Summer Research Fellowship Programme of India's Science Academies 2017
Remediation potential of select wetland plants
A B Gunjal
Indian Institute of Science, Bengaluru
Guided by
T.V. Ramachandra
Indian Institute of Science, Bengaluru
Abstract
Phytoremediation is the use of plants for the treatment of contaminated soils and water. The
macrophytes are more important in this regard as they have high biomass which helps for the
uptake of nutrients and also heavy metals. In this study, the wetland plants were sampled from
inlets and outlets of Jakkur lake, Bengaluru on weekly basis. The plants were identified using
taxonomic literatures and the physiology of the plants was also studied. The dry weights of the
plant samples were noted and the biomass was expressed as the dry weight (kg/m
2
). Further,
the remediation potential of select wetland plants was also studied. The powdered samples were
analysed for carbon and nitrogen using CHN analyser. The acid digested samples were also
analysed for phosphorus content by the stannous chloride method. Total seven different
wetland plant species were collected. The plants were identified to be Typha sp., Cyperus sp.,
Ludwigia sp., Pistia stratiotes, Polygonum sp., Alternanthera philoxeroides and Spirodela sp.,
where all the plants are perennial. The plants viz., Typha sp., Cyperus sp., Pistia stratiotes and
Spirodela sp. are monocots, and the plants Ludwigia sp., Polygonum sp. and Alternanthera
philoxeroides are dicots. All the wetland plants showed the ability for the uptake of the
nutrients, viz; carbon, nitrogen and phosphorus. The uptake of carbon and nitrogen was in the
range of 3045 % and 1–5 % respectively. The uptake of phosphorus was in the range of 0.9–
3.5%. The study showed that the wetland plants have the ability to uptake the nutrients, which
proves the plants have the remediation potential. The macrophytes can therefore be used in
constructed wetland systems to remove nitrogen from domestic and agricultural wastewater.
The wetland plants also showed the uptake of the heavy metals viz., cadmium, zinc, nickel,
copper, chromium and lead.
The uptake of Cu and Zn was more by young Typha sp. from inlet and also uptake of Cr was
more by young Typha sp. from outlet during the the 1st week of sampling. The uptake of Cu,
Pb, Zn and Ni was more by young Typha sp. shoot from inlet which was 1.4, 9.0, 9.4 and 10.0
mg/kg respectively during the 2nd week of sampling. Also, the Pb and Zn uptake was more by
mature Typha sp. from outlet which was 13.6 and 10.4 mg/kg respectively during the 3rd week
of sampling. Ni uptake was also more by mature Typha sp. from inlet which was 4.0 mg/kg
during the 3rd week of sampling. The uptake of Pb, Zn and Cd was more by mature Typha sp.
from outlet which was 12.2, 23.0 and 1.4 mg/kg respectively during the 4th week of sampling.
The uptake of Pb, Ni, Cr and Cd was more by medium Polygonum sp. from inlet which was
5.2, 2.4, 12 and 0.8 mg/kg respectively during the 1st week of sampling.
The uptake of Zn was more by mature Alternanthera philoxeroides from outlet which was 10.2
mg/kg during the 2nd week of sampling. The uptake of Cr was found to be more by mature
Alternanthera philoxeroides from outlet.which was 19.6 mg/kg during the 3rd week of
sampling. The uptake of Cu, Pb, Zn and Cr was more by young Alternanthera philoxeroides
form inlet which was 2.2, 9.8, 8.6 and 10.2 mg/kg respectively during the 4th week of sampling.
Similarly, the uptake of Cr was more by Spirodela sp. which was 13.6 mg/kg from outlet during
the 2nd week of sampling.
The uptake of Cu was more by medium Ludwigia sp. from outlet which was 2.4 mg/kg during
the 3rd week of sampling.
Keywords: Phytoremediation; Macrophytes; Physiology; Wetland; Nutrients; Perennial
1. Introduction
The soil and water are increasingly polluted nowadays which is damaging the environment.
