Heavy Metals: Soil Contamination and Its Remediation

Abstract

Heavy metals are the most important soil contaminants in the environment. Heavy metals are the integrated components of the biosphere and thus occur naturally in soils and plants. Heavy metals (Cd, Cr, Cu, Hg, Pb, and Zn) have occurred widely as a result of human, agricultural and industrial activities which is responsible for the contamination of soils. Some of these metals are micro-nutrients that are necessary for plant growth, such as Zn, Cu, Mn, Ni, and Co, while others have unknown biological functions, such as Cd and Pb. Agricultural activities involve the addition of inorganic fertilizers, insecticides, pesticides, and amendments to the soil for increasing productivity are responsible for soil contamination/pollution. Water used for irrigation and the release of industrial effluents in water resources pollute the soil with solid wastes, heavy metals, and several other organic and inorganic substances. Reclamation of such contaminated soils through the phytoremediation method was found to be the cheapest and an effective method for extraction or removal of pollutants from contaminated soils.

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© 2022 AATCC Review. All Rights Reserved.
Agriculture Association of Textile Chemical and Critical Reviews Journal (2022) 59-76
Review Article Open Access
Heavy Metals: Soil Contamination and Its Remediation
Risikesh Thakur1, S. Sarvade*2 and B.S. Dwivedi3
1(Soil Science), College of Agriculture, Balaghat, Jawaharlal Nehru Krishi Vishwa Vidyalaya, Jabalpur (M. P.) – India
2(Agro-forestry), College of Agriculture, Balaghat, Jawaharlal Nehru Krishi Vishwa Vidyalaya, Jabalpur (M. P.) – India
3Department of Soil Science, College of Agriculture, Jabalpur, Jawaharlal Nehru Krishi Vishwa Vidyalaya, Jabalpur (M. P.) – India
Abstract
Heavy metals are the most important soil contaminants in the environment. Heavy metals are the integrated
components of the biosphere and thus occur naturally in soils and plants. Heavy metals (Cd, Cr, Cu, Hg, Pb, and
Zn) have occurred widely as a result of human, agricultural and industrial activities which is responsible for
the contamination of soils. Some of these metals are micro-nutrients that are necessary for plant growth, such
as Zn, Cu, Mn, Ni, and Co, while others have unknown biological functions, such as Cd and Pb. Agricultural
activities involve the addition of inorganic fertilizers, insecticides, pesticides, and amendments to the soil for
increasing productivity are responsible for soil contamination/pollution. Water used for irrigation and the
release of industrial euents in water resources pollute the soil with solid wastes, heavy metals, and several
other organic and inorganic substances. Reclamation of such contaminated soils through the phytoremediation
method was found to be the cheapest and an eective method for extraction or removal of pollutants from
contaminated soils.
Keywords: Heavy Metals, Soil Pollution, Sink, Phytoremediation, and bioremediation.
*CorrespondingAuthor: S. Sarvade
E-mail Address: somanath553@gmail.com
DOI: https://doi.org/10.58321/AATCCReview.2022.10.02.59
© 2022 by the authors. e license of AATCC Review. is
article is an open access article distributed under the terms
and conditions of the Creative Commons Attribution (CC
BY) license (http://creativecommons org/licenses/by/4.0/).
14 August 2022: Received
19 September 2022: Revised
30 September 2022: Accepted
22 October 2022: Available Online
http://aatcc.peerjournals.net/
Introduction
Heavy metals are conventionally dened as elements
with metallic properties and an atomic number
is more than 20 [49] [27]. Soil pollution by heavy
metals has a harmful impact on biological activities
in soil because heavy metals are not biodegraded
easily. Some toxic heavy metals i.e.Pb, Co, Cd
cannot be biodegraded but it can be accumulated
in living organisms, which causes various diseases
and disorders even in relatively lower concentrations
[71]. ey are also known to have an eect on plant
growth, and ground cover and have a negative impact
on soil microora [77]. e development of industry,
intensive agricultural practices, the transport sector,
and other developmental activities in cities in the
last 150 years have been so rapid and extensive that
has resulted in soil alteration especially changes in
the soil constituents. e application of agricultural
inputs resulted in a change in the natural ecosystem.
e wild ora has been replaced by agricultural
activities. Productive soils are regularly being
dumped with chemicals and industrial materials.
is has deteriorated the soil, which has resulted
in a decline in soil productivity in several parts of
the country. When a trace of toxic elements from
soil or rock enters the environment, follows normal
biogeochemical cycles, being governed by air, water,
and gravity until it reaches a geochemical sink. Soil
is a universal sink for many substances, particularly
the metals that accumulate in soil and move under
the process of leaching, plant uptake, and erosion,
which temporarily form sink and on decay, the
elements are released into the soil. Accumulation of
heavy metals and metalloids through the disposal of
high metal wastes, mine tailings, leaded gasoline and
paints, application of fertilizers in the soil, animal
dung manures, sewage sludge, pesticides/herbicide,
coal combustion residues, toxic gases emission for
the industrial areas, wastewater irrigation, spillage
of petrochemicals, and atmospheric deposition
are responsible for the contamination of soil. e
most common heavy metals that constitute an ill-
dened group of inorganic chemical hazards found
at contaminated sites are lead, chromium, arsenic,

