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Continuous and Inexpensive Monitoring of Non-Purgeable Organic Carbon by Coupling High-Efficiency Photo-oxidation Vapor Generation to Miniaturized Point Discharge Optical Emission Spectrometry Shu Zhang, Yunfei Tian, Hongling Yin, Yubin Su, Li Wu, Xiandeng Hou, and Chengbin Zheng Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01064 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 14, 2017

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Environmental Science & Technology

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Continuous and Inexpensive Monitoring of Non-Purgeable

2

Organic Carbon by Coupling High-Efficiency

3

Photo-oxidation Vapor Generation to Miniaturized Point

4

Discharge Optical Emission Spectrometry

5

Shu Zhang1, Yunfei Tian3, Hongling Yin2, Yubin Su1, Li Wu3, Xiandeng Hou1,3, and

6

Chengbin Zheng1*

7 8 9 10 11

1

Key Laboratory of Green Chemistry & Technology of MOE, College of Chemistry,

Sichuan University, Chengdu, Sichuan 610064, China 2

College of Resources and Environment, Chengdu University of Information

Technology, Chengdu, Sichuan 610225, China 3

Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China

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*Corresponding author:

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Fax: +86 28 85412907; Phone: +86-28-85415810

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E–mail: [email protected] (C. B. Zheng)

1

ACS Paragon Plus Environment


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ABSTRACT

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Currently, no applicable analyzers are available to accomplish online and continuous

3

monitoring of organic pollution, which is one of the most important factors

4

contributing to water shortages around the world, particularly in developing countries.

5

In this work, a sensitive, miniaturized, inexpensive, on-line and continuous

6

non-purgeable organic carbon (NPOC) analysis system was developed for the

7

continuous monitoring of such organic pollution. This system consists of a specially

8

designed and high efficiency UV photo-oxidation vapor generation (HE-POVG)

9

reactor and a miniaturized, low power (7 W) point discharge microplasma optical

10

emission spectrometer (PD-OES). Organics present in sample or standard solutions

11

are pumped to the HE-POVG and efficiently converted into CO2, which is separated

12

and further transported to the PD-OES for NPOC analysis via the highly sensitive

13

detection of carbon atomic emission at 193.0 nm. Under optimal conditions, a limit of

14

detection (LOD) of 0.05 mg L-1 (as C) is obtained, with precision better than 5.0%

15

(relative standard deviation, RSD) at 5 mg L-1. This system overcomes many

16

shortcomings associated with conventional COD or TOC analyzers such as long

17

analysis time, use of expensive and toxic chemicals, production of secondary toxic

18

waste, requirement of large, power consuming and expensive instrumentation and

19

difficulties implementing continuous, online and continuous monitoring. The system

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was successfully applied to the sensitive and accurate determination of NPOC in

21

various water samples and for the continuous monitoring of such organic pollution in

22

tap water.

23

2


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INTRODUCTION

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Water scarcity is one of the toughest challenges facing the world today and about 600

3

million people are at risk of disease or death because of insufficient clean water.1,2

4

Besides population growth and climate change, organic pollution produced from

5

human activities exacerbates this scarcity.3-5

6

Chemical oxygen demand (COD), biochemical oxygen demand (BOD) and total

7

organic carbon (TOC) are the most used methodologies for assessing organic

8

pollution 6-8. The conventional COD and BOD methods usually involve tedious and

9

time-consuming reflux (2-3 hours) and incubation (5 days) processes, respectively.

10

Moreover, the COD methods require highly toxic (Cr6+ and Hg2+), expensive (Ag+) or

11

corrosive (H2SO4) reagents. Recently, many methods based on electrocatalytic,9,10

12

photocatalytic11-14 and photoelectrochemical15-18 oxidation principles have been

13

developed for COD analysis. These methods simplified analytical procedures and

14

minimized the consumption of toxic chemicals. It is noteworthy that both COD and

15

BOD are indirect methods and may be severely interfered by some oxidizable ions.

