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Removal of aquaculture therapeutants by carbon adsorption
Aquaculture 192 Ž2001. 249–264 www.elsevier.nlrlocateraqua-online
Removal of aquaculture therapeutants by carbon adsorption 2: Multicomponent adsorption and the equilibrium behaviour of mixtures S.J. Aitcheson, J. Arnett, K.R. Murray ) , J. Zhang Department of Mechanical and Chemical Engineering, Heriot-Watt UniÕersity, Edinburgh EH14 4AS, UK Received 17 December 1999; received in revised form 7 June 2000; accepted 7 June 2000
Abstract This paper presents experimental data on the batch equilibrium adsorption behaviour of binary and ternary mixtures of aquaculture therapeutants ŽMalachite Green, Chloramine-T, and Oxytetracycline. and dissolved organic carbon ŽDOC. onto the coal-based granular activated carbon 207EA ŽSutcliffe-Speakman.. These data indicate directly the relative strengths of adsorption of some of the components under the conditions studied and confirm that DOC is generally less strongly adsorbed than most of the therapeutants at the concentrations of interest. This paper also reviews existing multicomponent equilibrium adsorption models and concludes that, for aquaculture applications, the Ideal Adsorbed Solution Theory ŽIAST. with the Freundlich isotherm is the model that is most suitable. This model was then tested using the experimental mixture data. The results show that IAST with the Freundlich isotherm is very successful at predicting the reduction in adsorption of DOC caused by the presence of competing therapeutants. It tends, however, to underestimate the amounts adsorbed of the therapeutants, particularly when these are present at higher concentrations. In general, the drop in DOC removal caused by the presence of competing therapeutants is quite small. IAST is clearly a good model to use in the design of batch carbon adsorption treatments for therapeutant removal. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon adsorption; Chloramine-T; Malachite-green; Oxytetracycline; Dissolved organic carbon
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Corresponding author. Tel.: q44-131-449-5111 ext. 4719; fax: q44-131-451-3129. E-mail address: [email protected] ŽK.R. Murray..
0044-8486r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 0 0 . 0 0 4 4 9 - X
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1. Introduction Organic chemical therapeutants are widely used in aquaculture to treat diseases ŽBjorkland et al., 1991; Borgan et al., 1981; Coyne et al., 1994.. Recycle aquaculture systems are generally purged while such therapeutants are in use because of the adverse effect on the biological filters. Activated carbon filtration has been extensively used in portable water treatment after ozone or chlorine treatment and for organic colour removal ŽKiely 1996; Henry and Heinke, 1996.. It has been put to good use in intensive re-cycle aquaculture systems ŽRosenthal and Black, 1993; Hirayama et al., 1988., removing traces of these potential toxic constituents from the re-cycling waters. Organic chemical therapeutants are also effectively removed from the water column by adsorption onto activated carbon ŽAitcheson et al., 2000.. However, these therapeutants will be in competition with these other aquaculture constituents for the available adsorption sites on the carbon. This competition is expected to inhibit therapeutant removal, leading to rises in concentrations that may be harmful to the fish. Purging, too, results in the direct release to the environment of these mixed aquaculture effluents ŽDissolved Organic Carbon, ŽDOC. and therapeutants., which in themselves are highly polluting and cause concern in their fate ŽSamuelson et al., 1994; Nygaard et al., 1992; Samuelson 1989.. This paper reports the results of a study of carbon adsorption of therapeutant and DOC components of aquaculture effluents. The aim of the study was to acquire the data necessary for the design of carbon filters. This would remove the need for purging and the consequent environmental pollution. Furthermore, the design of effective filters offers recycle AclosedB systems the potential to retain the use of highly effective but ecotoxic therapeutants that are currently banned because of the difficulty of controlling their release to the environment. Aitcheson et al. Ž2000. presented equilibrium data on single component adsorption of mixed DOC Žwith a composition similar to that of a real effluent., D-glucose, and the commonly used therapeutants. Adsorption efficiencies were reported plus Freundlich and Langmuir isotherm parameters for a wide range of conditions, together with adsorption capacities for bulk solutions of the therapeutants. This paper discusses which equilibrium multicomponent adsorption models are appropriate to aquaculture effluents. The single component data of Aitcheson et al. Ž2000. are used to model multicomponent adsorption, and the performance of the modelling against adsorption data from multicomponent mixtures is tested. The multicomponent mixture data give direct information on the relative adsorption strengths of competing adsorbates. The paper considers only equilibrium adsorption onto activated carbon. If this can be modelled reasonably accurately for mixed Žtherapeutantq DOC. aquaculture effluents, then it is possible to design batch treatments for these effluents. 2. Adsorption models Aquaculture effluents are typically dilute oxic aqueous solutions of a variety of Žmainly. organic compounds ŽKamstra et al., 1998; Dumas et al., 1998.. These sub-
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stances include the normal waste products of fish metabolism, components dissolved from uneaten feed, and any therapeutants that happen to be in use. The AbackgroundB Ži.e. non-therapeutant. DOC components of real effluents are rarely characterised in any great detail and may be somewhat variable in composition. In practice, this uncertainty in initial effluent composition will limit the accuracy which it is worth trying to achieve with multicomponent models ŽSumari, 1982.. Two long-standing models in particular have been applied to equilibrium adsorption onto activated carbon for organic substances in dilute aqueous solution. These are the Polanyi adsorption potential theory ŽPolanyi, 1920a,b. and the Ideal Adsorbed Solution Theory ŽMyers and Prausnitz, 1965.. 2.1. Polanyi adsorption potential theory (PAPT) The Polanyi adsorption potential theory was modified ŽRosene and Manes, 1976. for the competitive adsorption onto activated carbon of dissolved solids in aqueous solution. However, there are some serious practical difficulties with the application of this to aquaculture effluents. In particular, the solubility of each component of the effluent must be known accurately and the substances should not be soluble in each other. Many of the therapeutants of interest in aquaculture have few or no published solubility data, and our measurements have shown that some of the therapeutants are mutually soluble ŽArnett and Zhang, unpubl. data.. Also, Urano et al. Ž1981. showed that the Polanyi theory could only be applied to substances whose solubilities were small. This is not true of some of the adsorbates which are likely to be present in aquaculture effluents, such as simpler sugars Že.g. the aqueous solubility of D-glucose can exceed 700 grl at room temperature, Seidell, 1941.. For these reasons it was considered that the Polanyi adsorption theory is inappropriate for modelling multicomponent adsorption from aquaculture effluents. 2.2. Ideal adsorbed solution theory (IAST) The ideal adsorbed solution theory ŽIAST. was originally developed by Myers and Prausnitz Ž1965. for gas mixtures and modified by Radke and Prausnitz Ž1972. for dilute liquid solutions. Given information on the mass of adsorbent, the volume of liquid, the initial concentrations of each component in the liquid and their single component isotherm parameters, IAST predicts the amounts of each component adsorbed from a mixture at equilibrium. For a straightforward exposition of the IAST method, see Crittenden et al. Ž1985. or Sorial et al. Ž1993.. A key assumption of IAST is that, at equilibrium, the adsorbed mixture forms an ideal solution at constant spreading pressure Ži.e. the spreading pressures of each component are the same as that of the mixture.. The spreading pressure is the pressure, which the component would exert as a gas if the solvent molecules were removed; and, so, is closely related to the concentration of the component. In their review, Myers et al. Ž1992. reported that in general the IAST model performs very well in multicomponent predictions Aup to about 50% of the adsorption of saturationB, but that as the amount adsorbed increases, the measured amount adsorbed tends to be higher than was predicted
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by IAST. Their suggested explanations for this phenomenon include non-ideality of the adsorbed phase, non-uniformity of the carbon surface Žor surface energy. and mutual interaction between the solvent and the adsorbate. Since IAST was first introduced, there have been refinements which address these problems ŽGamba et al., 1989; Sorial et al., 1993.. IAST can be formulated as a set of equations, two of which depend on which type of isotherm equation is used to describe the single component data. One of these two equations computes the spreading pressure of each component and, with a judicious choice of isotherm equation, the spreading pressure equation may easily be integrated; and the result combined with the other IAST equations to give an analytical expression for the amount adsorbed by each component. These expressions are then treated as simultaneous equations and solved for the amount adsorbed of each component in the mixture. 2.3. Isotherm equation selection Few types of isotherm equation lead to analytical IAST expressions for the amount adsorbed of each component. Those that do include the Freundlich isotherm equation: 1r n q s KCeq
Ž 1.
