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Determination of ammonia in wastewaters containing high concentrations of surfactants by flow injection potentiometry with on-line sample clean-up
Analytica Chimica Acta 367 (1998) 193±199
Determination of ammonia in wastewaters containing high concentrations of surfactants by ¯ow injection potentiometry with on-line sample clean-up Hongda Shen, Terence J. Cardwell*, Robert W. Cattrall Centre for Scienti®c Instrumentation, School of Chemistry, La Trobe University, Bundoora, Vic. 3083, Australia Received 27 November 1997; received in revised form 16 February 1998; accepted 17 February 1998
Abstract It is demonstrated that surfactants lead to problems in the analysis of ammonia in gas-diffusion ¯ow injection by wetting the membrane to allow transport of potentially interfering species to the detector. An on-line clean-up procedure, involving use of an activated carbon cartridge, is described for the removal of ionic and non-ionic surfactants (up to 1000 mg lÿ1) and other organic compounds from aqueous samples. Ammoniacal nitrogen was determined in synthetic samples by gas-diffusion ¯ow injection with a nonactin-based potentiometric sensor used as the detector for the ammonium ion. Analyses could be performed at a sampling rate of ca. 25 hÿ1 and the cartridge was effective for at least 1000 injections of 50 ml samples in the concentration range 1±180 mg lÿ1. The procedure was applied to the analysis of wastewaters and was found to be ef®cient in removing ionic and non-ionic surfactants from acidi®ed (pH 1) samples. # 1998 Elsevier Science B.V. Keywords: Ammonia; Surfactants; Wastewaters; Flow injection; Activated carbon; Potentiometry; Nonactin electrode
1. Introduction The widespread use of surfactants in domestic and industrial applications has stimulated studies of the in¯uence of surfactants on the determination of many species using various methods, especially in the analyses of wastewater and sewage which may contain high concentrations of surfactants and other organics. For example, in potentiometric methods, surfactants [1±5] and other organic species such as humic substances [6] have been found to have an adverse effect
*Corresponding author. Tel.: +61 3 9479 2536; fax.: +61 3 9479 1399; e-mail: [email protected] 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00168-8
on the performance of ion selective electrodes (ISEs). The effects of surfactants seem to be very complex and depend on the type of surfactant and the electrode system used [1,4]. It is also dif®cult to determine the ionic species in complex sample matrices which contain surfactants and many other organic compounds. Two of the most widely used methods for the determination of ammonia in natural and wastewater samples are based on ¯ow injection (FI) employing a gas-diffusion (GD) cell or potentiometry with the ammonia probe. However, in both of these procedures, surfactants and other organics can lower the performance of gas-diffusion membranes either by wetting the hydrophobic membrane [5,7±9] or by blocking the minipores of the membrane [10].
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To overcome surfactant and other organic interferences in potentiometry, several methods have been recommended in the literature. One of them involves headspace analysis, where the ammonia probe is suspended above the sample solution [5], but the detection limit is high (>110ÿ3 M) and the time to reach equilibrium is long. Another method uses liquid±liquid extraction [9], but the procedure is tedious and time consuming. Alternative methods employing alternate washing [8] and standard addition [10] are also reported, but these methods do not eliminate completely the interferences from surfactants and other organic compounds. This paper describes the use of activated carbon to remove surfactants and other organic substances on-line, prior to the determination of the ammoniacal nitrogen in wastewaters by gas-diffusion ¯ow injection (GD-FI) using the nonactin-based ammonium ion sensor. The method is simple and reliable, with no interferences from surfactants and other organic compounds.
glacial acetic acid (Mallinckrodt) and making it up to 1.0 l. Triton X-100 (BDH), Hyamine 2389 and sodium lauryl sulphate (both from Ajax) were used to prepare non-ionic, cationic and anionic surfactant solutions, respectively. Solutions of 400 mg lÿ1 n-decanol (Merck) and 1-chlorodecane (Aldrich) were made up in 1 g lÿ1 Triton X-100 medium because of their insolubility in water. Activated charcoal powder (Ajax) was used for adsorption of surfactants and other organics. As many commercial charcoal powders contain some impurities which may interfere in the determination of ammonia, they should be puri®ed before use. The puri®cation procedure involved several washings with acidi®ed water (pH 2±3), followed by several rinses with deionised water, before drying in an oven. The activated carbon cartridge consisted of an empty, stainless steel, liquid chromatography guard column (5 cm long, 4.6 mm i.d.) dry packed with activated carbon and conditioned for about 10 min with the acidic carrier stream.
2. Experimental
2.2. Procedures
2.1. Reagents and chemicals
The ammonium ion-selective electrode was prepared from a mixture consisting of 1 mg of nonactin (Fluka), 66 mg of bis(1-butylpentyl) adipate (Fluka) and 33 mg of polyvinyl chloride (Fluka). This was dissolved in 1 ml of puri®ed tetrahydrofuran (BDH), sonicated for 10 min and then coated on the silver contacts of the detector ¯ow-cell. The sensors were dried in air for 24 h and then conditioned in 0.1 M ammonium chloride for 24 h before use. To investigate the adsorption behaviour of surfactants on activated carbon off-line, the following procedure was used: 0.3 g of activated carbon powder was added to the solution (sample or standard), sonicated for 10 min, allowed to settle for 5 min, centrifuged at 3000 rpm for 20±30 min, and the supernatant withdrawn by syringe before injection into a GD-FI system without the carbon adsorption cartridge.
