Lead Contamination in Soils and How to Treat It

Lead is a toxic heavy metal that poses a significant risk to humans, wildlife and natural habitats, as heavy metals are not degraded in the environment and consequently accumulate in soils.

The lead-contamination of soil and its circulation in the soil–plant–animal–human biological food chain can cause chronic disease in humans and other living organisms.

Lead can enter the body through inhalation, by breathing in dust or fumes, and by ingestion, from consuming contaminated food or drinking contaminated water.

It is a cumulative poison, with small amounts gradually building up in the body, causing serious long-term health problems. Most of it accumulates in the bones and teeth. Some of that absorbed lead that is not excreted is exchanged with the blood, and soft tissues such as the liver, kidneys, lungs, brain, spleen, muscles, and heart.

What Health Problems Does Lead Exposure Cause?

Exposure to lead is linked to many adverse health effects in adults. Low blood levels can lead to decreased kidney function. Moderate levels are associated with increased blood pressure (hypertension) and incidences of essential tremor, a degenerative nervous system disorder characterised by tremors of the arms or hands during voluntary movements. High levels are also associated with cardiovascular effects, nerve disorders, and fertility problems, including delayed conception, lower sperm count and motility.

Children are even more vulnerable to lead toxicity than adults, as their developing brains and nervous systems are far more sensitive to the effects of lead, and are much more easily damaged, affecting their development and behavior, with many of the effects being permanent. Low levels of lead in maternal blood are associated with reduced foetal growth, while low levels in children may lead to lowered IQ scores and academic achievement, learning difficulties, increased behavioural problems and attention-related issues. Moderate blood levels of lead in children are also associated with delays in puberty, and decreases in hearing, cognitive performance, and growth in height.

Additionally, young children under five years of age are at greater risk of lead poisoning in contaminated environments as they often put their hands or other objects into their mouths, leading to greater levels of lead ingestion compared to adults. Pets may also be subjected to higher levels of lead intake due to increased contact with contaminated soils and licking their coats to groom themselves. Birds need to ingest grit for digestion, and this would make them particularly vulnerable in environments with lead-contaminated soils.

Where Does Lead Contamination of the Soil Come From?

Lead contamination of the soil is a fairly common problem in urban areas, particularly where garden beds are sited alongside painted walls that were painted with lead-based paints, where toxic lead compounds are used as pigments. When these paints begin peeling, flaking or chalking, they contaminate the ground below. Paints containing lead were generally banned in the 1970’s, though some governments (Australia) have permitted paints to contain small percentages of lead (< 1%) after that date, and since 1997 lowered the permissible level to 0.1%. Not all paints are lead-free!

Another common location prone to lead contamination are roadside verge plantings along the footpath on busy roads with lots of traffic. As horrific as it sounds, tetraethyl lead, an organolead compound which comprised of lead bonded to carbon atoms, making it far more absorbable by living things, was used as a fuel additive in automotive petrol (gasoline) until it was banned in the 1990s and early 2000s in most countries. It was added as an antiknock agent, allowing engines to run higher compression to produce more power. It also spread superfine particles of lead into the air that were capable of being breathed in and carried by the wind to contaminate the surrounding landscape.

The brake linings of cars, trains and trams also contain lead, creating a fine metal dust in locations where these vehicles come to a stop, which is readily spread by the wind and the air movement created by fast-moving vehicles.

Other sources of lead contamination are those from industrial activities, such as lead in fumes from metal smelting and battery manufacturing.

The contamination of water may be caused by the use of lead pipes, or lead-based solder used to join metal pipes. In fact, the chemical symbol for lead is Pb from the Latin name plumbum due to its use for water pipes in Ancient Rome, which is where the word ‘plumber’ is derived from. And yes, the Romans did end giving themselves lead poisoning this way!

Lead contamination in soil is highly problematic as lead is a metal, so it can’t be broken down any further to neutralise it, and it therefore persists in the soil indefinitely. It can form compounds of lead, which are also toxic. Therefore, it is crucial to understand how to treat lead-contaminated soil effectively.

There are several methods for treating lead-contaminated soil, and the following are some of the most commonly used approaches for treating lead-contaminated soil.

1. Physical Removal of Contaminated Soil

Physical removal of contaminated soil is the most straightforward method for treating lead-contaminated soil, which basically just involves digging up the contaminated soil and disposing of it in a secure landfill.

This may be an effective method for dealing with large areas of highly contaminated soil in industrial sites as long as the contaminated soil isn’t too widespread or deep. This approach may also be viable when demolishing a house and clearing the whole block, but it’s too expensive, time-consuming and impractical a method for regular gardens.

