Published on March 7, 2014
WAQAS AZEEM PAGF12E033 Dept. of Soil & Environmental Sciences UCA, UNIVERSITY OF SARGODHA
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)
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
SEDIMENT S FROM WASTE H2 O MUNICIPAL & INDUSTRIAL WASTE SOURCES OF HEAVY METALS LEACHATE FROM SOLID WASTE TREATMENT PLANT MINING WASTE
SOURCES OF HEAVY METALS Municipal and industrial waste Sediments from wastewater treatment plant
SOURCES OF HEAVY METALS Mining Waste Leachate from Solid Waste Treatment Plant
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.
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
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
i. Ex situ Bioremediation (away from the site) – Land farming – Composting – Biopiles – Bioreactors (Hambay, 2008).
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..,
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
WHY USE PHYTOREMEDIATION?
APPLICATIONS OF PHYTOREMEDIATION Heavy Metals Petroleum Hydrocarbons Radionuclides Applications of Phytoremediation Chlorinated Solvents Explosives Pesticides
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 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.
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 (40200) 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%)
AN OVERVIEW OF PLANTS USED FOR PHYTOREMEDIATION • trees yellow poplar various organics metals gum tree poplar willow (Pilon-Smits, 2005)
AN OVERVIEW OF PLANTS USED FOR PHYTOREMEDIATION Brassicaceae: • For inorganics Thlaspi • grasses Brassica juncea Alyssum (Pilon-Smits, 2005)
An Overview of Plants Used for for Phytoremediation various grasses for organics hemp buffalo grass red fescue for inorganics bamboo kenaf
An Overview of Plants Used for for Phytoremediation salicornia aquatic plants cattail parrot feather halophytes for inorganics for organics poplar, willow reed spartina
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 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
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
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
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