Remediation is to avoid all such environmental damage. Remediation can be achieved with the
help of various physico-chemical processes viz., ion-exchange, precipitation, evaporation and
chemical reduction; aquatic plants; and microorganisms (fungi, bacteria, yeasts).
In phytoremediation, the contaminated soils and water are treated in-situ with the help of plants
(Etim, 2012). The aquatic plants have gained more importance in this regard and hence, the
uptake mechanisms and rates of these plants in wetland settings are now more studied. The
plants are especially useful in phytoremediation. The value of metal-accumulating plants to
wetland remediation is gaining importance (Black, 1995).
The phytoremediation has application where the soil and water are less contaminated, where
the material which needs treatment is at shallow or medium depth and the area which has to be
treated is also large. Plants which are able to decontaminate soils or water carries the following
mechanisms: a) uptake of contaminant from soil particles or soil liquid into their roots; b) bind
the contaminant into their root tissue; and c) transportation of contaminant from roots into
shoots and thus, avoid the contaminant from leaching out of the soil (Paz-Alberto and Sigua,
2013).
Phytoremediation is classified into eight different types viz.,
phytoextraction/phytoaccumulation; rhizofiltration; phytovolatilization; phytostabilization;
phytodegradation/phytotransformation; hydraulic control; rhizodegradation/phytostimulation
and phytopumping (Gupta and Balomajumder, 2015). In phytoextraction, the plants uptake and
translocate metal contaminants in the soil with the help of their roots into the above ground
portions (USEPA, 2000). Rhizofiltration involves the adsorption or precipitation on the plant
roots or absorption of contaminants in the solution surrounding the root zone. Rhizofiltration
has application in ground water, surface water, or wastewater for the removal of metals and
inorganic compounds. In phytovolatilization, plants uptake the contaminants from the soil and
transform them into volatile forms (USEPA, 2000). Phytostabilization involves the use of
plants to immobilize contaminants in the soil and ground water through absorption and
accumulation by roots, adsorption onto roots, or precipitation within rhizosphere.
Phytodegradation is conversion of complex molecules to simple molecules and incorporation
of simple molecules into the plant tissues (Trap et al., 2005). Rhizodegradation is the
breakdown of contaminants by the microorganisms within the plant rhizosphere. In
phytopumping, plants act as the organic pumps for the uptake of contaminated water as the part
of transpiration process. In hydraulic control, the phreatophytic trees and plants control the
water table and the soil field capacity (Etim, 2012).
Some examples of plants used in phytoremediation are water hyacinth (Eichhornia crassipes);
poplar tree (Populus spp.); Typha spp; Phragmites spp; forage kochia (Kochia spp.); alfalfa
(Medicago sativa); Kentucky bluegrass (Poa pratensis); Scirpus spp., coontail (Ceratophyllum
demersum L.); American pondweed (Potamogeton nodosus); and the emergent common
arrowhead (Sagittaria latifolia), Brassica juncea and Brassica oleracea, grasses viz., Vetiver
grass (Vetiveria zizanioides), Cogon grass (Imperata cylindrica), Carabao grass (Paspalum
conjugatum), etc. (Herath and Vithanage, 2015).
Aquatic plants (macrophytes) are often used in phytoremediation. These plants are
important component of wetlands (Gupta et al., 2012). Macrophytes have the special ability,
i.e., adaptation to functioning in permanent contact with the surface water and groundwater
(Schulz et al., 2003). They have thin outer tissues and aerenchyma, through which air is
distributed to the parts of plant below the surface of the water. The macrophytes play role in
bioaccumulation and filtration in shallow water, littoral zone of rivers, canals, and lakes, and
streams (Bunn et al., 1998). Pollution reinforces studies of use of macrophytic vegetation to
remediate various pollutants from environment (Debusk et al., 2001). The macrophytes have
high biomass which helps to uptake nutrients and heavy metals. Typha spp. is a macrophyte
which has a considerably high nutrient uptake capacity (Maddison et al., 2009). Macrophytes
have a metabolic role in wastewater treatment due to their potential to release oxygen into the
rhizosphere which helps in nitrification and by direct uptake of nutrients (Greenway and
Woolley, 2001).
