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
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: q s KC
1r n
, 1
Ž .
eq
where q is the number of moles adsorbed per kg of carbon, C is the equilibrium molar
eq
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 lnC
values. This procedure attaches greater weight to the lower
eq
concentration data. The Langmuir isotherm is defined as:
q s q K C r 1 q K C
, 2
Ž .
Ž . Ž
.
m L
eq L
eq
where C and q are defined as before and where q
and K are empirically
eq m
L
determined constants. For each substance, the values of q and K
were obtained for
m L
each set of experimental conditions by unweighted least squares linear regression of measured C rq ratios on their corresponding C
values.
eq eq
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 Freundlich parameters
Langmuir parameters Adsorption
efficiency
2
pH Temp.
I K
Power R
q K
m L
Ž . Ž
. Ž
. 8C
mM s1r n
Malachite Green 8.5
10 20
146.97 0.7639
0.9638 3.47ey03
2.67eq06 95–98
8.5 20
20 3662
0.8812 0.8441
5.85ey03 5.75eq06
99–100 7
5 2
2.2921 0.5466
0.7487 2.09ey03
1.67eq06 78–96
7 10
2 41.497
0.7825 0.8898
3.64ey03 3.05eq05
71–88 7
20 2
1200.4 0.8822
0.9538 5.80ey03
1.68eq06 97–99
7 30
2 2.00eq14
2.9211 0.8509
y2.46ey04 y6.56eq05
44–85 7
10 0.2
1.3646 0.4716
0.7899 1.89ey03
8.71eq06 85–99
7 20
0.2 3.00eq10
1.952 0.9222
y6.44ey04 y4.90eq06
96–98 7
10 20
10.992 0.7443
0.9433 3.07ey03
1.38eq05 55–76
7 20
20 37.338
0.7368 0.9554
2.87ey03 9.72eq05
83–93 6
5 2
38.582 0.8098
0.8734 3.69ey03
1.68eq05 58–79
6 10
2 26.083
0.7277 0.9334
2.91ey03 6.88eq05
81–92 6
20 2
2.00eq10 2.0408
0.9939 y6.59ey04
y2.02eq06 88–96
6 30
2 3.8757
0.564 0.9332
2.16ey03 2.76eq06
87–97 6
10 0.2
52.285 0.7581
0.8928 3.32ey03
7.84eq05 86–94
6 20
0.2 7.6404
0.5768 0.8504
2.37ey03 5.02eq06
92–99 Chloramine-T
6 5
2 9466
1.3806 0.9747
y3.25ey03 y2.38eq04
24–45 7
5 2
1175.9 1.1495
0.9871 y1.43ey02
y1.22eq04 44–51
6 10
0.2 5.4689
0.5057 0.977
8.84ey03 2.15eq06
94–99 6
10 2
0.5413 0.4194
0.9805 6.02ey03
3.59eq05 64–96
7 10
0.2 3.7739
0.4826 0.9713
8.61ey03 2.21eq06
92–99 7
10 2
3.1232 0.5643
0.9571 7.95ey03
1.68eq05 71–92
7 10
20 5.8771
0.6232 0.9832
8.40ey03 1.29eq05
67–88 8.5
10 20
40.08 0.7961
0.9854 1.35ey02
7.28eq04 73–85
6 20
0.2 18.699
0.6868 0.9677
9.76ey03 1.79eq05
76–88 6
20 2
57.495 0.7911
0.9816 1.31ey02
8.50eq04 74–84
7 20
0.2 28.09
0.7364 0.9911
1.16ey02 9.80eq04
73–86 7
20 2
26.874 0.7419
0.9866 1.07ey02
9.42eq04 71–82
7 20
20 61.746
0.8429 0.9889
1.73ey02 2.90eq04
64–72 8.5
20 20
61.459 0.819
0.9783 1.51ey02
4.87eq04 70–81
6 30
2 720.28
1.0052 0.9801
y8.59ey02 y7.52eq03
73–79 7
30 2
275.03 0.9092
0.982 5.52ey02
1.68eq04 79–85
7 10
2 9.7028
0.6479 0.9617
70–92 7
20 2
28.104 0.7458
0.9883 67–82
7 20
2 6.6835
0.6351 0.9749
55–82 Oxytetracycline
6 20
0.2 778.87
0.9985 1
6.44ey02 1.24eq04
80 6
20 2
911.82 0.9992
0.9999 y9.75ey03
y9.37eq04 82
7 20
0.2 629.48
0.9863 1
8.21ey03 9.72eq04
79–80
Ž .
