Determination of phosphate in wastewaters by ion-exclusion chromatography with flow coulometric detection

Water Res. Vol. 16, pp. 719 to 723. 1982 Printed in Great Britain. All rights reserved

0043-1354/82/050719-05503.00/0 Copyrighl © 1982 Pergamon Press Ltd

DETERMINATION OF PHOSPHATE IN WASTEWATERS BY ION-EXCLUSION CHROMATOGRAPHY WITH FLOW COULOMETRIC DETECTION KAZUHIKOTANAKAand TOSHIO ISHIZUKA Government Industrial Research Institute, Nagoya 1-1. Hirate-machi, Kita-ku, Nagoya-shi, Aichi, 462, Japan (Received June 1981)

Abstract--lon-exclusion chromatography of PO~- in wastewaters has been investigated on a cation exchange resin in the H+-form. Phosphate ion was chromatographed by ion-exclusion as H3PO4 formed by cation exchange, and monitored with a flow coulometric detector based on the electrochemical reaction of H ÷ ion from HaPO4 and p-benzoquinone at a constant applied potential (+0.45 V vs Ag-Agl) or a conductometric detector. A reasonable separation of PO~- from strong acid anions (CI- SO~- and NO~) and CO 2- as coexisting anions could be accomplished by isocratic elution with 60~o (v/v) acetone-water. The calibration graph for PO~- with the flow coulometric detector was linear over the concentration range 1-150 ppm (slope = 0.982), but not with the conductometric detector. The agreement of the analytical results of PO~- between the ion-exclusion chromatography with the flow coulometric detector and the colorimetry (molybdenum-blue method) was excellent for some industrial wastewater and domestic sewage samples.

INTRODUCTION Phosphorus is a nutrient for algae and aquatic plants, which causes the eutrophication of aquatic environment. Phosphorus in wastewater and sewage mostly presents as orthophosphate (po3-). The determination of P O ] - in the wastewater and sewage has mostly been carried out colorimetrically by the molybdenum-blue method (APHA, 1975). This method, however, suffers from several chemical interferences and is time-consuming. In previous papers {Tanaka et al., 1977, 1978, 1980), we have reported that the separation of PO~- from strong acid anions such as CI-, SO ] - , NO~, and several condensed phosphates (P2074-, P30~o and P30 3-) could be accomplished by ion-exclusion chromatography (1EC) on a cation exchange resin in the H+-form by elution with an acetone-water and dioxane-water mixture. As the separation mechanism of the IEC by elution with water alone for numerous anions or their respective acids is based on the Donnan membrane equilibrium principle (ion-exclusion effect), IEC is a highly useful technique for the separation of nonelectrolytes such as carboxylic acids and H2CO 3 from electrolytes such as HCI and H2SO4 (Tanaka et al., 1979a). IEC has also been coupled to "ion chromatography" (Dionex ion chromatograph) to determine simultaneously both weak and strong acids (Dionex application note, 1979). The IEC separation of PO 3 - from the strong acid anions by elution with organic solvent-water described above is basedon this ion-exclusion effect and/or the partition effect between the cation exchange resin phase (water-rich) and the mobile phase (organic solvent-rich) owing to the hydration of the resin (Tanaka et al., 1980). Separ-

ated P O ] - has been monitored as the corresponding acid (H3PO,,) with a flow coulometric detector (FCD) for the detection of H + ion and a conductomeric detector (COND). The purpose of this work is to investigate the possibility of determining PO~- in some actual samples by the IEC on a cation exchange resin in H+-form by elution with acetone-water. This paper discusses the optimal chromatographic conditions for the separation of PO~- from CI-, SO~-, CO l -, etc, which are always present in the common wastewater and sewage samples, the FCD and C O N D responses, calibration graph, detection limit, and reproducibility of PO~-. The method was applied to the determination of P O ] - in industrial wastewater and domestic sewage samples.

EXPERIMENTAL

Apparatus

A Spectra-Physics 3500 B liquid chromatograph equipped with a FCD (Hitachi 630) and a COND (Yanagimoto C-202) was used for isocratic elution with acetonewater. In the FCD, the following electrochemical reaction was used to detect H" ion from HaPO4 eluted: p-benzoquinone + 2H ÷ + 2e- -* hydroquinone (+0.45 V vs AgAgl). Details for the FCD have been already described by Takata & Muto (1973). A glass-jacketted column packed with a Hitachi 2613 strongly acidic cation exchange resin in the H ÷-form was used at 30°C. The flow rate of eluent was I mlmin -~. Aqueous sample solutions containing PO~- and other anions (0.5-2 ml) were introduced onto the column with a variable-loop injector and chromatographed by ion-exclusion as the corresponding acids. A two-pen strip chart recorder and two digital integrators used in this work have already been described (Tanaka & Ishizuka, 1980).

