Removal of aquaculture therapeutants by carbon adsorption

Aquaculture 183 Ž2000. 269–284 www.elsevier.nlrlocateraqua-online

Removal of aquaculture therapeutants by carbon adsorption 1. Equilibrium adsorption behaviour of single components S.J. Aitcheson, J. Arnett, K.R. Murray ) , J. Zhang Department of Mechanical and Chemical Engineering, Heriot-Watt UniÕersity, Edinburgh EH14 4AS, UK Accepted 6 September 1999

Abstract This paper presents data on batch equilibrium adsorption onto the coal-based activated carbon 207EA ŽSutcliffe Speakman. of Malachite Green, formalin, Chloramine-T and Oxytetracycline. These substances are widely used in aquaculture to control fish parasites and disease, but few data were previously available on their adsorption behaviour. In addition, equilibrium adsorption data for carbon 207EA are presented for the mixed dissolved organic carbon ŽDOC. typically present in the water where fish are reared, as well as for D-glucose. Together, these data permit the design of carbon adsorption treatment units that will remove both therapeutants and DOC without causing stress to the fish stock. It further removes the need for land-based recycle systems to discharge these mixed effluents untreated to the environment. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Carbon-adsorption; Aquaculture; Chloramine-T; Formalin; Malachite Green; Oxytetracycline

1. Introduction Classical intensive recycle aquaculture systems commonly use nitrifying biological filtration to clean up the recirculating water ŽWesterman et al., 1993.. These filters also

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Corresponding author. Tel. q44-131-449-5111 ext. 4719; fax: q44-131-451-3129; e-mail: [email protected] 0044-8486r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 9 9 . 0 0 3 0 4 - X

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effectively remove suspended solids and control dissolved organic carbon ŽDOC. levels ŽBoller and Gujer, 1986.. Diseases are often controlled by the addition of organic chemical therapeutants, either directly to the water or in the fish feed. Unfortunately, these biological filters are not designed to remove therapeutants and the shock loading of the therapeutants on the filters may destroy the nitrifying bacteria and thereby lead to unacceptable stress to the fish stock. Therefore, it is common practice simply to purge recycle aquaculture systems once therapeutants have been added, and only to resume normal effluent treatment and recirculation once the therapeutants have been removed. ŽMurray and McEvoy, 1990.. As a classical unit operation for the removal of DOC and colour in potable water treatment, activated carbon filtration ŽHenry and Weinke, 1996; Kiely, 1997. has also been used extensively for post ozone or chlorine treatment. It is clear that it has the potential to effectively remove DOC and certain therapeutants, i.e., Malachite Green, from fish farm waste waters ŽAlderman, 1985.. This would effectively prevent the release of these pollutants to the environment whether as a purge from an intensive aquaculture system or, for that matter, any land-based operations for disease treatment, particularly as there is concern and doubt about their fate in the ecosystem ŽBjorkland et al., 1990; Coyne et al., 1994; Kerry et al., 1994; Smith, 1996; Herwig et al., 1997.. To design carbon filters, this requires fundamental experimental data on the equilibrium adsorption behaviour with respect to activated carbon of the main components of the effluent. However, while DOC removal using carbon filters is often reasonably well characterised empirically, very few adsorption data are available for therapeutants. This paper, therefore, presents experimental data on the batch equilibrium adsorption of the following commonly used therapeutants: Oxytetracycline, Malachite Green, formaldehyde and Chloramine-T, onto the coal-based activated carbon 207EA Žsupplied by Sutcliffe-Speakman.. Data are also presented for DOC for the same carbon. These data can be used directly to model multicomponent adsorption of DOC q therapeutant mixtures onto carbon filters. They may also indicate the likely competitive adsorption behaviour of the therapeutants onto carbonaceous matter in the environment, such as might occur in sediment close to fish cages moored in natural water bodies.

2. Materials and methods 2.1. Adsorbates studied in this work This study investigated the adsorption behaviour of the following substances. Ž1. Malachite Green ŽŽC 23 H 25 N2 . 2 P ŽC 2 H 2 O4 . 3 ; oxalate salt; molecular weight 929. is a triphenylmethane dye that is used extensively in aquaculture as a fungicide and as an ectoparasiticide. It is added directly to the water to give a concentration of up to about 2 ppm. A Malachite Green q formalin mixture Žin proportions of about 1:80. is used to treat external parasites and fin rot. According to Alderman Ž1985., Malachite Green dye produced industrially in the past often varied widely both in actual concentration and in its precise composition, so that it is difficult to compare earlier studies on

