Chemosphere 53 (2003) 487–494
www.elsevier.com/locate/chemosphere
Removal of heavy metals from a metaliferous water
solution by Typha latifolia plants and sewage sludge compost
T. Manios
a
a,*
, E.I. Stentiford b, P. Millner
c
School of Agricultural Technology, Technological and Educational Institute of Crete, Technical University of Crete,
Heraklion 71500, Crete, Greece
b
School of Civil Engineering, Leeds University, LS2 9JT Leeds, UK
c
School of Biochemistry and Molecular Biology, Leeds University, LS2 9JT, Leeds, UK
Received 2 December 2002; received in revised form 16 April 2003; accepted 9 May 2003
Abstract
Typha latifolia plants, commonly known as cattails, were grown in a mixture of mature sewage sludge compost,
commercial compost and perlite (2:1:1 by volume). Four Groups (A, B, C and D) were irrigated (once every two weeks)
with a solution containing different concentrations of Cu, Ni, and Zn, where in the fifth (group M) tap water was used.
At the end of the 10 weeks experimental period substrate and plants were dried, weighed and analysed for heavy metals.
The amounts of all three metals removed from the irrigation solution, were substantial. In the roots and leaves/stems of
T. latifolia the mean concentration of Zn reached values of 391.7 and 60.8 mg/kg of dry weight (d.w.), respectively. In
the substrate of Group D all three metals recorded their highest mean concentrations of 1156.7 mg/kg d.w. for Cu,
296.7 mg/kg d.w. for Ni and 1231.7 mg/kg d.w. for Zn. Linear correlation analyses suggested that there was a linear
relationship between the concentration of metals in the solutions and the concentration of metals in the substrates at the
end of the experiment. The percentage removal of the metals in the substrate was large, reaching 100% for Cu and Zn in
some groups and almost 96% for Ni in group D. The total amount of metals removed by the plants was considerably
smaller than that of the substrate, due mainly to the small biomass development. A single factor ANOVA test (5% level)
indicated that the build up in the concentration of metals in the roots and the leaves/stems was due to the use of
metaliferous water solution and not from the metals pre-existing in the substrate. The contribution of the plants (both
roots and leaves/stems) in the removing ability of the system was less than 1%.
Ó 2003 Elsevier Ltd. All rights reserved.
Keywords: Heavy metals; Wetlands; Wastewater; Sewage sludge; Compost; Typha latifolia
1. Introduction
The use of constructed wetlands (reed beds) is a lowcost, low-technology method, often used for the removal
of heavy metals from wastewater. In wetlands, metals are
removed from wastewater by plant uptake, chemical
precipitation, ion exchange and adsorption onto clay,
*
Corresponding author. Tel.: +30-2810-379456; fax: +302810-318204.
E-mail address: [email protected] (T. Manios).
organic and inorganic compounds (Martin and Johnson,
1995). The use of different substrates in order to achieve
heavy metals removal has been investigated in recent
years. Karathanasis and Thompson (1993), used six
materials, including peat moss with approximately 90%
organic matter, as wetlands substrate. This increase in the
content of organic mater in the substrate, was considered
to be the main explanation for the high removal performances (reaching values of 100%), achieved by the wetlands treating simulated acid mine drainage wastewater
(Karathanasis and Thompson, 1993). A similar link between organic matter and heavy metals removal was also
0045-6535/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0045-6535(03)00537-X
488
T. Manios et al. / Chemosphere 53 (2003) 487–494
reported by a number of other researchers (Eger, 1994;
Mitchell and Karathanasis, 1995; Chu et al., 2000).
The reasons why organic matter retains heavy metals
are complicated. According to Eger (1994), who treated
mine drainage water using wetlands with a substrate rich
in organic matter (peat), the heavy metals were retained
mainly due to exchange between the large number of
hydrogen ions on the peatÕs colloidal surface and the
metal cations. This phenomenon is correlated with the
cation exchange capacity (CEC) of the soil.
All the organic materials mentioned above (used in
wetlands treating metaliferous wastewater) contained
small amounts of metals before the trials. On the contrary sewage sludge compost is a waste derived material
with increased concentrations of heavy metals. Consequently, its use in agriculture and horticulture is problematic with strict regulations determining both the
amount and the method of application (EU Directive
86/278/EEC). The possible use of such material as a biofilter removing heavy metals from wastewater could
provide an important alternative application.