16

Compared to COD and BOD methods, TOC is the most suitable method for the

17

evaluation of organic pollution of water because it not only significantly reduces the

18

use of toxic chemicals but also directly express a measure of organic pollution.6

19

Almost all TOC measurements include oxidation of organics to CO2 by chemical

20

oxidation,19

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using noble metal catalysts (Pt or Pd), and detection of the generated CO2 with

22

nondispersive infrared (NDIR) absorption spectrometry,22,25 thermal conductivity,26

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electrolytic conductivity,27 titrimetry28 or ion chromatography29. Despite such

24

advantages of TOC, there still remain a number of shortcomings in both the oxidation

25

and the detection steps. Chemical and photo-oxidation usually lead to incomplete

26

oxidation of organics and underestimate the TOC value. Although HTC offers higher

27

oxidation efficiency than other methods, this technique is still handicapped by the use

28

of expensive catalysts and operation at high temperature. For CO2 detection,

29

gravimetry and titrimetry are not sensitive and electrolytic conductivity is frequently

UV photo-oxidation20,21 or high-temperature combustion (HTC)22-24

3



1

precluded by the ionic compounds produced from the degradation of the organics

2

(particular for nitrogen and sulfur containing compounds). Notably, almost all of the

3

TOC techniques require relatively large and expensive laboratory instrumentation

4

which makes them unaffordable in developing countries, whereas water crises caused

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by organic pollution typically occurs in these countries because of their rapid

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economic development. Moreover, there are currently no applicable analyzers for the

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online and continuous monitoring of TOC. Therefore, there is an urgent need to

8

develop an inexpensive and continuous TOC monitoring system. Recently, we have

9

coupled microwave assisted oxidation vapor generation (MOVG) to dielectric barrier

10

discharge (DBD) optical emission spectrometry (OES) for TOC analysis.30 MOVG

11

can significantly improve the oxidation efficiency of organics and reduce the analysis

12

time. However, conventional microwave oven heating is power consuming, expensive

13

and automatically turned off when the oven is overheated, preventing its use for

14

continuous monitoring. Meanwhile, careful operation and complex heating devices

15

are also required to obtain satisfactory analytical performance because of the low

16

excitation capacity of the DBD. Since purgeable organic carbon (POC) is also

17

removed during the acidification and purging procedure used to eliminate the

18

interference of total inorganic carbon (TIC)

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methods is non-purgeable organic carbon (NPOC).

20

In this work, an inexpensive and online system coupling a high-efficiency UV

21

photo-oxidation vapor generation (HE-POVG) reactor to a miniaturized point

22

discharge (PD) optical emission spectrometer was developed for the determination

23

and continuous monitoring of NPOC. The high-efficiency POVG was realized using a

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special photo-oxidation reactor with high UV transmittance from a low-pressure

25

mercury lamp wherein conversion of organics to CO2 was more efficient than

26

conventional photo-oxidation reactors.31 Moreover, the PD possesses higher

27

excitation capability than a DBD32 and thus offers higher sensitivity for the NPOC

28

detection. To the best of our knowledge, this is the first report of use of HE-POVG

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coupled to a PD-OES for NPOC analysis.

6,21

, the TOC determined by most of

4


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EXPERIMENTAL SECTION

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Instrumentation. The HE-POVG-PD-OES system is presented in Figure 1 and

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consists mainly of a flow injection (FI) or continuous flow HE-POVG reactor and a

4

point discharge optical emission spectrometer equipped with a commercial hand-held

5

charge coupled device (CCD) spectrometer (Maya 2000 Pro, Ocean Optics Inc.,

6

Dunedin, FL) with 200 - 400 nm of spectral range and 0.4 nm of spectral resolution.

7

The FI-HE-POVG reactor consisted of two channel peristaltic pumps (BT100-02,

8

Baoding Qili Precision Pump Company, Ltd., Baoding, China), a six port injection

9

valve (Genuine Rheodyne Co., USA) equipped with a 0.5 mL of sampling loop, a

10

high-efficiency photo-oxidation reactor (Beijing Titan Instruments Co., Beijing,

11

China), a quartz gas liquid separator (GLS) and a quartz dryer filled with CaCl2. The

12

photo-oxidation reactor was similar to the one reported in previous work,31 which

13

included a quartz reaction tube (93 cm length × 1.0 mm i.d. × 2.0 mm o.d.) and a 19

14

W low pressure mercury vapor UV lamp. The reaction tube was folded three times to