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dosage regimes and toxic effects. It is now known to be highly toxic to mammalian cells and to act as a liver tumour promoter ŽPanandiker et al., 1993.. In fish, it is absorbed rapidly through the gills and can persist for over a month in the kidneys and for several weeks in the liver ŽKasuga et al., 1992., where it is cytotoxic ŽZahn and Braunbeck, 1995.. It appears to have a strong affinity for organic matter and is adsorbed readily onto suspended organic matter in the water column ŽSagar et al., 1994.. Ž2. Chloramine-T ŽC 7 H 7 ClNO 2 SNa; N-chloro-p-toluene-sulfonamide sodium salt; molecular weight 227.6. is added directly to the water Žconcentration about 2–4 ppm. as a general external antibacterial treatment, but especially to treat Myxobacterial Gill Disease. It is an irritant and an oxidant. At the concentrations used in aquaculture, it has no reported carcinogenicity, but in water, its organic trihalomethane byproducts can be carcinogenic in experimental mammals. Neurotoxic effects Žby the inhibition of acetyl cholinesterase activity. of Chloramine-T have been reported in humans and amphibians ŽWang and Minami, 1996.. Ž3. Oxytetracycline ŽC 22 H 24 N2 O 9 P 2H 2 O, molecular weight 496.5. is widely used as an antibacterial agent and is usually administered in fish feed, from which it may leach into the water column. It appears to persist in the environment for many months ŽHansen et al., 1992; Pouliquen et al., 1992., mainly in sediment where it disrupts the normal community structure of bacteria and encourages the growth of Oxytetracycline-resistant bacteria ŽKerry et al., 1994.. Oxytetracycline is also widely used as an antibiotic in other veterinary applications and in human medicine, so that its indiscriminate release to the environment could eventually reduce its efficiency in these fields by increasing the Oxytetracycline resistance of human and animal pathogens. Oxytetracycline may also interfere with nitrification processes ŽKlaver and Matthews, 1994.. It appears to be relatively stable and thus most persistent in anoxic sediment ŽSamuelsen, 1988.. Ž4. Formaldehyde ŽCH 2 O, molecular weight 30.03. is used in the aqueous form Žformalin. as a fungicide in combination with, or instead of, Malachite Green. Both mixtures and pure formalin therapeutants are applied directly to the water. A typical concentration of formalin in the water would be 15–20 ppm when used alone ŽAustin, 1985.. A typical mixture would be 1 part Malachite Green to 80 parts formalin by weight. Formaldehyde is classed as a probable human carcinogen. It appears to be able to denature proteins or to form crosslinkages that prevent protein unfolding, and so can be cytotoxic or damage DNA. Ž5. Mixed DOC. For this study an artificial effluent was created by mixing together starch, fish oil and urea to give individual DOC contributions of 5.1, 0.7 and 1.3 mg Crl, respectively. The total carbon concentration of the mixture was 7.1 mgrl. This artificial effluent composition is the same as the average composition of a real effluent from the Heriot-Watt University fish farm, except that it omits the small carbon contribution Ž0.1 mg Crl. present from protein in the real effluent. The effluent was diluted to 3 and 5 ppm DOC for the adsorption experiments. Ž6. D-glucose ŽC 6 O6 H 12 , molecular weight 180.. The adsorption behaviour of this substance should indicate the behaviour of any simpler sugars present in an aquaculture effluent. Due to technical difficulties with uncharacterised DOC analysis, it was not possible to obtain adsorption data on urea, starch and fish oil components individually.

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2.2. Analytical methods The carbon used for this study was a coal-based granular activated carbon, 207EA, supplied by Sutcliffe Speakman, with a BET surface area of 1000 m2rg and bulk density of 0.46. The carbon was washed with distilled water, oven dried at 1008C for 24 h and crushed to a mesh size of 12 = 40 uus Ž0.6–1.7 mm.. Each single component 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 the activated carbon 207EA. Initial concentrations were 1–10 ppm for Malachite Green and Chloramine-T, 0.1–1 ppm for Oxytetracycline, 5–20 ppm for D-glucose, 3–5 ppm for DOC in the artificial effluent, and 1–20 ppm for formaldehyde. For some conditions, the Oxytetracycline data were extended to include an initial concentration of 20 ppm. This is much higher than the concentration that would occur in a real effluent, but permits comparison of its adsorption strength with that of other adsorbates at similar initial molar concentrations. Temperature was controlled to within "0.58C by immersing the experimental flask in a thermostatically controlled shaker water bath. pH and ionic strength were controlled by means of buffer solutions comprising various mixtures of KH 2 PO4 , Na 2 HPO4 , Na 2 B 4 O 7 , NaCl and HCl. The experiments were done at pH values of 6, 7 and 8.5, temperatures of 5, 10, 20 and 308C, and ionic strengths of 0.2, 2 and 20 mM. These experimental conditions cover most of the range of pH, temperature and ionic strength likely to occur in real aquaculture effluents. Maximum adsorption capacities were obtained by extending single component isotherms for bulk solutions Ži.e., the substance of interest dissolved in distilled water. to very high initial concentrations Žseveral thousands ppm.. The procedure was similar to the single component isotherms experiments described above, except that no buffers were used in these experiments. The bulk solutions of the therapeutants have pH values Žmeasured at 50 ppm. as follows: Malachite Green, pH 5.3; Oxytetracycline, pH 3.9; Chloramine-T, pH 6.7; and formaldehyde, pH 6.1. Concentrations of Malachite Green, Chloramine-T, Oxytetracycline and D-glucose were obtained from the UV absorbance measured by a Pye Unicam SP1700 UV Spectrophotometer, which had previously been calibrated for each substance ŽArnett and Zhang, 1994.. Separate calibrations for the three different pH values were used for Malachite Green measurements because the buffers had a significant effect on absorbance. A single calibration line could be used for each of the other substances as the buffers had no observable effect on their UV absorbance. To obtain the D-glucose concentration, the sample was first reacted with a mixture of phenol and H 2 SO4 for 10–20 min at 258C and the UV absorbance of the product was measured. UV absorbance was measured at the following wavelengths: l s 615 nm for Malachite Green; l s 200 nm for Oxytetracycline and for Chloramine-T; and l s 488 nm for D-glucose. Errors in equilibrium concentrations measured by UV spectrophotometry were 0.032–0.052 ppm for Chloramine-T, 0.052 ppm for Oxytetracycline, 0.094–0.7 ppm for D-glucose, and 0.01–0.03 ppm ŽpH 6., 0.043 ppm ŽpH 7. and 1.052 ppm ŽpH 8.5. for Malachite Green. Errors in amounts adsorbed per kg of carbon were: 0.056–0.063 grkg ŽChloramine-T.; 0.061 grkg ŽOxytetracycline.; 0.070–0.075 grkg ŽD-glucose.;

