Temperature was the parameter that had the largest effect on the adsorption efficiency Ž
. of the therapeutants, with the lowest adsorption efficiencies
50 or lower only
Ž .
observed at the lowest temperature 58C . 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 Chlo- ramine-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 corre- sponds 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.
Ž .
The formaldehyde isotherm not 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
Ž .
valid Urano 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
Ž .
Freundlich parameters form an extremely coherent group of results Figs. 1 and 2 . For these reasons, and because the use of the Freundlich isotherm considerably simplifies
Ž .
some multicomponent adsorption modelling calculations Crittenden 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 Ž
. appropriate for those isotherms that constrain adsorption capacity e.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
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 manifesta- Ž
. tions of the well-known ‘‘Characteristic Curve’’ of Polanyi 1920a; b ; where, for a
Ž .
particular adsorbent, all the Freundlich K values or adsorbate volumes
and the adsorption potentials lie on a single correlation curve, irrespective of temperature or
Ž .
which components are considered. Urano et al. 1981 made more explicit the connec- tion 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:
E s RT ln C rC . 6
Ž .
Ž .
eq s
Ž .
The expression for number of moles adsorbed Roginsky, 1948 :
E
max
Q s f E d E.
7
Ž . Ž .
H
E
eq
Ž . The hypothetical exponential adsorption energy E distribution:
f E s Ae
ya E
. 8
Ž . Ž .
The resulting expression is analogous to the Freundlich isotherm equation: A
a RT
Q s C
9
Ž .
a RT
ž
aC
s
with: A
1rn s aRT and K s .
a RT
aC
s
In the above expressions, A and a are constants, R is the gas constant, T is absolute temperature, C is the solubility of the adsorbate, C is the equilibrium concentration of
s
adsorbate in the liquid and E is the adsorption potential in an infinitely dilute
max
solution. For single component adsorption, rearranging the expression for K gives:
1rn s y 1rln C ln K y ln Ara
10
Ž .
Ž .
Ž .
s
or: T ln C s 1raR ln AraK .
11
Ž . Ž
. Ž .
s
To obtain the straight-line relationship between 1rn and ln K with this formulation would require the following to be true:
Ž 1. a and A are constant i.e., each molecule ‘‘sees’’ the same adsorption energy
. distribution ; and
2. either the solubility is approximately constant over the temperature range considered, Ž
. or 1rlnC is a linear function of T requiring K to be approximately constant .In
s
addition, this formulation implies that: Ž
. 3. The slope of the graph of 1rn vs. ln K should be y 1rlnC
and the intercept
s
Ž .
should correspond to yln 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.
Ž The values of ‘‘a’’ in the expression for the adsorption energy distribution where
. ‘‘a’’ is the width of the distribution
can be computed directly from each fitted Ž
. Freundlich 1rn value Table 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 Ž
adsorption. On a histogram with site types listed in order of their adsorption energy not .
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
Ž strength of adsorption of a single component expressed as a proportion of the substance
. that is adsorbed at equilibrium is dependent on the initial concentration, C , as well as
on the Freundlich parameters. Meaningful comparisons between the inherent adsorbabil- ity of different components require this proportion to be computed at the same initial
Ž .
molar concentration C 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 C and q:
eq
q s KC
1r n
the Freundlich isotherm , i
Ž .
Ž .
eq
q s C y C rload,
ii
Ž .
Ž .
eq
Ž Ž
.. Ž Ž ..
where load s mass of activated carbon kg r volume of liquid l in the experiment.
Finally, the estimate of q is used to compute the proportion of the substance that will
U
Ž .
be adsorbed, i.e., load qrC .
Ž . Ž .
The aboved Eqs. i and ii are usually not soluble analytically, but they can very Ž .
easily be solved graphically by plotting q as a function of C using Eq. i and then Eq.
eq
Ž . Ž
Ž . ii and observing where the two curves intersect. This is also true if Eq. i is the
. Langmuir isotherm rather than the Freundlich isotherm . As the Freundlich power 1rn
Ž Ž
. Ž
Ž . .
Ž approaches unity i.e., 1rn
™1 , then C ™C r 1q load K and q™KC r 1q
eq
Ž . .
load K , i.e., and the graphs of C and q vs. C
become straight lines. This is
eq
approximately true of single component Oxytetracycline behaviour. These considerations, plus the adsorption efficiencies observed in the experiments
Ž .
Table 1 , indicate that for the typical concentrations of components present in aquacul- ture 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 Ž
. Ž
. field observations of Smith et al. 1994 that by 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
Ž .
organic components in the sediment Bjorkland et al., Coyne et al., Samuelson would explain its relatively high concentration there and its relatively rapid disappearance from
Ž .
the water column. This, together with its stability Pouliquen 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
Ž .
matter reported by Sagar et al. 1994 .
5. Conclusions