where K and Ž1rn. are empirically determined constants, and the Myers isotherm equation: Ceq s Ž qrH . exp Ž KCeqp .
Ž 2.
where H, K and p are empirically determined constants. For isotherm equations that are not easily integrable, the spreading pressure equation must be evaluated numerically, adding considerably to the complexity of the IAST calculation Žthis is particularly true of the Langmuir isotherm.. Freundlich and Langmuir isotherms are easy to fit to single component data by least-squares linear regression, and parameters for these types of isotherm are routinely reported in the literature. To fit Myers isotherms requires a numerical non-linear regression procedure, and relatively few Myers isotherm parameters have been published to date. The region of low concentration, the Henry region, is described differently by the different isotherm equations, so that the results of multicomponent IAST modelling will be sensitive to which isotherm equation is picked, even if all types of isotherm equation fit the data equally well ŽRota et al., 1993.. Unfortunately, this low concentration region is not always accurately described by experiments. Further sensitivity to low concentration data can be introduced by the regression method used to fit the single-component isotherms to the experimental data ŽRichter et al., 1989.. One theoretical problem with the Freundlich isotherm is that it does not reduce to Henry’s Law at low concentrations, whereas both the Langmuir and Myers isotherms do. A further theoretical difference between isotherm equations is that the use of the Langmuir equation assumes an energetically homogeneous surface, whereas the Freundlich equation can be related to the distribution of energy ŽMyers et al., 1992..
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Finally, one of the most important features of the isotherm equation is that it should fit well the experimental single component data. The Freundlich isotherm equation generally fitted the single component isotherm data for chemical therapeutants adequately, with only a few exceptions at low temperatures, while the Langmuir isotherm equation did not ŽAitcheson et al., 2000.. From the above considerations, it is concluded that the Freundlich isotherm is a reasonable choice of isotherm equation for the application of IAST to aquaculture effluents. Freundlich isotherm parameters are commonly reported in the literature, thus its use will make it easy to include additional aquaculture effluent components into the modelling where published isotherm parameters already exist. The uncertainties in the initial composition of real effluents are such that the thermodynamic inconsistency of the Freundlich isotherm does not introduce a great deal of extra error into the predictions. In this paper the formulation proposed by Crittenden et al. Ž1985. of IAST is used with the Freundlich isotherm to predict the amounts adsorbed from binary and ternary mixtures.
3. Materials and methods The carbon used in all of the experiments was the coal-based granular activated carbon 207EA supplied by Sutcliffe Speakman. The experimental procedures and the analytical methods follow those described in Aitcheson et al. Ž2000., except that batch equilibrium adsorption experiments were carried out for mixtures, rather than for single components. The carbon was washed with distilled water, oven dried at 1008C for 24 h and crushed to a mesh size of 0.6–1.7 mm. Each isotherm experiment involved allowing time Žnominally 20–30 h. for equilibration of 100 " 0.1 ml of bulk solution with 0.5 " 0.01 g of activated carbon. Temperature was controlled to within "0.58C by immersing the experimental flasks in a thermostatically controlled shaker water bath. The pH and ionic strength were controlled by means of buffer solutions comprising various mixtures of KH 2 PO4 , Na 2 HPO4 , Na 2 B 2 O 7 , NaCl and HCl ŽAitcheson et al., 2000.. The experiments were conducted at pH values of 6, 7 and 8.5, temperatures of 58C, 108C and 208C, and ionic strengths 0.2, 2 and 20 mM, while D-glucose ŽC 6 O6 H 12 . and mixed Dissolved Organic Carbon ŽDOC. were introduced to simulate any simpler sugars or background organics that may be present in an aquaculture effluent, see Aitcheson et al. Ž2000.. The concentrations of Malachite Green, Chloramine-T, Oxytetracycline and D-glucose were measured by UV spectrophotometry ŽPye Unicam SP1700., but with a correction for the presence of the other components ŽZhang, unpubl. data.. The optical absorbance Ž A o . of the dilute binary and ternary mixtures of therapeutants was measured at the wavelength of maximum absorption. The maximum absorption peak Ž l s 615 nm. was unchanged for Malnachite Green in the presence of other therapeutants. The measurement of oxtetracycline was only affected at solution concentrations above 0.5 ppm; while absorbance of Chloramine-T was strongly dependent on the
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Table 1 The wavelengths of therapeutants at maximum absorption peaks in binary and ternary mixtures ŽZhang, unpubl. data. Therapeutants
Conc. Žppm.
Component 1 max wave length Žnm.
Component 2 max wave length Žmm.
Component 3 max wave length Žnm.
Malachite Green ŽMG. Chloramine-T ŽCT. Oxytetracycline ŽOXY. MGqCT MGqOXY CTqOXY MGqCTqOXY
2 2 2 2 2 2 2
615 – – 615 615 – 615
– 220 – 210 – 220 210
– – 275:350 – 310 270 260
mixture concentrations and account had to be taken by measuring absorbance at wavelength l s 275 nm, as well as at 200 nm. A detailed study of the influence of absorbance of Malachite, Chloramine-T and Oxytetracycline in the binary and ternary mixtures resulted in a summary, ŽTable 1. of the wavelengths at the maximum absorption peaks ŽZhang, unpubl. data.. This was for
Table 2 Summary of adsorption experiments onto activated charcoal of therapeutants, D-glucose and mixed DOC Ža. Binary Mixtures
Equal starting concentrations Žppm.
Variable pH, ionic strength ŽmM. and temperature Ž8C.
Malachite Green, Chloramine-T Chloramine-T, Oxytetracycline
1–10 ppm 1–10 ppm
pH 6–8.5, 0.2–20 mM, 5–208C
Binary mixtures
Fixed ionic strength ŽmM. and D-glucose Conc. Žppm.
Variable pH, therapeutant conc. Žppm. and temperature Ž8C.
Malachite Green, D-glucose Chloramine-T, D-glucose Oxytetracycline, D-glucose
2.0 mM and 20 ppm 2.0 mM and 20 ppm 2.0 mM and 20 ppm
pH 6–7, 1–10 ppm, 10–208C pH 6–7, 1–10 ppm, 10–208C pH 6–7, 1–10 ppm, 10–208C
Ternary mixtures two therapeutants plus either D-glucose or mixed DOC
Fixed ionic strength ŽmM., temperature Ž8C. and pH
Fixed concentration ppm of therapeutants, D-glucose or DOC
D-Glucose, Malachite Green, Chloramine-T DOC, Malachite Green, Chloramine-T D-Glucose, Chloramine-T, Oxytetracycline
2 mM, 208C and pH 7
20 ppm, 10 ppm, 10 ppm
2 mM, 208C and pH 7
20 ppm, 10 ppm, 10 ppm
2 mM, 208C and pH 7
20 ppm, 10 ppm, 10 ppm
Žb.
