Abhayawardhana et al. /Journal of Tropical Forestry and Environment Vol. 9, No. 02 (2019) 65-77

_____________________________________________
*Correspondence: dnudilini@gmail.com
ISSN 2235-9370 Print/ISSN 2235-9362 Online © University of Sri Jayewardenepura

Removal of Heavy Metals and Nutrients from Municipal Wastewater

using Salvinia molesta and Lemna gibba

M.L.D.D. Abhayawardhana1*, N.J.G.J. Bandara1 and S.K.L.S. Rupasinghe2

1Department of Forestry and Environmental Science, University of Sri Jayewardenepura, Sri Lanka

2National Water Supply and Drainage Board, Ratmalana

Date Received: 26-03-2019 Date Accepted: 12-12-2019

Abstract

The present study was focused on the investigation of the abilities and efficiencies of Salvinia

molesta and Lemna gibba to remove selected heavy metals (Cr, Cu, Fe, Ni and Pb) and excess nutrients

from wastewater taken from the Moratuwa-Ratmalana municipal wastewater treatment plant. The

wastewater samples were analysed for pH, Temperature, for N-Nitrates, N-Nitrites, ammonia Nitrogen,

Phosphates, and selected heavy metals, Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand

(COD) and Total Kjeldhal Nitrogen. Then, the wastewater samples were treated with S. molesta and L.

gibba separately for a period of 7 days and analysed for N-Nitrates, N-Nitrites, Ammonia Nitrogen,

Phosphates and five selected heavy metals at 24 hour intervals. BOD, COD and Total Kjeldhal Nitrogen

were analysed at 7 days intervals. The average Total Nitrogen removal efficiencies of S. molesta and L.

gibba were 73.3% and 62.1% whereas the average Total Phosphate removal efficiencies of S. molesta and

L. gibba were 72.6% and 77.2% respectively. The average Cr, Cu, Fe, Ni and Pb removal efficiencies of

S. molesta were 81.6%, 69.8%, 65.2%, 66.3% and 74.8% respectively. The average Cr, Cu, Fe, Ni and Pb

removal efficiencies shown by L. gibba were 86.9%, 69.7%, 73.1%, 61.8% and 85.7% respectively. The

Bio Concentration factors of S. molesta for Cr, Cu, Fe, Pb and Ni were 823, 698, 652, 663 and 748

respectively and the Bio Concentration factors of S. molesta for Cr, Cu, Fe, Pb and Ni were 870, 698, 731,

618 and 857 respectively. According to the obtained results in the present study S. molesta and L. gibba

can be considered as suitable candidates for the polishing of municipal wastewater.

Keywords: Heavy metals, Lemna gibba, nutrients, Salvinia molesta

1. Introduction

Wastewater contains Biodegradable organics, Nutrients; mainly nitrogen and phosphate

compounds that lead to eutrophication, Organic pollutants, heavy metals and dissolved inorganics such as

sodium, calcium and sulphates etc. Almost all the human activities generate wastewater. Nature has an

assimilation capacity to cope with small amounts of wastewater and pollution associated with it, but

nature has no capacity to handle the huge amount of wastewater that is generated every day after being

subjected to human consumption and several other uses. Therefore, the treatment of this water and

returning clean and safe water into the waterways is essential to ensure the safety of people and the

environment. The proper treatment of wastewater is an environmental challenge since the wastewater is

required to be treated and disposed safely in an efficient manner. Further, the presence of some

components in the water poses a challenge in the wastewater treatment process.

DOI: https://doi.org/10.31357/jtfe.v9i2.4469

Enrichment of large quantities of nitrogen and phosphate compounds in wastewater is one of the

main causes of eutrophication that negatively affects many natural water bodies. Heavy metals are

considered as priority pollutants due to their acute toxicity. Heavy metals in wastewater cause detrimental

effects on environment and human health. The heavy metals in wastewater have the potential of bio-

accumulation which can cause adverse impacts on environment and human health.

