Characterization of intracellular & extracellular algae organic matters (AOM) of Microcystic aeruginosa and formation of AOM-associated disinfection byproducts and odor & taste compounds

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Characterization of intracellular & extracellular algae organic matters (AOM) of Microcystic aeruginosa and formation of AOM-associated disinfection byproducts and odor & taste compounds Lei Li a,b, Naiyun Gao a,*, Yang Deng c, Juanjuan Yao d, Kejia Zhang e a

State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai City 200092, China Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720, USA c Department of Earth and Environmental Studies, Montclair State University, Montclair, NJ 07043, USA d Key Laboratory of the Three Gorges Reservoir Regions Eco-Environment, Ministry of Education, Chongqing University, Chongqing 400045, China e Department of Civil Engineering, Zhejiang University (Zijingang Campus), Hangzhou 310058, China b

article info

abstract

Article history:

Algae organic matters (AOM), including intracellular organic matters (IOM) and extracel-

Received 21 June 2011

lular organic matters (EOM), are causing numerous water quality issues, among which

Received in revised form

formation of disinfection byproducts (DBPs) and odor & taste (O&T) compounds are of

7 December 2011

particular concern. In this study, physiochemical properties of IOM and EOM of Microcystic

Accepted 10 December 2011

aeruginosa under an exponential growth phase (2.01
1011/L) were comprehensively

Available online 16 December 2011

characterized. Moreover, the yields of DBPs during AOM disinfection and O&T-causing compounds were quantified. Hydrophilic organic matters accounted for 86% and 63% of

Keywords:

DOC in IOM and EOM, respectively. Molecular weight (MW) fractions of IOM in <1 kDa,

Microcystic aeruginosa

40e800 kDa, and >800 kDa were 27%, 42%, and 31% of DOC, respectively, while EOM

Algae organic matters

primarily contained 1e100 kDa molecules. Besides, a low SUVA (0.84 L/mg m) and the

Disinfection byproducts

specific fluorescence spectra suggested that AOM (especially IOM) was principally

Odor and taste compounds

comprised of protein-like substances, instead of humic-like matters. The formation

Intracellular organic matters

potentials of chloroform, chloroacetic acid, and nitrosodimethylamine were 21.46, 68.29

Extracellular organic matters

and 0.0096 mg/mg C for IOM, and 32.44, 54.58 and 0.0189 mg/mg C for EOM, respectively. Furthermore, the dominant O&T compound produced from EOM and IOM were 2-MIB (68.75 ng/mg C) and b-cyclocitral (367.59 ng/mg C), respectively. Of note, dimethyltrisulfide became the prevailing O & T compound following anaerobic cultivation. ª 2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Algae -inducing eutrophication frequently occurs in lakes and reservoirs, and causes water quality degradation on a global level (Boyd et al., 2000; Yang et al., 2008). Among a variety of

algae, cyanobacteria (e.g. Microcystic aeruginosa) is one of the most common algae responsible for fresh water blooming (Fahnenstiel et al., 2008; Yang et al., 2008; Zhang et al., 2010, 2011). Typically, the undesirable algae organic matters (AOM)-associated contaminants of major concern include: (1)

* Corresponding author. Tel.: þ862165982691. E-mail address: [email protected] (N. Gao). 0043-1354/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2011.12.026