15

increase the residence time of sample solution in UV irradiation zone within the body

16

of the mercury lamp. The reactor was enclosed in aluminum foil to protect the

17

operator from exposure to UV irradiation and reflect the UV back to the reaction

18

solution, further improving the oxidation efficiency. It should be emphasized that the

19

photo-oxidation reactor is quite different from that previously reported19,21 because its

20

reaction tube does not surround the exterior of the UV lamp but is inserted and sealed

21

within the lamp discharge, which significantly improves the transmission of vacuum

22

UV light and allows highly efficient and uniform UV irradiation of the sample

23

solution from all directions. This reactor generates both 254 nm and 185 nm of UV

24

light, significantly improving the generation of strong oxidants (i.e., hydroxyl radical)

25

for increasing the oxidation efficiencies of organics. To demonstrate the superior

26

performance of the HE-POVG reactor, a conventional photo-oxidation reactor

27

consisting of a coiled quartz tube (90 cm length × 2.0 mm i.d. × 3.0 mm o.d.) wrapped

28

around a 20 W low pressure mercury vapor UV lamp (Philips, Holland) was also used

29

for NPOC analysis. The GLS was connected to the HE-POVG reactor and the 5



1

PD-OES via silica tubing. The setup of the PD-OES is the same as that reported in the

2

previous work32, which is briefly described together with the setup of the DBD-OES

3

in Section 1 and 2 of the Supporting Information (SI). A picture of FI-HE-POVG-OES

4

system is presented in Figure S1 (See section 3 of the SI). The continuous HE-POVG

5

eliminated the six port injection valve but added the quartz GLS for online removal of

6

the generated CO2 from any inorganic carbon before the photo-oxidation of organics.

7

The full instrumental setup for continuous HE-POVG-PD-OES is described in Figure

8

S2.

9

The accuracy and practicality of this approach was validated via comparison of the

10

analytical results obtained using the proposed system and a HTC-NDIR (Analytikjena

11

multi N/C 2100 analyzer, Germany). The typical operation parameters for the

12

HTC-NDIR analysis are: furnace temperature, 680 oC; sample volume, 200 µL;

13

oxygen purge time, 180 s; and oxygen gas flow rate, 200 mL min-1.

14

Reagents. All chemicals were of at least analytical-reagent grade. 18 MΩ cm

15

deionized water (DIW) produced from a water purification system (Chengdu

16

Ultrapure Technology Company, Ltd., Chengdu, Sichuan Province, China.) was

17

boiled to remove the residual CO2 prior to its use for preparation of all solutions. A

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10000 mg L-1 of NPOC (as C) standard stock solutions were prepared by dissolving

19

analytical-reagent grade potassium hydrogen phthalate (KHP, C8H5KO4, Kelong

20

Reagent Company, Chengdu, China) and stored in refrigerator at 4 oC before analysis.

21

Standard solutions containing various concentrations of NPOC were prepared daily by

22

diluting the stock solution with DIW. Argon (99.99%) obtained from Qiaoyuan Gas

23

Company (Chengdu, Sichuan Province, China) was simultaneously employed as a

24

carrier and discharge gas.

25

Sample Collection. Water samples were collected from artificial lakes located in 9

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municipal parks and rivers of Chengdu city. The sampling map and sites of the lakes

27

are shown in Figure S3. All the samples were collected in 250 mL glass bottles and

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stored in an ice box. These samples were analyzed with the proposed system and the

29

HTC-NDIR instrument as soon as possible after being transported to the laboratory.

30

Detailed information about sample preparation is presented in Section 6 of the SI. 6


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Sample Analysis by FI-HE-POVG-PD-OES. The procedure for flow injection

2

NPOC analysis is described as follows. The standard or sample solution was acidified

3

using 10% phosphoric acid and purged for 3 min with 200 mL min-1 of Ar carrier gas

4

prior to NPOC analysis. The pretreated solution was initially pumped to a 0.5 mL

5

sample loop through a six-port valve. Then the valve was activated to pass DIW

6

carrier solution so as to flush the solution together with oxidizing solution (30% (m/v)

7

of Na2S2O8) through the photo-oxidation reactor for UV irradiation. Organics were