Ammonium chloride (BDH) was used to prepare standard solutions of the analyte. 0.1 M lithium chloride (Ajax) was used as the reference stream in the potentiometric detector. In the gas-diffusion systems, 0.5 M sodium hydroxide (Mallinckrodt) containing 0.05 M EDTA (BDH) was used as the donor stream to convert ammoniacal nitrogen to ammonia, and the receiving stream was a buffer of 0.02 M tris(hydroxymethyl)aminomethane (Aldrich) adjusted to pH 7.2 with hydrochloric acid (BDH). For on-line removal of surfactants, the carrier stream through the carbon cartridge was composed of 0.1 M HNO3/0.02 M Pb2 when samples contained anionic surfactants, or 0.1 M HNO3 when samples were known to contain surfactants other than anionic ones. All solutions were prepared using >15 M cm NANO pure water (Barnstead, Dubuque, IA). Potassium chloride (Mallinckrodt) solutions were used for testing the wettability of the gas-diffusion membrane. An acetate buffer (pH 5.5) was prepared by mixing 4.2 g of lithium hydroxide (BDH) with 6.8 ml of
2.3. Apparatus Two GD-FI systems were used, as shown in Fig. 1. System A (Fig. 1(A)) consists of a 4-channel Gilson Minipuls 2 peristaltic pump for delivery of all ¯owing
H. Shen et al. / Analytica Chimica Acta 367 (1998) 193±199
195
Fig. 1. (A) GD-FI system with nonactin-based ammonium ion potentiometric detector. (B) GD-FI system for on-line sample clean-up. GDM ± gas-diffusion module; PD ± pulse dampener; AC column ± activated carbon cartridge; IV ± injection valve; W ± waste.
streams, a Rheodyne 5041 injection valve, a Perspex gas-diffusion module and the potentiometric detector. In System B (Fig. 1(B)), used for on-line removal of surfactants and organics, the sample introduction line consists of a conventional liquid chromatography
pump (Altex), a high pressure injection valve (Rheodyne 7125) and the stainless steel cartridge packed with activated carbon. In some FI experiments, a ¯uorescence detector (Hitachi, Model F-1050) was used to monitor the non-ionic surfactant.
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The gas-diffusion module was similar in design to the one described in a previous paper [11], and PTFE plumber's tape was used as the membrane. It is essential that the ¯ow-rates on each side of the membrane are identical so to avoid rupture of the membrane. Thus, in both GD-FI systems, the carrier and 0.5 M sodium hydroxide (also containing 0.05 M EDTA) stream ¯ow-rates were 0.9 and 0.7 ml minÿ1, respectively, and the receiving solution had a ¯ow-rate of 1.6 ml minÿ1. The detector in the manifold was an improved version of our coated-wire multi-ion potentiometric detector [11]. This consists of two Perspex blocks separated by a PTFE gasket. The bottom block contains the silver wire contacts which are coated with nonactin-based membrane mixture to form the ammonium ion-selective coated-wire sensors. Several sensors of the same type were made to allow for redundancies. This detector had a separate reference stream of 0.1 M lithium chloride ¯owing at 0.9 ml minÿ1. The potentiometric detector was interfaced to a 486 DX-33 PC through a 16-bit A/D card (MetraByte, Taunton, MA). FCS software (A-Chem Technologies, Melbourne) was used for data acquisition and processing.
Anionic and neutral surfactants had little effect on the slope of the calibration plot and the response time for the ammonium ion in the range 110ÿ2± 110ÿ5 M, but all electrode potentials decreased by about 10 mV over this concentration range. Furthermore, the cationic surfactant interfered so strongly that a calibration plot could not be obtained for the ammonium ion in the presence of 500 mg lÿ1 Hyamine; the electrode potentials remained constant as the ammonium ion concentration was varied. Thus, the nonactin-based sensor could not be used directly in samples containing surfactants, which makes the differential pH procedure [12] without gas diffusion nonviable for these samples. It is interesting to note that the electrode recovers completely from exposure to high concentrations of surfactants for extended periods of time. After running a 1000 mg lÿ1 Hyamine, Triton X-100 or sodium lauryl sulphate solution as carrier continuously for 30 min across the electrode surface and then ¯ushing the entire system with water, the performance of the electrode returned to normal (based on a calibration plot and response time).
3. Results and discussion
A GD-FI system was investigated for the determination of ammoniacal nitrogen in wastewaters containing high concentrations of surfactants. However, this raised another problem, namely the ability of surfactants to wet the gas-diffusion membrane. When this happens, the membrane loses its ability to restrict the transport of non-gaseous species across the membrane into the receiving stream. It was thus of interest to study the wettability of a gas-diffusion membrane by surfactants. This was carried out by making use of the known interference of K on the response of the nonactinbased ammonium ion-selective membrane. Under normal operating conditions, K in an aqueous sample are not transported through a gas-permeable membrane, but they will be, if the membrane becomes wetted. Therefore, K can be added to a sample and their detection by the sensor in the receiving stream can be used to indicate changes in the hydrophobicity of the membrane. In the present study, the wettability of the membrane by the non-ionic surfactant was
3.1. Effect of surfactants on the nonactin-based ammonium ISE In a recent paper, a differential pH procedure was described for the determination of ammoniacal nitrogen using a nonactin-based ammonium ion sensor in FI analysis without gas diffusion [12]. The method was able to compensate for interference from moderate concentrations of other cations. In an extension of this work, it was considered of interest to investigate the application of the differential pH procedure to the analysis of industrial wastewaters containing moderate to high concentrations of surfactants. For these experiments, solutions containing 500 mg lÿ1 surfactant (Hyamine, Triton X-100 or sodium lauryl sulphate) in ammonium standards of different concentrations were injected into an FI system without gas diffusion and the sensor responses were recorded.