719

720

KAZUI-tlKt) TANAKA a n d TOSHIO ISHIZUKA

A Shimadzu UV-210 recording spectrophotometer was used for the determination of PO~- in the actual samples by the molybdenum-blue method {APHA, 1975}.

Reagent.~ All stock solutions of PO 3 and the other anions (CI-, SO~-. NO~ and CO~ ) used were prepared by dissolving the reagent-grade sodium salts in deionized, distilled water. Acetone and the other chemicals were of reagent grade.

Preparation o/ actual samples To remove suspended substances in the actual samples, each sample solution was filtered with a 0.45/~m Milliporefilter paper in a Swinny holder, and PO]- concentration in the filtrates was determined by the IEC.

RESULTS AND DISCUSSION

Separation of PO 3,- from some diverse anions by elution with water Tanaka et al. (1979a) have reported that in IEC, numerous anions or their respective acids can be separated gel chromatographically by elution with water on a cation exchange resin in the H ÷-form. The distribution coefficient (Kd) values of most acids calculated from the conventional equation for IEC were between 0 and I depending on their first dissociation constants (pKt) in the range about 1--7, and were 0 for strong acid anions (CI-, S O ,z- and NO~), 0.09 for PO 3-, and 1.0 for C O ~ - . These results indicated that all of the strong acid anions are ion-excluded completely from the resin phase and P O ,3- and CO 2 - are not completely ion-excluded from the resin phase, depended on the Donnan membrane equilibrium. Figure 1 shows the chromatogram of the mixture of CI-, SO~-, N O 3 , PO 3- and C O 2- of each 10ppm obtained with F C D by elution with water. As can be expected from their K,, values described above, P O ,3could be successfully separated from CO32 , but not from the strong acid anions owing to the similar K~ values. Hence, the peak resolution (R,) between PO~and the strong acid anions was insufficient for the quantitation of PO 3 .

Separation (~/ PO~- ./rom .some diverse a~ions by elutiott with acetone-water In order to conveniently separate PO 3- from the strong acid anions, the effect of acetone on the retention volumes of strong acid anion (CI-) and PO43was investigated in 0-60% (v/v) concentration acetone as eluent. The retention volume of PO 3- increased with increase in the concentration of acetone in the eluent, whereas, that of CI- hardly varied and corresponded to the column void volume over the concentration range examined (K~ = 0, complete ion-exclusion). The K,~ of PO 3- by elution with 60% acetone-water increased from 0.09 to 0.2. Thus, the R~ between CIand PO~- increased in proportion as the Kd or the retention volume of P O ,3- increased. In contrast, that of CO~- decreased with increase in concentration of acetone and the K,~ of CO~- by elution with 600,~,

acetone water then decreased from 1.0 to 0.60. Figurc 2 shows the chromatogram of a mixture of 10 ppm of CI-. SO 2-. N O 3 , PO 3- and CO 2- obtained with the F C D by elution with 60",, acetone water. As can bc seen from Fig. 2. PO~.- could be separated from the strong acid anions and CO 2-. The R~ value between the strong acid anions and PO~. - was about 1.7. This R~ value suffices for the quantitation of PO 3 by the peak area measurement with a computing integrator used in this work. This chromatogram indicates that both PO 3- and C O 2- penetrate into the resin phase and CI- is ion-excluded completely from the resin phase because the H ' ion of HCI dissociates completely in 60,°0 acetone, water. Details for the behavior of the increasing retention volume of PO 3 - caused by increasing the acetone concentration in the eluent has been already described by Tanaka & lshizuka (1980). This behavior is based on the increasing effect of the pKt value of PO 3 owing to the decreasing dielectric constant of the eluent and/or the partition effect between the resin phase (water-rich) and the mobile phase lacetone-rich) owing to the hydration of the resin as a side effect in IEC. In Fig. 2. an unknown peak was observed between PO~- and C O ~ . This peak was due to water in the sample solution introduced into the chromatograph. However, the peak did not interfere with the quantitation of PO 3- .

/

c:, soJ-,

/

NO3-

Po43-

C032-

t 10 20 Time, rain

t 317

Fig. I. Ion-exclusionchromatogram of mixture of lOppm each of CI-, SO~-, NO~, PO,s- and CO~- obtained by elution with water alone, l:)¢tectcdwith F C D (lOOmV full scale).

