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American Journal of  Environmental
Economics (AJEE) 

An Academic Review on Heavy Metals in the Environment: Effects on Soil, Plants
Human Health, and Possible Solutions

Paul Obokparo Ewubare1*, Sarah Onosteike Aliyu1, Ikioukenigha Michael2, Joy Osebhajimede Ejakhaegbe1, Justice Obomejero3

Sunday Osarodion Okoro4, Oluwaseyi Joseph Olukayode1

Volume 3 Issue 1, Year 2024
ISSN: 2833-7905 (Online)

DOI: https://doi.org/10.54536/ajee.v3i1.3261
https://journals.e-palli.com/home/index.php/ajee

Article Information ABSTRACT

Received: July 03, 2024

Accepted: August 06, 2024

Published: August 10, 2024

Heavy metal uptake by plants and successive accumulation in human tissues and 
biomagnifi cation through the food chain cause signifi cant concerns for both human health 
and the environment. Human activities, including industrial, agricultural, traffi c, domestic, and 
mining processes, have increased the toxic levels of  these metals beyond those contributed 
by natural rock-forming processes. Heavy metals are potentially toxic to plants, resulting in 
chlorosis, weak growth, yield depression, reduced nutrient uptake, metabolic disorders, and 
diminished nitrogen-fi xation ability. Utilization of  food crops contaminated with heavy metals 
is a major food chain route for human exposure. The cultivation of  plants in contaminated 
soil poses a potential risk since vegetal tissues can accumulate heavy metals. Owing to their 
toxicity and potential for bioaccumulation, these compounds should be subject to mandatory 
monitoring, particularly in soil and plants, to prevent their entry into the human food system. 
Furthermore, studies have shown that phytoremediation and microbial remediation are 
promising techniques for mitigating the negative effects of  heavy metals contamination. These 
methods are environmentally friendly and economically effective, making them applicable 
globally. This review paper summarizes the effects of  heavy metals in our environment by 
examining relevant works related to the topic. To achieve this, databases such as Google 
Scholar, Frontier in Microbiology, African Journals Online (AJOL), Scopus, Web of  Science, 
ScienceDirect, and Directory of  Open Access Journals (DOAJ) were explored to identify 
studies on the effects on soil, plants, human health and managing heavy metals in the 
environment.

Keywords

Heavy Metals, Bioaccumulation, 
Environmental Contamination, 
Human Health, Phytoremediation, 
Microbial Remediation

1 University of  Benin, Edo State, Nigeria
2 Igbinedion University, Okada, Edo State, Nigeria
3 Imo State University, Imo State, Nigeria
4 Auchi Polytechnic, Edo State, Nigeria
* Corresponding author’s e-mail: paulewubare9@gmail.com

INTRODUCTION
Heavy metals are serious environmental pollutants, 
with their toxicity becoming increasingly signifi cant for 
ecological, evolutionary, nutritional, and environmental 
reasons (Jaishankar et al., 2013; Nagajyoti et al., 2010). 
Metals are naturally present in the earth’s crust, and 
their levels in the environment can vary widely across 
different regions, leading to spatial variations in 
background concentrations. The distribution of  metals 
in the environment is infl uenced by the properties of  
the metals themselves and various environmental factors 
(Khlifi  and Hamza-Chaffai, 2010). Heavy metals enter the 
environment through both natural and human activities. 
Sources include natural weathering of  the earth’s crust, 
mining, soil erosion, industrial discharge, urban runoff, 
sewage effl uents, pesticides and disease control agents 
used on plants, and air pollution fallout, among others 
(Ming-Ho, 2005). While some individuals are primarily 
exposed to these contaminants in the workplace, most 
people encounter these toxic elements through their 
diet (food and water). The contamination cycle of  heavy 
metals typically follows this path: industry, atmosphere, 
soil, water, food, and humans. Although the toxicity 
and resulting threat to human health depend on the 
concentration of  the contaminant, it is well-known that 

chronic exposure to low levels of  heavy metals and 
metalloids can have harmful effects (Castro and Méndez, 
2008).
Heavy metals taken up by plants can accumulate in human 
tissues and biomagnify through the food chain, posing 
concerns for both human health and the environment 
(Wong and Selvam, 2006). These metals exert toxic 
effects on soil microorganisms, leading to changes in 
the diversity, population size, and overall activity of  soil 
microbial communities (Ashraf  and Ali, 2007). High 
levels of  lead (Pb) in soils can decrease soil productivity, 
and even low concentrations of  Pb can inhibit crucial 
plant processes such as photosynthesis, mitosis, and water 
absorption. Symptoms of  toxicity in plants include dark 
green leaves, wilting of  older leaves, stunted foliage, and 
brown short roots (Bhattacharya et al., 2008). The uptake 
of  heavy metals from soils at high concentrations poses 
a signifi cant health risk through the food chain (Jordao 
et al., 2006). Consumption of  food contaminated with 
heavy metals can severely deplete essential nutrients in the 
body, leading to decreased immune defenses, intrauterine 
growth retardation, malnutrition-related disabilities, and 
higher rates of  upper gastrointestinal cancer (Khan et al., 
2008). This academic review paper aims to assess heavy 
metal contamination in our environment, focusing on its 



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effects on soil, plants, and human health, and to propose 
possible solutions for managing these pollutants.

