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Remediating heavy metal toxicity in soil system for safe cultivation of crops: To lock or not to lock?

Dr. Anandkumar Naorem
Dr. Anandkumar Naorem
Transfer of heavy metals through food chain
Transfer of heavy metals through food chain

1. Why Heavy Metal Pollution is a concern?

With the advent of the Anthropocene, the soil system has been exploited for several anthropogenic activities including crop cultivation, in order to supply the food demand for an ever-increasing human population. Land use conversions from natural ecosystems to cultivated land have considerably influenced the physicochemical and biological properties of the soil system, thus threatening agricultural sustainability. Moreover, the increasing rate of industrialization has posed threats to the soil ecology due to the high loading of toxic contaminants into the soil system via indiscriminate waste disposal practices. 

As soil is one of the largest sinks of these contaminants, it, directly and indirectly, affects the health of plants, animals and humans viathe food chain. One of the prominent soil pollutants is heavy metal, which is defined as those metals and metalloids that have a relatively high density (having a specific density of more than 5 g cm-3) and can be toxic even at low concentration levels. Some common examples of heavy metals are zinc, lead, chromium, nickel, arsenic, copper etc. The presence of these heavy metals in the human body above a threshold level causes a variety of diseases. Accumulation of heavy metals in the human body disrupts the metabolic functions in vital organs such as the heart, kidney, liver, brain etc. In addition, it also displaces essential minerals from their original places. This has increased the global concern of several health authorities. Tragic cases such as Minamata disease in Japan are derived through mercury poisoning. Arsenic water pollution has sparked concerns over several parts of Punjab and West Bengal. Several countries have reported an increase in such cases and have emphasized that millions of people are at risk from heavy metal poisoning.

2. Myths and Reality:

Before unravelling some complexities involved in heavy metal contamination in the soil, there are some of the myths that need to be addressed to have a better understanding in this regard.

2.1 All heavy metals are toxic:

Not all heavy metals are toxic at low concentrations. For example, zinc is heavy metal but is essential for the normal functioning of the human body. It is important for the functioning of many vital human organs as well as tissue repair. The deficiency of zinc in the human body can be fatal if it falls below the threshold concentration. On the other hand, excessive zinc can be very harmful and can lead to several types of pathological consequences such as pulmonary inflammations and flu-like symptoms. Similarly, iron, cobalt, copper are required by humans for normal functioning and growth.

2.2 Heavy metals affect only human beings:

Plants can also face oxidative stress upon exposure to heavy metals leading to cellular damage. These disruptions often divert the resources of the plant into detoxification processes, thus can affect crop productivity and survival. However, several plants have developed specialized detoxification mechanisms based on chelation and subcellular compartmentalization.

2.3 All crops grown on heavy metal contaminated soil can accumulate high amounts of heavy metals in the plant tissues

All crops have different mechanisms to detoxify or accumulate certain levels of heavy metals in their plant parts. For example, if two different crops A and B (both are fruit crops) are planted in a heavy metal contaminated soil, there are high chances that either one of them (say A) may accumulate a meagre amount or zero concentration of heavy metal in its fruit part. This can be related to the translocation factor, which is the ratio of heavy metal concentration in the plant to the heavy metal concentration in the soil system. Therefore, based on these results, a recommendation can be made that if A and B are similarly profitable to the farmer, then A must be planted rather than B if no remediating techniques are not followed.

2.4 Heavy metals are man-made:

There are two main sources of heavy metals or metalloids in the soil system: Geogenic (natural) and anthropogenic (man-made). Most of these heavy metals are naturally occurring and their release is due to the weathering or breaking down of soil parent materials. Arsenic is a metalloid and its source is geogenic. The arsenic content of some igneous rocks is averaged between 2-3 mg per kg. However, the dramatic increase of heavy metal loading is contributed by anthropogenic activities, primarily the disposal of domestic and industrial waste and long term sewage irrigation. Due to the unavailability of water resources or good quality irrigation water, sewage water is often used as an irrigant in several parts of the world. It partially fulfils the water and nutrient demand of the crops. However, untreated sewage water contains a high amount of heavy metals. The long term sewage irrigation leads to a considerable increase in the heavy metal pools in the soil system, thus affecting human health through the food chain. Among agricultural practices, fertilization is another route of delivering heavy metals into the soil system. Phosphate fertilizers contain a range of heavy metals or metalloids, especially cadmium through cadmium containing phosphate fertilizers. Similarly, copper is also loaded in high quantities into the soil system through pesticides or insecticides or biosolid applications.