Determination of phosphate in wastewaters

721 3O

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~25

.o

2O

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$

..~15

!

g Q.

e~

o

//t ,

o

i

,

u. i

0

O5 10 15 20 Sample volume, mI

Fig. 3. Relationships between FCD responses and sample volume. Sample: Mixture of 10ppm each of CI- and POl-. ~43CO32-

&_ I0

20

3O

T i m e , rain

Fig. 2. Ion-exclusion chromatosram of mixture of 10 ppm each of CI-, SO~-, NO~-, PO~- and CO 2- obtained by e]ution with 60% acetone-water. Detected with FCD

(lO0 mV full scale). From the above experimental results, it was concluded that 60% acetone-water is a suitable eluent for IEC separation of PO~- from strong acid anions and co~ -

Effect qf sample volume In the IEC determination of PO~-, the detection limit (concentration) can be lowered by going to a larger sample volume. The effect of sample volume on the FCD response of PO~- and the R, between CIand PO~- was examined on the sample volume range 0.5--2.0ml. Figure 3 shows the relationship between the sample volume and the peak area or peak height of PO~-. The peak area or peak height of PO~linearly increased with increase in the sample volume. Figure 4 shows the relationship between the R, and the sample volume. Despite the increase of the chromatogram peak width for both CI- and PO~- resulting from increasing the sample volume, a reasonable R, (1.2) between both anions could be obtained, even with 2-ml-injection by elution of 60~o acetone-water. By using this technique, it is possible to increase the sensitivity of PO~-.

Effect of acetone on detector responses As is well-known, the pK and the conductance of acids in organic solvent-water mixture solution are

dependent on the concentration of organic solvent. The FCD and COND responses for strong acid anion (CI-), weak acid anion (PO34-), and very weak acid anion (CO~-) were therefore investigated in the concentration range 0-60% acetone-water as eluent. The detector responses (peak area) of these anions (1-mlinjection of each 10ppm) obtained with the FCD and COND "were defined as 100 for CI- at 0% acetone-water eluent. In Fig. 5, the C O N D responses of each anion by elution with water alone decreased in order of decreasing acidities or equivalent conductances of HCI, H3PO4, and H2CO3. The C O N D responses of each anion linearly decreased with increase in concentration of acetone in the eluent, and the peak of C O l - was no longer detectable with 40?/o acetonewater. This is one serious disadvantage of COND. On the other hand, remarkable differences among the FCD responses for each anion by elution with water alone were not observed, as shown in Fig. 6. This experimental result indicates that the ratio of the response for CI-, P O ] - and C O l - per micromole was 1:2:1. This is the most striking characteristic of 20 °1,5

I .~ 10

~0~5

o

o;~ ,io

,is

I

~o

Sample volume, ml Fig. 4. Relationship between peak resolution and sample volume. Sample: Mixture of ]0ppm each of CI- and po ~,-.

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Fig. 5. Effect of acetone on COND responses of CI . PO4s- and CO.~- Sample concn.: 10ppm each.

5

1 Conch

FCD. This means that the electrochemical reaction of 2H + ions in the 1st stage dissociation (pK1 = 2.1) and 2nd stage dissociation (pK2 = 7.2) of HaPOa or H + ion in the 1st stage dissociation (pKl = 6.4) of H2CO3 and p-benzoquinone in F C D proceeds stoichiometrically the same as that of H ÷ ion from HCI (pK = - 7 ) and p-benzoquinone. Hence, it is evident that the both H ÷ ions in the 3rd stage dissociation (pK2 = 11.9) of H3PO4 and 2nd stage dissociation (pK2 = 10.3) of H2CO 3 are not detectable with FCD. In FCD, the response of CI- did not vary at all over the concentration range 0-60% acetone--water because the H ÷ ion of HCI dissociates completely, even with 60% acetone-water. The F C D responses of P O ~ - and C O ~ - did not remarkably decrease in comparison with the results obtained with C O N D under the same elution conditions. This indicates that the electrochemical reaction of H " ions from these weak acids and p-benzoquinone in the F C D proceeds considerably, even with 60% acetone-water. Such behavior has also been observed in IEC with F C D of PO~- and several condensed phosphates by elution with dioxane-water (Tanaka & lshizuka, 1980). The above experimental results indicate that F C D is a

10C

o-----c

c,

of POz, 3-. ppm

Fig. 7. Relationship between FCD or COND response and concentration of PO s-.

more suitable detector than C O N D for the IEC determination of weak acid or very weak acid anions by elution with water or acetone-water.