METHODOLOGY
The methodology for this study involved sourcing 
data primarily from previous research and secondary 
sources such as published studies, articles, and relevant 
materials. Databases including Google Scholar, Frontier 
in Microbiology, African Journals Online (AJOL), 
Scopus, Web of  Science, ScienceDirect, and Directory 
of  Open Access Journals (DOAJ) were systematically 
searched using keywords such as “heavy metals in the 
environment,” “heavy metal and human health,” “heavy 
metal and soil,” and “heavy metal and plants.” Sources 
were selected based on their relevance, recency, and the 
credibility of  the publication. By synthesizing insights 
from scholarly articles and reports, this research aims 
to address issues related to heavy metals and explore 
potential management strategies.

DISCUSSION
Heavy metals naturally occur in the environment and 
are vital for survival, but they may become hazardous 
when they accumulate in organisms. A few of  the most 
frequent heavy metals that contaminate the environment 
include mercury, cadmium, arsenic, chromium, nickel, 
copper, and lead (Hazrat et al., 2019). Heavy metals in 
the environment can be absorbed by soil, which then 
contaminates plants growing in that soil. These plants, 
when consumed by humans, transfer the heavy metals, 
potentially leading to health issues.

be considered high. Furthermore, the interconnected 
cycle above highlights the importance of  monitoring 
and managing soil quality to prevent the accumulation 
of  harmful substances in the food chain. Explicitly, the 
Soil acts as a reservoir for heavy metals, infl uencing 
their availability and uptake by plants, while the Plants 
absorb heavy metals from the soil, which can accumulate 
in edible parts. On the other hand, Humans consume 
plants containing heavy metals, leading to potential health 
risks. The Venn diagram visually emphasizes the overlap 
and interaction between these three components in the 
context of  environmental contamination.

Heavy Metals
Heavy metals are a unique group of  metals characterized 
by their high densities, atomic numbers, and atomic 
weights on the periodic table (Pourret et al., 2021). 
These metals, including mercury (Hg), cadmium 
(Cd), lead (Pb), chromium (Cr), and arsenic (As), are 
particularly concerning due to their toxicity, even at low 
concentrations (Di et al., 2023). Heavy metals typically 
have a specifi c gravity of  5 g/cm³ or higher (Sparks et 
al., 2003) and are non-biodegradable, meaning they can 
persist in the environment for decades. Human activities 
such as industrial processes, agriculture, traffi c, domestic 
activities, and mining have signifi cantly increased the 
levels of  these toxic metals in the environment compared 
to natural sources like rock weathering (Pam et al., 2013). 
Metals like Pb, Cd, Hg, Cr, and As are widespread in the 
environment, and they are non-essential elements with 
no benefi cial effects on humans. They are considered the 
most toxic to humans and animals due to their harmful 
health effects, even at low concentrations (Draghici et al., 
2010). On the other hand, metals like iron (Fe), zinc (Zn), 
nickel (Ni), manganese (Mn), and copper (Cu) are essential 
for plant growth and development at lower concentrations, 
playing crucial roles in physiological and biochemical 
functions, such as participating in redox reactions and 
enzymatic activities (Nagajyoti et al., 2010; Vieira et al., 2011). 
In the natural environment, heavy metals exist in various 
chemical forms and exhibit different behaviors regarding 
chemical interactions, mobility, biological availability, and 
potential toxicity (Osu et al., 2014).

Cadmium, Mercury, and Lead 
Cadmium (Cd) is released into the atmosphere through 
both natural processes and human activities. It can enter 
the aquatic environment via absorption, industrial waste, 
and surface runoff  into soils and sediments. Humans 
and animals can be exposed to cadmium by ingesting 
contaminated food, breathing polluted air, or drinking 
contaminated water. Cadmium does not support plant 
growth or metabolic processes (Hayat et al., 2018). 
Mercury (Hg) is another highly toxic heavy metal found 
in the biosphere. Human activities have increased its 
presence in the atmosphere, where it can transform into 
highly toxic methylmercury upon contact with aquatic 
sediments (Gworek et al., 2020). Methylmercury can 

Figure 1: Interconnections of  Soil, Plants, and Humans
Source: Author’s Creation

In a study by Omokaro et al. (2023), the concentration 
of  copper in Okra (Abelmoschus esculentus) cultivated 
on dumpsite soils in Benin City, Nigeria was investigated. 
The highest uptake of  copper (36.0 mg/kg) was observed 
in residential land, while the lowest uptake (23.3 mg/kg) 
was recorded in farmland. However, the World Health 
Organization (WHO, 1996) sets a permissible limit of  
10 mg/kg for copper in vegetables, suggesting that the 
copper content in Okra grown in dumpsite soils may 



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enter the human body through the consumption of  
contaminated fi sh, seafood, and wildlife, leading to various 
neurological problems (Rice et al., 2014). More so, Lead 
(Pb) is a non-biodegradable metal naturally present in low 
amounts, but its levels are continuously increasing due to 
human activities like manufacturing, mining, and fossil 
fuel burning. Lead exposure is particularly dangerous to 
children, as they are more susceptible to lead poisoning, 
especially from environmental dust laden with lead (Loh 
et al., 2016).