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0.053–0.057 grkg ŽMalachite Green at pH 6.; 0.059–0.063 grkg ŽMalachite Green at pH 7.; and 0.260–0.264 grkg ŽMalachite Green at pH 8.5. ŽArnett and Zhang.. Formaldehyde was measured using a Hach DR2000 spectrophotometer and following the MBTH method: 3-methyl-2-benzothiazoline hydrazone ŽMBTH. was added in excess to the sample containing formaldehyde. MBTH reacts with the formaldehyde to form an azine, then excess MBTH is oxidised by addition of a developing solution Žsulphuric acid plus ferric chloride.. The oxidised MBTH reacts with the azine to form a species with an intense blue colour, with intensity proportional to the original concentration of formaldehyde. The method is extremely sensitive: errors in measured equilibrium concentrations of formaldehyde were 1 ppb or lower. Errors in the amount of formaldehyde adsorbed were 0.050–0.057 grkg of carbon. Adsorption of DOC in the artificial mixed effluent was quantified as the difference in DOC concentration of two aliquots of each sample, where both aliquots had experienced identical conditions, except that one was equilibrated with the activated carbon while the other was not exposed to the adsorbent. DOC concentrations in the artificial effluent were measured using a Shimanzu TOC-500 carbon analyser with a combustion system and NDIR detector. Uncertainties Žexpressed as 2 = standard error. in total carbon concentration measurements were 1–1.7 ppm.

3. Results 3.1. Single component isotherm parameters Table 1 gives the fitted Freundlich parameters for the therapeutants and for D-glucose for a range of experimental conditions. Langmuir isotherms were fitted to some of the data and the Langmuir parameters are also given in Table 1. The full list of experimental data to which these isotherms were fitted is available on request from the authors. The Freundlich isotherm is defined as: 1r n , q s KCeq

Ž 1.

where q is the number of moles adsorbed per kg of carbon, Ceq is the equilibrium molar concentration of the liquid, and K and 1rn are constants obtained empirically. For each substance, the values of K and 1rn were obtained for each set of experimental conditions by unweighted least squares linear regression of five measured ln q values on their corresponding lnCeq values. This procedure attaches greater weight to the lower concentration data. The Langmuir isotherm is defined as: q s Ž qm K L Ceq . r Ž 1 q K L Ceq . ,

Ž 2.

where Ceq and q are defined as before and where qm and K L are empirically determined constants. For each substance, the values of qm and K L were obtained for each set of experimental conditions by unweighted least squares linear regression of measured Ceq rq ratios on their corresponding Ceq values.

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Table 1 Freundlich and Langmuir parameters fitted to experimental isotherm data for single components of aquaculture effluents adsorbed onto carbon 207EA Isotherms were fitted to five datapoints in each case, except for the combined DOC dataset, which comprised 18 datapoints. R 2 is the correlation coefficient referring to the goodness of fit of the linearised Freundlich isotherm equation to the isotherm data. Experimental conditions pH

Temp. Ž8C.

Freundlich parameters

Langmuir parameters

I ŽmM.

