Review
A Review on Remediation Technology and the Remediation
Evaluation of Heavy Metal-Contaminated Soils
Lei Xu 1,2, *, Feifei Zhao 1 , Xiangyu Xing 3 , Jianbiao Peng 4 , Jiaming Wang 4 , Mingfei Ji 4 and B. Larry Li 5,6
1
2
3
4
5
6
*
Citation: Xu, L.; Zhao, F.; Xing, X.;
Peng, J.; Wang, J.; Ji, M.; Li, B.L. A
Review on Remediation Technology
and the Remediation Evaluation of
Henan Province Engineering Research Center of Environmental Laser Remote Sensing Technology and
Application, Nanyang Normal University, Nanyang 473001, China; terry__xp@163.com
Collaborative Innovation Center of Water Security for Water Source Region of Mid-Line of South-to-North
Diversion Project of Henan Province, Nanyang Normal University, Nanyang 473001, China
Non-Major Foreign Language Teaching Department, Nanyang Normal University, Nanyang 473001, China;
xxyxxy880902@163.com
College of Water Resources and Modern Agriculture, Nanyang Normal University, Nanyang 473001, China;
pjb126com@163.com (J.P.); wangjmecology@163.com (J.W.); jimfdy@gmail.com (M.J.)
Ecological Complexity and Modeling Laboratory, Department of Botany and Plant Sciences, University of
California–Riverside, Riverside, CA 92521, USA; bai-lian.li@ucr.edu
International Joint Laboratory of Watershed Ecological Security and Collaborative Innovation Center of Water
Security for Water Source Region of Middle, Nanyang Normal University, Nanyang 473001, China
Correspondence: hedaxl@163.com
Abstract: With the rapid development of industry and agriculture, soil contamination has become a
significant environmental issue, and the heavy metal contamination of soils is an important part of it.
The main methods for the remediation of heavy metal-contaminated soils include physical methods,
chemical methods, biological methods, and combined remediation methods have been proposed as
research deepens. However, the standards and evaluation methods for the remediation of heavy metalcontaminated soils are still not well-established. This article discusses the sources and contamination
status of heavy metals in soils, the advantages and disadvantages of remediation technology for
heavy metal-contaminated soils, remediation standards, and post-remediation evaluation methods.
It also proposes scientific issues to be addressed in future research and provides an outlook on future
development, hoping to assist in subsequent remediation studies of heavy metal-contaminated soils.
Keywords: soil; heavy metal contamination; remediation technology; remediation standard; remediation
evaluation
Heavy Metal-Contaminated Soils.
Toxics 2024, 12, 897. https://doi.org/
10.3390/toxics12120897
Academic Editor: Łukasz
Chrzanowski
Received: 22 November 2024
Revised: 2 December 2024
Accepted: 9 December 2024
Published: 10 December 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1. Introduction
As one of the natural and essential resources for human survival, the soil bears
the burden of providing food and other renewable resources for humanity. However,
with the development of industries, agricultural production, and the advancement of
modern technology, heavy metal contamination, especially soil heavy metal contamination,
has become one of the major issues impairing global environmental quality as well as
human survival and development [1]. The characteristics of soil contamination, such as its
concealment, long-term nature, and irreversibility, coupled with the difficulty in degrading
heavy metal elements by soil microorganisms, mean that, once the soil is contaminated by
heavy metals, restoring its original ecological functions will consume a significant amount
of manpower and resources [2,3]. Therefore, the remediation and improvement of heavy
metal-contaminated soils have become an important research area in soil science, ecology,
and environmental science. In recent years, the remediation of heavy metal-polluted soils
has become a hot topic of research [4,5]. However, the establishment of standards and
evaluation for the remediation of heavy metal-contaminated soils are influenced by a
variety of factors, such as the background values of the soil, the technological methods used
in the remediation process, and ecological toxicological assessments of the contamination.
Toxics 2024, 12, 897. https://doi.org/10.3390/toxics12120897
https://www.mdpi.com/journal/toxics
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This creates an awkward situation of how to clearly define the extent of soil remediation
that can be considered as clean and the level at which it will not pose a threat to subsequent
reuse. Therefore, conducting this work scientifically and orderly can further promote the
steady development of soil remediation work.
This study focuses on analyzing the sources and hazards of heavy metals in soils, the
remediation methods for heavy metal-contaminated soils, the standards for the remediation
of heavy metal-contaminated soils, and the evaluation after remediation. Through this
research, feasible plans for the future remediation of heavy metal-contaminated soils will be
provided, and a methodological basis for establishing a systematic study of contaminated
soil remediation benchmarks will be offered, promoting the establishment of an ecological
security system.
2. Sources and Hazards of Soil Heavy Metals
The sources of heavy metals in soils are extensive, and in addition to being related
to natural factors such as the parent material, soil formation processes, and volcanic
activities, human activities are the most important sources of soil heavy metal pollution [6,7].
Irrigation with wastewater is a feasible way to solve the water needs of crops in arid areas,
but improper irrigation measures and the introduction of a large amount of wastewater
exceeding heavy metal standards have caused serious pollution to agricultural soils [8,9].