3.2. Wettability of a gas-diffusion membrane by surfactants
H. Shen et al. / Analytica Chimica Acta 367 (1998) 193±199
investigated by repetitively injecting 100 ml of a 110ÿ2 M K solution into the GD-FI system A (Fig. 1(A)) followed by ten injections of 1000 mg lÿ1 Triton X-100 solution. The sensor response was measured after each injection of the K solution. A signal for potassium was observed after about 30 injections of Triton X-100, indicating wetting of the membrane. After about 120 injections of surfactant, the gasdiffusion membrane became transparent and the signal due to K ions reached a value greater than 10 mV. The wettabilities of the three different surfactants were compared in the GD-FI system (Fig. 1(A)) by injecting 100 ml of 0.1 M K solution periodically into a carrier stream of 1000 mg lÿ1 surfactant solution. The electrode potential was plotted against the time of exposure of the membrane to the continuously ¯owing surfactant carrier stream (Fig. 2). The detection of K almost instantly indicates that Triton X-100 wets the membrane very rapidly in comparison to the cationic and anionic surfactants. These results are not surprising as it would be expected that the non-ionic surfactant would interact more ef®ciently than the hydrophilic ionic surfactants with the hydrophobic PTFE membrane. All of the above studies indicate that surfactants and other organic compounds should be removed or at least reduced to low levels if ammoniacal nitrogen is
197
to be determined either with a polymer membranebased ammonium ion sensor or using a gas-diffusion membrane. Investigations were made to determine if activated carbon was suitable for removing the surfactants and other organic compounds prior to the measurement of ammoniacal nitrogen. 3.3. Adsorption of surfactants and other organics by activated carbon Initially, a series of experiments was conducted using a ¯uorescence detector in a FI system without a gas-diffusion module. Various amounts of activated carbon were added to 10 ml of 1000 mg lÿ1 Triton X-100 solution, the mixtures were sonicated and centrifuged, and the supernatant solutions were injected into the FI system to detect the presence of the non-ionic surfactant. For alkylphenol ethoxylates (e.g. Triton X-100), the excitation and emission wavelengths used were (ex)277 nm and (em)298 nm, respectively [13]. It was established that 0.2 g of activated carbon reduced the level of surfactant to 0.1 mg lÿ1. To ensure complete removal of organic compounds including surfactants, solutions of composition 400 mg lÿ1 1-decanol or 1-chlorodecane dissolved in 1000 mg lÿ1 Triton X-100 solution were treated with 0.3 g of activated carbon per 10 ml solution. The pretreated solutions were used as carriers in the GD-FI system A (Fig. 1(A)). After running continuously for 2 h, the gas-diffusion membrane was not wetted. No signal was produced when 0.1 M K was injected into the system, whereas injection of ammonium standards gave peak heights identical to those obtained when water was used as carrier. Similar results were obtained for the two ionic surfactants used, viz. Hyamine and sodium lauryl sulphate. 3.4. On-line removal of surfactants from synthetic samples containing ammonium ions
Fig. 2. Wetting effect of ionic and non-ionic surfactants. Conditions: GD-FI, system A; carrier, 1000 mg lÿ1 surfactant; injected sample, 0.1 M K. SLS ± sodium lauryl sulphate.
As shown above, the off-line treatment of aqueous samples with activated carbon is effective in reducing the levels of surfactants in solution to those which cause negligible wetting of the membrane in a GD-FI system. However, this procedure is tedious and timeconsuming. Hence, the GD-FI system was modi®ed to carry out the clean-up on-line by incorporation of a
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stainless steel cartridge packed with activated carbon into the FI system (Fig. 1(B)). Because of the high back-pressure generated by this cartridge, a high pressure liquid chromatography pump was required to deliver the acid carrier stream in this section of the GD-FI system B. Low impedance microcolumns for use with low pressure pumps proved to be inef®cient for multiple injections of samples containing high concentrations of surfactants. The data presented in Fig. 2 demonstrate that nonionic surfactants wet a hydrophobic membrane more rapidly than ionic surfactants. Hence, Triton X-100 was used to test the clean-up ef®ciency of the carbon cartridge in the GD-FI system in the analysis of synthetic samples containing high concentrations (1000 mg lÿ1) of the surfactant and known concentrations of ammonium ion. The results are presented in Table 1. The good recoveries of the ammonium ion indicate that the use of the carbon cartridge on-line in a GD-FI system gives reliable results for samples containing a non-ionic surfactant. It was observed that the membrane was not wetted during any of these experiments and there was no response from the detector when 0.1 M K solution was injected. Similar results were obtained for samples containing the cationic surfactant Hyamine. After repeated injections of synthetic samples containing anionic surfactants, the FI peak becomes broader and the peak height decreases followed by a slow return of the detector response to baseline. As this is observed only in the presence of anionic surfactants, it is postulated that adsorption of the surfactant on the activated carbon leads to a change Table 1 Analytical data for synthetic samples containing Triton X-100 using GD-FI with on-line activated carbon clean-upa Sample
NH 4 added (mg lÿ1)
Triton X-100 (mg lÿ1)
b NH 4 detected (mg lÿ1)
1 2 3 4 5 6
1.8 1.8 9.0 9.0 18.0 18.0
200 1000 200 1000 200 1000
1.7 1.8 9.0 9.4 17.1 17.3
a
(1.6) (3.1) (0.8) (1.2) (1.8) (2.8)
Each sample contained 400 mg lÿ1 1-decanol. Conditions were as in Fig. 1(B). b Relative mean deviation (%) in parentheses (n3).
