Biosorption of phenolic compounds by the brown alga Sargassum
muticum.
Eugenia Rubín, Pilar Rodríguez*, Roberto Herrero and Manuel E. Sastre de
Vicente.
Departamento de Química Física e Enxeñería Química I, Universidade da
Coruña, Alejandro de la Sota 1, 15008 A Coruña, Spain.
*Corresponding author e-mail: prbarro@udc.es; Phone: (34) 981 167000
(ext.2197); Fax: (34) 981 167065
Abstract
Phenol, 2-chlorophenol (2-CP), and 4-chlorophenol (4-CP) biosorption on
Sargassum muticum, an invasive macroalga in Europe, has been investigated.
The efficiency of this biosorbent was studied measuring the equilibrium uptake
using the batch technique. A chemical pre-treatment with CaCl2 has been
employed in this study in order to improve the stability as well as the sorption
capacity of the algal biomass. The influence of pH on the equilibrium binding
and the effect of the algal dose were evaluated. The experimental data at pH=1
have been analysed using Langmuir and Freundlich isotherms. It was found
that the maximum sorption capacity of chlorophenols, qmax=251 mg g-1 for 4-CP
and qmax=79 mg g-1 for 2-CP, as well as that of a binary mixture of both
chlorophenols, qmax=108 mg g-1, is much higher than that of phenol, qmax=4.6
mg g-1. Moreover, sorption kinetics have been performed and it was observed
1
that the equilibrium was reached in less than 10 h. Kinetic data have been fitted
to the first order Lagergren model, from which the rate constant and the sorption
capacity were determined. Finally, biosorption of the phenolic compounds
examined in the present study on Sargassum muticum biomass was observed
to be correlated with the octanol-water partitioning coefficients of the phenols.
This result allows us to postulate that hydrophobic interactions are the main
responsible for the sorption equilibrium binding.
Keywords: phenol; monochlorophenol; seaweed; biosorption; adsorption
isotherms; adsorption kinetics, hydrophobicity.
1. INTRODUCTION
Phenol and chlorophenols are long-lived pollutants frequently found in
industrial effluents. Phenols are widely used for the commercial production of a
wide variety of resins including phenolic resins, epoxy resins and adhesives,
and polyamide for various applications. Chlorophenols are used extensively as
fungicides, herbicides, insecticides, pharmaceuticals, as preservative for wood,
glue, paint, vegetable fibbers, and leather, and intermediates in chemical
synthesis.1 Chlorophenols may also be formed by the degradation of
chlorinated pesticides, by the reaction of chlorinated water supplies with phenol
in the environment, and during incineration of organic material in the presence
of chloride. Phenolic compounds impart objectionable taste and odour to
drinking water at concentration as low as 0.005 mg dm -3.2 Besides, they are
harmful to organisms and toxic to humans.
In order to keep waters at acceptable quality levels, different purification
2
methods were developed for the treatment of wastewaters. Adsorption onto
activated carbon is commonly utilised to remove phenolic compounds from
wastewater3 but the high cost of activated carbon led to investigate the use of
cheaper materials. Dry activated sludge and fly ash, 4 bentonite,5 eggshell
membranes,6 papermill sludge,7 rice husk8 and chitosan9 are some of the
sorbents investigated.
Brown algae have been successfully used as biosorbent for heavy
metals10-14 and their high efficiency to remove methylene blue has been recently
shown.15 Ion exchange plays a predominant role in sorption mechanism in the
heavy metal adsorption process and hydrophobic interactions seem to be
responsible for the alga-dye binding. In the case of phenolic compounds
biosorption, hydrophobic7,16 and donor acceptor4 interactions, have been
suggested to drive phenols sorption processes.
This paper reports the results of a preliminary evaluation of the use of the
macroalga Sargassum muticum as biosorbent for the removal of phenol, 2chlorophenol, 4-chlorophenol and a mixture of both chlorophenols from
wastewaters. The study includes the effect of pH and algal dose on the sorption
process as well as the sorption isotherms and the sorption kinetics for these
three phenolic compounds.
2 EXPERIMENTAL
2.1 Preparation of the sorbent
The sorbent used was alga Sargassum muticum collected in A Coruña
(Galicia, NW Spain). Algae were washed with generous amounts of distilled
3
water and dried in an oven at 60C overnight. Then, they were ground in an
analytical mill IKA A 10 and sieved in the size pore range from 0.5 to 1 mm.