Different plants have been used to achieve maximum
heavy metals removal from metaliferous wastewater, in
recent years. Mitchell and Karathanasis (1995) used
bulrush plants (Scipus validus), Ye et al. (1997b) used
Phragmites australis, while Mungur et al. (1997) studied
the accumulation of heavy metals in the root systems
of four different plants (P. australis, Typha latifolia,
Schoenoplectus lacustris, Iris pseudacorus). The concentration of heavy metals in the plant tissue of cattails
(Typha spp) is highly dependent on a number of parameters, like the solution pH, the original heavy metals
concentration in both the solution and the substrate and
the exposure time (Karathanasis and Thompson, 1993).
Up to now all relevant studies aimed to determine the
ability of the T. latifolia to remove metals from wastewater (Ye et al., 1997a). In all cases the results indicated
that there is a considerable ability of the plants to do so.
However the concentration of heavy metals in the substrates used in such experiments was small. The presence
of metals in the substrate in conjunction with a metaliferous wastewater, and how that affects the removal
ability of T. latifolia has not been studied.
Therefore the objectives of this study were:
(a) determining the potential use of sewage sludge compost in a wetlandÕs substrate, treating wastewater
containing heavy metals, and
(b) determining the effect of such substrate, in the heavy
metals absorbing ability of T. latifolia plants, which
are often used in wetlands.
The above two objectives will be tackled by evaluating:
(i) the ability of the compost to remove metals, since
due to previous adsorption of metal ions on the sur-
face of the compost particles (including colloidal
particles) and/or metals already exchanged with hydrogen ions (or other cations) its ability to absorb
metals might be reduced,
(ii) the possible saturation limit of the substrate, considering the existing high concentration of metals
in the sludge, and
(iii) the amount of metals which could be retained by
the plantsÕ tissue from the wastewater.
2. Materials and methods
The main component of the substrate was mature
sewage sludge compost. The compost was produced by
Thames Water Plc using windrows. Sewage sludge was
mixed with straw on a 1:1 by volume basis (v/v). The
chemical characteristics of the sewage sludge and the
produced compost are shown in Table 1. The final material used in the pots was a mixture of this compost with
commercial peat based compost (25% v/v) and perlite
(25% v/v). The use of peat and perlite was considered as
necessary in order to avoid any phytotoxic phenomena
from the compost. It is well established that mixtures
containing more than 50% v/v mature compost of any
origin can produce some kind of phytotoxicity (Manios
et al., in press). The pH of the mixture was 7.1 and the
concentrations of Cu, Ni and Zn were 567 34.71,
47 6.82 and 745 21.50 mg/kg dry weight (d.w.), respectively ( standard deviation for three samples, measured with atomic absorption spectrophotometer, AAS).
A large number of young and healthy T. latifolia
plants (approximately 10 to 20 cm high) were gathered
from a local lagoon. In each pot a single plant was introduced and was left in the substrate for six weeks to
adjust. At the end of that period 30 plants were selected
Table 1
Typical characteristics of the sewage sludge and the produced
compost (supplied by Thames Water Plc)
Parameters
Dry matter (% w.w.)
Volatile solids (% d.w.)
pH
Sewage sludge
Compost
25.2
66.4
6.7
31
65
7.9
Total-P (% d.w.)
2.3
2.6
Cu (mg/kg d.w.)
Zn (mg/kg d.w.)
Ni (mg/kg d.w.)
Cd (mg/kg d.w.)
Pb (mg/kg d.w.)
Cr (mg/kg d.w.)
Hg (mg/kg d.w.)
As (mg/kg d.w.)
Se (mg/kg d.w.)
599
728
99
1.2
191
134
2.5
2.5
2.0
525
825
68
1.5
189
118
2.6
1.9
1.9
T. Manios et al. / Chemosphere 53 (2003) 487–494
for the experiment. The plants were separated into five
groups with six replicates (pots) in each. The selection
was based in the height and the number of leaves of each
plant in order to achieve uniformity among groups.
Groups A, B, C and D were the groups, which would be
watered with the heavy metals solution, and Group M
which would be used as blank and watered with tap water.
Table 2 shows the concentrations of metals used for
the different groups. These concentrations are multiples
of those found in domestic wastewater (Tchobanoglous
and Burton, 1996) and are considerably higher than
those found in other types of wastewater, for example,
acid mine drainage (Eger, 1994). Such high concentrations would be sufficient to reach the potential saturation limit of the substrates in the relatively short
experimental period.