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S. Sarvade et al., / AATCC Review (2022)
cadmium, mercury, and nickel [113] [43] .
e dierent agricultural inputs like inorganic
fertilizers and micronutrients are manufactured
products containing certied amounts of plant
nutrients to facilitate crop growth when applied to
cultivated lands. In addition to the active ingredients,
however, fertilizers and micronutrients may contain
trace elements such as cadmium and lead that are
potentially harmful to consumers of the harvested
products. When the phosphatic fertilizers and micro-
nutrients were applied in the soil, the amounts
applied invariably exceeded the amounts taken up by
plants. In addition, parts of plant biomass would be
reincorporated into the soil aer the crop harvests,
thus recycling part of the nutrients and contaminants
[78]. erefore, active ingredients of phosphatic
fertilizers, along with micro-nutrient ingredients
namely phosphorus, zinc, iron, and manganese
are expected to accumulate in soil receiving
routine applications. e phosphorus contents
of the cultivated soils would invariably increase
in proportion to the amount of fertilizers used in
cultivated lands. Fe and Mn are abundant in soils
and increases in their concentrations could not easily
be distinguished from the already high background
levels. However, the concentration of Zn in the soil
would be sensitive to the inputs and may be used as
an indicator of micro-nutrient inputs. e content of
lead and cadmium which are present in soil are either
geogenic or added through applications of phosphatic
fertilizers. e cadmium content of phosphatic
fertilizers in Indonesia is 35-255 g mt-1 [2]. Single
super phosphate was used in India, and contains
Cd in the range of 2-200 mg kg-1 (Foy et al., 1978).
Heavy metals and other elements are more toxic soil
contaminants that came from sewage sludge. us,
the soils treated with the repeatedly or large amount
of sewage sludge it may accumulate heavy metals and
consequently become unable to even support plant
life [60] .
Agricultural soils in many parts of the world are
contaminated by heavy metal toxicity such as Cd,
Cu, Zn, Ni, Co, Cr, Pb, and As. is could be due to
the continuous application of phosphatic fertilizers,
sewage sludge application, dust from smelters,
industrial waste, and bad irrigation practices in
agricultural lands [9] [82] [68] .
Materials and Methods
e present study is based on dierent forms
of review literature, which is collected from the
published in various national and international
journals, books and book chapters, conferences/
seminars proceedings, searched online documents
from Research Gate, CeRA, and Google Scholar,
etc. e keywords i.e. heavy metals in soil, heavy
metals in plants, the toxic eect of heavy metals,
phytoremediation, phytotoxicity, rhizoltration
used for the completion of the information to the
formulation of this manuscript.
Sources of heavy metals in soil
Heavy metals occur naturally in the soil environment
through the weathering process of the parent
material. It can be found in the form of hydroxides,
oxides, sulphides, sulphates, phosphates, silicates,
and organic compounds. e most common heavy
metals are Pb, Ni, Cr, Cd, As, Hg, etc. [32]. e
dierent man-made activities disturb and accelerate
the geochemical cycles of metals in the soils of rural
and urban environments, which may accumulate
various toxic heavy metals to cause risks to plant
health and the ecosystem [18] . Dierent sources of
heavy metals are presented in Table 1 and illustrated
in gure 1.
e improper and continuous use of herbicides,
pesticides, and fungicides to protect the crops from
pests, fungi, etc. but alters the basic composition of
the soils and makes the soil toxic for plant growth.
Organic insecticides like DDT, aldrin, benzene
hexachloride, etc. are used against soil-borne pests
and they accumulate in the soil because they do not
degrade or are slowly degraded by the microorganisms
(bacteria) in soil. Consequently, they have a very
deleterious eect on plant growth, stunting their
growth and reducing the yield and size of fruit [60].
erefore, intensication of agricultural production
by practices of irrigation (causes salination), excessive
fertilizers, pesticides, insecticides etc. have created
the problems of soil pollution.
e use of dierent inorganic fertilizers is required
for the adequate supply of essential nutrients i.e. NPK
for crop growth under intensive farming systems and
it content a trace amount of heavy metals (Cd and
Pb) as impurities particularly phosphatic fertilizers.
e continuous application of inorganic fertilizers
in excess amounts may increases signicantly the
heavy metals concentration in the soil [40].It can be
adversely aected plant growth, because these metals
interfere with metabolic functions in plants, including

61 © 2022 AATCC Review. All Rights Reserved.
S. Sarvade et al., / AATCC Review (2022)
physiological and biochemical processes, inhibition
of photosynthesis and respiration, and degeneration
of cell organelles and leading to plant death [26] [80]
[81]. Contamination of soils by heavy metals may
also change the composition of microbial activities in
the soil and adversely aect soil characteristics [46]
[48].
Table 1: Sources of heavy metals
Heavy met-
als Sources References
Cadmium
Geogenic sources, anthropo-
genic activities, metal smelting
and rening, fossil fuel burn-
ing, application of phosphate
fertilizers, sewage sludge.
[66] [2] [41]
Chromium Electroplating industry, sludge,
solid waste, tanneries. [45]
Lead
Mining and smelting of metal-
liferous ores, burning of leaded
gasoline, municipal sewage,
industrial wastes enriched in
Pb, paints.
[29]
Nickel
Volcanic eruptions, land ll,
forest re, bubble bursting and
gas exchange in ocean, weath-
ering of soils and geological
materials.
[45]
Figure 1: Natural and anthropogenic sources of
heavy metals
Anthropogenic source of heavy metals
e dierent industrial activities, agriculture
activities, wastewater (sewage sludge), mining,
manufacturing, and the use of synthetic products
can result in heavy metal contamination of urban
and agricultural soils. us, anthropogenic sources
of heavy metals are toxic beyond the natural uxes
for some metals. Some important anthropogenic
sources which signicantly contribute to the heavy
metal contamination in the environment include
automobile exhaust which releases lead; smelting
which releases arsenic, copper, and zinc; insecticides
which release arsenic and burning of fossil fuels
which release nickel, vanadium, mercury, selenium,
etc. Potentially contaminated soils may also occur
at the past application of wastewater or municipal
sludge, areas in or around mining waste piles and
tailings, industrial areas where chemicals may have
been dumped on the ground, or in areas downwind
from industrial sites.
Further, the incorrect way of chemical waste
disposal from dierent types of industries can cause
contamination of soil. Human activities like disposal
of industrial waste, heavy metals, toxic chemicals,
oil, and fuel, etc. have also been found to contribute
more environmental pollution due to the everyday
manufacturing of goods to meet the demands of the
large population [31] . e concentration of heavy
metals in any soil depends initially on the nature of
the parent materials. e ndings of [24] observed
that the heavy metals concentration was maximum
in basaltic rocks and minimum in granite rocks and
similarly, [104] also reported that the heavy metal
composition in coal ash (Table 2).
Table 2: Heavy metals in some typical igneous rocks
and coal ash.
S.
No.
Trace
elements
(mg kg-1)
Rock types Coal ash
Granite Basalt
Mean
coal ash
content
Crustal
abun-
dance
1. Cr 9.0 16.03 246 100
2. Cd 0.010 0.067 11 0.2
3. Cu 10.7 22.4 217 55
4. Ni 6.4 18.0 171 75
5. Pb 28.7 18.0 287 13
6. Zn 74.9 132.0 572 70
Fertilizers and soil amendments
e excess use of nitrogenousfertilizers leads to the
acidication of soil and contaminates the agricultural

