Contamination of heavy metals results in soil acidification and subsequently affects other soil properties. Contamination of heavy metals causes a decline in the specific adsorption of other cations through an increase in saturation or oversaturation of the cation exchange sites by heavy metal cations, thus displacing the protons into the soil solution, which results in a significant drop in soil pH. Three different pathways in which enzyme activities are inhibited by heavy metals: masking of catalytically active groups; denaturation of protein conformation; and competition with heavy metals for enzyme–substrate complexes. Soluble forms of heavy metals (Ag, Cu, Hg and Zn) are considered to be more toxic to enzyme activities (urease, dehydrogenase and acid phosphatase) due to their high bioavailability.

1. HEAVY METAL POLLUTION IN SOIL
ECOSYSTEM

2. HEAVY METAL:
 The term 'Heavy metals' is used for a group of metals and metalloids having relatively higher density and
pose toxic effects, and are the major inorganic pollutants.
 A list of heavy metals according to their density and which are more common in our everyday life are:
• Titanium
• Vanadium
• Chromium
• Manganese
• Iron
• Cobalt
• Nickel
• Copper
• Zinc
• Arsenic
• Molybdenum
• Silver
• Cadmium
• Tin
• Platinum
• Gold
• Mercury
• Lead

3. SOIL POLLUTION:
 1)Lower horizon of the soil and thus lower ground water may not be readily contaminated by heavy
metals.
 2) Surface layers of the soil may accumulate large amounts of heavy metals, which subsequently affect
sensitive plants growing in the soil.
Deliberate pollution : It includes wastewater irrigation, pesticides, animal manures, fertilisers, leaded paint,
mine ore waste (mine tailing), sewage sludge, spillage of petroleum distillates, coal combustion residues,
waste dumpings.
Nondeliberate pollution : It may be brought about through flooding of seas and rivers which brings
sewage and contaminated water to the land and accidents involving vehicles transporting toxic chemicals

4. HEAVY METALS SOURCES IN THE SOIL:
Geogenic Accumulation:
 Parent material a geogenic source is responsible for accumulation of heavy metals in soils and nature of parent
material is responsible for the differential levels of these metals in soils.
Anthropogenic Accumulation:
 Anthropogenic sources of trace metals include metal smelting industries, industrial effluents, sewage sludges,
municipal solid wastes, burning of fossil fuels, rain water etc.

5. SOIL ENHANCERS:
 Soil amendments are important in replenishing nutrients in the soil and keeping it suitable for
agriculture hence the use of bio-solids, sewage effluents and fertilizers.
 Bio-solids are organic materials present in animal waste, sewage sludge and industrial waste such as
pulp sludge from the production of paper.
 Bio-solids do not only contain soil enrichment nutrients but they also contain heavy metals such as Pb,
Ni, Cd, Cr, Cu and Zn, which are toxic in high concentration.

7. EFFECTS ON SOIL HEALTH:
 Contamination of heavy metals results in soil acidification and subsequently affects other soil properties.
 Contamination of heavy metals causes a decline in the specific adsorption of other cations through an
increase in saturation or oversaturation of the cation exchange sites by heavy metal cations, thus
displacing the protons into the soil solution, which results in a significant drop in soil pH.
 Three different pathways in which enzyme activities are inhibited by heavy metals:
 masking of catalytically active groups;
 denaturation of protein conformation; and
 competition with heavy metals for enzyme–substrate complexes.
 Soluble forms of heavy metals (Ag, Cu, Hg and Zn) are considered to be more toxic to enzyme activities
(urease, dehydrogenase and acid phosphatase) due to their high bioavailability.

8. INDICATORS OF SOIL CONTAMINATION:
 Chemical indicators (total/recoverable content, available/extractable amount and fractionation);
 Biochemical indicators (enzyme activity and FDA hydrolysis);
 Microbial indicators (biomass, quotient, specific respiration, metabolic quotient and community
structure);
 Soil animal indicators (earthworm-quantity and variety); and
 Plant indicators (biomass yield, uptake of metals and metal accumulation in edible parts).

9. REMEDIATION TECHNIQUES:

10. 1)Surface Capping:
 It is basically a prevention technique and involves covering the soil surface with layers to prevent
exposure to contaminated soil. It can be referred to as a prevention technique and not the treatment
technique.
2)Encapsulation:
 It is a similar process as surface and involves containing the polluted soil to prevent the spread of heavy
metals. Unlike surface capping, where only horizontal sheets are used, it involves vertical sheets to avoid
the spreading as well.
3)Electrokinetics:
 It is the process of removal of heavy metals from the soil by applying electrolysis process on the solution
of soil. The ions migrate to respective electrodes, and the separation of ions is carried out in this way.

11. 4)Soil flushing:
 This soil remediation technique involves passing a fluid through the soil. After the fluid extracts the
heavy metals, it is further treated and disposed off.
5)Immobilization:
 It is the process of immobilizing the heavy metals in the soil. In this process, the heavy metals are not
removed from the ground. Instead, they are immobilized so that they cannot migrate to other locations
where they can be harmful such as in food crop location.
6)Phytoremediation and Bioremediation:
 Phytoremediation and Bioremediation are the processes of cultivating plants and microorganisms on the
contaminated soils, respectively. In these processes, heavy metals are adsorbed out of the ground in
these green sources, and these are considered as the most feasible methods for in-situ remediation of
the soil.

12. 7)Landfilling:
 Landfilling is an ex-situ soil remedy technique where the contaminated soil is removed from its location
and is dumped in an engineered landfill.
8)Soil washing:
 It is a process, similar to soil flushing, but is ex-situ where the soil is removed from the parent site, and a
treating solution is passed through it to remove heavy metals out of it.
9)Solidification:
 It involves the removal of contaminated soil from the site and mixing it with a binding agent to form a
structure which prevents the mobilization of heavy metals.

13. REMOBILIZATION OF HEAVY METALS BY MANGROVE LEAVES
CONCLUSIONS :
We demonstrate that mangroves remobilize
metals buried deep in the soil into the environment
by shedding the senescing leaves into the surface
soil, which could either enter in food chains via leaf
consumption or act as an important source of metals
to the adjacent ecosystems depending on
biogeochemical conditions and processes. Therefore,
when used as phytoremediators, the role of
mangroves as heavy metal sinks through retention in
the woody parts has to be evaluated taking into
account the remobilization through leaves shedding.