Phytoremediation is the use of both aquatic and terrestrial plants for the treatment of contaminated

water and soils (Ali et al., 2013). According to previous studies, some aquatic macrophytes have shown a

great promise in wastewater treatment, both in the removal of nutrients (Phosphates, Nitrogen

compounds) and heavy metals. S. molesta and L. gibba are two preferable candidates which had been

used for the treatment of wastewater in previous studies. Low cost and easy maintenance make the aquatic

plant system preferable to use. In an aqueous solution, metals are available in soluble form. Therefore, the

accumulation by the aquatic plants can be achieved much more easily and more efficiently than using

terrestrial plants.

S. molesta is a free floating aquatic plant which spreads rapidly by vegetative reproduction. The

species is well-known for its phytoremediation potential (Koutika and Rainey, 2015). L. gibba is a rooted

free-floating aquatic plant consisting of small fronds. Due to the high growth rate and large potential for

the uptake of heavy metals and nutrients, members of Genus Lemna have appeared as suitable candidates

for the phytoremediation of heavy metal contaminated wastewater (Verma and Suthar, 2015).

According to previous studies, the influent of Moratuwa-Ratmalana wastewater treatment plant

contains Total nitrogen and total phosphates higher than the upper limits given in the standards. The

presence of several heavy metals i.e. Cu, Cr, Pb, Fe, and Ni is also recorded (Danushika et al., 2017).

Since the sludge disposed by the treatment plant constitutes of heavy metals, the final disposal of sludge

has posed problematic issues. The study is carried out to determine the efficiency of S. molesta and L.

gibba as candidates for polishing of the influent of Moratuwa-Ratmalana wastewater treatment plant. The

potential of using S. molesta and L. gibba for the removal of nutrients (nitrogen compounds, phosphates)

and selected heavy metals (Cr, Cu, Ni, Pb, Fe) is determined in the study, thus investigating the capability

of these two aquatic macrophytes to be used for the treatment of municipal wastewater.

2. Methodology

2.1 Plant acquisition and acclimatisation

S. molesta was collected from a fresh water body near the University. L. gibba was collected from

Moratuwa-Ratmalana wastewater treatment plant. Healthy mature plants were selected for the experiment

and rinsed with tap water in order to remove adhering mud particles or epiphytes. The plants were left for

an adaptation period of 10 days, growing in the containers inside the Green house of Department of

Forestry and Environmental Science, University of Sri Jayewardenepura to be adapted for the

experimental conditions.

2.2 Sample collection

Wastewater samples were collected from the inlet of the Moratuwa-Ratmalana wastewater

treatment plant. Samples were collected weekly. After the collection, the samples were checked for pH,

Temperature, Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Nitrates, Nitrites,

Ammonia Nitrogen, Phosphates and Total Kjeldhal Nitrogen and heavy metals (Cr, Cd, Cu, Mn, Pb, Ni,

and Fe).

The pH was measured using the pH meter and the temperature was measured using a thermometer

at the time of sample collection. BOD was measured using Winkler method. COD was determined using

closed reflux method. Ammonia Nitrogen was measured by comparative Ammonia method. Total

Kjeldhal Nitrogen concentration was measured by Micro Kjeldhal method. The concentrations of Nitrates,

Nitrites and Phosphates were measured using HACH 890 meter by Powder Pillow method. The

Abhayawardhana et al. /Journal of Tropical Forestry and Environment Vol. 9, No. 02 (2019) 65-77

concentrations of the heavy metals were measured by Atomic Absorption Spectroscopy (AAS) method.

The wastewater samples were collected in polypropylene bottles and were transferred to the green house

immediately.

2.3 Experimental design

Eight rectangular shaped glass aquariums with the dimensions of 1 ̍×6 ̎×6 ̎ were used to perform

the experiment. Six containers were taken and 3l of collected wastewater was added to each tank. To three

tanks, 15 g of S. molesta was introduced. To other three tanks, 15 g of L. gibba was introduced. The other

two tanks were filled with distilled water. 15 g of S. molesta was added to one tank and 15 g of L. gibba

was added to the other tank. These two tanks with distilled water were used as control samples in order to

compare the relative growth of the plants in wastewater and the control samples. Each experiment was

carried out for 7 days. 40 ml of wastewater samples were withdrawn from each tank at 24 hour intervals.

