Remediation Potential of Select Wetland Plants

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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

). 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 30–45 % 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

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

) (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 (1–2 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

)

.4H

O in 175 ml DW.

2. Add 280 ml conc. H

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

.2H

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

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

690

0.1

3 drops

0.784

0.2

0.871

0.3

0.977

0.4

1.214

0.5

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.