Table 1 continued Experimental conditions
Freundlich parameters Langmuir parameters
Adsorption efficiency
2
pH Temp.
I K
Power R
q K
m L
Ž . Ž
. Ž
. 8C
mM s1r n
Oxytetracycline 7
20 2
682.19 0.991
0.9999 1.26ey02
6.36eq04 79–80
7 20
20 653.26
0.99 0.9997
1.76ey02 4.38eq04
79–80 8.5
20 20
659.48 1.001
0.9998 9.64ey02
6.75eq03 76–77
6 10
0.2 738.5
0.9819 0.9997
7.87ey03 1.28eq05
83–84 6
10 2
757.97 0.9836
0.9993 5.05ey03
2.02eq05 82–84
7 10
0.2 810.55
0.9786 0.9997
6.74ey03 1.75eq05
85–86 7
10 2
1023.2 0.9985
1 7.10ey02
1.48eq04 84
7 10
20 738.5
0.9819 0.9997
7.87ey03 1.28eq05
83–84 8.5
10 20
810.55 0.9786
0.9997 6.74ey03
1.75eq05 85–86
6 30
2 1249.5
0.9932 0.9994
4.46ey02 3.14eq04
87–88 7
30 2
1430.4 0.9986
1 7.79ey02
1.88eq04 88
6 5
2 353.42
0.9892 1
8.72ey03 4.86eq04
67–68 7
5 2
412.17 0.9982
1 4.65ey02
9.14eq03 68
Formaldehyde 6
10 0.2
86.313 0.7442
0.8305 73–92
7 20
2 2.3136
0.3849 0.4686
74–99
D
-glucose 7
20 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
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
Ž Ž ..
Ž Ž ..
Fig. 1 a and for the Chloramine-T data Fig. 1 b .
Let this relationship be expressed as: 1rn s X ln K q Y ,
3
Ž .
where X and Y are constants equal to the slope and intercept of the graph, respectively. The Freundlich isotherm can be rewritten to:
w x
1rn s ln q y ln K rln C . 4
Ž .
eq
Equating both expressions for 1rn gives: ln K s ln q y Y ln C
r 1 q X ln C .
5
Ž .
Ž . Ž
.
eq eq
Thus, if X and Y are known, a single measurement of q and C is sufficient to
eq
determine the relevant Freundlich K parameter, and from this the corresponding value of 1rn.
Ž . Fig. 1. Freundlich parameters obtained for adsorption onto carbon 207EA of Malachite Green a , Chloramine-T
Ž . Ž .
b and Oxytetracycline c under a range of experimental conditions.
The Freundlich parameters fitted to the Oxytetracycline data vary little with experi- Ž
Ž .. mental conditions Fig. 1 c : 1rn is always close to unity and K is generally between
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
and q , for every data set for every adsorbate all lie on a
L m
Ž .
single correlation curve Fig. 3 . Thus, a single measurement of q and C is sufficient
eq
to obtain the appropriate K and q
values for modelling adsorption under a particular
L m
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.
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 Fre-
undlich 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. Ž
. Ž
. Temp. 8C
pH I
Relative adsorbability strongest to weakest 5
6 2
Malachite Green GOxytetracyclineChloramine-T DOC 5
7 2
Malachite Green OxytetracyclineChloramine-T DOC 10
6 0.2
Chloramine-T Malachite Green Formaldehyde Oxytetracycline DOC
10 6
2 Chloramine-T G Malachite Green Oxytetracycline DOC
10 7
0.2 Chloramine-T Malachite Green Oxytetracycline DOC
10 7
2 Malachite Green GOxytetracyclineGChloramine-T DOC
10 7
20 OxytetracyclineGChloramine-T Malachite Green DOC
10 8.5
20 Malachite Green OxytetracyclineChloramine-T DOC
20 6
0.2 Malachite Green Chloramine-T Oxytetracycline DOC
20 6
2 Malachite Green OxytetracyclinesChloramine-T DOC
20 7
0.2 Malachite Green Chloramine-T GOxytetracycline DOC
20 7
2 FormaldehydeChloramine-T GOxytetracycline
Malachite Green G
D
-glucoses DOC 20
7 20
Malachite Green OxytetracyclineChloramine-T DOC 20
8.5 20
Malachite Green OxytetracyclineGChloramine-T DOC 30
6 2
Malachite Green OxytetracyclineChloramine-T DOC 30
7 2
OxytetracyclineChloramine-T G Malachite Green DOC
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.
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
