were much higher than those from chloramination, which is well consistent with the results of Fang et al. (2010b) when chlorinating and chloraminating cells of Microcystis. The lower concentration of DCAN in chloramination than in chlorination suggested that organic nitrogen content rather than inorganic nitrogen may play an important role in the formation of DCAN. This observation could be further proved by the good linear relationships (R2 ¼ 0.76e0.98) between DON and DCAN during chlorination and chloramination (see Fig. S6, Supporting Information). TCNM formation followed the same trends and chlorination of EOM and IOM generated larger amounts of TCNM than chloramination. It has been reported that TCNM formed from chlorination or chloramination of aliphatic amines and amino acids (Joo and Mitch, 2007; Hu et al., 2010). Other study found that another relevant TCNM formation pathway involves chlorination of nitrite to form a nitrating agent ClNO2 (Shah and Mitch, 2012). Meanwhile, whether monochloramine contributes to the nitrogen source of TCNM remains unknown (Yang et al., 2011). Of note, NDMA formation showed the opposite trends and the average concentrations of NDMA

Fig. 3 e Molecular weight distribution of (a) EOM before and after pre-ozonation, (b) IOM before and after pre-ozonation.

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Chlorination

(b)

Chloramination

100

80 781%

60

330%

40 20 0 -20 -40 -60 TCM

DCAA

TCAA

DCAN

TCNM

NDMA

DBP Yield variation after ozonation (%)

DBP Yield variation after ozonation (%)

(a)

Chlorination

Chloramination

100

80 438%

60

147%

40 20 0 -20 -40 -60 TCM

DCAA

TCAA

DCAN

TCNM

NDMA

Fig. 4 e The percentage changes in C-DBPs and N-DBPs yields of (a) EOM and (b) IOM after pre-ozonation during subsequent chlorination and chloramination.

Fig. 3 presents the effect of pre-ozonation on the molecular weight distribution of EOM and IOM. Before pre-ozonation, the major molecular weight fractions of EOM were found in the ranges of 0.3e10 KDa, and 20e1000 KDa. After pre-ozonation, the EOM was shifted to smaller molecular weight fractions. A decrease in the 20e1000 KDa ranges resulted in an increase in the fraction of 0.3e10 KDa. Similar results were observed in IOM. High-MW fractions (>30 KDa) were shifted to low-MW fractions of 1e10 KDa. These results were well supported by other studies in algae (Plummer and Edzwald, 2001) that EOM showed a significant increase in the low apparent molecular weight fractions after ozonation, with a corresponding decrease in the higher apparent molecular weight fractions. Fig. 4 shows the percentage changes of DBPs yields after pre-ozonation during subsequent chlorination and chloramination. The TCAA and DCAN yields formed by chloramines were less than 1.0 mg/mg C, thus the data are not shown. In general, results showed that O3eCl2/NH2Cl increased DBP

yields to different degrees compared to Cl2/NH2Cl alone. Of note, the fact that an opposite trend was observed for DCAA and TCAA yield suggested that DHAAs and THAAs form from different precursors and reaction pathways (Hua and Reckhow, 2013). Furthermore, in this study, pre-ozonation increased the NDMA yields by 19.1e27.3% and 36.4e51.8% in chlorination and chloramination, respectively, which was consistent with the results of other study (Wert and RosarioOrtiz, 2013). There are two probable interpretations for this phenomenon. The first interpretation is that ozone shifted high-MW fractions to low-MW fractions, which would contribute to the net increase in TCM, DCAA, THAA, DCAN, TCNM and NDMA formation. As shown in Fig. 5, the MW < 10 KDa fractions generally had greater DBP yields than the MW > 100 KDa fractions during chlorination and chloramination, except for C-DBPs from chlorination of IOM. Meanwhile, the MW > 100 KDa fractions of EOM and IOM were found to be

Fig. 5 e DBP yields from MW < 10 KDa and MW > 100 KDa fractions of (a) EOM and (b) IOM during chlorination and chloramination.

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shifted to MW < 10 KDa fractions after pre-ozonation as seen in Fig. 3. Therefore, it can be easily noted that the DBP yields with pre-ozonation were obviously greater than that without pre-ozonation. The second interpretation is that algal organic matter have much low SUVA values (i.e., 0.95 and 1.91 L mg1 m1 for EOM and IOM, respectively) in the study, indicating that EOM and IOM lack the degree of aromaticity associated with terrestrially derived organic matter. It was reported that ozone was not effective in destroying THM and THAA precursors in waters with low SUVA values (<2 L mg1 m1) (Hua and Reckhow, 2007). As a result, negligible reduction in DBP yields may be expected when using ozone pre-treatment. Overall, it is possible that ozone reacted with EOM/IOM and produced secondary byproducts containing more C-DBP and N-DBP precursor sites, which led to net increases in these DBP concentrations during subsequent chlorination and chloramination.

3.4.

Implications for drinking water treatment

The results of this study suggest that low-MW (<1 KDa) and high-MW (>100 KDa) fractions of EOM and IOM contain significant amounts of C-DBP and N-DBP precursors. In drinking water treatment, high-MW fractions of algal organic matter can be easily removed by conventional drinking water treatment processes (e.g., coagulation, sedimentation, and filtration) (Pivokonsky et al., 2012). However, the removal of lowMW fractions by coagulation is likely to be negligible (Pivokonsky et al., 2012; Safarikova et al., 2013), and thus they will largely pass through the water treatment processes and result in AOM-derived DBP problems with drinking water. Therefore, advanced treatment technologies, such as powdered or granular activated carbon adsorption, nanofiltration (NF) and biological treatment processes that target the removal of low-MW fractions would be ideal to minimize the formation potential of C-DBPs and N-DBPs (Hnatukova et al., 2011; Pivokonsky et al., 2014; Velten et al., 2011). During pre-treatment of algal-rich water, ozone as an oxidant has been increasingly used in drinking water treatment. In this study, it should be noted that pre-ozonation is not effective in destroying the DBP precursors. Instead, preozonation may substantially increase the DBP yields to different degrees, especially for NDMA. For this reason, the use of ozone for treating algal-rich water should be carefully considered.

4.

Conclusion

The EOM and IOM of Microcystis aeruginosa were fractionated into different molecular weight fractions and their contributions to the formation potentials of C-DBPs and N-DBPs were comprehensively investigated. The effects of pre-ozonation on the DBPs yields were also evaluated during subsequent chlorination and chloramination. The following conclusions can be drawn. (a) EOM and IOM were mainly distributed in low-MW (<1 KDa) and high-MW (>100 KDa) fractions.

(b) EOM contained more proteins in low-MW fraction and polysaccharides in high-MW fraction, while IOM had more proteins in high-MW fraction and polysaccharides in low-MW fraction. (c) Low-MW and high-MW fractions of EOM and IOM contributed to a significant amount of C-DBPs and N-DBPs precursors during chlorination and chloramination. (d) Pre-ozonation shifted the high-MW fraction to lower MW fraction and led to net increases of DBP yields.

Acknowledgment This research work was jointly supported by the National Major Project of Science & Technology Ministry of China (No. 2012ZX07403-001), National Natural Science Foundation of China (No. 51178321, 51378366), and the research and development Project of Ministry of Housing and Urban-Rural Development (No. 2009-K7-4).

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2014.11.023.

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