Žc.
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prepared solutions of equal therapeutant concentrations, 2 ppm, in distilled water. This work has been extended to cover a range of mixture options, staring from a primary concentration of a particular therapeutant Ž2 ppm. and increasing another therapeutant’s concentration from zero to 2 ppm. A paper outlining the measurement of these low concentrations of mixed therapeutants by UV spectrophotometry and the problems experienced in obtaining meaningful data is in preparation ŽZhang in prep... The range of mixtures of various starting concentrations ŽCo., ionic strengths ŽmM. equilibrium temperatures and pH values are summarised briefly in Table 2, while a complete data set of the experiments is available from the author.
Fig. 1. Comparison of isotherms of amount adsorbed Ž q . on activated carbon Ž c . versus initial concentration Ž Ceq ŽM.. Žmolesrl. for Chloramine-T and Oxytetracycline in binary mixture experiments and in single component experiments.
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4. Results and discussions 4.1. Binary mixtures of therapeutants Figs. 1 and 2 illustrate the isotherms for each component in the binary mixtures of therapeutants described in Table 2a. For comparison, these figures also show the isotherms measured for the single components under the same conditions. The single
Fig. 2. Comparison of isotherms also for Chloramine-T and Malachite Green in binary mixture experiments and in single component experiments.
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component oxytetracycline isotherms were originally measured only for initial concentrations up to 1 ppm, and are straight lines when plotted on a log–log scale. These have been extrapolated to the higher concentration region of the figures Žbroken lines in Fig. 1.. The extent to which the isotherm from the mixture is shifted downwards compared with that of the single component is a crude measure of the strength of adsorption of that
Fig. 3. Comparison of Freundlich parameters for single therapeutants ŽAitcheson et al., 2000. with each of the same components in a binary mixture of therapeutants.
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component: the component whose isotherm is shifted the more by the introduction of a second component is the one which is the more weakly adsorbed ŽLo and Alok, 1996.. Temperature, pH and ionic strength variables do not appear to create much of an overall weakening effect on the adsorption of oxytetracycline except at neutral pH. This is more pronounced for Chloramine-T, where, relative to the adsorption potential of the single component, there is a marked weakening in the adsorption potential at pH 7. Overall in the Chloramine-Tq Oxytetracycline mixtures ŽFig. 1. the data indicate that under most conditions Oxytetracycline is more strongly adsorbed onto carbon 207EA than Chloramine-T. In the Chloramine-Tq Malachite Green mixtures, the relative strengths of adsorption of the two components is more variable ŽFig. 2.. Adsorption of both components is enhanced at pH 6 and 58C, while Malachite Green is likewise more strongly adsorbed than Chloramine-T at pH 7 and 108C. At higher temperature Ž208C., Chloramine-T seems to be at least as strongly adsorbed as Malachite Green. These suggest that under most conditions the order of adsorption strength of these three therapeutants is Oxytetracycline) Chloramine-TG Malachite Green.
Table 3 Single-component Freundlich parameters used in ideal adsorbed solution theory ŽIAST. modelling of adsorption from binary and ternary mixtures. I s ionic strength. K and 1r n are emperically determined constants Conditions
Adsorbate
Freundlich K
Freundlich 1r n
R2
pH 6, I s 2 mM, 58C
Chloramine-T Malachite Green Oxytetracycline Chloramine-T Malachite Green Oxytetracycline D-glucose Chloramine-T Malachite Green Oxytetracycline Chloramine-T Malachite Green Oxytetracycline D-glucose Chloramine-T Malachite Green Oxytetracycline D-glucose Mixed DOC Chloramine-T Malachite Green Oxytetracyline D-glucose Chloramine-T Malachite Green Oxytetracycline
9466 38.582 353.42 0.5413 26.083 757.97 15 225 18.699 7.6404 778.87 57.495 2.00Eq10 911.82 15225 3.1232 41.497 1023.2 15 225 6.5216 28.104 1200.4 682.19 15 225 61.459 3662 659.48
1.3806 0.8098 0.9892 0.4194 0.7277 0.9836 1.4886 0.6868 0.5768 0.9985 0.7911 2.0408 0.9992 1.4886 0.5643 0.7825 0.9985 1.4886 0.6426 0.7458 0.8822 0.991 1.4886 0.819 0.8812 1.001
0.9747 0.8734 1.0 0.9805 0.9334 0.9993 0.9986 0.9677 0.8504 1.0 0.9816 0.9939 0.9999 0.9986 0.9571 0.8898 1.0 0.9986 0.6011 0.9883 0.9538 0.9999 0.9986 0.9873 0.8441 0.9998
pH 6, I s 2 mM, 108C
pH 6, I s 0.2 mM, 20 8C
pH 6, I s 2 mM, 208C
pH 7, I s 2 mM, 108C
pH 7, I s 2 mM, 208C
pH 8.5, I s 20 mM, 108C
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Aitcheson et al. Ž2000. showed that all the single-component Freundlich parameters obtained under any conditions for a particular therapeutant and a particular carbon define a straight-line trend. The Freundlich parameters obtained for Chloramine-T and Malachite Green when a second therapeutant is present also fall on the straight-line trends defined by the single component data ŽFig. 3., while for Oxytetracycline the single-component data were tightly clustered on the 1rn versus ln K plot, but the data from binary mixtures define a straight-line trend that passes through this cluster of the single-component data. Collectively, these parameters define a strong linear relationship that is essentially independent of experimental conditions.
Fig. 4. Performance of the ideal adsorbed solution theory ŽIAST. in prediction of amounts adsorbed Ž q . from a binary mixture of Chloramine-T and Oxytetracycline for various conditions and initial adsorbate concentrations.
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4.2. Other mixture experiments The amounts of the therapeutants adsorbed in the binary therapeutantq D-glucose experiments ŽTable 2b. were compared with the amounts adsorbed when no other components were present, i.e. single component isotherms Figs. 1 and 2. The resultant isotherms showed that the presence of 20 ppm D-glucose had no significant effect on the adsorption of any of the therapeutants, which supports the notion that the D-glucose is much more weakly adsorbed than the therapeutants.
Fig. 5. Performance of the ideal adsorbed solution theory ŽIAST. in prediction of amounts adsorbed Ž q . from a binary mixture of Chloramine-T and Malachite Green for various conditions and initial adsorbate concentrations.
