🧫 Bioremediation and Phytoremediation

This lesson explains key concepts in a structured way and connects them to practical agricultural applications and exam-oriented understanding.

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Bioremediation

Bioremediation is the use of living organisms — primarily bacteria, fungi, and algae — to degrade, transform, or immobilize environmental contaminants in soil and water. It is an eco-friendly, cost-effective alternative to physical and chemical remediation methods.

Types of Bioremediation

1. In-situ bioremediation — treatment of contaminated soil in place without excavation

   • Biostimulation — addition of nutrients (N, P) and oxygen to stimulate the growth and activity of indigenous soil microorganisms that degrade pollutants
   • Bioaugmentation — introduction of specific pollutant-degrading microbial strains into the contaminated soil
   • Bioventing — supplying oxygen to the subsurface to enhance aerobic biodegradation of petroleum hydrocarbons

2. Ex-situ bioremediation — contaminated soil is excavated and treated elsewhere

   • Land farming — excavated soil spread in thin layers on a prepared surface; periodically tilled, irrigated, and fertilized to enhance microbial degradation
   • Biopiles — excavated soil heaped with aeration, moisture, and nutrient systems
   • Bioreactors — contaminated soil slurried in controlled vessels with optimized conditions

Microbial Agents

• Pseudomonas species — degrade hydrocarbons, pesticides, and aromatic compounds
• Bacillus species — degrade hydrocarbons and heavy metal resistant
• Trichoderma and Aspergillus — fungal species that degrade pesticides and aromatic compounds
• Mycorrhizal fungi — enhance plant uptake of heavy metals in phytoremediation systems
• White-rot fungi (Phanerochaete chrysosporium) — produce lignin-degrading enzymes (laccase, peroxidase) that break down a wide range of persistent organic pollutants

Phytoremediation

Phytoremediation uses plants and their associated rhizosphere microorganisms to remove, contain, or detoxify contaminants from soil and water.

Mechanisms

1. Phytoextraction — plants absorb contaminants (especially metals) through roots and accumulate them in shoots and leaves; the harvestable biomass is then removed and disposed of safely
2. Phytostabilization — plants immobilize contaminants in the root zone through absorption, adsorption, or precipitation, preventing their spread by erosion, leaching, or wind
3. Phytodegradation — plants and associated microbes metabolize organic contaminants within plant tissues or in the rhizosphere
4. Rhizofiltration — plant roots absorb, concentrate, or precipitate contaminants from water flowing through the root zone
5. Phytovolatilization — plants absorb contaminants and release them as volatile forms through transpiration (e.g., selenium, mercury)

Hyperaccumulator Plants

Hyperaccumulators are plant species that can accumulate extraordinarily high concentrations of metals in their shoots without showing toxicity symptoms:

Metal	Hyperaccumulator Species	Threshold Concentration
Nickel	Alyssum bertolonii, Berkheya coddii	> 1,000 mg/kg
Zinc	Thlaspi caerulescens	> 10,000 mg/kg
Cadmium	Thlaspi caerulescens, Sedum alfredii	> 100 mg/kg
Arsenic	Pteris vittata (brake fern)	> 1,000 mg/kg
Lead	Brassica juncea (Indian mustard)	> 1,000 mg/kg

Indian mustard (Brassica juncea) is particularly notable as it is a familiar crop in India with well-developed agronomic practices and moderate phytoextraction capability for Pb, Cd, and Zn.

Biochar

Biochar is a carbon-rich material produced by the pyrolysis (thermal decomposition in the absence of oxygen) of biomass such as crop residues, wood, manure, or agricultural waste at 300–700 degree C.

Role in Soil Remediation

• Heavy metal immobilization — biochar has a high cation exchange capacity and large surface area that adsorbs heavy metals (Cd, Pb, Cu, Zn), reducing their bioavailability and plant uptake
• Pesticide sorption — biochar adsorbs organic pollutants, reducing their mobility and bioavailability
• pH amendment — biochar is typically alkaline and can raise the pH of acid soils, reducing Al and Mn solubility
• Improved soil health — increases water holding capacity, supports microbial communities, and enhances nutrient retention
• Carbon sequestration — biochar is highly resistant to decomposition and can sequester carbon in soil for hundreds to thousands of years

Application Rates

Typical biochar application rates range from 5 to 20 tonnes per hectare, depending on the contaminant level, soil type, and biochar properties. Research at Indian agricultural universities has demonstrated the effectiveness of rice husk biochar, sugarcane bagasse biochar, and wood biochar in improving contaminated soils.

Biochar-based remediation is gaining significant attention as a sustainable, low-cost approach for managing contaminated soils, particularly in developing countries where advanced remediation technologies are often unaffordable.

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Summary Cheat Sheet

Key Recall Points

• Bioremediation relies on microbes; phytoremediation relies on plants for pollutant risk reduction.
• Method choice depends on contaminant type, depth, and project time horizon.
• Integrated biological approaches can reduce remediation cost and ecological damage.

Exam Traps

• Fast removal is not always feasible with biological methods; timelines vary widely.
• Hyperaccumulator performance is species- and site-specific.
• Biological remediation still needs monitoring of residual contamination.

References

3 sources • [1] [2] [3]

[1]

Environmental Biotechnology References on Bioremediation Mechanisms

Book

[2]

Phytoremediation Literature for Metal and Organic Pollutants

Journal

[3]

ICAR and University Notes on Biological Remediation in Agriculture

Official