The seven day experiment was repeated for six times.

2.4 Sample analysis

The samples withdrawn from the tanks at 24 hour intervals were checked for pH, temperature,

nitrates, nitrites, ammonia nitrogen and phosphates. The collected water samples were filtered using

Whatmann No.1 filter papers and were analysed using atomic absorption spectroscopy (AAS) method to

determine Cr, Cu, Pb, Ni and Fe concentrations at 24 hour intervals. At the end of the experiment, on the

7th day, the samples were checked for BOD, COD and total Kjeldhal nitrogen.

2.5 Analysis of plant materials

On the 7th day, plant materials were harvested and their fresh weights were determined. Then the

plant materials were oven dried at 800° C for 48 hours and the dry weights were determined. Then the

dried biomass was digested according to dry digestion method (Kalagbor and Opusunju, 2015) and were

analysed for concentrations of Cr, Cu, Pb, Ni, and Fe using Atomic Absorption Spectroscopy.

2.6 Calculations

The following parameters were calculated using the mean values of the data obtained through the

experiments.

Relative growth

Relative growth values were calculated for S. molesta and L. gibba using the initial fresh weight

and final fresh weight values.

Relative growth=
Final fresh weight (g)

Initial fresh weight (g)

(Thayaparan et al., 2013)

Removal efficiency

The removal efficiencies for total Nitrogen, total Phosphates, COD, BOD and selected heavy

metals by S. molesta and L. gibba were calculated using initial and final values.

Removal efficiency =
Initial concentration -Final concentration

Initial concentration
×100

Metal uptake capacity

The uptake capacities for each metal for S. molesta and L. gibba were calculated.

Metal uptake capacity (mg/kg)=
Metal concentration of the dried biomass (

mg
l

) ×Total diluted volume(ml)

Dry weight (g)

(1)

(2)

(3)

(Lokuge, 2016)

Bio Concentration Factor (BCF)

The Bio Concentration Factors for each metal for S. molesta and L. gibba were calculated.

BCF=

Metal concentration of the dried biomass (
mg
kg

)

Initial concentration of the metal in external solution (
mg
l

)

(Uysal, 2013)

2.7 Statistical Analysis

The data were statistically analysed using Minitab 18 software. The significant differences among

the parameters were determined by One-way Anova test. The treatment means were compared using

Tukey’s 95% simultaneous confidence intervals test.

3. Results and Discussion

3.1 Nutrient removal by S. molesta and L. gibba

Total nitrogen

In the present study, the final total nitrogen concentrations in the tanks with S. molesta and L.

gibba were 31.82 mg l-1 and 51.81 mg l-1 respectively, whereas the initial Total Nitrogen concentration

was 136.27 mg l-1 S. molesta showed a significantly higher potential (p<0.05) in the uptake of total

nitrogen than L. gibba in the present study.

Figure 1. The mean values of initial and final concentrations of Total Nitrogen in the tank with S. molesta

and in the tank with L. gibba. Bars indicate mean±SD (Standard Deviation), where n=6.

According to Ng and Chan (2017), the nitrate concentration in palm oil mill effluent treated by S.

molesta has increased until day 12 and then has slightly decreased in the end. In a free floating system, the

total nitrate concentration is controlled by nitrification and denitrification, in addition to plant and

microbial uptake. In the present study also, the nitrate concentration of the tank with S. molesta had

remained constant from day 3 to day 4. The nitrate concentration in L. gibba had increased slightly from

day 3 to day 4. These are due to the increase of nitrates as a result of nitrification. However, in the present

study, the increase of nitrates by nitrification is not much high. nitrification/ denitrification can be caused

by nitrifying bacteria which are attached to flocculates (Korner and Vermaat, 1998). But, the flocculates

settled to the bottom of the tanks due to the small depth of the tanks. Therefore, the nitrification/

denitrification by suspended bacteria can be neglected.