Manganese, Chromium, and Cobalt 
Manganese (Mn) is abundant in nature and can 
be released into the air during the combustion of  
methylcyclopentadienyl manganese tricarbonyl (MMT), a 
gasoline additive. While essential for various physiological 
activities, excessive manganese can be toxic (Loranger and 
Zayed, 1995; O’Neal and Zheng, 2015). Chromium (Cr) 
exists in two stable oxidation states in the environment: 
chromium (III) and chromium (VI). Chromium (III) is 
less hazardous than chromium (VI), which can convert to 
the less toxic form during industrial processes. However, 
chromium (VI) remains highly toxic and carcinogenic, 
posing signifi cant risks in areas with high industrial 
emissions (Coetzee et al., 2020; Kimbrough et al., 1999). 
Cobalt (Co) is widely distributed in the environment 
and used in alloy production. While small amounts of  
cobalt are typically harmless, large discharges into the 
environment can be fatal (Domingo, 1989).

Nickel, Copper, and Zinc
Nickel (Ni) is naturally abundant and extensively used in 
industry. It is released from both natural and anthropogenic 
sources into the atmosphere and can cause allergies, nasal 
and lung cancer, and kidney and cardiovascular diseases 
due to inhalation of  contaminated air (Genchi et al., 2020; 
Lu et al., 2005). Copper (Cu) is an essential micronutrient 
for living organisms, playing a crucial role in the normal 
physiological functions of  plants, such as chlorophyll 
formation, photosynthesis, and carbohydrate and protein 
metabolism. A lack of  copper can disrupt these important 
metabolic processes, while excessive exposure can lead to 

toxicity (Schwartz et al., 2003). Zinc (Zn) is a fundamental 
and widespread metal, involved in numerous enzymatic 
reactions by acting as a cofactor. The toxicity of  zinc 
depends on the manner and quantity of  exposure. Major 
sources of  zinc in the environment include smelting and 
mining. Signifi cant amounts of  zinc are released into the 
environment through mineral processing activities, which 
can impact ecosystems and living organisms (Zhang et al., 
2012).

Antimony and Thallium
Antimony is a toxic element present in nanogram 
amounts in the air. Its emissions into the atmosphere can 
result from natural events such as volcanic activity and 
weathering, as well as human activities (He et al., 2019). 
Thallium is found in the environment in various forms 
and is highly toxic to biological organisms. Its toxicity 
surpasses that of  other heavy metals. Monovalent thallium 
ions appear in natural water and can be released into 
the air through aqueous routes (Peter & Viraraghavan, 
2005). Additionally, industrial emissions signifi cantly 
contribute to increased thallium levels in the atmosphere. 
Exposure to thallium poses severe health risks to humans 
(Kazantzis, 2000).

Sources of  Heavy Metals into the Soil
Heavy metals are released into the environment from 
various sources including mining, urbanization, chemical 
industry, sewage plants, pesticide plants, biomedical and 
unsafe agricultural practices as described in Figure 2 below. 
The United Nations Environment Program (UNEP/
GPA) and the Global Plan of  Action (GPA) recognize 
electronic waste (e-waste) which includes devices like 
mobile phones, tablets, computers, and smartwatches 
as a major threat to the environment and human well-
being. This is primarily due to the presence of  heavy metals 
like Hg, Cd, and Pb in electronic devices, which can pose 
serious risks to both the environment and human health if  
not properly disposed of  according to UNEP/GPA (2006) 
and Tchounwou et al. (2012). More so, major unrestricted 
and monitored dumpsites are also major source of  heavy 
metal into soils and underground water.

Figure 2: A schematic diagram illustrating the origins of  heavy metal pollution
Source: Das et al. (2023)



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The pollution levels of  these heavy metals into the 
environment are infl uenced by industrial activities, 
geographic locations, regulatory oversight, and diverse 
sources. For instance, Hg primarily emanates from coal 
combustion, electric/light bulb, wood preservatives, 
leather tanning, ointments, thermometers, adhesives and 
paints (Bradl, 2005; Mukherjee et al., 2008), in most areas 
in Africa there are industrial activities with no regulatory 
activities or policies governing their activities in most 
areas. Cd often originates from industries like battery 
manufacturing, paint pigments, pesticides, galvanized 
pipes, plastics, polyvinyl and copper refi neries. Pb, 
an extremely toxic metal, commonly originates from 
substances like Pb-based paints, gasoline and mobile 
batteries. Cr is emitted from a variety of  industrial 
activities, including petroleum refi ning, electroplating, 
leather tanning, textile manufacturing, and pulp 
processing. As, a naturally occurring element in the 
Earth’s crust, is released into the environment through a 
variety of  human activities, including mining, agricultural 
practices, automobile exhaust and industrial dust, wood 
preservatives, and dyes (Choppala 2013; Velusamy et al., 
2021; Rees and Fuller, 2020; Jiang et al., 2023). Human 
activities through industrial, agricultural, traffi c, domestic, 
mining and other human activities processes have 
contributed to increase the toxic levels of  these metals 
when compared to those contributed from rock forming 
processes (Pam et al., 2013).