Mining and smelting industrial processes generate a large amount of heavy metal elements,
and some enterprises directly discharge untreated waste into the environment, causing
nearby soils to be enriched with heavy metals [10,11]. Erin LeGalley used isotope tracing to
study street sediments in Ohio and found that the sediments contained a large amount of
heavy metals such as Cu, Cr, Co, and Pb, which mainly come from the yellow lines on the
roads and automobile exhaust [12]. The issue of soil heavy metal contamination caused by
the long-term application of organic and chemical fertilizers is also increasingly receiving
attention. Studies have found that organic fertilizers and phosphate fertilizers contain
higher levels of heavy metals, and the long-term application of phosphate fertilizers is a
major factor affecting the accumulation of heavy metals in soils [13–15]. Recent studies
have increasingly found that atmospheric deposition is becoming a significant source of
soil heavy metal contamination [16,17]. This is due to the fact that the heavy metals emitted
into the atmosphere from activities such as fossil fuel combustion, vehicle exhaust, and
non-ferrous mining and smelting can impact ecosystems and human health, both directly
and indirectly, through multi-pathway transportation [18]. The typical sources of heavy
metals in soils are listed in Table 1.
Soil heavy metal contamination can have a severe impact on ecosystem stability
and human health. Firstly, soil heavy metal contamination can hinder the growth and
development of crops, thereby reducing crop yield and quality [19]. For example, under
the stress of high concentrations of Pb and Cd, wheat height decreases and secondary
roots are reduced, ultimately leading to a decrease in wheat yield [20,21]. At the same
time, Hg, Cd, and Pb are the most serious heavy metal elements exceeding standards in
vegetables in suburban areas of our country [22,23]. Moreover, the accumulation of heavy
metals in soil may also lead to changes in soil properties, thereby changing the supply of
nutrient elements and fertility characteristics. For example, when there is an excessive
accumulation of heavy metals in soil, the adsorption of soil phosphorus, the mineralization
of organic nitrogen, and the change in the potassium form all hinder the retention and
supply of N, P, and K in the soil, thereby affecting crop yield and quality [24–26]. Heavy
metals exceeding a certain concentration have a significant impact on the activity and
quantity of soil microorganisms [27]. Long-term positioning experiments have shown
that, when the soil is contaminated by Zn, Cd, Cu, Ni, Pb, and Cr, the nitrogen-fixing
activity of cyanobacteria can be reduced by 50%, and their numbers are also significantly
reduced [28]. Studies have found that heavy metal stress affects soil enzyme activity [29],
and soil urease activity decreases with the increase in mercury pollution concentration [30].
The difference in soil urease activity among different pollution levels is highly significant.
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When the amount of mercury added to the soil reaches 12mg/kg, the soil urease activity is
only 34% compared to the control, indicating that soil urease is very sensitive to mercury
pollution [31]. In addition, crops in heavy metal-contaminated soils will absorb and utilize
heavy metals and partially enrich them in edible parts, entering the human body through
the digestive tract via the food chain, causing a series of diseases such as cancer and
hypertension [32]. The typical effects of heavy metals on ecosystems and human health are
listed in Table 1.
Table 1. The sources and hazards of typical heavy metals in soils.
Major Existence Forms a
Environmental Toxicity
Irrigation (60%), atmospheric
deposition (35%), livestock and
fertilizer (5%) [33]
Residual Hg (12.3%), organic-bound
Hg (86.2%), oxidized Hg (1.5%) [34]
Affects seed germination and plant
morphogenesis; causes sensory
abnormalities, ataxia, delayed intellectual
development, and language and hearing
impairments in humans [35]
Cd
Irrigation (67%), atmospheric
deposition (32%), livestock and
fertilizer (1%) [33]
Exchangeable Cd (39.2%),
carbonate-bound Cd (4.32%), Fe–Mn
oxide-bound Cd (21.9%), organic
matter-bound Cd (7.58%), residual Cd
(27.0%) [36]
Disrupts the protein structure of soil
microorganisms, causing them to die due
to protein denaturation, resulting in soil
compaction [37]; excessive intake of
copper may cause acute and chronic
poisoning in the human body [38]
Pb
Irrigation (13%), atmospheric
deposition (84%), livestock and
fertilizer (1%) [33]
Exchangeable Pb (33.4%),
carbonate-bound Pb (13.2%), Fe–Mn
oxide-bound Pb (13.8%), organic
matter-bound Pb (9.55%), residual Pb
(30.0%) [39]
Affects the physiological systems of the
human body, including the urinary
system, reproductive system,
gastrointestinal system, endocrine
system, and cardiovascular system [40]
As
Irrigation (71%), atmospheric
deposition (20%), livestock and
fertilizer (9%) [33]
Water-soluble As (5.25%),
surface-adsorbed As (14.4%),
iron–aluminum oxide-bound As
(15.2%), acid-extractable As (4.95%),
residual As (60.2%) [41]
Disrupts the structure of soil microbial
communities [37]; causes acute or chronic
poisoning, and can also induce skin
cancer, lung cancer, and bladder cancer,
etc. [42]
Zn
Irrigation (26%), atmospheric
deposition (72%), livestock and
fertilizer (2%) [33]
Exchangeable Zn (4.11%),
carbonate-bound Zn (1.15%), Fe–Mn
oxide-bound Zn (3.06%), organic
matter-bound Zn (1.06%), residual Zn
(90.6%) [43]
Excessive zinc can lead to disorders in
the oxidative metabolism of myocardial
cells, simple osteoporosis, and atrophy of
the brain tissue [44]
Cu
Irrigation (62%), atmospheric
deposition (35%), livestock and
fertilizer (3%) [33]
Exchangeable Cd (20.7%),
carbonate-bound Cd (10.6%), Fe–Mn
oxide-bound Cd (14.6%), organic
matter-bound Cd (18.8%), residual Cd
(30.2%) [36]
It inhibits chlorophyll synthesis [45];
reduces the number of bacteria, fungi,
and actinomycetes in the soil [46]; and
decreases the activity of urease and
nitrate reductase [47]
Cr
Irrigation (22%), atmospheric
deposition (72%), livestock and
fertilizer (6%) [33]
Exchangeable Cr (3.07%),
carbonate-bound Cr (7.43%), Fe–Mn
oxide-bound Cr (21.6%), organic
matter-bound Cr (11.6%), residual Cr
(56.3%) [36]
It affects the activity of soil urease and
catalase; it induce respiratory diseases
such as bronchial asthma; long-term
contact with the skin can cause erythema,
eczema, and other skin diseases; and it is
highly likely to cause cancer [48]
Types
Hg
Source
a There are various methods to evaluate the forms of metal elements present in soil. The method used in this
study was the one commonly employed by many researchers. Additionally, the speciation distribution of heavy
metal elements varies significantly across different soils. The data provided in this study are for reference only.