100

120

140

160

Initial L.gibba S.molesta

T
o
ta

l
N

it
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g
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c
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a
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(

m
g
/l

)

Sample

(4)

Abhayawardhana et al. /Journal of Tropical Forestry and Environment Vol. 9, No. 02 (2019) 65-77

In the present study, volatilisation of ammonium ions as ammonia gas can be neglected since the

surface of the tank is covered with the dense mat of plant materials. Also, the pH value of the wastewater

had remained close to neutral values throughout the experiment. Therefore, only a small amount of

ammonium ion may be present as ammonia gas.

The concentrations of nitrites, nitrates, ammonia nitrogen and phosphates in the tank with S.

molesta had decreased daily in the seven day period. The total nitrate concentration did not show a

gradual decrease. The concentration of nitrates had remained constant from day 3 to day 4. The

concentrations of nitrites, nitrates, ammonia nitrogen and phosphates in the tank with L. gibba had

decreased in the period of 7 days. The concentration of nitrates has shown a slight increase from day 3 to

day 4.

Figure 2. The daily variation of the mean values of concentrations of nitrites, nitrates, ammonia nitrogen

and phosphates in the tank with S.molesta where n=6.

The total nitrogen removal efficiencies of S. molesta and L. gibba were 73.35% and 62.18%

respectively. S. molesta has shown a significantly greater (p<0.05) efficiency in the removal of Total

Nitrogen than L. gibba. According to Žaltauskaitė et al., (2014), the efficiency of L. gibba in the removal

of Nitrogen from wastewater is between 42%-62% of total nitrogen, depending on initial nitrogen

concentrations. The present study has also shown similar results. According to Mkandawire and Dudel,

(2005), the total Nitrogen removal efficiency by Lemna spp. is 50%. In the present study, L. gibba has

demonstrated an efficiency higher than that.

3.1.2 Total phosphate

Phosphorous is an essential macronutrient for plants which is required for the synthesis of

Adenosine Diphosphate (ADP), Adenosine Triphosphate (ATP) and nucleic acids. Therefore, free

orthophosphates are specifically absorbed from wastewater by plant systems. In the present study, the

initial concentrations of Total Phosphates in the tanks with S. molesta and L. gibba were 1.65 mg l-1 and

1.5 mg l-1 respectively, whereas the initial concentration was 6.17 mg/l . The final Total Phosphate

concentrations in the two tanks were not significantly different (p>0.05).

0 1 2 3 4 5 6 7 8

C
o
n

c
e
n

tr
a
ti

o
n

(
m

g
/l

)

Sampling Interval (Days)Nitrate Nitrite Ammonia N Phosphate

Figure 3. The daily variation of the mean values of concentrations of nitrites, nitrates, ammonia nitrogen

and phosphates in the tank with L. gibba where n=6.

Figure 4. The mean values of initial and final concentrations of total phosphate in the tank with S. molesta

and in the tank with L. gibba. Bars indicate mean±SD, where n=6.

Mohedano et al., (2012) showed a final total phosphate concentration of 5.2 mg l-1 on day seven,

whereas the initial concentration was 215 mg l-1 in which the swine waste was treated by L. gibba. In this

study, the effluent has been sent through a system consisting of a storage pond and two duckweed ponds.

Therefore, the removal potential is higher than the present study as the effluent has gone through the

Duckweed pond twice in a single treatment and because of the increased surface area for the treatment.

The total Phosphate removal efficiencies of S. molesta and L. gibba were 72.63% and 77.29%

respectively which were not significantly different (p>0.05). According to Žaltauskaitė et al., (2014), the

total phosphate removal efficiency from municipal wastewater by L. minor is 100%. However,

Mkandawire and Dudel, (2005) shows that the efficiency of Total Phosphate removal by Lemna spp. is

between 50-60%. In the present study, L. gibba has demonstrated a greater efficiency than that.

