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LRA, Vol. 12, pp. 157–163 157 Photochemical Oxidation and Flow Injection Conductivity Determination of Dissolved Organic Carbon in Estuarine and Coastal Waters Kanayathu Koshy and Melchior Mataki School of Pure and Applied Sciences, University of the South Pacific, Suva, Fiji Received 10 October 1999 ABSTRACT: A modified version of the flow injection method for the determination of dissolved organic carbon (DOC) developed by Koshy et al. is described. This method is capable of DOC measurements in the presence of high levels of chloride and has a method detection limit of 0.8 mg C/L, a linear response range of 0–35 mg/L, carbon recovery of 98 9%, and a sample throughput of 60 h 1 . 2000 John Wiley & Sons, Inc. Lab Robotics and Automation 12:157–163, 2000 INTRODUCTION An innovative wet chemical oxidation (WCO) method involving flow injection analysis (FIA) for the determination of dissolved organic carbon (DOC) was described by Koshy et al. [1]. In this method, the DOC was oxidized by an alkaline solution of perox- oydisulfate (persulfate) to CO 2 with spectrophoto- Correspondence to: K. Koshy, School of Pure and Applied Sci- ences, University of South Pacific, P.O. Box 1168, Suva, Fiji. Contract Grant Sponsor: University of the South Pacific Re- search Committee. Contract Grant Number: 6845-1321-70766-00. 2000 John Wiley & Sons, Inc. metric detection. Although this is an excellent method for DOC determination in fresh water, it suf- fers from chloride interference in estuarine and ma- rine waters. In this article, we present details of the nature of chloride interference and different approaches that may be adopted to minimize its effects. We give de- tails about the use of inorganic carbon (IC) masking as the easiest option to measure DOC levels in coastal and estuarine samples. EXPERIMENTAL Chemicals Analytical grade chemicals, UNIVAR, and Ajax Chemicals were used throughout this work. Distilled water was deionized before use. Reagents The photo-oxidation reagent was prepared by dis- solving accurately measured amounts of 17 g sodium tetrahydroborate and 20 g of potassium persulfate in 500 mL of distilled deionized water (DDW). The so- lution will henceforth be referred to as the 100% ox- idant strength solution.

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Page 1: Photochemical oxidation and flow injection conductivity determination of dissolved organic carbon in estuarine and coastal waters

LRA, Vol. 12, pp. 157–163

157

Photochemical Oxidationand Flow InjectionConductivity Determinationof Dissolved Organic Carbonin Estuarine and CoastalWaters

Kanayathu Koshy and Melchior Mataki

School of Pure and Applied Sciences, University of the South Pacific, Suva, Fiji

Received 10 October 1999

ABSTRACT: A modified version of the flow injectionmethod for the determination of dissolved organiccarbon (DOC) developed by Koshy et al. is described.This method is capable of DOC measurements in thepresence of high levels of chloride and has a methoddetection limit of 0.8 mg C/L, a linear response rangeof 0–35 mg/L, carbon recovery of 98 � 9%, and asample throughput of 60 h�1. � 2000 John Wiley &Sons, Inc. Lab Robotics and Automation 12:157–163,2000

INTRODUCTIONAn innovative wet chemical oxidation (WCO)method involving flow injection analysis (FIA) forthe determination of dissolved organic carbon (DOC)was described by Koshy et al. [1]. In this method, theDOC was oxidized by an alkaline solution of perox-oydisulfate (persulfate) to CO2 with spectrophoto-

Correspondence to: K. Koshy, School of Pure and Applied Sci-ences, University of South Pacific, P.O. Box 1168, Suva, Fiji.

Contract Grant Sponsor: University of the South Pacific Re-search Committee.

Contract Grant Number: 6845-1321-70766-00.� 2000 John Wiley & Sons, Inc.

metric detection. Although this is an excellentmethod for DOC determination in fresh water, it suf-fers from chloride interference in estuarine and ma-rine waters.

In this article, we present details of the nature ofchloride interference and different approaches thatmay be adopted to minimize its effects. We give de-tails about the use of inorganic carbon (IC) maskingas the easiest option to measure DOC levels in coastaland estuarine samples.

EXPERIMENTALChemicalsAnalytical grade chemicals, UNIVAR, and AjaxChemicals were used throughout this work. Distilledwater was deionized before use.