Sources of  Heavy Metals in Soil
Soil is essential for supporting terrestrial ecosystems 
and biodiversity. However, heavy metals are prevalent 
pollutants in the soil environment and can negatively 
impact microorganisms, plants, and animals. The 
European Environment Agency (EEA) has set limit 
values for soil pollutant levels of  various heavy metals, 
including Hg (0.20 ppm), Cd (0.44 ppm), Pb (0.48 ppm), 
Cr (0.20 ppm), and As (0.11 ppm) (European Union, 
2002; Baritz et al., 2023). Heavy metals can accumulate 
in soils to toxic levels due to the long-term application 
of  untreated liquid and solid waste, industrial activities, 
and fertilizers (Papafi lippaki et al., 2008). When soils are 
irrigated with these wastes, heavy metals can accumulate 
on the surface. As the soil’s capacity to retain heavy 
metals diminishes with repeated waste application, these 
metals can leach into groundwater or remain available 
for plant uptake. Wastewaters often contain signifi cant 
concentrations of  organic and inorganic nutrients, 
including nitrogen, phosphate, micronutrients, and heavy 
metals (Alshammary & Qian, 2008; Mojiri & Amirossadat, 
2011). Urban and industrial effl uents are major sources 
of  heavy metals in wastewater, which can build up to 
toxic levels in soils with long-term use (Tabari et al., 2008). 
Given that wastewater is an uncommon water source, its 
application in agriculture needs careful management to 
maximize benefi ts and prevent environmental and health 
risks (Asgari et al., 2007).

Contamination of  soils by heavy metals like Cd, Ni, 
Zn, Pb, and Cu has increased dramatically over the past 
few decades due to activities such as mining, smelting, 
manufacturing, the use of  agricultural fertilizers and 
pesticides, municipal waste, traffi c emissions, and 
industrial effl uents (Morgan, 2013; Chibuike and Obiora, 
2014). This contamination is now widespread (Al-Nagger 
et al., 2013), causing signifi cant land degradation and 
adversely affecting the environment and ecosystems 
worldwide (Li et al., 2013). Heavy metals in wastewater-
irrigated soils can contaminate food, posing hazards to 
humans and animals (Jolly et al., 2013). These metals are 
poorly soluble in water and cannot be degraded, leading 
to accumulation in soils and plants (Ghoneim et al., 
2014). They persist in soil, leaching into groundwater 
and potentially inducing enhanced antioxidant enzymatic 
activities in plants or becoming adsorbed onto solid soil 
particles (Iannelli et al., 2002).

Mechanism of  Heavy Metals in Soil
Heavy metal pollution in soil not only affects plant quality 
and yield but also changes the size, composition, and 
activity of  microbial communities (Yao et al., 2003). Heavy 
metals are a major source of  soil pollution, adversely 
affecting soil biological and biochemical properties. Soil 
characteristics like organic matter, clay content, and pH 
signifi cantly infl uence the impact of  metals on these 
properties (Speira et al., 1999). Heavy metals indirectly 
affect soil enzymatic activities by altering microbial 
communities that synthesize enzymes (Shun-hong et al., 
2009). They exhibit toxic effects on soil biota, affecting key 
microbial processes and reducing the number and activity 
of  soil microorganisms. However, long-term exposure 
can increase the tolerance of  bacterial communities and 
fungi like arbuscular mycorrhizal (AM) fungi, which play 
a crucial role in restoring contaminated ecosystems (Mora 
et al., 2005). 
Heavy metals cause a decrease in bacterial species richness 
and an increase in soil actinomycetes or a decrease in both 
the biomass and diversity of  bacterial communities (Chen 
et al., 2010). Different metals infl uence enzyme activities in 
various ways due to their different chemical affi nities in the 
soil system. For example, Cd is more toxic to enzymes than 
Pb because of  its greater mobility and lower affi nity for soil 
colloids, and Cu inhibits β-glucosidase activity more than 
cellulose activity (Karaca et al., 2010). Soil pH signifi cantly 
affects the solubility or retention of  metals in soils, with 
heavy metals exhibiting toxic effects on soil biota by 
affecting key microbial processes and reducing the number 
and activity of  soil microorganisms (Ghosh and Singh, 
2005). Heavy metals have low environmental mobility, 
making a single contamination event potentially lead to 
long-term exposure to humans, microbes, fauna, fl ora, and 
other soil communities (Adeleken and Abegunde, 2011). 
Farming on dumpsites can negatively impact human health 
due to high concentrations of  trace metals in plants, which 
subsequently enter the food chain (Elaigwu et al., 2007).



