Heavy metal pollution in soil and its

possible mitigation aspect

Speaker: Dr.Tarik Mitran

Agricultural Chemistry and Soil Science

What is a Heavy Metal?

The term heavy metal refers to any metallic chemical element that has a

relatively high density and is toxic or poisonous at low concentrations. 

Examples of heavy metals include mercury (Hg), cadmium (Cd), arsenic

(As), chromium (Cr), thallium (Ti), and lead (Pb).

Heavy metals are those having densities five times greater than water,

and the light metals are those having lesser densities.

Humans consume metallic elements through both water and food.

 Source: Brady(1994)

 

It should be noted that the content of metals in tissue generally builds up from left to right, indicating the vulnerability of humans to heavy metal toxicity

Sources of heavy metals and their cycling in the soil-water-air organism ecosystem

Rocks in Earth’scrust

Air

Soil

Water

Plants

Birds

Domesticanimals

Fish

Humans

Industrial ProductsBurned fuelFertilizersPesticides

Anthropogenic sources of heavy

metals in soil

Tannery

BatteryDistillery

Steel Fly ash

Electroplating Mining

Smelting

Heavy metalHeavy metal

Transition metals

Ia IIaIIIa IVa Va VIa VIIa

Ib IIb

Lanthanides

Actinides

Chemical Major uses and sources of soil contamination

Arsenic Pesticides, plant desiccants, animal feed additives, coal and petroleum, mine tailings and detergents

Cadmium Electroplating, pigments for plastics and paints, plastic stabilizers and batteries, fertilizers

Chromium Stainless steel, chrome–plated metals, pigments and refractory brick manufacture

Lead Combustion of oil, gasoline, and coal; iron and steel production

Mercury Pesticides, catalysts for synthetic polymers, metallurgy, thermometers

Nickel Combustion of coal, gasoline, and oil; alloy manufacture, electroplating, batteries

 Source: Moore and Ramamoorthy (1984)

Sources Annual Emission (T/year)

Natural SourcesWindblown dustForest firesVolcanogenic particlesVegetationSea salt spray

10012

5202001

Anthropogenic sources

Mining, non-ferrous metals

Primary non-ferrous metal Production

Cd

Cu

Pb

Zn

Secondary non-ferrous metal production

Iron and steel production

Industrial applications

Coal combustion

110

1600

200

2800

600

70

50

60

Table: Emission of Cadmium to the atmosphere

Source: Nriagu (1980) and Vogeli Lange (1989)

ElementConcentration

range (mg/kg)

Regulatory limit

(mg/kg)

Lead 1-6900 600

Cadmium 0.1-345 100

Arsenic 0.1-102 20

Chromium 0.005-3950 100

Mercury 0.001-1800 270

Copper 0.03-1550 600

Zinc 0.15-5000 1500

Table: 2 Heavy metals prevailing in soils and their regulatory limits

Source: Salt et al (1994)

Heavy Metals Limit in Soil

Source: Sherman, 2002

Heavy Metals according to their Toxicity

Cd, As

Hg, Pb, Ni, F

B, Cu, Mn, Zn

Extremely poisonous

Moderately poisonous

Relatively less poisonous

Behavior of Hg in the Environment

Source: US EPA, (1997)

Hg species Bio –chemical property

Hg0 It is relatively inert and non-toxic.in vapour form it is toxic.

Hg2+ Toxicity is low

RHg+(Organo-mercurial)

Highly toxic cause’s irreversible nerve and brain damage can be stored in tissue. Abortion of baby affecting foetal .Borned baby suffer mental fat retardation, convulsion cerebral palsy

R2Hg(diorgano-mercurials)

Low toxicity but in acid medium converted to organo mercurials

HgS Insoluble, adsorbed in soil

Table: species of Mercury and their toxicity

Source: Das, 2005

Fate of selenite in soil (1:10 water extract)Heavy metal load field experiment: Dr. Imre Kádár, Nagyhörcsök

0

100000

200000

300000

400000

500000

600000

700000

800000

900000

1000000

0 10 20duration [min]