ReagentsThe photo-oxidation reagent was prepared by dis-solving accurately measured amounts of 17 g sodiumtetrahydroborate and 20 g of potassium persulfate in500 mL of distilled deionized water (DDW). The so-lution will henceforth be referred to as the 100% ox-idant strength solution.

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158 Koshy and Mataki

Figure 1. The FIA manifold showing the major components.

A substitute for synthetic seawater (SSW) wasprepared by dissolving 35 g of sodium chloride in 1L of DDW (35‰ NaCl solution) [2].

Instrumentation

The flow injection manifold used in this work isshown schematically in Figure 1. Two Ismatec 5Apumps were used for the carrier, sample and reagentpumping. A 600 lL sample was injected with a Rheo-dyne six-way valve. 0.3 mm i.d., Poly(tetrafluoro-ethylene) (PTFE) tubing, was used throughout themanifold.

The UV photo-oxidation system consisted of a 2m PTFE tubing wound in the form of figure eightaround a UV lamp regulated at 360 nm. Because ofthe alkaline conditions in the UV chamber, the car-bon dioxide produced was present as carbonate.

The acid mixing coil was placed in water bathmaintained at 58�C to increase the rate of hydrolysisof the carbonate produced in the oxidation chamber,decrease the solubility of the carbon dioxide in thecarrier stream, and enhance gas diffusion.

Gas diffusion was achieved using a Tecatorchemifold V with a TBA permatite GT gas plumbingtape (i.d. 0.2 mm).

The conductivity of the acceptor stream wasmeasured by a custom-built conductivity detector.

The detector consisted of two square perspex blocksseparated by a Teflon plate that had a passage to al-low the flow of the acceptor stream. Each of theblocks had two circular platinum electrodes that de-tected the change in conductivity as the acceptor so-lution passed between them. Using a 600 lL loop, asingle run took about 45 seconds.

Sample Collection and AnalysesThe coastal water samples were collected in acid-washed poly(vinyl chloride) (PVC) bottles and fil-tered using 0.45 lm filters.

The analysis for DOC had two parts to it. First,the IC was determined under nonoxidizing condi-tions. Second, the water samples were reanalyzedwith the oxidant in place. The resulting carbon di-oxide was a measure of the total carbon (TC) con-sisting of IC naturally present and the IC generatedfrom the oxidation of DOC. After correcting for chlo-ride interference using the equation in Figure 2, thelevel of DOC was calculated by the difference.

RESULTS AND DISCUSSIONSQuantification of Chloride InterferenceThe interference by chloride (%Clox) was establishedby measuring the amount of chloride oxidized (IC

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Photochemical Oxidation and Flow Injection Conductivity Determination 159

TABLE 1. The Interference by Chloride on the Measurement of Carbon (IC) Standards

[C] mg/LDDW Peak area (�1SD)

n � 10 (Peak area � 107)35‰ NaCl solution Peak

area (�1SD) n � 10 %Clox (�2SD) [Cl]/[IC] � 103

0 1.30(0.01)1 0.38(0.03) 1.02(0.04) 168(16) 211.2 0.59(0.02) 1.21(0.01) 105(5) 181.8 0.95(0.03) 1.64(0.02) 73(6) 122 0.96(0.02) 1.36(0.02) 42(3) 103 1.27(0.04) 1.70(0.08) 34(7) 74 1.78(0.07) 2.38(0.05) 34(7) 58 3.46(0.02) 3.63(0.06) 5(2) 3

16 6.32(0.09) 6.60(0.03) 4(1) 124 8.84(0.09) 8.94(0.01) 1(1) 1

Note: %Clox � % difference in carbon signal caused by chloride.� [(35‰ NaCl solution � DDW)] � (DDW/100).� A measure of chloride interference.[Cl]/[IC] � The concentration ratio between chloride and inorganic carbon in a standard.

TABLE 2. Recovery Tests of 50% Oxidant and50% Oxidant Containing Catalyst on VaryingOrganic Carbon Levels Represented by KHP (DDW)Standards

[C] KHP mg/L

Recovery with50% Oxidant

(�1SD)n � 4

Recovery with50% Oxidant

� Catalyst(�1SD)n � 4

Quantity ofCatalyst/g

25 92(2) 96(2) 0.004815 81(2) 70(2) 0.010915 84(2) 87(2) 0.00485 89(2) 103(2) 0.00482 89(2) 102(2) 0.0048

Figure 2. Chloride interference (%Clox) versus [IC] in the35‰ NaCl solution.