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S. Sarvade et al., / AATCC Review (2022)
Table 3: Concentration of heavy metal in dierent inorganic fertilizers and soil amendments.
Fertilizers Heavy Metals (mg kg-1)
Zn Cu Fe Mn B Mo Pb Cr Cd As Ni
Urea (1) 4.0 0.6 36 0.5 1.0 5.3 4.0 6.0 1.0
Ammonium Nitrate (2) - <0.60 - - - - <0.40 - <0.20 <0.401<0.201
Calcium Ammonium Nitrate (1) 7.6 2.8 407 24.8 9.0 56 116 9.0 6.0 - -
Ammonium Sulphate Nitrate (1) 54.7 1.9 409 53.8 6.5 5 - - - - -
Ammonium Sulphate (1) 11.3 0.8 23 3.5 6.0 6 - - - - -
Triple Super Phosphate (1) 418 49.8 3483 75.0 21.5 270 11.1-
13.2 88.9 5.0-
6.2
15.3-
16.2
15.6-
25.2
Single Super Phosphate (1) 165 15.5 4050 8900 133 335 487 88 187 - -
Rock Phosphate (1) 187 32.0 19917 975 71.5 555 962 184 303 16.5-
20.5
16.8-
50.4
Potassium Chloride (1) 10.0 3.12 110 3.5 16.3 26 - - - - -
Potassium Sulphate (1) 2.0 7-10 - 2.2-13 4.0 0.2 - - - - -
Nitro Phosphate 15-15-15 (1) 40 14.0 36 532 144 133 285 54 89 - -
Nitro Phosphate 10-10-10 (1) 45.5 5.4 4507 120 134 140 - - - - -
Amm. Phos. Sulp. 10-10-10 (1) 164 9.4 2425 52 242 249 - - - - -
NPK 12-32-16 (1) 114 16.4 9358 230 207 91.5 - - - - -
NPK 10-26-26 (1) 38.0 13.3 7750 116.5 176 88 0 0 - - -
Diammonium Phosphate (1) 112 7.2 11275 307 396 75.3 195 81 109 - -
Dairy manure (mean) (2) - 18 - - - - 7.5 - 0.7 6.8 9.6
Poultry manure (3) 330-456 48-78 - - - - 6.0-
8.4
<1.0-
7.7
0.20-
0.30 - 7.1-9.0
Swine manure (3) 540-1200 250-
600 - - - - 7.0-
11
2.2-
1.3
0.50-
0.82 - 11.33
Source: [8] [74] [110]
soils. Excess and imbalance use of fertilizers are
regularly to soils in intensive farming systems to
provide adequate N, P, and K for crop growth. e
compounds used to supply these elements contain
trace amounts of heavy metals (e.g., Cd and Pb) as
impurities and continuous application of fertilizers
in soils may signicantly increase the heavy metals
content in the soil [40] . Further, for the proper
development and complete the life cycle, plants must
acquire not only macronutrients (N, P, K, S, Ca, and
Mg), but also micronutrients or heavy metals (i.e.
Co, Cu, Fe, Mn, Mo, Ni, and Zn) that are essential
for healthy plant growth [49] , and crops may be
supplied with these as an addition to the soil or as
a foliar spray. e concentration of heavy metals in
fertilizers and soil amendments is presented in table
3.
Pesticides / Herbicides
Pesticides (DDT, Aldrin, and Dieldrin) are synthetic
toxic chemicals that kill dierent types of pests and
insects causing damage to agriculture, but it has
many ecological repercussions. For example copper-
containing fungicidal sprays such as Bordeaux mixture
(copper sulphate) and copper oxychloride [40] . Lead
arsenate was used in fruit orchards for many years
to control some parasitic insects. Compared with
fertilizers, the use of such materials has been more
localized, being restricted to particular sites or crops
[57] . ey are generally insoluble in water and non-
biodegradable and these chemicals will be accumulated
in the soil. us, it will aect human health through
many physiological and metabolic disorders. However,
the herbicides like sodium arsenite (Na3AsO3),
sodium chlorate (NaClO3), etc. can decompose in few
months. Also, they aect the environment and are not
environmentally friendly. Further, dierent ndings
suggested that spraying herbicides cause more insect
attack and diseases of plants in comparison to manual
weeding.
Bio-solids and Manures
e use of dierent bio-solids (e.g., composts,
livestock manures, and municipal sewage sludge)
to land inadvertently leads to the accumulation of
heavy metals such as As, Cd, Cr, Cu, Pb, Hg, Ni, Zn,

