Ž .
effectively remove suspended solids and control dissolved organic carbon DOC levels Ž
. Boller 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.
Ž
. Murray and McEvoy, 1990 .
As a classical unit operation for the removal of DOC and colour in potable water Ž
. treatment, activated carbon filtration Henry 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,
Ž .
from fish farm waste waters Alderman, 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, Ž
particularly as there is concern and doubt about their fate in the ecosystem Bjorkland 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, formal-
Ž dehyde and Chloramine-T, onto the coal-based activated carbon 207EA supplied 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.
Ž . ŽŽ
. Ž
. .
1 Malachite Green C H N
P C H O
; oxalate salt; molecular weight 929
23 25
2 2 2
2 4 3
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
Ž .
about 2 ppm. A Malachite Green q formalin mixture in proportions of about 1:80 is Ž
. used to treat external parasites and fin rot. According to Alderman 1985 , Malachite
Green dye produced industrially in the past often varied widely both in actual concentra- tion and in its precise composition, so that it is difficult to compare earlier studies on
dosage regimes and toxic effects. It is now known to be highly toxic to mammalian cells Ž
. and to act as a liver tumour promoter Panandiker 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 Ž
. Ž
weeks in the liver Kasuga et al., 1992 , where it is cytotoxic Zahn and Braunbeck, .
1995 . It appears to have a strong affinity for organic matter and is adsorbed readily onto Ž
. suspended organic matter in the water column Sagar et al., 1994 .
Ž . Ž
2 Chloramine-T C H ClNO SNa; N-chloro-p-toluene-sulfonamide sodium salt;
7 7
2
. Ž
. molecular weight 227.6 is added directly to the water concentration 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
Ž carcinogenic in experimental mammals. Neurotoxic effects by the inhibition of acetyl
. cholinesterase activity of Chloramine-T have been reported in humans and amphibians
Ž .
Wang and Minami, 1996 . Ž .
Ž .
3 Oxytetracycline C H N O P 2H O, molecular weight 496.5 is widely used as
22 24
2 9
2
an antibacterial agent and is usually administered in fish feed, from which it may leach Ž
into the water column. It appears to persist in the environment for many months Hansen .
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
Ž .
bacteria Kerry 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
Ž .
interfere with nitrification processes Klaver and Matthews, 1994 . It appears to be Ž
. relatively stable and thus most persistent in anoxic sediment Samuelsen, 1988 .
Ž . Ž
. 4
Formaldehyde CH O, molecular weight 30.03 is used in the aqueous form
2
Ž .
formalin 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
Ž concentration of formalin in the water would be 15–20 ppm when used alone Austin,
. 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.
Ž . 5 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
Ž .
contribution 0.1 mg Crl present from protein in the real effluent. The effluent was diluted to 3 and 5 ppm DOC for the adsorption experiments.
Ž . Ž
. 6
D
-glucose C O H , molecular weight 180 . The adsorption behaviour of this
6 6
12
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.
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 m
2
rg and bulk density of 0.46. The carbon was washed with distilled water, oven dried at 1008C for 24
Ž .
h and crushed to a mesh size of 12 = 40 uus 0.6–1.7 mm . Each single component Ž
. isotherm experiment involved allowing time nominally 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 PO , Na HPO ,
2 4
2 4
Na B O , NaCl and HCl. The experiments were done at pH values of 6, 7 and 8.5,
2 4
7
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
Ž .
isotherms for bulk solutions i.e., the substance of interest dissolved in distilled water to Ž
. very high initial concentrations several 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
Ž .
measured 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
Ž Spectrophotometer, which had previously been calibrated for each substance Arnett 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 ab- sorbance. 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 SO for
2 4
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
Ž .
Ž .
Ž ppm for
D
-glucose, and 0.01–0.03 ppm pH 6 , 0.043 ppm pH 7 and 1.052 ppm pH .
8.5 for Malachite Green. Errors in amounts adsorbed per kg of carbon were: 0.056–0.063 Ž
. Ž
. Ž
. grkg Chloramine-T ; 0.061 grkg Oxytetracycline ; 0.070–0.075 grkg
D
-glucose ;
Ž .
Ž 0.053–0.057 grkg Malachite Green at pH 6 ; 0.059–0.063 grkg Malachite Green at
. Ž
. Ž .
pH 7 ; and 0.260–0.264 grkg Malachite Green at pH 8.5 Arnett and Zhang .
Formaldehyde was measured using a Hach DR2000 spectrophotometer and following Ž
. the MBTH method: 3-methyl-2-benzothiazoline hydrazone
MBTH 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
Ž .
sulphuric 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 concentra-
tion of formaldehyde. The method is extremely sensitive: errors in measured equilibrium concentrations of formaldehyde were 1 ppb or lower. Errors in the amount of formal-
dehyde 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
Ž .
and NDIR detector. Uncertainties expressed as 2 = standard error in total carbon concentration measurements were 1–1.7 ppm.
3. Results