3. Remediation Technologies for Heavy Metal-Contaminated Soil
Based on current research, the remediation methods for heavy metal-contaminated
soils mainly include physical remediation, chemical remediation, biological remediation,
and combined remediation. Single physical and chemical remediation methods are costly
and may cause secondary pollution, making them difficult to implement in large-scale
soil remediation projects. As an emerging remediation technology, phytoremediation
has the advantages of low cost, good effect, minimal environmental disturbance, and no
destruction of landscape ecology, and has attracted much attention and application [49,50].
Combined remediation technology involves the combined application of two or three of
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physical, chemical, and biological remediation methods to achieve soil remediation and
ecological restoration, and has a broad prospect in future application and research [51,52].
The commonly used heavy metal soil remediation technologies and the advantages and
disadvantages of these technologies are listed in Table 2.
Table 2. Commonly used remediation technologies for heavy metal-contaminated soils.
Remediation Technology
Mechanism
Influence
Factor
Advantage
Disadvantage
References
Effectively isolating
contaminated soil
to reduce its impact
on the environment
Labor-intensive, with
high transportation
costs over long
distances, and only
suitable for the
remediation of severely
contaminated soils in
small areas
[53,54]
The volatility of
heavy metal
elements in
soils
It can effectively
reduce the
concentration of
volatile heavy
metals in the soil
It requires a significant
amount of energy,
making it unsuitable for
large-scale remediation.
Improper collection of
heavy metals can lead
to secondary pollution,
and it can disrupt soil
moisture and organic
matter
[55]
Solidification
Excessive power current
is used to gradually
heat the contaminated
soils to reach the
melting temperature
Temperature
and soil
conductivity
It is very effective in
removing large
amounts of heavy
metalcontaminated waste
and can be used for
the remediation of
large quantities of
soils. The vitrified
material can be
recycled and reused
as aggregates and
clean fillers
It cannot be used when
the soil has poor
conductivity. It can only
be applied to in situ
solidification in moist
soils with a low alkali
content. The cost is
prohibitive for
large-scale field
applications
[56]
Stabilization
Adding stabilizers to
alter the
physicochemical
properties of the soil, or
directly interacting with
heavy metals through
precipitation,
adsorption,
coordination,
complexation, and
redox reactions to
change the form of
heavy metals, thereby
reducing their
concentration, mobility,
and bioavailability
Soil particle
size and
porosity, heavy
metal
concentration
and speciation
The remediation
cost is moderate,
and the treatment
effect is good
It does not truly achieve
the removal of heavy
metals; once activated,
heavy metals can
pollute the soil again
[57,58]
Leaching
Using leaching
solutions containing
chemical reagents
(acids/bases,
surfactants, chelating
agents, salts, or redox
agents) to transfer
metals from the soil into
aqueous solutions
Heavy metal
type,
concentration,
speciation, and
soil property
It can completely
remove heavy
metals from the soil,
with a high removal
efficiency and easy
operation
Commonly used
leaching agents are
either difficult to
degrade or can destroy
the physicochemical
properties of the soil,
and degradable
leaching agents are
relatively expensive
[59]
Tilling and
soil
replacement
Thermal
treatment
Physical
remediation
Chemical
remediation
Replacing or partially
replacing contaminated
soils with
uncontaminated soils to
dilute the concentration
of pollutants in the soil
Removing volatile
heavy metals from soils
by heating
Quantity of soil
replacement
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Table 2. Cont.