K

Power s1r n

R

Malachite Green 8.5 10 8.5 20 7 5 7 10 7 20 7 30 7 10 7 20 7 10 7 20 6 5 6 10 6 20 6 30 6 10 6 20

20 20 2 2 2 2 0.2 0.2 20 20 2 2 2 2 0.2 0.2

146.97 3662 2.2921 41.497 1200.4 2.00eq14 1.3646 3.00eq10 10.992 37.338 38.582 26.083 2.00eq10 3.8757 52.285 7.6404

0.7639 0.8812 0.5466 0.7825 0.8822 2.9211 0.4716 1.952 0.7443 0.7368 0.8098 0.7277 2.0408 0.564 0.7581 0.5768

Chloramine-T 6 5 7 5 6 10 6 10 7 10 7 10 7 10 8.5 10 6 20 6 20 7 20 7 20 7 20 8.5 20 6 30 7 30 7 10 7 20 7 20

2 2 0.2 2 0.2 2 20 20 0.2 2 0.2 2 20 20 2 2 2 2 2

9466 1175.9 5.4689 0.5413 3.7739 3.1232 5.8771 40.08 18.699 57.495 28.09 26.874 61.746 61.459 720.28 275.03 9.7028 28.104 6.6835

Oxytetracycline 6 20 6 20 7 20

0.2 2 0.2

778.87 911.82 629.48

2

Adsorption efficiency Ž%.

qm

KL

0.9638 0.8441 0.7487 0.8898 0.9538 0.8509 0.7899 0.9222 0.9433 0.9554 0.8734 0.9334 0.9939 0.9332 0.8928 0.8504

3.47ey03 5.85ey03 2.09ey03 3.64ey03 5.80ey03 y2.46ey04 1.89ey03 y6.44ey04 3.07ey03 2.87ey03 3.69ey03 2.91ey03 y6.59ey04 2.16ey03 3.32ey03 2.37ey03

2.67eq06 5.75eq06 1.67eq06 3.05eq05 1.68eq06 y6.56eq05 8.71eq06 y4.90eq06 1.38eq05 9.72eq05 1.68eq05 6.88eq05 y2.02eq06 2.76eq06 7.84eq05 5.02eq06

95–98 99–100 78–96 71–88 97–99 44–85 85–99 96–98 55–76 83–93 58–79 81–92 88–96 87–97 86–94 92–99

1.3806 1.1495 0.5057 0.4194 0.4826 0.5643 0.6232 0.7961 0.6868 0.7911 0.7364 0.7419 0.8429 0.819 1.0052 0.9092 0.6479 0.7458 0.6351

0.9747 0.9871 0.977 0.9805 0.9713 0.9571 0.9832 0.9854 0.9677 0.9816 0.9911 0.9866 0.9889 0.9783 0.9801 0.982 0.9617 0.9883 0.9749

y3.25ey03 y1.43ey02 8.84ey03 6.02ey03 8.61ey03 7.95ey03 8.40ey03 1.35ey02 9.76ey03 1.31ey02 1.16ey02 1.07ey02 1.73ey02 1.51ey02 y8.59ey02 5.52ey02

y2.38eq04 y1.22eq04 2.15eq06 3.59eq05 2.21eq06 1.68eq05 1.29eq05 7.28eq04 1.79eq05 8.50eq04 9.80eq04 9.42eq04 2.90eq04 4.87eq04 y7.52eq03 1.68eq04

24–45 44–51 94–99 64–96 92–99 71–92 67–88 73–85 76–88 74–84 73–86 71–82 64–72 70–81 73–79 79–85 70–92 67–82 55–82

0.9985 0.9992 0.9863

1 0.9999 1

6.44ey02 y9.75ey03 8.21ey03

1.24eq04 y9.37eq04 9.72eq04

80 82 79–80

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Table 1 Žcontinued. Experimental conditions pH

Temp. Ž8C.

Freundlich parameters

Langmuir parameters 2

qm

KL

1.26ey02 1.76ey02 9.64ey02 7.87ey03 5.05ey03 6.74ey03 7.10ey02 7.87ey03 6.74ey03 4.46ey02 7.79ey02 8.72ey03 4.65ey02

6.36eq04 4.38eq04 6.75eq03 1.28eq05 2.02eq05 1.75eq05 1.48eq04 1.28eq05 1.75eq05 3.14eq04 1.88eq04 4.86eq04 9.14eq03

Adsorption efficiency Ž%.

I ŽmM.

K

Power s1r n

R

Oxytetracycline 7 20 7 20 8.5 20 6 10 6 10 7 10 7 10 7 10 8.5 10 6 30 7 30 6 5 7 5

2 20 20 0.2 2 0.2 2 20 20 2 2 2 2

682.19 653.26 659.48 738.5 757.97 810.55 1023.2 738.5 810.55 1249.5 1430.4 353.42 412.17

0.991 0.99 1.001 0.9819 0.9836 0.9786 0.9985 0.9819 0.9786 0.9932 0.9986 0.9892 0.9982