8

efficiently converted to CO2 in this process. In our previous work, the reaction

9

solution had to pass through a complex ice bath to ensure no condensed liquid

10

droplets were transported to the DBD-OES, whereas it was directly swept into the

11

GLS for the separation of CO2 in this work, simplifying the instrumental setup. The

12

CO2 passed through the dryer before entering the PD-OES for the detection of carbon

13

atomic emission

14

Online and Continuous Monitoring of NPOC. The main purpose of this work is to

15

develop a continuous monitoring system for the real time monitoring of NPOC in

16

water system. In this case, the sampling tube was directly inserted into the pumped

17

water flow and then the water sample was pumped to mix with 10% of phosphoric

18

acid in a coil reaction tube (40 cm length × 2.0 mm i.d. × 3.0 mm o.d.) for the

19

efficient reaction. The reaction mixture was swept into the first GLS wherein an Ar

20

carrier gas was introduced to separate the generated CO2. It is worthwhile to note that

21

a magnetic stir bar was put in the first GLS to stir the mixture for the completely

22

removal of the CO2 from the TIC in the water sample. The mixture was further

23

pumped into the photo-oxidation reactor together with the Na2S2O8 solution to

24

produce CO2. Then CO2 was separated by the second GLS and finally detected by

25

PD-OES.

26

RESULTS AND DISCUSSION

27

Preliminary Studies and Practicality of UV-POVG-PD-OES. The simple and

28

inexpensive determination of TOC can be obtained by coupling MOVG to

29

miniaturized DBD-OES.30 However, the MOVG-DBD-OES cannot be easily used for 7



1

continuous monitoring of NPOC because of the drawbacks of a conventional

2

microwave heating device. Despite its low oxidation capability, a conventional UV

3

photo-oxidation reactor is frequently used for the online and continuous degradation

4

of organics.20,33,34 As a consequence, an initial experiment was undertaken to use the

5

conventional photo-oxidation reactor as an alternative to the MOVG to realize the

6

continuous NPOC analysis with DBD-OES. A 0.5 mL aliquot of 50 mg L-1 (as C)

7

KHP solution was analyzed; results are shown in Figure 2a. Unexpectedly, the

8

specific carbon emissions at either 193.0 or 247.8 nm were not detected by the

9

DBD-OES system. It is possible that both the low oxidation efficiency of the

10

conventional photo-oxidation reactor and the weak excitation capability of the DBD

11

microplasma caused this result. Since the PD microplasma has higher excitation

12

energy compared to the DBD,32 it replaced the DBD source and was coupled with the

13

conventional photo-oxidation reactor for NPOC analysis. Interestingly, the specific

14

carbon atomic emission (Figure 2a) was obviously observed with this setup, although

15

the sensitivity is still much lower than that reported in previous work using the

16

MOVG-DBD-OES. Sensitivity should be further improved if photo-oxidation

17

efficiencies of organics are enhanced by using the high-efficiency photo-oxidation

18

reactor. Tao et al.31 reported a home-made photo-oxidation reactor with remarkably

19

high oxidation capability. This reactor could completely decompose persistent organo-

20

arsenic compounds (e.g. arsenobetaine) to As (V) in 42 s, even in the absence of

21

added oxidant, whereas over 6 h was needed for this complete decomposition with a

22

conventional photo-oxidation reactor. Thus, a similar HE-POVG reactor was used to

23

couple with the PD-OES for NPOC analysis. As can be seen from Figure 2a and b, the

24

intensities of the carbon atomic emission spectra are enhanced about 18-fold

25

compared to those obtained using the conventional photo-oxidation reactor. According

26

to previous work31, this remarkable improvement arises primarily due to the vacuum

27

ultraviolet light at 185 nm and its high transmittance to the sample in this system.

28

Actually, the photo-oxidation efficiency is increased by increasing the reaction

29

temperature.35 To gain further insight into the HE-POVG reactor, a handheld infrared

30

thermometer was used to measure the temperatures of the reaction tubes of the 8


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photo-oxidation reactors. The temperature of the HE-POVG reactor (50 ± 5 °C) was

2

much higher than that of conventional photo-oxidation reactor (30 ± 2 °C), suggesting

3

that the significant improvement of oxidation efficiency arises not only because of the

4

vacuum ultraviolet irradiation, but also due to the high reaction temperature.