Figure 3. DOC data for the coastal sites.

TABLE 3. The Effect of Varying Concentrations ofHydroxylaminehydrochloride (NH2OH.HCl) on theChloride Signal Under 100% Oxidant StrengthSolution

35‰ NaCl solution �NH2OH.HCl [mg/L]

Peak height/mm(�1SD) n � 3

35‰ NaCl solution 200(1)35‰ NaCl solution � 21,000 37(1)35‰ NaCl solution � 15,000 37(1)35‰ NaCl solution � 12,000 36(1)35‰ NaCl solution � 9,000 39(1)35‰ NaCl solution � 3,000 33(1)35‰ NaCl solution � 900 41(1)35‰ NaCl solution � 1000 36(1)35‰ NaCl solution � 300 46(1)35‰ NaCl solution � 100 41(1)Mean peak height 38(3)

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160 Koshy and Mataki

Figure 4. DOC data for the offshore sites.

Figure 5. Standard deviations versus DOC concentrationfor the DOC raw data.

standards prepared in the 35‰ NaCl solution) in thepresence of varying concentrations of IC with respectto IC standards prepared in DDW. The results for thispart of the work are given in Table 1 and Figure 2.The interference by chloride on the carbon signalrapidly decreased as the IC level increased. The di-minishing %Clox was a function of the decreasing ra-tio of [chloride]/[IC], albeit the large differences intheir respective concentrations.

On the other hand, the peak area (1.30) due tochloride was not constant as the concentration of ICincreased. Therefore, it is also probable that IC couldhave partially prevented the oxidation of chloride.Furthermore, because of the increasing level of IC,more carbon dioxide was produced, consequently atthe diffusion membrane; the carbon dioxide mole-

cules might have hampered the diffusion of the chlo-rine molecules.

Minimization of Chloride Interference

Because the chloride interference arises mainly fromthe oxidation of the chloride to chlorine, efforts weremade to reduce the power of the oxidant progres-sively to see how it affects the carbon recovery. It wasfound that a 50% oxidant strength solution gave 80%carbon recovery from a potassium hydrogen phthal-ate (KHP) standard (5 mg/L) in comparison to its re-covery using the 100% oxidant solution.

Use of Titanium Dioxide

To compensate for this loss, 0.0048 g of TiO2 wasused as catalyst [3] in a suspension form. Table 2gives the summary of CO2 recovery results from the5 mg/L KHP standard using the 50% oxidant strengthsolution and 50% oxidant strength solution plus thecatalyst. The catalyst was able to boost the CO2 re-covery slightly above 100% for the 2 and 5 mg/L KHPstandards and not for the 15 mg/L standard. Lineclogging resulted when higher amounts of the cata-lyst were used.

The interesting observation was that the combi-nation of the catalyst and the 50% oxidant strengthsolution boosted not only the carbon recovery butalso the chloride oxidation as well, rendering the ap-proach almost unsuitable for our purpose.

Use of Hydroxylaminehydrochloride

Hydroxylaminehydrochloride (NH2OH.HCl) is a re-ducing agent used by Schreurs [4] for the reductionof chlorine to chloride. When we employed this so-lution in our trials, the chloride signal was reducedby 81 � 3%. The use of the reducing agent in thepresence of IC yielded %Clox levels similar to thoseshown by IC alone (see Table 3). Thus, the reducingagent’s capacity to minimize chloride interference onits own was not any more than the masking capacityof IC alone. However, when combined with IC con-centrations greater than 10 mg/L, the FIA methodwould only experience %Clox in the range of 2 to 3%.

