Plants for cleaning and restoring contaminated soils

Plants for cleaning and restoring contaminated soils

Phytoremediation, phytostabilization and biological restoration of the territory

Phytoremediation is the use of plants, their roots, and associated microorganisms to reduce the hazards of contaminated soil, water, or sediment.

It's important to understand that plants don't always completely remove pollutants. Depending on the substance, they may:

extract it from the soil;

retain in the root zone;

reduce mobility;

prevent the spread of dust;

stimulate microbial decomposition;

intercept contaminated water;

create conditions for the gradual restoration of the ecosystem.

For heavy metals, extraction and stabilization are most often used. For oil, diesel fuel, some pesticides, and organic compounds, microbial degradation in the root zone plays an important role.

1. Basic mechanisms of phytoremediation

The main principle:

Metals cannot be biodegraded, but can be removed, bound, or sequestered; organic pollutants can sometimes be degraded.

2. Three groups of plants according to their function

2.1. Extractor plants

They are used when the pollutant is sufficiently accessible to the roots and is transferred to the aboveground part.

Examples:

Sarepta mustard;

amaranth;

sunflower;

certain types of rapeseed;

specialized hyperaccumulators;

some species of willow.

2.2. Stabilizer plants

Their main task is to cover the surface and stop the spread of contaminated soil.

Examples:

fescue;

ryegrass;

fieldfare;

cocksfoot;

rump;

miscanthus;

sedges;

cane;

willow;

birch;

pine.

2.3. Plants as ecosystem restorers

They return organic matter, soil biology, pollinators and natural succession.

Examples:

alfalfa;

clover;

fireweed;

chicory;

yarrow;

local grains;

shrubs;

willow and birch.

3. Alfalfa is a biological soil engine

Alfalfa isn't the strongest metal hyperaccumulator. Its uniqueness lies in its combination of several functions.

The main properties of alfalfa

Why Alfalfa Is Good for Oil Pollution

It doesn't absorb oil as a ready-made substance. Most of the work occurs around its roots.

Root secretions:

feed oil-degrading bacteria;

support the microbial community;

improve soil structure;

increase oxygen availability;

stimulate the decomposition of some hydrocarbons.

Alfalfa works best when mixed with cereals:

fescue;

ryegrass;

rump;

hedgehog team.

Cereals have a very dense network of fine roots, while alfalfa provides deep penetration and a nitrogen-fixing component.

Alfalfa and heavy metals

It can absorb and retain:

cadmium;

zinc;

copper;

nickel;

part of chromium;

part of lead.

But its role usually consists of a combination of:

partial extraction;

retention of pollutants in the roots;

creation of a stable rhizosphere;

restoration of soil structure.

In lead contamination, alfalfa is more of a stabilizer than a strong extractor.

Alfalfa limitations

She doesn't tolerate:

long-term flooding;

stagnation of water;

strong acidity;

very high salinity;

heavy anaerobic soil;

extremely high pollution.

Alfalfa biomass from a contaminated site cannot be used as feed, food, grass feed or regular compost.

4. Fireweed is a plant of the first restoration

Fireweed, or Ivan-tea, is a typical pioneer plant.

Its main function is to quickly return vegetation to areas where the previous ecosystem has been destroyed.

He appears on:

fires;

clearings;

construction embankments;

disturbed forest lands;

quarries;

dumps;

roadsides;

displaced soil.

Unique properties of fireweed

Role in phytoremediation

Fireweed is not considered a universal hyperaccumulator. Its main functions are:

covering the contaminated surface;

dust reduction;

fixing the top layer;

support of microorganisms;

accumulation of organic matter;

initiation of natural succession.

A natural sequence might look like this:

bare ground → fireweed → mixed grass → shrubs → young forest.

For oil and heavy metals, fireweed is best used after the acute toxicity has been reduced and the surface has been stabilized.

Fireweed from a contaminated area should not be collected for tea, food, medicine or cosmetics.

5. Amaranth - high biomass in one season

Amaranth combines:

rapid growth;

large above-ground mass;

developed root system;

heat resistance;

the ability to absorb some metals.

It doesn't necessarily accumulate record metal concentrations in every kilogram of tissue. Its advantage is the large overall yield of contaminated biomass.

