WAQAS AZEEM

PAGF12E033

Dept. of Soil & Environmental

Sciences

UCA, UNIVERSITY OF SARGODHA

Heavy Metals

Specific Gravity is

greater than 5.0 g/cm-3

Poisonous in nature

Elements having At.wt. b/w 63.54 &

200.59

They can damage living things at low conc. and tend to accumulate in the food chain. (USEPA, 2000)

HEAVY METALS IN THE FOOD CHAIN

HM in Earthworms after application of sewage sludge concentrate Cd, Zn

Animal uptake of soil (not via plant)!

Up to 30% of diet is soil for sheep and goats Up to 18% for cattle Depends on management how much the animals

get soil. Direct ingestion of soil particles may increase

uptake of HM

AN OVERVIEW OF ANIMALS UPTAKE OF SOIL

SOURCES OF

HEAVY METALS

SEDIMENTS FROM WASTE

H2O

MINING WASTE

LEACHATE FROM SOLID

WASTE TREATMENT

PLANT

MUNICIPAL &

INDUSTRIAL WASTE

Municipal and industrial waste Sediments from wastewater treatment plant

SOURCES OF HEAVY METALS

SOURCES OF HEAVY METALS

Mining Waste Leachate from Solid Waste Treatment Plant

Caused by the presence of xenobiotic chemicals or other alteration in the natural soil environment.

Typically caused by industrial activity, agricultural chemicals, or improper disposal of waste.

SOIL CONTAMINATION

Heavy Metal Toxicity

Excessive accumulation of HM can be toxic to

many plants leading to..

Reduce seed germination, Biomass

formation

Root elongation

Inhibition of Chlorophyll biosynthesis

HEAVY METAL TOXICITY

There are several techniques to rehabilitate contaminated soils. Some of them are as under.

– Biological– Chemical– Physical

Bioremediation

i. In situ Bioremediation (at the site) – Bioventing– Biostimulation– Biosparging– Bioaugmentation– Phytoremediation

TECHNIQUES TO REHABILITATE CONTAMINATED SOIL

i. Ex situ Bioremediation (away from the site) – Land farming– Composting– Biopiles– Bioreactors

(Hambay, 2008).

Microbial/ Biological MeasuresThese approaches are ecological and economically sound but physical removal/ cleaning up of contaminants does not occurs as contaminants remain in the soil system

Chemical MeasuresChemical extraction procedures have been suggested but they are not cost effective.

So, these constraints have forced the researcher to think of using plants for cleaning up their own support system which will eco-friendly and cost effective. This new approach is..,

NEED FOR THE NEW REMEDIATION TECHNIQUE

PHYTOREMEDIATION

“Phyto”= Plant (in Greek)

“Remediare”= To remedy (in Latin)

Phytoremediation can be defined as the use of green plants to remove the contaminants from the environment or to render them harmless.

An innovative clean-up technology by the use of various plants for treatment of contaminated soil and water.

Cont.

The basic principle behind Phytoremediation is that plant roots either break the contaminant down in the soil, or suck the contaminant up, storing it in the stems and leaves of the plant.

PROCESS OF PHYTOREMEDIATION

(www.epa.gov/superfund/sites

Cont.

WHY USE PHYTOREMEDIATION?

APPLICATIONS OF PHYTOREMEDIATION

Applications of Phytoremediation

Heavy Metals

Petroleum Hydrocarbons

Chlorinated Solvents

Pesticides

Explosives

Radionuclides

FACTORS AFFECTING THE PHYTOREMEDIATION

There are mainly three factors which

affect phytoremediation of

soil.

Plant Factors

Soil Factors

Metal Factors

Plant Factors; PLANT RESPONSE TO HEAVY METALS

Metal Excluders

• Prevent metals from entering their aerial parts.

Metal Indicators

• Actively accumulate metals in their tissues and reflect metal level in soil.

Metals

Accumulat

ors

• Concentrate metals in their aerial parts, to levels far exceeding than soil.

UPTAKE OF HM BY CORN FROM SEWAGE SLUDGE

CONCENTRATION OF Pb AND As IN PLANTS

Roots > leaves> fruits and seeds

Root skin is higher than inner flesh--

Roots absorb but do not transport Pb

Apples and apricots contain low Pb and As

HYPERACCUMULATORS A plant that absorbs toxins, such as heavy metals, to a

greater concentration than that in the soil in which it is growing.

A hyperaccumulator will concentrate more than

100 ppm for Cd

1,000 ppm for Co and Pb

10,000 ppm for Ni.

Arsenic toxicity threshold level for most of plants is (40-200) mg As per kg

Criteria for Designing a Plant as Hyperaccumulator

Shoots metal conc. (oven dry basis) should be more than 1% for Mn and Zn; 0.1% for Cu, Ni & Pb; and 0.01% for Cd and As.