Remediation Technology
Mechanism
Using chelating action,
cell wall precipitation,
compartmentalization,
and other detoxification
mechanisms of plants to
absorb and concentrate
heavy metals from the
Phytoremediation
soil into the
above-ground parts of
the plants, and then
harvesting the
above-ground parts to
achieve the removal of
heavy metals
Bioremediation
Fauna
remediation
By absorbing and
concentrating heavy
metals themselves,
thereby reducing the
content of heavy metals
in the soil; by improving
the activation capacity
of heavy metals in the
soil through their own
activities, promoting the
enrichment of heavy
metals by plants
Microbial
remediation
Utilizing the adsorption,
immobilization,
methylation, and redox
capabilities of
microorganisms
towards heavy metal
ions, or the biopolymers
produced by
microorganisms to
chelate or precipitate
heavy metal ions,
forming complexes and
reducing the toxicity of
heavy metals
Combined remediation technology
Combining different
remediation
technologies for the
remediation of heavy
metal-contaminated
soils
Influence Factor
Advantage
Disadvantage
References
Speciation of
heavy metals and
soil environment
It costs the least,
and the plants can
be used for
bioenergy
production after
phytoremediation,
with no secondary
pollution and of
easy operation
It is only effective
for recovering
certain metals (Cd,
Ni, Cu, and Zn);
metal capture
kinetics are very
slow; metal-laden
biomass must be
managed after plant
growth
[60,61]
Soil environment
and speciation of
heavy metals
It can improve the
physicochemical
properties of the
soil, promote plant
growth, enhance
the ecological
environment, and
does not produce
secondary pollution
Limited tolerance
range, highly
susceptible to
environmental
influences, strong
specificity, and for
large areas of
contaminated soil,
its remediation
capacity may be
limited
[62]
Soil environment
and temperature
Low cost, low
energy
consumption,
capable of
absorbing multiple
heavy metals,
reducing the
remediation time,
participating in the
regulation of
nutrient cycles in
the soil, and
increasing the
transformation rate
of heavy metals
The microbial
environment is
strict, and the
process is difficult
to control. Some
microorganisms can
immobilize heavy
metals, reducing the
absorption of heavy
metals by plants
[63,64]
It can improve the
efficiency of
remediation and
compensate for the
shortcomings of
single soil
remediation
technologies
The application
scope is limited,
and current
research mainly
focuses on
phytochemicalcombined
remediation and
phytomicrobecombined
remediation
technologies
[65,66]
Types and
speciation of soil
heavy metals
3.1. Physical Remediation
The physical remediation of heavy metal-contaminated soils refers to the use of single
physical methods for soil improvement, aiming to reduce and stabilize heavy metals in the
soil. Common methods include soil replacement, washing, solidification and landfilling,
thermal treatment, and electrokinetic remediation.
3.1.1. Tilling and Soil Replacement
Tilling involves mechanically turning over the soil deeply to disperse and transfer
heavy metals concentrated in the surface layer to deeper levels, thereby diluting them [67].
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Soil replacement involves adding a large amount of clean soil to the contaminated soil and
mixing it with the original soil to reduce pollutant concentrations below critical hazard
levels [68]. Tilling and soil replacement are effective for treating lightly contaminated
soils and can achieve the established remediation goals in the short term [53]. However,
they require a significant amount of labor and resources, with high investment costs, and
may reduce soil fertility and primary productivity, making them unsuitable for large-scale
promotion [69].
3.1.2. Thermal Treatment
High-temperature thermal treatment technology refers to the method of volatilizing
some volatile heavy metals such as Hg and As from the soil by heating. This method
can remove 99% of PAHs and volatile pollutants from the soil [70], and also has a certain
fixation effect on some non-volatile heavy metals and radioactive elements, reducing their
environmental risks [71,72]. However, high-temperature pyrolysis can destroy soil organic
matter and structural water, and it requires a large amount of energy consumption, leading
to limited practical application [73].
3.1.3. Solidification
Solidification refers to the addition of stabilizing agents to heavy metal-contaminated
soils to change its physical and chemical properties, reducing the mobility and bioavailability of heavy metals through adsorption and co-precipitation [74]. Stabilizing agents include
single agents such as lime, cement, and fly ash, as well as composite stabilizing agents,
which are mixed in various proportions to improve and enhance engineering technical
performance. Solidified soils not only reduce the mobility and bioavailability of heavy metals but can also be used in building materials and roadbed construction, killing two birds
with one stone [75]. However, solidification technology has inherent shortcomings, such as
soil destruction, significant disturbance to the ecosystem, and the need for a large amount
of stabilizing agents, leading to high costs. Therefore, it is only suitable for small-area
pollution control, and further research is needed for large-area application.
3.2. Chemical Remediation
Chemical remediation is an in situ remediation technology that involves adding
amendments or inhibitors to the soil to cause a series of chemical reactions in heavy metals,
reducing their solubility, mobility, and bioavailability in the soil, thereby achieving the
purpose of governance and remediation [76,77]. Currently, the application of amendments
to the soil is more common, and the ideal amendment should have the characteristics of
being pollution-free, low-cost, effective, and durable. Commonly used amendments such
as lime, limestone powder, apatite, hydroxyapatite, and calcium–magnesium phosphate
fertilizer have certain remediation effects on the soil. Surface-active agents and heavy metal
chelating agents can also be added to achieve improvements.