63 © 2022 AATCC Review. All Rights Reserved.
S. Sarvade et al., / AATCC Review (2022)
and so forth, in the soil [7]. Certain animal wastes
such as poultry, cattle, and pig manures produced
in agriculture are commonly applied to crops and
pastures either as solids or slurries [92]. Pig and
poultry manures have also contaminated the soils by
the addition of heavy metals i.e. Cu and Zn because
these metals added as diets and growth promoters
for pig/poultry health products [92]. e manures
produced from animals on such diets contain high
concentrations of As, Cu, and Zn and, if repeatedly
applied to restricted areas of land, can cause the
considerable buildup of these metals in the soil in
the long run. e dierent heavy metal contents in
municipal solid waste and its standards are presented
in table 4. Bio-solids (sewage sludge) are primarily
organic solid products, produced by wastewater
treatment processes that can be benecially recycled
[100]. e bio-solid materials applied in soil are a
common practice in many countries that allow the
reuse of bio-solids produced by urban populations
[109]. e bio-solids term is more common in place
sewage sludge because it is thought to reect more
accurately the benecial characteristics inherent
to sewage sludge [90]. Bio-solids content various
heavy metals (Pb, Ni, Cd, Cr, Cu& Zn), and these
metals concentrations are governed by the nature
and industrial activity, as well as the type of process
employed during the bio-solids treatment [55]. Under
certain conditions, metals added to soils through
bio-solids can be leached downwards through the
soil prole and can have the potential to contaminate
groundwater [56].
Table 4: Heavy metals content in municipal solid
waste (MSW) and its standards.
Heavy Metals
Municipal Solid
Waste German Standards
(mg ka-1)
Lead (Pb) 420 150
Chromium (Cr) 107 150
Nickel (Ni) 84 50
Cadmium (Cd) 2.8 3.0
Mercury (Hg) 1.9 3.0
Copper (Cu) 222 150
Zinc (Zn) 919 500
Waste Disposal
e application of industrial and wastewater-related
euents to land are a common practice in many parts
of the world [75]. Worldwide, it is estimated that 20
million hectares of arable land are irrigated with waste
water. e ndings of the various studies suggested that
agriculture based on wastewater irrigation accounts
for 50 percent of the vegetable supply to urban areas
[12]. In general, farmers are not bothered about
harmful eects of these contaminants and they are
only interested in getting more yields and prots from
intensive agricultural practices. Further, the long-term
or continuous irrigation of lands through wastewater
euents can accumulate the concentration of heavy
metals in the soil.
Basic Chemistry of Heavy Metals
In order of abundance of heavy metals i.e. Pb, Cr, As,
Zn, Cd, Cu are found at contaminated soils [101]
and they are capable of reducing crop production
due to the risk of bioaccumulation in the food chain.
Basic chemistry is necessary for understanding the
bioavailability and remedial options of these heavy
metals in the soil because the fate and chemistry in soil
depend signicantly on the chemical form of the heavy
metals (Table 5). Once, heavy metals are adsorbed on
soil colloids it is redistributed into various chemical
forms with varying bioavailability, mobility, and
toxicity [14]. However, the distribution is believed to
be governed by reactions of heavy metals in soils such
as mineral precipitation and dissolution, ion exchange,
adsorption, and desorption, aqueous complexation,
biological immobilization and mobilization, and
uptake by the plants [50].
Table 5: Basic chemistry of the heavy metals.
S.
No.
Heavy
Metals
Chemistry of Heavy metals
Atomic
Num-
ber
Atomic
Weigh t
Den-
sity (g
cm-3)
Melting
Point
(oC)
Boiling
Point
(oC)
1.
Chro-
mium
(Cr)
24.0 52.0 7.19 1875.0 2665.0
2. Nickel
(Ni) 28.0 58.7 8.90 1455.0 2913.0
3. Copper
(Cu) 29.0 63.5 8.96 1083.0 2595.0
4. Zinc
(Zn) 30.0 65.4 7.14 419.5 906.0
5. Arsenic
(As) 33.0 75.0 5.72 817.0 613.0
6. Mercu-
ry (Hg) 80.0 200.6 13.6 –38.8 357.0
7.
Cad-
mium
(Cd)
48.0 112.4 8.65 320.9 765.0
8. Lead
(Pb) 82.0 207.2 11.40 327.4 1725.0

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S. Sarvade et al., / AATCC Review (2022)
Impact of Heavy Metals Toxicity in Plants
e uptake of dierent heavy metals by the plant
through soil solution or easily solubilized by root
exudates [13]. Also plants required certain heavy
metals for their development/growth and the excess
quantity of these metals in soils can contaminated
the soils and ultimately shows toxicity to plants.
Further, the concentration of these heavy metals in
plants exceeds from their optimal levels, they can
adversely aect plant growth.e toxic eect of heavy
metals on the plants growth varies according to the
particular heavy metal involved in the process. e
data presented in table 6 shows a summary of the toxic
eects of specic metals on the growth, biochemistry,
and physiology of various plants [16]. e growing of
dierent plants on heavy metals contaminated soils
can reduceplant growth.
Nickel
e phytotoxic eects of Ni have been known for a
long time [94]. Apart from a decrease in growth, the
symptoms of Ni toxicity include chlorosis, stunted
Table 6: Eect of heavy metal toxicity on plants.
Heavy
metal Plant Toxic eect on plant Source
As
Rice (Oryza sativa)Reduction in seed germination; decrease in seedling height; reduced leaf area
and dry matter production [1]
Tomato ( Lycopersicon
esculentum)Reduced fruit yield; decrease in leaf fresh weight [19]
Cd
Wheat (Trit icumsp.) Reduction in seed germination; decrease in plant nutrient content; reduced
shoot and root length [112]
Garlic (Allium sativum) Reduced shoot growth; Cd accumulation [39]
Maize (Zea mays) Reduced shoot growth; inhibition of root growth [106]
Cr
Wheat (Trit icumsp.) Reduced shoot and root growth [67]
Tomato ( Lycopersicon
esculentum)Decrease in plant nutrient acquisition [62]
Onion (Allium cepa) Inhibition of germination process; reduction of plant biomass [65]
Cu Bean (Phaseolus
vulgaris)Accumulation of Cu in plant roots; root malformation and reduction [17]
Hg Rice (Oryza sativa)Decrease in plant height; reduced tiller and panicle formation; yield reduc-
tion; bioaccumulation in shoot and root of seedlings [44]
Tomato ( Lycopersicon
esculentum)
Reduction in germination percentage; reduced plant height; reduction in
owering and fruit weight; chlorosis [85]
Heavy
metal Plant Toxic eect on plant Reference
Mn
Spearmint (Mentha
spicata)
Decrease in chlorophyll a and carotenoid content; accumulation of Mn in
plant roots [4]
Pea (Pisumsativum)Reduction in chlorophylls a and b content; reduction in relative growth rate;
reduced photosynthetic O2 evolution activity and photosystem II activity [20]
Tomato ( Lycopersicon
esculentum)Slower plant growth; decrease in chlorophyll concentration [86]
Ni
Pigeon pea (Cajanus
cajan)
Decrease in chlorophyll content and stomatal conductance; decreased en-
zyme activity which aected Calvin cycle and CO2 xation [87]
Wheat (Trit icumsp.) Reduction in plant nutrient acquisition [6]
Rice (Oryza sativa) constraintfor root growth [51]
Pb Maize (Zea mays)Reduction in germination percentage; suppressed growth; reduced plant
biomass; decrease in plant protein content [36]
Oat (Avena sativa) Inhibition of enzyme activity which aected CO2 xation [64]
Zn
Cluster bean (Cyamopsis
tetragonoloba)Reduction in germination percentage; reduced plant height and biomass;
decrease in chlorophyll, carotenoid, sugar, starch, and amino acid content [54]
Pea (Pisumsativum)Reduction in chlorophyll content; alteration in structure of chloroplast; re-
duction in photosystem II activity; reduced plant growth [21]