in the behaviour of the carbon to that of a cation exchanger, leading to the ammonium ion being slightly retained by the carbon cartridge. Carriers containing a doubly charged cation, Mg2, Ca2, Pb2 or Ba2, were investigated to elute singly charged ions, including NH 4 , from the cartridge. For reasons unknown at this stage, the best recoveries were achieved for samples containing up to 1000 mg lÿ1 sodium lauryl sulphate with a carrier of composition 0.1 M HNO3/0.02 M Pb2. Furthermore, this carrier composition is also suitable for analysis of samples containing neutral and cationic surfactants. A cartridge containing ca. 0.6 g of activated carbon is useful for more than 1000 injections of 50 ml sample or standard solutions containing 1000 mg lÿ1 surfactant at a ¯ow-rate of 1 ml minÿ1. When exhausted, the cartridge can be regenerated by ¯ushing with ethyl acetate for ca. 1 h at a ¯ow-rate of 1 ml minÿ1, but it is recommended, as this process is slow and incomplete, that the cartridge be discarded and replaced by a freshly packed one. A sampling rate approaching 30 hÿ1 was achieved at low concentrations with a precision of about 2%. The linear concentration range is 1±180 mg lÿ1 NH 4 with a detection limit of 0.15 mg lÿ1. In this system, the lifetime of the nonactin-based sensor was three months. 3.5. Analyses of effluent samples Industrial ef¯uent samples of variable composition (suspected to contain many organics such as chlorinated hydrocarbons, oil, grease, alcohols up to 12C, and surfactants) were analysed by GD-FI and on-line sample clean-up. The results are presented in Table 2. The satisfactory recoveries indicate that the method is reliable and that complex samples containing surfactants and various organic compounds are easily cleaned up using an on-line activated carbon cartridge in the sample introduction stream of a GD-FI system. 4. Conclusions Injected surfactants wet the gas-permeable membrane in a GD-FI system so that non-gaseous species pass through the membrane and non-ionic surfactants
H. Shen et al. / Analytica Chimica Acta 367 (1998) 193±199
199
Table 2 Analytical data for effluent samples Samplea
NH 4
ÿ1
(mg l ) Relative mean deviationb (%) Recovery (%)
a b
A
B
C
D
E
F
G
H
I
J
3.6 1.4 101.5
6.2 2.5 101.8
5.0 1.9 102.4
2.0 1.8 98.3
11.1 3.1 103.0
19.4 2.2 95.3
38.4 2.7 97.0
6.7 2.6 103.8
5.6 2.0 104.5
0.5 3.6 102.2
Samples were obtained from Probe Analytical, NSW, Australia, acidifed to pH 1 after collection. n3.
wet the PTFE membrane more rapidly than ionic surfactants. Serious interference in the determination of the ammonium ion using a nonactin-based sensor can be caused by cationic surfactants present in the sample and by K transported across a membrane that is wetted by a surfactant. It is therefore essential that surfactants are removed from samples before their potentiometric analysis by GD-FI. It has been demonstrated that an activated carbon cartridge is very effective for on-line removal of surfactants and other organic compounds from acidi®ed synthetic and ef¯uent samples in the analysis of ammonium ions by the GD-FI method. In the case of the ef¯uent samples, satisfactory recoveries were achieved for the ammonium ion in the range of 0.5 to ca. 40 mg lÿ1. Acknowledgements We thank the Australian Research Council for ®nancial support. HS is grateful to La Trobe University for the award of a Postgraduate Scholarship.