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algae toxins; (2) odor and taste (O & T)-causing compounds; and (3) other AOM, such as the proteins, peptides, amino sugars, and polysaccharose (Fang et al., 2010b; Henderson et al., 2008; Takaara et al., 2010). Aqueous AOM derive from algae metabolites that are generally categorized into extracellular organic matter (EOM) and intracellular organic matter (IOM). The AOM can cause such water treatment operational problems as a rising coagulant demand, membrane fouling, elevated total organic carbon (TOC), higher formation potentials of disinfection byproducts (DBPs), disagreeable O&T compounds, and other risks associated with health security of drinking water (Carmichael, 1994; Henderson et al., 2010; Lee et al., 2007; Tencalla et al., 1994). Understanding the physicochemical characteristics of the AOM is essential to design appropriate water treatment processes. Typical water quality parameters used to characterize the AOM include dissolved organic carbon (DOC), specific UV absorbance (SUVA), zeta potential, hydrophilicity, MW distribution, and fluorescence properties. Bernhardt et al. (1985) conducted the first EOM characterization study from a treatment perspective, and found that high-MW EOM could act as a flocculant, while lowMW EOM resulted in a colloidally stable suspension characterized by negatively charged particle surfaces. Henderson et al. (2008, 2010) reported that the EOM of Microcystis aeruginosa was dominated by polysaccharides and proteins, and its MW was characterized by a bimodal distribution with 55% >30 kDa and 38% <1 kDa. Additionally, the EOM of all algae species examined were largely hydrophilic (57%) and consisted of 33% protein, and 28% carbohydrate as C: C (Henderson et al., 2008), while algae cells were made up of 82% protein, and 14.5% carbohydrate (Hong et al., 2008), thus suggesting a difference in the molecular composition between EOM and algae cells. Although there is n need for understanding of the properties of EOM and IOM, few in-depth studies (e.g. characterization of AOM’s hydrophilicity and hydrophobicity) have been conducted to comprehensively analyze the two organic material types. Among the algaecausing water quality issues, the formation of DBPs and O & T compounds is particularly concerned. Previous findings in the yields of DBPs produced from AOMs were variable. The yields of THMs, DCAA, and TCAA of M. aeruginosa EOM ranged at 16.5e45.7, 14e42 and 18.7e25 mg/mg C, respectively (Fang et al., 2010b; Hong et al., 2008; Huang et al., 2009; Nguyen et al., 2005), apparently regardless of the cell growth phases (Huang et al., 2009; Nguyen et al., 2005). Moreover, the yields of TCM, DCAA, and TCAA produced from the algae cells were 22e61, 18e71, and 25e93 mg/mg C, respectively (Fang et al., 2010a; Huang et al., 2009). Of note, the cells were readily removed within a few minutes by coagulation and sedimentation. In contrast, the IOM released during pre-oxidation or hydraulic shearing of algae cells might exist throughout the entire water treatment. Therefore, research specifically on IOM, rather than the whole algae cell, might be more advisable to evaluate the DBP formation. Moreover, IOM, as high protein compounds, may have a high potential to produce N-containing DBPs, such as haloacetonitriles (HANs), halonitromethanes (HNMs), and nitrosamines (Lee et al., 2007; Mitch and Sedlak, 2002; Plewa et al., 2004). Unfortunately, the correlation of NDMA and other DBPs with the AOM’s hydrophilicity and hydrophobicity has not been studied.

On the other side, numerous algae compounds are able to contribute to the production of O&T compounds in water, such as geosmin, 2-methyl-isoborneol (2-MIB) (Lalezary et al., 1986), b-cyclocitral, and volatile sulfide (e.g. dimethyltrisulfide). Dimethyltrisulfide, with a septic and marshy smell, was found as the main O&T compound formed during a serious eutrophication-induced drinking water crisis in China, 2007 (Yang et al., 2008; Zhang et al., 2010). In fact, this O&T compound was also commonly detected in the drinking water as a result of algal contamination in many other countries (Khiari et al., 1997). However, a quantitative analysis of the formation of individual O&T compounds from algae EOM and IOM under different environmental conditions has not been reported. M. aeruginosa was chosen as the target pollutant as it is one of the most popular blue-green species found in fresh water bloom (Yang et al., 2008; Zhang et al., 2010). Besides, it has exhibited a higher DBPFP than other blue-green species such as Anabaena (Huang et al., 2009). The major objective of this study is to comprehensively characterize and compare physicochemical characteristics of EOM and IOM of M. aeruginosa using multiple analytical techniques, and evaluate the contributions of EOM and IOM to the formation of two objectionable pollutants, DBPs and O&T compounds, during water treatment.