5

It is noteworthy that although both carbon emission lines at 193.0 and 247.8 nm can

6

be observed with this system, emission at 193.0 nm was finally chosen for the NPOC

7

analysis because many strong emission bands in the spectral range 230−270 nm

8

(those belonged to the NO molecule emission bands) were detected,32,36 which lead to

9

serious spectral interference for the NPOC analysis. The linear coefficient obtained by

10

the proposed system after analysis of a series of standard solutions containing various

11

concentrations of KHP was better than 0.99 (Figure 2c), further supporting the

12

feasibility and practicality of the HE-POVG-PD-OES for NPOC analysis.

13

Optimization of Experimental Parameters and Investigation of Oxidation

14

Efficiencies. In order to improve the analytical performance of the proposed system,

15

both the operation conditions of the PD-OES (discharge voltage, discharge gap and

16

flow rate of carrier gas) and UV photo-oxidation system (flow rate of carrier solution

17

and concentration of Na2S2O8) were carefully optimized using 10.0 mg L-1 (as C)

18

NPOC standard solutions. Results are summarized in Table S1 (See sections 7 of the

19

SI).

20

Since the organics contained in real water samples are quite different, the oxidation

21

efficiencies of various organic compounds may severely affect the accuracy of the

22

method. Therefore, three approaches were utilized to investigate the effect of the

23

kinds of the organics in water on the oxidation efficiency of NPOC. Firstly, the

24

oxidation efficiencies of 20 organic compounds (all contain 50 mg L-1 of NPOC (as

25

C)) were investigated using the HE-POVG-PD-OES, as shown in Figure 3a. The

26

results show that the relative oxidation efficiencies compared to that of KHP (set to

27

100%) range from 88 to 110%, indicating that any difference between the oxidation

28

efficiencies of the tested organics is not evident. Secondly, a series of solutions

29

containing various concentrations (as C) of different organics were analyzed by the

30

proposed system. The results summarized in Figure 3b show a calibration curve with 9



1

a linear coefficient of 0.995, showing the proposed system is capable of oxidizing

2

various organic compounds uniformly. Actually, some organics in real water are more

3

complex than these 20 organics, for example humic acids. Finally, the oxidation

4

efficiencies of humic acid or NPOC in real river water were studied. Since the exact

5

concentrations of humic acid and NPOC value are unknown, the oxidation efficiency

6

(En) is thus defined as:

7

E

n

% 

I1

I n  100  I2  I3  I4  I5

8

Where In is the intensity (a.u.) of carbon atomic emission obtained from the n-th

9

photo-oxidation (n=1, 2, 3, 4 or 5). As shown in Figure S6 (See Section 8 of SI), the

10

oxidation efficiencies of humic acid and NPOC in river water from the first oxidation

11

are higher than 85%. Meanwhile, the oxidation efficiencies of 20 mg L-1 KHP were

12

also evaluated by this method and the results summarized in Figure S6 show that its

13

first oxidation efficiency is 96%. These results indicate that the oxidation efficiencies

14

for most of the dissolved organic matter even for humic acid are higher than 86%

15

compared to that of KHP, implying that the HE-POVG-PD-OES can be employed for

16

accurate NPOC analysis, regardless of the chemical forms of organic pollutants

17

present in water samples.

18

Effect of Acidified and Purged Procedures. In order to eliminate the interference

19

from inorganic carbon, acidification and purging steps are required prior to NPOC

20

analysis. A river water and a tap water were used to study the effect of inorganic

21

carbon on NPOC analysis via analyzing these samples with or without such steps. The

22

results are summarized in Figure 4 and show that the inorganic carbon present in the

23

sample provides a serious interference on NPOC analysis. Fortunately, a simple

24

pretreatment, including an acidification of sample using 10% (v/v) of phosphoric acid

25

and a 3 min purge with 200 mL min-1 of Ar is able to efficiently eliminate this

26

interference. These samples were also analyzed without acidification but with 3 min

27

of purge, as shown in Figure 4. The results show that there is no obvious difference

28

between the responses obtained with or without the purge, implying that the purgeable

29

organic carbon is negligible in many surface and ground waters. These results agree 10


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well with those reported in previous studies22 and that the NPOC obtained by the

2

proposed method can be substituted for TOC in these sample analyses.