The biomass was chemically modified by means of a pre-treatment with
CaCl2, which was carried out as follows.
A sample of 2.5 g of dried biomass was treated with 100 cm 3 of 0.2 mol
dm-3 CaCl2 solution, keeping the solution pH constant at a value of 5.0, because
it is the optimum pH value for calcium activation of biomass. The mixture was
shaken for 24 h on a rotary shaker at 175 rpm and room temperature. The
biomass was then filtered off followed by washing with deionised water to
remove the excess of calcium and it was dried in an oven at 60C for 24 h.17
Pre-treated biomass was kept in plastic containers refrigerated at 4C for
further use.
2.2 Preparation of phenolic solutions
The phenolic compounds used in this study were phenol, 2-chlorophenol
(2-CP) and 4-chlorophenol (4-CP). All reagents were supplied by Aldrich and
they were used without further purification.
A stock solution was prepared by dissolving 1.0 g of phenol, 2-CP or 4CP in 1 dm3 of deionised water. The phenolic solutions for the sorption
experiments were prepared by diluting the stock solution to give different
concentrations within the range 10 to 500 mg dm -3 for phenol and 10 to 1000
mg dm-3 for chlorophenols.
2.3. Sorption experiments
The effect of the solution pH on the sorption capacity of the alga was
examined for phenol, 2-CP and 4-CP. A constant mass of alga (0.1 g) was
4
weighed into a conical flask and 40 cm 3 of phenolic solutions of an initial
concentration of 200 mg dm-3 were added to it adjusting the pH value between
1 and 10 by the addition of dilute HCl or NaOH.
Experiments to determine the equilibrium binding capacity of Sargassum
muticum were carried out mixing the same mass of alga and volume of each
phenolic solution, whose concentration range from 10 to 1000 mg dm -3, and
adjusting the pH to a suitable value. In all experiments these mixtures, were
shaken for 24 h, to ensure equilibrium, at 175 rpm and room temperature
(estimated temperature range 20-25oC). The algal biomass was then filtered
through a 0.45 m pore size cellulose nitrate membrane filter and the phenol or
chlorophenol concentration was determined using 10 mm quartz cells in a
spectrophotometer UV/VIS (Varian Cary 100 Bio) at max. The maximum
absorbed ultraviolet wavelengths of phenol, 2-chlorophenol and 4-chlorophenol
were 270, 274 and 280 nm, respectively. The solutions involved were diluted to
proper concentrations, to give absorbances in the range 0.1-1, before making
the measurements. The concentration of each phenol was calculated by means
of the corresponding molar absorption coefficients obtained from standard
calibration curves whose values and correlation coefficients are listed in Table1.
The sorption capacity of the alga was determined from the concentration
difference of the solution, at the beginning and at equilibrium:
qe 
V C i  C e 
(1)
1000m
where Ci and Ce are the initial and the equilibrium solution concentrations (mg
dm-3), V is the volume of solution (cm3), and m is the mass of alga used (g).
5
In order to evaluate the dependence of the sorbent dose on phenols
removal, an algal mass ranged from 0.05 to 0.5 g was weighed into a conical
flask and 40 cm3 of a phenolic solution of an initial concentration of 200 mg dm-3
at pH=1 were added to it. The binding capacity of the sorbent was determined
as mentioned above.
The kinetic experiments were performed as follows: a constant mass of
alga (0.125 g) was weighed and transferred into a thermostatted cell at
25.00.1oC, 50 cm3 of phenolic solutions at pH=1 and Ci=200 mg dm–3 were
added to it, and the mixtures were shaken. Aliquots were withdrawn at various
time intervals for 24h, and concentration of phenols was determined as
indicated above. The amount of adsorption at time t, qt (mg g-1), was calculated
by:
qt 
VCi  C t 
(2)
1000m
where Ct (mg dm-3) is the concentration of phenols at time t.
Finally, to evaluate the performance of the adsorbent in a binary system,
a constant mass of alga (0.1 g) was weighed into a conical flask and 40 cm 3 of
an equimolecular mixture of 2-CP and 4-CP at pH=1, whose total concentration
range from 10 to 1000 mg dm-3, were added to it. The absorption spectrum of
the mixture of these two chlorophenols gives a wide band, which is the sum of
2-CP and 4-CP spectra. A separate determination of each solute was not
possible as the corresponding absorbance maxima are too close each other.