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In the ternary mixture experiments Table 2c, D-glucose adsorption was slightly lower in the presence of therapeutants than in similar experiments where no other component was present. This supports the likelihood of a reduction in DOC adsorption by activated carbon when therapeutants are also present. 4.3. Performance of the IAST For all of the mixture experiments, Table 1, the Crittenden et al. Ž1985. formulation of the IAST was used to predict the amounts adsorbed of the different components in the mixtures. The Freundlich isotherm parameters used in the calculations are listed in Table 3 and are based on values ŽAitcheson et al., 2000. for single components. Figs. 4 and 5 compare the observed amounts of each component adsorbed in binary mixtures of therapeutants with the amount predicted by IAST, for various experimental conditions. Most of the data points lie close to the prediction line in each figure, indicating good agreement between the predicted and observed values. In only a few cases do the observed and predicted amounts adsorbed differ by more than 20%. Discrepancies between observed and predicted values arise mainly for mixtures having higher initial concentrations Ž10 ppm.. In the Chloramine-T and Malachite Green mixtures, all discrepancies were under-predictions by IAST of the amount adsorbed, while in the Chloramine-T and Oxytetracycline mixtures the discrepancies included both over- and under-predictions, with more over-predictions for Chloramine-T than for Oxytetracycline. Fig. 6 shows comparisons between the observed amount adsorbed and that predicted by IAST for a single therapeutant in the presence of 20 ppm D-glucose. All but two data
Fig. 6. Performance of the Ideal Adsorbed Solution Theory ŽIAST. in prediction of amounts of therapeutants adsorbed Žq. from a binary mixture of one therapeutant Ž Co either 1 or ppm. plus D-glucose.
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Fig. 7. Performance of the ideal adsorbed solution theory ŽIAST. in prediction of amounts adsorbed Ž q . from ternary mixtures. Each mixture comprised two therapeutants plus either D-glucose or mixed DOC from the artificial effluent.
points lie Žwithin analytical error. on the prediction line. The discrepant values correspond to a fairly high initial concentration of Chloramine-T Ž10 ppm., which is two to five times the normal concentration used for general external antibacterial treatment in aquaculture. Fig. 7 compares the observed amount adsorbed with that predicted by IAST for components of ternary mixtures comprising two therapeutants, plus either D-glucose or mixed DOC. Again, there is some agreement between observed and predicted values, with all discrepancies erring on the side of underprediction by IAST of the extent of therapeutant adsorption.
5. Conclusions This study shows that, for mixed aquaculture effluents with low therapeutant concentrations Žless than 10 ppm., IAST with the Freundlich isotherm should predict reasonably accurately their multicomponent adsorption behaviour with respect to activated carbon ŽFigs. 6 and 7.. On the basis of these results, and given desired levels of therapeutants and DOC in the treated effluent, it should be possible to design a simple batch treatment procedure involving dosing a tank with an appropriate quantity of carbon and allowing effluent, which would otherwise have been purged, to equilibrate with the carbon. For example, taking the data of a ternary mixture of the two therapeutants Chloramine-T and Malachite Green, it could be predicted that an aquacul-
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ture effluent volume of 3 m3 containing 10 ppm levels of both therapeutants could be reduced to 3 ppm concentrations by the addition of 15 kg of activated carbon Ž207EA.. A further 20 kg of fresh activated carbon should reduce the concentration levels to approximately 0.1–0.2 ppm. For effluents with higher initial concentrations of adsorbates the results indicate that the total amount adsorbed from multicomponent aquaculture effluents is likely to be greater than predicted by IAST. This phenomenon seems to be quite general in the application of IAST to the adsorption of organic substances ŽMyers et al., 1992.. The enhancement of adsorption that was observed when any additional organic component was present applies mainly to the therapeutants. DOC adsorption seems to be reduced by the presence of therapeutants, but the size of this reduction seems to be relatively small. The enhancement beyond the expectations of IAST of adsorption when other components are present indicates that processes unaccounted for by the IAST model are occurring at the carbon surface. Sorial et al. Ž1993. found that greater than expected Žby IAST. adsorption of phenols occurred mainly under oxic conditions, and suggested that the presence of molecular oxygen was leading to polymerisation on the carbon surface. The therapeutants Malachite Green, Chloramine-T and Oxytetracycline are much more strongly adsorbed than DOC or D-glucose onto granular activated carbon. The presence of more than one component tends to enhance adsorption considerably, especially of therapeutants. The linear relationship between the Freundlich parameters 1rn and ln K observed for single therapeutants also holds for these substances when other components are present, even when these other components compete strongly for sites on the carbon. This fact is a valuable constraint on the equilibrium adsorption behaviour Žas described by isotherm parameters. of any adsorbate, which holds regardless of conditions or of which other components are present. The IAST combined with the Freundlich isotherm appears to be a good model for describing the equilibrium adsorption from dilute aqueous mixtures of DOC or D-glucose and one or more therapeutants. It is thus very suitable for modelling equilibrium adsorption treatment of aquaculture effluents when therapeutants are in use. IAST appears to underestimate adsorption of therapeutants at higher initial concentrations. Thus, for the purpose of environmental protection, IAST tends to err on the side of safety in predicting therapeutant removal.
Acknowledgements We gratefully acknowledge financial support from the Biology and Biotechnology Science Research Council Žcontract CTE01769. and from Heriot-Watt University.
References Aitcheson, S.J., Arnett, J., Murray, K.R., Zhang, J., 2000. Removal of aquaculture therapeutants by carbon adsorption. 1: Equilibrium adsorption behaviour of single components. Aquaculture 183, 269–284. Bjorkland, H.V., Rabergh, C.MI., Bylund, G., 1991. Residues of oxilinic acid and oxytetracycline in fish and sediments from fish farms. Aquaculture 97, 85–96.
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Borgan, A., Odegaard, S., Bergsjo, T., 1981. Temperature related absorption and excretion of sulphadimidine in Rainbow Trout. Acta. Vet. Scand. 22, 211–217. Coyne, R., Hiney, M., O’Connor, B., Kerry, J., Cazabon, D., Smith, P., 1994. Concentration and persistence of oxytetracycline in sediments under a marine salmon farm. Aquaculture 123, 31–42. Crittenden, J.C., Luft, P., Hand, D.W., Oravitz, J.L., Loper, S.W., Arl, M., 1985. Prediction of multicomponent adsorption equilibria using ideal adsorbed solution theory. Environ. Sci. Technol. 19, 1037–1043. Dumas, A., Laliberte, ´ G., Lessard, P., de la Noue, ¨ J., 1998. Biotreatment of fish farm effluents using cyanobacterium Phormidium bohneri. Aquacult. Eng. 17, 57–68. Gamba, G., Rota, R., Storti, G., Carra, ` S., Morbidelli, M., 1989. Adsorbed solution theory models for multicomponent adsorption equilibria. J. Am. Inst. Chem. Eng. 35, 959. Henry, J.G., Heinke, G.W., 1996. Environmental Science and Engineering. 2nd edn. Prentice Hall, Upper Saddle River, N.S. Hirayama, K., Mizuma, H., Mizue, Y., 1988. The accumulation of dissolved organic substances in closed recirculating culture systems. Aquacult. Eng. 7, 73–87. Kamstra, A., Van der Heul, J.W., Nijhof, M., 1998. Performance and optimisation of trickling filters on eel farms. Aquacult. Eng. 17, 175–192. Kiely, G., 1996. Environmental Engineering. McGraw-Hill Publishers, Maidenhead Berks., UK. Lo, I.M.C., Alok, P.A., 1996. Computer simulation of activated carbon adsorption for multicomponent systems. Environ. Int. 22, 239–252. Myers, A.L., Prausnitz, J.M., 1965. Thermodynamics of mixed-gas adsorption. J. Am. Inst. Chem. Eng. 11, 121–127. Myers, A.L., Belfort, G., Suzuki, M., 1992. Prediction of multicomponent adsorption equilibrium. Discussions on energy of adsorption in relation to adsorption equilibrium. Int. Chem. Eng. 32, 585–590. Nygaard, K., Lunstead, B.T., Hektoen, H., Berge, J.A., Hormazabal, V., 1992. Resistance to oxytetracycline, oxilinic acid and furazolidone in bacteria from marine sediments. Aquaculture 104, 31–36. Polanyi, M., 1920a. Neueres uber ¨ adsorption und ursache der adsorptionskrafte. ¨ Z. Elektrochem. 