65 © 2022 AATCC Review. All Rights Reserved.
S. Sarvade et al., / AATCC Review (2022)
root growth, and sometimes brown interval necrosis
and symptoms specic to the plant species. e
concentration of Ni varied from 50 to 100 mgg-1 (dry
weight basis) is indicated by its toxicity in plants. For
example, [69] reported toxicity symptoms in spring
wheat at 8 mg kg-1 but no yield loss in oats at 90 mg
kg-1.parameters due to a reduction in photosynthetic
activities, plant mineral nutrition, and reduced
activity of some enzymes [41].
Lead
e visual non-specic symptoms of Pb toxicity are
rapid inhibition of root growth, stunted growth of the
plant, and chlorosis [15]. Pbphyto-toxicity leads to
the inhibition of enzyme activities, disturbed mineral
nutrition, and water imbalance. ese disorders
distress the normal physiological activities of the
plant. At high concentrations Pb eventually may cause
to cell death [83]. Pb toxicity inhibits the germination
of seeds and retards the growth of seedlings. Lead
contents in soils decreases seed germination, root/
shoot growthand dry mass of roots/shoots [59].
High Pb concentrations caused decreased rice seeds
germination (14 to 30%)and reduced 13 to 45%
seedling growth [103].
Arsenic
High content of arsenic in plants inhibits the
metabolic process such as photosynthesis thereby it
inhibiting the growth of plants [97]. e threshold
value of As toxicity 40 mg kg-1 was established for crop
plants [88]. us, the higher concentration of arsenic
signicantly reduces the biomass production and
crop yields. In general, the accumulation of As in the
edible parts of most plants is low, which is attributed to
several reasons, including [108] (i) low bioavailability
of As in soil; (ii) restricted uptake by plant roots; (iii)
limited translocation of As from roots to shoots; and
(iv) phytotoxicity and subsequent premature plant
death at relatively low As concentrations in plant
tissues. Most plants do not accumulate enough As to
be toxic to animals and humans. Growth reductions
and crop failure are the main consequences of soil As
contamination [105] .
Cadmium
Cadmium contaminations in soils are mainly from
the application of bio-solids, use of phosphates
fertilizer, and euents from cadmium-using and
recycling industries. e toxic eect of Cd to an
induced reduction in the number of owers and in
vitro pollen germination, it stimulated tube growth,
decreased the number of ovules/pistil (ovules were
morphologically normal and receptive), inhibited the
number of pods and seeds. Cd treatment increased
protein content in physiologically matured seeds.
Further, the rate of Cd- enriched sewage sludge/
city compost resulted was obtained signicant yield
reduction in Alsol and Ultisol. e applied Cd
remained in the top 10 cm soil (87-96%) and resulted
in a lower recovery of Cd (8.3%) in the leachates [30].
Chromium
e Chromium toxicity in plants depends on its
valence state, Cr (VI) which is highly toxic and mobile
whereas Cr (III) is less toxic. Since plants lack a specic
transport system for Cr, it is taken up by carriers of
essential ions such as sulphate or iron. Eects of Cr
toxicity on plant growth and development include
alterations in the germination process as well as in the
growth of roots, stems and leaves, which may aect
total dry matter production and yield. Cr toxicity was
also had deleterious eects on plant physiological
processes such as photosynthesis, water relation,
and minerals nutrition, and alteration of metabolic
activities have also been described in plants by the
direct eect on enzymes or by its ability to generate
oxidative stress. e high levels of Cr in plants can
inhibit seed germination and subsequent seedling
growth [84].
Impact of Heavy Metals Toxicity in Soils
Heavy metals are considered one of the major soil
contaminants for creating soil pollution and these
metals shows the toxic eect on soil microorganism
by the change of the population size,bio-diversity, and
various activities of the soil microbial communities
[3]. e toxicity of heavy metals in soils is a very
serious issue due to their presence in the food chain,
thus, it destroying the entire ecosystem. ere are
various ways through which heavy metals present
risks to humans, animals, plants, and ecosystems as a
whole. Such ways include direct ingestion, absorption
by plants, food chains, consumption of contaminated
water, and alteration of soil pH, porosity, colour, and
its natural chemistry which in turn impact the soil
quality [61] [79].
e soil properties i.e. organic matter, clay contents,
and pH have major inuences on the extent of the
eects of metals on biological and biochemical