PVC pots were used, in all five groups, with an average diameter of 200 mm and a height of 200 mm, with
a usable volume of 5.0 l. The PVC trays, inserted under
each pot, were large enough to retain any water drained
from the pot, enabling its reabsorption from the substrate. Each pot of each group was given 1 l of the
groupÕs solution every two weeks. The solution was
added slowly to the surface of the soil taking care not to
spill any on the leaves or outside the pot. In total five
waterings (week 0, 2, 4, 6 and 8) with the heavy metal
solutions took place over a period of 10 weeks. This
amount for Group M is zero since the concentration of
metals in the tap water was below detectable levels.
At the end of the 10th week the plants were carefully
uprooted, washed thoroughly with water and soap, and
then were rinsed twice with distilled water in order to
489
wash off any soil particles. After washing, roots and
leaves/stem of each plant were separated, inserted in preweighed paper bags and dried (80 °C for 72 h) in order
to record their dry weight. The contents of each pot were
emptied into large plastic containers and weighed. A
representative sample from each pot was used to estimate moisture (dried at 105 °C for 24 h) and calculate
the dry weight of the substrate in each pot.
For determining the heavy metals concentration in
the roots and leaves/stem (separately) of the plants of
each group, six samples were used, one gram (d.w.) of
tissue in each sample. For the same analysis in the
substrate, six samples (five grams, d.w.) were used for
each group (one from each pot). All samples were introduced into special digestion tubes (Buchi 430 Digestor) with concentrated (97%) nitric acid, 25 ml for the
plant samples and 100 ml for the substrate samples.
Based on relevant literature (Sposito et al., 1983), the
digestion process included a 24 h period with the mixture at room temperature (left undisturbed), followed by
a four hour digestion at a range of high temperatures.
For the first hour the temperature was 100 °C, for the
second hour 150 °C and finally 200 °C for two hours The
remaining liquid, which most times was about one
quarter of the original acidÕs volume, was filtered using
Whatman GF/C filters. De-ionised water was added
until the new solution reached the volume of the acid
originally used (25 ml and 100 ml for the plantsÕ and
substratesÕ samples, respectively). The samples were then
analysed using an (AAS, Spectra AA-10). The mean
concentration of the three metals from the six samples of
each group are presented in Table 3, for the substrate,
Table 2
Concentration of heavy metals in each experimental groupÕs solution and the solutionÕs pH
Group
Group
Group
Group
Group
M
A
B
C
D
Cu (mg/l)
[Cu(NO3 )2 5H2 O]
Ni (mg/l)
[Ni(NO3 )2 6H2 O]
Zn (mg/l)
[Zn(NO3 )2 6H2 O]
pH
–
10
20
40
80
–
5
10
20
40
–
10
20
40
80
7.53
6.55
6.32
6.30
6.15
Table 3
Mean metals concentration (mg/kg d.w.) in the substrate of each group at the end of the experiment
Group
M
A
B
C
D
Cu
Ni
a
480.00 46.55
588.33 46.34b
646.67 225.88c
828.33 34.84d
1156.67 46.55e
Zn
a
40.00 10.00
48.33 13.44a
76.67 21.34b
140.00 25.16c
296.67 26.87d
758.33 41.39a
762.50 37.39a
903.33 79.72b
1063.33 72.72b
1231.67 128.12c
Note: six replicates per group, () standard deviation.
The mean values followed by different superscripts within each column indicate that they were significantly different at a probability
level of 0.05 according to ANOVA test.
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T. Manios et al. / Chemosphere 53 (2003) 487–494
Table 4
Mean metals concentration (mg/kg d.w.) in the roots of the
plants of each group at the end of the experiment
Group
Cu
Ni
Zn
M
A
B
C
D
40.00 14.14a
46.67 12.47b
45.00 9.58b
60.00 10.00c
93.33 12.47d
30.00 8.16a
38.33 12.13ab
45.00 15.00b
51.67 10.67b
55.00 9.57b
293.33 28.09a
300.00 20.00a
330.00 21.60b
361.67 36.25bc
391.67 19.51c
Note: six replicates per group, () standard deviation.
The mean values followed by different superscripts within each
column indicate that they were significantly different at a
probability level of 0.05 according to ANOVA test.
Table 5
Mean metals concentration (mg/kg d.w.) in the leaves/stems of
the plants of each group at the end of the experiment
Group
Cu
Ni
Zn
M
A
B
C
D
9.17 3.44a
10.83 4.48ab
10.83 5.34ab
14.17 4.48b
15.00 7.64b
17.50 6.92a
21.67 8.98ab
25.00 9.58ab
27.67 4.53b
27.50 3.82b
34.18 15.38a
48.33 12.14ab
58.33 10.68b
55.83 8.38b
60.83 13.04b
Note: six replicates per group, () standard deviation.
