Phytoremediation

Phytoremediation

Phytoremediation— is the use of plants and root-associated microorganisms to clean, bind, or reduce the toxicity of contaminated soil, water, and sediments. It is used against heavy metals, petroleum products, some pesticides, solvents, and other pollutants.hero.epa.gov)

It's important to understand: plants don't always "destroy" pollution. Depending on the technology, they can:

extract pollutants from soil;

keep it in the roots;

prevent the spread of dust and erosion;

stimulate microbial decomposition;

absorb pollution from water;

partially transform organic compounds.

Main types of phytoremediation

Phytoextraction actually reduces the total metal content in the soil only when the aboveground biomass is regularly removed from the site. Phytostabilization, on the other hand, does not remove the metal, but rather reduces its mobility and the risk of contact.19january2017snapshot.epa.gov)

1. Phytoextraction

The plant absorbs the metal through its roots, transfers it to its stems and leaves, and then the biomass is mown down and removed.

A suitable plant should combine:

resistance to pollution;

the ability to absorb the desired element;

transfer of the element to the above-ground organs;

rapid growth;

sufficient biomass;

possibility of mechanized harvesting.

Hyperaccumulators

Hyperaccumulators are plants that can accumulate unusually high concentrations of certain metals without dying.

Examples:

A disadvantage of hyperaccumulators is their often low biomass. Therefore, a plant may contain a lot of metal per kilogram of dry mass but extract little metal per hectare.

Sometimes, fast-growing plants with moderate accumulation are more effective than hyperaccumulators:

willow;

poplar;

mustard;

sunflower;

corn;

miscanthus;

millet;

ryegrass.

Current reviews emphasize that for field effectiveness, both the metal concentration in the plant and the total biomass yield must be considered. (ScienceDirect)

2. Phytostabilization

This is the most realistic method for many sites contaminated with lead, mine tailings, or mixed metals.

Vegetation:

covers bare ground;

reduces dusting;

stops water and wind erosion;

retains contamination in the root zone;

gradually forms an organic horizon;

reduces direct contact of people and animals with the soil.

For this purpose, use:

perennial cereals;

fescue;

ryegrass;

soup;

reed grasses;

alive;

poplar;

a little;

pine;

shrubs resistant to local conditions.

Phytostabilization is often enhanced by adding biochar, lime, phosphate materials, clay, zeolite, or pure compost. However, the additive is selected after laboratory testing: one material can bind one metal while simultaneously increasing the mobility of another.

3. Rhizodegradation of petroleum products

For organic pollutants, the main work is often done not by the plant itself, but by microorganisms around the roots.

The roots are distinguished by:

Sahara;

organic acids;

amino acids;

enzymes;

dying cells.

This increases the number of bacteria and fungi that can break down hydrocarbons.

Plants with a dense root system are suitable:

ryegrass;

fescue;

alfalfa;

clover;

millet;

miscanthus;

willow;

poplar.

Petroleum hydrocarbons are generally more biodegradable than heavy metals. However, weathered heavy fractions and PAHs decompose slowly, sometimes taking years.hero.epa.gov)

4. Willow and Poplar

Willows and poplars are often used in phytotechnology due to:

rapid growth;

high biomass;

deep and branched root system;

ability to consume a lot of water;

simple propagation by cuttings;

possibilities of growing in short rotations.

They can be used for:

interception of contaminated water;

stabilization of metals;

stimulation of decomposition of petroleum products;

creation of protective strips;

gradual restoration of the soil structure.

However, they are not universal "all-metal pumps." A significant portion of the contaminant may remain in the roots or soil. Efficiency depends greatly on the variety, pH, depth of contamination, and availability of the metal.

5. Mustard and sunflower

These plants are often referred to as all-purpose cleaners, but their capabilities are usually exaggerated.

Mustard

Advantages:

grows quickly;

forms sufficient biomass;

easy to clean;

can absorb cadmium, zinc, copper and some other elements.

Limitations:

efficiency depends on the shape of the metal;

poorly extracts tightly bound lead;

may die if heavily contaminated;

requires annual sowing and mass removal.

Sunflower

Advantages:

large biomass;

developed roots;

convenient cleaning;

suitability for rhizofiltration of some polluted waters.

However, field results for lead are often much weaker than popular claims. It should be considered a component of a test mixture, not a guaranteed cleanup method.

