This document discusses heavy metals contamination of soil and their uptake in the food chain. It provides details on various techniques used for remediation of contaminated soils, with a focus on phytoremediation. Phytoremediation uses plants and their associated microbes to remove, contain or render harmless contaminants in soil and water. Factors that affect phytoremediation like plant species, soil properties and metal properties are discussed. The use of hyperaccumulator plants for phytoremediation of heavy metals like arsenic is also described.

2. WAQAS AZEEM
PAGF12E033
Dept. of Soil & Environmental
Sciences
UCA, UNIVERSITY OF SARGODHA

3. Specific
Gravity is
greater than
5.0 g/cm-3
Heavy
Metals
Elements
having At.wt.
b/w 63.54 &
200.59
Poisonous in
nature
They can damage living things at low conc.
and tend to accumulate in the food chain.
(USEPA, 2000)

4. HEAVY METALS IN THE FOOD CHAIN

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

6. 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

7. AN OVERVIEW OF ANIMALS UPTAKE OF SOIL

8. SEDIMENT
S FROM
WASTE
H2 O
MUNICIPAL
&
INDUSTRIAL
WASTE
SOURCES
OF
HEAVY
METALS
LEACHATE
FROM SOLID
WASTE
TREATMENT
PLANT
MINING
WASTE

9. SOURCES OF HEAVY METALS
Municipal and industrial waste
Sediments from wastewater treatment plant

10. SOURCES OF HEAVY METALS
Mining Waste
Leachate from Solid Waste Treatment Plant

11. SOIL CONTAMINATION
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.

13. HEAVY METAL TOXICITY
Excessive accumulation
of HM can be toxic to
many plants leading to..
Heavy Metal
Toxicity
Reduce seed
germination, Biomass
formation
Root elongation
Inhibition of
Chlorophyll
biosynthesis

14. TECHNIQUES TO REHABILITATE CONTAMINATED SOIL
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

15. i.
Ex situ Bioremediation (away from the site)
– Land farming
– Composting
– Biopiles
– Bioreactors
(Hambay, 2008).

16. NEED FOR THE NEW REMEDIATION TECHNIQUE
Microbial/ Biological Measures
These approaches are ecological and economically sound but physical
removal/ cleaning up of contaminants does not occurs as contaminants
remain in the soil system
Chemical Measures
Chemical 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..,

17. 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.

18. 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.

19. PROCESS OF PHYTOREMEDIATION
(www.epa.gov/superfund/sites

23. Cont.

24. WHY USE PHYTOREMEDIATION?

25. APPLICATIONS OF PHYTOREMEDIATION
Heavy Metals
Petroleum
Hydrocarbons
Radionuclides
Applications of
Phytoremediation
Chlorinated
Solvents
Explosives
Pesticides

26. FACTORS AFFECTING THE PHYTOREMEDIATION
There are mainly
three factors which
affect
phytoremediation of
soil.
Plant
Factors
Soil
Factors
Metal
Factors

27. Plant Factors; PLANT RESPONSE TO HEAVY METALS
Metal
Excluders
Metal
Indicators
Metals
Accumulators
• Prevent metals from entering their aerial parts.
• Actively accumulate metals in their tissues and reflect metal
level in soil.
• Concentrate metals in their aerial parts, to levels far
exceeding than soil.

29. UPTAKE OF HM BY CORN FROM SEWAGE SLUDGE

30. 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

31. 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 (40200) mg As per kg

32. 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%)

33. AN OVERVIEW OF PLANTS USED FOR PHYTOREMEDIATION
• trees
yellow poplar
various organics
metals
gum
tree
poplar
willow
(Pilon-Smits, 2005)

34. AN OVERVIEW OF PLANTS USED FOR PHYTOREMEDIATION
Brassicaceae:
• For inorganics
Thlaspi
• grasses
Brassica juncea
Alyssum
(Pilon-Smits, 2005)

35. An Overview of Plants Used for for Phytoremediation
various grasses
for organics
hemp
buffalo grass
red fescue
for inorganics
bamboo
kenaf

36. An Overview of Plants Used for for Phytoremediation
salicornia
aquatic plants
cattail
parrot feather
halophytes
for inorganics
for organics
poplar, willow
reed
spartina

37. SOIL FACTORS
pH
Eh
Clay content
Organic Matter
CEC
Conc. of other
trace elements
Nutrient
Balance

38. 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+

39. 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.

40. 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

41. 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.

42. 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

43. Nutrient balance
Absorption of trace elements by roots is controlled
by the concentration of other elements and interactions
have 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 observed
on soils treated with arsenic pesticides

44. Concentration of other trace
elements in soils
Grasses take up less trace elements than fastgrowing 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

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

47. 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

48. 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.

49. Pteris vittata Study Results

50. Pityrogramma calomelanos study results

51. 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


52. 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