Lime and carbonate minerals are among the most commonly used heavy metal immobilization materials. Studies have shown that adding 0.2% lime to the soil can reduce
the available forms of Cu and Cd by 97% and 86%, respectively [78]. The main alkaline
materials for remediating heavy metals include calcium carbonate (CaCO3 ) and dolomite
[CaMg (CO3 )2 ]. After adding lime or calcium magnesium phosphate to contaminated soils,
the exchangeable form of Cd can be reduced significantly [79]. Phosphorus-containing
materials are one of the most effective heavy metal immobilization agents, and the main
phosphorus-containing immobilization materials used in soil heavy metal immobilization include apatite, calcium phosphate, superphosphate, and phosphorus-containing
sludge [80–82]. Silicon-containing materials have certain remediation effects on heavy
metals such as aluminum, iron, zinc, cadmium, and manganese [83]. Commonly used
silicon-containing materials include silicate fertilizers, calcium silicate, silicon-containing
sludge, fly ash, and silicate clay minerals [84]. Biochar is a carbon-rich material that has
been utilized to remediate heavy metal-contaminated farmland. It has been increasingly
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used in the remediation of heavy metal-contaminated soils due to its multiple benefits,
including reducing soil heavy metal contamination, improving soil fertility, and promoting
soil carbon sequestration [85,86]. Its application in soil remediation has shown promising
results in laboratory and field trials, and it is considered a cost-effective and environmentally friendly soil amendment. Meanwhile, utilizing the antagonistic effects of heavy metals
in the soil environment can reduce their toxicity. For example, increasing the supply level
of Ca can reduce the plant absorption of heavy metals, such as Cd, Ni, Pb, Zn, and Cr, or
alleviate the toxicity of heavy metals [87]. Therefore, adding calcium-containing substances
to the soil can also reduce the bioavailability of some heavy metals.
Chemical remediation is simple and easy to perform, and industrial by-products can
be used as amendments during the remediation process, making the cost relatively low
and suitable for large-area contaminated soil remediation [88]. However, single chemical
remediation cannot completely remove heavy metals from the soil, and they may be
released back into the soil when environmental conditions change [89]. It may also have
certain impacts on soil microorganisms, posing potential safety hazards. Therefore, when
applying chemical remediation, materials with stable effects and minimal impact on the
soil ecosystem should be considered as amendments.
3.3. Bioremediation
Bioremediation is divided into broad and narrow senses. The bioremediation we usually refer to is the broad sense of bioremediation, which means using organisms as the main
technology for the remediation of heavy metal contamination. Through their metabolic
activities, it reduces the content of heavy metals in the soil or reduces their toxicity by
changing their form, thus achieving the purpose of remediation. Common bioremediation
technologies include phytoremediation, fauna remediation, and microbioremediation.
3.3.1. Phytoremediation Technology
Phytoremediation technology refers to the use of plants and their rhizosphere microorganisms to extract, absorb, volatilize, transform, degrade, and fix pollutants in the
soil, thereby removing pollutants from the soil [90]. The mechanisms of phytoremediation
include phytoextraction, rhizodegradation, phytodegradation, phytostabilization, and phytovolatilization [91]. Among them, phytoextraction utilizes the absorption of heavy metals
by certain hyperaccumulator plants, and by properly handling the harvested aboveground
parts, it removes heavy metals from the soil, achieving the goal of reducing pollution
and eliminating toxicity [92]. This technology is widely used in current research on the
phytoremediation of heavy metal-contaminated soils.
As a technology for the remediation of heavy metal contamination, phytoremediation
depends on several aspects: the degree of soil pollution, the effectiveness of rhizosphere
microorganisms, and the ability of applied plants to intercept, absorb, concentrate, and
degrade heavy metals [93]. Therefore, choosing the right plants is crucial in pollution
remediation. Existing studies have shown that plants of the genus Alyssum have the ability
to hyperaccumulate Ni [94]; plants of the genus Thlaspi have the ability to hyperaccumulate
Cd [95]; Indian mustard (Brassica juncea) and corn show a hyperaccumulation of Pb under
EDTA enhancement, with the Pb content in the aboveground parts of Indian mustard
exceeding 15,000 mg/kg [96]; the highest concentration of As in the fronds of Pteris vittata
L of the Pteridaceae family can reach 22,630 mg/kg, and the bioconcentration factor (BF)
exceeds 10 [97]; and Sedum plumbizincicola is a hyperaccumulator of Cd and Zn [98].
Utilizing phytoremediation for heavy metal contamination also increases soil organic
matter content and soil fertility, and the increase in surface vegetation cover is beneficial
to the improvement in the ecological environment, and the cost is low [99,100]. Therefore,
how to use biotechnology to cultivate new hyperaccumulator plants has become a hot topic
in phytoremediation research.
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3.3.2. Soil Fauna Remediation
Narrowly defined, soil fauna refers to animals that spend their entire life history in
the soil, while broadly defined, soil fauna are animals that come into contact with the soil
surface or live in the soil during a certain period (or a certain season) of their life history. The
soil fauna remediation technology we usually refer to is the use of broad-sense soil animals.