2.

Methodology

2.1.

Algae cultivation and experimental procedures

M. aeruginosa, purchased from the Institute of Hydrobiology, Chinese Academy of Sciences, was cultured in BG11 media in an incubator at 25  C. The algae solution was harvested during its exponential period (number 2.01
1011/L), and then centrifuged at 10,000 rpm for 10 min. The supernatants were filtered through 0.45 mm cellulose acetate membranes, and the organic matters in the filtrate represented EOM. The cells separated during the centrifugation were washed three times and then re-suspended in Milli-Q water. The cells was then subjected to three freeze/thawing cycles (Daly et al., 2007) before filtered through 0.45 mm cellulose acetate membranes. The organic matters in the filtrate were referred as IOM. In the tests for THM and HAA formation potentials, samples were chlorinated with sodium hypochlorite (NaOCl) individually and incubatedat 25  1  C in dark for 7 days. The initial weight ratio of NaOCl (as Cl2) to DOC for was 5 (Xu et al., 2007). For anaerobic cultivation, the solution pH of 500 mL IOM and EOM extractions were pre-adjusted to 7.0  0.1 with 1 N HCl and NaOH, and then sealed in a 1-L glass flask at 30  1  C for 7-day incubation. After the tests were completed, the final dissolve oxygen (DO) was <0.5 mg/L.

2.2.

Analytical methods

2.2.1.

XAD and IRA resin fractionation

An XAD-8/XAD-4/IRA-958 (Amberlite, USA) column was used to fractionate the AOM into hydrophobic (HPO), transphilic (TPI), charged hydrophilic (CHPI) and neutral hydrophilic (NHPI) fractions according to the method developed by Carroll

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et al. (2000). The recovery rate of the fractionation were 82e92%. The presented data had been normalized to 100%.

2.2.2.

Fluorescence spectroscopy

Excitation-emission matrices (EEMs) (230e560 nm) of EOM and IOM were acquired using a fluorescence spectrophotometer (model F-4500, Hitachi, Japan). Data were analyzed using Matlab (MathWorks Inc., Natick, MA).

2.2.3.

Measurement of DBPs

Samples after 7 days incubation were extracted and then analyzed with a gas chromatograph (GC-2010, Shimadzu, Japan) equipped with an electron capture detector (ECD) (Xu et al., 2007). NDMA formation potential (NDMAFP) was measured using the method developed by Mitch and Sedlak (2002) as follows: All the tests were conducted in 1-L amber bottles containing 500 mL of 0.45 mm membrane filtered samples buffered at pH 6.8 with 10 mM phosphate buffer. Reaction was initiated by addition of monochloramine (2.0 mM). All the tests were performed at 25  1  C in dark to minimize the photolysis of NDMA formed. After 7 days, the reaction was quenched by addition of 5 mmol of ascorbic acid. NDMA in the samples were quantified using solid phase extraction (SPE) followed by UPLC/MS/MS (ESI source, Thermo Fisher Scientific Inc., USA) with a Thermos Hypersil GOLD C8 column (150 mm
2.1 mm, 3 mm, Thermo Fisher Scientific Inc., USA) (Liang et al., 2009; Plumlee et al., 2008). Packed cartridges with activated charcoal (Sigma, USA) was used in the extraction process with a SPE Vacuum Manifolds (Supelco, PA, USA). Methanol and 2 mM ammonium acetate buffer in Milli-Q water were used as the mobile phase at a flow rate of 0.15 mL/min.

2.2.4.

Measurement of O&T compounds

2-MIB, b-cyclocitral, geosmin, ionone, dimethyltrisulfide in the samples were quantified with headspace solid-phase microextraction (HSPME) coupled with a gas chromatograph (GC, Agilent 6890) and a mass spectrometer (MS, Agilent 5973). A commercially available fiber (30/50 mm DVB/CAR/PDMS No. 57348-U)) and a manual fiber holder (No. 57330-U), both purchased from Supelco (Bellefonte, PA, USA), were used for extraction of the odorants. The compounds trapped in the fiber were then determined using the GC/MS method described elsewhere (Lin et al., 2003, 2002).