3

Interferences. It is well-known that interferences arising from co-existing anions,

4

particularly from chloride ion, present a serious problem with conventional COD

5

methods. For example, Dharmadhikari et al.8 reported that chloride ions seriously

6

interfere with the COD measurement using Cr(VI) as oxidizing reagent at

7

concentrations higher than 2 g L-1, making these methods unsuitable for the COD

8

analysis of the sample containing high concentration of chloride ions, such as sea

9

water. Our work30 showed that the MOVG-DBD-OES could be directly utilized for

10

the TOC analysis of seawater because it possessed much higher resistance to

11

interference by Cl-, even at concentrations as high as 10 g L-1. However, UV

12

persulfate oxidation of organics is different from that using microwave assisted

13

oxidation. Therefore, the effects of some common anions (NO3-, PO43-, SO42-, and Cl-)

14

and cations (Na+, K+, Fe3+, Zn2+, Mn2+, Mg2+, Cu2+, Ag+, Fe2+, and Ca2+) on the NPOC

15

analysis by the proposed method were carefully investigated. As summarized in Table

16

S2, no significant interferences from the tested cations are evident, even at

17

concomitant concentrations as high as 500 mg L-1. Moreover, the results also

18

demonstrate that the interferences from 100.0 g L-1 of PO43- and SO42- are not present.

19

However, 100.0 g L-1 of NO3- and Cl- induced serious interference on the NPOC

20

analysis and the recoveries decreased to 73% and 65%, respectively. To further

21

explore the effects of NO3- and Cl-, different concentrations of NO3- and Cl- were

22

added to standard solutions containing 10.0 mg L-1 KHP (as C) and analyzed by the

23

proposed methods. As can be seen in Figure S7, recoveries were enhanced above 80%

24

when the concentration of NO3- and Cl- were lower than 10 g L-1, which is sufficient

25

for the routine analysis of NPOC in typical surface water samples.

26

Analytical Performance. Under optimal condition, a series of NPOC standard

27

solutions in the range of 0.5-5000 mg L-1 were analyzed by the HE-POVG-PD-OES.

28

Results are summarized in Figure 5a，which show that linear range can be extended to

29

200 mg L-1 and is limited by saturation of the CCD spectrometer. Thus, the calibration

30

curve was finally constructed by analyzing a series of standard solutions with the 11



1

NPOC ranging from 0.5-200 mg L-1 (based on 0.5 mL sample volume), as shown in

2

Figure 5b. The linear correlation coefficient (R2) for the constructed calibration curve

3

is better than 0.999. The limits of detection (LOD), defined as the analyte

4

concentration equivalent to three times the standard deviation of 11 measurements of

5

a blank solution (UP water) divided by the slope of the calibration curve, was 0.05 mg

6

L-1. Figure S8 illustrates steady-state signals obtained for the 11 replicate

7

measurements of a 5 mg L-1 NPOC standard solution using the proposed system,

8

yielding a precision of 5.0% relative standard deviation (RSD). Table S3 summarizes

9

the analytical performance of this method and compares it with some other methods.

10

The results indicate that the analytical performance of the proposed system is

11

comparable to or better than that obtained by other techniques. It is noteworthy that

12

although the sample consumption of the proposed system is reduced some 20-fold

13

compared to the MOVG-DBD-OES (10 mL of sample volume), the obtained LOD

14

remains comparable to that achieved by MOVG-DBD-OES, and the LOD is improved

15

20-fold when the HE-POVG reactor was used as an alternative to the conventional

16

photo-oxidation reactor. These may be a consequence of the high photo-oxidation

17

vapor generation efficiency of the proposed reactor coupled with the high excitation

18

capability of the PD microplasma. Moreover, this system can further provide several

19

unique advantages for online, continuous monitoring as it comprises an inexpensive

20

and miniature setup, requires low power consumption, reduces use of toxic chemicals

21

and achieves a high sample throughput (1.5 min per sample).