Metal ions cannot move across the cellular membranes due to their charge. Ion transport into
cells is mediated by transporters. The transmembrane structure facilitates the transfer of ions
from extracellular space through the hydrophobic environment of the membrane into the cell.
The ions associated with the roots are absorbed into the cells. A significant amount of ion is
adsorbed at the extracellular negatively charged sites of the root cell walls (Seuntjens, 2004).
The cell wall bound fraction is not translocated to the shoots. For phytoextraction, metals must
be transported from the root to the shoot.
The common aquatic plant species (Typha latifolia, Myriophyllum exalbescens, Potamogeton
epihydrus, Sparganium angustifolium, Myriophyllum spicatum and Sparganium
multipedunculatum) have been reported for aluminium (Al) phytoremediation (Gallon et al.,
2004). Parrot feather (Myriophyllum aquaticum), creeping primrose (Ludwigia palustris), and
water mint (Mentha sp.) have been reported for phytoremediation of iron (Fe), zinc (Zn),
copper (Cu), and mercury (Hg) from water (Kamal et al., 2004). The L. minor has been studied
for phytoremediation of Cu and cadmium (Cd) from contaminated soils (Hou et al., 2007).
Wetlands are shallow water, low dissolved oxygen (DO), and saturated soils. Wetlands are of
two types, natural and constructed wetlands. Natural wetlands act as ecosystem filters (Cheng
et al., 2002), while constructed wetlands are artificially engineered systems which acts as
biofilter by removing nutrients and heavy metals from the water. Wetland plants have the
ability to remove pollutants and excess mineral nutrients from the water and soils (Romero et
al., 1999).
The selection of plants for a wetland is important where phytoremediation is to be applied.
Wetland plants are divided into emergent, submerged and floating. The emergent plants are
rooted in the soil with basal portions, and leaves, stems and reproductive organs are aerial
(Herath and Vithanage, 2015). Emergent species are used for phytotranspiration,
phytoextraction, and phytovolatilization and are easy to harvest. The examples of emergent
plants are Phragmites australis, Typha domingensis, Typha latifolia, Phragmites karka, Juncus
pallidus, Empodisma minus, Phalaris arundinacea, Scirpus cyperinus, Aster novae-angliae,
Limonium carolinianum, Cephalanthus occidentalis and Rhizophora mangle. Submerged
plants are below the surface of water for their entire life cycle. The examples of submerged
plants are Ceratophyllum demersum, Vallisneria americana, Myriophyllum spicatum, Hydrilla
verticillata, Heteranthera dubia, etc. Submerged species provide more biomass for the uptake
and sorption of the contaminants through phytoextraction. Submerged plants have the ability
to accumulate more metals in their tissues in comparison to rooted emergent plants. This is
because in submerged plants the foliage is exposed to the water (Herath and Vithanage, 2015).
In floating plants, the leaves and stems float on the surface of water. The examples of floating
plants include Eichhornia crassipes, Pistia stratiotes, Salvinia herzogii, Wolffia columbiana,
Lemna valdiviana, Nymphaea spp., Nuphar advena, Juncus effusus, Phyllanthus fluitans, etc.
(Herath and Vithanage, 2015).
Permanent wetlands are always or nearly always flooded and dominated by aquatic plants viz.,
ribbon weed (Vallisneria sp.) and wavy marshwort (Nymphoides crenata). Semi-permanent
wetlands are flooded every year and characterised by sedges (e.g. Cumbungi and Cyperus sp.),
rushes (e.g. Juncus sp., Marsh clubrush), spike-rushes (Eleocharis sp.), water couch
(Paspalum distichum), common reed (Phragmites australis), etc. (Herath and Vithanage,
2015).
Ephemeral wetlands have irregular flooding and long dry periods. The examples of ephemeral
wetland plants are lignum (Muehlenbeckia cunninghamiana), river red gum, black box, and
other dry land species.