The mean values followed by different superscripts within each
column indicate that they were significantly different at a
probability level of 0.05 according to ANOVA test.
in Table 4, for the roots and in Table 5 for the leaves/
stems.
Linear regression was used for evaluating the effect of
the amount of metals in the watering solutions in the
mean metals concentration in the substrate and the
plantsÕ biomass. In order to evaluate statistically any
significant differences among mean values, a single factor ANOVA test was used. In all tests the significance
level at which we evaluated critical values differences was
5%.
3. Results and discussion
As the experiment progressed, the amount of metals
present in each pot increased in all groups, with the
exception of the blank, which remained the same and
equal to the original amount of metals pre-existing in the
substrate. Respectively, and according to Tables 3–5, the
mean concentration of Cu, Ni and Zn in the substrate,
the roots and the leaves, at the end of the experiment,
was larger in Groups A, B, C and D compared to Group
M. For Groups C and D this difference was significant
(5% level) according to a single factor ANOVA test,
among all five groups. For Groups A and B the difference was significant in most cases but not in all. Since
Table 6
Correlation coefficient ðr2 Þ from the linear correlation between
the amount of metals added in the four groups and the mean
metals concentration in the substrate, the roots and leaves/
stems of the four groups
Cu
Ni
Zn
Substrate
Roots
Leaves/stems
0.907
0.884
0.998
0.896
0.979
0.999
0.869
0.860
0.700
the use of different irrigation solutions was the only
notable difference among the groups, is safe to suggest
that the differentiation in the concentration of metals
should be correlated with the watering pattern. In order
to support this theory a linear regression test was used to
correlate the amount of each metal in the irrigation solutions with the relevant concentration in the substrate,
the roots and the leaves/stems, through the groups. The
results indicated that there was a strong linear relationship among these variables (Table 6).
Both sewage sludge compost and the peat based
commercial compost contained organic matter. These
two materials made up 75% of the substrateÕs volume,
which result in a high concentration of organic mater in
the final mixture. According to Stevenson (1994) and
Eger (1994) organic matter causes higher CEC, more
than any other soil component. This high CEC of the
substrate is likely to be the main reason for the retention
of such large amounts of heavy metals (Fig. 1), resulting
in the increased concentrations presented in Table 3.
This correlation between the presence of organic matter
and the removal of metals has been supported by a large
number of researchers (Karathanasis and Thompson,
1993; Eger, 1994; Mitchell and Karathanasis, 1995; Tam
and Wong, 1999). The only external parameter which
might decrease the ability of such rich in organic mater
substrate, to retain heavy metals would be an acidic
water solution, of a pH lower than 5.5 (Stevenson, 1994;
Mitchell and Karathanasis, 1995). The pH of the solutions used in this experiment varied (Table 2) but in all
cases remain above 6.0 and close to neutral, not affecting
the process.
Based in the above two statistical tests is safe to
suggest that the metals accumulated in the roots–leaves
origin mostly from the artificial wastewater for three
reasons: (a) the metals in the wastewater were in the easily
absorbable by the plants chemical form of diluted inorganic salts, (b) the majority of the metals in the compost
were retained by the colloids delaying their release in the
water solution, and (c) the short duration of the experiment did not allow the plants adequate time in order to
absorb metals from the substrate. The concentrations of
metals in the roots and leaves, as presented in Tables 4
and 5, are some of the larger recorded in literature for
cattail plants (Mungur et al., 1995; Ye et al., 1997a).
T. Manios et al. / Chemosphere 53 (2003) 487–494
491
Fig. 1. Comparison between the original and the final mean amount of Cu, Ni and Zn in the substrate of each group. Note: six
replicates per group, () standard deviation. The mean values followed by different superscripts indicate that they were significantly
different at a probabililty level of 0.05 according to ANOVA test.
The results presented in Tables 3–5 and their statistical analyses (Table 6) do prove that there was a substantial increase in the metals concentration in the
substrate, the roots and the leaves/stems of groups A, B,
C and D. However, what is of equal interest, is the
evaluation of the importance of this accumulation in the
overall ability of the system (substrate and plants) to
absorb and remove heavy metals from a metaliferous
water solution. In order to estimate this, is necessary to
calculate the amount of metals absorbed by the plants
and the substrate and compare them with the total
amount of metals added through the water solutions.