Actual pollutant removal

The plant's efficiency is determined not only by the metal concentration, but also by the volume of the collected mass:

metal concentration in dry biomass × dry biomass mass = total metal removal.

Therefore, a plant with a moderate concentration but a huge mass sometimes removes more metal than a small hyperaccumulator.

Amaranth potential

Benefits of amaranth

quickly covers the surface;

tolerates heat well;

suitable for seasonal cycles;

easy to grow;

can be used in experimental phytoextraction plots;

produces a large amount of measurable and collectable biomass.

Restrictions

Amaranth does not like:

prolonged waterlogging;

strong shadow;

extremely acidic soil;

very high toxicity.

In a contaminated area, its seeds, leaves, oil, flour or green mass cannot be used for food or feed purposes.

6. Mustard - A quick seasonal extractor

Sarepta or Indian mustard is especially interesting for phytoremediation.

Its uniqueness lies in its short cycle, rapid growth and relatively good transfer of some metals from the roots to the above-ground parts.

Features of mustard

The most promising pollutants

cadmium;

zinc;

copper;

nickel;

certain forms of chromium;

partly lead.

Why is mustard interesting for lead?

Mustard has been extensively studied in enhanced phytoextraction experiments. However, lead is poorly soluble and often remains in the roots.

To increase its availability, chelating agents were sometimes used. This can indeed increase Pb uptake by shoots, but it also increases the risk of dissolved lead leaching into groundwater.

Therefore, synthetic chelators cannot be used on a real site without engineering control of water, drainage and soil chemistry.

The practical role of mustard in Pb:

fast seasonal cover;

experimental partial extraction;

lead availability control;

additional culture in a complex system;

not a replacement for engineering stabilization.

7. Sunflower - a large biopump and rhizofilter

Sunflower is valuable due to the combination of:

large above-ground mass;

developed root surface;

intensive water consumption;

ability to absorb metals;

possibilities of use in water systems.

Unique qualities

Sunflower and metals

It is being investigated for:

cadmium;

zinc;

it is;

nickel;

lead;

uranium;

some radionuclides.

Of particular value for water purification

Sunflower is particularly interesting not only in soil but also in controlled tanks.

Roots can:

absorb dissolved metal;

retain particles;

bind the pollutant to cell walls;

partially absorb it.

After this, the root mass can be removed entirely, which is much easier than removing all the roots from the soil.

Sunflower and lead

In soil, Pb often remains primarily in the root system. Therefore, sunflowers are not always good field extractors of lead.

The most rational applications:

rhizofiltration of contaminated water;

experimental plots;

creation of large biomass;

metal availability assessment;

additional crop after site stabilization.

Do not use seeds, oil, cake, flowers or stems in contaminated areas.

8. Comparison of key plants

9. Lead is a special problem

Lead is one of the most dangerous and complex soil pollutants.

He:

does not decompose;

can persist for decades and centuries;

accumulates in the body;

especially dangerous for children;

damages the nervous system;

affects brain development;

disrupts hematopoiesis;

damages the kidneys;

increases cardiovascular risk;

may affect the reproductive system.

The main routes of lead exposure to humans

In practice, the danger often comes not from the absorption of lead by plants, but fromdirect exposure of contaminated soil and dust to the body.

10. Why are plants poor at extracting lead?

10.1 Low solubility

Lead binds to:

clay;

organic matter;

iron oxides;

manganese oxides;

carbonates;

phosphates;

sulfides.

Therefore, the total Pb content may be high, while the plant-available portion is relatively small.

10.2. Delay in the roots

Pb often:

is absorbed on the surface of the roots;

binds to cell walls;

precipitates in the rhizosphere;

remains inside the root;

poorly transferred to leaves and stems.

This makes plants good stabilizers but weak extractors.

10.3. The danger of artificial mobilization

If we chemically increase the solubility of lead, it will become more available to plants, but at the same time:

the risk of washout will increase;

drainage may be contaminated;

the metal can go deeper;

the risk of contamination of groundwater will increase.

Therefore, maximum plant absorption does not always mean increased safety.

11. Four Strategies for Working with Lead

Strategy 1: Removing Hot Spots

Applicable if the site contains:

battery waste;

pieces of slag;

metal shot;

remnants of old paint;

local areas with very high concentrations;

Pollution near children's and residential areas.