The main advantage of the phytoremediation is it is low cost and environmentally-friendly
technology (Vishnoi and Srivastava, 2008). Phytoremediation conserves the topsoil and also
the harmful products generated are minimized (Ensley, 2000). With this view, the following
objectives were taken for the study.
1.1. Objectives
Main objective of the current research is to assess remediation potential of select macrophytes.
This involved:
1. Sampling, identification of macrophytes samples.
2. Understanding the morphology and physiology of these plants.
3. Determination of biomass.
4. Assessment of nutrients in the samples.
5. Remediation potential of select wetland plants.
2. Materials and methods
Sampling, identification and physiology of select wetland plants: Wetland plants were
sampled from inlets and outlets of Jakkur lake, Bengaluru on weekly basis. The plants were
harvested in triplicates by the quadrat method (0.25 m
2
area). Collected macrophytes were
stored in polythene bags after species identification using taxonomic literatures (Cook, 1996).
The physiology of the wetland plants were studied using Devlin, 1969 and Salisbury and Ross,
1969.
Determination of biomass of wetland plants: The fresh weight of the plant samples was
noted. They were washed to eliminate sediments and epiphytes and separated into species.
Above ground and below ground parts were then separated and oven dried at 60°C for 2–3
days until constant weight. The dry weights of the plant samples were noted and the biomass
was expressed as the dry weight (kg/m
2
) (Costa and Henry, 2010).
Remediation potential of wetland plants: All the dried plant samples were powdered using
mortar and pestle, sieved (1 mm) to get fine powders and labelled properly. The powdered
samples were analysed for Carbon (C) and Nitrogen (N) using CHN analyser (LECO TruSpec).
The powdered plant sample (0.5 g) was weighed to which conc. nitric acid (5 ml) and perchloric
acid (1 ml) was added and kept for digestion in fume hood till a clear solution (12 ml)
remained. The solutions were cooled and filtered through Whatman filter paper no. 5 and the
volume was made to 100 ml with distilled water (DW) (APHA, 1995). The digested samples
were analysed for six heavy metals viz., cadmium (Cd), chromium (Cr), copper (Cu), nickel
(Ni), lead (Pb) and zinc (Zn) with reagent blanks and suitable standards using Flame Atomic
Absorption Spectrophotometry (GBC Avanta Version 1.31).
The acid digested samples were also analysed for phosphorus (P) content by the stannous
chloride method (APHA, 1995). The standard graph of phosphorus was constructed (Fig. 1).
Reagents:
Ammonium molybdate:
1. Dissolve 25 g (NH
4
)
6
Mo
7
O
24
.4H
2
O in 175 ml DW.
2. Add 280 ml conc. H
2
SO
4
to 400 ml DW.
The acid solution is cooled, add molybdate solution and dilute to 1 l with DW.
Stannous chloride: Dissolve 2.5 g SnCl
2
.2H
2
O in 100 ml glycerol. Heat the mixture to hasten
the dissolution. Prepare freshly.
Standard graph of phosphorus:
Standard solution: Dissolve 0.4394 g KH
2
PO
4
in 100 ml DW (1000 mg/lit).
Working stock : Dilute 1 ml of the above solution to 100 ml with DW (10 mg/lit).
Prepare the range of standards from 0.1–0.5 mg/lit.
Working stock : 10 mg/lit
Conc. P
(mg/lit)
Stock
(ml)
Make volume
with DW (ml)
Ammonium
molybdate (ml)
Stannous
chloride
Mix
A
690
0.1
5
50
2
3 drops
0.784
0.2
10
0.871
0.3
15
0.977
0.4
20
1.214
0.5
25
1.351
For phosphorus estimation of the acid digested plant samples:
25 ml sample + 1 ml ammonium molybdate reagent + 3 drops stannous chloride. Mix and
measure the absorbance at 690 nm.
The phosphorus was calculated using the formula;
P (%) = P (mg) in 50 ml final volume x 1000/ ml of sample
0 .5 x 100
Fig. 1. Standard graph of phosphorus. The data is mean of triplicates.