It refers to the use of soil animals and their intestinal microorganisms to decompose,
digest, and concentrate pollutants in polluted soil under artificially controlled or natural
conditions, thereby reducing or eliminating pollutants, which is a kind of bioremediation
technology [101]. The mechanisms of fauna remediation are roughly as follows: soil
animals absorb heavy metals and form metallothioneins [102]; peptides produced through
metabolism reduce the activity of heavy metals through chelation; under heavy metal stress,
a variety of heavy metal transport protein genes are produced to enhance resistance [103].
Due to the limitations of fauna remediation, there is less research on it, and the current
focus is on using earthworms, rodents, and other large soil animals for remediation. Its
application in actual remediation projects requires further research.
3.3.3. Microbial Remediation
Microbial remediation refers to the use of naturally occurring or specially cultured microorganisms to degrade organic pollutants in the soil under controllable environmental conditions, or to change the form of toxic elements through biosorption and bio-oxidation/reduction
reactions, thereby reducing their toxicity and ecological risk in the environment [104,105]. The
remediation mechanisms of different microorganisms vary greatly. Bacterial microorganisms
mainly change the form of heavy metals, while fungi reduce the concentration of free heavy
metal ions through chelation with metallothioneins in their bodies, thereby reducing their
toxicity [106,107]. Microbial remediation technology is flexible and can be carried out in situ,
ex situ, or a combination of in situ/ex situ remediation. Moreover, microbial remediation
has the characteristics of low cost and high efficiency, and has become a hot topic in heavy
metal remediation research in recent years [108]. However, there are many problems in the
application of microbial remediation, such as the fact that the added microorganisms may not
adapt to the environment or may not be able to survive due to competition with other microorganisms in the environment, and are greatly affected by environmental conditions [109].
Therefore, cultivating microorganisms with strong adaptability and broad-spectrum characteristics through modern biotechnology and applying them to pollution control will become a
trend in environmental remediation research.
3.4. Combined Remediation Technology
Due to the increasing complexity of soil pollution and the significant differences in pollution levels, soil types, regional conditions, and reuse requirements in different areas, a single
remediation method can no longer achieve the desired remediation effects. How to develop
compound remediation technologies according to local conditions has become a direction in
the research on the remediation of heavy metal-contaminated soils [110]. The commonly used
types of combined remediation technologies are plant–microorganism-combined remediation,
animal–plant-combined remediation, and physical–chemical–biological-combined remediation [111–113]. Plant–microorganism-combined remediation, as an intensified plant remediation technology, has gradually become a hot topic in domestic and international research [114].
According to the different forms of plant–microorganism-combined remediation of heavy
metal-contaminated soils, this combined remediation technology can be divided into two
forms: plant-combined remediation with specific strains and plant-combined remediation
with mycorrhizae [115]. Like other remediation methods, plant–microorganism combined
remediation technology is also affected by many factors during application, such as the degree
of soil heavy metal pollution, the availability of heavy metals, the characteristics of plants
themselves, and changes in the rhizosphere environment (pH, redox potential, rhizosphere
secretions, rhizosphere microorganisms, and rhizosphere minerals) [116–120].
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4. Remediation Standards and Evaluation of Heavy Metal-Contaminated Soil
The establishment of remediation and evaluation standards for heavy metal-contaminated
soils, from the perspective of ecological toxicology or human health protection, has become
a key component in the remediation of heavy metal pollution in some developed countries.
Most developed countries regulate the environmental management of contaminated sites
through legislation, forming a legal system and management system, and have developed a
relatively complete system of remediation technologies through a large number of research
and practices in remediation [121,122]. However, due to various factors, there is still relatively
little research in this area in China to date. Establishing regional- or even national-level
evaluation standards through scientific research and demonstration will have significant practical implications for China’s agricultural product safety, ecosystem health, and sustainable
development of land resources, and should be of high concerned by government departments
and environmental workers.
4.1. Remediation Standards for Heavy Metal-Contaminated Soil
In the process of formulating remediation standards for heavy metal-contaminated
soils, it is necessary to fully consider factors such as the level of economic development,
social factors, clean technology factors, regulatory factors, and political factors [123]. Therefore, the remediation standards vary greatly among countries, and even different provinces
or states within the same country have significantly different soil standards. Table 3 lists the
remediation standards for heavy metal contaminated soils in different countries and regions.
For example, New Jersey’s “Soil Remediation Standards” in the United States classify land
resource remediation standards into three categories: residential land, non-residential land,
and remediation standards for potential impacts on groundwater. Different land types
have corresponding pollutant types, health risk standards, and remediation standards [124].
Denmark’s soil standards are quite unique; different from general soil standards, they are
not based on a comprehensive basis such as groundwater and atmospheric standards for
soil standard setting, but are based on the differences in soil regions, according to soil quality benchmarks, pollution area pollutant reduction standards, pollution area groundwater
quality standards, and pollution area air quality standards for the establishment of soil
remediation standards [125]. The Netherlands established a brand-new set of soil environmental standards based on human health and ecological risk research after discontinuing
its original soil and groundwater standards in 1994. The new standards can effectively
indicate the pollution of soil and groundwater, aiming to build the country’s ultimate soil
quality goals [126]. China’s soil environmental quality standards (GB15618—2018) classify
soil environmental quality into three categories based on the application function and
protection objectives of the soil, which to some extent provide a basis for soil pollution
remediation work in China [127]. However, due to the large number of soil types in China,
the significant differences in properties, varying degrees of pollution in different regions,
and factors such as composite pollution, it cannot fully solve the problems in the actual
work. More importantly, the standards do not focus on people and do not propose standards related to places where people live, such as residential areas, commercial areas, and
industrial areas, which are closely related to human health and sustainable development;
so, they still need to be further improved.