Alternatively, a calibration curve was done with various equimolecular mixtures,
from which the total chlorophenol adsorbed was calculated.
6
All experiments were performed in duplicate. The accuracy was 0.5 mg
dm-3 and the detectability 5 mg dm-3.
3. RESULTS AND DISCUSSION
3.1. Characterization of the alga biomass
The BET surface area, 2.86 m2 g-1, and pore volume, 0.003 cm3 g-1, of
Sargassum muticum seaweed were determined from N2 adsorption and
desorption isotherms measured at 77 K using a Sorptomatic 1990 equipment.
The surface area of the adsorbent was also determined using the methylene
blue method, the values obtained are in the range 412-206 m2 g-1,15 which are
much higher than that found using the BET method. The reason for this
discrepancy may be a different sorption mechanism for nitrogen and dye
molecules since in the water-wet state, the alga is swollen and there is a waterfilled porous structure.
In order to identify the chemical groups present in the alga a Fourier
Transform Infrared analysis was done using a Bruker spectrophotometer (model
Vector 22). Figure 1 shows the FTIR spectrum of Sargassum muticum treated
with CaCl2. It exhibits absorption bands at 3421, 2926, 1635 and 1035 cm -1,
which indicate the presence of –OH, –COOH, C=O and C-O groups,
respectively.
The total amount of proton binding sites in Sargassum biomass was
estimated by a potentiometric titration of the protonated alga, which was carried
out with a NaOH solution in 0.05 mol dm -3 NaNO3.12 The total number of weak
acid groups found was 2.61 mmol g-1. Although sulphate groups are known to
7
be present in the Sargassum sp.;18 no evidence of their presence was found in
the titration curves.
3.2. Effect of pre-treatment on biomass stability
Preliminary tests were run to determine the sorption capacity of the raw
alga, but it was observed that untreated algae were easily decomposed and the
decomposition products interfere in the absorbance measurements and
consequently, in the determination of phenols in aqueous solution. This led us
to try a pre-treatment with CaCl2, since it has been shown that Ca2+ improves
the stability of alginate molecules by the formation of the characteristic alginate
arrangement known as the egg-box structure.18
Previous studies in our laboratory, with Sargassum muticum,12 have
shown that this chemical treatment presents a high effectiveness in stabilising
the algal biomass. This efficiency was determined in this study evaluating the
values of biomass losses in water during 3-4 h of stirring as well as the
percentages of weight loss due to treatment. It was found that calcium
treatment gives very low values for biomass losses and organic leaching,
compared to the ones for the acid, basic, formaldehyde, acetone, and methanol
treatments. For this reason, in the present work Sargassum muticum treated
with CaCl2 has been chosen to study the sorption of phenols.
3.3 Effect of adsorbent dose
As it is shown in Table 2, the percentage removal of phenols increases
when the adsorbent dose is increased from 0.05 to 0.5 g, while the adsorption
capacity at equilibrium, qe (mg g-1), decreases. The latter result can be
explained as a consequence of a partial aggregation, which occurs at high
8
biomass concentration giving rise a decrease of active sites.
In order to perform the sorption experiments, 0.1 g of Sargassum
muticum seaweed were selected because the value of qe obtained is higher
than those found for higher algal doses. Although the percentage removal of
phenols from aqueous solution increases almost linearly with sorbent
concentration until 0.1 g, after this value it appears to level off. Therefore, a
further increase in algal dose involves a loss of effectiveness of the biomass for
the retention of phenols, due to the aggregation phenomena mentioned above.
On the other hand, 0.1g is a small amount of biosorbent and it is enough to
carry out the sorption experiments.
3.4. Equilibrium of sorption
The sorption equilibrium binding has been described in terms of
Langmuir and Freundlich isotherms,19,20 which are given by equations (3) and
(4), respectively:
qe 
qmax bC e
1  bC e
qe  K F C1e/ n
(3)
(4)
where qmax and qe are the maximum amount of adsorption and the sorption
capacity of the sorbent at equilibrium (mg phenol/g alga), respectively; C e is the
solution concentration at equilibrium (mg dm-3); b is the affinity constant (mg-1
dm3), and KF [(mg g-1)(mg dm-3)n] and n are the Freundlich constants.