0.9999 0.9997 0.9998 0.9997 0.9993 0.9997 1 0.9997 0.9997 0.9994 1 1 1

Formaldehyde 6 10 7 20

0.2 2

86.313 2.3136

0.7442 0.3849

0.8305 0.4686

73–92 74–99

2

15225

1.4886

0.9986

44–58

All DOC data combined 6–8.5 5 to 30 0.2 to 20 6.5216

0.6426

0.6011

7–68

79–80 79–80 76–77 83–84 82–84 85–86 84 83–84 85–86 87–88 88 67–68 68

D-glucose

7

20

The mixed DOC results were rather scattered and did not define clear isotherms. However, the artificial effluent data-field straddles the D-glucose isotherm, suggesting that suitable Freundlich parameters for real effluent could be broadly similar to those for D-glucose. Fig. 1 shows log–linear plots of the fitted Freundlich 1rn parameters vs. their corresponding K value for three therapeutants for all of the experimental conditions. A strong linear relationship exists between ln K and 1rn for the Malachite Green data ŽFig. 1Ža.. and for the Chloramine-T data ŽFig. 1Žb... Let this relationship be expressed as: 1rn s X ln K q Y , Ž 3. where X and Y are constants equal to the slope and intercept of the graph, respectively. The Freundlich isotherm can be rewritten to: 1rn s w ln q y ln K x rln Ceq . Ž 4. Equating both expressions for 1rn gives: ln K s Ž ln q y Y ln Ceq . r Ž 1 q X ln Ceq . .

Ž 5.

Thus, if X and Y are known, a single measurement of q and Ceq is sufficient to determine the relevant Freundlich K parameter, and from this the corresponding value of 1rn.

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Fig. 1. Freundlich parameters obtained for adsorption onto carbon 207EA of Malachite Green Ža., Chloramine-T Žb. and Oxytetracycline Žc. under a range of experimental conditions.

The Freundlich parameters fitted to the Oxytetracycline data vary little with experimental conditions ŽFig. 1Žc..: 1rn is always close to unity and K is generally between

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about 600 and 900. Thus, there is no difficulty in picking reasonable Freundlich parameters for Oxytetracycline for any conditions. Fig. 2 plots 1rn vs. ln K for all the adsorbates together, including both therapeutants and DOC. Collectively, these parameters also define a strong linear relationship that is independent of experimental conditions. In general, the Langmuir isotherm did not fit the data as well as the Freundlich isotherm equation; but, as with the Freundlich isotherm parameters, the two empirically determined constants, K L and qm , for every data set for every adsorbate all lie on a single correlation curve ŽFig. 3.. Thus, a single measurement of q and Ceq is sufficient to obtain the appropriate K L and qm values for modelling adsorption under a particular set of conditions. 3.2. Strength of adsorption Table 1 gives the adsorption efficiency for each adsorbate under the different experimental conditions. The adsorption efficiency is expressed here as the percentage of the moles of the substance present at the start of the experiment that were found to be adsorbed at equilibrium. This quantity is a useful measure of how strongly the substance is adsorbed by the carbon used in the experiments.

Fig. 2. Freundlich parameters obtained for carbon 207EA for all the adsorbates in this study under all conditions. The inset shows the full range of values obtained, while the main part of the figure enlarges the region nearest the origin.

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Fig. 3. Langmuir isotherm parameters obtained for adsorption onto carbon 207EA for the therapeutants Malachite Green, Oxytetracycline and Chloramine-T under various experimental conditions. Like the Freundlich parameters, they fall on a single correlation curve for the carbon investigated.

Table 2 summarises the observed order of strength of adsorption for the substances studied under various experimental conditions. Under all conditions, the therapeutants

Table 2 Relative strength of adsorption onto carbon 207EA observed for components of aquaculture effluents I is ionic strength in mM and DOC is mixed DOC. Temp. Ž8C.

pH

5 5 10

6 7 6

10 10 10 10 10 20 20 20 20

6 7 7 7 8.5 6 6 7 7

2 0.2 2 20 20 0.2 2 0.2 2

20 20 30 30

7 8.5 6 7

20 20 2 2

I

Relative adsorbability Žstrongest to weakest. 2 2 0.2

Malachite GreenGOxytetracycline)Chloramine-T ) DOC Malachite Green)Oxytetracycline)Chloramine-T ) DOC Chloramine-T ) Malachite Green) Formaldehyde) Oxytetracycline) DOC Chloramine-T G Malachite Green)Oxytetracycline) DOC Chloramine-T ) Malachite Green)Oxytetracycline) DOC Malachite GreenGOxytetracyclineGChloramine-T ) DOC OxytetracyclineGChloramine-T ) Malachite Green) DOC Malachite Green)Oxytetracycline)Chloramine-T ) DOC Malachite Green)Chloramine-T )Oxytetracycline) DOC Malachite Green)OxytetracyclinesChloramine-T ) DOC Malachite Green)Chloramine-T GOxytetracycline) DOC Formaldehyde)Chloramine-T GOxytetracycline) Malachite GreenG D-glucoses DOC Malachite Green)Oxytetracycline)Chloramine-T ) DOC Malachite Green)OxytetracyclineGChloramine-T ) DOC Malachite Green)Oxytetracycline)Chloramine-T ) DOC Oxytetracycline)Chloramine-T G Malachite Green) DOC

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were much more strongly adsorbed than the mixed DOC or D-glucose. This indicates that in mixed effluents the therapeutants are likely to be preferentially adsorbed, leading to a reduction in DOC removal. Malachite Green was usually more strongly adsorbed than Oxytetracycline, and Oxytetracycline was usually as strongly adsorbed or more strongly adsorbed than Chloramine-T, except in some of the 108C experiments at relatively low pH and ionic strength.