2.2.5.

Other analysis

Dissolved inorganic nitrogen (DIN) (ammonia, nitrate, and nitrite) was measured with a spectrophotometer (Unico, USA). Dissolved organic nitrogen (DON) was the difference between total dissolved nitrogen (TDN) and DIN. Molecular weight distribution was analyzed by a LC-10ATVP gel permeation chromatograph (GPC) (Shimadzu, Japan) coupled with a TSK 4000GPC column (1000 kDa exclusion limit) and a RID-10A detector. Polyethylene glycol was used to calibrate. Microcystin-LR (MC-LR) was determined using a HPLC method (LC-2010AHT, Shimadzu, Japan) (Li et al., 2009). DOC was quantified using a TOC analyzer (TOC-VCPH, Shimazu, Japan). Algae were counted with microscope (YS100, Nikon, Japan). Chlorophyll was measured in vivo using a PHYTO-PAM phytoplankton analyzer (Walz, Germany). All the tests were

conducted at least in duplicate. The standard deviations (RSD) for different batches were normally <13% and up to 22% (it is relatively higher than normal chemical experiment as the algae growth can’t be identical from time to time). The RSD of the detection facility were <5%.

3.

Results and discussion

3.1.

Basic parameters of AOM

IOM extract solution was characterized by bright-blue color, which was as a result of a high concentration of phycocyanin (Ernst et al., 1992). In contrast, the EOM extract was characterized by light yellow, due to the presence of extracellular metabolites such as carbohydrates, amino acids, enzymes, hormonal substances and inhibitors (Fogg, 1983). Table 1 summarizes the basic water quality parameters of the EOM and IOM extract solutions. IOM extract solution had much higher DOC (100.5 mg/L) and algae toxin MC-LR (506.1 mg/L) than EOM (29.7 mg/L DOC and 141.2 mg/L MC-LR). IOM contained 3.2 mg/L chlorophyll-a, accounting for 3.15% of the DOC. Of note, the IOM had a DON/DOC of w0.23, in agreement with a previous finding (Fang et al., 2010a) and approximately four fold of the average DON/DOC in natural organic matters (NOMs) (w0.067) (Westerhoff and Mash, 2002; Xu et al., 2011), implying that IOM might have a high formation potential in nitrogen DBPs.

3.2.

MW distribution

MW distributions of IOM and EOM are shown in Fig. 1. Both of them exhibited a trimodal distribution pattern. As shown, IOM’s MW fractions in the ranges of <1 kDa (chlorophyl, algae toxin, O&T compounds, amino acids and other small molecules), 40e800 kDa, and >800 kDa (mostly phycocyanin and carbohydrates) (Henderson et al., 2008, 2010)) were w27%, 42%, and 31% of the total DOC peak areas, respectively. In contrast, the major EOM peak was located within 1e100 kDa, accounting for w85% of the overall peak area. Furthermore, the average MW of IOM was higher than that of EOM. Of note, our observation was entirely different from the findings of other studies in NOM (Xu et al., 2007, 2011) that <1 kDa accounted for 40e55% of the total DOC, while >30 kDa was less than 10%. It should be note that the detection may be slightly affected by BG11 cultivation media as the RID-10A is nonselective detector. The effect of high-MW AOM molecules on coagulation during water treatment has been previously studied, but the results were conflicting. Pivokonsky et al. (2006) reported that

Table 1 e Basic water quality parameters of the EOM and IOM extract solutions from exponential growth phase Microcystis aeruginosa solution (cell number 2.01 3 1011/L).