Use of Microporous Tubing

A microporous tubing (Accurel PP,type S6/2,0.2lmpore size, Enka) was placed before the water bath toeliminate the gases formed in the UV chamber. Thechloride interference was suppressed, through de-bubbling the chlorine, to an extent of about 90 to98%. The major setback of this approach was its pro-

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Photochemical Oxidation and Flow Injection Conductivity Determination 161

TABLE 4. Effect of the Microporous Tubing on Chloride Interference on Carbon Standards Prepared inDDW and 35‰ NaCl Solution

[C] mg/L

DDW (mm)(wo) (�1SD)

n � 10

35‰ NaClsolution (mm)(wo) (�1SD)

n � 10

DDW (mm)(w) (�1SD)

n � 10

35‰ NaClsolution (mm)

(w) (�1SD)n � 10

%Clox

(w) (�1SD)n � 10

%Clox (wo)(�1SD)

5 40.0(0.2) 48.4(0.3) 42.5(0.2) 44.4(0.2) 5(2) 21(2)10 73.7(0.1) 87.5(0.1) 71.6(0.1) 73.3(0.2) 2(1) 19(1)15 126.9(0.1) 140.6(0.1) 118.4(0.2) 128.5(0.1) 9(4) 11(2)

(wo), without micro-porous tubing; (w), with microporous tubing.

Figure 6. Standard deviations versus DOC concentrationfor the DOC transformed data.

Figure 7. Average FIA DOC data versus average Shimadzu2000-UV DOC data.

hibitive cost coupled with its unavailability in smallquantities. Furthermore, smooth continuous flow iscritical for the efficient operation of this FIA system.With the use of the microporous tubing, we foundthat “sweating” was a major practical problem, re-sulting in signal irregularities.

Baseline DOC Study of the Suva lagoon

By design, this was a baseline DOC study of selectedsites in the Suva lagoon and not a comprehensiveinvestigation of the temporal and spatial variation ofDOC in the Suva lagoon. The discussion has been

categorized into coastal and offshore sites to ensureclarity.

Coastal SitesThe DOC in each site sampled ranged from levels lessthan the detection limit of the FIA system to a max-imum of 35 mg/L (Figure 3). There was not any fixedpattern apart from the wide variation in DOC levelscaused by the input from the landmass of Suva. Thesamples from trips 1, 2, 3, and 7 exhibited higherlevels of DOC consistent with the wet weather thatpreceded the sampling period. On the other hand, theestuarine water samples that were collected on therest of the sampling trips showed lower DOC levels,less than 10 mg/L.

It is clear from our results that the input via therivers, storm water outlets, and creeks to the coastalwaters of the Suva lagoon significantly contributedto the DOC in the sites studied. The general trend wasthat fine weather was usually associated with DOClevels less than 10 mg/L, and wet weather could dou-ble or even triple the DOC levels in the sites. Randomdissolved oxygen (DO) measurements of the sites reg-istered DO levels in the range of 5–8 mg/L, indicatinga certain degree of DOC-related oxygen loss [5].

Offshore SitesThese sites were situated in the range of 0.1 to 5 kmfrom the landmass of Suva. The sites showed a widevariation in their DOC levels in the range of 0 to 35mg/L, just like the coastal sites (Figure 4). This alsoimplied that, the terrestrial organic input from theland significantly contributed to the observed DOClevels. Furthermore, increased surface runoff fromthe land initiated by rain generally resulted in ele-vated levels of DOC in most of the sites, see Figure4: sampling trips 1, 2, 5, and 6. One of the sites con-sistently registered a DOC level around 0.8 mg/L, al-beit its proximity to the land than most of the othersites. It became apparent that because of its locationin a passage where there was continuous water move-ment, the site maintained a nearly constant level ofDOC.

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162 Koshy and Mataki

TABLE 5. The Average DOC Values for the Sites Using the FIA Manifold Used in this Project and theAverage DOC Values Obtained from a Shimadzu 2000-UV HTCO Unit

Sites

FIA Method RawData [DOC] mg/L(�1SD) n � 32

FIA MethodTransformed

Data [DOC] mg/L(�1SD) n � 32

Shimadzu 2000-UVHTCO Unit Data

[DOC] mg/L(�1SD) n � 20

[FIA � HTCO] � 100HTCO

(�1SD) n � 10

Kinoya outfall 15(11) 13(4) 12(1) 8(4)200 m Kinoya outfall 18(14) 16(4) 11(1) 31(8)Nukulau passage 10(11) 9(3) 9(1) 0Centra hotel 0.8(0.2) 0.8(0.1) 0.8(0.1) 0200 m Nabukalou creek 19(10) 18(3) 1.0(0.4) 1700(6)Mosquito island 19(13) 18(3)Tradewinds 9(11) 8(2) 3(1) 167(4)Vugalei bridge 6(6) 5(1) 3(1) 67(2)Tamavua river 8(5) 8(1) 2(1) 300(2)Nabukalou creek 7(7) 7(1) 11(2) �36(4)Nasese 5(6) 4(1)Suva point 8(9) 7(2) 1.0(0.1) 600(5)

Note: With the Shimadzu 2000-UV HTCO unit, DOC data reported were averages of two sets by 5 replicate analyses. The Shimadzu2000-UV HTCO unit belonged to a Japanese Aid program based at Monfort Boys Town.