2. Stabilization or Solidification

Stabilization/solidification is another expensive industrial process that’s not really relevant to gardening. It involves mixing the contaminated soil with binders, such as cement or asphalt to create a solid, stable material that can be safely disposed of, which is capable of immobilising the lead in the soil and reducing the risk of leaching.

3. Phytoremediation

Phytoremediation, a term originating from the Greek prefix φυτο “phyto” – plant, attached to the Latin root “remedium” – to correct or remove, restoring balance, or remediating, is a method of using plants to remove, degrade, or immobilise contaminants in the soil.

This method can be effective for treating lead-contaminated soil, as certain plants are capable of accumulating lead from the soil and transferring it to their aerial (above-ground) parts, which can then be harvested and safely disposed of into landfill waste.

There are several plant species that have been found to be effective in removing lead from the soil, including sunflowers (Helianthus annuus) and Brassica species, such as mustard (Brassica juncea).

Poplar (Populus spp.) and willow (Salix spp.) trees, both from the Salicaceae family, and jatropha from the Euphorbiaceae (spurge) family are being for the dual purpose of phytoremediation as well as energy production.

Willows and poplars have many ecological uses, and one of those is to produce biomass which can be used to produce heat and electricity by direct combustion, or for biofuel production. The oil extracted from jatropha seeds is used in the production of biodiesel.

These plants are able to extract lead from the soil, while coping with high lead levels within their system.

Which Vegetable Crops Take Up the Most Lead from The Soil?

Crops can take up different amounts of lead from the soil, depending on several factors such as soil characteristics, plant species, and stage of plant growth.

Root crops, such as carrots, potatoes, turnips and radishes, tend to accumulate more lead from the soil than leafy vegetables or fruit and berries because they absorb more water and nutrients from the soil, which can result in a higher concentration of lead in the edible portion of the plant.

In a study titled “Bioremediation Methods for the Recovery of Lead-Contaminated Soils: A Review” it was found that among the leaf crops, which have a fairly low lead uptake, lettuce and spinach had the highest levels of lead. Tomatoes, which are a fruiting crop showed only a scarce accumulation capability. The study also showed that a plant’s ability to take up lead is influenced by the soil pH. Lead absorption in lettuce and carrots was shown to increase in low pH (<6) acidic soil conditions.

4. Bioremediation and Bioaugmentation

Microorganisms such as bacteria, fungi, and algae, which are capable of degrading or converting toxic compounds into less harmful substances, can be used to break down contaminants in the soil and water.

• Bioremediation is the process of using naturally occurring microorganism populations present in the environment, which may be a slow process if their numbers are low. This approach may require the addition of nutrients or other amendments to support the growth and activity of the microorganisms.
• Bioaugmentation, on the other hand, involves the addition of specifically selected microorganisms to the contaminated environment in high numbers to increase the rate of degradation and speed up the process.

Using microorganisms to treat lead-contaminated soil is an effective method, as certain bacteria have been shown to reduce the solubility and toxicity of lead in the soil.

Which Microorganisms Are Used for The Remediation of Lead Contamination and Where Are They Found?

Various bacteria and microorganisms have been identified as effective for the bioaugmentation of lead contamination. Some of the most commonly used microorganisms for this purpose include species of Bacillus, Pseudomonas, Klebsiella, and Aspergillus.

• Bacillus species, such as Bacillus subtilis and Bacillus cereus, are well-known for their ability to degrade lead and other heavy metals. These bacteria have the ability to produce chelating agents, such as citric acid, which can solubilize lead and make it available for degradation.
• Pseudomonas species, such as Pseudomonas fluorescens, are also effective for lead bioremediation. These bacteria are known to produce extracellular enzymes, such as phytases, that can solubilize lead and make it available for degradation.
• Klebsiella species, such as Klebsiella pneumoniae, are also commonly used for lead bioremediation. These bacteria are known to produce citric acid, which can solubilize lead and make it available for degradation.
• Aspergillus species, such as Aspergillus niger, are fungi that are also effective for lead bioremediation. These fungi produce organic acids, such as oxalic acid, which can solubilize lead and make it available for degradation.

These microorganisms can be obtained from a variety of sources, including compost, worm farm leachate, and specialized microbial cultures. Some of these microorganisms are naturally occurring in the environment, while others may be obtained from commercial sources that specialize in the production of microbial inoculants for bioremediation.