Methods:

selective extraction;

removal of large particles;

screening;

replacement of the top layer;

removal of contaminated material;

covering with clean soil.

For heavily polluted residential areas, this is more reliable than long-term phytoextraction.

Strategy 2. Phytostabilization

Main goal:

close the ground;

stop dusting;

reduce erosion;

retain Pb with roots;

reduce surface transfer;

limit direct human contact with the soil.

Suitable plants:

reed fescue;

red fescue;

ryegrass;

fieldfare;

cocksfoot;

rump;

miscanthus;

willow;

birch;

poplar;

sedges;

cane;

rug.

Strategy 3. Immobilization

The goal is to convert Pb into less soluble and less accessible forms.

Possible materials:

phosphate minerals;

hydroxyapatite;

lime materials;

biochar;

iron-containing sorbents;

zeolites;

bentonite;

some clay materials.

After this, the area is covered with vegetation.

Important: Dosage is determined by laboratory testing. For example, excess phosphates can create new environmental problems and affect the mobility of other pollutants.

Strategy 4. Phytoextraction

It is used where:

pollution is moderate;

some of the Pb is available to the roots;

biomass can be safely removed;

there is an opportunity to work for many seasons;

water and soil are controlled.

Possible crops:

mustard;

sunflower;

amaranth;

rapeseed;

willow;

mulberry;

certain species of Chenopodiaceae;

Corn as a non-food technical crop.

But for lead, phytoextraction is usually slow and should not be the only method of protection.

12. What is more important with lead: extraction or stabilization?

The main task with Pb is not to extract the maximum amount of metal at any cost, butfirst stop its flow to a person.

13. Optimal multi-level recovery system

Stage 1. Diagnostics

It is necessary to determine:

total pollutant content;

mobile fraction;

depth of contamination;

pH;

organic matter;

mechanical composition;

groundwater level;

drainage;

presence of hot spots;

content of pollutants in water.

The site is divided into zones:

clean;

moderately polluted;

heavily polluted;

wet;

dusty;

erosion-hazardous;

areas with local waste.

Step 2: Removing Concentrated Sources

The following are removed:

pieces of slag;

metal;

battery scraps;

contaminated building materials;

local areas with extreme concentrations.

Stage 3. Soil stabilization

If necessary, apply:

liming;

phosphate materials;

biochar;

mineral sorbents;

clean covering layer;

anti-erosion mats.

Step 4: Quickly close the surface

First mixture:

ryegrass;

reed fescue;

red fescue;

fieldfare;

cocksfoot.

Target:

stop dust;

to consolidate the soil;

reduce surface runoff;

create a root network.

Stage 5. Biological restoration

After stabilization, the following are introduced:

alfalfa;

clover;

chicory;

yarrow;

local herbs;

fireweed in suitable areas.

Stage 6. Perennial frame

Landing:

miscanthus;

willow;

poplar;

birch;

local shrubs;

sedges and reeds in damp places.

Stage 7. Experimental phytoextraction

In separate controlled plots:

mustard;

amaranth;

sunflower;

rapeseed;

short rotation willow.

After each cycle, the following is analyzed:

roots;

stems;

leaves;

seeds;

dry mass;

total pollutant removal;

residual soil;

mobile fraction;

drainage water.

Stage 8. Formation of a sustainable ecosystem

After risk mitigation:

preserve some of the local herbs;

plant bushes;

create forest belts;

form wet biofiltration zones;

gradually transfer the territory from technical reclamation to a sustainable ecosystem.

14. An example of functional division of plants

15. Plants for different types of pollution

Cadmium and zinc

Promising:

amaranth;

mustard;

certain types of rapeseed;

sunflower;

willow;

specialized hyperaccumulators.

Nickel

Promising:

specialized typesOdontarrhena;

certain types of mustard;

amaranth;

metal-resistant herbs.

Copper

Used:

mustard;

amaranth;

sunflower;

willow;

fescue;

Miscanthus.

Arsenic

A specialized plant is the bracken fern.Pteris vittataHowever, the chemical form of arsenic and soil conditions are crucial.

Oil and diesel

The main role belongs to rhizosphere microorganisms.

Suitable plants:

alfalfa;

ryegrass;

fescue;

clover;

miscanthus;

willow;

poplar.