References [1] A. Hulanicki, M. Trojanowicz, E. Pobozy, Analyst 107 (1982) 1356. [2] A. Craggs, G. Moody, J. Thomas, B. Birch, Analyst 105 (1980) 426. [3] A. Frend, G. Moody, J. Thomas, B. Birch, Analyst 108 (1983) 1072. [4] A. Hulanicki, in: E. Pungor (Ed.), Ion Selective Electrodes, vol. 3, Elsevier, Amsterdam, 1981, p. 103. [5] Orion Model 95-12 Ammonia Electrode Instruction Manual, Orion Research Inc., Boston, MA, 1990. [6] G. Csiky, G. Marko-Varga, J. Jonsson, Anal. Chim. Acta 178 (1985) 307. [7] D. Midgley, K. Torrance (Eds.), Potentiometric Water Analysis, 2nd ed., Wiley, Chichester, 1991, p. 374. [8] H. Hara, A. Mtoike, Anal. Chem. 59 (1987) 1995. [9] F. Clark, H. MacPherson, Analyst 104 (1979) 358. [10] J. Bernal, M. Nozal, L. Deban, I. Torremocha, Analyst 111 (1986) 631. [11] P.C. Hauser, S.S. Tan, T.J. Cardwell, R.W. Cattrall, I.C. Hamilton, Analyst 113 (1988) 1551. [12] H. Shen, T.J. Cardwell, R.W. Cattrall, Analyst 122 (1997) 89. [13] W. Miszkiewicz, J. Szymanowski, Crit. Rev. Anal. Chem. 25 (1996) 203.
Determination of ammonia in wastewaters containing high concentrations of surfactants by ¯ow injection potentiometry with on-line sample clean-up Hongda Shen, Terence J. Cardwell*, Robert W. Cattrall Centre for Scienti®c Instrumentation, School of Chemistry, La Trobe University, Bundoora, Vic. 3083, Australia Received 27 November 1997; received in revised form 16 February 1998; accepted 17 February 1998
Abstract It is demonstrated that surfactants lead to problems in the analysis of ammonia in gas-diffusion ¯ow injection by wetting the membrane to allow transport of potentially interfering species to the detector. An on-line clean-up procedure, involving use of an activated carbon cartridge, is described for the removal of ionic and non-ionic surfactants (up to 1000 mg lÿ1) and other organic compounds from aqueous samples. Ammoniacal nitrogen was determined in synthetic samples by gas-diffusion ¯ow injection with a nonactin-based potentiometric sensor used as the detector for the ammonium ion. Analyses could be performed at a sampling rate of ca. 25 hÿ1 and the cartridge was effective for at least 1000 injections of 50 ml samples in the concentration range 1±180 mg lÿ1. The procedure was applied to the analysis of wastewaters and was found to be ef®cient in removing ionic and non-ionic surfactants from acidi®ed (pH 1) samples. # 1998 Elsevier Science B.V. Keywords: Ammonia; Surfactants; Wastewaters; Flow injection; Activated carbon; Potentiometry; Nonactin electrode
1. Introduction The widespread use of surfactants in domestic and industrial applications has stimulated studies of the in¯uence of surfactants on the determination of many species using various methods, especially in the analyses of wastewater and sewage which may contain high concentrations of surfactants and other organics. For example, in potentiometric methods, surfactants [1±5] and other organic species such as humic substances [6] have been found to have an adverse effect
*Corresponding author. Tel.: +61 3 9479 2536; fax.: +61 3 9479 1399; e-mail: [email protected] 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00168-8
on the performance of ion selective electrodes (ISEs). The effects of surfactants seem to be very complex and depend on the type of surfactant and the electrode system used [1,4]. It is also dif®cult to determine the ionic species in complex sample matrices which contain surfactants and many other organic compounds. Two of the most widely used methods for the determination of ammonia in natural and wastewater samples are based on ¯ow injection (FI) employing a gas-diffusion (GD) cell or potentiometry with the ammonia probe. However, in both of these procedures, surfactants and other organics can lower the performance of gas-diffusion membranes either by wetting the hydrophobic membrane [5,7±9] or by blocking the minipores of the membrane [10].
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H. Shen et al. / Analytica Chimica Acta 367 (1998) 193±199
To overcome surfactant and other organic interferences in potentiometry, several methods have been recommended in the literature. One of them involves headspace analysis, where the ammonia probe is suspended above the sample solution [5], but the detection limit is high (>110ÿ3 M) and the time to reach equilibrium is long. Another method uses liquid±liquid extraction [9], but the procedure is tedious and time consuming. Alternative methods employing alternate washing [8] and standard addition [10] are also reported, but these methods do not eliminate completely the interferences from surfactants and other organic compounds. This paper describes the use of activated carbon to remove surfactants and other organic substances on-line, prior to the determination of the ammoniacal nitrogen in wastewaters by gas-diffusion ¯ow injection (GD-FI) using the nonactin-based ammonium ion sensor. The method is simple and reliable, with no interferences from surfactants and other organic compounds.
glacial acetic acid (Mallinckrodt) and making it up to 1.0 l. Triton X-100 (BDH), Hyamine 2389 and sodium lauryl sulphate (both from Ajax) were used to prepare non-ionic, cationic and anionic surfactant solutions, respectively. Solutions of 400 mg lÿ1 n-decanol (Merck) and 1-chlorodecane (Aldrich) were made up in 1 g lÿ1 Triton X-100 medium because of their insolubility in water. Activated charcoal powder (Ajax) was used for adsorption of surfactants and other organics. As many commercial charcoal powders contain some impurities which may interfere in the determination of ammonia, they should be puri®ed before use. The puri®cation procedure involved several washings with acidi®ed water (pH 2±3), followed by several rinses with deionised water, before drying in an oven. The activated carbon cartridge consisted of an empty, stainless steel, liquid chromatography guard column (5 cm long, 4.6 mm i.d.) dry packed with activated carbon and conditioned for about 10 min with the acidic carrier stream.