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Heavy Metals and Plants
The absorption of  heavy metals by plant roots is a 
primary route for these contaminants to enter the food 
chain (Jordoa et al., 2006). The uptake and accumulation 
of  heavy metals by plants can pose signifi cant threats to 
animal and human health (Sprynskyy et al., 2007). Some 
heavy metals, such as arsenic (As), cadmium (Cd), mercury 
(Hg), lead (Pb), and selenium (Se), are not essential for 
plant growth and do not serve any known physiological 
functions. Others, like cobalt (Co), copper (Cu), iron 
(Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), 
and zinc (Zn), are essential elements required for normal 
plant growth and metabolism. However, these elements 
can become toxic when their concentrations exceed 
optimal levels (Rascio and Izzo, 2011). Heavy metals can 
cause chlorosis, weak plant growth, yield depression, and 
reduced nutrient uptake, leading to metabolic disorders 
and decreased nitrogen fi xation in leguminous plants 
(Guala et al., 2010). The accumulation of  heavy metals 
in plants depends on various soil factors, including pH, 
electrical conductivity (EC), clay content, organic matter 
content, and physical and mechanical soil characteristics. 
Plants absorb heavy metals through mechanisms such 
as absorption, ionic exchange, and redox reactions 
(Zabalawy et al., 2015). The effi ciency of  different plants 
in absorbing metals is evaluated by plant uptake or soil-
to-plant transfer factors (Khan et al., 2008). Elevated 
levels of  lead (Pb) in soils can reduce soil productivity 
and inhibit vital plant processes, such as photosynthesis, 
mitosis, and water absorption, leading to symptoms like 
dark green leaves, wilting of  older leaves, stunted foliage, 
and short brown roots (Bhattacharya et al., 2008). The 
presence of  increasing concentrations of  lead (Pb) can 
delay seed germination, possibly due to mechanisms like 
leaching, chelation, metal binding, or accumulation by 
microorganisms that neutralize the toxic effects of  lead 
(Ashraf  and Ali, 2007).

Heavy Metals and Human Health
High concentrations of  heavy metals in plants can pose 
signifi cant health risks when these plants are consumed, 
considering the implications for the food chain. 
According to the World Health Organization (WHO) 
guidelines, acceptable levels of  heavy metal pollutants 
in drinking water are: Hg—0.001 ppm, Cd—0.005 ppm, 
Pb—0.05 ppm, Cr—0.05 ppm, and As—0.05 ppm 
(WHO, 2004). The Food and Agriculture Organization 
(FAO) of  the United Nations and WHO set maximum 
limits for heavy metal consumption through vegetables: 
Hg—0.05 mg/kg for all vegetables; Cd—0.2 mg/kg 
for leafy vegetables, 0.3 mg/kg for root vegetables, and 
0.1 mg/kg for other vegetables; Pb—0.15 mg/kg for 
all vegetables; Cr—0.1 mg/kg for all vegetables; and 
As—0.1 mg/kg for all vegetables (Wu, 2014; Kesson et 
al., 2015). The consumption of  food crops contaminated 
with heavy metals represents a signifi cant risk, as these 
metals can accumulate in plant tissues and subsequently 
enter the food chain (Jordao et al., 2006). Heavy metals 