The microorganisms can be added to the soil in high numbers by mixing them directly into the soil, such as when using compost. They can also be added using a soil drench when using worm farm leachate diluted 10:1, or when using compost tea for the purpose. The process of lead bioremediation or bioaccumulation can be accelerated by also adding natural fertilisers to provide nutrients that support the growth and activity of the microorganisms in the soil.

How Microorganisms Reduce Lead Availability in Soils

Soil microorganisms are able reduce the solubility and toxicity of lead in the soil by producing enzymes that convert lead into a less toxic form.

One of the key mechanisms of this process is the production of organic acids by bacteria, which can dissolve lead-bearing minerals in the soil, converting them into soluble lead complexes, which are taken up by the bacteria and reduced to elemental lead or lead-carbonate precipitates that are less soluble and therefore less available for plant uptake.

The bacteria can also produce extracellular polymeric substances (EPS), which can form a protective coating around lead particles and reduce the availability of lead to plants. EPS can also serve as a source of energy and nutrients for the bacteria, which can help to support the growth and activity of the bacteria in the soil.

5. Remediation with Soil Amendments

Soil amendments are materials that are added to soil to improve the physical and chemical properties, such as the texture, structure, fertility, and pH of the soil.

Some common soil amendments that have been used to treat lead-contaminated soil include compost, lime, iron filings and biochar.

These amendments can help to reduce the solubility and bioavailability of lead in the soil, reducing plant uptake or leaching into groundwater.

Using Compost to Treat Lead-Contaminated Soils

Compost is the product of the decomposition of plant materials, and as such contains high levels of organic matter that is rich in beneficial microorganisms.

The addition of compost to soil has been shown to be effective in reducing lead uptake by plants and increasing the uptake of other essential nutrients, such as phosphorus and nitrogen, that can also help to reduce the bioavailability of lead in soil.

According to a study published in the Journal of Environmental Quality, adding compost to lead-contaminated soil has been shown to reduce lead uptake by plants by as much as 70%.

Using Garden Lime to Treat Lead-Contaminated Soils

Garden lime, or limestone, is calcium carbonate (CaCO₃), a soil amendment that can be used to raise the soil pH, making it more alkaline.

A high pH level can reduce the availability of lead in soil and help to reduce the risk of lead uptake by plants. According to a study published in the journal Environmental Science and Technology, adding lime to lead-contaminated soil has been shown to reduce lead uptake by plants by as much as 90%.

In the soil, lead is typically found in the form of lead compounds, where it is present as lead (II) cations (Pb²⁺). At low pH levels, these cations are highly soluble and readily available for plant uptake. When the soil pH is raised, lead-carbonate complexes are formed, such as PbCO₃, which are less soluble and therefore less available for plant uptake.

Using Iron Filings to Treat Lead-Contaminated Soils

Iron filings are small particles of iron that contain iron (Fe) in the form of iron oxide (Fe₂O₃) or iron hydroxide (Fe(OH)₃), otherwise known as ‘rust’. When these materials are added to lead-contaminated soils, they can react with the lead (Pb²⁺) to form an insoluble complex, such as FePbO₂ or FePb(OH)₂, that is not readily available for plant uptake.

According to a study published in the journal Environmental Science & Technology, adding iron filings to lead-contaminated soil has been shown to reduce lead uptake by plants by as much as 50%.

Using Biochar to Treat Lead-Contaminated Soils

Biochar is charcoal produced for mixing into soil, it is a porous and carbon-rich solid material made by heating wood, agricultural and forestry waste or other plant material in an oxygen-deprived atmosphere.

Through the process by which it is produced, biochar has a highly porous structure and a very large total surface area, which allows it to interact with minerals, other soil organic matter, and soil biota. This structure allows it to effectively adsorb heavy metals such as lead from the soil.

Additionally, biochar also contains many hydroxyl, carboxyl, and other functional groups that are favourable for forming complexes with heavy-metal ions, such as lead (II) cations (Pb²⁺), which at low pH levels are highly soluble and readily available for plant uptake, as mentioned earlier. When the lead is bound up in complexes, it becomes unavailable to plants. The oxygen-containing functional groups and carboxyl groups of biochar are the main mechanism by which these complexes are formed.

Biochar is a cheap and effective remediation material. It reduces the risk of contaminants entering the food chain by binding or precipitating them in the soil. It can be used to immobilize heavy metals and organic pollutants in soil through adsorption, as well as my means of its ample organic and active functional groups, its inclusion of inorganic minerals, the microporous structure with its associated high surface area, as well as a high pH, cation exchange capacity (CEC), and carbon content.

References

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