22

Sample Analysis with FI-HE-POVG-PD-OES. The utility and accuracy of the

23

FI-HE-POVG-PD-OES system were validated by the determination of NPOC in real

24

water samples. The lake water samples were simultaneously analyzed by the proposed

25

system and by use of a commercial HTC-NDIR. Results are summarized in Table 1.

26

The t test shows that there are no significant differences between the analytical results

27

obtained by the proposed method and the HTC-NDIR at the 95% level of confidence.

28

Secondly, the river water samples were also used to validate the accuracy of the

29

proposed system via evaluation of the recoveries of the spiked NPOC. As can be seen

30

from the data in Table 2, recoveries calculated from spiking standards into the real 12


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samples were in range of 91−111%, further confirming the accuracy and practicality

2

of the proposed system for NPOC analysis.

3

Online, Continuous Monitoring of NPOC in Water System. The aim of this work

4

was to develop a simple, inexpensive, online and continuous system for the

5

continuous monitoring of NPOC. Therefore, the feasibility of online inorganic carbon

6

removal was firstly investigated by using a river water sample. The sample was

7

separately analyzed both without any pretreatment and after an offline or online

8

acidification using 10% (v/v) phosphoric acid and Ar purge. As can be seen from

9

Figure 6a, the response obtained with online acidification and purge is almost the

10

same as that obtained with the offline mode, indicating that inorganic carbon can be

11

efficiently removed with the online acidification and purge and its interference can be

12

online eliminated. Moreover, TIC can also be monitored online with this system via

13

the detection of the CO2 generated from the acidification and purging of the sample,

14

as shown in Figure 6a. The potential of online and continuous monitoring of NPOC

15

by the proposed system was further evaluated by continuously monitoring the NPOC

16

variation of a water sample flow, as shown in Figure 6b. In order to increase the

17

NPOC value of the sample, KHP solution was gradually added to the sample flow.

18

The results show that the signals quickly responded when the NPOC of sample flow

19

changed. A calibration curve with linear coefficient of 0.999 could be obtained based

20

on responses accorded by peak height, as shown in Figure S9. Moreover, the response

21

rapidly dropped to the blank value as 18 MΩ cm DIW was introduced, showing no

22

memory effect occurred in this system. These suggest that our developed system can

23

accomplish online and continuous monitoring of NPOC.

24

The practicality of long-term monitoring of real water was evaluated by continuous

25

monitoring the NPOC of tap water (Figure S10). Figure 6c shows the results obtained

26

after uninterrupted monitoring for more than 12 hours. After monitoring for 5.5 hours,

27

a sharp response change is detected. This is because a solution containing a high

28

concentration of glucose was gradually introduced into the flow of tap water, showing

29

the rapid response of this system. These results strongly confirm the practicality of the

30

HE-POVG-PD-OES for continuous and online monitoring of NPOC in real water. 13



1

In summary, a promising NPOC monitoring analyzer was developed for NPOC

2

analysis by coupling a specially designed photo-oxidation vapor generator and

3

miniaturized PD microplasma OES. Bestowed to the advantages of high oxidation

4

efficiencies of organics and continuous operation provided by the generator and high

5

excitation capability, inexpensive and simple setup, low power consumption offered

6

by the PD-OES, this system can be used to accomplish online, continuously and

7

inexpensive monitoring NPOC of real water. This will be very helpful to developing

8

countries, allowing the convenient determination and online control of water qualit.

9 10

ACKNOWLEDGEMENTS

11

We gratefully acknowledge the National Nature Science Foundation of China (Grant

12

Nos. 21575092 and 21622508) for financial support.

13

ASSOCIATED CONTENT

14

Supporting Information. Additional information as noted in text. This material is

15

available free of charge via the Internet at http://pubs.acs.org.

16

14


Page 14 of 28

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18


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1

Figure Captions

2

Figure 1. Schematic diagram of the FI-HE-POVG-PD-OES. GLS: gas liquid

3

separator.

4

Figure 2. (a) Optical emission spectrum obtained under different conditions; (b)

5

Comparison of the responses of carbon atomic emission at 193.0 nm using

6

HE-POVG-PD-OES and conventional POVG-PD-OES; and (c) Calibration curves

7

established using FI-HE-POVG-PD-OES. Experimental conditions: discharge voltage,

8

2.55 kV; discharge gap, 3 mm; Ar flow rate, 300 mL min-1; carrier solution flow rate,

9

3 mL min-1; the concentration of Na2S2O8 solution, 300 g L-1, and CCD integration

10

time, 100 ms.