become toxic when they are not metabolized by the 
body and accumulate in soft tissues (Sobha et al., 2007). 
Chronic ingestion of  toxic metals can have severe health 
impacts, often becoming apparent only after many years 
of  exposure (Khan et al., 2008). Specifi c heavy metals are 
hazardous to human health. For instance, cadmium (Cd) 
is a well-known toxicant with a specifi c gravity 8.65 times 
greater than water. Target organs for Cd toxicity include 
the liver, placenta, kidneys, lungs, brain, and bones (Sobha 
et al., 2007). Cd is emitted into the air by mines, metal 
smelters, and industries using cadmium compounds for 
alloys, batteries, pigments, and plastics, despite stringent 
controls in many countries (Harrison, 2001). Symptoms 
of  Cd exposure include nausea, vomiting, abdominal 
cramps, dyspnea, and muscular weakness, with severe 
exposure leading to pulmonary edema and death. Chronic 
inhalation of  Cd can cause pulmonary and renal effects 
(Duruibe et al., 2007).
Zinc (Zn) is relatively non-toxic, especially when taken 
orally, but excessive amounts can cause system dysfunctions 
affecting growth and reproduction. Clinical signs of  zinc 
toxicity include vomiting, diarrhea, bloody urine, icterus 
(yellow mucus membrane), liver failure, kidney failure, and 
anemia (Lalor, 2008). Copper (Cu), an essential element 
in mammalian nutrition, can be toxic at high levels, 
causing mucosal irritation, capillary damage, hepatic and 
renal damage, and central nervous system irritation (Stern 
et al., 2007). Nickel (Ni) exposure can cause skin irritation 
and damage to the lungs, nervous system, and mucous 
membranes (Argun et al., 2007). On the other hand, 
Lead (Pb) is physiologically and neurologically toxic to 
humans, with acute poisoning causing dysfunction in the 
kidneys, reproductive system, liver, and brain, potentially 
leading to sickness and death (Odum, 2000). Pb exposure 
occurs primarily through contaminated food, air, and 
drinking water (Ming-Ho, 2005). Mercury (Hg), which 
has no known function in human biochemistry, is highly 
toxic and can cause spontaneous abortion, congenital 
malformations, gastrointestinal disorders, and severe 
neurological disorders (Duruibe et al., 2007). Hg exposure 
can occur through agriculture, pharmaceuticals, industrial 
processes, and contaminated air, water, and soil (Zhang 
and Wong, 2007). High levels of  Hg exposure can cause 
permanent damage to the brain, kidneys, and developing 
fetus (ATSDR, 2003). Heavy metals like Hg and Cd can 
induce DNA damage, cause chromosome aberrations, 
and alter DNA replication and transcription, leading to 
serious health issues such as Alzheimer’s disease, lung 
damage, and skin ailments (Shekhawat & Meshram, 2020; 
Harischandra et al., 2021; Coetzee et al., 2020; Kothapalli, 
2021; Das et al., 2023).

Heavy Metal Concentration in Vegetables
Certain edible crops can accumulate heavy metals, even in 
minimal amounts. These heavy metals can enter the food 
chain, disrupt the food pyramid, and pose signifi cant 
health risks, including cancer and liver diseases. Vegetables 
such as brinjal, gourd, spinach, coriander, tomato, and 



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pumpkin are particularly susceptible to heavy metal uptake 
by their roots, which can then be transported to the edible 
parts of  the plant (Gupta et al., 2022; Minhas et al., 2022). 
Consequently, consuming vegetables containing heavy 
metals can be extremely hazardous to human health. 
Research by Alexander et al. (2006) involving vegetables 
grown in soil contaminated with heavy metals showed 
signifi cant variations in metal accumulation levels among 
different vegetables. For cadmium (Cd), lettuce exhibited 
higher accumulation (8.6 mg/kg dry matter) compared 
to spinach (5.8 mg/kg dry matter), onion (3.6 mg/kg dry 
matter), carrot (2.0 mg/kg dry matter), pea (0.29 mg/kg 
dry matter), and French bean (0.07 mg/kg dry matter). 
Lettuce also recorded the highest concentration of  lead 
(Pb), nearly double that of  onions, which had the second-
highest average value. The sequence was as follows: 
lettuce (14.6 mg/kg dry matter) > onion (7.5 mg/kg dry 
matter) > carrot (5.8 mg/kg dry matter) > spinach (1.8 
mg/kg dry matter) > pea (0.78 mg/kg dry matter) > 
French bean (0.34 mg/kg dry matter).
A study by Zhu et al. (2021) revealed that the concentration 
of  heavy metals in the edible parts of  vegetables varied, 
with leafy vegetables having the highest amounts, 
followed by stalk vegetables, root vegetables, solanaceous 
vegetables, legume vegetables, and melon vegetables. 
Previous reports have suggested that edible crops grown 
in industrial areas such as coal mines and petrochemical 
plants tend to contain higher levels of  heavy metals 
(Haque et al., 2021). Human exposure to heavy metals 
primarily occurs through the consumption of  edible 
crops, accounting for 90% of  the exposure, with the 
remaining 10% attributed to the inhalation of  polluted 
air particles (Khan et al., 2015). Excessive levels of  
heavy metals can harm various organs such as the brain, 
muscles, nerves, liver, kidneys, and heart. The European 
Protection Agency (EPA) has reported that prolonged 
exposure to heavy metals can result in severe cancer. 
Research by the WHO indicates that higher exposure to 

heavy metals puts 10% of  women at risk of  infertility 
(Abudawood et al., 2021; Bhardwaj et al., 2021).

Managing Heavy Metal Contamination in the 
Environment
Several techniques are used to decontaminate the 
environment from heavy metals and prevent their entry 
into the food chain, but these methods tend to be costly 
and have suboptimal effi cacy (Li et al., 2019; Qasem et 
al., 2021). For example, Omokaro (2024), who worked 
on Farmers’ Perceptions of  Pest and Disease Control 
Methods in South-South Nigeria suggested that Integrated 
Pest Management is an environmentally friendly approach 
that combines biological, cultural, and chemical methods 
to manage pests. The author further noted that the 
signifi cant use of  chemical pesticides indicates a reliance 
on chemical control, which can have environmental 
and health implications if  not managed properly. The 
increasing concerns about environmental contamination 
have driven the development of  technologies to assess 
the presence and mobility of  metals in soil, water, and 
wastewater. Both private and government institutions 
face technical challenges in removing contaminants 
from the environment. Phytoremediation has emerged 
as a popular and economical plant-based technology for 
effectively addressing environmental issues. This process 
uses plants to extract and remove elemental pollutants 
or reduce their bioavailability in soil or water (Li and 
Tran, 2023). Phytoremediation is widely accepted due to 
its eco-friendliness, affordability, and high effectiveness. 
It takes advantage of  the unique and selective uptake 
capabilities of  plant root systems, along with the 
translocation, bioaccumulation, and contaminant 
degradation abilities of  the entire plant body. Both 
aquatic and terrestrial plant species have been utilized 
to eliminate pollutants from the environment (Sharma, 
2021; Nedjimi, 2021; Huang et al., 2021).