66 © 2022 AATCC Review. All Rights Reserved.
S. Sarvade et al., / AATCC Review (2022)
properties [91]. Heavy metals indirectly aect
soil enzymatic activities by shiing the microbial
community which synthesizes enzymes [89]. Heavy
metal content also exhibits toxic eects on soil
biota by aecting important microbial processes
and decreasing the number and activity of soil
microorganisms. [42] reported that the enzyme
activities are inuenced in dierent ways by dierent
metals due to the dierent chemical anities of the
enzymes in the soil system and each soil enzyme
exhibits a dierent sensitivity to heavy metals.
e order of inhibition of urease activity generally
decreased according to the sequence Cr > Cd > Zn
>Mn>Pb. Diversity and activity of soil microbes play
signicant roles in the recycling of plant nutrients,
maintenance of soil structure, detoxication of
noxious chemicals and the control of plant pests and
plant growth communities are important indices of
soil quality.
It is important to investigate the functioning of soil
microorganisms in ecosystems exposed to long-
term contamination by heavy metals [107]. [3] also
reported that heavy metals exert toxic eects on soil
microorganisms hence resulting in the change in the
diversity, population size, and overall activity of the
soil microbial communities and observed that the
heavy metal (Cr, Zn and Cd) pollution inuenced the
metabolism of soil microbes in all cases. In general, the
concentration of heavy metals increases in the soil it
adversely aects soil microbial properties i.e.enzyme
activity, and respiration rate, which appear to be very
useful indicators of soil contaminants.
Impact of long-term application of inorganic
fertilizers on the accumulation of heavy metals in
Vertisols
e ndings of the long-term fertilizer experiment
with soybean-wheat cropping sequence in a
Vertisolsare presented in table 7 and revealed that
the hazardous heavy metals (Cd, Pb, Ni, and Cr)
and essential heavy metal (Zn, Fe, Mn, and Cu)
contents were found to accumulate signicantly due
to continuous application of dierent agricultural
inputs (inorganic fertilizers and organic manures)
to the soil. Further, the highest contents of Cd, Pb,
and Ni were found in the treatment receiving super
optimal dose fertilizers (150% NPK through urea,
SSP, and MOP, respectively) and the lowest values
were recorded in the control plot [96].
However, the essential heavy metals (Zn Cu, Fe, and
Mn) are not limiting factors even aer 35 years of
intensive cropping with continuous applications of
various inorganic fertilizers and organic manure. e
highest contents of essential heavy metals were noted
in 100% NPK+15 t FYM ha-1treatment plots, which
is obviously due to annual acceleration through farm
yard manure which contributes signicantly to the
availability of these essential heavy metals, followed
by 150% NPK treatments and the lowest values were
recorded in control plot [95].
Table 7: Eect of dierent treatments on hazardous
and essential heavy metals.
Treatments
Hazardous heavy metals content (mg
kg-1)
Cd Pb Ni Cr
50% NPK 0.032 1.733 0.357 0.134
100% NPK 0.034 1.809 0.388 0.156
150% NPK 0.042 1.886 0.405 0.180
100% NP 0.026 1.630 0.327 0.052
100% N 0.020 1.764 0.514 0.143
100% NPK + FYM 0.018 1.591 0.296 Trace
100% NPK – S 0.017 1.742 0.327 0.047
Control 0.018 1.425 0.195 Trace
CD (P = 0.05)0.01 0.15 0.05 0.01
Treatments
Essential heavy metals content (mg
kg-1)
Zn Fe Mn Cu
50% NPK 0.47 20.64 17.20 1.58
100% NPK 0.48 21.81 17.91 1.40
150% NPK 0.75 26.05 16.84 1.64
100% NP 0.53 28.73 19.65 1.46
100% N 0.50 19.71 12.56 1.25
100% NPK + FYM 0.92 31.04 16.58 1.82
100% NPK – S 0.49 26.89 15.00 1.32
Control 0.42 17.04 14.19 1.18
CD (P = 0.05)0.09 2.32 1.60 0.18
Permissible limits of heavy metals content in
water,soil and plant
e accumulation of heavy metals in agricultural soil
and plants from dierent sources of soil contaminates,
it may deteriorate the soil quality and that can increase
risks to human health. e maximum allowable limits
of heavy metals in irrigation water, soil, and vegetables
have been established by standard regulatory bodies
such as World Health Organization (WHO), Food
and Agricultural Organization (FAO), and Ewers U,
Standard Guidelines in Europe as shown in table 8

67 © 2022 AATCC Review. All Rights Reserved.
S. Sarvade et al., / AATCC Review (2022)
[8] and maximum allowable limits of heavy metals in
dierent countries are presented in table 9 [73].
Standard forcompost manure solid waste
Maximum allowable limit of heavy metals and other
standards for compost manure solid waste by Central
Pollution Control Board (CPCB), (Management and
Handling) Rules 2000, MEF [8] are presented in table
10.
Remediation of contaminated soils
Remediate means to solve a problem of soil
contamination and phytoremediation means to
use plants or plant parts to solve an environmental
problem such as contaminated/polluted soils or
contaminated groundwater. e concentration of
various heavy metals in contaminated soil is strongly
inuenced by the selection of the appropriate
remediation treatment approach. e contamination
of heavy metals in the soil should be characterized
by their type, amount, and distribution in the soil.
Once the site is characterized, the desired level of
each metal in the soil must be determined. Several
technologies exist for the remediation of metal-
contaminated soil [111]. e following methods
are used for the remediation of heavy metals from
Table 8: Maximum allowable limits of heavy metal in irrigation water, soils and vegetables.
Heavy metals Maximum permissible level in irrigation
water (μg ml-1)
Maximum permissible level
in soils (μg g-1)
Maximum permissible level in
vegetables (μg g-1)
Cadmium (Cd) 0.01 3 0.10
Lead (Pb) 0.065 100 0.30
Nickel (Ni) 1.40 50 67
Chromium (Cr) 0.55 100 -
Zinc (Zn) 0.20 300 100
Copper (Cu) 0.017 100 73
Iron (Fe) 0.50 50000 425
Manganese (Mn) 0.20 2000 500
Cobalt (Co) 0.05 50 50
Arsenic (As) 0.10 20 -
Table 9: Values of maximum allowable limits for heavy metals in dierent countries (mg kg-1).
Element Austria Canada Poland Japan Great Britain Germany
Cadmium(Cd) 5 8 3 - 3 2
Cobalt(Co) 50 25 50 50 - -
Chromium(Cr) 100 75 100 - 50 200
Copper(Cu) 100 100 100 125 100 50
Nickel (Ni) 100 100 100 100 50 100
Lead (Pb) 100 200 100 400 100 500
Zinc (Zn) 300 400 300 250 300 300
Table 10: Heavy metals standards for compost manure solid waste.
Heavy Metals Maximum acceptable concentration (mg kg-1)
Zinc (Zn) 1000
Copper (Cu) 300
Nickel (Ni) 50
Cadmium (Cd) 5
Chromium (Cr) 50
Lead (Pb) 100
Arsenic (As) 10
Mercury (Hg) 0.15