By combining the substrateÕs dry weight in each pot
of each group, together with the heavy metals concentration before and after the experiment (Table 3) was
possible to estimate the original and final mean amount
(mg/pot d.w.) of all three metals in the substrate of each
group (Fig. 1). The values of r2 , from the linear correlation between the amount of Cu, Ni and Zn, added
through the irrigation solution and the amount retained
in the substrates (Table 6), suggest a linear relationship
between those two variables. This pattern, was also
presented by Karathanasis and Thompson (1993), Eger
(1994), Mitchell and Karathanasis (1995) and Chu et al.
(2000). All the above researchers explained such results
by suggesting that the substrates did not became saturated by the externally added metals. This conclusion is
of great importance considering the original assumptions of this study. The existing concentration of metals
in the substrate was high (due to the presence of sewage
sludge), but this did not seemed to had any effect either
in the ability of the substrate to absorb metals or
reaching a possible saturation point. Possibly, the reason
for not reaching saturation, was the short experimental
period. However the large concentration of metals in the
watering solutions (compared to any type of wastewater)
suggests other wise.
When comparing the net amount of metals absorbed
by the substrate of each pot with the total amount of
metals introduced in each pot is possible to estimate the
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T. Manios et al. / Chemosphere 53 (2003) 487–494
Table 7
Absorbance percentage of heavy metals by the substrate
Group
Cu
Ni
Zn
A
B
C
D
32.96 2.79
58.99 5.87
97.01 9.47
112.8 9.07
5.05 0.43
44.24 4.40
69.20 6.75
95.59 7.68
29.29 2.47
118.05 11.75
118.67 11.58
93.4 7.51
Note: six replicates per group, () standard deviation.
removal percentage. Table 7 presents mean removal
percentage values of the substrate for each group for all
three metals. There is close similarity with relevant research published by Mitchell and Karathanasis (1995)
and Tam and Wong (1999). Cu, Ni and Zn showed
high percentage of absorbance, which was increasing as
the amount of these three metals increased in the watering solutions (from Group A to D). According to our
original assumption, we considered the substrate unable
to retain large amounts of externally added metals, due
to the high original concentration of its components
(Table 1). Based on that, it was expected to have the
higher values of percentage absorbance in Group A, due
to the small quantity of metals used in the watering
solution. This proved to be wrong. Additionally Zn in
Groups B and C, and Cu in Group D, showed percentages above 100%. This represents a mathematical
statistical error due to the complicated experimental
design and the materials used, which can be explained in
a satisfactory manner.
As watering was taking place the amount of metals
added in the pots was becoming more and more substantial compared to the amount of metals existing in
the substrate before the experiment. In Group A, due to
the addition of smaller amount of metals the effect of the
watering was not as substantial as in the other groups.
For example, the mean amount of Cu at the end of the
experimental period was in Group A 447.6 37.9 mg/
pot, where in the beginning was 431.09 36.5 mg/pot.
The addition in this pot of 50 mg was not substantial
enough to create a measurable effect in the percentage
absorbance of the substrate in this group. Additionally
the standard deviation values were almost equal to the
amount added, creating a mathematical abnormality in
the calculations of the absorbance percentage where
mean values were used. Respectively the amount added
in Group D for Cu (400 mg per pot) was large and al-
most equal to the mean amount of the metal existing in
the pot in the beginning of the experiment (433.4 36.5
mg/pot) and substantially larger than the standard deviation. This large amount of Cu had a considerable
effect in the substrate resulting to a mean amount at the
end of the experiment equal to 885.6 37.9 mg/pot. Due
to the high standard deviations in the original and final
amount of Cu in Group D it was possible to calculate a
mean percentage removal above 100%. The physical
meaning of this value represents the absorbance of the
great majority of Cu and Zn into the substrate.
Table 8 shows the mean dry weight of the roots and
leaves/stems (g/plant) for Groups A, B, C, D and M. By
combining these mean values with the mean heavy
metals concentrations recorded in the roots (Table 4)
and the leaves/stems (Table 5) of each group was possible to estimate the mean amount of metals absorbed by
both types of tissue, in each group, at the end of the
experimental period. However, these metals were absorbed from both the substrate and the irrigation solution. In order to estimate the net amount of metals
absorbed entirely from the metaliferous water, the mean
amount of metals, recorded in both tissues of Group M
plants (calculated by combining Tables 4, 5 and 8), was
deducted from the mean values of Groups A, B, C and
D plants (also calculated by combining Tables 4, 5 and
8), respectively (Fig. 2).