EOM IOM

DON (mg/L)

MC-LR (mg/L)

DOC (mg/L)

Chlorophyll-a (mg/L)

DON/ DOC

NA 23.1

141.2 506.1

29.7 100.5

NA 3.2

NA 0.23

NA: not analyzed.

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EOM

80

%

and filtration than hydrophobic fraction. Therefore, the residual hydrophilic portion subsequent to rapid filtration might increase the N-DBP formation potential during chlorination, and cause biological instability of the finished water as the organic matters are easily consumed by the bacteria existing in the water distribution system.

IOM

100

60 40

3.4.

20 0 0

2

4

6

8

Log(MW) Fig. 1 e Molecular weight distribution of IOM and EOM.

AOM with ～60 kDa appeared to form complexes with Al(III) and Fe(III)-based coagulant to increase the coagulant demands, while Bernhardt et al. (1985) demonstrated that high-MW AOM played a role in flocculation to reduce the coagulant dose. Although the impact of high-MW AOM on the coagulant’s dose was out of scope in this study, the presence of AOM would influence (reduce or increase) the coagulant demand during water treatment, considering that ～60 kDa organic matters accounted for a considerable portion of EOM and the most of IOM molecules were >100 kDa fraction.

3.3.

Hydrophilicity vs. hydrophobicity

Hydrophilic and hydrophobic properties of the EOM and IOM are shown in Table 2 .The fractions of HPO, TPI, NHPI and CHPI were 31%, 6%, 53%, and 10% in EOM, and 9%, 5%, 76%, and 10% in IOM, respectively. The results were almost in agreement with the finding of Henderson et al. (2008) that the fractions of HPO, TPI and HPI in the total DOC were 30%, 10% and 60% for EOM of M. aeruginosa, respectively. Of note, the complex of water matrix (i.e. the BG11 medium) with EOM may would result in the shift of hydrophilicity, which may account for the difference result reported by other researchers (Fang et al., 2010b). High percentage of hydrophilic fraction, which was mainly protein-like compounds (Pivokonsky et al., 2006), would decrease the water treatment efficiency as it is less readily removed by traditional coagulation, sedimentation

Fluorescence spectroscopy

EEMs of EOM and IOM are shown in Fig. 2. The fluorescence spectroscopic data can be explained by a fluorescence regional integration (FRI) method (Chen et al., 2003). As shown in Fig. 2(a), the most intense peak (9,275 mV) of IOM occurred at Ex/Em of 280/335 nm, just falling within a soluble microbial product (SMP)-like region. The second highest peak (3,113 mV) was observed at Ex/Em of 230/350 nm, representing an aromatic protein region. Moreover, the other two distinct peaks of IOM occurred at Ex/Em of 365/460 (2,241 mV) and 305/ 465(2,230 mV) nm, respectively, of which both were within a humic acid-like region. In contrast, in Fig. 2(b), two distinct peaks of EOM were observed at Ex/Em of 280/335 nm (7.44 mV) in a soluble microbial product-like region, and at 345/ 435(389 mV) in a humic acid-like region, respectively. In this study, hydrophobic fractions in the EOM and IOM were 31% and 9%, respectively. The SUVA values were very low in the EOM and IOM extract solutions (0.72 and 0.91 L/mg m, respectively) as shown in Table 3. Considering that humic substances (a type of hydrophobic materials) represent the principal precursors during the formation of THMs (Christman et al., 1983; Stevens et al., 1976), and SUVA is an index frequently used to predict the THMFP, our findings suggested AOM may has a relatively low risk than NOM in the THMs formation during chlorination. However, SMP-like substances were largely present in both IOM and EOM as shown in the EEM images. This organic compound type is primarily responsible for formation of haloacetamides, a type of highly toxic N-DBPs (Chu et al., 2010; Plewa et al., 2008). Therefore, it is possible that chloramination of the AOM produced N-containing DBPs. As a result, the formation of various DBPs should be further studied.

3.5.