Figure 2: An illustrative diagram elucidating bioremediation, highlighting the crucial roles of  plants, bacteria, and fungi
Source: Das et al. (2023)

Phytoremediation Process
Phytoremediation encompasses several processes, including 
phytoextraction, phytoaccumulation, phytovolatilization, 

phytostabilization, and phytotransformation (Liu and 
Tran, 2021). Phytoextraction is a technique that involves 
the absorption of  organic and inorganic pollutants 



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through the roots and stems. Besides, some particular 
plant species, like Brassica juncea, Cassia alata, Celosia 
argentea, Kummerowia striata, Helianthus annuus, 
Momordica charantia, Nicotiana tabacum, Salix 
mucronata, Salix viminalis, Solanum lycopersicum, 
Solanum melongena, Swietenia macrophylla, Pteris vittata, 
and Vigna unguiculata, have the potential to be used as 
suitable plant selections to enhance the phytoextraction 
process (Hazarika et al., 2022; Kumar et al., 2022). In 
phytostabilization, in this process, plants accumulate and 
immobilize heavy metals by binding with biomolecules. 
Miscanthus giganteus, Avena sativa, and Sinapis alba 
can also help to stabilize heavy metal compounds in 
the soil (Muthusamy et al., 2022; Hegedus et al., 2023). 
There are several processes by which plants can reduce 
contaminants which are further explained below.

Phytoextraction and Rhizofi ltration
Phytoextraction, also called phytoaccumulation, involves 
the accumulation of  heavy metals from earth land. In this 
method, the uptake and translocation of  contaminants 
by plants root into the aerial portions of  plants and 
deposited into vacuoles. The mechanism during the 
accumulation process is used to absorb and precipitate 
the toxic metals by metal-phytochelatin complex before 
translocating into the shoot, leaf  and stem parts of  
the plant. The hyperaccumulator species accumulate a 
higher concentration of  heavy metals (Asgari et al., 2019; 
Gul et al., 2021). More so, Rhizofi ltration involves the 
elimination of  heavy metals using plant roots. Though it 
is comparable to phytoextraction, in this process, plants 
remove contaminants from wastewater or groundwater 
rather than soil. In this process, plant roots assimilate 
or adsorb pollutants from wastewater, groundwater, 
or surface water. Generally, aquatic plant species are 
employed to eliminate pollutants through rhizofi ltration. 
Rhizofi ltration is effective for removing Cd, Pb, and Cr, 
which are primarily accumulated in the roots. Sunfl ower, 
tobacco, and spinach exhibit promising potential in 
removing Pb from water (Mohan et al., 2021).

Phytostabilization and Phytovolatilization 
Plant roots can limit the movement of  heavy metals by 
phytostabilization, a process that reduces toxic effects. This 
process involves the capture of  contaminants on the root 
surface using transport proteins or secondary metabolites. 
Furthermore, the process involves the breakdown of  
complex organic molecules into simpler ones by coupling 
them with protein, amino acid, and sugar derivatives. Black 
nightshade, sunfl ower, and cowpea are among the plant 
species that employ phytostabilization mechanisms (Li et 
al., 2020). However, the Phytovolatilization process entails 
the uptake of  contaminants by plants from the soil and 
their conversion into less toxic volatile compounds that are 
released into the atmosphere. The volatile compounds are 
primarily released from aerial plant parts such as stems and 
leaves. This mechanism is effective when the contaminants 
are less toxic (Pouresmaieli et al., 2022).

Phytodegradation (Phyto-transformation)
Phytotransformation, also known as this process, refers to 
the absorption of  contaminants by plants, which are then 
metabolized or broken down into less toxic compounds 
and translocated to various plant organs. The organic 
compounds are then degraded into non-toxic forms 
inside the plant tissue (Nebeská et al., 2021).