Fig. 4. Isotherms for single therapeutants dissolved in water at 238C with no buffers. These isotherms extend to very high concentrations and level off at a q-value that is interpreted to be the monolayer adsorption capacity of the carbon for that substance.

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Temperature was the parameter that had the largest effect on the adsorption efficiency of the therapeutants, with the lowest adsorption efficiencies Ž50% or lower. only observed at the lowest temperature Ž58C.. The effects of pH and ionic strength were much smaller, although adsorption efficiency seemed to be lowered by high ionic strengths. Adsorption efficiency of the therapeutants seemed to be greatest at 10–208C, pH 7 and an ionic strength of 0.2–2 mM. 3.3. Maximum adsorption capacities Fig. 4 shows isotherms obtained for single components for initial concentrations ranging from one to several thousand ppm. The amount adsorbed where the isotherm flattens off parallel to the concentration axis is interpreted to be the maximum monolayer adsorption capacity of the carbon for that substance. These maximum adsorption capacities differ greatly from substance to substance and are: 325 g or 0.35 molesrkg of carbon for Malachite Green; 614 g or 2.7 molesrkg carbon for Chloramine-T; and 60 g or 0.12 molesrkg of carbon for Oxytetracycline. The Oxytetracycline isotherm has a step in it, suggesting that multiple layers are forming at high concentrations. The maximum monolayer adsorption capacity reported above refers to the lower step. The upper step in the Oxytetracycline isotherm corresponds to a maximum adsorption capacity of about 99 g or 0.2 molesrkg carbon; this may be the ‘‘true’’ or multilayer adsorption capacity. The formation of multiple layers may indicate that the concentration is approaching the solubility of Oxytetracycline in water. The formaldehyde isotherm Žnot shown in Fig. 4. shows no sign of flattening off, and indicates a formaldehyde adsorption capacity of at least 79 kgrkg carbon or 1800 molesrkg carbon.

4. Discussion Aquaculture effluents are typically very dilute solutions of the adsorbates of interest, and so they fulfil one of the main conditions for which use of the Freundlich isotherm is valid ŽUrano et al., 1981.. In practice, the Freundlich isotherm appears to fit well to most of the single component experimental data acquired in this study, and the fitted Freundlich parameters form an extremely coherent group of results ŽFigs. 1 and 2.. For these reasons, and because the use of the Freundlich isotherm considerably simplifies some multicomponent adsorption modelling calculations ŽCrittenden et al., 1985., the Freundlich isotherm is preferred for describing adsorption of components of aquaculture effluents. The Langmuir isotherm equation fits the data less well, but would be more appropriate for those isotherms that constrain adsorption capacity Že.g., Fig. 4. by extending to very high liquid concentrations of the adsorbates. The Freundlich parameters presented in Table 1 for the different adsorbates together with the linear relationships between them allow appropriate Freundlich parameters to be selected for conditions other than those treated in this study with a minimum of extra information. These parameters can be used directly in multicomponent adsorption

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models to predict the equilibrium adsorption behaviour of any mixture. The apparent adherence of the DOC data to the linear relationship between all of the Freundlich parameters of all the adsorbates is particularly useful because it constrains which Freundlich parameters are reasonable for the DOC in real effluents, despite the fact that this DOC is usually very poorly characterised. The linear relationships between the Freundlich parameters appear to be manifestations of the well-known ‘‘Characteristic Curve’’ of Polanyi Ž1920a; b.; where, for a particular adsorbent, all the Freundlich K values Žor adsorbate volumes. and the adsorption potentials lie on a single correlation curve, irrespective of temperature or which components are considered. Urano et al. Ž1981. made more explicit the connection between the Freundlich parameters and the characteristic curve by combining the following expressions relating to single component adsorption: The Polanyi expression for the adsorption potential: Eeq s RT ln Ž CsrC . .

Ž 6.

The expression for number of moles adsorbed ŽRoginsky, 1948.: Qs

Emax

HE

f Ž E . d E.

Ž 7.

eq

The hypothetical exponential adsorption energy Ž E . distribution: f Ž E . s Aeya E .

Ž 8.

The resulting expression is analogous to the Freundlich isotherm equation: Qs

ž

A aCsa RT

/

C a RT

Ž 9.

with: 1rn s aRT and K s

A aCsa RT

.

In the above expressions, A and a are constants, R is the gas constant, T is absolute temperature, Cs is the solubility of the adsorbate, C is the equilibrium concentration of adsorbate in the liquid and Emax is the adsorption potential in an infinitely dilute solution. For single component adsorption, rearranging the expression for K gives: 1rn s y Ž 1rln Cs . ln K y ln Ž Ara .

Ž 10.

T ln Cs s Ž 1raR . ln Ž AraK . .