DBP yields of different AOM fractions

Chloroform (TCM) and chloroactic acids represented the levels of THMs and HAAs, respectively, because bromine was not

Table 2 e Characters of DOM fractions of EOM and IOM.

EOM

IOM

Fraction

DOC (mg/L)

C/N

SUVA L/mg m

THMFP (mg/mg C)

MCAAFP (mg/mg C)

DCAAFP (mg/mg C)

TCAAFP (mg/mg C)

THAAFP (mg/mg C)

HPO TPI NHPI CHPI HPO TPI NHPI CHPI

9.20 1.78 15.74 2.97 9.04 5.02 76.4 10.1

0.15 0.07 NA NA 0.28 0.17 0.23 0.21

0.51 2.13 0.45 1.87 0.98 0.86 0.83 1.93

25.30 82.35 17.52 48.39 19.53 64.09 14.19 27.67

2.81 2.12 3.51 2.65 3.40 2.50 3.61 3.11

37.24 34.12 43.21 40.87 36.21 32.50 46.89 52.10

11.42 23.35 15.20 17.80 7.80 10.24 18.90 19.24

51.47 59.59 61.92 61.32 47.41 45.24 69.40 74.44

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Fig. 2 e EEMs of (a) IOM and (b) EOM.

present in the solution. Results in TCM formation potential (TCMFP) is shown in Table 2. Four AOM fractions of EOM and IOM both followed the same order in terms of their specific TCMFP: TPI > CHPI > HPO > NHPI. This order is consistent with the data from Xu et al. (2007) that TCMFP order of the NOM fractions for Huangpu River water was CHPI > HPO > NHPI, while inconsistent with other report that specific DBPFP from HPO is slightly higher than that of HPI in normal surface water (Leenheer and Croue, 2003). Meanwhile, it was reported that no significant difference for the THM and THAA yields from the three hydrophobicity groups (HPO,TPI and HPI) of the low SUVA water (Hua and Reckhow, 2007). This discrepancy may be due to the different compositions of the water sources. For AOM, conjugated carbon double bond, a major branch of the precursor of THMs, was largely contained in phycocyanin (Ernst et al., 1992; Robertson et al., 1999), which is contained substantially in AOM and belongs to HPI/TPI fraction. For high SUVA water (e.g. most of the surface water), The THMs precursor was mainly exist in humic/fulvic acid (Krasner et al., 1996; Liang and Singer, 2003), which are a group of strong HPO compounds. Therefore, some previously found predictive relationships for NOM are unlikely to hold for systems that contain AOM (Henderson et al., 2008), DBPs seems depend on specific water properties and cannot be simply predicted merely by hydrophilicity or other single water index. The overall TCMFP of EOM and IOM were 32.44 and 21.46 mg/mg C, respectively (Table 3), which were considerably less than the typical TCMFP for NOM(w50e90 mg/mg C) (Fang et al., 2010a; Liang and Singer, 2003; Westerhoff and Mash, 2002; Xu et al., 2011). The results are aligned with our prediction based on the EEM results that the dominant protein-like fractions of IOM (e.g. amino acids) have a lower potential in the formation of chloroform than NOM, predominately characterized by complex humic and fulvic acids.