68 © 2022 AATCC Review. All Rights Reserved.
S. Sarvade et al., / AATCC Review (2022)
contaminated soils.
Phytoremediation
During recent years the concept of using plants to
remediate heavy metal-contaminated sites (called
phytoremediation) has received greater attention
[102] [38] ,. Phytoremediation may involve either
phytostabilization or phytoextraction (means the
use of plants to remove the contaminant from
contaminated soils). e concept of using plants
to accumulate metal for subsequent processing is
both technically and economically attractive. e
term“phytoremediation” is a Greek word phytonwhich
means plants and the Latin root remedium(to correct
or to remedy) [23] [98].It deals with the cleanup of
organic pollutants and heavy metal contaminants
using plants and rhizospheric microorganisms.
Dierent denitions of phytoremediation are given
by the dierent scientist which is presented in table
11.
Table 11: Denition of phytoremediation.
Denition of phytoremediation Researchers
e use of plants to improve degraded envi-
ronments [63]
e use of plants, including trees and grasses,
to remove, destroy or sequester hazardous
contaminants from media such as air, water,
and soil
[72] [25]
e use of plants to remediate toxic chem-
icals found in contaminated soil, sludge,
sediment, groundwater, surface water, and
wastewater
[76]
An emerging technology using specially
selected and engineered metal accumulating
plants for environmental cleanup
[52]
e use of vascular plants to remove pollut-
ants from the environment or to render them
harmless
[11]
e engineered use of green plant to remove,
contain, or render harmless such environ-
mental contaminants as heavy metals, trace
elements, organic compounds, and radioac-
tive compounds in soil or water. is deni-
tion includes all plant-inuenced biological,
chemical, and physical processes that aid in
the uptake, sequestration, degradation, and
metabolism of contaminants, either by plants
or by the free-living organisms that constitute
the plant rhizosphere
[33]
Phytoremediation is the name given to a set
of technologies that use dierent plants as
a containment, destruction, or extraction
technique. Phytoremediation is an emerg-
ing technology that uses various plants to
degrade, extract, contain, or immobilize
contaminants from soil and water
[98]
Phytoremediation in general implies the use
of plants (in combination with their associ-
ated microorganisms) to remove, degrade, or
stabilize contaminants
[28]
e phytoremediation technology is eco-friendly
and an ecient means for the restoration of polluted
environments especially those of heavy metals.
Nonetheless, the level of soil contamination, the
quantity of metal contaminant in the soil, as well as the
ability of plants to aggressively take up metals from the
soil, determine the success of phytoremediation at any
polluted site. Plants utilized in phytoremediation are
the hyper-accumulators with very high heavy metal
accumulation potential and little biomass eciency,
and non-hyper-accumulators, which possess lesser
extraction capacity than hyper-accumulators, but
whose total biomass yield is substantially higher and
are fast-growing species (Figure 2).
Figure 2: Phytoremediation strategies [70]
Heavy metals uptake mechanism through
phytoremediation
Generally, phytoremediation is an emerging
technology using selected plants to clean up
the contaminated environment from hazardous
contaminants to improve the environment quality.

69 © 2022 AATCC Review. All Rights Reserved.
S. Sarvade et al., / AATCC Review (2022)
e uptake mechanisms of both organic and inorganic
contaminants through phytoremediation technology
are illustrated in Figures 3 and 4. Forthe remediation
of organic contaminants phytostabilization,
rhizodegradation, rhizoltration, phytodegradation,
and phytovolatilization technology are used and
these mechanisms are not able to be absorbed
into the plant tissue. However, for the inorganic
contaminants, phytostabilization, rhizoltration,
phytoaccumulation, and phytovolatilization
mechanisms can be involved for the remediation of
contaminated soils. Phytoremediation is currently
divided into following areas [23] [99] :-
Phytoextraction: Phytoextraction is the process of
removal or extraction of heavy metal into harvestable
plant tissues from contaminated soils. e uptake/
absorption and translocation of heavy metal
contaminants through plant roots into the plant
shoots (above-ground portions of the plants) which
can be harvested and burned. For example, some
plant species i.e. Suterafodina, Dicomaniccolifera,
and Leptospermum Scoparium have been reported
to accumulate Cr content to high concentrations in
their tissues.
Phytostabilisation: e use of certain plant species
to immobilize the heavy metal contaminants from
the soil and groundwater through absorption and
accumulation in plant tissues. ese contaminate are
adsorbed onto roots or precipitation within the root
zone that can be preventing their migration in soil, as
well as their movement by erosion and deation.
Rhizoltration: e use of plant roots to remove
heavy metal contaminants from contaminated water
[22]. Rhizoltrationis the process of adsorption or
precipitation onto plant roots or absorption into and
sequesterization in the roots of contaminants that are
in solution surrounding the root zone by constructed
wetland for cleaning up communal wastewater. A
high level of Pb deposition is seen in corn root tips as
revealed by histochemical and electron microscopy
studies. [53] also showed that corn plants treated
with 10-3 M Pb accumulated 138, 430 mg of Pb per
kg of dry weight in root tips compared to 26,833
mg in the root basal part. Since the rst 8 mm of
the apical root accounts for approximately 50% of
the Pb accumulated by the entire root system [53],
it appears that the plant with a more branched root
system will take up more Pb and other heavy metals
compared to plants with longer and less branched
root systems. Generally, aquatic plants are growing
in contaminated water. Examples- Scirpuslecusteris,
Phragmiteskarka and Bacopamonnieri.
Figure 3: Uptake mechanisms on phytoremediation
technology [37].
Figure 4: Phytoremediation technology foruptake
oeavy metal by the plants [93].
Phytovolatilization: It refers to the uptake and
transpiration of contaminants, primarily organic
compounds, by plants. e contaminant, present in
the water taken up by the plant, passes through the
plant or is modied by the plant, and is released to the
atmosphere (evaporates or vaporizes).In this method
growing of trees and other plants take up water along
with the contaminants. Heavy metal contaminants
can pass through the plants to the leaves and