ICP

-MS

sig

nal [

CP

S]

0

5000

10000

15000

20000

25000

0 10 20duration [min]

ICP

-MS

sig

nal [

CP

S]

0

5000

10000

15000

20000

25000

0 10 20duration [min]

ICP

-MS

sig

nal [

CP

S]

1993 1993 2003

oxidationselenite

selenate

organic selenium

Conclusion: selenite will be oxidised in soil to selenate selenate more available for plants (it is analogous to sulphate) risk of leaching

selenite

selenate

Basic facts of Cr :

• Chromium (Cr) discovered first by the French chemist Vanquelin in 1798 from red lead ore in the Siberia.

• Cr is a transitional element in the group VI B.• Cr (VI) is most toxic usually occurs associated with

oxygen as chromate (CrO42-) or dichromate (Cr2O7

2-) oxyanions. Most mobile species.

• Cr (III) less toxic, less mobile & mainly found bound to organic matter in soil & aqueous environment.

• Besides organic Cr (III) are known to form complexes with inorganic ligands (OH-, SO2

-, NO3- and CO3

2-). Organically complexed (citric and fulvic acids) Cr (III) prevented from precipitation even upto pH 7.5.

• Hence in most natural systems Cr (VI) occurred as CrO4

2- and Cr (III) present as hydroxides and various organic complexes.

• Cr (VI) is a strong oxidising agent & reduced by appropriate electron donor

HCrO4- + 7H+ + 3e- → Cr3+ + 4H2O←

Reactions in Soil – Water System

• In most natural systems, hexavalent Cr6+ present as CrO4

2- & major trivalent Cr3+ species include hydroxides & various organic complexes.

• Cr undergoes various chemical & biological reactions in natural system that govern speciation & in turn, environmental behaviour.

• Important among these reactions includeOxidation or reductionPrecipitation or dissolution andAdsorption or desorption

Cr in water

Leachate

Kinetics of Redox TransformationCr3+ Cr6+

Precipitation Dissolution

Adsorption & Desorption

Water Cr3+ & Cr6+

Predicted Cr concentration

A simple scheme involving reactions of Cr in soil & water systems

Source: Rai et al., 1989

Effect of Cr on plant growth and development

Process Crop/plant Effect

Germination Bushbean, Lucerne, Mungbean, Sugarcane

Reduced germination percentage & reduced bud sprouting

Root growth Rice, Mungbean, Sorghum

Decrease in root length & dry weight, increase in root diameter & root hairs. Proportional variation in

cortical & pith tissue layers.

Shoot height Rice, Bushbean Reduction in plant height

Yield & Dry matter

production

Cauliflower, Cabbage, Radish, Bushbean,

Maize, Finger Millet

Upto 50% reduction in yield. Reduced number of flowers per

plant. Reduced grain weight. Increased seed deformity. Reduced

pod weight.

Source: Patel et al., 1992

Source: Sherman, 2002

Soil Heavy metal (ppm)

Available TOTAL

Pb Cf Cr Pb Hg Cd Cr

Surface 1.8 0.02 0.6 43.8 4.6 9.6 23.2

Sub surface 2.3 0.04 0.04 40.3 3.8 10.1 19.8

Table: Heavy Metal Pollution Through Sewage Water

Source: Som et al. (1994)

Feer‘s Disease (acrodynia) exanthema due to Hg Toxicity

Picture Source: www.who.int/ceh

Source of

Exposure

Methyl mercury

(MeHg) (μg/day)

Inorganic Hg Salts (Hg+

+) (μg/day)

Elemental Hg (Hg0)

(μg/day) Foods (non-fish) Negligible 0.9 Negligible

Commercial fish 6 <1 Negligible

Sport fish No population-based data

available

No population-based data

available

Negligible

Public supply

water Negligible <<4 Negligible

Private wells Negligible 0.4-4 0.006-0.03

Outdoor air Negligible Negligible 0.04 - 0.2

Indoor air Negligible Negligible No population based data

available Soil ingestion Negligible >3 Negligible

Dental amalgams Negligible Negligible .3-17

Table: 10 Sources and Estimates of Daily Human Exposures to Mercury

Source: Charles, 2000

Elements Plant species Max. reported

conc. (mg/kg)