26, 370–374. Polanyi, M., 1920b. Adsorption aus Losungen beschrankt stoffe. Z. Phys. 2, 111–116. ¨ ¨ loslicher ¨ Radke, C.J., Prausnitz, J.M., 1972. Thermodynamics of multisolute adsorption from dilute liquid solutions. J. Am. Inst. Chem. Eng. 18, 761–768. Richter, E., Schutz, W., Myers, A.L., 1989. Effect of adsorption equation on prediction of multicomponent adsorption equilibrium by the ideal adsorbed solution theory. Chem. Eng. Sci. 44, 1609–1616. Rosene, M.R., Manes, M., 1976. Application of the Polanyi adsorption potential theory to adsorption from solution on activated carbon. VII Competitive adsorption of solids from water solution. J. Phys. Chem. 80, 953–959. Rosenthal, H., Black, E.A., 1993. Recirculation systems in aquaculture. In: Wang, J.K. ŽEd.., Proceedings of an Aquaculture Engineering Conference, 21–30 June 1993. Am. Soc. of Agri. Eng., St. Joseph, MI. pp. 284–294. Rota, R., Gamba, G., Morbidelli, M., 1993. On the use of the adsorbed solution theory for designing adsorption separation units. Sep. Technol. 3, 230–237. Samuelson, O.B., 1989. Degragation of oxytetracycline in seawater at two different temperatures and light intensities, and the persistence of oxytetracycline in the sediment from a fish farm. Aquaculture 83, 7–16. Samuelson, O.B., Lunestad, B.T., Ervik, A., Fjelde, S., 1994. Stability of antibacterial agents in an artificial marine aquaculture sediment studied under laboratory conditions. Aquaculture 126, 283–290. Seidell, A., 1941. 3rd edn. Solubilities of Organic Compounds vol. 2, Van Nostrand, New York, 926 pp. Sorial, G.A., Suidan, M.T., Vidic, R.D., Maloney, S.W., 1993. Competitive adsorption of phenols on GAC. I: Adsorption equilibrium. J. Environ. Eng. 119, 1026–1043. Sumari, O., 1982. A report on fish-farm effluents in Finland. In: Alabaster, J.S. ŽEd..., Silkeborg. Report of the EIFAC workshop on fish-farm effluents, Denmark, 26–28 May 1981, EIFAC Tech. Paper No. 41. Food and Agriculture Organisation, Rome, pp. 21–27. Urano, K., Koichi, N., Nakazawa, Y., 1981. Equilibria for adsorption of organic compounds on activated carbons in aqueous solutions.1. Modified Freundlich isotherm equation and adsorption potentials of organic compounds. J. Coll. Inter. Sci. 81, 477–485.
Removal of aquaculture therapeutants by carbon adsorption 2: Multicomponent adsorption and the equilibrium behaviour of mixtures S.J. Aitcheson, J. Arnett, K.R. Murray ) , J. Zhang Department of Mechanical and Chemical Engineering, Heriot-Watt UniÕersity, Edinburgh EH14 4AS, UK Received 17 December 1999; received in revised form 7 June 2000; accepted 7 June 2000
Abstract This paper presents experimental data on the batch equilibrium adsorption behaviour of binary and ternary mixtures of aquaculture therapeutants ŽMalachite Green, Chloramine-T, and Oxytetracycline. and dissolved organic carbon ŽDOC. onto the coal-based granular activated carbon 207EA ŽSutcliffe-Speakman.. These data indicate directly the relative strengths of adsorption of some of the components under the conditions studied and confirm that DOC is generally less strongly adsorbed than most of the therapeutants at the concentrations of interest. This paper also reviews existing multicomponent equilibrium adsorption models and concludes that, for aquaculture applications, the Ideal Adsorbed Solution Theory ŽIAST. with the Freundlich isotherm is the model that is most suitable. This model was then tested using the experimental mixture data. The results show that IAST with the Freundlich isotherm is very successful at predicting the reduction in adsorption of DOC caused by the presence of competing therapeutants. It tends, however, to underestimate the amounts adsorbed of the therapeutants, particularly when these are present at higher concentrations. In general, the drop in DOC removal caused by the presence of competing therapeutants is quite small. IAST is clearly a good model to use in the design of batch carbon adsorption treatments for therapeutant removal. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon adsorption; Chloramine-T; Malachite-green; Oxytetracycline; Dissolved organic carbon
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Corresponding author. Tel.: q44-131-449-5111 ext. 4719; fax: q44-131-451-3129. E-mail address: [email protected] ŽK.R. Murray..
0044-8486r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 0 0 . 0 0 4 4 9 - X
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1. Introduction Organic chemical therapeutants are widely used in aquaculture to treat diseases ŽBjorkland et al., 1991; Borgan et al., 1981; Coyne et al., 1994.. Recycle aquaculture systems are generally purged while such therapeutants are in use because of the adverse effect on the biological filters. Activated carbon filtration has been extensively used in portable water treatment after ozone or chlorine treatment and for organic colour removal ŽKiely 1996; Henry and Heinke, 1996.. It has been put to good use in intensive re-cycle aquaculture systems ŽRosenthal and Black, 1993; Hirayama et al., 1988., removing traces of these potential toxic constituents from the re-cycling waters. Organic chemical therapeutants are also effectively removed from the water column by adsorption onto activated carbon ŽAitcheson et al., 2000.. However, these therapeutants will be in competition with these other aquaculture constituents for the available adsorption sites on the carbon. This competition is expected to inhibit therapeutant removal, leading to rises in concentrations that may be harmful to the fish. Purging, too, results in the direct release to the environment of these mixed aquaculture effluents ŽDissolved Organic Carbon, ŽDOC. and therapeutants., which in themselves are highly polluting and cause concern in their fate ŽSamuelson et al., 1994; Nygaard et al., 1992; Samuelson 1989.. This paper reports the results of a study of carbon adsorption of therapeutant and DOC components of aquaculture effluents. The aim of the study was to acquire the data necessary for the design of carbon filters. This would remove the need for purging and the consequent environmental pollution. Furthermore, the design of effective filters offers recycle AclosedB systems the potential to retain the use of highly effective but ecotoxic therapeutants that are currently banned because of the difficulty of controlling their release to the environment. Aitcheson et al. Ž2000. presented equilibrium data on single component adsorption of mixed DOC Žwith a composition similar to that of a real effluent., D-glucose, and the commonly used therapeutants. Adsorption efficiencies were reported plus Freundlich and Langmuir isotherm parameters for a wide range of conditions, together with adsorption capacities for bulk solutions of the therapeutants. This paper discusses which equilibrium multicomponent adsorption models are appropriate to aquaculture effluents. The single component data of Aitcheson et al. Ž2000. are used to model multicomponent adsorption, and the performance of the modelling against adsorption data from multicomponent mixtures is tested. The multicomponent mixture data give direct information on the relative adsorption strengths of competing adsorbates. The paper considers only equilibrium adsorption onto activated carbon. If this can be modelled reasonably accurately for mixed Žtherapeutantq DOC. aquaculture effluents, then it is possible to design batch treatments for these effluents. 2. Adsorption models Aquaculture effluents are typically dilute oxic aqueous solutions of a variety of Žmainly. organic compounds ŽKamstra et al., 1998; Dumas et al., 1998.. These sub-