PAHs and pesticides

Possible:

rhizodegradation;

phytodegradation;

joint work of plants and bacteria.

Suitable for:

cereals;

alfalfa;

willow;

poplar;

wetland plants.

Polluted waters

Used:

sunflower in controlled systems;

duckweed;

polyrhizome;

rug;

cane;

reed;

sedges;

Iva.

Radionuclides

The possibility of using plants depends on:

a specific isotope;

chemical form;

soil composition;

potassium and calcium content;

level of radiation hazard.

Working with radionuclides requires a special radiological project.

16. Treatment of contaminated wastewater

Example of a sequential biofiltration system:

settling tank for large particles;

mineral or sorption barrier;

cattail zone;

reed zone;

sedge and sedge zone;

willow strip;

polishing pond;

water sampling checkpoint.

For lead and other metals, plants should not replace technical filters. They act as an additional stage, stabilizing sediment and intercepting residual pollutants.

17. Disposal of contaminated biomass

Biomass from the remediation site cannot be:

to eat;

use as feed;

process into oil or flour;

make tea from it;

use in cosmetics;

compost with regular waste;

leave as mulch;

burn at the stake;

burn in a home stove;

use ash in the garden;

make homemade biochar.

Heavy metals do not disappear during combustion. They concentrate in the ash and can be carried away by smoke dust.

Possible ways of contacting:

controlled combustion with flue gas cleaning;

special burial;

ash stabilization;

extraction of metals from plant material;

transfer to a licensed organization.

18. Basic mistakes

Mistake 1: Thinking that any plant cleanses the soil

The presence of a plant in a contaminated area does not mean that it effectively removes the pollutant.

Mistake 2: Looking only at the concentration in the leaves

It is necessary to take into account the total dry mass and total removal from the site.

Mistake 3: Leaving contaminated biomass in place

The metal then returns to the soil after decomposition.

Mistake 4. Using chelators without supervision

It can increase metal leaching and pollute the water.

Mistake 5: Growing food plants in a remediation area

Even if the metal enters the fetus only slightly, there remains the danger of contaminated dust and soil.

Mistake 6: Using one plant

A system in which different species perform different functions works more reliably.

Mistake 7: Considering stabilization as a complete cleanup

Phytostabilization reduces the risk, but the contaminant remains in the soil.

19. Universal DREVO system

For a large disturbed area, a multi-stage model can be used.

The first level is security

analysis;

zoning;

hot spot removal;

access restriction;

dust prevention;

protection of waterways.

The second level is stabilization

mineral and organomineral sorbents;

dense perennial herbs;

anti-erosion coating;

buffer strips.

The third level is biological revitalization

alfalfa;

clover;

chicory;

local herbs;

beneficial bacteria;

mycorrhizal fungi.

Level 4 - Controlled Extraction

mustard;

amaranth;

sunflower;

willow;

other proven crops.

The fifth level is the formation of an ecosystem

fireweed;

local herbs;

shrubs;

willow;

birch;

poplar;

forest belts;

wet filtration zones.

20. Main conclusion

There is no single universal plant that cleans all types of dirt.

Each type performs its own task:

alfalfarestores soil biology, fixes nitrogen and creates a deep rhizosphere;

fireweedthe first to restore vegetation cover and initiate natural succession;

amaranthproduces high biomass and can extract cadmium and zinc;

mustardgrows quickly and is suitable for repeated seasonal cycles of phytoextraction;

sunflowerforms a powerful biomass and is especially interesting for water rhizofiltration;

cerealscreate a dense turf and stop pollination;

miscanthusprovides long-lasting non-food cover;

willow and poplarmanage water and form a perennial root system;

sedges, cattails and reedsoperate in wet filtration systems.

For lead, the most sensible strategy is:

remove concentrated sources → bind remaining metal → cover soil with dense vegetation → limit human contact → gradually extract the accessible portion → continuously monitor soil, water and biomass.

The goal of phytoremediation is not simply to grow plants on contaminated soil. It is to create a controlled system that consistently reduces toxicity, stops the spread of contamination, and restores the area to sustainable ecological function.

Andosol is already formed volcanic soil, while regolith is the initial loose mineral material from which the soil must still be created.