Figure 2 shows the sorption capacity against equilibrium concentrations
for each phenolic compound studied and for an equimolecular mixture of
chlorophenols at pH=1. The solid and dashed lines represent the analysis of the
9
experimental data according to equation (3) and to equation (4), respectively.
From these fits the sorption parameters were calculated, and their values
together with the correlation coefficients are summarised in Table 3. It is clear
that experimental data fit quite well to both models. Freundlich model may be
more suitable to describe the equilibrium binding since a saturation point is not
reached but Langmuir model gives the maximum adsorption capacity, which is
an useful parameter to compare the performance with that of other adsorbent
materials.
It can be noticed that the parameter n varies slightly for the different
solutes and pH. Moreover, the n value is different to unity, which implies that the
sorption process over the entire concentration range is complex, involving not
only a partitioning process (n=1) but also another mechanisms.21-23
It is clear that the maximum sorption capacity of chlorophenols is much
higher than that of phenol. Fly ash exhibit a similar trend as it is shown in Table
4, which compares the sorption of phenolic compounds using different sorbents.
In order to justify this behaviour, Aksu and Yener4 suggest the activation of the
aromatic ring of monochlorophenols by the chlorine, which favours the
existence of donor-acceptor interactions between the phenolic compounds and
the groups of the biosorbent surface. They also found that biosorption of 4-CP
is higher than that of 2-CP i.e., the position of the -Cl atom in the benzene ring,
in relation to phenolic -OH substituent, strongly influences the sorption uptake.
They explain this behaviour as a result of steric hindrance between the -Cl and
the -OH substituents in the case of 2-CP. However, other authors7,16,24,25 have
considered hydrophobic interactions to explain the sorption process of phenols
10
onto biomass. These latest studies, taken together with the fact that equilibrium
binding data are acceptably described using the Freundlich isotherm, led us to
try to correlate the sorption capacity of phenol and chlorophenols with the
octanol-water partitioning.
In addition, the evaluation of the influence of the pH of the bulk solution
on the sorption capacity of the phenolic compounds was done, as the
concentration of hydrogen ions in the solution affects the degree of ionisation of
phenolic sorbate and the surface properties of the sorbent.
Figure 3 shows the effect of pH in the sorption capacity of phenol and
chlorophenols. It is observed that the sorption capacity of phenol is slightly
affected by the pH of the solution and it always keeps a low value, which seems
to decrease at pH=10. The uptake of 2-chlorophenol and 4-chlorophenol
decreases from pH=1 to pH=4, increases from pH=4 to pH=6, and then
decreases until pH=10; in fact, the qe value determined for 2-CP at pH=10 is
experimentally negligible. Aksu and Yener4 found a similar behaviour for the
adsorption of chlorophenols on granular activated carbon and they suggested
that these compounds were adsorbed at the carbonyl oxygens on the activated
carbon surface according to a donor-acceptor complexation mechanism.
As mentioned above, the essential effect of pH is the change of the
degree of ionisation of the sorbates and biosorbent. The phenolic compounds
considered in this study, phenol, 2-chlorophenol and 4-chlorophenol, have a
pKa of 9.9, 8.3 and 9.2 respectively;26 so, they become anions only at high pH
values. The algal surface, whose pKa is 3.85 for a degree of dissociation of
0.5,12 is neutral at low pH values, and as the pH increases above this value, the
11
overall surface charge becomes negative. Taking into account these
considerations, the decrease of sorption capacity in the case of chlorophenols
as pH increases, at the pH range 6-10, can be explained as a result of higher
electrostatic repulsion when the sorbate and sorbent are both negatively
charged. However, for the chlorophenols studied, at pH values below 6, the
variation of sorption capacity with pH is not clear if only electrostatic interactions
are considered. To account for the observed behaviour, some additional types
of retention mechanisms should be contemplated, including donor-acceptor
interactions between the aromatic ring activated by the -OH and -Cl substituents
and the groups of the biosorbent surface, and interactions of phenols with the
algae by complex formation or the distribution of phenol and chlorophenols
which are neutral species, between the aqueous solution and the wall cell
structure of the algal cellular membranes.