BOD and COD

The mean initial BOD value was 259.22 mg l-1 whereas the mean final BOD values in the tank

with S. molesta and in the tank with L. gibba were 73.75 mg l-1 and 85.29 mg l-1 respectively which were

not significantly different (p>0.05). The mean initial COD value was 530.19 mg/l whereas the mean final

0 1 2 3 4 5 6 7 8

C
o
n
c
e
n
tr

a
ti

o
n
(

m
g
/l

)

Sampling Interval (Days)Nitrate Nitrite

Initial L.gibba S.molesta

T
o
ta

l
P

h
o
sp

h
a
te

s
c
o
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c
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tr

a
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(

m
g
/l

)

Sample

Abhayawardhana et al. /Journal of Tropical Forestry and Environment Vol. 9, No. 02 (2019) 65-77

COD values in the tank with S. molesta and in the tank with L. gibba were 200.80 mg l-1 and 111.89 mg l-

1 respectively which were significantly different (p<0.05). The COD reduction potential demonstrated by

L. gibba was higher than in that of S. molesta in the present study. In the present study, the BOD removal

efficiencies of S. molesta and L. gibba were 71.51% and 67.24% respectively which were not

significantly different (p>0.05). The COD removal efficiencies of S. molesta and L. gibba were 61.985%

and 78.957% respectively. L.gibba demonstrated a significantly higher efficiency (p<0.05) in the removal

of COD.

According to Mkandawire and Dudel (2005), the BOD removal efficiency and COD removal

efficiency of Lemna spp. are 60% and 30-40% respectively. But, in the present study, L. gibba has shown

greater efficiencies than that in the removal of both BOD and COD. Kumari and Tripathi (2014) reports

BOD and COD removal efficiencies by mixed culture of Eichhornia crassipes and Salvinia natans

accompanied by aeration in municipal wastewater as 84.5% and 83.2% respectively. The efficiencies are

greater than the BOD and COD removal efficiencies of S. molesta and L. gibba recorded in the present

study. The effect of aeration and the enhancement of removal caused by the use of mixed culture of

Eichhornia crassipes and Salvinia natans can be the reasons for higher efficiencies.

Figure 5. The mean values of initial and final values of BOD in the tank with S. molesta and in the tank

with L. gibba. Bars indicate mean ± SD, where n= 6.

Figure 6. The mean values of initial and final values of COD in the tanks with S. molesta and L. gibba.

Bars indicate mean±SD, where n=6.

100

150

200

250

300

Initial L.gibba S.molesta

B
io

c
h

e
m

ic
a
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O
x
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D
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(

m
g
/l

)

Sample

100

200

300

400

500

600

Initial L.gibba S.molesta

C
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(

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/l

)

Sample

3.2 Heavy metal removal by Salvinia molesta and Lemna gibba

Chromium (Cr)

The final Cr concentrations of the two tanks were not significantly different (p>0.05). The Cr

uptake capacity of L. gibba was significantly greater (p<0.05) than that of S. molesta. The Bio

Concentration Factor (BCF) of S. molesta and L. gibba were not significantly different (p>0.05) and both

were lower than 1,000. The initial Cr concentration was higher than tolerance limits for industrial and

domestic wastewater discharged into marine coastal areas (1 mg l-1). The final concentrations of Cr in

both tanks were lower than the tolerance limit. But, toxicity symptoms were not observed in the harvested

plant materials.

Table 1: The mean values of initial Cr concentration in wastewater samples, final Cr concentration after

being subjected to the treatment by plants, the metal uptake capacities of the two species and the

Bio Concentration Factors (BCF).

Characteristic
Control

Salvinia molesta
Control

Lemna gibba
Salvinia molesta Lemna gibba

Initial concentration

(mg l-1)
- - 1.58±0.08

A1
1.58±0.08

Final concentration

(mg l-1)
- - 0.29±0.07

B1
0.21±0.05

Metal uptake capacity

(mg kg
-1

)
- - 64.32±2.44

B2
68.55±2.64

BCF - - 823.2±40.78
A3

870.0±28.25
A3

The significant differences are indicated by superscripted letters.

The Cr removal efficiencies of S. molesta and L. gibba are 81.66% and 86.99% respectively,

which were significantly different (p<0.05). L. gibba demonstrated a higher Cr removal efficiency than S.

molesta in the present study.

Copper (Cu)

The final Cu concentrations of the two tanks were not significantly different (p >0.05). In

addition, the Cu uptake capacities of S. molesta and L. gibba were not significantly different (p>0.05).

The BCF values of the two plants for Cu were not significantly different (p>0.05) and the values were

lower than 1000.