Microbial Remediation Process
Microbial remediation is the process of  using living 
microorganisms such as bacteria, fungi, and archaea to 
break down and detoxify various chemical and metallic 
hazardous wastes from the environment (Yaashikaa et al., 
2022). Bioremediation involves the direct application of  
microorganisms to the polluted site in order to facilitate 
the degradation of  contaminants. Microorganisms are 
used in a variety of  remediation techniques, including 
bioaugmentation and biostimulation. In bioaugmentation, 
specifi c microorganisms are added to a contaminated 
site to enhance the breakdown of  contaminants. In 
biostimulation, the environmental conditions at the site 
are modifi ed to promote the growth and activity of  
naturally occurring microorganisms that can degrade 
contaminants. Physical and chemical treatments are 
conventional remediation methods that have drawbacks 
such as high cost, heavy machinery, logistical glitches, 
and potential environmental toxicity (Iwamoto & 
Nasu, 2001). In contrast, bioremediation technologies 
have seen signifi cant growth and development, making 
it a promising method for treating soil and water 
contamination. Among these methods, bioremediation 
of  oil spills is the most lucrative and environment-
friendly technique (Pete et al., 2021). Ewubare et al. (2023) 
posited that a reliable alternative to the use of  chemical 
fertilizers which also increase heavy metals in agricultural 
produce is microbial inoculants, which can act as 
biofertilizers, bioherbicides, biopesticides, and biocontrol 
agents. Microorganisms can promote plant growth, 
and controlling pests, diseases, and weeds. Microbial 
inoculants are benefi cial microorganisms applied either 
to the soil or the plant to improve productivity and crop 
health. These natural-based products are widely used to 
control pests and enhance soil and crop quality, thereby 
benefi ting human health. Microbial inoculants consist of  
a blend of  microorganisms that work with the soil and its 
inhabitants to improve soil fertility and health, ultimately 
benefi ting human health. They can minimize the negative 
impact of  chemical inputs, thereby increasing the quantity 
and quality of  farm produce.
Furthermore, fungi are used for the remediation of  
pollutants in myco-remediation, a type of  bioremediation. 
Fungi play a vital role in cleaning up contaminated sites 
in both soil and aquatic ecosystems (Kumar & Dwivedi, 
2021). These microorganisms, which are widely present 
in nature, can thrive in a diverse range of  environmental 
conditions. These microorganisms survive in extreme 
conditions and produce some extracellular ligninolytic 
enzymes like peroxidase and laccases. These enzymes 



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Am. J. Environ Econ. 3(1) 70-81, 2024

help fungi to transform pollutants into non-toxic forms. 
Pollutants can be adsorbed by extracellular enzymes (Shourie 
& Vijayalakshmi, 2022). Diverse fungal species such as 
Aspergillus sp., Bjerkandera adusta, Coriolus versicolor, 
Cryptococcus sp. Hirschioporus laricinus, Inonotus 
hispidus, Mucor sp., Penicillium sp., Phanerochaete 
chrysosporium, Phlebia tremellosa, Phanerochaete 
chrysosporium, Pleurotus sp., and Trametes versicolor, 
have been reported for bioremediation (Jebapriya & 
Gnanadoss, 2013; Singh et al., 2013). Microalgae are 
photosynthetic microorganisms that play a vital role in 
the bioremediation of  various types of  wastewaters. 
According to Omokaro and Nafula (2023), microalgae 
are effective in the removal of  nitrogen (N), phosphorus 
(P), and carbon (C), as well as in reducing biological 
oxygen demand (BOD) and removing heavy metals. 
These microorganisms have shown promise in producing 
biofuels, bioethanol, bioremediation agents, biofertilizers 
for agricultural production, and essential molecules like 
proteins. The integration of  microalgae into wastewater 
treatment processes can signifi cantly reduce the cost of  
wastewater treatment, lower the energy consumption 
footprint, and provide environmental sustainability 
compared to conventional wastewater treatment processes. 
This integration enhances the overall effi ciency of  
wastewater treatment by leveraging the natural capabilities 
of  microalgae to absorb and metabolize pollutants.

CONCLUSION
Metals occur naturally in the Earth’s crust, and their 
concentrations in the environment can vary between 
different regions, resulting in spatial variations of  
background levels. Heavy metal accumulation in soil 
causes pollution, while in plants, it can lead to slow growth 
and even death. In humans, it can result in severe health 
hazards. Several toxicity evaluations have indicated that 
heavy metals pose a substantial threat to non-targeted 
living entities. These metals are taken up by plants 
from the soil, reducing crop productivity by inhibiting 
physiological metabolism. The uptake of  heavy metals 
by plants and their subsequent accumulation in human 
tissues, along with biomagnifi cation through the food 
chain, raise signifi cant concerns for both human health 
and the environment. Heavy metals have been proven 
toxic to both human and environmental health. Studies 
have revealed that phytoremediation and microbial 
remediation are potential techniques for mitigating the 
negative effects of  environmental contamination. These 
methods are environmentally friendly and economically 
effective, making them applicable to both emerging 
and developed nations globally. Due to the toxicity of  
heavy metals and their potential for bioaccumulation, 
mandatory monitoring of  these compounds, particularly 
in soil and plants, is essential to prevent their entry into 
the human food system. Therefore, funding ongoing 
research and innovation in remediation technologies is 
crucial for addressing the expanding environmental issues 
of  the 21st century.

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