Cadmium Thlapsi caerulescens 500

Copper Ipomoea alpina, 12300

Cobalt Haumaniuastrum robertii 10200

Lead Thlapsi rotundifolium, Brassica juncea, Zea mays 8200

Nickel Alyssum lesbiacum, Sebertia acuminata 47500

Zinc Thlapsi caerulescens, Brassica oleracea, B.

campestris

51600

Selenium Brassica juncea, B. napus 900

Chromium Brassica juncea, Helianthus annus 1400

Table: 13 Important hyperaccumulators for metal remediation

Surce: Sarkar, 2005

Lead

Lead is by far the most common contaminant of soils.

Lead in soil is virtually a permanent resident. Organic matter,

especially, will bind and hold itself in other metals very

effectively.

Sources: Number one source contamination: lead-based paint

Other sources: gasoline exhaust

motor oil

automobile tires

industrial activity

coal combustion, and pesticides.

Mercury• Mercury occurs in two forms:

- organic

- inorganic

• Inorganic forms most often occur when mercury is combined

with chlorine, sulfur or oxygen.

• Organic forms occur when mercury combines with carbon.

Sources: Metallic forms of mercury are not absorbed by plants, but

are converted by microorganisms to organic forms such

as methyl mercury, which are taken up by plants.

Environmental sources include thermometers, pesticides,

metallurgy, and vapors from burning coal and other

fuels.

Mercury in Plants• The uptake of mercury:

- decreases growth

-induces disorientation of roots and shoots, plant tissue, and finally the cell wall.

• A major portion of Mercury is tightly bound and remains in the roots.

Lead in Plants• Lead in plants:

- Absorbed through roots

- Lead builds up in both leaf and root tissue

- Causes lower concentration of chlorophyll

- Lead compounds absorb UV light

- Plants biomass declines, which includes roots, shoots, and fruits.

Toxic Effect of Hg and Pb in Plants

Boreysza, Fabritus,Laures, 2006

Source: Arsenic is a new terror; Asit Kumar Roy; Desh, 2004

DRAWDOWN

Radius of influence

Cone of Depression

WTVadose zone

Arsenopyrite

Darcy’s Law

Pitticite

Effect of Arsenic Pollution

Crop Arsenic conc. (mg/kg) at harvest

Leaf Stem Root Eco. Produce

Elephant Foot yam

4.30 8.0 - 4.0

Green gram 5.10 4.9 4.7 4.3

Cowpea 4.91 5.1 5.2 2.1

Maize 3.30 6.2 5.2 2.6

Rice (boro) 10.2 5.7 5.9 10.0

Jute 3.5 8.0 6.8 4.0

Potato 3.9 9.3 - 5.9

Mustard 7.1 9.8 5.7 3.3

Ground nut 2.0 2.0 2.2 4.0

Sesame 2.0 2.0 4.0 0.6

Crops were subjected to irrigation with water containing 0.22 mg As / lit of water Soil had an Olsen-extractable arsenic content of 1.23 to 1.37 mg/kg of soil (initial)

Arsenic uptake by different plant parts of crops grown in Gotera, Chakdah

Source Source : Prof. S. K. Sanyal,2005 : Prof. S. K. Sanyal,2005

Then what is the way out???Then what is the way out???

PHYTOREMEDIATION

THE GREEN-CURE TECHNOLOGY

What is phytoremediation?

“Use of green plants to remove pollutants from the

environment or render them harmless.”

This concept has emerged from a broader philosophy

of Bioremediation where besides plants, soil micro-

organism are also used for amelioration of organic and

inorganic contaminants.