Plant should be fast growing with high rate of biomass production.

Should be able to accumulate metals even from low external metal conc.

Should be able to transfer accumulated metals from root to shoot (above ground) quite efficiently (often more than 90%)

• trees

AN OVERVIEW OF PLANTS USED FOR PHYTOREMEDIATION

various organicsmetals

poplar

willow

gum treeyellow poplar

(Pilon-Smits, 2005)

• For inorganics• grasses

Brassica juncea

Alyssum

Thlaspi

Brassicaceae:

(Pilon-Smits, 2005)

AN OVERVIEW OF PLANTS USED FOR PHYTOREMEDIATION

hemp

kenafbamboo

various grasses

red fescuebuffalo grass

for organics

for inorganics

An Overview of Plants Used for for Phytoremediation

parrot feather

poplar, willow spartina

halophytes

salicornia

reed

aquatic plantscattail

for organics

for inorganics

An Overview of Plants Used for for Phytoremediation

SOIL FACTORS

pH

Eh

Clay content

Organic Matter

CEC

Conc. of other trace elements

Nutrient Balance

pH

The solubility and availability/toxicity of heavy metals decreases as soil pH increases

(McLaughlin, 2002).

In the pH range 7.1-8.5, carbonate acts as a pH buffer. Mg2+, Zn2+, Cu2+, Fe2+ and Al3+ may replace Ca2+ on exposed surface lattice sites. The reactive surfaces of carbonates may adsorb soil contaminants such as Ba2+, Cd2+ and Pb2+

Redox Potential (Eh)

Metal solubility increases as redox potential decreases.

As redox potential decreases, trace elements become less available.

The uptake of Cd by rice seedlings is at a minimum at low Eh.

Clay Content

Metals are more available in sandy soils than in clayey soils, where they are firmly retained on the surface of clay minerals.

They may form types of complexes on clay surfaces: outer sphere ion-exchange complexes on the basal plane, and coordination complexes with SiOH or AlOH groups exposed at the edge of the silicate layers

Organic Matter

Organic matter in soil, e.g. humic compounds, bears negatively charged sites on carboxyl and phenol groups, allowing for metal complexation.

The presence of high amounts of insoluble organic matter in soil is negatively correlated with plant uptake, as often observed on peat soils with Cu.

Cation Exchange Capacity

Cation exchange capacity (CEC), a function of clay and organic matter content in soil, controls the availability of trace elements.

In general, an increase in CEC decreases uptake of metals by plants

Nutrient balance

Absorption of trace elements by roots is controlledby the concentration of other elements and interactionshave often been observed.

Macronutrients interfere antagonistically with up take of trace elements. Phosphate ions reduce the uptake of Cd and Zn in plants

(Haghiri, 1999; Smilde et al., 1992)

They also diminish the toxic effects of As, as observedon soils treated with arsenic pesticides

Grasses take up less trace elements than fast-growing plants, e.g. lettuce, spinach and carrots.

When grown in the same soil, accumulation of Cd by different plant species decreases in the order: leafy vegetables > root vegetables > grain crops

Concentration of other trace elements in soils

Cost

Phytoremediation is usually less costly than competing alternatives such as soil excavation, pump-and-treat, soil washing, or enhanced extraction.

METAL FACTORS

Different forms of a single metal also affects phytoremediation process significantly.

For e.g.

Arsenic is typically found in the soil in the following forms.. Arsenate, Arsenite, dimethyl arsenic acid and monomethyl

arsenic acid Inorganic forms arsenate, or As (V), and arsenite, or As

(III), most common in soil

Arsenate prevails under aerobic conditions, is less toxic and less mobile than arsenite, due to stronger soil sorption

WHY IS ARSENIC TOXIC FOR MOST PLANTS?

Arsenic toxicity threshold for most plants is (40-200) mg As per kg DW depending on soil conditions

Arsenate replaces phosphate when taken up, and disrupts production of ATP, which results in cell death

Arsenic is inhibitory towards cell function because it reacts with sulfhydryl enzymes and disrupts their activity.

Pteris vittata Study Results

Pityrogramma calomelanos study results

Disposal of Plant Biomass

Significant amounts of arsenic can leach from biomass (threat to groundwater)

Arsenite in biomass oxidizes back to arsenate

Marine algae capable of biotransforming arsenic into non-toxic forms

Biomass can NOT be burned, results in release of toxic As2O3

CONCLUSIONS

Phytoremediation is land-management technology

It is a low-cost, sustainable solution for contaminated land and waste-streams

Making the technology work relies on the ‘intelligent’ synergy of botany, microbiology and geochemistry

Revegetation, land stabilisation and phytoextraction are all working scenarios of phytoremediation

Thanks…