The different specific HAA formation potentials of the four AOM fractions are shown in Table 2. Dichloroacetic acid (DCAA) formation potentials of IOM and EOM were 45.76 and 40.56 mg/mg C, respectively, significantly greater than the trichloroacetic acid (TCAA) formation potentials (17.51 and 14.77 mg/mg C, respectively), which was in agreement with the report from Westerhoff and Mash, 2002 that high DON would increase the ratio of DCAAFP/TCAAFP. The DCAAFP values were close to the findings of Huang et al. (2009) that formation potentials of cells and EOM were 71 and 42 mg/mg C, respectively, Of note, result may vary with different water backgrounds and chlorination conditions (Fang et al., 2010b). The total chloroacetic acid formation potential of EOM was 68.29 mg/mg C, slightly higher than that of IOM (54.58 mg/mg C). There were no significant difference of HAAFP between HPI and HPO as the same reason discussed before (Hua and Reckhow, 2007). AOM exhibited relatively low DBPFP specific yields than NOM, but still risk the drinking water safety as it exceed the EPA and WHO standard (Xu et al., 2007). Furthermore, the celllysis resulted from chlorination should be paid great attention. Daly et al. (2007) reported that 4e5 mg/L chlorine could cause >99.9% M. aeruginosa cell lysis (algae concentration ¼ 1.05
109/L). Lin et al. (2009) found a similar effect of chlorine on algae integrity of Anabaena circinalis and M. aeruginosa. IOM such as algae toxins, O&T- caused compounds and other undesirable compounds would released accompanied with the cell lysis, thus increasing the DBP formation potential. In practice, chloramination is an alternative for chlorination since the former has low THM and HAA formation potentials as well as less destruction to the cell wall (Ding et al., 2010). However, one unintended consequence of chloramination is dichloroacetaldehyde and 1,1-dichloro-2propanone (Fang et al., 2010b) and some highly toxic DBPs

Table 3 e DBPs yields by EOM and IOM.

EOM IOM

DOC (mg/L)

SUVA (L/mg m)

THMFP (mg/mg C)

THAAFP (mg/mg C)

NDMAFP (ng/mg C)

29.7 100.5

0.72 0.91

32.44 21.46

54.58 68.29

18.91 9.65

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Table 4 e Yields of O & T compounds formed from EOM and IOM before and after anaerobic cultivation.a,b

EOM IOM Threshold level

Dimethyltrisulfide (ng/mg C)

2-MIB (ng/mg C)

b-cyclocitral (ng/mg C)

Geosmin (ng/mg C)

Ionone (ng/mg C)

7.12 (4.72) 47.87 (0.84) 10 ng/L

78.00 (68.75) 14.39 (6.75) 9 ng/L

0 (0) 113.11 (367.59) 500 ng/L

5.44 (1.07) 0.23 (0.30) 4 ng/L

6.19 (4.92) 1.18 (2.10) 7 ng/L

a Numbers in and out parentheses are the yields before and after anaerobic cultivation, respectively. b Bold font represents the concentrations above the threshold levels on the assumption of 3 mg/L DOC(typical level).

such as N-nitrosodimethylamine (NDMA) and dichloroacetamide. As shown in Table 3, the NDMA formation potentials (NDMAFP) of EOM and IOM were 18.91 and 9.65 ng/ mg C, respectively. This finding indicated that EOM and IOM could contribute to 56.73 and 28.95 ng/L NDMA, respectively, at 3 mg/L DOC (a typical level). The values were much greater than the current notification level of 10 ng/L and a public health goal (PHG) of 3 ng/L in California (Park et al., 2009). It would be noted that although IOM contained much more protein-like matters than EOM, the NDMAFP of IOM was only approximately half of that of EOM, indicating that NDMA precursors, originally existing more in EOM than IOM. The NDMAFP found in this study (41.95 ng/mg DON for IOM) was about one order of magnitude lower than that of NOM (331.9 ng/mg DON (Xu et al., 2011)), indicating DON probably could not serve as surrogate parameter for the NDMAFP prediction for different water categories. Overall, the findings implied that chlorination and chloramination are both not appropriate for disinfection of Microcystic aeruginosa e containing water.

3.6.