Ž 11 .

or:

To obtain the straight-line relationship between 1rn and ln K with this formulation would require the following to be true: 1. a and A are constant Ži.e., each molecule ‘‘sees’’ the same adsorption energy distribution.; and

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2. either the solubility is approximately constant over the temperature range considered, or 1rlnCs is a linear function of T Žrequiring K to be approximately constant..In addition, this formulation implies that: 3. The slope of the graph of 1rn vs. ln K should be yŽ1rlnCs . and the intercept should correspond to ylnŽ Ara.. 4. The higher values of 1rn should correspond to higher temperatures. In fact, only some of these conditions turn out to be physically true, suggesting that the Polanyi adsorption model andror the exponential energy distribution are only of limited use in describing the adsorption of aquaculture therapeutants. In particular, the observed solubilities of Malachite Green and Chloramine-T are considerably higher than those suggested by the slopes of the graphs and the relationship between 1rn and temperature is not monotonic. The values of ‘‘a’’ in the expression for the adsorption energy distribution Žwhere ‘‘a’’ is the width of the distribution. can be computed directly from each fitted Freundlich 1rn value ŽTable 1.. In theory, the larger the value of ‘‘a’’, the narrower is the adsorption energy distribution, i.e., the fewer are the types of site that are involved in adsorption. On a histogram with site types listed in order of their adsorption energy Žnot shown., most of the values of ‘‘a’’ for a particular therapeutant form a single narrow peak, indicating that for a particular adsorbate, ‘‘a’’ varies little with temperature. However, the computed mean values of ‘‘a’’ for different adsorbates differ by at least a factor of two, i.e., the distributions for different adsorbates cannot be strictly congruent. The Freundlich parameters presented here describe empirically the equilibrium partitioning of a substance between the adsorbed phase and the liquid. However, the strength of adsorption of a single component Žexpressed as a proportion of the substance that is adsorbed at equilibrium. is dependent on the initial concentration, C0 , as well as on the Freundlich parameters. Meaningful comparisons between the inherent adsorbability of different components require this proportion to be computed at the same initial molar concentration Ž C0 . for each substance and under the same conditions. In real situations, the initial concentration is usually known or assumed, but the equilibrium concentration is usually unknown. To compare strengths of adsorption for substances at initial concentrations outside the range examined in this study, one must specify the Freundlich parameters and the initial concentration, and then solve the following two simultaneous equations for Ceq and q: 1r n q s KCeq Ž the Freundlich isotherm. ,

Ž i.

q s Ž C0 y Ceq . rload,

Ž ii .

where load s Žmass of activated carbon Žkg..rŽvolume of liquid Žl.. in the experiment. Finally, the estimate of q is used to compute the proportion of the substance that will be adsorbed, i.e., loadU Ž qrC0 .. The aboved Eqs. Ži. and Žii. are usually not soluble analytically, but they can very easily be solved graphically by plotting q as a function of Ceq using Eq. Ži. and then Eq. Žii. and observing where the two curves intersect. ŽThis is also true if Eq. Ži. is the Langmuir isotherm rather than the Freundlich isotherm.. As the Freundlich power 1rn approaches unity Ži.e., Ž1rn 1., then Ceq C0rŽ1 q Žload. K . and q KC0rŽ1 q

™

™

™

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Žload. K ., i.e., and the graphs of Ceq and q vs. C0 become straight lines. This is approximately true of single component Oxytetracycline behaviour. These considerations, plus the adsorption efficiencies observed in the experiments ŽTable 1., indicate that for the typical concentrations of components present in aquaculture effluents, the therapeutants Malachite Green and Oxytetracycline are always, and Chloramine-T is usually, more strongly adsorbed onto the carbon than the background DOC. Thus, failure to include this feature in the design of adsorption treatment systems might result in impaired DOC removal when therapeutants are present. The prediction of relatively strong adsorption of Oxytetracycline finds support in the field observations of Smith et al. Ž1994. that Žby a rough mass balance calculation. most of the Oxytetracycline used on a real fish farm over a measured period was retained by the drum filter used for effluent treatment. Strong adsorption of Oxytetracycline to organic components in the sediment ŽBjorkland et al., Coyne et al., Samuelson. would explain its relatively high concentration there and its relatively rapid disappearance from the water column. This, together with its stability ŽPouliquen et al., 1992. in the anoxic conditions common within organic-rich sediment, could explain its persistence in natural sediments. The relatively strong adsorption of Malachite Green indicated by our experiments is also consistent with the strong affinity of Malachite Green for organic matter reported by Sagar et al. Ž1994.. 5. Conclusions This study has obtained a substantial body of data on the batch equilibrium adsorption behaviour of some of the commonest therapeutant components of aquaculture effluents under a wide range of conditions. The isotherm parameters reported here allow models of equilibrium multicomponent adsorption for activated carbon to account much more accurately than was hitherto possible for the presence of chemical therapeutants in aquaculture effluents. The observations show that the therapeutants are much more strongly adsorbed onto activated carbon than background DOC. Thus, accurate predictions of multicomponent adsorption are very important in designing adsorption units for effluent treatment that can maintain adequate DOC removal in the presence of therapeutants. Acknowledgements We gratefully acknowledge financial support from the Biology and Biotechnology Research Council Žthrough grant CTE01769 to J. Arnett. and from Heriot-Watt University. We thank M. Miller and E. McEvoy for technical assistance. References Alderman, D.J., 1985. Malachite Green: a review. J. Fish Dis. 8, 289–298. Arnett, J.G., Zhang, J., 1994. The analytical methods of organic therapeutants from aquaculture recycle systems. Interim Progress Report No. 1, BBRC Contract CTE01769, Department of Mechanical and Chemical Engineering, Heriot-Watt University, Riccarton Campus, Edinburgh, UK.