2. Experimental
2.2. Procedures
2.1. Reagents and chemicals
The ammonium ion-selective electrode was prepared from a mixture consisting of 1 mg of nonactin (Fluka), 66 mg of bis(1-butylpentyl) adipate (Fluka) and 33 mg of polyvinyl chloride (Fluka). This was dissolved in 1 ml of puri®ed tetrahydrofuran (BDH), sonicated for 10 min and then coated on the silver contacts of the detector ¯ow-cell. The sensors were dried in air for 24 h and then conditioned in 0.1 M ammonium chloride for 24 h before use. To investigate the adsorption behaviour of surfactants on activated carbon off-line, the following procedure was used: 0.3 g of activated carbon powder was added to the solution (sample or standard), sonicated for 10 min, allowed to settle for 5 min, centrifuged at 3000 rpm for 20±30 min, and the supernatant withdrawn by syringe before injection into a GD-FI system without the carbon adsorption cartridge.
Ammonium chloride (BDH) was used to prepare standard solutions of the analyte. 0.1 M lithium chloride (Ajax) was used as the reference stream in the potentiometric detector. In the gas-diffusion systems, 0.5 M sodium hydroxide (Mallinckrodt) containing 0.05 M EDTA (BDH) was used as the donor stream to convert ammoniacal nitrogen to ammonia, and the receiving stream was a buffer of 0.02 M tris(hydroxymethyl)aminomethane (Aldrich) adjusted to pH 7.2 with hydrochloric acid (BDH). For on-line removal of surfactants, the carrier stream through the carbon cartridge was composed of 0.1 M HNO3/0.02 M Pb2 when samples contained anionic surfactants, or 0.1 M HNO3 when samples were known to contain surfactants other than anionic ones. All solutions were prepared using >15 M cm NANO pure water (Barnstead, Dubuque, IA). Potassium chloride (Mallinckrodt) solutions were used for testing the wettability of the gas-diffusion membrane. An acetate buffer (pH 5.5) was prepared by mixing 4.2 g of lithium hydroxide (BDH) with 6.8 ml of
2.3. Apparatus Two GD-FI systems were used, as shown in Fig. 1. System A (Fig. 1(A)) consists of a 4-channel Gilson Minipuls 2 peristaltic pump for delivery of all ¯owing
H. Shen et al. / Analytica Chimica Acta 367 (1998) 193±199
195
Fig. 1. (A) GD-FI system with nonactin-based ammonium ion potentiometric detector. (B) GD-FI system for on-line sample clean-up. GDM ± gas-diffusion module; PD ± pulse dampener; AC column ± activated carbon cartridge; IV ± injection valve; W ± waste.
streams, a Rheodyne 5041 injection valve, a Perspex gas-diffusion module and the potentiometric detector. In System B (Fig. 1(B)), used for on-line removal of surfactants and organics, the sample introduction line consists of a conventional liquid chromatography
pump (Altex), a high pressure injection valve (Rheodyne 7125) and the stainless steel cartridge packed with activated carbon. In some FI experiments, a ¯uorescence detector (Hitachi, Model F-1050) was used to monitor the non-ionic surfactant.
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H. Shen et al. / Analytica Chimica Acta 367 (1998) 193±199
The gas-diffusion module was similar in design to the one described in a previous paper [11], and PTFE plumber's tape was used as the membrane. It is essential that the ¯ow-rates on each side of the membrane are identical so to avoid rupture of the membrane. Thus, in both GD-FI systems, the carrier and 0.5 M sodium hydroxide (also containing 0.05 M EDTA) stream ¯ow-rates were 0.9 and 0.7 ml minÿ1, respectively, and the receiving solution had a ¯ow-rate of 1.6 ml minÿ1. The detector in the manifold was an improved version of our coated-wire multi-ion potentiometric detector [11]. This consists of two Perspex blocks separated by a PTFE gasket. The bottom block contains the silver wire contacts which are coated with nonactin-based membrane mixture to form the ammonium ion-selective coated-wire sensors. Several sensors of the same type were made to allow for redundancies. This detector had a separate reference stream of 0.1 M lithium chloride ¯owing at 0.9 ml minÿ1. The potentiometric detector was interfaced to a 486 DX-33 PC through a 16-bit A/D card (MetraByte, Taunton, MA). FCS software (A-Chem Technologies, Melbourne) was used for data acquisition and processing.
Anionic and neutral surfactants had little effect on the slope of the calibration plot and the response time for the ammonium ion in the range 110ÿ2± 110ÿ5 M, but all electrode potentials decreased by about 10 mV over this concentration range. Furthermore, the cationic surfactant interfered so strongly that a calibration plot could not be obtained for the ammonium ion in the presence of 500 mg lÿ1 Hyamine; the electrode potentials remained constant as the ammonium ion concentration was varied. Thus, the nonactin-based sensor could not be used directly in samples containing surfactants, which makes the differential pH procedure [12] without gas diffusion nonviable for these samples. It is interesting to note that the electrode recovers completely from exposure to high concentrations of surfactants for extended periods of time. After running a 1000 mg lÿ1 Hyamine, Triton X-100 or sodium lauryl sulphate solution as carrier continuously for 30 min across the electrode surface and then ¯ushing the entire system with water, the performance of the electrode returned to normal (based on a calibration plot and response time).