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stances include the normal waste products of fish metabolism, components dissolved from uneaten feed, and any therapeutants that happen to be in use. The AbackgroundB Ži.e. non-therapeutant. DOC components of real effluents are rarely characterised in any great detail and may be somewhat variable in composition. In practice, this uncertainty in initial effluent composition will limit the accuracy which it is worth trying to achieve with multicomponent models ŽSumari, 1982.. Two long-standing models in particular have been applied to equilibrium adsorption onto activated carbon for organic substances in dilute aqueous solution. These are the Polanyi adsorption potential theory ŽPolanyi, 1920a,b. and the Ideal Adsorbed Solution Theory ŽMyers and Prausnitz, 1965.. 2.1. Polanyi adsorption potential theory (PAPT) The Polanyi adsorption potential theory was modified ŽRosene and Manes, 1976. for the competitive adsorption onto activated carbon of dissolved solids in aqueous solution. However, there are some serious practical difficulties with the application of this to aquaculture effluents. In particular, the solubility of each component of the effluent must be known accurately and the substances should not be soluble in each other. Many of the therapeutants of interest in aquaculture have few or no published solubility data, and our measurements have shown that some of the therapeutants are mutually soluble ŽArnett and Zhang, unpubl. data.. Also, Urano et al. Ž1981. showed that the Polanyi theory could only be applied to substances whose solubilities were small. This is not true of some of the adsorbates which are likely to be present in aquaculture effluents, such as simpler sugars Že.g. the aqueous solubility of D-glucose can exceed 700 grl at room temperature, Seidell, 1941.. For these reasons it was considered that the Polanyi adsorption theory is inappropriate for modelling multicomponent adsorption from aquaculture effluents. 2.2. Ideal adsorbed solution theory (IAST) The ideal adsorbed solution theory ŽIAST. was originally developed by Myers and Prausnitz Ž1965. for gas mixtures and modified by Radke and Prausnitz Ž1972. for dilute liquid solutions. Given information on the mass of adsorbent, the volume of liquid, the initial concentrations of each component in the liquid and their single component isotherm parameters, IAST predicts the amounts of each component adsorbed from a mixture at equilibrium. For a straightforward exposition of the IAST method, see Crittenden et al. Ž1985. or Sorial et al. Ž1993.. A key assumption of IAST is that, at equilibrium, the adsorbed mixture forms an ideal solution at constant spreading pressure Ži.e. the spreading pressures of each component are the same as that of the mixture.. The spreading pressure is the pressure, which the component would exert as a gas if the solvent molecules were removed; and, so, is closely related to the concentration of the component. In their review, Myers et al. Ž1992. reported that in general the IAST model performs very well in multicomponent predictions Aup to about 50% of the adsorption of saturationB, but that as the amount adsorbed increases, the measured amount adsorbed tends to be higher than was predicted
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by IAST. Their suggested explanations for this phenomenon include non-ideality of the adsorbed phase, non-uniformity of the carbon surface Žor surface energy. and mutual interaction between the solvent and the adsorbate. Since IAST was first introduced, there have been refinements which address these problems ŽGamba et al., 1989; Sorial et al., 1993.. IAST can be formulated as a set of equations, two of which depend on which type of isotherm equation is used to describe the single component data. One of these two equations computes the spreading pressure of each component and, with a judicious choice of isotherm equation, the spreading pressure equation may easily be integrated; and the result combined with the other IAST equations to give an analytical expression for the amount adsorbed by each component. These expressions are then treated as simultaneous equations and solved for the amount adsorbed of each component in the mixture. 2.3. Isotherm equation selection Few types of isotherm equation lead to analytical IAST expressions for the amount adsorbed of each component. Those that do include the Freundlich isotherm equation: 1r n q s KCeq
Ž 1.
where K and Ž1rn. are empirically determined constants, and the Myers isotherm equation: Ceq s Ž qrH . exp Ž KCeqp .
Ž 2.
where H, K and p are empirically determined constants. For isotherm equations that are not easily integrable, the spreading pressure equation must be evaluated numerically, adding considerably to the complexity of the IAST calculation Žthis is particularly true of the Langmuir isotherm.. Freundlich and Langmuir isotherms are easy to fit to single component data by least-squares linear regression, and parameters for these types of isotherm are routinely reported in the literature. To fit Myers isotherms requires a numerical non-linear regression procedure, and relatively few Myers isotherm parameters have been published to date. The region of low concentration, the Henry region, is described differently by the different isotherm equations, so that the results of multicomponent IAST modelling will be sensitive to which isotherm equation is picked, even if all types of isotherm equation fit the data equally well ŽRota et al., 1993.. Unfortunately, this low concentration region is not always accurately described by experiments. Further sensitivity to low concentration data can be introduced by the regression method used to fit the single-component isotherms to the experimental data ŽRichter et al., 1989.. One theoretical problem with the Freundlich isotherm is that it does not reduce to Henry’s Law at low concentrations, whereas both the Langmuir and Myers isotherms do. A further theoretical difference between isotherm equations is that the use of the Langmuir equation assumes an energetically homogeneous surface, whereas the Freundlich equation can be related to the distribution of energy ŽMyers et al., 1992..
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Finally, one of the most important features of the isotherm equation is that it should fit well the experimental single component data. The Freundlich isotherm equation generally fitted the single component isotherm data for chemical therapeutants adequately, with only a few exceptions at low temperatures, while the Langmuir isotherm equation did not ŽAitcheson et al., 2000.. From the above considerations, it is concluded that the Freundlich isotherm is a reasonable choice of isotherm equation for the application of IAST to aquaculture effluents. Freundlich isotherm parameters are commonly reported in the literature, thus its use will make it easy to include additional aquaculture effluent components into the modelling where published isotherm parameters already exist. The uncertainties in the initial composition of real effluents are such that the thermodynamic inconsistency of the Freundlich isotherm does not introduce a great deal of extra error into the predictions. In this paper the formulation proposed by Crittenden et al. Ž1985. of IAST is used with the Freundlich isotherm to predict the amounts adsorbed from binary and ternary mixtures.
3. Materials and methods The carbon used in all of the experiments was the coal-based granular activated carbon 207EA supplied by Sutcliffe Speakman. The experimental procedures and the analytical methods follow those described in Aitcheson et al. Ž2000., except that batch equilibrium adsorption experiments were carried out for mixtures, rather than for single components. The carbon was washed with distilled water, oven dried at 1008C for 24 h and crushed to a mesh size of 0.6–1.7 mm. Each isotherm experiment involved allowing time Žnominally 20–30 h. for equilibration of 100 " 0.1 ml of bulk solution with 0.5 " 0.01 g of activated carbon. Temperature was controlled to within "0.58C by immersing the experimental flasks in a thermostatically controlled shaker water bath. The pH and ionic strength were controlled by means of buffer solutions comprising various mixtures of KH 2 PO4 , Na 2 HPO4 , Na 2 B 2 O 7 , NaCl and HCl ŽAitcheson et al., 2000.. The experiments were conducted at pH values of 6, 7 and 8.5, temperatures of 58C, 108C and 208C, and ionic strengths 0.2, 2 and 20 mM, while D-glucose ŽC 6 O6 H 12 . and mixed Dissolved Organic Carbon ŽDOC. were introduced to simulate any simpler sugars or background organics that may be present in an aquaculture effluent, see Aitcheson et al. Ž2000.. The concentrations of Malachite Green, Chloramine-T, Oxytetracycline and D-glucose were measured by UV spectrophotometry ŽPye Unicam SP1700., but with a correction for the presence of the other components ŽZhang, unpubl. data.. The optical absorbance Ž A o . of the dilute binary and ternary mixtures of therapeutants was measured at the wavelength of maximum absorption. The maximum absorption peak Ž l s 615 nm. was unchanged for Malnachite Green in the presence of other therapeutants. The measurement of oxtetracycline was only affected at solution concentrations above 0.5 ppm; while absorbance of Chloramine-T was strongly dependent on the
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Table 1 The wavelengths of therapeutants at maximum absorption peaks in binary and ternary mixtures ŽZhang, unpubl. data. Therapeutants
Conc. Žppm.