Sources of heavy metal loading in the soil system
Sources of heavy metal loading in the soil system

3. How does heavy metal contamination differs from organic pollutants?

Organic pollutants can be acted upon by microbial or chemical decomposition. Unlike these organic pollutants, heavy metals are resistant to such degradation and therefore the total residence time of these heavy metals in the soil system is generally high after its loading into the soil. The presence of heavy metals disturbs the normal functioning of the soil microbial structure and activities, thus affecting the soil biological health. It alters the chemical properties of the soil so extensively that plant growth and crop productivity are limited.

4. Are there any remediation techniques for the safe cultivation of crops?

There are several techniques through which heavy metals toxicity can be remediated in the soil system. The traditional method includes removal of the polluted soil and replacing it with clean soil, which is often laborious and quite cost-prohibitive. Therefore, other methods are been advocated by several researchers in which soil amendments are applied to the soil system to alter the bioavailability of the heavy metals.  Bioavailability is referred to how much of these heavy metals can be taken up by plants or can be used by soil biota for their nutrition and metabolism. The bioavailability of these heavy metals can be restricted or enhanced through both chemical and biological approaches.

There are mainly two ways through which heavy metal toxicity can be reduced in the soil system: Mobilization and Immobilization. Mobilization refers to the enhancement of the bioavailability of these heavy metals so that they can be taken up by phytoremediation plants and dispose of these plants later, thus decreasing the total amount of heavy metals in the soil. On the other hand, immobilization refers to stabilizing or restricting the bioavailability of these heavy metals by forming insoluble complexes, precipitates and compounds so that crops grown in these heavy metal contaminated soil could not take up the heavy metals into the plant system. For example, in simple terms, mobilization means opening the gate of a dam so that the excess water can be drained out and on the other hand, immobilization means locking the gate of the dam. There have been several discussions in the scientific community concerning which of these modes is more effective.

5. Soil amendments for remediating heavy metal toxicity in the soil system:

Several soil amendments have been reported by several researchers. Some of them are meant to immobilize heavy metals and some are best mobilizers. The mechanisms involved in the mobilization technique is to enhance the bioavailability of the heavy metal and release them into the soil solution which is further removed by specific plants. On the other hand, immobilization or stabilization technique refers to the removal of the target heavy metal from the soil solution through adsorption, precipitation, complexation. The addition of phosphate fertilizers to arsenic-contaminated soils have been found to increase the mobility of heavy metals. Similarly, the application of hydroxyapatite results in an increase of chromium (VI) and arsenic (V) in the soil solution. The phosphate fertilizers work through the competition of adsorption sites between the phosphate ions and the target heavy metal ion. In addition, there are certain chelating agents such as EDTA, DTPA that have a high affinity for the heavy metal ions and increase the solubility of these heavy metal ions through the formation of soluble chelates. Chelating agents have been well recommended in remediating iron, copper and zinc toxicity in the soil system. 

Simple representation of mobilization (left) and immobilization (right) technique
Simple representation of mobilization (left) and immobilization (right) technique

Certain organic amendments such as green waste compost not only enhance soil health and improve plant growth but also have immobilizing effects on the heavy metals through the production of humic acids that bind the heavy metals in the soil. However, the immobilizing capacity of these organic amendments largely depends on the type, quality, source of the organic amendment, the nature of the heavy metal and soil conditions.

There is another term ‘microbial immobilization’, which refers to a reduction of heavy metal toxicity by using soil microbes. Microbes such as Escherichia coli expresses metal-binding proteins and help in immobilizing the heavy metals in the soil.

6. What are the common mobilizing as well as immobilizing agents?

Mobilizing agents: EDTA, EDDS, DTPA, Citric acid, Humic acid, NTA, Tartaric acid, poultry manure, sewage sludge, manure, compost, rice straw etc.

Immobilizing agents: Apatite, phosphate, lime, red mud, zeolite, calcium hydroxide, biosolid, compost, manure, biochar etc.

7. Future scope of  researchers

Heavy metal contamination in the soil system is still a huge concern on a global level. The rate of research in this aspect has been growing in the scientific community. However, there are certain research gaps and questions that need to be addressed. Future studies are required in areas such as the response of the plants treated with both immobilizing and mobilizing agents, standardization of threshold levels of heavy metals in soil and plant system, identification of crops that naturally restricts the accumulation of heavy metals in its plant parts, enhancement of immobilization techniques and development of cost-effective multifaceted soil amendment that can reduce the toxicity of several heavy metals with a high rate of effectiveness

About Author: 

Dr. Anandkumar Naorem
Head (I/C) and Scientist (Soil Sciences)
ICAR-Central Arid Zone Research Institute, RRS-Bhuj

Contact Details:

Mobile: +91-8240244565
Email: Anandkumar.naorem@icar.gov.in/naoremanand@gmail.com
LinkedIn: https://www.linkedin.com/in/dr-anandkumar-naorem-593778195/

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