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Table 3. Soil remediation standards for heavy metal contamination in different regions.
Regulations
Type of Heavy Metal
Remediation Standard
(mg/kg)
America
Soil Remediation Standards,
2004
Hg
Cd
Pb
As
Zn
Cu
270
100
600
20
1500
600
Australia
National Environment
Protection (Assessment of
Site Contamination)
Measure,
NEPM, 1999
Hg
Cd
Pb
As
Zn
Cu
15
20
300
100
7000
1000
Canada
Canadian Soil Quality
Guidelines, CSQG, 1997
Hg
Cd
Pb
As
Zn
Cu
6.6
64
70
12
200
63
China
Soil Environmental
Quality—Risk Control
Standards for Soil Pollution
of Agricultural Land (Trial)
(GB 15618-2018)
Hg
Cd
Pb
As
Zn
Cu
1.3
0.3
70
40
200
50
Contaminated Soil Act, 2000
Hg
Cd
Pb
As
Zn
Cu
1.0
0.5
40
20
500
500
Soil Remediation
Circular.2006
Hg
Cd
Pb
As
Zn
Cu
0.3
0.8
85
29
140
36
Region
Denmark
The Netherlands
Influence Factor
Economic development
level, social factors,
clean technology
factors, regulatory
control factors, and
political factors
[128–130]
The establishment of remediation standards will be affected by various factors, such as
soil background values, technical methods used in remediation, the instrumental detectable
level of pollutants, the adjustability of standards, and ecological toxicological evaluation [131,132]. Therefore, in the process of formulating standards, the existing classification
methods should not be limited. The ultimate utilization objectives of soil remediation
should be fully considered, integrating the issues of reuse after soil remediation and the
risks to human health [133]. It is essential to formulate soil remediation standards that are
suitable for different regions, types, and levels of pollution, tailored to local conditions [134].
4.2. Remediation Evaluation of Heavy Metal-Contaminated Soil
In the remediation work of heavy metal-contaminated soils, observing the soil after
remediation and evaluating it through scientific methods can clarify whether the remediated soil meets the standards, whether the threat of pollution to human health and the
ecosystem has been eliminated, and whether it can be reused. This is the final and even the
most critical step. There are many evaluation methods for heavy metal contaminated soil
remediation, and Table 4 lists the commonly used evaluation criteria.
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Table 4. Post-remediation monitoring and ecological assessment methods of contaminated soils.
Monitoring Methods
Principles
Laboratory monitoring
Ecotoxicological monitoring
of remediated soil samples in
the laboratory
In situ monitoring
Performing ecological and
toxicological diagnostics at
the remediation site to assess
the effectiveness of
contaminated soil
remediation
Biochemical toxicology monitoring
Selecting indicators at the
molecular and cellular levels
to measure the impact of
pollutants on organisms
Ecological indication
monitoring
Plant symptomatology
and growth
measurement method
Assess the degree of heavy
metal pollution in soil
through the growth status
and biomass of plants
Sensitive animal
indicator method
Assessing the degree of soil
contamination by the
number and survival status
of sensitive animals such as
earthworms in the soil
Biomarker method
The characterization of
exposure to one or more
chemical contaminants or
their effects through
biochemical, cellular,
physiological, behavioral, or
energetic changes measured
in body fluids, tissues, or the
whole organism
Soil enzyme indicator
method
Evaluating remediation
effectiveness by monitoring
enzyme activity in soils
Ecological Assessment
Steps
References
Risk factor
identification—
exposure analysis—risk
characterization
[135–139]
4.2.1. Post-Remediation Monitoring of Contaminated Soil
The post-remediation monitoring of contaminated soils is to observe the damage to
different biological components in the soil ecosystem after remediation, and to qualitatively
or quantitatively evaluate the risk of the remediated soils to the ecosystem and human
health [140]. According to the difference in monitoring sites, it can be divided into laboratory monitoring and in situ monitoring. According to the restoration target function
recovery indicators, it can be divided into biochemical toxicological monitoring and ecological indicator monitoring. According to the difference in monitoring time, it can be divided
into short-term monitoring and long-term monitoring [141–143]. Laboratory monitoring
is to collect soil samples from the remediation site and conduct ecological toxicological
monitoring indoors. Laboratory monitoring has certain limitations and is therefore affected
in its application, while in situ monitoring is to evaluate the remediation effect through the
ecological toxicological diagnosis of the remediation site, which can more objectively and
comprehensively evaluate the remediation effect [144].