3. Results and discussion
A GD-FI system was investigated for the determination of ammoniacal nitrogen in wastewaters containing high concentrations of surfactants. However, this raised another problem, namely the ability of surfactants to wet the gas-diffusion membrane. When this happens, the membrane loses its ability to restrict the transport of non-gaseous species across the membrane into the receiving stream. It was thus of interest to study the wettability of a gas-diffusion membrane by surfactants. This was carried out by making use of the known interference of K on the response of the nonactinbased ammonium ion-selective membrane. Under normal operating conditions, K in an aqueous sample are not transported through a gas-permeable membrane, but they will be, if the membrane becomes wetted. Therefore, K can be added to a sample and their detection by the sensor in the receiving stream can be used to indicate changes in the hydrophobicity of the membrane. In the present study, the wettability of the membrane by the non-ionic surfactant was
3.1. Effect of surfactants on the nonactin-based ammonium ISE In a recent paper, a differential pH procedure was described for the determination of ammoniacal nitrogen using a nonactin-based ammonium ion sensor in FI analysis without gas diffusion [12]. The method was able to compensate for interference from moderate concentrations of other cations. In an extension of this work, it was considered of interest to investigate the application of the differential pH procedure to the analysis of industrial wastewaters containing moderate to high concentrations of surfactants. For these experiments, solutions containing 500 mg lÿ1 surfactant (Hyamine, Triton X-100 or sodium lauryl sulphate) in ammonium standards of different concentrations were injected into an FI system without gas diffusion and the sensor responses were recorded.
3.2. Wettability of a gas-diffusion membrane by surfactants
H. Shen et al. / Analytica Chimica Acta 367 (1998) 193±199
investigated by repetitively injecting 100 ml of a 110ÿ2 M K solution into the GD-FI system A (Fig. 1(A)) followed by ten injections of 1000 mg lÿ1 Triton X-100 solution. The sensor response was measured after each injection of the K solution. A signal for potassium was observed after about 30 injections of Triton X-100, indicating wetting of the membrane. After about 120 injections of surfactant, the gasdiffusion membrane became transparent and the signal due to K ions reached a value greater than 10 mV. The wettabilities of the three different surfactants were compared in the GD-FI system (Fig. 1(A)) by injecting 100 ml of 0.1 M K solution periodically into a carrier stream of 1000 mg lÿ1 surfactant solution. The electrode potential was plotted against the time of exposure of the membrane to the continuously ¯owing surfactant carrier stream (Fig. 2). The detection of K almost instantly indicates that Triton X-100 wets the membrane very rapidly in comparison to the cationic and anionic surfactants. These results are not surprising as it would be expected that the non-ionic surfactant would interact more ef®ciently than the hydrophilic ionic surfactants with the hydrophobic PTFE membrane. All of the above studies indicate that surfactants and other organic compounds should be removed or at least reduced to low levels if ammoniacal nitrogen is
197
to be determined either with a polymer membranebased ammonium ion sensor or using a gas-diffusion membrane. Investigations were made to determine if activated carbon was suitable for removing the surfactants and other organic compounds prior to the measurement of ammoniacal nitrogen. 3.3. Adsorption of surfactants and other organics by activated carbon Initially, a series of experiments was conducted using a ¯uorescence detector in a FI system without a gas-diffusion module. Various amounts of activated carbon were added to 10 ml of 1000 mg lÿ1 Triton X-100 solution, the mixtures were sonicated and centrifuged, and the supernatant solutions were injected into the FI system to detect the presence of the non-ionic surfactant. For alkylphenol ethoxylates (e.g. Triton X-100), the excitation and emission wavelengths used were (ex)277 nm and (em)298 nm, respectively [13]. It was established that 0.2 g of activated carbon reduced the level of surfactant to 0.1 mg lÿ1. To ensure complete removal of organic compounds including surfactants, solutions of composition 400 mg lÿ1 1-decanol or 1-chlorodecane dissolved in 1000 mg lÿ1 Triton X-100 solution were treated with 0.3 g of activated carbon per 10 ml solution. The pretreated solutions were used as carriers in the GD-FI system A (Fig. 1(A)). After running continuously for 2 h, the gas-diffusion membrane was not wetted. No signal was produced when 0.1 M K was injected into the system, whereas injection of ammonium standards gave peak heights identical to those obtained when water was used as carrier. Similar results were obtained for the two ionic surfactants used, viz. Hyamine and sodium lauryl sulphate. 3.4. On-line removal of surfactants from synthetic samples containing ammonium ions
Fig. 2. Wetting effect of ionic and non-ionic surfactants. Conditions: GD-FI, system A; carrier, 1000 mg lÿ1 surfactant; injected sample, 0.1 M K. SLS ± sodium lauryl sulphate.