3.5 Biosorption kinetics
Sorption studies were carried out for 24 h in order to determine the effect
of time on sorption. Figure 4 plots the adsorption capacity of each phenolic
compound, qt (mg g-1), versus time. It is seen that equilibrium was reached in 810h for every adsorbate.
The experimental data were fitted to Lagergren equation27 in the form:

qt  qe 1 e k1 t

(5)
where qe and qt are the sorption capacity at equilibrium and at time t,
respectively (mg g-1) and k1 is the rate constant of pseudo-first order sorption (h1).
The fit to equation (5) is represented in Figure 4 by solid lines, which
12
shows that sorption of the phenolic compounds studied on Sargassum muticum
treated with CaCl2 follows pseudo-first order kinetics. The kinetic parameters
were calculated using the non-linear Marquardt algorithm since a non-linear fit
has the advantage of providing a value for qe, which has to be fixed in the case
of using the linearised form of equation (5). The values obtained and the
corresponding correlation coefficients are listed in Table 5.
3.6. Relationship
between
adsorption capacity and
octanol-water
partitioning
Biosorption of the phenolic compounds examined in the present study on
Sargassum muticum biomass was observed to be correlated with the octanolwater partitioning coefficients of phenols reported by Kishino and Kobayashi. 26
The mathematical relationship was determined using the qe values calculated
from the Freundlich equation at various equilibrium concentrations and pH=1.
The log qe was plotted vs. log Kow for each phenolic compound and data were
fitted to the following equation:
log qe  A log K ow  B
(6)
The results obtained are summarised in Table 6.
It can be noted that qe increases when Kow increases, i.e. more
hydrophobic organic sorbates tend to result in a higher equilibrium sorption
capacity. The data resulting from the present study follow the same trend as the
data previously reported in the literature. In a similar plot of log q e vs. log Kow,
Antizar-Ladislao and Galil16 found a slope of 0.47 for the sorption of phenol and
chlorophenols onto biomass developed in sandy aquifers under bioremediation
13
conditions; Kennedy et al.24 found a slope of 0.65 for the sorption of
chlorophenols by sludge granules; Tsezos and Bell25 also investigated this
relationship for the sorption of organochlorine and organophosphorus pesticides
on live biomass, and they obtained slopes of 1.51 and 0.532 for the biosorption
of pollutants on live activated sludge and live R. arrhizus, respectively. In our
case, the slope of the linear plot of log qe vs. log Kow, shows practically the
same value, close to 1.3, for every concentration of phenols. This result can be
interpreted as a consequence of the importance of hydrophobic interactions in
the equilibrium binding.
4. CONCLUSIONS
The invasive macroalga in Europe, Sargassum muticum, can be used as
biosorbent material to remove phenolic compounds from aqueous solutions.
The algal dose and the solution pH affect the sorption of phenols on Sargassum
muticum. It was found that 0.1g of alga and pH=1 are suitable values to carry
out the sorption experiments. Moreover, it was seen that this biosorbent is also
efficient for removing chlorophenols from binary systems. On the other hand,
biosorption kinetics have shown that the equilibrium was reached in less than
10h. Besides, a pseudo-first order empirical model, from which the
corresponding kinetic parameters were obtained, can describe the sorption
process. The equilibrium binding was described in terms of Langmuir and
Freundlich isotherms. It was observed that the sorption capacity of 2-CP and 4CP by the algal biomass is much higher than that of phenol. Finally, biosorption
of phenol and chlorophenols on Sargassum muticum biomass was correlated
14
with the corresponding octanol-water partition coefficients of the phenolic
compounds, and it was found that qe increases when Kow increases, i.e., more
hydrophobic organic sorbates tend to result in a higher equilibrium sorption
capacity. This behaviour can be interpreted as a consequence of the
importance of hydrophobic interactions in the equilibrium binding.
The results obtained in this study demonstrate the suitability of the alga
Sargassum muticum for industrial use as biosorbent, although additional
experiments have to be carried out for its practical applicability.
ACKNOWLEDGEMENTS
This work was funded by the projects BQU2002-02133 (from the Ministerio de
Ciencia y Tecnología of Spain) and PGDIT02TAM10302PR (from the Xunta de
Galicia).
15
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19
TABLES
Table 1. Molar absorption coefficients for phenol, 2-CP and 4-CP.