Calibration graphs The calibration graphs of P O 3- with F C D and C O N D were constructed for the concentration range 1-150 ppm under the conditions of 60% acetone--

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_ P

O CO3~-

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Fig. 6. Effect o f acetone on F C D responses o f C ] - , PO 3 and C O l - Sample concn.: 10 ppm each.

i

o

1o

20

o

IO

,

20

Time, rain

Fig. 8. Ion-exclusion chromatograms of industrial w a t e r FCD: 50 mV full scale.

waste-

Determination of phosphate in wastewaters

723

Table 1. Analytical results for actual samples Sample Domestic sewage (Influent water) Domestic sewage (Effluent water) Industrial wastewater (Influent water) Industrial wastewater (Effluent water)

PO~- concentration, ppm Present method* Molybdenum-blue method 5.1

5.2

4.5

4.2

23.6

24.4

1.4

1.4

* Determined with FCD. water as eluent and sample injections of 2 mi. Figure 7 shows the relationships between the concentration of PO~- and the peak areas obtained with both detectors. The calibration graph obtained with FCD has a slope of 0.982 and was nearly linear. However, the graph obtained with COND was not linear. This is caused by the incomplete dissociation of HaPO4 in 60yo acetone-water. Such behavior has also been observed in the determination of NH~ with FCD based on the electrochemical reaction of O H - ion from NH4OH (pk b = 4.7) and hydroquinone and with COND in IEC of weak base cations on an anion exchange resin in the OH--form (Tanaka et al., 1979b). From the above experimental results, it was concluded that FCD was a more powerful detector than COND for the IEC determination of weak acid anion such as PO~- by elution with acetone-water mixture. Reproducibility of PO~ - measurement The reproducibility of the chromatogram of PO~obtained with FCD were examined with 1-ml-inject/on of the mixture of CI- and PO~- of each 10 ppm. The coefficient of variation for the peak area of PO,~obtained was 2.2°,0 (n = 6). Reproducible chromatograms were obtained during repeated chromatographic runs. Detection limit of PO~The detection limit of POa3- with the FCD was determined with 60Yo acetone-water as eluent and l-ml-injection of 1 ppm PO~-. A detection limit of 26.8 ppb (26.8 ng) was predicted at twice the observed background noise. Determination of PO~- in some actual samples The determination of PO~- in several domestic sewage and industrial wastewater samples was carried out by IEC. Figure 8 shows the chromatograms of influent and effluent in the coagulating sedimentation process for industrial wastewater. Reasonable separation of PO~- in the industrial wastewater could be accomplished by elution with 60~ acetone-water. The analytical results obtained by the present method

WR

16.5

o

with FCD are shown in Table 1 together with the results obtained by the molybdenum-blue method. The agreement of the analytical results between both methods was excellent for all samples actually examined. CONCLUSION

IEC on a cation exchange resin in the H +-form by elution with acetone-water is a useful technique for the separation of PO~- in some industrial wastewater and domestic sewage samples. FCD for the detection of H + ion is a more powerful detector than COND for the IEC determination of PO 3- by elution with acetone water. In addition, FCD may be readily applicable to ion chromatography (Dionex ion chromatograph) for the determination of various anions. Acknowledgement--The authors wish to thank Professor H. Sunahara, Department of Industrial Chemistry, Hiroshima University, for many helpful discussions and suggestions during the course of this research. REFERENCES

APHA (1975) Standard Methods for the Examination of Water and Wastewaters, 14th Edition. American Public Health Association, Washington, DC. Dionex application note (1979) Analysis of strong and weak acids in coffee extracts, No. 19, Dionex corporation: Ion chromatography systems. Takata Y. & Muto G. (1973) Flow coulometric detector for liquid chromatography. Analyt. Chem. 45, 1864. Tanaka K. & Ishizuka T. (1980) Ion-exclusion chromatography of condensed phosphates on a cation exchange resin. J. Chromatogr. 190, 7. Tanaka K. & Sunahara H. (1978) Ion exclusion chromatography of phosphate, phosphite and hypophosphite ions on a hydrogen-form cation exchange resin with flow coulometric detection. Bunseki Kagaku 27, 95. Tanaka K., lshizuka T. & Sunahara H. (1979a) Elution behaviour of acids in ion-exclusion chromatography using a cation-exchange resin. J. Chromatoqr. 174, 153. Tanaka K., lshizuka T. & Sunahara H. (1979b) Ion-exclusion chromatography of the ammonium ion on an anion-exchange resin. J. Chromatoyr. 177, 21. Tanaka K., Nakajima K. & Sunahara H. (1977) Ion exclusion chromatography of phosphate ion using cation exchange resin. Bunseki Kagaku 26, 102.