Source: Salt et al.(1998)

Phytoextraction Phytodegradation Rhizofiltration Phytostabilization Phytovolatilization

Phytoremediation

D i f f e r e n t a p p r o a c h e s o f p h y t o r e m e d i a t i o n

A c c u m u l a t io n o f m e t a l s i n s h o o t t i s s u e s f o l l o w e d b y h a r v e s t i n g

U s e o f p l a n t s a n d a s s o c i a t e d m i c r o b e s t o d e g r a d e o r g a n i c p o l l u t a n t s

U s e o f p l a n t r o o t s t o a b s o r b a n d a d s o r b m e t a l s f r o m a q u e o u s w a s t e s t r e a m

R e d u c t i o n i n l e a c h i n g , r u n o f f , s o i l e r o s i o n a n d b i o a v a i l a b i l i t yo f t o x i c m e t a l s

U s e o f p l a n t s t o v o l a t i l i z e p o l l u t a n t s

Pteris vittata (Chinese brake) – a reported hyper-accumulator for arsenic

Disposal of hyper-accumulator plant refuse

Harvest

Incineration

Controlled disposal of ash to underground – away from root zone and aquifer

Phytomining

Jade green alkaloid from cut stem of Phyllanthus palawanensis contains 88,580 µg Ni g-1 dry weight

Contents of some heavy metals in fertilizers and sludges

Source Metal mg/kg dry material

Cd Cr Cu Pb Zn

Ammonium Niitrate (A/N)

1.1 2.5 3.6 5.4 11.7

SSP 16.6 157.0 22.6 20.6 244.0

Compound 8-10-8

4.9 54.3 8.3 3.2 97.5

Sewage Sludge 20.0 500.0 250.0 700.0 3000.0

Source: Pain et al., 1991

Accumulation of Heavy Metals in Soil and Plant (mg/g)  Zn Cu Pb Cd Cr

Soils of Dhapa 1038-1256 154-196 79-113 0.38-0.52 9.1-17.0

Spinach 320-340 60-72 60-82 0.8-2.2 6.5-15.8

CauliflowerHead

300-1100 20-30 30-90 Trace 5.2-5.7

Source: Gupta et al., 1997

Parts Zn Cu Pb Cd Cr

Brain 3.2 trace 2.9 0.4 11.3

Muscle 29.1 3.4 2.4 0.5 0.9

Liver 53.1 79.5 3.3 6.1 3.2

Kidney 62.6 8.9 11.9 12.3 14.9

Source: Bhattacharyya, 1997

Accumulation of Heavy Metals in Rohu fish (1.5 kg) on dry weight Basis (g/g)

Effect of Heavy Metal Pollution on Microbial Diversity in soil (experiment conducted in

Portugal, 2006)

Source: Oliveira et al. (2006)

Micro-organism Contaminated soil Uncontaminated soil

2003 2004 2003 2004

Aerobic heterotrophic

bacteria (106 cfu/g)3.9 1.5 7.4 4.6

Actinomycetes (104

cfu/g)8.3 6.6 10.4 9.6

Fungi (105 cfu/g) 2.8 1.2 4.3 1.8

Asymbiotic Nitrogen

fixer (104 cfu/g)2.5 0.8 9.5 3.5

Some of the cultivated species can also act as

efficient metal hyper-accumulators

Brassica juncea

Hordeum vulgare

Avena sativa

Remedial / Mitigation Options1. Optimum conjunctive use of ground & surface water

[ e.g. harvested rainwater]

2. Irrigation with pond-stored groundwater – decontamination facilitated by rainfall and sedimentation

3. Recharge groundwater resource

4. Enhance water use efficiency (optimum water management)

5. Prefer low-water requiring farmer-attractive cropping sequences (especially for the lean period)

6. Increased use of FYM and other manures + green manure crops, inclusion of pulses/other legumes as well as application of appropriate amendments (Zn/Fe salts as and where applicable)

7. Cost-effective phytoremediation options

8. Creation of general awareness: Mass campaigning, holding of farmers’ day, field demonstrations, socioeconomic factors

(Source: Sanyal, 2008)

The chemicals to which life is asked to make

its adjustment are . . . the synthetic creations

of man's inventive mind, brewed in his

laboratories, and having no counterparts in

nature.

Rachel CarsonSilent Spring