Date post: | 06-May-2015 |
Category: |
Education |
Upload: | waqas-azeem |
View: | 1,350 times |
Download: | 5 times |
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…