The r2 values from the linear correlation between the
amount of Ni and Zn, added through the irrigation and
the amount absorbed by the roots, did suggest a linear
relation. As with the substrate, this could be regarded as
an evidence, that the maximum absorbing ability of the
roots was never reached (saturation level). The correlation between the concentration of the metals in leaves/
stems (Fig. 2) and the amount of metals added could not
be easily characterised as linear for any of the three
metals (Table 6). All three values are lower than 0.9 with
the r2 value for Zn, as low as 0.7, indicating a possible
saturation state. In simpler terms this means that the
leaves/stems were unable to transfer and accumulate any
additional metal cations, from the roots, regardless of
the increasing metalÕs build up in the roots (Table 4).
This theory is furthermore supported by the high concentration values of Cu, Ni and Zn in the roots and
the leaves/stems of the T. latifolia plants, some of the
larger ever recorded (Mitchell and Karathanasis, 1995;
Mungur et al., 1995; Ye et al., 1997a).
Table 8
Mean dry weight of roots and leaves/stems (g) per plant of each group
Leaves/stems
Roots
A
B
C
D
M
1.65 1.35
4.44 1.29
1.68 1.29
3.83 2.61
2.49 1.29
2.78 1.60
1.69 0.87
4.13 1.93
1.08 0.64
1.94 0.92
Note: six replicates per group, () standard deviation.
T. Manios et al. / Chemosphere 53 (2003) 487–494
493
Fig. 2. Net mean amount of metals removed by the roots and accumulated in the leaves/stems from the artificial wastewater together
with the removal percentages recorded by both plantsÕ tissues. Note: six replicates per group, () standard deviation.
It is difficult, though, to support undoubtedly, if
these high concentrations could be related and in what
extent with the metals either in the irrigation solution or
the metals all ready existing in the substrate. According
to Sims and Kline (1991) and Manios and Stentiford
(1997), the available fraction of Cu, Ni and Zn, in
sewage sludge compost is less than 5–10% of the total
concentration. The metals added through the irrigation
were all in water soluble form, and as such directly
available to the plants. Considering all the above it
could be suggested that the presence of large amounts of
metals in the substrate did not had a significant effect in
the absorbing-accumulating ability of the plants. This is
supported by the statistical tests used for evaluating the
origin of the metals responsible for the increased concentration recorded in the roots and leaves/stems, and
which are presented in the beginning of this section.
By comparing the net amount of metals absorbed by
each plant of each group (mg/plant d.w.) with the
amount of metals introduced in each pot is possible to
calculate the mean absorbance percentage achieved by
the plantsÕ tissue of each group. Fig. 2 shows the absorbance percentage of metals from the watering solution by, the plants. Most percentages were less than 1%
for the roots whereas in the leaves/stems were less than
0.1%. These values are in agreement with similar work
of Karathanasis and Thompson (1993), Mitchell and
Karathanasis (1995), Ye et al. (1997a,b) and Scholes
et al. (1998). Based in these values the ability of the
plants to remove metals from metaliferous water was
considerably smaller than that of the substrate. It could
be argued though that these values do not indicate the
real potential ability of the plants since they were calculated using a single plantÕs dry weight per pot. In full scale
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T. Manios et al. / Chemosphere 53 (2003) 487–494
wetlands the number of plants is considerably larger, as
larger is their total biomass produced per area unit.
4. Conclusions
The presence of organic mater in the substrate,
through the use of both sewage sludge and peat resulted
in a mixture with high CEC. This high CEC was mainly
responsible for removing by the substrate, large
amounts of heavy metals from the metaliferous water
solution which was used for the irrigation of T. latifolia.
The percentage removal for Cu, Ni and Zn reached very
high values. However, these percentages varied, depending on the metal and the concentration in the watering solution. The results suggest that the substrate
never reached a saturation point, where no additional
amount of metals could be removed. In summary, sewage sludge compost could be an important component
of a substrate in a subsurface flow wetland, designed to
remove heavy metals from wastewater. The use of
compost did not seem to effect substantially the heavy
metal absorbing ability of T. latifolia. For the leaves/
stems a saturation point was probably reached, due to
the high metal concentration in the watering solution,
but the roots removed considerably larger amounts of
metals, without reaching a saturation state. It could be
suggested that if a wetland is sufficiently vegetated by
metal absorbing plants like T. latifolia, even though
harvesting will not occur, the overall treating ability of
the system should be increased.
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