The Cu removal efficiencies of S.molesta and L.gibba were 69.81% and 69.78% respectively. The

efficiencies were not significantly different (p>0.05).

Table 2: The mean values of initial Cu concentration in wastewater samples, final Cu concentration after

being subjected to the treatment by plants, the metal uptake capacities of the two species and the

Bio Concentration Factors (BCF).

Characteristic Control
Salvinia molesta

Control
Lemna gibba

Salvinia molesta Lemna gibba

Initial concentration

(mg l
-1

)
- - 0.302±0.13

A1
0.302±0.13

Final concentration

(mg l
-1

)
- - 0.097±0.07

B1
0.094±0.05

Metal uptake capacity

(mg kg
-1

)
- - 10.283±3.78

A2
10.408±4.14

BCF - - 698.029±72.86
A3

698.421±64.80
A3

The significant differences are indicated by superscripted letters.

Iron (Fe)

Abhayawardhana et al. /Journal of Tropical Forestry and Environment Vol. 9, No. 02 (2019) 65-77

The final Fe concentrations of the two tanks were not significantly different (p >0.05). The Fe

uptake capacities of S. molesta and L. gibba were not significantly different (p>0.05). In addition, the

BCF values of the two plants for Fe were not significantly different (p>0.05) and the values were lower

than 1000. The Fe removal efficiencies of S. molesta and L. gibba were 65.27% and 73.10% respectively.

The efficiencies were not significantly different (p>0.05).

Nickel (Ni)

The final Ni concentrations in both tanks were not significantly different (p>0.05). The Ni uptake

capacities of S.molesta and L.gibba were not significantly different (p>0.05). The BCF values of the two

plants for Ni were not significantly different (p>0.05) and the values were lower than 1000.

Table 3: The mean values of initial Fe concentration in wastewater samples, final Fe concentration after

being subjected to the treatment by plants, the metal uptake capacities of the two species and the

Bio Concentration Factors (BCF).

Characteristic
Control

Salvinia molesta

Control

Lemna gibba
Salvinia molesta Lemna gibba

Initial concentration

(mg l-1)
- - 0.124±0.02A1 0.124±0.02A1

Final concentration

(mg l-1)
- - 0.044±0.01B1 0.036±0.02B1

Metal uptake

capacity (mg kg-1)
- - 4.008±1.79A2 4.408±1.74A2

BCF - - 652.7±53.71A3 731.0±120.16A3

The significant differences are indicated by superscripted letters.

The Ni removal efficiencies of S. molesta and L. gibba were 66.39% and 61.87% respectively. The

efficiencies were not significantly different (p>0.05).

Table 4: The mean values of initial Ni concentration in wastewater samples, final Ni concentration after

being subjected to the treatment by plants, the metal uptake capacities of the two species and the

Bio Concentration

Factors (BCF).

The significant differences are indicated by superscripted letters.

Lead (Pb)

The final Pb concentrations of the two tanks were not significantly different (p>0.05). The Pb

uptake capacities of S. molesta and L. gibba were not significantly different (p>0.05). The BCF values

were lower than 1000 and the BCF value of L. gibba was significantly higher (p<0.05) than that of S.

molesta. The Pb removal efficiencies of S. molesta and L. gibba were 74.85% and 85.74% respectively.

The Pb removal efficiency by L. gibba was significantly higher (p<0.05) than that of S. molesta.

Characteristic
Control

Salvinia molesta

Control

Lemna gibba
Salvinia molesta Lemna gibba

Initial concentration

(mg l-1)
- - 0.217±0.04A1 0.217±0.04A1

Final concentration
(mg l-1)

- - 0.072±0.01B1 0.081±0.01B1

Metal uptake

capacity (mg kg-1)
- - 7.258±1.79A2 6.783±1.74A2

BCF - - 663.9±69.63A3 618.7±51.32A3

Table 5: The mean values of initial Pb concentration in wastewater samples, final Pb concentration after

being subjected to the treatment by plants, the metal uptake capacities of the two species and the

Bio Concentration Factors (BCF).