11

Figure 3. (a) Oxidation efficiencies of different organic compounds relative to KHP

12

NPOC standard; and (b) Calibration curves established by analyzing sample

13

containing different organics. Experimental conditions: discharge voltage, 2.55 kV;

14

discharge gap, 3 mm; Ar flow rate, 300 mL min-1; carrier solution flow rate, 3 mL

15

min-1; and the concentration of Na2S2O8 solution, 300 g L-1.

16

Figure 4. Effect of acidification and purging on NPOC analysis.

17

Figure 5. (a) Plots of carbon atomic emission intensity as a function of NPOC

18

concentration from 0.5 to 5000 mg L−1, showing good linearity in the range 0.5 to 200

19

mg L−1; and (b) Typical calibration curve using FI-HE-POVG-PD-OES. Experimental

20

conditions: discharge voltage, 2.55 kV; discharge gap, 3 mm; Ar flow rate, 300 mL

21

min-1; carrier solution flow rate, 3 mL min-1; and the concentration of Na2S2O8

22

solution, 300 g L-1.

23

Figure 6. (a) Comparison of the removal of inorganic carbon using online and offline

24

acidification and purging; (b) Feasibility of analysis of NPOC by continuous

25

HE-POVG-PD-OES; and (c) Continuous monitoring of NPOC in tap water using the

26

proposed system. Experimental conditions: discharge voltage, 2.55 kV; discharge gap,

27

3 mm; Ar flow rate, 300 mL min-1; carrier solution flow rate, 3 mL min-1; the

28

concentration of Na2S2O8 solution, 300 g L-1; and the concentration of phosphoric

29

acid, 10% (v/v).

30 19



1

2 3

Figure 1.

4

20


Page 20 of 28

Page 21 of 28


1

2 3 4

Figure 2.

5

21



1

2 3 4

Figure 3.

5

22


Page 22 of 28

Page 23 of 28


1 2 3

Figure 4.

4

23



1

2 3

Figure 5.

24


Page 24 of 28

Page 25 of 28


1 2

Figure 6.

3

25



Page 26 of 28

1

Table 1. Sample Detection Sample

Sampling location

UV-PVG-PD-OES -1 a

HTC-NDIR

(mg L )

(mg L-1)

1

Southwest Jiaotong University

7.03 ± 0.50

7.12 ± 0.06

2

Huanhuaxi Park-1

4.81 ± 0.22

5.22 ± 0.28

3

Huanhuaxi Park-2

1.92 ± 0.15

2.03 ± 0.21

4

Chenghua Park

1.97 ± 0.08

2.25 ± 0.20

5

Xinhua Park

5.24 ± 0.59

5.52 ± 0.43

6

Renmin Park

5.26 ± 0.25

5.41 ± 0.08

7

Wangjiang Park

1.37 ± 0.16

1.52 ± 0.03

8

Donghu Park

5.51 ± 0.48

5.81 ± 0.09

9

Tazishan Park

7.98 ± 0.20

8.15 ± 0.16

10

Mingyuan Lake

5.32 ± 0.34

5.56 ± 0.18

a

Average ± standard deviation of three trials.

2 3 4

26


Page 27 of 28


1

Table 2. Sample Detection Samples

Added, mg L-1

Found, mg L-1a

Recovery, %

Drinking water

0.00

0.83 ± 0.13

-

2.00

2.88 ± 0.07

103

5.00

6.31 ± 0.37

110

0.00

6.37 ± 0.72

-

5.00

11.93 ± 0.77

111

0.00

4.53 ± 0.06

-

5.00

9.10 ± 0.53

91

0.00

5.30 ± 0.26

-

5.00

10.03 ± 0.95

95

0.00

12.40 ± 0.36

-

5.00

17.70 ± 0.46

106

10.00

22.30 ± 0.95

99

Fu river

Nan river

Funan river-1

Funan river-2

a

Average ± standard deviation of three trials.

2 3

27



For TOC only

1

2 3

28


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