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Austin, B., 1985. Antibiotic pollution from fish farms: effects on aquatic microflora. Micro. Sci. 2, 113–117. Bjorkland, H.V., Bondestam, J., Bylund, G., 1990. Residues of oxytetracycline in wild fish and sediments from fish farms. Aquaculture 86, 359–367. Boller, M., Gujer, W., 1986. Nitrification in tertiary trickling filters followed by deep-bed filters. Water Res. 20 Ž11., 1363–1373. Coyne, R., Hiney, M., O’Conner, 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. Hansen, P.K., Lunestad, B.T., Samuelsen, O.B., 1992. Effects of oxytetracycline, oxolinic acid and flumequine on bacteria in an artificial marine fish farm sediment. J. Microbiol. 38, 1307–1312. Henry, J.G., Weinke, G.W., 1996. Environmental Science and Engineering, 2nd edn. Prentice-Hall. Herwig, R.P., Gray, J.P., Weston, D.P., 1997. Antibacterial resistant bacteria in surficial sediments near salmon net-cage farms in Puget Sound, Washington. Aquaculture 149, 263–283. Kasuga, Y., Hishida, M., Tanahashi, N., Arai, M., 1992. Studies on the disappearance of Malachite Green in cultured rainbow trout. J. Food Hyg. Soc. Jpn. 33, 539–542. Kerry, J., Hiney, M., Coyne, R., Cabazon, D., NicGabhainn, S., Smith, P., 1994. Frequency and distribution of resistance to oxytetracycline in microorganisms isolated from marine fish farm sediments. Aquaculture 123, 43–54. Kiely, G., 1997. Environmental Engineering. McGraw-Hill, ISBN 0-07-709127-2. Klaver, A., Matthews, R.A., 1994. Effects of oxytetracycline on nitrification in a model aquatic system. Aquaculture 123, 237–247. Murray, K.R., McEvoy, E., 1990. Disease treatment procedures in the intensive re-cycle salmon hatchery at Heriot-Watt. COSHH Technical Report, ‘Therapeutants’, Aquaculture Group, Department of Mechanical and Chemical Engineering, Heriot-Watt University, Edinburgh. Panandiker, A., Fernandes, C., Rao, T.K.G., Rao, K.V.K., 1993. Morphological transformation of Syrian-hamster embryo cells in primary culture by Malachite Green correlates well with the evidence for formation of reactive free radicals. Cancer Lett. 74, 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 ¨ Pouliquen, H., LeBris, H., Pinault, L., 1992. Experimental study of the therapeutic application of oxytetracycline, its attenuation in sediment and sea-water, and implications for farm culture of benthic organisms. Mar. Ecol.: Prog. Ser. 89, 93–98. Roginsky, S.S., 1948. Adsorption and catalysis on heterogeneous surfaces. USSR Academy of Sciences, Moscow. Sagar, K., Smyth, M.R., Wilson, J.G., McLaughlin, K., 1994. High-performance liquid-chromatographic determination of the triphenylmethane dye, Malachite Green, using amperometric detection at a carbon-fibre microelectrode. J. Chromatogr., A 659, 329–336. Samuelsen, O.B., 1988. Degradation 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. Smith, P., 1996. Is sediment deposition the dominant fate of oxytetracycline used in marine salmonid farms: a review of available evidence. Aquaculture 146, 157–169. Smith, P., Donlon, J., Coyne, R., Cabazon, D.J., 1994. Fate of oxytetracycline in a fresh-water fish farm — influence of effluent treatment systems. Aquaculture 120, 319–325. Urano, K., Koichi, Y., 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. Wang, Z.Y., Minami, M., 1996. Effects of chloramine on neuronal cholinergic factors — further case studies of toxicity mechanism suggested by an unusual case record. Biog. Amines 12, 213–223. Westerman, P.W., Losardo, T.M., Wildhaber, M.L., 1993. Evaluation of various bilfilters in an intensive recirculating fish production facility. Techniques for Modern Aquaculture. Proceedings of an Aquacultural Engineering Conference, 21–23 June 1993, Spokane, WA. ASAE, St. Hoseph, MI, pp. 315–325. Zahn, T., Braunbeck, T., 1995. Cytotoxic effects of sublethal concentrations of Malachite Green in isolated hepatocytes from rainbow trout Ž Onchorhyncus mykiss .. Toxicology 9, 729–741, in vitro.