Comparison of DOC Data Obtained fromthe Modified FIA Method Used Here witha Shimadzu 2000-UV HTCO UnitThe raw average DOC data obtained from the FIA sys-tem was transformed so that the large variations interms of the standard deviations quoted for the rawaverages could be reduced, consequently making thedata statistically acceptable for comparison [6]. Thelarge variations in the raw averages indicated thelarge influence of land-based sources of DOC on theDOC present in the Suva lagoon. The transformedDOC data was essentially the same as the raw averagedata according to an F-test performed on the two setsof data. The only marked difference in the raw andtransformed data was the generally low standard de-viations quoted for the transformed data. The simi-larity between the sets of data was also evident fromthe similar correlation coefficients (�R2 � 0.7) cal-culated for the individual plots of raw and trans-formed averages against their standard deviations(Figures 5 and 7).

The variation of the DOC data with respect to theindividual methods (FIA and Shimadzu 2000-UV) ofDOC determination used in this work was statisti-cally insignificant. The coastal sites are essentiallyestuarine, generally having low salinity in the rangeof 23 to 34 practical salinity units (PSU).

Furthermore, there was a weak but positive cor-relation between the average DOC data obtained fromthe FIA system and the Shimadzu 2000-UV HTCOunit (see Figure 6).

Also, the data from both methods (FIA system

and Shimadzu 2000-UV HTCO unit) were correctedfor the blanks (reagent, water, and instrument) interms of the Y intercepts of the calibration standardcurves used. The Y intercepts of the calibrationcurves are reliable estimates of the total blank value[7].

CONCLUSION

The modified FIA method used in this work had amethod detection limit (MDL) of 0.8 mg C/L, a carbonrecovery of 98 � 9%, and a sample throughput of 60samples per hour. These results compare very wellwith those of the FIA method described by Koshy [1].

The modified FIA method also suffered fromchloride interference, but the interference (%Clox)was masked to levels as low as 1% with IC levelsgreater than 8 mg/L. Because of the high levels of ICin most coastal and marine waters [8], the modifiedFIA method could be used for the determination ofDOC in these types of water samples because chlo-ride interference could be reduced significantly.

The baseline DOC study in this work indicatedthat the Suva lagoon received a significant amount ofDOC from the landmass of Suva. Factors such as rain,tide, and current movements in the lagoon signifi-cantly influenced the concentration of DOC presentin the water columns of the sites monitored.

The modified FIA method measured DOC levelsthat were comparable to the DOC levels obtainedfrom an established Shimadzu 2000-UV HTCO unit.

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Photochemical Oxidation and Flow Injection Conductivity Determination 163

REFERENCES[1] Koshy, K.; McKelvie, R. T.; Ferret, I. D.; Hart, B. T.;

Bapat, J. B. Anal Chim Acta 1992, 261, 287–294.[2] Aleprin, M. J.; Martens, C. S. Mar Chem 1993, 41,

135–143.[3] Matthews, R. W.; Abdullah, M.; Low, G. K. C. Chem

Australia1990, 57, 85–89.[4] Schreurs, W. Hydrobiol Bull 1978, 12, 137–142.

[5] Libes, S. M. An Introduction to Marine Biogeochem-istry; Wiley & Sons: New York, 1992.

[6] Damon, R. A. Jr.; Harvey, W. R. Experimental Design:ANOVA and Regression; Harper & Row: NewYork, 1987.

[7] Skoog, D. A.; Leary, J. J. Principles of InstrumentalAnalysis; Harcourt Brace College Publishers: Syd-ney, 1992.

[8] Manahan, S. E. Environmental Chemistry; CRC Press:Boca Raton, FL, 1994.