Control

Salvinia molesta

Control

Lemna gibba
Salvinia molesta Lemna gibba

Initial concentration

(mg l-1)
- - 0.292±0.07A1 0.292±0.07A1

Final concentration

(mg l-1)
- - 0.073±0.02B1 0.043±0.01B1

Metal uptake

capacity (mg kg-1)
- - 10.917±2.83A2 12.45±3.05A2

BCF - - 749.0±34.78B3 857.4±26.61A3

The significant differences are indicated by superscripted letters.

According to Al-Khafaji et al. (2017) the Cr, Ni and Pb removal efficiencies of Lemna minor are

32.26%, 74.48%, and 79.1% respectively. However, in the present study, L.gibba has reported higher

efficiencies for the removal of Cr and Pb and lower efficiency for the removal of Ni. According to Verma

and Suthar (2015), Pb removal efficiency of L.gibba at pH 7 and 2 mg l-1 Pb concentrations is 93.8%

which is higher than the present study. Dhir and Srivastava (2011) report Cu, Ni and Cr removal

efficiencies in a multi-metal solution by Salvinia natans respectively as 73.8%, 56.8%, and 41.4%. In the

present study, S.molesta has shown higher efficiencies for the removal of Ni and Cr and lower efficiencies

for the removal of Cu.

Bio Concentration Factor (BCF)

Bio Concentration Factor (BCF) is an index that provides information about the potential of the

plants for the uptake of heavy metals. If the BCF value exceeds 1,000, the particular plant can be

considered as a useful candidate for the uptake of heavy metals (Zayed et al. (1998). In the present study,

none of the BCF values exceeded 1,000. According to Thayaparan et al. (2013), the BCF values of Azolla

pinnata for Pb had increased with the increase in the initial Pb concentration and the BCF value is higher

than 1,000 when the initial Pb concentration exceeds 4 mg l-1. Lokuge (2016) reports a reduction in BCFs

of Cr, Cd, Ni and Pb when they were presented in altogether. According to the author, the competition of

the metals for the uptake sites and the interactive effects of the metals are the reasons for the reduction of

the BCF values.

According to Ranjitha et al. (2016), S. molesta has shown its potential in the uptake of Cu, Cr, Pb

and Cd and any change in growth regulation is not observed. According to the observations of the present

study also, S. molesta and L. gibba had grown healthily with the accumulation of these heavy metals.

Toxicity symptoms caused by heavy metals were not observed in the harvested plant materials. However,

the tolerance limits of the plants for the heavy metals are not discovered. Zayed et al. (1998) reports the

BCF values of Duckweeds as 500 to 800 for Cu, 400 to 700 for Cr, and 50 to 450 for Pb at low supply

concentrations (1 mg l-1). In the present study, similar BCF values are recorded for Cu, but higher BCF

values are recorded for Pb and Cr.

3.3 Plant material analysis

The relative growth values of the two control samples were not significantly different (p>0.05).

The relative growth values of S. molesta and L. gibba were significantly different (p<0.05). The relative

growth value of S. molesta was the highest. The relative growth value of L. gibba was the second highest.

The relative growth values of both control samples were lower than the S .molesta and L. gibba samples.

The dry weights of the two control samples were not significantly different (p>0.05) and the dry

weights of S. molesta and L. gibba were not significantly different (p>0.05). The fresh weights of S.

molesta and L. gibba were significantly different (p<0.05). The dry weights of the two control samples

Abhayawardhana et al. /Journal of Tropical Forestry and Environment Vol. 9, No. 02 (2019) 65-77

were not significantly different (p>0.05). The final fresh weight of S. molesta plants was the highest

because of its high moisture content. However, the dry weight of S. molesta was lower than the dry

weight of L. gibba.

Table 6: The mean final fresh weights, mean relative growth values and the mean dry weights obtained

from the tank with Salvinia molesta, the tank with Lemna gibba, the control tank with Salvinia

molesta and the control tank with Lemna gibba.

Control
Salvinia molesta

Control

Lemna gibba
Salvinia molesta Lemna gibba

Fresh weight (g) 17.828±2.16C1 16.479±1.37C1 34.918±3.67A1 24.197±3.67B1

Relative growth 1.188±0.14C2 1.099±0.09C2 2.191±0.37A2 1.613±0.24B2

Dry weight (g) 0.536±0.20B3 0.543±0.12B3 1.091±0.19A3 1.135±0.35A3

n=6. The significant differences are indicated by superscripted letters.