The primary goal of remediating contaminated soil is to restore its function as a habitat
(plants and soil animals). Ecological indicator monitoring is based on this goal, introducing
some sensitive organisms (plants, soil animals, and microorganisms) into the remediated
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soil, and judging the effect of soil remediation by monitoring their physiological and ecological changes [145]. There are several methods: plant symptom and growth method;
sensitive animal indicator method; biomarker method; broad bean root tip micronucleus
test method; and soil enzymology indicator method [146–150]. Ecological indicator monitoring can intuitively identify the ecological safety of soil pollutants, closely linking the
cleanliness of the soil and the effect of remediation, and has certain practicality in the
evaluation of contaminated soil remediation. Biochemical toxicological monitoring is based
on the potential hazards of residual pollutants in the soil to the physiological and biochemical aspects of the environment and evaluates the size of the risk through molecular- or
cellular-level measurements, thereby evaluating the remediation effect [137]. This method
has the advantages of high measurement sensitivity and short measurement cycle.
4.2.2. Ecological Risk Assessment of Reused Remediated Soil
Risk assessment is a process that predicts the possibility of pollutants in the environment having adverse ecological effects on the entire ecosystem or parts of it [151]. Soil
ecological risk assessment focuses on the potential impacts of environmental pollutants
entering the soil, including at least two aspects: human health assessment, with human
health as the core goal, and ecological health assessment aimed at the stability of the soil
ecosystem or its components [152]. Different countries or regions have certain differences
in the starting points and steps of soil ecological risk assessment. For example, the one
adopted by the United States is based on human health, and its assessment consists of
three steps: problem formation, problem analysis, and risk characterization [153]. The one
adopted by the United Kingdom is based on the national sustainable development strategy,
and is carried out in several steps, including risk identification, consequence determination,
risk perception, risk assessment, risk management, and risk monitoring [154].
China’s soil ecological risk assessment started relatively late, and experts and scholars
have proposed a general method for soil ecological risk assessment in China based on the
introduction and introduction of foreign theories and practical results. Risk identification,
exposure analysis, and risk characterization are often used as metrics for soil ecological
risk assessment [155]. Risk factor identification is the first step in soil ecological risk
assessment, which lays the foundation for soil ecological risk assessment by hypothesizing
the ecological effects produced by human activities, identifying risk sources, and then
assessing the process. Exposure analysis is the process of using technical research on data
to summarize the potential contact and symbiotic relationships between risk sources and
various elements of the soil ecosystem [155]. Risk characterization is the final stage of soil
ecological risk assessment, which is a comprehensive analysis of the results of the first two
steps. The purpose of risk characterization is to assess the stress caused by pollutants to the
soil ecosystem or some of its components through the relationship between soil pollutants
and their concentrations and the resulting ecological effects, explain the risk assessment,
and report the results [156].
5. Conclusions and Prospect
In conclusion, the future research and application of soil remediation technologies
should focus on developing efficient, low-cost, and potential soil remediation technologies,
such as microbial remediation and phytoremediation. The sole application of physicochemical technologies for the remediation of heavy metal-contaminated soils is not only
costly but also impractical for large-scale soil improvement. Moreover, it may lead to
issues such as soil structure destruction, fertility degradation, and a decline in biological
activity. Phytoremediation, as an emerging and efficient bioremediation technology, has
been widely applied. However, it also has drawbacks such as low biomass, strong selectivity, and the subsequent treatment of plants enriched with heavy metals after harvesting.
Therefore, to address the issue of soil heavy metal pollution, it is essential to strengthen
research on the ecological chemical behavior of heavy metal elements and remediation
technologies, especially plant and microbial remediation technologies, which have broad
Toxics 2024, 12, 897
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application prospects. Actively exploring the combined application of various remediation
technologies, leveraging their strengths and compensating for their weaknesses, will better
achieve the remediation of heavy metal-contaminated soils.
It is also necessary to establish a comprehensive soil remediation assessment system
to reduce the occurrence of over-remediation or under-remediation. Based on domestic
and international research technologies, the development of efficient stabilization agents
for the simultaneous disposal of multiple heavy metals is crucial. Additionally, interdisciplinary knowledge should be integrated to improve the efficiency of high-biomass
economic plants in remediating heavy metal-contaminated soils. The establishment of
heavy metal-contaminated soil remediation standards is not a simple concept or action
but is based on a clear understanding of the ecological toxicological critical levels of pollutants, comprehensively considering various factors such as economic development level,
social factors, political factors, clean technology level, and regulatory controls. China’s
current “Soil Environmental Quality Standards” (GB15618-2018) can no longer meet the
requirements of soil pollution remediation work. Formulating a new, society developmentadapted, and comprehensive soil environmental quality standard is key to solving soil
pollution issues at present. The remediation evaluation of heavy metal-contaminated
soils is a complex process involving multiple disciplines such as ecology, ecotoxicology,
environmental science, geography, soil science, and disaster science. There are significant
deficiencies in both basic theoretical research and practical work, which should be paid
sufficient attention in future research.
Author Contributions: Writing—review and editing, L.X. and F.Z.; investigation, X.X., J.P. and J.W.;
methodology, B.L.L.; resources, M.J. All authors have read and agreed to the published version of
the manuscript.
Funding: This research was supported by the PhD Special Project of Nanyang Normal University
(2018ZX018), the National Natural Science Foundation of China (42377039), Key Research and
Development Projects of Henan Province (221111520600), and the Higher Discipline Innovation and
Talent Introduction Base of Henan Province (CXJD2019001).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data used to support the findings of this study are available from
the corresponding author upon request.
Conflicts of Interest: The authors declare no conflicts of interest.
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