Component 1 max wave length Žnm.
Component 2 max wave length Žmm.
Component 3 max wave length Žnm.
Malachite Green ŽMG. Chloramine-T ŽCT. Oxytetracycline ŽOXY. MGqCT MGqOXY CTqOXY MGqCTqOXY
2 2 2 2 2 2 2
615 – – 615 615 – 615
– 220 – 210 – 220 210
– – 275:350 – 310 270 260
mixture concentrations and account had to be taken by measuring absorbance at wavelength l s 275 nm, as well as at 200 nm. A detailed study of the influence of absorbance of Malachite, Chloramine-T and Oxytetracycline in the binary and ternary mixtures resulted in a summary, ŽTable 1. of the wavelengths at the maximum absorption peaks ŽZhang, unpubl. data.. This was for
Table 2 Summary of adsorption experiments onto activated charcoal of therapeutants, D-glucose and mixed DOC Ža. Binary Mixtures
Equal starting concentrations Žppm.
Variable pH, ionic strength ŽmM. and temperature Ž8C.
Malachite Green, Chloramine-T Chloramine-T, Oxytetracycline
1–10 ppm 1–10 ppm
pH 6–8.5, 0.2–20 mM, 5–208C
Binary mixtures
Fixed ionic strength ŽmM. and D-glucose Conc. Žppm.
Variable pH, therapeutant conc. Žppm. and temperature Ž8C.
Malachite Green, D-glucose Chloramine-T, D-glucose Oxytetracycline, D-glucose
2.0 mM and 20 ppm 2.0 mM and 20 ppm 2.0 mM and 20 ppm
pH 6–7, 1–10 ppm, 10–208C pH 6–7, 1–10 ppm, 10–208C pH 6–7, 1–10 ppm, 10–208C
Ternary mixtures two therapeutants plus either D-glucose or mixed DOC
Fixed ionic strength ŽmM., temperature Ž8C. and pH
Fixed concentration ppm of therapeutants, D-glucose or DOC
D-Glucose, Malachite Green, Chloramine-T DOC, Malachite Green, Chloramine-T D-Glucose, Chloramine-T, Oxytetracycline
2 mM, 208C and pH 7
20 ppm, 10 ppm, 10 ppm
2 mM, 208C and pH 7
20 ppm, 10 ppm, 10 ppm
2 mM, 208C and pH 7
20 ppm, 10 ppm, 10 ppm
Žb.
Žc.
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prepared solutions of equal therapeutant concentrations, 2 ppm, in distilled water. This work has been extended to cover a range of mixture options, staring from a primary concentration of a particular therapeutant Ž2 ppm. and increasing another therapeutant’s concentration from zero to 2 ppm. A paper outlining the measurement of these low concentrations of mixed therapeutants by UV spectrophotometry and the problems experienced in obtaining meaningful data is in preparation ŽZhang in prep... The range of mixtures of various starting concentrations ŽCo., ionic strengths ŽmM. equilibrium temperatures and pH values are summarised briefly in Table 2, while a complete data set of the experiments is available from the author.
Fig. 1. Comparison of isotherms of amount adsorbed Ž q . on activated carbon Ž c . versus initial concentration Ž Ceq ŽM.. Žmolesrl. for Chloramine-T and Oxytetracycline in binary mixture experiments and in single component experiments.
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4. Results and discussions 4.1. Binary mixtures of therapeutants Figs. 1 and 2 illustrate the isotherms for each component in the binary mixtures of therapeutants described in Table 2a. For comparison, these figures also show the isotherms measured for the single components under the same conditions. The single
Fig. 2. Comparison of isotherms also for Chloramine-T and Malachite Green in binary mixture experiments and in single component experiments.
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component oxytetracycline isotherms were originally measured only for initial concentrations up to 1 ppm, and are straight lines when plotted on a log–log scale. These have been extrapolated to the higher concentration region of the figures Žbroken lines in Fig. 1.. The extent to which the isotherm from the mixture is shifted downwards compared with that of the single component is a crude measure of the strength of adsorption of that
Fig. 3. Comparison of Freundlich parameters for single therapeutants ŽAitcheson et al., 2000. with each of the same components in a binary mixture of therapeutants.
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component: the component whose isotherm is shifted the more by the introduction of a second component is the one which is the more weakly adsorbed ŽLo and Alok, 1996.. Temperature, pH and ionic strength variables do not appear to create much of an overall weakening effect on the adsorption of oxytetracycline except at neutral pH. This is more pronounced for Chloramine-T, where, relative to the adsorption potential of the single component, there is a marked weakening in the adsorption potential at pH 7. Overall in the Chloramine-Tq Oxytetracycline mixtures ŽFig. 1. the data indicate that under most conditions Oxytetracycline is more strongly adsorbed onto carbon 207EA than Chloramine-T. In the Chloramine-Tq Malachite Green mixtures, the relative strengths of adsorption of the two components is more variable ŽFig. 2.. Adsorption of both components is enhanced at pH 6 and 58C, while Malachite Green is likewise more strongly adsorbed than Chloramine-T at pH 7 and 108C. At higher temperature Ž208C., Chloramine-T seems to be at least as strongly adsorbed as Malachite Green. These suggest that under most conditions the order of adsorption strength of these three therapeutants is Oxytetracycline) Chloramine-TG Malachite Green.
Table 3 Single-component Freundlich parameters used in ideal adsorbed solution theory ŽIAST. modelling of adsorption from binary and ternary mixtures. I s ionic strength. K and 1r n are emperically determined constants Conditions
Adsorbate
Freundlich K
Freundlich 1r n
R2
pH 6, I s 2 mM, 58C
Chloramine-T Malachite Green Oxytetracycline Chloramine-T Malachite Green Oxytetracycline D-glucose Chloramine-T Malachite Green Oxytetracycline Chloramine-T Malachite Green Oxytetracycline D-glucose Chloramine-T Malachite Green Oxytetracycline D-glucose Mixed DOC Chloramine-T Malachite Green Oxytetracyline D-glucose Chloramine-T Malachite Green Oxytetracycline
9466 38.582 353.42 0.5413 26.083 757.97 15 225 18.699 7.6404 778.87 57.495 2.00Eq10 911.82 15225 3.1232 41.497 1023.2 15 225 6.5216 28.104 1200.4 682.19 15 225 61.459 3662 659.48
1.3806 0.8098 0.9892 0.4194 0.7277 0.9836 1.4886 0.6868 0.5768 0.9985 0.7911 2.0408 0.9992 1.4886 0.5643 0.7825 0.9985 1.4886 0.6426 0.7458 0.8822 0.991 1.4886 0.819 0.8812 1.001
0.9747 0.8734 1.0 0.9805 0.9334 0.9993 0.9986 0.9677 0.8504 1.0 0.9816 0.9939 0.9999 0.9986 0.9571 0.8898 1.0 0.9986 0.6011 0.9883 0.9538 0.9999 0.9986 0.9873 0.8441 0.9998
pH 6, I s 2 mM, 108C
pH 6, I s 0.2 mM, 20 8C
pH 6, I s 2 mM, 208C
pH 7, I s 2 mM, 108C
pH 7, I s 2 mM, 208C
pH 8.5, I s 20 mM, 108C
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Aitcheson et al. Ž2000. showed that all the single-component Freundlich parameters obtained under any conditions for a particular therapeutant and a particular carbon define a straight-line trend. The Freundlich parameters obtained for Chloramine-T and Malachite Green when a second therapeutant is present also fall on the straight-line trends defined by the single component data ŽFig. 3., while for Oxytetracycline the single-component data were tightly clustered on the 1rn versus ln K plot, but the data from binary mixtures define a straight-line trend that passes through this cluster of the single-component data. Collectively, these parameters define a strong linear relationship that is essentially independent of experimental conditions.