In the control sample, distilled water was used as the substrate for the plant growth. At the end of

the experiment, on the 7th day, a brown coloration in leaves was observed in the control sample of S.

molesta. But, S. molesta plants grown in the wastewater tank did not show a discoloration. The most

likely reason for the brown colour observed in the control sample of S. molesta is the lack of nutrients.

However, in the control sample of L. gibba, no discoloration was observed. The L. gibba plants

showed an increase in growth even in the control sample as well as in the wastewater sample. Therefore,

the potential of L. gibba species to survive in adverse conditions can be identified.

According to Zayed et al. (1998), growth reduction of Duckweeds is reported only at 10 mg l-1 Ni

concentration. Also, Ranjitha et al. (2016) reports that there is no change in the growth rate of S. molesta

when exposed to Cr, Cu, Cd and Pb containing wastewater where the particular heavy metals were present

in concentrations lower than 2.5 mg l-1. Therefore in the present study, the low concentration of heavy

metals may be the reason for the absence of growth reduction in the plants grown in wastewater tanks.

Srivastav et al. (1994) report the relative growth values of Salvinia spp. at 1 ppm of Cr

concentration as 1.15 and at 1 ppm Ni concentration as 1.17. Also he reports the relative growth values of

Spirodela spp. at 1 ppm of Cr concentration and at 1 ppm Ni concentration respectively as 1.16 and 1.13.

In the present study, S. molesta and L. gibba plants grown in wastewater and control sample of S. molesta

recorded greater relative growth values. The most likely reason is the low concentrations of heavy metals

in wastewater. Only Cr was present in a concentration higher than 1 ppm in the wastewater used in the

present study. The moisture content of fresh S. molesta may be the reason for the high relative growth

value in control sample.

4. Discussion

According to the obtained results, S. molesta and L. gibba have proven their potential for the

removal of nutrients and heavy metals from municipal wastewater. S. molesta has performed well in the

removal of Total Nitrogen, Phosphates, BOD and COD reporting the removal efficiencies respectively as

73.35%, 72.63%, 71.51% and 61.98%. Also, L. gibba demonstrated a significant removal of Total

Nitrogen, Phosphates, BOD and COD reporting the removal efficiencies respectively as 62.18%, 77.28%,

67.24% and 78.96%. S. molesta was more efficient in the removal of Total Nitrogen and L. gibba showed

a greater removal efficiency for COD.

The average Cr, Cu, Fe, Ni and Pb removal efficiencies of S. molesta were 81.66%, 69.81%,

65.26%, 66.39% and 74.85% respectively. The average Cr, Cu, Fe, Ni and Pb removal efficiencies shown

by L. gibba were 86.99%, 69.77%, 73.10%, 61.87% and 85.74% respectively. L. gibba performed greater

removal efficiencies in the removal of Cr and Pb when compared to S. molesta. But, the Bio

Concentration Factors recorded by S. molesta and L. gibba in the uptake of selected heavy metals were

always lower than 1000 in the present study. Visual toxicity symptoms were not observed in the plant

materials grown in the wastewater tanks. However, both S. molesta and L. gibba can be considered as

suitable candidates for the removal of nutrients and these heavy metals (Cr, Cu, Fe, Ni and Pb) from

wastewater even at low heavy metal concentrations.

5. Conclusion

This study was carried out to investigate the nutrient and heavy metal removal potential and the

efficiencies of S. molesta and L. gibba for the removal of nutrients and selected heavy metals (Cr, Cu, Fe,

Ni and Pb) from municipal wastewater samples collected from the influent of Moratuwa-Ratmalana

wastewater treatment plant.

These aquatic plants have the potential of phytoaccumulation of nutrients and heavy metals from

water. This strategy is a low cost and eco-friendly technology for the treatment of wastewater. The

invasive nature of S. molesta and the fast growth rate of L. gibba are the practical issues that may arise in

the practical use of these plants.

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