Adsorbate
 (M-1 cm-1)
R2
Phenol
1646±9
0.9996
2-Chlorophenol
2028±11
0.9991
4-Chlorophenol
1796±5
0.9998
Table 2. Effect of the algal dose on the percentage of adsorbate removed and
adsorption capacity of Sargassum muticum treated with CaCl2 for different algal
dose at pH=1 and Ci=200 mg L-1 at room temperature.
qe (mg g-1)
Adsorbate removed (%)
ms (g)
Phenol
2-CP
4-CP
Phenol
2-CP
4-CP
0.05
2.2
15.6
20.2
3.8
26.1
35.0
0.1
3.4
22.7
30.6
2.9
19.3
25.9
0.2
4.3
29.0
38.7
1.8
12.2
16.2
0.5
4.9
34.3
48.2
0.8
5.8
8.2
20
Table 3. Adsorption parameters calculated from Langmuir and Freundlich
isotherm equations at room temperature.
Langmuir
Adsorbate
qmax (mg g-1)
b (mg-1 dm3)
R2
Phenol (pH=1)
4.6±0.3
0.006±0.001
0.984
Phenol (pH=2.5)
5.7±0.7
0.007±0.002
0.945
Phenol (pH=4.5)
13±3
0.002±0.001
0.965
2-CP
79±11
0.0019±0.0005
0.971
4-CP
251±45
0.0007±0.0002
0.992
2-CP + 4-CP
108±18
0.0014±0.0004
0.986
KF
n
R2
Phenol (pH=1)
0.0320.004
1.30.2
0.931
Phenol (pH=2.5)
0.160.02
1.80.1
0.973
Phenol (pH=4.5)
0.250.04
1.90.1
0.971
2-CP
0.580.24
1.50.1
0.971
4-CP
0.250.02
1.120.02
0.995
2-CP + 4-CP
0.160.04
1.10.1
0.954
Freundlich
21
Table 4. The maximum sorption capacity of phenols on various biosorbents at
optimum pH values and room temperature.
qmax (mg g-1)
Sorbent
Phenol
2-chlorophenol
4-chlorophenol
Activated carbon
602.328
380.24
422.14
Activated sludge
236.829
281.129
287.229
Fly ash
3.8530
98.74
118.64
S. muticum
4.6*
79*
251*
Rice husk
42.28
___
___
Bentonite
1.7125
___
___
Eggshell membranes
___
___
11.26
* Present work
22
Table 5. Kinetic parameters obtained from the first order equation for the
sorption of phenols on Sargassum muticum treated with CaCl2 at pH=1, Ci=200
mg dm-3 and 25oC.
Adsorbate
qe (mg g-1)
k1 (h-1)
R2
Phenol
2.8±0.1
0.49±0.07
0.967
2-CP
22.0±0.4
0.37±0.02
0.996
4-CP
24.3±0.6
0.52±0.03
0.994
Table 6. Parameters calculated from equation (6) at different equilibrium
concentrations of solute.
Ce (mg dm-3)
A
B
R2
100
1.2±0.2
1.9±0.5
0.9681
200
1.3±0.2
1.7±0.3
0.9846
500
1.30±0.06
1.5±0.1
0.9976
800
1.24±0.03
1.14±0.06
0.9995
23
FIGURE CAPTIONS
Figure 1. FTIR spectrum of Sargassum muticum treated with CaCl2.
Figure 2. Sorption isotherms for phenol, 2-CP, 4-CP and an equimolecular
binary mixture (2-CP + 4-CP) by Sargassum muticum treated with CaCl2 at
pH=1. Symbols represent experimental points (□ phenol, o 2-CP,  4-CP, ◊
equimolecular mixture 2-CP + 4-CP) and solid and dashed lines are modelled
results according to Langmuir and Freundlich isotherms, respectively.
Figure 3. Effect of pH on sorption of phenolic compounds (□ phenol, o 2-CP, 
4-CP) by Sargassum muticum treated with CaCl2 at Ci=200 mg dm-3.
Figure 4. Sorption kinetics for phenol, 2-CP and 4-CP by Sargassum muticum
treated with CaCl2 at pH=1 and Ci=200 mg dm-3. Symbols represent
experimental points (□ phenol, o 2-CP,  4-CP) and solid lines are modelled
results according to equation (5).
24