70 © 2022 AATCC Review. All Rights Reserved.
S. Sarvade et al., / AATCC Review (2022)
volatilize into the atmosphere at comparatively low
concentrations.
Phytodegradation: Use of plants and associated
micro-organisms to degrade organic pollutants.
e contaminants can be absorbed by the root
to be subsequently stored or metabolised by the
plant. Degradation of contaminants in the soil by
plant enzymes exuded from the roots is another
phytoremediation mechanism [58]. For many
contaminants, passive uptake via micro-pores in the
root cell walls may be a major route into the root,
where degradation can take place [33].
Hyperaccumulation: In this method dierent plant
species is classied as hyperaccumulator when it takes
heavy metals against their concentration gradient
between the soil solution and cell cytoplasm, thus
acquiring capacity of accumulating a very high metal
concentration in tissues without much diculty in
carrying out growth and metabolic functions. Some
important heavy metal hyper accumulators are given
below. e data of the bioaccumulation coecient
of various brassica species are presented in table 12
which is used as indicator for hyper accumulation
[10].
Table 12: Bioaccumulation coecients of various
Brassica species at maturity stage.
Species Heavy metals (mg kg-1)
Zn Cu Ni Pb
B. juncea 6.83 3.08 3.21 12.86
B. compestris 12.48 2.59 13.61 2.56
B. carinata 11.89 1.94 12.30 17.72
B. napus 9.87 1.14 8.98 9.37
B. nigra 6.56 2.04 8.66 7.94
• Brassica juncea (mustard)
• Streptanthuspolygaloides Gray (Brassicaceae)
• Silene spp. (Cryophyllaceae) – S. vulganis, S.
burchelli, S. cobaltica, S. nata, S. diocia – ese
varieties of silene produce the highest dry matter
yield and also remediate > 10000 ppm Ni from
the polluted soils.
• laspi spp. (Brassicaceae) - T. cacrulescens, T.
montanum, T. ochlecum.
• Alyssum spp. (Brassicaceae) -A. argentum, A.
corsicum, A. euboeum, A. heldrechii, A. murale, A.
cnium, A. troodii [5]
Advantages of phytoremediation technology
• e technology is less disruptive than current
techniques
• e eectiveness in contaminant reduction
• It is low-cost technology as compared to other
treatment methods.
• It is applicable to a wide range of contaminants
• It is environmentally friendly and aesthetically
pleasing to the public.
• It works on a variety of organic and inorganic
compounds.
• It can be either in situ / ex-situ.
• is technology is very easy to implement and
maintain.
• It reduces the number of wastes to be landlled.
Limitation of phytoremediation technology
• e time-consuming method (may take several
years to remediate).
• e amount of produced biomass.
• Restricted to sites with shallow contamination
with the rooting zone.
• e impacts of contaminated vegetation
• It may depend on the climatic condition.
• Possible eect on the food chain.
Chelation
Results from chelation experiments indicate that
Pb concentration in the shoot can be increased
dramatically when the soil Pb concentration is
increased by adding a synthetic chelate to the
contaminated soil. Synthetic chelate EDTA forms
a soluble complex with many metals, including Pb
[47], and can solubilize Pb from soil particles [102].
Application of EDTA to Pb-contaminated soils has
been shown to induce the uptake of Pb by plants
causing Pb to accumulate more than 1% (w/w) of

71 © 2022 AATCC Review. All Rights Reserved.
S. Sarvade et al., / AATCC Review (2022)
the shoot dry biomass [34] [35]. Large Pb particles
cannot easily cross the casparian strip due to their
size and charge characteristics but once they form a
complex with chelators such as EDTA, their solubility
increases, the particle size decreases, and they become
partially ‘invisible’ to those processes that would
normally prevent their unrestricted movement such
as precipitation with phosphates and carbonates, or
binding to the cell wall through mechanisms such as
cation exchange [38]. It is important to point out that
the addition of chelates to the soil has to be done in
a carefully controlled manner so as not to mobilize
Pb into groundwater or otherwise promote its o-site
migration [35].
Soil scraping
Replacement of the uppermost contaminated soil
(0-15 cm depth) from the cultivated eld has been
possible. e maximum amount of lead was absorbed/
adsorbed by soil in clay–humus complexes. By
scraping of contaminated soil highest quality heavy
metals can be removed from the soil and become
suitable from growing crops. One to two times further
cleaning by phytoremediation or rhizoltration leads
to the remove the traces amount of heavy metals from
soil and then crop produce becomes edible for animal
consumption.
Conclusion
Environmental pollution through heavy metal
ions is the current world growing problem. It is
increasing due to the increase in urbanization and
industrialization. Soil is the nal sink for most
of the contaminants/pollutants. e discharge of
heavy metal ions as a byproduct of various human
activities are accompanied with large-scale water and
soil pollution. ese contaminants reduce microbial
activities and ultimately deteriorate the soil quality. e
toxicity of heavy metals in plants shows by a reduction
in growth due to changes in their physiological and
biochemical activities. e remediation of heavy
metals from contaminated soils is necessary to
reduce the associated risks or available resources for
agricultural production and human health. In this
context, phytoremediation technology is frequently
listed among the best available technologies for
cleaning up heavy metal-contaminated soils. e
phytoremediation and rhizoltration technology
is used to extract or removal of pollutants (heavy
metals) from contaminated soil and water. us, this
technology is environmental friendly and potentially
cost-eective.
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