6. Why is lead particularly difficult?

Lead often binds tightly to soil particles and organic matter. Therefore, its concentration in leaves can remain low even when it is present in high levels in the soil.

For lead it is usually more rational:

cover the soil with vegetation;

eliminate dusting;

adjust pH;

bind metal with sorbents;

do not grow food crops;

If the risk is high, remove or isolate the top soil.

Attempts to artificially increase lead availability through chelation may increase its migration into water and create a risk of groundwater contamination. This technology is only permissible as a controlled engineering measure.

7. The role of soil acidity

The mobility of many metals increases in acidic soil.

As the pH decreases, the following may become more available:

cadmium;

zinc;

nickel;

manganese;

copper.

Therefore, liming sometimes reduces metal uptake by plants. However, with phytoextraction, excessive metal binding reduces the rate of its removal.

There is a fundamental difference:

forsecurity and stabilizationmetals generally tend to bind;

forplant extractionssome accessibility needs to be maintained;

Increasing availability is permitted only if leaching and plant condition are monitored.

Absorption is also affected by organic matter content, clay type, moisture content, competing ions, and the chemical form of the metal itself. (mdpi.com)

8. Working with contaminated biomass

This is a mandatory part of the project.

After phytoextraction, plants are not considered as ordinary green mass, but as potentially contaminated material.

It is forbidden:

leave the mown mass on the field;

compost it with food compost;

use as mulch;

feed to animals;

make fertilizer out of it;

burn in a household stove;

processed into food or cosmetics.

Possible ways:

specialized combustion with flue gas cleaning;

controlled pyrolysis treatment;

disposal as contaminated material;

extraction of metals from ash;

phytomining of metals when economically feasible.

Even after combustion, metals do not disappear – they are concentrated in the ash.

9. Practical layout of the site

Stage 1. Examination

Soil samples are taken:

on a regular grid;

separately from suspicious spots;

at different depths;

near drains, buildings, roads and storage areas for materials.

Not only the total concentration is analyzed, but, if possible, the mobile fraction of pollutants.

Stage 2. Mapping

The site is divided into zones:

clean;

light pollution;

moderate;

strong;

local hot spots.

Heavily soiled stains are often more cost-effective to remove or isolate than to spend decades cleaning them with plants.

Stage 3. Trial plots

Before planting the entire area, several plots are made, for example:

perennial cereal mix;

willow;

mustard;

sunflower;

locally native plant;

control without plants.

After the season, they analyze:

soil;

roots;

stems;

leaves;

the amount of biomass obtained.

Stage 4. Main cycle

One cycle includes:

sowing → growing → analysis → removal of all contaminated mass → safe disposal → re-analysis of the soil.

For metals, many such cycles may be required. Phytoremediation is usually most suitable for mild to moderate surface contamination, where there is no immediate threat to people or groundwater. For acute risks, it is too slow.Nature)

10. Example of the DREVO plant system

First defensive tier

fescue;

ryegrass;

fieldfare;

lotus;

Clover - only if it tolerates pollution well.

Objective: to cover the soil and stop erosion.

Second extraction tier

In separate plots:

mustard;

sunflower;

specialized hyperaccumulators;

local metal-resistant plants.

Objective: To check the actual extraction of each metal.

The third tree tier

willow;

poplar;

birch;

alder on suitable soils.

Objective: deep stabilization, water interception and microclimate formation.

Microbial layer

local rhizosphere bacteria;

mycorrhizal fungi;

wood chips for organic contamination;

clean mature compost in a controlled dosage.

What can you grow after cleaning?

The transition to food plants is determined not by the number of years that have passed, but by repeated analyses.

Until the site is officially confirmed to be safe, it is best to use it for:

technical grasses;

ornamental plants;

energy wood;

protective forest belts;

seed material only if safety is confirmed;

biodiversity without grazing animals.

Particularly undesirable on contaminated soil:

leafy greens;

roots;

mushrooms;

medicinal raw materials;

forage plants;

Plants for baby food.

The main conclusion

Phytoremediation is not a one-time planting of a “cleansing plant,” but a controlled engineering-biological system:

Pollution analysis → mechanism selection → plant selection → water and soil monitoring → regular biomass removal → safe disposal → repeat analyses.

Most reliable application:

stabilization of large, slightly polluted areas;

dust and erosion control;

gradual extraction of cadmium, zinc and nickel;

restoration of oil-contaminated soils in conjunction with microorganisms;

creating a vegetation barrier around more polluted areas.