As shown above, the off-line treatment of aqueous samples with activated carbon is effective in reducing the levels of surfactants in solution to those which cause negligible wetting of the membrane in a GD-FI system. However, this procedure is tedious and timeconsuming. Hence, the GD-FI system was modi®ed to carry out the clean-up on-line by incorporation of a
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stainless steel cartridge packed with activated carbon into the FI system (Fig. 1(B)). Because of the high back-pressure generated by this cartridge, a high pressure liquid chromatography pump was required to deliver the acid carrier stream in this section of the GD-FI system B. Low impedance microcolumns for use with low pressure pumps proved to be inef®cient for multiple injections of samples containing high concentrations of surfactants. The data presented in Fig. 2 demonstrate that nonionic surfactants wet a hydrophobic membrane more rapidly than ionic surfactants. Hence, Triton X-100 was used to test the clean-up ef®ciency of the carbon cartridge in the GD-FI system in the analysis of synthetic samples containing high concentrations (1000 mg lÿ1) of the surfactant and known concentrations of ammonium ion. The results are presented in Table 1. The good recoveries of the ammonium ion indicate that the use of the carbon cartridge on-line in a GD-FI system gives reliable results for samples containing a non-ionic surfactant. It was observed that the membrane was not wetted during any of these experiments and there was no response from the detector when 0.1 M K solution was injected. Similar results were obtained for samples containing the cationic surfactant Hyamine. After repeated injections of synthetic samples containing anionic surfactants, the FI peak becomes broader and the peak height decreases followed by a slow return of the detector response to baseline. As this is observed only in the presence of anionic surfactants, it is postulated that adsorption of the surfactant on the activated carbon leads to a change Table 1 Analytical data for synthetic samples containing Triton X-100 using GD-FI with on-line activated carbon clean-upa Sample
NH 4 added (mg lÿ1)
Triton X-100 (mg lÿ1)
b NH 4 detected (mg lÿ1)
1 2 3 4 5 6
1.8 1.8 9.0 9.0 18.0 18.0
200 1000 200 1000 200 1000
1.7 1.8 9.0 9.4 17.1 17.3
a
(1.6) (3.1) (0.8) (1.2) (1.8) (2.8)
Each sample contained 400 mg lÿ1 1-decanol. Conditions were as in Fig. 1(B). b Relative mean deviation (%) in parentheses (n3).
in the behaviour of the carbon to that of a cation exchanger, leading to the ammonium ion being slightly retained by the carbon cartridge. Carriers containing a doubly charged cation, Mg2, Ca2, Pb2 or Ba2, were investigated to elute singly charged ions, including NH 4 , from the cartridge. For reasons unknown at this stage, the best recoveries were achieved for samples containing up to 1000 mg lÿ1 sodium lauryl sulphate with a carrier of composition 0.1 M HNO3/0.02 M Pb2. Furthermore, this carrier composition is also suitable for analysis of samples containing neutral and cationic surfactants. A cartridge containing ca. 0.6 g of activated carbon is useful for more than 1000 injections of 50 ml sample or standard solutions containing 1000 mg lÿ1 surfactant at a ¯ow-rate of 1 ml minÿ1. When exhausted, the cartridge can be regenerated by ¯ushing with ethyl acetate for ca. 1 h at a ¯ow-rate of 1 ml minÿ1, but it is recommended, as this process is slow and incomplete, that the cartridge be discarded and replaced by a freshly packed one. A sampling rate approaching 30 hÿ1 was achieved at low concentrations with a precision of about 2%. The linear concentration range is 1±180 mg lÿ1 NH 4 with a detection limit of 0.15 mg lÿ1. In this system, the lifetime of the nonactin-based sensor was three months. 3.5. Analyses of effluent samples Industrial ef¯uent samples of variable composition (suspected to contain many organics such as chlorinated hydrocarbons, oil, grease, alcohols up to 12C, and surfactants) were analysed by GD-FI and on-line sample clean-up. The results are presented in Table 2. The satisfactory recoveries indicate that the method is reliable and that complex samples containing surfactants and various organic compounds are easily cleaned up using an on-line activated carbon cartridge in the sample introduction stream of a GD-FI system. 4. Conclusions Injected surfactants wet the gas-permeable membrane in a GD-FI system so that non-gaseous species pass through the membrane and non-ionic surfactants
H. Shen et al. / Analytica Chimica Acta 367 (1998) 193±199
199
Table 2 Analytical data for effluent samples Samplea
NH 4
ÿ1
(mg l ) Relative mean deviationb (%) Recovery (%)
a b
A
B
C
D
E
F
G
H
I
J
3.6 1.4 101.5
6.2 2.5 101.8
5.0 1.9 102.4
2.0 1.8 98.3
11.1 3.1 103.0
19.4 2.2 95.3
38.4 2.7 97.0
6.7 2.6 103.8
5.6 2.0 104.5
0.5 3.6 102.2
Samples were obtained from Probe Analytical, NSW, Australia, acidifed to pH 1 after collection. n3.
wet the PTFE membrane more rapidly than ionic surfactants. Serious interference in the determination of the ammonium ion using a nonactin-based sensor can be caused by cationic surfactants present in the sample and by K transported across a membrane that is wetted by a surfactant. It is therefore essential that surfactants are removed from samples before their potentiometric analysis by GD-FI. It has been demonstrated that an activated carbon cartridge is very effective for on-line removal of surfactants and other organic compounds from acidi®ed synthetic and ef¯uent samples in the analysis of ammonium ions by the GD-FI method. In the case of the ef¯uent samples, satisfactory recoveries were achieved for the ammonium ion in the range of 0.5 to ca. 40 mg lÿ1. Acknowledgements We thank the Australian Research Council for ®nancial support. HS is grateful to La Trobe University for the award of a Postgraduate Scholarship.
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