Fig. 4. Performance of the ideal adsorbed solution theory ŽIAST. in prediction of amounts adsorbed Ž q . from a binary mixture of Chloramine-T and Oxytetracycline for various conditions and initial adsorbate concentrations.
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4.2. Other mixture experiments The amounts of the therapeutants adsorbed in the binary therapeutantq D-glucose experiments ŽTable 2b. were compared with the amounts adsorbed when no other components were present, i.e. single component isotherms Figs. 1 and 2. The resultant isotherms showed that the presence of 20 ppm D-glucose had no significant effect on the adsorption of any of the therapeutants, which supports the notion that the D-glucose is much more weakly adsorbed than the therapeutants.
Fig. 5. Performance of the ideal adsorbed solution theory ŽIAST. in prediction of amounts adsorbed Ž q . from a binary mixture of Chloramine-T and Malachite Green for various conditions and initial adsorbate concentrations.
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In the ternary mixture experiments Table 2c, D-glucose adsorption was slightly lower in the presence of therapeutants than in similar experiments where no other component was present. This supports the likelihood of a reduction in DOC adsorption by activated carbon when therapeutants are also present. 4.3. Performance of the IAST For all of the mixture experiments, Table 1, the Crittenden et al. Ž1985. formulation of the IAST was used to predict the amounts adsorbed of the different components in the mixtures. The Freundlich isotherm parameters used in the calculations are listed in Table 3 and are based on values ŽAitcheson et al., 2000. for single components. Figs. 4 and 5 compare the observed amounts of each component adsorbed in binary mixtures of therapeutants with the amount predicted by IAST, for various experimental conditions. Most of the data points lie close to the prediction line in each figure, indicating good agreement between the predicted and observed values. In only a few cases do the observed and predicted amounts adsorbed differ by more than 20%. Discrepancies between observed and predicted values arise mainly for mixtures having higher initial concentrations Ž10 ppm.. In the Chloramine-T and Malachite Green mixtures, all discrepancies were under-predictions by IAST of the amount adsorbed, while in the Chloramine-T and Oxytetracycline mixtures the discrepancies included both over- and under-predictions, with more over-predictions for Chloramine-T than for Oxytetracycline. Fig. 6 shows comparisons between the observed amount adsorbed and that predicted by IAST for a single therapeutant in the presence of 20 ppm D-glucose. All but two data
Fig. 6. Performance of the Ideal Adsorbed Solution Theory ŽIAST. in prediction of amounts of therapeutants adsorbed Žq. from a binary mixture of one therapeutant Ž Co either 1 or ppm. plus D-glucose.
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Fig. 7. Performance of the ideal adsorbed solution theory ŽIAST. in prediction of amounts adsorbed Ž q . from ternary mixtures. Each mixture comprised two therapeutants plus either D-glucose or mixed DOC from the artificial effluent.
points lie Žwithin analytical error. on the prediction line. The discrepant values correspond to a fairly high initial concentration of Chloramine-T Ž10 ppm., which is two to five times the normal concentration used for general external antibacterial treatment in aquaculture. Fig. 7 compares the observed amount adsorbed with that predicted by IAST for components of ternary mixtures comprising two therapeutants, plus either D-glucose or mixed DOC. Again, there is some agreement between observed and predicted values, with all discrepancies erring on the side of underprediction by IAST of the extent of therapeutant adsorption.
5. Conclusions This study shows that, for mixed aquaculture effluents with low therapeutant concentrations Žless than 10 ppm., IAST with the Freundlich isotherm should predict reasonably accurately their multicomponent adsorption behaviour with respect to activated carbon ŽFigs. 6 and 7.. On the basis of these results, and given desired levels of therapeutants and DOC in the treated effluent, it should be possible to design a simple batch treatment procedure involving dosing a tank with an appropriate quantity of carbon and allowing effluent, which would otherwise have been purged, to equilibrate with the carbon. For example, taking the data of a ternary mixture of the two therapeutants Chloramine-T and Malachite Green, it could be predicted that an aquacul-
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ture effluent volume of 3 m3 containing 10 ppm levels of both therapeutants could be reduced to 3 ppm concentrations by the addition of 15 kg of activated carbon Ž207EA.. A further 20 kg of fresh activated carbon should reduce the concentration levels to approximately 0.1–0.2 ppm. For effluents with higher initial concentrations of adsorbates the results indicate that the total amount adsorbed from multicomponent aquaculture effluents is likely to be greater than predicted by IAST. This phenomenon seems to be quite general in the application of IAST to the adsorption of organic substances ŽMyers et al., 1992.. The enhancement of adsorption that was observed when any additional organic component was present applies mainly to the therapeutants. DOC adsorption seems to be reduced by the presence of therapeutants, but the size of this reduction seems to be relatively small. The enhancement beyond the expectations of IAST of adsorption when other components are present indicates that processes unaccounted for by the IAST model are occurring at the carbon surface. Sorial et al. Ž1993. found that greater than expected Žby IAST. adsorption of phenols occurred mainly under oxic conditions, and suggested that the presence of molecular oxygen was leading to polymerisation on the carbon surface. The therapeutants Malachite Green, Chloramine-T and Oxytetracycline are much more strongly adsorbed than DOC or D-glucose onto granular activated carbon. The presence of more than one component tends to enhance adsorption considerably, especially of therapeutants. The linear relationship between the Freundlich parameters 1rn and ln K observed for single therapeutants also holds for these substances when other components are present, even when these other components compete strongly for sites on the carbon. This fact is a valuable constraint on the equilibrium adsorption behaviour Žas described by isotherm parameters. of any adsorbate, which holds regardless of conditions or of which other components are present. The IAST combined with the Freundlich isotherm appears to be a good model for describing the equilibrium adsorption from dilute aqueous mixtures of DOC or D-glucose and one or more therapeutants. It is thus very suitable for modelling equilibrium adsorption treatment of aquaculture effluents when therapeutants are in use. IAST appears to underestimate adsorption of therapeutants at higher initial concentrations. Thus, for the purpose of environmental protection, IAST tends to err on the side of safety in predicting therapeutant removal.
Acknowledgements We gratefully acknowledge financial support from the Biology and Biotechnology Science Research Council Žcontract CTE01769. and from Heriot-Watt University.
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