Odor and taste yields of AOM

As shown in Table 4, the yields of O & T compounds, including dimethyltrisulfide, 2-MIB, b-cyclocitral, geosmin, and ionone were strongly dependent of IOM or EOM. The dominant O&T compounds were 2-MIB (68.75 ng/mg C) from EOM, and b-cyclocitral (367.59 ng/mg C) from IOM, separately. Subsequent to a 7-day anaerobic cultivation, dimethyl trisulfide for IOM could be dramatically increased from 0.84 to 47.87 ng/ mg C, and became prevailing, probably due to bio-degradation of amino acids, methionine and cysteine in IOM (Yang et al., 2008). All the other O&T compounds were also increased following the anaerobic cultivation, except b-cyclocitral, geosmin, and ionone of IOM that dropped from 367.59 to 113.11 ng/mg C, from 0.30 to 0.23 ng/mg C, and from 2.10 to 1.18 ng/mg C, respectively. In particular, the yields of dimethyltrisulfide from IOM (47.87 ng/mg C) and 2-MIB from EOM (78.00 ng/mg C) were almost 15 and 27 times as their threshold levels, respectively, at 3 mg/L (a typical DOC level). These findings suggest that eutrohpication, which typically reduces and even depletes dissolved oxygen (DO) in natural water, can significantly increase the production of numerous O&T compounds via anaerobic degradation. For example, in the May of 2007, the municipal drinking water supply was shut a few days in the Wuxi city, China, (population > 4 million) as a result of eutrpohication occurring in the water source. As high as 11,39 ng/L dimethyltrisulfide was detected in raw

water (Yang et al., 2008), most likely caused by anaerobic decomposition of algae IOM according to our findings. Therefore, to maintain an adequately high DO in water appears to be a strategy to alleviate the formation of O&T compounds from AOM.

4.

Conclusion and implications

This study characterized the physicochemical properties of AOM extracted from M. aeruginosa (number 2.01
1011/L) under an exponential growth phase, and evaluated the formation of DBPs and O&T-causing compounds under different experimental conditions and disinfection methods. IOM and EOM were mainly composed of hydrophilic organic matters. Low SUVA (0.84 L/mg m) and specific fluorescence spectra suggested that AOM (especially IOM) was principally comprised of protein-like substances, instead of humic-like matters. DBPs study showed IOM accounted for 77% of total DOC, 78% of MC-LR, 70% of THMFP, 81% of HAAFP and 63% of NDMAFP. Therefore, to maintain the cell integrity and prevent the release of IOM is essential for algae-polluted water treatment. High formation potentials of THMs and HAAs during chlorination, and of N-DBPs (e.g. NDMA) during chloriamination imply that both of the disinfection technologies are not appropriate for algae-containing water treatment. Alternative (e.g. ozonation) should be considered for water safety. During field remediation of an eutrophicated water body, algae inactivation is not sustainable for water body remediation as it may deteriorate water quality as a result of the cell damage and subsequent release of IOM. In the current available technologies, algae harvest may be the most promising and sustainable way in which a large amount of nutrient can be extracted from the polluted water in the form of the algae biomass (i.e. IOM and cells wall). To prevent O&Ts crisis such as the event in Taihu Lake in China in 2007, avoid the anaerobic decomposition of the algae cells might be essential and critical. Although the algae used in this study was pure M. aeruginosa solution, our study may provide valuable information regarding biochemical and O&T relevant characteristics of other algae species. The biochemical characteristics likely to be shared include low SUVA, high protein/DON content and hydrophilicity (Henderson et al., 2008). Similarly, the O&Ts problem (e.g. dimethyltrisulfide) is not derived from specific components in M. aeruginosa, but typically from anaerobic decomposition of amino acids methionine and cysteine

w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 1 2 3 3 e1 2 4 0

protein metabolite, which are commonly found by almost all the algae species (Yang et al., 2008). Therefore, the O&T relevant issues revealed in this study may be found in other algal species. As to specific/detail AOM characterization, DBPFPs and O&Ts evaluation for Microcystic aeruginosa during other growth phase, or for other algae species, further study is warranted.

Acknowledgment This project was supported by the National Major Science and Technology project of China (No. 2008ZX07421-002 and 2008ZX07421-004), National High Technology Research and Development Program of China (863 Program) (No. 2008AA06A412), Research and Development Program of Housing and Urban-Rural Development Ministry (No. 2009K7-4), and China Scholarship Council.

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